Effect of Water on the Physical Properties of Quaternary Ammonium Salt–(dl)-Lactic Acid Deep Eutectic Solvents

Deep eutectic solvents (DESs) are new green solvents, and quaternary ammonium salt acid DESs have been widely used in the field of analytical separation in recent years due to their low cost and environmental friendliness. In this work, the density, viscosity, and electrical conductivity of binary mixtures containing three quaternary ammonium salt–(dl)-lactic acid DESs with H2O were determined. The average molar volume, excess molar volume, and viscosity deviation of the DESs' aqueous solutions trends were computed. The viscous flow activation energy of DESs' aqueous solutions was derived using the Arrhenius equation. The DESs' physical properties are significantly influenced by their structure, temperature, and water content. The excess molar volume and viscosity deviation are negative over the entire concentration range, indicating a strong interaction between DESs and water. The variation of viscosity and electrical conductivity of the binary system with temperature conforms to the Vogel–Fulcher–Tammann (VFT) equation and shows an opposite trend, i.e., viscosity decreases and electrical conductivity increases with increasing temperature. The VFT equation can be conveniently applied to the aqueous solutions of DESs.