Phase coexistence in [C₂₂/C₁MIm]⁺[NO₃]^- ionic-liquid mixtures and first-order phase transitions from homogeneous liquid to smectic B by varying the cation ratio

We perform molecular dynamics simulations to investigate the transition processes of [C₂₂/C₁MIm]⁺[NO₃]^- binary mixtures by varying the cation ratio of C₂₂ to C₁ at a fixed temperature of 400 K. The cation ratio is tuned by ranging C₂₂ percentage from 0% to 100% with a fixed number of 4096 total simulated ion pairs. Our simulated-annealing results indicate that, at 400 K, pure C₁ is a homogeneous liquid whilst pure C₂₂ is an ionic liquid crystal (ILC) of smectic-B (SmB) type. With increasing C₂₂ percentage, the system goes through a first-order phase transition from homogeneous liquid to nano-fragment liquid in the range from 15% to 17.5%, during which some of the individual cationic alkyl side chains locally aggregate to form small bundles "floating" in the polar "solvent" composed of anions and cationic head groups. Although the side chains in each bundle are parallelly aligned, the bundles distribute randomly without a global orientation. As the C₂₂ percentage further increases, another first-order phase transition occurs to bring the system into the SmB ILC phase. Particularly, when the C₂₂ percentage is in the range from 45% to 50%, the SmB phase coexists with the liquid phase containing both individual and bundled alkyl side chains.