London forces interfacial tension

The interfacial tension between two phases is subject to the resultant force field made up of components arising from attractive forces in the bulk of each phase and the forces, usually the London dispersion forces, operating accross the interface itself (Fowkes, 1965 Adamson 1990) (see Appendix, this Chapter). 

EXAMPLE 6.5 Estimation of Interfacial Tensions Using the Girifalco-Good-Fowkes Equation. The following are the interfacial tensions for the various two-phase surfaces formed by n-octane (O), water (W), and mercury (Hg) for n-octane-water, y = 50.8 mJ m 2 for n-octane-mercury, y = 375 mJ m 2 and for water-mercury, y = 426 mJ m 2. Assuming that only London forces operate between molecules of the hydrocarbon, use Equation (100) to estimate y d for water and mercury. Do the values thus obtained make sense Take y values from Table 6.1 for the interfaces with air of these liquids. 

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... 

L) values for water and mercury have been determined by measuring the interfacial tension of these liquids with a number of liquid-saturated hydrocarbons. The inteimolecular attraction in the liquid hydrocarbons is entirely due to London-van der Waals dispersion forces for all practical purposes. Yjd was derived from contact angle measurements. 

Combining the Fowkes (22) equation for the Interfacial tension, Ya between two phases with Young s equation for the contact angTe, 6, of a liquid, l, and a solid, s, when only London forces operate across the Interface, a relationship Is obtained between the equilibrium contact angle, 9, and the various tensions ... 

Debye and Keesom forces together with London Dispersion forces are known coiiec-tively as van der Waals forces. See Lifshitz-van der Waals forces for a further discussion. They play a significant role in the Adsorption theory of adhesion and in surface phenomena such as Contact angles and interfacial tension. 

The use of van der Waals or dispersion forces to describe cohesive and adhesive phenomena is ubiquitous. As discussed above, Fowkes has shown that a substantial amount of the interfacial tension is due to these forces.The origin of these forces is the instantaneous dipole formed when the electrons in an atom, or molecule, are asymmetrically distributed around the nucleus or nuclei. London derived an expression for the interparticle attraction between two such instantaneous dipoles, shown in Eq. (92),... 

Input from the intermolecular forces is useful in numerous direct or indirect applications in colloid and surface science. For example, the geometric mean mle typically used for the combining mle of the energy parameter in equations of state and in theories for the interfacial tension has its origin in the geometric mean mle for the intermolecular potential, as derived from the dispersion (London) forces. Concepts from intermolecular forces wUl be used in, for example, theories for interfacial tensions and in the study of colloid stability. 

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