The Henry adsorption constant is the constant appearing in the linear adsorption isotherm, which formally resembles Henry's law; therefore, it is also called Henry's adsorption isotherm. It is named after British chemist William Henry. This is the simplest adsorption isotherm in that the amount of the surface adsorbate is represented to be proportional to the partial pressure of the adsorptive gas:
Contents
- Application at a permeable wall
- Application at a static membrane
- Application at an impermeable wall
- Choice of the dividing surface
- References
where:
For solutions, concentrations, or activities, are used instead of the partial pressures.
The linear isotherm can be used to describe the initial part of many practical isotherms. It is typically taken as valid for low surface coverages, and the adsorption energy being independent of the coverage (lack of inhomogeneities on the surface).
The Henry adsorption constant can be defined as:
where:
Application at a permeable wall
If a solid body is modeled by a constant field and the structure of the field is such that it has a penetrable core, then
Here
Introducing "the surface of zero adsorption"
where
and
we get
and the problem of
Taking into account that for Henry absorption constant we have
where
where
Application at a static membrane
If a static membrane is modeled by a constant field and the structure of the field is such that it has a penetrable core and vanishes when
We see that in this case the
Application at an impermeable wall
If a solid body is modeled by a constant hard-core field, then
or
where
Here
For the hard solid potential
where
Choice of the dividing surface
The choice of the dividing surface, strictly speaking, is arbitrary, however, it is very desirable to take into account the type of external potential
First,
Second. In the case of weak adsorption, for example, when the potential is close to the stepwise, it is logical to choose
In the case of pronounced adsorption it is advisable to choose
Conversely, if
Thus, except in the case of static membrane, we can always avoid the "negative adsorption" for one-component systems.