Solid Film Formation at the Tetradecane/Aqueous Hexadecyltrimethylammonium Bromide Solution Interface Studied by Interfacial Tensiometry and X-ray Reflectometry

The Effect of oil on the condensed film formation in adsorbed film of hexadecyltrimethylammonium bromide (C16TAB) at the tetradecane (C14)/water (W) interface was examined by interfacial tension and X-ray reflectivity measurements. The interfacial tension vs. temperature curves have break point due to the expanded – condensed phase transition of the adsorbed film. The partial molar entropy of C16TAB at the interface changes discontinuously while the interfacial density almost continuously at the phase transition point. The electron density profile normal to the interface manifested that the condensed film is regarded as 2D solid rotator phase in which C16TAB and C14 molecules are densely packed with perpendicular orientation. Combining the interfacial tension and X-ray reflectivity data, the mixing ratio of C16TAB to C14 in the solid film was determined to be 2/3, and thus the film is enriched in oil molecules than surfactant ones. Furthermore, the partial molar entropy change of C14 associated with solid...