SUM-FREQUENCY SPECTROSCOPY OF SURFACTANTS ADSORBED AT A FLAT HYDROPHOBIC SURFACE

Infrared-visible sum-frequency spectroscopy (SFS) has been used to obt ain vibrational spectra in the C-H stretching region of surfactants ad sorbed at the solid-water interface All of the surfactants formed mono layers at a hydrophobic substrate with the hydrocarbon chains of the s urfactant oriented away from the aqueous phase. Surfactants with a wid e range of structures have been examined, including cationic, anionic, nonionic, and zwitterionic head groups and single, double, and branch ed alkyl chains. Features in the C-H stretching region originate princ ipally from the hydrocarbon chains and not from the polar head groups. The packing density and the orientational order of the surfactant mon olayers is deduced from the relative strengths of the chain methylene and terminal methyl modes. The structure of the monolayers adsorbed at a flat, hydrophobic surface is compared to surfactant aggregates in b ulk aqueous solution, at the air-water interface and in the solid crys talline state.