MONTE-CARLO AND POISSON-BOLTZMANN STUDY OF ELECTROLYTE EXCLUSION FROM CHARGED CYLINDRICAL MICROPORES

The exclusion of model electrolytes from charged cylindrical capillaries is studied using the grand canonical Monte Carlo method and the Poisson-Boltzmann equation. Concentration profiles inside the capillary, Donnan exclusion coefficients, and particle number fluctuations were evaluated for 2:1 (divalent co-ions, monovalent counterions) electrolytes and 1:2 electrolytes (monovalent co-ions, divalent counterions) in the restrictive primitive model approximation for a range of electrolyte concentrations and surface charge densities. The Poisson-Boltzmann equation appears to be a good approximation for 2:1 electrolytes (monovalent counterions) but breaks down completely for 1:2 (divalent counterions) electrolytes in the capillary. The Donnan exclusion coefficient GAMMA is not an increasing function of the surface charge density, as predicted by the Poisson-Boltzmann equation; rather, it passes through a maximum and decreases with further increase in the surface charge. The same behavior has been observed previously for 2:2 electrolytes. The concentration fluctuations in the capillary, studied for the first time in this work, are much smaller than in the bulk electrolyte solution due to the strong correlations in the electrical double layer. As expected, the concentration fluctuations decrease by increasing the charge on the surface. Finally, a more realistic electrolyte model which fits bulk properties of lithium and cesium chloride solutions very well has also been studied as a function of the surface charge in the capillary. The differences in the electrolyte exclusion between lithium and cesium salt can be explained in view of the short-range forces due to the restructing of water molecules around the ions.