EVALUATION OF HEAT-CONDUCTION MICROCALORIMETRY IN PHARMACEUTICAL STABILITY STUDIES .4. THE INFLUENCE OF MICROCRYSTALLINE CELLULOSE ON THE HYDRATION RATE OF ANHYDROUS LACTOSE

Microcalorimetry at 25.0-degrees-C, differential scanning calorimetry (DSC) and water uptake measurements were used to monitor the influence of water vapour on a 50/50 mixture of microcrystalline cellulose (MCC) and anhydrous lactose (26% alpha- and 74% beta-lactose), and on MCC alone, after storage at various relative humidities for different lengths of time. It was shown that MCC protects anhydrous lactose when surrounded by abundant water vapour by slowing down the rate of hydration. In a limited amount of water vapour, it was shown that MCC can redistribute its absorbed water through the vapour phase. This extends the duration of incorporation of hydrate water in anhydrous lactose over that seen when anhydrous lactose is alone. In the evaluation of the microcalorimetric technique it was shown that if a process is in progression before the first microcalorimetric measurement in a series, important information can be lost and the result may be difficult to interpret. However, together with other analytical techniques, the microcalorimetric result can give a valuable indication of how the process has proceeded.