Modeling of coupled transport of ions and zwitterions across porous ion exchange membranes

Abstract Mathematical models are derived to describe the active and facilitated transport of physiologically active zwitterions (proteins and amino acids) across ion exchange membranes possessing carboxylic groups. These phenomena have been observed in cells with membranes separating acidic and alkaline solutions. Literature findings are used to show the great diversity of physical mechanisms supporting interactions between the 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 and dissociation of carboxylic groups, recharging of the transported zwitterions, electromigration (electrophoresis) of solutes in the electric field arising from ion diffusion and, finally, diffusiophoresis of proteins regarded as weakly charged spheres with radii comparable with their Debye screening length. Important contributions of carboxylic groups to amino acid active transport and of diffusiophoresis and surface diffusion to protein facilitated transport are shown. It is also found that chemical reactions produce a thin transient layer in the membrane 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 transport and improving its selectivity by varying the pH values of the solutions.