JOURNAL OF COLLOID AND INTERFACE SCIENCE 181, 399–412 (1996) ARTICLE NO. 0397 Charge Adsorption and Zeta Potential in Cyclopore Membranes J. I. CALVO, A. HERNA ´ NDEZ, 1 P. PRA ´ DANOS, AND F. TEJERINA Departamento de Termodina ´mica y Fı B sica Aplicada, Facultad de Ciencias, Universidad de Valladolid, 47071 Valladolid, Spain Received July 7, 1995; accepted February 26, 1996 charge when they are left in contact with a polar medium. This The surface electrochemistry of several microporous membranes leads to the formation of an ‘‘electrical double layer’’ (EDL) is studied. Concretely, we deal here with six types of track-etched that restores the electroneutrality in the solution. microporous filters from Cyclopore whose surface charges are ob- Among the parameters that characterize this EDL, one of tained, within the framework of the so-called microcapillary pore the most relevant is the electrostatic potential at the no-slipping model, from typical streaming potential experiments when bathed plane, usually termed ‘‘electrokinetic’’ or ‘‘zeta potential.’’ in diluted LiCl solutions. The dependence of the surface charge The importance of zeta potential in describing and quantifying densities and the zeta potentials on the bulk concentration is stud- particle – particle and particle – surface interactions is increas- ied by assuming that an adsorption process of anions is the cause ingly recognized. A relevant example can be taken from the of these charges. The adsorption isotherm data are treated ac- cording to two adsorption mechanisms, a heterogeneous Freund- strong dependence of protein fouling in microfiltration pro- lich adsorption followed by a homogeneous Langmuir adsorption cesses with the zeta potential of the surface and the charge of for higher bulk concentrations. Both stages allow us to obtain the protein (3, 4). The zeta potentials of porous materials are important parameters concerning the membrane material:the mo- usually evaluated from electrokinetic experiments, as elec- lar free energy of specific adsorption, the maximum number of troosmosis and streaming potential, applying the well-known sites accessible to anions, and the actual surface charge density at Smoluchowski equation (5, 6). Nevertheless, this corresponds the solid surface (or proper surface charge density). All these to a simplification of the complete picture underlying the EDL parameters are quite similar for all the membranes analyzed, show- and the definition of the zeta potential. A more accurate evalua- ing significative differences depending on the adsorption step; in tion of that parameter relies on solving the Poisson– Boltzmann particular, feweradsorption sites and higheradsorption free energ- equation for the electric potential profile into the pores (7). ies appear for high concentrations while the proper charge is posi- Moreover, the influence of both the adsorbed charges and those tive but low and acts only forvery low bulk concentrations. This two-step adsorption model gives a very approximate prediction of previously existing at the membrane surface on the characteris- actual zeta potentials as well.  1996 Academic Press, Inc. tics of the EDL and, consequently, the values of the zeta poten- Key Words: microporous membranes;electrokinetic phenomena; tial has been frequently missing. charge adsorption; streaming potential; zeta potential. In effect, if we suppose that membrane walls have a proper charge given by a surface charge density s 0 , some of the ions present in the solution are prone to interact strongly INTRODUCTION with the pore surface charges by an adsorption process. So they can be considered afterward as being fixed on it. Conse- Membranes cannot be understood simply as sieves (1), quently, if s 0 is low it should be effective, in relation with rejecting solute particles greater than the mean pore size. electrokinetic phenomena, only for low surface densities of Complex solution and membrane interactions (such as sol- adsorbed counterions (i.e., ions whose charge sign is oppo- ute–membrane, solute–solvent, and solute–solute relations site to that of the pore surface ) . In this case the electrokinetic (2)) play an important role in the effectiveness of the actual behavior of the membrane could experience a radical qualita- membrane separation process. tive change depending on the extension of adsorption, i.e., In particular, the interaction between a charged membrane on the concentration of the electrolytical solution. and a solution containing some polyelectrolytes (as usually Among the usual electrokinetic parameters, it has been found in biotechnological processes) , which is pushed through proved that the streaming potential is particularly sensitive the membrane pores due to the application of a pressure or to concentration changes. The streaming potential is the ratio electric potential gradient, causes some complex processes of the measured electric potential drop over the applied pres- commonly grouped under the so-called electrokinetic phenom- sure difference, in zero current conditions: ena. Membranes, like most materials, acquire an electrical n p Å  Df Dp  IÅ0 . [1] 1 To whom correspondence should be addressed. 399 0021-9797/96 $18.00 Copyright  1996 by Academic Press, Inc. All rights of reproduction in any form reserved. AID JCIS 4277 / 6g12$$$361 06-26-96 08:14:45 coidal AP: Colloid