(553a) Quantifying the Role of Polarizability in the Self-Organization of Ionic Liquid Crystals

At the intersection of ionic liquids and liquid crystals exists an intriguing class of materials which incorporate the properties of both. These ionic liquid crystals (ILCs) have many potential applications, such as tunable conductive materials or greener solvents. ILCs are typically comprised of bulky ionic groups with at least one species having mesogenic “tails”, and are reminiscent of surfactant molecules. The typically layered liquid crystalline orderings segregate the aliphatic and ionic moieties, allowing for directed ion transport separated by hydrophobic groups. By adjusting the structure of these features, these materials can be tuned to specific applications. In particular, modifications of the tail or sidechain length can change where the phase transitions occur, though a full understanding of the mechanisms of these phase changes is still lacking. We focus here on the phase behavior of a typical ionic liquid crystalline material family of 1-alkyl-3-methylimidazolium nitrate using a polariable atomistic model in molecular dynamics simulations, with an emphasis on the structure, stability, and transport of these ionic liquid crystal phases.

While fixed-charge atomistic models have been shown to be able to capture the qualitative phase behavior of ionic liquid crystals, phase transition temperatures are often overestimated by a hundred kelvin or more. We hypothesize that this is due to improperly balanced Coulomb and van der Waals forces arising from pragmatic charge-scaling of molecules described by generalized force fields, and test this by simulating the phase transitions using the AMOEBA force field, a rigorous implementation with responsive multipole moments applied to each atomic site. The method of polarizability and the additional protocols for fitting the force field allow for a more physical representation of the balance between short- and long-range interactions. We will further discuss implications for anisotropic ordering of charged materials, and mesophase orderings within them.