Mesoscale modelling studies of microemulsions

Mesoscale simulations of microemulsions have been carried out by molecular dynamics using simple single site representations of the oil and water mole cules. The surfactant molecules were represented by dimers composed of the water and oil moieties. The focus of the work is on the ternary phase diagr am and the extent to which a simple model can reproduce the main features. The simulations explored some of the generic factors that determine the sel f-assembling characteristics of these three components. Key features of rea l microemulsion systems were reproduced by this very simple coarse-grained model. Simulations carried out at the centre of the triangular phase diagra m followed the species spontaneously self-assemble into a bicontinuous phas e. The balance, even in this part of the phase diagram, could be shifted to wards micelle formation with surfactant molecules of sufficiently small rad ius of curvature, which caused the formation of discrete water swollen reve rse micelles, and an increasing number of free surfactant molecules or wate r-less reverse micelles. In the more dilute limit where the oil is the majo r phase, water-swollen inverse micelles were observed to form in the simula tions. On decreasing the radius of curvature of the surfactants, an increas ing number of smaller micelles were produced. There was some evidence of fi nite-size effects in the computer model, in that some of the systems had a tendency to form rod-like micelles (the periodic boundary conditions remove d the necessity for 'end-capping' which occur in real systems). These becam e spherical micelles when larger systems with the same relative number of e ach component were considered. Also for the infinite radius of curvature su rfactants, the swollen micelles were found to be below 'optimum' size as fa r as surfactant interfacial coverage was concerned (there were insufficient surfactant molecules in the system to cover the single water-swollen rever se 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 f or the size of the spherical micelles, based on a modification of the class ical treatment, which takes into account the volume of the headgroup. This gives much improved agreement with the computed micelle size.