Mechanisms for gas adsorption and desorption in silica aerogels: the effect of temperature.

We present a theoretical study of the adsorption and desorption mechanisms of fluids in silica aerogels, focusing on the effect of temperature. We adopt a coarse-grained lattice description in which the gel structure is generated by a diffusion-limited cluster-cluster aggregation algorithm and the fluid configurations are computed using local mean-field (i.e., density functional) theory. Our calculations reproduce qualitatively the changes in the shape of the hysteresis loops observed with (4)He in gels of varying porosity. We study in detail the morphology of the condensation and evaporation events that correspond to the irreversible processes (avalanches) which are at the origin of the hysteresis. Depending on porosity and temperature, these avalanches may be localized, involve regions that extend beyond the gel correlation length, or even span the entire sample. This makes difficult the characterization of aerogels based on analyzing sorption isotherms.