Adsorption Isotherms II

BET theory extends beyond Langmuir's monolayer assumption to account for multilayer adsorption on homogeneous solid surfaces. While Langmuir assumes only a single layer of gas molecules can adsorb, BET allows subsequent layers to form on top of previous ones, held by van der Waals forces. This makes BET more applicable to Type II isotherms and provides a more realistic picture of actual adsorption processes.

Van der Waals forces are weak attractive forces that hold each adsorbed layer to the layer beneath it in BET multilayer adsorption. Each new layer can only form after the previous layer has covered the surface, with successive layers bonded through these intermolecular attractions. This layering mechanism allows gas molecules to accumulate beyond the first monolayer.

In BET theory, the first layer experiences a constant, higher adsorption energy due to direct interaction with the solid surface. Subsequent layers have adsorption energy equal to the energy of gas liquefaction, the energy released when gas condenses into liquid. This distinction reflects that molecules in upper layers interact primarily with other adsorbate molecules rather than the substrate.

A BET plot is a straight line generated by plotting relative adsorption pressure against the BET function. The slope and intercepts of this linear regression line yield the monolayer adsorption volume and the BET constant c. From these values, researchers can calculate the number of molecules forming a monolayer and determine the surface area per unit mass of the solid sample.

BET theory is most accurate for pressures up to one-third of the saturation pressure. At higher pressures, significant deviations occur because the assumptions of uniform adsorption layers and distinct monolayer formation no longer hold true. Additionally, the assumption of equal adsorption energy for all layers beyond the first may not accurately reflect real adsorption behavior at elevated pressures.

The isosteric heat of adsorption decreases significantly as surface coverage increases. This occurs because energetically favorable binding sites are filled first, and repulsions between adsorbed molecules increase as more molecules occupy the surface. This behavior reflects the heterogeneous nature of adsorption sites and molecular interactions on the solid surface.

As pressure increases, additional layers of gas molecules can form on the solid surface beyond the initial monolayer. Each new layer requires the previous layer to be substantially covered before formation begins. The cumulative effect of multiple layers forming at higher pressures results in greater total gas adsorption, making pressure a key variable in multilayer adsorption processes.