Transition from a foam-like to an onion-like nanostructure in water-rich L₃ phases

In dilute water-surfactant systems L-3 phases are found in which bilayers interconnect to form a sample-spanning sponge-like structure. From our previous study of the system water/NaCl-AOT (sodium bis(2-ethylhexyl) sulfosuccinate) we know that a transition of this sponge-like structure to an oil-continuous foam-like structure occurs upon addition of minute amounts of oil (about 3 wt%, alpha = 0.03) in the L-3 channel at a constant surfactant mass fraction of gamma = 0.15 and T = 25 degrees C. The aim of the present study was to verify if the same transition occurs at gamma = 0.25. To achieve this goal, we determined the relevant part of the phase diagram and studied the electrical conductivities and viscosities within the narrow one-phase L-3 channel. Although the electrical conductivities and viscosities change qualitatively like those observed at gamma = 0.15 we did not observe a sponge-like structure at gamma = 0.25 in the oil-free system (alpha = 0) with freeze-fracture electron microscopy (FFEM) and freeze fracture direct imaging (FFDI). Together with the FFEM/FFDI images and SANS/SAXS curves we provide experimental evidence for a structural transition with decreasing oil content from a thermodynamically stable foam-like to a thermodynamically stable onion-like nanostructure at gamma = 0.25 rather than to a sponge-like structure as is the case at gamma = 0.15.