Interfacial tension intermolecular forces

One fascinating feature of the physical chemistry of surfaces is the direct influence of intermolecular forces on interfacial phenomena. The calculation of surface tension in section III-2B, for example, is based on the Lennard-Jones potential function illustrated in Fig. III-6. The wide use of this model potential is based in physical analysis of intermolecular forces that we summarize in this chapter. In this chapter, we briefly discuss the fundamental electromagnetic forces. The electrostatic forces between charged species are covered in Chapter V. 

Many of these features are interrelated. Finely divided soHds such as talc [14807-96-6] are excellent barriers to mechanical interlocking and interdiffusion. They also reduce the area of contact over which short-range intermolecular forces can interact. Because compatibiUty of different polymers is the exception rather than the rule, preformed sheets of a different polymer usually prevent interdiffusion and are an effective way of controlling adhesion, provided no new strong interfacial interactions are thereby introduced. Surface tension and thermodynamic work of adhesion are interrelated, as shown in equations 1, 2, and 3, and are a direct consequence of the intermolecular forces that also control adsorption and chemical reactivity. 

Fig. 8-9. Illustration of intermolecular forces as they affect interfacial tension.

At the interface between two liquids there is again an imbalance of intermolecular forces but of a lesser magnitude. Interfacial tensions usually lie between the individual surface tensions of the two liquids in question. 

The short-range intermolecular forces which are responsible for surface/interfacial tensions include van der Waals forces (in particular, London dispersion forces, which are universal) and may include... 

Consider the molecules in a liquid. As shown in Figure 3.1, for a liquid exposed to a gas the attractive van der Waals forces between molecules are felt equally by all molecules except those in the interfacial region. This imbalance pulls the latter molecules towards the interior of the liquid. The contracting force at the surface is known as the surface tension. Since the surface has a tendency to contract spontaneously in order to minimize the surface area, droplets of liquid and bubbles of gas tend to adopt a spherical shape this reduces the total surface free energy. For two immiscible liquids a similar situation applies, except that it may not be so immediately obvious how the interface will tend to curve. There will still be an imbalance of intermolecular forces resulting in an interfacial tension and the interface will adopt a configuration that minimizes the interfacial free energy. 

Fowkes FM. (1963). Additivity of intermolecular forces at interfaces. L Determination of the contribution to surface and interfacial tensions of dispersion forces in various liquids. / Phys Chem 67 2538-2541. 

Like surface tension, viscosity is a manifestation of the physical interaction between molecules. The uniquely shallow slope of the viscosity-temperature curve of PDMS (20) is due in part to low intermolecular forces. The interfacial viscosity of PDMS ( 10 xg/s) is the lowest known for a polymer film (9) and is also partly the result of low methyl-methyl interactions dominating polymer-polymer interactions in PDMS. 

The use of the ADR method may not always provide accurate vehicle compositions for a given solute since intermolecular forces are dependent on structural characteristics of the solvent and solute that are not expressed by It is possible, and perhaps desirable, to substitute other measures of cosolvent polarity, such as solubility parameter, surface or interfacial tension, etc., for e when blending solvents, although inaccuracies in vehicle predictions will generally continue to exist. 

The advantage of using interfacial tension as a measure of solute-solvent interactions is that it can be measured for substances whose intermolecular forces are quite different from each other. Thus, it is useful for estimating solubilities for systems that are highly irregular. In contrast, regular solution approaches are useful when solute and solvent polarities are similar and the interfacial tensions are immeasurable. 

The interfacial tension is obviously an important property of a liquid because it gives a direct indication of the magnitude of intermolecular forces. As a result of interfacial tension a liquid which is not in contact with another condensed phase, such as a water droplet in air, assumes the shape which has minimum area. It turns out that this shape is a sphere. As a result, there are no elliptical or square water droplets By maintaining a spherical shape, the area-to-volume ratio, and the number of molecules at the surface are their lowest possible values. One is not surprised by this fact on the basis of experience. 

The failure to describe the interfacial tension adequately in terms of the total surface tensions in interfaces involving materials with strong polar interactions led to the idea that the total free energy at a surface should be expressed as the smn of the contributions from the different types of intermolecular forces at the surface 19] ... 

The above discussion for a liquid-gas interface is also applicable to a liquid-liquid interface between two immiscible liquids with an interfacial tension acting at the interface. As before, there is an imbalance of intermolecular forces, although smaller. The magnitude of the interfacial tension usually lies between the surface tensions of each liquid. 

Surface tensions, interfacial tensions, and contact angles can be used as laboratory tools for the evaluation of the various intermolecular forces that determine cohesion in a single phase or adhesion between two dissimilar materials at an interface. By the use of these tools, considerable information about the magnitude of various intermolecular forces may become available. Basic understanding of means to determine interfacial forces has sprung from the principle of additivity of intermolecular forces at surfaces and interfaces [4, 6]. 

The logical next step in the process of extending the utility of theory to practical systems is to include polar molecular interactions. For this step, Fowkes suggested that the intermolecular forces contributing to surface and interfacial tensions, and subsequent phenomena such as wetting, could be broken down into independent and additive terms. For example, a polar molecule such as an ester would have two terms making up its surface tension—dispersion forces (d) and dipolar interactions (p)—so that... 

We note that the approach described above can be used for thin films and other nonuniform systems where density or composition varies rapidly with position at equilibrium (see Chapter 2). It contrasts sharply with the classical treatment of interfacial thermodynamics givrai in Section 3, which deals with overall properties of the interfacial region without considering density or concentration profiles or even the existence of molecules and intermolecular forces. An advantage of the earlier approach is that its results are independent of molecular properties. The principal disadvantage is that values of interfacial tension cannot be predicted. Hence the role of molecular theory is, as in other areas of thamodynamics, to provide information beyond that obtainable with the classical approach. 

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