In a mixture of solvents (A) and (B), the interactive character of (B) is considered strongly polar, and that of solvent (A) strongly dispersive. For example, solvent (A) might be n-heptane and solvent (B) might be ethyl acetate.

Let a solution of the polar solvent, at a concentration of (c) g/ml, be in equilibrium with the silica gel surface. Under the conditions of equilibrium, let a fraction (a) of the surface be covered with a mono-layer of the polar solvent (B) and, of that fraction (a), let a fraction (b) be covered by a second layer of the polar solvent (B).

Let the number of molecules striking and adhering to the surface covered with a mono-layer of polar solvent (A) and that covered with a mono-layer of solvent (B) per unit time be (n') and be (n") respectively and let the number of molecules of solvent (A) leaving the mono-layer surface and the bi-layer surface per unit time be n₁and n₂ respectively.

Now, under conditions of equilibrium,

n' = n₁

and n" = n₂

n" = aca(1-b)