  In order to understand the nature and dynamics of interfacial water molecules
on the surface of complex systems, large scale molecular dynamics simulations
of an aqueous micelle of cesium perfluorooctanoate surfactant molecules have
been carried out. The lifetime and the intermolecular vibrational frequencies
of the hydrogen bonds that the water molecules form with the polar headgroups
of the surfactants, are calculated. Our earlier classification of the
interfacial water molecules, based on structural and energetic considerations,
into bound and free type is further validated by their dynamics. Lifetime
correlation functions of the water-surfactant hydrogen bonds show the long
lived nature of the bound water species. The water molecules that are singly
hydrogen bonded to the surfactants have longer lifetime than those that form
two such hydrogen bonds. The free water molecules that do not form any such
hydrogen bonds behave similar to bulk water in their reorientational dynamics.
A few water molecules that form two such hydrogen bonds are orientationally
locked in for durations of the order of few hundreds of picoseconds. The
intermolecular vibrational frequencies of these interfacial water molecules
shows a significant blue shift in the librational band apart from a similar
shift in the near neighbor bending modes, relative to water molecules in bulk.
These blue shifts suggest an increase in rigidity in the structure around
interfacial water molecules. This is in good agreement with recent incoherent,
inelastic neutron scattering data on macromolecular solutions. The results of
the present simulations should be relevant to the understanding of dynamics of
water near any hydrophilic surface.
