Unexpected observation of an intermediate hexagonal phase upon fluid-to-gel transition: SDS self-assembly in glycerol

Understanding morphological transformations upon temperature-induced mesophase transitions offers mechanistic insights into the self-assembly process. We have recently reported the unexpected formation of a microfibrillar lamellar gel in SDS-glycerol mixtures above a critical gelation concentration (CGC) as low as ~2 wt%. The gel phase comprised a fibrillar structure on the microscale and a lamellar structure on the nanoscale. Here, the nanoscopic structure of the gel as a function of temperature was probed with small-angle neutron scattering (SANS). The gel underwent a gel-to-fluid transition at a critical gelation temperature, TGC = 45 °C, forming cylindrical micelles at elevated temperatures. Upon cooling, a hexagonal phase formed at ~TGC, evident from the SANS Bragg peaks. This hexagonal phase upon the fluid-to-gel transition sheds light on the gelation mechanism, in which self-assembled SDS micelles undergo a cylindrical-to-lamellar morphological transition via a hexagonal phase. This unprecedented observation also highlights the complexity of self-assembly in nonaqueous hydrogen-bonding rich media.