Variation of Density, Viscosity, and Electrical Conductivity of the Deep Eutectic Solvent Reline, Composed of Choline Chloride and Urea at a Molar Ratio of 1:2, Mixed with Dimethylsulfoxide as a Cosolvent

Abstract

© 2020 American Chemical Society. We investigated the variation of density (ρ), dynamic viscosity (η), and electrical conductivity (κ) of mixtures of the deep eutectic solvent (DES) reline, composed of choline chloride and urea at a molar ratio of 1:2, with dimethylsulfoxide (DMSO) covering the entire miscibility range. Data for these properties were obtained as a function of temperature in the range 308.15 K ≤ T ≤ 363.15 K for ρ and η, whereas κ was recorded between 308.15 and 338.15 K. While ρ depended linearly on T, η and κ were best fitted by the empirical Vogel-Tammann-Fulcher (VFT) equation. The ρ(x1) and κ (x1) data - with x1 as the DES mole fraction - were used for calculating excess molar volumes (VE) and excess logarithmic viscosities (ln ηE) of the studied system. With respect to volumetric properties, reline/DMSO mixtures deviate largely from ideality, possessing negative VE values over the entire range of T and x1. The temperature dependence of VE suggests that nonspecific interactions (packing effects) are mainly responsible for the system contraction. However, specific interactions (including H-bonding) cannot be excluded as ln ηE values are positive and exhibit a temperature dependence characteristic for systems with strong interspecies interactions. Similar to conventional electrolytes and to mixtures of ionic liquids with a molecular solvent, κ as a function of x1 shows a maximum at x1 ≈ 0.25, indicating a concentration at which rising viscosity and the formation of ion pairs and larger aggregates compensate the conductivity rise caused by increasing charge carrier density. Changes in the VFT parameters point at a transition from electrolyte solution-like behavior to molten salt-like behavior at x1 ≈ 0.6.