Solubility Determination, Correlation, and Thermodynamic Analysis of 3,5-Dimethylbenzoic Acid in 11 Pure Solvents and a Binary Solvent (Ethanol + Ethyl Acetate) from 293.15 to 333.15 K

The solubility of 3,5-dimethylbenzoic acid (3,5-DMBA) in 11 pure solvents and ethanol + ethyl acetate binary mixtures was measured using the static equilibrium method combined with gravimetric analysis at temperatures from 293.15 to 333.15 K. Solubility increased with temperature in all systems. In pure solvents, the order was cyclohexanone > n-butanol > isopropanol > acetone > n-propanol > ethanol > isobutanol > n-propyl acetate > ethyl acetate > isopropyl acetate > acetonitrile. In binary mixtures, solubility exhibited a maximum at an ethanol mass fraction of 0.4. Experimental data were correlated with the modified Apelblat, van’t Hoff, λh, NRTL, Wilson, and UNIQUAC models; the modified Apelblat equation yielded the best fit. KAT-LSER analysis indicated the contributions of α, β, π*, and δH to the solubility of 29.48%, 4.04%, 39.44%, and 27.04%, respectively. Thermodynamic functions of mixing derived from the Wilson model showed endothermic mixing in acetonitrile, isopropyl acetate, and cyclohexanone, and exothermic mixing in the others. ΔSmix increased with temperature, ΔGmix remained negative (spontaneous), and the dissolution driving force shifted from entropy- to enthalpy-dominated with increasing temperature. The order of ΔGmix was inverse to solubility. These findings offer theoretical support for optimizing industrial crystallization of 3,5-DMBA and solvent-temperature selection.