Quantitative physicochemical analysis of equilibria on chemically modified silica surfaces

Abstract

Quantitative physicochemical analysis (QPCA) enables the determination of the stoichiometric compositions and physicochemical parameters of species in equilibrium systems proceeding from the composition-property dependencies. The paper discusses modifications to the routine QPCA procedures required to characterize properties of reagents fixed on surfaces of silica-organic hybrid materials. The cooperative effects and the energetic heterogeneity of fixed reagents are especially important in this context. It follows that the main peculiarities of silica surfaces chemically modified by aliphatic amines are (a) the pronounced energetic heterogeneity of reagents caused by the non-random surface topography, (b) the decrease of the bacisity of amines induced by their interactions with residual surface silanols, and (c) the expressed sensibility of reactions in the near-surface layer to the state of its hydration. The interaction of grafted organic bases with metal ions results in the preferred formation of bis metal-ligand coordination compounds. Stability constants of complexes are decreased as a consequence of fixation and depend on not only donor but also acceptor ability of a solvent. Also, the denticity of polydentate ligands may decrease as a result of grafting. The changes of protolytic and complexing properties in the case of grafting of weak acids and phosphorus-containing complexons are due to their interactions with other surface groups and the influence of hydration effects in the near-surface layer.