The structure of ionic aqueous solutions at interfaces: an intrinsic structure analysis.

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Surface Science

Background:

• Understanding the structure of ionic solutions at interfaces is crucial for various chemical and physical processes.
• Previous studies often relied on averaged profiles, potentially obscuring intricate interfacial details.

Purpose of the Study:

• To investigate the intrinsic interfacial structure of alkali halide solutions in water.
• To analyze the impact of ion size and concentration on interfacial structure and electrostatic potential.
• To introduce and validate a computational approach for removing interfacial capillary wave effects.

Main Methods:

• Molecular dynamics simulations using point charge ion models.
• A novel computational method to eliminate interfacial capillary wave averaging.
• Analysis of intrinsic interfacial structure and electrostatic potential.

Main Results:

• A complex, alternating double layer structure was observed, dependent on cation and anion size.
• Small changes in ion diameter significantly altered the double layer, favoring anion adsorption or cation density enhancement.
• The intrinsic interfacial electrostatic potential exhibited strong oscillations, with a surface minimum approximately 4 times stronger than the bulk potential.
• The SPC/E water model showed a surface potential of ~-2 V (~80 kBT), stronger than previously reported.

Conclusions:

• The intrinsic surface technique provides a more accurate representation of ionic solution interfaces than methods affected by thermal fluctuations.
• The findings highlight the significant role of ion size and solvation shells in shaping interfacial structure and electrostatics.
• This work offers a refined understanding of aqueous interfaces and a robust method for future investigations.