Insights into Non-Ideal Behavior of Double Salt Ionic Liquids with Common Cation: Volumetric Behaviour, Molecular Dynamics Simulations and NMR Experiments

The designer concept of ionic liquid (ILs) allows combination of different types of cation with anion to manipulate desire sets of physico-chemical properties. The additional degree of freedom to improve these properties is addition of second IL, i. e. mixture of two pure ILs, known as “Double Salt Ionic Liquids (DSILs)”. Depending on variation in physico chemical properties, DSILs could be ideal or non-ideal solution. The present study was explores non-ideal behaviour of three novel combination of imidazolium based DSILs using physico-chemical properties. The proposed DSILs are homogeneous mixtures of two pure ILs with common cation and two different anions: ethylsulfate and bis(trifluoromethylsulfonyl)imide / trifluoromethanesulfonate / thiocyanate. The densities, ρ and speeds of sound, u for proposed DSILs were determined at different temperatures. The excess molar volumes V_m ^E and excess molar isentropic compressibilities K_S,m ^E were calculated and discussed in view of chemical structures of pure ILs, nature of anions and type of interaction between ions. The V_m ^E were found to be positive for all cases, whereas positive or negative K_S,m ^E were observed for studied DSILs. The behaviour of DSILs was explained by molecular dynamics simulations in terms of interaction energies and hydrogen bonds between components of DSILs as well as ¹H NMR results. The results indicate random distribution of anions around imidazolium cation and structural changes during formulation of DSILs.