Phase equilibria of a near-critical ionic system. Critical exponent of the order parameter

Abstract

The density ρ of coexisting phases of the ternary system water+1,4-dioxane+potassium chloride was investigated along the liquid–liquid–solid coexistence curve near the critical end-point using a vibrating tube densimeter. By visual determination, this lower critical end-point was located at 311.026±0.010 K with a mole ratio dioxane (D) to water (W) rc=nD/nW=0.418±0.004. Density measurements were carried out in the range 0.01 K<(T−Tc)<31.5 K (i.e., 3×10−5<t=(T−Tc)/Tc<0.1). The obtained coexistence curve displays an apparent sharp crossover at reduced temperatures t≃10−2 from a nonclassical (Ising) to a classical (mean field) exponent. In ancillary experiments, the critical salt concentration was determined. Reduced critical values were calculated and compared with those predicted by the simplest ionic model (RPM, Restricted Primitive Model). The relation between critical behavior and reduced critical parameters in the present and other non-Coulombic systems is discussed.

Universality class of the critical point in the restricted primitive model of ionic systems

“…Earlier simulation results for the RPM in continuum space were not sufficiently accurate to allow for definite conclusions concerning the universality class, however. Moreover, some early experimental results confirmed the Ising criticality [8,9], whereas other results were in conformity with the mean-field criticality [10,11,12] or indicated a crossover to the Ising-type critical exponents unusually close to the critical point [13,14]. In ternary aquaous solutions crossover to multicritical behavior was reported [15], and a tricritical, rather than a critical point was indeed observed in simulations [16,17,18] and predicted theoretically [5,16,19,20,21,22,23] in the RPM with the locations of the ions restricted to the sites of the simple-cubic lattice.…”

Lattice models of ionic systems

“…On the other hand, a number of organic salts in appropriate solvents, typically of low dielectric constant, were found to exhibit classical or close-to-classical behavior 5 , and have been called Coulombic, stressing the importance of the dominant electrostatic interactions. Moreover, in sodium-ammonia solutions 6 (and some other systems: see 7,8 and references therein), crossover from classical to Ising behavior had been observed, but at a reduced temperature t ≡ (T − T c )/T c ≃ 0.6 × 10 −2 , unusually close to the critical point. This led to the idea 2 that the true asymptotic critical behavior of ionic fluids is always of Ising character but that crossover from nonasymptotic, close-toclassical behavior occurs at scales that may sometimes be experimentally inaccessible 2,7 .…”

Phase diagrams in the lattice restricted primitive model: From order-disorder to gas-liquid phase transition

“…For a symmetric 1:1 electrolyte system, for example, recent experiments [1,2,3,4,5] and simulations [6,7,8,9, 10] strongly support three-dimensional Ising-like criticality as the asymptotic behavior. One of the most basic and successful models of ionic fluids is the restricted primitive model (RPM), in which the ions are viewed as equisized hard spheres carrying positive and negative charges of the same magnitude.…”