A boundary between two phases is called a surface or interface. The two words are often used synonymously, although interface is preferred for the boundary between two condensed phases and in cases where the two phases are named explicitly, e.g. the solid/gas interface2, but the surface of a solid. In some in stances the word surface is limited to its geometrical meaning while interface is used to describe the thin three dimensional layer (surface layer or interfacial layer, see §1.1.3) between the phases in contact. The area of the surface or interface is denoted by , or (but not ); the symbol may be used to avoid confusion with the Helmholtz energy or with the entropy . For curved surfaces the area depends on the choice of the surface defining the boundary.
When the area of the interface between two phases is proportional to the mass of one of the phases (e.g. for a solid adsorbent, for an emulsion or for an aerosol), the specific surface area (, or preferably ) is defined as the surface area divided by the mass of the relevant phase.