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

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, ETN 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.