Thermodynamic study on levulinic acid in NaCl, (C2H5)4NI and mixed MgCl2/NaCl and CaCl2/NaCl aqueous solutions at T = 298.15 K

Highlights: • Calorimetry and potentiometry to determine fundamental thermodynamic parameters. • Debye-Hückel, SIT, van't Hoff equations to satisfactorily model I and T dependence. • Chemical speciation of levulinate predicted under different experimental conditions. • Entropy is the prevailing contribution to ΔG° in the proton binding reaction. • Carbonyl group is not involved in the formation of the HL and ML species. -- Abstract: Some thermodynamic properties of levulinc acid (LA) were studied under different experimental conditions. The partition coefficient between 2-methyl-1-propanol/pure water (or NaCl aqueous solutions up to I = 1.00 mol dm⁻³) was measured at T = 298.15 K by means of potentiometric titrations. The protonation constant of levulinic acid was also determined potentiometrically at different ionic strengths in NaCl_(aq), (C₂H₅)₄NI_(aq) as well as in mixed MgCl₂/NaCl and CaCl₂/NaCl aqueous solutions. The protonation enthalpy was determined by calorimetric titrations in the same conditions of temperature and ionic strength. The linear fitting of the distribution ratio (log K_D) determined at different salt concentrations vs. the salt concentration allowed for the determination of the distribution coefficient at infinite dilution ($\text{log}{\text{K}}_{\text{D0}}^{\text{0}}$ = −0.14 ± 0.02), the Setschenow (k_m = 0.35 ± 0.03) and the activity coefficient of the neutral species (log γ_HL = k_m·m_salt). The activity coefficient of the charged L⁻ species was also computed by means of the Specific Interaction Theory (SIT) approach by keeping constant the value of the activity coefficient for the neutral HL species. The medium effect on the protonation constant was also studied using the weak interaction model, namely by comparing the protonation constants obtained in NaCl_(aq), (C₂H₅)₄NI_(aq) as well as in mixed aqueous MgCl₂/NaCl and CaCl₂/NaCl mixtures by considering the formation of a NaL_(aq), CaL⁺_(aq), and MgL⁺_(aq) species.