THE USE OF ISOTHERMAL MICROCALORIMETRY IN THE STUDY OF CHANGES IN CRYSTALLINITY INDUCED DURING THE PROCESSING OF POWDERS

Isothermal microcalorimetry has been used to follow recrystallisation of amorphous regions of powder surfaces. Lactose monohydrate was taken as a model powder, and was processed by spray drying and micronisation. Spray drying produced an amorphous powder (as shown by X-ray diffraction), which was found to recrystallise when exposed to humidities over 50% RH. The recrystallisation process was extremely cooperative, with the entire sample recrystallising almost instantaneously, rather than a gradual process over the period of exposure to the water vapour. Similar results were noted when micronised material was investigated. The amount of amorphous material produced during micronisation was directly proportional to the intensity of the process. It proved possible to quantify the % amorphous content of powder sample with a resolution of at least 1%, which is considerably better than other techniques. The amorphous regions of the lactose crystallised as either alpha- or beta-lactose. The difference between these samples could be detected by X-ray diffraction, and also could be seen by isothermal calorimetry, as the beta-regions mutarotated to alpha-lactose. The application of isothermal microcalorimetry to studies of crystal properties of powders provides a quantitative characterisation of many aspects of crystallinity and crystal transition. The data obtained can subsequently be used to characterise the properties of the material, and to show.how and when crystallisation will occur, and to aid predictions of the product of the crystallisation process. The demonstration of these applications provides a huge potential for the use of isothermal microcalorimetry in this field of study.