Synthesis, Spectroscopic and Calorimetric Characterization of Amino Acid Based Deep Eutectic Solvents

Deep eutectic solvent (DES) is the new generation of greener and sustainable solvents, formulated by the complex of hydrogen bond acceptor (HBA) and hydrogen bond donor (HBD) molecules. They are typically mixed at a certain molar ratio to form the optimum number of nonbonding interactions among the constituents. These optimum interactions help the solvents to achieve a significantly lower freezing point compared to its components. The long-range and numerous nonbonding interactions among the components assist in freezing-point depression of the DES. Compared to traditional DESs, Amino acid based Deep Eutectic Solvent (AADES) present the potential for greener, biodegradable, non-toxic options with a vast range of potential applications. In this research, various amino acids (Proline, Lysine, and Tyrosine) DESs were synthesized with glycerol due to its high propensity to form hydrogen bonds with amino acids. The formation of the DESs were confirmed by FTIR experiments. In the samples analyzed, multiple shifts from the original components where noticed. In proline-glycerol DES, the OH stretching shifted slightly with decreased in intensity. In addition, 2 bands at 2879 cm-1 and 2932 cm-1 pertaining to C-H stretching were also shifted. Similar pattern was also noticed when DESs were synthesized with Lysine and Tyrosine. Beside FTIR studies, differential scanning calorimetry (DSC) experiments were conducted which revealed the phase transitions within the DES samples. As expected of a stable liquid, no endothermic transition was seen in the thermogram of Tyrosine-Glycerol DES within the temperature range studied, although exothermic changes did take place. The exothermic transition in Tyrosine-Glycerol DES, which peaks at -75 oC, may be attributed to crystallization or freezing of the DES. Similar pattern was also detected for other amino acid based DESs. The DSC studies revealed that all the amino acid-based DES synthesized in this study showed good freezing point depression as expected of a deep eutectic solvent