Crystalline solids are generally thought to adsorb water because of minimal pathways for water to penetrate the particle bulk. However, it is possible that some of the critical distinctions in crystalline materials arise from defects or imperfect
crystallinity, which leads to water absorption in addition to adsorption. In this context, the theoretical analysis should not be limited to or assume only adsorption. It is in fact unnecessary and certainly undesirable to make such limiting assumptions
in the theoretical analysis.
The chemical potential (μ) of water vapor can be readily calculated from its vapor pressure (p). Themost common standard state for sorption studies is water vapor at its saturation vapor is
pressurep
0
. Using this standard state, the chemical potential