Polydimethylsiloxane Membranes for

Mar 18, 2013 - (5) used PDMS as filler for their nanosilicalite membrane for butanol/water separations, creating supported membranes with approximatel...
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Nanocomposite Silicalite-1/Polydimethylsiloxane Membranes for Pervaporation of Ethanol from Dilute Aqueous Solutions Amit Yadav, Mary Laura Lind,* Xiaoli Ma, and Y. S. Lin School for Engineering of Matter, Transport and Energy, P.O. Box 876106, Tempe, Arizona 85287, United States ABSTRACT: Separation of alcohols from dilute aqueous solutions is important to enable continuous microbial production of biofuels. Pervaporation is a process with great potential for continuous extraction of alcohols from biological fermentation broths. Here, we report on the synthesis and pervaporation performance of thin, supported pure polydimethylsiloxane (PDMS) and silicalite-1/PDMS nanocomposite membranes with nanoparticle contents of 10, 20, and 30 wt %. Using a batch pervaporation system, we measured flux and separation factors of the membranes for solutions of 4 wt % ethanol in water at 25 °C, 50 °C, and 65 °C. With increased nanosilicalite loadings in the nanocomposite membranes, we obtained both increased flux and increased alcohol separation factors. Moerman’s membranes had limited fluxes (maximum 340 g m−2 h−1) as a result of the high membrane thickness. Liu et al.5 used PDMS as filler for their nanosilicalite membrane for butanol/water separations, creating supported membranes with approximately 75 wt % silicalite content and films 300 nm in thickness. However, this membrane is essentially an inorganic membrane with the macrovoids in the inorganic structure filled with a polymer, rather than a mixed-matrix membrane. In this work, we evaluated the impact of silicalite nanoparticles on the pervaporation performance of thin (less than 50 μm) PDMS films. Here, we report for the first time on 20− 40 μm thick silicalite-1/PDMS nanocomposite thin films formed through dip coating onto a porous alumina support for ethanol/water separation. In this research, we found that increased silicalite loading in our thin PDMS films yielded membranes with increased fluxes and ethanol separation factors compared to our pure PDMS control membranes.

1. INTRODUCTION Among the major applications of pervaporation membrane processes, alcohol separation from dilute aqueous solutions is increasingly important as a method for the recovery of biologically produced ethanol.1 Many hydrophobic materials such as poly(1-(trimethylsily)-1-propyne), poly(methyl phenylsiloxane), and polysiloxaneimide2−4 have been studied for the removal of alcohols from water. Polydimethylsiloxane (PDMS) is the benchmark material for hydrophobic pervaporation membranes for separation of alcohols and VOCs from dilute aqueous solutions because it is an elastomeric material which exhibits excellent film-forming ability, thermal stability, and chemical and physiological inertness.1 The rapid chain segment motion in PDMS leads to a large free volume that favors the diffusion of the permeating molecules.5 Silicalite-16 (referred to as silicalite throughout this paper) is the aluminum-free analogue of the hydrophobic zeolite ZSM5.7 Both silicalite-1 and ZSM-5 preferentially adsorb alcohols from aqueous mixtures.1 In 1987, Te Hennepe et al. published a seminal work on ethanol-selective mixed matrix pervaporation membranes made from silicalite-filled silicone rubber; these mixed matrix membranes showed significant increases in pervaporation flux and separation factor compared to the pure polymer.8 Since then, there have been numerous publications investigating mixed matrix membranes consisting of micron-sized silicalite incorporated into polydimethylsiloxane for pervaporation of alcohol/water solutions.9−13 These mixed matrix membranes uniformly have a higher alcohol separation factors (βethanol/water > 20) than the pure polymer membranes (βethanol/water ≈ 8). Additionally, all of these membranes had composite films greater than two micrometers in thickness because of the casting method. The development of molecular sieve nanoparticles provides the opportunity to increase the interfacial surface area for sorption of selective components as well as produce thinner composite films. Moermans et al.14 prepared 200−400 μm thick free-standing PDMS membranes incorporating 70 nm silicalite nanoparticles with up to 30 wt % loading. These membranes had ethanol/water separation factors ranging from 9 to 16 for 6 wt % aqueous ethanol solutions. However, © 2013 American Chemical Society

2. EXPERIMENTAL SECTION Commercially available alumina membranes (Anopore, Whatman Co.) were used as supports; hereafter we refer to them as “Anodiscs.” Anodisc membranes have an overall thickness of approximately 60 μm and a 2-layer structure with straight and cylindrical pores. The top 2 μm of the Anopore has 20 nm diameter pores. The bottom 58 μm of the Anopore membranes have pores with a diameter of 200−250 nm. Anodisc was chosen as support because the straight pore structure provides low resistance to transport and has uniform small pores. Figure 1a presents scanning electron microscopy (SEM) images of a cross-section of the Anopore structure and Figure 1b shows the SEM view of the bottom surface of the Anodisc. The Anodisc membrane has a diameter of 25 mm and has a polypropylene ring attached to the outer edge of the membrane to facilitate handling. Prior to pervaporation performance testing, the Received: Revised: Accepted: Published: 5207

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octane/RTV B solution and the entire solution was stirred for 15 min at 25 °C. The overall composition of the PDMS/ nanosilicalite casting solution for a membrane with 30 wt % nanosilicalite was 90 wt % solvent, 7 wt % polymer (RTV A + RTV B) and 3 wt % nanosilicalite-1. The casting solution (solvent, polymer, nanosilicalite) was then heated to 65 °C with continuous mixing for 180 min to allow partial polymerization of PDMS similar to pure PDMS membrane casting. After 180 min the casting solution was cooled to 25 °C and used directly for dip-coating as described for pure PDMS membranes. 2.3. Composition and Morphology of the Membranes. The chemical composition of the synthesized membranes was studied with attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR, Nicolet 4700). The morphology of the membranes was evaluated through scanning electron microscopy (SEM, XL30 ESEMFEG). Cross-sections of the membrane samples for SEM were prepared by freezing the sample in liquid nitrogen and then fracturing the sample. For conductivity, the fractured surface was sputter-coated with gold. 2.4. Pervaporation Performance. The raw separation performance of the as-synthesized membranes was evaluated through pervaporation of dilute ethanol/water solutions using the system described by O’Brien−Abraham et al.15 The pervaporation cell was heated using a heating jacket obtained from HTS Amptek (Stafford, TX). The feed had a composition of 4 wt % ethanol in deionized water. Pervaporation performance of the membranes was measured at 25, 50, and 65 °C. The permeate side of the cell was kept under vacuum of 0.2 kPa and the permeate condensed into a liquid nitrogen cold trap. The mass of the condensed permeate was used to calculate the overall flux through the membrane according to the equation: J = W/(At), where W is the weight of permeate (grams), A is the membrane area (m2), and t is the time (h) for the sample collection.16 The composition of ethanol in the feed and permeate was measured with a gas chromatograph (GC, Model 8610C, SRI Instruments, Menlo Park, CA). The separation factor is defined as β = (Yethanol/(1 − Yethanol))/(Xethanol/(1 − Xethanol)), where Xethanol and Yethanol denote the mass fraction of ethanol in the feed and permeate sides, respectively.16 To measure the concentration of ethanol in the condensate, the permeate was diluted with a known quantity of water prior to analysis in the GC. Permeability and selectivity of the membranes were calculated from the measured flux and separation factor values following the methodology recommended by Baker et al.17 These calculations normalize the measured membrane performance for the ethanol concentration, the temperature, and the thickness. The permeability is calculated from

Figure 1. Images of straight pore alumina membrane. SEM images of (a) cross-section of anodizc and (b) bottom surface of anodizc.

excess support ring was removed to ensure a proper fit in the testing cell. 2.1. Synthesis of Silicalite-1. Nanoparticles of silicalite-1 were synthesized based on a method presented by Grose and Flanigen.6 Five grams of fumed silica were added to a solution containing 25 mL of a templating agent, tetrapropylammonium hydroxide solution (TPAOH, Sigma−Aldrich), and 0.35 g of sodium hydroxide (NaOH, Sigma−Aldrich). The reaction mixture was prepared in a glass jar and then transferred into a Teflon vessel. The reaction took place in an autoclave at 120 °C under autogenous pressure for 12 h. After washing (centrifugation and decanting of supernatant) with deionized water until the pH of the solution was below 8, the nanosilicalite was dried under vacuum to remove excess water in between individual silicalite nanoparticles. To remove the template from the silicalite pores, the template filled nanoparticles were calcined for 6 h in air at 550 °C in a furnace (Muffle Furnace 3-550, Prosource Scientific) with a ramp speed of 0.5 °C/min. 2.2. Membrane Preparation. Prior to PDMS casting, the Anodiscs were sonicated in deionized water for 10 min to remove any impurities that were physically adsorbed on the surface; then the Anodiscs were soaked in deionized water for 1 h to fill the pores with water. This prevented intrusion of PDMS solution into the pores of the Anodisc. Pure PDMS thin films supported on Anodiscs were synthesized from RTV A (prepolymer) and RTV B (crosslinker) obtained from Fischer Scientific in a solvent of isooctane. The mass ratio of RTV A to RTV B was 10:1. The composition of the final casting solution was 90 wt % solvent and 10 wt % polymer (RTV A + RTV B). The solution consisting of the solvent, prepolymer, and cross-linker was stirred in a 125 mL conical flask at 65 °C for 180 min. After 180 min the solution was allowed to cool to 25 °C by standing at room temperature. The Anodisc was removed from the water bath and taped to a holder. Excess water on the top of the Anodisc was removed with filter paper. The Anodisc was dipped into the PDMS solution for 5 s and withdrawn. After drying at 25 °C for 10 min, the dip-coating process was repeated. After two dip coating layers were applied to the anodizc, the membrane was dried at room temperature for 24 h. Finally, the PDMS/Anodisc membrane was held at 70 °C for 3 h under 5 in Hg vacuum to ensure complete cross-linking of the PDMS. Nanocomposite films consisting of 10, 20, and 30 wt % nanosized silicate-1 were synthesized in a similar fashion to the pure PDMS membranes. Prior to polymerization of the PDMS, nanosilicalite-1 particles were dispersed in iso-octane for 180 min by sonication at 25 °C. Next, RTV B was added to the isooctane/silicalite solution and the solution was mixed at 25 °C for 15 min. Then, RTV A was added to the silicalite/iso-

Ji = Pi(γi0χi0 pisat − pil )/L 0

(1)

In eq 1 Ji is the molar flux of component i [kmol s−1 m−2] and Pi is the permeability of component i [kmol m−1 s−1 kPa−1]. In eq 1 the subscript 0 represents the feed liquid while the subscript 1 represents the permeate side; the activity coefficient of component i in the feed is γi0, the mole fraction of component i in the feed is χi0. Antonie’s equation is used to calculate the vapor pressure of pure component i in the feed is psat i0 [kPa]. The vapor pressure of component i on the permeate side of the membrane is pil [kPa]. Finally, L [m] is the membrane thickness. 5208

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the Si-CH3 umbrella mode.20 The peak at 1255 cm−1 is the result of CH3 vibrations.20 The Si−O−Si structure20 of the pure nanosilicalite-1 results in broad characteristic peaks centered at 1072 and 545 cm−1; these peaks are also present in the composite material, confirming the successful incorporation of nanosilicalites into the composite membranes. Also, in the nanocomposite membranes the intensity of the peaks at 700−830 cm−1 and 1025−1080 cm−1 increased compared to the pure PDMS membranes because of the additional Si-O-Si bonds present in the silicalite nanoparticles. 3.2. Morphological Characterization of the Nanosilicalite Particles and Membranes. The nanosilicalite-1 particle size was approximately 100 nm and was confirmed through scanning electron microscopy (SEM). Figure 3 presents a SEM image of the synthesized nanosilicalite particles. The nanosilicalite structure was confirmed through X-ray diffraction. Figure 4a shows a SEM cross-section for a pure PDMS coated anodizc, here the thickness of the PDMS membrane is

The selectivity of the membrane for ethanol over water, αew [dimensionless], is the ratio of the ethanol permeability to the water permeability.17 We compare the permeability and selectivity of our nanocomposite membranes to that of mixed matrix membranes as reported in the literature. Activity coefficients used in our calculations were determined with the universal functional activity coefficient (UNIFAC) model.18

3. RESULTS 3.1. Characterization of the Nanosilicalite-PDMS Composite Membrane. Figure 2 presents representative

Figure 2. ATR-FTIR spectra of anodizc support, silicalite particles, PDMS coated anodizc, and silicalite/PDMS nanocomposite membrane. Figure 4. SEM cross-sections of (a) pure PDMS membrane and (b) 20 wt % nanosilicalite-PDMS membrane.

ATR-FTIR spectra of the pure anodics, pure silicalite-1, an anodizc coated with pure PDMS, and the PDMS/silicalite-1/ anodizc nanocomposite membranes. The PDMS sample exhibited strong peaks at 800−880 cm−1 and 1260 cm−1. The multiple peaks between 700 and 830 cm−1 were from the methyl (CH3 group) rocking and the Si-C stretching vibrations in the Si-CH3 group.19 The twin peaks at 1025 and 1080 cm−1 originated from the asymmetric stretching of the Si-O-Si and

approximately 7 μm. Figure 4b shows an SEM of the crosssection of a silicalite/PDMS nanocomposite membrane, here the thickness of the nanocomposite membrane is approximately 28 μm. The greater thickness of the nanocomposite film compared to the pure PDMS film ensures that the membrane is free from possible cracks or pinholes.

Figure 3. SEM of as-synthesized nanosilicalite-1 particles. 5209

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3.3. Effect of Silicalite Loading on Ethanol and Water Permeabilities at 25 °C. Figure 5a shows the fluxes and

membranes was 2.1 times greater than that of pure PDMS membranes. 3.4. Comparison with Previously Reported Membranes. Table 2 summarizes reported permeability and selectivity for mixed matrix ethanol selective membranes. Vane et al.13 found that the ethanol permeability increased by a factor of 2.8 for membranes with 50 wt % 0.3−0.75 μm ZSM (particles with a Si/Al ratio of 137 and NH4+ counterions) compared to pure PDMS for a 5% ethanol solution. However, Vane et al. found that the water permeability did not change significantly between the pure PDMS membrane and the mixed matrix membrane with 50 wt % ZSM; their membrane had a selectivity of 2.13.13 Hennepe et al.8 reported that the fluxes (for a 5 wt % ethanol solution) increased as the content of 0.5− 5 μm diameter silicalite particles was increased from 0 to 60 wt %. In the silicalite/PDMS membranes of Hennepe et al., the ethanol and water permeabilities increased 3.3 fold and 1.5 fold, respectively, compared to the ethanol and water permeabilities of the pure PDMS membranes.8 The selectivity of the silicalite/ PDMS 60 wt % membrane was 1.71.8 In comparison, for a 4 wt % ethanol feed solution, our membranes with 30 wt % nanosilicalite at 25 °C had a 3.7 fold increase in ethanol permeability and a 1.8 fold increase in water permeability compared to the pure PDMS membrane. The selectivity increased to 1.22 from 0.58 for the same nanosilicalite loading. Our membranes exhibited good selectivity and high permeability at a lower silicalite loadings and membrane thicknesses. 3.5. Effect of Temperature on Pervaporation Performance. We also measured the pervaporation performance of our nanocomposite membranes at 50 and 65 °C. At these higher temperatures, we observed higher overall fluxes through the membrane (compared to the performance measured at 25 °C). The higher fluxes at higher temperatures are the result of two factors: increased driving force and increased diffusivity of ethanol. First, at higher temperatures the vapor pressure of the feed solution is increased, which, in turn, increases the overall driving force (vapor pressure difference) for transport across the membrane.22 Second, at higher temperatures the ethanol swells the PDMS and polymer chain mobility increases; this can increase the diffusivity of ethanol and water within the membrane. Both of these factors, higher driving force and polymer swelling, contribute to higher fluxes through the membranes above room temperature.

Figure 5. Pervaporation performance of PDMS/silicalite-1 nanocomposite membranes for 4 wt % ethanol feed solution: (a) total permeate flux and (b) ethanol/water separation factor.

Figure 5b shows the separation factors for pervaporation of a 4 wt % ethanol/water solution as a function of membrane silicalite content and feed temperature. All samples were synthesized and tested in triplicate, if error bars are not visible, the error is smaller than the symbol. The ethanol−water separation factor increased with increasing silicalite loading in the nanocomposite membranes compared to the pure PDMS. At 25 °C our pure PDMS membranes had a separation factor of 8.03 and the 30% silicalite/PDMS membrane had a separation factor of 16.5. In addition to increased ethanol selectivity with increased silicalite content, the overall flux through the membrane also increased with increasing silicalite content. These results are in agreement with the results presented in literature for other PDMS/silicalite composites with micrometer-sized silicalite particles.8,9,21 Table 1 presents the calculated permeability and selectivity for our nanocomposite membranes at all three temperatures. At 25 °C the permeability of ethanol in the membranes with 30 wt % nanosilicalite was 3.7 times greater than that in the pure PDMS membranes, while the water permeability of the 30 wt % membranes was only 1.8 times that of the pure PDMS membranes. Additionally, the ethanol/water selectivity of the 30 wt % nanocomposite

Table 1. Permeabilities and Selectivities for Nanocomposite Membranesa pervaporation temp (°C)

nanosilicalite content (wt %)

25.0

0 10 20 30 0 10 20 30 0 10 20 30

50.0

65.0

a

PEtOH (× 1012 kmol m m−2 s−1 kPa−1) 4.34 6.65 11.06 16.33 2.81 4 6.67 9.74 2.69 3.42 5.31 7.90

± ± ± ± ± ± ± ± ± ± ± ±

0.4 0.35 0.94 0.92 0.26 0.25 0.36 0.37 0.15 0.24 0.31 0.11

Pwater (× 1012 kmol m m−2 s−1 kPa−1) 7.48 9.16 11.44 13.36 5.06 6.50 8.24 9.47 4.88 5.99 7.35 8.06

± ± ± ± ± ± ± ± ± ± ± ±

0.61 0.71 0.90 0.97 0.30 0.47 0.57 0.45 0.25 0.37 0.25 0.23

selectivity, α 0.58 0.73 0.97 1.22 0.56 0.61 0.81 1.03 0.55 0.57 0.72 0.98

± ± ± ± ± ± ± ± ± ± ± ±

0.12 0.09 0.12 0.09 0.11 0.1 0.09 0.06 0.07 0.09 0.07 0.03

PEtOH = permeability of EtOH; Pwater = permeability of water. 5210

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Table 2. Comparison of Permeabilities and Selectivities for PDMS-Molecular Sieve Membranes permeability (× 1012 kmol m m−2 s−1 kPa−1)

a

membrane thickness (μm)

temp (°C)

zeolite loading (wt %)

type of particle

particle size (μm)

water

ethanol

selectivity, αa

100 85 120 150 200 25 200−400

50 50 50 22.5 25 25 35

60 65 40 60 30 30 30

silicalite-116 high silica ZSM-513 silicalite-110 silicalite8 TP-902326 nanosilicalite (this work) nanosilicalite14

1 0.35−0.7 unknown 0.5−5 10 0.1 0.07

2.38 13.63 4.79 17.00 140.00 13.36 12.38

8.69 42.65 12.78 29.00 180.00 16.33 13.77

3.65 3.12 2.67 1.71 1.29 1.22 1.11

EtOH/water.

The variation of the total flux with temperature follows an Arrhenius relationship:23 J = J0 exp( −Ea /RT )

be attributed to the higher silicalite permeability for ethanol compared to the PDMS permeability for ethanol.20 A slight reduction in water activation energy can be seen with increasing zeolite loading. We hypothesize this is the result of void formation at polymer/zeolite interface. We observed a slight reduction in selectivity of the nanocomposite membranes with increasing temperature (compared to room temperature) for all of the silicalite loadings. This is contrary to results found in the literature for PDMS composites with microsized molecular sieves.3,9,14 We hypothesize that there are two reasons for decreased selectivity of nanocomposite membranes at higher pervaporation temperatures: (1) decreased ethanol sorption capacity in silicalite at higher temperatures and (2) void space at the silicalite/polymer interface. Previously, Klein and Abraham found that the ethanol sorption capacity of silicalite decreased with increasing temperature.24 Barrer and James demonstrated that adhesion problems occurred at the polymer/zeolite interface when mixed matrix membranes were prepared.25 Additionally, at higher temperatures, increased polymer chain mobility could result in more void space at the interface between the polymer and the inorganic phases. In our nanocomposite membranes, there is increased silicalite/polymer interfacial contact area (compared to a composite membrane made with micron sized particles) and thus more opportunity for nonselective voids to appear.

(2)

−2 −1

where J [kg m s ] is the flux, J0 is the exponential factor, Ea is the apparent activation energy of permeation for ethanol (kJ/ kmol), R is the gas constant (kJ K−1 kmol−1), and T is the feed temperature (K). Figure 6 plots ln J vs 1/T for our membranes.

Figure 6. Arrhenius plot for membranes with different silicalite content based on (a) ethanol flux and (b) water flux.

4. CONCLUSIONS We have presented a method for fabricating supported nanocomposite silicalite/PDMS membranes with silicalite nanoparticle content up to 30 wt %. These are the thinnest nanocomposite films of silicalite/PDMS reported to date. In comparison with other PDMS−silicalite membranes reported in literature, our membranes exhibited high selectivity and high permeability for ethanol−water mixtures at a low silicalite loading of 30 wt % and at a low membrane thickness of 25 μm. This can be attributed to increased ethanol mobility because of the larger surface area provided by nanosized silicalite-1 particles. Additionally, nanocomposite membranes exhibited higher fluxes and higher separation factors, and corresponding higher permeabilities and higher selectivities, for ethanol−water mixtures compared to the pure polymer membranes. However, the nanocomposite membranes had slightly lower selectivities at 50 and 65 °C than at 25 °C. We conclude that the loss in selectivity is the result of decreased ethanol adsorption by nanosilicalite particles and void formation at the particle/ polymer interface. In future work it is important to further understand how to disperse nanoparticles within polymeric thin

Table 3. Activation Energies for Ethanol and Water Permeation at Various Silicalite Loadings activation energy (kJ/mol) silicalite content (wt %) 0 10 20 30

ethanol 36.60 31.53 29.78 29.80

± ± ± ±

0.66 0.78 1.37 0.88

water 36.65 34.94 34.21 33.17

± ± ± ±

0.81 1.56 1.55 0.93

Table 3 shows the activation energies for permeation of ethanol and water through our PDMS/silicalite membranes at different zeolite loadings. From these data we see that the activation energies for ethanol and water are nearly identical in pure PDMS membranes. However, the activation energy for ethanol decreases considerably with silicalite addition. This means that less energy is required for ethanol molecules to transport across the nanocomposite membrane at the same conditions. This can 5211

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films and how to control the nature and quality of the interfacial contact between the polymer and the nanoparticles.

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(15) O’Brien-Abraham, J.; Kanezashi, M.; Lin, Y. S. A comparative study on permeation and mechanical properties of random and oriented MFI-type zeolite membranes. Microporous Mesoporous Mater. 2007, 105 (1−2), 140−148. (16) Yi, S.; Su, Y.; Wan, Y. Preparation and characterization of vinyltriethoxysilane (VTES) modified silicalite-1/PDMS hybrid pervaporation membrane and its application in ethanol separation from dilute aqueous solution. J. Membr. Sci. 2010, 360 (1−2), 341− 351. (17) Baker, R. W.; Wijmans, J. G.; Huang, Y. Permeability, permeance and selectivity: A preferred way of reporting pervaporation performance data. J. Membr. Sci. 2010, 348 (1−2), 346−352. (18) Wittig, R.; Lohmann, J.; Gmehling, J. Vapor−liquid equilibria by UNIFAC group contribution. 6. Revision and extension. Ind. Eng. Chem. Res. 2003, 42 (1), 183−188. (19) Smith, B. C. Infrared Spectral Interpretation: A Systematic Approach; CRC Press: Boca Raton, FL, 1999. (20) Larkin, P. In Infrared and Raman Spectroscopy: Principles and Spectral Interpretation; Elsevier: Boston, 2011. (21) Hennepe, H.; Smolders, C. A.; Bargeman, D.; Mulder, M. H. V. Exclusion and tortuosity effects for alcohol water separation by zeolitefilled PDMS membranes. Sep. Sci. Technol. 1991, 26 (4), 585−596. (22) Wijmans, J. G.; Baker, R. W. The solution-diffusion modelA review. J. Membr. Sci. 1995, 107 (1−2), 1−21. (23) Feng, X. S.; Huang, R. Y. M. Estimation of activation energy for permeation in pervaporation processes. J. Membr. Sci. 1996, 118 (1), 127−131. (24) Klein, S. M.; Abraham, W. H. Adsorption of ethanol and water vapors by silicalite. AIChE Symp. Ser. 1983, 79 (230), 53. (25) Barrer, R. M.; James, S. D. Electrochemistry of crystal-polymer membranes. 1. Resistance measurements. J. Phys. Chem. 1960, 64 (4), 417−421. (26) Lue, S. J.; Chien, C.-F.; Mahesh, K. P. O. Pervaporative concentration of ethanol−water mixtures using heterogeneous polydimethylsiloxane (PDMS) mixed matrix membranes. J. Membr. Sci. 2011, 384 (1−2), 17−26.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 1+(480) 727-8613. Fax: 1+(480) 727-9321. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS SEM imaging was performed at the Arizona State University LeRoy Eyring Center for Solid State Science. REFERENCES

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