Solubility of collagen-forming amino acids in 1-ethyl-3-methylimidazolium propionate ionic liquid at various temperatures

A comprehensive understanding of the solubility of amino acids across different solvents and environmental conditions enables improved extraction and purification strategies, also providing insight into molecular interactions and supporting the development of more accurate modelling approaches. In this work, the solubility of seven collagen-forming amino acids (glycine, β -alanine, L-proline, trans -4-hydroxy-L-proline, L-glutamic acid, L-aspartic acid and L-arginine) in the ionic liquid 1-ethyl-3-methylimidazolium propionate, and its mixture with water at equal mass proportions, was measured at temperatures ranging from 298.15 K to 328.15 K and atmospheric pressure. Compositions were determined from density–composition calibration curves. Acidic amino acids showed the highest solubility in the pure ionic liquid, followed by proline and trans-4-hydroxy-L-proline, key amino acids for collagen stabilization. Using water as a co-solvent increased the solubility on a mass basis of L-proline and β -alanine but decreased that of the other amino acids. Solubility increased with temperature across all systems. A comparison with other imidazolium salts suggests that increasing the ionic liquid’s hydrophobicity, whether by elongating the alkyl chain of the anion or cation, leads to a decrease in solubility. The van’t Hoff equation was found to accurately describe solubility across the studied temperature range and was used to calculate apparent thermodynamic properties of dissolution. All the apparent properties were found to be positive, indicating and endothermic and entropy-favoured process. Finally, the experimental data were correlated using the Non-Random Two-Liquid (NRTL) model.