BET theory - Huaqiang Electronic Network

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The basic assumptions of the model used in the derivation of the BET equation for the specific surface area analyzer are:
1. The solid surface is uniform and multiple layers of adsorption occur;
2. The heat of adsorption of the other layers except the heat of adsorption of the first layer is equal to the heat of liquefaction of the adsorbate. Both methods of deriving both thermodynamic and dynamic angles are based on this assumption.
It can be seen from the hypothesis that in the derivation of the BET equation of the specific surface area measuring instrument, the adsorption at the beginning of the second layer is regarded as the aggregation of the adsorbate itself, and the relationship between the adsorption outside the first layer and the solid adsorbent itself is not considered (detailed BET II) Constant and three-constant equations, the derivation of thermodynamics and kinetics, if available). A large number of experiments have also confirmed that the difference in solid adsorbent causes its own surface energy to be different and the influence on the adsorption outside the first layer of adsorbate is very weak. For the low-temperature nitrogen adsorption method, nitrogen is used as the adsorbate, and the BET equation is established under the condition that the partial pressure of nitrogen is required to be 0.05-0.35. The reason is also due to these two assumptions (the multilayer physics cannot be established when the relative pressure is less than 0.05). Adsorption equilibrium, even single-molecule physical adsorption is far from being formed; when the relative pressure is greater than 0.35, the pore structure makes the effect of capillary condensation prominent, quantitative and linear.