What is a surface excess

J. Willard Gibbs in his pioneering work on the influence of surfaces discontinuity upon the equilibrium of heterogeneous masses suggested for the measurement of the quantities of a system a geometrical surface dividing the interfacial layer. Surface excess is the difference between the amount of a 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 the arbitrary chosen but precisely determined in position dividing surface.