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 to 5.7 have been studied by fine-discretization Monte Carlo methods. Normalizing via the length scale sigma (+/-) = 1/2(sigma (+) + sigma (-)), relevant for the low densities in question, both T-c* (= k(B)T(c)sigma (+/-)/q(2)) and rho (c)* (= rho (c)sigma (3)(+/-)) decrease rapidly (from similar or equal to0.05 to 0.03 and 0.08 to 0.03, respectively) as h increases. These trends, which unequivocally contradict current theories, are closely mirrored by results for tightly tethered dipolar dimers (with T-c* lower by similar to0%-11% and rho (c)* greater by 37%-12%).