Polymer-Induced Structural Transitions in Microemulsions

We predict, qualitatively, the structural transitions in microemulsion systems induced by nonionic polymers, by virtue of their ability to associate with the surfactant assemblies. The predictions are made possible by linking (a) theoretical calculations of the domains of existence of oil-in-water (O/W) and water-in-oil (W/O) droplet microemulsions with (b) the experimental observation that on increasing the alcohol-to-surfactant ratio, an O/W droplet microemulsion phase inverts to a W/O droplet microemulsion, often via a bicontinuous microemulsion. We predict five distinct types of polymer-induced structural transitions: (I) an OIW droplet microemulsion remains an O/W droplet system but the amount of oil solubilized and the radius of the droplet are both reduced; (11) a bicontinuous microemulsion transforms into an O/W droplet microemulsion; (111) a WIO droplet microemulsion inverts to an O/W droplet microemulsion with the droplet radius either decreasing in size (IIIA) or increasing in size (IIIB); (IV) a WIO droplet microemulsion transforms into a bicontinuous microemulsion; (V) a WIO droplet microemulsion remains a W/O system but the droplet radius and the amount of water solubilized are both increased. We also predict that the addition of polymers cannot lead to the following types of transitions: (VI) an OIW droplet microemulsion cannot invert to a W/O droplet microemulsion; (VII) an 01 W droplet microemulsion cannot transform into a bicontinuous microemulsion; (VIII) a bicontinuous microemulsion cannot transform into a W/O droplet microemulsion. From a practical point of view, the above predictions open up an effective approach to the manipulation of microemulsion structures using nonionic-polymer molecules as additives.