Studies of Liquid-Liquid Interfaces

Two experiments have been performed in which surface light scattering and a mechanically generated capillary wave technique have been used to study the interface between two liquids. In the first experiment, surface light scattering was used to obtain quantitative values of the effective surface tension between two miscible liquids were obtained as a function of time. The system studied was a near-critical binary liquid mixture of isobutyric acid and water (IBW), originally at two-phase equilibrium slightly below its critical temperature T_{c}. When the IBW temperature was increased to slightly above T_{c}, the two phases slowly began to interdiffuse. It was found that the effective surface tension decreased very slowly, much more slowly than would be expected using the one-phase equilibrium diffusion constant. In separate measurements it was found that when the temperature of the IBW system was raised from one two-phase equilibrium temperature to another two -phase equilibrium temperature closer to T_ {c}, the IBW system reached its new equilibrium surface tension very quickly, about as quickly as it reached its new equilibrium temperature. In the second experiment, films of polystyrene -b-poly(methacrylic acid) (PS-PMAA) diblock copolymer were studied at the toluene/water interface. Values for the surface film mechanical parameters (surface tension and transverse viscosity) were obtained as a function of the overall diblock molecular weight M_{w }, and the amount of diblock copolymer at the interface. Measurements of the complex wave number obtained using a mechanically generated capillary wave technique, and measurements of the complex frequency obtained using surface light scattering, were combined over a large frequency range to obtain "best-fit" interfacial viscoelastic parameters. Good agreement was found between the results obtained using the two methods.