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 bromide (C(14)TAB) on the surface of water in the presence of benzoate ions have been obtained by infrared-visible sum-frequency spectroscopy (SFS) with three different polarization combinations. The sum-frequency spectra of the benzoate ions arise solely from those ions bound to the monolayer and not from ions in the diffuse double layer or in the bulk solution. From an analysis of the intensity of the nu(2) C-H stretching mode, values for the second-order nonlinear susceptibility are derived. Interpretation of these susceptibilities in terms of the molecular hyperpolarizability and the orientational distribution of the benzoate anions demonstrates that the ions are oriented nearly perpendicular 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.