Journal of Membrane Science 441 (2013) 83–92

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Journal of Membrane Science

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Synthesis and characterization of hybrid membranes using chitosanand 2-(3,4-epoxycyclohexyl) ethyltrimethoxysilane for pervaporationdehydration of isopropanol

P.S. Rachipudi a, A.A. Kittur b, A.M. Sajjan a, M.Y. Kariduraganavar a,n

a Department of Chemistry, Karnatak University, Dharwad 580 003, Indiab Department of Chemistry, SDM College of Engineering & Technology, Dharwad 580 002, India

a r t i c l e i n f o

Article history:Received 5 May 2012Received in revised form13 February 2013Accepted 17 March 2013Available online 2 April 2013

Keywords:Chitosan2-(3,4-Epoxycyclohexyl)ethyltrimethoxysilanePervaporationSelectivityActivation energy

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esponding author. Tel.: +91 836 2215286x23;ail address: mahadevappak@yahoo.com (M.Y.

a b s t r a c t

Chitosan based hybrid membranes were prepared by incorporating 2-(3,4-epoxycyclohexyl) ethyltri-methoxysilane (ETMS) into chitosan matrix using a sol–gel technique. The resulting membranes werestudied using Fourier transform infrared spectroscopy (FTIR), wide-angle X-ray diffraction (WAXD),differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). After measuring theswelling data at different mass% of water, membranes were employed for pervaporation separation ofwater–isopropanol mixtures in a temperature range of 30–50 1C. The experimental results demonstratedthat membrane containing 40 mass% of ETMS showed the highest separation selectivity of 17,990 with aflux of 2.92�10−2 kg/m2h at 30 1C for 10 mass% of water. The values of total flux and flux of water arefound to be overlapping particularly for hybrid membranes, suggesting that the developed hybridmembranes could be effectively used to break the azeotropic point of water–isopropanol mixtures. Fromthe temperature dependent diffusion and permeation values, the Arrhenius activation parameters wereestimated. The activation energy values obtained for water permeation (EPw) are two times lower thanthose of isopropanol permeation (EPIPA), suggesting that the developed membranes have higherseparation efficiency for water–isopropanol system. The estimated Ep and ED values were rangedbetween 21.64 to 33.26, and 22.62 to 33.49 kJ/mol, respectively. The negative heat of sorption (ΔHS)values was observed in all the hybrid membranes, indicating that Langmuir's mode of sorption ispredominant.

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1. Introduction

Membrane separation processes have been offered manyadvantages over the existing separation processes. Pervaporation(PV) is one such type of membrane separation processes with awide range of uses such as solvent dehydration and separation ofaqueous organic mixtures. It has significant advantages particu-larly in azeotropic systems where traditional distillation is onlyable to recover pure solvents with the use of entrainers. Theremoval of entrainers takes additional separation step and tediousas well [1,2]. Therefore, PV is accepted as environment-benign andenergy-saving technology, and has become a promising alternativeto conventional technologies in separation of liquid mixtures dueto its high separation efficiency coupled with energy savings [3].

The dehydration of isopropanol (IPA) has received a greatattention from industries due to the escalation cost of isopropanol

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and its importance in various industries. It is one of the importantsolvents used on a large scale in chemical industries as well as inpharmaceutical laboratories. It is generally used as solvent for oilsand gums, as a cleaning agent in the semi-conductor and liquidcrystal display industries. However, IPA forms an azeotrope at 12.4mass% of water and hence, the separation of water-IPA mixture byconventional methods such as solvent extraction and rotavapor orby distillation could prove uneconomical [4–6]. To overcome this,polymeric materials are overwhelmingly explored as PV mem-branes because of easy fabrication, reasonable separation perfor-mance and low cost. Based on the solution-diffusion model,a highly selective PV membrane could be developed if membranepossesses high selective sorption and diffusion. As far as dehydra-tion of organic solvent is concerned, the ideal materials are thelong chain rigid-network polymers, which have active groups ofestablishing some interactions with water (ion-dipole or hydrogenbonding) [7]. In the light of this, several hydrophilic polymers suchas poly(vinyl alcohol) (PVA), poly(acrylic acid) (PAA) and poly-saccharides like hydroxyl ethyl cellulose (HEC), sodium alginateand chitosan [8–11], demonstrated strong affinity towards water.

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P.S. Rachipudi et al. / Journal of Membrane Science 441 (2013) 83–9284

Among them, chitosan [poly-β (1-4)-D-glucosamine] hasattracted a great interest as a basic membrane material becauseof its natural occurrence, high abundance, hydrophilicity, chemicalresistance, good membrane forming properties, functional groupsthat can be easily modified, and ease of processing [12]. However,membranes developed using this type of materials posed a lack ofmechanical strength and chemical stability in aqueous solutionsmainly due to excessive swelling. This could be overcome byintroducing crosslinks in the membrane matrix [13], blendingchitosan with other polymers [14,15], casting chitosan on anotherpolymer substrate to form composite membrane [16,17], incor-poration of selective zeolite into membrane matrix [18] and thedevelopment of organic–inorganic hybrid membranes [19,20].Among these, organic–inorganic hybrids, especially silica based,are an emerging class of innovative nano-structured materialswith tailored properties and unparallel performances suitable forwide range of practical applications [21–23]. This is mainlybecause; silica-hybrid materials have synergistic effects of stabilityof inorganic material and film forming ability of a polymer.Therefore, these materials offer consistent and unique opportu-nities to obtain the specific transport properties by combiningorganic and inorganic materials, so that membranes with highselectivity and good flux can be achieved [24,25].

Keeping this in mind, we have prepared novel organic–inor-ganic hybrid membranes via hydrolysis of 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane followed by condensation with chitosanusing a sol–gel reaction. The 2-(3,4-epoxycyclohexyl) ethyltri-methoxysilane is used as a coupling agent and its crosslinkingwith chitosan chains can effectively reduce the degree of swellingof chitosan and expected to increase the water permselectivity.The physico-chemical properties of the resulting membranes werestudied by Fourier transform infrared, wide-angle X-ray diffrac-tion, differential scanning calorimetry and thermogravimetricanalysis. The membranes were successfully employed for PVseparation of water-isopropanol system at different compositions.The values of permeation flux and separation selectivity weredetermined. The diffusion coefficient and Arrhenius activationparameters were also estimated. The results were discussed interms of PV separation efficiency of membranes.

2. Experimental section

2.1. Materials

Chitosan (Mw�200,000; N-deacetylation degree 75–85%) and2-(3,4-epoxycyclohexyl) ethyltrimethoxysilane (ETMS) were pur-chased from Sigma-Aldrich Chemicals, USA. Isopropanol and aceticacid were procured from s. d. fine Chemicals Ltd., Mumbai,India. All the chemicals were of reagent grade and used withoutfurther purification. Double distilled water was used throughoutthe study.

2.2. Membrane preparation

Chitosan (2.5 g) was dissolved in 100 ml of deaerated-distilledwater containing 2% of acetic acid with a constant stirring forabout 24 h at room temperature. The solution was filtered througha fritted glass disc filter to remove the undissolved residueparticles. It was then kept in an ultrasonic bath at a fixedfrequency of 38 kHz (Grant XB6, UK) for 30 min so as to breakthe possible aggregated molecules and also to remove the airbubbles. The resulting homogeneous solution was then spreadonto a glass plate with the aid of a casting knife in dust-freeatmosphere. After being dried at ambient temperature for about

48 h, the membrane was peeled-off and the membrane thusobtained is designated as M-1.

To prepare hybrid membrane, a known amount of ETMS and1 ml conc. HCl were added into a homogenous chitosan solutionand stirred for 8 h at room temperature. Before reaction proceeds,added HCl helps to hydrolyze the ETMS. The resulting solution waspoured onto a glass plate and the rest of the procedure wasfollowed as similar to M-1. The amount of ETMS with respect tochitosan was varied as 10, 20 and 40 mass%, and the membranesthus obtained were designated as M-2, M-3 and M-4, respectively.We have also prepared membranes with higher mass% of ETMS,but they failed to withstand pervaporation due to their brittlenature and hence, we have restricted only up to 40 mass% of ETMS.The thickness of the membranes was measured at different pointsusing Peacock dial thickness gauge (Model G, Ozaki MFG Co. Ltd.,Japan) with an accuracy of72 μm and it was found to be4072 μm.

2.3. Membrane characterization

2.3.1. Fourier transform infrared (FTIR) spectroscopyThe crosslinking reaction between chitosan and ETMS, was

confirmed by FTIR spectrometer (Nicolet, Impact-410, USA).Membrane samples were ground well to make KBr pellets underhydraulic pressure of 400 kg/cm2 and spectra were recorded in therange of 500–4000 cm−1. In each scan, the amount of membranesample and KBr were kept constant in order to know the changesin the intensities of the characteristic peaks with respect to theamount of ETMS.

2.3.2. Wide-angle X-ray diffraction (WXRD)The crystallinity of pure chitosan and its hybrid membranes

was studied at room temperature using a Brucker's D-8 advancedwide-angle X-ray diffractometer. The X-ray source was Ni-filteredCuKα radiation (40 kV, 30 mA). The dried membrane of uniformthickness (4072 μm) was mounted on a sample holder and thepattern was recorded in the reflection mode at an angle 2θ over arange of 5–451 at a speed of 8 deg/min.

2.3.3. Differential scanning calorimetry (DSC)Thermal properties of chitosan and its hybrid membranes were

studied using a differential scanning calorimeter (DSCQ 20, TAInstruments, Waters LLC, New Castle, Delawave, USA). Sampleweights ranged from 9–10 mg and heated from ambient to 400 1Cat a heating rate of 10 1C/min.

2.3.4. Thermogravimetric analysisThermal stability of the membranes was investigated using a

thermogravimetric analyzer (SDTQ 600, TA Instruments WatersLLC, New Castle, Delawave, USA). The sample weights ranged from9–10 mg were heated from ambient to 650 1C at a heating rate of10 1C/min.

2.4. Swelling measurements

The equilibrium sorption experiments were performed indifferent compositions of water–isopropanol mixtures using anelectronically controlled oven (WTB Binder, Germany). The massesof the dry membranes were first determined and kept forequilibrium by soaking in different compositions of the feedmixtures in sealed vessels at 30 1C for 24 h. The swollen mem-branes were weighed as quickly as possible after careful blottingon a digital microbalance (Mettler, B204-S, Toledo, Switzerland)within an accuracy of70.01 mg. The percent degree of swelling

Fig. 1. (A). Schematic representation of pervaporation apparatus: (1) pervaporation cell; (2) vacuum control valves; (3) permeate cold trap; (4) moisture cold trap;(5) pressure sensor; (6) vacuum pump. and (B). Schematic diagram of pervaporation cell: (1) water inlet; (2) water outlet; (3) feed inlet; (4) thermometer jacket; (5) stirrer;(6) O-ring; (7) membrane; (8) sintered disk; (9) permeate outlet.

P.S. Rachipudi et al. / Journal of Membrane Science 441 (2013) 83–92 85

(DS) was calculated as:

DS ð%Þ ¼ Ws−WdWd

� �� 100 ð1Þ

where Ws and Wd are the masses of the swollen and drymembranes, respectively.

2.5. Pervaporation experiments

PV experiments were performed using the in-house designedapparatus illustrated in Fig. 1(A and B). The effective surface areaof the membrane in contact with the feed mixture was 34.23 cm2

and the capacity of the feed compartment was about 250 cm3.The vacuum in the downstream side of the apparatus wasmaintained [1.333224�103 Pa (10 Torr)] using a two-stagevacuum pump (Toshniwal, Chennai, India). The water compositionin the feed mixture was varied from 10 to 30 mass%. Whilemaintaining the temperature, the test membrane was allowed toequilibrate for about 2 h in the feed compartment with a knownvolume of feed mixture before performing the PV experiment.After a steady state was attained, the permeate was collected in atrap immersed in the liquid nitrogen jar on the downstream sideat fixed intervals of time.

The experiments were carried out at 30, 40 and 50 1C. The fluxwas calculated by weighing the collected permeate on a digitalmicrobalance. The composition of water and isopropanol in the

permeate was estimated by measuring the refractive index of thepermeate within an accuracy of70.0001 units using an Abbe'srefractometer (Atago-3 T, Japan), and by comparing it with astandard graph that was established previously with the knowncompositions of water-isopropanol mixtures. All the experimentswere performed at least three times, and the results wereaveraged. The results of permeation for water–isopropanol mix-tures during the pervaporation were reproducible within anadmissible range.

From the PV data, separation performance of the membraneswas assessed in terms of total flux (J), separation selectivity (α) andpervaporation separation index (PSI). These were respectivelycalculated using the following equations:

J ¼ WA t

ð2Þ

α¼ Pw=PIPAFw=FIPA

ð3Þ

PSI¼ Jðα−1Þ ð4Þ

where W is the mass of permeate (kg); A, the effective membranearea (m2); t, the permeation time (h); Pw and PIPA are the masspercent of water and isopropanol in the permeate, respectively;Fw and FIPA are the respective mass percent of water and iso-propanol in the feed.

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3. Results and discussion

3.1. Synthesis of hybrid membranes

The hydrolysis of ETMS and its possible reaction with chitosanare respectively illustrated in Fig. 2(A and B). From the Fig. 2A it isnoticed that trimethoxy groups of ETMS underwent hydrolysis inthe presence of acid catalyst, and yielding silanol groups. Thesehydroxyl groups were condensed with hydroxyl groups of chit-osan. Similarly, under the same acidic condition, the epoxy rings ofETMS were opened and the resulting –OH groups subsequentlyreacted with hydroxyl groups of chitosan to form C–O–C bonds(Fig. 2B). The reaction possibility of –OH groups of silanol orterminated by epoxy groups with –NH2 groups of chitosan is ruledout since all the –NH2 groups were converted into –NH3+ by theaddition of acetic acid. During the formation of siloxanes, theco-condensation reaction was occurred between the silanols and/or hydroxyls by dehydration. The flexible –OH groups arising fromETMS could bridge between the silica particles and chitosan chain,thereby making the hybrid matrix more rigid and compact.

Fig. 3. FTIR spectra of pure chitosan and its crosslinked membranes: (M-1) 0 mass%; (M-2) 10 mass%; (M-3) 20 mass%; (M-4) 40 mass%; of 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane content.

3.2. Membrane characterization

3.2.1. FTIR studiesThe chemical structure of the chitosan–siloxane hybrid mem-

branes was analyzed by FTIR spectrometer and the spectra thusobtained are presented in Fig. 3. A characteristic strong and broadband appeared at around 3400 cm−1 in all the membranescorresponds to O–H stretching vibrations of the hydroxyl groups.The bands appeared at around 1650 and 1570 cm−1 are respec-tively assigned to amide I (CQO) and amide II (NH) functionalgroups of chitosan [26,27]. The multiple bands appeared between900 and 1200 cm−1 were assigned to C–O stretchings. Uponcondensation of silanol or epoxy terminated –OH groups withchitosan matrix, the intensity of C–O stretching was increased andthis has become predominant with increasing the amount of ETMSas clearly demonstrated from the spectra of M-2 to M-4. This is

Fig. 2. (A and B). Synthesis of chitosan-based hybrid membrane.

Fig.4. Wide-angle X-ray diffraction patterns of pure chitosan and its crosslinkedmembranes: (M-1) 0 mass%; (M-2) 10 mass%; (M-3) 20 mass%; (M-4) 40 mass%; of2-(3,4-epoxycyclohexyl) ethyltrimethoxysilane content.

expected due to Si–O stretching of silanol as it overlaps at thesame frequency of C–O [28]. In addition to this, the –OH bandappeared at around 3400 cm−1 was also correspondingly shifted tolower frequency with increasing the content of ETMS, whichsupports the enhanced hydrogen bonding occurring betweenETMS and the chitosan. All these evidences confirm the incorpora-tion of silanol and its interaction with the chitosan.

P.S. Rachipudi et al. / Journal of Membrane Science 441 (2013) 83–92 87

3.2.2. WAXD studiesTo study the effect of crosslinking on the membrane morphol-

ogy of chitosan, X-ray diffraction was employed and the patternsthus obtained for the chitosan and its hybrid membranes arepresented in Fig. 4. The diffraction pattern of pure chitosanmembrane exhibits two sharp peaks at around 2θ¼10 and 151,and a broad peak at around 211. The sharp peaks correspond tosemicrystalline part while a broad peak corresponds to amorphouspart [29]. Upon increasing the content of ETMS, the intensity ofbroad peak was decreased gradually owing to the formation ofcrosslinks in the membrane matrix. This becomes responsible fordecreasing the crystallinity. Further, it is observed that a broadpeak appeared at 2θ¼211 was shifted towards higher degree withincreasing the content of ETMS and this is very well supported bythe calculated d-spacing values, which were decreased from 2.14 Åto 1.93 Å. The decreased d-spacing values give a clear evidence ofshrinkage in cell size or inter-segmental spacing, which wouldimprove the selective permeation of water through the hybridmembranes.

3.2.3. DSC studiesAny change in physical properties in the polymer membranes

due to crosslinking can be reflected in glass transition temperature(Tg) and melting temperature (Tm). This has prompted us tomeasure the DSC of pure chitosan and its hybrid membranes ata heating rate of 10 1C/min in nitrogen atmosphere and theresulting thermograms are illustrated in Fig. 5. The DSC thermo-gram of virgin chitosan membrane shows a Tg at 70.1 1C. It isevident from the thermograms that Tg values of hybrid mem-branes were increased with increasing the ETMS content in themembrane. This is due to increased crosslinking density by theincorporation of ETMS content, resulting to decreased free-volumein the membrane matrix. This is clearly reflected in the d-spacingvalues, which are calculated from the WAXD data. The DSCpatterns also reveal about the melting temperature Tm, whichwere gradually shifted from 205 to 181 1C by the incorporation ofETMS into a chitosan matrix. The shifts in melting temperature are

Fig. 5. Differential scanning calorimetry patterns of pure chitosan and its cross-linked membranes: (M-1) 0 mass%; (M-2) 10 mass%; (M-3) 20 mass%; (M-4) 40mass%; of 2-(3,4-epoxycyclohexyl) ethyltrimethoxysilane content.

ascribed to reduce the crystallinity of chitosan through cross-linking reaction with ETMS, which breaks the molecular symmetryof chitosan matrix to marginally. Thus, by incorporation of ETMScontent into a chitosan matrix, the mechanical property ofchitosan was enhanced while suppressing the crystallinity andwhich are responsible for increasing the overall PV performance.

3.2.4. TGA studiesThe thermal stability and degradation behavior of chitosan–

siloxane hybrid membranes were investigated under nitrogenatmosphere using TGA and the resulting thermograms are dis-played in Fig. 6. It is observed that all the membranes underwentdecomposition in two stages. The first stage of decompositionoccurred between ambient and 200 1C is accounted to around 12to 16% weight loss, corresponding to dehydration of membranes.The second stage of decomposition which starts from around 200to 550 1C was attributed to a weight loss due to the decompositionof polymeric network. When we compared the TGA patterns of allthe membranes, it is clear that the weight loss of hybrid mem-branes at the first stage was correspondingly decreased withincreasing the content of ETMS. Similarly, the weight loss of hybridmembrane at the second stage was drastically reduced. Thissuggests that water retention capacity of the hybrid membraneswas significantly affected by the crosslinks. This is clearly reflectedin both swelling and pervaporation study. This signifies thatincorporation of crosslinks was highly influenced on the thermalstability of the chitosan matrix.

3.3. Effects of feed composition and ETMS content on membraneswelling

The characteristics of the sorption (swelling) mechanismdepend on the structure of the membrane, the affinity of thepermeants towards membrane and the mutual interactionbetween the permeants [30–32]. In PV process, sorption propertyof a membrane plays a key role in achieving good separationperformance, which is generally evaluated by studying the mem-brane swelling in different feed mixtures. Fig. 7 shows the swellingbehavior of all the membranes in different mass% of water–IPAmixtures at 30 1C. It is observed that the degree of swelling wasincreased remarkably for all the membranes with increasing themass% of water in the feed. This is because of increased interaction

Fig. 6. Thermogravimetric analysis of pure chitosan and its crosslinked mem-branes: (M-1) 0 mass%; (M-2) 10 mass%; (M-3) 20 mass%; (M-4) 40 mass%; of2-(3,4-epoxycyclohexyl) ethyltrimethoxysilane content.

Fig. 7. Variation of degree of swelling with different mass% of water in the feed forall the membranes.

Fig. 8. Variation of total pervaporation flux with different mass% of water in thefeed for all the membranes.

Fig. 9. Variation of total flux, and fluxes of water and IPA with 10 mass% of water inthe feed for all the membranes.

Fig. 10. Variation of selectivity with different mass% of water in the feed for all themembranes.

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between water molecules and the membrane, owing to thepresence of large number of hydrophilic groups (–NH3+ and–OH) in the membrane matrix. This is expected since water beingmore polar than IPA, which preferentially interacts with mem-brane resulting to increased degree of swelling. On the contrary,the degree of swelling was decreased with increasing the contentof ETMS (M-1 to M-4), and this trend remains same throughoutthe investigated feed compositions. This is mainly because ofincreased crosslinks between siloxane and chitosan matrix, whichminimize the free-volume in the membrane matrix. Secondly, theestablishment of hydrogen bonds also responsible for decreasingthe membrane swelling as it was increased owing to the increaseof silanol and epoxy terminated –OH groups in the membrane.This was clearly demonstrated in both WAXD and TGA patterns.

3.4. Effects of feed composition and ETMS content on pervaporation

Fig. 8 illustrates the effects of feed composition and ETMS contenton the total permeation flux for all the membranes at 30 1C. It isobserved that total permeation flux was increased for all themembranes with increasing the amount of water in the feed. This ismore predominant at higher composition of water. The increased fluxwith increasing mass% of water in the feed is not merely dependenton the hydrophilic groups that are present in the membrane, but alsoon the increased chemical potential. This is expected since thecomposition of water increases the vapor pressure, and hence thedriving force due to chemical potential. As a result of both interactivegroups and chemical potential, the permeation flux was increasedwith increasing the water composition in the feed.

On the other hand, the permeation flux was decreased asexpected from membrane M-1 to M-4. This is due to increasedcrosslinking and hydrogen bonding as discussed in swelling study.In order to see the extent of permeation of individual components,we have plotted the individual fluxes as a function of mass% ofETMS at 10 mass% of water in the feed as shown in Fig. 9. From theplot, it is clearly noticed that except for pure chitosan membrane(M-1) the values of total flux and water flux are almost over-lapping each other while suppressing the flux of IPA, signifyingthat the crosslinked hybrid membranes developed in the presentstudy are highly water selective.

The overall selectivity of a membrane in PV process is generallydescribed on the basis of interaction between membrane and thepermeating molecules, molecular size of the permeating species and

Fig. 12. Variation of pervaporation separation index with 10 mass% of water in thefeed for all the membranes.

Table 1Diffusion coefficients of water and IPA at different mass% of water in the feed fordifferent membranes.

P.S. Rachipudi et al. / Journal of Membrane Science 441 (2013) 83–92 89

pore diameter of the membrane. Fig. 10 displays the effects of watercomposition and ETMS content on the selectivity of all the mem-branes. It is observed that the selectivity was decreased almostexponentially for all the membranes with increasing the waterconcentration in the feed. This is because, at higher concentrationof water in the feed a small amount of water dissolves in themembrane matrix, which in turn acts as plasticizer for the mem-brane leading to a greater flexibility of polymeric chains in thematrix. As a consequence of this, the swollen and plasticizedupstream surface of a membrane allows some IPA molecules alongwith the selective permeants, leading to decreased selectivity.However, the selectivity was dramatically increased with increasingthe content of ETMS as can be seen frommembrane M-1 to M-4. Thedecrease in free-volume due to an increase of crosslinking andhydrogen bonding increases the rigidity and compactness of themembrane, which becomes responsible for the increase of mem-brane's selectivity as the content of ETMS increases. This is furtherdemonstrated from the Fig. 11, in which flux and selectivity wereplotted as a function of ETMS content in the membranes at 10 mass%of water in the feed. Generally, as the packing density of themembranes increases either due to increase of crosslinks or due tothe incorporation of fillers into the membrane matrix, the permea-tion flux decreases and selectivity increases [33,34]. Obviously, thesame trend was observed due to the existence of a trade-offphenomenon in PV process.

Mass % of water Dw�108 (cm2/s) DIPA�1010 (cm2/s)

M-1 M-2 M-3 M-4 M-1 M-2 M-3 M-4

10 11.4 8.40 7.20 5.90 0.79 0.30 0.21 0.1015 11.3 9.16 7.93 6.33 1.10 0.53 0.41 0.2220 11.2 9.83 8.33 6.06 1.49 0.96 0.54 0.4025 10.7 9.04 7.84 6.17 2.11 1.49 1.03 0.6830 10.3 8.31 7.92 5.73 4.36 2.59 1.57 0.65

3.5. Pervaporation separation index

The pervaporation separation index is the product of totalpermeation flux and separation factor, which determines themembrane separation ability. To implement the PV process, thisindex is being used as a relative guideline for choosing the suitablemembranes based on the optimal combination of flux and selec-tivity. To demonstrate this, the resulting PSI values were plotted asa function of mass% of ETMS at 30 1C for 10 mass% of water in thefeed (Fig. 12). It shows that all the hybrid membranes have shownhigher PSI values than that of a pure chitosan membrane M-1.This is because of remarkable selectivity demonstrated by thehybrid membranes. Among the hybrid membranes, membranecontaining 40 mass% of ETMS exhibited an excellent overall PVperformance.

Fig. 11. Variation of flux and selectivity with 10 mass% of water in the feed for allthe membranes.

3.6. Diffusion coefficient

The mass transport of binary liquid molecules through apolymer matrix is generally explained by a solution-diffusionmechanism, which occurs in three steps: sorption, diffusion andevaporation [35]. Thus, the permeation rate and selectivity aregoverned by the solubility and diffusivity of each component ofthe feed to be separated. In PV process, because of the establish-ment of fast equilibrium distribution between bulk feed and theupstream surface of a membrane, the diffusion step controls thetransport of permeants [33,36]. To understand the mechanism ofmolecular transport, it is therefore important to estimate thediffusion coefficient, Di of permeating molecules.

From Fick's law of diffusion, the diffusion flux can be expressedas [37]:

Ji ¼ −DidCidx

ð5Þ

where J is the permeation flux per unit area, D is the diffusioncoefficient, C is the concentration of permeant, subscript i standsfor water or isopropanol, and x is the diffusion length. Forsimplicity, it is assumed that the concentration profile along thediffusion length is linear. Thus, diffusion coefficient, Di can becalculated with the following modified equation [38]

Di ¼JiδCi

ð6Þ

where δ is the membrane thickness. The calculated values of Di at30 1C are presented in Table 1. Similar to PV study, the diffusioncoefficients of both water and IPA molecules were decreased frommembrane M-1 to M-4 at all feed compositions. This is because of

Table 2Pervaporation flux and separation selectivity at different temperatures for differentmembranes at 10 mass% of water in the feed.

Temp. 1C J�102 (kg/m2h) αsep

M-1 M-2 M-3 M-4 M-1 M-2 M-3 M-4

30 5.60 4.14 3.55 2.92 4727 8991 11,241 17,99040 6.20 5.03 4.04 3.33 4491 8172 10,201 12,84750 7.80 7.39 6.88 6.11 4081 6419 8991 9991

Fig. 13. Variation of log J with temperature for all the membranes at 10 mass% ofwater in the feed.

Fig. 14. Variation of log D with temperature for all the membranes at 10 mass% ofwater in the feed.

Table 3Arrhenius activation parameters for permeation and diffusion, and heat of sorption.

Parameters (kJ/mol) M-1 M-2 M-3 M-4

EP 21.64 23.07 28.77 33.26ED 22.62 23.79 29.13 33.49EPw 21.01 23.43 29.70 33.61EPIPA 43.16 51.43 58.63 66.18EDw 22.62 23.89 30.24 34.88ΔHs −1.02 −0.72 −0.36 −0.23

P.S. Rachipudi et al. / Journal of Membrane Science 441 (2013) 83–9290

the establishment of crosslinking and hydrogen bonding in themembrane matrix, which results to decreased free-volume andincreased both rigidity and compactness. On the other hand, as weincreased the water concentration in the feed for all the mem-branes, the diffusion coefficients of IPA were increased, but nosystematic trend was observed for diffusion of water. Probably, thismight be due to hindrance caused by the IPA molecules during thediffusion. In spite of this, the magnitude of diffusion coefficientsfor water is quite high in comparison with IPA, suggesting that thehybrid membranes developed in the present study have higherseparation selectivity towards water even at higher concentrationof water in the feed.

3.7. Effect of temperature

The effect of operating temperature on the pervaporationperformance for water–isopropanol mixtures was studied for allthe membranes at 10 mass% of water in the feed and the valuesthus obtained are presented in Table 2. As we increase thetemperature from 30 to 50 1C, the permeation flux was increasedwhile decreasing the separation factor. This was explained on thebasis of increased driving force and decreased viscosity of per-meating molecules. As the temperature increases the vaporpressure at the feed side increases, but the pressure at thepermeate side is not affected. This led to increase the drivingforce for the mass transport across the membrane. In addition,viscosity of the feed solution decreases with increasing thetemperature. All these led to easier transportation of moleculesand thereby increasing the permeation flux. Secondly, an increaseof temperature decreases the interaction between membrane andthe permeants, and thereby membrane allows IPA molecules alongwith selective water molecules, resulting to a decreased selectivity.This effect prompted us to estimate the activation energies for

permeation and diffusion using the Arrhenius type equation [35]:

X ¼ Xoexp−ExRT

� �ð7Þ

where X represents permeation (J), or diffusion (D). Xo is a constantrepresenting pre-exponential factor of Jo or Do. Ex representsactivation energy for permeation or diffusion depending uponthe transport process under consideration, and RT is the usualenergy term.

Arrhenius plots of log J and log D versus 1/T are shown inFigs. 13 and 14, for the temperature dependence of permeationflux and diffusion, respectively. In both the cases, a linear behaviorwas observed, suggesting that permeability and diffusivity followan Arrhenius trend. From the least-squares fits of these linearplots, the activation energies for total permeation (EP) and totaldiffusion (ED) were estimated. Similarly, we have also estimatedthe activation energies for permeation of water (EPw) and iso-propanol (EPIPA), and diffusion of water (EDw), but the plots are notgiven to avoid the crowdness. The values thus obtained arepresented in Table 3.

From Table 3, it can be seen that the apparent activation energyvalues of water (EPw) are almost two times lower than those ofisopropanol (EPIPA), suggesting that membranes have higherseparation efficiency towards water. The activation energy valuesof water (EPw) and total permeation (EP) are almost close to eachother, signifying that coupled–transport of both (water and iso-propanol molecule) is minimal as due to higher selective nature ofmembranes. The activation energy values were increased frommembrane M-1 to M-4, and this is expected due to increasedrigidity and compactness in the membrane owing to increasedcrosslinking density and hydrogen bonding. The estimated Ep and EDvalues ranged between 21.64 and 33.26, and 22.62 and 33.49 kJ/mol,

P.S. Rachipudi et al. / Journal of Membrane Science 441 (2013) 83–92 91

respectively. Using these values, we have further estimated theheat of sorption as:

ΔHs ¼ Ep−ED ð8ÞThe resulting ΔHs values are included in Table 3. The ΔHs values

give the additional information about the transport of moleculesacross the polymer membrane. It is a composite parameterinvolving the contributions of both Henry's and Langmuir's typesof sorption [39]. The Henry's type of sorption requires both theformation of a site and the dissolution of chemical species intothat site. The formation of a site involves an endothermic con-tribution to the sorption process. However, Langmuir's type ofsorption requires the pre-existence of a site in which sorptionoccurs only by a hole filling mechanism, giving an exothermiccontribution. The ΔHs values obtained in the present study arenegative for all the membranes, suggesting that Langmuir's sorp-tion is predominant, giving an exothermic contribution.

4. Conclusions

Using a sol–gel technique, chitosan based hybrid membraneswere developed by varying a silica precursor such as 2-(3,4-epoxycyclohexyl) ethyltrimethoxysilane. The membranes were suc-cessfully employed to separate water–isopropanol mixtures at 30,40 and 50 1C. The amount of ETMS added in the membraneimproved the separation ability of the membranes. The membranecontaining 40 mass% of ETMS showed the highest separationselectivity of 17,990 with a flux of 2.92�10−2 kg/m2h at 30 1C for10 mass% of water in the feed. The PV separation index data alsosupport that membrane with higher amount of ETMS demonstratedan excellent PV performance. Experimental data also reveal that thetotal flux and flux of water are overlapping each other particularlyfor the silica precursor incorporated membranes, suggesting thathybrid membranes developed here are highly selective towardswater and this is, in accordance with the diffusion data as well.

Temperature effect study shows that the permeation flux wasincreased while decreasing the selectivity when the temperature wasincreased. This is because of increased driving force and decreasedselective interaction between permeants and the membrane. Themembranes exhibited almost two times lower activation energyvalues for water permeation (EPw) than that of isopropanol permea-tion (EPIPA), again suggesting that membranes developed here havehigher separation ability towards water. The estimated EP and EDvalues were ranged between 21.64 and 33.26, and 22.62 and33.49 kJ/mol, respectively. All the membranes exhibited negativeΔHs values, indicating that sorption is mainly dominated by theLangmuir's mode of sorption, giving an exothermic contribution.

Acknowledgments

Authors wish to acknowledge the UGC, New Delhi, MajorResearch Project (F. no. 37-245/2009, SR) for providing thefinancial support. One of the authors (Padmeswary S. Rachipudi)wishes to acknowledge the UGC, New Delhi, for awarding theResearch Fellowship under meritorious category. Authors sincerelythank the Department of Physics, Indian Institute of Science,Bangalore, for extending wide-angle X-ray diffraction facility.

Nomenclature

Mw molecular weightA effective membrane area (m2)DS degree of swelling (%)

Do pre-exponential factor for diffusionED activation energy for diffusion (kJ/mol)EDw activation energy for diffusion of water (kJ/mol)Ep activation energy for permeation (kJ/mol)Epw activation energy for permeation of water (kJ/mol)EDIPA activation energy for diffusion of IPA (kJ/mol)Ex activation energy for permeation or diffusion (kJ/

mol)ΔHs heat of sorption (kJ/mol)IPA isopropanolJ total flux (kg/m2h)Jo pre-exponential factor for permeationPSI pervaporation separation indexP and F mass percent of permeate and feedR gas constantt permeation time (h)T temperature (K)W mass of permeate (kg)Ws and Wd mass of the swollen and dry membranes

Greek letters

δ membrane thickness (40 μm)αsep separation factor

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Synthesis and characterization of hybrid membranes using chitosan and 2-(3,4-epoxycyclohexyl) ethyltrimethoxysilane for...IntroductionExperimental sectionMaterialsMembrane preparationMembrane characterizationFourier transform infrared (FTIR) spectroscopyWide-angle X-ray diffraction (WXRD)Differential scanning calorimetry (DSC)Thermogravimetric analysis

Swelling measurementsPervaporation experiments

Results and discussionSynthesis of hybrid membranesMembrane characterizationFTIR studiesWAXD studiesDSC studiesTGA studies

Effects of feed composition and ETMS content on membrane swellingEffects of feed composition and ETMS content on pervaporationPervaporation separation indexDiffusion coefficientEffect of temperature

ConclusionsAcknowledgmentsReferences