Cooperative molecular motions in water: The liquid-liquid critical point hypothesis

We discuss the hypothesis that, in addition to the known critical point in water (below which two fluid phases — a lower-density gas and a higher-density liquid — coexist), there exists a “second” critical point at low temperatures (below which two liquid phases — a higher-density liquid and a lower-density liquid — can coexits). We also discuss briefly some of the evidence relating to this hypothesis. This evidence is rather tentative at the present time, and is largely based on a growing number of computer simulations using the ST2 and TIP4P intermolecular potentials. We also discuss selected experimental results that are consistent with this hypothesis.