Molecular-Based Description of the Osmotic Second Virial Coefficients of Electrolytes: Rigorous Formal Links to Solute-Solvent Interaction Asymmetry, Virial Expansion Paths, and Experimental Evidence

Preferential Solvation Phenomena as Solute-Induced Structure-Making/Breaking Processes: Linking Thermodynamic Preferential Interaction Parameters to Fundamental Structure Making/Breaking Functions

In this work, we identify the explicit macroscopic-to-microscopic rigorous links between existing thermodynamic preferential interaction parameters Γ Q α Q β ( χ i ) and microstructural descriptors based on total correlation function integrals, leading to their unambiguous characterization in terms of fundamental structure making/breaking functions S α β . First, we provide the statistics mechanical framework to identify a universal molecular-based signature for the preferential solvation P S phenomenon involving solutes at infinite dilution in mixed-solvent environments and discuss its fundamental properties. Then, we characterize the S α β functions relevant to the P S process, identify the microscopic markers for the existing preferential interaction parameters Γ Q α Q β ( χ i ) in terms of the S α β functions, and test their compliance with a pair of essential microstructural constraints linked to the properties of the universal P S signature. Moreover, we illustrate the analysis by probing the behavior of a representative ternary system comprising the solubility of methane in aqueous 1,4-dioxane mixed-solvent environments under ambient conditions. Finally, we discuss some relevant issues surrounding the statistical mechanical (microstructural) interpretation of the thermodynamic (macroscopic) preferential interaction parameters, review some pitfalls in their evaluation from molecular simulation, and provide an outlook.

On the Elusive Links between Solution Microstructure, Dynamics, and Solvation Thermodynamics: Demystifying the Path through a Bridge over Troubled Conjectures and Misinterpretations

We build a fundamentally based bridge between the solute-induced microstructural perturbation of the species environment and the dynamic as well as thermodynamic responses of the fluid system, regardless of the state conditions, composition, nature of the solvent, and either the magnitude or the type of solute-solvent intermolecular-interaction asymmetries. For that purpose, we advance a fluctuation-based solvation formalism of fluid mixtures to provide meaningful descriptors of solvation phenomena, the microstructural signatures of their solute-solvent intermolecular interaction asymmetry, and the thermodynamic manifestations linked to the solution nonideality. The rigorous foundations afford us to address some crucial issues frequently invoked in the literature including the microstructural perturbation domain, its proper identification and molecular-based meaning toward the interpretation of the solvation process, and the potential impact of the local differential behavior between anions and cations on the actual salt-induced perturbation of the solvent microstructure. Indeed, we link the precisely characterized species solvation behavior to fundamental thermodynamic residual-property relations, and the dynamics associated with either the viscous flow or diffusive behavior of the solvent, to finally illustrate their outcome with experimental data of aqueous electrolyte solutions from the available literature. Ultimately, this effort provides a highly desirable unambiguous identification of the cause-effect connections between the microstructurally perturbed domains and the experimentally measured macroscopic solvation properties, including their effect on the dynamics of the solvent environment. More importantly, it lends a well-established solvation framework to bridge rigorously the microstructural details of the mixture, its dynamics, and its solvation thermodynamics to enhance our understanding of well-defined ranked Hofmeister series, i.e., by avoiding ad hoc conjectures and unsupported microscopic interpretations of solvation phenomena.