The analytical development definition of BET Isotherm considers it to be a theory that deals with the amount of absolute that is absorbed onto the surface of an absorbent at a certain pressure and temperature. Currently there are five bet isotherm types and they all have different characteristics based on bet analysis and bet analysis instrument. Let?s take a closer look at these bet isotherm types:
The first type of bet isotherm is a monolayer adsorption which is usual depicted in a graph using Langmuir adsorption isotherm. In this type of isotherm all of the adsorbed molecules are in direct contact with the surface layer of the absorbent. Based on analytical development scientist, if the absorbent is multilayered than the absorption space will generally accommodate for more than one layer of the molecules. However not all of the adsorbed molecules will be in direct contact with the surface layer of the absorbent.
The second type shows more a deviation from the langmuir model of absorption where the flat area within the isotherm has a very close similarity to monolayer formation explained above. An example of this type of absorption is nitrogen adsorbed on iron catalyst and nitrogen adsorbed on silicon gel.
This version of the bet isotherm types also has a major deviation from the langmuir model. BET analysis proves that it shows the formation of multilayer adsorption and is very curvy without a flat portion in the curve itself. The monolayer formation in this type is missing completely. An example of type 3 absorption is bromide on silica gel or maybe even iodine on silica gel.
Type 4 absorption on a graph looks very similar to type 2 at the lowest pressure. It also shows formation of monolayer as well as multilayer. The gas in this type of absorption are condensed into the very small capillary pores of the adsorbent at a pressure well under the gases saturation pressure. A prime example of type 4 adsorption isotherm would be benzene on iron oxide and absorption of benzene in silica gel.
On a graph, type 5 absorption is very similar to type 4 however in his case it is a absorption of water on charcoal. Like type 4, type 5 also shows massive capillary of the condensation of gas.