Ion Distribution and Hydration Structure at Solid-Liquid Interface between NaCl Crystal and Its Solution.

Area of Science:

• Physical Chemistry
• Materials Science
• Computational Chemistry

Background:

• Understanding the structure of electrolyte solutions at interfaces is crucial for various chemical and physical processes.
• The sodium chloride (NaCl) crystal-solution interface is a fundamental system with implications in geochemistry, materials science, and electrochemistry.

Purpose of the Study:

• To investigate the detailed structural characteristics of the saturated NaCl solution-crystal interface at 298 K.
• To elucidate the coordination behavior of sodium (Na+) and chloride (Cl-) ions with water molecules and each other at the interface.

Main Methods:

• Molecular dynamics (MD) simulations were employed to model the NaCl-water interface.
• Simulations were conducted at saturated concentration and a temperature of 298 K.

Main Results:

• Complex fine structures were observed at the NaCl-solution interface.
• Near the crystal surface, Na+ ions primarily coordinate with water, while Cl- ions coordinate with both Na+ and water.
• Ions coordinating with more water molecules are positioned further from the crystal's epitaxial lattice sites. The first hydration shells show increased ordering, but water molecule dipole orientation becomes more disordered compared to the bulk solution.
• The hydration shell of Na+ is less influenced by adjacent Cl-, whereas the Cl- hydration shell is significantly affected by nearby Na+.

Conclusions:

• The NaCl-water interface exhibits distinct structural and dynamic properties compared to the bulk solution.
• The differing coordination environments and hydration shell behaviors of Na+ and Cl- ions at the interface are key findings.
• These results provide atomic-level insights into interfacial phenomena in ionic solutions.