Thermodynamics of amorphous starch-water systems. 2. Concentration fluctuations

This paper presents a theoretical and experimental study of the sigmoidal water sorption isotherms of amorphous starch. Sorption isotherms have been measured by gas chromatography at infinite dilution of water in starch and by isothermal and isosteric sorption experiments in an extended concentration and temperature range in which the biopolymer behaves either as a brittle glass or as a rubbery melt. A maximum value of the isothermal activity coefficient of water is observed at a composition corresponding to the glass transitions measured by calorimetry. Therefore, the partial derivatives of the activity coefficient of water with respect to concentration and temperature are positive in glasses and negative in melts. A transition from sigmoidal to Flory type sorption is estimated to occur at 175 degrees C, which is lower than the glass transition of dry starch. The distribution of water molecules in glasses and melts is analyzed with the Kirkwood theory of solutions. In glasses, water shows large negative excluded volumes typical of an antiplasticizer, as reflected also in the density increase observed at low water concentrations. In melts, water shows positive excluded volumes typical of a plasticizer having recovered its motional freedom restricted in the glassy state. Up to 80 degrees C, the self-clustering functions of water in melts diverge at higher water contents. These functions only take finite values above this temperature once a full melting of the starch-starch hydrogen bond interaction has occurred. The sigmoidal water sorption isotherms are analyzed with a new explicit relationship combining the generalized Freundlich adsorption model and the Flory model of polymer solutions. A restricted translational and rotational freedom is predicted for the adsorption water, and a clustering tendency is predicted for the solution water. The Freundlich-Flory sorption model provides a consistent description of the solvation, the swelling, and the dissolution of hydrophilic polymer glasses in a solvent like water whereas the Brunauer-Emmet-Teller model is only physically meaningful for the adsorption of nonsolvents such as oxygen or nitrogen gases.