The equipment doesn´t work properly. The sample can be saturated when you increase the pressure of gas (you get a horizontal line such as in microporous materials), but it cannot desorb the gas as in your case.
Hi,
As above. Also check your sample is out-gassed properly before the measurement as water etc. can also cause problems with the instrument. You also need to make sure your kit is calculating the Free space properly (based on sample mass, volume of cell etc..)
James
Dear Peter Alaba,
I really agree with all the previous explanations.
This type of "non-logical" shape for low pressure adsorption (increase of the amount adsorbed foollowed by a decrease) is typical in the case you do not have enough total surface in your sample cell (in m2). In this case, it might be possible that the amount of material is too low or the specific surface area (in m2.g-1) is too low too. The amount of N2 adsorbed at each point is too low to be accurate when reaching the plateau. A maximum of 24-25 STP cc/g is quite low, and, in this case, we can assume that the sample do not have high specific surface area (it is just an estimation based on "rule of thumb"). Thus, having only a small amount of material inside of the analysis cell leads to high uncertainties. It can also be due to a mistake while entering the mass of the sample (if it is the case, it will not make this strange behaviour disappear but it could reduce its amplitude).
Concerning the non-closing hysteresis, this can either be due to an entrapment of the gas inside of the material (as explained in first answer) or to a change of "cold" and "warm" volume during the measure. I personnaly encountered both before when dealing with high-pressure adsorption. If the lab temperature is slightly changing or oscillating during the day you will create an artificial and unrealistic hysteretic behaviour ("warm" and "cold" volume might have changed). If your material is too dense or is swelling while adsorbing N2, there is a possiblity of entrapment and thus also of hysteretic behaviour.
Best regards,
Sébastien
Check the free volume used in the isotherm. These results are typical when you use a value very high of Could Free Volume. The big problem is when you use a value very low. In this situation the isotherm increase excessively and this effect is more difficult to estimate.
¿What methods are you use to determine the free volume? You have more information in the Application Note of Micromeritics Nº 104