Dipolar Interactions London, Keesom, and Debye Forces

1 Dipolar Interactions London, Keesom, and Debye Forces 

Van der Waals forces arise from dipole-dipole interactions. In a dipolar molecule, there is a separation of positive from negative charge. For example, if the molecule contains charges -Fq and —q separated by a distance vector r, then the molecule has a dipole moment of u = rq. Dipolar moments are often measured in units of the Debye D, where 1 D = 3.336 X 10 coulomb-meter. While alkanes have no permanent dipole moments, 

The electric field created by a dipole (ui) either attracts or repels a neighboring dipole (U2) depending on their relative orientation—that is, the sign of Ui U2. In a liquid, where the molecules can rotate freely, the dipoles tend to align themselves with respect to each other to maximize attractive dipolar interactions and minimize repulsive ones. Of course, in an isotropic liquid, no permanent molecular orientations result from these interactions, only correlations between the orientation of one dipole and that of its neighbors. Because of these correlations, there is a net attractive force among molecules. This is the attractive force that holds most liquids together. 

Even molecules such as alkanes (which are short oligomeric versions of polyethylenes see Fig. 2-7) that possess no permanent dipole have fluctuating dipoles because of the motion of the electronic cloud around each molecule. Because the dipoles communicate with each other through their electric fields, they are able to correlate their fluctuating magnitudes and orientations to produce a net attractive force, called the London force. Thus, the electric field E produced by one temporary dipole can induce a dipole moment Uind in another molecule, where Ui d = aoE and ao is the molecule s polarizability. Since these fluctuating 

Liquids containing permanent dipoles have additional attractive interactions called Keesnm forces, which are caused by the tendency of the permanent dipoles to align anti-parallel with each other. Finally, there are also Debye or induction interactions between permanent dipoles and fluctuating ones. The dispersion interactions are the most important of the three types, however, because they occur in all materials, and are usually stronger than the Keesom and Debye interactions, when the latter are present at all. 

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