Henry adsorption constant

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:

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:

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 ${\displaystyle x'}$ is the position of the dividing surface, ${\displaystyle u=u(x)}$ is the external force field, simulating a solid, ${\displaystyle u_{0}}$ is the field value deep in the solid, ${\displaystyle \beta =1/k_{B}T}$, ${\displaystyle k_{B}}$ is the Boltzmann constant, and ${\displaystyle T}$ is the temperature.

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