Primitive and non-primitive model electrolytes: Comparing ion-related Helmholtz energies using molecular simulations

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Chemical Physics

Background:

• Electrolyte solutions are described by non-primitive and primitive models.
• Primitive models simplify solvent interactions, using equations like Debye-Hückel (DH) and Mean Spherical Approximation (MSA).

Purpose of the Study:

• To evaluate the accuracy of primitive model approaches for electrolyte solutions.
• To compare primitive model Helmholtz energies with molecular simulation data from non-primitive models.

Main Methods:

• Molecular simulations of electrolyte solutions using Lennard-Jones particles.
• Thermodynamic integration to isolate Helmholtz energy contributions.
• Simulations conducted at varying temperatures, densities, charges, dipole moments, and ion mole fractions.

Main Results:

• Primitive model expressions offer qualitative but not quantitative agreement for electrolyte solutions.
• Systematic underestimation of ion-solvent and ion-ion interaction energies by primitive models.
• Empirical adjustment of the Born radius is required, yielding radii larger than actual ion sizes.

Conclusions:

• Primitive models require significant empirical adjustments, questioning their fundamental applicability.
• This study provides benchmarks for MSA, DH, and Born theories against molecular simulation data.
• Findings offer insights for refining existing models and developing new equations of state for electrolyte solutions.