Only the following surfaces are referred to as interfaces in the narrower sense :
- between liquid and liquid phases
- between liquid and solid phases
- between solid and solid phases,
while the area
- between solid and gaseous phases
- between liquid and gaseous phases
is usually referred to as a surface ( solid surface ).
The distinction between boundary and surface areas is historical and partly arbitrary. The numerous discontinuities in the earth's crust and mantle show that the third case mentioned above (interfaces within a solid) is also important .
Boundary and surface
A surface can be defined less clearly than one might assume. The geometric definition as the totality of the surfaces that delimit a body from the outside becomes problematic from the microscopic level. The "surface atoms" are not sharply delimited spheres and there can be gaps between them that can be much larger than the atoms themselves. The size of a "boundary or surface" is scale-dependent , ie it depends on the observation scale used.
Therefore, there is also a distinction between the outer and inner surface of all porous materials: When viewed "from the outside", these form a compact body, but are filled with numerous cavities. For numerous physical and chemical processes, however, these inner surfaces are just as relevant as the outer surface. The inner surface often exceeds the outer surface by many orders of magnitude .
Temperature jumps occur at the interface between gases and solids when gas and solids have different temperatures.
Typical examples are:
- on earth in particular the surface of the earth or the water level of the sea and water bodies
- in the case of stars, the photosphere
- the free surface of a liquid under gas pressure
- In technology, the surface of a workpiece to be machined is noted as a shape deviation from the ideal body in the surface information
- a thin layer on an object, see also surface finishing (coating)
- the measured variable for powder , heaps and the like, the specific surface as total volume or mass-related surface
- Human body surface area , assessment formulas for a scale-dependent variable
Within a phase, atoms or molecules interact in all spatial directions with atoms or molecules in their vicinity. The same interaction forces ( cohesions ) act here in all directions . This is not the case near the interface. At the interface, in the direction of the neighboring phase, there may be no or only completely different neighbors available for an interaction. It is usually energetically unfavorable for atoms or molecules to be in the vicinity of the interface. This results in an interfacial tension , in the case of liquid-gaseous the surface tension .
- Surface-active substances such as surfactants ( emulsifiers ) reduce the surface tension and can promote the mixing of two phases to form an emulsion or dispersion .
- At interfaces between solid and gaseous phases, it usually comes to an adsorption of substances from the gas phase. This phenomenon is used in many technical processes and plays a role in heterogeneous catalysis .
- An electrochemical double layer forms at the phase boundary between an electron conductor (solid) and an electrolyte (liquid) . This double layer is an important phenomenon in electrochemistry .
Phase boundary in the phase diagram
- Interface in the lexicon of physics on Spektrum.de
- Entry to interfaces. In: Römpp Online . Georg Thieme Verlag, accessed on December 16, 2019.
- JT Davies: Stable contact potentials at the oil-water interface . In: Journal of Electrochemistry and Applied Physical Chemistry . tape 55 , no. 6 , 1951, ISSN 0005-9021 , p. 559-560 , doi : 10.1002 / bbpc.19510550618 .
- Wolfgang H. Binder: Supramolecular arrangement (sic!) Of nanoparticles at liquid-liquid interfaces . In: Angewandte Chemie . tape 117 , no. 33 , 2005, ISSN 1521-3757 , pp. 5300-5304 , doi : 10.1002 / anie.200501220 .