Charge compensation at the interface between the polar NaCl(111) surface and a NaCl aqueous solution

Area of Science:

• Computational materials science
• Physical chemistry
• Electrochemistry

Background:

• Periodic supercell models for electric double layers face finite size errors and require careful treatment of electrostatic interactions.
• Previous work introduced finite field methods to address these issues using a simple point charge model of NaCl solution between charged walls.

Purpose of the Study:

• To extend finite field methods to model the interface between a polar NaCl (111) crystal surface and a concentrated NaCl aqueous solution.
• To investigate the charge compensation mechanism and determine the Helmholtz capacitance at this interface.

Main Methods:

• Application of finite field methods to a rigid NaCl (111) crystal surface in contact with a high-concentration NaCl electrolyte.
• Verification of ionic charge compensation against the Tasker 1/2 rule for polar surfaces.
• Calculation of generalized Helmholtz capacitance (C_H) by varying the applied electric field.

Main Results:

• The excess electrolyte ionic charge at the interface conforms to the Tasker 1/2 rule.
• The calculated generalized Helmholtz capacitance (C_H) is 8.23 μF cm⁻² for the polar (111) surface.
• This value is significantly higher than the 4.23 μF cm⁻² found for the non-polar NaCl (100) surface.

Conclusions:

• Finite field methods provide a robust approach for modeling electric double layers at polar crystal-electrolyte interfaces.
• The higher capacitance at the polar (111) surface is attributed to ions losing solvation shells and forming contact ion pairs.
• The study provides a quantitative measure of capacitance for polar surfaces, relevant for understanding interfacial phenomena in electrochemical systems.