If the particles were spherical with only one diameter d (µm), then the external surface area should read S (m2/g) = 6/(d D), where D (g/cm3) is the density of the particles. In your case, given the composition of the particles (from which an estimation of the density can be made), and given their size, the measured surface is much higher than the geometrical surface you may expect. The value is quite realistic, but suggests that the particles are porous. CO2 is a gas able to penetrate very small pores (having a diamter less than 1 nm), so it is one of the best probes for investigating narrow porosity. You don't need many narrow pores to have a strong impact on the surface area. You therefore measured the total surface area of slightly porous particles, nothing surprising.

Alain

Thanks a lot Alain,

If I understand your comment well, the CO2 gas method only able to measure the external surface area besides some internal surface area in the micropores but this not mean that the measured value in current case resemble the total surface area. Because the total surface area (usually measured with Ethylene Glycol; EGME) of smectite-rich materials varies from 400-800 m2/g.

Thank you so much Behzad,

Actually I have only the results shown in the previous attached file where the P/P0 have been measured till 0.04 as well as I haven't not experienced in this topic.

Regards,

46 m2/g is quite reasonable for a clay sample. Typical values for samples composed out of 100% smectite are 30-90 m2/g, for kaolinite 5-15 m2/g, illite 5-20 m2/g (personal experience). This is the area of the particles in your

Dear Pieter Bertier,

Thank you so much for your comment. It is very helpful.

I had'nt open the file with the isotherm, so I just did it now. I confirm that the shape of the hysteresis loop is strange. I see 2 possibilities for this. Either you have a leak in your system, so that the loop can never be closed even at low relative pressure (so the measurements are wrong and need to be repeated). This is the first thing to be checked, as the problem happens sometimes. After you have checked that an excellent vacuum can be maintained for a long time, the loop should close. Or, if the loop is maintained after that, then, as Pieter suggested, there is not only adsorption but CO2 penetrated the material and remained partly trapped inside. This phenomenon is not uncommon when water vapour is adsorbed on slightly hydrophylic materials: there is no pure adsorption, but absorption as well, hence the non-closing loop.

Alain

Thanks for your reply, Alain.

I think a leak can be excluded here, since I see the same in my measurements, and other people have shown it to me from their measurements. It would surprise me if all our setups leak.

Absorption might indeed explain this, though for this material I'd rather see it as chemisorption. A sequence of consecutive isotherms, without intermediate outgassing could confirm that.

Yasser,

I also had another look at your isotherm. The higher part of the adsorption branch is considerably steeper. I looks like you measured a lot of points there in a narrow p/p0 range. This would mean that the total equilibration time at that p/p0 was longer, i.e. hint at a time dependency. It would be worth remeasuring your sample with a stricter equilibration criterion/longer equilibration time to check for a time dependency. You might be underestimating your area from this measurement.

regards,

Pieter