The region of space comprising and adjoining the phase boundary within which the properties of matter are significantly different from the values in the adjoining bulk phases, is called the surface layer or interfacial layer, as shown schematically in Figure 1 (p. ).
In addition it may be expedient to be more explicit and to define a surface or interfacial layer of finite thickness () bounded by two appropriately chosen surfaces parallel to the phase boundary, one in each to the adjacent homogeneous bulk phases; a layer of this kind is sometimes called a Guggenheim layer. For very highly curved surfaces (radii of curvature of the same magnitude as ) the notion of a surface layer may lose its usefulness.
Quantities referring to the surface layer are indicated by the superscript (e.g. the volume of the interfacial layer is ).