Improved coarse-grain model to unravel the phase behavior of 1-alkyl-3-methylimidazolium-based ionic liquids through...

Imidazolium-based ionic liquids (ILs) in water exhibit a surfactant-like behavior that is only partially characterized by experimental techniques with molecular dynamic (MD) simulations emerging as a complimentary tool to study their phase behavior. However, while atomistic models suffer of time and size scale limitations, higher-level models (e.g. coarse-grain) are still of limited applicability, accuracy, and transferability.

A robust and transferable CG model for 1-alkyl-3-methylimidazolium halides [C_nmim][X], using the MARTINI forcefield (FF), was proposed and validated against all-atom (AA) simulations and existing experimental data. A systematic study on the effect of the alkyl chain length, IL concentration, and temperature on the phase behavior of [C_nmim][Cl] aqueous solutions was performed.

At low amphiphile concentrations, the micellar regime extends from the critical micellar concentration (cmc) up to 10-25 wt%, depending on the alkyls chain length, where a sphere-to-rod transition is observed. The aggregation numbers of the spherical micelles were found to be in good agreement with experiments and, as the concentration was increased, a variety of mesophases was observed, providing useful insights into these systems. Furthermore, the segregation of IL moieties into polar and non-polar domains in ILs, possessing short alkyl tails, was demonstrated.