When two ionic liquids are mixed, a new ionic fluid is formed, composed of three or four different ions. This new ionic fluid possesses different physico-chemical properties, sometimes not easily estimated from those of the original salts. The properties of the multiple-ion fluid can be explained by a balance between the interactions (electrostatic and dispersive) of the different ions and by the microscopic structure of the new liquid phase. We have studied the molecular interactions and the structure of the multiple ionic fluid formed by the mixture of two ionic liquids using calorimetric measurements and molecular simulation. The enthalpy of mixing of imidazolium and pyrrolidinium-based ionic liquids with different anions was measured using an isothermal titration nanocalorimeter. The measured enthalpies of mixing are always positive indicating that the molecular interactions in the presence of unlike cations are less favorable than in the pure salts. Comparison across the homologous series CnC1Im+ sharing the same anion shows that the unfavorable interactions are more significant as the difference between the alkyl chain lengths of the cations in the mixture increases. Ionic fluids containing two anions and a common cation also exhibit endothermic mixing, often with higher mixing enthalpies than in the multiple-ion fluids with only one anion.The molecular structure and the ion-ion interactions in the ionic liquid mixtures were studied using molecular dynamics simulation. Significant differences in the microscopic structure were found depending on the imidazolium and pyrrolidinium-based ionic liquids originally mixed. These differences could explain the thermodynamics of mixing and also the diffusivities of the different ions in the mixture that were calculated from the simulation data.

Mixtures of ionic liquids: Energetics of mixing and mass transport

248th National Meeting (2014)

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