Vlasova N. Quantitative estimation of adsorption equilibria as complexation reactions onto silica - aqueous solution interface

The adsorption of a number of chemical compounds of various nature is considered in terms of the surface complextion theory. A constructive approach to the choice of complex formation reactions is developed, which is based on the comparison of the state of the solute and the functional groups of the silica surface, depending on the pH, ionic strength and concentration of the components of the solution. Using the basic Stern model and the GRFIT program, the ionization constants of the silanol surface groups and the equilibrium constants of complex formation reactions between molecules and ions and neutral or ionized silanol are determined. It is shown that substances of the basic nature are sorbed predominantly in the form of cations with the formation of outer-sphere complexes. Adsorption of low soluble substances is due to hydrophobic repulsion from the aqueous solution, which is facilitated by a decrease of dielectric permittivity of water near the surface of oxide. Transition metal ions form on the silica surface inner- sphere complexes, and the presence of organic compounds in solutions leads to the formation of ternary (mixed ligand) complexes in which silanol groups act as ligands. Stability constants of these adsorption complexes are determined. Their structures are confirmed by spectroscopic technique. The obtained results allow predicting the adsorption properties of silica in relation to chemicals of a different nature. Using the dependence of adsorption on physicochemical parameters, it is possible to predict conditions for the creation of materials with predetermined properties, for example, drugs of prolonged action with controlled release of active components.