Enhancement of the Ion-Transport Selectivity of Layered

Chem. Mater. 2001, 13, 2641-2648

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Enhancement of the Ion-Transport Selectivity of Layered Polyelectrolyte Membranes through Cross-Linking and Hybridization Jacqueline L. Stair, Jeremy J. Harris, and Merlin L. Bruening* Department of Chemistry, Michigan State University, East Lansing, Michigan 48824 Received February 9, 2001. Revised Manuscript Received June 4, 2001

Layer-by-layer deposition of polyelectrolytes on porous supports is a convenient method for producing ultrathin membranes with moderate ion-transport selectivities. This study shows that the capping of poly(styrene sulfonate) (PSS)/poly(allylamine hydrochloride) (PAH) membranes with a few layer pairs of poly(acrylic acid) (PAA)/PAH results in dramatic increases in anion-transport selectivity. Deposition of less than eight total layer pairs yields reliable Cl-/SO42- and Cl-/Fe(CN)63- selectivities up to 150 and 3000, respectively. Fieldemission scanning electron microscope images show that the surface structure of PAA/PAH is altered by deposition on PSS/PAH, and we surmise that the PAA/PAH on these hybrid films is less strongly hydrated than the surface of pure PAA/PAH membranes. This would result in a higher surface charge density and more Donnan exclusion of multiply charged ions. Termination of membranes with PAH rather than PAA greatly reduces (in some cases reverses) Cl-/SO42- selectivities, demonstrating that selectivity is indeed largely due to Donnan exclusion at the membrane surface. Cross-linking of pure PAA/PAH membranes through heat-induced amidation can increase Cl-/SO42- selectivity as much as 4-fold. Because of their minimal thicknesses, hybrid polyelectrolyte films can be selective without greatly hindering the flux of monovalent ions. Thus, these films are attractive for applications such as water softening.

Introduction Alternating deposition of polycations and polyanions on porous supports is an extremely versatile method for synthesizing thin separation membranes.1-3 However, the gas or ion selectivity of these membranes is usually modest unless many layers are deposited on the support.4-11 In this paper, we show that deposition of one to three capping bilayers12 of poly(acrylic acid) (PAA)/poly(allylamine hydrochloride) (PAH) on top of five bilayers of poly(styrene sulfonate) (PSS)/PAH yields remarkable (up to 30-fold) increases in anion-transport selectivities. Cross-linking of PAA/PAH films also en* Author to whom correspondence should be addressed. E-mail: [email protected]. Phone: (517) 355-9715 ext. 237. Fax: (517) 353-1793. (1) Decher, G.; Hong, J. D. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1430. (2) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210/ 211, 831. (3) Decher, G. Science 1997, 277, 1232. (4) Stroeve, P.; Vasquez, V.; Coelho, M. A. N.; Rabolt, J. F. Thin Solid Films 1996, 284-285, 708. (5) Leva¨salmi, J.-M.; McCarthy, T. J. Macromolecules 1997, 30, 1752. (6) Krasemann, L.; Tieke, B. Langmuir 2000, 16, 287. (7) Krasemann, L.; Tieke, B. Mater. Sci. Eng. C 1999, 8-9, 513. (8) Krasemann, L.; Tieke, B. J. Membr. Sci. 1998, 150, 23. (9) van Ackern, F.; Krasemann, L.; Tieke, B. Thin Solid Films 1998, 327-329, 762. (10) Quinn, R.; Laciak, D. V. J. Membr. Sci. 1997, 131, 49. (11) Zhou, P.; Samuelson, L.; Alva, K. S.; Chen, C.-C.; Blumstein, R. B.; Blumstein, A. Macromolecules 1997, 30, 1577. (12) There is substantial interpenetration between alternating layers of polyelectrolytes; thus, the terms “bilayer” and “layer” are used loosely. (See refs 3 and 35.)

hances transport selectivity. The high monovalent/ divalent ion selectivity of these membranes makes them attractive candidates for water purification. Separation of monovalent and divalent ions in solution is an important part of applications such as water softening,13,14 purification of groundwater,15-19 and production of edible and raw salt from seawater.20 For example, reductions in SO42- concentrations allow for human consumption of salt produced from seawater and prevent scaling of membranes and seawater reservoirs through precipitation of compounds such as CaSO4.20,21 Although membranes are available for such applications, increased ion selectivity will make these processes much more effective. Many ion-separation membranes consist of ionexchange materials that exhibit moderate selectivities among either cations or anions.22-27 Specific modifica(13) Brett, S. W.; Gaterell, M. R.; Morse, G. K.; Lester, J. N. Environ. Technol. 1999, 20, 1009. (14) Mika, A. M.; Childs, R. F.; Dickson, J. M. Desalination 1999, 121, 149. (15) Bader, M. S. H. J. Hazard. Mater. 2000, B73, 269. (16) Amor, Z.; Malki, S.; Taky, M.; Bariou, B.; Mameri, N.; Elmidaoui, A. Desalination 1998, 120, 263. (17) Hell, F.; Lahnsteiner, J.; Frischherz, H.; Baumgartner, G. Desalination 1998, 117, 173. (18) Sata, T.; Yamaguchi, T.; Matsusaki, K. J. Chem. Soc., Chem. Commun. 1995, 1153. (19) Oldani, M.; Killer, E.; Miquel, A.; Schock, G. J. Membr. Sci. 1992, 75, 265. (20) Saracco, G.; Zanetti, M. C. Ind. Eng. Chem. Res. 1994, 33, 96. (21) Sata, T.; Mine, K.; Higa, M. J. Membr. Sci. 1998, 141, 137. (22) Sata, T. J. Membr. Sci. 2000, 167, 1. (23) Mizutani, Y. J. Membr. Sci. 1990, 54, 233.

10.1021/cm010166e CCC: $20.00 © 2001 American Chemical Society Published on Web 07/11/2001

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tions of these membranes designed to increase selectivity include deposition of a highly charged surface layer,28,29 cross-linking of the membranes’ interiors,30 and incorporation of hydrophobic/hydrophilic groups within the film and on the surface.25,31,32 Selective transport through these films is based on anion size, charge, hydration energy, and specific chemical interactions with the membranes. Layered polyelectrolyte films are especially attractive membrane materials because of their ease of deposition and wide variability. The minimal thickness of these films should also allow for high fluxes. Deposition of layered polyelectrolyte films, first demonstrated by Decher and co-workers,1 simply involves the alternating adsorption of polycations and polyanions to form multilayer assemblies held together by electrostatic interactions. Recently, Schlenoff and co-workers showed that this process can be performed by sequential spraying of polyelectrolytes onto desired substrates.33 Film buildup occurs by charge overcompensation,34-36 which provides coverage of defects and inhomogenities in previous layers.3,37,38 This mechanism also allows for the formation of composite membranes through the coverage of porous supports with defect-free films.39 Significantly, polyelectrolyte deposition occurs from aqueous solutions, rendering this procedure more environmentally friendly than most film-formation techniques. The versatility of alternating polyelectrolyte deposition is evident from the large number of substrates and polyelectrolytes that can be used to form these films.40-47 Synthesis can occur on most charged or polar43,48 (24) Sata, T.; Kawamura, K. J. Membr. Sci. 2000, 171, 97. (25) Eyal, A.; Kedem, O. J. Membr. Sci. 1988, 38, 101. (26) Sata, T. J. Membr. Sci. 1994, 93, 117. (27) Andre´s, L. J.; Riera, F. A.; Alvarez, R.; Audinos, R. Can. J. Chem. Eng. 1994, 72, 848. (28) Takata, K.; Yamamoto, Y.; Sata, T. J. Membr. Sci. 2000, 179, 101. (29) Takata, K.; Yamamoto, Y.; Sata, T. Bull. Chem. Soc. Jpn. 1996, 69, 797. (30) Sata, T.; Nojima, S. J. Polym. Sci. B: Polym. Phys. 1999, 37, 1773. (31) Sata, T.; Shimokawa, Y.; Matsusaki, K. J. Membr. Sci. 2000, 171, 31. (32) Sata, T.; Yamaguchi, T.; Matsusaki, K. J. Phys. Chem. 1996, 100, 16633. (33) Schlenoff, J. B.; Dubas, S. T.; Farhat, T. R. Langmuir 2000, 16, 9968. (34) Caruso, F.; Donath, E.; Mo¨hwald, H. J. Phys. Chem. B 1998, 102, 2011. (35) Schlenoff, J. B.; Ly, H.; Li, M. J. Am. Chem. Soc. 1998, 120, 7626. (36) Joanny, J. F. Eur. Phys. J. B 1999, 9, 117. (37) Ladam, G.; Schaad, P.; Voegel, J. C.; Schaaf, P.; Decher, G.; Cuisiner, F. Langmuir 2000, 16, 1249. (38) Ramsden, J. J. Thin Solid Films 1995, 254, 246. (39) Harris, J. J.; Stair, J. L.; Bruening, M. L. Chem. Mater. 2000, 12, 1941. (40) von Klitzing, R.; Mo¨hwald, H. Thin Solid Films 1996, 284285, 352. (41) Han, S.; Lindholm-Sethson, B. Electrochim. Acta 1999, 45, 845. (42) Ichinose, I.; Mizuki, S.; Ohno, S.; Shiraishi, H.; Kunitake, T. Polym. J. 1999, 31, 1065. (43) Delcorte, A.; Bertrand, P.; Wischerhoff, E.; Laschewsky, A. Langmuir 1997, 13, 5125. (44) Mo¨hwald, H. Colloids Surf. A: Physicochem. Eng. Aspects 2000, 171, 25. (45) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117. (46) Russell, R. J.; Sirkar, K.; Pishko, M. V. Langmuir 2000, 16, 4052. (47) Caruso, F.; Niikura, K.; Furlong, D. N.; Okahata, Y. Langmuir 1997, 13, 3422. (48) Laschewsky, A.; Wischerhoff, E.; Kauranen, M.; Persoons, A. Macromolecules 1997, 30, 8304.

Stair et al.

substrates, as well as some polymer substrates,4,43,49-51 and is independent of substrate size and topology.1,2 Materials capable of forming layered polyelectrolyte films include dendrimers,52-54 proteins,45,55-59 alumosilicates,60,61 nucleicacids,62-64 conductingpolymers,55,65-67 semiconductor nanoparticles,68 dyes,40,48 light-emitting molecules,69 and inorganic ions.70 This variety of materials should afford control over selective permeation through these films. Additionally, alteration of deposition parameters (i.e., pH,71-75 polymer concentration,76 number of bilayers,77 supporting salt concentration,37,71,78-82 and drying procedures37,83) allows for the tailoring of film characteristics such as thickness, density, charge density, and pore size. One means of utilizing polyelectrolytes for membrane modification is deposition of a single layer of polyelectrolyte to modify an existing membrane.28,84-86 Tsuru (49) Phuvanartnuruks, V.; McCarthy, T. J. Macromolecules 1998, 31, 1, 1906. (50) Hsieh, M. C.; Farris, R. J.; McCarthy, T. J. Macromolecules 1997, 30, 8453. (51) Fadeev, A. Y.; McCarthy, T. J. Langmuir 1998, 14, 5586. (52) Watanabe, S.; Regen, S. L. J. Am. Chem. Soc. 1994, 116, 8855. (53) Tsukruk, V. V.; Rinderspacher, F.; Bliznyuk, V. N. Langmuir 1997, 13, 2171. (54) Tsukruk, V. V. Adv. Mater. 1998, 10, 253. (55) Kotov, N. A.; Haraszti, T.; Turi, L.; Zavala, G.; Geer, R.; De´ka´ny, I.; Fendler, J. H. J. Am. Chem. Soc. 1997, 119, 6821. (56) Lvov, Y.; Haas, H.; Decher, G.; Mo¨hwald, H.; Mikhailov, A.; Mtchedlishvily, B.; Morgunova, E.; Vainstein, B. Langmuir 1994, 10, 4232. (57) Onda, M.; Lvov, Y.; Ariga, K.; Kunitake, T. Biotechnol. Bioeng. 1996, 51. (58) Hodak, J.; Etchenique, R.; Calvo, E.; Singhal, K.; Bartlett, P. N. Langmuir 1997, 13, 2708. (59) Caruso, F.; Niikura, K.; Furlong, N.; Okahata, Y. Langmuir 1997, 13, 3427. (60) Kotov, N. A.; Magonov, S.; Tropsha, E. Chem. Mater. 1998, 10, 886. (61) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. Langmuir 1996, 12, 3038. (62) Montrel, M. M.; Sukhorukov, G. B.; Shabarchina, L. I.; Apolonnik, N. V.; Sukhorukov, B. I. Mater. Sci. Eng. C 1998, 5, 275. (63) Lvov, Y. M.; Zhongqing, L.; Schenkman, J. B.; Xiaolin, Z.; Rusling, J. F. J. Am. Chem. Soc. 1998, 120, 4073. (64) Lvov, Y.; Decher, G.; Sukhorukov, G. Macromolecules 1993, 26, 5396. (65) Fou, A. C.; Rubner, M. F. Macromolecules 1995, 28, 7115. (66) Cheung, J. H.; Fou, A. F.; Rubner, M. F. Thin Solid Films 1994, 244, 985. (67) Ferreira, M.; Cheung, J. H.; Rubner, M. F. Thin Solid Films 1994, 244, 806. (68) Kotov, N. A.; De´ka´ny, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065. (69) Wu, A.; Yoo, D.; Lee, J.-K.; Rubner, M. F. J. Am. Chem. Soc. 1999, 121, 4883. (70) Joly, S.; Kane, R.; Radzilowski, L.; Wang, T.; Wu, A.; Cohen, R. E.; Thomas, E. L.; Rubner, M. F. Langmuir 2000, 16, 1354. (71) Harris, J. J.; Bruening, M. L. Langmuir 2000, 16, 2006. (72) Mendelsohn, J. D.; Barrett, C. J.; Chan, V. V.; Pal, A. J.; Mayes, A. M.; Rubner, M. F. Langmuir 2000, 16, 5017. (73) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 4213. (74) Yoo, D.; Shiratori, S. S.; Rubner, M. F. Macromolecules 1998, 31, 4309. (75) Blaakmeer, J.; Bo¨hmer, M. R.; Stuart, C. M. A.; Fleer, G. J. Macromolecules 1990, 23, 2301. (76) Ferreira, M.; Rubner, M. F. Macromolecules 1995, 28, 7107. (77) Lvov, Y.; Decher, G.; Mo¨hwald, H. Langmuir 1993, 9, 481. (78) Bo¨hmer, M. R.; Evers, O. A.; Scheutjens, J. M. H. M. Macromolecules 1990, 23, 2288. (79) Kolarik, L.; Furlong, D. N.; Joy, H.; Struijk, C.; Rowe, R. Langmuir 1999, 15, 8265. (80) Ho, P. K. H.; Friend, R. H. Synth. Met. 1999, 102, 1021. (81) Steitz, R.; Leiner, V.; Siebrecht, R.; v. Klitzing, R. Colloids Surf. A: Physicochem. Eng. Aspects 2000, 163, 63. (82) Lo¨sche, M.; Schmitt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K. Macromolecules 1998, 31, 8893. (83) Decher, G.; Lvov, Y.; Schmitt, J. Thin Solid Films 1994, 244, 772. (84) Tsuru, T.; Nakao, S.-i.; Kimura, S. J. Membr. Sci. 1995, 108, 269.

Ion-Transport Selectivity of Polyelectrolyte Membranes

and co-workers showed that one polyelectrolyte layer deposited on an oppositely charged commercial reverse osmosis membrane provides selective transport of monovalent ions over divalent ions.84-86 Deposition of multilayer polyelectrolyte films on porous supports differs from this procedure in that it forms a defect-free “skin” on a highly permeable support. The use of layered polyelectrolyte films also provides additional versatility in membrane formation, as mentioned above. Two recent studies showed that membranes consisting of polyelectrolyte multilayers exhibit monovalent/divalent ion-transport selectivities.6,39 This paper focuses on enhancing the ion-transport selectivity of layered polyelectrolyte membranes. Deposition of hybrid PSS/PAH/PAA membranes combined with cross-linking of PAA/PAH films yields average Cl-/ SO42- selectivities as high as 360. Additionally, we utilized field-emission scanning electron microscopy (FESEM) and Fourier transform infrared external reflection spectroscopy (FTIR-ERS) to qualitatively investigate the chemistry behind these selectivity enhancements. Experimental Section Materials. Poly(allylamine hydrochloride) (Aldrich, Mw ) 70 000), poly(sodium 4-styrenesulfonate) (Aldrich, Mw ) 70 000), poly(acrylic acid) (Alfa Aesar, Mw ) 90 000, 25 wt % solution), MnCl2 (Acros), NaBr (Aldrich), NaCl (Spectrum), KCl (Baker), MgCl2 (Spectrum), K2SO4 (CCI), K2Ni(CN)4 (Aldrich), and K3Fe(CN)6 (Mallinckrodt) were used as received. Porous alumina membrane filters (Whatman Anodisc, 0.02 µm surface pore diameter) were used as received except for cases where films were cross-linked by heating. In those cases, the polymeric support ring surrounding the alumina substrate was removed by heating at 400 °C for 18 h. This prevents the polymer ring from melting into the substrate pores during heat treatments. Martin and co-workers previously demonstrated the utility of porous alumina for forming and investigating ultrathin membranes.87-89 Silicon (100, p-type) wafers were sputter coated with aluminum (∼200 nm) and used for ellipsometry measurements and reflectance FTIR spectra. Synthesis of Polyelectrolyte Films. Prior to film deposition, aluminum-coated silicon wafers and porous alumina supports were UV/O3 cleaned for 15 min (Boekel UV-Clean model 135500). Porous alumina supports were then placed in an O-ring holder that exposed only the top of the substrate to solutions in an effort to restrict deposition to the filtrate side of the alumina. Polyelectrolyte deposition began by immersion of the positively charged Anodisc or aluminum-coated slide in a polyanion solution: PSS (0.02 M, pH 2.1, 0.5 M MnCl2) for 2 min or PAA (0.02 M, pH 4.5, 0.5 M NaCl) for 5 min. Polymer molarites are always given with respect to the repeating unit. The surface was rinsed for 1 min with deionized water (MilliQ, 18.2 MΩ cm) after deposition of each polyelectrolyte layer. After being rinsed, the surface was immersed in a solution of PAH (0.02 M) for 5 min. Two solutions of PAH (pH 2.3, 0.5 M NaBr and pH 4.5, 0.5 M NaCl) were prepared. The former PAH solution was used for PSS/PAH films, while the latter was used for PAA/PAH films. Both solutions were used for hybrid films containing PSS, PAH, and PAA. Hybrid films were synthesized through deposition of a 5-bilayer PSS/PAH precursor film followed by deposition of a PAA layer or PAA/PAH bilayers. (85) Tsuru, T.; Urairi, M.; Nakao, S.; Kimura, S. Desalination 1991, 81, 219. (86) Urairi, M.; Tsuru, T.; Nakao, S.; Kimura, S. J. Membr. Sci. 1992, 70, 153. (87) Chen, W. J.; Aranda, P.; Martin, C. R. J. Membr. Sci. 1995, 107, 199. (88) Chen, W. J.; Martin, C. R. J. Membr. Sci. 1995, 104, 101. (89) Liu, C.; Martin, C. R. Nature 1991, 352, 50.

Chem. Mater., Vol. 13, No. 8, 2001 2643 The PAH solution adjusted to pH 2.3 was used at the interface between PSS and PAA layers unless otherwise stated. After deposition of all layers, each film was dried with N2 and stored at room temperature. After film deposition and drying, membranes and wafers to be cross-linked were placed in a flask that was then purged with N2 for 15 min. The flask was slowly heated to the desired temperature (approximately 45-min ramping time) and then heated further for 2 h. All heating was done under a small flow of N2. Film Characterization. Ellipsometry measurements on aluminum-coated silicon wafers were made using a rotating analyzer ellipsometer (model M-44, J. A. Woollam) and WVASE32 software.71 At least three separate areas on each sample were measured. Reflectance FTIR spectra of polyelectrolyte films on aluminum-coated silicon wafers were obtained using a Nicolet Magna-IR 560 spectrometer. A Hitachi S-4700 FESEM instrument (accelerating voltage of 6-8 kV) was used to image polyelectrolyte films deposited on porous alumina membranes, and samples were sputter-coated with ∼5 nm of Au prior to imaging. Ion-Permeability Studies. The permeability apparatus consists of two glass cells (volumes of 90 mL) connected by an O-ring in the middle of a 2.5-cm-long neck. The membrane separates the feed and permeate cells of the apparatus with an exposed area of 1.7-2.0 cm2. The polyelectrolyte films were exposed to the feed cell, which contained 0.1 F salt solutions of KCl, K2SO4, K2Ni(CN)4, K3Fe(CN)6, NaCl, MgCl2, or KCl and K2Ni(CN)4. The receiving cell was filled with deionized water, and the ion concentration in this cell was monitored using a conductivity meter (Orion model 115). After an initial conductivity reading of the receiving cell, both sides were vigorously stirred, and the permeate conductivity was recorded every 10 minutes for 90 min. Normalized conductivities were calculated by dividing the receiving-phase conductivities by the final source-phase conductivity (dividing by the final source-phase conductivity rather than the initial source-phase conductivity changes the normalized conductivity value by less than 10%). Permeability experiments using K2SO4, K2Ni(CN)4, MgCl2, K3Fe(CN)6, and KCl and K2Ni(CN)4 through hybrid films showed an average transfer of about 7, 8, 5, 10, and 5 mL of water, respectively, from the receiving phase to the source phase over the 90-min period. Between permeability experiments with different ions, both sides of the apparatus were rinsed and filled with deionized water. The cells were allowed to equilibrate with deionized water for an additional 20 min and then rinsed once more. Most membrane permeability experiments were run in the order KCl, K2SO4, K3Fe(CN)6, and KCl. Only those results from the first KCl run are reported. The second KCl run was performed to test the integrity of the film. Most films show a change of less than 15% in Clflux between the first and second trials. However, hybrid membranes with more than 1.5 bilayers of PAA/PAH showed 25-50% decreases in the second Cl- flux. A decrease in Clflux39 might be due to Fe(CN)63- binding to the films, as we observed little decrease (