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.

See: Gibbs adsorption

Source:

PAC, 1986, 58, 967 (Reporting data on adsorption from solution at the solid/solution interface (Recommendations 1986)) on page 969

Green Book, 2nd ed., p. 63