A model for the temperature-dependent interactions in uncharged droplet microemulsions

Microemulsions consisting of uncharged droplets generally behave as hard spheres at the solubilisation phase boundary (SPB) where the actual droplet size is close to the preferred droplet size. As the temperature is changed to move inside the one-phase microemulsion region, experimental data suggest that the droplets may either grow or cluster together due to increasing interdroplet attraction. We present a model in which it is assumed that droplet growth is absent and that the temperature dependence of the interdroplet interactions arises primarily from the energy associated with bending the stabilising surfactant monolayer. At temperatures close to the SPB, the actual curvature of the monolayer coating the droplets is close to its preferred value and the energy required to deform the interface is high, with the result that the droplets behave as hard spheres. Away from the SPB, where the actual monolayer curvature becomes higher than the preferred value, the energy required for deformation decreases and the droplets behave as 'soft' spheres. This results in an apparent increase in attractive interactions between the droplets. The model shows reasonable agreement with experimental turbidity data for uncharged microemulsions.