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 micronisatio n. Spray drying produced an amorphous powder (as shown by X-ray diffra ction), which was found to recrystallise when exposed to humidities ov er 50% RH. The recrystallisation process was extremely cooperative, wi th the entire sample recrystallising almost instantaneously, rather th an 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 dir ectly proportional to the intensity of the process. It proved possible to quantify the % amorphous content of powder sample with a resolutio n 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 b y 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 powder s provides a quantitative characterisation of many aspects of crystall inity and crystal transition. The data obtained can subsequently be us ed to characterise the properties of the material, and to show.how and when crystallisation will occur, and to aid predictions of the produc t of the crystallisation process. The demonstration of these applicati ons provides a huge potential for the use of isothermal microcalorimet ry in this field of study.