# Activity coefficients of individual ions in LaCl$_{3}$ from the II+IW theory

## TL;DR

This paper models the activity coefficients of ions in LaCl₃ by combining ion-ion and ion-water interactions without adjustable parameters, using Monte Carlo simulations and experimental data, achieving accurate predictions.

## Contribution

It introduces a parameter-free model that combines Monte Carlo simulations and Born's solvation theory to predict ion activity coefficients in LaCl₃.

## Key findings

- The model accurately predicts ion activity coefficients.
- Large opposing ion-ion and ion-water contributions balance each other.
- No adjustable parameters are needed, only experimental inputs.

## Abstract

We investigate the individual activity coefficients of ions in LaCl$_{3}$ using our theory that is based on the competition of ion-ion (II) and ion-water (IW) interactions (Vincze et al., J. Chem. Phys. 133, 154507, 2010). The II term is computed from Grand Canonical Monte Carlo simulations on the basis of the implicit solvent model of electrolytes using hard sphere ions with Pauling radii. The IW term is computed on the basis of Born's treatment of solvation using experimental hydration free energies. This agreement is remarkable considering the facts that (i) the result is the balance of two terms that are large in absolute value (up to $20 kT$) but opposite in sign, and (ii) that our model does not contain any adjustable parameter. All the parameters used in the model are taken from experiments: concentration dependent dielectric constant, hydration free energies, Pauling radii.