Intrusion and extrusion of water in highly hydrophobic mesoporous materials: effect of the pore texture

Understanding of the physics of confined fluids is of major theoretical and practical interest. In the field of materials science, the most popular techniques used to characterise the texture of mesoporous supports are based on the analysis of the properties of a confined fluid: thermoporometry, adsorption technique and mercury intrusion porosimetry. In particular, in the last two techniques the pressure parameter is related to the dimension of the pores. The present study is dedicated to an original analysis of the forced-intrusion of a non-wetting liquid (water) in hydrophobic mesoporous materials presenting different pore topologies. Among these supports, MCM-41 type materials allowed to point out distinct mechanisms for the intrusion and the extrusion of water. Whereas the intrusion process obeys to the Laplace law of capillarity, the extrusion is found to be preferentially governed by the formation of the vapor phase by a mechanism such as nucleation. Therefore, the conventional interpretation given for the hysteresis observed in mercury porosimetry, based on wetting hysteresis and pore-blocking effects is not directly transposable to the case of water intrusion experiments. Data obtained on other supports such as silica gels and Controlled Pore Glass supports are also reported and discussed taking into account the results obtained in MCM-41 type materials. Finally, the mechanisms leading to hysteresis in both phenomena (sorption of a wetting fluid and forced intrusion of a non-wetting liquid) are shown to present strong analogies in model materials as well as in disordered and interconnected porosities. (C) 2004 Published by Elsevier B.V.