Wettability of clay minerals

Advancing contact angles of water are compared to those of tetrabromoethane on freshly prepared surfaces of tetrahedral-octahedral-tetrahedral clay minerals, to examine the wettability characteristics of the latter and determine if they fall within the category of high-, medium-, or low-energy surfaces. The hydrophobicity of tale and pyrophyllite is explained on the basis of the electronic properties of the exposed cleavage planes. It is proposed that these reflect the influence of a canonical structure of the siloxane group which does not display electron-donor properties, the latter being necessary for hydrogen-bond formation with the water molecule. Vermiculite, with ions in its cleavage surface, is hydrophilic as expected. Tetrabromoethane interacts more strongly with hydrophilic vermiculite than with hydrophobic tale or pyrophyllite. It is proposed that this is due either to a contribution of oxygen lone-pair donor electrons to the London dispersion force, or to tetrabromoethane's possibly acting as a Lewis acid on this donor pair. It is apparent that the high-, medium-, or low-energy classification system is not well suited to predicting the wettability characteristics of clay mineral cleavage surfaces, although those consisting exclusively of siloxane groups behave as conventional low-energy surfaces.