Differential scanning calorimetry (DSC) assay

The freezing point of each sample was determined using a differential scanning calorimeter (Diamond DSC, PerkinElmer, Waltham, MA). Indium was used as the calibration standard. Each sample was prepared by placing a 20-μL aliquot in an aluminum calorimeter pan. The capped pan was then placed in the chamber and subjected to a thermal cycle. The thermal cycle consisted of the following: (1) heat to 50°C at 10°C/min; (2) hold at 50°C for 1 min; (3) cool to −40°C at 10°C/min; (4) heat to 50°C at 10°C/min; (5) hold for 1 min at 50°C; and (6) cool to −40°C at 10°C/min. The melting/freezing point was determined during the heating phase from −40°C to 50°C. Data were analyzed by soft Origin.