ABSTRACT
I. INTRODUCTION Wetting refers to the macroscopic manifestations of molecular interaction between liquids and solids in direct contact at the interface between them. Such manifestations include the formation of a contact angle at the solid/ liquid/fluid interline, the spreading of a liquid over a solid surface (displacing the fluid initially in contact with that surface), and the wicking of a liquid into (and displacement of a fluid from) a porous solid medium. The interactions of interest in wetting exclude the dissolution or swelling of the solid by the liquid or chemical reactions between the phases that change the system composition. The various wetting phenomena may be characterized without ambiguity in terms of macroscopic thermodynamic properties, which we term the "wetting parameters." We thus begin with a set of definitions of these parameters and examine the ways in which they may be determined by direct measurement. Since wetting phenomena are traceable to molecular interactions, we must consider the origin and nature of these molecular properties and the methods by which the macroscopic wetting parameters may be computed in terms of them. It has long been recognized that molecular interactions in condensed phases, or across interfaces between condensed phases, may be split into those of a physical and those of a chemical nature. Although this view is at least as old as the arguments between Dolezalek and van Laar [1], near the turn of the century, regarding the relative importance of these contributions to solution nonideality, recent years have seen refinements in the prevailing view of each of them. We seek to examine these refinements and their implications for the evaluation of the wetting parameters. The physical molecular interactions are those referred to collectively as the "van der Waals" forces and these are dealt with first. Wetting parameters are evaluated for systems subject only to these types of interactions and compared to direct data obtained for systems of this type. It is the thesis of this chapter that most of the chemical interactions relevant to wetting phenomena, as defined above, are effectively described as acid-base interactions. Acid-base chemistry is briefly reviewed, and the methods currently proposed for describing its influence on the macroscopic wetting
parameters is considered in detail. Particular attention is given to methods for measuring the strength of the acid-base interactions and correlating the results for different liquids and solids. Finally, the prospects for predicting the effect of acid-base interactions on the macroscopic wetting parameters are examined. We conclude by describing some of the experimental techniques used for the acid-base characterization of solid surfaces.
