Reentrant Phase Separation in Microemulsions.

We propose a mechanism for reentrant phase separation in globular microemulsions based on the combined effects of a shape transition and attractive interactions. Long cylindrical globules can phase separate at relatively low interglobular attractions. A transformation from elongated globules to compact spherical drops alters the balance between the entropy and effective interglobule interactions, leading to the remixing of the globular system. Our theory qualitatively explains the closed-loop coexistence regions seen in recent experiments on nonionic surfactant microemulsions.