This is due to capillary condensation in so-called "ink-bottle" shaped pores, i.e. having a entrance diameter smaller than the size of the pore body. Condensation occurs progressively in such pores until the pores are completely filled by the condensable vapour at high relative pressure. But when the relative pressure decreases, the liquid needs to go out of the pores and therefore needs to pass the narrow necks. This occurs at a lower relative pressure than what was required for filling the pores, and when the critical relative pressure is reached, the pores empty abruptly. Hence the loop in the isotherms.

Alain

You can find the answer of your question here:

https://www.researchgate.net/post/Why_does_mesoporous_material_have_hysteresis_loop_in_adsorption-desorption_N2_curve

You can also read this paper for more information:

http://www.uni-leipzig.de/~gfp/sites/default/files/27-dr._sergej_naumov/dissertation.pdf

Dear Dr. Quasin:

Hysteresis is the name of generic phenomena with a same class of characteristic: after a stimulus some information about or generated by the stimulus is maintained. In this sense, as an example, it is possible to mention thermal hystheresis, ferroelectric hysteresis and magnetic hysteresis, obviously each one of them with a proper physical-chemistry stimulus.

The hysteresis of Adsorption/desorption of N2 is correlated with condensation of N2 at mesoporo.  During step of adsorption N2 is compressed in the porous, in this sense, under pressure the N2 gas turn a liquid. In the desorption, the pressure is decreasing gradually but N2 is now a liquid and undergo the effects of capillarity, (major action) turning a gas again at different P/Po ratio. Here, it is possible a more deep insight, but seems not necessary.

The area and shape of hysteresis depend on the area and pore geometry. This phenomenon was studied by several reseachers but is more commonly reported from BET Isotherm theory,  Sthephen Brunaeur, Paul Emmet and Edward Teller, Brunaer/Emmet/Teller, BET. A priori, from BET isotherm is possible to derive the surface area of solids, but a serie of parameters, as mentioned it is possible to determine the geometry of pores.

kind regards.

Marcos Nobre (M. A. L. Nobre)

The answers above give a very good description of the physical basis. If you want more elaborate explanation, look in the literature for work by Matthias Thommes.

please look two useful references :

Pure & Appl. Chem., 577,1985,603

and

J.Am. Chem. Soc. 62,1940, 1723

 A hysteresis loop is formed by  adsorption-desorption N2 curve in mesoporous materia.

in this case ,the hysteresis of Adsorption/desorption of N2 is correlated with condensation of N2 at mesoporos. During step of adsorption N2 is compressed in the porous, in this sense,at a certain p/p0  (i.e 0.45  to 0.50 ) pressure the N2 gas turn a liquid and  it behave works as a capillary condensation in with in the pore like  "ink- bottle" .when the N2 gas turn to liquid its  entrance diameter smaller than the size of the pore body. Condensation occurs progressively in such pores until the pores are completely filled by the condensable vapour at high p/p0 relative  pressure.

But during the desorption when the relative pressure decreases, the liquid needs to go out of the pores and therefore needs to pass the narrow necks. This occurs at a lower relative pressure than that is  required for filling the pores, finally  when the critical relative pressure is reached, the pores empty abruptly. this is called  the loop in the isotherms.This is due to capillary condensation in so-called "ink-bottle" shaped pores, i.e. having a entrance diameter smaller than the size of the pore body. Condensation occurs progressively in such pores until the pores are completely filled by the condensable vapour at high relative pressure. But when the relative pressure decreases, the liquid needs to go out of the pores and therefore needs to pass the narrow necks. This occurs at a lower relative pressure than what was required for filling the pores, and when the critical relative pressure is reached, the pores empty abruptly. This is due to capillary condensation in so-called "ink-bottle" shaped pores, i.e. having a entrance diameter smaller than the size of the pore body. Condensation occurs progressively in such pores until the pores are completely filled by the condensable vapour at high relative pressure. But when the relative pressure decreases, the liquid needs to go out of the pores and therefore needs to pass the narrow necks. This occurs at a lower relative pressure than what was required for filling the pores, and when the critical relative pressure is reached, the pores empty abruptly. Hence,  this is formed  loop in the isotherms.

Basically the hystesis loop formed due to the capillary condensation of N2 inside the pore system at high relative pressure. Whatever the type of pores of mesoporous materials. Which may be Cyliderical, ink-bottle, slit-type, cone-shapped type of pores.

Where the N2 gas converted into liquid by the effect of high relative pressure P/Po.

The N2 uptake required to remove the physically adsorbed N2 gas inside the pores or at the surface during desorption process is much higher that taken during the adsorption process. 

All answers are very descriptive and helpfull, but as Heba Gobara says, capillary condensation occurs whatever the mesopore shape is. The hysteresis loop is the consequence of the two different mechanisms that happen during adsorption-desorption of vapors in mesopores: while adsorption mechanism involves the creation of a mesopores-surface coverage layer by layer, that finlally yields to a delayed condensatation, desorption occurs from the evaporation of liquid in mesopores from the meniscus (as Kelvin equation describes). Ink-bottle pores are not necesary for hysteresis, but they can include pore-blocking or  even cavitation phenomena, which reduce the P/P0 value where adsorption and desorption branches mathches (lower than the usuall 0.6 value).

You may also read: Pure & Appl. Chem., Vol. 61, NO. 11, pp. 1845-1852, 1989 iupac.org/publications/pac/61/11/1845/pdf/; https://hal.archives-ouvertes.fr/docs/.../la051030o.pdf and http://scholarworks.umass.edu/cgi/viewcontent.cgi?article=1015&context=open_access_dissertations

Thanks all of you. I was nice and much descriptive replied by all of you.

Useful discussion

Good and useful answers. Thanks !

An introduction in the subject and you can go as deep as you desire.

I am using porous material for the BET analysis and the free space difference is negative (-0.0984 cm3) is there something wrong with the machine or the used protocol?

The isotherm also didn't close. What is the reason for the open-loop in N2-Adsorption/Desorption isotherm? I didn't use the filler tube and my sample is in the form of solid chunks is that causing a problem?

Hello Honey Gupta

1. Read the answer given by Alain Celzard. If your sample is very porous, and not dry at a high temperature before the BET test, the weight will go down during the evacuation for the BET test. So, maybe the weight is not enough, try to increase the amount/weight. I see from your data weight of the sample is 0.0355g, which is too low. Use about 0.1g

2. I faced a similar problem recently, and the curves were improved by increasing the weight but not avoided totally. Have a look

Article Bismuth-impregnated aluminum/copper oxide-pillared montmoril...

Hi Alemtsehay Tesfay Reda

I have already degassed my samples for 3 days and I could see the significant change in the weight of the sample post degassing process. Since the samples are protein-based I can't degas them at a higher temperature than 50C due to the chances of degradation. I did run the same sample twice and the results are the same.

I have increased the sample amount to repeat the experiment to see if that works.

Thanks.