Liquid polyamorphism and the anomalous behavior of water

We present evidence from experiments and computer simulations supporting the hypothesis that water displays polyamorphism, i.e., water separates into two distinct liquid phases. This concept of a new liquid–liquid phase transition is finding potential application to other liquids as well as water, such as silicon and silica. Here we review the relation between changes in dynamic and thermodynamic anomalies arising from the presence of the liquid–liquid critical point in (i) Two models of water, TIP5P and ST2, which display a first order liquid–liquid phase transition at low temperatures; (ii) Two spherically symmetric two-scale potentials known to possess a liquid–liquid critical point, in which the competition between two liquid structures is generated by repulsive and attractive ramp interactions; and (iii) A Hamiltonian model of water where the idea of two length/energy scales is built in. This model also displays a first order liquid–liquid phase transition at low temperatures besides the first order liquid-gas phase transition at high temperatures. We find a correlation between the dynamic fragility crossover and the locus of specific heat maxima Cmax P (“Widom line”) emanating from the critical point. Our findings are consistent with a possible relation between the previously hypothesized liquid-liquid phase transition and the transition in the dynamics recently observed in neutron scattering experiments on confined water. More generally, we argue that this connection between Cmax P and the dynamic crossover is not limited to the case of water, a hydrogen bonded network liquid, but is a more general feature of crossing the Widom line, an extension of the first-order coexistence line in the supercritical region