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Schlagwörter:
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Zusammenfassung:
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 application to other liquids as well as water, such as silicon
and silica. Specifically, we investigate, 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) the Jagla model, a
spherically symmetric two-scale potential 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 CmaxP (“Widom line”) emanating from the critical point.
Our findings are consistent with a possible relation between the previously
hypothesised 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 CmaxP 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.
