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

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 [formula presented] the maximum separation between the ions that compose the dimer. This parameter can vary from [formula presented] that corresponds to the tightly tethered dipolar dimer model to [formula presented] 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 [formula presented] by grand-canonical Monte Carlo techniques. Our results show that this dependence is smooth but nonmonotonic and converges asymptotically towards the free ion case for relatively small values of [formula presented] 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.