Vapor-Liquid Equilibria of the Ionic Liquid 1-Hexyl-3-methylimidazolium Triflate (C6 mimTfO) with n-Alkyl Alcohols

Isobaric vapor-liquid equilibria (VLE) of binary mixtures of the ionic liquid (IL) 1-hexyl-3-methylimidazolium trifluoromethanesulfonate (C(6)mimTfO) and the n-alkyl alcohols methanol, ethanol, propan-l-ol, and butan-l-ol at three pressures are reported. Measurements were carried out at pressures of p = 1000, 700, and 500 mbar, which allowed to cover compositions in the range of 0.24-0.35 <= x (n-alkyl alcohol) <= 1.0 and temperatures in the range of 321.8 K < T < 423.6 K. The experimental data are described applying two theoretical models: (1) the nonrandom two-liquid (NRTL) model as an example of a descriptive model for calculating the excess free Gibbs energy G(E) and (2) the perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state (EoS) as an example of a model based on statistical thermodynamics. Both approaches allow a description of the phase equilibria with good accuracy (AARD(VLE,NRTL) <= 0.3% and AARD(VLE,PC-SAFT) <= 1.1%) and redraw its details, partially quantitative, when applied as a fitting algorithm (NRTL) or when applying the PC-SAFT theory. The present work is a continuation of an ongoing study, which aims at developing the structure-property relationships of ILs. With the present results, a first systematic view on the impact of the alkyl side chain length of the C(x)mim+ (x = 2, 4, 6) cation on the boiling behavior of binary IL (C(x)mimTfO)/n-alkyl alcohol mixtures is provided by scaling the boiling temperatures of the mixtures with that of the pure alcohol master plot results. Our data reveal that increasing the alkyl chain length of the C-x mim(+) cation causes an enhancement of the attractive forces between the IL and the n-alkyl alcohol moieties, resulting in an increase in the relative boiling temperature of the binary mixture.