Physicochemical Properties and Empirical Polarity Parameters of Lithium Salt-Based Deep Eutectic Solvents

Abstract

Physicochemical properties of deep eutectic solvents (DESs) constituted of lithium bis(trifluoromethylsulfonyl)imide (LiTf₂N) as the H-bond acceptor (HBA) and three different H-bond donors (HBDs)─urea, acetamide, and 2,2,2-trifluoroacetamide─in (1:2), (1:3), and (1:4) mole ratios are investigated. The density of these DESs decreases linearly with increasing temperature within the 288–358 K range; it is higher for LiTf₂N/2,2,2-trifluoroacetamide in comparison to LiTf₂N/urea and LiTf₂N/acetamide. Density decreases linearly with increasing HBD per mole of LiTf₂N. The dynamic viscosity is highest for LiTf₂N/urea and lowest for LiTf₂N/2,2,2-trifluoroacetamide; it follows Vogel–Fulcher–Tammann (VFT) temperature dependence. With increasing HBD concentration, the dynamic viscosity of the DES decreases monotonically. Electrical conductivity, surface tension, and refractive index of these DESs at 298 K exhibit considerable variation with constituent and composition. The empirical solvent polarity parameter, E_T^N for all DESs within the investigated temperature range is close to or higher than one, implying these DESs to possess considerably high dipolarity/polarizability and/or HBD acidity. Kamlet–Taft empirical parameters─dipolarity/polarizability (π*), HBD acidity (α), and HBD basicity (β)─reveal that the LiTf₂N/urea DES exhibits the highest π*, followed by LiTf₂N/2,2,2-trifluoroacetamide and LiTf₂N/acetamide; the α is considerably higher for acetamide DES in comparison. The β values of these DESs are fairly low.