Various properties of water and aqueous solutions are reviewed within the framework of the 'strong/fragile liquids' classification. The peculiar situation presented by pure water and dilute aqueous solutions, is discussed, then the more concentrated and easily glass-forming solutions involving salts, molecular components, and polymeric components are reviewed. In the electrolyte cases the properties of solutions in the glass-forming domain up to the ice-saturated limit prove to be those of very fragile liquids. This behavior is incompatible with the properties of pure water near the glass transition of vitreous ice (amorphous solid waters; ASW), supporting the notion that the latter is a distinct and separate phase of H2O. In the molecular solute cases, particularly with sugar components, the liquids appear to become stronger with increasing water content, whereas with water-plasticized polymers the position is not yet clear. Evidence on methacrylate polymers and proteins suggested that they may be rather strong. With this background the effects of solution fragility on the kinetics of longer time scale processes is discussed. In particular, the freezing of chemical reactions at temperatures well above Tg is considered in relation to the fluid (solvent) fragility, and accounted for in terms of a simple diffusion model.
ASJC Scopus subject areas
- Food Science