Alkylimidazolium-based ionic liquid Mixtures: The case of [EMIM][BF4] + [EMIM][MeSO3]

• Near-ideal density mixing progression. • Non-linear viscosity with temperature variation. • Exothermic mixing with negative enthalpy. • Hydrogen bonding through H1 site in imidazolium. • Molecular dynamics reveal hydrogen bond dynamics. Ionic liquids (ILs) have emerged as a promising class of materials due to their unique properties compared to conventional (volatile) organic solvents. This research explores the potential and properties of binary ionic liquid mixtures, also known as double salt ionic liquids (DSILs), obtained through the combination of two ILs, leading to mixtures containing a single type of cation and two types of anions. The focus of this study is the investigation of a specific (archetypical) DSIL system composed of 1-ethyl-3-methylimidazolium cation ([EMIM] + ) combined with tetrafluoroborate ([BF4] − ) and methanesulfonate ([MeSO3] − ) anions, experimentally obtained as a liquid mixture of [EMIM][BF 4 ] + [EMIM][MeSO 3 ]. A multiscale approach integrating thermophysical measurements and molecular modeling (Molecular Dynamics simulations) is employed to achieve a comprehensive understanding of the DSIL’s properties. The properties of the systems are analyzed in terms of their deviations of ideality when considered as a mixture of two compounds, and its relationships with the developed intermolecular forces, considering their role in the mixed liquid (nano)structuring, ionic clustering and fluid’s dynamics. This knowledge is crucial for unlocking the potential applications of ILs beyond those containing just and anion and a cation, through the possibility of tuning their properties including several types of ions, thus leading to fluids with more complex structures which can in various technological fields.