I have been trying to make heads and tails of this topic. Some explained that it depends on geometrical effect of meniscus of the condensed adsorbate while another raw explanation was that it requires more pressure to push and adsorb molecules deep inside the pore, however it is easy to desorb. Kindly help me and thanks in advance.
The origin of the hysteresis is the difference in the mechanism between pore filling (adsorption) and pore emptying (desorption). Upon physiosorption a liquid-like layer (a few atoms thick) is being formed on the surface of the solid. The thickness of this liquid-like layer increases slowly with increasing pressure of the adsorbate. This will increase the curvature of the liquid phase, and thus its pressure. At a particular curvature, the pressure in the liquid will become higher than the condensation pressure and the pore will fill up with liquid.
Pore emptying is also contributed by curvature, but now of the curvature of the meniscus formed at the pore end. The difference in the curvature results in the formation of the hystresis.
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