Thermophysical Properties of Imidazolium-Based Binary Ionic Liquid Mixtures Using Molecular Dynamics Simulations

Until very recently, task-specific ionic liquids have been designed by altering the chemistry of either the cation, anion, or both. An alternative approach, that is gaining considerable momentum, is to consider ionic liquids that are derived from mixing two different ionic liquids and varying the molar composition of such blends to exert precise control over the desired physicochemical and biological properties. As the number of ionic liquids that result from mixing is projected to be close to a billion, it is highly desirable to predict a priori whether ionic liquid mixtures can be considered as ideal solutions of their pure analogues. Toward this end, we employ molecular dynamics simulations to predict the density, molar volumes, excess molar volumes, self-diffusion coefficients, and ionic conductivities for 11 ionic liquid mixture systems as a function of mole fractions spanning the entire range of compositions of the constituent ionic liquids. The ionic liquid mixtures investigated here are 1-n-butyl-3-methylimidazolium [C(4)mim](+) chloride Cl- paired with [C(4)mim](+) acetate [CH3COO](-)/[OAC](-), [C(4)mim](+) trifluoroacetate [CF3COO](-)/[TFA](-) and [C(4)mim](+) trifluoromethanesulfonate [CF3SO3](-)/[TFS](-), and [C(4)mim][OAC] combined with [C(4)mim][TFA] and [C(4)mim][TFS]. The effect of change in the alkyl chain length on the thermophysical properties of ionic liquid mixtures containing anions as Cl--methylsulfate [MeSO4](-), and Cl--bis(trifluoromethanesulfonyl)imide [NTf2](-) is evaluated by coupling with 1-ethyl-3-methylimidazolium [C(2)mim](+), 1-n-hexyl-3-methylimidazolium [C(6)mim](+), and 1-n-octyl-3-methylimidazolium [C(8)mim](+) cations. The deviation of the property trend from the linear mixing rule is discussed in terms of the difference in the properties of pure ionic liquid analogues.