Isothermal microcalorimetry and inverse phase gas chromatography to study small changes in powder surface properties.

It is known that processing can alter the surface energetics of powders. In this study a sample of drug has been processed by use of different drying techniques. The samples were then assessed using inverse phase gas chromatography. It was seen that the original material had a highest surface energy and the tray-dried sample had the lowest energy surface, other samples were intermediate. The use of isothermal microcalorimetry to study water sorption to the powders revealed that the surface of the original material was unstable, as the water sorption response changed on repeat cycling. The tray-dried sample did have a stable surface which gave the same sorption response on repeat exposure to water vapour. It was concluded that the drug had minor variations in surface energy, with the as received material being in a high energy unstable state, which could be due to it being partially amorphous. The tray-dried sample had a lower energy stable surface. In certain applications differences in surface energetics could be expected to lead to changes in processing nature of the powder, so these vapour sorption techniques offer a good way of providing an assurance of the same surface energy between batches of nay material which may be at risk.