Theoretical study of the electrokinetic and electrochemical behaviors of two-layer composite membranes

A theoretical study concerning the effect of structure (porosity, pore radi us and layer thickness) and surface characteristics (zeta potential) of two -layer composite membranes on global streaming potential (SPg), membrane po tential (Em(g)) and membrane conductivity (lambda (g)) is presented. To thi s end, each layer of the composite membrane (composed of a support layer an d a filtering layer) was modeled as a bundle of identical capillary tubes w ith connections between pores of the two layers (the pores in the filtering layer being smaller than those of the support layer). The global parameter s SPg, Em(g) and lambda (g) were calculated by using the theory of thermody namics of irreversible processes and a space charge model. SPg, Em(g) and l ambda (g) were expressed as a function of the individual parameters of each layer SP(i), Em((i)) and lambda ((i)), respectively, the length fraction o f the support layer, the porosity and pore radius ratios, it was shown that the electrokinetic (streaming potential and membrane conductivity) and ele ctrochemical (membrane potential) behaviors of such composite membranes var y between that of single layers. For streaming potential, the results indic ate that the contribution of the filtering layer to the global streaming po tential is very little influenced by zeta potentials of both types of pores . It appears that the individual streaming potential of the filtering layer greatly dominates the global streaming potential. This is due to the fact that the streaming potential of the filtering layer is weighted by the pore radius ratio which is a predominant parameter in determining the global st reaming potential. In contrast to the streaming potential. the contribution of the filtering layer to the global membrane potential (Emg) or membrane conductivity (lambda (g)) depends more or less on the zeta potentials of bo th kinds of pores and the corresponding electrokinetic radii as well. As to the membrane potential, the contribution of the filtering layer to Emp is all the more sensitive to the zeta potentials than the electrokinetic radii are small. The filtering layer greatly dominates the global membrane poten tial when its pores are narrow (with regard to the Debye length) and strong ly charged. For the electrolyte conductivity inside pores, the smaller pore s (inside the filtering layer) have an effect all the more dominant on the apparent membrane conductivity than their zeta potential is low and that of larger pores (inside the support layer) is high. (C) 2001 Elsevier Science B.V. All rights reserved.