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

Abstract

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 [C(4)mim][NTf2] with those for 1-butyl-3-methylimidazolium tetrafluoroborate ([C(4)mim][BF4]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([C(4)mim][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.

Liquid–vapor equilibrium and critical parameters of the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate from molecular dynamics simulations

“…In addition to the results specific to [bmim][PF 6 ], the corresponding-states properties are of interest because they seem to be rather generic for room temperature ionic liquids (RTILs) and do not depend too sensitively on the chemical identity of the RTIL. In good agreement with the data reported by Rai and Maginn [16,17] and by Rane and Errington [18,19] for other ionic liquids (see below), the critical compressibility factor Z c of 0.03-0.05 is found to be markedly smaller than for non-ionic fluids (Z c ≈ 0.3 for simple fluids), but not far from the estimate of Z c = 0.029 for the simplest generic model of ionic fluids, the restricted primitive model (RPM), which consists of equisized charged hard spheres [18,47]. Guggenheim's ratio, relating the apparent enthalpy of vaporization (obtained as the supposedly constant slope of a plot of lnP vs. 1/T) to the critical temperature, Δ vap H app /(RT c ), is about 10, significantly larger than the value of 5-6 found for simple fluids, and it thus turns out to be in the range estimated for ionic liquids from experimental data and for the generic ionic fluid model RPM from simulations [15].…”

“…Guggenheim's ratio measures the apparent enthalpy of vaporization in relation to the critical temperature, Δ vap H app /(RT c ). For simple fluids, this ratio is typically 5-6; the value of 9.9 ± 0.5 obtained here is significantly larger, but in the range encompassed for RTILs (9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) based on experimental data [15].…”

“…The vapor pressure of RTILs was studied by means of different variants of Monte Carlo (MC) simulations by Rai and Maginn [16,17] using direct Gibbs ensemble MC (GEMC) and by Rane and Errington [18,19] using temperature expanded ensemble (TEE) MC techniques allowing them to obtain data also at low and intermediate temperatures. (To the best of our knowledge, there have been no attempts to obtain vapor pressures directly from MD simulations, perhaps because this approach is restricted to very high temperatures for RTILs.)…”

“…Furthermore, we estimate the correlation length from the thickness of the liquid-vapor interface. Subsequently, we calculate several corresponding-states properties and compare them to those of generic ionic and non-ionic model fluids [15] and to the results for molecular models of RTILs evaluated by means of Monte Carlo simulation [16][17][18][19] to test the applicability of different methodologies.…”

Novel 2-alkyl-1-ethylpyridinium ionic liquids: synthesis, dissociation energies and volatility

“…50 The non-electrostatic potential is expected to increase nearly linearly along the alkyl chain length. [80][81][82] The graphic representations of the standard molar enthalpies and entropies of vaporization at reference temperature, T = 460 K, as a function of the total number of carbon atoms in the two alkyl side chains of the cation, N, are presented in Fig. 3 50,76 ionic liquid families decreases, being however in all the cases higher, indicating that the electrostatic interactions contribution should be slightly higher in the pyridinium based ILs than in the imidazolium ILs.…”