Molecular dynamics study of the ionic liquid 1-n-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([C6C1im][FAP]): Force field development and the effect of [FAP]− isomer content on properties

A new all-atom force field for the tris(pentafluoroethyl)trifluorophosphate ([FAP]${}^{-}$) anion was developed and paired with an existing force field for 1-n-hexyl-3-methylimidazolium ([C${}_6$C${}_1$im]${}^{+}$). Molecular dynamics simulations were employed to compute the density, shear viscosity, self-diffusivity, local dynamics, and liquid structure of [$C_6$$C_1$im][FAP]. Computed densities, isothermal compressibility, isobaric expansivity, and shear viscosities all agree well with experimental data. The octahedral geometry of the [FAP]${}^{-}$ anion results in facial ([fFAP]${}^{-}$) and meridional ([mFAP]${}^{-}$) isomers. Simulations with varying ratios of the two [FAP]${}^{-}$ isomers show modest but non-negligible differences in properties as a function of isomer composition.