Adsorption theory and model of aerogel

1. Difference in adsorption layers

-Woqin theory: Assuming that adsorption only occurs on a single molecular layer on the surface of a solid, it no longer continues to adsorb after formation.

-BET theory: extended to multi-layer adsorption, the first layer is adsorbed by chemical bond or strong van der Waals force, and the subsequent layer is dominated by intermolecular force, which is suitable for medium and low pressure physical adsorption (such as multi-layer adsorption in the pores of aerogel).

2. Essence of adsorption mechanism

-Woqin theory: Based on chemisorption, the adsorption sites are uniform and irreversible, and dynamic equilibrium exists between adsorption and desorption.

-BET theory: based on physical adsorption, the first layer has strong interaction with the surface, and the subsequent layers have weak intermolecular interaction, which is applicable to the specific surface area and pore size analysis of porous materials (such as aerogel).

3. Comparison of applicable scenarios

-Woqin theory: applicable to single-layer adsorption systems (such as chemical bonding or low adsorption site materials), but cannot describe the complex adsorption behavior of porous structures.

-BET theory: specially designed for multi-layer adsorption, especially suitable for materials with high specific surface area such as aerogels. The fitting effect is best under medium relative pressure (P/P ₀=0.05 – 0.35), which can accurately calculate the specific surface area and analyze the pore distribution.

4. Comparison of core assumptions

-Woqin Theory:

-Uniform surface and equivalent adsorption sites;

-There is no interaction between adsorbed molecules;

-Only applicable to single-layer reversible adsorption.

-BET theory:

-The first layer of adsorption is similar to Woqin, and the subsequent layers are dominated by intermolecular forces;

-The adsorption heat remains constant from the second layer onwards (equivalent to the heat of liquefaction);

-Calculate the monolayer adsorption capacity through linear fitting (p/V (p ₀ - p) vs. p/P ₀), and then derive the specific surface area.

5. Practical application in aerogel

The high specific surface area and porous structure of aerogel make its adsorption behavior more consistent with BET theory:

-Calculation of specific surface area: The BET equation is used to fit the nitrogen/carbon dioxide adsorption isotherm, combined with the molecular cross-sectional area to obtain the result, which is the core indicator for material characterization.

-Pore distribution analysis: BET theory is often combined with BJH, DFT and other models to reveal the microporous (<2nm) and mesoporous (2-50nm) structures of aerogels.

-Experimental optimization: Attention should be paid to the pressure range (preferably 0.05-0.35) to avoid single layer unsaturation under low pressure or capillary condensation interference under high pressure.

6. Woqin in Special Scenarios

theory application

Despite the dominance of BET theory, Woqin still applies in the following situations:

-Low pressure adsorption (single-layer unsaturated);

-Functionalized surface (uniform adsorption sites);

-Weak intermolecular interactions (primarily physical adsorption)