Fine probing the effect of replacing [PF6]− with [PF3(C2F5)3]− on the local structure and nanoscale organization of [bmim]+-based ionic liquids using MD simulation

Comparative all-atom molecular dynamics simulations are used to study the microscopic local structure and interionic interactions of two ionic liquids (ILs) composed of the 1-butyl-3-methylimidazolium cation, [bmim]+, coupled with the hexafluorophosphate, [PF6]−, or tris(pentafluoroethyl)trifluorophosphate, [FAP]−, anions. Respective distribution functions clearly reveal that the structural correlations between the cation and anion decrease when (i) replacing [PF6]− with [FAP]−, (ii) scaling the partial atomic charges, and (iii) considering the anion's structural flexibility versus rigidity. Replacement of [PF6]− with [FAP]− expands the nonpolar domains totally and causes the decreasing of the three-dimensional polar networks as well as the diminishing of the nano-aggregation of cation side chains. Current simulations show that with increasing the anion size and its charge delocalization, the probability of the in-plane cation–anion conformation, its related hydrogen bond acceptor ability, and the cation–cation π–π interaction decreases in accordance with the fluidity enhancements of the corresponding imidazolium-based IL. Hence, structural findings can explain and justify rationally the origins of the observed trends in the simulated dynamical properties of these ILs in our previous report. A complete understanding of the microscopic structure of ILs is necessary to control the outstanding properties of ILs as designer solvents that will support experimentalists for the best engineering design and a breakthrough efficiency of IL-related processes.