Role of ion hydration in the charge regulation at the surface of porous silica gel

Abstract

By using potentiometric titration of a precisely characterised porous silica gel sample in solutions of different electrolytes, an attempt has been made to detect a possible manifestation of the spatial inhomogeneity caused by electrostatic saturation in the hydration potential of ions in the vicinity of a charged interface. Experimentally, an appreciable difference between the titration curves for different background electrolytes is observed. To determine whether this can be attributed to the difference in the hydration potentials of different ions and to obtain a theoretical estimate of the magnitude of the effect expected, the titration curves were simulated using a charge-regulation model based on the non-linear Poisson–Boltzmann equation with dielectric saturation being either neglected or allowed for. The distributions of surface charge density and surface potential over pore sizes for different pH and ion strengths have been calculated. It has been found that, although ‘switching on’ the dielectric saturation results in a noticeable reduction in the equilibrium surface charge density, the summed effect is hardly observable by potentiometric titration because of a concomitant increase in the amount of ion pairs. This suggests that the difference in titration curves may be attributed to a difference in the intrinsic association constants, characteristic of individual ion pairs.