Surface energy interfacial tension points

At this point is must be emphasized again that a surface or interfacial tension is a tensor by its physical origin (cf O Chap. 4) and hence it may not be split into additive scalar components in general. Such additivity holds for the energy, and referring again to OEq. 6.11 for the specific excess free Helmholtz energy for a surface of phase A in equilibrium with its vapor phase... 

When three different phases make contact with each other, where three surfaces intersect at a triple point, we obtain three contact angles and three interfacial tension values, as can be seen in Figure 3.7. We can obtain contact angle equilibria when we place an immiscible drop on a liquid or solid in air or a vapor phase there are many applications of contact angle measurement in industry and surface science (see Chapter 9). In these conditions, the total excess surface internal energy can be written from Equation (201) so that... 

The values of the evaporation and sublimation heats are usually quite close to each other, as well as the densities of solid substances and their melts, measured at the melting point. Consequently, the values of the surface energy at the liquid-vapor, aLV, and at the solid-vapor, osv, interfaces are nearly identical. Oppositely, the interfacial energy oSL at the interface between the solid phase and its melt is usually low oSL values normally do not exceed 1/10 of surface tension values of melt (note that the heats of melting are also on the order of -10% of those of evaporation). 

Several workers [e.g.. Refs. 5,45,46)] have pointed out that solvent adsorption energy per unit of surface (and therefore solvent strength, from its definition) is related to the interfacial tension between solvent and... 

The interfacial tension of a stable, two-phase system is always positive, otherwise the two phases would spontaneously mix since they lower their free energy by making more and more interface. Therefore, near the critical point for phase separation, where the two coexisting phases become indistinguishable, one expects the surface tension between the two phases to vanish. The addition of a third interfacially active component to a two-component mixture with a tendency to phase separate can also result in an effectively negative tension (related to the chemical potential of the third component) which can cause the two components to spontaneously form a dispersion with an amount of internal interface related to the amount of the interfacially active component. Such systems are described in Chapter 8. 

The film formation process is extremely complex, and there are a number of theories — or more accurately, schools of theories — to describe it. A major point of difference among them is the driving force for particle deformation surface tension of the polymer particles. Van der Waals attraction, polymer-water interfacial tension, capillary pressure at the air-water interface, or combinations of the above. These models of the mechanism of latex film formation are necessary in order to improve existing waterborne paints and to design the next generation. To improve the rate of film fonnation, for example, it is important to know if the main driving force for coalescence is located at the interface between polymer and water, between water and air, or between polymer particles. This location determines which surface tension or surface energies should be optimized. 

Gauthier and colleagues have pointed out that polymer-water interfacial tension and capillary pressure at the air-water interface are expressions of the same physical phenomenon and can be described by the Young and Laplace laws for surface energy [5]. The fact that there are two minimum film formation temperatures, one wet and one "dry," may be an indication that the receding polymer-water interface and evaporating interstitial water are both driving the film formation (see Section 3.4). 

Before leaving this topic of various optimum conditions, it should be pointed out that these have been derived in terms of (the surface free energy of the liquid, in case of adhesives, it signifies the surface free energy of the liquid adhesive) but Dyckerhoff etal found that it is not the surface free energy of the liquid adhesive which is important, rather the surface free energy of the adhesive in the solid form decides the optimum adhesion. They concluded that for minimum interfacial tension, the surface free energy of the hardened adhesive is equal to that of the adherend. 

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