# Viscosity as a Smoking Gun for Complex Formation in Solution: Fe2+ and Mg2+ Chlorides as Examples

## TL;DR

This paper shows that viscosity can reveal complex formation in concentrated ionic solutions, using FeCl2 and MgCl2 as examples.

## Contribution

A novel method to determine complexation in concentrated solutions using viscosity measurements is proposed.

## Key findings

- Viscosity is a reliable indicator of complexation in concentrated electrolyte solutions.
- FeCl2 solutions exhibit higher viscosity due to increased complexation compared to MgCl2.
- X-ray absorption and neutron scattering experiments support the conclusion of complexation differences.

## Abstract

Electrolyte solutions at high concentration are indispensable
and
yet poorly understood. In particular, the extent of speciationthe
formation of complexes composed of multiple speciesin concentrated
ionic solutions is very challenging to obtain theoretically and experimentally,
but can have a strong effect on solution properties. The literature
is rife with contradictory estimates of speciation from experiments.
We find that speciation affects transport properties and is therefore
a prerequisite to accurately model concentrated solutions. We turn
this to our advantage by showing that the viscosity can be used to
determine the extent of complexation in concentrated aqueous solutions.
Results of simulations as well as experimental measurements are presented.
The atomistic Madrid-2019 force field is extended to model FeCl2. Solutions of FeCl2 and MgCl2 are compared,
and the observed difference in viscosity is explained by more complexation
in the former, a conclusion supported by recently reported X-ray absorption
and neutron scattering experiments.