Topological investigations of molecular interactions in binary ionic liquid mixtures with a common ion: Excess molar volumes, excess isentropic compressibilities, excess molar enthalpies and excess molar heat capacities

• The VE, HE  , κSE and CPE data have been measured over entire mole fraction at 4 temperatures.
• The VE, HE  , κSE and CPE data have been fitted to Redlich-Kister equation.
• The observed data have been analyzed in terms of Graph theory.
• The VE, HE  , κSE and CPE values predicted by Graph theory compare well with experimental values.

The densities, ρ, speeds of sound, u, and molar heat capacities, CP, of 1-butyl-2,3-dimethylimidazolium tetrafluoroborate (i) + 1-butyl-3-methylimidazolium tetrafluoroborate (j); 1-butyl-3-methylimidazolium tetrafluoroborate (i) + 1-ethyl-3-methylimidazolium tetrafluoroborate (j) and 1-butyl-2,3-dimethylimidazolium tetrafluoroborate (i) + 1-ethyl-3-methylimidazolium tetrafluoroborate (j) mixtures at temperatures (293.15, 298.15, 303.15, 308.15) K and excess molar enthalpies, HE, for the same mixtures at temperature (298.15) K have been measured over entire range of mole fraction under atmospheric pressure. Using experimental data, excess molar volumes, VE, excess isentropic compressiblities, κSE, excess molar enthalpies, HE and excess molar heat capacities, CPE have been determined. The VE, κSE, HE and CPE data have been fitted to Redlich-Kister equation to estimate the binary adjustable parameters and standard deviations between experimental and calculated values. The topology of the constituent molecules (Graph theory) has been utilized to predict VE, κSE, HE and CPE data of the studied mixtures. The comparison of VE, κSE, HE and CPE values determined by Graph theory with their corresponding experimental values lends additional support to the proposed molecular entities existing in mixtures along with various processes involved in the (i + j) mixtures formation. Further, IR studies and quantum mechanical calculations support the presence of proposed molecular entities in the studied mixtures.