Statistical Thermodynamics

10Adsorption

In this chapter, we apply the grand canonical‐ensemble formulation to adsorption phenomena. The latter are widely observed on a solid surface, on a liquid surface, and at the interface between two liquid phases. After reviewing the adsorption phenomena in Section 10.1, we learn two models of adsorption in the next two sections. The first is a Langmuir isotherm, and the second a Brenauer, Emmett, and Teller (BET) isotherm. In Section 10.4, we look at a more complicated situation in which diatomic molecules dissociate into atoms upon adsorption.

10.1 Adsorption Phenomena

When a gas is in contact with a surface, some of the gas molecules may adsorb onto the surface. The surface is called adsorbent, and the adsorbed molecules are adsorbate. If the surface is flat, the amount of the adsorbate is usually too small to change the pressure of the gas. However, if a finely divided material is used as an adsorbent, the surface can adsorb a sufficient number of molecules to allow measurement of a pressure drop or a mass increase of the adsorbent.

Finely divided materials include fine powders of metal, activated carbon powders, and porous silica particles (see Figure 10.1a–c). Fine powders are usually also porous. The smaller the pore diameter, the greater the surface area. For example, 1 g of silica gel of pore size less than 10 nm has typically 200–500 m² of surface area, ready to adsorb water; silica gels are widely used as desiccant. A 1 g of nickel or its alloy in powder ...