Volumetric Properties of Amino Acids in Aqueous Protic Ionic Liquid Solutions: Probing Molecular Interactions

Volumetric properties of amino acids (glycine and l-valine) in aqueous solutions of protic ionic liquids (PIL) (diethylammonium acetate [DEAAc] and ethanolammonium acetate [EAAc]) were investigated at different temperatures. The apparent molar volume (V2,ϕ), infinite dilution partial molar volume (V2°), infinite dilution partial molar volume of transfer (ΔtV2°), partial molar compression at infinite dilution (Ks,2°), and partial molar isentropic compression of transfer at infinite dilution (ΔtKs,2°) were determined from density/sound velocity data to investigate the nature of interactions between amino acid and cosolute (PIL) in water. Partial molar expansibilities (E°ϕ), Hepler’s constant ((∂2V2° /∂T2)P), and hydration number (NH) were also derived to assess the solvation behavior. Transfer volumes for both amino acids in [EAAc] solutions were positive, whereas positive and negative transfer volumes were observed for l-valine, and only negative transfer volumes were observed for glycine in the presence of [DEAAc]. Positive transfer volumes indicate the dominance of hydrophilic interactions among ions of PIL and charged functional groups (amino or carboxylic) of amino acids. Hydrophobic interactions contributed toward negative transfer volumes. The volumetric analysis offers insight into specific interactions between the ions of PILs and amino acids, helping to elucidate molecular forces that contribute to solute−solvent behavior and stabilization mechanisms in such systems.