# How antagonistic salts cause nematic ordering and behave like diblock copolymers

## TL;DR

This paper uses simulations and theory to show how antagonistic salts induce nematic ordering and complex structures in binary fluids, linking electrostatic interactions to the formation of lamellar phases and their dynamics.

## Contribution

It introduces a theoretical framework connecting antagonistic salt effects to the Ohta-Kawasaki model, predicting structure sizes and morphologies in binary fluid mixtures.

## Key findings

- Spinodal decomposition is arrested with small electrostatic interactions.
- Lamellar structures exhibit nematic order and grow over time following a salt-dependent power law.
- High salt concentrations lead to structure dissolution due to vanishing surface tension.

## Abstract

We present simulation results and an explanatory theory on how antagonistic salts affect the spinodal decomposition of binary fluid mixtures. We find that spinodal decomposition is arrested and complex structures form only when electrostatic ion-ion interactions are small. In this case fluid and ion concentrations couple and the charge field can be approximated as a polynomial function of the relative fluid concentrations alone. When the solvation energy associated with transfering an ion from one fluid phase to the other is of the order of a few k_BT, the coupled fluid and charge fields evolve according to the Ohta-Kawasaki free energy functional. This allows us to accurately predict structure sizes and reduce the parameter space to two dimensionless numbers. The lamellar structures induced by the presence of antagonistic salt in our simulations exhibit a high degree of nematic ordering and the growth of ordered domains over time seems to follow a power law. This power law carries a time exponent proportional to the salt concentration. We reproduce and interpret neutron scattering data from previous experiments of similar systems. The dissolution of structures at high salt concentrations observed in these experiments agrees with our simulations and we explain it as the result of a vanishing surface tension due to electrostatic contributions. We conclude by presenting preliminary 3D results showing the same morphologies as predicted by the Ohta-Kawasaki model as a function of volume fraction and suggesting that our findings from 2D systems remain valid in 3D.