疏水涂层

Regular ArticleOne-step fabrication of highly stable,superhydrophobic composites from controllable and low-cost PMHS/TEOS sols for efficient oilcleanupPing Guo,Shangru Zhai ⇑,Zuoyi Xiao,Qingda An ⇑Faculty of Light Industry a r t i c l e i n f o Article history:Received 9December 2014Accepted 22January 2015Available online 28January 2015Keywords:Separation Oil sorption Hybrid coating PMHS Durability Low-costa b s t r a c tFacing the issues of significant increase of industrial oily wastewater and frequent accident of oil spills,the developing of efficient and affordable absorbents for improving oil pollution is of practical signifi-cance.Herein,several superhydrophobic and superoleophilic materials,utilizing filter paper,filter cloth and polyester sponge as substrates,through facile coating of hybrid SiO 2colloid particles from control-lable PMHS–TEOS sol system were presented.These methyl-modified particles not only provided hierar-chical micro/nano-scale structure with distinct roughness,but also largely lowered the surface energy of the coated substances,leading to excellent superhydrophobic and superoleophilic surfaces.The modified filter cloths could be applied for oil/water separation owing to the flexible and foldable property;sponges could efficiently absorb oil or organic solvents in situ on account of its low density and high porosity,and meanwhile the absorbed oil could be easily recollected by simple squeezing.It is worth mentioning that both modified filter cloths and sponges exhibited excellent selectivity,high efficiency,outstanding rapid-ity and remarkable recyclability.More importantly,after treatment of 100abrasion cycles with metal scalpel and strongly acidic and basic water droplets,the whole WCA values of resultant filter cloths still maintained superhydrophobic character (>150°),illuminating the charming mechanical and chemical stability of sol–gel processed coating with hierarchical roughness and covalently bonded methyl bining controllable fabrication process and cheap raw precursors,this method enables scalable man-ufacturing of stable and superhydrophobic substances,which are promising in practical applications involved in oil/water separation and oil sorption.Ó2015Elsevier Inc.All rights reserved.1.IntroductionAccompanying with the progressing industrialization,the increased oily wastewater as well as frequent oil spill accidents have caused severe environment and ecological problems.The Gulf of Mexico oil spill in 2010not only seriously damaged the marine water,animals and plants,but also was a great loss of energy [1–3].Various techniques including oil skimmers,oil containment booms,in situ burning,dispersants,solidifiers and absorbents have been employed to solve the problem of oil pollution [4–7].Amongst them,removing oil by low-cost and efficient absorbing materials is convenient and attractive,as oil pollutants can be transformed from liquid into absorbents,realizing the collection/10.1016/j.jcis.2015.01.0620021-9797/Ó2015Elsevier Inc.All rights reserved.⇑Corresponding authors.E-mail addresses:zhaisr@ (S.Zhai),anqingda@ (Q.An).and elimination of oil from water system.Nevertheless,employed absorbents,including inorganic mineral products[8,9],organic natural products[10,11]and synthetic polymers[12],commonly suffer from certain shortages such as low separation efficiency, insufficient buoyancy,slow biodegradability,poor selectivity and low sorption rate,etc[13].To this end,it is of practical significance to explore facile,low-cost yet highly efficient approaches to fabri-cate oil-absorbing materials with excellent selectivity and sorption capacity[14–17].Inspired by the self-cleaning and water-repellent character of lotus leaf and water strider legs,nowadays,worldwide researchers are strongly interested in designing superhydrophobic surfaces with a water contact angle(WCA)greater than150°by controllable fabrication protocol[18–20].Within these contexts,it has been demonstrated that superhydrophobic surfaces can be achieved by combining hierarchical micro/nano-structure and low surface energy[21–24].Accordingly,if those functional materials with superhydrophobic and superoleophilic surface were successfully designed,they would have promoted oilflow through selectively and repelled water strongly,possibly leading to high efficiency and excellent selectivity to oily substances[25–29].So far,aiming for idea,many functional materials with special wetting surface,such as fabrics/textiles[30–32],filter papers [33],films[34,35],sponges[36–39]and other devices[40]have been prepared and employed in separation of oil/water mixture. For example,Zhou and co-workers[30]prepared the superhydro-phobic fabric coating using polydimethylsiloxane elastomer, hydrophobic silica nanoparticles andfluorinated alkyl silane;as-prepared fabrics showed remarkable durability against repeated machine washes,severe abrasion,strong acid or base,and boiling water treatment.Cheng et al.[40]showed a multifunctional device by combining electroless metal deposition with self-assembled hydrophobic monolayers,which exhibited high selectivity,high separation yield,reproducibility and suitability for various oils and organic solvents.However,certain issues such as expensive raw materials,relatively poor stability and durability,complicated equipment,sophisticated process,rigorous experiment conditionIt was shown in our previous studies that polymethylhydrosi-loxane(PMHS),a commercially cheap polymer with active Si-H bonds and methyl groups,can be employed as multiple role reagent(e.g.precursor,in situ reductant and surface modifying agent)to fabricate sol–gel processed hybrid substances[45–48]. Indeed,taking advantage of the fascinating reaction character andflexible linear chain,not only can PMHS effectively take part in the sol–gel process together with traditional organosilanes (e.g.TEOS,TMOS and BTME,etc),but also can desirably tailor the textural characteristics and surface chemistry of resulting materi-als by anchoring onto newly formed colloid particles through co-condensation process[49,50].In this respect,it is highly possible that superhydrophobic surface reminiscent of lotus leaf,i.e.hierar-chical micro/nano-structured roughness and significantly low sur-face energy,can be single-step achieved by modifying commercial substrates with the one-pot prepared sol system with PMHS and TEOS;this maybe open a door to fabricate oil-absorbing materials with practical requirements of low-cost,scalable,ease of operation and mechanically stable.Herein,in continuation of previous work on employing PMHS to design sol–gel functional materials for various applications [51],a single-step sol–gel coating techniquefitting for producing various porous superhydrophobic materials withfilter cloth and polyester sponge as substrates was presented(see Scheme1). Through the comparison and analysis with our previous work, we could see that this process is more simple and time-saving, and polystyrene is not necessary for creating superhydrophobic surfaces.With being simply modified by sol system of polymeric PMHS and TEOS,the hydrophobic characteristics of modified substrates were investigated by WCA method thoroughly.Addi-tionally,the oil-separating/absorbing performance,mechanical stability and reusability of modified substances have also been fully studied,to demonstrate the potentiality of the proposed method.2.ExperimentalScheme1.Preparation and application of superhydrophobic materials. 156P.Guo et al./Journal of Colloid and Interface Science446(2015)155–1622.2.Preparation of superhydrophobic surface on various substratesFirstly,filter cloths and polyester sponges were ultrasonic washed with ethanol and distilled water to remove any possible impurities,and then the cleaned substrates were obtained after drying at60°C for2h.On the other hand,thefilter paper was used directly.Typically,0.1mL anhydrous ethylenediamine was added into 60mL tetrahydrofuran solution and stirred for0.5h at room tem-perature.Immediately,0.3mL PMHS was dipped into the solution by stirring for0.5h.After then,2.6mL TEOS was slowly added into the system and continuously stirred for12h.After that,0.3mL H2O was dipped into the system and vigorously stirred for3h,to allow the occurrence of hydrolysis-condensation reaction between PMHS and TEOS[49].At this stage,piece offilter paper,filter cloth and polyester sponge can be immersed in the sol system for coating for-mation on the porous substrates with duration of6h.Finally,the modified materials were obtained after being simply dried in air.2.3.Oil/water separationA sensitive method was used to qualitatively demonstrate the separation efficiency of preparedfilter cloth.As shown in Fig.2 and Movie S2,a piece of modifiedfilter cloth was closed to thefil-ter cone with folding into‘‘V’’shape.A mixture of n-hexane colored with Sudan red and0.1M NaCl aqueous solution colored with methyl blue was poured onto the sample.After separation,n-hex-ane and aqueous solution were collected and added into0.1M AgNO3aqueous solution,respectively.If there were tiny amount of NaCl aqueous solution penetrated through thefilter cloth together with n-hexane,NaCl would have reacted with AgNO3to generate a white precipitate,for which can be visibly evaluate the separation efficiency of modifiedfilter cloth.2.4.Oil sorption form water and the amount of oil absorbedModified polyester sponge was put into the mixture of kerosene colored with Sudan red and0.1M NaCl aqueous solution dyed with methyl blue.After a few seconds,the absorbed kerosene was col-lected by squeezing the saturated polyester sponge.Soon after, 0.1M AgNO3was added into the collected oil and the rest of NaCl aqueous solution,respectively,to check out the oil-absorbing per-formance of modified polyester sponge.To measure the amount of oil absorbed(k),a piece of as-prepared sample was left submerged in the oils and organic solvents(without water)for5min to reach sorption equilibrium.Then the sample was removed with tweezer and immediately measured the weight of saturated sponge to avoid evaporation of solvents or oils.In consideration of the recy-clability,the used sponge was simply squeezed and then rinsed with ethanol for three times to remove adherent oil species.There-after the sample was dried under natural condition so as to use to repeat the above cycles.2.5.CharacterizationMicroscopic morphologies of thefilter cloths surfaces were studied by scanning electron microscopy(SEM,JSM-6460LV,JEOL, Japan)equipped with EDS unit at an acceleration voltage of20kV. Fourier transform infrared(FT-IR,Perkin-Elmer,USA)spectra were acquired by excluding air background.Static water contact angle (WCA)values were measured by JYSP-360CA apparatus at envi-ronment temperature.For WCA studies,8l L distilled water was used and WCA values were calculated from at leastfive different places of the samples.Abrasion tests were completed by dragging across a surface with metal scalpel.3.Results and discussion3.1.Optimum preparation conditions of sol system for coatingFirst of all,employing highly hydrophilicfilter paper as sample substrate,the optimum preparation conditions of sol system was investigated.Accordingly,the discriminative surface wettability of the originalfilter paper and modifiedfilter was shown in Fig.S1and Movie S1.It was clear that the original paperfilter couldcloths surfaces:(a)before modification;(b)after modification;(c)recycle after5times;(d)recycle after10times.be immediately wetted by the dying water droplets at the moment,indicating the highly hydrophilic character.On the contrary,the dying water droplets retained spherical in shape on the modified filter paper with a WCA value about 153°;more interestingly,the water drops could easily roll away from the surface with nearly no hindrance.It was noticeable that the common filter paper turned into superhydrophobic substance after simple treatment with the sol of PMHS and TEOS,revealing the potential of employed sol system toward the fabrication of superhydrophobic coating on porous substrates.As far as the preparative conditions of sol system are concerned,the dependence of the WCA values of as-fabricated filter papers on the water dosage and the mass ratio of PMHS/TEOS were investi-gated (see Fig.S2).As shown in Fig.S2(a),the WCA increased with the dosage of water and,at the water of 0.3mL,the WCA reached the highest value of 153°,indicating superhydrophobic character of the prepared filter paper at this point.However,as the water increased beyond 0.3mL,the reduction in WCA was observed.It was speculated that PMHS proceeded hydrolysis-condensation together with TEOS upon addition of water,subsequently yielding hybrid colloid particles attached with hydrophobic methyl groups [50].On the one hand,these particles provided hierarchical micro/nano-scale rough structure on the surface of filter paper;on the other hand,the surface energy of the substance was significantly lowered due to the presence of methyl groups on the particles.There was no sufficient water to participate in hydrolysis-conden-sation when the dosage of water was lower;however,if were the amount of water excessive,the rate of hydrolysis-condensation would have been highly accelerated,thereby affecting the particlesize of SiO 2aggregates and in-turn the micro/nano scale roughness,for which was a determining factor for the superhydrophobic surfaces.In addition,altering the amount of TEOS introduced in the reac-tion system while maintaining the amount of PMHS constant,it was also found that the mass ratio of PMHS/TEOS played a signif-icant effect on the WCA of the filter paper (Fig.S2b ).Except the diverse methyl content produced an effect on the surface energy,it was considered that the variation of TEOS might also lead to the considerable change of particle size of the sol system,and accordingly the fluctuant variation of the WCA value of modified filter paper.Consequently,for the preparation of superhydrophobic coating surface on potential substrates with the sol system derived from PMHS and TEOS,the conditions of 0.3mL water and the mass ratio of PMHS/TEOS of 1/8are suitable.3.2.Modified filter cloths for efficient oil/water separationFig.1showed SEM images of the filter cloth before and after surface modification.It was clear that the original smooth surface of filter cloth distinctly turned into hierarchical micro/nano-scale structure with significant roughness after being simply modified by the sol system.Accordingly,on such rough surface,air could be trapped into the interspace of aggregated particles,by which would greatly reduce the actual liquid/solid contact area and thus significantly increasing the contact angle [52].In addition,appar-ent distinctions could be also provided by EDS observations;that is,original filter cloth did not display Si peak (see inset in Fig.1a),and contrarily modified filter cloth showed an obviousSiseparation of n -hexane (dyed with Sudan red for easy observation)and 0.1M NaCl aqueous solution (dyed with methyl obtained filter cloth;(c)collected n -hexane and NaCl aqueous solution;(d)0.1M AgNO 3aqueous solution were addedpeak at1.87keV(the inset in Fig.1b).Itparticles with hydrophobic methyl moietiescoated on thefilter cloth surfaces.showed the SEM images of modifiedfilterten times separation–abstersion cycles,the surfaces still maintained distinctyses confirmed that the existence of surfacefilter cloth even after10cycles.It was athe mechanical stability of employed hybridonto thefilter cloth was highly acceptable,clability toward oil/water separation couldIn addition,just as displayed in Fig.S3,ginalfilter paper,the distinct peaks at1273cmcoatedfilter paper could be attributable to thethis also supported the fact that hybrid SiO2lently bonded methyl groups had beenthe surfaces of cleanedfilter cloths.As is known,filter cloth is very soft andenvisaged as a promising candidate forfore,a piece of modifiedfilter cloth was closedthe separation of the mixture of n-hexane/NaCl aqueous solution, to see how it would respond as an effective separator(see Fig.2and Movie S2).Fig.2(a)was the mixture of n-hexane and0.1M NaCl aqueous solution,and n-hexane keeping above the NaCl aque-ous solution for lower density.Interestingly,when the mixture was poured into the simple device continuously,n-hexane quickly passed through thefilter cloth and dropped into the beaker under-neath,while the NaCl aqueous solution was entirely kept onto the filter cloth due to the superhydrophobicity and superoleophilicity of the modifiedfilter cloth(see Fig.2b).Next,the retained NaCl aqueous solution could be easily collected by another breaker.It is well known that Ag+is very sensitive to ClÀions,even if a tiny amount of NaCl aqueous would have quickly reacted with AgNO3 to generate a visible white precipitate.As expected,when0.1M AgNO3aqueous solution was added into the collected n-hexane, no white precipitate could be detected even for a while,whereas the added solution presented the inherent layering.On the con-trary,clear white precipitate was formed instantly in the breaker of collected NaCl aqueous solution(Fig.2d).that the n-hexane/water mixture could bemodifiedfilter cloth.Meanwhile,it was alsomodifiedfilter cloth still maintainedeven though it was wetted by oil or organicIn general,most of the biomimeticshow poor stability toward the abrasion andment(such as strongly acidic or basic solution),rioration of the topography and surfacecriteria for practical applications,themodifiedfilter cloth against various harshmechanical abrasion and liquid with diverseinvestigated.The variation of WCA valueusing a metal scalpel as the abrasionFig.3.Clearly,the superhydrophobicity wasout apparent reduction even after100cycles,mechanical stability of the hybrid SiO2coatingter cloths.Moreover,considering the importantstability toward corrosion resistance forthe coatedfilter cloth was further treatedand basic water droplets.As shown in Fig.S4,displayed hemisphere on the originalfilteramount of water could permeate thefiberconditions.This can be associated to themicro/nano-scale rough structure on thetrast,all corrosive droplets could distinctlyon the resultant superhydrophobicfilter corresponding WCA values exceeded150°over a wide range of pH values from1to14.This clearly revealed that these modified superhydrophobic coatings derived from sol–gel reaction pathway exhibited excellent chemical stability toward studied corrosive liq-uids with wide pH range,revealing good potentiality of it toward practical processes.Lastly,in view of energy saving and environmental protection, the reusability is of practical significance to meet potential applica-tion requirement.At this regard,the sample was washed with eth-anol for three times after each use,and then dried under natural condition.Just as shown in Fig.4,not only did the WCA values almost unchange with cycle numbers increased from0to8times but also held a much high level at10cycle times,clearly revealing the excellent reutilization potentiality of modifiedfilter cloths. Interestingly,the sample subjected to10cycles of separation–cleaning could be regained by simply immersing used sample into the sol system again.Overall,it had been clearly suggested that, along with the efficient separation of oil/water mixture,the as-fab-ricated superhydrophobicfilter cloth possessed excellent reusabil-and stability toward the abrasion and corrosive liquid,making promising to participate in the complicated practical conditions.3.Variation of WCA values of coatedfilter cloths with abrasion cycles. inserts in lower right-hand corner is the picture of abrasion with metal scalpel.Fig.4.Variation of WCA values of coatedfilter cloths with separation cycles.P.Guo3.3.Modified polyester sponges for in-situ oil sorptionConsidering the increased accident of oil spills,commercial polyester sponges with low density and developed porosity is fur-ther chosen as a typical substrate to prepare superhydrophobic functional materials than can be effectively applied for in-situ oil sorption,and,at the same time,to demonstrate the universality of the sol system of PMHS and TEOS.Firstly,Fig.5illustrated the discrepant wettability of the raw and modified polyester sponges.It was clear that,after being place on the surface of water,the modified products could smoothlyfloat on the water surface without submergence,while the raw sponge totally sank into the water surface within few seconds.The water-repellent behavior of the samples was further confirmed by the photographs in Fig.5b.Noticeably,water droplet could easily per-meate into the interior of the unmodified sponge due to the strong hydrophilicity and capillarity of developed network.By contrast, the water droplet could stably stay on the modified surface in a nearly spherical shape.More interestingly,the water droplet could be removed completely from the sponge surface without any trace, clearly revealing the superhydrophobic character endowed by the simple coating treatment with the sol system.Subsequently,Fig.6showed the oil sorption and collection pro-cess using the modified sponge.It was noticeable that the coated sponge could quickly absorb the oil layerfloating on the water sur-face,and at this stage it could be easily wetted with red oil soon after being touched with the mixture of kerosene and0.1M NaCl aqueous solution,which was closely dependent on the superhy-drophobicity/superoleophilicity and capillary pressure of the as-prepared porous sponge(see Movie S3in supporting information). After a moment or two,the whole oil layer could be absorbed spon-taneously to the coated sponge at this physical trapping,leading to the clear cut recovery of the water surface.More desirably,the absorbed oil could be released out through a simple mechanical squeezing process.To present a more direct visualization of the effi-cient oil-absorbing process for modified sponge,0.1M AgNO3aque-ous solution was added into the collected kerosene and the rest of NaCl aqueous solution,respectively;clearly,there was no any white precipitate appeared in the collected oil system,directly indicating a complete sorption of the oil phase from the mixture of oil and NaCl aqueous solution.This further demonstrated that commercial hydrophilic polyester sponge could be easily changed into excellent oil-absorbing materials by single-step coating of PMHS and TEOS sol system with ease of operation,avoiding using any expensivepure and modified sponge placed on the water surface;(b)photographs of dyed water droplet on the surfaces of originalsponge could be removed without any residual dye.160P.Guo et al./Journal of Colloid and Interface Science446(2015)155–162Additionally,the oil sorption capacities (k )of the modified sponge for typical kerosene and n -hexane were listed in Fig.7,and k was calculated with the following formula:k ¼ðM 1ÀM 0Þ=M 0ð1Þwhere M 1was the weight of the sorption saturated sponge with oil (g),M 0was the weigh of sponge before sorption (g).On the one hand,the sorption capacities of the samples varied from 44for ker-osene to 26for n -hexane,which could be contributed to the differ-ent density of the oily pollutant.On the other hand,the contaminated sponge could be retrieved after mechanical extrusion and rinsing with ethanol.More noticeably,there was only slight reduction of uptake capacities after ten times of sorption–extru-sion-rinse cycles,confirming the excellent recyclability and fasci-nating selectivity of the modified sponge for in situ oil sorption.4.ConclusionIn summary,a facile,low-cost and single-step sol–gel approach for fabricating robust water-repellent coatings on various porous substrates of filter paper,filter cloth and polyester sponge was pre-sented.It was found that all fabricated materials displayed super-hydrophobicity and superoleophilicity,excellent mechanical and chemical stability.More significantly,the modified flexible filter cloths could separate oil/water mixture rapidly and selectively,and the coated polyester sponges could efficiently absorb various oils and organic solvents in situ,without being wetted by any water.It was thus believed that this universal technology is prom-ising for oil-containing industrial wastewater treatment and oil spill cleanup,by virtue of the multi-advantages of ease of operation process,commercially cheap raw materials and excellent recycla-bility and mechanical/chemical stability of covalently cross-linked hierarchical and hydrophobic coatings.AcknowledgmentsFinancial support from the National Natural Science Foundation of China (21446001),the Program for Liaoning Innovative Research Team in University (LT2013012)and the Program for Liaoning Excellent Talents in University (LJQ2014056)is highlyappreciated.sorption of dyed kerosene from dyed 0.1M NaCl aqueous solution surface by using the coated sponge;(c)collected aqueous solution was added into (c).5101520253035404512345678910Reycle numberskerosenen-hexane Variation of oil sorption capacities (k )of the superhydrophobic sponges with sorption cycles.Appendix A.Supplementary materialSupplementary data associated with this article can be found,in the online version,at /10.1016/j.jcis.2015.01.062. 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