# Underscreening in concentrated electrolytes

## TL;DR

This paper challenges conventional beliefs by demonstrating that in concentrated electrolytes, the screening length increases with ion concentration and Bjerrum length, supported by surface force measurements and a proposed scaling model.

## Contribution

It provides experimental evidence and a theoretical scaling model showing the non-traditional increase of screening length with ion concentration in concentrated electrolytes.

## Key findings

- Screening length increases with ion concentration in concentrated electrolytes.
- Surface force measurements support the scaling of screening length with Bjerrum length.
- A simple scaling conjecture explains the concentration dependence based on solvent molecules as charge carriers.

## Abstract

Screening of a surface charge by electrolyte and the resulting interaction energy between charged objects is of fundamental importance in scenarios from bio-molecular interactions to energy storage. The conventional wisdom is that the interaction energy decays exponentially with object separation and the decay length is a decreasing function of ion concentration; the interaction is thus negligible in a concentrated electrolyte. Contrary to this conventional wisdom, we have shown by surface force measurements that the decay length is an increasing function of ion concentration and Bjerrum length for concentrated electrolytes. In this paper we report surface force measurements to test directly the scaling of the screening length with Bjerrum length. Furthermore, we identify a relationship between the concentration dependence of this screening length and empirical measurements of activity coefficient and differential capacitance. The dependence of the screening length on the ion concentration and the Bjerrum length can be explained by a simple scaling conjecture based on the physical intuition that solvent molecules, rather than ions, are charge carriers in a concentrated electrolyte.