London component

Next let us consider the situation in which a second condensed phase B adjoins the reference phase A. The new consideration in this case is the London component of attraction of the molecules in condensed phase B for the A molecules in the interface. This A-B attraction partially overcomes the A-A attraction that opposes the movement of an A molecule to any interface. As a consequence, there is a difference in the energy that must be expended to bring... 

It is argued that only the London component of intermolecular attractions operates across the interface to decrease the work required to bring a particle to the surface. Suppose we define as that fraction of the surface tension due to London forces. Next we use the sort of geometric mixing rule that was employed in Chapter 3, Section 3.4b, to estimate AES as follows ... 

The assignment of a geometric mean rather than an arithmetic mean or some other function of the two 7 terms is justified primarily on the basis of the successful use of this type of averaging in the theory of nonelectrolyte solubility. Only the London component of y is used since it is the part of y that crosses phase boundaries. 

Finally, we turn our attention to the third contribution to van der Waals attraction, London (or dispersion) forces between a pair of induced dipoles. It will be noted that (at least) one permanent dipole is needed for the preceding sources of attraction to be operative. No such restriction is present for the London component. Therefore this latter quantity is present between molecules of all substances, whether or not they have a permanent dipole. These are the same forces that we considered in Chapter 6 when we discussed the Girifalco-Good-Fowkes equation. 

We focus on the three-body forces in Fig. 5.21 where the SCF portion is compared with its components extracted before (Heitler-London, HL) and after (deformation, def) the wave functions of the monomers are perturbed by one another. It is apparent that the latter SCF-def forces are largely responsible for the anisotropy of the SCF three-body terms. The Heitler-London component is weaker, and resembles a mirror of SCF-def, with a maximum where SCF-def contains a minimum. The extremum at 20° corresponds to a configuration... 

Assuming that the work of adhesion between a saturated hydrocarbon and a second phase Is equal to the free energy of desorption per mole of CHj groups, Dorris and Gray (5 ) proposed the equation for the estimation of the London component of the surface free energy of the adsorbent ... 

Table III. Equilibrium Spreading Pressures on n-Nonane on Carbon Fibers and the London Component of the Carbon Fiber Surface Free...

London component of surface free energy of liquid (mN/m). 

London Component of the Surface Energy of Heated Treated Silicas. Surface energy is usually considered as the sum of two components the London component (y ), steming from London forces, and the specific component (y p), originating from all other types of forces (polar, H-bonding, metallic, etc). Two methods are commonly used for the measurement of surface energies wettability and adsorption techniques. 

Figure 2. Variation of the London Component ( y ) of precipitated (P) and fumed (A 200) silicas upon heat treatment...

Table I London Component of the Surface Energy of Silica (PI and A130) Before and After Methyl (Cj) and Hexadecyl (Cjg) Esterification...

Figure 1. Variation of the London component measured at 80°C (O), and of the number of silanol groups ( ) upon heat treatment temperarure (HTT) of a fumed silica sample.

Figure 2. Variation of the London component (7 ) of AI2O3 (1) and boehmite (2), measured at 60°C, after heat treatment at different temperatures.

London Component of the Surface Free Energy of Heat-Treated Silicas. Figure 1 shows the evolution of 7sd for the different types of silicas versus heat-treatment temperature. The origin of the sample as well as the thermal treatments applied are important in determining 7sd. 

London Component of the Surface Free Energy of Heat-Treated Silicas. 381... 

First Polymer Series. The first series of polymers was investigated, in order to limit the interaction between an AFM probe and polymer surfaces to only the dispersion (London) component of the Van der Waals force. Adhesional forces were measured between a S10,t probe and a set of nonpolar polymers that provided a range of refractive indexes (as measured) polystyrene (1.582), isotactic polypropylene (1.501), poly(vinylidene fluoride) (1.407), and poly(tetrafluoroethylene-co-hexafluoropropylene) (1.348). The histograms of the pull-off forces, measured with a SiOx probe, are shown in Figure 2 and tabulated with the calculated values for adhesion energy in Table 1. 

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