Keesom

Keesom interaction, or Keesom force, is a type of intermolecular force that arises between permanent dipoles in polar molecules. It is named after the Dutch physicist Willem Hendrik Keesom, who studied the orientational dependence of dipole–dipole interactions.

Mechanism and characteristics: The force results from electrostatic interactions between the permanent dipole moments of two molecules. Because polar molecules rotate thermally, the average interaction energy over all orientations at a given temperature is attractive and becomes weaker with increasing temperature. In practical terms, the Keesom interaction scales approximately as the inverse sixth power of the separation distance and is inversely proportional to temperature, reflecting the orientational averaging that occurs in liquids.

Relation to other van der Waals forces: The Keesom interaction is one category within van der Waals forces. It specifically involves permanent dipoles. Other categories include Debye forces, which arise from a permanent dipole and an induced dipole, and London dispersion forces, which result from instantaneous induced dipoles (induced dipole–induced dipole interactions). Together, these forces help explain the behavior of non-bonded molecules in condensed phases.

Significance: Keesom forces contribute to the physical properties of polar substances, such as higher boiling points and distinctive liquid structure, especially in the absence of strong hydrogen bonding. They are weaker than covalent or hydrogen bonds but are an important component of molecular cohesion in polar liquids.