Formation of micelles in aqueous solutions of a room temperature ionic liquid: a study using coarse grained molecular dynamics

Abstract

A coarse grained force field model has been developed to study aqueous solutions of the room temperature ionic liquid 1-n-decyl-3-methylimidazolium bromide using molecular dynamics (MD) simulations. Spontaneous self-aggregation of amphiphilic cations to form quasi-spherical micelles has been observed in dilute solutions. Two types of growth mechanism have been observed during the MD simulations, including the fusion of small micelles to form larger ones. Aggregates are found to be poly-disperse and the aggregation number, as determined from the MD simulations, compares well with results from small angle neutron scattering. The dynamics of micelles is discussed. Formation of the hexagonal columnar phase at a concentration of 37% (w/w) of water has been observed with a spacing of the columns in good agreement with the experimentally determined value. The vapor–liquid interface of an aqueous ionic liquid solution has been investigated and the structure at the interface has been characterized. The surface area per cation agrees with the experimentally determined value for a similar ionic liquid containing the chloride anion.

Keywords:

• ionic liquids
• molecular dynamics
• self-assembly
• ionic surfactants
• micelles
• fusion of micelles

Acknowledgements

We thank Professor Sundaram Balasubramanian and Dr. Russell DeVane for helpful discussions and the Ras Al Khaimah Center for Advanced Materials for supporting this work through a research fellowship named after His Highness Sheikh Saqr Bin Mohammed Al Qasimi. The simulations were performed in part using Teragrid resources.