title: Experimental evidence for a liquid-liquid crossover in deeply cooled confined water.
abstract: 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 SiO2 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.
authors.affiliation: University of Palermo, Department of Physics and Chemistry, via Archirafi 36, 90123 Palermo, Italy.
authors.forename: Antonio
authors.initials: A
authors.lastname: Cupane
authors.affiliation: University of Palermo, Department of Physics and Chemistry, via Archirafi 36, 90123 Palermo, Italy.
authors.forename: Margarita
authors.initials: M
authors.lastname: Fomina
authors.affiliation: University of Palermo, Department of Physics and Chemistry, via Archirafi 36, 90123 Palermo, Italy.
authors.forename: Irina
authors.initials: I
authors.lastname: Piazza
authors.affiliation: Institut Laue Langevin, 71 avenue des Martyrs, F-38000 Grenoble, France, Université Joseph Fourier, F-38041 Grenoble Cedex 9, France, CNRS, CEA-Institut de Biologie Structurale, 71 avenue des Martyrs, 38000 Grenoble, France.
authors.forename: Judith
authors.initials: J
authors.lastname: Peters
authors.affiliation: CNRS-Institut de Biologie Structurale, 71 avenue des Martyrs, 38000 Grenoble, France.
authors.forename: Giorgio
authors.initials: G
authors.lastname: Schirò
mesh_terms: D002152:Calorimetry, Differential Scanning; D003080:Cold Temperature; D006859:Hydrogen / Q000737:chemistry*; D057927:Hydrophobic and Hydrophilic Interactions; D008956:Models, Chemical; D033363:Neutron Diffraction / Q000379:methods; D044367:Phase Transition; D012822:Silicon Dioxide / Q000737:chemistry; D014867:Water / Q000737:chemistry*; D014872:Water Movements
chemical_list: D014867:Water; D012822:Silicon Dioxide; D006859:Hydrogen