On density-based modeling of dilute non-electrolyte solutions involving wide ranges of state conditions and intermolecular asymmetries: Formal results, fundamental constraints, and the rationale for its molecular thermodynamic foundations

Abstract

We present a novel molecular thermodynamic framework for the unambiguous assessment of the reliability of modeling approximations, their internal consistency, and the compliance with fundamental limiting behaviors in the description of solvation phenomena of species at infinite dilution in fluid systems over wide ranges of state conditions and solute-solvent intermolecular asymmetries. The proposed aprioristic approach does not rely on any type of regression technique; its accuracy rests on the theoretical validity and self-consistency of the underlying core solvation formalism and the first principles of chemical thermodynamics already established for model systems for which we have available exact formal results, and its application leads to the identification, as well as the isolation of the sources of modeling deficiencies. To that end, we (i) set the foundations for a rigorous description of the solvation behavior of solutes at infinite dilution at the microscopic and concomitant macroscopic levels, (ii) advance a set of thermodynamic limiting conditions as required constraints for any model of infinitely dilute solutions, (iii) identify the microscopic signatures of the solvation phenomena in terms of solute-induced perturbations of the solvent microstructure, and (iv) invoke an exact fluctuation formalism of solutions to assess the predictive capability of some current density-based models of aqueous non-electrolyte solutions. According to the findings of this analysis, we suggest a novel fully molecular-based alternative for the thermodynamic description of solutions at infinite dilution involving wide ranges of state conditions and solute-solvent intermolecular asymmetry, including the ideal gas solute and the solute behaving as another solvent species. This approach, which functions at a self-consistent integral plus derivative level, would automatically comply with all required thermodynamic constraints.