COOPERATIVE MOLECULAR MOTIONS IN WATER - THE LIQUID-LIQUID CRITICAL-POINT HYPOTHESIS

We discuss the hypothesis that, in addition to the known critical poin t in water (below which two fluid phases - a lower-density gas and a h igher-density liquid - coexist), there exists a ''second'' critical po int at low temperatures (below which two liquid phases - a higher-dens ity liquid and a lower-density liquid - can coexist). We also discuss briefly some of the evidence relating to this hypothesis. This evidenc e is rather tentative at the present time, and is largely based on a g rowing number of computer simulations using the ST2 and TIP4P intermol ecular potentials. We also discuss selected experimental results that are consistent with this hypothesis.