The thermodynamic systems consisting of a liquid and a lyophobic porous matrix, that is a porous solid and a non wetting liquid, have the property to accumulate, transform, restore or dissipate energy. 1-3 By submitting these systems to an increasing hydrostatic pressure, the intrusion of the liquid into the pores of the solid is observed when the pressure P becomes equal to the capillary pressure P L 1 which can be expressed by the LaplaceWashburn relation where σ is the surface tension, r the radius of the pores, and θ the contact angle between the liquid and the solid (θ . 90°).During the intrusion of a nonwetting liquid into a porous material, a large interface Ω carrying the surface free energy is created. The development of this surface (∆Ω > 0), leads to an increase in the Gibbs energy (∆G > 0) which may be written as To minimize this energy (∆G < 0), the system spontaneously evolves and decreases its interface (∆Ω < 0) by extrusion of the liquid out of the cavities of the solid. Depending on the external pressure, the molecules of the liquid can penetrate or are expelled from the cavities. 2,4 Therefore, there is a reciprocal transformation of mechanical energy into interfacial energy. When the stress is suppressed, the compressed heterogeneous system spontaneously expands as a result of the extrusion of the liquid and then constitutes a real molecular spring. This property could be used in engineering for devices where short-range molecular forces occur to induce important efforts and large displacements 3-5 such as the spreading of solar panels of satellites.The originality of the present thermodynamic study results in the choice of the constituents of the heterogeneous systems. Water is well suited as the mobile phase; it is a polar liquid, not polluting, easy to obtain as a pure phase, not expensive, and characterized by a high surface tension. The very small water molecules, which are comparable to small spheres of 2.8 Å of diameter, are able to access very small micropores. The choice of water imposes the solid phase to be a hydrophobic porous material. Crystallized microporous solids such as zeolites should be good candidates. These materials show a large variety of structures with variable chemical compositions and are characterized by the presence of micropores with very well defined shape and size. During its intrusion in the zeolitic channels, liquid water disperses as clusters, constituted by few water molecules. Two types of attractive interactions can be involved: (i) the dispersion forces between the water molecules and the pore walls, (ii) the interactions between the water molecules themselves. In the case of these hydrophobic solids, the apparent repulsive interaction between the pore walls and water clusters is probably due to the fact that the latter interaction is stronger than the former.In the present work, the behavior of several "water-zeolite" systems was studied. In particular those including purely siliceous zeolites (zeosils), which present a strong hydrophobic character, for ...

We report a joint experimental and molecular simulation study of water condensation in silicalite-1 zeolite. A sample was synthesized using the fluoride route and was found to contain essentially no defects. A second sample synthesized using the hydroxide route was found to contain a small amount of silanol groups. The thermodynamics of water condensation was studied in these two samples, as well as in a commercial sample, in order to understand the effect of local defects on water adsorption. The molecular simulation study enabled us to qualitatively reproduce the experimentally observed condensation thermodynamics features. A shift and a rounding of the condensation transition was observed with an increasing hydrophilicity of the local defect, but the condensation transition was still observed above the water saturation vapor pressure P0. Both experiments and simulations agree on the fact that a small water uptake can be observed at very low pressure, but that the bulk liquid does not form from the gas phase below P0. The picture that emerges from the observed water condensation mechanism is the existence of a heterogeneous internal surface that is overall hydrophobic, despite the existence of hydrophilic "patches". This heterogeneous surface configuration is thermodynamically stable in a wide range of reduced pressures (from P/P0 = 0.2 to a few thousands), until the condensation transition takes place.