Effect of temperature and composition on density and dynamic viscosity of (lanthanide metal salts plus urea) deep eutectic solvents

Deep eutectic solvents (DESs) are rapidly becoming solvent media of prominence due not only to their unique properties but also to their promising environmentally-benign nature. For effective utilization of these neoteric media, assessment of their physical properties is essential. Density and dynamic viscosity of a class of DESs constituted of a hydrated lanthanide salt and urea at varying composition in the temperature range 293.15 to 363.15 K are presented. Specifically, different possible eutectic mixtures of lanthanum nitrate hexahydrate: urea (named La : Urea), cerium nitrate hexahydrate: urea (named Ce : Urea) and gadolinium nitrate hexahydrate: urea (named Gd : Urea) are investigated. DES (Gd : Urea) at 1: 2 mole ratio exhibits the highest density (1.98783 g.cm(-3) at 293.15 K) of all the DESs investigated; density is found to decrease with increasing urea. Density of these DESs decreases linearly with increasing temperature. Dynamic viscosity of these DESs also decrease with increasing urea content. The temperature dependence of the dynamic viscosity follows Vogel - Fulcher - Tammann (VFT) expression. While the activation energy of viscous flow (E-a,E-eta) for these (lanthanide salt : Urea) DESs is closer to those of (choline chloride + H-bond donor) DESs, their viscosity-temperature dependence is similar to that of common imidazolium ionic liquids. (C) 2022 Elsevier B.V. All rights reserved.

Automated summary

Deep eutectic solvents (DESs) are solvent media with unique properties and promising environmental benefits. This study investigates the density and dynamic viscosity of DESs composed of hydrated lanthanide salts and urea at different compositions and temperatures. The results show that density decreases with increasing urea content and temperature, and dynamic viscosity also decreases with increasing urea content. The viscosity-temperature relationship follows Vogel - Fulcher - Tammann (VFT) expression. The DESs studied have similar viscosity-temperature dependence to common imidazolium ionic liquids.