Overview

The Mixed Solvent Electrolyte (MSE) framework, developed and maintained by OLI Systems, is a robust thermodynamic modeling approach designed to accurately simulate aqueous and mixed solvent systems. Central to its architecture is the treatment of solvation phenomena, which can be represented through two distinct methodologies: implicit solvation and explicit solvation. Implicit solvation—used as the default approach—captures the effects of solute-solvent interactions through activity coefficients without introducing separate solvated species. In contrast, explicit solvation is applied in specialized cases where nuanced chemical behavior demands the inclusion of defined solvated complexes. This article explores the rationale behind these two approaches, with a particular focus on the explicit incorporation of species such as NH₄OH to ensure predictive fidelity in complex systems.

Solvation in MSE

In the Mixed Solvent Electrolyte (MSE) framework used by OLI, solvation of species in water can be modeled in two ways:

1. Implicit Solvation (Default Approach):
   • Most solvation effects are captured indirectly through activity coefficients.
   • These coefficients reproduce interactions between a dissolved solute and water.
   • No special solvated species are added to the model. Instead, the thermodynamic behavior is calibrated to reproduce known experimental data.
2. Explicit Solvation (Special Cases):
   • In certain systems, explicit solvation species are introduced to achieve high fidelity with experimental data across a wide concentration range.
   • A classic example is the hydronium ion (H₃O⁺), which explicitly represents the solvated proton in aqueous systems.
   • Another is NH₄OH, which acts as a solvated form of ammonia (NH₃).

Why NH₄OH Is Explicitly Included

• The MSE model introduces NH₄OH explicitly as a distinct species.
• This was necessary because:
  • Ammonia-water mixtures show complex non-ideal behavior, especially at high concentrations.
  • An explicit NH₄OH species helps match experimental data (like vapor-liquid equilibria and osmotic coefficients) more accurately than relying solely on implicit activity coefficient models.
• This ensures the model’s validity across the full concentration range, which is essential for applications in industries such as chemical manufacturing, environmental modeling, and power systems.

Conclusion

The inclusion of explicit solvation species within the MSE framework is a strategic enhancement reserved for cases where conventional implicit modeling cannot sufficiently replicate empirical data. The decision to model NH₄OH explicitly reflects both the complexity of ammonia-water interactions and OLI’s commitment to delivering accurate, high-fidelity thermodynamic predictions across broad operational conditions. By accommodating such specialized behavior, the MSE model continues to support critical industrial applications with a level of precision and reliability that reinforces OLI’s position as a leader in electrolyte chemistry modeling.