Cold Water As a Novel Supercritical-Fluid Solvent: A Two-State Model

Water above its vapor-liquid critical point is commonly used as a supercritical-fluid solvent, in particular in the destructive oxidation of organic waste.  However, many scientists believe that deep in the supercooled region, not directly accessible to bulk-water experiments, there exists a critical point of liquid-liquid separation.  We have applied the concepts of critical phenomena and thermodynamics of supercritical fluids to explore the possibility of viewing supercooled water as a novel supercritical solvent.  Using a phenomenological mean-field “two-state” model, we clarify the nature of the phase separation in a polyamorphic single-component liquid.  Considering supercooled water as a mixture of two structures, controlled by thermodynamic equilibrium, we describe water as an athermal “solution” with phase separation driven by the non-ideal entropy of mixing.  We then elaborate on this model with respect to the addition of solutes.  The addition of a solute generates critical lines emanating from the pure-water liquid-liquid critical point.  Because of the anomalously significant change of pressure with respect to temperature along the liquid-liquid transition line, the value of the Krichevskii parameter, which determines the selectivity of supercritical-fluid solvent to fine variations of temperature and pressure, may be very large even if the critical temperature weakly depends on the solute concentration.  Physically, it means that even a very small addition of the solute may significantly affect the properties of cold water and aqueous solutions.