On the anomalous behavior of aqueous solutions: Rigorous formal results and the unraveling of their microstructural origin

Abstract

We unravel the formal, unambiguous relationships between the evolution of the solute-induced perturbation of the solvent environment and the consequent macroscopic manifestation of the anomalous (aka non-monotonous composition) behavior of the second and third derivatives of the excess Gibbs free energy of a binary mixture. We address some relevant issues regarding the anomalous behavior of aqueous solutions, including the identification of their molecular-based manifestation and the rigorous interpretation according to explicit cause–effect relations between the thermodynamic evidence and the molecular-based behavior in terms of the relevant fundamental structure-making/structure-breaking functions and their temperature/pressure derivatives. Because this novel approach involves no restrictions on the type of interactions and the nature of the species in solution, it provides a powerful tool to gain understanding of the role played by solute–solvent intermolecular interaction asymmetries either in the presence or absence of hydrogen bonding interactions. Finally, we illustrate the formalism through a detailed analysis of the anomalies of aqueous ethanol solutions and test some conjectured hypotheses underlying the anomalies that point to their plausible connection to changes in the hydrogen bonding between species.