The liquid–liquid phase transition in ionic solutions: Coexistence curves of tetra-n-butylammonium pricrate in alkyl alcohols

Abstract

We report coexistence curves of the liquid–liquid phase transition in ionic solutions. The phase diagrams of tetra-n-butylammonium pricrate solutions in a series of alkanols (2-propanol, 1-decanol, 1-dodecanol, 1-tridecanol and 1-tetradecanol) are determined either by measuring the refractive index in the two phase region in one sample of near-critical composition as a function of the temperature, or by direct observation of the composition dependent phase separation temperatures. With the exception of the 2-propanol system, the critical points are in accordance with the predictions by the restricted primitive model. The coexistence curves are analyzed in terms of different composition variables, of which the volume fraction seems to be the most appropriate one. For the volume fraction, deviations from asymptotic Ising behavior are observed which are equally well described by a critical exponent slightly different from the Ising value or by Wegner corrections. Although the deviations are quite small, they show a systematic increase with decreasing dielectric constant of the solvent, thus suggesting an approach to the mean-field case. The significance of this finding is, however, weakened by the fact that the corrections to scaling are also affected by the choice of the composition variable. For all investigated systems, the diameter of the coexistence curve shows a pronounced nonanalytic temperature dependence.

Coexistence and Criticality in Size-Asymmetric Hard-Core Electrolytes

“…The most studied system is the restricted primitive model (RPM) that consists of equisized hard spheres, half carrying a charge 1q and half 2q. Recent simulations [4][5][6][7] agree with respect to the critical temperature and density for the vapor-liquid transition, finding the remarkably low values T ‫ء‬ c Ӎ 0.05 and r ‫ء‬ c Ӎ 0.07 [6]; see (1). By contrast to the RPM, the effects of charge and size asymmetry have not been extensively analyzed either theoretically or via simulation.…”

“…In summary, our fine-discretization simulations of hardcore 1:1 electrolyte models have provided unequivocal evidence that increasing the size asymmetry, measured by the diameter ratio l s 2 ͞s 1 , leads to sharp, monotonic drops in appropriately scaled critical temperatures and densities [see (1)]. Tightly tethered dipolar dimers (or dumbbells [15]) display broadly similar behavior but with relatively larger critical densities, and critical temperatures that decrease faster with increasing l. These trends are in severe disagreement with current theories [2,3] and present what appear to be deep challenges to our theoretical understanding even at a qualitative level.…”

“…These trends, which unequivocally contradict current theories, are closely mirrored by results for tightly tethered dipolar dimers (with T ‫ء‬ c lower by ϳ0% 11% and r ‫ء‬ c greater by 37% 12%). The formation of coexisting fluid phases of different electrolyte concentrations in ionic solutions has been the topic of numerous recent experimental [1], theoretical [2,3], and simulation studies [4][5][6][7]. The simplest models for electrolytes -the "primitive models"-treat the solvent as a uniform dielectric continuum.…”

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

“…Subsequent precise experiments by Schröer, Wiegand and others [25,26,27,28,29,30,31,32] showed that by careful identification and elimination of various additional effects (chemical instability, background scattering) Ising criticality is obtaind in all the systems for which MF criticality or tricriticalty, or a delayed crossover were reported previously. The authors of Ref.…”

Crossover behavior in fluids with Coulomb interactions

“…The analysis of experimental data for various ionic solutions confirmed that such systems generally exhibit crossover or, at least a tendency to crossover from the Ising behavior asymptotically close to the critical point, to the mean-field behavior upon increasing distance from the critical point [34]. Moreover, the systematic experimental investigations of the ionic systems such as tetran-butylammonium picrate, Bu 4 NPic, (for tetra-n-butylammonium picrate we will follow the notations from [5,6]) in long chain n-alkanols with dielectric constant ranging from 3.6 for 1-tetradecanol to 16.8 for 2-propanol suggest an increasing tendency for crossover to the mean-field behavior when the Coulomb contribution becomes essential [5,6,35]. They also indicate that the "Coulomb limit" reduced temperature of the RPM T c ≃ 0.05 is valid for the almost non-polar long chain alkanols [6,35].…”