Investigation on 2-Mercapto-5-methyl-1,3,4-thiadiazole Solubility in Twelve Kinds of Organic Monosolvents Based on Models, Thermodynamic Analysis, and Solvent Effect

Solid–liquid equilibrium solubilities of 2-mercapto-5-methyl-1,3,4-thiadiazole (MMTD) in 12 kinds of monosolvents (i.e., methanol, ethanol, n-propanol, i-propanol, n-butanol, i-butanol, ethyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, and amyl acetate) were determined by a gravimetric method with the temperature range from 283.15 to 323.15 K. In alcoholic solvents, the solubility order is ethanol > methanol > n-propanol > i-propanol > n-butanol > i-butanol. In ester solvents, the solubility of MMTD ranking is methyl acetate > ethyl acetate > propyl acetate > butyl acetate > amyl acetate > ethyl formate. The solubility of MMTD increased with increasing temperature. Four thermodynamic models (i.e., the modified Apelblat model, the λh model, the Wilson model, and the Jouyban model) were selected to correlate the solubility data. The RAD and RMSD values were less than 0.0254 and 0.000536, respectively. The Wilson model had the best-fitting effect. The KAT-LSER model and Hansen solubility parameters were used to explain the solvent effect in this work. The Hirshfeld surface, molecular electrostatic potential surface, and free energy of solvation were used to explain the interactions between molecules. Dissolution properties of MMTD were calculated by the Wilson model. The dissolution process of MMTD is endothermic, entropy driven, and spontaneous.