Image featured in Physical Review Focus.
This picture shows a typical ion configuration in an electrolyte solution near its critical point. In this so-called restricted primitive model, all ions are monovalent and have the same diameter (in the figure, positive ions are red and negative ions are blue), and the solvent is simply modeled as a medium with a given dielectric constant.
One of the big outstanding fundamental questions in ionic solutions is the issue of the nature of the critical point: It is known that the universal critical properties depend on the range of the interactions between the constituents of the system. Phase separation (into phases of low and high ionic concentration) is driven by long-ranged Coulombic forces, so one might expect critical properties that differ from those encountered near a conventional critical point of systems with a scalar order parameter (in this case the concentration difference between the two phases). On the other hand, the concept of electrostatic screening suggests that the effective interactions might actually be short-ranged.
In a recent numerical study (Physical Review Letters 88, 185701 (2002)) we have now, for the first time, been able to demonstrate in an unbiased fashion that the restricted primitive model indeed exhibits short-range critical behavior, i.e., with the same universal properties as the critical point of uni-axial ferromagnets or conventional gas-liquid phase separation.
Along with this fundamental result, several more generally applicable results have been established, such as the issue which estimators can be used to most efficiently determine the critical point in systems with a highly asymmetric coexistence curve (a notorious feature of electrolyte solutions, but also of polymeric systems).
The picture below shows a typical near-critical ion configuration. As can be observed, there is already a high degree of association at this temperature. The large energy required to break a dipolar pair is one of the hurdles that have to be overcome in order to obtain high-precision numerical results.
Image featured in Physical Review Focus.
© Copyright 2001 Erik Luijten --- University of Illinois at Urbana-Champaign
This page was last updated on April 22, 2002.