How Does the Addition of a Third Ion Affect the Molecular Interactions and the Thermodynamic Properties of Acetate-Based Ionic Liquids?

Abstract

The effect of the addition of a third ion to the ionic liquid 1-butyl-3-methylimidazolium acetate [C₄C₁Im][OAc] was studied through the measurement of the enthalpy of mixing and of the excess molar volume of its mixtures with 1-butyl-3-methylimidazolium trifluoroacetate [C₄C₁Im][CF₃CO₂], 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C₄C₁Im][NTf₂], and tetrabutylphosphonium acetate [P₄₄₄₄][OAc]. Negative enthalpies of mixing (Δ_mixH < 0) and positive excess molar volumes (V^E > 0) were observed in all cases. The infrared and NMR studies of the pure ionic liquids and their mixtures show that the presence of a third ion with a weaker affinity with the common counterion contributes to prevailing the more favorable hydrogen-bond, herein always between the imidazolium cation and the acetate anion. Both radial and spatial distribution functions calculated by molecular simulation confirm this behavior. The remarkable enhancement of the viscosities of the [C₄C₁Im][OAc] + [P₄₄₄₄][OAc] mixtures could be discussed in light of the calculated friction coefficients.