Pressure Dependence of Fragile-to-Strong Transition and a Possible Second Critical Point in Supercooled Confined Water

By confining water in nanopores of silica glass, we can bypass the crystallization and study the pressure effect on the dynamical behavior in deeply supercooled state using neutron scattering. We observe a clear evidence of a cusplike fragile-to-strong (FS) dynamic transition. Here we show that the transition temperature decreases steadily with an increasing pressure, until it intersects the homogenous nucleation temperature line of bulk water at a pressure of 1600 bar. Above this pressure, it is no longer possible to discern the characteristic feature of the FS transition. Identification of this end point with the possible second critical point is discussed.

Figure 1

(a) and (c) (left panels) show QENS spectra measured at $Q=0.58{\AA }^{-1}$, at two pressures, 800 bar and 1600 bar, and at a series of temperatures. (b) and (d) (right panels) show the RCM analysis of one of the spectra from each pressure. The resolution function in each case is shown by a dashed line.

Figure 2

Temperature dependence of $⟨{\tau }_T⟩$ plotted in $\log (⟨{\tau }_T⟩)$ vs $T_0/T$ or $1/T$. Data from ambient pressure, 400, 800, 1200, 1600, and 2000 bars are shown in panels (a), (b), (c), (d), (e), and (f), respectively.

Figure 3

The pressure dependence of the FS dynamic transition temperature, $T_L$, plotted in the $P\mathrm{\text{−}}T$ plane (solid circles). Also shown are the homogeneous nucleation temperature line, denoted as $T_H$ [25], crystallization temperatures of amorphous solid water [26], denoted as $T_X$, and the temperature of maximum density line, denoted as TMD [27].