Detection of amorphous-amorphous phase separation in small molecular co-amorphous mixtures with SEM-EDS

Amorphicity is one possible way to increase the solubility of poorly water soluble drugs. However, amorphous solids are thermodynamically unstable and tend to recrystallize with material-specific kinetics. Crystallization is not the prime phenomenon in the whole process, although it is the easiest to measure. The primary phenomenon prior to the crystallization of glass is phase separation, the detection of which is very rarely reported among small molecular compounds. In the present study, a scanning electron microscope with energy dispersive X-ray spectrometer (SEM-EDS) was used to detect very early stage amorphous-amorphous phase separation in co-amorphous drug mixtures. Miscibility was calculated for five studied mixtures based on the Flory-Huggins method and four immiscible pairs and one partial miscible pair were selected for the laboratory experiments. Co-amorphous samples (n = 3) were prepared by melt-quench method and stored at the elevated temperature to induce the separation of amorphous phases. Each sample was stored at the same relative percentage temperature between glass transition temperature T_g and melting temperature T_m. Immediately after the sample preparation, the full amorphousness was verified with polarizing light microscopy. Before SEM-EDS analysis, the samples were fractured into two pieces and measurements were done from cross-section (from the bulk sample). All five pairs phase separated during two days of storage at the elevated temperature. The study proved that SEM-EDS was able to detect a very small phase separated regions in the amorphous sample, as amorphous-amorphous phase separation was detected in four out of five pairs. However, the surface roughness could affect the analysis and give a false indication of phase separation. SEM-EDS also supported calculation results, since every studied pair showed phase separation during study, as was predicted on the grounds of Flory-Huggins miscibility calculation.