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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.