Authors:Libby Yoerg, M. Ellen Matthews, Lakshmi Kaza, Naullage Indika Perera, David W. Ball, John Moran, and Alan T. Riga
Three aldohexose monosaccharides, d-glucose, d-mannose, and d-galactose, were examined by scanning temperature dielectric analysis (DEA) from ambient temperatures through their melts. Phase transitions, including glass transition (Tg) and melting temperature (Tm), were evaluated by differential scanning calorimetry (DSC). The monosaccharides were found to exhibit thermally-induced dielectric loss spectra in their amorphous-solid phase before melting. Activation energies for electrical charging of each of the monosaccharides were calculated from an Arrhenius plot of the tan delta (e″/e′, dielectric loss factor/relative permittivity) peak frequency versus reciprocal temperature in Kelvin. The DEA profiles were also correlated with the DSC phase diagrams, showing the changes in electrical behavior associated with solid–solid and solid–liquid transitions.
Authors:Shravan Singh Thakur, Manik Pavan Kumar Maheswaram, Dhruthiman Reddy Mantheni, Lakshmi Kaza, Indika Perara, David W. Ball, John Moran, and Alan T. Riga
Thermal mechanical analysis (TMA) of crystalline drugs and excipients in their pre-melt temperature range performed in this study corroborate their newly found linear dielectric conductivity properties with temperature. TMA of crystalline active pharmacy ingredients (APIs) or excipients shows softening at 30–100 °C below the calorimetric melting phase transition, which is also observed by dielectric analysis (DEA). Acetophenetidin melts at 135 °C as measured calorimetrically by DSC, but softens under a low mechanical stress at 95 °C. At this pre-melting temperature, the crystals collapse under the applied load, and the TMA probe shows rapid displacement. The mechanical properties yield a softening structure and cause a dimensionally slow disintegration resulting in a sharp dimensional change at the melting point. In order to incorporate these findings into a structure–property relationship, several United States Pharmacopeia (USP) melting-point standard drugs were evaluated by TMA, DSC, and DEA, and compared to the USP standard melt temperatures. The USP standard melt temperature for vanillin (80 °C) , acetophenetidin (135 °C) , and caffeine (235 °C)  are easily verified calorimetrically via DSC. The combined thermal analysis techniques allow for a wide variety of the newly discovered physical properties of drugs and excipients.