The following definitions are useful only when can be assessed unequivocally on the basis of a physica1 model of the interfacial layer, or when can be taken as negligibly small.

*Interfacial energy* () is defined by

*Interfacial entropy* () is defined by

*Interfacial Helmholtz energy* () is defined by

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

*Enthalpy and Gibbs energy.* When the state of a system
depends upon more than one pair of conjugate mechanical (or electrical) variables, i.e. more than , then it is possible to derive several sets of functions having the character of enthalpies and Gibbs energies. These functions are related in the following way

Energy [r]^-xY [d]^-TS &
Enthalpy[d]^-TS

Helmholtz energy [r]^-xY & Gibbs energy

Helmholtz energy [r]^-xY & Gibbs energy

where is an intensive mechanical (or electrical) variable and the conjugate extensive variable.

The properties of interfacial layers depend on both and
. In defining an enthalpy in terms of the corresponding energy either ,
or
may be subtracted from the energy function. There are thus three possible definitions of *interfacial enthalpy*:

and three definitions of the

No distinguishing names have been suggested for these different functions.

A possible nomenclature, if one is needed, could be

-enthalpy | |||

-enthalpy | |||

-enthalpy | |||

-Gibbs energy | |||

-Gibbs energy | |||

-Gibbs energy |

Of these and are not often used.