Structural and dynamical aspects of extremely swollen lyotropic phases

Abstract

In this contribution, the composition-temperature phase diagram of a swollen microemulsion system was investigated using high resolution small-angle X-ray scattering to explore the morphological landscape over the selected region of interest. As the oil content was increased, the morphology transformed from a bicontinuous microemulsion, XD < 0.6, to a disordered emulsified-microemulsion, XD > 0.6, consisting of a dense packing of microemulsion droplets, and a hexagonal phase at XD = 0.6. During the heating from room temperature up to the highest temperature, T = 304 K, each composition transitioned into the disordered emulsified-microemulsion morphology, however, while cooling more complex structural changes were observed. Interestingly, for each composition the same sequence of phase transitions was observed during the cooling process, first there was a transition to the emulsified hexagonal phase and then to the bicontinuous microemulsion, likely due to further expulsion of oil, resulting in a composition that would favour a bicontinuous morphology. The dynamics of the microemulsion droplets were probed using X-ray photon correlation spectroscopy, which revealed two relaxation modes, typically found in dense colloidal systems below their glass transition. These results demonstrate the importance of a combined structural and dynamical approach in the studies of dense microemulsion systems, relevant for many industrial applications.