2.7.1. Solubility Test

The solubility measurements were performed under water (pH 7.0), hydrochloric acid buffer (pH 1.2), acetate acid-sodium acetate buffered solution (pH 4.5), and phosphate buffer solution (pH 6.8), respectively. An excess number of samples were added to a test tube containing 10 mL of solvent and the tube was shaken in an orbital shaker (37 ± 0.5 ◦C) until reaching the equilibrium condition (48 h). The solution was filtered through a 0.45 μm mixed cellulose ester membrane and then analyzed by HPLC (LC-20, Shimadzu, Japan) equipped with a SP-20 ultraviolet detector at 254 nm wavelength. Quantitative tests were performed on a YMC C8 column (250 × 4.6 mm, 5 μm) with an external standard method. The mobile phase consisted of acetonitrile and water (containing 0.1% trimethylamine) (65:35, *v*/*v*), with a flow rate of 1.0 mL/min. Each test was performed in triplicate.
