Bionic creation of nano-engineered Janus fabric for selective oil/organic solvent absorption

Abstract

In this study, we present a self-driven and tunable hydrophobic/oleophilic, wettability-modified Janus fabric composed of a cellulosic substrate engineered with nanofibers via facile electrospinning technique that exhibits one-step selective oil absorption capacity from water. A nano-fibrous porous non-woven mat of polyvinylidene fluoride (PVDF) is coated on the cellulosic substrate with and without inclusion of silicon carbide (SiC) nanoparticles. PVDF and nano-SiC particles facilitate the hydrophobicity (WCA 113° ± 1.6°), and at the same time the porous structure and high aspect ratio of the nano-fibrous mat support the superoleophilicity (WCA 0°). Morphological analysis and air permeability studies revealed the corroboration of porous fine interconnected nanofibers. The retrieved Janus fabric efficaciously separated the oil/solvent from water with a constant selective oil absorption capacity up to 8.6 times, 5.9 times, and 5.5 times against its own weight for machine oil, toluene and ethanol respectively. It is also observed that the SiC nanoparticles augmented the absorption capacity in the Janus structure. Furthermore, the engineered Janus fabric can be reused up to 10 times during the oil/solvent recovery. The reported Janus fabric possesses the advantage of scalable fabrication, high separation efficiency, stable recyclability, excellent durability, time saving, and has strong potential for industrial applications in oil spill management.

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1. Introduction

A wide range of ecosystems subsist in the marine environment. A gigantic quantity of spilled oil (∼1.7–8.8 million tons) is reported each year, which is a major issue for the sensitive aquatic environment.^1,2 These oil spills are not only devastating the environment but also traumatize the ecological system.^3,4 Marshlands, chemical toxicity, light and oxygen deficiency to aquatic organisms, physical smothering of birds and mammals are some of the repercussions of oil spills.^5,6 Therefore, the direct impact of catastrophic oil spills needs to be addressed to save the marine ecosystem.^7,8 Physical, chemical and biological systems are primarily being used for oil/water separation.⁹ Physical adsorption techniques, such as activated carbon,¹⁰ foam,¹¹ fine metal mesh,¹² textile substrates¹³ and fibers¹⁴ have limited absorption capacity.¹⁵ Presently, multifarious systems are available for selective oil extraction, such as selective etching of surfaces, layer by layer assembling of block copolymers, air flotation, chemical and electrical treatments and electrospinning of fibres.¹⁶ In all these diverse techniques, highly effective absorbents with extensive porosity are required for oil/solvent separation.¹⁷ Modification of extensively porous absorbents along with membranes that have superhydrophobic and superoleophilic layers, can effectively enhance the ability of oil spill removal.^18–20

Bionic structures mimic nature (plants, animals) and can incorporate desired attributes.²¹ Various plant and animal surfaces manifest superhydrophobicity having a water contact angle (WCA) greater than 150° and sliding angle less than 10°, such as the Nelumbo nucifera (lotus) leaf surface, and this has inspired other bionic creations that have unique wettability structures.^22–24 Liquids with lower surface tension than the lotus leaf show the immediate spreading and penetration.²⁵ The surfaces with surface tension less than water (72.8 mN m⁻¹) and larger than oil and organic solvents (<30 mN m⁻¹), possess the heterogeneous wettability towards the oil/organic solvents and water.²⁶ This concept has been derived from ‘Janus particles’ which belong to a unique family of nanoparticles, in which half of its surface behaves as hydrophobic and the other half behaves as hydrophilic, thereby imparting a dual functionality to these nanoparticles. The name ‘Janus’ is inspired from the Roman god Janus, who has two opposite faces, depicting the past and future.^26,27 Fabric that possesses two or more distinct functional properties (in our case hydrophobic and oleophilic) can be termed as a ‘Janus fabric’.

Materials possessing a Janus structure with a dual wettability phenomenon are desirable for oil/water separation due to their oil affinity which can curtail hazardous secondary pollutants. The hydrophobic surfaces predominantly exhibit the oleophilic nature because of their enhanced surface roughness.^21,22,28 B. N. Sahoo et al., have demonstrated that hydrophobicity can be incorporated on surfaces by creating surface roughness or by altering the surface chemistry with inherently hydrophobic materials.^29–31 Changing the geometry of the surfaces includes various coating processes such as surface instigated polymerization, layer by layer techniques,³² chemical vapor deposition,³³ electrospinning³⁴ and so on. Electrospinning can provide ultra-thin membranes and porous structures with a simple one-step approach.^34,35 Electrospun nanofibers possessing high permeability with an evenly distributed porous hierarchical network-like structure coupled with hydrophobic and oleophilic characteristics effectively improve oil absorbance and water repellency.^36,37 Kim et al., have reported a superhydrophobic membrane with a WCA up to 156°, by the electrospinning of PVDF and silane functionalized PVDF.³⁸ Apart from the use of a hydrophobic polymer, inclusion of hydrophobic nanoparticles (NPs) such as TiO₂,³⁹ ZnO,⁴⁰ and SiO₂,⁴¹ in the polymer solution can further enhance the hydrophobicity of the nanofibers. Dhanshetty and Balasubramanian have fabricated a Janus fabric from a wettable heterogeneous nanocomposite possessing unidirectional water penetration properties by a facile electrospinning technique. Their Janus fabric was engineered with nylon-6/TiO₂ nanofibers having hydrophobic (WCA 99.7° ± 1.7°) and hydrophilic (WCA 30.86° ± 1.5°) layers.³⁹ Wang et al., have described the fabrication of a polystyrene nano-fibrous superhydrophobic mat incorporating silica nanoparticles yielding a WCA as high as 157°.⁴² For the facile separation of oil/water, a nano-fibrous membrane can be supported by solid substrates like metal mesh⁴³ or textiles,^44–46 which can also impart mechanical strength and durability in the material. Considering its inherently porous structure, abundant presence of water loving hydroxyl groups in its cellulosic chain and biocompatibility coupled with thermal and chemical resistance, cotton is an efficient absorbent material.^47–50 PVDF is generally utilized in the commercial level construction of microfiltration and ultrafiltration membranes, and it is well known for its mechanical and thermal stability, high organic selectivity, excellent membrane separation attributes as well as hydrophobicity, with a WCA of 138° ± 1°.^51,52 Ahmed et al.,⁵³ demonstrated the fabrication of a cellulose/PVDF-HFP membrane for oil/water separation via electrospinning with a great separation efficiency up to 99.98%.⁵⁴ On the other hand, silicon carbide (SiC) is also a strong, lightweight, and inherent hydrophobic material with a WCA of 130° ± 2°.⁵⁵

In the present work, we demonstrate the fabrication of a nano-engineered Janus fabric possessing tunable wettability. The fabric is a sandwich structure of a cellulosic substrate and nano-fibrous membrane of PVDF/nano-SiC for efficient oil/water separation via a facile electrospinning technique. However, to our best knowledge the fabrication of a sandwich structured cotton fabric engineered with PVDF/nano-SiC nano-fibrous membrane has rarely been reported to date. A comparative study has been carried out for pristine PVDF and nano-SiC incorporated PVDF samples. The morphological features and an air permeability study revealed the formation of nanofibers and porous interconnected architecture of the non-woven nano-fibrous mat. The wettability towards oil and water, along with the recyclability of the modified Janus substrates, was investigated for machine oil and organic solvents. The intrinsic hydrophobic nature of the PVDF and SiC, interconnected network, porous structure of nanofibers and woven cellulosic fabric play an important role in imparting the heterogeneous wettability effect (hydrophobic–oleophilic) in the coated substrates. The engineered Janus fabric can be recycled up to 10 times with a slight decrease in the absorption capacity. The simple manufacturing technique, re-usability and versatility for the distinct oil/organic solvent absorption create a successful, proficient and scalable oil spill absorbent material modified with a Janus structure.

2. Experimental section

2.1 Materials

Polyvinylidene fluoride (PVDF) ((C₂H₂F₂)_n), (M_w ∼ 530 000 g mol⁻¹, melting point 177 °C, density 1.78 g cm⁻³), silicon carbide nano powder (∼50 nm, specific surface area of 70–90 m² g⁻¹) and N,N-dimethyl formamide (DMF) were procured from Sigma Aldrich Pvt. Ltd., India. Plain woven cotton fabric was obtained from the local market. For the oil absorption study, machine oil, ethanol and toluene were procured from Sigma Aldrich Pvt. Ltd., India. De-ionized water was obtained from a Millipore Milli-Q system. All reagents were of analytical grade and were utilized as received without any further refinement.

2.2 Preparation of PVDF/nano-SiC solution

A dispersed solution of 20 w/w% PVDF was prepared by dissolving PVDF in DMF with a magnetic stirrer (100 rpm) at room temperature. Next, 5 w/w% nano-SiC was added in the already prepared PVDF solution and was sonicated for 10 minutes followed by stirring for 30 min at room temperature. The plain PVDF solution and nano-SiC incorporated PVDF solutions were used separately for further electrospinning.

2.3 Electrospinning

Electrostatic jetting was performed with a horizontal electrospinning set up which was accompanied by a glass syringe (9.14 mm diameter) fitted needle, syringe pump with a high voltage cathode supply (∼15 kV) and anode connected collector plate. A solution flow rate of 5 µL min⁻¹ was maintained with a constant work distance of 15 cm from the needle tip to collector plate. Room temperature and relative humidity were maintained as per the standards. For the fabrication of the Janus structure, a 10 × 10 cm² cotton fabric was cleaned by acetone and DI water and then it was air dried, followed by conditioning prior to spinning; further, the cleaned cotton fabric was fixed over the collector plate. Synthesized homogeneous solutions of PVDF and PVDF/SiC were loaded separately in to the glass syringe. An electrode was attached to the needle and on the other side a cathode was connected to the fabric covered collector plate. A constant voltage supply (∼15 kV) was maintained throughout the experiment. The nanofibers of PVDF and PVDF/SiC precursor solutions were collected on the front and back side of the cotton fabric. In order to obtain the desired properties in the nanofibers prepared via an electrospinning technique, various parameters like voltage, needle-tip to collector plate distance, and flow rate were varied, but the solution viscosity is a paramount factor which renders the melt strength to nanofibers.^56–58 In this context, we have extensively exploited various formulations and found that 5 wt% loading of nano-SiC results in the desired solution viscosity. It has been observed that when the loading of nano-SiC is below 5 wt%, the solution simply gets sprinkled on the collector plate without any nanofibers, whereas the loading beyond 5 wt% resulted in the formation of large sized beads along the length of the nanofibers. Therefore, the 5 wt% loading of nano-SiC in the PVDF was decided as the optimum loading for the electrospinning experiment. The engineered Janus fabric with a non-woven nano-fibrous membrane having heterogeneous wettability was retrieved on both sides of the cellulosic substrate. After electrospinning the fabric was dried at room temperature for 24 hours and then subjected to further assessment.

2.4 Oil/solvent absorption analysis

The maximum oil (uptake) absorption capacity of the modified Janus fabric was investigated using an immiscible oil/water mixture by following a simple and novel reported technique.²⁴ A known weight of conditioned samples of PVDF and PVDF/nano-SiC were taken and kept immersed in the oil/water mixture for the complete saturation up to 24 hours and the maximum oil absorption capacity in weight% (M_a) was calculated using the following equation:where, W₁ = weight of fabric before dipping, W₂ = weight of fabric after dipping.

2.5 Recyclability

The recyclability of the PVDF and PVDF/nano-SiC engineered cellulosic substrate for oil/solvent absorption was investigated from the periodic observations of the oil/solvent uptake capacity. A series of experiments for the same sample were carried out with a time interval of 1 hour, and thereafter squeezing it with the maximum hand pressure. Samples were re-used and squeezed until it showed a constant weight; the change in absorption capacity with constant time was noted.

3. Characterization

Field emission scanning electron microscopy (FE-SEM) (Carl Zeiss AG, Germany) was utilized to scrutinize the morphologies of the pristine cotton fabric and the modified Janus fabric samples. Wettability analysis was carried out by the static contact angle measurement instrument (Kruss DSA100, Kruss GmbH Germany) with 2 µL droplet volume. To assess the change in the porosity, air permeability of the cellulosic substrate was investigated before and after electrospinning according to the IS standard 11056 by a Textest FX 3300 Air permeability tester at 100 Pa pressure. All the fabric testing was carried out in the standard atmospheric conditions (27 °C ± 2 °C and 65% RH).

4. Results and discussion

4.1 Morphological analysis

The fabrication of the Janus fabric via electrospinning as described in the Experimental section is illustrated in Fig. 1. Field emission scanning electron microscopy (FE-SEM) was utilized to scrutinize the morphology of the as received virgin PVDF, PVDF nano-fiber coated and PVDF/nano-SiC coated fabrics (Fig. 2). The plain weaved structure of the as received cotton fabric possesses alternate interlacement of the twisted warp and weft yarns. In order to introduce hydrophobicity, the fabric was coated on the front and back side with PVDF and PVDF/nano-SiC by a simple one-step electrospinning technique. The highly entwined and interconnected structure of the nanofibers imparts porosity in the modified Janus structure. The porous structure enhances the hydrophobicity by forming an air cushion on the coated surface of the fabric that restricts the contact between the water droplet and the nano-fibrous membrane.^39,59 In the Janus fabric architecture, the arrangement of the warp and weft in the woven fabric structure and the nano-fibrous porous coating act in a synergistic manner to form air pockets, which ultimately results in hydrophobic or superhydrophobic surfaces.³⁹ In Fig. 2(b), the fibers are expanded and the bulkier elliptical structures, highlighted with the red doted circle, are known as “beads”. Thermo-physical properties of the precursor and the process variables are the key factors for controlling the magnitude and appearance of the beads. The network like fine porous membrane (pore size ranging from few microns to nanometer) of randomly distributed PVDF/nano-SiC non-woven nanofibers with the bead-on-string structure is clearly observed on the fabric surface (Fig. 2(b)). The presence of beads in the nanofibers is generally unwelcome, but for the hydrophobic structures, beads are beneficial and do not greatly affect the coherence of the membrane.^60–64

Fig. 1 Fabrication of the modified Janus fabric via electrospinning, (A) solution preparation for the electrospinning, (B) electrospinning process: coating of nano-fibrous mat on the front and back side of the cotton fabric, (C) modified Janus fabric for oil/water separation.

Fig. 2 (a) FE-SEM image of the pristine cotton fabric, (b) FE-SEM image of PVDF/nano-SiC coated cotton fabric (with red doted circle showing bead-on-string structure of the nanofiber membrane).

Ma et al., have reported that a beaded nanofiber membrane provides a higher water contact angle than the bead free nanofiber membrane. Beads exhibit higher surface roughness than the cylindrical bead free fibers, as per the Cassie–Baxter equation, and the water contact angle is higher for spheres than a cylindrical surface equipped with identical radius.^59,65

4.2 Thermal analysis

Thermal analysis of the fabricated samples was done using thermogravimetric analysis (TGA) as shown in Fig. 3. It has been observed that the virgin PVDF polymer samples are stable up to 410 °C, and after this temperature the degradation occurs until 475 °C, with a weight loss of 40%. In the first degradation stage i.e. from 410 °C to 475 °C, the breaking of C–H, and C–C bonds occurs. In the same first zone the evaporation of water and removal of oligomers and –CH₂OH can also take place. Then in the second degradation stage i.e. 474 °C to 733 °C, the breaking of C–F bonds occurs, since it has higher bond energy, i.e. 485 kJ mol⁻¹, compared to C–H and C–C bonds. The loading of nano-SiC can also help by creating a protective layer on the Janus fabric by forming a Si–O bond between the cellulosic cotton substrate and the PVDF/nano-SiC nanofibers which acts as a thermally insulating layer when it crosses the degradation temperature.^66,67

Fig. 3 TGA graphs of the samples.

In electrospun PVDF nano-fiber samples, the thermal degradation occurs in three stages, where the second and third degradation stages are similar to virgin PVDF samples. The first degradation, occurring with a weight loss of 8% up to 396 °C, is attributed to the evaporation of volatile DMF solvent, which is entrapped in the PVDF polymer chains. As the polymer chains start relaxing with progressing temperature, it creates a free volume for DCM solvent molecules.^67,68

For the PVDF nano-SiC sample, in the first and second degradation stages the offset temperature occurs at 471 °C and 645 °C respectively, with a 4.71% remaining residue which is un-combusted. Generally, inorganic material does not combust on heating. Since, nano-SiC is the inorganic material which is added with a 5 wt% loading in the Janus fabric, the corresponding residue of 4.71% weight is attributed to nano-SiC. The Janus fabric contains a substrate of cotton which is mostly composed of organic cellulose material, and therefore its thermal degradation takes place in four stages. The main degradation occurs at 267 °C, 358 °C and 436 °C.

Table 1 shows the differential scanning calorimetry data (DSC) of the samples. It has been observed that the virgin PVDF sample possesses the highest melting temperature i.e. 163 °C, as these samples have high crystallinity compared to other samples which have been processed using the dispersed solution.

Table 1 Differential scanning calorimetry data^67,68

Samples	Crystallization			Melting
	T c (°C)	ΔHC (J g^−1)	T c onset (°C)	T m (°C)	ΔHm (J g^−1)	T m onset (°C)
Virgin PVDF	131	32.4	138	163	24.15	150
PVDF nanofibers	131	31.55	138	159	22.12	149
PVDF + SiC	132	32	140	161	21.56	150
Janus fabric	133	2.93	139	159	1.54	150
Cotton	—	—	—	—	—	—

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The cotton samples did not show any change in enthalpy over a temperature range of 20 °C to 160 °C. The crystallization temperatures of all the samples are the same with a slight variation in crystallization enthalpy i.e. ΔH_C, except for the Janus fabric and cotton samples. The onset temperatures of crystallization and melting regions are almost constant at 138 °C and 150 °C respectively. There is a noticeable difference in the crystallization and melting enthalpies of the Janus fabric sample compared with other PVDF samples. The Janus fabric has a large content of cellulosic cotton substrate by weight percent, and therefore most probably, the heat released and taken during the crystallization and melting is directly dispersed in the cellulosic cotton substrate.^68,69

4.3 FT-IR analysis

Fourier transform infrared (FT-IR) analysis was done for the fabricated samples as shown in Fig. 4. In the given FT-IR graph, the peak at 876 cm⁻¹ corresponds to the Si–C bond present in the silicon carbide nanoparticles i.e. nano-SiC. Two strong peaks observed at 1025 cm⁻¹ correspond the bending vibration of C–O bonds present in the cellulosic cotton substrate, the peaks at 1319 and 1105 cm⁻¹ also belong to the same family. The peaks observed at 1278 cm⁻¹ and 1396 cm⁻¹ belong to the β phase present in the PVDF polymer, and other small peaks observed between 876 to 1278 cm⁻¹, are also attributed to the same phase of PVDF samples.^38,70–72

Fig. 4 FT-IR analysis of the samples.

4.4 WAXD analysis

In Fig. 5, the XRD plots show the presence of α and β peaks in virgin PVDF, which have been observed at 2θ = 18.6° (α), 2θ = 20.2° (β), and 2θ = 26.8° (α) respectively. The α phase corresponds to the chain conformation of TGTG (020) (021) (Trans-Gauche-Trans-Gauche) and the β phase corresponds to the chain conformation of TTTT (200) (Trans-Trans-Trans-Trans) having planar zigzag structure.^73,74

Fig. 5 XRD characterization of the samples.

The sharp peaks of α and β phases at 2θ = 18.6° and 2θ = 20.2° respectively, are absent in PVDF and PVDF/SiC nanofibers. The broad peak observed in the range from 2θ = 25° to 2θ = 37° corresponds to the presence of an amorphous phase in PVDF and PVDF/SiC nanofibers and it is attributed to the action of DMF which is a polar solvent. The polar nature of the DMF solvent has helped in reducing the intermolecular cohesive forces between the chains, which leads to the chain relaxation in the polymer, thereby decreasing the orientation i.e. crystallinity, was manifested in the original PVDF polymer. The reduction in intermolecular cohesive forces is necessary, because it helps to in preparing the required dispersed solution, thereby facilitating processability during the electrospinning process.^70–74

4.5 Wettability behaviour of the nano-fibrous membranes

The wettability of the pristine cotton fabric, PVDF coated and PVDF/nano SiC coated fabrics was studied in detail by contact angle measurements for water as well as for oil. The pristine cotton fabric was readily wetted by oil and water, while the modified Janus structure manifested wettability towards the oil and water. Fig. 6(a) and (b) are images obtained during the contact angle measurement experiment. Water and oil droplets of about 2 µL were dropped on the surface of the substrates separately, and contact angles were calculated by the static method. The contact angles of the modified Janus fabric samples were measured for the front and back sides of the fabric. Water contact angles (WCA) of 113° ± 1.6° (Fig. 6(a)) and 96° ± 2° were observed for the PVDF coated and PVDF/nano-SiC coated fabrics respectively. The oil contact angle (OCA) of 0° (Fig. 6(b)) of the oil droplet has been confirmed for both the coated substrates by immediate spreading, and saturated thoroughly (within 7 seconds) on the surface of the substrates.^24,39 Fig. 6(c) and (d) are the images of the oil and water droplets on the Janus fabric surface. Fig. 6(e) displays the rolling behaviour of the water droplets after tilting the substrate.

Fig. 6 (a) Water contact angle of the PVDF/nano-SiC coated cotton fabric, (b) oil contact angle of PVDF/nano-SiC coated cotton fabric, (c) hydrophobic modified Janus nanocomposite, (d) oleophilicity of the modified Janus fabric, (e) sliding water droplets from the modified Janus fabric.^24,39

According to Wenzel’s theory, roughness on the homogeneous surface is the driving factor for imparting the hydrophobicity, which is explained using the following equation as:⁷⁵

where, θ_w is the contact angle of homogeneous rough surface, θ is the Young’s contact angle formed on smooth surface, and r is the surface roughness factor which is a ratio of authentic and projected surface area.⁷⁵

The surface of the Janus fabric can be considered as the heterogeneous rough surface, because it possesses a porous nature and roughness, since the nanofibers coated onto the cellulosic cotton substrate are randomly oriented. The hydrophobicity of the heterogeneous rough surface is explained by the Cassie–Baxter theory, which states that the hydrophobicity is governed by the porous surface which traps air pockets between the water droplets and the substrate surface, which helps in reducing the water contact angle. The Cassie–Baxter equation is given as:⁷⁶

cos θ_c = f₁ cos θ₁ − f₂ (3)

where, θ_c is the apparent contact angle, f₁ and f₂ are the surface fractions of phase 1 and phase 2 respectively and θ₁ is the contact angle on phase 1.⁷⁶

Considering the porous structure, and the intuitive hydrophobic nature of the PVDF nanofiber membrane, the water droplet rests on to the surface of the Janus fabric. The water repellent properties of the silicon carbide nanoparticles have significantly contributed to enhance the hydrophobicity of the Janus fabric.^24,39,76

4.6 Oil absorption analysis

The results show that the modified Janus substrates have dual wettability performance with rapid oil absorption rate with the help of its porous structure, capillary action, and the elemental hydrophobic nature of PVDF and nano-SiC particles that promote it as an ideal and promising candidate for a role of an oil spill absorbent.³⁹

In order to prove the dual wettability of Janus fabric, a few drops (∼5 mL) of machine oil were mixed in water as shown in Fig. 7(A). The light weight oil started floating on the surface of the water quickly, and then the known weight PVDF and PVDF/nano-SiC coated samples were brought in to contact with the oil/water mixture. Samples were consistent with the expected results, because of the hydrophobic–oleophilic attribute. Oil and solvent were rapidly absorbed by the PVDF and PVDF/nano-SiC coated samples in a few minutes because of its network like framework, interconnected voids and capillary action. Fig. 7(A) and (B) show that no trace of oil/solvent is present in the remaining water after the absorption, and then the wet samples were taken out for further weight analysis. Further, toluene and ethanol were also tested in the above mentioned way, and the absorption capacity was investigated from the weight analysis of the absorbent before and after submerging, using the formula elucidated in the Experimental section.

Fig. 7 Images of the engineered Janus fabric separating (A) oil/water mixture, (B) solvent/water mixture (water dyed with red color).

The PVDF nano-fiber coated samples showed an absorbency towards machine oil of 4.1 times compared to its initial weight, while the PVDF/nano-SiC coated samples showed a higher absorbency than the PVDF nano-fiber coated samples for machine oil of 8.6 times compared to its initial weight. The given Table 2 shows the comparative oil absorption capacities of nano-PVDF and PVDF/nano-SiC (5%) coated fabrics for machine oil and organic solvents.

Table 2 Comparison of the absorption capacity between PVDF/nano-SiC coated fabric and PVDF coated cotton fabric

Absorbent	Oil/organic solvent	Absorption capacity (against initial weight)
PVDF/nano-SiC coated fabric	Machine oil	8.6 times
	Toluene	5.9 times
	Ethanol	5.5 times
PVDF coated fabric	Machine oil	4.1 times
	Toluene	3.8 times
	Ethanol	3.6 times

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Fig. 8 shows the absorption capacity of the fabricated samples with respect to composition. From the results it has been observed that the nano-SiC particles have greatly improved the oil uptake capacity as compared with the pristine PVDF nano-fiber coated samples.

Fig. 8 Absorption capacity of the pristine nano-PVDF and PVDF/nano-SiC coated fabrics for oil and solvents.

The reticulated architecture of the porous membrane voids within the interconnected micro/nano fibrous framework helps in advancing the capillary force and expanding the specific surface area by providing oleophilic attributes, which further enhance the oil absorption capacity of the modified Janus fabric. In the oil uptake process of the oil uptake process of the PVDF and PVDF/nano-SiC coated fabrics, an oleophilic approach and van der Waals force acting on the oil/solvent, coupled with the nano-fibrous membrane, contribute significantly to reduce the surface tension. The reduced contact angle corresponds to the minimum energy obstruction for the oil/solvent which penetrate in to the pores of the nano-fibrous composite membrane.^70,71 Direct linear variations between the effective specific surface area, capillary force and oil/solvent contact area on the fabric promote the selective wettability characteristics in the modified Janus structure.^75–78

4.7 Air permeability analysis

Fig. 9 represents the air permeability values for pristine cotton, PVDF nanofiber coated fabric, and PVDF/nano-SiC coated fabric. Porosity of the membrane also plays an important role by imparting the hydrophobic properties, as the existence of the porous structure was revealed from the morphological analysis. The presence of porosity has also been confirmed by the air permeability study by testing all the samples at constant air pressure (100 Pa). It is observed from Fig. 2(d), that after a dense coating of highly entangled bead-on-string structured PVDF and PVDF/nano-SiC nano-fibrous membrane, the modified Janus fabric is still air permeable, though the results reveal the decrease in the air permeability value. The oil separation performance occurs from the synergistic effect of the surface characteristics and the porosity, and also the hydrophobicity and oleophilicity can decrease if the porosity is destroyed.^24,43,77

Fig. 9 Air permeability of the pristine cotton fabric and the modified Janus fabric.^24,43,77

4.8 Absorption mechanism

Fig. 10 represents the possible mechanism for the interaction between oil/water and the Janus fabric surface. The hydrophobicity concept of the Cassie–Baxter theory as described in Section 4.5 can suitably be applied for explaining the absorption mechanism.⁷⁶ The improved oil absorption capacity is predominantly exploited from the porosity.^77–80 The electrospun nanofibers possess a porous structure with high aspect ratio, which combined with the surface roughness phenomenon, exhibits a heterogeneous wettability giving hydrophobicity and the oil loving attributes. The wettability of the Janus fabric is recognized not only by its surface phenomenon but also with its porous structure because of the contribution of capillary action. When the oil comes in contact with the electrospun nano-fibrous porous mesh it gets absorbed by the physical trapping of the nanofiber surfaces followed by the filling of inter-fiber voids. Subsequently, the oil diffuses in to the porous structure rapidly via capillary action. These porous membranes show heterogeneous selective wettability (hydrophobic/oleophilic) properties with strong capillary effects, which are due to the high internal surface area and network-like highly interconnected architecture of the membrane. Cellulosic substrates support the nanofiber membrane by providing additional porosity, stability, durability and recyclability.^81–85

Fig. 10 Wetting behaviour of the Janus fabric surface with oil/water interfaces.^22,23,39,76

4.9 Time dependent absorption analysis

The time dependent absorption analysis of the modified Janus fabric was carried out at time intervals of 1 hour to 8 hours, 12 hours and 24 hours, until the decline in the absorption capacity of the fabric was observed as shown in Fig. 11.

Fig. 11 Time dependent absorption capacity of the PVDF and PVDF/nano-SiC coated fabrics.

The fabric samples taken for the study were squeezed after each interval from 1 hour to 8 hours and then the oil absorption capacity was measured. The absorption capacity of the same samples was also evaluated in an identical manner after 12 and 24 hours. The absorption capacity was found to diminish slightly after the intervals, because of the decrease in the concentration gradient. After 24 hours, the oil uptake capacity of the PVDF and PVDF/nano-SiC coated Janus fabric has been observed as 7.2 and 3.6 times its own weight respectively, which shows a decrease in the absorption capacity by half for machine oil after 10 absorption cycles.

5. Conclusion

In summary, we have demonstrated a facile approach to engineer a Janus fabric of heterogeneous wettability for oil/water and organic solvent/water separation. The nanocomposite sandwich structure was fabricated by coating the front and back sides of a cellulosic cotton fabric with a highly porous hierarchical PVDF and PVDF/nano-SiC non-woven nano-fibrous membrane via a facile electrospinning technique. Loading of 5 wt% nano-SiC in the PVDF solution effectively enhanced the oil absorption capacity of the Janus fabric. The porous nature of the structure was investigated by morphological and air permeability studies. Three dimensional fine reticulated structures, the porous and fine membrane of the nanofibers, and the intrinsic attributes of the PVDF and nano-SiC resulted in a synergetic hydrophobic–oleophilic effect with tunable wettability towards oil and organic solvents. These structures can easily sunder and absorb the oil from the oil/water mixture even after 10 absorption cycles. The reported nanocomposite reveals the advantage of a versatile oil and organic solvent absorbent with scalable fabrication, high separation efficiency, stable recyclability, excellent durability, time saving and also has a strong potential for industrial applications.

Acknowledgements

The authors would like to thank Dr Surendra Pal, Vice-Chancellor, Defence Institute of Advanced Technology (DU), Pune, for the support and DIAT-DRDO Programme on Nanomaterials (EPIPR/1003883/M/01/908/2012/D (R&D))/1416 Dated: 28.03.2012, DRDO-HQ, New Delhi, for the financial assistance. Also authors would like to acknowledge Mr Dhananjay Gunjal for his help with FE-SEM characterisation and technical support, and Mr Shankar Kalletla and Dr Baloji Naik (NMRL, DRDO, Ambernath) for help with thermal characterization.

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