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Abstract:
Water’s behavior differs from that of normal fluids, having more than sixty anomalies.
Simulations and theories propose that many of these anomalies result from the coexistence of
two liquid phases with different densities. Experiments in bulk water confirm the existence of
two local arrangements of water molecules with different densities, but, because of inevitable
freezing at low temperature T , cannot ascertain whether the two arrangements separate into two
phases. To avoid the freezing, new experiments measure the dynamics of water at low T on the
surface of proteins, finding a crossover from a non-Arrhenius regime at high T to a regime that
is approximately Arrhenius at low T . Motivated by these experiments, Kumar et al (2008 Phys.
Rev. Lett. 100, 105701) investigated, by Monte Carlo simulations and mean field calculations
on a cell model for water in two dimensions (2D), the relation of the dynamic crossover with
the coexistence of two liquid phases. They show that the crossover in the orientational
correlation time τ is a consequence of the rearrangement of the hydrogen bonds at low T , and
predict that: (i) the dynamic crossover is isochronic, i.e. the value of the crossover time τL is
approximately independent of pressure P; (ii) the Arrhenius activation energy EA(P) of the
low-T regime decreases upon increasing P; (iii) the temperature T ∗(P) at which τ reaches a
fixed macroscopic time τ ∗ τL decreases upon increasing P; in particular, this is true also for
the crossover temperature TL(P) at which τ = τL.
Here, we compare these predictions with recent quasi-elastic neutron scattering (QENS)
experiments performed by Chu et al on hydrated proteins at different values of P. We find that
the experiments are consistent with these three predictions.
