Principles and Practice of Chromatography - Factors Affecting the Magnitude of the Distribution Coefficient (K) > Molecular Forces > Dispersion Forces > Page 22

(n) is the number of molecules per unit volume.

If (r) is the density of the medium and (M) is the molecular weight, then the number of molecules per unit volume is where (N) is Avogadro's number,

Thus,

where (P) is called the Molar Polarizability.

It is seen that the Molar Polarizability is proportional to , the molar volume; consequently dispersive forces (and thus "hydrophobic" or "lyophobic forces") will be related to the 'molar volume' of the interacting substances. A diagrammatic representation of dispersive interactions is shown in figure 4.

Figure 6 Dispersive Interactions

Dispersive interactions are not the result of a localized charge on any part of the molecule, but from a host of fluctuating, closely associated charges that, at any instant, can interact with instantaneous charges of an opposite kind situated on a neighboring molecule.