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 approximately 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 approximately 0%-11% and rho(*)(c) greater by 37%-12%).

Criticality and crossover in accessible regimes

The near-critical behavior of (d = 3)-dimensional Ising-model ferromagnets or simple lattice gases with equivalent first, second, and third nearest-neighbor interactions is studied through Monte Carlo simulations using histogram reweighting techniques and comparisons with series expansions. By carefully analyzing numerical data from relatively small finite systems using scaling and extrapolation methods, it is demonstrated that one can reliably estimate critical exponents, critical temperatures, and universal amplitude ratios, thereby distinguishing convincingly between different "nearby" universality classes and revealing systematic crossover effects. This study is preparatory to extending similar techniques to study criticality in continuum fluid models lacking symmetries, with Coulomb interactions, etc.

The yang-yang anomaly in fluid criticality: experiment and scaling theory

Yang and Yang proved that the divergence of C(V)(T) at a gas-liquid critical point implies that either d(2)p/dT(2) identical withp(")(sigma) or d(2)&mgr;/dT(2) identical with&mgr;(")(sigma) or both diverge when T-->T(c)- on the phase boundary sigma. They queried the lattice-gas prediction that &mgr;(")(sigma) remains finite. Analysis of two-phase heat-capacity data provides, for the first time, evidence for such a Yang-Yang anomaly (&mgr;(")(sigma)-->+/-infinity) in propane and suggests an anomaly of opposite sign in CO (2). A revision of standard scaling theory for fluid criticality is demanded: specifically, p-p(c) must appear in the ordering field. The coexistence diameter hence gains a |T-T(c)|(2beta) term.