Pleas explain what the difference is between total surface area and BET surface area. Which one is more accurate?
Hi,
Total surface area is the actual "total" surface area regarding the substance you might be talking of, and as in BET, it provides you with the surface area that it calculates based on the adsorption of a gas on the substance exposed surface, withholding some assumptions:
1. the adsorption is monolayer,
2. the adsorption is specific to one molecule per adsorption site,
3. all the adsorption sites have equal accessibility to the adsorbing gas molecules and the adsorption of all molecules is identical, and so on.
..it is only because of these assumptions the total "actual" surface area may deviate from the determined BET surface area, but as far as the physical restrictions to find out the surface area of samples, BET has been proven to be very close to the actual value and dearly accurate.
adding to what Mr. adhidesh kawat has said,
BET surface area is equal to total surface area only if the pores are > 2nm and if the pores are open.
Actually, BET SSA refers to external surface area which is mainly equal to total surface area of > 2nm pores. The total SSA including external surface area and internal surface area (which refers to the SSA of less than 2nm pores).
The BET SSA and total SSA are controled by the employed methods.
For the materials without micropores, the BET SSA should be equal to the total SSA which is used and accepted widely in sediments and sedimentary rocks.
BET should be fine for meso- to macroporous materials, that is, materials with pores size above 2nm. For micro-porous materials, such as activated carbon, other data evaluations than BET should be made, these are based on thickness curves for the adsorption layer.
Provided that you measure in the BET applicable meso/macroporous pore size region the only difference between the Total and the BET surface area is the closed porosity, the part of the porosity that is not accessible from the outside. Crushing the material may open up such closed porosity.
If adsorption and desorption isotherms of a sorbent follow the same path, they are the same. This is a feature of fully microporous solids (Type I of IUPAC Classification). On the contrary, if does not follow the same path they are different. This is a fature of solids having advanced micro- and particularly mesoporous (Type II and Type IV)that occurs capillary condensation.
I am not clear, what is meant by Total surface area. Actually there are two type of adsorption are knwon. One is mono layer and another one is multilayer. If you calculate the surface area from multilayer adsorption, that is BET surface area. Adsorption isotherms with respect to pressure are of five to six types.
Please see "Sing, K.S.W., Everett, D.H., Haul, R.A.W., Moscou, L., Pierotti, R.A., Rouquerol, J., Siemieniewska, T., 1985. Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity, Pure Appl. Chem. 54, 603-619."
Total surface area is a concept, and when the size of the pores start narrowing below micropores, it becomes more complicated.
Can you explain what you mean by 'total surface area'? Which method are you using to measure it? Based on your feedback, it is easier for the community to reply more accurately.
On the other hand, the concept of BET surface area is widely known and most answers given here are correct.
Mostly the suggested answers are very helpful. However, the idea of the application of BET model just for pores bigger than 2nm is not acccurate and a bit obsolete. It has been proved that the stimation of SSA by BET model is satisfying for microporous materials like MOFs or COFs, if the pressure-points range for the BET equation is correctly selected for the current material (linear fitting and C>0). However, it is highly recommended to provide both BET and Langmuir SSA for comparisson. Please, check:
Article Cavitation in Metastable Liquid Nitrogen Confined to Nanoscale Pores
For porous materials the external surface area is negligible comparing with the internal one. Therefore total surface area has no sense for such materials. Closed pores have no relevance for surface phenomena. Just for density. Nitrogen adsorption and BET is the only way to get the surface area of porous materials, taking into account that it gives overvalues when micropores are present.
In this paper, "Measurement of airborne nanoparticle surface area using a filter-based gas adsorption method for inhalation toxicology experiments"
P.688
Does the total surface area means the area which got from the DFT method area?
Why Langmuir surface area > BET surface
area for mesoporous of activated carbon and what is the different between them ?
How can measurements or calculate of Specific Surface Area for particles m2/m3??
I need to know the protocol for measurements Specific Surface Area laboratory for sugarcane bagasse
Regarding Langmuir vs BET: Langmuir isother assumes all gas molecules goes into one single layer on the surface, while BET isotherm assumes the formation of several layers, on top of each other, and from the model deduces the number of gas molecules in a single layer. That is why Langmuir always gives larger value for SSA than BET.
in mine data BET surface area is less than 50 and pore width 40 A what does it means
Assuming no experimental error, it seems that you have a basically non-porous solid with just few micropores.
The total surface area is specific for the external surface, while the BET surface area is calculated with whole the surface such as surface of pores, external surface of material and calculated by the adsorption of nitrogen gas on the surface of materials. The BET surface area is generally higher than the total surface area and it is relative to the total surface area. The surface area can be measured in m2/g or m2/kg.
Firstly, specific means per gram sample.
Secondly, open porosity should be counted in the surface area. If the pores are too small for molecules to adsorb in them then they will not be measured. That means, with ultramicroporous materials different gases will adsorb to yield different measurements of surface area.
Thirdly, the BET is notoriously bad for measuring surface area. It is one of the few theories I know of for which one throws away 70% of the data. It is imprecise, inaccurate, contradictory, prone to anomaly, cannot properly measure porosity, and prone to producing an imaginary energy of adsorption. It really can only be fit to 30% of a Type II isotherm.
The low pressure (UHV) data for isotherms definitely disproves the validity of the BET and all other "Henry's" law type theories for physical adsorption, since a phase transition is detected at what is referred to as the threshold pressure. For ceramics this threshold is in the 10^-6 torr range but for low energy adsorbent it might be as high as 1 torr.
Fourthly, there are superior ways of analyzing the data. The quantum mechanical derivation which is consistent with the ESW thermodynamic approach. They properly use nearly 100% of the data, if you have paid attention to temperature control (shielding and shielding) , and are statistically superior even if restricted to the BET range.
The QM/ESW derivations yield simple equations, to determine the monolayer equivalence and the adsorption energy. For simple systems, homogeneous and single pore size, the equations are simple enough to do with a hand calculator that's 30 years old. For heterogeneous energies and distribution of pores you might need a laptop computer. The calorimetric heats of adsorption can be calculated correctly from the isotherm. There is a high degree of confidence for the calculations ultramicroporosity, microporosity and mesoporosity. From the pure gas isotherms, the binary phase diagrams may be calculated, which is ideal for screening studies - it saves a LOT of work.
These proposals have been around for 40 years, but little attention has been paid to them. This is mainly due to the consensus that the issue was long settled and publication was extremely difficult. Some of the researchers that did the early work are now dead and myself and Juegen Adolphs are the only ones that I know of who are still living.
If you are interested I can send you a report on the subject, or better yet you could buy or ask the library for an interlibrary loan for the book I wrote. The new edition for Elsevier is the 2nd edition, Get that edition on loan if possible.
But whatever you do, stop using the BET. The DRK, as bad as it is theoretically at least gives you an approximation of the porosity.