The properties of ionic liquid (1-ethyl-3-methylimidazolium BF4 and 1-butyl-3-methylimidazolium BF4) + solvents (water, ethylene glycol or dimethylformamide) mixtures are studied in the full composition range as a function of mixture composition and temperature. These mixed fluids are characterized by selected physical properties and microscopic studies using density functional theory and molecular dynamics simulations. The reported results showed large non-ideal mixtures, which are caused by strong anion (BF4) – molecular solvent hydrogen bonding. Likewise, the interaction of ions, evolves from anion-cation pairs solvated by molecular solvents, for mixtures rich in molecular solvent, to large ionic aggregates for ionic liquid rich mixtures separated by a transitional composition regime. The large non-linearity of the evolution of microscopic properties with mixture composition is the origin of macroscopic thermodynamic deviations from ideality.
