# Scaling analysis of the screening length in concentrated electrolytes

## TL;DR

This paper presents a scaling theory explaining the anomalous increase of screening length with ion concentration in concentrated electrolytes, supported by experiments and linking to activity coefficients.

## Contribution

The authors develop a simple scaling theory that accounts for the increased decay length in concentrated electrolytes and validate it through experiments and theoretical analysis.

## Key findings

- Decay length increases linearly with Bjerrum length.
- The theory collapses decay lengths of various salts onto a single curve.
- Measured decay length correlates with activity coefficients.

## Abstract

The interaction between charged objects in an electrolyte solution is a fundamental question in soft matter physics. It is well-known that the electrostatic contribution to the interaction energy decays exponentially with object separation. Recent measurements reveal that, contrary to the conventional wisdom given by classic Poisson-Boltzmann theory, the decay length increases with ion concentration for concentrated electrolytes and can be an order of magnitude larger than the ion diameter in ionic liquids. We derive a simple scaling theory that explains this anomalous dependence of the decay length on ion concentration. Our theory successfully collapses the decay lengths of a wide class of salts onto a single curve. A novel prediction of our theory is that the decay length increases linearly with the Bjerrum length, which we experimentally verify by surface force measurements. Moreover, we quantitatively relate the measured decay length to classic measurements of the activity coefficient in concentrated electrolytes, thus showing that the measured decay length is indeed a bulk property of the concentrated electrolyte as well as contributing a mechanistic insight into empirical activity coefficients.