I measured the surface area and pore distribution for a polymer with N2 under 77K using ASAP2020. But I found the isotherm looks bad and surface area value is quite small. Then I changed to Argon and measured under 87K. The isotherm plot is completely different (see attached). What's the reason that causing this huge difference?
Thank you very much.
200 mg of material is not too much if the surface area is really low. Assuming that it is equal to 1 m2/g, it means that your device will have to measure 0.2 m2. This is below the accuracy of the machine, which is typically 10 m2 when using N2 as probe molecule. Hence my suggestion of using Kr at 77K, with which measuring surface areas close to 1 m2/g and even lower becomes quite possible. Ultra narrow pores can be probed by using CO2 at 0°C: its size is close to that of N2 but its diffusion is dramatically faster (and easier) at such temperature. As a result, CO2 at 0°C penetrates narrower pores than N2 can do at 77K, and you can indeed measure something if such pores exist. If they don't exist, Kr at 77K is the best (and the last) option.
Alain
It appears that not only your material should have an extremely low surface area (in that case better using krypton adsorption), but you have not enough material in your measurement cell. Your "isotherm" is indeed a straight line, and looks like the calibration curve of an empty tube. If your polymer is not porous enough, you should not waste your time trying to measure a surface area: this has no interest.
Alain
Dear, Alain
Thank you for answering my question. I put more than 0.2g sample into the sample tube and it's much higher than the minimum required (0.1g). Is it possible the porous are too small that N2 cannot get in, but the higher temperature allows argon can be adsorbed?
200 mg of material is not too much if the surface area is really low. Assuming that it is equal to 1 m2/g, it means that your device will have to measure 0.2 m2. This is below the accuracy of the machine, which is typically 10 m2 when using N2 as probe molecule. Hence my suggestion of using Kr at 77K, with which measuring surface areas close to 1 m2/g and even lower becomes quite possible. Ultra narrow pores can be probed by using CO2 at 0°C: its size is close to that of N2 but its diffusion is dramatically faster (and easier) at such temperature. As a result, CO2 at 0°C penetrates narrower pores than N2 can do at 77K, and you can indeed measure something if such pores exist. If they don't exist, Kr at 77K is the best (and the last) option.
Alain
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