Thermodynamic and Acoustic Mapping of Glutathione–Poly(ethylene glycol) Interactions in Water: A Volumetric, Ultrasonic, and Viscometric Investigation

This study systematically investigates the volumetric, ultrasonic, and viscometric properties of glutathione (GSH) and poly(ethylene glycol) (PEG) in aqueous solutions (0.01 to 0.05 mol·kg–1) across a temperature range of 293.15 to 318.15 K. To elucidate molecular interactions, density measurements were used to derive key volumetric parameters, including the limiting apparent molar volume (VØ0) and transfer volume (ΔtrVØ0). Results for GSH in lower concentrations of PEG (0.01 to 0.03 m) indicated a predominance of solute–solute interactions, evidenced by increasing VØ0 and positive experimental slopes (Sv). Conversely, higher PEG concentrations (0.04 to 0.05 m) suggested stronger solute–solvent interactions. Ultrasonic data, including isentropic compressibility (Ks), apparent molar isentropic compressibility (Ks,Ø), and hydration numbers (nh), corroborated the coexistence of these competing forces. Viscometric analysis via Jones–Dole coefficients (A, B) and the temperature derivative (∂B/∂T) identified both solutes as structure makers within the aqueous medium. Finally, thermodynamic activation parameters, namely, Gibbs free energy of activation (ΔG*), enthalpy of activation (ΔH*), and entropy of activation (ΔS*) were calculated to provide a comprehensive profile of the viscous flow and molecular mechanism. These findings offer critical insights into the existing molecular interaction and stability of GSH in the presence of PEG.