Glass surfaces Waals interactions

FIG. 2 Interaction forces between glass surfaces upon compression in ethanol-cyclohexane mixtures. The dashed and solid lines represent the van der Waals force calculated using the nonretarded Hamarker constants of 3 X 10 1 for glass/cyclohexane/glass and 6 X 10 J for glass/ethanol glass, respectively. 

The Hamaker constant A can, in principle, be determined from the C6 coefficient characterizing the strength of the van der Waals interaction between two molecules in vacuum. In practice, however, the value for A is also influenced by the dielectric properties of the interstitial medium, as well as the roughness of the surface of the spheres. Reliable estimates from theory are therefore difficult to make, and unfortunately it also proves difficult to directly determine A from experiment. So, establishing a value for A remains the main difficulty in the numerical studies of the effect of cohesive forces, where the value for glass particles is assumed to be somewhere in the range of 10 21 joule. 

Experimentally, a sample of the nanofiber mat sealed in a chamber is maintained at a constant temperature well below its glass transition temperature and the chamber is evacuated. An adsorbate gas (usually N2 or a N2/He mixture, although other gases can be used) is admitted into the chamber in several increments until the entire surface of the porous mat is covered by a mono-layer of gas. To allow sufficient gas molecules to be adsorbed by the weak van der Waals interactions, however, the sample has to be cooled, normally to the boiling point of the gas (for N2 gas to 77.35 K). 

Figure 20.7. A van der Waals interaction plot measured by using the MASIF between a glass surface and a cellulose-coated glass surface. The fit is only valid at larger separations below about 10 nm, the surfaces are in an unstable jump and the data no longer reflect the true interaction (105). The Hamaker constant was not a fitting parameter but was calculated by Lennart Bergstrom (Institute for Surface Chemistry, Stockholm) from the complete Lifshitz theory...

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