Insight Into Water Structure at the Surfactant Surfaces and in Micro-Emulsion Confinement (original) (raw)
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 infrared (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 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 head group. The same feature is observed in IR spectra of reverse micelles after deconvoluting the interfacial vs. bulk spectral contributions. We performed orientational analysis on these water molecules utilizing the polarization dependent SFG, which shows the average tilt angle of the OH-stretch of surfactant bound water molecules ~ 155˚ with respect to the surface normal.
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