Quantitative assessments of powder crystallinity: Estimates of heat and mass transfer to interpret isothermal microcalorimetry data

Isothermal microcalorimetry is used to study small quantities of amorphous materials in crystalline powders. The aim of this work is to better understand the isothermal microcalorimetry measurement with regard to the quantification of amorphous contents of materials. Amorphous lactose was crystallized in a sealed ampoule in an isothermal microcalorimeter at a range of temperatures (25-60 degrees C) and humidities. Identical heat changes for crystallization were observed at all humidities at 25 degrees C; however, the measured heat varied with humidity at higher temperatures. The heat measured by isothermal microcalorimetry was approximately the difference between the heat of crystallization and the heat of vaporization of the desorbed water. The isothermal microcalorimetry output for this process is now better understood and it can be seen that, in order to obtain quantitative data for crystallinity, it is necessary to have a slow supply of vapor. As the measured heat change is related to the extent of water desorption, care must be taken when using microcalorimetry to quantify the amorphous content of powders, especially when comparing data generated at different environmental conditions. (C) 1998 Elsevier Science B.V.