Unprecedented High Probe-Reported Polarity of Deep Eutectic Solvents Composed of Lanthanide Salts and Urea

Deep eutectic solvents (DESs) have emerged as viable alternatives to the toxic organic solvents. The most intriguing aspect of these solvents is perhaps the widely varying physicochemical properties emerging from the changes in the constituents that form DESs along with their composition. Based on the constituents, a DES can be hydro-philic/polar or hydrophobic/non-polar rendering vastly varying spectrum of polarity a possibility. DESs formed by mixing urea (U) with hydrated lanthanide salts, lanthanum nitrate hexahydrate (La : U), cerium nitrate hexahydrate (Ce : U), and gadolinium nitrate hexahydrate (Gd : U), respectively, exhibit very high polarity as manifested via probe-reported empirical parameters dipolarity/polarizability (π*). The highest π* of 1.70 exhibited by the DES (Gd : U) in 1 : 2 molar ratio is unprecedented. The π* ranges from 1.50 to 1.70 for these DESs, which is almost the highest reported for any solvent system. The π* decreases with increasing urea in the DES, however, the anomalous trends in H-bond donating acidity (α) and H-bond accepting basicity (β) appear to be due to the hydrated water of the lanthanide salt. The emission band maxima of fluo-rescence probe of “effective” dielectric constant (εeff) of the solubilizing media, py-rene-1-carboxaldehyde (PyCHO), in salt-rich DESs reflect higher cybotactic region di-polarity than even that offered by water. Probe Nile red aggregates readily in these DESs to form non-fluorescent H-aggregates; a characteristic of highly polar solvents. Behavior of probe pyranine also corroborates these outcomes as the (lanthanide salt : urea) DES system supports the formation of deprotonated form of the probe in the excited-state. The (lanthanide salt : urea) DES system offers solubilizing media of exceptionally high polarity, which is bound to expand their application potential.