Molecular-level interactions governed by temperature and composition in double salt ionic liquid-water systems

An equimolar mixture of two ionic liquids (ILs), 1-butyl-3-methylimidazolium tetrafluoroborate, [C₄mim]BF₄ and 1-butyl-3-methylimidazolium methyl sulfate, [C₄mim][MeSO₄], formed a double salt ionic liquid (DSIL), [C₄mim](BF₄)_0.5[MeSO₄]_0.5. Volumetric and viscometric analyses revealed the thermophysical and dynamic properties of ILs, DSIL, and DSIL-water binary systems throughout a considerable range of temperatures (293.15-328.15 K). The [MeSO₄]^- anion engages in more pronounced directional ion-dipole and hydrogen-bond associations with the cation in comparison with the BF₄ ^- anion. The positive excess molar volume, V _m ^E and the reduction in energy barrier, E for viscous flow, revealed that the formation of the DSIL from its parent ILs transforms a rigid, strongly correlated ionic network into a flexible, dynamically disordered structure. The decreased free energy change, ΔG and increased entropy change, ΔS unveil the coexistence of weakly coordinating BF₄ ^- and strongly hydrogen-bonding [MeSO₄]^- anions in DSIL disrupt uniform packing and enhance free volume. A non-ideal behavior of the binary system was observed across all mole fractions. A less structured form of the mixture is observed in both water-rich (x _DSIL = ∼0.1) and DSIL-rich (x _DSIL = ∼0.9) regions than in the intermediate mole fraction regions of DSIL (x _DSIL = 0.5-0.8) from the volumetric study. The dynamic viscosity (η) profiles, analyzed by using the Vogel-Fulcher-Tammann (VFT) model, showed a sharp decrease in η of pure DSIL with temperature, while the temperature sensitivity diminished upon addition of water. The E for viscous flow is the highest at 15.57 kJ mol^-1 near 0.8 mole fraction of DSIL, indicating significant cluster formation. Analysis of viscosity deviation (Δη) revealed negative deviations at low DSIL mole fractions, shifting to positive deviations beyond 0.8, highlighting the role of hydrogen bonding and cluster formation. Dynamic light scattering (DLS) suggested three distinct cluster sizes, which are small (0.6-1.0 nm), intermediate (1.3-68.1 nm), and large (4150.0-5560.0 nm), and the intermediate-sized mixed clusters diminish above 0.8 mole fraction. This study provides insight into DSIL-water molecular interactions, crucial for optimizing DSIL applications in green chemistry, energy storage, and drug delivery systems.