Structure and Property of a Model Water-in-Oil Microemulsion

In this article, we have reviewed some of the basic work on the investigation of the structure and properties of a three component water-in-oil microemulsion containing AOT. It was found that the suucture of the system consisted mainly of polydisperse droplets, whose mean radius was given by the water-to-AOT ratio, and is mostly independent of the volume fraction of the dispersed phase. It is also found to be fairly insensitive to temperature changes. By neutron spin echo studies, it was found that the surfactant layer was reasonably stiff with a splay elastic constant of the order of a few k_BT. The natural tendency of the surfactant is to bend the interface towards the water phase when they are tightly packed together, owing to its bulky tail strucure. There is apparently a short range attractive interaction between the droplets, and this interaction is thought to be responsible for the observed critical point and the phase behaviour for the system. We have described the various experiments that lead to this description, and we are fairly confident of the fact that the droplet structure of the system is very stable in the neighbourhood of the oil corner of the ternary phase diagram. Because of his stable structure, AOT microemulsions were often used as model hardsphere liquids for the study of the scattering structure factor, glass transition, percolation phenomena, droplet dynamics, and a host of other interesting physical properties. This review will survey some of the fundamental work of the basic structure and interaction of this model 3-component system, and will try to illustrate how this information may be used to understand the behaviour of microemulsions.