Experimental evidence for a liquid-liquid crossover in deeply cooled confined water

In this work we investigate, by means of Elastic Neutron Scattering (ENS), the pressure dependence of Mean Square Displacements (MSD) of hydrogen atoms of deeply cooled water confined in the pores of a 3-dimensional disordered SiO2 xerogel; experiments have been performed at 250 and 210K from atmospheric pressure to 1200 bars. The "pressure anomaly" of supercooled water (i.e. an MSD 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 (210K) than at higher (250K) temperature. ENS results are complemented by differential scanning calorimetry data that put in evidence, besides the glass transition at about 170K, a first order-like endothermic transition occurring at about 230K 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 230K (at ambient pressure) from a liquid phase predominant at 210K to another liquid phase predominant at 250K; therefore, they are fully consistent with the Liquid-Liquid Transition hypothesis.