At low concentrations, the entire surface of the silica gel adsorbs. The total surface is composed from pores and the macroscopically visible, very much smaller, surface of the particles. The entire surface is permeated with silanol groups, capable of binding bipolar water molecules via hydrogen bridges.
Figure 6.3.2.1
This process, described as chemical adsorption, leads to a monomolecular layer. The image of the water initially adsorbed at the surface is best presented through the structural diagram. In this way, depending on the size of the specific surface, about 10 - 30 % of the maximum possible loading is bound. We owe to Langmuir one of the first theories concerning adsorption. Langmuir based his theory on the image of a dynamic equilibrium between gaseous phase and the adsorbed phase, and thus arrived at the following isothermal equation, in which the following quantities are in principle interdependent :
Formula 6.3.2.1
V the adsorbed quantity at pressure p
Vm the adsorbed quantity in the monomolecular layer
p the vapour pressure
K a constant
The Langmuir isothermal can be directly applied to the formation of a monomolecular layer on the assumption that the heat of adsorption has the same value for each water molecule adsorbed. However, in most cases the heat of adsorption is a function of the degree of coverage.
