Influence of the base size and strength on the acidic properties of silicagel and monodispersed silica beads: Interest of impedance measurements forthe in situ monitoring of the ionization process

The reactivity of silica gel and Stober silica toward molecular and anionic bases of various size and strength was examined in aqueous medium. The ext ent of the deprotonated silanol groups was discussed with respect to the si ze, strength, and concentration of the reactants, as well as to the nature of the silica. Measurements of the complex impedance performed on the decan ted samples were found to be a rapid, sensitive, and nondestructive way for the in situ characterization of the surface silica ionization process in a queous alkaline solutions. The apparent equilibrium constant for the deprot onation reaction was found to be strongly affected by the degree of dissoci ation of both silica samples, so that the complete neutralization was never observed. The molecular-sieving properties of the Stober silica was demons trated experimentally by using bases of various dimensions: while the ioniz ation of silica gel was nearly unaffected by the base size, that of the mon odispersed silica beads was strongly influenced by the ability or inability of the base to enter the bulk. of the beads. The use of reactants larger t han 0.3 nm totally prevents accessibility to the internal silanols. Reactiv ity of bases of critical dimensions ranging from 0.2 to 0.3 nm, though they were able to diffuse freely within the porous structure, was found to be h indered by volume exclusion effects.