Transition from a sponge-like to a foam-like nanostructure in a water-rich L3 phase: A confirmation

Abstract

Early studies on water - n-alkane - ionic surfactant microemulsions provide first hints for the possible existence of a foam-like nanostructure, i.e. a dense packing of polyhedral nanometer-sized water droplets separated by a thin layer of a continuous oil phase. Indeed, we found a foam-like structure in the system water/NaCl - hexyl methacrylate (C₆MA) - dioctyl sulfosuccinate sodium salt (AOT). We were able to locate an isotropic one-phase channel, the L₃ phase, emanating from the pseudo-binary system water/NaCl - AOT at ambient temperature and extending towards lower NaCl content with increasing oil content. We showed in our previous work that already upon addition of small amounts of oil to the L₃ phase the conductivities become very low and the viscosities very high. Freeze fracture electron microscopy allowed us to visualize the anticipated foam-like nanostructure. To complement our previous work, we investigated the structural transition in the L₃ channel by NMR self-diffusion measurements. The new data unambiguously confirm the existence of a foam-like structure. Based on this confirmation we offer an explanation for the topological transition to a foam-like structure, which one can also consider as a “super-swollen reverse micellar phase” – the first of its kind reported so far.