(239g) Drying Behaviour of Complex Amorphous Solids

Daryl R. Williams

Surfaces and Particle Engineering Laboratory, Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK

(d.r.williams@imperial.ac.uk+44 (0) 0207 594 5611)

In fully understanding drying behavior of solids, it can be of enormous fundamental importance to understand the process by which water is initially sorbed. The uptake of water by amorphous or partially amorphous organic powders is a remarkably ubiquitous phenomena, yet one which is surprisingly poorly understood. Although this phenomena has been studied by notable scientists for 100 years, the literature, including many current publications, is awash with papers which misunderstand fundamentally the water sorption behaviour of solids which have significant amorphous content. This includes freeze dried and spray dried solids, as well as many natural materials such as cotton and other cellulosic materials, keratin and other cellular solids a well as many polymeric materials Its importance in solids engineering is that moisture sorption is often the precursor event to powder caking, poor powder flow, batch to batch variability’s, phase changes, changes in T_g, re-crystallisation and a myriad of other often undesirable property changes in solid state materials systems.

The paper will commence by reporting on a detailed experimental study of water vapor sorption in freeze dried solids as well as a range of natural solid materials. The kinetics and equilibrium uptake of moisture by these materials will then be analysed using a range of polymer-solution based thermodynamics models, starting with the classic Flory-Huggins model and then leading onto the more recent lattice fluid and non-equilibrium lattice fluid theories. These models are far superior and fundamentally sounder than the surface adsorption models such as GAB and BET which are commonly (and erroneously) used to model sorption isotherms in amorphous powders. These thermodynamic models provide a highly relevant theoretical framework which can allow the complexities of water sorption within amorphous solids to be understood on a fundamentally sounder basis. Finally this paper highlight how this new understanding of the importance of water-amorphous solid state thermodynamics will help us gain a sounder understanding of how water sorbs and dissolves in amorphous organic powders.