Composition and Temperature Dependence of Excess Properties of Binary Mixtures of Imidazolium Based Ionic Liquids: II ([C n mim][PF6]) + Propylamine

Abstract

The experimental data of densities, ρ, speeds of sound, u, and refractive indices, $$ n_{\text{D}} $$ , 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 $$ V_{\text{m}}^{\text{E}} $$ , excess molar isentropic compressibilities $$ K_{S,\text{m}}^{\text{E}} $$ , and refractive index deviations $$ \Delta_{\varphi } n_{\text{D}} $$ were derived and correlated with the Redlich–Kister polynomial equation. The values of $$ V_{\text{m}}^{\text{E}} $$ and $$ K_{S,\text{m}}^{\text{E}} $$ are negative and decrease with increase in temperature whereas the $$ \Delta_{\varphi } n_{\text{D}} $$ are positive and increase with temperature for all studied systems. $$ V_{\text{m}}^{\text{E}} $$ and $$ K_{S,\text{m}}^{\text{E}} $$ 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. $$ V_{\text{m}}^{\text{E}} $$ was also predicted from the Prigogine–Flory–Patterson theory.