Structural Relaxation and Thermodynamics of Viscous Aqueous Systems: A Simplified Reappraisal

The attainment of true equilibrium conditions is a dynamic process that encompasses a time span. For slow relaxing systems, non-quilibrium steady states can often look like equilibrium states. This is the case of viscoelastic systems, whose properties reflect their thermo-rheological history. After a summary of the seminal woks by Eyring, Adam & Gibbs and Angell, and mention of promising recent approaches that imply updated theoretical and experimental techniques, the paper suggests a simplified approach for aqueous systems, through a modified expression of the chemical potential of water and use of the “dynamic” phase diagram, so far proposed by Slade and Levine. For homogeneous systems (aqueous solutions), an extra term in the expression of the chemical potential accounts for the energy related to the residual strains produced during the thermo-rheological history of the system. This approach allows estimation of the effect of viscosity on the observed freezing point of polymer solutions. For heterogeneous systems (hydrogels, colloidal glasses), changes of the phase boundaries in the phase diagram explain the gel/sol hysteresis and the syneresis process as the result of water exchange between hosting meshes and trapped aqueous solution. Finally, physical hurdles that hinder inter-phase water displacements and/or the access to the headspace of the system can lead to the coexistence of aqueous phases with different aW within the same heterogeneous system.