  It has been known for a long time that the adsorption and condensation of gas
cause elastic deformation of the porous matrix. The reversible formation of an
adsorbed film, which precedes capillary condensation, results in an extension
of the porous material while, in the hysteresis region, the negative liquid
pressures under the concave menisci contract the porous matrix. The elastic
deformation exhibits an hysteresis loop in the same pressure region as the
adsorption phenomenon. These deformations have been neglected in practically
all the theoretical treatments of adsorption. We develop a thermodynamic
approach which includes the elastic energy of the solid. This approach is
generic to all porous materials. Thermodynamics of adsorption is directly
connected to the elastic properties of the porous solid. We find that the
variation of the surface free energy related to the elastic deformation is an
important component of the total free energy. It is shown that the condensation
branch represents the more stable states and that an energy barrier exists to
evaporation which depends essentially on the elastic deformation. The pores
interact through the deformation of the walls. Based on this interaction
mechanism and on the shape of the scanning curves which are common to materials
with interconnected pores such as porous glass or noninterconnected pores such
as p+-type porous silicon and SBA-15, we propose a scenario for the filling and
emptying of these porous materials.
