text: 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