The adsorption isotherm for a meso porous material like activated carbon shows an hysteresis loop at P/P0 around 0.4. Can anybody from this platform can explain what could be the reason?
Because the mechanism of adsorption (monolayer/multilayer model) is essentially different to the mechanism of desorption where molecules desorb from a meniscus present in the pore that has been previously filled.
This is due to capillary effects which make the pores fill in differently from emptying them. Of course, this applies to higher values of relative pressure p/po.