The Born model can accurately describe electrostatic ion solvation

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Electrochemistry

Background:

• Accurate modeling of ion solvation free energies is vital for understanding electrolyte solutions.
• The Born model, a key tool for solvation free energies, is considered flawed due to its prediction of linear water response.
• This linear response contradicts the observed asymmetry in water's solvation of anions versus cations.

Purpose of the Study:

• To investigate the source of asymmetry in water's response to ionic charge in simulations.
• To reassess the validity of the Born model for ion solvation free energies.
• To explore alternative definitions for the center of a water molecule in computational models.

Main Methods:

• Utilized classical molecular dynamics simulations.
• Re-evaluated the Born model by shifting the center of the water molecule model.
• Analyzed the electrostatic response of water to varying ionic charges.

Main Results:

• The observed asymmetry in water's response is an artifact of the chosen water molecule center, not an intrinsic property.
• Shifting the water molecule's center by 0.5 Å towards hydrogen atoms yields a charge-symmetric response.
• This adjustment aligns the simulation results with the Born model's predictions and improves surface potential calculations.

Conclusions:

• The asymmetry in ion solvation by water is not an intrinsic electrostatic property but arises from specific short-range repulsive interactions.
• The Born model's failure to capture this asymmetry is not a limitation of the model itself but a consequence of the simulation's water representation.
• A revised definition of the water molecule's center in simulations can reconcile simulation results with the Born model, enhancing the accuracy of solvation free energy calculations.