Salt exclusion from charged and uncharged micropores

Abstract

The exclusion of electrolytes from charged and uncharged capillaries is studied using (a) the Poisson-Boltzmann equation and (b) Grand Canonical Monte Carlo computer simulations. The thermodynamics and structure are examined as a function of the charge fixed on the inner capillary surfaces. The results for 2:2 electrolytes indicate that the classical Poisson-Boltzmann equation breaks down under these conditions, and in fact its predictions are in serious disagreement with the simulation results for the same system over the whole range of surface charge densities. An additional interesting result of this study concerns the ability of microporous materials to exclude electrolytes. The salt exclusion first increases by increasing the charge density, passes through a maximum, and then decreases with further increase of the surface charge. This behavior (completely missed by the Poisson-Boltzmann approximation) can be explained by the influence of the interionic correlations on the distribution of electrolyte inside the capillary.