Molecular Simulation of Mixtures Containing Imidazolium- and Pyridinium-Based Ionic Liquids and 1-Butanol

A molecular simulation study of mixtures containing 1-butanol and various imidazolium and pyridinium-based ionic liquids that form liquid-liquid equilibrium is reported. Solutions of 1-butanol and 1-n-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF(4)]), 1-n-butyl-3-methylimidazolium bis(trifluoromethane-sulfonyl) imide ([bmim][Tf(2)N]), 1-n-butyl-3-methylpyridinium tetrafluoroborate ([bmpy][BF(4)]), and 1-n-butyl-3-methylpyridinium bis(trifluoromethane-sulfonyl) imide ([bmpy][Tf(2)N]) are examined using isothermal-isobaric molecular dynamics simulations. Quantities computed include molar volumes, self-diffusivities, radial distribution functions, local composition functions, and interaction energies. The simulations help explain recent experimental results in which [bmpy][BF(4)] was found to have a lower upper critical solution temperature (UCST) with 1-butanol than [bmim][BF(4)]. This is due to weaker cation/anion association in the [bmpy] mixture, which in turn allows stronger alcohol/anion and alcohol/cation association in this system relative to [bmim][BF(4)]. The simulations also help explain why [bmim][Tf(2)N] has a lower UCST with 1-butanol than [bmpy][Tf(2)N]. This behavior is attributed to weak cation/anion association in both systems and a stronger alcohol/cation interaction with the [bmim] cation relative to the [bmpy] cation.