Another look at the interfacial interaction parameter

Interfacial energy γ_1,2 of a liquid-liquid or solid-liquid system is of paramount importance in colloid and interface science and its applications. To assess the dependence of γ_1,2 on the surface energies γ₁ and γ₂ of two materials in contact, Girifalco and Good proposed their venerable equation involving the interfacial interaction parameter ϕ. Subsequently, values of ϕ have been experimentally determined for various material pairs. Here, we show that, the data of ϕ closely follow a unique relationship ϕ = (1 - γ_1,2/γ₁)^½ for all pairs where the other material is non-polar. Theoretically, this curve describes the smallest possible ϕ. However, we also show that substituting this relationship into the Girifalco-Good equation reduces it to Antonov's rule γ_1,2 = γ₁-γ₂. Such a simplistic approach is inaccurate, and we conclude that the plotting of ϕ vs. γ_1,2 has contributed to overestimating the applicability of the Girifalco-Good Equation.