The aggregation behavior of cellulose micro/nanoparticles in aqueous media

Abstract

Cellulose microparticles (CMPs) and cellulose nanoparticles (CNPs) were obtained from cotton microcrystalline cellulose (MCC) using a NaOH/urea system and sulfuric acid hydrolysis, respectively. The comparative analysis of the morphology and surface charge density between the CMP and CNP is carried out. Moreover, the effect of ionic strength on the aggregation behavior of the CMP and CNP in aqueous media has been investigated by means of small-angle light scattering during rheological characterization (rheo-SALS). The aqueous CMP suspensions present more pronounced particle aggregation than the CNP suspensions at the same concentration. Moreover, the dimensions of the CNP aggregates can be tuned by changing ionic strength, while the CMP aggregates are stable and do not reorganize themselves with ionic strength. Both the aqueous CMP and CNP suspensions exhibit a shear-thinning behavior, and the SALS patterns indicate the aqueous suspensions remain isotropic with no preferential alignment in the shear direction. With the addition of NaCl, the viscosity of CNP increases significantly over the shear rate range investigated, while that of CMP is less sensitive. The different responsive behavior to the addition of NaCl between the two types of suspensions is ascribed to the difference in the amount of surface charge, which governs the interparticle forces.

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Introduction

Nanocrystalline cellulose, an organic material not synthesized from molecular or atomic components but rather typically extracted from plants, has seen growing interest.^1,2 Nanocrystalline cellulose can be produced by different routes and from a variety of cellulose sources. The treatments of native cellulose samples using strong acid^3–5 or NaOH/urea^6,7 can disintegrate the microfibrils, yielding nanocrystalline cellulose suspensions. The preparation parameters affect the resulting cellulose properties, such as size, aspect ratio, surface charge and thus their potential application.

It has been shown that nanocrystalline cellulose can be used as stabilizing agents in emulsions, also called Pickering emulsions, which are ultrastable even at very low concentrations.^8–11 The high stability of the emulsions is attributed to the nanoparticle irreversible adsorption associated with a networking organization. The formation of the networking originates from the aggregation ability of nanocrystalline cellulose. It is well established that under specific processing conditions (temperature, pressure, salinity and pH), nanocrystalline cellulose can self-assemble into different kinds of aggregates.^12–14 The aggregates of nanocrystalline cellulose represent a new class of colloids with fascinating but not yet completely understood properties. In many nanocrystalline cellulose systems, the formation and characteristics of the aggregates is a result of long-range repulsive electrostatic interactions, which is larger than the attractive interactions that typically governed by van der Waals interactions.¹⁵ Stable aqueous nanocrystalline cellulose suspensions have been achieved through chemical modification on the cellulose surface via steric stabilization by the formation of polymer brushes^16,17 or electrostatic stabilization by the introduction of surface charged groups.¹⁸ So far, most studies have only focused on the aggregation of nanocrystalline cellulose prepared from strong acid hydrolysis, and very few studies have made comparison on the aggregation properties of the cellulose prepared with different methods. The preparation conditions govern the morphologies and surface properties of the individual nanocrystalline cellulose, and hence the aggregation behavior.

Considering the fact that the cellulose aggregates are highly sensitive to the processing conditions, understanding the rheology will be critical for their application. In the present study, we compare the aggregation behavior of two types of nanocrystalline cellulose with different particle morphologies and surface charge density. The two types of nanocrystalline cellulose were prepared from cotton microcrystalline cellulose. One is quasi-spherical after NaOH/urea treatment, and the other is rod-like after sulfuric acid hydrolysis. The rheological properties and microstructure of the cellulose suspensions have been investigated using rheology-small angle light scattering (rheo-SALS) method. The utilization of SALS with rheological characterization will allow for real time analysis of the structure formation of the cellulose aggregates and their correlation with changes in viscosity. The changes in the aggregation behavior upon the addition of salt are described and discussed. To our knowledge, no previously published study has compared the aggregation properties of the cellulose micro/nanoparticles prepared using NaOH/urea system and sulfuric acid hydrolysis, respectively. Compared with the extensive research on cellulose nanoparticle at nanoscale, this work probes the formation of the cellulose aggregates under shear at larger length scales. This work is representative of ongoing efforts to provide a basis on understanding the macroscopic aggregation properties of cellulose micro/nanoparticles.

Materials and methods

Chemicals

Sodium hydroxide (NaOH), urea, sulphuric acid (H₂SO₄), and sodium chloride (NaCl) were purchased from Sinopharm Chemical Reagent CO., Ltd, and used without further purification. A commercial cotton microcrystalline cellulose (MCC) was purchased from Sangon Biotech (Shanghai) Co., Ltd., with a particle size about 100 μm.

Dissolution of MCC

The MCC powder was added to a 50 mL centrifuge tube, and then deionized water was added to wet the cellulose powder, followed by the addition of the precooled aqueous solution of NaOH/urea (−7 °C) or preheated aqueous solution of H₂SO₄ (45 °C). Homogeneous cellulose suspensions were obtained by a vortex and then incubated for durations of up to 5 h. The %transmittances of the cellulose suspensions at 610 nm were measured using a spectrophotometer and used as indexes to show the extent of cellulose dissolution (the higher the %transmittance, the better the dissolution).¹⁹ The effect of NaOH/urea or H₂SO₄ concentration on the dissolution of cellulose was studied in this way.

Preparation of CMP

The aqueous solution containing NaOH/urea/H₂O at the ratio of 7 : 12 : 81 by weight was used as a solvent for MCC. The solvent was precooled to below −7 °C. Then the MCC sample in the desired amount was added immediately into it at an ambient temperature of below 10 °C. The MCC was completely dissolved within 2 min with stirring at 10 000 rpm. The dissolved cellulose was regenerated by adding 10 times v/v deionized water, followed by precipitation and separation of the regenerated cellulose by centrifugation at 6000 rpm for 10 min. The remnants of NaOH and urea from the precipitated cellulose were removed by further washing five times with deionized water.

Preparation of CNP

The hydrolysis of MCC was performed in 60 wt% sulfuric acid at 45 °C while stirred within 3 h. The suspensions were diluted 10-fold to stop the reaction immediately following hydrolysis, and then washed with deionized water using repeated centrifuge cycles. The centrifuge step was stopped after at least five washings until the supernatant became turbid. Dialysis against deionized water was performed to remove any free acid molecules from the dispersion.

Preparation of aqueous CMP and CNP suspensions

The suspensions were obtained at a constant concentration of 1.34 wt% by osmotic compression using cellulose dialysis tubing and a 10 wt% aqueous solution of poly(ethylene glycol) 20 000. The resulting suspensions were followed by sonication. Different concentration of NaCl (0, 10 and 100 mM) was added to the aqueous CMP and CNP suspensions in order to study their effects on the aggregation of CMP and CNP.

Characterization of MCC, CMP, CNP and their suspensions

The average degree of polymerization (DP) was determined by viscosity (25 °C) of cellulose solution in cupri ethylenediamine solution using Ubbelohde viscometer. The efflux time was measured and the viscosity ratio, η_r, was calculated based on the efflux time. The intrinsic viscosity, [η], in mL g⁻¹ was calculated by using the equation:^20–22 log[(η_r − 1)/c] = log[η] + 0.13[η]c, where c is the concentration of the test solution in g mL⁻¹. The DP was then determined from the formula: DP^0.905 = 0.75[η].

The morphology of the MCC particles was investigated by scanning electron microscope. The instrument is a Hitachi model S-4800 equipped with a field emission source and operating at a high voltage (1.0 kV) (Hitachi Tokyo, Japan). The samples were gold coated in an ion sputter coater for 2 min before observation. Morphology of the CMP and CNP was analyzed using a JEOL JEM-2100 transmission electron microscope (TEM) at 80 kV acceleration voltage. The CMP and CNP suspension was deposited on a TEM grid immediately after sonication and dried under ambient conditions. Image J software was used to measure the physical dimensions of CMP and CNP.

The amount of surface sulfate groups for the CMP and CNP was determined by conductometric titration of the suspensions with 0.01 M NaOH. The crystal structures of the CMP and CNP were collected using an X-ray diffractometer (D8 Adrance, Bruker AXS, Germany) equipped with Cu Kα radiation (λ = 1.5418 Å) in a powder measurement mode over the range of 2θ from 8 to 80° at a scan rate of 2° min⁻¹. The crystallinity index was determined by Segal's empirical method.²³

Malvern Instruments Zetasizer (Nano ZS) was used to measure the zeta potential and equivalent hydrodynamic size of the CMP and CNP suspensions as functions of ionic strength. All experiments were carried out in dilute CMP and CNP suspensions (0.01 wt%) where the CMP and CNP randomly oriented. The zeta potential and size of the CMP and CNP suspensions were measured over a range of NaCl concentration (0–100 mM) at 25 °C.

The rheological properties and microstructure of the aqueous CMP and CNP suspensions were investigated by rheo-SALS. Rheology and SALS data were collected simultaneously using an AR-G2 rheometer (TA Instruments, New Castle, DE) with the commercially available SALS attachment. The experimental apparatus has already been described elsewhere.^24–27 A transparent, quartz parallel-plate configuration (40 mm diameter) with a 1 mm gap was used for all rheology tests. The measurements were made at shear rates from 1 to 400 s⁻¹, and temperatures from 25 to 50 °C at a rate of 5 °C min. A single viscosity and scattering pattern are reported per measured shear rate. A He–Ne laser (λ = 635 nm) was used and SALS images were recorded for each of the samples. The accessible range of scattering vectors q is from 1.38 to 6.10 μm⁻¹, where q = (4πn₀/λ)sin(θ/2) depends on the distance between sample and screen, θ is the scattering angle, n₀ is the refractive index of water, and λ is the wavelength of the laser. The SALS images were analyzed using ImageJ with standard protocols for subtracting the background and removing the beam stop from the raw images.^28,29 The normalized mean intensity is defined as the ratio of the average intensity of the image (after removal of the beam stop) to the intensity if all pixels in the image were saturated (a value of 255 for the 8-bit camera used). The characteristic length (a_c) of the CMP and CNP aggregates was determined using Debye–Bueche plots (I^−0.5 vs. q²).^30,31 A linear fit to the data plotted in the Debye–Bueche format was calculated in the flow or vorticity direction using the method of least squares. The slope and intercept of the linear fit were used to calculate a characteristic length [a_c = (slope/intercept)^0.5] for the aggregates at various conditions. In addition, the distortion of the aggregates was characterized by an aspect ratio, which was determined by taking the ratio of the characteristic length values in the flow and vorticity directions.

Results and discussion

Solubility characteristics of MCC in aqueous solution of NaOH/urea (−7 °C) or preheated aqueous solution of H₂SO₄ (45 °C) are given in Fig. 1. The addition of urea to NaOH solution can significantly improve the solubility of cellulose, as shown in Fig. 1a. With further increase of urea concentration to 12 wt%, the cellulose can be completely dissolved in the 7 wt% NaOH solution, as evidenced by the values in %transmittance. The dissolution of cellulose also depends on sulfuric acid concentration, as shown in Fig. 1b. With the increase of sulfuric acid concentration from 52 to 60 wt%, the %transmittance of the cellulose solution increases at all cellulose/sulfuric acid ratios. The solubility of cellulose is thus improved significantly with the increase of sulfuric acid concentration, and the cellulose can be completely dissolved in the 60 wt% sulfuric acid solution. According to the Fig. 1, the optimum solvents for MCC are NaOH/urea/H₂O at the ratio of 7 : 12 : 81 by weight and 60 wt% sulfuric acid.

Fig. 1 Effects of reaction conditions on dissolution of MCC using NaOH/urea (a), and sulfuric acid (b).

In order to further observe the morphology and size distribution of the obtained CMP and CNP as well as the MCC, SEM/TEM and dynamic light scattering (DLS) were carried out. The samples show substantially different shapes in the SEM/TEM images (Fig. 2). The image of MCC (Fig. 2a), reveals that MCC presents mainly rod-like particles with size of 50 ± 10 μm long and 7 ± 3 μm wide, giving aspect ratio in the range of 5–14. The image of CMP produced by NaOH/urea treatment is shown in Fig. 2b, and that of CNP produced by sulfuric acid hydrolysis is shown Fig. 2c. The images reveal that CMP presents in the form of quasi-spherical particles with diameter of 316 ± 18 nm, while CNP is predominantly elongated rod-like particles with size of 218 ± 25 nm long and 8 ± 3 nm wide, giving aspect ratio in the range of 16–30. The size distribution of the CMP, CNP and MCC, which is assessed by DLS, corresponds well with the SEM/TEM images.

Fig. 2 SEM/TEM image and size distribution of MCC (a), CMP (b) and CNP (c).

It is likely that the crystalline domains of cellulose may be disrupted during the dissolution, whereas some might be realigned and converted during the regeneration. The properties, such as yield, instrinsic viscosity, degree of polymerization (DP) of the CMP, CNP and MCC, are shown in Table 1. The yield of CMP (69.8%) is slightly higher than that of CNP (62.3%). The instrinsic viscosity of MCC is 486.91 mL g⁻¹, while that of CMP and CNP decreases to 115.32 mL g⁻¹ and 78.24 mL g⁻¹, respectively. The average DP decreases significantly from 620 to 90 during the preparation of CNP, while there is a relatively small decrease in DP from 620 to 138 during the preparation of CMP. These results reflect that during the production of CNP, the hydrolysis reaction not only removed the amorphous region of MCC but also partly destroyed their crystalline region.

Table 1 Crystallinity index (CI), instrinsic viscosity, degree of polymerization (DP) and yield values of the MCC, CMP and CNP^a

Sample	Yield (%)	Intrinsic viscosity [η] (mL g^−1)	DP	CI (%)
a Data are represented as mean ± standard derivation (n = 3).
MCC		486.91 ± 6.03	678 ± 10	67.8 ± 3
CMP	69.8 ± 1.0	115.32 ± 2.89	138 ± 4	57.6 ± 2
CNP	62.3 ± 0.7	78.24 ± 1.49	90 ± 2	71.5 ± 2

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The WXRD spectra of CMP and CNP are shown in Fig. 3. Depending on the treatment methods, different WXRD patterns are obtained. MCC exhibits a sharp high peak at 2θ = 22.5° and three overlapped weaker diffraction peaks at 2θ = 15.5, 16.6 and 34.5°, assigned to (002), (101), (101̄) and (400) crystallographic planes of cellulose I, respectively.²¹ The CI of MCC is 67.8%. The diffraction pattern of CMP is typical of cellulose II, with the diffraction peaks at 20.0 and 22.2° corresponding to the (110) and (200) lattice planes.¹⁹ Compared with CMP, CNP exhibits a sharp high peak at 2θ = 22.5° and two overlapped weaker diffraction peaks at 2θ = 15.5 and 16.6°, assigned to (002), (101), and (101̄) crystallographic planes of cellulose I, respectively.²¹ The CI of CMP is calculated to be 57.6%, which is less than that of CNP to be 71.5%. The less crystallinity of CMP implies a more amorphous structure is formed after the treatment by NaOH/urea, which may be due to the disruption of the cellulose crystalline domains during the dissolution. For the sulfuric acid hydrolysis, however, the acid can diffuse preferentially into the amorphous regions, hydrolyze the accessible glycosidic bonds and transversely release individual crystallites. Consequently, CNP shows greater CI than MCC and CMP, as indicated by the WXRD measurement. Conductometric titration of the CMP and CNP suspensions reveals that CNP has a surface charge of 65 mmol kg⁻¹ due to the introduction of sulfate groups, while that of CMP is undetectable. The degree of sulfation on the surface of CMP and CNP determines their surface charge density, which in turn affects the stabilization of CMP and CNP suspensions. Based on the above results, it can be found that the CMP and CNP have different particle morphologies and surface charge density. This difference results in a great difference in aggregation behavior as discussed below.

Fig. 3 X-ray diffraction patterns of MCC, CMP and CNP.

Given the different physico-chemical properties of CMP and CNP, the CMP and CNP suspensions with the constant concentration of 1.34 wt% and different concentration of NaCl (0, 10 and 100 mM) have been prepared to study the effect of NaCl addition on the aggregation behavior of CMP and CNP in aqueous media independently. The degree of transparency of the CMP and CNP indicates the status of their dispersion. As shown in Fig. 4, in the absence of NaCl, the CNP suspension is nearly optically transparent while the CMP suspension is turbid, which signals the presence of large aggregates. Moreover, the addition of NaCl to the aqueous phase doesn't affect the aggregation of CMP but changed drastically that of CNP. The different dispersion results obtained with CMP and CNP may be due to specific interactions existing between the particles. The zeta potential and size of the CMP and CNP suspensions have been measured to explore their colloidal stability with different NaCl concentration. The zeta potential and size values measured for the CMP and CNP suspensions as a function of NaCl concentration from 0 to 100 mM are plotted in Fig. 5. At low NaCl concentrations, the absolute value of zeta potential of CNP is more than 30 mV, thus repulsive forces dominate and little aggregation occurs. With the increase of NaCl concentration, the zeta potential of CNP becomes less negative and the equivalent hydrodynamic size of the CNP aggregates increases. This is commonly observed with most colloidal particles in aqueous suspension. The presence of NaCl lowers the absolute value of zeta potential of CNP which is due to the adsorption of Na⁺ counter-ions on the negatively charged CNP surfaces thereby compressing the double layer surrounding the particles. As compared with CMP, CNP is more easily tended to aggregate and the size of CMP is much larger than that of CNP. The zeta potential of CMP, however, is less sensitive to the addition of NaCl over the same concentration range, which is mainly due to the weaker repulsive forces between particles for CMP.

Fig. 4 Macroscopic appearance of CMP and CNP suspensions with different concentration of NaCl.

Fig. 5 Zeta potential and size of CMP and CNP as a function of NaCl concentration.

It was reported that the addition of electrolyte to aqueous CMP and CNP suspensions can affect the microstructure of the suspensions and also the critical concentrations of liquid crystal phase formation.¹² The changes in the microstructure can thus directly affect the rheological behavior of CMP and CNP suspensions. In this study, the shear viscosities for the CMP and CNP suspensions have been investigated as a function of shear rate and temperature, as shown in Fig. 6. All suspensions show a large decrease of viscosity with the increase of shear rate. The viscosity of the CMP suspension, which is independent of temperature, is much larger than that of CNP. The effects of adding NaCl to aqueous CMP and CNP suspensions have also been evaluated at different NaCl concentration. For the CMP suspension, the viscosity is independent of ionic strength suggesting that the NaCl does not affect the aggregation of CMP. For the CNP suspension, however, the addition of NaCl results in extensive aggregation in the system and thus increases the viscosity of the samples.

Fig. 6 Rheology of CMP and CNP suspensions with different NaCl concentration. (a and b) Viscosity as a function of shear rate. (c and d) Viscosity as a function of temperature.

The information about the microstructure of the CMP and CNP aggregates is provided from two-dimensional SALS images (Fig. 7) as a function of shear rate and temperature. Overall, the scattering pattern shows isotropic ring pattern patterns, which suggest the formation of nearly spherical aggregates (i.e., they do not have a preferential orientation after shear). During the aggregation process, the entropic stresses may be relatively more significant, leading to an overall stronger resistance to flow. As a result, the CMP and CNP aggregates become disordered and more random, and a nearly circular droplet on the micrometer scale can be formed. The characteristic length and aspect ratio of the shear-induced aggregates are derived from the radically averaged intensity using Debye–Bueche plots. The aggregates of the CMP and CNP at different NaCl concentration range in size from 1.12 to 3.30 μm and the aspect ratio remains fairly constant around a value of 1, as shown in Fig. 8. The characteristic length of the CMP aggregates remains near 3.2 μm. The CNP aggregates in the absence of NaCl appear to have size of about 1.1 μm. By adding NaCl to the suspension, the size of the CNP aggregates increases significantly. The distinct scattering patterns of CMP and CNP are consistent with the rheological behavior.

Fig. 7 Small-angle light scattering images of the aggregates for CMP and CNP as a function of shear rate and temperature. The flow direction is from left to right in all images.

Fig. 8 Characteristic length (a) and aspect ratio (b) of the aggregates for CMP and CNP as a function of NaCl concentration.

The interesting observation necessaries a discussion on the primary driving force for the CMP and CNP aggregation behavior. Interparticle forces can be classified into two main categories: repulsive and attractive. From the above results, there is a net attractive force between the particles for the CMP suspension, compared to the CNP suspension. According to the DLVO theory, colloidal interactions are related to the balance between the repulsive electrostatic forces and the van der Waals attratcive forces. A range of surface forces between different cellulose surfaces under aqueous conditions were measured and interpreted in terms of van der Waals and electrostatic interactions.^32–34 Hence, the different aggregation modes of the CMP and CNP can be explained by differences in repulsive electrostatic forces and attractive forces between the spheres (CMP) and the rods (CNP). With their higher aspect ratio compared to the spheres, the rods in general have a higher surface area than the spheres, so should also have more attachment than the spheres. On the contrary, we find that the rods, experience less aggregation than the spheres. We speculate that the difference in particle surface charge should have a pronounced effect on the aggregation of the CMP and CNP. It was reported that an important aspect of nanocrystalline cellulose prepared by sulfuric acid hydrolysis is leaving anionic sulfate ester groups on the cellulose surface.^35–37 Consequently, the CNP with more surface charges is expected to have a stronger mutual repulsion, while the CMP with less surface charges should possess a relatively weaker electrostatic repulsion with each other. The aqueous CMP suspensions thus present more pronounced particle aggregation than the CNP suspensions at the same concentration. With the addition of NaCl, the adsorption of cation counterion Na⁺ on negatively charged CNP surfaces will reduce the electrostatic repulsion between the particles due to the electrostatic screening effect of Na⁺, leading to a more enhanced CNP aggregation and thus a higher viscosity.

Conclusion

The comparative analysis of morphology and structure between the CMP and CNP has been carried out. CMP presents in the form of quasi-spherical particles, while CNP is predominantly elongated rod-like particles. The DP and yield of CNP are lower than those of CMP. CMP lost its crystallinity during the dissolution, while CNP is of high crystallinity. CNP presents higher charge density than CMP due to the anionic sulfate ester groups left on CNP surface by sulfuric acid hydrolysis. At the same concentration, the CMP suspension presents more pronounced particle aggregation than the CNP suspension. The electrostatic repulsion for CNP is much higher than that for CMP. The addition of NaCl shows more prominent influence on the aggregation behavior of CNP than on that of CMP. The screening effect of NaCl reduces electrostatic repulsion between the particles, resulting in an enhanced aggregation of CNP.

Acknowledgements

The work was supported by the National Natural Science Foundation of China (no. 20906039), the Key Laboratory of Food Colloids and Biotechnology (no. JDSJ2013-04) and Collaborative Innovation Center for Food Safety and Quality Control.

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