Gas molecules are deposited at the boundary surfaces of a solid body. This process is defined as molecular adsorption on the surfaces of solids. Depending on the type of link established, one differentiates between chemical and physical adsorption. Chemical adsorption is based on ionic (electrostatic) convalent or co-ordinative links, physical adsorption on Van der Waal’s forces. The adsorption process is represented in the form of an adsorption thermal, similar to a diagram registering the adsorbate as a function of the equilibrium concentration of the substance to be adsorbed. The form or type of the isothermal depends on the interacting forces of the individual reaction partners. With porous substances, such as silica gel, two types of isothermals apply.
Figure 6.3.1.1
Type 1 is characterised by a steep rise in the adsorption curve at low concentrations. Type 2 adsorbs mainly at high concentrations. Type 1 is characterised by a steep rise in the adsorption curve at low concentrations. Type 2 adsorbs mainly at high concentrations.
Figure 6.3.1.2
In order to clarify the adsorption process and the origin of the differing types of isothermals, three different, overlapping phenomena must be considered in the presence of pores:
- Chemical adsorption
- Condensation
- Capillary condensation
These are explained by means of the example with water as adsorbate.
