Preparation of organic–inorganic hybrid cation-exchange membranes via blending method and their electrochemical characterization

Journal of Membrane Science 328 (2009) 23–30

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Preparation of organic–inorganic hybrid cation-exchange membranes via blending method and their electrochemical characterization Xingtao Zuo a , Shuili Yu a,b,∗ , Xia Xu a , Ruiling Bao a , Jun Xu a , Wenming Qu c a

State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China State Key Laboratory of Pollution Control and Resource Reuse, Tongji University, Shanghai 200092, China c No. 2 Oil Production Company of Daqing Oilfield Company LTD., Daqing 163414, China b

a r t i c l e

i n f o

Article history: Received 28 May 2008 Received in revised form 2 July 2008 Accepted 10 August 2008 Available online 19 August 2008 Keywords: Organic–inorganic hybrid membranes Cation-exchange membranes Blending process Poly(vinylidene fluoride) SiO2 nanoparticles

a b s t r a c t Organic–inorganic hybrid membranes based on poly(vinylidene fluoride)-SiO2 nanoparticles were prepared by blending method and cation-exchange groups in the membrane matrix were introduced by the reaction of epoxy groups with fuming sulfuric acid at 25 ◦ C. Various membranes were prepared with different content of SiO2 nanoparticles. These membranes were extensively characterized for their surface morphology, thermal stability, water content and surface charge property using SEM, FTIR, TGA, water uptake and ion-exchange capacity measurements. Membrane potential measurements along with membrane surface fixed charge density have been carried out with different counter-ions to investigate the relationship between ionic migration and SiO2 content in the membrane forming material. Membrane conductance performed in NaCl solution was interpreted not only by the counter-ion diffusion coefficient in the membrane phase but also by phenomenology coefficients. It was found that physicochemical and electrochemical properties of these membranes were found to be highly dependent on the SiO2 nanoparticles content in the membrane matrix. Furthermore, membranes with SiO2 nanoparticles in 2.0% weight ratio of PVDF exhibited very good selectivity, water content, ion-exchange capacity and moderate membrane conductivity, which may be used for their application in electro-membrane processes. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Separation membranes have been widely studied and utilized industrially in various fields. Among these membranes, ionexchange membrane has mostly been used in solutions containing multiple components, including electrodialytic demineralization of saline water, treatment of industrial effluents containing metal ions, and desalination of cheese whey solution [1–3]. For the purpose of these processes, the development of chemically and thermally stable ion-exchange membrane with good physicochemical and electrochemical properties is highly desired [4–7]. Among all organic macromolecule polymer materials, poly(vinylidene fluoride) (PVDF) is one of the excellent materials that can form asymmetric membrane. PVDF-based membranes show outstanding oxidative, thermal and hydrolytic stability as well as good mechanical and film-forming properties. Thus, PVDF membranes are widely used in many separation processes through various modifications. It is well documented [8–10] that PVDF is a common ultrafiltration, microfiltration and pervaporation membrane material because of its excellent chemical resistance and ∗ Corresponding author. Tel.: +86 451 8628 2101. E-mail address: [email protected] (S. Yu). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.08.012

thermal stability. Studies of PVDF blending modification focus on inorganic materials blending. Organic–inorganic composites offer the possibility for new generation of nanostructure materials with diversified applications such as catalysts, electronic or photonic devices, and sensors. Furthermore, the incorporation of inorganic material on a nanoscale can enhance the retardancy and mechanical strength of polymers. It has been reported that introduced nanoparticles to polymer membranes may be silica [11], Al2 O3 [12], Fe3 O4 [13], ZrO2 [14], TiO2 [15,16] and polymeric nanoparticles [17]. It is believed that the addition of nanoparticles may favour the formation of complexes with the polymer and in turn act as a cross-linking center for the polymer by reducing the tension of the polymer for self-organization. Also, large surface area of the nanoparticles prevents the polymer chains to recrystallize and in addition acts as a support matrix for the polymer [18]. Several reports are available concerning the preparation of new generation of ion-exchange membrane based on inorganic and organic materials. Negatively charged hybrid ion-exchange membranes were prepared by endcapping polyethylene oxide with silane containing secondary amine and trialkoxysilane groups [19]. Ion-exchange membranes were prepared through sol–gel process of PEO-[Si(OCH3 )3 ]2 and quarteramination with C2 H5 Br thereafter, respectively [20,21]. Nagarale et al. [22] had prepared the polyvinyl

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X. Zuo et al. / Journal of Membrane Science 328 (2009) 23–30

alcohol–silica hybrid heterogeneous anion-exchange membranes and electrochemical transport properties of these membranes were found to be very good. But little work has been done on the preparation and application of PVDF-nanostructure inorganic composite material as ion-exchange membranes. In this study, therefore, it was to develop a new type of organic–inorganic composite ion-exchange membranes based on PVDF-SiO2 nanoparticles via blending method and the ionexchange groups were introduced by the chemical reaction of epoxy group. These membranes with different content of SiO2 nanoparticles were extensively characterized by physicochemical and electrochemical studies. 2. Experimental 2.1. Materials PVDF(FR904) (MW: 600,000) were obtained from Shanghai 3F New Materials Co., Ltd., China. Glycidyl methacrylate (GMA) was obtained from Shanghai YuanJi Chemical Ltd., China. SiO2 particles of size of 30 nm were used as received (Zhejian Mingri Chemicals Co., China). Dimethylacetamide (DMAC ≥ 99%), divinyl benzene (DVB), benzoyl peroxide (BPO), fuming sulfuric acid, HCl, NaOH, NaCl, MgCl2 , CaCl2 (AR Grade, Tianjin Chemical Reagents Plant, China) were used as obtained. Double distilled water was used for the preparation of all the solutions. 2.2. Membrane preparation Fig. 1. Membrane preparation process.

A given amount of SiO2 nanoparticles was added to DMAC, and the solution was vibrated by an ultrasonator for 20 min. After standing of the solution for 20 min, the mixture was vibrated for 20 min to obtain an optimal dispersion of the particles in the solution. PVDF was dissolved in the solution, and then GMA, DVB, and BPO were also added to the mixture with constant stirring at room temperature for at least 6 h to get a gel. The gel was deposited in a no sun light place for 24 h to remove air bubbles from it. The resulting gel was cast on a Plexiglas plate to a desired thickness and then the cast polymer solutions were heated for 2 h at 120 ◦ C for effecting cross-linkage in the membrane matrix. These membranes were immersed in fuming sulfuric acid at room temperature for 3 days to induce cation-exchange groups, and then washed with distilled water until last trace of acidity was removed. The membrane preparation procedure was demonstrated in Fig. 1. The obtained membranes before being used for electrochemical studies were conditioned with 0.1 mol dm−3 HCl solution and 0.1 mol dm−3 NaOH solution alternately several times and then equilibrated with experimental solution for further characterization. Heterogeneous cation-exchange membranes with different weight ratio of SiO2 /PVDF were prepared and named as SiO2 /PVDFX, where X is the weight ratio of SiO2 nanoparticles and PVDF in the membrane forming materials.

For the determination of the volume fraction of water, the membrane was immersed in distilled water for 24 h, the surface was wiped and the wet membrane weighed. The wet membrane was dried under vacuum at 60 ◦ C (100 mm Hg column) until a constant weight was attained. The thickness of wet and dry membranes was determined by means of a digital micrometer. The water uptake (ϕw ) in wet membrane was estimated from the following equation.

2.3. FT-IR

ϕw =

FTIR spectra of the hybrid membranes were recorded using FTIR spectrometer (PerkinElmer, Japan) with a resolution of 2 cm−1 and a spectral range of 4000–500 cm−1 .

where w is the weight difference between wet and dry membrane and wd is the weight of dry membrane, dw and dp are the densities of water and dry membrane, respectively. The membrane porosity  (volume of free water within membrane per unit volume of wet membrane per unit volume of wet membrane) can be obtained by the following equation [23].

2.4. Thermal stability (TGA) The degradation process and the thermal stability of the membranes were investigated using thermogravimetric analysis (TGA) (NETZSCHSTA 449 C, German) under a nitrogen atmosphere using

a heating rate of 10 ◦ C min−1 from 50 to 800 ◦ C. Nitrogen gas was used as the carrier gas at a flow rate of 200 ml min−1 . 2.5. Scanning electron microscope (SEM) Membrane cross-sections were obtained after breaking the membranes under liquid nitrogen. SEM samples were coated with an Au-layer to reduce sample charging under the electron beam. The surface morphology of these membranes and cross-section structures were recorded by using S4800HSD (Japan) under dry condition at room temperature. 2.6. Water uptake and ion-exchange capacity (IEC) measurements

=

w/dw w/dw + wd /dp

V 1 + V

(1)

(2)

X. Zuo et al. / Journal of Membrane Science 328 (2009) 23–30

25

where V is the volume variation of the membrane upon absorption of the water per unit of dry membrane volume, which may be estimated by using the following equation. V =

wdp dw Wd

(3)

The IEC was measured using the classical titration technique. The membrane was equilibrated in 1 mol dm−3 HCl solution to convert the membrane into the H+ form. The membrane was then washed with distilled water to remove excess of HCl, and then equilibrated with 100 ml of 0.1 mol dm−3 NaCl solution for 24 h. The IEC was determined from the increase in acidity, which in turn determined by back titration. 2.7. Membrane conductance and membrane potentional measurements The experimental cell used for membrane potential measurements [24] had two compartments separated by a membrane of circular shape with an effective area of 7.065 cm2 . To minimize the effect of boundary layers on potential, the solutions in both the compartments were vigorously stirred by magnetic stirrers. The developed potential across the membrane was recorded with the help of a multimeter using saturated calomel electrodes and salt bridges. For membrane potential measurements, the relationship between electrolyte concentrations of the higher (C1 ) and the lower side (C2 ) was as follows: C1 /C2 = 10, C/CS = (C1 − C2 )/[(C1 + C2 )/2]. Membrane conductance measurements were carried out using a clip cell [24]. This cell was composed of two black graphite electrodes. The two compartments of the cell were separated by the membrane. The active area of electrodes as well as that of membrane was 0.785 cm2 . Before the experiments, the membrane was sandwiched in experimental solution. Membrane conductance measurements were performed using a potentiostatic two-electrode mode with alternating current (AC). Membrane resistance (Rm ) was estimated by subtraction of electrolyte conductance (Rsol ) without a membrane from membrane resistance equilibrated in electrolyte solutions (Rcell ). The membrane conductance was measured with the help of a digital conductivity meter, up to ±0.01 mS reproducibility. 3. Results and discussion 3.1. Membranes preparation As mentioned in Section 2, the cation-exchange membranes were prepared by copolymerization of epoxy acrylate monomers (GMA) with DVB subsequent sulfonation with fuming sulfuric acid. The FTIR spectra of these hybrid membranes were recorded and shown in Fig. 2. It can be seen that the absorption bands at ∼1100 cm−1 were all quite strong. These bands might include the Si–O–Si stretching formed during the cross-linking process. The band at ∼940 cm−1 may be due to the Si–OH stretching. However, it seemed that there was no considerable change in the intensity of these absorption bands as the content of SiO2 nanoparticles increased. This was possibly due to the complexity of the spectra: Si–O–Si and C–O–C which are all in the region of 1050–1150 cm−1 , and the contribution of newly formed Si–O–Si groups cannot be well identified. The intensities of peaks related to epoxy ring of GMA at ∼1254, ∼909 and ∼821 cm−1 were decreased, indicating that epoxy ring had opened partially and many of them kept intact. Since characteristic peaks of –SO3 H groups were in the region ∼1100 and ∼1200 cm−1 , the characteristic peaks of –SO3 H groups overlap with Si–O–Si stretching bands and they cannot be distinguished in Fig. 2.

Fig. 2. FTIR spectra of (a) SiO2 /PVDF-0; (b) SiO2 /PVDF-1.0; (c) SiO2 /PVDF-2.0.

The ion-exchange capacity discussed in the following section will confirm the formation of –SO3 H groups during the sulfonation process. 3.2. TGA studies Thermal stability of membrane materials is highly essential for better performance of membranes even at elevated temperature. The thermal stability of these membranes was investigated by means of TGA and the diagrams obtained were shown in Fig. 3. The thermal stability of these membranes increased with the inorganic silica content as indicated in Fig. 3. The first weight loss occurred around 100 ◦ C, which was attributed to the loss of absorbed water molecules in the membrane matrix. The second weight loss region from 300 to 400 ◦ C corresponded to the loss of –SO3 H. In the third weight loss region at a temperature over 500 ◦ C, the polymeric matrix was further degraded, which corresponded to the decomposition of the main chains of the PVDF. Furthermore, all the membranes resulted in similar types of TGA curves with a varied amount of absorbed water, and these membranes were kept in boiling water for a long time and no weight loss or dimensional change was observed. This study revealed that these types of organic–inorganic hybrid cation-exchange membranes were sta-

Fig. 3. Thermogravimetric analysis curves of hybrid membrane.

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X. Zuo et al. / Journal of Membrane Science 328 (2009) 23–30

Fig. 4. Surface SEMs of the hybrid membranes: (a) SiO2 /PVDF-0; (b) SiO2 /PVDF-1.0; (c) SiO2 /PVDF-2.0. SEMs of cross-section of the hybrid membranes: (d) SiO2 /PVDF-0; (e) SiO2 /PVDF-1.0; (f) SiO2 /PVDF-2.0.

ble up to 250–300 ◦ C without losing their mechanical strength and functional properties. 3.3. SEM studies Scanning electron micrographs (SEMs) of the surfaces and crosssection of SiO2 /PVDF-0, SiO2 /PVDF-1.0 and SiO2 /PVDF-2.0 were presented in Fig. 4.The effect of SiO2 nanoparticles on the membrane morphology was clearly observed in these membranes. With the increase of SiO2 content in the membrane matrix, an increase in the membrane compactness was obtained, but overall quite homogeneous blending was received for all blending ratios. Also no phase separation of the membrane surface could be observed, indicating that these hybrid polymeric membranes were homogeneous and dense in nature. 3.4. Water content and IEC studies IEC values of all the prepared membranes were given in Table 1, it can be seen that IEC was in the range of 1.251–1.997 meq. g−1 , which increased with the increase in SiO2 nanoparticles content. The increase of IEC may be attributed to increase the loading of SiO2 in the membrane phase resulting in improving the structure of the membrane matrix to some degree. The ion-exchange capacity provides information on the density of ionizable functional groups present in the membrane matrix, which are responsible for the charged nature of the membrane and the membrane conductivity. So it was inferred that with the addition of SiO2 , the physicochemical properties of these membranes were improved, which was

discussed in the following sections and thus the sulfonation degree was enhanced. As can be seen in Table 1, an increase in SiO2 nanoparticles content in the membrane matrix initially led to a substantial increase in the water content. In general, membranes having the some degree of composition absorb the same amount of water, where the density of ionizable groups is the same throughout the membrane matrix [25,26]. An increase in hydrophilic species such as ion group concentration with the loading of SiO2 nanoparticles was responsible for the observed variation in water content. Further, incorporation of more silica in the membrane matrix will also lead to an increase in pore volume and thus water uptake as can be seen from the values of  in Table 1. Hydrogen oxygen groups increased in the membrane network because of the loading of SiO2 and epoxy opening after sulfonation, which may be contributed to interpret the increase of hydrophilic property of the membrane network. With the increase in the loading of SiO2 nanoparticles in the membrane matrix, optiTable 1 Thickness (l), porosity () and water uptake in the membrane phase (ϕ (w)) and ion-exchange capacity (IEC) values for SiO2 /PVDF-X membranes. SiO2 /PVDF-X*

l (mm)

ϕ (w)

IEC (meq. g−1 )



SiO2 /PVDF-0.0 SiO2 /PVDF-0.5 SiO2 /PVDF-1.0 SiO2 /PVDF-1.5 SiO2 /PVDF-2.0

0.198 0.202 0.196 0.197 0.201

0.100 0.190 0.220 0.254 0.262

1.251 1.617 1.854 1.982 1.997

0.133 0.148 0.156 0.179 0.195

* Membranes with different weight ratio of SiO2 were prepared, where X is the weight ratio of SiO2 nanoparticles and PVDF in the membrane forming materials.

X. Zuo et al. / Journal of Membrane Science 328 (2009) 23–30

mal water content is observed, which improves their dimensional stability and applicability as cation-exchange membrane with the higher ion-exchange capacity. 3.5. Membrane conductance studies Membranes conductance was measured for different composite membranes equilibrated with NaCl solution of concentrations ranging from 0.001 to 0.20 mol dm−3 . The effective membrane resistance determined experimentally as a function of solution concentration in equilibrium with the membrane, may in principle be used for the estimation of membrane specific conductivity [27]. The specific conductivity of the membrane is given by the following equation [24]. m =

l ARm

(4)

where l is the thickness of the wet membrane, A is its area, and Rm is its electrical resistance. The variation of specific membrane conductivity m with the NaCl solution conductivity was shown in Fig. 5 for these hybrid membranes. Examination of the data clearly showed that the m value increased with the equilibrating solution concentration. Furthermore, membrane conductivity increased with the loading of SiO2 nanoparticles as indicated in Fig. 5, and this result is the same as that observed for water uptake and IEC behaviour. The increase of membrane conductivity may be attributed to three factors: (i) increase in ion-exchange capacity of these membranes with high SiO2 content, (ii) increase in water uptake of the membrane matrix with the increase in SiO2 content and (iii) increase the hydrophilic property of the membrane phase and thus increase in membrane conductivity with the increase in SiO2 content. It seemed that all these factors together were responsible for the increase in membrane conductivity as SiO2 nanoparticles content in the membrane matrix increased. According to the model proposed by Gnusin [28] and developed by Zabolosky and Nikonenko [29], the microheterogeneous model represents an ion-exchange membrane as a multiphase system containing at least two phases, a gel phase and an integral phase with volume fractions f1 and f2 , respectively, f1 + f2 = 1. The gel phase represents a nanopore medium, globally electroneutral including fixed

27

Table 2 m Isoconcentration (Ciso ), isoconductivity (iso ) and counter-ion diffusion coefficient (Dim ) values for different composite cation-exchange membranes. Membrane

Ciso (mol dm−3 )

m iso (mS cm−1 )

Dim × 107 (cm2 s−1 )

SiO2 /PVDF-0.0 SiO2 /PVDF-0.5 SiO2 /PVDF-1.0 SiO2 /PVDF-1.5 SiO2 /PVDF-2.0

0.0023 0.0049 0.0071 0.0084 0.0092

0.204 0.398 0.609 0.816 0.966

0.3706 0.5601 0.7500 0.9466 1.1161

and mobile ions, polymer matrix and water. The properties of the integral phase are assumed to be the same as the outer equilibrium solution, which fills the inner parts of meso- and macropores as well as fissures and cavities [30]. It is reported [31] that the microheterogeneous structure of the membrane phase is the main factor determining the concentration dependence of membrane transport properties such as electrical conductivity, diffusion permeability and counter-ion transport number. To describe the conductivity behaviour of ion-exchange membranes taking into account of their microheterogeneity, the following equation was used [32]. f1 f2 m = iso s

(5)

where iso is the value of the conductivity in the isoconductivity point, and s is the conductivity of the solution. The conductivity of the membrane phase and the solution phase become equal in the isoconductivity point. The slopes of ln m –ln s plot give m and C the values of f2 . iso iso were obtained from the intercept of the curves drawn in Fig. 5. Concentrations of counter-ions in the m and C membrane phase at the iso iso for these composite membranes with different weight ratio of SiO2 /PVDF were presented in m were found to be increased due to the Table 2. The values of iso increase in the loading of SiO2 nanoparticles content in the membrane phase. IEC and water content also increased with an increase of SiO2 content, which confirmed the increase in hydrophilic nature of the membrane with the loading of SiO2 . The isoconductivity and corresponding electrolyte concentration allow us to predict the concentration range over which the membranes will be most efficient in the electro-driven separation process. The diffusion coefficient (Dim ) of the counter-ion in the membrane phase can be determined from the following equation [33]. Dim =

m RT iso 2 F Q

(6)

where R is the gas constant, T is the absolute temperature, and F is the Faraday constant, Q is the ion-exchange capacity of the joint gel phase and can be calculated from the magnitude of the membrane capacity Qm through the equation. Q =

Qm f1

(7)

As seen from Table 2, the calculated values of Dim for different composite ion-exchange membranes followed a trend similar to m , which also supported the explanation of m . iso iso 3.6. Membrane potentional and permselectivity studies

Fig. 5. Specific conductivity values for SiO2 /PVDF-X hybrid membranes with different concentration of NaCl solution: (- - - -) solution; () SiO2 /PVDF-0; (䊉) SiO2 /PVDF-0.5; () SiO2 /PVDF-1.0; () SiO2 /PVDF-1.5; () SiO2 /PVDF-2.0.

When the membrane separates solutions of unequal concentrations of an electrolyte, an electrical potential develops across the membrane due to the different mobility of counter-ion and co-ion. The magnitude of the membrane potential depends on the electrical characteristics of the membrane in addition to the nature and concentration of electrolyte solutions used [29]. Membranes potential of these different membranes were recorded in electrolyte solutions such as NaCl, CaCl2 and MgCl2 with 0.055 mol dm−3 . The

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X. Zuo et al. / Journal of Membrane Science 328 (2009) 23–30 Table 3 Permselectivity (Ps ) and fixed charge concentration (Xm ) in NaCl solutions of different concentration for different cation-exchange membranes. Membranes

[NaCl] (mol dm−3 ) Xm

Ps

SiO2 /PVDF-0 SiO2 /PVDF-0.5 SiO2 /PVDF-1.0 SiO2 /PVDF-1.5 SiO2 /PVDF-2.0

0.055

0.11

0.165

0.055

0.11

0.165

0.879 0.921 0.966 0.974 0.976

0.855 0.895 0.948 0.963 0.966

0.845 0.887 0.934 0.956 0.961

0.203 0.260 0.411 0.473 0.493

0.483 0.580 0.849 1.008 1.057

0.521 0.634 0.863 1.076 1.145

equation [35]. Xm =

Fig. 6. Dependence of counter-ion transport numbers on SiO2 /PVDF weight ratio in the membranes phase for different solutions with Cs = 0.055 mol dm−3 .

counter-ion transport number in the membrane phase for different types of membranes was estimated from membrane potential data using the TMS approach [33]. m E m = (2t+ − 1)

RT a1 ln nF a2

(8)

where a1 and a2 are the mean activities of electrolyte solutions m values estimated by and n is the electrovalence of counter-ion. t+ Eq. (8) were presented in Fig. 6 for different experimental conditions. For SiO2 /PVDF composite membranes, membrane selectivity increased with SiO2 nanoparticles content up to 1.5% of PVDF and with further increase in the loading of SiO2 , a litter increment m values for in the membrane cation selectivity was observed. t+ + Na initially increased with rapid manner with the increase of the SiO2 content in the membrane materials. For bivalent counter-ions m values increased very slowly. It was observed that (Ca2+ , Mg2+ ), t+ incorporation of silica in the cation-exchange membrane matrix resulted in the increase in the Donnan exclusion of co-ions, and its cation selectivity increased with the increase in the loading of SiO2 . Counter-ion transport numbers attended limiting values because of the decrease in the Donnan exclusion of co-ion at a higher loading of SiO2 . Also SiO2 /PVDF-2.0 composite membranes m = 0.98, 0.93, 0.92 for Na+ , Ca2+ , exhibited very good selectivity (t+ Mg2+ , respectively) which is comparable to the available commercial membranes and may be suitable for their practical application as cation-exchange membranes. Membranes selectivity quantitatively can be expressed in terms of permselectivity. Permselectivity (Ps ) is a measure of the characteristic difference in the membrane permeability of counter-ion and co-ion, which may be expressed in the following equation [34]. Ps =

m−t ) (t+ + (1 − t+ )

2Cs Ps



1 − Ps

(10)

where Cs is the mean electrolyte concentration. Permselectivity values for different counter-ions in different experimental solution were presented in Table 3. For Na+ , permselectivity values increased in slow manner with an increase in SiO2 nanoparticles content in the membrane matrix. Xm values revealed that concentration of fixed charges on the membrane matrix increased in rapid manner with the increase of SiO2 content. m , P and Xm . Increase in electroA similar trend is exhibited by t+ s chemical properties of these membranes may also be attributed to increase in IEC values due to the loading of SiO2 nanoparticles. Permselectivity for other types of electrolyte is also estimated, which varied corresponding to NaCl solution for these hybrid cation-exchange membranes. It is reported [36] that permselectivity and concentration of fixed charge on the membrane surface for the some type of membranes decrease with the increase in Stokes radius of the hydrated counter-ions. The Stokes radius for different counter-ions follows the trend Na+ < Mg2+ < Ca2+ [37], while the opposite trend is observed for permselectivity and concentration of fixed charges on the membrane surface as indicated in Fig. 7. Membrane potentional measurements in combination with membrane conductance data can be used with advantage for the estimation of phenomenological coefficients. When a membrane separates an electrolyte solution with different concentrations, an electrical potential develops across the membrane. The current flow (I) and solute flux (Js ) may be expressed in terms of the potential

(9)

where t+ is the counter-ion transport number in the solution phase. The permselectivity arises due to the nature of the membrane for discrimination between counter-ion and co-ion. This type of discrimination arises because of the nature and magnitude of the charge that the membrane matrix carries because of fixed charge concentration, the so-called concentration of fixed charges on the membrane surface in addition to interaction between counter-ion and membrane matrix [27]. The concentration of fixed charges on the membrane surface (Xm ) has expressed in terms of permselectivity in the following

Fig. 7. Variation of fixed charge density with weight ratio of SiO2 /PVDF in the membrane phase for different electrolyte solution with 0.055 mol dm−3 .

X. Zuo et al. / Journal of Membrane Science 328 (2009) 23–30

Fig. 8. Variation of phenomenological coefficient for SiO2 /PVDF-X composite cation-exchange membranes in NaCl solutions of different concentration (mol dm−3 ): ( ) 0.055; (䊉 ) 0.11; ( ♦) 0.165.

difference () and the concentration difference (C) by means of phenomenological equation [38]. I = L11  + L12 RT

C C

Js = L12  + L22 RT

C C

(11) (12)

Lij (i,j = 1,2) are phenomenological coefficients. Eq. (11) clearly indicates that current flow is not only driven by the potential difference but also by the concentration difference. Similarly solute flow may take place because of the existence of difference of concentration as well as difference of potential across the membrane [33]. According to Eq. (11), L11 can be expressed by Eq. (13)

 I  

= L11

(13)

29

dense, and the dispersion between organic and inorganic phase was homogeneous. Further, SEM confirmed that SiO2 particles were in nanoscale dispersed in the polymer matrix. It was found that membrane hydrophilic nature, water content and transport properties of different counter-ions characterized are strongly dependent on the SiO2 nanoparticles weight fraction in the hybrid cation-exchange membrane. As the weight of SiO2 nanoparticles increased, IEC of these membranes increased from 1.251 to 1.997 meq. g−1 . It was also observed that the content of SiO2 in the membrane matrix contributed to the increase in membrane conductivity and water content. A rapid increase in membrane selectivity was observed with the increasing SiO2 contents. This may be because of the incorporation of silica in the ion-exchange membrane matrix resulted in the increase in the Donnan exclusion of co-ions and its cation selectivity increased with the increase in the loading of SiO2 . Counter-ion diffusion coefficient values and phenomenological coefficient estimated from membrane conductivity and membrane potential also supported the explanation. Furthermore, these membranes, especially SiO2 /PVDF-2.0 hybrid cation-exchange membranes exhibited very good selectivity, water content, ion-exchange capacity and moderate membrane conductivity, which may be used for their application for electrochemical processes. Acknowledgements Financial supports from the National High Technology Research and Development Program of China (863 program) (No. 2006AA06Z303), Key Projects in National Science & Technology Pillar Program (No. 2006BAJ08B09), National Natural Science Foundation of China (No. 50778050) and Foundation for Innovative Research Groups of China (No. 50821002). The author would like to thank Dr. RuiLing Bao for his kind of proof-reading the manuscript and his colleagues (Dr. Xia Xu, Dr. Jun Xu and Ms. Juan Wang) in the author’s lab. Specific thanks will be given to professor Shuili Yu for constant encouragement and constructive suggestions.

C=0

L11 obviously is the membrane conductance. Also, when current I = 0, then ()I=0 = −

L12 C RT L11 C

(14)

L11 and L12 values obtained using Eqs. (13) and (14) for different composite membrane in different concentration of NaCl were presented in Fig. 8. It was observed that L11 and L12 values increased with the increase in SiO2 nanoparticles content, meanwhile, phenomenological coefficients increased with the equilibrating solution concentration. These indicated that the solute flux for Na+ across the membrane matrix increased as silica content in the membrane increased. They also followed a trend similar to the m , and supported the explanation of the increase in permselectivity with the loading of silica. 4. Conclusion Organic–inorganic hybrid membranes based on PVDF-SiO2 nanoparticles were prepared by the blending method and cation selectivity was introduced in the membrane. These membranes extensively characterized by FTIR, TGA, SEM as well as the conventional ion-exchange measurements such as ion-exchange capacity, water content and conductivity. FTIR results revealed that SiO2 nanoparticles were incorporated with the polymeric matrix. It was observed that the thermal behaviour was illustrated by TGA analysis, which showed that the membranes doped with silica could undergo a curing temperature up to 300 ◦ C. Morphology observations showed that the hybrid membranes were compact and

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