Water insertion in hydrophobic porous oxides D. Carrière, S. Sidis, K. Lahlil, M. Moreau, P. Barboux, J.-P. Boilot Physique Matière Condensée, UMR CNRS 7643, Ecole Polytechnique, 91128 Palaiseau, France ABSTRACT We have studied the synthesis of porous hydrophobic systems made from silica and zirconia. The silica systems have been grafted with alkyl-chlorosilanes. Alternative hydrophobic systems were also obtained by synthesis of colloidal monoclinic zirconia grafted with various alkyl phosphonates. The mechanism and the density of grafting were studied by solid state MAS NMR and nitrogen adsorption isotherms. The bulk thermodynamic properties of water are strongly dominated by the interfacial interactions at the surface of these porous solids. The penetration of water in these porous systems was studied by high pressure intrusion of water (water porosimetry between 0 and 100 MPa) into the hydrophobic pores. The amazing mechanical behavior of such water-porous body mixtures can find interesting applications for mechanical energy storage and dissipation. INTRODUCTION Hydrophobic porous systems can find various applications in the domain of selective chromatography [1], preparation of non-wetting surfaces and mechanical energy storage or dissipation [2,3]. Indeed, the pressure required to force a non wetting liquid into a spherical pore of radius r is given by the Laplace-Washburn equation r P LV L θ γ cos 2 - = , where θ is the contact angle between the liquid and the solid and γ LV is the surface tension of the liquid (73 10 -3 N/m 2 for water). The volume V of pore invaded by water at pressure P L can allow a reversible mechanical energy storage E = P.V whereas the hysteresis observed in the intrusion-extrusion cycle, ∆P= P int -P ext , may allow energy dissipation. This property can find valuable applications in high density energy shock absorbers. Different porous systems have been proposed ranging from conventional grafted silica [3] to mesoporous organized MCM41-type systems [4] as well as zeolites [5]. Smaller pore sizes lead to higher working pressures, limited hysteresis and dissipation [4] but to lower intrusion volumes. We have been trying to increase the energy capacity of such systems. In this work, we present a study on grafted silica for which we compare different experimental techniques (adsorption, thermoporometry, intrusion curves) based on previous studies [6]. We also describe new systems based on porous zirconia grafted with long- chain phosphate esters. EXPERIMENTAL DETAILS Synthesis : Silica : We only present in this paper the results on commercial silica. Commercial silica (Davisil 60Å, Davisil 150Å and Merck 100Å purchased from Aldrich, the commercial size always refer to pore diameters whereas we will use radii in the following) was first washed in a diluted HCl solution then dried at 180°C for 12 hours. Grafting was performed by reacting 10 g of silica with 0.055 mol of chlorodimethyloctylsilane in 200 ml of toluene, in the presence of DD3.7.1 Mat. Res. Soc. Symp. Proc. Vol. 755 © 2003 Materials Research Society