An electrostatic interaction between molecules that possess permanent electric dipole moments, such as water or hydrogen chloride. The force arises from the tendency of these dipoles to align head-to-tail, resulting in a net attraction. This interaction is temperature-dependent, as thermal motion disrupts the alignment. The orientation-averaged potential energy varies as [latex]V \propto -\frac{1}{r^6 k_B T}[/latex].
The Keesom force, or permanent dipole-dipole interaction, is one of the three components of Van der Waals forces, specifically describing the interaction between two polar molecules. A polar molecule has a permanent separation of positive and negative charge, creating a permanent electric dipole moment. When two such molecules approach each other, their dipoles interact electrostatically. The lowest energy configuration is an attractive, head-to-tail alignment. However, molecules in a fluid are in constant thermal motion (rotating and translating), which tends to randomize their orientations.
Willem Keesom’s contribution was to perform a statistical mechanical average over all possible orientations, weighted by their Boltzmann factor. The result showed that, on average, attractive orientations are slightly more probable than repulsive ones, leading to a net attractive force. The strength of this interaction is uniquely dependent on temperature; as temperature increases, thermal energy more effectively disrupts the alignment, and the Keesom force weakens. This is reflected in the [latex]1/T[/latex] term in the potential energy equation. This force is crucial for understanding the behavior of many common substances, particularly water, whose properties are dominated by strong dipole-dipole interactions known as hydrogen bonds.
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