Interfacial tension macroscopic determination

Interfacial Tension. The interfacial energy a between a crystal and an aqueous solution cannot (at least in general) be measured by macroscopic methods. But it may be deduced from homogeneous nuclea-tion data (20-24). For the purpose of determining the edge energy Y = a CT one may either take the individual value determined on the actual substance (if it is determined) or use the general correlation with the solubility cs, expressed for instance by (10,18)... 

Basically, all the methods for measuring interfacial tensions described so far have in common that the Helmholtz energy for extending an interface is determined. Upon this extension, the interfacial tension should not vary, otherwise the quantity y would become ill-defined. One of the changes that might be incurred could result from strong curving of the interface. In the present chapter this issue was avoided because we have only considered macroscopic interfaces with radii of curvatures above 0(10-100 nm). Already in sec. 1.2.23c we showed that y is then still independent of curvature. 

Perhaps the most striking property of a microemulsion in equilibrium with an excess phase is the very low interfacial tension between the macroscopic phases. In the case where the microemulsion coexists simultaneously with a water-rich and an oil-rich excess phase, the interfacial tension between the latter two phases becomes ultra-low [70,71 ]. This striking phenomenon is related to the formation and properties of the amphiphilic film within the microemulsion. Within this internal amphiphilic film the surfactant molecules optimise the area occupied until lateral interaction and screening of the direct water-oil contact is minimised [2, 42, 72]. Needless to say that low interfacial tensions play a major role in the use of micro emulsions in technical applications [73] as, e.g. in enhanced oil recovery (see Section 10.2 in Chapter 10) and washing processes (see Section 10.3 in Chapter 10). Suitable methods to measure interfacial tensions as low as 10 3 mN m 1 are the sessile or pendent drop technique [74]. Ultra-low interfacial tensions (as low as 10 r> mN m-1) can be determined with the surface light scattering [75] and the spinning drop technique [76]. 

A macroscopic liquid film between a solid and air is subject to the sum of the interfacial tensions 7 + 75/.- What is the situation in the case of a thin film in which the corrective term P e) plays a role To determine the corresponding tension 7/i/m(e), we will consider the change in energy when the surface area of the film varies by an amount dA. But contrary to what we did in the previous case, we will this time implement the transformation keeping the number of molecules constant, that is to say, at constant volume Ae ... 

Surface tension and contact angle phenomena play a major role in many practical things in life. Whether a liquid will spread on a surface or will break up into small droplets depends on the above properties of interfaces and determines well-known operations such as detergency and coating processes and others that are, perhaps, not so well known, for example, preparation of thin films for resist lithography in microelectronic applications. The challenge for the colloid scientist is to relate the macroscopic effects to the interfacial properties of the materials involved and to learn how to manipulate the latter to achieve the desired effects. Vignette VI provides an example. 

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