Keesomovy

Keesomovy síly, also known as Keesom forces or Keesom interaction, are orientation-dependent intermolecular forces between permanent dipoles in polar molecules. The term is named after Dutch physicist Willem Hendrik Keesom, who studied dielectric properties of gases and liquids and described how the thermal orientation of permanent dipoles contributes to attractive interactions between polar species. These forces are one component of van der Waals interactions, alongside Debye (dipole-induced dipole) and London (dispersion) forces.

Mechanism and characteristics: Keesom forces arise from electrostatic interactions between permanent dipole moments of two molecules. In a real system at finite temperature, molecular orientations are random, and averaging over these orientations yields an effective attraction that decays rapidly with distance, roughly as the sixth power of separation (1/r^6), and increases as temperature decreases. The strength scales with the magnitudes of the dipole moments of the interacting molecules and becomes more significant for molecules with larger dipoles.

Role and applications: These forces are especially important for polar liquids and gases, influencing properties such as boiling and melting points, phase behavior, and dielectric constants. They contribute to the cohesive energy of polar liquids and to the arrangement of molecules in condensed phases. Keesom forces are typically most relevant at separations of a few angstroms and lessen with rising temperature.

History and usage: The concept is widely discussed in physical chemistry and molecular physics. In Czech sources, the term Keesomovy síly is used to refer to these orientation-based dipole–dipole interactions; in English-language literature, the standard terms are Keesom forces or Keesom interaction.