First-Order Phase Transition in Mixed Monolayers of Hexadecyltrimethylammonium Bromide and Tetradecane at the Air−Water Interface

Mixed monolayers of CTAB (hexadecyltrimethylammonium bromide) and tetradecane were formed at the air−water interface by addition of small amounts of tetradecane to the surface of a CTAB solution below the critical micelle concentration. Ellipsometry and sum-frequency spectroscopy were used to demonstrate that these mixed monolayers exhibit a first-order phase transition from a conformationally disordered to a conformationally ordered state as the temperature is lowered. The phase transition occurs ca. 11 °C above the bulk melting point of tetradecane.