Understanding the Liquid Structure of [N1888][TFSI] via Polarizable Molecular Dynamics
Shehan Parmar, Georgia Institute of Technology
Ionic liquids (ILs) are favorable electrochemical reaction and gas separation media given their low volatility, high conductivity, and many other useful properties. In such applications, ILs encounter vacuum or solid interfaces in which complex structural phenomena, from orientational preference to cation-anion layering, arise. Because interfacial behavior is not easily predicted by classical theory (e.g., Gouy-Chapman-Stern theory for electrical double layers), molecular dynamics (MD) simulations play a crucial role in elucidating IL nanostructure. In this work, the bulk-phase structure of a unique, quaternary ammonium-based IL, [N1888][TFSI], is investigated. Unlike typical imidazolium-based ILs, [N1888][TFSI] a computationally demanding task of simulating a very viscous ( about 600 to 700 mPa⋅s) and bulky (cation and anion molecular volumes of 724 and 248 Å3, respectively) system. To this end, GPU-accelerated, polarizable, MD simulations were employed using the SAPT-FF ab initio force field realized in OpenMM. To mitigate the possibility of finite-size effects, a 12.0 x 12.0 x 12.0 nm domain of 1600 cation-anion pairs was generated to simulate bulk systems at varied temperatures for upwards of 50 ns. Calculated neutron scattering structure factors showed excellent agreement with experimental results. Peaks at about 0.25 Å-1 and 0.75-1.0 Å-1 indicate the formation of "polarity" domains due to a polar alkyl chains present in the [N1888] cation and long-range "charge alternation" domains, respectively. Radial and spatial distribution functions further illustrate two dominant spatial motifs, where the [TFSI] anion nitrogen and oxygens show preferential coordination with the cationic nitrogen. Further visualization provided insights into the complex nature of the near-liquid crystalline behavior of [N1888][TFSI] and its influence on long-range ordering.
Abstract Author(s): Shehan Parmar, Jesse McDaniel