SURFACE VIBRATIONAL SPECTROSCOPY OF ORGANIC COUNTERIONS BOUND TO A SURFACTANT MONOLAYER

Sum-frequency vibrational spectroscopy has been used to determine the polar orientation of organic counterions bound to a charged surfactant monolayer physisorbed at the solid-liquid interface and to investigat e the effect of counterion binding on the conformational order of the surfactant. The resonances that appear in the sum-frequency spectra of tosylate, benzoate, and salicylate anions bound to a monolayer of tet radecyltrimethylammonium cations are assigned to methyl and aromatic m odes of the counterions. The phase of the resonant sum-frequency signa l indicates that the ions adopt a preferential orientation with their hydrophobic end toward the solid surface. Polar, uncharged aromatic mo lecules are also oriented by the monolayer but generally only at highe r concentrations than the anions. In contrast to the behavior of surfa ctant aggregates in the bulk phase, there is no evidence that the bind ing of aromatic counterions increases the packing density of surfactan t molecules adsorbed at the solid-liquid interface.