multilayerAdsorption

Multilayer adsorption refers to the accumulation of adsorbate molecules on a solid surface in more than one molecular layer. This occurs at relative pressures where the first layer is strongly bound to the surface, while additional layers form by condensation of the adsorbate, approaching bulk liquid/gas equilibrium. Multilayer adsorption is a central mechanism in gas adsorption on many solids and underpins methods for measuring surface area and porosity.

The Langmuir model describes monolayer adsorption on a homogeneous surface, but it fails for multilayer adsorption. The Brunauer-Emmett-Teller (BET) theory extends Langmuir to multilayer adsorption by assuming that the first layer has a distinct heat of adsorption, while all subsequent layers are held at a heat of condensation similar to the bulk gas. The BET equation allows the determination of the monolayer capacity V_m and a constant C from the adsorption isotherm, enabling calculation of the specific surface area.

Experimentally, multilayer adsorption is characterized by nitrogen or argon adsorption isotherms measured at low temperatures (commonly 77 K for N2). Isotherms showing Type II or Type IV shapes, with hysteresis in the desorption branch, indicate multilayer adsorption and/or capillary condensation in mesopores. Pore analyses such as BJH can be applied to derive pore size distributions from the isotherm data. Limitations include reduced reliability for microporous materials where pore filling dominates and for systems where the BET assumptions do not hold.

Applications include evaluation of adsorbent materials (activated carbon, zeolites, metal-organic frameworks), catalysts supports, and materials science studies where surface area and porosity influence performance in adsorption, separations, and catalysis.