Ionic Behavior in Highly Concentrated Aqueous Solutions Nanoconfined between Discretely Charged Silicon Surfaces

Area of Science:

• Physical Chemistry
• Surface Science
• Computational Nanoscience

Background:

• Understanding ion behavior in confined systems is crucial for electrochemical applications.
• The discrete nature of surface charges significantly impacts electric double-layer structures.

Purpose of the Study:

• To investigate ionic behavior in concentrated sodium chloride (NaCl) solutions confined between discretely charged silicon surfaces.
• To analyze the effects of surface charge density, bulk concentration, and confinement on electric double-layer structure and ion distribution.

Main Methods:

• Molecular dynamics (MD) simulations were employed.
• Thermal vibration of surface atoms was considered.
• Simulations covered NaCl concentrations from 0.2 to 4.0 M.

Main Results:

• Electric double-layer structure is sensitive to surface charge density and distribution.
• Charge inversion was observed, dependent on surface charge density, concentration, and confinement.
• Ion accumulation layers remained stable but increased in peak values with concentration.
• Alternating layered distributions of Na(+) and Cl(-) ions appeared at high concentrations (4.0 M).
• Confinement significantly affected ion distribution, with ions and water squeezed out as surfaces approached.
• Ionic hydration analysis showed reduced coordination numbers for Na(+) in confined spaces.

Conclusions:

• The discreteness of surface charges leads to complex electric double-layer structures and charge inversion in confined electrolytes.
• High ion concentrations and confinement induce ordered, layered ion distributions, potentially explaining experimental observations of solidification.
• Confinement plays a critical role in modifying ion hydration and distribution within nanoscale gaps.