Surface energy

The surface energy is a measure of the energy that is necessary for breaking the chemical bonds when a new surface of a liquid or a solid is produced. It is defined as the energy that must be expended to generate the surface per unit area:

The SI unit of surface energy J/m2.

Although most of the term "surface energy " is used, the free energy of the surface must be considered at temperatures above absolute zero actually. However, the difference is small and may often be neglected.

The surface energy is always positive

Because energy is required to break bonds. As the thermodynamically stable state of a system is the one with the lowest ( free ) energy that each system has a tendency to prevent, or surface of high surface energy to be minimized. It follows, for example, that materials of high surface energy are slightly covered by materials with low surface energy ( wetting), but not vice versa. As a rough rule of that materials with strong ties (these usually have a high melting and high boiling point ) higher surface energies than weakly bound materials.

Measurement

For liquids, the surface energy equal to the surface tension and the measurement is easily accessible.

However, the surface energy of solids can hardly be directly measured, because it is not possible to create a new surface without also power for other processes will be used (for example, deformation of the body ). Therefore, it is often argued that the most accurate values ​​for the surface energy of those from quantum mechanical calculations (using the density functional theory ) are. However, these values ​​can sometimes still have errors of about 20 %.

In solids, the surface energy of the orientation of the surface is dependent on:

• Those directions of metals have the lowest surface energy at which the atoms are in a plane as close as possible and therefore, many bonds to other atoms in the surface and below have, so that only a few missing binding partner. These are: in the face centered cubic lattice, the surface having the Miller indices (111)
• In body-centered cubic metals, the (110 ) face
• In the hexagonal lattice, the (0001 ) plane.

• With covalently bound solids such as semiconductors often occur so-called surface reconstruction, to reduce the surface energy.

For indirect measurement of the surface energy of solid bodies, the contact angle is determined, which is formed between the fixed body and one or more liquids of known surface tension on the phase boundary. Young's equation describes this relationship between the contact angle, the surface tension of the liquid, the interfacial tension between the two phases and the surface energy of the solid. Various models for calculation of the surface energy from contact angle data differ in the description of the interactions that are responsible for the respective stress components at the phase boundaries.

Swell

• Thermodynamics
• Surface Physics