Transition Mechanism from the Metastable Two-Dimensional Gel to the Stable Three-Dimensional Crystal of Imidazolium-Based Ionic Liquids

Oneimportant quest for making high quality materialswith amphiphilesis to understand how a disordered self-assembly changes to a stablecrystalline state. Herein, we addressed the basic question by investigatingthe phase transition mechanism of imidazolium-based ionic liquid(IL) [C(16)mim]Br, using time-resolved small- and wide-angleX-ray scattering (SAXS-WAXS), differential scanning calorimetry, andFourier transform infrared spectroscopy techniques. Totally, a hexagonalphase, two lamellar-gel phases, and three lamellar-crystalline phaseswere observed, showing the special polymorphism of the system. Itwas demonstrated that at low concentrations the two-dimensional gelphase (L-& beta;1) transforms into the most stable lamellar-crystalphase (L-c3) through two intermediate crystalline phasesL(c1) and L-c2. At high concentrations, the L-& beta;1 phase changes to a condensed lamellar gel phase (L-& beta;2) before changing to L-c2 and eventuallyto L-c3. Comparative studies using [C(16)mim]Cland [C(16)mim]NO3 unveiled that the interactionsbetween the counterions and the headgroups of the IL, as well as thedehydration process, govern the nucleation process of L-c3 and thus the formation of the crystal. The in-depth investigationon the transition mechanism and the phase polymorphism in the presentwork advances our understanding of the crystallization of amphiphilicionic liquids in dispersions and would promote future applications.