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In physics and materials science, the interface or phase boundary is the area between two phases , for example the area between two immiscible liquids such as oil and water.

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 .

Surface chemistry and surface physics deal with the properties of interfaces in the broader sense .

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:


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 .

  • 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 .

Phase boundary in the phase diagram

The phase boundary is represented in the phase diagram of a substance by the phase boundary line as a function of temperature and kinetic pressure. This behavior is a characteristic of the substance.

See also

Web link

Individual evidence

  1. 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 .
  2. 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 .