Another look at the interfacial interaction parameter.

Interfacial energy γ1,2 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 γ1,2 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 - γ1,2 /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 γ1,2  = γ1 -γ2 . Such a simplistic approach is inaccurate, and we conclude that the plotting of ϕ vs. γ1,2 has contributed to overestimating the applicability of the Girifalco-Good Equation.