Effects of a Nonvacuum Medium

The equation for the total van der Waals interaction between two atoms or molecules [Eq. (4.40)] includes a factor for corrections due to changes in the dielectric characteristics of an intervening medium other than vacuum. That aspect of the theory can be of great importance both quantitatively and qualitatively and has significant ramifications in practical systems. A full discussion of the theoretical aspects of the effects of medium on van der Waals interactions is beyond the scope of this book, but the reader is referred to the work by Israelachvili for further enlightenment. From a practical standpoint, however, several important points arise from an analysis of the dispersion force equation for media of differing dielectric constants. The relevant points include the following  

for interactions in a vacuum, the ionization potential I (or hi for most materials is much greater than kT for v 0, the dispersion contribution to the total interaction is usually greater than the dipolar contributions for r = 0. This agrees with the results given in Table 4.4. 

The magnitudes of the van der Waals interactions are greatly reduced in the presence of a medium other than vacuum. For example, in the case of two nonpolar molecules, the magnitude of their interaction in vacuum may be reduced by an order of magnitude in the presence of an intervening medium. 

Dispersion force contributions in a medium other than vacuum may be either attractive or repulsive, depending on the relative ionization potentials of the materials involved. 

In some cases where the interaction between two molecules in a solvent is very small, such as the lower molecular weight alkanes in water, the interaction becomes dominated, not by the dispersion force, but by an entropic term of the form 

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