Novel amino acid-based ionic liquid analogues: neutral hydroxylic and sulfur-containing amino acids

Deep eutectic solvents (DESs) have shown potential promise for various industrial applications. Abundant research works were carried out on Type III DES employing choline chloride as the quaternary ammonium salt in combination with a variety of hydrogen bond donors (HBDs). Despite the fact that they are cheap, biodegradable, and have a wide application in food, polymer, and pharmaceutical industries, amino acids have not been utilized as HBD for the synthesis of DES. In this work, different ammonium-based salts are used to synthesize different amino acid-based DES systems. Four different amino acids are considered, two of which are neutral amino acids, namely L-serine and L-threonine, while the other two are based on the sulfur-containing amino acids, namely L-methionine and L-cysteine. The successful DESs were characterized in terms of their physical properties. The melting point, density, viscosity, surface tension, refractive index, conductivity, and pH were measured for the successful DES systems at different temperatures ranging from 303.15 to 353.15 K. The introduction of such novel DES opens the door for performing more work on utilizing them for various chemical, biochemical, and pharmaceutical applications.