We
introduce a molecular-based route to the evaluation of the osmotic
second virial coefficients of dissociative solutes in dilute binary
solutions, according to a general molecular thermodynamic solvation
formalism of electrolyte solutions. We discuss the underlying solvation
fundamentals and derive rigorous expressions leading to (i) the functional
relationship among the osmotic second virial coefficients and the
limiting composition behavior of the non-Coulombic contribution to
the Kirkwood–Buff integral of the solute–solute interactions,
the corresponding composition slope of the mean activity coefficient
of the electrolyte solute, and a precisely defined solute–solvent
intermolecular interaction asymmetry that characterizes unambiguously
the solution non-ideality; (ii) the self-consistent calculation of
the osmotic second virial coefficients of electrolytes as defined
by the composition expansion along different thermodynamic paths and/or
composition variables; (iii) the microstructural interpretation of
Hill’s isobaric-isothermal osmotic second virial coefficient
in terms of Kirkwood–Buff correlation function integrals and
its relationships to other osmotic coefficients from composition expansions
along alternative thermodynamic paths; and (iv) the identification
of drawbacks in the implementation of previous methods, originally
intended for non-electrolyte systems, to systems involving dissociative
solutes. The proposed formalism provides the fundamentally based foundations
to the determination of the osmotic second virial coefficients of
any type of electrolyte solute, whose thermodynamic expressions converge
naturally to the non-electrolyte ones by setting to unity the solute
stoichiometric coefficient ν. Following the formal results,
we illustrate the formalism with the calculation of a variety of osmotic
second virial coefficients involving a wide selection of aqueous solutions
at ambient conditions and comprising a wide range of anion–cation
type combinations characterized by 2 ≤ ν ≤ 6.
Finally, we interpret the behavior of the resulting osmotic virial
coefficients in terms of the solute–solvent intermolecular
interaction asymmetry, discuss the experimental data requirements
for the accurate evaluation of the osmotic second virial coefficients,
and provide some observations as well as their modeling implications.