Hydration free energies of the silica silanol groups in the density functional theory
Abstract
The results of ab initio hydration and acidity parameters calculations of the model silica system are presented. The density functional theory B3LYP/6-31++G(d,p), 6-311++G(d,p) and COSMO model of solvation effects have been used. It was demonstrated that even for the simplest model, with the presence of only isolated silanol groups, various water complexes arose. Systematical addition of water molecules to silanole system leads to increase of degree of freedom. Consequently It leads to the dispersion in the silica surface pKa values. By evaluation of pKa values which is obtained by using different approximations it was concluded that the structure with isolated OH groups is quite representative. Hence such a structure can be used for description of protolytic properties of silica surface.
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