The technical details of the specific surface area analyzer are based on a fundamental model known as the BET (Brunauer-Emmett-Teller) equation. This model is widely used in gas adsorption analysis, particularly for determining the surface area of solid materials. The derivation of the BET equation relies on several key assumptions:
1. The solid surface is considered to be uniform, and adsorption occurs in multiple layers.
2. The heat of adsorption for all layers beyond the first is assumed to be equal to the heat of liquefaction of the adsorbate.
These assumptions form the basis for both thermodynamic and dynamic approaches in deriving the BET equation. According to this model, the formation of the second layer is treated as a physical process similar to the condensation of the adsorbate itself, without considering the interaction between the adsorbate and the solid surface beyond the first layer. This simplification leads to the development of different forms of the BET equation, such as the two-constant and three-constant models, which are used in various experimental conditions.
Experimental studies have shown that differences in the surface energy of solid adsorbents have minimal impact on the adsorption behavior of the second and subsequent layers. For the low-temperature nitrogen adsorption method, nitrogen is commonly used as the adsorbate. The BET equation is applied under specific pressure conditions, typically when the relative pressure of nitrogen is between 0.05 and 0.35. This range is chosen because at lower pressures (below 0.05), the adsorption process does not reach equilibrium, and even monolayer adsorption is incomplete. On the other hand, at higher pressures (above 0.35), capillary condensation within the pores becomes significant, making it difficult to maintain linear and quantifiable results.
In summary, the BET theory provides a reliable framework for analyzing surface area, but its accuracy depends on the validity of the underlying assumptions and the proper selection of experimental conditions.
Jiangmen Hongli Energy Co.ltd , https://www.honglienergy.com