The Phase Diagram of the API Benzocaine and Its Highly Persistent, Metastable Crystalline Polymorphs

The availability of sufficient amounts of form I of benzocaine has led to the investigation of its phase relationships with the other two existing forms II and III using adiabatic calorimetry, powder X-ray diffraction, and high-pressure differential thermal analysis. The latter two forms were known to have an enantiotropic phase relationship with form III stable at low-temperature and high-pressure, while form II is stable at room temperature. Using adiabatic calorimetry data, it can be concluded, that form I is the stable low-temperature, high-pressure form, which also happens to be stable at room temperature; however, form II, due to its persistence at room temperature, is still the most convenient polymorph to use in formulations. Form III presents a case of overall monotropy and does not possess any stability domain in the pressure-temperature phase diagram. Heat capacity data for benzocaine have been obtained by adiabatic calorimetry from 11 K up to 369 K above its melting point, which can be used to compare to results from in-silico crystal structure prediction.