J. Coat. Technol. Res. https://doi.org/10.1007/s11998-018-00177-z Methyltrichlorosilane functionalized silica nanoparticles-treated superhydrophobic cotton for oil–water separation Love Dashairya, Dibya Darshan Barik, Partha Saha American Coatings Association 2019 Abstract Water pollution due to oil spills has become a significant concern in recent times for the marine ecosystem. The use of oleophilic/hydrophobic sorbents for oil–water separation has gained a lot of attention as an economical and environment-friendly solution. Herein, we developed a superhydrophobic/superoleophilic cotton by silica nanoparticles ( 800 nm) treatment followed by surface functionalization with methyltrichlorosilane (MTCS). X-ray diffraction, field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Fourier-transform infrared spectroscopy studies reveal that the formation of pseudo-amorphous SiO2 NPs and a C–Si–O coverage on cotton fiber render it superhydrophobic with increased surface roughness. The MTCS/SiO2-treated cotton exhibited contact angles of 173 ± 2 and 0 on the water–cotton and oil–cotton interface, respectively. Moreover, the MTCS/SiO2-treated cotton demonstrated superhydrophobicity over the entire pH range, with excellent absorption capacities for various oil–water mixtures ranging from 30 to 40 times its weight. Keywords Superhydrophobicity, Absorption capacity, Contact angle, Methyltrichlorosilane Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11998-018-00177-z) contains supplementary material, which is available to authorized users. L. Dashairya, D. D. Barik, P. Saha (&) Department of Ceramic Engineering, National Institute of Technology, Rourkela, Odisha 769008, India e-mail: sahapartha29@gmail.comsahap@nitrkl.ac.in Introduction Water pollution due to oil spills, by the accidental dumping of crude oil, petroleum by-products, and hazardous organic solvents from chemical industries into oceans, has become a significant concern for the marine ecosystem.1–3 The negative impact of oil spills is enormous and associated with an adverse effect on the ecosystem.4 Oil spills during oil extraction, transportation, and storage account for more than one-third of the accidental release of oil in the marine ecosystem.1,5 The above factors have led to an increasing demand for an effective solution to remove oil from oily wastewater. Traditional practices like gravitational separation, skimming, flotation, centrifugation, flocculation, and coagulation have its shortcoming.6 The major drawbacks including low separation efficiency, prolonged separation time, high energy requirement, and complicated steps warrant new technology for separation of oil. In this regard, porous sorbents are considered an economic and viable option for oil recovery owing to their ease of operation and availability. A porous sorbent must have a high degree of separation efficiency and low processing cost.7 However, preliminary research targeted to develop hydrophobic/oleophilic sorbents demonstrated low separation efficiency and concomitant absorption of oil and water and calls for further research.6,8 Cotton, a natural plant fiber that is cheap and possesses a three-dimensional macroporous structure, is ideal from a sorbent’s point of view.6,9 Therefore, there is opportunity to fabricate cotton-based hydrophobic/oleophilic sorbents for oil–water separation.10 However, pristine cotton generally covered with a wax layer has a low surface roughness which impairs its hydrophobic/oleophilic behavior. It is to be borne in mind that low surface energy and high surface roughness are essential parameters for sorbents to develop Recently, hydrophobic/oleophilic properties.11,12 J. Coat. Technol. Res. Fe2O3/Cr2O3 nanoparticles and TiO2-coated cotton fabrics demonstrated surface roughness and hydrophobicity, showing water contact angles greater than 150.4,13 Silica nanoparticles-impregnated sponge has shown high surface roughness and low surface energy for developing sorbents for oil–water separation.7,9,14 Silica nanoparticles offer many benefits such as ease of functionalization due to the presence of silanol groups and compliance with a variety of surfaces (cotton, fabrics, sponge).14,15 Qinwen et al. developed hydrophobic cotton and polyester fabrics by silica sol nanoparticles and nonfluorinated alkyl silane, which showed excellent water repellency with a water contact angle of 155 on cotton and 143 on polyester.16 Recently, Liu et al.9 modified cotton surface by silica nanoparticle and octadecyltrichlorosilane demonstrating high oil absorption capacity (AC). However, many synthesis routes use expensive 1H,1H,2H,2H-perfluorooctyltriethoxysilane for surface modification which limits the commercial aspects and leads to the development of low-cost sorbents for oil spill cleanup.17 Also, usage of expensive and toxic fluorinated long alkyl silane has its own drawbacks and poses a high risk to human health and the environment due to the emission of fluorinated compounds during synthesis.18,19 Methyltrichlorosilane (MTCS) is an inexpensive and fluorine-free silane that possesses critical surface tensions below 35 dynes/cm and demonstrates outstanding superhydrophobicity properties.20,21 Khoo et al.22 studied MTCS-functionalized glass and SiO2 substrates, which exhibited contact angle 160. To the best of our knowledge, MTCS-functionalized silica nanoparticles-treated cotton fibers have not been reported for oil–water separation. In the present work, we present a facile method to fabricate superhydrophobic/superoleophilic MTCS-functionalized silica nanoparticle (MTCS/SiO2)-treated cotton. The cotton is initially treated with stöber silica nanoparticles via solgel route and subsequently modified with MTCS through a simple dip coating method demonstrating selective absorption of oils up to 30–42 times its weight. Experimental Materials All the chemicals were analytical reagents grade and used without further purification. Deionized (DI) water was used throughout the experiments. Cotton balls were purchased from a local medical shop. Tetraethylorthosilicate (C8H20O4Si, 99.9%) was purchased from Alfa Aesar. Ammonia solution (NH4OH, 25%) was purchased from RFCL Ltd. Cyclohexane (C6H12, 99%) was purchased from Emplura, Merck Ltd. 2-Propanol (C3H8O, 99%) was purchased from Rankem. Methyltrichlorosilane (CH3Cl3Si, 98%) was purchased from Avra Synthesis Pvt. Ltd. Sodium chloride (NaCl), magnesium chloride (MgCl2), citric acid, sodium sulfate (Na2SO4), and sodium hydroxide pellets (NaOH) were acquired from Loba Chemie. Engine oil, pump oil, kerosene, diesel, vegetable oil, and hydraulic oil were obtained from local stores. Pretreatment of cotton The raw cotton was ultrasonically washed with DI water. A 2% (wt./wt.) NaOH aqueous solution was prepared in a beaker. The cotton washed with DI water was placed in the NaOH aqueous solution for 10 min. Then, the cotton was taken out from the NaOH solution and washed several times with DI water until the pH of the filtrate reached 7. This pretreated cotton was dried at 50C for 12 h.9 Preparation of SiO2 NPs-treated cotton A solgel route was used to achieve SiO2 NPs-treated cotton fibers. A solution containing 2-propanol (45 mL), tetraethyl orthosilicate (5 mL), and 5 mL of DI water (volume ratio—9:1:1) was prepared in a beaker. Then, 5 mL of ammonia solution was added dropwise to the mixture at room temperature under magnetic stirring for 45 min. The pretreated cotton ( 10 g) was immersed in the solution and kept for 1 h. After the hydrolysis and condensation reactions were over, the SiO2-treated cotton was removed and washed with 2-propanol and dried in a vacuum oven at 50C for 12 h.9 Preparation of MTCS/SiO2-treated cotton The SiO2-treated cotton was dipped in a cyclohexane solution of 2% (v/v) MTCS for 1 h and then dried at 80C for 12 h to prepare superhydrophobic MTCS/ SiO2-treated cotton.21 A schematic representation of the preparation of superhydrophobic MTCS/SiO2treated cotton is presented in Fig. S1. Absorption capacity Three different quantities were calculated during the absorption capacity tests. The quantities are absorption capacity (AC), collection capacity (CC), and recovery (%). AC, CC, and recovery (%) are defined as follows: AC g g1 ¼ ðWf Wi Þ=Wi ; CC g g1 ¼ ðWr Wi Þ=Wi ; Recoveryð%Þ ¼ ðWf Wr Þ=ðWr Wi Þ 100; J. Coat. Technol. Res. where Wi denotes the initial (dry) weight of the MTCS/SiO2-treated cotton, Wf denotes the weight of the MTCS/SiO2-treated cotton after absorption of oil, and Wr denotes the weight of the MTCS/SiO2-treated cotton after oil desorption by mechanical squeezing. To evaluate the AC, an equal weight ( 0.105 g) of cotton was taken to conduct the experiments. The weight of the cotton before oil absorption was noted as the initial weight (Wi). An oil–water mixture (volume ratio = 1:5) was prepared into which the modified cotton was immersed for oil absorption, and the weight was noted as residual weight (Wr). Further, the oil-loaded MTCS/SiO2-treated cotton was mechanically squeezed to remove oil, and the residual weight was noted as Wr.6,23 This absorption/ desorption process was carried out for 10 cycles with two cottons to get an accurate value of the reported data. The flux (Lm2 h1 atm1) of oil using MTCS/SiO2treated cotton was determined by calculating the volume of oil passed in unit time from the oil–water mixture using the following equation.24 F¼ VL A:P:t where F is the volume flux of oil, VL (liter) is the volume of oil passing through the MTCS/SiO2-treated cotton, A is the effective area of cotton (m2), P is the suction pressure (1 atm), and t is the separation time (s). Also, Vi and Vf are the volume of the oil before and after the oil–water separation test, respectively. Furthermore, the separation efficiency (g) was calculated according to given equation: g% ¼ Vi 100 Vf Results and discussion Reaction mechanisms A natural plant wax layer is present on the surface of the pristine cotton.9 This wax layer renders the surface of cotton fiber quite smooth.25 Therefore, the SiO2 NPs cannot attach themselves to the surface of the fiber owing to its smoothness.26 The natural plant wax layer was removed by pretreatment with an NaOH solution. Pretreatment exposes many longitudinal corrugations and interspaces (–OH groups) on the cotton fiber surface. Further, a solgel route was adopted to form stöber SiO2 NPs ( 800 nm) where the reactions proceeded by the hydrolysis of TEOS followed by the condensation of silanol [Si–(OH)4] groups. The condensation reaction occurs with the formation of either alcohol or water and the condensed siloxane ( ” Si–O–Si ” ) species as the end products.27,28 The corrugations on the cotton fiber surface help the SiO2 sol to permeate into the cotton. Consequently, the SiO2 NPs get attached with the cotton fiber through a chemical interaction between the hydroxy groups of the cotton fiber and hydrolyzed TEOS.9,29 These hydroxy groups on the cotton fiber play a key role in the evolution of modified cotton fibers.30,31 Moreover, the SiO2 NPs treatment (in the form of crosslinked siloxane ” Si–O–Si ” and HO–Si) on the cotton fiber generates a rough surface texture.7,10 The SiO2-treated cotton further modified with MTCS develops superhydrophobicity (see Scheme 1). The alkyl groups present in MTCS react with the hydroxy groups (siloxane ” Si–O–Si ” and HO–Si) present on the SiO2treated fiber and formed a rough surface with formation of polymethylhydrosiloxane and become water repellent completely.10 Microstructural analysis Material characterizations The raw cotton, SiO2 NPs, SiO2-treated cotton, and MTCS/SiO2-treated cotton were characterized by Xray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), and energy-dispersive X-ray spectroscopy (EDX). XRD was performed using a Philips Rigaku Ultima IV system with Cu-Ka radiation (kCu = 1.5418 Å) within the 2h range of 5–65. FESEM was performed using Nova NanoSEM FEI450 operating at 10 kV. FTIR analysis was done with PerkinElmer Spectrum version 10.4.00 in wave number range of 400–3600 cm1 at room temperature using KBr disks. A drop shape analyzer (Model DSA 4, Kruss, Germany) was used for contact angle measurement. The FESEM images of the raw cotton illustrated in Fig. 1a display the smooth fiber surface of natural cotton. Figure 1b represents the full-frame EDX analysis of raw cotton showing the presence of only C and O. The SiO2 NPs, prepared by the solgel process, were able to attach on the pretreated cotton fiber easily as shown in Fig. 1c. The SiO2 NPs acted as substrates for MTCS and allowed the cotton surface modification as discussed earlier. As seen in the FESEM image of SiO2-treated cotton, the spherical SiO2 NPs have a smooth surface. Upon treatment with MTCS, the cotton surfaces become rough (see Figs. 1d, 1e, and 1f), which makes it superhydrophobic and superoleophilic. High-magnification FESEM image of MTCS/SiO2-treated cotton represented in Fig. 1f revealed the presence of SiO2 NPs on the cotton fibers along with a rough surface. Moreover, magnified FESEM images in Fig. S2a–d clearly describe the J. Coat. Technol. Res. Stable water droplet OH OH OH OH H O H O OH Cotton Cotton Cotton OH H O Pre-treatment OH C6H7O2(OH)3 (cotton) OH OH OH OH OH OH MTCS modification Treated by SiO2 NPs SiO2 NPs Cotton fibre Cotton fibre Cotton fibre OH OH OH OH OH OH H C O CH2OH H OH C H C C OH H O OH H C C H OH O C C H H MTCS coating OH H OH Si OH O CH3-Si H H O H O O CH2OH OH Si OH O O H n n Abundant hydroxyl groups present on cotton fibres Hydrophilic cotton NaOH solution SiO2 on hydroxyl groups of cotton fibres Hydrophilic cotton O OH OH C C H O O CH3-Si Si O O Si O OH O H H O O O H n MTCS reacts with hydroxyl groups of SiO2 treated cotton Superhydrophobic cotton Scheme 1: The proposed mechanism for the synthesis of superhydrophobic MTCS/SiO2-treated cotton surface roughness difference between SiO2-treated cotton and MTCS/SiO2-treated cotton. Figure S3 (electronic supplementary information) represents the full-frame EDX analysis of SiO2-treated cotton where a high percentage of oxygen and silicon is present. This is in agreement with the fact that SiO2 NPs were attached to the cotton fibers. In addition, a small amount of carbon was also evident from EDS analysis. This could be explained by the presence of cellulose in the cotton fiber.32 Figure 1g represents the full-frame EDX analysis of MTCS/SiO2-treated cotton. It was observed that silicon, oxygen, and carbon percentages increased by a considerable amount, which can be explained by the formation of crosslinked polymethylhydrosiloxane on the SiO2-treated cotton fibers from the reaction between MTCS and the SiO2 NPs.10 Phase and structural analysis The XRD pattern (see Fig. 2a) of the raw cotton demonstrated four Bragg’s diffraction peaks at 14.82, 16.58, 22.9, and 34.5 due to the monoclinic structure of cellulose [(C6H10O5)x] phase.6,33 The XRD pattern of SiO2 NPs showed a single broad peak centered at 22.9 2h value suggesting the formation of pseudo-amorphous or nanocrystalline colloidal SiO2 phase.34,35 The single amorphous peak is due to the smaller particle size effect and incomplete inner structure of the spherical SiO2 NPs.34,36,37 The XRD profile of the SiO2 NPstreated cotton showed no additional peaks except the XRD pattern of the raw cotton (see Fig. 2a). The XRD profile of MTCS/SiO2-treated cotton did not divulge any unwanted peaks. The absence of any unwanted peaks in the SiO2-treated cotton and MTCS/SiO2treated cotton indicated no new crystalline phase formed during chemical modification of the cotton surface. The monoclinic structure of cellulose is retained after MTCS surface modification. FTIR analysis was performed to understand the surface evolution of raw cotton, SiO2-treated cotton, and MTCS/SiO2 NPs-treated cotton, respectively (see Fig. 2b). FTIR identified the chemical bonding in the raw cotton and SiO2 NPs. The absorption bands at 461, 799, 950, and 1090 cm1 were observed due to characteristic vibrations of Si–O out-ofplane deformation, Si–OH stretching, Si–O bending, and Si–O–Si stretching, respectively.9 It was also observed that a significant absorption band at 1632 cm1 was present corresponding to C–O bending vibration.7,10 This can be explained by the complete reaction between TEOS and ammonia solution and corroborates the presence of siloxane ( ” Si–O–Si ” ) coatings.32 FTIR analysis shows additional absorption bands at 786 and 1273 cm1 due to MTCS treatment attributed to the characteristic vibrations of Si–O–Si and C–Si asymmetric stretching present in the C–Si–O units.3,38,39 The absorption peak around J. Coat. Technol. Res. Fig. 1: FESEM images of (a) raw cotton, (b) EDS spectra showing the elemental distribution of raw cotton, (c) SiO2-treated cotton, (d–f) MTCS/SiO2-treated cotton, and (g) EDS spectra showing the elemental distribution of MTCS/SiO2-treated cotton Intensity (a.u.) MTCS@SiO2-coated cotton SiO2-coated cotton SiO2 nanoparticles Transmittance (a.u.) (b) (a) MTCS/SiO2-treated cotton 786 cm-1 1273 cm-1 SiO2-treated cotton 2852 cm-1 461 cm-1 1090 cm-1 2923 cm-1 SiO2 nanoparticles 1632 cm-1 3367 cm-1 799 cm-1 950 cm-1 Raw cotton Raw cotton 10 15 20 25 30 35 40 2θ (deg.) 45 50 55 60 65 3500 3000 2500 2000 1500 1000 500 −1 Wavenumber (cm ) Fig. 2: (a) XRD patterns and (b) FTIR spectra of raw cotton, SiO2-treated cotton, and MTCS/SiO2-treated cotton 2923 and 2852 cm1 corresponds to stretching and bending vibration of –CH2 of the cotton surface, whose intensity becomes sharp after successful grafting of MTCS compared to that of the pristine cotton, likely due to attachment of –CH3 groups of MTCS.4,7 However, the peaks were absent in the FTIR spectrum J. Coat. Technol. Res. of the bare SiO2 NPs. Also, the absorption peak at 3367 cm1 was present in FTIR spectra for entire cotton samples due to –OH groups.4,7 Superhydrophobicity and wettability Wettability of any sorbents is an essential parameter to determine the hydrophobicity and oleophilicity toward the oil–water separation. The digital images of raw cotton and MTCS/SiO2-treated cotton illustrated the physical change in color after MTCS modification (see Fig. 3a). Superhydrophobicity of the as-synthesized MTCS/SiO2-treated cotton was determined by a simple immersion test as shown in Figs. 3b and 3c. It was found that the raw cotton was hydrophilic and remained completely immersed in water due to the large liquid contact area of solid–liquid interface and abundant hydrophilic surface groups. However, MTCS/SiO2-treated cotton remained floating on water and absorbed oil quickly due to the formation of the Cassie state with low surface energy and high surface roughness.40 The general perception of hydrophobicity or hydrophilicity was examined by the extent to which a sessile water droplet remains static or spreads quickly upon addition on the surface of the material (see Fig. 3c). Moreover, SiO2-treated cotton shows hydrophilic nature upon immersion in water and shows immediate absorption of both oil–water as displayed in Fig. S4a–b. The spreading action takes place due to the difference in the surface tension of the liquid–solid interface. The extent to which a static water droplet spreads on a surface is measured by a parameter known as contact angle (CA) as shown in Fig. 3d. Since the surfaces are rarely homogeneous, we employed here the Cassie equation applicable to the heterogeneous rough surface with low surface energy exhibiting superhydrophobicity.40 cos hc ¼ f ðcos h þ 1Þ 1 where f is the fraction of the solid surface in contact with liquid, h and hc represent the water CA on smooth and rough surfaces, respectively. Fabrication of a proper rough surface with a low surface energy can render a surface superhydrophobic/superoleophilic since the surface tension of oil are generally much lower than that of water.41 Microtextured or micropatterned surfaces with hydrophobic tendencies can exhibit apparent CA greater than 150 (Cassie state) and are associated with superhydrophobicity or the ‘‘lotus effect.’’42 The CA of a static droplet of water ( 8 lL) on the MTCS/SiO2-treated cotton was measured 173.3 ± 2 at room temperature. Also, the interfacial surface tension of MTCS/SiO2-treated cotton was calculated 23.5 mN/m, less than 35 mN/m threshold limit showing superhydrophobicity.20 Furthermore, CA of a water droplet-in air on the SiO2treated cotton was measured 75.1 at room temperature, which shows the hydrophilic nature of SiO2treated cotton as shown in Fig. S4c. Fig. 3: Optical images of (a) raw cotton and MTCS/SiO2-treated cotton, (b) hydrophilicity, hydrophobicity, and oleophilicity test of MTCS/SiO2-treated cotton, (c) digital image showing stable water droplet and oil absorbed in superhydrophobic MTCS/SiO2-treated cotton, and (d) CA measurement of stable water droplet on the surface of MTCS/SiO2-treated cotton J. Coat. Technol. Res. Selectivity tests The oil–water selectivity of the MTCS/SiO2-treated cotton was examined using two simple tests. First, colorless aqueous solutions of silver nitrate (AgNO3) and sodium chloride (NaCl) were taken in two different beakers. AgNO3 (Ag+ ions) solution has a high sensitivity toward chloride ions (Cl) as shown in Fig. S5 (electronic supplementary information). If a small amount of Cl ions comes into contact with Ag+ ions, a white precipitate of silver chloride (AgCl) forms and changes the color of the solution to milky gray.43 To verify whether MTCS/SiO2-treated cotton is indeed superhydrophobic or not, an AgCl test was performed and it was expected that white precipitate would not form when absorbed oil squeezed into an NaCl solution using MTCS/SiO2-treated cotton. Since MTCS/SiO2-treated cotton is superhydrophobic, it did not consume any water along with soluble AgNO3 salt which may turn the colorless NaCl solution into milky gray by the formation of AgCl (see Fig. S5a–d).43 However, a similar experiment with raw cotton demonstrated that NaCl solution immediately forms a white precipitate due to the formation of AgCl. In the second test, a suction pump was used to check the oil– water selectivity toward MTCS/SiO2-treated cotton under negative pressure ( 100 Torr) condition. The experiment was performed using an MTCS/SiO2treated cotton attached to one end of a tube and the other end connected with a pump while a conical flask attached with septum/stopper was placed in between acting as a trap such that anything pumped in through the cotton would fall into the flask. The MTCS/SiO2treated cotton was placed directly on the surface of an oil–water mixture. It was observed that only oil appears into the conical flask after few seconds while the water despite being pumped-in under negative pressure did not pass through the cotton surface (see Fig. S6 electronic supplementary information).43 Oil/water separation tests The superoleophilicity of MTCS/SiO2-treated cotton arose from the open pores that exist and allowed oil to permeate into the channels by virtue of capillary force. Hence, the oil selectivity of the MTCS/SiO2-treated cotton increased with time due to capillary action. This superoleophilic property of the MTCS/SiO2-treated cotton was quantified by determining the AC in the presence of oil–water mixtures. The cotton exhibited excellent oil selectivity and water repellent behavior when exposed to the oil–water mixtures (see Video 1 in electronic supplementary material accessible via the link: https://vimeo.com/279800203). The modified cotton exhibited AC of 30–42 times of its weight for different oils (see Fig. 4a). The AC of MTCS/SiO2treated cotton was reasonably better than other cottonbased absorbent materials reported in the literature (see Table 1). Thus, it can be inferred that as-synthe- sized MTCS/SiO2-treated cotton may serve as an alternative sorbent for oil spill cleanup. Further, the AC of the cotton was determined at 20, 40, and 60 min holding time after reaching the saturation value. Four sets of oil-absorbed MTCS/SiO2-treated cotton were placed separately in petri dish allowing oil to permeate out due to gravitational force. It was observed that a low amount of oil was released from the MTCS/SiO2treated cotton when kept aside in a petri dish with a maximum fall of 22%, 14%, 10%, 18%, 14%, 12%, for pump oil, engine oil, diesel, hydraulic oil, vegetable oil, and kerosene, after 60 min, respectively (see Fig. 4b). This behavior could be explained by the surface tension that develops between the oil and the petri dish which facilitates oil coming out from the sorbent with time. Reusability of MTCS/SiO2-treated cotton The MTCS/SiO2-treated cotton was subjected to 10 cycles of continuous absorption–desorption test with various oils. AC and CC were calculated for MTCS/ SiO2-treated cotton as shown in Fig. 4c. The MTCS/ SiO2-treated cotton was found to possess first cycle AC of 34.55, 41.64, 31.31, 35.56, 35.76, and 31.7 g g1 for hydraulic oil, engine oil, kerosene, pump oil, vegetable oil, and diesel, respectively. For each cycle, the AC and CC were calculated. However, there was a gradual drop in AC in the subsequent cycles due to the presence of oil residue after mechanical squeezing the MTCS/SiO2-treated cotton which cannot be desorbed completely without drying. It was observed that at the tenth cycle AC value did not fall below 75% of the first cycle AC value of corresponding oil (see Table 2). It should also be borne in mind that CC values did not vary much between the cycles and the maximum deviation between the fifth and tenth cycle CC was 0.15 g g1. Hence, it can be inferred that CC remains quite stable between the cycles. The average recovery of the modified cotton from the first to ten cycles was observed between 86% and 89% demonstrating that MTCS/SiO2-treated cotton could be reused effectively for ten cycles without compromising its sorption behavior (see Fig. 4d). In the real world, the MTCS/SiO2-treated cotton would be subjected to different pH of water or saline water. Therefore, AC was determined under the harsh conditions. Oil–water mixtures were prepared with varying pH (2, 5, 7, 9, and 12) of water. It was observed that AC remains stable ( 29.02 to 28.77 g g1) with pH as shown in Fig. 5a. Further, AC of the MTCS/SiO2-treated cotton was evaluated with different salts and alkali added kerosene–water mixture (1:5 v:v ratio) as a model oil–water mixture. The concentration of the salts and alkali solution was taken as 1 molar in each experiment. The oil–water separation results showed AC deviation of 9.71%, 11.29%, 7.92%, 4.48%, 8.87%, 6.39% between first J. Coat. Technol. Res. 20 31.7 31.31 35.56 30 35.76 41.64 40 10 Hydraulic oil 40 20 0 Engine oil Kerosene Pump oil Vegetable oil Diesel Diesel AC 60 AC0 AC40 AC 40 20 0 AC 40 20 0 AC 40 20 0 AC 40 20 0 AC Vegetable oil CC Pump oil CC Kerosene CC Engine oil CC AC60 40 30 20 10 0 Pump oil (d) CC 40 20 0 AC20 50 Oil recovery(%) Sorption capacity (g g−1) (c) 0 Absorption capacity (g g−1) (b) 50 34.55 Absorption capacity (g g−1) (a) Hydraulic oil CC 90 60 30 0 90 60 30 0 90 60 30 0 90 60 30 0 90 60 30 0 90 60 30 0 1 2 3 4 5 6 7 8 9 10 Hydraulic Vegetable Kerosene oil oil Diesel Diesel (Average = 89.9%) Vegetable oil (Average = 87.95%) Pump oil (Average = 88.54%) Kerosene (Average = 89.08%) Engine oil (Average = 89.55%) Hydraulic oil (Average = 86.21%) 0 0 Engine oil 1 2 3 4 5 6 7 8 9 10 No. of cycles No. of cycles Fig. 4: (a) AC of different oils after the first cycle, (b) AC retention with time (min) using superhydrophobic MTCS/SiO2treated cotton, (c) AC and CC for different oil after 10 cycles, and (d) percent oil recovery after 10 cycles Table 1: Comparison of selective absorption behavior of cotton-based superhydrophobic sorbents Absorbent materials Synthesis route Absorption type Performance Reduced graphene oxidecoated cotton Graphene-coated cotton Dip coating of polydimethylsiloxane Hydrothermal method Organic solvents and oils Oil Reduced graphene oxide cotton Superhydrophobic cotton fabrics Acylated cotton fibers with a fatty acid Polydimethylsiloxane (PDMS)-coated cotton MTCS/SiO2 cotton Thermal reduction Oil Polydimethylsiloxane and ZnO coating Cellulose acylation using microwave radiation Chemical vapor deposition Oil Oil absorption 11 to 25 times its weight Oil absorption 30 times its weight Oil absorption 22 to 45 times its weight Higher water rejection Dip coating of methyltrichlorosilane and third cycle for NaCl, MgCl2, NaOH, Na2SO4, hot water, and cold water solution in kerosene–water mixture (see Fig. 5b). Further, gravity-driven oil–water separation test was performed using MTCS/SiO2-treated cotton.24 The purpose of the gravity-driven oil–water separation test is to understand the filtration behavior of MTCS/SiO2treated cotton as an oleophilic membrane. The MTCS/ Oil Oil Oil Oil absorption 11 to 20 times its weight Oil absorption 30 to 65 times its weight Oil absorption 30 to 42 times its weight References 44 43 33 45 46 47 Present work SiO2-treated cotton was pressed in a circular shape with 50 mm diameter and fixed between two glass tubes of filtration unit as shown in Figs. 6a, 6b, and 6c. Kerosene–water mixture-filled glass tube filtration unit was attached at the tilted position so that kerosene–oil mixture remained in contact with the MTCS/SiO2treated cotton and oil could pass quickly through the open pores in the cotton at atmospheric pressure (see J. Coat. Technol. Res. Table 2: AC and CC of MTCS/SiO2-treated cotton with various oils over multiple cycles Hydraulic oil Engine oil Kerosene Pump oil Vegetable oil Diesel CC at 1st cycle (g g1) AC at 1st cycle (g g1) 34.55 41.64 31.31 35.56 35.76 31.70 2.4 3.2 2.8 2.1 2 2.3 ± ± ± ± ± ± 3.95 3.96 2.74 3.82 3.28 2.77 ± ± ± ± ± ± AC retention after 5th cycle (%) CC variation after 5th cycle (g g1) AC retention after 10th cycle (%) CC variation after 10th cycle (g g1) 85.79 93.25 84.40 95.78 92.36 89.38 0.08 0.02 0.18 0.01 0.16 0.20 75.58 86.82 74.84 88.15 79.97 85.24 0.05 0.03 0.12 0.06 0.05 0.05 0.44 0.21 0.17 0.20 0.33 0.17 (b) 40 Kerosene-water mixture 35 28.77 20 31.86 30.51 25 32.76 30 29.02 Absorption capacity (g g−1) (a) 15 10 5 0 1 2 3 4 5 6 7 8 9 10 11 12 13 Absorption capacity (g g−1) Oils 45 Cycle 1 Cycle 2 Cycle 3 Kerosene-water mixture 40 35 30 25 20 15 10 5 0 Sodium Magnesium Sodium Sodium Hot Water Chloride Chloride Hydroxide Sulphate pH Water Fig. 5: AC with (a) variable pH and (b) different salts/alkali added kerosene–water mixture using superhydrophobic MTCS/ SiO2-treated cotton (a) (b) (c) Water Kerosenewater mixture Kerosene 100 1000 80 800 60 600 40 400 200 20 Volume flux Separation efficiency 0 0 0 50 100 150 Time (S) 200 250 Absorption capacity (g g−1) (e) 1200 Separation efficiency (%) Volume flux (Lm−2h−1atm−1) (d) 40 30 Hydraulic oil Engine oil Kerosene Pump oil Vegetable oil Diesel 20 10 0 0 5 10 15 20 25 30 Time (S) Fig. 6: (a–c) Gravity-driven oil–water separation setup, (d) variation of kerosene oil volume flux and separation efficiency measured with time, and (e) absorption kinetics behavior of MTCS/SiO2-treated cotton with time for different oils J. Coat. Technol. Res. Video 2 in electronic supplementary material accessible via the link: https://vimeo.com/279800581). Figure 6d shows the volumetric flux and separation efficiency of kerosene with time.48 It can be observed that within 270 s, 94% kerosene can be recovered from the kerosene–water mixture using MTCS/SiO2treated cotton. Further, the reduction in volume flux and increase in separation efficiency (%) of kerosene with time confirmed that hydrocarbon could permeate easily through the superhydrophobic MTCS/SiO2treated cotton under gravity. Absorption kinetics for sorption mechanism A highly interconnected porous structure that exists in the MTCS/SiO2-treated cotton helps oils become absorbed quickly with time. However, the absorption kinetics found that the sorption characteristics also depend on the oil viscosity. While the MTCS/SiO2treated cotton achieved its saturation AC with low viscous oils (diesel and kerosene) within 2 s or less, the saturation AC for oils with higher viscous oils took almost 10 s (see Fig. 6d and Table 3). The low viscous oils have a relatively faster diffusion which enables them to rapidly enter the interconnected porous structure of the MTCS/SiO2-treated cotton. In the case of the higher viscous oils, the oils tend to remain on the surface before permeating into the pores of the cotton. To understand the kinetic behavior, a pseudosecond-order kinetic model was employed2: for highly viscous oils (see Table 3). Therefore, it can be concluded that sorption behavior of various oils in the MTCS/SiO2-treated cotton depends not only on the quality of sorbent but also on intrinsic viscosity. Conclusions A superhydrophobic/superoleophilic MTCS/SiO2-treated cotton was fabricated using a simple dip coating method. A low concentration of MTCS solution [2% (v/v)] was used for surface modification and gave rise to superior superhydrophobic properties. Since cotton is reasonably cheap and readily available, the cost of fabrication becomes low. FESEM, EDX, FTIR, and XRD analyses confirmed the formation of –C–Si–O– units on the cotton fiber surface. The –C–Si–O units grew layer by layer onto the MTCS/SiO2-treated cotton surface and developed superhydrophobicity on the cotton surface. This modified cotton exhibited CA of 173 ± 2 which satisfies the conditions for a Cassie state. It showed AC of 30–40 g g1 for various oils and was found to be reusable for ten cycles with minimal loss in its sorption behavior. Acknowledgment Dr. Partha Saha gratefully acknowledges the financial support received from Department of Science and Technology, Science and Engineering Research Board (DST-SERB) (Grant Number ECR/2016/000959). 1 1 ¼ K:t Qs Qt Qs where t denotes the sorption time, Qs denotes the saturation AC, Qt indicates the amount of oil absorbed at time t, and K is a viscosity-dependent sorption constant. Qs, K, and t were calculated using the pseudo-second-order kinetic model. The experimental data were fitted in this model, and an excellent agreement was achieved with the regression coefficient (R2) value greater than 0.95.49–52 The K value for less viscous oils like diesel and kerosene was found to be 0.652 and 0.627 s1, respectively. 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