Intermolecular force

Main types and characteristics

• London (dispersion) forces: Universal, arising from instantaneous charge fluctuations; important for nonpolar substances and increase with molecular size.
• Dipole–dipole interactions: Occur between permanent molecular dipoles and favour specific orientations when molecules approach.
• Hydrogen bonding: A special, directional interaction involving hydrogen bound to electronegative atoms (often O, N, or F); crucial in water structure and biomolecules.
• Ion–dipole and electrostatic interactions: Stronger, longer‑range attractions between charged species and polar molecules; central to salts in solution.
• Hydrophobic effect (entropic): Not a simple pairwise force but an emergent tendency for nonpolar groups to aggregate in water, driven largely by solvent organization.

These interactions differ in strength and spatial decay: electrostatic forces persist over longer distances than dispersion forces, which fall off rapidly as separation increases. Typical interaction energies are far smaller than covalent bond energies, but collectively they can produce substantial stability.

History, models and measurement

Concepts of intermolecular attraction were formalized in the 19th century and later modelled by potentials such as the Lennard‑Jones form and van der Waals descriptions. Experimental evidence comes from calorimetry, spectroscopy, X‑ray and neutron scattering, and single‑molecule probes like atomic force microscopy; these tools reveal how noncovalent contacts control packing, dynamics and energetics.

Practical consequences are wide: intermolecular forces set melting and boiling points, dictate solubility and miscibility, create surface tension and viscosity, and determine biomolecular folding, enzyme specificity and nucleic acid base pairing. Engineers and chemists exploit them in formulation, polymer design, drug binding and self‑assembly.

It is essential to distinguish intermolecular from intramolecular interactions: the latter hold atoms together within a molecule, while the former act between separate entities. Understanding both the qualitative types and quantitative models of these forces remains central to physical chemistry and molecular science.

For further reading see sources on physics, chemistry, and biology that cover intermolecular interactions in different contexts.