surface excess, n σ
For an interface, the adsorption or surface excess of a given component is defined as the difference between the amount of component actually present in the system, and that which would be present (in a reference system) if the bulk concentration in the adjoining phases were maintained up to a chosen geometrical dividing surface (Gibbs dividing surface). For a solid/liquid interface in which no component of the liquid phase penetrates into the solid, the surface excess (or adsorption) of component i is defined as:
n i σ = n i − V l c i l
where n i is the total amount of i in the system, V l is the volume of an arbitrarily chosen amount of bulk liquid (in the framework of the so-called algebraic method) and c i l is its bulk concentration in the liquid.
PAC, 1986, 58, 967 (Reporting data on adsorption from solution at the solid/solution interface (Recommendations 1986)) on page 969