Whilst the BET equation can be applied computationally to any data set, the derived value has no physical meaning as the type 3 isotherm is indicating that a complete monolayer does not form across the surface. Type III (3) are extremely rare when using e.g. N2/77K and Ar/87K but are not unusual when making water vapor sorption measurements on chemically hydrophobic surfaces like pure activate carbons (graphitic or graphenic surfaces with no hydrophilic surface functional groups). Krypton's advantage only refers to measuring low surface areas, and not avoiding type III. Indeed, krypton on polymers might also generated a type III because of extremely weak gas-solid interactions.

The only solution, using adsorption, is to find an adsorbate (gas/vapor/liquid) which will "wet" to the surface (and form a complete monolayer before patchy multilayer adsorption accumulates).Regarding carbons, iodine and certain dyes have been used in this way (but from solution rather than from the vapor phase).

Some type II (2) isotherms can masquerade as type III when the surface related adsorption is very small compared to the rise at higher p/p0, and not enough data points at low p/p0 have been measured to show the knee. It's easy enough to find out by applying the BET equation and looking at the c-constant.

Hope that helps.

dear Phuwadej Pornaroontham

Greetings ..,,,

as i understand from your question that u r using the BET

SO MOST OF THE bet give the report with excel file for the material that you analysis and the surface area taken from Single point surface area at P/Po

after calculating of Langmuir surface area as shows that in the report or in the excel file

example ,,,,,,,,,,,,

Summary Report

Surface Area

Single point surface area at P/Po = 0.201057939: 535.0903 m²/g

BET Surface Area: 556.3385 m²/g

Langmuir Surface Area: 771.4949 m²/g

t-Plot Micropore Area: 62.7649 m²/g

t-Plot External Surface Area: 493.5736 m²/g

BJH Adsorption cumulative surface area of pores

between 1.7000 nm and 300.0000 nm diameter: 502.353 m²/g

BJH Desorption cumulative surface area of pores

between 1.7000 nm and 300.0000 nm diameter: 540.6836 m²/g

and there is paper for how u can calculate the H1,H2,H3,H4 isotherm plot with explanation i will send it to you

I hope this paper can be useful,

doi:10.1016/j.cej.2010.07.040

Esmeralda García Díaz Esmeralda García Díaz

Thank you very much for your kind suggestion.

Yes that paper answer my question, since isotherm type 3 has no knee point to calculate monolayer volume which was needed for surface area calculation.

In that paper said, a correction is needed to apply to BET equation.

I will try that way

Sincerely

Ashraf Alattar

Thank you for your reply

That is beneficial, I would try reading it.

Sincerely

The BET eq. can handle an type 3 isotherm, but the obtained monolayer volume, and therefore the BET-Specific surface area, are determined by the BET model and do not have physical relevance. The C-BET parameter is rather smal and should be mentioned to inform the reader about the absence of a 'knee' .The BET area (plus C-value) is therefore just a kind of 'fingerprint' that can be compared with BET-areas of comparable materials (that also have a type 3 isotherm).

Whilst the BET equation can be applied computationally to any data set, the derived value has no physical meaning as the type 3 isotherm is indicating that a complete monolayer does not form across the surface. Type III (3) are extremely rare when using e.g. N2/77K and Ar/87K but are not unusual when making water vapor sorption measurements on chemically hydrophobic surfaces like pure activate carbons (graphitic or graphenic surfaces with no hydrophilic surface functional groups). Krypton's advantage only refers to measuring low surface areas, and not avoiding type III. Indeed, krypton on polymers might also generated a type III because of extremely weak gas-solid interactions.

The only solution, using adsorption, is to find an adsorbate (gas/vapor/liquid) which will "wet" to the surface (and form a complete monolayer before patchy multilayer adsorption accumulates).Regarding carbons, iodine and certain dyes have been used in this way (but from solution rather than from the vapor phase).

Some type II (2) isotherms can masquerade as type III when the surface related adsorption is very small compared to the rise at higher p/p0, and not enough data points at low p/p0 have been measured to show the knee. It's easy enough to find out by applying the BET equation and looking at the c-constant.

Hope that helps.