Effect of silane coupling agents on the performance of RO membranes

Journal of Membrane Science300(2007)224–231Effect of silane coupling agents on the performance of RO membranesNowon Kim b,Dong Ho Shin a,Yong Taek Lee a,∗a Department of Chemical Engineering&Advanced Materials Engineering,College of Environment and Applied Chemistry,Kyung Hee University,Gyeonggi-do,Republic of Koreab Department of Environmental Engineering,College of Engineering,Dong-Eui University,Busan,Republic of KoreaReceived19October2006;received in revised form23March2007;accepted29May2007Available online13June2007AbstractThis study investigates the effect of silane coupling agents on the performance of reverse osmosis(RO)membranes on the basis of sol–gel coating method.The surfaces of the RO membranes were chemically modified with four different alkoxysilanes in order to reduce their hydrophilicity. The objective of this study is to superpose hydrophobic polysiloxane layer on the surface of a polyamide TFC RO membrane and to increase the extent of salt rejection by the modified membranes.A commercial composite RO membrane(SWC1)was treated with silane coupling agents in ethanol at three different concentrations:1.0,1.5,and2.0%(w/v).The silane coupling agents contain one alkyl or phenyl and three alkoxy groups (e.g.,methyltriethoxysilane,octyltriethoxysilane,octadecyltrimethoxysilane and phenyltriethoxysilane).In addition,the effect of alkyl or phenyl group hydrophobicity on the permeability and salt rejection of the modified membrane was examined.The surfaces of the modified membranes were characterized by SEM,AFM,contact angle analyzer,and XPS in order to confirm successful sol–gel methods.The modified membranes showed significantly enhanced salt rejection without a decrease influx.From the surface analysis results,we can observe the changes in the surface roughness,elemental composition,electron energy,and hydrophilicity.©2007Elsevier B.V.All rights reserved.Keywords:Polyamide;Reverse osmosis membrane;Silane coupling agents;Surface modification;High rejection1.IntroductionReverse osmosis(RO)membrane processes are economical methods for converting feed water into a purified permeate. These processes exhibit various advantages such as high per-meate quality,ease of operation,minimal chemical addition, and low energy requirement[1,2].Conventional studies on RO membrane development focused on enhancing the permeabil-ity and selectivity of membranes.One of the most successful RO membranes is the aromatic polyamide thin-film-composite (TFC)membrane.Cadotte prepared an aromatic polyamide film—the interfacial reaction product of an aromatic polyamine having at least two primary amine groups as substituents and an aromatic acyl halide having at least three acyl halide groups as substituents[3].A number of studies have been conducted to improve the salt rejection ratio andflux of RO membranes based on the aromatic polyamide TFC membrane ∗Corresponding author.Tel.:+82312012577;fax:+82312021946.E-mail address:yongtlee@khu.ac.kr(Y.T.Lee).structure[3–8].Such improved RO membranes facilitate the reduction of the economic costs incurred in water treatment processes.In recent years,a new scheme of RO seawater desalination, referred to as the brine conversion two-stage process,has been introduced[9].In this process,pretreated seawater is fed to the first stage,and a permeate recovery of approximately40%is obtained;the concentrate from thefirst stage is introduced to the second stage,yielding more permeate.Consequently,the water recovery ratio increases from40to60%.This two-stage process permits a small plant size and low water production costs.How-ever,this process requires considerable improvement in the RO membrane performance because the feed water concentration at the second stage is up to60,000ppm and the operating pressure is almost60kg f/cm2.Generally,the seawater desalination process is requested the quality of water under350ppm after desalination treatment. When the feed water concentration is up to60,000ppm,the process becomes possible through the development of a new composite membrane,which has higher salt rejection than com-mercial seawater membranes.0376-7388/$–see front matter©2007Elsevier B.V.All rights reserved. doi:10.1016/j.memsci.2007.05.039N.Kim et al./Journal of Membrane Science300(2007)224–231225Fig.1.The general mechanism of sol–gel methods for surface modification with trialkoxyalkylsilane compounds.Most of RO membranes are prepared by the interfacial poly-merization of the active thinfilms on the microporous support layer.For the interfacial polymerization,diamines and acyl chlo-rides were used.A number of methods were reported to enhance theflux and salt rejection of membranes,using various diamines,acyl chlorides,amine salts and polar organic solvents.This study is concerned with the high rejection membranes based on the sol–gel condensation of silane coupling agents,which is not induced by the additives during interfacial polymerization but the silane coupling agents on the polyamide surface.We previously reported a method to improve salt rejection by treating polyamide TFC RO membrane through sol–gel reaction [10].Treatment of polyamide TFC RO membranes with MeTES (methyltriethoxysilane)or OcTES(octyltriethoxysilane)signif-icantly increased salt rejection.The essential step is the sol–gel methods of silane coupling reagents on the polyamide surface. The general mechanism of sol–gel methods with trialkoxyalkyl-silane compounds to form polysiloxane is shown in Fig.1[11].We have found trialkoxyalkylsilane chemicals are suitable for second coating on the polyamide surface.This paper focuses on the relationship between the structure of silane coupling agents and the properties of RO membranes.We select two silane coupling agents,OdTMS(octadecyltrimethoxysilane) and PhTES(phenyltriethoxysilane).OdTMS has a long-chain aliphatic functional group andflexible carbon chain structure. PhTES has a phenyl functional group and rigid structure.Both silanes are much more hydrophobic than MeTES or OcTES. In this study,OdTMS or PhTES treated membranes are com-pared their permeability and selectivity with those of MeTES or OcTES treated membranes.We examined the influence of the alkyl or aryl hydrophobic factor on water permeabilities and solute selectivities in modified membranes based on extensive surface characterization experiments.2.Experimental2.1.MaterialsCommercially available seawater RO membranes were obtained from Hydranautics Corporation.MeTES(Sigma–Aldrich,98%),OcTES(Sigma–Aldrich,96%),OdTMS (Sigma–Aldrich,98%)and PhTES(Sigma–Aldrich,98%) were used without further purification.Absolute ethanol (Sigma–Aldrich,99.9%)was used to prepare the silane cou-pling solution.Potassium metabisulfite(Sigma–Aldrich,99%)and potassium persulfate(Sigma–Aldrich,99%)were used to etch the polyamide surface of the RO membranes.Sodium chlo-ride(NaCl,Duksan Chemical Co.,98%)was used to test the performance of the RO membranes.Deionized water was used in all the membrane preparation processes and performance tests.2.2.Preparation of silane-coated TFC membranesCommercial membrane modules were subject to an autopsy, and their sheets were washed with deionized water.The mem-branes were immersed in an aqueous solution containing5% (w/v)of propyleneglycol for30min followed by draining of the excess solution.The wet membranes were dried at70◦C in a vacuum oven.Then,the membranes were taped on a glass plate,immersed in an aqueous solution containing0.01M potassium metabisulfite and0.01M potassium persulfate for 30min,and rinsed with deionized water to remove any residues. Each of the aqueous silane solutions containing a catalytic amount of nitric acid was coated on the membranes and cured at70◦C for10min.Four different silanes were used and the following abbreviations were used to denote each silane-coated membrane:MeTES n(methyltriethoxysilane),OcTES n (octyltriethoxysilane),OdTMS n(octadecyltrimethoxysilane) and PhTES n(phenyltriethoxysilane).The subscript n denotes the %(w/v)concentration of the prepared silane solution(Fig.2).2.3.Membrane characterizationRO membrane performance was tested using a laboratory-scale batch test unit with sixflat cells.The apparatus comprised a40l feed reservoir,three parallel stainless steel test cells with an effective membrane area of27.01cm2,and needle valves to control the hydraulic pressure.The feed solution was adjusted to 35g/l of the aqueous NaCl solution.The permeability test was conducted at an operating pressure of55kg f/cm2,feed solution flow rate of4.5l/min,and temperature of25◦C.The water per-meability and salt rejection of membranes were measured by collecting the permeate for30min after stabilization for30min. All membranes were tested in duplicate,and the results were averaged.The performance of the RO membranes is reported in terms of salt rejection and permeateflux.A digital conductivity meter (460CP,Istek electronics,Korea)was used for measuring the conductivities of the feed solution and permeate sample.The salt rejection(SR)and permeateflux are calculated by the following equations:SR(%)=1−permeate conductivityfeed conductivity×100(1)flux(MMD)=permeate volume(m3)membrane area(m2)×time(day)(2)It is extremely difficult to apply conventional analysis tech-niques to the structural characterization of silane-treated mem-branes.The silane condensation was carried out on a polyamide film layer.Polyamides have a crosslink structure;hence,it is insoluble in most polar solvents.Moreover,its polymerization226N.Kim et al./Journal of Membrane Science 300(2007)224–231Fig.2.Silane coupling agents:(a)MeTES,(b)OcTES,(c)OdTMS,and (d)PhTES.on the surface pores of the porous support was very thin and tight [12].It is almost impossible to separate polysilox-ane from a polyamide thin film for membrane characterization purposes.The images of the surface of the membranes were observed with an atomic forced microscope (AutoProbe CP,Thermo Microscope Inc.,USA)used in the non-contact mode and scan-ning electron microscope (SEM:Lica,stereoscan 440,USA).X-ray photospectroscopy (ESCA2000,VG Microtech,GB)was performed to obtain information on the composition of the mem-brane surface layer.An Al K ␣monochromated X-ray source (h =1486.6eV)was used to stimulate photoelectron emission.The measurements were conducted at a take-off angle of 90◦,power of 170W,and chamber pressure of 4.0×10−9Torr.The hydrophilicity of the membranes was observed with a contact angle analyzer (Phoenix300,First-ten Angstrom,USA).The contact angle measurements were carried out with a 6-␮l drop of deionized water.Images were captured 5s after introducing the drop and the contact angles were calculated.In order to remove siloxane oligomers,all the membranes subject to surface analy-sis were rinsed with deionized water using a test cell apparatus operated at a pressure of 4kg f /cm 2,flow rate of 4.5l/min,and temperature of 25◦C.3.Results and discussion 3.1.Membrane performanceSilane coupling agents can be used to modified membrane surfaces containing organic or inorganic materials [10,13,14].The silanes selected in this study contain one alkyl or aryl and three alkoxy functional groups.The alkoxy groups were con-sumed during the sol–gel reaction methods,and the resulting polysiloxane contained only one alkyl or aryl group.Four differ-ent silane coupling agents were used,which contained methyl,octyl,octadecyl or phenyl functional groups.Using a seawater desalination test procedure,the membrane performance for the commercial composite RO membranes (SWC1)and silane-treated membranes was evaluated and their results are summarized in Table 1.As the silane concentra-tion increased,the water flux decreased and salt rejection increased.The MeTES n membranes showed some decrease in the flux;however,the MeTES 2.0membrane showed increased salt rejection.The OdTMS n membranes also showed increased salt rejection up to 99.7%;however,they showed a consider-able decrease in the flux.The decrease in flux of the PhTES n and OcTES n membranes was less than that of the MeTES n or OdTMS n membranes.In particular,the PhTES 2.0membrane shows 99.6%salt rejection at a water flux of 0.673m 3/(m 2day),and the PhTES 1.5membrane shows 99.5%salt rejection at a water flux of 0.739m 3/(m 2day).3.2.Contact angle analysisThe membrane surfaces have been physically characterized using a contact angle analyzer.The contact angle measurement between solids and liquids is a commonly used technique to study hydrophobicity of the surface.Although changes in the contact angle do not provide direct evidence about the polysilox-ane structure,they provide a worthwhile explanation of the extent of polysiloxane modified on the membrane surface.Fig.3shows the images of the contact angles for the SWC1,MeTES n ,OcTES n ,OdTMS n and PhTES n membranes.Differences are observed in their hydrophobic properties based on their alkyl or aryl group structure and silane solution concentration.The rela-tionships between hydrophobicity and permeate performance of membranes are shown in Table 1.The MeTES 1.5and MeTES 2.0membranes showed almost similar contact angles,and even theN.Kim et al./Journal of Membrane Science 300(2007)224–231227Table 1Results of performance and contact angle measurement Membrane n (wt.%)Salt rejection a (%)Flux a (m 3/(m 2day))Contact angle b (◦)SWC1–99.20.84852.1MeTES n1.098.90.55040.51.599.00.55153.12.099.50.55055.8OcTES n1.099.50.795701.599.60.652942.099.50.509110OdTMS n1.099.30.420821.599.50.3061042.099.70.249128PhTES1.099.10.80966.51.599.50.739682.099.60.67368a All results were obtained with 35g/l NaCl feed solution at an operating pressure of 55kg f /cm 2,flow rate of 4.5l/min,and temperature of 25◦C.bMeasurements were performed 10s after introducing the water drop.MeTES 1.0membrane showed a decreased contact angle.All the MeTES n membranes showed similar permeate fluxes;how-ever,the salt rejection increased with the silane concentration.The contact angles of the OcTES n and OdTMS n membranes are remarkably proportional to the silane concentration.Both membranes contain flexible long-chain carbon groups and the hydrophobicity of the membranes depends on their carbon chain number.After surface modification,the fluxes of the OcTES n and OdTMS n membranes decreased.Although their fluxes decreased,all the silane-coated membranes showed improved salt rejection.In particular,the OcTES 1.0membrane shows high salt rejection with a small decrease in flux.The PhTES n membranes contain rigid and hydrophobic phenyl group so that the hydrophobicity of the membranes increased.But the PhTES n membranes show smaller increase in hydrophobicity than the OcTES n and OdTMS n membranes.As a result,the PhTES n membranes show small decreases in flux but significant increases in salt rejection.It is supposed that phenyl group of the PhTES n membranes not only acts as hydrophobic environment but also affords a small decrease influx.Fig.3.Images of contact angle with various silane coupling agent concentrations:(a)SWC1,(b)n =1.0,(c)n =1.5,and (d)n =2.0.228N.Kim et al./Journal of Membrane Science 300(2007)224–231Fig.4.SEM images of silane-coated membrane surface at 30,000×:(a)SWC1,(b)MeTES 2.0,(c)OcTES 2.0,(d)OdTMS 2.0,and (e)PhTES 2.0.3.3.Surface structure analysis by SEMSEM micrographs were used to compare the surface structure of SWC1with that of the silane-coated membranes (n =2.0),as shown in Fig.4.The image of SWC1shows a confused peak-and-valley and nodular structure.The peak-and-valley structure is a general feature of polyamide RO membranes and nodu-lar structure is observed in high salt rejection RO membranes [15].Meanwhile,the surfaces of the silane-coated membranes show well-developed peaks and valleys filled with polysiloxane.The image of MeTES 2.0shows a well-developed peak-and-valley structure,while the OcTES 2.0membrane shows a small and loose peak-and-valley structure.The OdTMS 2.0membrane shows a small and dense peak-and-valley structure.These obser-vations are in good agreement with the number of carbon chains substituted in the polysiloxane.Because the MeTES 2.0mem-brane has small methyl group,it is free from steric hindrance in polymerization.But the OcTES 2.0membrane or OdTMS 2.0membrane may be interrupted by their long-chain aliphaticcarbon in polymerization.The PhTES 2.0membrane shows a completely different surface structure to that of the OcTES 2.0or OdTMS n membranes.The PhTES 2.0membrane shows a surface structure similar to that of MeTES 2.0;however,the sharpness of the images is considerably less.In the MeTES 2.0and PhTES 2.0membrane,they contain a rigid methyl or phenyl group.Further-more,the stacking interaction between phenyl groups affords parallel array in sol–gel condensation,PhTES 2.0membrane shows a huge peak-and-valley structure.3.4.Surface structure analysis by AFMUsing AFM analysis,we can obtain the 3D images and height histograms from a specific projection area.In order to mini-mize the sample damage and maximize the resolution,AFM was operated in the non-contact mode.The 3D images of the four membranes are shown in Fig.5,and the statistical values of sur-face roughness and area of all membranes are listed in Table 2.The scales of the X -and Y -axes are 2␮m ×2␮m,while theN.Kim et al./Journal of Membrane Science300(2007)224–231229Fig.5.AFM images of silane-coated membrane surface at20,000×:(a)SWC1,(b)MeTES2.0,(c)OcTES2.0,(d)OdTMS2.0,and(e)PhTES2.0.Z-axis is automatically scaled by the instrument.The statistical parameters are peak-to-valley distance(R p–v),average rough-ness(R a),root-mean-square roughness(R rms),and surface area. From the statistical values listed in Table3,a similar surface morphology for all the samples except SWC1can be observed. It is assumed that the change in roughness is caused by an overlap of polysiloxane on the polyamide surface.The surface areas of all the silane-treated membranes decreased.The surface areas Table2Surface roughness and area values of modified RO membranesMembranes n Surface roughness(nm)Surface area(nm2)R a R rms R p–vSWC1–63.580.1887192MeTES n 1.068.687.57051311.557.772.96001262.054.668.6535124OcTES n 1.047.759.94711221.560.275.76041282.073.494.7805140OdTMS n 1.072.393.38981351.571.989.25731372.059.874.9533128PhTES n 1.081.51015851191.593.41137081592.092.9117847155of the MeTES n and OdTMS n membranes decreased with an increase in silane concentration.However,the surface area of the OcTES n and PhTES n membranes increased with the silane concentration.A similar tendency was observed in the rough-ness.The R a,R rms,and R p–v values of MeTES n and OdTMS n membranes decreased with an increase in the silane concentra-tion;however,the roughness values of the OcTES n and PhTES n membranes increased.In previous studies,there is a close cor-relation of the surface area and roughness with the permeation flux;theflux increases with the surface area and roughness[5]. The behavior of the OdTMS n membranes was in good agree-ment with this rule,and thefluxes of the MeTES n membranes were already saturated at1.0%(w/v)concentration.However, the OcTES n and PhTES n membranes showed a relatively lesser surface area and roughness,but greaterflux than the MeTES n membranes and particularly the OdTMS n membranes.Theflux behavior may be attributed to the differences in the polymer development between the silanes.It is considered that the car-bon chain number of the silanes is affected by the morphological changes during sol–gel methods for surface modification.3.5.Surface analysis by XPSThe SWC1membrane comprised a polyamide thinfilm, porous polysulfone support layer,and non-woven polyester.The silane solutions were deposited on the SWC1membrane,which was polymerized at70◦C;then,membranes were obtained with230N.Kim et al./Journal of Membrane Science300(2007)224–231Table3Elemental analysis of the membrane surface using XPSMembranes n Atomic contents(%)Atomic ratioC N O Si N/C O/C Si/C Si/O SWC1–69.110.520.4–0.1520.295––MeTES n 1.070.69.7416.7 2.960.1380.2400.0420.1771.565.98.5619.8 4.720.1290.3000.0700.2382.069.98.0317.83.470.1150.2550.0500.194 OcTES n 1.070.810.316.3 2.620.1450.2300.0370.1601.572.47.6316.1 2.640.1050.2200.0360.1642.073.64.3514.9 6.720.0610.2020.0910.452 OdTMS n 1.071.47.5317.8 3.310.1060.2490.0470.1861.578.6 5.2311.7 3.830.0660.1490.0490.3282.081.53.6111.0 3.530.0440.1340.0400.322 PhTES n 1.072.69.9116.50.990.3160.2270.0140.0601.572.58.9516.9 1.710.1230.2330.0240.1012.071.98.7717.0 2.270.1210.2360.0320.134an integrally skinned structure.Although the contact angles, SEM images,and AFM data provide substantial information regarding the surface,they cannot provide direct evidence regarding the successful occurrence of sol–gel method for surface modification.XPS analysis is a suitable method for investigating the skin layer in order to gain a more thorough understanding of the nature of the surface composition.The sur-face compositions of the SWC1,MeTES n,OcTES n,OdTMS n and PhTES n membranes are listed in Table3.Very little silicon was observed at the surface of the SWC1membrane.In con-trast,all the silane-treated membranes contained silicon.There is an obvious decrease in nitrogen content with an increase in the value of n.Furthermore,the structure of the silane agents also affects the nitrogen content;the OdTMS n membranes,which have a long-chain alkyl group,show a significant decrease in nitrogen content as compared to the MeTES n membranes.The decrease in nitrogen content of the PhTES n and MeTES n mem-branes was less than that of the OcTES n or OdTMS n membranes. With regard to the carbon concentration,the interpretation of the XPS results is considerably complex as compared to that of nitrogen or silicon because the carbon content is associated with both the polyamide and polysiloxane.The carbon content ratio of MeTES and PhTES are less than that of polyamide. This implies that the carbon content of MeTES n and PhTESn Fig.6.XPS spectra for the SWC1,MeTES2.0,OcTES2.0,OdTMS2.0and PhTES2.0membranes:(a)Si2p3/2,(b)C1s,(c)O1s,and(d)N1s.N.Kim et al./Journal of Membrane Science300(2007)224–231231decreased with an increase in the silane concentration.On the contrary,the carbon content ratios of OcTES and OdTMS are greater than that of polyamide.The carbon content of OcTES n showed a slight increase and that of OdTMS n showed a sig-nificant increase with the silane concentration.Especially,N contents of surface layer are a strong evidence of their rela-tionship between the membrane performance and the silane concentration.The silane compounds do not have N element but polyamide has.Therefore,the decrease of N contents by sol–gel condensation means that the polysiloxane layer shields polyamide layer and the shield effect is proportional to the silane concentration.From the results of XPS analysis,the carbon and nitrogen contents can be changed with the silane concentration and chemical structure of polysiloxane.Fig.6shows the C1s,O1s,N1s,and Si2p3/2spectra for the SWC1,MeTES2.0,OcTES2.0,OdTMS2.0and PhTES n membranes.The shapes of the C1s,O1s,N1s,and Si2p3/2 spectra were similar for all the membranes.In case of the MeTES2.0,OcTES2.0,and OdTMS2.0membranes,no differ-ences were observed in the binding energy of C1s,and N1s, Si2p3/2,except O1s.Although the MeTES2.0,OcTES2.0,and OdTMS2.0membranes have different substituted alkyl groups, the chemical structure and shielding effect do not change the binding energy of each atom.However,the binding energy of O1s showed a shift of0.5–1.0eV.The oxygen of the carboxyl group attached to the polyamide can be stabilized by electrostatic attraction with the silicon atom.The difference in the binding energy depends on the molar concentration of the silane solu-tion used in sol–gel method for surface modification.MeTES has the highest molar concentration of silicon at the same% (w/v)concentration.Furthermore,the methyl group of MeTES is rigid and has a shorter carbon chain as compared to the octyl group of OcTES and octadecyl group of OdTMS.This explains the small shift in the O1s binding energy in the MeTES2.0mem-brane.The PhTES2.0membrane shows a significant change in the binding energy of Si2p3/2.The increased binding energy of Si2p3/2is due to the chemical structure of PhTES2.0membrane, the stacking interaction between phenyl groups stabilizes the electron state of silicon atom,and then the binding energy of Si 2p3/2of the PhTES2.0membrane showed a shift of0.5–1.0eV.4.ConclusionIn this study,a novel method for the preparation of high salt rejection membranes was investigated using long-chain alkyl substituted silane agent and rigid phenyl substituted silane agent.Measurements for initial performance were carried out using a batch-scaled test cell unit.The relativeflux of the silane-coated membranes decreased,but their selectivity toward NaCl increased to99.6%.In particular,the PhTES2.0mem-brane showed99.6%salt rejection at a water permeability of0.673m3/(m2day).The characteristics of the silane-coated membranes were analyzed using FE-SEM,XPS,and contact angle analyzer.Although the results of the analyses did not exactly match the silane solution concentration,we found that the contents of coated polysiloxane were significantly propor-tional to the silane concentration.The main conclusion of this study is that the modified of hydrophobic polysiloxane on the surface of a polyamide TFC RO membrane can enhance NaCl salt rejection.AcknowledgementThis research was supported by a grant(4-4-2)from Sustain-able Water Resources Research Center of21st Century Frontier Research Program.References[1]R.J.Petersen,J.E.Cadotte,Thinfilm composite reverse osmosis mem-branes,in:M.E.Porter(Ed.),Handbook of Industrial Membrane Technology,Noyes Publication,Park Ridge,NJ,1990,p.307.[2]R.J.Petersen,Composite reverse osmosis and nanofiltration membranes,J.Membr.Sci.83(1993)81.[3]J.E.Cadotte,Interfacially synthesized reverse osmosis membrane,USPatent4,277,344(1981).[4]J.-Y.Koo,N.Kim,Composite polyamide reverse osmosis membrane andmethod of producing the same,US Patent6,015,495(1998).[5]M.Hirose,H.Ito,Y.Kamiyama,Effect of skin layer surface structures ontheflux behavior of RO membranes,J.Membr.Sci.121(1992)209. [6]J.E.Tomaschke,Interfacially synthesized reverse osmosis membrane con-taining an amine salt and processes for preparing the same,US Patent 4,872,984(1989).[7]N.N.Li,M.A.Kuehne,R.J.Petersen,Highflux reverse osmosis membrane,US Patent6,162,358(2000).[8]M.Hirose,H.Ito,M.Maeda,K.Tanaka,Highly permeable compositereverse osmosis membrane,method of producing the same,and method of using the same,US Patent5,614,099(1997).[9]M.Taniguchi,M.Kurihara,S.Kimura,Behavior of a reverse osmosis plantadopting a brine conversion two-stage process and its computer simulation, J.Membr.Sci.183(2001)249.[10]D.H.Shin,N.W.Kim,Y.T.Lee,Surface modification of reverse osmosismembrane skin layer by silane compound,Membr.J.16(2006)106. [11]T.Sano,M.Hasegawa,S.Ejiri,Y.Kawakami,H.Yanagishita,Improvementof the pervaporation performance of silicalite membranes by modification with a silane coupling reagent,Micropor.Mater.5(1995)179.[12]I.J.Roh,J.-J.Kim,S.Y.Park,Mechanical properties and reverse osmosisperformance of interfacially polymerized polyamide thinfilms,J.Membr.Sci.197(2002)199.[13]E.Ferjani,M.Mejdoub,M.S.Roudesli,M.M.Chehimi,D.Picard,M.Delamar,XPS characterization of poly(methylhydrosiloxane)-modified cellulose diacetate membranes,J.Membr.Sci.165(2000)125.[14]Y.K.Kim,N.W.Kim,Y.T.Lee,Silane–polyamide composite membraneand method there of,KR Patent10-0583136-0000(2006).[15]S.-Y.Kwak,D.W.Ihm,Use of atomic force microscopy and solid-stateNMR spectroscopy to characterize structure–property–performance corre-lation in high-flux reverse osmosis(RO)membranes,J.Membr.Sci.158 (1999)143.。