Mesoscale modelling studies of microemulsions

Abstract

Mesoscale simulations of microemulsions have been carried out by molecular dynamics using simple single site representations of the oil and water molecules. The surfactant molecules were represented by dimers composed of the water and oil moieties. The focus of the work is on the ternary phase diagram and the extent to which a simple model can reproduce the main features. The simulations explored some of the generic factors that determine the self-assembling characteristics of these three components. Key features of real microemulsion systems were reproduced by this very simple coarse-grained model. Simulations carried out at the centre of the triangular phase diagram followed the species spontaneously self-assemble into a bicontinuous phase. The balance, even in this part of the phase diagram, could be shifted towards micelle formation with surfactant molecules of sufficiently small radius of curvature, which caused the formation of discrete water swollen reverse micelles, and an increasing number of free surfactant molecules or water-less reverse micelles. In the more dilute limit where the oil is the major phase, water-swollen inverse micelles were observed to form in the simulations. On decreasing the radius of curvature of the surfactants, an increasing number of smaller micelles were produced. There was some evidence of finite-size effects in the computer model, in that some of the systems had a tendency to form rod-like micelles (the periodic boundary conditions removed the necessity for ‘end-capping’ which occur in real systems). These became spherical micelles when larger systems with the same relative number of each component were considered. Also for the infinite radius of curvature surfactants, the swollen micelles were found to be below ‘optimum’ size as far as surfactant interfacial coverage was concerned (there were insufficient surfactant molecules in the system to cover the single water-swollen reverse micelle). The micelles were on average spherical but showed significant departures from spherical symmetry over short periods on the time scale of the density fluctuations in the system. We derive a simple analytic model for the size of the spherical micelles, based on a modification of the classical treatment, which takes into account the volume of the headgroup. This gives much improved agreement with the computed micelle size.