Interactions of volatile organic compounds with the ionic liquid 1-ethyl-3-methylimidazolium tetracyanoborate

Abstract

Interactions of volatile organic compounds with the ionic liquid (IL) 1-ethyl-3-methylimidazolium tetracyanoborate [EMIM][B(CN) 4] were explored through systematic GLC retention measurements. Infinite dilution activity coefficients γ 1 ∞ and gas–liquid partition coefficients K L of 30 selected solutes in [EMIM][B(CN) 4] were determined at five temperatures in the range from (318.15 to 353.15) K and respective dissolution enthalpies H ¯ 1 E, ∞ and entropies S ¯ 1 E, ∞ were derived. Reliability of our results was substantiated by additional measurements we carried out using the inert gas stripping technique, IL samples from two different batches, and careful purity checks. Since two similar experimental studies on [EMIM][B(CN) 4] have emerged during the progression of this work, a unique possibility has arisen for direct inter-laboratory comparison of results. While the present values and those of Yan et al. [P.F. Yan, M. Yang, X.M. Liu, C. Wang, Z.C. Tan, U. Welz-Biermann, J. Chem. Thermodyn. 42 (2010) 817–822] show disparities as large as 30% in γ 1 ∞ and 6 kJ · mol −1 in H ¯ 1 E, ∞ , the present values and those of Domanska et al. [U. Domanska, M. Krolikowska, W.E. Acree, G.A. Baker, J. Chem. Thermodyn. 43 (2011) 1050–1057] exhibit almost perfect agreement. The Linear Free Energy Relationship (LFER) analysis of the results was performed to disclose molecular interactions operating between the IL and the individual solutes. [EMIM][B(CN) 4] was identified as a rather weakly cohesive IL. Comparison with [EMIM][NO 3] and [EMIM][MeSO 3], two [EMIM] based ILs with hard anions we studied recently, reveals much lower capability of [EMIM][B(CN) 4] to accept hydrogen bonds and interact with lone electron pairs of solutes. In contrast with the former ILs, [EMIM][B(CN) 4] was found to possess however appreciable proton donating capability. This striking observation demonstrates that the behavior of [EMIM] cation is affected by its counterion: while in case of the former ILs the acidic hydrogen in position 2 on the [EMIM] ring is blocked interacting with the hard anions, in case of [EMIM][B(CN) 4] this hydrogen atom is made more available for hydrogen bonding with basic solutes thanks to the soft character of [B(CN) 4] anion.