Coexistence and Criticality in Size-Asymmetric Hard-Core Electrolytes

Liquid-vapor coexistence curves and critical parameters for hard-core 1:1 electrolyte models with diameter ratios $\lambda ={\sigma }_{-}/{\sigma }_{+}=1\mathrm{}$ to 5.7 have been studied by fine-discretization Monte Carlo methods. Normalizing via the length scale ${\sigma }_{\pm }=\frac{1}{2}({\sigma }_{+}+{\sigma }_{-})$, relevant for the low densities in question, both $T_c^{*}$ ( ${=k}_B{T}_c{\sigma }_{\pm }{/q}^2$) and ${\rho }_c^{*}$ ( $={\rho }_c{\sigma }_{\pm }^3$) decrease rapidly (from $\simeq 0.05$ to 0.03 and 0.08 to 0.04, respectively) as $\lambda$ increases. These trends, which unequivocally contradict current theories, are closely mirrored by results for tightly tethered dipolar dimers (with $T_c^{*}$ lower by $\sim 0\%–11\%$ and ${\rho }_c^{*}$ greater by $37\%–12\%$).