Moisture sorption isotherm

The relationship between water content and equilibrium relative humidity of a material can be displayed graphically by a curve, the so-called moisture sorption isotherm.

[2] BET theory (Brunauer-Emmett-Teller) provides a calculation to describe the physical adsorption of gas molecules on a solid surface.

Because of the complexity of the process, these calculations are only moderately successful; however, Stephen Brunauer was able to classify sorption isotherms into five generalized shapes as shown in Figure 2.

On the horizontal axis they provide relative humidity or water activity of the air presented to the material.

Such a set of adsorption isotherms is provided in Figure 3 as measured by Dini on a Type V silica gel.

Moisture sorption isotherm
Classification of Sorption Isotherms by Brunauer Type I through V.
Figure 3. Type V Isotherm Curve Set