Surface energy, molecular tension

The above kinetic equations, developed based on the thermodynamic approach of Gibbs (1928), Volmer and Weber (1926), and Becker and Doring (1935), belong to the so-called classical nucleation theories. They have been criticized for the use of surface energy (interfacial tension), cr, which is probably of little physical significance when applied to small molecular assemblies of the size of critical nucleus. 

This effect assumes importance only at very small radii, but it has some applications in the treatment of nucleation theory where the excess surface energy of small clusters is involved (see Section IX-2). An intrinsic difficulty with equations such as 111-20 is that the treatment, if not modelistic and hence partly empirical, assumes a continuous medium, yet the effect does not become important until curvature comparable to molecular dimensions is reached. Fisher and Israelachvili [24] measured the force due to the Laplace pressure for a pendular ring of liquid between crossed mica cylinders and concluded that for several organic liquids the effective surface tension remained unchanged... 

Coran and Patel [33] selected a series of TPEs based on different rubbers and thermoplastics. Three types of rubbers EPDM, ethylene vinyl acetate (EVA), and nitrile (NBR) were selected and the plastics include PP, PS, styrene acrylonitrile (SAN), and PA. It was shown that the ultimate mechanical properties such as stress at break, elongation, and the elastic recovery of these dynamically cured blends increased with the similarity of the rubber and plastic in respect to the critical surface tension for wetting and with the crystallinity of the plastic phase. Critical chain length of the rubber molecule, crystallinity of the hard phase (plastic), and the surface energy are a few of the parameters used in the analysis. Better results are obtained with a crystalline plastic material when the entanglement molecular length of the... 

Van der Waals further finds a relation between the temperature coefficient of surface tension and the molecular surface energy which is in substantial agreement with the Eotvos-Ramsay-Shields formula (see Chapter V.). He also arrives at a value for the thickness of the transition layer which is of the order of magnitude of the molecular radius, as deduced from the kinetic theory, and accounts qualitatively for the optical effects described on p. 33. Finally, it should be mentioned that Van der Waals theory leads directly to the conclusion that the existence of a transition layer at the boundary of two media reduces the surface tension, i.e., makes it smaller than it would be if the transition were abrupt—a result obtained independently by Lord Rayleigh. 

Sugden J.G.S. cxxv. 1177, 1924 cxxvii. 1525, 1868, 1925) has compared the molecular volumes of substances under conditions such that they possess identical surface tensions and has shown that they are determined by the molecular constitutions of the substances. In obtaining the parachor P Sugden makes use of the approximate relationship between free surface energy and density noted by Macleod Trans. Farad. Soc. xix. 38, 1923) a = c(pi- p y... 

The interface between two fluids is in reality a thin layer, typically a few molecular dimensions thick. The thickness is not well defined since physical properties vary continuously from the values of one bulk phase to that of the other. In practice, however, the interface is generally treated as if it were infinitesimally thin, i.e., as if there were a sharp discontinuity between two bulk phases (LI). Of special importance is the surface or interfacial tension, a, which is best viewed as the surface free energy per unit area at constant temperature. Many workers have used other properties, such as surface viscosity (see Chapter 3) to describe the interface. 

In this section we outline the molecular origins of the Debye, Keesom, and London forces and discuss the strengths of these forces relative to each other. In addition, we also outline how macroscopic properties and behavior (such as the heat of vaporization of materials, nonideality of equations of state, and condensation of gases) can be traced to the influence of the above van der Waals forces and illustrate these through specific examples. Another example of the van der Waals forces, namely, the relation between the surface tension (or surface energy) of materials and the London force, is discussed in Section 10.7. 

The surface tension and viscosity.—The surface tension <7, and the molecular surface energy, (Mv)l, of R. Eotvos was determined for liquid hy.drogen chloride by D. McIntosh and B. D. Steele 8 ... 

The surface tension, a, and molecular surface energy, /x, of the liquid at various temperatures have been found 12 to be ... 

Solids also have surface tension because molecules on the surface of a solid particle are subject to fewer attractive forces than molecules in the bulk of the solid. Measurements of the surface tension of solids (usually called the surface energy) are difficult because solids are rarely pure and smooth on the molecular scale. 

A liquid forms an interface with another fluid. At the surface, the molecular layers are different in density than the bulk of the fluid. This results in siuface tension and interfacial phenomena. The surface tension coefficient, a, is the force per unit length of the circumference of the interface, i.e., N/m, or the energy per unit area of the interface area. 

Variations in chemical composition and molecular structure can also have important repercussions on the surface energy of the material. Several techniques have been proposed for the experimental determination of surface tension, the sessile and pendant drop methods being the most promising and commonly used... 

In an attempt to lower their surface energy, solids almost always will adsorb small molecules, such as H2O or O2, on their surfaces. Molecules which form molecular solids have much smaller surface tensions than other solids. The study of surfaces is a difficult one, it can be seen. Fortunately a number of very special experimental methods have been developed, and surfaces may be studied in great detail. Such studies are useful in many important practical areas, such as adhesion, lubrication, corrosion and adsorption. However, the most important area is probably heterogeneous catalysis. 

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