PHASE-EQUILIBRIA OF POLYMER-CONTAINING DROPLET MICROEMULSIONS

We study theoretically the behaviour of stable microemulsion droplets under addition of polymer to the dispersed phase. For long enough polymers we assume that the structure consists of a "necklace" of droplets strung along the chain. The system is then viewed in an effective three-component picture (polymer/droplet "necklaces", polymer-free droplets, and the continuous solvent phase) with a single effective droplet-droplet interaction parameter. A simple (Flory-Huggins) estimate for the free energy is derived and the resulting phase equilibria are investigated. For attractive interactions among droplets, the added polymer can destabilise the system by reducing the translational entropy of the droplets. For the water-in-oil droplet system AOT/water/heptane, we estimate the interaction parameter as a function of droplet size and use this to predict the shift in the droplet gas/droplet liquid phase boundary under addition of polyethylene glycol to the water pool. Comparison with recent experiments suggests that the behaviour for this system is rather more complicated than predicted by our simple model, perhaps suggesting for small droplet sizes a relatively strong local deformation of the droplet structure upon addition of polymer.