Dissolving salt is not equivalent to applying a pressure on water

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Materials Science

Background:

• Salt water is vital in geology and physiology, yet its structural impact on water remains debated.
• Understanding ion-water interactions is key to various scientific disciplines.

Purpose of the Study:

• To investigate the structural effects of dissolved ions on water using advanced computational methods.
• To clarify whether ions in salt water induce pressure-like changes in water structure.

Main Methods:

• Utilized machine learning-based molecular dynamics simulations grounded in density functional theory.
• Modeled sodium chloride, potassium chloride, and sodium bromide solutions across various concentrations.
• Validated simulation results against experimental neutron diffraction data for reciprocal-space structure factors.

Main Results:

• Achieved quantitative agreement between simulated structure factors and experimental neutron diffraction data.
• Demonstrated that ions primarily affect water within their first solvation shells.
• Observed minimal changes in the bulk water structure beyond the immediate ionic vicinity.

Conclusions:

• Ions in salt water do not induce a global, pressure-like distortion of the water structure.
• Structural modifications are localized to the ionic hydration shells, with limited impact on the bulk hydrogen bond network.
• The concept of a "pressure-like effect" in Hofmeister series requires re-evaluation based on these findings.