Exploration of the Solid–Liquid Equilibrium Characteristics of Oxalic Acid in 12 Solvents: Investigation into Solubility, Model Correlation, and Molecular Simulation

The solubility of oxalic acid in 12 solvents (methanol, ethanol, n-propanol, isopropanol, n-butanol, ethylene glycol, methyl acetate, ethyl acetate, acetonitrile, N,N-dimethylformamide, methyl methacrylate(MMA), and DI water) was quantified via gravimetry across 283.15–323.15 K. Our results indicate that the equilibrium solubility of oxalic acid in all investigated solvents exhibits a positive correlation with temperature. At 298.15 K, ethylene glycol and acetonitrile demonstrated maximum dissolution capacities in alcohol and nonalcohol categories, respectively, whereas n-butanol and MMA exhibited minimal values. Four thermodynamic models (modified Apelblat, λh, van’t Hoff, NRTL) were applied for data correlation, with the modified Apelblat model achieving optimal fit. Molecular electrostatic potential and Hirshfeld surface analyses revealed intermolecular interaction energies, while thermodynamic parameters (Δ_mixG < 0, Δ_mixH > 0, Δ_mixS > 0) confirmed spontaneous, entropy-driven dissolution with endothermic characteristics. Atomic-level molecular dynamics simulations further decoded the solute–solvent affinity through radial distribution function analysis. These findings establish predictive models for crystallization optimization and mechanistically interpret solid–liquid equilibrium variations across solvent systems.