Interaction behavior and solvation effects of aliphatic amino acids (glycine and L-alanine) in synthesized cholinium butanoate ionic liquid

Abstract

Ionic liquids are promising sustainable alternatives to conventional chemicals and solvents for green products and chemical and biochemical applications. In this investigation, a choline-based ionic liquid, cholinium butanoate ([Chl][But]), was synthesized and subsequently employed to determine the density (ρ) and ultrasonic velocity (u) of glycine and L-alanine in aqueous media. A series of thermophysical parameters were evaluated across varying amino acid and ([Chl][But]) concentrations at equidistant temperatures ranging from 288.15 to 318.15 K. The partial molar volume of transfer (∆Vϕ⁰) and the partial molar isentropic compressibility of transfer (∆Kϕ,s⁰) were calculated to elucidate the nature of solute–solvent interactions—whether hydrophobic–hydrophobic, hydrophilic–hydrophilic, or hydrophilic–hydrophobic—within the ternary system (amino acid + H₂O + [Chl][But]). Additionally, pair and triplet interaction coefficients were derived from ∆V ϕ⁰ and ∆K ϕ,s⁰ to gain further insight into molecular interactions. The structure-making or structure-breaking tendencies of the aliphatic amino acids in aqueous [Chl][But] solutions were analyzed using Hepler’s constant. Furthermore, the influence of alkyl chain length and temperature on parameters such as V ϕ , V ϕ⁰ , ∆V ϕ⁰ , and K ϕ,s was systematically examined. Complementary physicochemical quantities, including isentropic compressibility (β s ), relative association (RA), specific acoustic impedance (Z), relaxation strength (r), Wada’s constant (W), and Rao’s constant (R), were also computed. The limiting apparent molar volumes (Vφ⁰) obtained ranged from ~ 42 to 44 × 10⁻⁶ m³ mol⁻¹ for glycine and from ~ 58 to 64 × 10⁻⁶ m³ mol⁻¹ for L-alanine across the studied temperature and ([Chl][But]) concentration range, confirming systematically stronger solute–solvent interactions for L-alanine. Finally, FT-IR spectroscopy was employed to corroborate the interaction patterns and validate the observed experimental findings.