Interfacial films and wetting behavior of the air/hexadecane/aqueous solution of a surfactant system

The tension of the three interfaces and the dihedral angle of the lens of t he air/hexadecane lens/aqueous solution of tetramethylammonium dodecyl sulf ate (TMADS) system were measured as a function of the surfactant concentrat ion. The air/water surface tension vs concentration curves both with and wi thout an oil lens exhibited a break point corresponding to the phase transi tion of the adsorbed films. From the thermodynamic analysis, it was shown t hat the phase transition drives the spreading of hexadecane molecules into the adsorbed film. The dihedral angle decreased very rapidly from about 50 degrees at zero concentration to about 6 degrees at the phase transition po int and then increased slowly to about 20 degrees at the critical micelle c oncentration (cmc), suggesting the very drastic change in wetting propertie s at the phase transition point. The plausible gap was observed between the dihedral angles measured and those calculated from the interfacial tension values at the concentration range from just above the phase transition to near the cmc. This suggests a case where the Neumann relation does not hold even when a lens exists stably. One of the possibilities may be that the w etting film is rather thick just above the phase transition point.