Adsorption on Silica Gels
Like zeolites, silica gel functions on the basis of physisorption. Adsorption occurs
due to van der Waals interactions and capillary condensation at high humidity. The
adsorption force is less than for zeolites, resulting in a lower adsorption capacity
at low concentrations of adsorbents.
On the other hand, at higher concentrations of adsorbents, the adsorption capacity
is higher than for zeolites due to the higher internal pore volume. Certain grades
of silica can adsorb up to 1.2 times their own weight of water.
Due to the lower adsorption forces, silica gel is also
easier to regenerate.
Due to this behavior, silica gel is the material of choice when only moderately low
dew points need to be achieved and the high adsorption capacity and / or good regeneration
behavior of silica gel can fully be exploited. The suitability of silica gels for
the adsorption of liquids or gases and vapors varies based on the pore size distribution.
In addition, narrow pore and wide pore silica gels exhibit different degrees of stability
against liquid water. Liquid water can lead to a destruction of the silica gel pores
in case of a narrow pore silica gel, while wide pore silica gel is not affected
by the presence of liquids.
Grace Davison offers silica gel adsorbents in granular or beaded form, but is it
also possible to produce polymer-bound desiccants and monoliths/miniliths. Silica
gels are also widely used in drying sachets.