Adsorption Theory and Models of Aerogel

1. Differences in Adsorption Layers

woqin Theory: Adsorption occurs only in a monomolecular layer on the solid surface; once the molecular layer is formed, adsorption ceases.

BET Theory: Applicable to multilayer adsorption. The first layer is governed by chemical bonds or strong van der Waals forces, while subsequent layers are primarily governed by intermolecular forces. This theory is applicable to low- and medium-pressure physical adsorption, such as multilayer adsorption within the pores of aerogels.

2. Nature of Adsorption Mechanisms

Woqin Theory: Based on chemical adsorption. Adsorption sites are uniform and irreversible, with a dynamic equilibrium between adsorption and desorption.

BET Theory: Based on physical adsorption. Surface interactions are strong in the first layer, while intermolecular forces in subsequent layers are weak. It is suitable for surface area and pore size analysis of porous materials such as aerogels.

3. Comparison of Applicable Scenarios

Woqin Theory: Applicable to single-layer adsorption systems (such as chemically bonded or low-site density materials), but cannot describe complex adsorption within porous structures.

BET Theory: Designed for multilayer adsorption. It is well-suited for high-surface-area materials such as aerogels. It performs best at moderate relative pressures (P/P₀ = 0.05–0.35), enabling accurate calculation of surface area and analysis of pore size distribution.

4. Comparison of Core Assumptions

Woqin Theory:

Surface uniformity and equal adsorption sites

No interaction between adsorbed molecules

Applicable only to reversible monolayer adsorption

BET Theory:

The first layer is similar to the Langmuir model, with subsequent layers primarily governed by intermolecular forces

The heat of adsorption remains constant from the second layer onward (equal to the heat of liquefaction)

The monolayer adsorption capacity is obtained by linearly fitting p/V(p₀−p) vs. p/P₀p / V(p₀-p) vs. p/P₀p/V(p₀−p) vs. p/P₀p

5. Practical Applications of Aerogels

The high surface area and porous structure of aerogels make their adsorption behavior more consistent with the BET theory:

Specific surface area calculation: The BET equation is fitted to nitrogen or carbon dioxide adsorption isotherms; the result is derived from the molecular cross-sectional area—a key to materials characterization.

Pore Distribution Analysis: BET is often used in conjunction with BJH, DFT, and other models to reveal micropores (<2 nm) and mesopores (2-50 nm).

Experimental Optimization: A pressure range of 0.05-0.35 is recommended to avoid undersaturation at low pressures or capillary condensation at high pressures.

6. woqin Adsorption in Special Scenarios

Although the BET theory is dominant, Langmuir adsorption is still applicable to:

Low-pressure adsorption (monolayer undersaturation)

Functionalized surfaces (uniform adsorption sites)

Weak intermolecular interactions (primarily physical adsorption)