Infrared band frequency shifts and dipole interactions at surfaces of inorganic and organic clay intercalates

Abstract

Molecules adsorbed at clay surfaces are often observed to have the frequencies of their infrared (IR) absorption bands altered by the proximity of the surfce. Using a simplified theory of dipole induction, it is shown that dispersion forces give rise to shifts or splitting of requencies which are proportional to the polarizability of the interacting dipoles and to the reciprocal of the cube of the distance between them. Additivity of bond polarizabilities makes this relationship easy to used, and it is able to account for the frequency shifts in the following four systems: hectorite and degraded phlogopite reacted with inorganic cations, dimethyl sulfoxide— and dimethyl selenoxide—kaolinite intercalates, anilinium—vermiculite intercalate, and alkylammonium—vermiculite intercalates. In the last system, the theory explains changes in the NH deformation frequencies of angled and perpendicular CN bonds that have been observed in these intercalates by showing up differences between the NH groups interacting with the clay surface. The theory can be used to determined polarizabilities if interatomic distances and IR band shifts are known. Hence it may be of potential use to comment on charges of interacting groups giving different frequencies shifts, since polarizabilities vary to some extent with charge; this is illustrated using the interaction of NH vibrations with three clays of differing charge. The theory can predict interatomic distances at surfaces from IR band shifts and polarizabilities.