Activity coefficients of aqueous electrolytes from implicit-water molecular dynamics simulations

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Materials Science

Background:

• Accurate prediction of electrolyte activity coefficients is crucial for understanding solution behavior.
• Molecular dynamics (MD) simulations offer a powerful tool for studying electrolyte solutions.
• Implicit-water models simplify simulations by representing water as a continuous medium, but their accuracy for electrolytes is debated.

Purpose of the Study:

• To evaluate the accuracy of implicit-water molecular dynamics simulations for calculating activity coefficients of NaCl and KCl.
• To compare two approaches: constant relative permittivity versus concentration-dependent relative permittivity.
• To identify limitations of implicit-water models in capturing electrolyte solution behavior.

Main Methods:

• Implicit-water molecular dynamics simulations at 298.15 K and 1 bar.
• Two approaches: constant relative permittivity (εr) and concentration-dependent εr.
• Calculation of individual ion activity coefficients (IIACs) and mean ionic activity coefficients (MIACs) via gradual ion insertion.

Main Results:

• Simulations with constant εr yielded MIACs in reasonable agreement with experimental data.
• Simulations incorporating concentration-dependent εr showed significant negative deviations in both IIACs and MIACs.
• Even with constant εr, the relative positioning of individual ions was inaccurate for NaCl.

Conclusions:

• Implicit-water simulations have severe limitations in accurately predicting aqueous electrolyte activities.
• Correcting for concentration-dependent relative permittivity does not compensate for the lack of explicit hydration structure.
• Findings impact the development of electrolyte theories and the interpretation of implicit-solvent simulation results.