Equation for the interfacial tension between demixed polymer solutions

The theory of Cahn and Hilliard is used to derive an equation for the interfacial tension (free energy) between some demixed polymer solutions, applying a simple solution model treated by Debye in his theory of light scattering near the critical solution temperature. For a (symmetrical) system containing two polymers in a common solvent it is found that the interfacial tension is given by σ = (l/12^½)Ω_φp ²σ_r, where l is the Debye l parameter for the range of molecular interaction—here equal to (2S² )^½, where (S² )^½ is the radius of gyration of both polymers, Ω is a heat of mixing parameter for polymer-polymer interaction, φ_p is the total volume fraction of polymer and σ_r is a function of the ratio of temperature and critical solution temperature. The equation is in qualitative agreement with experiments of Langhammer and Nestler.