Aliphatic amino acids as possible hydrogen bond donors for preparing eutectic solvents

Since their first introduction in the literature about two decades ago, deep eutectic solvents have competed with ionic liquids due to their lower production cost, simple synthesis, and favorable biodegradability. Many studies showed their suitability in wide-ranging applications involving mass and heat transfer, catalysis as well as in the pharmaceutical industries. Very few amino acids-based eutectic solvents have been reported including neutral and acidic-basic amino acids. In this work, a group of four aliphatic-type amino acids were successfully coupled as hydrogen bond donors with tetrabutylammonium chloride forming new eutectic solvents at different molar compositions. Three different salts to amino acid molar ratios were prepared and characterized in terms of their phase-change behavior and basic physicochemical properties within the temperature range of 313.15–353.15 K. Besides, their thermal phase change nature, as well as the ionic strength and infra-red spectra, have also been studied. Results showed an inverse linear relation for density, surface tension, refractive index, and pH. While an inverse exponential relation was noticed for the viscosity and proportional exponential relation for the conductivity with the increase of temperature. However, the molar ratio trends did not follow a specific pattern.