MODELING OF COUPLED TRANSPORT OF IONS AND ZWITTERIONS ACROSS POROUS ION-EXCHANGE MEMBRANES

Mathematical models are derived to describe the active and facilitated transport of physiologically active zwitterions (proteins and amino a cids) across ion exchange membranes possessing carboxylic groups. Thes e phenomena have been observed in cells with membranes separating acid ic and alkaline solutions. Literature findings are used to show the gr eat diversity of physical mechanisms supporting interactions between t he solute fluxes through a membrane. The models take into account pore and surface diffusion of solutes, chemical reactions in the membrane, such as mutual neutralization of the acidic and alkaline solutions an d dissociation of carboxylic groups, recharging of the transported zwi tterions, electromigration (electrophoresis) of solutes in the electri c field arising from ion diffusion and, finally, diffusiophoresis of p roteins regarded as weakly charged spheres with radii comparable with their Debye screening length. Important contributions of carboxylic gr oups to amino acid active transport and of diffusiophoresis and surfac e diffusion to protein facilitated transport are shown. It is also fou nd that chemical reactions produce a thin transient layer in the membr ane which may show a sharp peak of electric field, in particular. The calculations are carried out for the limiting cases of ''wide'' and '' narrow'' pores. The analysis suggests ways of intensifying membrane tr ansport and improving its selectivity by varying the pH values of the solutions.