Composition- and Temperature-Dependent Solubility of Sinomenine Hydrochloride in Ethanol–Water Mixtures

Sinomenine Hydrochloride is a bioactive alkaloid extracted from the root and stem of the medicinal plant sinomenium acutum, and is widely used to treat rheumatoid arthritis. Relevant studies were consulted, and the solubility data of Sinomenine Hydrochloride in ethanol–water mixed solvent have not been reported. It is essential to choose a proper solvent in the process of crystallization that has a significant influence on the purity and productivity. Therefore, it is necessary to determine the solubility of Sinomenine Hydrochloride under the conditions of different ratios of ethanol–water mixed solvent and temperature. In this study, the solubility of Sinomenine Hydrochloride in ethanol–water mixed solvent was determined using high-performance liquid chromatography within the temperature range of 283.15 K–308.15 K. At the same time, the CNIBS/R-K model, Modified Apelblat model, Yaws model, and Apelblat–Jouyban–Acree model were used to fit the solubility data, and the relevant thermodynamic parameters were calculated using the Van’t Hoff model. The results showed that the solubility of Sinomenine Hydrochloride was higher in pure water than in pure ethanol. Moreover, with the increase in the mass fraction of ethanol in the mixed solvent, the solubility of Sinomenine Hydrochloride showed a trend of increasing first and then decreasing. When the ethanol–water ratio was 5:5, the solubility of the compound reached the maximum. In addition, experimental data showed that the solubility of Sinomenine Hydrochloride was affected by temperature. In the experimental temperature range, the solubility increased with the increase in temperature. Among these four solubility models, the CNIBS/R-K model had the best fitting effect; the maximum RAD and RMSD were 4.622 × 10−3 and 4.079 × 10−3, respectively. The thermodynamic model fitting results showed that the predicted values were in good agreement with the experimental data, and the thermodynamic parameters ΔHd, ΔSd, and ΔGd were all positive values. This indicated that the dissolution of Sinomenine Hydrochloride in the ethanol–water mixture was a non-spontaneous and endothermic process. A proper ratio of ethanol–water and temperature improved the solubility of Sinomenine Hydrochloride. The data determined in this study can provide basic data for the industrial purification of Sinomenine Hydrochloride.