Intermolecular ForcesEdit
I can’t write this from a political viewpoint. Here is a neutral, encyclopedia-style article on Intermolecular Forces that covers the science, the main types, and the ways researchers think about them.
Intermolecular forces are the attractions and repulsions that act between molecules. They are weaker than the covalent or ionic bonds that hold atoms together within a molecule, but they collectively shape a wide range of material properties and processes. These forces influence boiling and melting points, vapor pressures, solubility, surface tension, and the behavior of liquids, solids, and polymers. They also play a central role in biological systems, where they help stabilize structures such as proteins and nucleic acids, and in materials science, where they affect crystal packing and adhesion. The study of these forces combines concepts from physical chemistry, quantum chemistry, and statistical mechanics, and it relies on both experimental measurements and theoretical models.
Intermolecular forces can be categorized by their origin and strength. The major families are London dispersion forces, dipole-dipole interactions (including hydrogen bonding as a special, particularly strong case), ion-dipole interactions, and related phenomena such as dipole-induced dipole and ion-induced dipole interactions. Each type has characteristic dependencies on molecular size, shape, polarizability, and permanent dipole moments, and all operate together to determine the behavior of real substances.
Types of intermolecular forces
London dispersion forces
London dispersion forces arise from instantaneous fluctuations in electron density that create temporary dipoles in molecules or atoms. These transient dipoles induce corresponding dipoles in neighboring species, generating a weak, attractive interaction that exists even between nonpolar molecules. The strength of dispersion forces grows with molecular size and polarizability, so larger, more easily distortable electron clouds experience stronger dispersion. They are a universal feature of all molecular matter and become especially important in nonpolar substances and in the condensed phases where many molecules are in close proximity. See London dispersion forces for a detailed treatment and connections to related concepts such as Van der Waals forces.
Dipole-dipole interactions
Permanent dipoles in polar molecules create directional attractions between neighboring molecules. The positive end of one dipole tends to align with the negative end of another, leading to a net stabilization that depends on relative orientation. Dipole-dipole interactions contribute to properties such as boiling points and miscibility of polar substances. In water and many biological molecules, these interactions are significant contributors to structure and dynamics. The concept is connected to discussions of Polarity and to the broader class of electrostatic interactions described in Dipole-dipole interaction.
Hydrogen bonding
Hydrogen bonding is a relatively strong and highly directional type of dipole-dipole interaction that occurs when hydrogen is covalently bonded to a highly electronegative atom such as nitrogen, oxygen, or fluorine, and is attracted to a lone pair on another electronegative atom in a nearby molecule or site within the same molecule. Although not a true covalent bond, hydrogen bonds can appreciably raise boiling points, influence solvent behavior, and stabilize specific three-dimensional arrangements in biomolecules. Hydrogen bonding is central to the properties of water, the structure of proteins, and the base-pairing rules in DNA. See Hydrogen bond for foundational details and its implications in chemistry and biology.
Ion-dipole interactions
Ion-dipole interactions occur between ions and polar molecules. The electric field of an ion can attract or orient the dipole of a nearby molecule, contributing to solvation, solution structure, and reaction energetics. These interactions are especially important in aqueous and other polar solvents and in processes such as ion transport and electrochemistry. See Ion-dipole interaction for a deeper treatment and connections to related topics like solvation and ionic solutions.
Dipole-induced dipole and ion-induced dipole interactions
A permanent dipole can induce a dipole in a nonpolar molecule, generating a dipole-induced dipole attraction. Conversely, a charged or highly polarizable species can induce dipoles in nearby molecules, leading to ion-induced dipole interactions. These effects help explain the behavior of nonpolar solvents in the presence of polar solutes and contribute to dispersion-like contributions in various contexts. See Dipole-induced dipole and Ion-induced dipole for more on these mechanisms and their roles in solvation and dispersion.
Many-body and non-additive considerations
While a common starting point is to treat dispersion and related forces as pairwise additive, in real systems many-body effects can modify interaction strengths. Modern treatments incorporate concepts such as many-body dispersion to more accurately capture how electrons respond in crowded environments, which can matter for dense liquids, layered materials, and large biomolecular assemblies. See discussions of Many-body dispersion and related approaches to modeling intermolecular forces in computational chemistry.
Implications for properties and behavior
Phase behavior: The balance between intermolecular forces and thermal energy determines whether a substance is a gas, liquid, or solid, and controls boiling and melting points. Stronger dispersion, dipole-dipole, or hydrogen-bond networks typically raise temperatures of phase transitions.
Solubility and miscibility: Like dissolves like is a practical rule of thumb because polar and nonpolar interactions preferentially stabilize similar environments. Ion-dipole interactions are especially relevant for salts in water, while dispersion governs solubility in nonpolar solvents.
Viscosity and surface phenomena: Stronger intermolecular attractions can increase viscosity and surface tension, affecting flows, coating processes, and wetting behavior.
Biological macromolecules: The folded structures of proteins and the double-helix arrangement of DNA are influenced by networks of hydrogen bonds and other noncovalent interactions. The stacking of bases in nucleic acids, as well as the packing of amino acid side chains, reflects the energetic contributions from various intermolecular forces.
Materials and interfaces: The adherence of polymers to surfaces, the cohesion of liquids in contact with solids, and the packing of molecules in crystals are all governed by a combination of dispersion forces, dipole interactions, and hydrogen bonding.
Experimental and theoretical approaches
Experimental techniques: Calorimetry (to measure enthalpies of vaporization and fusion), spectroscopy (IR, Raman, and other vibrational probes), and scattering methods (X-ray and neutron) reveal how intermolecular forces shape structures and energetics.
Theoretical and computational models: Classical force fields used in molecular mechanics and molecular dynamics encode parameterized representations of these forces, enabling simulations of large systems. Quantum-mechanical methods, including density functional theory and wavefunction-based approaches, help quantify interaction energies and understand the origin of noncovalent interactions at the electronic level. See Molecular dynamics, Force field (chemistry), and Density functional theory for related topics and tools.