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Excess molar enthalpies, H E of 1-ethyl-3-methylimidazolium tetrafluoroborate (1) + pyrrolidin-2-one or 1-methyl pyrrolidin-2-one (2) binary mixtures at 298.15 K, densities, ρ, and speeds of sound, u, data of the same mixtures at (293.15, 298.15, 303.15, and 308.15) K have been measured over entire mole fraction range. The observed densities and speeds of sound data have been utilized to determine their excess molar volumes, V E, and excess isentropic compressibilities, κ S E. The topology of the constituents of mixtures (graph theory) has been employed to calculate the V E, H E, and κ S E of ionic liquid mixtures. The analysis of the measured data in terms of graph theory suggests that investigated (1 + 2) mixtures are characterized by interactions forming a 1:1 molecular complex, and V E, H E, and κ S E values calculated by graph theory are close to the experimental ones.

Densities, ρ 123 , and speeds of sound, u 123 , of 1-ethyl-3-methylimidazolium tetrafluoroborate (1) + 2-methylaniline (2) + pyridine or α-, β-, or γ-picoline (3) ternary mixtures have been measured over the entire range of composition at 293.15 K, 298.15 K, 303.15 K, and 308.15 K using density and speed of sound analyzer (Anton Paar DSA 5000). The heat capacity, C p , of pure liquids pyridine and α-, β-, and γ-picoline have also been measured at 293.15 K, 298.15 K, 303.15 K, and 308.15 K using differential scanning calorimeter. The measured data have been employed to determine excess molar volumes, V 123 E and excess isentropic compressibilities,(κ S E ) 123 . The V 123 E and(κ S E ) 123 data have been fitted to Redlich−Kister equation to calculate ternary adjustable parameters and standard deviations. The thermodynamic properties of the ternary mixtures have been analyzed in terms of Graph (which deals with the topology of the constituent of mixtures) and Prigogine-Flory-Patterson (PFP) theories. It has been observed that Graph theory describes well V 123 E and (κ S E ) 123 data of the studied ternary ionic liquid mixtures. However, PFP theory correctly predicts the sign of V 123 E and (κ S E ) 123 data of the studied mixtures.

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