Low-Temperature Dynamics of Water Confined in Unidirectional Hydrophilic Zeolite Nanopores

International audienceThe dynamical properties of water molecules confined in the hydrophilic nanopores of AlPO4-54 zeolite investigated with Quasi-Elastic Neutron scattering as a function of temperature down to 2 K. Water molecular diffusion into the pore is measured down to 258 K. Diffusion follows a jump mechanism with a jump distance increasing with temperature and an activation energy of Ea = (20.8 ± 2.8) kJ/mol, in agreement with previous studies on similar confining media. Water rotational diffusion is instead measured down to temperatures (118 K) well below the water glass transition. The rotational time scale shows a non-Arrhenius behavior down to the freezing of water diffusion, while it has a feeble temperature dependence below. This fast molecular reorientation (fractions of nanoseconds) is believed to take place in the dense, highly disordered amorphous water present in the pore center, therefore indicating its plastic amorphous nature

Strongly Modified Mechanical Properties and Phase Transition in AlPO₄‑17 Due to Insertion of Guest Species at High Pressure

The porous aluminophosphate AlPO4-17 with a hexagonal erionite structure, exhibiting very strong negative thermal expansion, anomalous compressibility, and pressure-induced amorphization, was studied at high pressure by single-crystal and powder X-ray diffraction in the penetrating pressure transmitting media N2, O2, and Ar. Under pressure, these guest species were confirmed to enter the pores of AlPO4-17, thus completely modifying its behavior. Pressure-induced collapse in the xy plane of AlPO4-17 no longer occurred, and this plane exhibited close to zero area compressibility. Pressure-induced amorphization was also suppressed as the elastic instability in the xy plane was removed. Crystal structure refinements at a pressure of 5.5 GPa indicate that up to 28 guest molecules are inserted per unit cell and that this insertion is responsible for the reduced compressibility observed at high pressure. A phase transition to a new hexagonal structure with cell doubling along the a direction was observed above 4.4 GPa in fluid O2

Mechanism of H₂O Insertion and Chemical Bond Formation in AlPO₄‑54·<i>x</i>H₂O at High Pressure

The insertion of H₂O in AlPO₄-54·<i>x</i>H₂O at high pressure was investigated by single-crystal X-ray diffraction and Monte Carlo molecular simulation. H₂O molecules are concentrated, in particular, near the pore walls. Upon insertion, the additional water is highly disordered. Insertion of H₂O (superhydration) is found to impede pore collapse in the material, thereby strongly modifying its mechanical behavior. However, instead of stabilizing the structure with respect to amorphization, the results provide evidence for the early stages of chemical bond formation between H₂O molecules and tetrahedrally coordinated aluminum, which is at the origin of the amorphization/reaction process

New experimental set-ups for studying nanoconfined water on the AILES beamline at SOLEIL

Three ensembles designed to investigate condensed matter in complex environments have been developed recently on the AILES beamline at SOLEIL. They have been exploited for studies aiming at understanding the properties of water molecules and their network in various confining systems, namely:- a hydration and temperature-controlled cell for the study of water confined in nanoporous Vycor,- a high pressure set-up allowing the study of the evolution of water molecules network trapped in Faujasite through the pressure-induced amorphisation of the matrix material,- a temperature resolved electrochemical cell used to record FIR difference spectra of metalloproteins interacting with water molecules.By combining the high infrared flux and collimation of the AILES beamline with these optimized sample environments, it is possible to measure the infrared and THz spectra for minute quantities of samples in precise physical conditions

High-Pressure Phase Transition, Pore Collapse, and Amorphization in the Siliceous 1D Zeolite, TON

The siliceous zeolite TON with a 1-D pore system was studied at high pressure by X-ray diffraction, infrared spectroscopy, and DFT calculations. The behavior of this material was investigated using nonpenetrating pressure-transmitting media. Under these conditions, a phase transition from the <i>Cmc</i>2₁ to a <i>Pbn</i>2₁ structure occurs at close to 0.6 GPa with doubling of the primitive unit cell based on Rietveld refinements. The pores begin to collapse with a strong increase in their ellipticity. Upon decreasing the pressure below this value the initial structure was not recovered. DFT calculations indicate that the initial empty pore <i>Cmc</i>2₁ phase is dynamically unstable. Irreversible, progressive pressure-induced amorphization occurs upon further increases in pressure up to 21 GPa. These changes are confirmed in the mid- and far-infrared spectra by peak splitting at the <i>Cmc</i>2₁ to <i>Pbn</i>2₁ phase transition and strong peak broadening at high pressure due to amorphization