title: Structure and dynamics of water in nonionic reverse micelles: a combined time-resolved infrared and small angle x-ray scattering study.
abstract: We study the structure and reorientation dynamics of nanometer-sized water droplets inside nonionic reverse micelles (water/Igepal-CO-520/cyclohexane) with time-resolved mid-infrared pump-probe spectroscopy and small angle x-ray scattering. In the time-resolved experiments, we probe the vibrational and orientational dynamics of the O-D bonds of dilute HDO:H(2)O mixtures in Igepal reverse micelles as a function of temperature and micelle size. We find that even small micelles contain a large fraction of water that reorients at the same rate as water in the bulk, which indicates that the polyethylene oxide chains of the surfactant do not penetrate into the water volume. We also observe that the confinement affects the reorientation dynamics of only the first hydration layer. From the temperature dependent surface-water dynamics, we estimate an activation enthalpy for reorientation of 45 ± 9 kJ mol(-1) (11 ± 2 kcal mol(-1)), which is close to the activation energy of the reorientation of water molecules in ice.
authors.affiliation: Van 't Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands. T.H.vanderLoop@uva.nl
authors.forename: Tibert H
authors.initials: TH
authors.lastname: van der Loop
authors.forename: Matthijs R
authors.initials: MR
authors.lastname: Panman
authors.forename: Stephan
authors.initials: S
authors.lastname: Lotze
authors.forename: Jing
authors.initials: J
authors.lastname: Zhang
authors.forename: Thomas
authors.initials: T
authors.lastname: Vad
authors.forename: Huib J
authors.initials: HJ
authors.lastname: Bakker
authors.forename: Wiebke F C
authors.initials: WF
authors.lastname: Sager
authors.forename: Sander
authors.initials: S
authors.lastname: Woutersen
mesh_terms: D003510:Cyclohexanes / Q000737:chemistry*; D008823:Micelles; D056004:Molecular Dynamics Simulation; D015394:Molecular Structure; D053838:Scattering, Small Angle; D013055:Spectrophotometry, Infrared; D013501:Surface-Active Agents / Q000737:chemistry*; D013997:Time Factors; D014867:Water / Q000737:chemistry*; D014961:X-Ray Diffraction
chemical_list: D003510:Cyclohexanes; D008823:Micelles; D013501:Surface-Active Agents; D014867:Water