The inhomogeneous space region intermediate between two bulk phases in contact, and where properties are significantly different from, but related to, the properties of the bulk phases. Examples of such properties are: compositions, molecular density, orientation or conformation, charge density, pressure tensor, electron density, etc. The interfacial properties vary in the direction normal to the surface. Complex profiles of interfacial properties occur in the case of multicomponent systems with coexisting bulk phases where attractive/repulsive molecular interactions involve adsorption or depletion of one or several components. This interfacial region may also be regarded as a distinct, though not autonomous, phase and be called the interphase.
PAC, 1986, 58, 437. 'Interphases in systems of conducting phases (Recommendations 1985)' on page 439 (https://doi.org/10.1351/pac198658030437)
PAC, 1994, 66, 1667. 'Thin films including layers: terminology in relation to their preparation and characterization (IUPAC Recommendations 1994)' on page 1673 (https://doi.org/10.1351/pac199466081667)