Liquid Water: A State Between Two Critical Points

Liquid water remains a puzzle. Unlike ordinary substances, one can regard low-temperature water (near the triple point and in the supercooled region) and water near the vapor-liquid critical point as “the same substance – two different liquids”. Highly-compressible, low-dielectric-constant, near-critical water is commonly used as a supercritical-fluid solvent. On the low-temperature side of the phase diagram, water is an almost incompressible, high-dielectric constant solvent with some mysterious properties. In this region, some of the puzzles of liquid water can be explained by the existence of the metastable (supercooled) liquid-liquid critical point. Fluctuations of entropy, diverging at the liquid-liquid critical point, may be associated with anomalous sensitivity (“susceptibility”) of water structure to external perturbations and may also be responsible for mysterious behavior of some nonelectrolyte aqueous solutions. By stabilizing the fluctuations of water structure, through self-assembly of small organic molecules in aqueous solutions, one can create novel smart materials and devices.