Chemistry, Physics and Technology of Surface

ISSN 2518-1238 (Online), ISSN 2079-1704 (Print)

Despite the large number of experimental and theoretical works devoted to the structure and properties of silica surface known in the literature, as well as the extensive research on the structure of hydrated ionic pairs, the mechanism of adsorption of hydrated ions of electrolytes on silica surface has not yet been studied sufficiently. The paper presents a review of experimental works devoted to this problem, as well as their analysis at the atomic-molecular level. In order to study the interface "the silica surface - a solution of electrolyte", a number of papers used a method of density functional theory (DFT) using the exchange-correlation functional В3LYP and the basis set 6-31++G(d,p). The environmental impact modeling was taken into account within the framework of the supermolecular approach using a continuum solvent exposure model (PCM). The structure of the double electric layer and the mechanism of proton exchange in the near-surface layer of the silica surface are considered, as well as the value of the Gibbs energy of the silanol group deprotonation reaction, from which, in turn, the value of pK_a2 was obtained. The results of calculations have shown that, with increasing size of oligomer molecules of silicic acid, the degree of localization of charges on hydrogen atoms and oxygen of the silanol group increases, the length of the O-H bond increases and the pK_a2 decreases.

It is shown that when adsorbed on a silica surface, alkali metal cations, in contact with silanol groups, can change the protolytic equilibrium of surface groups, due to redistribution of electron density, thereby increasing their acidity.

The processes of ion exchange on silica surface and the dependence of the pK_Me value on the size of ions and pH are considered. In a strongly acidic medium on silica surface, the formation of a cationic form of the silanol group is probable due to the proton transfer from the ion of hydroxonium to the oxygen atom of the silanol group. The deprotonation constant pK_a1 of the cationic form of the silanol group depends on the nature of the anion and increases as an absolute value with anion radius increase. The models considered allow us to find the theoretical value of the point of zero charge of silica surface, which corresponds to the experimental data.

Calculations using the constructed models of adsorption complexes of alkali hydroxides for the molecular state and for the state with separated charges with the participation of water molecules and silanol groups of silica surface indicate a possibility of ionization of the silanol group. The calculated Gibbs free energy changes are used to determine the values of pK_Me, which increase in the series Li<Na<K, which correlates with the experimentally found adsorption values.

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