A thermodynamic model of a fluid/liquid interface based on the redistribution of "elastic" energy as a consequence of the contact between formerly isolated phases is proposed. The interface consists of two sub phases adjacent to their respective bulks. Each sub-phase is capable of storing elastic potential energy. The adsorption isotherms are reproduced in the usual way, equalizing the chemical potential of the adsorbent between the bulk phases and the interface. In this formalism, the interfacial energy results from a sum of two terms each belonging to a subphase and can be expressed in terms of the activity of one component of the system in one bulk phase and at the interface.