ION PARTITIONING BETWEEN CHARGED MICROPORES AND BULK ELECTROLYTE SOLUTION - MIXTURES OF MONOVALENT AND DIVALENT COUNTERIONS AND MONOVALENT CO-IONS

The exclusion of mixed electrolytes from charged and uncharged cylindr ical capillaries is studied using the grand canonical Monte Carlo meth od and the Poisson-Boltzmann equation. Concentration profiles inside t he capillary, the Donnan exclusion coefficient, the separation factor, and concentration fluctuations were evaluated for a system with a mix ture of divalent and monovalent counterions and monovalent co-ions. Th e electrolyte is treated in the restrictive primitive model approximat ion. All these quantities are studied as a function of the composition of the bulk external electrolyte. The calculations apply to a range o f surface charge densities and to two values of the total ionic streng th. The simulation results are compared with the predictions of the Po isson-Boltzmann approximation and the ideal Donnan theory. Both approx imate theories grossly overestimate the ability of a micropore to excl ude electrolytes. For higher concentrations of divalent counterions pr esent in the system the Poisson-Boltzmann theory yields incorrect resu lts. We present a comparison with the experimental data for charged me mbranes immersed in a mixed KCl/CaCl2 solution.