Ion association in low-polarity solvents: comparisons between theory, simulation, and experiment

TL;DR

This study combines simulations, theory, and experiments to analyze ion pairing in low-polarity solvents, revealing reliable estimators and excellent agreement across methods for understanding ion association at very low concentrations.

Contribution

It introduces specialized simulation techniques and compares ion-pairing theory with experimental data, improving accuracy in low-polarity solvent ion association analysis.

Findings

Conventional distance-based clustering criteria can be misleading.

Reliable estimators for ion association are proposed.

Theoretical predictions match experimental results with high accuracy.

Abstract

The association of ions in electrolyte solutions at very low concentration and low temperature is studied using computer simulations and quasi-chemical ion-pairing theory. The specific case of the restricted primitive model (charged hard spheres) is considered. Specialised simulation techniques are employed that lead to efficient sampling of the arrangements and distributions of clusters and free ions, even at conditions corresponding to nanomolar solutions of simple salts in solvents with dielectric constants in the range 5-10, as used in recent experimental work on charged-colloid sus- pensions. A direct comparison is effected between theory and simulation using a variety of clustering criteria and theoretical approximations. It is shown that conventional distance-based cluster criteria can give erroneous results. A reliable set of theoretical and simulation estimators for the degree of association is proposed. The ion-pairing theory is then compared to experimental results for salt solutions in low-polarity solvents. The agreement is excellent, and on this basis some calculations are made for the screening lengths which will figure in the treatment of colloid-colloid interactions in such solutions. The accord with available experimental results is complete.