Forces and Adsorption at Solid Surfaces

Abstract

Essentially all interactions between atoms, molecules and ions spring from the electrical charges borne by the nuclei and electrons of which they are constituted. Two broad classes of interaction are distinguished by the terms “chemical” and “physical,” the former referring to those cases wherein some of the electrons of the two participants have become shared, with the consequent formation of a chemical bond. Such chemical forces have the property of saturation, are of very short range, and are usually associated with the comparatively large binding energies of a few electron volts. Some of the more important types of physical interactions, listed more or less in order of decreasing energy, are: ion-ion, ion-dipole, dipole-dipole, ion-induced dipole, induced dipole-induced dipole, etc. Those not involving ions are often grouped together under the term “van der Waals forces,” and generally have energies of only a fraction of an electron volt. The (long-range) ion-ion interactions (“ionic bonds”) are mainly responsible for the stability of ionic crystals, and thus may have energies comparable with those of chemical bonds.

An adsorbed molecule interacts very nearly additively with all the constituent atoms and ions of the adsorbent surface, and with other adsorbed molecules as well. The collective effect may be described through the “energy of adsorption,” which will generally be a function of the amount adsorbed. In physical adsorption, multilayer films and capillary condensation may occur at gas pressures approaching saturation.

The small size and high dipole moment of the water molecule make it strongly adsorbed on ionic substrates. The lamilar structure of the clay minerals furnishes enormous surface areas for adsorption. Investigation of these interactions at the molecular level provides a fundamental basis for a more complete understanding of clay-water systems.