Underscreening and hidden ion structures in large scale simulations of concentrated electrolytes

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Materials Science

Background:

• Debye-Hückel theory predicts decreasing electrostatic screening length with increasing ionic strength.
• Experimental studies show an increasing screening length in concentrated electrolytes, termed underscreening.
• Underscreening is attributed to ion-ion correlations and short-range forces like excluded volume interactions.

Purpose of the Study:

• To investigate the phenomenon of underscreening in concentrated electrolytes using Brownian Dynamics simulations.
• To explore the role of ion-ion correlations and excluded volume interactions on electrostatic screening.
• To analyze the emergence of large-scale ion structures and their impact on charge correlation.

Main Methods:

• Brownian Dynamics simulations of a Restrictive Primitive Model for binary electrolytes.
• Analysis of charge-charge correlation functions in bulk electrolytes.
• Comparison of simulation results with experimental data and weak coupling theories.

Main Results:

• Simulations confirm underscreening, where screening length increases with concentration due to ion-ion correlations and excluded volume.
• At high concentrations, excluded volume interactions dominate charge-charge correlation oscillations, with periods related to ion diameter.
• Formation of like-charge ion clusters observed at high concentrations, transitioning from negative to positive nearest-neighbor correlation.

Conclusions:

• The formation of like-charge ion clusters increases local charge density, contributing to underscreening.
• Topological constraints of charged surfaces can further enhance underscreening effects.
• Simulation results provide a microscopic explanation for experimentally observed underscreening in concentrated electrolytes.