Thermodynamics of wetting: information brought by microcalorimetry

The wetting state of a solid/fluids system depends on the relative value of the surface tensions acting at the different interfaces. Whereas the free energy of a given solid/fluid interface is not directly measurable, the internal energy part can be relatively easily determined in the case of powders by microcalorimetry. In this paper, the various energy data that can thus be determined and the corresponding suitable setups are reviewed with a special attention to a new method that was developed to study porous media. Both heat and pressure-volume data related to the wetting process can be determined in a lame range of pressure and temperature. Wetting phenomenology is characterised by contact angle hysteresis between the advancing and receding of the fluid/fluid interface on the solid surface. It is shown here how the heat dissipated during intrusion/extrusion of a nonwetting fluid inside a porous media can be related to hysteresis of contact angle and/or to the presence of metastable states. When the temperature is increased the amplitude of the hysteresis phenomenon is decreased. These developments are illustrated by examples of water intrusion/extrusion experiments in porous hydrophobic silica in the range 0-70 MPa and 25-100 degreesC. Possible implications for wetting phenomena occurring in complex systems such as oil reservoirs are proposed. (C) 2004 Elsevier B.V. All rights reserved.