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 studparticle-particle and particle-surface interactions is increasied 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 according to two adsorption mechanisms, a heterogeneous Freund-strong dependence of protein fouling in microfiltration prolich 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 mousually evaluated from electrokinetic experiments, as eleclar 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, showand the definition of the zeta potential. A more accurate evaluaing significative differences depending on the adsorption step; in tion of that parameter relies on solving the Poisson-Boltzmann particular, fewer adsorption sites and higher adsorption free energequation 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 for very low bulk concentrations. This two-step adsorption model gives a very approximate prediction of previously existing at the membrane surface on the characterisactual zeta potentials as well.