Dear Lida hadidi

Please find attached herewith attached file & link

regards,

Prem Baboo

https://www.researchgate.net/post/Why_does_mesoporous_material_have_hysteresis_loop_in_adsorption-desorption_N2_curve?_tpcectx=profile_highlights

If the desorption branch doesn't reach the adsorption branch, it can means that the equilibrium pressure is not established due to the slow diffusion rate of nitrogen in the micropores of your carbon materials. Repeat again the mesure by increasing the equilibrium time during the desorption phase.  

You can find further discussion in the following thread

https://www.researchgate.net/post/How_to_interpret_the_following_N2_adsorption_desorption_isotherms

 Linda, please, can you send one example? And more information about the material and the experimental procedure? Carbon material there are excessive amount, and whit your information all answer are speculative.

could you pl. attache the isotherm graph

For microporous samples such as activated carbons there can be an activated entry effect at the lowest pressures which inhibits the nitrogen from filling all of the micropores. However, these pores are filled as the relative pressure increases. Thus, when you perform the desorption branch of the isotherm you will observe the low pressure hysteresis that you find. If you increase the equilibrium time you may observe that the hysteresis loop is a Little closer since with longer times there may be more adsorption. Nitrogen adsorption at liquid nitrogen temperature means that there is Little kinetic energy. For this reason some workers use CO2 adsorption at 0°C to get a more accurate picture of the first part of the isotherm. Alternatively, if you increase the equilibrium times and the difference between the adsorption branch and the desorption branch increases then you have a slight but cumulative leak in your apparatus.

Does anybody know why two distinct loops might occur? Why would a desorption curve cross the adsorption curve and finish lower than the adsorption curve?

Perhaps one of the following causes would be to explore:

- A cumulative error: too many experimental points are requested and we must not forget that the errors are cumulative;

- The sample is warming up: frequent for long experiments. The limitation of the number of experimental points should allow to reduce the duration of analysis.

Dr Max may be you have leaks. I recommend to make one isotherm whit an empty tube, whit a short number of points, but a long equilibrium time. I have isotherms that last a week and close without problems

(I suppose a volumetric apparatus is being used).

It might just be an error in the calibration values of the different dead volumes. It is a quite common cause of hysteresis loops that do not close with the desorption branch running above or below the adsorption branch. Please, do an isotherm with an empty cell - no adsorbent - and see how near cero are the calculated amounts adsorbed.