Open Access Article
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Advancement of bottom-up precipitation synthesis and applications of barium sulphate nanoparticles

Megawati Zunita*, Dendy Adityawarman, Eveline Iskandar and Rahadian Rizky Ramadhan
Chemical Engineering Department, Faculty of Industrial Technology, Institut Teknologi Bandung, Jalan Ganesha 10, Bandung, West Java 40132, Indonesia. E-mail: m.zunita@itb.ac.id

Received 13th April 2025 , Accepted 2nd July 2025

First published on 22nd July 2025


Abstract

Barium sulphate nanoparticles (BaSO4 NPs) have garnered considerable attention owing to their distinctive physicochemical characteristics and varied uses in the polymer industry and medicine. Their synthesis by the bottom-up precipitation process is significantly affected by capping agents, which are crucial for regulating shape and size. BaSO4 nanoparticles exhibit significant potential in medical imaging, reinforcing of bone cement, and administration of cancer therapeutics, while simultaneously improving the tensile strength, thermal stability, and impact resistance of polymers. This review paper examines the influence of diverse capping agents, including polymers, surfactants, and organic compounds, on the shape and size modulation of nanoparticles. It also analyses reactor modifications, such as rotating packed beds, T-mixers, membrane reactors, and spinning disc reactors, to enhance size distribution. This review also emphasises the prospective applications of BaSO4 nanoparticles in medical imaging, targeted medication administration, energy storage, and composite materials. Green synthesis techniques and ionic liquid-assisted methods are examined to enhance the sustainable manufacture of BaSO4 nanoparticles.


1. Introduction

Recently, nanotechnology has undergone many developments in its implementation in various industrial sectors. In general, nanotechnology can be defined as an applied science specifically implemented to manipulate materials on a nanometer scale.1 The main objective of nanotechnology is to design, enhance, and employ structures at nanometer scale to develop novel materials or improve the efficiency of currently available technologies.2,3 The development of nanotechnology is mainly related to the synthesis and modification of nanomaterials. Nanomaterials can be composed of metals, ceramics, or in polymer composites.4–6 The petite size of nanomaterials causes significant changes from the base materials physical, chemical, and mechanical properties.7,8 These distinctive properties of nanomaterials are leveraged to create more sophisticated technologies. In addition, to the extensively researched carbon nanotubes and fullerenes, the domain of nanomaterials includes a wide variety of forms and compositions, each displaying distinct properties and functionalities. Nanomaterials, with dimensions within the nanoscale range (1–100 nm), exhibit unique properties due to their enhanced surface area to volume ratio and quantum mechanical effects. These properties are attributed to the increased ratio of surface atoms to inside atoms. Nanoparticles, nanowires, and nanotubes are examples of materials with desirable properties at the nanoscale. Nanomaterials can be categorized based on dimensionality, chemistry, and shape. One-dimensional nanomaterials, such as thin films and surface coatings, are used for surface modification and protection. Two-dimensional nanomaterials, like nanowires and nanorods, are used in electronic devices and sensors. Zero-dimensional nanomaterials, like quantum dots and colloidal nanoparticles, are used in drug distribution, bioimaging, and catalysis.

Barium sulfate nanoparticles are one of the metal-based nanomaterials that has been widely explored and developed, proven by the increment of number of studies on barium sulphate nanoparticles every year. The graph in Fig. 1 sums up the quantity of studies conducted to examine barium sulphate nanoparticles.


image file: d5ra02597d-f1.tif
Fig. 1 Number of publications related to barium sulfate nanoparticles bottom-up synthesis and barium sulfate nanoparticles applications indexed by Scopus, Queries (TITLE-ABS-KEY (“synthesis of barium sulfate nanoparticles”), TITLE-ABS-KEY (“green synthesis of barium sulfate nanoparticles”), and TITLE-ABS-KEY (“barium sulfate nanocomposites”)); accessed on: August 21st, 2024.

Barium sulfate is an unique material due to its physicochemical properties, such as relatively high specific gravity (4.5), opaqueness to X-rays, and inertness.9,10 Barium sulfate nanoparticles have up-and-coming development prospects in various technological applications. Barium sulphate can be used as an important filler material in the plastics, rubber, printing inks, and paint industries.11–14 To gain more benefit from this material, barium sulfate has been developed as a nanomaterial through various physical and chemical methods. Barium sulfate nanoparticles have highly promising prospects for development in various technological applications, including as an inert contrast agent and shielding material in advanced medical imaging technology and targeted alpha-therapy, battery efficiency improvement, energy storage systems, and composite material additives.15–20 This applications is achievable because barium sulfate nanoparticles tend to have a large surface area with diverse morphology depending on its production method.21–23

Generally, nanoparticle synthesis can be divided into top-down and bottom-up route.24,25 As shown in Fig. 2, bottom-up approach entails building larger nanostructures from smaller atoms and molecules whereas the top-down approach involves scaling down materials to the nanoscale.26 Top-down synthesis route typically using physical method to obtain nanosize particles from bulk materials, such as ball milling and laser ablation.27,28 These top-down methods have high production batch capacity and usually no need to chemically purify the products. However, it requires more sophisticated technology and consumes a lot of electricity. In contrast, bottom-up approach to nanoparticle synthesis is favored for creating nanoscale materials due to its ability to yield more uniform sizes, shapes, and distributions compared to the products of top-down synthesis methods.29


image file: d5ra02597d-f2.tif
Fig. 2 Illustration of top-down and bottom-up approaches of nanoparticles synthesis.

The bottom-up synthesis of barium sulphate nanoparticles is mainly classified to two methods: microemulsions and precipitation. The microemulsions method involves the synthesis of nanoparticles within nanodroplets of a water-in-oil (w/o) microemulsion system consists of water, oil (typically cyclohexane), and surfactants. The method utilizes the unique properties of microemulsions as nanoreactors where two reactants, i.e. BaCl2 and K2SO4 would be mixed at the droplet interfaces, leading to the precipitation of nanoparticles.30 As illustrated in Fig. 3a, the production of barium sulfate nanoparticles in a semi-batch stirred reactor involves two microemulsions: a stock microemulsion containing encapsulated K2SO4 and a feed microemulsion containing BaCl2. The interaction between these microemulsions leads to droplet fusion, which serves as the chemical reaction step for the formation of liquid BaSO4. When the number of molecules reaches a critical threshold (Ncrit) necessary for stability through nucleation in the fission step, the particle growth rate is controlled by the consumption of liquid BaSO4 molecules (C).


image file: d5ra02597d-f3.tif
Fig. 3 (a) Scheme of BaSO4 nanoparticles production process using microemulsions system; TEM of BaSO4 nanoparticles with various microemulsions concentration: set 1 with K2SO4 = 0.1 mol L−1 and BaCl2 = (b) 0.1 mol L−1, (c) 0.075 mol L−1, (d) 0.05 mol L−1, (e) 0.025 mol L−1, (f) 0.01 mol L−1, (g) 0.005 mol L−1; set 2 with BaCl2 = 0.1 mol L−1 and K2SO4 = (h) 0.1 mol L−1, (i) 0.075 mol L−1, (j) 0.05 mol L−1, (k) 0.025 mol L−1, (l) 0.01 mol L−1, (m) 0.005 mol L−1 (reproduced from ref. 30 with permission from Otto-von-Guericke Universität Magdeburg, copyright 2007).

This method has several benefits, including refined size and distribution control due to droplet fusion and fission reaction in surfactant's protective monolayer, high interfacial area, and efficient tuning by adjusting surfactant concentration, stirring rates, and reactant ratios.31 Based on the observation of the TEM results of BaSO4 nanoparticles gained from various concentration ratio in Fig. 3b–m, the decrement of K2SO4 or increment of BaCl2 would affect the primary chemical reaction stoichiometry condition and the particles' morphology. If the reaction occurs at relatively balanced stoichiometric feed ratio, the particle size of BaSO4 would be smaller (6 nm). Conversely, BaSO4 particles formed under a non-stoichiometric reaction system would have larger mean diameter (up to 31 nm) due to lower supersaturation condition. This deviation in stoichiometric feed ratio would alter the shape of obtained BaSO4 nanoparticles from spherical to cubical structure. Hence, this method has more precise parameters for tuning particle size and shape. However, microemulsions method of nanoparticles synthesis also has some drawbacks, such as complexity of phase behavior, dependency on the stability of surfactant systems, narrow operating condition windows, and limitation of size tuning in highly-viscous mixture systems that may affect the product's quality in large-scale synthesis. On the other hand, the precipitation method has been widely used for bottom-up synthesis of barium sulfate nanoparticles with high purity.32 This method involves the direct mixing of barium and sulfate solutions under constant stirring to yield BaSO4, which has low solubility in water, as represented by the following reaction:

Ba2+(aq) + SO42−(aq) → BaSO4(s)

Similar to the microemulsion method, several key parameters can be adjusted to prevent excessive growth or agglomeration of nanoparticles, such as concentration ratio, temperature, mixing rate, and pH. Therefore, the precipitation method is preferred for its simplicity and economic feasibility compared to other approaches.33,34 This method is also relatively more flexible to be established on a commercial scale for continuous production.35,36 Hence, the precipitation method is more commonly utilized in research on barium sulfate nanoparticles compared to other methods. This statement is supported by the data of the number of publications regarding to various nanoparticle synthesis methods, as presented in the subset of Fig. 1.

Nanoparticles precipitation usually requires a capping agent with specific functional group to prevent overgrowth and agglomeration of nanoparticles.37 Some of these experiments implies that synthesis using different capping agents leads to alteration of nanoparticle structures.38–40 Capping agent also influence the dispersion characteristics of barium sulfate nanoparticles in solvents.41 Some studies revealed that different concentration of capping agent and pH would results in some variety of size and morphology of nanosize final products.42–45 Nanoparticle synthesis can be conducted by using specific ionic liquid (IL). Ionic liquid developed as an alternative template in the synthesis of barium sulfate nanoparticles to pursuit an environmentally friendly approach.46

Nanoparticles synthesis with the precipitation method can be carried out in various types of reactors. Several types of reactors have been widely developed and modified for barium sulfate nanoparticle precipitation, including capillary microreactor, membrane reactor, spinning disk reactor, and rotating packed bed (RPB).32,47–51 The difference in reactor types affects the size of the barium sulfate nanoparticles produced from the precipitation process. This differences occur mainly because of the different types and sizes of reactors affect the homogeneity and diffusion of raw materials during the barium sulfate nanoparticle manufacturing process.52

In this paper review, several synthesis using precipitation methods to obtain barium sulfate nanoparticles (BaSO4 NPs) with capping agents have been summarized. We compared and discussed how the influence of variations in capping agents and types of reactors affect the morphology and mean particle diameter of barium sulfate nanoparticles. We also emphasized the advantages and applications of these nanoparticles in various sectors, including enhanced mechanical strength, improved thermal stability, and other beneficial features in the medical and polymer industry. Some green synthesis methods are mentioned in separated chapter of this paper review to enhance the discussion about environmentally friendly barium sulphate nanoparticles synthesis method, which is considered a prominent topic in recent research related to chemical synthesis.

2. Synthesis of BaSO4 nanoparticles with precipitation method

Precipitation is one of the most common methods to synthesize barium sulfate nanoparticles. This synthesis technique has many advantages, such as the simplicity of the mechanism, cost and energy effectiveness, easier to scaled-up, and faster reaction.53,54 However, precipitation method has several drawbacks, including difficulty to maintain quality control over particle size and distribution and excessive agglomeration due to van der Waals force on the nanoparticles.55,56 The precipitation method is carried out by dissolving a constituent material in a solvent before added with another feed solution and stirred continuously at certain rate. This action would bring the solution into saturated state as a precursor, leading to nucleation of specific nanoparticles precipitate according to the reactants.57 The precipitation of nanoparticles usually involves a compound to tune and stabilize the nucleation and growth of the nanoparticles. This compound is known as capping agent. Some capping agents often used in precipitation of barium sulfate nanoparticles are surfactants and various organic polar compounds. These capping agents have the capability to hinder further growth of barium sulfate particles so that their particle size distribution becomes narrower and smaller while also influencing their morphology.58

2.1 BaSO4 nanoparticles precipitation involving organic compounds

Several organic compounds have been utilized as precipitating and capping agents in barium sulfate nanoparticle synthesis. Bala et al. (2005) reported BaSO4 nanoparticles synthesis in a water–ethanol medium. This experiment was done by preparing 20 mL of 0.5 M BaCl2 solution and 20 mL of absolute ethanol before adding 10 mL of 1.0 M (NH4)2SO4 into the solution.59 The self-dispersed BaSO4 nanoparticles obtained from this precipitation method have an elliptical shape and a mean particle size of 36 nm, as presented in Fig. 6a. These nanoparticles can disperse with hydrodynamic diameter of 24.3 nm in ethanol suspension and can expand by dispersion in water to 86.5 nm. The morphology of BaSO4 nanoparticles from this study is not completely round as it have some mesopores around 6–8 nm, giving some leverage up to 1.12%-wt of ethanol absorption and consequently rising their ability to disperse in organic solution.60 The inhibition of BaSO4 crystal growth was due to ethoxide group linkage from ethanol on their surface which forms a protective layer.61

A later study conducted by Ramaswamy et al. (2011) confirms that compositions of ethanol/water mixed solvents in barium sulfate nanoparticles precipitation would significantly influence the average size of nanoparticles.62 The result shows declining tendency of BaSO4 nanoparticle size from 85 nm to 54 nm due to increment of ethanol in water–ethanol solvent from 30% to 70%. A more spherical shape of BaSO4 nanopowder is achieved by increasing the ethanol composition in the solvent. The same group also investigated different approaches of BaSO4 nanoparticles precipitation in water-benzene solvent system.63 This approach was chosen in consideration that benzene is more difficult to dissolve the reactants (BaCl2 and Na2SO4 solution) than water, thus the crystallization in this reaction is more reachable.64 The optimal condition for BaSO4 nanoparticles in this experiment is achieved by addition of 95% benzene in water mixed solvent system with an average particle size of 35.995 nm. Although the particle size is still bigger than the nanoparticles obtained from pure water solvent (30.466 nm), 95%-benzene in water solvent system still chosen as the optimal ratio since it has better agglomeration inhibition and homogeneous distribution of the BaSO4 nanoparticles. This statement is supported by visual analysis using FESEM images of BaSO4 nanoparticles produced with addition of pure water solvent and 95%-v/v benzene–water mixed solvent in Fig. 6b and c, respectively.

Meanwhile, Boguslavskii et al. (2015) developed an barium sulfate precipitation technique using water–tetrahydrofuran (THF) solvent system.65 In a nonequilibrium system of water–THF solvent, 0.01 mol per L Na2SO4 were introduced to Ba(NO3)2 to create an interphase transition layer until their final concentration is at 7.7 × 10−2 mol L−1 and 4.35 × 10−3 mol L−1, respectively. As illustrated in Fig. 4, the precipitation reaction mechanism involves several key points of nanoparticles dynamics in the transition layer, including the diffusion between solvent and the reactants, leading to the occurrence and dynamic movements of Bernard cells where the nucleation of BaSO4 is bound to happen. In the context of precipitation reaction of BaSO4, Bernard cells refer to molecules that undergo convective motion in a specific vortex-like pattern due to the temperature gradient in a system, instigate alteration of the particles' morphology.66,67 Interphase transition layer stabilization in this reaction is also affected by intensified mass transfer because of surface tension gradient of the interface layer between two phases, known as Marangoni effect.68–70 These surface tension gradients would generate a protective property, leading to selective reaction of BaSO4. As shown in Fig. 6d, the optimal molar ratio of H2O/THF to obtain homogenous BaSO4 nanoparticles (67 nm) with smallest deviation is at 1[thin space (1/6-em)]:[thin space (1/6-em)]1.


image file: d5ra02597d-f4.tif
Fig. 4 BaSO4 nanoparticle precipitation scheme in water–THF system.

In order to enhance the inhibiting effect of nanoparticles synthesis additives, Shen et al. (2007) developed a barium sulfate nanoparticles modifier by evaluating several organic acids.71 This BaSO4 modifier is prescribed with three approaches. The first approach was carried out by adding 0.1 M Na2SO4 directly to 0.1 M BaCl2 both in ethanol–water solvent system with ethanol/water volume ratio 1[thin space (1/6-em)]:[thin space (1/6-em)]1, then inserting a specific amount of stearic acid. As a result, the optimal amount of stearic acid to minimize the BaSO4 nanoparticles' size is obtained at 3%-wt. The second approach is done by tuning the pH of BaCl2 to 8–9 before added by Na2SO4 solution dropwise to attained better dispersion while the third approach was carried out by the similar condition to second approach with exception the pH of Na2SO4 is also adjusted to the same amount with BaCl2. This adjustment was done to gain smaller BaSO4 nanoparticles by tuning the supply of Ba2+ ion in the reaction system. The final result of the 3rd approach reaction can be seen in Fig. 6e.

In this precipitation reaction, organic acids act as an inducer. The addition of sulfate ion in the solution would make the barium ion surplus, thus it is also the main driving force of the precipitation while organic acid induces barium sulfate nanoparticles formation by attracting barium ions through electrostatic forces.72 These organic acids is also remarkably inhibit the nanoparticles' growth due to steric hindrance. When the pH adjustment was done by adding ammonia to the solution, organic acids would emit their RCOO linkage which then absorbed on the surface to prevent agglomeration of BaSO4 nanoparticles. The overall optimal reaction scheme in third approach as illustrated in Fig. 5 can be described as follows:

 
SO42− + Ba2+ → BaSO4 (1)
 
RCOO + Ba2+ → P(RCOO)2Ba (2)
 
RCOOH + NH3·H2O → RCOO + NH4+ + H2O (3)


image file: d5ra02597d-f5.tif
Fig. 5 Scheme of BaSO4 nanoparticles precipitation modified by organic acids (reproduced from ref. 71 with permission from Springer, copyright 2007).

The most suitable amount of tetradecanoic acid, hexadecanoic acid, and stearic acid to garnered the smallest nanoparticles by the third approach with average size of 25 nm, 20 nm, and 16 nm is 6%-wt, 4%-wt, and 3%-wt, respectively.

Another experiment conducted by Jones et al. (2001) shows that both nitrilotriacetic acid (NTA) and nitrilotrimethylenephosphonic acid (NTMP) can be utilized as barium sulfate complexing agents. 0.017 mM NTMP mixed into 0.25 mM BaCl2 and Na2SO4 solution in a system with pH of 5.6 would form thin and rounded BaSO4 nanoparticles with NTMP molecule[thin space (1/6-em)]:[thin space (1/6-em)]barium atoms ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]15.73 Meanwhile, adding 0.078 mM NTA to the same system would produce BaSO4 nanoparticles with NTA molecule[thin space (1/6-em)]:[thin space (1/6-em)]barium atoms ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]3 and morphology transformation at a carboxylate[thin space (1/6-em)]:[thin space (1/6-em)]barium ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]5. NTA performance as a inhibitor shows more particles homogeneity in terms of their monodispersity and adsorption on their surface. In attempt to enhance their beneficial properties, further study was done by Jones et al. (2006) to investigate BaSO4 nanoparticles formation involving nitrilotriacetic acid (NTA) in the precipitation process.74 Based on this study, NTA effectively inhibits barium sulfate precipitation at moderate pH levels, specifically barium complexion below 10% for pH < 7. The morphology transformation of the obtained BaSO4 nanoparticles happens at carboxylate[thin space (1/6-em)]:[thin space (1/6-em)]Ba ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]10 with an average particle size of 208 nm. The presence of NTA increases the induction time of barium sulfate precipitation, suggesting a decrease in nucleation rate, which is likely due to an increase in interfacial free energy.75 SEM image of BaSO4 in Fig. 6f reveals that the morphology of barite particles synthesized in the presence of NTA exhibits significant alterations as the temperature increases from 25 °C to 60 °C. At the lower temperature, the particles are approximately spherical. However, as the temperature rises, they transform into rhombic shapes, akin to the morphology observed in barite formed under conditions of very low supersaturation.76 Hence, the optimal temperature for BaSO4 inhibition must be lower than 60 °C.


image file: d5ra02597d-f6.tif
Fig. 6 (a) TEM images of BaSO4 nanoparticles in ethanol–water medium (reproduced from ref. 59 with permission from Elsevier, copyright 2005); (b) and (c) FESEM images of BaSO4 nanoparticles using pure water solvent and 95%-v/v benzene–water mixed solvent, respectively (reproduced from ref. 63 with permission from VBRI Press, copyright 2012); (d) TEM image of BaSO4 particles at phase interface with H2O[thin space (1/6-em)]:[thin space (1/6-em)]THF ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (reproduced from ref. 65 with permission from Springer, copyright 2015); (e) TEM images of BaSO4 nanoparticles obtained from 3rd approach with addition of 3%-wt stearic acid (reproduced from ref. 71 with permission from Springer, copyright 2007); (f) TEM images of BaSO4 nanoparticles formation in the presence of NTA (reproduced from ref. 74 with permission from the Royal Society of Chemistry, copyright 2006); (g) TEM images of BaSO4 nanoparticles obtained from synthesis with EDTA at pH = 4.5 (13 nm); (h) pH = 6.5 (23 nm); (i) pH = 9.5 (73 nm); (j) pH = 11.5 (89 nm) (reproduced from ref. 43 with permission from Elsevier, copyright 2003).

Aside from the mentioned organic compounds above, ethylenediaminetetraacetic acid (EDTA) also garnered interest of many researchers to facilitate BaSO4 precipitation in order to gain more controlled particle size of BaSO4. EDTA is a water-soluble chleating agent that has high intensity of interaction with metal ions (i.e. Ba2+) by using its unique structure which consists of four carboxylate and two amine groups.77 Li et al. (2003) synthesized BaSO4 nanoparticles by mixing 2.19 g of BaCl2 and 3.21 g of EDTA in 100 mL of distilled water then adding 100 mL of 0.1 mol per L Na2SO4 to obtain white BaSO4 precipitate.43 This precipitation reaction can be described as follows:

 
Ba2+ + EDTA ↔ Ba-EDTA (4)
 
Ba-EDTA + SO42− ↔ BaSO4 + EDTA (5)

The nanoparticles' growth is controlled by forming Ba-EDTA intermediate complex. This intermediate presence would directly reduce free barium ions that can bond with sulfate ion to stabilize the nuclei. EDTA also acts as an inhibitor by building a barrier to ensure the steric hindrance of the nanoparticles in order to prevent agglomeration with steric energy approximately −56.38 kcal mol−1.78,79 From this experiment, it is concluded that one of the most significant factors in BaSO4 nanoparticles synthesis using EDTA is pH. Reaction with higher pH leads to limitation of free barium ions so that the nanoparticle growth mechanism is more dominant than nucleation.80 This statement is supported by characterization of BaSO4 nanoparticles obtained from synthesis with EDTA at varying pH using TEM as shown in Fig. 6g–j.

Chen et al. (2008) conducted a similar co-precipitation experiment by using 4.0 mmol per L Ba(NO3)2, 4.0 mmol per L K2S2O8 and 8.0 mmol per L EDTA with N2-bubbling apparatus and γ-ray radiation.81 The final product is BaSO4 microspheres with an average particle size of 2–3 μm. These microspheres are composed of many quasi-spherical nanoparticles and nanorods. In this precipitation method, water molecules were irradiated so that they generated hydrated electrons, which would reduce peroxydisulfate ion to escalate the sulfate ion rate constant.10,82 Meanwhile, the barium ion rate constant was tuned by EDTA complex. This result confirms the previous study of BaSO4 nanoparticles precipitation using BaS and Na2SO4 as precursor and EDTA as modifying agent.83 The BaSO4 nanoparticles obtained from this simple direct precipitation have an average size of 16 nm.

Romero-Ibarra et al. (2010) developed a novel approach of BaSO4 nanoparticles synthesis with EDTA solvent system and dimethyl sulfoxide (DMSO).45 The morphology of BaSO4 aggregates varied significantly with pH and EDTA concentration. At pH 4, spherical agglomerates of around 500 nm were observed, while at higher pH, the agglomerate size decreased, and the spherical shape was deconstructed. This phenomenon happens due to EDTA dissociation in a system with high alkalinity, ensuring the surplus of anion for further reaction by surface absorption to increase the intensity of repulsive force, which is beneficial for particle size tuning purposes.84 It should be noted that the effectiveness of EDTA as a chelating agent can be diminished because of alterations in its pKa values, since EDTA in a system with low pH are under the influence of carboxylic functional group.85 BaSO4 synthesis in 5% water–DMSO system in presence of EDTA produces quasi monodisperse fibers with an average diameter less than 200 nm. These observations suggest that water plays a crucial role as a pH buffer in enhancing barium sulfate nanoparticle surface absorption mechanism by EDTA, consequently transforms their morphology from spherical to mesocrystalline fibres.22 HRTEM images revealed that the fibers are composed of several oriented primary particles, suggesting a hierarchical organization along the [001] and [010] directions. All BaSO4 samples were crystallized in an orthorhombic structure, with crystallite sizes estimated to be about 4 nm for spherical agglomerates and 12 nm for fibres.

Based on this results, Romero-Ibarra et al. proposed a mechanism based on multipolar attractions and a brick-by-brick organization of primary particles, facilitated by the preferential adsorption of EDTA on certain surfaces of the BaSO4 nanoparticles as follows:

 
2Cl(ac) + Ba(ac)2+ + EDTA(ac)4− + 2Na(ac)+ + 2H(ac)+ ↔ [Ba-EDTA](ac)2− + 2Na(ac)+ + 2Cl(ac) + 2H(aq)+ (6)
 
EDTA4˙ + jH+ ↔ HjEDTA4˙j (7)
 
[Ba-EDTA]2˙ + H+ ↔ [BaHEDTA] (8)
 
image file: d5ra02597d-t1.tif(9)

The significance of pH value in BaSO4 precipitation with EDTA was later confirmed by further study conducted by Akyol and Cedimagar (2016).42 Direct precipitation of BaSO4 nanoparticles using BaCl2 and Na2SO4 with addition of 0.1 mol per L EDTA in a system with pH = 6 would produces BaSO4 crystals with a mean particle size of 0.44 ± 0.09 μm, relatively smaller than the products gained from the same system with pH = 4 at an average diameter of 0.91 ± 0.36 μm. Further increment of pH value to 8 would results in smaller nanoparticles with an average size of 0.28 ± 0.05 μm. These results is aligned with EDTA tendency to has strong affinity to barium ions and endothermically adsorbed on the particles' surfaces.86

2.2 BaSO4 nanoparticles precipitation involving polymers

Aside from organic compounds, many BaSO4 nanoparticles precipitation experiments have been conducted with addition of various polymers. In these experiments, the steric effect of the polymer is mainly used to prevent the reaction of other particles with the nuclei.87 Qi et al. (2000) reported the effects of double-hydrophilic block copolymers to the morphology and size of BaSO4 nanocrystals.39,88 This synthesis of BaSO4 nanoparticles using 0.5 M Ba(OAc)2, 0.5 M (NH4)2SO4, and 1 g L−1 of various polymers shows significantly different shape and size. Fig. 7b reveals that poly(methacrylic acid) (PMAA) addition in the system would makes the final product have a rod-like shape with average length of 0.8 μm, composed of 22 nm crystallites due to its carboxylic acid group function hydrogen bond with sulfate ion.89 These products has significantly different size and morphology compared to non-additive products in Fig. 7a. The modification of polymer to PEG-b-PMAA would results in morphology changes to peanut-like shape with particle length of 0.8 m consists of 14 nm crystallites at pH 9 meanwhile peaches-like BaSO4 particles were obtained with average size of 2 μm composed of 18 nm crystallites at pH 5. Fig. 7c and d show that substituting the polymer to PEG-b-PMAA-Asp and PEG-b-PEIPA would produce different BaSO4 products consisting of 27 nm and 19 nm crystallites, respectively. The interaction between phosphonate and carboxylic acid group function of PEG-b-PMAAP polymer chain leads to stronger BaSO4 nanofibers bonds with length of some tens of micrometers and an average diameter 20–30 nm, as shown in Fig. 7g and h.90 This morphology variation tendency is related to the bonds between copolymers which would increase in higher pH system owing to the domination of carboxylate groups ionization, hence the increasing repulsive electrostatic bonds.91 Meanwhile, BaSO4 synthesis with PEG-b-PEIPSA produces 10 μm BaSO4 particles with flower-like shape consists of ten petals from secondary crystalline layers. Although the BaSO4 particles is still relatively bigger in size, this discovery becomes the basis for the development of barium sulphate nanoparticles synthesis in the presence of polymer templates. The same research team continued the study of BaSO4 nanofibres synthesis method using PEG-b-PMAA-PO3H2.40 SEM and TEM images in Fig. 7e and f shows that BaSO4 synthesis experiment with similar method as their previous study produces long BaSO4 nanofibres which composed of nanofilaments with average diameter 20–30 nm, same as their experiment using PEG-b-PMAA. The block structure of PEG-b-PMAA-PO3H2 improves the stabilization of precursor particles by keeping the binding polyelectrolyte segment distinct from the hydrophilic PEG block core shell.92 This separation allows each component to perform its specific function more effectively, leading to increased overall stability.
image file: d5ra02597d-f7.tif
Fig. 7 TEM of BaSO4 particles synthesized in presence of (a) no additives, (b) PMAA, (c) PEG-b-PEIPA, and (d) SEM BaSO4 particles synthesized in presence of PEG-b-PMAA-Asp (reproduced from ref. 39 with permission from the American Chemical Society, copyright 2000); (e) SEM and (f) TEM images of BaSO4 particles formed in presence of PEG-b-PMAA-PO3H2 (reproduced from ref. 40 with permission from Wiley-VCH, copyright 2001); (g) SEM and (h) TEM of BaSO4 fibres and nanofilaments in presence of PEG-b-PMAAP (reproduced from ref. 88 with permission from Wiley, copyright 2000).

Further experiment conducted by Wang et al. (2005) involves a non-ionic block copolymer consists of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO), namely PEO–PPO–PEO, to control the crystal growth of BaSO4 nanoparticles.93 Different amounts of copolymer added to the reactants would generates various shape of BaSO4 particles. BaSO4 precipitates obtained from 1 g per L copolymer have a lamellar star-like morphology with four petals and an average length of 8 μm. On the other hand, BaSO4 precipitates from 2 g per L copolymer solution are obtained in 50 nm square flakes shape. Furthermore, if the copolymer solution concentration increased to 3 g L−1, the obtained BaSO4 particles would be obtained in snowflake-like shape with four main branches, each branches have many twigs with average length around 1 μm. This copolymer-assisted BaSO4 precipitation method involve two important features. The first feature is face-selective adsorption which would makes some face have expanded growth, hence seem longer than the other faces.94 This phenomenon happens regarding to the different characteristics of the hydrophobic PPO block which by envelops the BaSO4 nuclei to create hydrophobic cores, stimulating the steric effect while the hydrophilic block PEO stretch into the solution, stabilize and maintain the structure of the crystal.95 The effectivity of this physical adsorption by PEO–PPO–PEO copolymer is highly related to BaSO4 particle size. Smaller particle size would enhance surface area and porosity. Higher porosity implies more spaces within the BaSO4 morphology, leading to faster adsorption rates.96 The other feature is self-assembled by the copolymer template mechanism itself.97 The core of the micelles highlighting the PPO block function, effectively inhibits the growth of BaSO4 particles from the core–shell micelles.

Another experiment conducted by Saraya and Bakr (2011) modify BaSO4 nanoparticles obtained from precipitation method with polycarboxylates.98 In difference from the experiment in the previous study, Saraya and Bakr reacted 0.1 M Ba(NO3)2 and 0.1 M (NH4)2SO4 in a water-soluble organic polymer modifying agent. The obtained BaSO4 nanoparticles have particle size ranging from 6–26 nm, with a mean particle size of 18 nm. The nanoparticles have uniformly elliptical shape with mesopores cavity 6–8 nm in diameter. This unique porous structure of BaSO4 nanoparticles is exhibited due to Ostwald ripening, where the continuous supersaturated condition of the solution, leading to the growth and stabilization of nanoparticles in closed system driven by interfacial energy.99 The porous structure of nanoparticles also indicates their better dispersibility due to increased surface area, consequently, have more capability to adsorb the modifying agent onto their surface.100 This result is aligned with the conclusion from the other experiment by Shen et al. (2007), which stated that in this precipitation method, polycarboxylate serves as inducer and inhibitor.71 This experiment was later developed by Nandakumar and Kurian (2012) who synthesized BaSO4 nanoparticles using 3% polyvinyl alcohol (PVA) as a polymeric template.101 Monodispersed porous spherical BaSO4 nanopowder with low impurity and an average size of 23 nm was achieved according to Scherrer formula at calcination temperature 600 °C. Meanwhile, the calcination at 400 °C still ineffective to gain nanoparticles with high purity whereas calcination at 800 °C leads to excessive agglomeration of BaSO4 nanoparticles since its protective layer already removed from the nuclei.102 Additionally, BaSO4 obtained from direct precipitation in absence of PVA have higher tendency to agglomerate and developed heterogeneous flaky structure due to higher surface energy and poor solubility.103

Later, Sun et al. (2014) investigated further the effects of sodium polyacrylate (PAAS) in BaSO4 precipitation reaction.104 The initial intermediates were still in enourmous particles before gradually reduced to spherical particles with a mean particle size of 200 nm. Further precipitation would increase the nanoparticles' growth to 20 nm. The final products were obtained with an average size around 30 nm and satisfactory dispersity. As confirmed in previous study, PAAS would emits polyacrylate ions that reacted with barium ion via ionic bonding, forming barium polyacrylate intermediate.40 Since the intermediate is more soluble in water than PAAS owing to its chain length and weight fraction, the reaction system materialised in form of suspension.105 Then, the sulfate ion from ammonium sulfate addition into the suspension would bonds with free barium ion to form BaSO4. As the precipitation time elapses, the intermediates are slowly dissolved so that nanoparticle growth can occur. The released polyacrylate ions would rearranged as an adlayer to inhibit the agglomeration of BaSO4 nanoparticles.106

Zhao and Liu (2006) analyzed BaSO4 nanoparticles obtained from precipitation method by using 0.1 grams of sodium hexametaphosphate ((NaPO3)6) as stabilizer, BaS and Na2SO4 as main reactants.83 The result shows that BaSO4 nanoparticles obtained from synthesis with Na(PO3)6 have bigger average size of 203 nm, indicating weaker influence to inhibit the growth of nanoparticles. However, Gupta et al. (2010) got different outcomes by dissolving 10 grams of Na(PO3)6 in 80 mL before mixed with 10 mL of 1 M Na2SO4 and 10 mL of 1 M Ba(NO3)2 solution.107 The obtained BaSO4 precipitates size is around 30–55 nm. This difference pinpoints the importance of determinating the amount of modifying agent involved in synthesis to gain the optimal size of nanoparticles.

2.3 BaSO4 nanoparticles precipitation involving surfactants

BaSO4 nanoparticles precipitation also has been experimented by adding surfactants, either the non-ionic or ionic surfactants. The interest of this experiment is increasing owing to the hydrophilic and hydrophilic configuration in surfactant that can be utilized to control the growth of nanoparticles.108 Li et al. (2011) investigated in influence of dodecyltrimethylammonium bromide (DTAB) to morphology and particle size of BaSO4 synthesized with direct precipitation method using BaCl2 and Na2SO4 as precursor.44 The BaSO4 precipitates obtained from a synthesis without DTAB have four-leaves-shaped morphology with an average size of 4.85 μm. As the concentration of DTAB is increased to 10.0 mmol L−1, flower-shaped structures with petals begin to form with an average dimension of 180 nm × 100 nm. Increment of DTAB concentration to 12.5 mmol L−1 would produce flower-like aggregates composed of nanorods with a mean particle size about 190 nm length and 80 nm width. Further addition of DTAB concentration to 15.0 mmol L−1 would result in aggregates with eight petals flower-shaped morphology. Each petals have a mean size of 1.15 μm × 1.00 μm. Synthesis of BaSO4 with 25 mmol per L DTAB would produces similar aggregates to the one obtained in absence of DTAB with size reduction to 3 μm. Based on this experiment, Li et al. proposed a two-step BaSO4 nanoparticles growth mechanism. The first step is the formation of BaSO4 nuclei and DTAB adsorption to the surface of BaSO4 crystals which increased by increasing electrostatic interaction frequency of the positive charge carried by the polar head groups DTAB and sulfate anions.109 The second step is the construction of a flower-shaped structure using the formed nanorods. Meanwhile, another experiment conducted by Jha et al. (2019) using cetyltrimethylammonium bromide (CTAB) proves its less effectiveness to control BaSO4 particles size as concluded from their mean particle size of 98.12 nm.38 This statement is supported by previous study by Zhao et al. (2011) which shows that the presence of CTAB in the ion-exchange reaction between solid-state barium carbonate (BaCO3) and aqueous sodium sulfate (Na2SO4) had a minimal impact on the morphological control of the resulting BaSO4 crystals.110 The formation schemes of BaSO4 nanoparticles with several DTAB concentrations are shown in Fig. 8.
image file: d5ra02597d-f8.tif
Fig. 8 BaSO4 nanoparticles growth mechanism with different DTAB concentration.

Octadecyl dihydrogen phosphate (n-C18H37OPO3H2, ODP), another surfactant was proposed by Bala et al. (2006) as a modifying agent in BaSO4 precipitation method.111 The liquid solvent medium was prepared by dissolving ODP to 0.1 M KOH before mixed with 20 mL 0.5 M BaCl2. 20 mL 0.5 M (NH4)2SO4 then added dropwise into the solution. The end product garnered from this precipitation reaction with weight ratio of ODP/BaSO4 2%-wt is spherical with an average size of 76 nm. These nanoparticles are equipped with core–shell structured protective layer (barium alkyl phosphates, Ba-ODP or Ba-2ODP) with thickness about 3–8 nm and cavities with an average diameter approximately 5–16 nm. This method is proven to successfully modify the surface tension of BaSO4 nanoparticles with ODP amount above 1.85%-wt to make it more hydrophobic. The modifying mechanism can be estimated by FTIR analysis data that the P–OH bonds is only detected in ODP spectra, which means that the barium ion reacts with ODP, producing hydrophobic salt to enhance the surface tension of nanoparticles.

Li et al. (2012) had used different approach to precipitate BaSO4 nanoparticles in presence of dodecyl benzene sulfonic acid (DBSA) in ethanol–water system with BaCl2 and Na2SO4 as precursor.112 The addition of 1%-wt DBSA is proven to decrease particle size drastically from 1000 nm to 56 nm. Further increment of DBSA concentration to 5%-wt would produces well-dispersed rounded 46 nm nanoparticles. The products with higher DBSA composition (5%-wt) have higher total weight loss about 6.32%-wt due to decomposition of DBSA at an optimal temperature of 454 °C. This data implies that further addition of DBSA would results in thermal instability of obtained BaSO4 nanoparticles. The summary of raw materials and additives used for BaSO4 nanoparticles synthesis and the mean particle size produced from several studies is listed in Table 1. The evaluation of each additive's advantages and setbacks in barium sulfate nanoparticles precipitation is presented in Table 2.

Table 1 Lists of raw materials and particles size gained from BaSO4 nanoparticles precipitation
Main reactants (precursors) Solvent d50 (nm) Ref.
BaCl2, (NH4)2SO4 Ethanol 36 59
BaCl2, (NH4)2SO4 Ethanol–water 54 62
BaCl2, Na2SO4 Water–benzene 35.995 63
Ba(NO3)2, Na2SO4 Water–THF 67 65
BaCl2, Na2SO4 Ethanol–water in presence of stearic acid 16 71
BaCl2, Na2SO4 NTMP N/A 73
BaCl2, Na2SO4 NTA 208 74
BaCl2, Na2SO4 Water + EDTA 13 43
BaS, Na2SO4 Water in presence of Na(PO3)6 203 83
Ba(NO3)2, Na2SO4 Water in presence of Na(PO3)6 30–55 107
Ba(NO3)2, K2S2O8 Water + EDTA 2 × 103, consists of quasi-spherical and rod-like nanoparticles 81
BaCl2, Na2SO4 Water–DMSO in presence of EDTA 4 (nanospherical), 12 (nanofibers) 45
BaCl2, Na2SO4 Water–EDTA 280 42
BaCl2, Na2SO4 Water in presence of DTAB 190 × 80 44
BaCl2, Na2SO4 Water in presence of CTAB 98.12 38
BaCl2, (NH4)2SO4 Water in presence of ODP 76 111
BaCl2, Na2SO4 Ethanol–water in presence of DBSA 56 112
Ba(NO3)2, (NH4)2SO4 Water in presence of polycarboxylates 18 98
BaCl2, (NH4)2SO4 Water in presence of PVA 23 101
BaCl2, (NH4)2SO4 Water–PAAS ∼30 104
BaCl2, Na2SO4 Water–PEO–PPO–PEO 50 93
Ba(OAc)2, (NH4)2SO4 PMAA 22 (primary crystallites) 88
Ba(OAc)2, (NH4)2SO4 PEG-b-PMAA 14 (primary crystallites) 88
Ba(OAc)2, (NH4)2SO4 PEG-b-PMAA-Asp 19 (primary crystallites) 88
Ba(OAc)2, (NH4)2SO4 PEG-b-PEIPA 27 (primary crystallites) 88
Ba(OAc)2, (NH4)2SO4 PEG-b-PMAAP 20–30 (nanofibres) 88
Ba(OAc)2, (NH4)2SO4 PEG-b-PMAA-PO3H2 20–30 (nanofibres with an average length of several hundred micrometers) 39 and 40


Table 2 Comparison of BaSO4 nanoparticles precipitation involving organic compounds, polymers, and surfactants
  Advantages Setbacks
Organic compounds • Organic capping agents control particle size, spheric morphology, and improve dispersion effectively by forming protective layers or steric hindrance • The vast choices of organic compound can impacts particle morphology, different kind of organic compound would produces irregular shaped nanoparticles
• Fine-tuning of nanoparticle size can be done simply by solvent composition or pH adjustments • High pH systems for several organic acids can lead to sporadic and uneven particle growth due to limited barium ion availability
Polymers • Different polymer types offer a wide range of size and morphology control, such as rod-like, blocks, or flower-shaped structures • High polymer concentrations may lead to excessive stabilization, hindering controlled growth
• Polymers-based agents would produce nanoparticles with uniform sizes and porous structures • Some polymers performance is highly dependent to pH and temperature. This can result in inconsistent particle size
  • Limited availability
Surfactants • Surfactants can improve control over nanoparticle size and structure through selective surface adsorption • The thermal stability of the product is highly affected by surfactant's concentration in the system
• Surfactants in certain concentration can affects the hydrophilic or hydrophobicity of the system to enhance nanoparticles' dispersibility • Ionic surfactants are more likely exhibit less control over particle size and morphology compared to non-ionic surfactants


2.4 BaSO4 nanoparticles precipitation involving ionic liquid

Ionic liquid is a salt solution formed by combining organic cations and organic or inorganic anions with relatively stable liquid properties at room temperature.113 Ionic liquids also have a remarkable ability to resist external forces and enhance catalytic processes due to their high conductivity and viscosity. The heat-resistant and electrical conductivity properties qualified ionic liquids application in proton exchange membrane fuel cells in order to enhance its durability and power density up to 500 kW by altering the IL's structure with a template to create protic ionic liquid (PIL) polyelectrolyte, which has excellent thermal stability and higher proton conductivity.114

The cationic proton head and anionic alkyl chains of ionic liquid have distinct characteristics at microscopic scale. With their long alkyl chains, ionic liquids emphasize their hydrophobic features and improve the dissolution of nanoparticles precursors, thereby increasing their thermal stability. Hence, some ionic liquids can act as stabilizers to improve the performance and stability of a nanomaterial.115 The hydrophobic properties have become a key factor in the development of microwave-assisted ionic liquid synthesis method, which is used to produce corrosion inhibitors and solvents for biomass conversion.116 Meanwhile, in general, the hydrophilic head group influences solubility and surface activity of the micelles in the solution.117,118

Ionic liquid has gained interest as a green solvent for its high solubility in water and thermal properties.119 Hence, many researchers have developed various synthesis route using this relatively new and non-toxic solvent for many purposes, including the utilization of environment-friendly IL-based electrolyte with wide electrochemical window in supercapacitor energy storage system, lignocellulosic biomass conversion in presence of 1-alkyl-3-methyl imidazolium-based IL as green catalyst, reuseable green solvent and catalyst for plastic degradation process in relatively low heat, green IL-based corrosion inhibitor with simple manufacturing method, and effective solvent system in synthesis of nanoparticles.120–124

Kowacz et al. (2011) analyze the most probable mechanism of BaSO4 nanoparticles synthesis using precipitation method with various ionic liquids and its effects on the crystallization process.125 In this study, BaSO4 precipitation was carried out by mixing 5 mL solution consists of 0.04 M Na2SO4 and 0.05 M of ionic liquid with 100 μL of 0.1 M BaCl2 in a reactor, then quickly quenched after the mixture is in highly saturated state. Based on this experiment, BaSO4 nanoparticles size in different ionic liquid has significantly dependent to their electrical conductivity and diffusivity in water. Therefore, ionic liquid viscosity is also closely related to the obtained BaSO4 nanoparticle size and diffusion coefficient according to Stokes–Einstein equation for mass transfer.126 Usually, nucleation kinetics correlate with the diffusion of constituent units, and according to Stokes' law, higher viscosity slows the mobility (μ) of charged particles.127 However, the experimental data revealed that the nucleation rate of ionic crystals increased with arising solvent viscosity, as shown in Fig. 9a and b, thus this phenomenon contradicts Stokes' law and the conventional colloidal system crystallization.


image file: d5ra02597d-f9.tif
Fig. 9 Correlation curve between BaSO4 crystal size and (a) limiting molar conductivity (Λo); (b) diffusion in water (Do/D) (reproduced from ref. 125 with permission from the American Chemical Society, copyright 2011).

The diffusion rate of the barium and sulfate ions needed to ensure BaSO4 nucleation is directly dependent on the frequency of water exchange in the solution interface, which increased by electrostatic attraction and solute's affinity dependent to its polarity in the water–IL system.128,129 Since this reaction happen in an ionic liquid system, the attraction between water's partial charge and ions from IL compound would decrease the water molecules' potential energy. The hydrophilic nature of ILs causes them to disrupt the cohesive forces of water and bind its molecules to BaSO4 particles, which leads to a higher rate of diffusion.130 This condition would ensure the stabilization of the solution and consequently increase the residence time of the reaction. Conversely, hydrophobic feature of IL would enhance the reactivity of hydroxide group in order to support the release of water molecules and the separation of precipitate.131

The solution stability degree relies on background ions dipole moment distribution in the solution.132 Closer ions have less impactful electric fields, thus reducing their influence on the water molecules. Moreover, the dynamic between hydrophilic ions and hydrophobic ions would significantly influence the movements of water molecules. Positive hydration occurs when ions have a strong affinity for water molecules so that they overcome the water cohesion, creating a hydration shell around them.133 This ions is generally known as hydrophilic ions. Positive hydration mechanism would result in a more ordered water shell around the ion, which limits the mobility of water molecules. On the other hand, negative hydration occurs when ions have a weaker affinity for water molecules compared to water cohesion in the vicinity of sufficiently large single-charged ions, hence increases the water molecules mobility.134 This effect is observed when hydrophobic alkyl chains attached to imidazole rings reduce interactions with polar surface sites due to their nonpolar nature. These hydrophobic chains promote the formation of a more compact, water-repellent layer on the metal surface, thereby enhancing its corrosion inhibition efficiency.135

The other parameter correlated to the obtained BaSO4 particle size is electrical conductivity. The conductivity of an electrolyte solution is determined by diffusion of ions, as described by the Nernst–Einstein equation.136 However, at higher concentrations, the tendency of ions association can lead to the formation of neutral droplet, thus lowers the electrical conductivity.137 The experiment results indicated that the increment of solvent viscosity and the association of IL ions in a solution environment enhance the nucleation rate, leads to higher conductivity and smaller final products. The charge density of these ILs is strongly related to hydration enthalpy. Hydrogen enthalpy is the combination of ligation enthalpy and dispersion enthalpy.

The effect of ILs on barite nucleation follows different trends based on the hydration characteristics of the IL anions. ILs with negatively hydrated anions like [SCN] and halides (Cl, Br) show a trend with crystal size around 620–750 nm and the conductivity approximately ranging from 130–170 mS cm−1 mol−1, while those with positively hydrated anions like [Ac] and tetracyanoborate [(CN)4B] show another trend with crystal size around 620–670 nm and smaller conductivity around 90–120 mS cm−1 mol−1. Sulfonate anions, which have mixed hydration properties, define an intermediate trend between the positive and negative hydrated anions. In solutions of ILs with anions of mixed hydration characteristics, two distinct regimes can be distinguished with respect to the effect on barium sulfate nucleation. ILs with longer chains (C3–C4) have smaller particle size compared to the nuclei formed in ILs with positively hydrated anions. This suggests that the net hydration of the anion is dominated by the hydrophobicity of the carbon chain in the C3–C4 region. The smallest particle size of BaSO4 from precipitation is achieved by using [EMIM]C4SO3. A more advanced synthesis of BaSO4 nanoparticles with IL [3,4-DCBSMIM]Cl and Ba(NO3)2 using solution combustion system (SCS) in a muffle furnace preheated at 400 °C results in much smaller BaSO4 particles with an average size approximately 11.6 nm.21 In addition, BaSO4 nanoparticles synthesis can also gained from robust precipitation with 0.56 gram Ba(NO3)2·2H2O in 40 mL H2O and 4 M NaOH as main reactant and 0.05 gram monocationic IL [(MIM)C2COOH]Cl in 10 mL pure water as capping agent.46 This solution is added by 0.27 gram of ammonium persulfate in 30 mL pure water as reaction initiator. This synthesis would produced porous fish-skeleton like BaSO4 nanostructure which has excellent characteristics to paired with poly aminophenol (POAP) to create nanofillers for supercapacitors. The additional information about physical properties of ILs used in barium sulfate nanoparticles synthesis are provided in Table 3.

Table 3 Characteristics of ILs used for BaSO4 nanoparticles synthesis (T = 25 °C)
ILs Chemical structure ρ (g cm−3) μ (cP) Ref.
Cations Anions
[EMIM]+ [(CN)4B] image file: d5ra02597d-u1.tif image file: d5ra02597d-u2.tif 1.0363 19 125, 138 and 139
[EMIM]+ [Ac] image file: d5ra02597d-u3.tif image file: d5ra02597d-u4.tif 1.03 91–162 125 and 140
[Ch]+ [Ac] image file: d5ra02597d-u5.tif image file: d5ra02597d-u6.tif 0.995 55.6 (in CH3COOH with molar ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]2) 125, 141 and 142
[EMIM]+ [SCN] image file: d5ra02597d-u7.tif image file: d5ra02597d-u8.tif 1.1162 22 125, 138 and 139
[EMIM]+ Br image file: d5ra02597d-u9.tif image file: d5ra02597d-u10.tif 1.48 N/A 125 and 139
[EMIM]+ Cl image file: d5ra02597d-u11.tif image file: d5ra02597d-u12.tif 1.186 3530 125 and 139
[Ch]+ Cl image file: d5ra02597d-u13.tif image file: d5ra02597d-u14.tif 1.103984 (ChCl/2H2O, 100 wt%) 37.760 (ChCl/2H2O, 100 wt%) 125 and 143
[Ch]+ [C1SO3] image file: d5ra02597d-u15.tif image file: d5ra02597d-u16.tif N/A N/A 125
[EMIM]+ [C2SO3] image file: d5ra02597d-u17.tif image file: d5ra02597d-u18.tif 1.203 200 125 and 144
[EMIM]+ [C3SO3] image file: d5ra02597d-u19.tif image file: d5ra02597d-u20.tif N/A N/A 125
[EMIM]+ [C4SO3] image file: d5ra02597d-u21.tif image file: d5ra02597d-u22.tif N/A N/A 125
[3,4-DCBSMIM]+ Cl image file: d5ra02597d-u23.tif image file: d5ra02597d-u24.tif N/A N/A 102
[(MIM)C2COOH]+ Cl image file: d5ra02597d-u25.tif image file: d5ra02597d-u26.tif N/A N/A 46


3. Synthesis of BaSO4 nanoparticles in modified reactors

Although the synthesis of nanoparticles via chemical precipitation is the easiest to scale up for industrial production, the use of conventional stirring methods suffers from large and widely distributed particle sizes due to poor macromixing and micromixing efficiency which is vital in providing the necessary supersaturated conditions.145 Therefore, several modifications to the reactor technology were developed to support the production needs on an industrial scale while improving the homogeneity and dispersion of nanoparticles that significantly affect the final product yield. A couple of factors that can be observed in nanoparticle reactors are flow rate, stirrer rotation speed, and the operating temperature inside the reactor vessel.

3.1 Rotating packed bed (RPB) reactor

Packed bed reactor, also known as fixed bed reactor, is a column containing catalysts or packing in a solid phase with well-distributed shape and size. These packings usually are utilized to provide contact in a chemical process, usually between gas–liquid, vapor–liquid, and liquid–liquid phase.146 This unit can be enhanced by modifying it with a rotating device to enhance overall efficiency of the process, mainly to reduce the required size of the reactor needed in production system while also increase the gravitational force by tuning its angular velocity.147,148 In nanoparticle preparation method using RPB, the precursors flows through the rotor's inner cavity before sprayed using nozzle on the inner edge of the rotor. This process would make the small droplets of reactants contacts easier due to the increasement of surface area.149 The liquid is contacted with porous packing to ensure the dispersion and optimal mass transfer in the reactor.150 As a result, the final products would be more homogeneous in terms of the size and morphology of the nanoparticles. The design of horizontal RPB and modified RPB can be seen in Fig. 10a and b, respectively.
image file: d5ra02597d-f10.tif
Fig. 10 (a) Sketch of horizontal-axis RPB (consists of: (1) liquid feed inlet; (2) liquid outlet; (3) vapor inlet; (4) vapor outlet; (5) packing; (6) motor) (reproduced from ref. 149 with permission from the American Chemical Society, copyright 2004); (b) sketch of RPB with radial sampling tubes (consists of: (1) shell; (2) electrical motor; (3) rotor; (4) packing; (5) sampling tubes; (6) liquid inlets; (7) liquid outlet) (reproduced from ref. 152 with permission from Elsevier, copyright 2010).

An experiment conducted by Fang et al. (2021) using BaCl2 solution and a mixture of methanol and H2SO4 on an RPB with a speed of 1500 rpm and a component flow rate of 300 mL min−1 under operating temperature conditions of 60 °C was able to produce BaSO4 nanoparticles with an average diameter of 13 nm.48 Meanwhile, the BaSO4 nanoparticle synthesis process carried out at rotational speeds of 500 rpm and 2500 rpm produced particles with a diameter of 17 nm and 10 nm, respectively. Thus, an increase in RPB rotational speed is inversely proportional to particle growth. A similar statement was made in the experimental results of CaCO3 nanoparticle synthesis with a rotating fluidised bed.151 They state that to produce smaller nanoparticles with large interparticle cohesive forces, higher rotational speeds are required to overcome these forces and achieve effective fluidization. Other experiments conducted by Yang et al. (2010) also led to the conclusion that the decreased growth of BaSO4 nanoparticles in RPB is due to the greater centrifugal force that increases the relative velocity between the nozzle and packing flow.152 As a result, there is an increase in micromixing efficiency and homogeneous supersaturation conditions occur in the RPB reactor.

3.2 T-Mixer and Y-mixer reactor

T-Mixer is a type of fluid mixing device where two liquid streams meet at a T-junction. T-mixer consists of two inlet streams and single outlet stream, as shown in Fig. 11a. The mixing process involves the injection of a solution containing a dissolved substance through one inlet and a liquid antisolvent through the other inlet.153 The two liquid streams meet at the T-junction, resulting in planar, swirling folds and simultaneously causing fluid turbulence as the Reynolds number increases.154 This process allows for efficient mixing of the two liquids as well enhancing the mass transfer, increase reaction rate and higher selectivity. T-mixers can be scaled up or down, depending on the specific requirements of the synthesis process. This advantages makes them versatile tools for both laboratory and industrial applications.155,156

Driven by its versatility, there have been several studies on the precipitation of BaSO4 nanoparticles using a T-Mixer reactor. The study on the synthesis of BaSO4 nanoparticles using a T-Mixer reactor conducted by Schwarzer and Peukert (2002) analyzed the precipitation results from the reaction of BaCl2 and H2SO4 at a constant temperature of 25 °C.157 The results of the study showed that an increase in the inlet flow rate and an increase in the rotation speed per minute would trigger a driving force in the form of ionic supersaturation of dissociated barium and sulfate ions, as illustrated in Fig. 11d. This condition is corresponds to the Von-Weimarn rule which states an inverse relationship between supersaturation level and particle size.158 The smallest product size was obtained at a flow rate of 5 mL s−1 with a diameter of about 80 nm. This phenomenon directly affects the nucleation rate and growth of nanoparticles, so the size of the monocrystalline precipitate becomes smaller with the increase in rotation speed. However, during this precipitation process, agglomeration occurs, which can hinder the optimization of the barium sulfate nanoparticle size. Further study proceeded by Gradl and Peukert (2010) were carried out with initial feed concentration of 0.5 M BaCl2 and 0.33 M H2SO4 in a T-mixer constructed of two feed tubes of 0.5 mm and pressure drop along the pipeline is kept constant.159 Correlation graphs in Fig. 11c show that the size of BaSO4 nanoparticles decreased with the increase of Reynold's number. This trends occur due to variable of nucleation rate and growth of nanoparticles as a function of solubility and interfacial tension.160 Supersaturation condition of BaSO4 triggers the nucleation rate to be higher than the growth rate.161 This condition implies that smaller nanoparticles can be obtained by installing the T-mixer with higher power input intensity to increase the feed flow rates and consequently optimize the liquid–liquid contact frequency.


image file: d5ra02597d-f11.tif
Fig. 11 (a) Sketch of a T-mixer reactor (reproduced from ref. 171 with permission from Springer, copyright 2019); (b) scheme of a modified T-mixer reactor with a capillary microreactor for microflow precipitation of BaSO4 nanoparticles (reproduced from ref. 32 with permission from Elsevier, copyright 2023); (c) correlation curve of volume density distributions of BaSO4 nanoparticles and particle size at various Reynold number and (d) correlation curve of BaSO4 nanoparticles size and power input of T-mixer reactor (reproduced from ref. 159 with permission from Springer, copyright 2010); particle morphology of precipitated barium sulfate with Y-mixer at Sa = 350 and initial free lattice ion ratios (R): (e) R = 0.1, (f) R = 1, (g) R = 10, and (h) R = 100 (reproduced from ref. 72 with permission from Elsevier, copyright 2006).

Similar results were obtained based on further research by Pieper et al. (2011) with BaCl2 reactants that were combined with dispersing agent Melpers 0030 before reacted with K2SO4 in a T-Mixer reactor.162 The 30%-wt dispersing agent played a role in preventing agglomeration that could occur during the continuous precipitation reaction. Experimental results at various reactant flow rates showed a similar trend to the findings of Schwarzer and Peukert,157 with the residence time decreasing as the Reynolds number increased. Additionally, an increase in supersaturation with the addition of reactant concentration significantly shortened the residence time, from 24 ms to 1.66 × 10−2 ms for reactant concentrations of 0.05 mol L−1 and 0.128 mol L−1, respectively. This condition causes particles to easily undergo nucleation. However, further increasing the reactant concentration to 0.25 mol L−1 was less effective in reducing particle size. The increment of operating temperature during nanoparticle synthesis resulted the increasing diffusion coefficient in water, indicating that inhibition of nanoparticles growth is more likely to happens at lower temperatures.163 The optimal size of barium sulfate nanoparticles from this experiment was obtained at 105 nm at a temperature of 20 °C with a flow rate of 900 mL min−1. Meanwhile, the concentration of the antisolvent not significantly affect particle size obtained from this experiment. Thus, small-sized BaSO4 nanoparticles can be obtained by increasing the mixing rate and lowering the temperature to support nucleation in this condition.

Another experiment performed by Kockmann et al. (2008) suggests a convective mixing precipitation method simulation model by using a T-Mixer reactor, 0.5 M BaCl2 and 0.33 M H2SO4 as reactants, combined with diffusive transport and thermodynamic precipitation theory.164 This experiment shows the importance of Reynold number, which must be higher than 350 for the condition above to ensure nanoparticle synthesis with particle size lower than 100 nm. Similar results is gained through an experiment of barium sulfate nanoparticle synthesis using Y-mixer with supersaturation level at 350 and free lattice ion ratio ranging from 0.1 to 100.72 Y-Mixer also provides efficient mixing, but it may not be as effective as the T-mixer in certain situations. Y-Mixer can create a more complex flow pattern, which leads to better mixing in some cases but may also introduce additional turbulence, potentially affecting nanoparticle synthesis due to the occurrence of stagnant region.165,166 At first, suspension with R < 1 indicates heterogenous nucleation with two peaks of particle size distribution. These peaks would gradually diminished into a monomodal curve with mean particle size approximately 1–1.6 μm due to aggregation prevention by repulsive electrostatic forces caused by the adsorption of barium ions on the particles' surface.167 SEM images in Fig. 11e–h reveals that the morphology of BaSO4 nanoparticles changes from nearly rectangular at a low R value to star-shaped at R = 1 and 10, and to a fir cone-like shape at R = 100.

Zhang et al. (2023) modified the conventional T-mixer reactor by combining it with a spiral capillary microreactor with an inner diameter of 0.8 mm at the outlet stream to enhance the quality of the final product.32 The modified reactor configuration is illustrated in Fig. 11b. This experiment uses BaS and H2SO4 with an initial concentration of 1 M for both feed material and 5 M NaOH was utilized to absorbed the exhausted H2S gas, a by-product from this reaction. The result shows a similarity with previous studies that the increment of Reynold number from 41.8 to 664 would decrease the particle size from 53.5 nm to 35.7 nm due to intensified heat and mass transfer.168 Meanwhile, the increment of residence time would decrease the flow rate and significantly reduce the coefficient of variation, leads to narrower particle size distribution in accordance to residence time distribution (RTD) theory even if it is slightly tends to trigger an increase in the growth rate of nanoparticles.169,170 These variables are the key factors to determine the optimal flow rate for nanoparticles production.

3.3 Spinning disc reactor

Spinning disc reactor (SDR) is initially developed to enhance heat transfer efficiency on a rotating surface and increase the conversion of a liquid or gas reactions through high gravity fields.50,172 SDR is a continuous-flow stirred reactor that utilize centrifugal force to exerts thin films on a rotating disc, typically around 50 μm for liquid compounds similar to water.34 This reactor needs lower energy than other CSTR equipment with higher shear force to increase the mixing rate dan mass transfer frequency.173 As shown in the scheme of SDR in Fig. 12a, the feed streams is located at the center and radially flows outward affected by centrifugal force of the spinning disc, with high rotating speed usually spanning from 300 rpm to 2000 rpm.174,175 The shear force implemented at the thin films of reactant provide chemical reaction on the disc surface under specific hydrodynamic condition.176
image file: d5ra02597d-f12.tif
Fig. 12 Scheme of (a) spinning disc reactor (SDR) (reproduced from ref. 34 with permission from Elsevier, copyright 2008); (b) double spinning disc reactor (DSDR) (reproduced from ref. 49 with permission from Elsevier, copyright 2017); and (c) high-speed spinning disk reactor (HSSDR) (reproduced from ref. 178 with permission from the American Chemical Society, copyright 2019).

An experiment conducted by Cafiero et al. (2002) shows that the reaction of 0.04 M BaCl2 and Na2SO4 to create BaSO4 nanoparticles at SDR with supersaturation degree 2000, disc diameter 0.5 m, rotating speed 900 rpm, and reactants flow rate 1.33 × 10−6 m3 s−1 have a larger amount of particles around 3.2 × 109 cm−3 compared to T-mixer with number of particles approximately between 2.2 × 108 and 4.0 × 108 cm−3.173,177 This condition also promotes lower specific dispersed power consumption only around 115 W kg−1 and rapid mixing time at 0.9 ms compared to the T-mixer at rotating speed 900 rpm. Similarly, another experiment conducted by Dehkordi and Vafaeimanesh (2009) proves that BaSO4 nanoparticles synthesis reaction in SDR with disc diameter 15 cm and 20 cm and supersaturation level 400–800 undergoes heterogeneous nucleation mechanism, whereas synthesis with supersaturation higher than 800 shows increasement of nucleation rate indicated by sharply declining particle diameter up to 100 nm at supersaturation degree of 2000.47 The average diameter of nanoparticles obtained from this experiment is relatively higher than those achieved from experiment conducted by Cafiero et al.173 operating at same condition. This could be caused by larger disc diameter that provides more stringent agitation to effectively prevent particles agglomeration. They also proposed that the mixing time of SDR at rotating speed above 750 rpm of 20-cm disk diameter generates smaller particles less than 100 nm at 1000 rpm compared to those achieved from 15-cm disk diameter.

Farahani et al. (2017) developed a new SDR design with 20 cm diameter aluminum double spinning disc reactor (DSDR) mounted horizontally in a polycarbonate chamber, as shown in Fig. 12b.49 This configuration introduces a new variable affecting the conversion results, namely the distance between the disks. This DSDR is designed to be operated at range of 0–4750 rpm. Based on this experiments, it is concluded that DSDR can produces smaller BaSO4 nanoparticles with average diameter 23,4 nm at higher rotating speed (4750 rpm) for both discs, smaller distance between the discs at 0.1 mm, and feed concentration ratio of BaCl2[thin space (1/6-em)]:[thin space (1/6-em)]Na2SO4 equals to 1.3[thin space (1/6-em)]:[thin space (1/6-em)]3.1 with flow rate 0.4 L min−1. Meanwhile, nanoparticle with diameter ∼60 nm can be achieved at operating rotating speed 500 rpm, lower than conventional SDR.

Recently, a novel modified configuration of a high-speed spinning disk reactor (HSSDR) was proposed as a medium of BaSO4 nanoparticle synthesis reaction.178 HSSDR model has lower cost and less complex than DSDR. The scheme of HSSDR can be seen in Fig. 12c. The most fine particle size produced from HSSDR is reported to be 16.4 nm with condition operation at rotational disk speed 15[thin space (1/6-em)]000 rpm, flow rate 450 mL min−1, BaCl2 feed concentration 1.40 mol L−1, Na2SO4 feed concentration 3.30 mol L−1, and supersaturation degree at 3362. Similar as the other experiments, the increasing rotational disk speed from 5000 rpm to 15[thin space (1/6-em)]000 rpm would significantly reduce the particle size from 83.1 nm to 47.6 nm. Additionally, variating the feed entrance radius from 10 mm to 70 mm plays important role in particle size reduction up to 10 nm for both rotating speed 5000 rpm and 15[thin space (1/6-em)]000 rpm.

3.4 Membrane reactor

Many process intensifying reactor technologies have been proposed to enhance nanoparticles synthesis efficiency. However, the particles achieved from these methods still have relatively poor dispersion and difficult to be upscaled to industrial scale of production. Membrane reactor is suggested as an option of nanoparticle synthesis method to obtain more disperse particle. Membrane reactor integrate membrane separation process and the principal of chemical reaction simultaneously.179 Generally, there are two methods of nanoparticle preparation using membrane reactor, namely the template method and membrane contactor process.180 The membrane template method is the most common technique to gain nanoparticle conversion in nanotubes, for example gold nanotubes.181 Membrane contactor process is carried out in two phases, the first phase inserted through the pores while the second phase flows parallel to the membrane surface.182 Furthermore, the membrane acts as an nonselective interface so the two phases can be separated and in contact simultaneously.183 Another way to produce nanoparticles with a membrane is in an emulsion system where dispersion occurs in a continuous phase without chemical reactions.184 Based on research from the past few years, the synthesis of barium sulfate nanoparticles has more frequently been conducted using the membrane contactor process, which involves dispersion in a chemical reaction. The membrane contactor working principle is described in Fig. 13a.
image file: d5ra02597d-f13.tif
Fig. 13 (a) Scheme of membrane contactor principle process (reproduced from ref. 180 with permission from Global Science Books, copyright 2007); (b) microscope images of microbubbles and (c) TEM images of BaSO4 nanoparticles at FG = 300 mL min−1 (reproduced from ref. 187 with permission from Elsevier, copyright 2013).

In an experiment conducted by Zhiqian and Zhongzhou (2002), BaSO4 nanoparticles with a diameter of 70 nm were produced using a semi-batch hollow fiber ultrafiltration membrane reactor made from PS, PDC, and PES materials.51 The driving force, in the form of pressure within the membrane lower than the ambient pressure, was able to increase the nucleation rate of these nanoparticles. Further experiments performed by Jia et al. (2003) concluded that the added additives to BaCl2, such as methyl alcohol and ethanol, effective to reduce and control the growth of BaSO4 nanoparticles size to 15 nm due to its higher polarities.185 Another experiment by Chen et al. (2004) using 20% ethyl alcohol in water as a solvent in a nickel membran reactor with both BaCl2 and Na2SO4 feed concentration 0.1 M results in particle size reduction from 69.3 nm to 22.6 nm.186 Du et al. (2013) proposed a novel barium sulfate preparation method by dispersion in double membrane reactor with 0.3 MPa nitrogen gas microbubbles injected into the system to improve mixing performance of the synthesis.187 The microbubbles have an important role to increase the pressure and enhance dispersion within the reactor. Fig. 13b and c show N2 microbubbles and the final product in this reaction, respectively. The BaSO4 synthesis reaction using nitrogen gas microbubbles which flows at 300 mL min−1 with 2.12 mol per L BaS and 0.32 mol per L Na2SO4 would produces nanoparticles with a mean size approximately around 40 nm.

Porous hollow fiber membrane is easier to be up-scaled to industrial scale and more compact compared to other micromixer reactor.188 The conversion rates of membrane reactor can be increased by adding more fibers or using devices in parallel.189 However, membrane reactors have the potential to experience fouling caused by blockages in the fiber layer due to the crystallization of particles resulting from reactions within the reactor.190 Moreover, membrane reactor has relatively higher maintenance cost than the other kind of reactors. The summary of precursors, solvents, operating parameters, and dimension of reactors used in BaSO4 nanoparticles synthesis are provided in Table 4.

Table 4 Types and condition to obtained BaSO4 nanoparticles at various mean particle size
Reactor Reactor dimensions Precursors Velocity & reaction time d50 (nm) Ref.
Rotating packed bed (RPB) BaCl2, H2SO4 in presence of methanol as solvent and stearic acid (60 °C) 2500 rpm, 18 s 10 48
Rotating packed bed (RPB) • Inner Diameter: 60 mm BaCl2, Na2SO4 in deionized water (25 °C) 800 rpm, typically ∼0.1 ms 45 152
• Outer Diameter:
446 mm
• Axial Depth:
50 mm
• Inlet from inner edge radius: 10 mm
• Liq. distributor:
• 2 × 10 mm
• Nozzles radius: 1 × 2 mm & 4 × 3 mm
T-Mixer BaCl2, H2SO4 in pure water 5 mL s−1 70 157
T-Mixer • Feed tubes diameter: 2 × 0.5 mm BaCl2, H2SO4 in demineralized water (25 °C)   45 159
• Cross section: 1 mm × 10 mm
• Outlet tube length: 3 mm
T-Mixer • 2 Reactant inlets (opposite position) and 1 product outlet diameter: 2.4 mm BaCl2, K2SO4 in deionized water + dispersing agent (Melpers 0030, polyethercarboxylate) 3.3 m s−1, 0.03 ms 75 162
• Mixing channel length: 5 mm
T-Mixer • Inlet channel cross-section: 300 × 300 μm2 BaCl2, H2SO4 in demineralized water (25 °C) 100–400 μs 80 164
• Mixing channel cross-section: 600 × 300 μm2
T-Mixer • Parallel syringe: 2 × 5 mL Ba(NO3)2, BaCl2, Na2SO4, H2SO4 in demineralized water (25 °C) 0.5–1.0 ms 1 × 103–1.5 × 103, consists of many fines under 1 μm 177
• Inlet length: 10 mm
• Inlet diameter: 3.4 mm
• Mixing arm length: 3 mm, 9 mm, 90 mm
• Outlet diameter: 1.9 mm
Modified T-junction – capillary microreactor • Sprial PTFE microtube inner diameter: 0.8 mm BaS black ash, H2SO4 in distilled water 0.017–0.27 m s−1; 3.75–15 s 33.8–53.5 32
• T-Junction inner diameter: 0.8 mm
Y-Mixer • Inlet tubes diameter: 2 × 0.5 mm BaCl2, H2SO4 in demineralized water (25 °C) 26 μs 50 72
• Outlet tube diameter: 2 mm
Spinning Disc Reactor (SDR) • Feed inlet radius: 50 mm BaCl2, Na2SO4 in demineralized water (25 °C) 900 rpm; 0.90 ms 700 173
• Disc diameter: 50 cm
Spinning Disc Reactor (SDR) • Feed inlet radius: 50 mm BaCl2, Na2SO4 in demineralized water (25 °C) 1500 rpm; 1.25 ms 38 47
• Disc diameter: 15–20 cm
• Inlet tubes length: 10 cm
• Inlet tubes diameter: 2 × 3 mm
• Inlet tubes distance: 2 × 5 mm
• Tubes-disc distance: 10 mm
Double coaxial spinning disc reactor (DSDR) • Feed inlet radius: 70 mm BaCl2, Na2SO4 in demineralized water (25 °C) ±4750 rpm; <0.5 ms 23.4 49
• Inlet nozzle: 1 mm for BaCl2 solution and 2 mm for Na2SO4 solution
• Disc diameter: 2 × 20 cm
• Discs distance: 0.1 mm
High-Speed Spinning Disc Reactor (HSSDR) • Discs diameter: 20 cm BaCl2, Na2SO4 in demineralized water (25 °C) 15[thin space (1/6-em)]000 rpm; 0.93–2.88 ms 16.4 178
• Discs distance: 5 cm
• Feed nozzle diameter: 2 × 5 mm
• Nozzles distance: 4–140 mm
• Nozzles-disc distance: 5 mm
• Feed entrance radius: 70 mm
Membrane reactor • Inner diameter: 1 mm BaCl2, Na2SO4 in demineralized water (25 °C) 0.589 m s−1, 480 min 70 51
• MWCO:
PES/PDC (1000 Da)
PS/PDC (30[thin space (1/6-em)]000 Da)
• Membrane length: 13 cm
  • UF membrane: BaS, Na2SO4 in presence of methyl alcohol 0.92 m s−1 15 185
Inner diameter: 10 mm
Effective length: 13 cm
PS/PDC (30[thin space (1/6-em)]000 Da)
  • Microfiltration membranes: BaCl2, Na2SO4 in presence of ethyl alcohol 22–24 mL min−1 20 186
5 μm stainless steel
0.9 μm Ni
0.2 μm Ni
• Active membrane area: 12.5 mm2
N2 microbubbles-aided membrane reactor • 5 μm stainless steel membrane BaS, Na2SO4 120 mL min−1, 2.4 ms 40 187
• Average pore size: 0.5 μm
• Active membrane area: 12, 5 mm2
• Porosity: 65%
• Main channel dimension: 20 mm × 2 mm × 0.5 mm


4. Barium sulfate nanoparticle application in various industries

4.1 Medical applications of barium sulfate nanoparticles

Recently, researchers are developing novel nanoparticle and nanocomposite materials for the advancement of the medical industry. Barium sulfate nanoparticles have garnered significant interest from researchers in the medical sector because of their distinctive properties, such as inertness, high dispersion rate, substantial X-ray attenuation, insolubility, biocompatibility, and colloidal stability in aqueous solutions.191,192 Hence, BaSO4 nanoparticles have since investigated as superior material to increase accuracy of medical imaging technique, mainly as X-ray contrast agent. Furthermore, X-ray shielding material is also needed to minimize the exposure of ionizing radiation to human cells. Some barium sulfate based nanocomposite have been proposed as a non-toxic X-ray shielding material, replacing lead based material which is toxic and highly hazardous to the ecological system.193

Meagher et al. (2013)41 synthesized 40 nm monodisperse BaSO4 nanoparticles in water-in-oil emulsion system, which encapsulated by dextran as a stabilizer.16 This synthesis mechanism produces well-dispersed in water BaSO4 nanoparticles with high X-ray attenuation (100–200 Hounsfield Unit (HU)) compared to soft tissues (20–80 HU). Larsson et al. (2015) optimized an imaging technique using BaSO4 and gadolinium as a dual-modality contrast agent component for detecting and monitoring diseases.194 This method involves macrophages combined with gadolinium and barium sulfate nanoparticles. Specifically, mouse alveolar macrophages are loaded with BaSO4–Gd at a ratio of 2[thin space (1/6-em)]:[thin space (1/6-em)]4 million cells to serve as a contrast agent in X-ray and magnetic resonance imaging (MRI). Lopresti et al. (2020) investigating a novel epoxy resins composite with barium sulfate as a supplementary substance for X-ray shielding purposes. This composite is coated by 0.75 grams stearic acid to reduce sedimentation and increase its dispersibility.195 Another nanocomposite designed by Abdolahzadeh et al. (2023) for X-ray shielding consists of polyethylene, tungsten oxide, bismuth trioxide, barium sulfate, and graphene oxide, as shown in Fig. 14a.196 The optimal composition to gains the maximum thermal resistance of the X-ray shield is achieved at 90%-wt HDPE and 10%-wt BaSO4–GO. This nanocomposite has a higher melting point of 132.1 °C while maintaining its crystallinity at 60.1%. Furthermore, the addition of 25%-wt WO3/Bi2O3/GO has a transmission factor of 10% at 50 kVp with 6 mm thickness, aligns with the compatibility standards of Pb-based protective apron with thickness of 5.5 mm while reducing its mass to 4.10 times lighter than Pb-based apron.


image file: d5ra02597d-f14.tif
Fig. 14 (a) Nanocomposite structure on nanographene oxide sheets for X-ray shielding (reproduced from ref. 196 with permission from International Journal of Radiation Research, copyright 2023); (b) SEM images of MWCNT-coated BaSO4 surface (reproduced from ref. 199 with permission from Wiley, copyright 2024).

Further experiment conducted by Kilian et al. (2023) developed a dual-modality contrast agent consists of barium sulfate and cyanine tetrafluoroborate salt pigments for both X-ray and photoacoustic imaging (PAI) in gastrointestinal tract.197 Result shows that combination of 40%-w/v barium sulfate and 5 mg per mL pigment can be used as contrast agent of Nd–YAG nonablative laser at a wavelength of 1064 nm, which has been widely used in PAI and relatively safe for on skin treatment.198 Meanwhile, Tokonami et al. (2024) modifies BaSO4 nanoparticles properties by coating them with multiwalled carbon nanotubes (MWCNTs) to increase the interfacial bonds and tensile modulus.199 Multiwall carbon nanotubes consist of multiple layers of graphite sheets rolled into a cylindrical shape, with an interlayer spacing of 3.4 Å and mainly synthesized through chemical vapor deposition (CVD) route.200,201 PS is considered to be used for matrix formation of BaSO4–MWCNTs nanocomposite due to its solubility in organic solvent.202 The PS/BaSO4–MWCNTs composite, as shown in Fig. 14b, does not exhibit fracture traces at the interface, indicating that cohesive failure occurs as a result of stress concentration within the matrix. This observation suggests that the PS resin bonds with the MWCNT structure through π-bond opening on the BaSO4 surface,203 thereby reduces interfacial slip between the PS and BaSO4 components.204 The modification of BaSO4 nanoparticles was proven did not disrupt the X-ray attenuation.

Aside from its main usage as contrast agent, BaSO4 nanoparticles also have been proposed to improve orthopedic advancement, i.e. analysis of bone structure and bone cement filler. Leng et al. (2005) suggested barium sulfate nanoparticles as contrast agent of micro-computed tomography (μCT) for bone microstructure analysis.205 Microdamage on the bone structure is an condition arising from continuous fatigue or overloading.206 Generally, there are two types of microdamages, namely linear microcracks and diffuse damage.207 The most common microdamage is linear microcracks with length around 30–100 μm.208 In that case, μCT scan with BaSO4 as contrast agent is one of the most prominent solution to locate the bone microcracks before determine further treatment.

Leng et al. (2005) prepared BaSO4 nanopowder with 1 M BaCl2 and 0.1 M Na2SO4, resulting an average particle size approximately 50 nm.205 The particles were injected into cortical bone tissue and coated the inner walls of the holes, resulting in a deposited thickness of BaSO4 nanoparticles which are smaller than the hole diameter. The measured hole diameter and volume with deposited nanoparticles appeared larger than its actual size and volume, indicating an approximately eightfold discrepancy in volume measurement. Further evaluation done by Leng et al. (2008) regarding to their previous experiment proves that the use of barium sulfate contrast agent allows for the imaging of microdamage without destroying the specimens, enabling repeated measurements and studies.209 The relative volume of the stained region, indicative of microdamage accumulation, was found to be amplified with an increasing number of loading cycles in accordance to the power law.210

Fang et al. (2014) modified BaSO4 nanoparticle synthesis to be implemented in bone cements by using 0.25 M K2S2O8 with 50 mL of 0.5 M KOH, BaCl2, and difunctional surface modification agent of (2-(methacryloyloxy) ethyl dimethyl-(3-sulfopropyl)) ammonium hydroxide (MSAH) with MSAH/BaCl2 molar ratio ranging from 0 to 0.128.211 The produced 10–30 nm spherical BaSO4 nanoparticles integrated with polymethyl methacrylate (PMMA) matrix to strengthen the bone cements. MSAH-functionalized BaSO4 nanoparticles effectively improved the bending modulus of the bone cement from 2293 MPa to 3100 MPa while the compressive strength gradually increased from 96 MPa to 132 MPa due to the increased MSAH coating and decreased particle size. The increment of the MSAH-functionalized BaSO4 nanoparticles to 20%-wt would result in the improvement of radiopacity and did not affect the biocompatibility of the bone cements.

Barium sulfate nanoparticles modification method also have been through some advancement for cancer treatment. Nowadays, several methods have been proposed to cure cancer and tumors, such as stem cells for lymphoma treatment, immune-stimulating vaccines for renal cell cancer (RCC), radiofrequency ablation for liver metastasis, and nanoparticle-based drug delivery system.212–215 Targeted alpha therapy utilized nanoparticles to deliver radionuclide which would emits alpha particles to treat the targeted tumor cells.216 Targeted alpha therapy have shorter route which establish higher linear energy transfer (LET) and relative biological effectiveness (RBE).217,218 Targeted alpha therapy is considerably less toxic compared to other kind of radiation methods. Hence, it garnered more interest in the development of cancer treatment technologies.

Reissig et al. (2019) developed a novel one-pot approach for the synthesis of barium sulfate nanoparticles as a carrier to bond Ra with the targeted molecule.219 This new route is designed to accommodate targeted alpha therapy. The carriers were built by the addition of 6.6 mg (NH4)2SO4 feeding solution to 61 mg BaCl2 and 46 mg alendronate stock solution under stirring at 1000 rpm and with Ba2+/SO42− ratio of 6[thin space (1/6-em)]:[thin space (1/6-em)]1. This precipitation produces barium sulfate nanoparticles with a mean diameter of 140 ± 50 nm. Alendronate serves as a binding agent between BaSO4 nanoparticles and the target molecule, featuring a phosphonate group and a peptide linkage.220 These synthesized alendronate-containing [133Ba]BaSO4 and [224Ra]BaSO4 nanoparticles exhibited a very low activity release of less than 5%, BaSO4 nanoparticles exhibited a very low activity release of less than 5% and their reactivity are confirmed by the result of active ester coupling, which are favoured for theragnostic purposes. However, this method only has an binding efficiency of 20% 224Ra with the BaSO4 nanoparticles. This discovery became the basis for further research conducted by Reissig et al. (2020), which involved a two-step precipitation method to produce BaSO4 nanoparticles with smaller size around 9.1 nm and more homogenous spherical orthorhombic geometry.221 These BaSO4 nanoparticles act as carriers for radium, barium, lutetium, indium, and zirconium with high radionuclide retention (>90%) and higher 224Ra radiochemical yields of 31%.

Aside from targeted alpha therapy, transcatheter arterial embolization is also considered as an alternative to cancer medication. Transcatheter arterial embolization (TAE) can be defined as a non-invasive therapy by inserting a catheter into a blood artery, commonly employed to remedy unresectable and neuroendocrine tumors, and liver cancer.222–224 However, the lack of inherent radiopacity in conventional embolic materials limits the real-time tracking and precise positioning of these agents, leading to challenges in controlled delivery and efficacy assessment. Wang et al. (2015) introduces a novel embolic agent consisting of barium alginate (ALG) microspheres loaded with in situ synthesized BaSO4 nanoparticles, prepared via a one-step droplet microfluidic technique combined with external ion-crosslinking.225 The BaSO4/ALG microspheres is more durable and have excellent X-ray visibility and embolic efficacy compared to the commercially available calcium alginate microspheres, as confirmed by in vitro and in vivo assays. The addition of 5–9 wt% BaSO4 nanoparticles in this admixture improve the thermal resistance below 220 °C and also proven to enhance the visibility under X-ray up to 14 days.

Another tumor theragnostic supporting material is developed by Shukla et al. (2023) using carrageenan-linked BaSO4 nanoparticles.226 The 200–300 nm Ba-linked iota carrageenan have better fluorescence activity than the powered Ba-carrageenan. The cancer cell selectivity observed through the escalation of pKa while some Ba2+ ions released in acidic environment. This ionization mechanism would disrupt cells homeostasis, consequently trigger the lysis of the cancer cells. Meanwhile, investigation on the cytotoxicity of the BaSO4 nanoparticles synthesized via one-step hydrothermal method shows viability decreasement of the cancer cell implanted in a mouse at a dose of 823.8 μg mL−1.227

4.2 Barium sulfate nanoparticles application in polymer industry

In recent years, the incorporation of barium sulfate nanoparticles into the polymer industry has sparked major interest due to their exceptional properties and versatile applications. BaSO4 nanoparticles have abundant advantages to improve polymers' characteristics, including high density, chemical inertness, thermal resistance, and unique optical properties.228 Consequently, barium sulfate nanoparticles have garnered significant attention from researchers and manufacturers to escalate the performance and functionality of polymer-based products. Furthermore, increasing consumption of polymer-based fibers for textile industry and coating materials also plays a role in high demand of better polymer products.

Chen et al. (2009) analyze the modification of high density polyethylene (HDPE) with BaSO4 nanoparticles as an additive, especially in terms of its mechanical properties for anti-wave applications in marine environments.229 40 nm BaSO4 nanoparticles and sodium stearate were mixed to provide organic functional bonds on their surface before melded with HDPE at 210 °C to produce BaSO4/HDPE nanocomposite. Sodium stearate alter the interstitial particle spaces to ensure better dispersion of barium sulfate so it have strong adhesion with the polymer matrix.230,231 From this experiment, it is concluded that the addition of BaSO4 nanoparticles should be at the maximum amount of 2%-wt due to lower interface binding strength caused by excess doping material in the polymer matrix structure. The differences in the bond strength between the nanocomposites at varying concentrations can be seen visually from SEM images in Fig. 15a–d. Increment of half crystallization time and peak crystallization temperature was observed with the addition of BaSO4 content. The highest half crystallization time at 0.55 min and peak crystallization temperature at 114.1 °C was achieved by 5%-wt BaSO4 nanoparticles addition into the nanocomposite. This phenomena happened due to unstable state caused by faster heterogenous nucleation rate and crystallization rate of the nanocomposite, identified by its narrow peak and smaller supercooling degree at 11 °C.232,233 However, further aggregation of BaSO4 nanoparticles above 1%-wt would defects and reduce the toughening effect of the product. Additionally, the tensile yield stress of HDPE/BaSO4 nanocomposites is mainly affected by the interface stress transfer. This interfacial binding is also increase 10.4% storage modulus for 5%-wt BaSO4 nanoparticles at 25 °C. Thus, the optimal mechanical properties for anti-wave material is achieved by addition of 1.0 wt% BaSO4 nanoparticles which reached the highest impact strength of 63.6 kJ m−2 compared to pure HDPE impact strength which is around 8.5–9.34 kJ m−2,234,235 as in this case impact strength is a critical factor for materials to withstand fractures when subjected to stress from wave forces.236


image file: d5ra02597d-f15.tif
Fig. 15 SEM micrographs of HDPE nanocomposites at various BaSO4 content: (a) 0%-wt; (b) 1%-wt; (c) 2%-wt; and (d) 4%-wt (reproduced from ref. 229 with permission from SAGE Publications, copyright 2009); (e) SEM image of BaSO4/PP nanocomposite fibers (reproduced from ref. 237 with permission from World Scientific, copyright 2012); (f) SEM images of 2%-wt blank BaSO4/PET and (g) 2%-wt modified BaSO4/PET nanocomposites (reproduced from ref. 242 with permission from Elsevier, copyright 2011); (h) SEM image of PVDF/BaSO4/nanoclay with mixing concentration/gram ratio of 5[thin space (1/6-em)]:[thin space (1/6-em)]0.68[thin space (1/6-em)]:[thin space (1/6-em)]0.10 (reproduced from ref. 247 with permission from Springer, copyright 2017).

Another experiment conducted by Li et al. (2012) to make BaSO4/polypropylene (PP) nanocomposite using similar method from Chen et al. (2009) with dodecyl benzene sulfonic acid (DBSA) as compatibilizer agent.229,237 DBSA was chosen due to its non-hygroscopic anionic properties and its chemical structure which consists of long dodecyl chain attached to a benzene ring and hydrophilic sulfonic group that would create protective layer and stabilize the particles by the steric hindrance.238 The presence of BaSO4 nanoparticles inversely affects materials' consistency at various temperature compared to the flow index, suggesting that these nanoparticles create strong bonds with polymers, thereby impeding the slippage among polymer molecules.239,240 Therefore, the BaSO4 content must be adjusted in terms of shear thinning to maintain the mobility of the PP molecules to facilitate spinning to form fibres. The effectiveness of interface interaction is proven to escalate the mechanical properties of the BaSO4/PP fibers. Enhancing the draw ratio of fibers leads to an improved preferential orientation along the fiber chain axis.241 This tendency makes the storage modulus of fibers increase at a higher amount of 60 wt% BaSO4. Thermogravimetry analysis of the BaSO4/PP hybrid fibers shows an escalation of the onset of degradation temperatures from nanocomposite fibers with 40%-wt and 60%-wt of BaSO4, approximately 10–20 °C higher than the pure PP fiber. The SEM image of BaSO4/PP nanocomposite fibers is presented in Fig. 15e.

Similar experiment was conducted by Gao et al. (2011) with precipitation method using stearic acid as hydrophobic agent to support the formation of 100–150 nm elliptical BaSO4 nanoparticles before combined with poly (ethylene terephthalate) to create BaSO4/PET nanocomposites.242 TGA data between pure PET and 2%-wt modified BaSO4/PET shows increasing decomposition temperature at 445.18 °C. This condition indicates that the integration of BaSO4 into the PET matrix results in increased thermal decomposition temperatures, thereby gave better thermal stability of PET. On the other hand, excessive agglomeration of doped BaSO4 would reduce thermal stability of this nanocomposite due to the weakened chains in polymer matrix and localized overheating. The final product with well-dispersed molecules and the best mechanical properties is obtained by 2%-wt modified BaSO4/PET nanocomposites at supercooling degree of 16.36 °C. SEM images of blank and modified 2%-wt BaSO4/PET nanocomposites which are respectively presented in Fig. 15f and g support the above statement. The same group then proceeded to do further research to analyze the thermal stability and crystallization tendency by using in situ polymerization of TPA, EG, and BaSO4 nanoparticles modified with stearic acid.243 TGA data revealed that BaSO4/PET nanocomposites maintain excellent thermal stability by showing no substantial mass loss below 360 °C. Based on Friedman method calculation, the activation energy for thermal decomposition of nanocomposites is higher compared to pure PET sample. Thus, the inclusion of BaSO4 nanoparticles hinders char formation and prevents the escape of volatile byproducts during thermal decomposition.

Kulkarni et al. (2013) proposed a novel approach with in situ synthesis method of poly(styrene–butylacrylate–acrylic acid) latex/BaSO4 nanocomposite.244 40 nm BaSO4 nanoparticles were produced via micellar solution spray process and sonicated with SDS and sorbitan monolaurate before stirred at 70 °C to combine them with styrene, butyl acrylate and acrylic acid monomers to create the desired nanocomposite. This method ensured a homogeneous dispersion of the nanoparticles within the latex matrix, as shown on the TEM images of BaSO4 nanoparticles. Atomic Force Microscopy (AFM) images confirmed that the uniform dispersion of BaSO4 nanoparticles must be at the maximum loadings of 2%-wt BaSO4 to avoid excessive agglomerates. Maximum tensile strength and elongation at break were achieved at a BaSO4 loading of 1.5%-wt, with values reaching 7.9 MPa and 387%, respectively. The enhancement in mechanical properties is attributed to the strong interfacial adhesion between the BaSO4 nanoparticles and the PSBA latex matrix. Incorporation of 2%-wt BaSO4 nanoparticles sets the onset and endset decomposition temperature at the higher value of 392 °C and 456 °C respectively, indicating improved thermal stability of this nanocomposite. The BaSO4 nanoparticles in this composite film also notably lower water absorbance percentage from 25% to approximately 5% in 72 hours, demonstrating their superior ability to enhance moisture resistance compared to the pure PSBA latex film. Therefore, this composite is has the potential to improve the quality of PBSA latex for various coating applications.

Another synthesis route for barium sulfate nanoparticles to modify acetal resin using a thermoplastic injection system is proposed by Ahmed and Hasan (2017).245 The mean force required to cause failure decreased from 12.551 kN for the control sample to 10.096 kN for the modified BaSO4/acetal nanocomposite. The agglomeration of BaSO4 nanoparticles would break the binding interface of the polymer matrix, resulting in a slight reduction in nanocomposite material strength. The optimal amount of BaSO4 nanoparticles required for acetal resin doping material to maintain its radio-opacity properties is 3%-wt. This nanocomposite has radio-opacity equivalent to 2 mm thickness of aluminum without significantly altering the color or flexibility of the acetal resin while ensuring adequate radio-opacity for dental applications. A similar approach was explored by Romero-Ibarra et al. (2012), who successfully produced nanocomposites consisting of 1%-wt of BaSO4 nanoparticles and polyurethane using a melt extrusion technique.246 This composite is less brittle than the other one containing 40%-wt of BaSO4. The BaSO4/PU nanocomposite shows potential for biomedical tracking of microplastics inside the human body. In a related study, Li et al. (2022) successfully modified a thermoplastic polybutylene terephthalate (PBT)/polyethylene terephthalate (PET) composite by incorporating BaSO4 nanoparticles to enhance heat resistance and impact tolerance.15 The surface of BaSO4 nanoparticles were adjusted with aluminate before being added to the blend. The optimal composition of the PBT/PET/BaSO4 composite was found to be PBT[thin space (1/6-em)]:[thin space (1/6-em)]PET at 90[thin space (1/6-em)]:[thin space (1/6-em)]10%-wt with 4%-wt of BaSO4 nanoparticles. This formulation increased the heat distortion temperature to 174.4 °C, 22.6% higher than pure PBT/PET, and enhanced tensile strength to 59.72 MPa, 11.6% higher compared to unmodified PBT/PET blend.

Aside from their leverages in terms of thermal stability and opaqueness, BaSO4 nanoparticles also considered as a potential material to construct UV-Vis absorbing feature in polymer. Agarwal et al. (2017) developed PVDF nanocomposites with alkyl quaternary ammonium bentonite montmorillonite nanoclay (MMT) and BaSO4 nanoparticles, showcasing its improved thermal stability and UV-shielding properties.247 25–33 nm BaSO4 nanoparticles were synthesized using an in situ deposition method with polyethylene glycol (PEG) as stabilizer surfactant. Then, BaSO4 nanoparticles were mixed with PVDF and nanoclay in DMSO solvent to produce nanocomposite which can be seen in Fig. 15h. TGA data shows that this nanocomposite has higher decomposition temperature >600 °C, whereas the average primary decomposition temperature of pure PVDF is ranging from 400–510 °C.248 The exothermic melting peak at 160–170 °C is caused by presence of MMT which act as heterogeneous nucleating agent, hence it is lower than the pure PVDF melting point at 174.8 °C.249 The UV-Vis spectroscopy data revealed that UV-Vis absorbance is mainly influenced by BaSO4 content, which escalated from 20% to 60% when the BaSO4 is increased twofolds. Moreover, the optimal mixing concentration/gram ratio of PVDF/BaSO4/Nanoclay to obtain the most effective UV-Vis absorbing composite material is 5[thin space (1/6-em)]:[thin space (1/6-em)]0.68[thin space (1/6-em)]:[thin space (1/6-em)]0.10. Further study conducted by Ahmad et al. (2021) investigated the effects of barium sulfate addition in polyvinyl acetate (PVA), especially their UV-Vis absorbance properties.250 From this experiment it is concluded that the maximum UV-Vis light absorption at a wavelength of 600 nm can be achieved by adding 2%-wt of BaSO4 to create the PVA/BaSO4 nanocomposite.

In addition to its utilization in polymer structure as a UV-Vis absorber, BaSO4 nanoparticles with high dispersibility have also been proposed as an environmentally friendly alternative for cooling systems. Wu et al. (2023) designed a BaSO4–epoxy resin composite film with radiative cooling feature.251 This composite constructed of spherical BaSO4 clusters formed using directly precipitated PVP-modified BaSO4 nanoparticles as their template. The obtained BaSO4–epoxy resin composite demonstrated a solar reflectance of 71% and the ability to cool down the temperature up to 13.5 °C with excellent amount of tensile strength, which is 16.6% higher than conventional radiative cooling films. All of these innovative applications of BaSO4 nanoparticles are summarized in Table 5.

Table 5 Applications of BaSO4 Nanoparticles
Application Nanocomposites Special features Ref.
Medication BaSO4 nanoparticles stabilized with dextran High X-ray attenuation (100–200 HU), well-dispersed in water 16
BaSO4–Gd contrast agent Dual-modality contrast for X-ray and MRI 194
Epoxy resin composite with BaSO4 NP and stearic acid Improved X-ray shielding, reduced sedimentation 195
HDPE–BaSO4–GO nanocomposite X-ray shielding, high thermal resistance, lightweight alternative to Pb-based aprons 196
BaSO4 with cyanine tetrafluoroborate salt pigments Dual-modality contrast agent for X-ray and PAI (photoacoustic imaging) 197
PS/BaSO4–MWCNTs composite Increased interfacial bond strength, improved tensile modulus, and cohesive failure prevention without disrupting X-ray attenuation 199
BaSO4 nanoparticles for μCT imaging Accurate bone microcracks locations detection, improved microdamage visualization 205 and 209
MSAH-functionalized BaSO4–PMMA composite Strengthened bone cements, improved bending modulus and compressive strength, increased radiopacity 211
Radium-binded BaSO4 nanoparticles Targeted alpha therapy for cancer treatment, high radionuclide retention (>90%) 219 and 221
BaSO4/ALG microspheres Enhanced X-ray visibility up to 14 days, improved embolic efficacy for TAE (transcatheter arterial embolization) 225
Carrageenan-linked BaSO4 nanoparticles Fluorescence activity, cancer cell selectivity via pKa escalation 226
Hydrothermally synthesized BaSO4 nanoparticles High cytotoxicity for cancer cell elimination 227
Polymer BaSO4/HDPE Enhanced mechanical properties for anti-wave applications; optimal impact strength at 1%-wt BaSO4; increased crystallization temperature and modulus at 5%-wt BaSO4 229
BaSO4/PP Increased fiber orientation; onset of degradation temperature increased by 10–20 °C 237
BaSO4/PET Increased thermal stability and decomposition temperature at 2%-wt BaSO4 242
TPA/EG/BaSO4 Increased thermal stability and reduced thermal weight loss 243
BaSO4/PSBA latex Improved dispersion, tensile strength, and elongation at break; enhanced thermal stability; reduced water absorbance (from 25% to ∼5%) 244
BaSO4/acetal resin Maintains radio-opacity equivalent to 2 mm aluminum; optimal doping at 3%-wt without altering color or flexibility 245
BaSO4/PU Less brittle than low BaSO4 content composites; potential for biomedical tracking applications 246
BaSO4/PBT-PET Enhanced heat resistance (+22.6%) and tensile strength (+11.6%) 15
BaSO4/PVDF/MMT Improved thermal stability (>600 °C); enhanced UV-Vis absorbance 247
BaSO4/PVA Maximum UV-Vis absorption at 600 nm achieved at 2 wt% BaSO4 250
BaSO4/epoxy resin High solar reflectance; radiative cooling effect; increased tensile strength (+16.6%) 251


5. Green synthesis of BaSO4 nanoparticles

New approaches to the green synthesis of nanomaterials are currently receiving a great deal of attention in the field of nanomaterial engineering.252 Many studies have developed the synthesis process of barium sulfate nanoparticles, for example via chemical precipitation, hydrothermal, and solution combustion.50,227,253 However, these methods have generated new problems in environmental pollution due to hazardous and toxic waste. Therefore, researchers are focusing on solutions to find nanomaterials synthesizing method that can optimize industrial processes and medical benefits to make them more sustainable and environmentally friendly. One of the most prominent routes to environmentally friendly nano-sized products is through green synthesis using plant extracts. This approach fosters the development of less toxic nanomaterials and reduces production costs.

Chen et al. (2016) investigated the characteristics of barium sulfate nanoparticles obtained from precipitation with soluble biomolecules from fruit extracts.254 Fig. 16a and b show that BaSO4 synthesis without fruit extract would produces larger final products which are more difficult to disperse. The initial nanoparticles obtained with kiwifruit extracts is found in leaf-shaped crystals with toothed edge, as revealed in Fig. 16c and d, while BaSO4 nanoparticles with 2–4 μm thorn spherical were gained in tomato extract, which is validated by SEM images in Fig. 16e and f. Fig. 16g and h show that rod-shaped or quasi-spherical BaSO4 crystals, ranging from a few hundred nanometers to a few micrometers can be produced in addition of orange juice. In contrast, carrot juice yielded quasi-spherical BaSO4 nanocrystals, as shown in Fig. 16i and j. Thus, carrot extract has the most potential to inhibit crystal growth. This synthesis mechanism involves aggregate formation by binding Ba2+ ions with biomolecules, which then adsorbed onto BaSO4 crystal surfaces gained from nucleation, and subsequently dictating the structure of the final products.255–258


image file: d5ra02597d-f16.tif
Fig. 16 SEM images of BaSO4 as final product: (a and b) without any extracts; (c and d) with kiwifruit extracts; (e and f) with tomato extracts; (g and h) with orange extracts; (i and j) with carrot extracts (reproduced from ref. 254 with permission from EDP Sciences, copyright 2016); (k) FEG-SEM of BaSO4 nanoparticles synthesized using Azadirachta indica under magnification 100 nm (reproduced from ref. 38 with permission from Springer, copyright 2019); (l) and (m) SEM images of BaSO4 nanorods synthesized using Azadirachta indica (reproduced from ref. 265 with permission from Environmental Nanotechnology Society, copyright 2024); FESEM analysis of BaSO4 nanoparticles at pH = 7 in presence of (n) starch and (o) PAC (reproduced from ref. 276 with permission from Islamic Azad University, copyright 2019).

Another experiment conducted by Jha et al. (2019) investigated a new method ultrasonic-assisted green synthesis of BaSO4 nanoparticle synthesis by using neem (Azadirachta indica) leaf extract.38 Neem leaf extracts consists of many phytocomponents, such as azadirachtin, nimbin, margosin, and several types of flavonoid, that can be utilized as nucleating agent and growth modifying agent in the main reaction of nanoparticle synthesis.259–262 Agglomeration reduced by tuning the ultrasonication mixing which consequently would increase the temperature in reaction system and nucleation rate.263,264 The average crystalline size of barium sulfate nanoparticles obtained from this experiment with 0.05 M BaCl2 and 0.05 M Na2SO4 is ∼55.6 nm, as presented in Fig. 16k. These BaSO4 nanorods have high thermal stability up to 310 °C with weight loss only around 7.69%. Further experiment using neem leaves extract by Saravanan et al. (2024) involves similar co-precipitation method with the same reactants as previous attempt by Jha et al.38,265 Instead of incubating the precipitate at 37 °C for 24 hours before centrifugated, Saravanan et al. drying the obtained BaSO4 particles in a hot-air oven at 100 °C for 2 hours. As shown in Fig. 16l and m, BaSO4 nanoparticles product from this experiment have a mean particle size of 71.34 nm with rod-like shape.

Aside from plant extracts, several compounds can also be considered as an alternative to enable green synthesis route for nanoparticles synthesis. For example, glycerol is a hydrophilic organic triol compound, usually found as a byproduct of lipid hydrolysis.266–268 This compound is biodegradable, non-toxic, and cheaper than other available reducing agents.269,270 Hosseini et al. (2019) developed a novel auxiliary solution consists of 5 mL pure glycerol, 3 mL concentrated HCl, 1 mL isopropanol, 4 grams NaCl, and 30 mL deionized water to stabilize barium sulfate nanoparticle in precipitation synthesis method with 0.04 M Na2SO4 and 0.04 M BaCl2.271 The results show that the optimal concentration ratio for this method is 1[thin space (1/6-em)]:[thin space (1/6-em)]1 and increment of feed concentration would produce more homogenous and well-dispersed particles. Furthermore, the addition of 5 mL of the auxiliary solution as steric and electrostatic stabilizer results in better uniformity and significantly reduce particle size.272–274 They hypothesized that the nanoparticle stabilization involves glycerol hydrophilic matter and liposome localization near the sulfate anions to change the chains configuration and subsequently prevent agglomeration.275 Moreover, lower operation temperature at 0 °C is more suitable to minimize the growth of nanoparticles. BaSO4 nanoparticles obtained from the optimal state at mixing rate of 900 rpm have an mean particle size of 12 nm.

Manteghian & Sameni (2019) synthesized BaSO4 nanoparticles using starch and polyanionic cellulose (PAC) as stabilizer.276 Starch is a polysaccharide composed of D-glucose residues with α-(1→4) linkages in linear amylose and approximately 5% α-(1→6) branch linkages in amylopectin, which are intertwined in starch granules that can expand and leach in heated water.277,278 Polyanionic cellulose (PAC) is a derivative product of cellulose compound which is more superior over carboxymethyl cellulose (CMC) in terms of filtration reduction, anti-salt, anti-collapse and thermal stability.279 PAC is mainly used as a renewable viscosifier additive in oil drilling.280 PAC and starch stabilize BaSO4 nanoparticles by carboxyl and hydroxyl groups formation in polymeric chains. Synthesis of BaSO4 nanoparticles with 300 mg/25 cc starch by adding 21.3 mg/25 cc BaS and 135 mg/25 cc Na2SO4 as reactant would result in wider range of particle size between 33–140 nm and mean particle size of 58.7 nm, as shown in Fig. 16n, whereas SEM image in Fig. 16o reveals that BaSO4 nanoparticles obtained with PAC at the same condition have particle sizes ranging from 10–58 nm with mean particle size of 18 nm. Due to longer chemical structure chains than starch, PAC would induce greater steric effect, hence have thicker protective layer surrounding BaSO4 nanoparticles that more effectively passivating their surface to prevent any contact with the covered particle.281,282

6. Conclusions and future outlooks

Barium sulfate has gained interests due to its unique characteristics. This argument is proven by increasing barium sulfate production per year to ensure abundant supply in the industrial market, mainly for petroleum and polymer industry. Barium sulfate nanoparticles were developed to increase its market value while expanding the range of sectors that can be optimized with barium sulfate. Barium sulfate nanoparticles have been used in medical imaging technology and as polymer additives to increase its thermal stability. With its economically beneficial advantages in mind, bottom-up production of barium sulfate nanoparticles by direct precipitation method is the most widely developed synthesis route to this date. However, direct precipitation of barium sulfate by only using two main reactants as precursor without any additives and pH tuning often leads to higher agglomeration rate. To solve this issue, capping agent are added during precipitation to control the nucleation rate and growth of nanoparticles. These conditions directly causes the shift in optimal pH conditions and necessary amount of all reactants and additives in reaction. Based on the comparison of several capping agents used to produce BaSO4 nanoparticles by precipitation method, surfactants such as DTAB, CTAB, ODP, and DBSA tend to produce larger particle sizes (56 nm to 289 nm) compared to polymers, which produce smaller particles ranging from 14 nm to 30 nm. This observation suggests that polymers might provide better control over nucleation and growth during particle synthesis, whereas the organic compound solvents have larger particle sizes, ranging from 16 nm to 208 nm, due to the different physical and chemical properties of each compound, including dynamic viscosity, polarity, and functional groups, which affect the interaction between the two phases in the chemical reaction. Nonetheless, this method still have some issues, such as the utilization of hazardous and toxic precipitating and capping agent. Ionic liquid proposed as an alternative capping agent that reduce the toxic waste material from barium sulfate nanoparticle precipitation. The selection of ionic liquid as capping agent for the precipitation of BaSO4 nanoparticles must consider several factors, including the hydrophobic and hydrophilic nature of ionic liquid, its molar conductivity, and its diffusivity in water. Another approach to developed green synthesis is proposed by using natural resources for conventional capping agent substitute. Some natural compounds, such as neem leaf extract, starch, polyanionic cellulose, and glycerol, have been reported to enhance the performance of BaSO4 nanoparticle precipitation, resulting in a mean particle size ranging from 10 nm to 72 nm, relatively similar to that of BaSO4 nanoparticles obtained from precipitation with other compounds. These natural compounds should be examined further to evaluate its reproducibility and stability in BaSO4 precipitation reaction.

Furthermore, barium sulfate nanoparticles direct precipitation scale-up is explored by designing various types of reactors with their own capacity and characteristics. In a Rotating Packed Bed (RPB) reactor, the BaSO4 nanoparticles precipitation reaction conducted at a rotating speed three times higher and involving organic solvents would dramatically decrease the obtained BaSO4 particle size from 45 nm to 10 nm. Meanwhile, BaSO4 nanoparticles precipitation in a T-mixer typically produces nanoparticles with a mean particle size around 70–80 nm, unless modified by using a capillary microreactor or adjusting the inlet tube angle to resemble a Y-mixer. These modifications increase the contact frequency and surface area, resulting in BaSO4 nanoparticles with a smaller mean particle size of around 33–53 nm. Another type of reactor, namely spinning disc reactor (SDR), tends to highly depend on its high rotating speed and small disc diameter to produce fine BaSO4 nanoparticles. On the same note, a membrane reactor produces fine particles ranging from 15 nm to 70 nm but requires more reaction time, as it works within a specific range of transmembrane pressure and has limited selectivity. Hence, reactor design, dimensions, flow rate, and rotating speed should be carefully considered before scaling-up this reaction process for specific industrial purposes.

Based on the comparisons data presented in this review, we conclude that further development of barium sulfate nanoparticles will more likely to continue the use of precipitation method in the liquid phase. With the external demand to find environmentally friendly synthesis routes, the most potential capping agents for further research are those based on organic compounds given their high availability and great dispersibility. The design of scale-up reactors that have prominent shear forces feature such as spinning disk reactors can be optimized to break up aggregates while maintaining the nucleation rate. Meanwhile, the performance of T-mixer and Y-mixer reactors as one of the most common reactor types can be improved by investigating the effect of solution flow rate and the length of pipelines on the rheology of the system which is related to the agglomeration rate and particle size distribution. Analysis of the effect of pressure variation on the nucleation rate in a closed reactor with a certain temperature can also be conducted to obtain a smaller final product with a narrower size distribution. Baffles may be used in a barium sulfate precipitation reactor to prevent vortex formation, significantly increasing the probability of achieving a more uniform distribution of reactants and resulting in a final product with an improved particle size distribution.

Barium sulfate nanoparticles have great prospects in the medical and polymer industries. Their surface-modifying capability can enhance both the base material's mechanical strength and its beneficial features to improve the base material's functionality, such as reduced water absorbance, a higher threshold for thermal decomposition, UV absorption, and serving as a tool to increase its economic worth. In conclusion, barium sulfate nanoparticles have significant potential in various biomedical and industrial applications due to their unique properties and functional versatility. Future research should focus on optimizing their synthesis and exploring innovative applications to fully harness their utilization in various sectors.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts to declare.

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