Anomalously high solubility behavior of methanethiol in alkylimidazolium–based ionic liquids

•Measurement solubility of CH3SH in four [C4mim] – based ionic liquids.•Evaluation of Henry’s constants for solubility of CH3SH in the ionic liquids.•Correlation of experimental data by the extended Henry’s law and Pitzer model.•Correlation of experimental data by the Redlich-Kwong equation of State.•The surprisingly higher solubility of CH3SH compared to H2S is discussed in molecular level using DFT calculations. In this work, the absorption capacity of four 1-butyl-3-methylimidazolium ([C4mim]+) ionic liquids containing tetrafluoroborate ([BF4]–), hexafluorophosphate ([PF6]–), trifluoromethanesulfonate ([OTf]–), and bis(trifluoromethyl)sulfonylimide ([Tf2N]–) anions for methanethiol has been experimentally and theoretically studied. The experimental measurements were conducted below 100 kPa and temperatures ranging from (303.15 to 343.15) K. Henry’s law constants, as well as the thermodynamic functions for dissolution of methanethiol in the ionic liquids were evaluated using the experimental solubility data and compared with the corresponding data for the solubility of hydrogen sulfide reported in the literature. The solubility of CH3SH was found to be unexpectedly higher (1.3 to 2.9 times) than that of H2S in the ionic liquids studied in this work. The comparison of Henry’s constants showed that the solubility of CH3SH in the ionic liquids follows the order: [C4mim][Tf2N] > [C4mim][PF6] ≈ [C4mim][BF4] > [C4mim][OTf]. The measured data for the solubility of CH3SH in the ionic liquids were correlated using both the γ – φ approach, based on the Pitzer’s activity coefficient model, and the φ – φ approach, on the basis of the generalized Redlich-Kwong cubic equation of state (GRK EoS). Surprisingly, the GRK EoS indicated a better correlative accuracy compared to the Pitzer’s model for the four CH3SH + IL mixtures studied in this work. Quantum chemical density functional theory (DFT) as well as atoms-in-molecules (AIM) calculations, based on M06-2X level of theory, using the 6–311++G(2d,2p) basis set, was employed to explore the irregularly high solubility behavior of CH3SH compared to H2S in the ionic liquids studied in this work. According to DFT calculations CH3SH establishes a stronger interaction with the ionic liquids compared to H2S. In addition, the interaction of the cation with the anions diminishes in the presence of CH3SH compared with H2S, which results in greater void volume available to CH3SH guest molecules compared to H2S.