Intrusion/Extrusion of water into organic grafted SBA-15 silica materials for energy storage.

Mesoporous SBA-15 silica materials were grafted with trialkylsilyl compounds having short (C1) and long (C8) carbon chain and characterized by XRD, N2 physisorption analysis, 29Si MAS-NMR and contact angle (CA) measurements. A drastic enhancement of the hydrophobic property after grafting was observed by forced intrusion water; it occurred in two steps and with quite high intrusion pressures (mean values - 10 and - 15 MPa). The hydrophobic nature of both internal and external surface area was confirmed by 29Si MAS-NMR and CA measurements, respectively. After contact with water, materials displayed a partial hydrophobic behaviour with uncompleted spontaneous extrusion. The energies absorbed during water intrusion correspond to 4.3 and 6.1 J x g(-1) for C1 and C8 grafted species, respectively.

Pure silica chabazite molecular spring: a structural study on water intrusion-extrusion processes.

Water intrusion-extrusion isotherms performed at room temperature on hydrophobic pure silica chabazite show that the water-Si-CHA system displays real spring behavior. However, differences in pressure-volume diagrams are observed between the first and the other intrusion-extrusion cycles, indicating that some water molecules interact with the inorganic framework after the first intrusion. (29)Si and especially (1)H solid-state NMR showed the creation of new defect sites upon the intrusion-extrusion of water and the existence of two kinds of water molecules trapped in the supercage of Si-CHA: a first layer of water strongly hydrogen bonded with the silanols of the framework and a subsequent layer of liquidlike physisorbed water molecules undergoing interaction with the first layer. This hydrogen bonding scheme is also supported by X-ray powder diffraction.

Thermodynamics of water intrusion in nanoporous hydrophobic solids.

We report a joint experimental and molecular simulation study of water intrusion in silicalite-1 and ferrerite zeolites. The main conclusion of this study is that water condensation takes place through a genuine first-order phase transition, provided that the interconnected pores structure is 3-dimensional. In the extreme confinement situation (ferrierite zeolite), condensation takes place through a continuous transition, which is explained by a shift of both the first-order transition line and the critical point with increasing confinement. The present findings are at odds with the common belief that conventional phase transitions cannot take place in microporous solids such as zeolites. The most important features of the intrusion/extrusion process can be understood in terms of equilibrium thermodynamics considerations. We believe that these findings are very general for hydrophobic solids, i.e. for both nonwetting as well as wetting water-solid interface systems.