The influence of additives on the recrystallisation of amorphous spray dried lactose

Amorphous material in crystals can constitute reactive ‘hot spots’, which can be centres for chemical degradation or physical transitions, leading to product instability. Problems have been encountered in studying small amounts of amorphous content for powdered systems, due to poor sensitivity of the majority of techniques. Isothermal microcalorimetry has been shown to have good resolution for cases where the amorphous content of the powder can be made to recrystallise in the instrument. In this study amorphous lactose has been investigated, being recrystallised by exposure to air at 75% RH. The lactose has been studied in two layers separated by varying amounts of glass beads (inert carrier), magnesium stearate (hydrophobic excipient), or microcrystalline cellulose (hygroscopic excipient). Significant differences were observed in the time needed to cause recrystallisation when amorphous material was separated by these different additives. Glass beads had only a small effect, but magnesium stearate caused an increased lag time prior to the recrystallisation event. In both these cases the lactose all recrystallised at one time, even though it was divided into two physically separated regions. A layer of microcrystalline cellulose between two layers of amorphous lactose resulted in a long lag time prior to recrystallisation, as it removed considerable amounts of water vapour from the atmosphere, thus preventing saturation of the lactose. By varying the weight of amorphous lactose in the upper and lower layers, and comparing data with the results obtained for homogeneous mixtures, it was possible to postulate a mechanism for the cooperative recrystallisation process. In essence, the water vapour is absorbed into the upper layers of the sample, and then transferred away yielding a concentration gradient through the entire sample in the cell. As the water content gradually increases in the lower layers, the rate of water absorption can become more rapid than the rate at which water is transferred away from the surface. After this time the surface saturates, starts to recrystallise thus liberating a great excess of water vapour, which is sufficient to cause the lower layers of powder to become saturated and also to recrystallise.