Different Water Networks Confined in Unidirectional Hydrophilic Nanopores and Transitions with Temperature

The structure and vibrational properties of water molecules confined in unidirectional hydrophilic nanopores of AlPO4-54·xH2O were investigated from room temperature down to 10 K by single-crystal synchrotron X-ray diffraction, neutron pair- distribution function analysis, incoherent inelastic neutron scatter- ing, far- and mid-infrared spectroscopy, ab initio molecular dynamics, and grand canonical Monte Carlo simulations. The ensemble of results indicates that water confined in AlPO4-54·xH2O nanopores does not crystallize down to 10 K and points at the existence of two different types of water networks, whose local arrangement and dynamical behavior become more and more distinguished when lowering the temperature below 150 K. Upon cooling, water close to the zeolite pore wall shows a highly ordered local arrangement induced by the pore wall, with a more defined site occupancy and lower density with respect to bulk water. Conversely, water in the pore core shows a denser, more disordered, and orientationally distorted arrangement and a glassy like behavior down to the lowest investigated temperature.