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We present a measurement of molecular orientation of water at charged surfactant aqueous interfaces as a function of surface charge density. The polarization dependent spectral line shapes of the water bend mode were measured by vibrational sum-frequency generation at the positively charged surfactant cetyltrimethylammonium bromide (CTAB)/water interface and negatively charged surfactant sodium dodecyl sulfate/water interface. Orientational analysis using the water bend mode as a vibrational probe, within the electric dipole approximation, reveals structural differences between these surfaces and quantifies how different hydrogen bonded species re-orient around the surfactant head groups as the surface charge density changes. As the concentration of the positively charged surfactant (CTAB) increases, the surface water molecules with free-OH groups reorient their hydrogen away from the bulk water and the C2v axis closer to the surface normal. This suggests that these free-OH molecules are in general located above the positively charged head groups of CTAB, and thus, the charge-dipole interaction pulls their oxygen "down" and pushes hydrogen "up." On the contrary, water molecules with two donor hydrogen bonds re-orient their hydrogen toward the bulk water, likely because most of these molecules are below the CTAB surfactant head groups. [ABSTRACT FROM AUTHOR]

Interactions with surfactant molecules can significantly alter the structure of interfacial water. We present a comparative study of water-surfactant interactions using two different spectroscopic approaches: water at planar surfactant monolayers by sum frequency generation (SFG) spectroscopy and interfacial water confined in reverse micelles formed by the same surfactants using IR absorption spectroscopy. We report spectral features in the OH-stretching region (3200-3700 cm-1) that are observed in both IR and SFG spectra, albeit with different relative amplitudes, for ionic surfactant sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT) and nonionic surfactant polyoxyethylene(4)lauryl ether (Brij L-4) reverse micelles in hexane and the corresponding monolayers at the air/water interface. A prominent feature in the SFG spectra of the OH stretch at 3560 cm-1 is attributed to water molecules that have a weak donor hydrogen bond to the surfactant headgroup. The same feature is observed in the IR spectra of reverse micelles after deconvoluting the interfacial versus bulk spectral contributions. We performed an orientational analysis of these water molecules utilizing the polarization-dependent SFG spectra, which shows an average tilt angle of the OH stretch of surfactant-bound water molecules of ~155° with respect to the surface normal. [ABSTRACT FROM AUTHOR]