Energetics behind the slope change in vaporisation enthalpy versus cation alkyl chain length of [Cnmim][PF6] ionic liquids: an experimental and computational study.

The vaporisation enthalpy as a function of alkyl chain length was studied for 1-alkyl-3-methylimidazolium hexafluorophosphate ([Cnmim][PF6]) ionic liquids (ILs) with both an experimental technique and molecular dynamics (MD) simulations. It was found that the vaporisation enthalpy increases linearly with increasing alkyl chain length, but with different slopes depending on the length of the cation alkyl chain. The enthalpy versus chain length slope for ILs with short alkyl chain lengths of n ≤ 10 is smaller than that for the ILs with long alkyl chain lengths n > 10. Energetic analyses based on MD simulations reveal that the electrostatic contribution to the vaporisation enthalpy is larger than the van der Waals (vdW) contribution for short chain ILs, whereas the vdW contribution becomes larger than the electrostatic contribution for the long chain ILs. The transition between electrostatic and vdW dominance is the reason for the slope change observed in the vaporisation enthalpy. However, this transition is not strong enough to cause any dramatic changes in the liquid structure.