Vibrational spectra of seven surfactants adsorbed at the surface of aqueous solutions have been obtained by IR-VIS sumfrequency 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. d C,,-betaine 0.50 NR 44 2.0 " NR = neutron reflectivity, XRD = X-ray diffraction, y = surface tension. Error limits in NR measurements are i -0.02 nm'. ' Solubility at 298 K. Not well established: concentration of 0.34 mmol 1-' used in experiments. Estimated from bulk lamellar phase.

A new experimental arrangement is described for acquiring sum-frequency (SF) spectra from surfactants at the oil-water interface. The key features of this approach are the stabilization of a thin oil film between a sapphire prism and an aqueous phase and the use of total internal reflection to enhance the total signal and discriminate against signals from other interfaces in the system. With this new methodology, the first SF vibrational spectra of surfactant monolayers at an alkane-water interface were obtained. Surface tensiometry was used to characterize the monolayers further. The structure of monolayers of the cationic surfactant hexadecyltrimethylammonium bromide (CTAB) at the hexadecane-water interface was determined by sumfrequency spectroscopy (SFS) and compared with monolayers of CTAB at the air-water interface. At low concentrations, CTAB/hexadecane showed the expected features in the C-H stretching region, characteristic of a conformationally disordered monolayer. As the bulk concentration approached the critical micelle concentration, the spectra changed to one characteristic of a more ordered, upright conformation. Ellipsometric measurements supported this conclusion. This qualitative structural change is not observed in analogous monolayers at the air-water interface or CCl 4 -water interface or in surfactant solutions in contact with a hydrophobic solid surface.

Sum-frequency vibrational spectroscopy has been used to determine the structure of monolayers of the cationic surfactant, hexadecyltrimethylammonium p-tosylate (C16TA+Ts-), at the surface of water. Selective deuteration of the cation or the anion allowed the separate detection of sum-frequency spectra of the surfactant and of counterions that are bound to the monolayer. The p-tosylate ions are oriented with their methyl groups pointing away from the aqueous subphase and with the C 2 axis tilted, on average, by 30−40° from the surface normal. The vibrational spectra of C16TA+ indicate that the number of gauche defects in the monolayer does not change dramatically when bromide counterions are replaced by p-tosylate. The ends of the hydrocarbon chains of C16TA+ are, however, tilted much further from the surface normal in the presence of p-tosylate than in the presence of bromide. A quantitative analysis of the sum-frequency spectra requires a knowledge of the molecular hyperpolarizability tensor:  the role of ab initio calculations and Raman spectroscopy in determining the components of this tensor is discussed.

Sum-frequency (SF) spectroscopy and ellipsometry have been used to study monolayers of the cationic surfactants C n TAB (CH3(CH2) n -1N+(CH3)3Br-) (n = 12, 14, 16, and 18) at the air-water interface at a constant surface area of 44 Å2/molecule. The SF spectra of all four surfactants are very similar and yield a mean chain tilt of 58° near the methyl terminus. The ellipsometric data suggest that the density of the chain region in the monolayers is close to that of a liquid hydrocarbon, which is consistent with the chain tilt derived from the sum-frequency spectra. These results are in quantitative agreement with detailed structural profiles of C12TAB and C16TAB determined by Thomas and co-workers by neutron reflection.

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