Dipolar origin of the gas-liquid coexistence of the hard-core 1:1 electrolyte model

TL;DR

This study investigates how ion pairing influences the gas-liquid phase transition in electrolyte models, revealing a smooth, non-monotonic dependence on ion separation and describing the transition as between two dimerized phases.

Contribution

It introduces a systematic analysis of dipolar dimer models with variable ion separation, connecting tightly bound and free ion models to explain phase behavior.

Findings

Coexistence curve depends smoothly on ion separation parameter R_c.

Critical parameters converge to free ion values at small R_c.

Gas-liquid transition can be viewed as a transition between two dimerized phases.

Abstract

We present a systematic study of the effect of the ion pairing on the gas-liquid phase transition of hard-core 1:1 electrolyte models. We study a class of dipolar dimer models that depend on a parameter R_c, the maximum separation between the ions that compose the dimer. This parameter can vary from sigma_{+/-} that corresponds to the tightly tethered dipolar dimer model, to R_c --> infinity, that corresponds to the Stillinger-Lovett description of the free ion system. The coexistence curve and critical point parameters are obtained as a function of R_c by grand canonical Monte Carlo techniques. Our results show that this dependence is smooth but non-monotonic and converges asymptotically towards the free ion case for relatively small values of R_c. This fact allows us to describe the gas-liquid transition in the free ion model as a transition between two dimerized fluid phases. The role of the unpaired ions can be considered as a perturbation of this picture.