The area of contact between two immiscible phases of a dispersion which may involve either the same or different states of matter. Five types are possible: (1) solid-solid (carbon black−rubber), (2) liquid-liquid (water-oil), (3) solid-gas (smoke-air), (4) solid-liquid (clay-water), (5) liquid-gas (water-air). At a fresh surface of either liquid or solid the molecular attraction exerts a net inward pull. Hence the characteristic property of a liquid is surface tension and that of a solid surface is adsorption. Both have the same cause, namely, the inward cohesive forces acting on the molecules at the surface. These phenomena provide to some degree the fundamental mechanism for many industrially important processes (catalysis, emulsification, mixing, alloying) and products (detergents, adhesives, lubricants, paints). Such properties as wettability of solid powders, spreading coefficients of liquids, and protective action of colloidal substances are intimately associated with interfacial behavior.See Surface; Surface Tension; Catalysis; Emulsion; Detergent; Wetting Agent.
