The effect of tetrahedral substitution of Si by Al on the surface acidity of the oxygen plane of clay minerals

Abstract

Oxygen planes of clay minerals are formed by a continuous linkage of [SiO₄]^4- tetrahedra. This plane determines many of the clay colloidal and surface properties. The d_π-p_π bond between O and Si in Si-O-Si and Si-O-Al groups is described here according to the valence bond treatment. Partial π interaction between O and Si causes the O atom to serve as a weak base. Tetrahedral substitution of Si by Al leads to changes in the clay surface properties, resulting from an increase in the basic strength of the siloxanes. Substitution of Mg by Al in octahedral sheets increases the polarization of the OH planes. Interlayer hydrogen bonds between oxygens and hydroxyls were detected in substituted serpentines and chlorites. Wettability of tetrahedral-octahedral-tetrahedral (TOT) clay minerals was determined from advancing contact angles of water. This angle decreased with increasing degree of tetrahedral substitution, indicating an increase in strength of hydrogen bonds between water and the oxygen plane, due to an increase in its basic strength. Hydrogen bonded water may serve as bridges between parallel layers and, consequently, the swelling of TOT minerals with tetrahedral substitution is prevented to some extent. Interactions occurring between the oxygen plane and acid sites in the interlayer space of smectites and vermiculites affect the bulk surface acidity of the interlayer and the type of reactions which occur between the clay and adsorbed organic acids and bases. Clays with tetrahedral substitution form π bonds by donating electrons to aromatic cations and radicals.