ORIENTATION OF BENZOATE IONS BOUND TO A MONOLAYER OF A CATIONIC SURFACTANT AT THE SURFACE OF WATER

Vibrational spectra of a mo no layer of tetradecyltrimethylammonium br omide (C(14)TAB) on the surface of water in the presence of benzoate i ons have been obtained by infrared-visible sum-frequency spectroscopy (SFS) with three different polarization combinations. The sum-frequenc y spectra of the benzoate ions arise solely from those ions bound to t he monolayer and not from ions in the diffuse double layer or in the b ulk solution. From an analysis of the intensity of the nu(2) C-H stret ching mode, values for the second-order nonlinear susceptibility are d erived. Interpretation of these susceptibilities in terms of the molec ular hyperpolarizability and the orientational distribution of the ben zoate anions demonstrates that the ions are oriented nearly perpendicu lar to the air-water interface, with an upper limit of 28 degrees for the mean tilt of the C-2 axis from the surface normal.