Jumping into Metastable 1:1 Urea–Succinic Acid Cocrystal Zone by Freeze-Drying

Aqueous solutions with molar ratios between urea and succinic acid from 0.3:1 to 3:1 were evaporated at room temperature, and products were pure or mixtures of stable 2:1 urea-succinic acid cocrystals, urea or succinic acid. By freeze-drying, metastable 1:1 urea–succinic acid cocrystal formed. The different mixtures of the 1:1 cocrystals reveal several “hidden” metastable zones in a ternary phase diagram of the 2:1 cocrystal. The formation of the 1:1 cocrystal indicated that the solution composition points in the phase diagram “jump” over the stable zone into the metastable zones.