The Gibbs adsorption isotherm for multicomponent systems is an equation used to relate the changes in concentration of a component in contact with a surface with changes in the surface tension, which results in a corresponding change in surface energy.  For a binary system, the Gibbs adsorption equation in terms of surface excess is:
where
: is the surface tension,
: is the surface excess concentration of component i,
: is the chemical potential of component i.