Experimental and Modeling Investigation of Solid–Liquid Equilibria and Thermodynamic Properties of Vanillic Acid and Gallic Acid in Water and Organic Solvents at Temperatures from 273.15 to 313.15 K

High Resolution Image Download MS PowerPoint Slide Vanillic acid, gallic acid, and their derivatives share key structural motifs with lignin, making them essential model compounds for solubility studies. In this study, the solubilities of vanillic acid and gallic acid are experimentally determined in water and six organic solvents (ethanol, tetrahydrofuran, γ-valerolactone, dimethyl sulfoxide, dimethylformamide, and cyclohexene) from 273.15 to 313.15 K, using an isothermal glass-cell apparatus. In addition to the experimental solubility data, the melting properties were measured using differential scanning calorimetry. Solid-phase studies were conducted using X-ray diffraction. The experimentally determined solubilities were correlated using five empirical models and predicted using two theoretical models. The correlational values showed good agreement with the experimental data set within the studied temperature range, with the nonrandom two-liquid (NRTL) model exhibiting the least relative average deviation from the experimental solubilities. In contrast, the predictive models exhibited unreliable solubility results, characterized by a high relative average deviation. The excess enthalpy of solution was calculated from the experimental solubilities to further understand solute–solvent interactions. Moreover, for dimethyl sulfoxide and dimethylformamide, the excess enthalpy of solution is negative, indicating strong solute–solvent interactions. These findings provide valuable insight and can support the design and optimization of lignin-related extraction, purification, and biorefining processes.