Surface energy and Hamaker constant

In order to calculate the surface energy of molecular crystals, we imagine the following experiment A crystal is cleaved in two parts which are separated by an infinite distance (Fig. 6.3). The work required per unit area is w = A/// 2n/92, where D0 is the distance between two atoms. Upon cleavage, two fresh surfaces are formed. With the surface energy 7,9 the work required is 275. Equating the results leads to 

For Teflon we calculate a surface energy of 16-28 mJ/m2 [127] using an atomic spacing of 1.7 A and a Hamaker constant of 3.4-6.0x 10-20 J. 

However, such calculation should be seen with caution. The application of a continuum view of matter (manifested by the use of a Hamaker constant) at distances at which a molecular graininess is expected to become important is certainly optimistic. Furthermore, rearrangement of surface atoms upon contact or separation might occur. Nevertheless, it was found that in many cases reasonable estimates of the surface energy can be obtained when an interatomic distance of 1.7 A is used. 

A frequently used method to measure the Hamaker constant of solids is based on the idea that the interaction of a solid surface with nonpolar liquids will mainly occur through the London dispersion interaction. One can therefore define a dispersive surface tension 7° that contains only the dispersive contribution. The value of Ys can be determined from contact angle measurements with nonpolar liquids using the Fowkes equation [83] 

0 is the contact angle (see Section 5.3) and Yl is the surface tension of the nonpolar liquid. By plotting cos 0 versus l/ / for several nonpolar liquids, one obtains from the slope of a linear fit. The Hamaker constant Ai vi for material 1 interacting with itself over a gap in vacuum is 

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