Ionic Rearrangements Lead to Like-Charge Attraction in a Nanoslit

Area of Science:

• Physical Chemistry
• Colloid and Surface Science
• Computational Physics

Background:

• Like-charge attraction is a known phenomenon in various scientific processes.
• The underlying mechanism of ionic electrostatic correlations causing this attraction has been established.
• The precise translation of these correlations into reduced forces has remained less explored.

Purpose of the Study:

• To systematically analyze the phenomenon of like-charge attraction between parallel walls.
• To investigate the influence of varying surface charge densities and electrolyte properties on this attraction.
• To elucidate the relationship between charge inversion and like-charge attraction.

Main Methods:

• Over 70,000 classical density functional theory (DFT) calculations were performed.
• A primitive model of charged, hard sphere electrolytes was employed.
• Systematic analysis across a wide range of wall surface charge densities and electrolyte properties (ion concentration, valence, size) was conducted.

Main Results:

• Ion rearrangements were observed within the narrowing slit, with non-monotonic changes in electrochemical potential components.
• Initial increases in co-ion concentration were followed by expulsion from the slit.
• Counterion redistribution led to decreased edge concentrations and a reduction in disjoining pressure.

Conclusions:

• The study provides deeper insight into how ionic correlations and electrochemical potential changes drive like-charge attraction.
• Ion behavior, specifically co-ion expulsion and counterion redistribution, is key to force reduction.
• The findings offer greater clarity on the connection between charge inversion and like-charge attraction.