# The Mechanism of Chromatographic Retention - Solvent/Solute Interactions with Adsorbent Surfaces > Bi-layer Adsorption > Page 48

and solving for (a),

_{ }

Now the total area covered (S), expressed as fraction of the total area will be ,

S = a + ab

Substituting for (b) and simplifying,

Substituting for (a) ,

or

Simplifying, (30)

Equation (30)
is the Langmuir equation for the two layers of solvent. The function that
describes the total concentration of polar solvent on the surface (C_{g/g})
which, in fact, is that usually measured when determining the adsorption
isotherm is given by,