Densities and viscosities of binary mixtures containing 1-butyl-3-methylimidazolium tetrafluoroborate with water over the salt-rich region concentrations were measured at a temperature range of (283.15 to 373.15) K. The excess molar volumes were calculated using the measured experimental data and fitted to the Redlich-Kister equation. Excess viscosities were determined as deviations from the log-mixing rule. The results show that the densities and viscosities strongly depend on the water content and temperature. The temperature dependence of density and dynamic viscosity of aqueous mixtures could be described by an empirical, linear relationship and by the Vogel-Fucher-Tammann (VFT) equation, respectively. Importantly, we identified a new correlation between the excess mixture viscosity and the excess molar volume in the ionic liquid rich regime that is consistent with free volume theory. This correlation enables predicting the viscosity of ionic liquids with added water directly from density measurements.
The densities and viscosities data obtained were also discussed in terms of the interactions between cations and anions of the ionic liquid and the water molecules. Ions of ionic liquids are associated with water molecules with the addition of water into the system. The hydration of the ionic liquid results in the reduction of attraction between cations and anions. Small angle neutron scattering (SANS) measurements were used to detect the microstructure changes of the system with varying water content. Structures are revealed in the ionic liquid aqueous solutions, indicating that inverse micelles form from the excess water once the ions are hydrated.
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