The traditional preparation process by isothermal method is as follows [5], [6] and [7]. The cylindrical double-jacketed glass vessel was controlled to a constant temperature with continuous stirring, in which the temperature was controlled to be constant with a precision of ±0.05 K through a thermostatic bath (Type 501 A, China). Then, excess solute was added to the solvent. After the temperature of the water bath was kept constant for at least 4 h, the stirring was stopped, and the solution was kept still for 3 h. A portion of the upper clear solution was filtered with the membrane (0.45 μm).
In terms of this method a long dissolution time (4 h) has an increased risk of l-Gn degradation. Therefore, an experiment to find a proper dissolution time is carried out. Excess solute was added to the solvent, the concentration of suspension was measured by weighing method (described later) at different dissolve times. Fig. 2 gives the concentration variation of suspension along with dissolution time. The suspension can reach the dissolution phase equilibrium in short term,