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 presence of different concentrations of alkali and tetraalkylammonium nitrates. The charge developed for the Stober silicas in the presence of alkali ions 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-porous Aerosil, a small increase of the charge density is observed. The charge increase on the non-porous silica has been attributed to the effect of specific adsorption of the tetraalkylammonium ions, whereas the charge reduction for Stober silica must be a consequence of porosity. The degree of dissociation of hydroxyls inside the porous volume of oxidic materials depends on the ability of counterions to penetrate into the porous phase. This principle has been applied to characterise the porosity of silica. For monodisperse silicas, three regions of charge formation have been distinguished: the outer surface and two kinds of pores. On the outer surface, the charge is 10-18 times smaller than that inside the porous volume, but of the same order of magnitude as the surface charge of the non-porous Aerosil OX50. The large pores ("mesopores") are accessible to both alkali and tetraalkylammonium 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 determined by the dimensions of the pores only; also, the nature of the counterions is relevant. (C) 1998 Elsevier Science B.V. All rights reserved.