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Microstructure dependent diffusion of water–ethanol in swollen poly(vinyl alcohol): A molecular dynamics simulation study
Chemical Engineering Science 64 (2009) 334 -- 340
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Microstructure dependent diffusion of water–ethanol in swollen poly(vinyl alcohol): A molecular dynamics simulation study Qiu Gen Zhang, Qing Lin Liu ∗ , Yu Chen, Jian Yang Wu, Ai Mei Zhu National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, Department of Chemical and Biochemical Engineering, College of Chemistry & Chemical Engineering, Xiamen University, Xiamen 361005, China
A R T I C L E
I N F O
Article history: Received 8 July 2008 Received in revised form 6 October 2008 Accepted 20 October 2008 Available online 30 October 2008 Keywords: Poly(vinyl alcohol) Molecular simulation Diffusion Pervaporation Swelling properties
A B S T R A C T
Molecular dynamics (MD) simulation was used to study the swelling properties of poly(vinyl alcohol) (PVA) in ethanol solutions containing 15, 30 and 45 wt% water. The characteristics of the swollen PVA, intrinsic relation between the microstructure of the swollen PVA and the diffusion of water and ethanol in the PVA matrix were analyzed. It was found that the free volume of the swollen PVA reduced with reductions in the degree of crystallinity was accompanied by an increase in the mobility of PVA chains. Water located mostly in the hydrophilic region of the hydroxyl groups of PVA chains; and hydrogen bonding formed between water and PVA. It was also noted water clusters form in the swollen PVA, whose size increased with increasing degree of swelling, whereas ethanol molecules disperse almost individually in the PVA matrix. The diffusion coefficients of water and ethanol in the swollen PVA are predicted to increase linearly with increasing swelling. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction Pervaporation is a novel and promising membrane separation technology which is chosen because of its high selectivity, easy implementation and energy saving qualities compared to other experimental techniques used for separating similar boiling points mixtures and azeotropic mixtures (Feng and Huang, 1997; Liu et al., 2005; Vane, 2005). The separation membrane is the key element in pervaporation separation equipment. Most membranes are fabricated from polymer materials owing to their low cost and good filmforming abilities. Polymeric membranes are however limited. They suffer from swelling in certain solvents which leads to a change in the structure of the membranes over time and hence a decrease in the mechanical strength and the permselectivity (Rao et al., 2006). Clearly quantitatively assessing the swelling properties of a membrane material is one of the key factors to evaluate the separation performance of membranes in designing new polymeric membrane materials. To date there is a large body of published experimental work on the swelling properties of polymeric membranes already in existence. Generally those works, however, focus on the degree of swelling and sorption selectivity of membranes at a macroscopic
∗
Corresponding author. Tel.: +86 592 2183751; fax: +86 592 2184822. E-mail address: [email protected] (Q.L. Liu).
0009-2509/$ - see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2008.10.028
level (Devi et al., 2005; Baker, 2004; Uragami et al., 2002; Zhang et al., 2007a). Review of the literature indicates more studies are required to be understood at a molecular level how solvents affect the structure of polymeric membranes and how they disperse in the polymeric membranes. Additionally, the diffusion characteristics of the solvent molecules through the swollen membranes are difficult and expensive to be measured by experiments and require further future exploration. Recently, researchers have investigated the characteristics of swollen polymers using molecular dynamics (MD) simulation and gained some useful insights into solvent-membrane interactions. Urata et al. (2005a,b) studied the swollen perfluorinated and perfluorosulfonic ionomers membranes containing different solvent (methanol or/and water) to elucidate the solvent effects on membrane material morphologies. The properties studied included: (1) the size of solvent cluster; (2) solvent location; (3) polymer structure and (4) the mobility of solvent and polymer chains. Vishnyakov and Neimark (2001) studied microphase segregation in the Nafion perfluorinated membranes at different water contents and observed the formation of water clusters containing up to 100 water molecules. Kucukpinar and Doruker (2004) reported oxygen transport in dry and hydrated amorphous ethylene–vinyl alcohol copolymers, the diffusion coefficient of oxygen in the swollen membranes was noted ¨ to increase with increasing water content. Muller-Plathe (1996a,b) calculated benzene diffusion coefficients in benzene–polystyrene systems and found that their composition dependence not only showed good agreement with experiment but also followed quite
Q.G. Zhang et al. / Chemical Engineering Science 64 (2009) 334 -- 340
well the predictions by lattice models. Entrialgo-Castaño et al. (2006) investigated water swollen olyglycolide and poly-(l-lactide) membranes at a detailed atomistic level. Xiang and Anderson (2005) explored the distribution of water and the effects on molecular mobility in poly(vinylpyrrolidone) glasses. As mentioned above, MD simulation is an effective and a convenient method to study the structure and performance of swollen polymers. Poly(vinyl alcohol) (PVA) often finds applications as a membrane material or hydrogel in the textile industries due to its excellent chemical stability, film-forming capacity, barrier properties and high hydrophilicity (Alghezawi et al., 2005; DeMerlis and Schoneker, 2003; Semenova et al., 1997; Will and Lichtenthaler, 1992). However, the swelling of PVA membranes in an aqueous solution results in a decrease in the mechanical and separation properties of PVA membranes. It is our purpose here to further develop the understanding of solvent interactions in swollen PVA by using MD simulations to understand the effect of solvents on the structure ¨ and performance of swollen PVA. Muller-Plathe (1998a,b) reported the concentration dependence of water diffusivities in PVA systems with 3–100 vol% water. By investigating swollen PVA in an aqueous ethanol solution, PVA was also found to be purely hydrophilic ¨ by Muller-Plathe and van Gunsteren (1997). Karlsson et al. (2004) studied the diffusion kinetics and mechanisms of oxygen and water in dry and water-containing amorphous PVA at low water contents ( < 5.2 wt%). Tamai et al. (1996a,b), Tamai and Tanaka (1998a,b, 1999) concluded that the diffusion of oxygen and nitrogen in PVA hydrogel increased with increasing water content. As described previously most solvent-membrane studies have focused on swollen polymers in one-component solvent and gas diffusion in swollen membranes, only a few researches have reported on properties of swollen polymers in multi-component solvent. In this paper, we have studied the swelling properties of PVA in water–ethanol mixtures by MD simulation. The characteristics of the swollen PVA, the distribution of water and ethanol in the PVA matrix, intrinsic relation between the microstructure of the swollen PVA and the diffusion of water and ethanol in the PVA matrix were all investigated.
2. Methodology 2.1. Simulation details In our previous work (Zhang et al., 2007a,b, 2008), the swelling behavior of PVA membranes in ethanol (and isopropanol) solutions with different water content was investigated. With increasing water content, the degree of swelling of the PVA membranes was noted to increase. Water and ethanol diffusion coefficients increased rapidly, whereas water sorption selectivity decreased. The degree of swelling of the PVA membranes in ethanol solutions containing 15, 30 and 45 wt% water was 22.13, 39.10 and 84.21%. The corresponding water concentration in the PVA membrane-adsorbed mixture is 76.61, 76.85 and 81.30 wt%, respectively. As an extension of this previous work, the characteristics of swollen PVA in water–ethanol mixtures containing 15, 30 and 45 wt% water were investigated by MD simulation in this study. All simulations were performed on the SGI workstation using material studio (MS) software (Accelrys Inc.) at 323.2 K and 1 atm, and a COMPASS force field was employed. The van der Waals interactions were calculated by the “atom based' method with the cut-off distance 9.5 Å, and the Coulombic interactions were taken into account using the “Ewald” method. Pressure was controlled using the Berendsen method (Berendsen et al., 1984), and temperature was controlled by the Andersen thermostat in NPT MD simulation. Nose's method (Nosé, 1984) with Q = 1.0 was used to control temperature
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to acquire dynamics properties in NVT MD simulation. A time step of 0.5 fs was used for all MD simulation. The syndiotactic PVA chain consisting of 50 repeating units was constructed. Then, the cubic simulation cells containing PVA chains, water and ethanol molecules, were constructed by the amorphous cell modules at an initial density of 0.3 g/cm3 , whose parameters are listed in Table 1. For each system 20 independent configurations were initially generated, and three configurations with minimum energy were selected for further simulations. These configurations were performed on a 10 000 step energy minimization to relax the unfavorable overlaps, and then subjected to 700 ps NPT and 200 ps NVT MD simulations to equilibrate the system. Subsequently 1500 ps NVT MD simulation was then carried out to investigate the diffusion properties of PVA chains, water and ethanol molecules. 2.2. Data analysis The mobility of atoms (molecules) in the dry and swollen PVA membranes is investigated using mean square displacement (MSD), which can be computed by the following equation MSD = |ri (t) − ri (0)|2
(1)
where r(0) is the initial positional coordinate of atom i (or molecules), ri (t) denotes the coordinates at time t. The diffusion coefficients D of atoms (or molecules) can be calculated from the slope of MSD for sufficient long time by Einstein relationship: |ri (t) − ri (0)|2 6t t→∞
D = lim
(2)
Free volume, as indicated earlier, is one of important factors to characterize the structure of polymeric membranes and can be determined accurately by molecular simulation. In MD simulations, atoms are represented by hard spheres with van der Waals radius in the simulation atomistic models (Pan et al., 2008). Fig. 1 shows the sketch of van der Waals surface and Connolly surface. The van der Waals surface intersects with the van der Waals radii of the atoms in the structure and Connolly surface is at the boundary between the Connolly probe and the atoms. Free volume is defined as the volume circled by the Connolly surface. Fractional free volume (FFV) was calculated by the ratio of free volume to the total volume of model. 3. Results and discussion 3.1. Characteristics The equilibrium structures of the dry and swollen PVA are obtained after MD simulation for refinement; their properties are listed in Table 1. The densities of the swollen PVA decreased with increasing swelling. This agreed with findings of other studies on dif¨ ferent swollen polymers (Muller-Plathe and van Gunsteren, 1997; Vishnyakov and Neimark, 2001). This is due to an increase in the degree of disorder and the mobility of PVA chains in the swollen PVA, resulting in an increase of the PVA matrix volume. This result was confirmed by the following analysis of X-ray scattering and MSD for backbone carbon atoms. Fig. 2 displays the simulated X-ray scattering pattern of the dry and swollen PVA. From the pattern, it can be observed that the characteristic peak of PVA appeared near at 2 = 20◦ and its intensity decreased with increasing degree of swelling. The position of the peaks shifts to the left from systems 1–4, indicating the crystalline region in the swollen PVA was becoming small owing to an increase in the kinetic energy of PVA chains. The degree of crystallinity for
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Table 1 Parameters of simulation systems. System
Cell component
1 2 3 4
4 3 3 2
PVA PVA PVA PVA
Weight fraction of components
chains chains, 62 H2 O, 7 ETOH chains, 110 H2 O, 13 ETOH chains, 164 H2 O, 15 ETOH
Cell after refinement
PVA
Water
Ethanol
Density
Size (Å)
100 82.02 71.93 54.74
0 13.86 21.56 36.69
0 4.12 6.51 8.57
1.1526 1.1338 1.1036 1.0735
23.3341 22.7632 24.0065 23.1833
`2 PVA chains, 164 H2 O, 15 ETOH' means that the simulation cell consists of three PVA chains, 62 water molecules, and seven ethanol molecules.
Fig. 1. Definition of van der Waals surface and Connolly surface.
Fig. 3. MSD for backbone carbon atoms in the dry and swollen PVA.
Fig. 2. Simulated X-ray scattering pattern of the dry and swollen PVA.
Fig. 4. FFV distributions of the dry and swollen PVA.
each system was calculated from Eq. (3): Xc,2 =
20 Ic (2)d(2)
20 Ia (2)d(2) + 20 Ic (2)d(2)
× 100%
(3)
where Xc,2 is the degree of crystallinity, Ic (2) and Ia (2) are the Xray scattering intensity of crystalline region and amorphous region. The calculated degree of crystallinity for systems 1–4 was 38.43, 36.25, 35.17 and 34.96%, respectively. Fig. 3 shows the MSD of backbone carbon atoms in the dry and swollen PVA. It is clear that the backbone motion increases from systems 1 to 4. The self-diffusion coefficient D of backbone carbon atoms
can be estimated sing the Einstein relationship. D (10−8 cm2 s−1 ) of backbone carbon atoms was calculated to be 1.58, 2.95, 10.83 and 22.77 for systems 1–4, respectively. This suggests that the mobility of backbone chains increased rapidly with increasing swelling. 3.2. Microstructure Free volume and its distribution in the dry and swollen PVA was investigated, as shown in Fig. 4. When the free volume cavity diameter was around 1.0–2.8 Å, the FFV for both the dry and the swollen
Q.G. Zhang et al. / Chemical Engineering Science 64 (2009) 334 -- 340
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Fig. 5. Simulated morphology of holes in the dry and swollen PVA excluding water and ethanol molecules (white circle denotes atoms; green circle denotes Connolly surface).
Fig. 6. Fraction of hole's volume in the dry and swollen PVA excluding water and ethanol molecules.
Fig. 7. Radial distribution function g(r) for O (water)–O (PVA) in the swollen PVA.
PVA can be seen to differ slightly. The FFV in the swollen PVA is remarkably less than that in the dry PVA when the diameter is in the range of 2.8–5.0 Å. This is because the free volume cavities in the swollen PVA matrix are inhabited by water and ethanol molecules. The typical size of water and ethanol molecules are in the range 2.8–3.2 Å (Grazianoa, 2005) and 4.3 Å (Yang et al., 2007), respectively. In the presence of water molecules, the free volume cavities are
about 2.8–3.2 Å in diameter and in the swollen PVA the diameters are less than those in the dry PVA. It is worth noting that any additional increase in swelling of the swollen PVA does not further reduce the free volume cavity diameter beyond a certain limit. This can possibly be attributed to any increase in the amorphous region and the mobility of PVA chains leading to a decrease in the compact
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region in PVA matrix. For large cavities with a diameter greater than 3.2 Å, the FFV in the swollen PVA decreases with increasing swelling. This is explained by the fact that the large cavities are easily occupied by water and ethanol molecules. In order to investigate the configuration of PVA chains in the swollen PVA, water and ethanol molecules inside the cells had firstly to be removed. The fraction of hole volume in the remaining structures which consist of only PVA chains was then calculated. The simulated morphology of holes in the PVA matrix is displayed in Fig. 5. With increasing swelling the holes grew rapidly and the throughpores formed in system 4. This suggests that the morphology of PVA chains is less compact in the swollen PVA than in the dry one and large holes are formed in between PVA chains. These larger holes favor the diffusion of water and ethanol molecules into the PVA matrix. To illustrate quantitatively, the fraction of hole's volume in the PVA matrix and swollen PVA is shown in Fig. 6. It can be observed that the fraction of holes in the PVA matrix increased remarkably in quantity and size with increasing degree of swelling. More specifically, the holes with diameter of 9.0–11.0 Å formed and occupied 0.36% volume in system 4.
3.3. Compartmentation of water and ethanol molecules The water distribution in the PVA matrix can be investigated using the radial distribution function g(r). In the swollen PVA, water dispersed in the PVA matrix could form hydrogen bonding with the hydroxyl groups of PVA chains, as noted by some researchers ¨ (Muller-Plathe and van Gunsteren, 1997; Tamai et al., 1996a, b). Fig. 7 shows the g(r) between water oxygen and the oxygen in PVA chains, a strong peak at 2.8 Å can be observed. This reveals that water located mostly in the hydrophilic region of the hydroxyl groups in PVA chains, and hydrogen bonding formed between water and PVA chains. And the hydrophilic region is defined to cover the inner region up to the first peaks of g(r) of water oxygen atoms around ¨ the hydroxyl groups (Muller-Plathe, 1996a). Water dispersed in the polymeric network could form clusters, as reported in some hydrogels and swollen polymers (Urata et al., 2005a,b; Vishnyakov and Neimark, 2001). In the swollen PVA, water ¨ clusters were also observed by other teams (Muller-Plathe and van Gunsteren, 1997; Tamai et al., 1996a). For the swollen PVA in an aqueous ethanol solution, water clusters are confirmed by the g(r) between water oxygen, and the peak around 2.7 Å can be observed, as displayed in Fig. 8. This indicates that water has ordered structure at short range. To elucidate compartmentation of water clusters in the PVA matrix intuitively, the distribution morphology of water and ethanol excluding PVA chains is shown in Fig. 9. It was found that most of water formed water clusters in the swollen PVA and grew larger with increasing degree of swelling. However, ethanol dispersed almost individually in the PVA matrix. This is possibly due to the size restrictions of ethanol (greater than water) molecules in polymer network making it more difficult to diffuse and congregate than water.
3.4. Diffusion of water and ethanol molecules
Fig. 8. Radial distribution function g(r) for O (water)–O (water) in the swollen PVA.
In swollen polymeric membranes, the mobility of backbone chains and the locations inhabited by solvents in the membranes increased, which favored the diffusion of solvent molecules in the polymeric matrix. The same result was observed in the swollen PVA. Fig. 10 shows the MSD of water and ethanol molecules in the swollen PVA, the mobility of water and ethanol molecules increased rapidly with increasing degree of swelling. Diffusion coefficients of water and ethanol were calculated by the Einstein relationship,
Fig. 9. Distribution pattern of water and ethanol molecules in the swollen PVA excluding PVA chains (green circle denotes ethanol molecules; the other denotes water molecules).
Q.G. Zhang et al. / Chemical Engineering Science 64 (2009) 334 -- 340
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Fig. 11. The degree of swelling-dependent diffusion coefficient of water and ethanol in the swollen PVA.
the diffusion coefficients of water and ethanol in the PVA matrix increased. The reason may be that the locations inhabited by water and ethanol molecules in the PVA matrix are available for the diffusion of solvent molecules through the PVA matrix. Water and ethanol can displace and diffuse in the locations inhabited by water and ethanol molecules, and the locations in the swollen PVA increased in size and quantity with increasing degree of swelling (Figs. 5 and 6). This results in a significant increase in the diffusion coefficients of water and ethanol. 4. Conclusion
Fig. 10. MSD of water (A) and ethanol (B) in the swollen PVA.
Table 2 Diffusion coefficients D (m2 /s) of water and ethanol molecules in the swollen PVA. System
2 3 4 a
Water
Ethanol
Sim
Expa
Sim
Expa
1.892×1010 4.502×1010 1.556×109
4.815×1010 3.368×109 1.267×108
5.412×1011 9.348×1011 4.340×1010
7.970×1011 1.006×109 6.013×109
Experiment value (Zhang et al., 2007a).
as listed in Table 2. It can be found that the simulated diffusion coefficient is slightly less than the experimental value (Zhang et al., 2007a). The reason is that chemical potential differences, resulting from the difference in pressure and concentration between the feed and the permeate sides, accelerated the diffusion of water and ethanol through membranes in experimental measurements. And the diffusion coefficients of water and ethanol increased linearly with increasing degree of swelling (Fig. 11). The diffusion of solvent molecules in polymeric membranes can be generally described using the solution-diffusion model, and membrane microstructure plays an important role in the diffusion of molecules through the membranes. In the swollen PVA, the free volume decreased with increasing degree of swelling (Fig. 4), however,
MD simulation technique was applied to investigate the static and dynamics properties of the swollen polyvinyl alcohol (PVA) in an aqueous ethanol solution. Calculations were carried out for the dry PVA and three swollen PVA in ethanol solutions containing 15, 30, and 45 wt% water. The characteristics of the swollen PVA, distribution of water and ethanol molecules in the PVA matrix, intrinsic relation between the microstructure of the swollen PVA and the diffusion of water and ethanol in the PVA matrix were studied. In the swollen PVA, the free volume reduced as the degree of crystallinity of the PVA decreased, and the mobility of PVA chains increased. Large holes inhabited by water and ethanol formed between PVA chains, the holes increased in quantity and size with increasing degree of swelling. Specifically, the holes with diameters around 9.0–11.0 Å occupied 0.36% volume in system 4. Most of water located in the hydrophilic region of the hydroxyl groups of PVA chains, and hydrogen bonding formed between water and PVA. Water clusters formed in the swollen PVA, and became larger with increasing degree of swelling. However, ethanol dispersed almost separately in PVA matrix. Diffusion coefficients of water and ethanol increased linearly with increasing degree of swelling. Acknowledgements The support of National Nature Science Foundation of China (no. 50573063), the Program for New Century Excellent Talents in University and the research fund for the Doctoral Program of Higher Education (no. 2005038401) in preparation of this article is gratefully acknowledged. The computational resource provided by 985 computing center at Xiamen University is acknowledged. The authors also wish to thank Dr. Ian Broadwell (University of Nottingham,
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UK) for his technical assistance in editing this manuscript for publication. References Alghezawi, N., Sanl, O., Aras, L., Asman, G., 2005. Separation of acetic acid–water mixtures through acrylonitrile grafted poly(vinyl alcohol) membranes by pervaporation. Chemical Engineering and Processing 44, 51–58. Baker, R.W., 2004. Membrane Technology and Applications. Wiley, Chichester. Berendsen, H.J.C., Postma, J.P.M., van Gunsteren, W.F., DiNola, A., Haak, J.R., 1984. Molecular dynamics with coupling to an external bath. Journal of Chemical Physics 81, 3684–3690. DeMerlis, C.C., Schoneker, D.R., 2003. Review of the oral toxicity of polyvinyl alcohol (PVA). Food and Chemical Toxicology 41, 319–326. Devi, D.A., Smitha, B., Sridhar, S., Aminabhavi, T.M., 2005. Pervaporation separation of isopropanol/water mixtures through crosslinked chitosan membranes. Journal of Membrane Science 262, 91–99. Entrialgo-Castaño, M., Lendlein, A., Hofmann, D., 2006. Molecular modeling investigations of dry and two water-swollen states of biodegradable polymers. Advanced Engineering Materials 8, 434–439. Feng, X., Huang, R.Y.M., 1997. Liquid separation by membrane pervaporation: a review. Industrial & Engineering Chemistry Research 36, 1048–1066. Grazianoa, G.J., 2005. Comment on “Hydrophobic effects on partial molar volume” [J. Chem. Phys. 122, 094509 (2005)]. Chemical Physics 123, 167103. Karlsson, G.E., Gedde, U.W., Hedenqvist, M.S., 2004. Molecular dynamics simulation of oxygen diffusion in dry and water-containing poly(vinyl alcohol). Polymer 45, 3893–3900. Kucukpinar, E., Doruker, P., 2004. Effect of absorbed water on oxygen transport in EVOH matrices: a molecular dynamics study. Polymer 45, 3555–3564. Liu, Y.L., Hsu, C.Y., Su, Y.H., Lai, J.Y., 2005. Chitosan–silica complex membranes from sulfonic acid functionalized silica nanoparticles for pervaporation dehydration of ethanol–water solutions. Biomacromolecules 6, 368–373. ¨ Muller-Plathe, F., 1996a. Local structure and dynamics in solvent-swollen polymers. Macromolecules 29, 4782–4791. ¨ Muller-Plathe, F., 1996b. Solvent dynamics in swollen polymers. Chemical Physics Letters 252, 419–449. ¨ Muller-Plathe, F., 1998a. Diffusion of water in swollen poly(vinyl alcohol) membranes studied by molecular dynamics simulation. Journal of Membrane Science 141, 147–154. ¨ Muller-Plathe, F., 1998b. Microscopic dynamics in water-swollen poly(vinyl alcohol). Journal of Chemical Physics 108, 8252–8263. ¨ Muller-Plathe, F., van Gunsteren, W.F., 1997. Solvation of poly(vinyl alcohol) in water, ethanol and an equimolar water–ethanol mixture: structure and dynamics studied by molecular dynamics simulation. Polymer 38, 2259–2268. Nosé, S., 1984. A molecular dynamics method for simulations in the canonical ensemble. Molecular Physics 52, 255–268. Pan, F., Peng, F., Lu, L., Wang, J., Jiang, Z., 2008. Molecular simulation on penetrants diffusion at the interface region of organic–inorganic hybrid membranes. Chemical Engineering Science 63, 1072–1080. Rao, P.S., Smitha, B., Sridhar, S., Krishnaiah, A., 2006. Preparation and performance of poly(vinyl alcohol)/polyethyleneimine blend membranes for the dehydration of 1,4-dioxane by pervaporation: comparison with glutaraldehyde cross-linked membranes. Separation and Purification Technology 48, 244–254.
Semenova, S.I., Ohya, H., Soontarapa, K., 1997. Hydrophilic membranes for pervaporation: an analytical review. Desalination 110, 251–286. Tamai, Y., Tanaka, H., 1998a. Permeation of small penetrants in hydrogels. Fluid Phase Equilibria 144, 441–448. Tamai, Y., Tanaka, H., 1998b. Dynamic properties of supercooled water in poly(vinyl alcohol) hydrogel. Chemical Physics Letters 285, 127–132. Tamai, Y., Tanaka, H., 1999. Effects of polymer chains on structure and dynamics of supercooled water in poly(vinyl alcohol). Physical Review E 59, 5647–5654. Tamai, Y., Tanaka, H., Nakanishi, K., 1996a. Molecular dynamics study of polymer–water interaction in hydrogels. 1. Hydrogen-bond structure. Macromolecules 29, 6750–6760. Tamai, Y., Tanaka, H., Nakanishi, K., 1996b. Molecular dynamics study of polymer–water interaction in hydrogels. 2. Hydrogen-bond dynamics. Macromolecules 29, 6761–6769. Uragami, T., Okazaki, K., Matsugi, H., Miyata, T., 2002. Structure and permeation characteristics of an aqueous ethanol solution of organic–inorganic hybrid membranes composed of poly(vinyl alcohol) and tetraethoxysilane. Macromolecules 35, 9156–9163. Urata, S., Irisawa, J., Takada, A., Shinoda, W., Tsuzuki, S., Mikami, M., 2005a. Molecular dynamics simulation of swollen membrane of perfluorinated ionomer. Journal of Physical Chemistry B 109, 4269–4278. Urata, S., Irisawa, J., Takada, A., Shinoda, W., Tsuzuki, S., Mikami, M., 2005b. Molecular dynamics study of the methanol effect on the membrane morphology of perfluorosulfonic ionomers. Journal of Physical Chemistry B 109, 17274–17280. Vane, L.M., 2005. A review of pervaporation for product recovery from biomass fermentation processes. Journal of Chemical Technology and Biotechnology 80, 603–629. Vishnyakov, A., Neimark, A.V., 2001. Molecular dynamics simulation of microstructure and molecular mobilities in swollen nafion membranes. Journal of Physical Chemistry 105, 9586–9594. Will, B., Lichtenthaler, R.N., 1992. Comparison of the separation of mixtures by vapor permeation and pervaporation using PVA composite membranes. Part I. Binary alcohol–water systems. Journal of Membrane Science 68, 119–125. Xiang, T.X., Anderson, B.D., 2005. Distribution and effect of water content on molecular mobility in poly(vinylpyrrolidone) glasses: a molecular dynamics simulation. Pharmaceutical Research 22, 1205–1214. Yang, J.Z., Liu, Q.L., Wang, H.T., 2007. Analyzing adsorption and diffusion behaviors of ethanol/water through silicalite membranes by molecular simulation. Journal of Membrane Science 291, 1–9. Zhang, Q.G., Liu, Q.L., Jiang, Z.Y., Chen, Y., 2007a. Anti-trade-off in dehydration of ethanol by novel PVA/APTEOS hybrid membranes. Journal of Membrane Science 287, 237–245. Zhang, Q.G., Liu, Q.L., Chen, Y., Chen, J.H., 2007b. Dehydration of isopropanol by novel poly(vinyl alcohol)–silicone hybrid membranes. Industrial & Engineering Chemistry Research 46, 913–920. Zhang, Q.G., Liu, Q.L., Jiang, Z.Y., Ye, L.Y., Zhang, X.H., 2008. Effects of annealing on the physico-chemical structure and permeation performance of novel hybrid membranes of poly(vinyl alcohol)/-aminopropyl-triethoxysilane. Microporous and Mesoporous Materials 110, 379–391.
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Chemical Engineering Science journal homepage: w w w . e l s e v i e r . c o m / l o c a t e / c e s
Microstructure dependent diffusion of water–ethanol in swollen poly(vinyl alcohol): A molecular dynamics simulation study Qiu Gen Zhang, Qing Lin Liu ∗ , Yu Chen, Jian Yang Wu, Ai Mei Zhu National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, Department of Chemical and Biochemical Engineering, College of Chemistry & Chemical Engineering, Xiamen University, Xiamen 361005, China
A R T I C L E
I N F O
Article history: Received 8 July 2008 Received in revised form 6 October 2008 Accepted 20 October 2008 Available online 30 October 2008 Keywords: Poly(vinyl alcohol) Molecular simulation Diffusion Pervaporation Swelling properties
A B S T R A C T
Molecular dynamics (MD) simulation was used to study the swelling properties of poly(vinyl alcohol) (PVA) in ethanol solutions containing 15, 30 and 45 wt% water. The characteristics of the swollen PVA, intrinsic relation between the microstructure of the swollen PVA and the diffusion of water and ethanol in the PVA matrix were analyzed. It was found that the free volume of the swollen PVA reduced with reductions in the degree of crystallinity was accompanied by an increase in the mobility of PVA chains. Water located mostly in the hydrophilic region of the hydroxyl groups of PVA chains; and hydrogen bonding formed between water and PVA. It was also noted water clusters form in the swollen PVA, whose size increased with increasing degree of swelling, whereas ethanol molecules disperse almost individually in the PVA matrix. The diffusion coefficients of water and ethanol in the swollen PVA are predicted to increase linearly with increasing swelling. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction Pervaporation is a novel and promising membrane separation technology which is chosen because of its high selectivity, easy implementation and energy saving qualities compared to other experimental techniques used for separating similar boiling points mixtures and azeotropic mixtures (Feng and Huang, 1997; Liu et al., 2005; Vane, 2005). The separation membrane is the key element in pervaporation separation equipment. Most membranes are fabricated from polymer materials owing to their low cost and good filmforming abilities. Polymeric membranes are however limited. They suffer from swelling in certain solvents which leads to a change in the structure of the membranes over time and hence a decrease in the mechanical strength and the permselectivity (Rao et al., 2006). Clearly quantitatively assessing the swelling properties of a membrane material is one of the key factors to evaluate the separation performance of membranes in designing new polymeric membrane materials. To date there is a large body of published experimental work on the swelling properties of polymeric membranes already in existence. Generally those works, however, focus on the degree of swelling and sorption selectivity of membranes at a macroscopic
∗
Corresponding author. Tel.: +86 592 2183751; fax: +86 592 2184822. E-mail address: [email protected] (Q.L. Liu).
0009-2509/$ - see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2008.10.028
level (Devi et al., 2005; Baker, 2004; Uragami et al., 2002; Zhang et al., 2007a). Review of the literature indicates more studies are required to be understood at a molecular level how solvents affect the structure of polymeric membranes and how they disperse in the polymeric membranes. Additionally, the diffusion characteristics of the solvent molecules through the swollen membranes are difficult and expensive to be measured by experiments and require further future exploration. Recently, researchers have investigated the characteristics of swollen polymers using molecular dynamics (MD) simulation and gained some useful insights into solvent-membrane interactions. Urata et al. (2005a,b) studied the swollen perfluorinated and perfluorosulfonic ionomers membranes containing different solvent (methanol or/and water) to elucidate the solvent effects on membrane material morphologies. The properties studied included: (1) the size of solvent cluster; (2) solvent location; (3) polymer structure and (4) the mobility of solvent and polymer chains. Vishnyakov and Neimark (2001) studied microphase segregation in the Nafion perfluorinated membranes at different water contents and observed the formation of water clusters containing up to 100 water molecules. Kucukpinar and Doruker (2004) reported oxygen transport in dry and hydrated amorphous ethylene–vinyl alcohol copolymers, the diffusion coefficient of oxygen in the swollen membranes was noted ¨ to increase with increasing water content. Muller-Plathe (1996a,b) calculated benzene diffusion coefficients in benzene–polystyrene systems and found that their composition dependence not only showed good agreement with experiment but also followed quite
Q.G. Zhang et al. / Chemical Engineering Science 64 (2009) 334 -- 340
well the predictions by lattice models. Entrialgo-Castaño et al. (2006) investigated water swollen olyglycolide and poly-(l-lactide) membranes at a detailed atomistic level. Xiang and Anderson (2005) explored the distribution of water and the effects on molecular mobility in poly(vinylpyrrolidone) glasses. As mentioned above, MD simulation is an effective and a convenient method to study the structure and performance of swollen polymers. Poly(vinyl alcohol) (PVA) often finds applications as a membrane material or hydrogel in the textile industries due to its excellent chemical stability, film-forming capacity, barrier properties and high hydrophilicity (Alghezawi et al., 2005; DeMerlis and Schoneker, 2003; Semenova et al., 1997; Will and Lichtenthaler, 1992). However, the swelling of PVA membranes in an aqueous solution results in a decrease in the mechanical and separation properties of PVA membranes. It is our purpose here to further develop the understanding of solvent interactions in swollen PVA by using MD simulations to understand the effect of solvents on the structure ¨ and performance of swollen PVA. Muller-Plathe (1998a,b) reported the concentration dependence of water diffusivities in PVA systems with 3–100 vol% water. By investigating swollen PVA in an aqueous ethanol solution, PVA was also found to be purely hydrophilic ¨ by Muller-Plathe and van Gunsteren (1997). Karlsson et al. (2004) studied the diffusion kinetics and mechanisms of oxygen and water in dry and water-containing amorphous PVA at low water contents ( < 5.2 wt%). Tamai et al. (1996a,b), Tamai and Tanaka (1998a,b, 1999) concluded that the diffusion of oxygen and nitrogen in PVA hydrogel increased with increasing water content. As described previously most solvent-membrane studies have focused on swollen polymers in one-component solvent and gas diffusion in swollen membranes, only a few researches have reported on properties of swollen polymers in multi-component solvent. In this paper, we have studied the swelling properties of PVA in water–ethanol mixtures by MD simulation. The characteristics of the swollen PVA, the distribution of water and ethanol in the PVA matrix, intrinsic relation between the microstructure of the swollen PVA and the diffusion of water and ethanol in the PVA matrix were all investigated.
2. Methodology 2.1. Simulation details In our previous work (Zhang et al., 2007a,b, 2008), the swelling behavior of PVA membranes in ethanol (and isopropanol) solutions with different water content was investigated. With increasing water content, the degree of swelling of the PVA membranes was noted to increase. Water and ethanol diffusion coefficients increased rapidly, whereas water sorption selectivity decreased. The degree of swelling of the PVA membranes in ethanol solutions containing 15, 30 and 45 wt% water was 22.13, 39.10 and 84.21%. The corresponding water concentration in the PVA membrane-adsorbed mixture is 76.61, 76.85 and 81.30 wt%, respectively. As an extension of this previous work, the characteristics of swollen PVA in water–ethanol mixtures containing 15, 30 and 45 wt% water were investigated by MD simulation in this study. All simulations were performed on the SGI workstation using material studio (MS) software (Accelrys Inc.) at 323.2 K and 1 atm, and a COMPASS force field was employed. The van der Waals interactions were calculated by the “atom based' method with the cut-off distance 9.5 Å, and the Coulombic interactions were taken into account using the “Ewald” method. Pressure was controlled using the Berendsen method (Berendsen et al., 1984), and temperature was controlled by the Andersen thermostat in NPT MD simulation. Nose's method (Nosé, 1984) with Q = 1.0 was used to control temperature
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to acquire dynamics properties in NVT MD simulation. A time step of 0.5 fs was used for all MD simulation. The syndiotactic PVA chain consisting of 50 repeating units was constructed. Then, the cubic simulation cells containing PVA chains, water and ethanol molecules, were constructed by the amorphous cell modules at an initial density of 0.3 g/cm3 , whose parameters are listed in Table 1. For each system 20 independent configurations were initially generated, and three configurations with minimum energy were selected for further simulations. These configurations were performed on a 10 000 step energy minimization to relax the unfavorable overlaps, and then subjected to 700 ps NPT and 200 ps NVT MD simulations to equilibrate the system. Subsequently 1500 ps NVT MD simulation was then carried out to investigate the diffusion properties of PVA chains, water and ethanol molecules. 2.2. Data analysis The mobility of atoms (molecules) in the dry and swollen PVA membranes is investigated using mean square displacement (MSD), which can be computed by the following equation MSD = |ri (t) − ri (0)|2
(1)
where r(0) is the initial positional coordinate of atom i (or molecules), ri (t) denotes the coordinates at time t. The diffusion coefficients D of atoms (or molecules) can be calculated from the slope of MSD for sufficient long time by Einstein relationship: |ri (t) − ri (0)|2 6t t→∞
D = lim
(2)
Free volume, as indicated earlier, is one of important factors to characterize the structure of polymeric membranes and can be determined accurately by molecular simulation. In MD simulations, atoms are represented by hard spheres with van der Waals radius in the simulation atomistic models (Pan et al., 2008). Fig. 1 shows the sketch of van der Waals surface and Connolly surface. The van der Waals surface intersects with the van der Waals radii of the atoms in the structure and Connolly surface is at the boundary between the Connolly probe and the atoms. Free volume is defined as the volume circled by the Connolly surface. Fractional free volume (FFV) was calculated by the ratio of free volume to the total volume of model. 3. Results and discussion 3.1. Characteristics The equilibrium structures of the dry and swollen PVA are obtained after MD simulation for refinement; their properties are listed in Table 1. The densities of the swollen PVA decreased with increasing swelling. This agreed with findings of other studies on dif¨ ferent swollen polymers (Muller-Plathe and van Gunsteren, 1997; Vishnyakov and Neimark, 2001). This is due to an increase in the degree of disorder and the mobility of PVA chains in the swollen PVA, resulting in an increase of the PVA matrix volume. This result was confirmed by the following analysis of X-ray scattering and MSD for backbone carbon atoms. Fig. 2 displays the simulated X-ray scattering pattern of the dry and swollen PVA. From the pattern, it can be observed that the characteristic peak of PVA appeared near at 2 = 20◦ and its intensity decreased with increasing degree of swelling. The position of the peaks shifts to the left from systems 1–4, indicating the crystalline region in the swollen PVA was becoming small owing to an increase in the kinetic energy of PVA chains. The degree of crystallinity for
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Table 1 Parameters of simulation systems. System
Cell component
1 2 3 4
4 3 3 2
PVA PVA PVA PVA
Weight fraction of components
chains chains, 62 H2 O, 7 ETOH chains, 110 H2 O, 13 ETOH chains, 164 H2 O, 15 ETOH
Cell after refinement
PVA
Water
Ethanol
Density
Size (Å)
100 82.02 71.93 54.74
0 13.86 21.56 36.69
0 4.12 6.51 8.57
1.1526 1.1338 1.1036 1.0735
23.3341 22.7632 24.0065 23.1833
`2 PVA chains, 164 H2 O, 15 ETOH' means that the simulation cell consists of three PVA chains, 62 water molecules, and seven ethanol molecules.
Fig. 1. Definition of van der Waals surface and Connolly surface.
Fig. 3. MSD for backbone carbon atoms in the dry and swollen PVA.
Fig. 2. Simulated X-ray scattering pattern of the dry and swollen PVA.
Fig. 4. FFV distributions of the dry and swollen PVA.
each system was calculated from Eq. (3): Xc,2 =
20 Ic (2)d(2)
20 Ia (2)d(2) + 20 Ic (2)d(2)
× 100%
(3)
where Xc,2 is the degree of crystallinity, Ic (2) and Ia (2) are the Xray scattering intensity of crystalline region and amorphous region. The calculated degree of crystallinity for systems 1–4 was 38.43, 36.25, 35.17 and 34.96%, respectively. Fig. 3 shows the MSD of backbone carbon atoms in the dry and swollen PVA. It is clear that the backbone motion increases from systems 1 to 4. The self-diffusion coefficient D of backbone carbon atoms
can be estimated sing the Einstein relationship. D (10−8 cm2 s−1 ) of backbone carbon atoms was calculated to be 1.58, 2.95, 10.83 and 22.77 for systems 1–4, respectively. This suggests that the mobility of backbone chains increased rapidly with increasing swelling. 3.2. Microstructure Free volume and its distribution in the dry and swollen PVA was investigated, as shown in Fig. 4. When the free volume cavity diameter was around 1.0–2.8 Å, the FFV for both the dry and the swollen
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Fig. 5. Simulated morphology of holes in the dry and swollen PVA excluding water and ethanol molecules (white circle denotes atoms; green circle denotes Connolly surface).
Fig. 6. Fraction of hole's volume in the dry and swollen PVA excluding water and ethanol molecules.
Fig. 7. Radial distribution function g(r) for O (water)–O (PVA) in the swollen PVA.
PVA can be seen to differ slightly. The FFV in the swollen PVA is remarkably less than that in the dry PVA when the diameter is in the range of 2.8–5.0 Å. This is because the free volume cavities in the swollen PVA matrix are inhabited by water and ethanol molecules. The typical size of water and ethanol molecules are in the range 2.8–3.2 Å (Grazianoa, 2005) and 4.3 Å (Yang et al., 2007), respectively. In the presence of water molecules, the free volume cavities are
about 2.8–3.2 Å in diameter and in the swollen PVA the diameters are less than those in the dry PVA. It is worth noting that any additional increase in swelling of the swollen PVA does not further reduce the free volume cavity diameter beyond a certain limit. This can possibly be attributed to any increase in the amorphous region and the mobility of PVA chains leading to a decrease in the compact
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region in PVA matrix. For large cavities with a diameter greater than 3.2 Å, the FFV in the swollen PVA decreases with increasing swelling. This is explained by the fact that the large cavities are easily occupied by water and ethanol molecules. In order to investigate the configuration of PVA chains in the swollen PVA, water and ethanol molecules inside the cells had firstly to be removed. The fraction of hole volume in the remaining structures which consist of only PVA chains was then calculated. The simulated morphology of holes in the PVA matrix is displayed in Fig. 5. With increasing swelling the holes grew rapidly and the throughpores formed in system 4. This suggests that the morphology of PVA chains is less compact in the swollen PVA than in the dry one and large holes are formed in between PVA chains. These larger holes favor the diffusion of water and ethanol molecules into the PVA matrix. To illustrate quantitatively, the fraction of hole's volume in the PVA matrix and swollen PVA is shown in Fig. 6. It can be observed that the fraction of holes in the PVA matrix increased remarkably in quantity and size with increasing degree of swelling. More specifically, the holes with diameter of 9.0–11.0 Å formed and occupied 0.36% volume in system 4.
3.3. Compartmentation of water and ethanol molecules The water distribution in the PVA matrix can be investigated using the radial distribution function g(r). In the swollen PVA, water dispersed in the PVA matrix could form hydrogen bonding with the hydroxyl groups of PVA chains, as noted by some researchers ¨ (Muller-Plathe and van Gunsteren, 1997; Tamai et al., 1996a, b). Fig. 7 shows the g(r) between water oxygen and the oxygen in PVA chains, a strong peak at 2.8 Å can be observed. This reveals that water located mostly in the hydrophilic region of the hydroxyl groups in PVA chains, and hydrogen bonding formed between water and PVA chains. And the hydrophilic region is defined to cover the inner region up to the first peaks of g(r) of water oxygen atoms around ¨ the hydroxyl groups (Muller-Plathe, 1996a). Water dispersed in the polymeric network could form clusters, as reported in some hydrogels and swollen polymers (Urata et al., 2005a,b; Vishnyakov and Neimark, 2001). In the swollen PVA, water ¨ clusters were also observed by other teams (Muller-Plathe and van Gunsteren, 1997; Tamai et al., 1996a). For the swollen PVA in an aqueous ethanol solution, water clusters are confirmed by the g(r) between water oxygen, and the peak around 2.7 Å can be observed, as displayed in Fig. 8. This indicates that water has ordered structure at short range. To elucidate compartmentation of water clusters in the PVA matrix intuitively, the distribution morphology of water and ethanol excluding PVA chains is shown in Fig. 9. It was found that most of water formed water clusters in the swollen PVA and grew larger with increasing degree of swelling. However, ethanol dispersed almost individually in the PVA matrix. This is possibly due to the size restrictions of ethanol (greater than water) molecules in polymer network making it more difficult to diffuse and congregate than water.
3.4. Diffusion of water and ethanol molecules
Fig. 8. Radial distribution function g(r) for O (water)–O (water) in the swollen PVA.
In swollen polymeric membranes, the mobility of backbone chains and the locations inhabited by solvents in the membranes increased, which favored the diffusion of solvent molecules in the polymeric matrix. The same result was observed in the swollen PVA. Fig. 10 shows the MSD of water and ethanol molecules in the swollen PVA, the mobility of water and ethanol molecules increased rapidly with increasing degree of swelling. Diffusion coefficients of water and ethanol were calculated by the Einstein relationship,
Fig. 9. Distribution pattern of water and ethanol molecules in the swollen PVA excluding PVA chains (green circle denotes ethanol molecules; the other denotes water molecules).
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Fig. 11. The degree of swelling-dependent diffusion coefficient of water and ethanol in the swollen PVA.
the diffusion coefficients of water and ethanol in the PVA matrix increased. The reason may be that the locations inhabited by water and ethanol molecules in the PVA matrix are available for the diffusion of solvent molecules through the PVA matrix. Water and ethanol can displace and diffuse in the locations inhabited by water and ethanol molecules, and the locations in the swollen PVA increased in size and quantity with increasing degree of swelling (Figs. 5 and 6). This results in a significant increase in the diffusion coefficients of water and ethanol. 4. Conclusion
Fig. 10. MSD of water (A) and ethanol (B) in the swollen PVA.
Table 2 Diffusion coefficients D (m2 /s) of water and ethanol molecules in the swollen PVA. System
2 3 4 a
Water
Ethanol
Sim
Expa
Sim
Expa
1.892×1010 4.502×1010 1.556×109
4.815×1010 3.368×109 1.267×108
5.412×1011 9.348×1011 4.340×1010
7.970×1011 1.006×109 6.013×109
Experiment value (Zhang et al., 2007a).
as listed in Table 2. It can be found that the simulated diffusion coefficient is slightly less than the experimental value (Zhang et al., 2007a). The reason is that chemical potential differences, resulting from the difference in pressure and concentration between the feed and the permeate sides, accelerated the diffusion of water and ethanol through membranes in experimental measurements. And the diffusion coefficients of water and ethanol increased linearly with increasing degree of swelling (Fig. 11). The diffusion of solvent molecules in polymeric membranes can be generally described using the solution-diffusion model, and membrane microstructure plays an important role in the diffusion of molecules through the membranes. In the swollen PVA, the free volume decreased with increasing degree of swelling (Fig. 4), however,
MD simulation technique was applied to investigate the static and dynamics properties of the swollen polyvinyl alcohol (PVA) in an aqueous ethanol solution. Calculations were carried out for the dry PVA and three swollen PVA in ethanol solutions containing 15, 30, and 45 wt% water. The characteristics of the swollen PVA, distribution of water and ethanol molecules in the PVA matrix, intrinsic relation between the microstructure of the swollen PVA and the diffusion of water and ethanol in the PVA matrix were studied. In the swollen PVA, the free volume reduced as the degree of crystallinity of the PVA decreased, and the mobility of PVA chains increased. Large holes inhabited by water and ethanol formed between PVA chains, the holes increased in quantity and size with increasing degree of swelling. Specifically, the holes with diameters around 9.0–11.0 Å occupied 0.36% volume in system 4. Most of water located in the hydrophilic region of the hydroxyl groups of PVA chains, and hydrogen bonding formed between water and PVA. Water clusters formed in the swollen PVA, and became larger with increasing degree of swelling. However, ethanol dispersed almost separately in PVA matrix. Diffusion coefficients of water and ethanol increased linearly with increasing degree of swelling. Acknowledgements The support of National Nature Science Foundation of China (no. 50573063), the Program for New Century Excellent Talents in University and the research fund for the Doctoral Program of Higher Education (no. 2005038401) in preparation of this article is gratefully acknowledged. The computational resource provided by 985 computing center at Xiamen University is acknowledged. The authors also wish to thank Dr. Ian Broadwell (University of Nottingham,
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UK) for his technical assistance in editing this manuscript for publication. References Alghezawi, N., Sanl, O., Aras, L., Asman, G., 2005. Separation of acetic acid–water mixtures through acrylonitrile grafted poly(vinyl alcohol) membranes by pervaporation. Chemical Engineering and Processing 44, 51–58. Baker, R.W., 2004. Membrane Technology and Applications. Wiley, Chichester. Berendsen, H.J.C., Postma, J.P.M., van Gunsteren, W.F., DiNola, A., Haak, J.R., 1984. Molecular dynamics with coupling to an external bath. Journal of Chemical Physics 81, 3684–3690. DeMerlis, C.C., Schoneker, D.R., 2003. Review of the oral toxicity of polyvinyl alcohol (PVA). Food and Chemical Toxicology 41, 319–326. Devi, D.A., Smitha, B., Sridhar, S., Aminabhavi, T.M., 2005. Pervaporation separation of isopropanol/water mixtures through crosslinked chitosan membranes. Journal of Membrane Science 262, 91–99. Entrialgo-Castaño, M., Lendlein, A., Hofmann, D., 2006. Molecular modeling investigations of dry and two water-swollen states of biodegradable polymers. Advanced Engineering Materials 8, 434–439. Feng, X., Huang, R.Y.M., 1997. Liquid separation by membrane pervaporation: a review. Industrial & Engineering Chemistry Research 36, 1048–1066. Grazianoa, G.J., 2005. Comment on “Hydrophobic effects on partial molar volume” [J. Chem. Phys. 122, 094509 (2005)]. Chemical Physics 123, 167103. Karlsson, G.E., Gedde, U.W., Hedenqvist, M.S., 2004. Molecular dynamics simulation of oxygen diffusion in dry and water-containing poly(vinyl alcohol). Polymer 45, 3893–3900. Kucukpinar, E., Doruker, P., 2004. Effect of absorbed water on oxygen transport in EVOH matrices: a molecular dynamics study. Polymer 45, 3555–3564. Liu, Y.L., Hsu, C.Y., Su, Y.H., Lai, J.Y., 2005. Chitosan–silica complex membranes from sulfonic acid functionalized silica nanoparticles for pervaporation dehydration of ethanol–water solutions. Biomacromolecules 6, 368–373. ¨ Muller-Plathe, F., 1996a. Local structure and dynamics in solvent-swollen polymers. Macromolecules 29, 4782–4791. ¨ Muller-Plathe, F., 1996b. Solvent dynamics in swollen polymers. Chemical Physics Letters 252, 419–449. ¨ Muller-Plathe, F., 1998a. Diffusion of water in swollen poly(vinyl alcohol) membranes studied by molecular dynamics simulation. Journal of Membrane Science 141, 147–154. ¨ Muller-Plathe, F., 1998b. Microscopic dynamics in water-swollen poly(vinyl alcohol). Journal of Chemical Physics 108, 8252–8263. ¨ Muller-Plathe, F., van Gunsteren, W.F., 1997. Solvation of poly(vinyl alcohol) in water, ethanol and an equimolar water–ethanol mixture: structure and dynamics studied by molecular dynamics simulation. Polymer 38, 2259–2268. Nosé, S., 1984. A molecular dynamics method for simulations in the canonical ensemble. Molecular Physics 52, 255–268. Pan, F., Peng, F., Lu, L., Wang, J., Jiang, Z., 2008. Molecular simulation on penetrants diffusion at the interface region of organic–inorganic hybrid membranes. Chemical Engineering Science 63, 1072–1080. Rao, P.S., Smitha, B., Sridhar, S., Krishnaiah, A., 2006. Preparation and performance of poly(vinyl alcohol)/polyethyleneimine blend membranes for the dehydration of 1,4-dioxane by pervaporation: comparison with glutaraldehyde cross-linked membranes. Separation and Purification Technology 48, 244–254.
Semenova, S.I., Ohya, H., Soontarapa, K., 1997. Hydrophilic membranes for pervaporation: an analytical review. Desalination 110, 251–286. Tamai, Y., Tanaka, H., 1998a. Permeation of small penetrants in hydrogels. Fluid Phase Equilibria 144, 441–448. Tamai, Y., Tanaka, H., 1998b. Dynamic properties of supercooled water in poly(vinyl alcohol) hydrogel. Chemical Physics Letters 285, 127–132. Tamai, Y., Tanaka, H., 1999. Effects of polymer chains on structure and dynamics of supercooled water in poly(vinyl alcohol). Physical Review E 59, 5647–5654. Tamai, Y., Tanaka, H., Nakanishi, K., 1996a. Molecular dynamics study of polymer–water interaction in hydrogels. 1. Hydrogen-bond structure. Macromolecules 29, 6750–6760. Tamai, Y., Tanaka, H., Nakanishi, K., 1996b. Molecular dynamics study of polymer–water interaction in hydrogels. 2. Hydrogen-bond dynamics. Macromolecules 29, 6761–6769. Uragami, T., Okazaki, K., Matsugi, H., Miyata, T., 2002. Structure and permeation characteristics of an aqueous ethanol solution of organic–inorganic hybrid membranes composed of poly(vinyl alcohol) and tetraethoxysilane. Macromolecules 35, 9156–9163. Urata, S., Irisawa, J., Takada, A., Shinoda, W., Tsuzuki, S., Mikami, M., 2005a. Molecular dynamics simulation of swollen membrane of perfluorinated ionomer. Journal of Physical Chemistry B 109, 4269–4278. Urata, S., Irisawa, J., Takada, A., Shinoda, W., Tsuzuki, S., Mikami, M., 2005b. Molecular dynamics study of the methanol effect on the membrane morphology of perfluorosulfonic ionomers. Journal of Physical Chemistry B 109, 17274–17280. Vane, L.M., 2005. A review of pervaporation for product recovery from biomass fermentation processes. Journal of Chemical Technology and Biotechnology 80, 603–629. Vishnyakov, A., Neimark, A.V., 2001. Molecular dynamics simulation of microstructure and molecular mobilities in swollen nafion membranes. Journal of Physical Chemistry 105, 9586–9594. Will, B., Lichtenthaler, R.N., 1992. Comparison of the separation of mixtures by vapor permeation and pervaporation using PVA composite membranes. Part I. Binary alcohol–water systems. Journal of Membrane Science 68, 119–125. Xiang, T.X., Anderson, B.D., 2005. Distribution and effect of water content on molecular mobility in poly(vinylpyrrolidone) glasses: a molecular dynamics simulation. Pharmaceutical Research 22, 1205–1214. Yang, J.Z., Liu, Q.L., Wang, H.T., 2007. Analyzing adsorption and diffusion behaviors of ethanol/water through silicalite membranes by molecular simulation. Journal of Membrane Science 291, 1–9. Zhang, Q.G., Liu, Q.L., Jiang, Z.Y., Chen, Y., 2007a. Anti-trade-off in dehydration of ethanol by novel PVA/APTEOS hybrid membranes. Journal of Membrane Science 287, 237–245. Zhang, Q.G., Liu, Q.L., Chen, Y., Chen, J.H., 2007b. Dehydration of isopropanol by novel poly(vinyl alcohol)–silicone hybrid membranes. Industrial & Engineering Chemistry Research 46, 913–920. Zhang, Q.G., Liu, Q.L., Jiang, Z.Y., Ye, L.Y., Zhang, X.H., 2008. Effects of annealing on the physico-chemical structure and permeation performance of novel hybrid membranes of poly(vinyl alcohol)/-aminopropyl-triethoxysilane. Microporous and Mesoporous Materials 110, 379–391.