Considering MOFs porosity as a type of skeleton-like cavities instead of a solid walls-delimited pores, is it possible they show hsyteresis loop in the desorption branch despite the crystallographically determined cavity size is slightly lower than the mesoporous size? In addition, structure of materials where I showed this behaviour is constructed by big cavities conected by smaller cavities, in a kind of ink-bottle neck structure (RhoZMOF, similar to zeolite RHO) and value for the conection point between adsorption and desorption branches are around P/P0 = 0.4, lower than 0.6. Could this be cavitation?
A friend of mine had a similar situation with zeoloites. The explanation for his hysteresis at a relative pressure between 0.2 and 0.3 was the fact that the N2 molecules arrange in different configurations (packing structures). This effect causes a sharp step in the adsorption branch as well as a sharp step in the desorption branch and due to the meta-stable configuration, a hysteresis was observed between the two.
Closing of the hysteresis loop between adsorption and desorption at relative pressures around 0.4 is on the other hand observed in numerous materials. Think of the number of N2 molecules, which would represent the corresponding surface curvature. You are getting down to a very low number of molecules and as such, the classical explanation of surface tension/surface energy do no longer hold under those conditions.
Thank you very much for your replies, they are very helpfull. Respect to the isotherm, a documment is attached whit a comparisson between the N2-adsorption-desorption behaviour of a RhoZMOF material (metal-organic framework with a RHO-zeolite-like structure) and ion-exchanged materials with (hydrated-) Li+. HDMA+ charge-compensating organocations are substituted by smaller metallic cations than the pristine compensating agent and increases the void volume in cavities. Thank you for your interest.
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