Source:
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  is the position of the dividing surface,  is the external force field, simulating a solid,  is the field value deep in the solid, ,  is the Boltzmann constant, and  is the temperature.
Introducing "the surface of zero adsorption"
where
and
we get
and the problem of  determination is reduced to the calculation of .
Taking into account that for Henry absorption constant we have
where   is the number density inside the solid, we arrive at the parametric dependence
where