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“Surface chemistry” and “surface physics” have, over the past half-century, gained increasing identity and stature as major subspecialties of the parent sciences of chemistry and physics, primarily because of the recognition that important physical, chemical, and electrical properties of matter confined to phase boundaries are often profoundly different from those of the same matter in bulk. For many multiphase systems, the fraction of the total mass which is localized at phase boundaries is so small that the contribution of this “abnormal” boundary material to the macroscopic behavior and properties of the system can, for all practical purposes, be neglected. There are, however, large numbers of important situations wherein either (1) the phase-boundary area is so large relative to the volume of the system that a substantial fraction of the total mass of the system is present at boundaries (for example, colloidal dispersions, emulsions); or (2) phenomena occurring at phase boundaries are so unusual relative to the expected interactions between the bulk phases present that the entire behavior of the system is governed by phase-boundary processes (for example, heterogeneous catalysis, corrosion, detergency, flotation). In both of these two types of situation an understanding of the causes of anomalous behavior of matter at surfaces or interfaces, and of the variables which influence such behavior, is clearly essential to accurate prediction and effective control of the properties and practical applications of such systems. The analysis of physicochemical phenomena occurring at phase boundaries, and the development of interrelationships between such phenomena and known properties of atoms and molecules, constitute the core of the science of surface chemistry and physics.
A phase boundary is obviously formed in any system wherein two phases can coexist. In common parlance, we refer to such a boundary as a “surface” when one of two phases is gaseous, and as an “interface” when the two phases are liquid or solid. It is intuitively evident that the existence of phase boundaries is due solely to those factors which make the coexistence of two immiscible phases possible; clearly, if all matter existed only in the gaseous state, there could be no coexistence of phases, and thus no surfaces or interfaces. An important and far-reaching corollary of this postulate is that all special properties of surfaces or interfaces arise fundamentally from precisely the same forces that are responsible for the existence of matter in condensed (that is, liquid or solid) states. Therefore, consideration of the phenomena responsible for the formation and stability of the liquid and solid states provides a major point of departure for gaining insight into the structure and properties of phase boundaries.
Michaels, Alan S.
Professor of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Mass.