Densities of the binary mixtures of organic solvents (acetic acid (HAc), n‐butanol (n‐BuOH) and n‐ butyl acetate (n‐BuAc)) with 1‐ethyl‐3‐methylimidazolium hydrogen sulphate {[EMIM][HSO4]: IL1}, 1‐ethyl‐3‐methyimidazolium ethyl sulphate {[EMIM][EtSO4]: IL2}, 1‐butyl‐3‐methylimidazolium acetate {[BMIM][OAc]: IL3}, 1‐butyl‐3‐methylimidazolium bis (trifluoromethylsulfonyl)imide, {[BMIM][NtF2]: IL4}} and/or Deep Eutectic Solvents (DESs) {choline chloride‐acetic acid [ChCl][AA]: DES1}, choline chloride‐glycerol [ChCl][Gly]: DES2), were measured over the entire composition range at T (293.15–343.15) K and 1 atm. The volumteric properties and derived thermodynamic properties of mixed solvents were calculated from the experimental data in order to understand the specific interactions and/or non‐specific interactions between mixed solvents during mixing at 298.15 K and 1 atm. Further, the molecular polarity of all the studied components were analyzed using COSMO‐RS model.

Liquid densities of deep eutectic solvents (DESs) such as choline chloride + glycerol {[ChCl]­[Gly]}, choline chloride + acetic acid {[ChCl]­[AA]}; ionic liquids (ILs), such as 1-ethyl-3-methylimidazolium hydrogen sulfate {[EMIM]­[HSO4]}, 1-ethyl-3-methylimidazolium ethyl sulfate {[EMIM]­[EtSO4]}, and 1-butyl-3-methylimidazolium acetate {[BMIM]­[Ac]}; and mixture of DESs + ILs as well as organic mixtures, such as n-butyl acetate + butanol and n-butyl acetate + acetic acid were measured at different temperatures from 293.15 to 343.15 K with an increase of 5 K. From this measured density, isobaric expansivity, excess molar volume, partial molar volume, excess partial molar volume, and apparent molar volume were calculated to understand and characterize the solution behavior as well as solvent–solvent interactions for the entire mole fraction at different temperatures. The excess molar properties showing positive deviation for DESs + ILs system, which indicates a creation of volume in the binary mixtures and disruption of H-bonds in the solution environment. Further, the sign and magnitude of all of the studied binary systems gave a good estimate of the attractive and disruptive interactions, chemical aggregation, H-bond formation, salt (choline chloride)–H (imidazolium) interaction, and salt (choline chloride)–O (acetate anion) interaction between those two components in the binary mixtures. The Redlich–Kister equation was used to fit the excess molar volume data and their corresponding regression coefficients (R 2) were also found. The σ-profile and σ-potential for the compounds in the present study have been generated and analyzed by means of specific molecular interactions between all of the studied compounds using the conductor like screening model for real solvents model.

The solution thermodynamic properties of mixed ionic liquids such as density, excess molar volume, partial molar volume and apparent molar volume are highly influential on the design of an effective separation unit and in the optimization of operating parameters like pressure, temperature and concentration for the separation processes. Therefore, it could be better to do a prior experimental study on of the solution thermodynamic properties of mixed ionic liquids at different temperatures for the whole mole fractions rather than doing separation characterization studies like selectivity, efficiency, distribution coefficient and performance index. In addition, the recovery and regeneration of ionic liquids also challenge researchers and separation scientists in several fields of applications. Therefore, in this chapter, density of pure 1-butyl-3-methyl imidazolium bis (trifluoromethylsulfonyl) imide {[BIMIM][NtF 2 ]}, 1-ethyl-3-methylimidazolium ethyl sulfate {[EMIM][ESO 4 ]}, 1-ethyl-3-methylimidazolium hydrogen sulfate {[EMIM][HSO 4 ]} and 1-butyl-3-methylimidazolium acetate {[BMIM][OAc]} and its binary mixtures have been measured at T (293.15-343.15) K. From the measured densities, isobaric expansivity, excess molar volume, partial molar volume, excess partial molar volume, and apparent molar volume have been calculated. Results were discussed in terms of physical interaction, chemical interaction and structural orientation at molecular level and their temperature and composition dependency.