Solid aqueous solutions

In most treatments of aqueous "solutions" it is taken for granted that water is present in the liquid state and in excess, with the concentrations of solutes limited by their saturation solubilities. Hence, the thermodynamic reference states are defined as the infinitely dilute solution, a solution of unit molality, etc. In recent years supersaturated aqueous solutions have been receiving increasing attention, mainly because of their importance in the life sciences and in diverse branches of technology. The physical chemistry of structures, dynamics and interactions in aqueous mixtures in which the concentration of water is of the order of 0-5% by weight is largely unexplored. Such mixtures can be produced from unsaturated aqueous solutions either by evaporative drying or freeze-drying. Being thermodynamically unstable, they rely for their (kinetic) stability on their extremely high viscosities. These systems, like other amorphous solids, exhibit the phenomenon of glass transitions with the residual moisture acting as plasticiser, reducing the glass temperature of the solid substrate. The themmechanical properties of amorphous, water-sensitive solids are important determinants in many technological processes, but also in the acclimation of living organisms to environmental stresses, such as freezing, saline conditions and desiccation.