Pressure Effects in Supercooled Water: Comparison between a 2D Model of Water and Experiments for Surface Water on a Protein

Abstract

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 $T$, can not ascertain whether the two arrangements separate in 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. [Phys. Rev. Lett. 100, 105701 (2008)] investigated, by Monte Carlo simulations and mean field calculations, the relation of the dynamic crossover with the coexistence of two liquid phases in a cell model for water and predict that: (i) the dynamic crossover is isochronic, i.e. the value of the crossover time $\ddot{I}{„}_{\mathrm{L}}$ is approximately independent of pressure $P$; (ii) the Arrhenius activation energy $E_{\mathrm{A}}(P)$ of the low-$T$ regime decreases upon increasing $P$; (iii) the temperature $T^{\ast }(P)$ at which $\ddot{I}„$ reaches a fixed macroscopic time $\ddot{I}{„}^{\ast }\ge \ddot{I}{„}_{\mathrm{L}}$ decreases upon increasing $P$; in particular, this is true also for the crossover temperature $T_{\mathrm{L}}(P)$ at which $\ddot{I}„=\ddot{I}{„}_{\mathrm{L}}$. Here, we compare these predictions with recent quasi elastic neutron scattering (QENS) experiments performed by X.-Q. Chu {\it et al.} on hydrated proteins at different values of $P$. We find that the experiments are consistent with these three predictions.

18 pages, 5 figures, to appear on J. Phys.: Cond. Mat