Solid-Liquid Equilibrium Solubility Determination and Correlation of 5-Hydroxy-1-methylpyrazole in 12 Kinds of Pure Solvents

The solid–liquid equilibrium behavior of 5-hydroxy-1-methylpyrazole (HM) in 12 pure solvents including alcohol solvents (i.e., methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, and sec-butanol) and ester solvents (i.e., methyl acetate, ethyl acetate, propyl acetate, butyl acetate, and amyl acetate) was measured by the gravimetric method within the temperature range of 278.15–318.15 K. The solubility of HM is positively correlated to temperature in the above solvents. HM was more soluble in alcohols than in esters. Thermodynamic models, i.e., the modified Apelblat model, the Yaws model, and the Wilson model, were used to fit the solubility data. The relative average deviation and root-mean-square deviation values of the three models were no larger than 4.35 × 10–2 and 17.85 × 10–4. The selected theoretical models were able to correlate the solubility data of HM in the selected solvents well. Molecular electrostatic potential energy surface was used to analyze the molecular interactions. KAT-LSER model and Hansen solubility parameters were used to investigate the solvent effect. Furthermore, the dissolution process thermodynamic properties of HM were calculated on the basis of the Wilson model. The dissolution process of HM is spontaneous, endothermic, and entropy increasing.