*Surface excess energy* (
) is defined by

and are the energy densities in the two bulk phases where and are the mean molar energies and and are the mean molar volumes of the two phases.

*Surface excess entropy* (
) is defined by

and are the entropy densities in the two bulk phases, where and are the mean molar entropies of the two phases.

*Surface excess Helmholtz energy* (
) is defined by

*Surface excess enthalpy* (
) is defined by

*Surface excess Gibbs energy* (
) is defined by

When the thermodynamics of surfaces is discussed in terms of excess quantities, . There is thus only one way of defining the excess surface enthalpy and excess surface Gibbs energy (cf. case B, below).

The corresponding excess quantities per unit area may be denoted by lower case letters:

*Relative (excess) surface energy* (with respect to
component 1)

*Reduced (excess) surface energy*

Analogous equations hold for , , and .