For an interface, 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).
Note: 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; \(n_{i}^{\upsigma} = n_{i} - V^{\rm{l}} {c}_{i}^{\rm{l}}\), where \(n_{i}\) is the total amount of \(i\) in the system, \(V^{\rm{l}}\) is the volume of an arbitrarily chosen amount of bulk liquid (in the framework of the so-called algebraic method), and \({c}_{i}^{\rm{l}}\) is its bulk concentration in the liquid.
Source:
PAC, 2020, 92, 1781. 'Glossary of methods and terms used in surface chemical analysis (IUPAC Recommendations 2020)' on page 1816 (https://doi.org/10.1515/pac-2019-0404)