Robert N. Ward

SUM-FREQUENCY VIBRATIONAL SPECTROSCOPY OF SOLUBLE SURFACTANTS AT THE AIR/WATER INTERFACE

Vibrational spectra of seven surfactants adsorbed at the surface of aqueous solutions have been obtained by IR–VIS sum-frequency generation. From these spectra the degree of conformational disorder and the angle of the terminal methyl group are inferred. In general, the number of gauche conformations increases as the area per chain increases. The angle of the methyl group, which is an indicator of the tilt of the hydrocarbon chains, is not simply related to the area per chain. Comparison of surfactants with the same chain length and area per molecule shows that the structure of the chain region of the monolayer is sensitive to the nature of the head group and not just to the packing density. Quantitative models to explain peak intensities in sum-frequency spectra of surfactants are discussed critically.

Orientation of benzoate ions bound to a monolayer of a cationic surfactant at the surface of water

Vibrational spectra of a monolayer of tetradecyltrimethylammonium bromide (C14TAB) 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 ν2 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° for the mean tilt of the C2 axis from the surface normal.

Sum frequency vibrational spectroscopy of the solid-liquid interface

Sum-frequency spectroscopy (SFS) has been used to elucidate the structure of surfactant monolayers at the hydrophobic solid-water interface. The polar orientation and conformational order of a series of surfactant molecules has been determined from the vibrational spectra of their hydrocarbon chains in the C-H stretching region. The SF spectra of inorganic (thiocyanate and cyanide) and aromatic (tosylate, benzoate, and salicylate) counterions indicate that they bind to a monolayer of charged surfactant with a net polar orientation. The spectra of the aromatic anions bound to a surfactant monolayer at the air-water interface are also presented. The interactions between co-adsorbed surfactant and polymer has been examined-- the spectra of the surfactant indicate that an oppositely charged polymer causes a monolayer of the surfactant to adsorb at much lower bulk concentrations than in the absence of the polymer. In one case, the bound polymer has a SF-active resonance indicating that the polymer is also orientationally ordered at the solid-water interface.