Composition and temperature dependence of excess properties of binary mixtures of imidazolium based ionic liquids: II ([C <inf>n</inf> mim][PF<inf>6</inf>]) + propylamine

Abstract The experimental data of densities, ρ, speeds of sound, u, and refractive indices, $$ n-{\text{D}} $$ n D, for pure ionic liquids, 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim][PF6]), 1-hexyl-3-methylimidazolium hexafluorophosphate ([C6mim][PF6]) and 1-octyl-3-methylimidazolium hexafluorophosphate ([C8mim][PF6]) and their binary mixtures with propylamine were measured by an Anton Paar DSA 5000 densimeter and Abbemat 300 refractometer, respectively. The measurements were performed as a function of concentration and at temperatures between 293.15 and 313.15 K and at 0.1 MPa. From these measured values, the excess molar volumes $$ V-{\text{m}}^{\text{E}} $$ V m E, excess molar isentropic compressibilities $$ K-{S,\text{m}}^{\text{E}} $$ K S, m E, and refractive index deviations $$ \Delta-{\varphi } n-{\text{D}} $$ Δ φ n D were derived and correlated with the Redlich-Kister polynomial equation. The values of $$ V-{\text{m}}^{\text{E}} $$ V m E and $$ K-{S,\text{m}}^{\text{E}} $$ K S, m E are negative and decrease with increase in temperature whereas the $$ \Delta-{\varphi } n-{\text{D}} $$ Δ φ n D are positive and increase with temperature for all studied systems. $$ V-{\text{m}}^{\text{E}} $$ V m E and $$ K-{S,\text{m}}^{\text{E}} $$ K S, m E decrease with increasing alkyl chain length of ILs. The variations in excess properties are discussed with respect to increases in temperature, addition of -CH2 groups in alkyl chain of ILs and the strength of solute-solvent interactions existing in the ILs + amine mixtures. $$ V-{\text{m}}^{\text{E}} $$ V m E was also predicted from the Prigogine-Flory-Patterson theory.