(95d) Atomistic Simulation of Ionic Liquid Crystals

Ionic liquids and liquid crystals have been a popular research subject, reflecting their many potential applications in technology and industry, though the intersection of these two fields has seen much less attention. Recently this has changed, as ionic liquid crystals (ILCs) have demonstrated promising behavior as tunable conductive materials and green solvents. These systems are composed of bulky ionic groups attached to mesogenic “tails” in a way which resembles common surfactants; the charge and tail structure are tunable and useful for applications in green solvents or charge transport media. As the liquid crystal behavior gives rise to local ordering, the ionic moieties form mesostructures that are conducive to ion transport by separating charged regions and aliphatic apolar regions. Layers form in these materials that effect an anisotropy in molecular mobility and hence ion transport.

Mesophases have long been observed in molten salt systems, but tuning the molecules to reach liquid crystal phases closer to room-temperature conditions has been of more recent interest. While it is known that the organization and thermal stability of ILC phases may be altered by changing the headgroup and tail length, comprehensive information about the structure of this phase is still lacking. Here we discuss our examination of a typical ILC material (1-alkyl-3-methylimidazolium nitrate) utilizing fully atomistic molecular dynamics simulations to elucidate the structure, phase stability and charge transport of smectic ionic liquid crystal phases. In investigating the phase transition points, we employ advanced sampling techniques to minimize hysteresis in the system. We discuss in particular our examination of a novel, apparently metastable phase that arises in this system, and the implications this has for technological applications of ILCs. We further discuss our efforts to improve accuracy in the force fields available for these systems, so that representative dynamics and thermodynamics may be obtained from simulations.