Solubility and Thermodynamic Modeling of Schiff-Base Ligands and Their Vanadium Oxo Complexes in Organic Solvents

The solubilities of two Schiff-base ligands H 2 L 1 and H 2 L 2 (L 1 =N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-diphenylethane-1,2-diamine;L 2 =N,N′-bis(3,5-di-tert-butylsalicylidene)propane-1,2-diamine) and their corresponding oxovanadium(IV) complexes (VOL 1 and VOL 2 ) were systematically determined in selected organic solvents using the static equilibrium method over the temperature range of 278.15 - 313.15 K. Solid-liquid equilibrium data were correlated using several empirical equations (Apelblat, van’t Hoff, Yaws, polynomial, and λ h) as well as activity-coefficient models (Wilson, NRTL, and UNIQUAC). All systems exhibited a monotonic increase in solubility with temperature, indicating endothermic dissolution behavior. Among the investigated models, UNIQUAC provided the most accurate correlation for H 2 L 1 , NRTL showed superior performance for H 2 L 2 , while the polynomial empirical model yielded the best overall fit for both vanadium complexes. Thermodynamic analysis revealed that the dissolution processes are predominantly enthalpy-driven. Furthermore, Hansen solubility parameters were employed to rationalize solvent-solute interactions and explain solvent-dependent solubility trends. The experimental data and thermodynamic modeling presented in this work provide a reliable basis for solvent screening, solubility prediction, and process-relevant applications of Schiff base vanadium complexes in homogeneous systems. The present data thus provide quantitative guidance for solvent selection and operating condition design in homogeneous catalytic processes and other solution-phase applications.