Analysis of the Pressure-Induced Potential Arising through Composite Membranes with Selective Surface Layers Anthony Szymczyk,* Mohammed Sbaı ¨, and Patrick Fievet Laboratoire de Chimie des Mate´ riaux et Interfaces, Universite´ de Franche-Comte´ , 25030 Besanc¸ on Cedex, France Received June 29, 2004. In Final Form: October 22, 2004 When a pressure gradient is applied through a charged selective membrane, the transmembrane electrical potential difference, called the filtration potential, results from both the applied pressure and induced concentration difference across the membrane. In this work we investigate the electrokinetic properties relative to both active and support layers of a composite ceramic membrane close to the nanofiltration range. First, the volume charge density of the active layer is obtained by fitting a transport model to experimental rejection rates (which are controlled by the active layer only). Next, the value of the volume charge density is used to compute the theoretical filtration potential through the active layer. For sufficiently high permeate volume fluxes, the concentration difference across the active layer becomes constant, which allows assessing the membrane potential of the active layer. Experimental measurements of the overall filtration potential arising through the whole membrane are performed. The contribution of the support layer to this overall filtration potential is put in evidence. That implies that the membrane potential of the active layer cannot be deduced directly from the overall filtration potential measurements. Finally, the contribution of the support layer is singled out by subtracting the theoretical filtration potential of the active layer from the experimental filtration potential measured across the whole membrane (i.e., support + active layers). The amphoteric behavior of both layers is put in evidence, which is confirmed by electrophoretic measurements carried out with the powdered support layer and by recently reported tangential streaming potential measurements. 1. Introduction It is now well established that filtration membranes cannot be viewed simply as sieves rejecting solutes according to a purely steric exclusion mechanism. Indeed, membranes used in nanofiltration (NF) and low ultra- filtration (UF) processes possess active layers with pores ranging from ∼1 to a few nanometers in diameter. The combination of such narrow pores with electrically charged materials leads to a strong overlap of electrical double layers inside pores. This latter is responsible for the so-called Donnan exclusion that is connected with the development of an interfacial electrostatic potential bar- rier pumping counterions through the membrane pores while repulsing coions. The free energy of ion transfer from bulk solution into pores of nanometric dimensions is likely to be affected also by structural changes of water in a confined medium and by the interaction of ions with the polarization charges that are produced at the interface between media char- acterized by different dielectric constants (i.e., the mem- brane matrix and the solution filling pores). 1 This latter phenomenon is usually described as the production of image forces since the interaction between an ion and the polarized interface is formally equivalent to the interaction with a fictitious image charge located at the other side of the interface at the same distance from it as the ion. 2 This dielectric effect is strongly affected by the presence of a fixed charge on the pore walls, which screens the interac- tion between the ion and the polarized surface. 2,3 It is then of great interest for a better understanding of membrane performance to characterize the electroki- netic properties of active (i.e., selective) layers. Among usual techniques, the transmembrane streaming potential measurement has become, thanks to its experimental simplicity, the most commonly used tool for assessing the electrokinetic properties of porous membranes. The transmembrane (or transversal) streaming potential is defined as the pressure-induced electrical potential dif- ference arising between pore ends under zero electrical current condition and no concentration difference across the membrane. This attractive technique has been used to assess the electrokinetic properties of a broad range of porous membranes 4-7 even in the case of membranes with high ionic retention properties. 8-10 However, in the case of membranes with selective layers, the streaming po- tential is no more the only pressure-induced component of the overall transmembrane electrical potential differ- ence and the term filtration potential should be used instead. 11 Since such membranes have usually a multi- layer structure, the porous sublayers are likely to con- tribute to the overall pressure-induced potential. 12,13 That * Corresponding author. Tel: 33.3.81.66.20.32. Fax: 33.3.81.66.20.33. E-mail: anthony.szymczyk@univ-fcomte.fr. (1) Dukhin, S. S.; Churaev, N. V.; Shilov, V. N.; Starov, V. M. Russ. Chem. Rev. 1988, 57, 572. (2) Yaroshchuk, A. E. Adv. Colloid Interface Sci. 2000, 85, 193. (3) Yaroshchuk, A. E. Sep. Pur. Technol. 2001, 22-23, 143. (4) Nystro¨m, M.; Lindstro¨m, M.; Matthiason, E. Colloids Surf. A 1989, 36, 297. (5) Nystro¨m, M.; Pihlajama¨ ki, A.; Ehsani, N. J. Membr. Sci. 1994, 87, 245. (6) Pontie´ , M.; Chasseray, X.; Lemordant, D.; Laine, J. M. J. Membr. Sci. 1997, 129, 125. (7) Szymczyk, A.; Fievet, P.; Aoubiza, B.; Simon, C.; Pagetti, J. J. Membr. Sci. 1999, 161, 275. (8) Huisman, I. H.; Pradanos, P.; Hernandez, A. J. Membr. Sci. 2000, 178, 55. (9) Elmarraki, Y.; Persin, M.; Sarrazin, J.; Cretin, M.; Larbot, A. Sep. Pur. Technol. 2001, 25, 493. (10) Condon, S.; Cretin, M.; Persin, M.; Sarrazin, J.; Larbot, A. Desalination 2002, 149, 447. (11) Yaroshchuk, A. E.; Boiko, Y. P.; Makovetskiy, A. L. Langmuir 2002, 18, 5154. 1818 Langmuir 2005, 21, 1818-1826 10.1021/la048399i CCC: $30.25 © 2005 American Chemical Society Published on Web 02/02/2005