(780c) The Water Sorption Behaviour of Amorphous Solids: Experimental and Theoretical Insights Using a Model Protein BSA

This paper considers the water sorption behaviour of complex amorphous solids using advanced experimental investigations as well as a number of thermodynamics models for the formation of solid- solute solutions. The water sorption behaviour of amorphous materials is especially complex kinetically as the loss or gain of water vapour by the solid is controlled by two nominally independent kinetic phenomena. The first is the diffusion kinetics of moisture sorption and the second factor is the relaxation kinetics of the glassy amorphous state.

Experimental evidence for a glassyâ??rubbery hydration mechanism of bovine serum albumin (BSA) is reported where BSA is used as an amorphous model protein substrate. Experimental results corroborate that the hydration mechanism in amorphous protein materials are more complex than anticipated. We have experimental evidence indicating that BSA can be experimentally trapped into distinct conformational states during water desorption. In considering the dimension of time during hydration or dehydration of a protein, a significant problem in determining and interpreting a water sorption isotherm is posed by the â??sample historyâ??, a time effect, of a lyophilised amorphous biological products. Its influence, both prior to the sorption experiment as well as produced during the actual measurement, has not been previously clarified.

A water vapour sorption experiment is typically carried out in one of the three modes: (a) interval sorption,where the solvent activity is subsequently raised in a stepwise fashion through one continuous measurement; (b) integral sorption, where the solvent activity is raised from zero up to one specific value and the mass uptake is recorded (typically with a new specimen for each measurement); or (c) differential sorption, where an activity ramp is applied in one continuous measurement. It should be understood that each of these three sorption modes creates a different experiment-induced time dependent behaviour. This paper will provide recommendations in best practise for determined water sorption properties of amorphous solids.

The final section of this paper will introduce modern equilibrium and non-equilibrium thermodynamics models for analysing water sorption isotherm behaviour for amorphous solids.