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Water Mobility in Reverse Micelles Studied by Quasielastic Neutron Scattering and Molecular Dynamics Simulation

Published online by Cambridge University Press:  26 February 2011

Branka Ladanyi  and
Nancy Levinger
Branka Ladanyi
Affiliation:
bl@lamar.colostate.edu, Colorado State University, Chemistry, Department of Chemistry, Fort Collins, Colorado, 80523-1872, United States, 970-491-5196, 970-491-3361
Nancy Levinger
Affiliation:
Levinger@lamar.colostate.edu, Colorado State University, Chemistry, United States
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Abstract

Reverse micelles (RMs) are aggregates in which nanoscale droplets of a polar liquid, usually water, are surrounded by a surfactant layer in a nonpolar continuous phase. They are widely used as media for reactions in which the extent of confinement or the presence of a surfactant interface play a central role. We have used molecular dynamics (MD) computer simulation and quasielastic neutron scattering (QENS) and to investigate the mobility of water molecules in reverse micelles. The contribution of water to the QENS signal is enhanced by deuterating the surfactant and the nonpolar phase. Our studies of water mobility have focused on the effects of water pool size, determined by the water/surfactant mole ratios w0, as well as on the properties of the water-surfactant interface. Specifically, we have examined the effects of varying w0 and of substituting other alkali ions for the usual Na+ counterion of the anionic surfactant AOT (bis (2-ethylhexyl) sulfosuccinate)). We find good agreement between the QENS signal and its prediction from MD simulation. This allows us to obtain additional insight into water mobility by analyzing the MD self-intermediate scattering function (ISF) of water hydrogens in terms of contributions from molecular rotation and translation and from molecules in different interfacial layers. MD data indicate that the translational ISF decays nonexponentially due to lower water mobility close to the interface and to confinement-induced restrictions on the range of translational displacements. Rotational relaxation also exhibits nonexponential decay. However, rotational mobility of O-H bond vectors in the interfacial region remains fairly high due to the lower density of water-water hydrogen bonds in the vicinity of the interface.

Keywords

neutron scatteringsimulationnanostructure
Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 899: Symposium N – Dynamics in Small Confining Systems VIII , 2005 , 0899-N05-01
Copyright
Copyright © Materials Research Society 2006

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