Curvature effects

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On strongly curved surfaces, special physical effects can occur due to the special conditions on such surfaces. In particular, the smallest liquid droplets or nanoparticles have a relatively large surface area in relation to their volume , so that the surface energy or the surface tension can affect the properties. Important examples of such effects, called curvature effects, are for the surface of liquids :

• A relatively large surface energy must be applied to form the finest droplets . Therefore, there is a higher saturation vapor pressure over their curved surfaces than over a flat liquid surface . The more the surface is curved, the smaller the droplet and the greater the vapor pressure in the environment must be so that the droplet does not evaporate. This relationship between curvature and vapor pressure is described with the Kelving equation .
• The other way around, the vapor pressure behaves with concave surfaces, e.g. B. in capillaries , or with gas bubbles in a liquid. Here the vapor pressure is reduced with the decreasing diameter of the capillary or the bubble, see also under Kelving equation .
• Due to the effect of surface tension, there is an increased pressure inside a liquid droplet . The smaller the drop, the larger it is. This is described with the Young-Laplace equation .
• The solubility of the smallest liquid droplets in an emulsion is higher than that of large ones.

Such effects occur not only with liquids, but also with solids . The relationships are more complicated with them, not only because differently oriented surfaces of a crystal have different surface energies. In addition, a distinction must be made between the surface energy or surface generation work and the elastic interfacial tension (English: surface stress ). Nevertheless, there are corresponding effects also with solids:

• The solubility, which usually does not depend on the degree of division , increases for the smallest particles.
• If the solid can sublime , the equilibrium vapor pressure over very small particles (nanoparticles) is greater than over large ones.
• There is increased pressure inside a single nanoparticle. The decisive factor is the elastic interfacial tension.
• The reduction potential in the electrochemical deposition of small metal particles is more negative than that in the deposition of the coarsely crystalline metal.
• The melting point of nanoparticles is lower than that of large crystals.

The effects mentioned also lead to eastern forest ripening , which means that if the particles are of different sizes, the smallest particles become smaller and disappear because they evaporate or dissolve while the larger ones grow.

literature

• Adamson and Gast, Physical Chemistry of Surfaces , 6th edition, (1997)

Individual evidence

1. ↑ Prof. Dr. Rainer Birringer: Interfacial tension in nanocrystalline solids. (No longer available online.) Saarland University, September 21, 2010, archived from the original on December 30, 2014 ; accessed on December 23, 2014 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.