gamma-APTES modified silica gels: The structure of the surface layer

Making use of the MM2 method of molecular mechanics and the AM1 method of quantum chemistry (in the cluster approximation), various chemical structures at the surface of gamma-APTES-modified silicas have been examined with respect to the existence of hydrogen bonding between amino groups and other polar groups at that surface. Only in few cases the geometrical factors appear to be auspicious for hydrogen bonding. Hydrogen bonds between amino groups and residual silanols can arise for sprawling aminopropyl tails on the surface, or in places of coupling of silica globules, Hydrogen bonds between neighboring aminogroups are enabled by the sintering of anchor silanols, which makes the amino groups come close together. Hydrogen bonding is facilitated by the molecular roughness of the amorphous silica surface, In the presence of adsorbed water, which promotes the transferring of protons from silanols to amino groups, and especially in contact with an aquatic environment, the surface acquires the character of a macroscopic zwitterion, This conclusion has been confirmed by the charge titration experiment. In treatment of the experimental data from charge titration, allowance has been made for the solubility of silica. (C) 1996 Academic Press, Inc.