Structured ion layers explain decay length increase in highly concentrated electrolyte solutions
Structured ion layers explain decay length increase in highly concentrated electrolyte solutions lead image
Electrolytes in solution interact with charged surfaces in a wide range of physical systems, from cell membranes to lithium-ion batteries. The electrolytes shield the electric field of the surface, creating an exponentially decaying field with a characteristic decay length.
In low electrolyte concentrations, the electric field decay length decreases with increasing concentration, in agreement with Debye-Huckel theory. At high concentrations, however, experiments show that this trend reverses. For many electrolytes, decay lengths begin to increase with increasing concentration above around one molar.
Anousheh et al. proposed that enhanced steric correlations between ions can explain this strange behavior. Using molecular dynamics simulations of a primitive model electrolyte, the authors showed that in confinement and at high concentrations, steric forces between ions produce layered ionic structures which lead to increasing decay lengths.
The model explored by the group involves ions confined between two charged parallel planes.
“We simulated a variety of systems by changing the ionic size, the confinement, the surface charge density,” said author Vikram Jadhao. “Just the scale of the exploration of the design space needed to be that big to really understand what was going on.”
The group found two distinct regimes of screening behavior. At low electrolyte concentrations, electrostatic forces dominate, leading to Debye screening. At high concentrations, steric forces between ions lead to the formation of alternating layers of anions and cations, which in turn increase the decay length with increasing concentrations.
While this simple mechanism reproduced strange screening behavior at high concentration, the authors hope further research will allow them to isolate the effects of steric correlations between ions from the effects due to solvent-mediated interactions.
Source: “Ionic structure and decay length in highly-concentrated confined electrolytes,” by Nasim Anousheh, Francisco J. Solis, and Vikram Jadhao, AIP Advances (2020). The article can be accessed at https://doi.org/10.1063/5.0028003 .
