Transition Mechanism from the Metastable Two-Dimensional Gel to the Stable Three-Dimensional Crystal of...

One important quest for making high quality materials with amphiphiles is to understand how a disordered self-assembly changes to a stable crystalline state. Herein, we addressed the basic question by investigating the phase transition mechanism of imidazolium-based ionic liquid (IL) [C₁₆mim]Br, using time-resolved small- and wide-angle X-ray scattering (SAXS-WAXS), differential scanning calorimetry, and Fourier transform infrared spectroscopy techniques. Totally, a hexagonal phase, two lamellar-gel phases, and three lamellar-crystalline phases were observed, showing the special polymorphism of the system. It was demonstrated that at low concentrations the two-dimensional gel phase (L_β1) transforms into the most stable lamellar-crystal phase (L_c3) through two intermediate crystalline phases L_c1 and L_c2. At high concentrations, the L_β1 phase changes to a condensed lamellar gel phase (L_β2) before changing to L_c2 and eventually to L_c3. Comparative studies using [C₁₆mim]Cl and [C₁₆mim]NO₃ unveiled that the interactions between the counterions and the headgroups of the IL, as well as the dehydration process, govern the nucleation process of L_c3 and thus the formation of the crystal. The in-depth investigation on the transition mechanism and the phase polymorphism in the present work advances our understanding of the crystallization of amphiphilic ionic liquids in dispersions and would promote future applications.