Thermodynamics and selectivity of separation based on activity coefficients at infinite dilution of various solutes in ionic liquid [HMMIM][BF4]

Highlights: • Measurements of activity coefficients at infinite dilution using GLC. • The excess thermodynamic functions, entropies, and Gibbs energies were calculated. • The solubility parameter of the IL [HMMIM][BF₄] was determined. • Selectivity and capacity for hexane (i)/benzene (j), cyclohexane (i)/benzene (j). • The linear free energy relationship (LFER) analysis of the results was performed. The activity coefficients at infinite dilution (${\gamma }_i^{\infty }$), gas–liquid partition coefficients ($K_L$) of organic solutes (n-alkanes, alkenes, alkyl benzenes, acetonitrile, acetone, tetrahydrofuran, ethyl acetate, chloromethanes, alcohols and 1,4-dioxane) in the ionic liquid 1-Hexyl-2,3-dimethylimidazolium tetrafluoroborate ([HMMIM][BF₄]) have been measured using the gas–liquid chromatographic method (GLC) at five temperatures: 313.15 K, 323.15 K, 333.15 K, 343.15 K, 353.15 K. The density of [HMMIM][BF₄], as a function of temperature, was determined in atmosphere. The values of the partial molar excess enthalpies at infinite dilution (${\bar{H}}_i^{\text{E}{\text{,}}_{\infty }}$) were derived from the temperature dependence of the In ${\gamma }_i^{\infty }$ values. The entropies ($T_{\mathrm{ref}}{\bar{S}}_i^{\text{E}{\text{,}}_{\infty }}$) and Gibbs energies (${\bar{G}}_i^{\text{E}{\text{,}}_{\infty }}$) of organic solutes in [HMMIM][BF₄] at a reference temperature $T_r\mathrm{ef}$ = 298.15 K were calculated from the ${\gamma }_i^{\infty }$ values. The Hildebrand's solubility parameters of the IL [HMMIM][BF₄] were determined by the regular solution theory (RST) combined with Flory ''combinatorial'' equation. Selectivity ($S_{\mathrm{ij}}^{\infty }$) and capacity ($k_j^{\infty }$) at infinite dilution at 323.15 K have been determined for n-hexane ($i$)/benzene (j), cyclohexane ($i$)/benzene (j). The results were analyzed in comparison to literature data for alkylimidazolium ionic liquids (ILs) with ${\left[{\text{BF}}_4\right]}^{-}$. The interaction between the IL and the individual solutes can be disclosed by the linear free energy relationship (LFER) analysis.