Solvent-induced ion clusters generate long-ranged double-layer forces at high ionic strengths.

Area of Science:

• Physical Chemistry
• Colloid and Surface Science
• Computational Chemistry

Background:

• Surface Force Apparatus (SFA) experiments reveal anomalous underscreening in electrolytes, deviating from established theories.
• Anomalous underscreening suggests electrostatic interaction ranges increase with salt concentration beyond a threshold (approx. 1 M).

Purpose of the Study:

• To theoretically investigate anomalous underscreening using an extended restricted primitive model (RPM).
• To incorporate short-ranged pair potentials of mean force (sPMF) to mimic ion hydration changes.
• To compare simulation results with experimental SFA data.

Main Methods:

• Utilized an extended restricted primitive model (RPM) with added short-ranged pair potentials of mean force (sPMF).
• Employed grand canonical simulations to predict surface forces.
• Adjusted sPMF strength for realistic saturation concentration (4-7 M for 1:1 salts).

Main Results:

• Simulated models showed significant ion clustering above 1 M salt concentration.
• Observed substantial double-layer repulsion exceeding standard RPM predictions.
• Screening length saturated at a high value, rather than increasing with concentration.
• Long-ranged interactions correlated with ion cluster formation and cluster-cluster interactions.

Conclusions:

• The modified RPM with sPMF successfully explains anomalous underscreening and experimental SFA data.
• Ion clustering and associated steric interactions are crucial for understanding long-ranged electrostatic behavior in concentrated electrolytes.
• The model provides insights into the complex interplay of hydration, clustering, and electrostatic forces.