Excess Properties of Aqueous Dilutions of Ammonium- and Phosphonium-Based Deep Eutectic Solvents

This work focused first on the measurement at 25 °C and ambient pressure of the thermal conductivities (k), thermal diffusivities (α), and heat capacities (C_p) of aqueous dilutions of two deep eutectic solvents (DESs). The first one composed of choline chloride (ChCl) and urea (U) in a 1:2 molar ratio (ChCl:2U), and the second one composed of methyltriphenylphosphoniumbromide (M3PhPBr) and glycerol (Gly) in a 1:3 molar ratio (M3PhPBr:3Gly). Then, we compared the deviation from ideality of the thermal conductivity (Δk) with those experienced by other excess properties, including excess molar volume (V ^E), excess viscosity (ln η^E), and excess adiabatic compressibility (Δβ_S). The aim was finding some correlation among them in the sign of the deviation and/or the specific DES content at which the maximum deviation from ideality is reached. Moreover, we have compared the hypersonic velocities (ν_H) obtained from Brillouin spectroscopy with regular ultrasonic velocities (ν_U) to identify the DES content for which, in terms of relaxation, the mixture moves from a purely dissipative fluid to an "elastic medium". Finally, we also investigated whether ¹H NMR spectroscopy could also be used to identify the DES content for which the deviation of ideality was the maximum.