Spontaneous liquid-liquid phase separation of water

We report a molecular dynamics simulation demonstrating a fast spontaneous liquid-liquid phase separation of water and a subsequent slow crystallization to ice. It is found that supercooled water separates rapidly into low- and high-density domains so as to reduce the surface energy in the rectangular simulation cell at certain thermodynamic states. The liquid-liquid phase separation, which is about two orders of magnitude faster than the crystallization, suggests a possibility to observe this phenomenon experimentally.

Figure 1

Isotherms of the pressure $P$ as a function of density. Note that the pressure is higher for the higher temperature at the highest density, 1.16 g cm${}^{-3}$, while it is not at other densities because of the anomaly of liquid water, i.e., the existence of the density maximum. (a)–(c) show the local excess densities $\Delta \rho (z,t)$ for $\langle \rho \rangle =0.94$ g cm${}^{-3}$ at $T$ = 250, 240, and 235 K, respectively. (d) and (e) are $\Delta \rho (z,t)$ for $\langle \rho \rangle =0.88$ and 1.04 g cm${}^{-3}$ at $T$ = 240 K.

Figure 2

Simulation results for $T$ = 235 K and $\langle \rho \rangle =0.98$. (a) Local excess density $\Delta \rho (z,t)$, which is defined by Eq. (1). The region where the density is comparable to the average value is white while the high- and low-density regions are green and blue, respectively. The temperature jumps from 300 to 235 K at $t$ = 0 ns. (b) Averaged density profiles along the $z$ axis for $-0.5\mathrm{ns}<t<0$ ns, 0 ns < $t$ < 0.5 ns, 10 < $t$ < 10.5 ns, and 20 ns < $t$ < 20.5 ns.

Figure 3

(a) Long time behavior of $\Delta \rho (z,t)$ for $T$ = 235 K and $\langle \rho \rangle =0.98$ g/cm${}^3$. The strange behavior at $t$ = 800 ns arises from the fact that a single crystal of ice ${\mathrm{I}}_{\mathrm{c}}$ spans across the shortest edge of the box and it grows rapidly. (b) Number of water molecules belonging to the ${\mathrm{I}}_{\mathrm{c}}$ fragments and ${\mathrm{I}}_{\mathrm{h}}$ fragments. (c) The $Q_6$ parameter of the system.

Figure 4

Time evolutions of $\Delta \rho (z,t)$ after the removal of walls at $T$ = 240 K. (a)–(c) show $\Delta \rho (z,t)$ for $\langle \rho \rangle =0.88$, 0.94, and 1.04 g/cm${}^3$, respectively. The two walls which separate the system into the low- and high-density parts are removed at $t$ = 0 ns.