Temperature-Dependent Empirical Parameters for Polarity in Choline Chloride Based Deep Eutectic Solvents

Deep eutectic solvents have emerged as a promising and economically viable alternative to address our need for environmentally benign media. Efficient utilization of deep eutectic solvents in various fields will require adequate knowledge about their physicochemical properties. In this work, independent polarity scales pertaining to solvent polarizability, dipolarity, acidity and basicity have been determined for some commonly used deep eutectic solvents based on choline chloride. The three eutectic mixtures selected for the study include reline (1:2 mixture of choline chloride and urea), ethaline (1:2 mixture of choline chloride and ethylene glycol), and glyceline (1:2 mixture of choline chloride and glycerol). The variation of the probe response as a function of temperature revealed some interesting details about the polarity of neoteric solvents being investigated. Our observations indicate that while the polarity parameters for ethaline and glyceline are easily determined, the instability of probe molecules in reline poses a potential challenge for the determination of its polarity parameters. Attempts to correlate the empirical polarity parameters with solvatochromic response of Reichardt's dye (ETN parameter) also led to surprising observations. Use of alternative probes to measure the polarity in reline indicated that the polarity scales using different probes need to be normalized for DESs. The results emphasize the extent of applicability of currently employed polarity scales to determine the polarity of DES, the need to develop appropriate probe molecules, and the factors limiting our understanding of these extremely interesting class of solvents.