title: Two Types of Water at the Water-Surfactant Interface Revealed by Time-Resolved Vibrational Spectroscopy.
abstract: 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.
authors.affiliation: Max Planck Institute for Polymer Research , Ackermannweg 10, 55128 Mainz, Germany.
authors.forename: Ruth A
authors.initials: RA
authors.lastname: Livingstone
authors.affiliation: Max Planck Institute for Polymer Research , Ackermannweg 10, 55128 Mainz, Germany.
authors.forename: Yuki
authors.initials: Y
authors.lastname: Nagata
authors.affiliation: Max Planck Institute for Polymer Research , Ackermannweg 10, 55128 Mainz, Germany.
authors.forename: Mischa
authors.initials: M
authors.lastname: Bonn
authors.affiliation: Max Planck Institute for Polymer Research , Ackermannweg 10, 55128 Mainz, Germany.
authors.forename: Ellen H G
authors.initials: EH
authors.lastname: Backus