Thermodynamically dominant hydration structures of aqueous ions

Area of Science:

• Computational chemistry
• Physical chemistry
• Chemical physics

Background:

• Hydration free energy is crucial for understanding ion-solvent interactions.
• Previous models often simplify the complex interplay between ions and water molecules.

Purpose of the Study:

• To investigate the thermodynamic contributions of different coordination states to ion hydration.
• To analyze the role of the ion in structuring the surrounding water molecules.

Main Methods:

• Employing a molecular aufbau approach within a classical molecular mechanics framework.
• Separating hydration free energy into chemical, packing, and long-range correction terms.
• Analyzing coordination states X[H(2)O](n) for Na(+), K(+), and F(-) ions.

Main Results:

• The chemical contribution to hydration free energy is dominated by water molecules within the most probable coordination state (ñ).
• Ion influence on water structure is local, with only a subset of water molecules (n ≤ ñ) significantly impacting hydration thermodynamics.
• Specific dominant coordination states were identified for Na(+), K(+), and F(-).

Conclusions:

• The ion's effect on solvent structuring is primarily local.
• A limited number of water molecules near the ion are most critical for hydration thermodynamics.
• Adding or removing water molecules beyond the dominant coordination state has significant energetic consequences.