SPONGE PHASES AND BALANCED MICROEMULSIONS - WHAT DETERMINES THEIR STABILITY

We discuss the criteria for the thermodynamic stability of the L-3 (sp onge) phase and its monolayer analogue, the balanced microemulsion, in the framework of the flexible surface model. In particular, we emphas ize that a parameter which plays a controlling role in the behavior of these phases is the monolayer spontaneous curvature, which, depending on the system being considered, may be tuned by ionic strength, tempe rature or interfacial composition. With this connection made, the phas e behavior of many experimental systems can be understood. In the case of the sponge, this phase is preferred over its competitors (for exam ple, lamellae) in a very narrow region of the phase diagram where it b est meets the energetic constraint of matching the negative monolayer spontaneous curvature. The balanced microemulsion, in contrast, is fou nd at zero spontaneous curvature, and the criteria for its stability c an then be argued from a consideration of the scaling behavior of the free energy. (C) 1997 Elsevier Science B.V.