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

Area of Science:

• Physical Chemistry
• Solution Chemistry
• Materials Science

Background:

• High-concentration electrolyte solutions are crucial in various applications but remain poorly understood.
• Quantifying speciation (complex formation) in these solutions is theoretically and experimentally challenging, with conflicting literature reports.
• Speciation significantly influences solution properties, including transport phenomena.

Purpose of the Study:

• To investigate the relationship between speciation and transport properties in concentrated electrolyte solutions.
• To establish viscosity as a reliable method for determining the extent of complexation.
• To model and compare the behavior of FeCl2 and MgCl2 solutions.

Main Methods:

• Utilized atomistic simulations with the extended Madrid-2019 force field.
• Performed experimental viscosity measurements on concentrated FeCl2 and MgCl2 solutions.
• Integrated simulation results with experimental data and recent X-ray absorption and neutron scattering findings.

Main Results:

• Demonstrated that viscosity measurements can accurately quantify complexation in concentrated aqueous solutions.
• Extended the Madrid-2019 force field to effectively model FeCl2.
• Observed and explained differences in viscosity between FeCl2 and MgCl2 solutions, attributing them to greater complexation in FeCl2.

Conclusions:

• Speciation is a prerequisite for accurate modeling of concentrated solutions.
• Viscosity serves as a powerful tool to determine complexation extents.
• The enhanced Madrid-2019 force field provides accurate simulations for FeCl2 solutions, aligning with experimental observations.