Structure and dynamics of water in nanoscopic spheres and tubes.
We study the reorientation dynamics of liquid water confined in nanometer-sized reverse micelles of spherical and cylindrical shape. The size and shape of the micelles are characterized in detail using small-angle x-ray scattering, and the reorientation dynamics of the water within the micelles is investigated using GHz dielectric relaxation spectroscopy and polarization-resolved infrared pump-probe spectroscopy on the OD-stretch mode of dilute HDO:H2O mixtures. We find that the GHz dielectric response of both the spherical and cylindrical reverse micelles can be well described as a sum of contributions from the surfactant, the water at the inner surface of the reversed micelles, and the water in the core of the micelles. The Debye relaxation time of the core water increases from the bulk value τ(H2O) of 8.2 ± 0.1 ps for the largest reverse micelles with a radius of 3.2 nm to 16.0 ± 0.4 ps for the smallest micelles with a radius of 0.7 nm. For the nano-spheres the dielectric response of the water is approximately ∼6 times smaller than expected from the water volume fraction and the bulk dielectric relaxation of water. We find that the dielectric response of nano-spheres is more attenuated than that of nano-tubes of identical composition (water-surfactant ratio), whereas the reorientation dynamics of the water hydroxyl groups is identical for the two geometries. We attribute the attenuation of the dielectric response compared to bulk water to a local anti-parallel ordering of the molecular dipole moments. The difference in attenuation between nano-spheres and nano-cylinders indicates that the anti-parallel ordering of the water dipoles is more pronounced upon spherical than upon cylindrical nanoconfinement.
Van 't Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, Netherlands.
Tibert H
TH
van der Loop
FOM Institute for Atomic and Molecular Physics, Science Park 104, 1098 XG Amsterdam, The Netherlands.
Niklas
N
Ottosson
FOM Institute for Atomic and Molecular Physics, Science Park 104, 1098 XG Amsterdam, The Netherlands.
Stephan
S
Lotze
Jülich Centre for Neutron Science, JARA-FIT and Peter Grünberg Institute, Forschungszentrum Jülich, 52425 Jülich, Germany.
Emmanuel
E
Kentzinger
Institut für Textiltechnik, RWTH Aachen University, Otto-Blumenthal-Str. 1, 52074 Aachen, Germany.
Thomas
T
Vad
Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons and Peter Grünberg Institute, Forschungszentrum Jülich, 52425 Jülich, Germany.
Wiebke F C
WF
Sager
FOM Institute for Atomic and Molecular Physics, Science Park 104, 1098 XG Amsterdam, The Netherlands.
Huib J
HJ
Bakker
Van 't Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, Netherlands.
Sander
S
Woutersen
D058266:Dielectric Spectroscopy; D008823:Micelles; D054118:Nanospheres / Q000737:chemistry*; D043942:Nanotubes / Q000737:chemistry*; D013501:Surface-Active Agents / Q000737:chemistry*; D014867:Water / Q000737:chemistry*
D008823:Micelles; D013501:Surface-Active Agents; D014867:Water