Surface and volume charge densities of monodisperse porous silicas

Potentiometric proton titrations of a series of monodisperse Stober silicas with different radii and of non-porous Aerosil OX50 are performed in the p resence of different concentrations of alkali and tetraalkylammonium nitrat es. The charge developed for the Stober silicas in the presence of alkali i ons is extremely high as compared to that of the non-porous silica. In the presence of tetraalkylammonium ions, the charge density of Stober silica is much lower than that in the presence of alkali ions, whereas for the non-p orous Aerosil, a small increase of the charge density is observed. The char ge increase on the non-porous silica has been attributed to the effect of s pecific adsorption of the tetraalkylammonium ions, whereas the charge reduc tion for Stober silica must be a consequence of porosity. The degree of dis sociation of hydroxyls inside the porous volume of oxidic materials depends on the ability of counterions to penetrate into the porous phase. This pri nciple has been applied to characterise the porosity of silica. For monodis perse silicas, three regions of charge formation have been distinguished: t he outer surface and two kinds of pores. On the outer surface, the charge i s 10-18 times smaller than that inside the porous volume, but of the same o rder of magnitude as the surface charge of the non-porous Aerosil OX50. The large pores ("mesopores") are accessible to both alkali and tetraalkylammo nium ions. The small pores ("micropores") are only accessible to the alkali ions. For the particle sizes investigated, the charge developed inside the mesopores is of the same order of magnitude as that on the outer surface. The distinction between the meso- and micropores is not necessarily determi ned by the dimensions of the pores only; also, the nature of the counterion s is relevant. (C) 1998 Elsevier Science B.V. All rights reserved.