How is a solid-gas interface different from other interfaces between phases of matter?

In the context of surface analysis, the term "surface" is specifically reserved for the interface between a solid and a gas or between a solid and a vacuum. While all interfaces represent a thin layer where properties differ from the bulk material on either side, the solid-gas interface is distinct because it defines the region of a solid that interacts with its environment.12

Other interfaces—such as those between two solids, two liquids, a solid and a liquid, or a liquid and a gas—are studied separately from surface analysis. A key characteristic of the solid-gas interface is the phenomenon of adsorption, where gas molecules are attracted to and held on the solid surface. Unlike absorption, which involves a substance penetrating the interior of a material, adsorption is strictly a surface phenomenon. Furthermore, because the surface layer of a solid often reacts with its environment, the atoms or molecules at this interface frequently differ from those in the bulk material, necessitating operational definitions to determine the depth of the "surface" region.134