ABSTRACT
The role of solid-solid, solid-liquid, and solid-gas interfaces is presently well recognized in numerous phenomena, especially in chromatographic processes, and in numerous applications of divided solid surfaces such as fine-powdered mineral oxides used as fillers in elastomers or paints, catalysts supports, and thixotropic agents. For instance, the rheological behavior of powder mixes, either in the dry state or in suspension in liquids, clearly depends on the particle morphology, size, and size distribution, but also on the physicochemical interactions between particles themselves or with their environment. Similarly, the stability of colloidal dispersions is influenced by a complex balance between various forces (attractive and repulsive van der Waals, electrostatic forces, hydrophobic forces, acid-base interaction forces) that act in the dispersion. Efficient gas adsorption phenomena that allow high selectivity in gas chromatography, and hence fine separation, are essentially due to specific interactions between adsorbent and adsorbate. Major efforts have consequently been spent to reach a better insight into and description of those forces. However, the experimental evaluation of the interaction potential of a solid surface involving those forces is far from being evident, especially for powders. The development of the surface force apparatus [1] and, more recently, of near field electron microscopes [2], as well as new theories [3], has given new impetus in this research domain. These methods apply fairly well to “model” surfaces such as cleaved muscovite in the case of both the surface force apparatus and the atomic force microscope, but so far divided solids cannot be simply examined with these methods. The most familiar ways of determining the surface energy (potential for reversible or physical interactions) are wetting or contact angle measurements. Here, drops of liquids of known characteristics are deposited on a flat and nonporous solid surface and the contact angles are evaluated using well-accepted procedures leading to the determination of the surface energy value [4], Such a procedure hardly applies to powders, even when they are compressed in a
platelet form on which liquid drops may be deposited, because it is quite impossible to prepare surfaces of adequate smoothness. In all cases, careful and time-consuming verifications of the surface smoothness are required [5] before reaching meaningful conclusions.
