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

Abstract

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 scattering, 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. The dependence of temperature of the infrared spectroscopy points at two transitions at 250 and 150 K. Upon cooling, water close to the zeolite pore wall shows a highly ordered local arrangement induced by the pore wall, with 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.

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

“…Due to the larger size of the MCM-41 pores, water in the pore center crystallizes when cooling below the ice homogeneous nucleation temperature ( T H ), , while water in AlPO 4 -54· x H 2 O does not crystallize even at the lower investigated temperature and show a much slower diffusive dynamics. Lowering the temperature zeolite inner water forms a highly disordered and dense amorphous state similar to high density amorphous water (HDA) . On the other hand, the zeolite crystal framework contributes to the proximal water ordering, acting as an orientational template due to the hydrogen bonding with the structural water linked to the AlPO 4 units .…”

“…Since AlPO 4 - n unidirectional zeolites have both hydrophobic and hydrophilic groups, it is expected that water flow would be larger for AlPO 4 -54 than the flux observed in pure alumina pores. However, molecular dynamics simulations did not observe any superflow phenomenon for this material at ambient pressure and room temperature ( T ), such as the one observed in carbon nanotubes, where the single-line arrangement of the water molecules into the pores gives rise to a flow orders of magnitude faster than in the bulk. , Contrary to the case of carbon and alumina nanotubes, the surface of the AlPO 4 -54 pore is not ≪atomically smooth≫, and the structural organization of the inner water molecules is much denser and disordered . How all this affects water diffusive dynamics is the object of the present study.…”

“…The material framework is built up of 4-, 6-, and 18-membered rings of alternating AlO 6 , AlO 4 , and PO 4 polyhedra. When water is inserted in AlPO 4 -54 pores, one-third of the aluminum welcomes two H 2 O molecules in their coordination sphere, this “structural water” is strongly linked to the AlPO 4 -54 structure and can be extracted only under secondary vacuum treatment, together with heating. − Remaining water is inserted in the pore and forms a disordered hydrogen-bonded network in its center at ambient conditions. When the temperature is lowered, pore water crystallization is prevented due to the high degree of confinement, and the water network differentiates into two distinct networks whose local arrangement and dynamical behavior depend on the proximity with the pore walls .…”

“…When water is inserted in AlPO 4 -54 pores, one-third of the aluminum welcomes two H 2 O molecules in their coordination sphere, this “structural water” is strongly linked to the AlPO 4 -54 structure and can be extracted only under secondary vacuum treatment, together with heating. − Remaining water is inserted in the pore and forms a disordered hydrogen-bonded network in its center at ambient conditions. When the temperature is lowered, pore water crystallization is prevented due to the high degree of confinement, and the water network differentiates into two distinct networks whose local arrangement and dynamical behavior depend on the proximity with the pore walls . Upon cooling below the ice homogeneous nucleation temperature, water close to the zeolite pore surface assumes a highly ordered local arrangement induced by the pore wall, with a more defined site occupancy and lower density with respect to bulk water.…”

The Structure of Water in Silica Mesopores – Influence of the Pore Wall Polarity

“…Therefore, we performed a deconvolution of the observed absorbance by a least-square fit of three Gaussian profiles. [7,[63][64][65][66] These three distinct vibration modes can be assigned as follows: [67][68][69][70] i) a weak band at ca. 3600 cm −1 corresponds to 'dangling' OH groups of the first monolayer of surface-adsorbed water molecules and/or surface silanol groups.…”