Making Sense of Enthalpy of Vaporization Trends for Ionic Liquids: New Experimental and Simulation Data Show a Simple Linear Relationship and Help Reconcile Previous Data

Abstract

Vaporization enthalpy of an ionic liquid (IL) is a key physical property for applications of ILs as thermofluids and also is useful in developing liquid state theories and validating intermolecular potential functions used in molecular modeling of these liquids. Compilation of the data for a homologous series of 1-alkyl-3-methylimidazolium bis(trifluoromethane-sulfonyl)imide ([C(n)mim][NTf2]) ILs has revealed an embarrassing disarray of literature results. New experimental data, based on the concurring results from quartz crystal microbalance, thermogravimetric analyses, and molecular dynamics simulation have revealed a clear linear dependence of IL vaporization enthalpies on the chain length of the alkyl group on the cation. Ambiguity of the procedure for extrapolation of vaporization enthalpies to the reference temperature 298 K was found to be a major source of the discrepancies among previous data sets. Two simple methods for temperature adjustment of vaporization enthalpies have been suggested. Resulting vaporization enthalpies obey group additivity, although the values of the additivity parameters for ILs are different from those for molecular compounds.

Structure of protic (HCnImNTf2, n=0–12) and aprotic (C1CnImNTf2, n=1–12) imidazolium ionic liquids: A vibrational spectroscopic study

“…Also, the change in slope after the propyl side chain systems in both PILs and APILs maybe resulted from a delicate competition between intramolecular and inter-ionic interactions i.e., I+ induction effect (APILs) and π-π interactions (PILs). Similar results were shown by Rocha et al [52] and Verevkin et al [53] who calculated the enthalpy and entropy changes for vaporization (ΔΗ vap and ΔS vap ) of the aprotic ionic liquids C 1 C n ImNTf 2 as a function of the alkyl chain length. Both found changes in the slope of the quantities as a function of alkyl chain length and attributed to the decrease of the electrostatic interactions and increase of van der Waals interactions.…”

Vapor pressures of 1,3-dialkylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids with long alkyl chains

“…The most recent data of The trends detected on the thermodynamic properties of vaporization of [C N-1 C 1 im][NTf 2 ] which presents an experimental evidence of the so-called nanostructuration/segregation of ionic liquids has been intensively discussed. 35 Despite of the efforts to clarify the existing discrepancy on the enthalpies of vaporization values published in the literature, the question remains yet to be solved. Nevertheless, the effect of nanostructuration/segregation of ionic liquids on the static and dynamic properties of these compounds has been established in previous works.…”

“…33,34,[36][37][38][39][40][41][42] A critical analysis of the enthalpies of vaporization for the [C N-1 C 1 im][NTf 2 ] ionic liquid series and the dependency of the data with the chain length of the alkyl group of the cation has been recently published. 35 Therefore, and considering the fact that literature data for this new ionic liquids are not available, in this work the discussion will be made focusing on the data obtained based on experimental vapor pressures. In Figure 2 This effect is related with the predominant orientation of the terminal methyl group of the alkyl chain to the imidazolium ring and their influence in the cation-anion interaction.…”

“…This findings support the previous interpretation of the experimental thermophysical results published on the literature. 33,34 Despite the controversy around the so called trend shift on the enthalpies of vaporization, 35 the effect nanostructuration in ionic liquids was already detected in several other properties. 33,34,[36][37][38][39][40][41][42] This work presents vapor pressure data for the 1, )] spectrometer, and the solvent used was DMSO D6.…”

Room temperature ionic liquids: A simple model. Effect of chain length and size of intermolecular potential on critical temperature

“…30 The RTIL [C 1 mim][BF4] and the restricted primitive model were used to test the method. 29 Two communications by Verevkin et al 31 reconciling the experimental and simulation enthalpy of evaporation and a report of vapor pressure by Rocha et al 32 make a thorough revision of the published data up to date.…”