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.