Influence of 2-hydroxyethylammonium acetate-based protic ionic liquids on the thermophysical properties of aqueous DL-alanine solutions

Abstract Understanding the hydration behavior of amino acids is fundamental to gaining insights into protein solvation mechanisms. Within this framework, examining the solvation properties of amino acids in aqueous media containing protic ionic liquids (PILs) a novel class of environmentally friendly solvents is crucial for characterizing their hydration dynamics. This research focuses on the influence of ammonium-based PILs, specifically mono-, bis-, and tris-(2-hydroxyethyl) ammonium acetate, on the hydration characteristics of DL-alanine. By employing COSMO computational analysis alongside thermophysical property measurements, the study evaluates the hydration energies and σ-profiles of DL-alanine and the PILs. The thermophysical behavior of DL-alanine in aqueous solutions containing varying concentrations of protic ionic liquids (PILs) was systematically analyzed using experimental data and modeling approaches. The density, speed of sound, viscosity, and refractive index were measured for ternary solutions of DL-alanine, water, and PILs across a temperature range of (298.15 to 318.15) K under atmospheric pressure. The standard partial molar volume ( $$\:{{V}_{\phi\:}}_{}^{0}$$ ) measured over the studied temperature range indicates that the solute-solvent interactions between [2-HEA]Ac and DL-alanine in the presence of water is 63.119 cm3·mol-1. Additionally, the hydration layer surrounding DL-alanine is notably influenced by temperature, as higher temperatures lead to the release of more water molecules relative to PIL-containing solutions. This temperature-dependent effect is especially pronounced in the presence of (2-hydroxyethyl) ammonium acetate, underscoring its significant impact on the hydration behavior of DL-alanine.