Saturation Properties of 1‑Alkyl-3-methylimidazolium Based Ionic Liquids

We
study the liquid–vapor saturation properties of room temperature
ionic liquids (RTILs) belonging to the homologous series 1-alkyl-3-methylimidazolium
bis­(trifluoromethylsulfonyl)­imide ([Cnmim]­[NTf2]) using Monte Carlo simulation. We examine the
effect of temperature and cation alkyl chain length n on the saturated densities, vapor pressures, and enthalpies of vaporization.
These properties are explicitly calculated for temperatures spanning
from 280 to 1000 K for RTILs with n = 2, 4, 6, 8,
10, and 12. We also explore how the identity of the anion influences
saturation properties. Specifically, we compare results for [C4mim]­[NTf2] with those for 1-butyl-3-methylimidazolium
tetrafluoroborate ([C4mim]­[BF4]) and 1-butyl-3-methylimidazolium
hexafluorophosphate ([C4mim]­[PF6]). Simulations
are completed with a recently developed realistic united-atom force
field. A combination of direct grand canonical and isothermal–isobaric
temperature expanded ensemble simulations are used to construct phase
diagrams. Our results are compared with experimental data and Gibbs
ensemble simulation data. Overall, we find good agreement between
our results and those measured experimentally. We find that the vapor
pressures and enthalpies of vaporization show a strong dependence
on the size of the alkyl chain at low temperatures, whereas no particular
trend is observed at high temperatures. Finally, we also discuss the
effect of temperature on liquid phase nanodomains observed in RTILs
with large hydrophobic groups. We do not observe a drastic change
in liquid phase structure upon variation of the temperature, which
suggests there is not a sharp phase transition between a nanostructured
and homogeneous liquid, as has been suggested in earlier studies.