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Two Types of Water at the Water–Surfactant Interface Revealed by Time-Resolved Vibrational Spectroscopy

Livingstone, Ruth A., Nagata, Yuki, Bonn, Mischa, Backus, Ellen H. G.
Journal of the American Chemical Society 2015 v.137 no.47 pp. 14912-14919
detergents, energy transfer, hydrogen bonding, industrial applications, models, sodium dodecyl sulfate, spectroscopy, surfactants
The surfactant sodium dodecyl sulfate (SDS) is widely used as a detergent for both domestic and industrial applications. It forms a self-assembled monolayer on the surface of water. We report a microscopic model for the interaction between the surfactant and water and between water molecules at the interface, revealed using static and time-resolved two-dimensional sum frequency generation spectroscopy. Two distinct subensembles of water in the presence of this negatively charged SDS surfactant have been identified: those close to the SDS headgroup having fairly isolated O–H groups, i.e., localized O–H stretch vibrations, and those whose O–H stretch vibrations are delocalized, i.e., shared between multiple O–H bonds. The two subensembles are coupled, with subpicosecond energy transfer occurring between them. This is markedly different from O–H bonds at the air–water interface, which are less heterogeneous, and indicates that the water molecules that interact with the surfactant headgroups have hydrogen-bonding properties different from those of water molecules interacting with the other water molecules.