Structural Properties of [N1888][TFSI] Ionic Liquid: A Small Angle Neutron Scattering and Polarizable Molecular Dynamics Study.

Abstract

In this study, we investigate the quaternary ammonium-based ionic liquid (QAIL), methyltrioctylammonium bis(trifluoromethylsulfonyl)imide, [N₁₈₈₈][TFSI], utilizing small angle neutron scattering (SANS) measurements and polarizable molecular dynamics (MD) simulations to characterize the short- and long-range liquid structure. Scattering structure factors show signatures of three length scales in reciprocal space indicative of alternating polarity (k ∼ 0.44 Å^-1), charge (k ∼ 0.75 Å^-1), and neighboring or adjacent (k ∼ 1.46 Å^-1) domains. Excellent agreement between simulation and experimental scattering structure factors validates various simulation analyses that provide detailed atomistic characterization of the different length scale correlations. The first solvation shell structure is illustrated by obtaining radial, angular, dihedral, and combined distribution functions, where two dominant spatial motifs, N⁺···N^- and N⁺···O^-, compete for optimal packing around the polar head of the [N₁₈₈₈]⁺ cation. Intermediate and long-range structures are governed by the balance between local electroneutrality and octyl chain networking, respectively. By computing the charge-correlation structure factor, S_ZZ, and the spatial extent of the octyl chain network using graph theory, the bulk-phase structure of [N₁₈₈₈][TFSI] is characterized in terms of electrostatic screening and apolar domain formation length scales.