ROLE OF ION HYDRATION IN THE CHARGE REGULATION AT THE SURFACE OF POROUS SILICA-GEL

By using potentiometric titration of a precisely characterised porous silica gel sample in solutions of different electrolytes, an attempt h as been made to detect a possible manifestation of the spatial inhomog eneity caused by electrostatic saturation in the hydration potential o f ions in the vicinity of a charged interface. Experimentally, an appr eciable difference between the titration curves for different backgrou nd electrolytes is observed. To determine whether this can be attribut ed to the difference in the hydration potentials of different ions and to obtain a theoretical estimate of the magnitude of the effect expec ted, the titration curves were simulated using a charge-regulation mod el 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 diff erent pH and ion strengths have been calculated. It has been found tha t, although 'switching on' the dielectric saturation results in a noti ceable 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 th e difference in titration curves may be attributed to a difference in the intrinsic association constants, characteristic of individual ion pairs.