Specific surface free energy

Capillary Rise. In the absence of external forces, a body of liquid tends to assume a shape of minimum area. It is normally prevented from assuming spherical shape by the force of gravity, as well as by contact with other objects. When a liquid is in contact with a solid surface, there exists a specific surface free energy for the interface, or interfacial tension yi2- A solid surface itself has a surface tension 72. vvhich is often large in comparison with the surface tensions of liquids. Let a liquid with surface tension 7i be in contact with a solid with surface tension yj, with which it has an interfacial tension 7i2- Under what circumstances will a liquid film spread freely over the solid surface and wet it This will happen if, in creating a liquid-solid interface and an equal area of liquid surface at the expense of an equal area of solid surface, the free energy of the entire system decreases ... 

Further work [86] showed that the specific surface free energy of silver particles was the same whether they were evaporated on carbon or silver bromide, but was lowered by coating with gelatin. The latter is given as the reason for the stabilizing effect of gelatin on latent image specks. 

In contrast to liquids, two different volumes of a solid phase can not be merged together upon contact. Since the mobility of molecules within solid phases is low, the differences in the bulk and surface structure of these volumes can not disappear spontaneously. Thus, even at the closest contact possible, the real physical interface having its own characteristic value of the specific surface free energy a is present between the two solid phases. For the two solid crystals, a is referred to as the specific surface energy of the grain boundary, agb. For nonpolar solids -1/2 Umo, (b) is less than the surface energy, a, i.e. -Umo[ (b)= = 2a-a. ... 

Methods Used for the Determination of the Specific Surface Free Energy... 

The specific surface free energy and the surface tension, a, numerically equal to it can be accurately determined at the gas-liquid and liquid-liquid interfaces. In this section we describe the general principles on which various methods for the determination of surface tension are based. In general, these methods can be classified as static, semi-static and dynamic. This classification, however, may be further extended with the dynamic versions of some methods listed below as static or semi-static exist. Extensive review of the methods used to determine the surface tension is given in [6,16-18]. 

According to Rehbinder and Shchukin the above condition has a simple physical meaning if the particles are of colloidal dimensions, and the interfacial tension is low, the spontaneous separation of particles from a macroscopic phase becomes possible, since the work required to form a new surface is compensated by the gain in energy upon the involvement of the newly formed particles in thermal motion. There is a critical value of the specific surface free energy, ocr, below which the spontaneous dispersion process becomes possible. This critical value is given by... 

If we create the new surface by adding atoms from rhe bulk, the specific surface free energy G is mdepciidcni of the surface area dC /ti(i)T p ... 

In other words, Ihe surface tension U equal to the specific surface free energy for a onc-eomfK>nent system These terms arc frequently used intcrehangcably in the literature. However, for solids at low tempemtures. cold working of the material can lead to (he formation of a new surface by strain that is not relieved because of negligible mobility of the surface species. (Consider, for example, stretched polymer chains that mtcHock )... 

Equation 2 if TTg is appreciable) together with Equation 7, are the specific surface free energies of the solid—e.g.,... 

In order for a silica sol to lower its specific surface free energy by particle growth or aggregation to form a sol of lower surface area, it is necessary to reverse the ionization and return the adsorbed ions to the intermicellar liquid. According to Yates the free energy change will be yS, where y is the value of the specific interfacial free energy between the sihca surface and water. The latter is a function of particle size and thus of S. 

Subtracting Eq. (7) from Eq. (5), we obtain dF = d(YA)+d nfVi j Integrating this equation and dividing by the area A, an important definition of the system s specific surface free energy, results ... 

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