关键词: 氨基己内酰胺, 溶解度, 模型拟合, 溶解热力学性质

Abstract: As an important organic chemical intermediate, the determination of the solubility of α-amino-ε-caprolactam is a thermodynamic basis for the development of solution crystallization purification technology and product quality control. The solubility of α-amino-ε-caprolactam in various organic solvents such as N,N-dimethylformamide, acetone, 1,4-dioxane, trioctyl phosphate, toluene, p-xylene, 1,3,5-trimethylbenzene, methyl tert-butyl ether, cyclohexane, and n-hexane was measured by the static equilibrium method. The results indicated that within the temperature range of 278.15~323.15 K, the solubility of α-amino-ε-caprolactam in different solvents increased with the rise of temperature, and the solubility in different solvents generally followed the trend that it increased with the increase of the polarity of the solvent. However, there were also some exceptional cases. For instance, although methyl tert-butyl ether was polar and could form electrostatic interaction with the solute, the steric hindrance restricted its interaction with α-amino-ε-caprolactam, resulting in a relatively low solubility. In contrast, the delocalized π-electron clouds of aromatic molecules could generate π-π interactions with the conjugated regions of α-amino-ε-caprolactam, thereby enhancing the solubility. These observations indicated that dissolution behavior was influenced not only by solvent polarity but also by intermolecular interactions such as π-π stacking. The solubility data were fitted by the Apelblat, λh, quadratic polynomial, and Van't Hoff models. The correlation coefficients (R2) of all models were greater than 0.95. Among them, the quadratic polynomial model showed the best fitting effect, with an average relative deviation ranging from 0 to 1.8403% and a root mean square deviation ranging from 0 to 0.3190%. Thermodynamic parameters of the dissolution process were analyzed by the Van't Hoff equation. It was shown that ΔdisH, ΔdisS, and ΔdisG of α-amino-ε-caprolactam were positive values in both n-hexane and cyclohexane, two non-polar solvents. It indicated that the dissolution process was a non-spontaneous, endothermic process with an increase in entropy. The calculated relative contribution of enthalpy ζH was less than the relative contribution of entropy ζTS, indicating that ΔdisH contributed less to ΔdisG than ΔdisS during the dissolution process in these 10 solvents.

Key words: α-amino-ε-caprolactam, solubility, model fitting, dissolution thermodynamic properties