Abstract
Contact angles and interfacial tensions have been measured in three-phase systems containing a nonpolar solid, water, and either oil or air, both in the presence and in the absence of surfactants. For all systems investigated, the sum of the wetting energy and the interfacial tension between the fluids ( o, w) is a constant: γ owcosθ+γ ow=C where o stands for any nonpolar fluid, including air, and w for the aqueous phase. The value of C is independent of the nature and of the amount of surfactant added. Its value can be predicted from the properties of the pure solutes and the nonpolar solid with an accuracy of 2 mN m −1 or better. Thus, the value of the contact angles in the presence of surfactants can be predicted when γ ow is known, provided that the values of γ ow or C are not very small.
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