Experimental Evidence for a Liquid-Liquid Crossover in Deeply Cooled Confined Water

In this work we investigate, by means of elastic neutron scattering, the pressure dependence of mean square displacements (MSD) of hydrogen atoms of deeply cooled water confined in the pores of a three-dimensional disordered ${\mathrm{SiO}}_2$ xerogel; experiments have been performed at 250 and 210 K from atmospheric pressure to 1200 bar. The “pressure anomaly” of supercooled water (i.e., a mean square displacement increase with increasing pressure) is observed in our sample at both temperatures; however, contrary to previous simulation results and to the experimental trend observed in bulk water, the pressure effect is smaller at lower (210 K) than at higher (250 K) temperature. Elastic neutron scattering results are complemented by differential scanning calorimetry data that put in evidence, besides the glass transition at about 170 K, a first-order-like endothermic transition occurring at about 230 K that, in view of the neutron scattering results, can be attributed to a liquid-liquid crossover. Our results give experimental evidence for the presence, in deeply cooled confined water, of a crossover occurring at about 230 K (at ambient pressure) from a liquid phase predominant at 210 K to another liquid phase predominant at 250 K; therefore, they are fully consistent with the liquid-liquid transition hypothesis.

Figure 1

Inset: Temperature dependence of molar specific heat ($C_P$) of water confined in a silica hydrogel at $h=0.42$. The dashed line is a linear fit to the low-temperature behavior. Main: Temperature dependence of $C_P$ (black line) and its derivative (red line) after subtraction of the linear extrapolation. The vertical lines indicate the midpoint temperatures of the glass transition and of the specific heat maximum (liquid-liquid transition). Note the close similarity of our $C_P$ data with the calculations of Ref. [40].

Figure 2

(a) MSD and (b) $R(P)=\mathrm{MSD}(P)/\mathrm{MSD}(P=20\mathrm{bar})$ as a function of pressure. Black open circles, $T=210\mathrm{K}$; red circles, $T=250\mathrm{K}$. Dashed lines are linear fits.