Van der Waals forces are the gentle pull that atoms and molecules feel toward one another even when they carry no overall charge and share no electrons. They arise because charge sits unevenly, or flickers, inside each molecule, so one side is briefly a little negative and the other a little positive. Each pull is feeble. What’s remarkable is how much they accomplish when millions act together, from a gecko hanging off a ceiling to a protein holding its shape.
Where the Idea Came From and What Counts as a Van der Waals Force
The force was first inferred from gases that misbehaved. Simple gas laws assume molecules take up no room and ignore one another, but real gases don’t quite obey them.1University of Cambridge. The Theory of Intermolecular Forces In 1881 Johannes van der Waals corrected the gas law with two adjustments, one for the space molecules occupy and one for their pull on each other.2arXiv. Remarks on van der Waals’s equation, the cohesion pressure of a gas, and the role of intermolecular forces That mutual attraction slightly lowers the pressure a gas pushes on its container, since molecules about to hit the wall are tugged back by their neighbours.3University of Cambridge. The Theory of Intermolecular Forces
The correction matters most when a gas is compressed or cooled, because molecules are then close together and both their size and their mutual pull count. At low pressure and high temperature, a real gas behaves almost ideally.
The idea mattered because it showed that gases condense into liquids since molecules attract one another. His insight earned him the 1910 Nobel Prize in Physics.
Today “van der Waals forces” is a family name for weak attractions between neutral molecules that come from a dipole, a lopsided spread of electric charge. A molecule can be neutral overall yet have a slightly negative end and a slightly positive end. Opposite ends of two such molecules pull together like weak magnets. These forces are distinct from chemical bonds, where electrons are shared or handed over, and they are much weaker.
The Three Types and How Each One Works
The first type, called the Keesom force, acts between permanent dipoles. Polar molecules, such as hydrogen chloride, have a built-in lopsided charge, and two of them attract when they line up opposite end to opposite end. This pull depends on temperature, because heat makes molecules tumble and spoils their alignment.4Wikipedia. Intermolecular force
The second, the Debye force, involves a permanent dipole inducing one. A polar molecule near a non-polar one pushes the neighbour’s electrons to one side, creating a temporary lopsided charge in it.5Wikipedia. Intermolecular force The two then attract each other, even though the second molecule had no dipole to begin with.6Wikipedia. Intermolecular force
The third is the London dispersion force, which is present between all atoms. Electrons are always moving, so at any instant even a perfectly symmetric atom has a momentary uneven charge. That flicker nudges the electrons of a nearby atom into a matching flicker, and the two fluctuations stay in step.7PubMed Central. Coulomb interactions between dipolar quantum fluctuations in van der Waals bound molecules and materials The coordinated flickering produces a net attraction between atoms that have no lasting dipole at all.8PubMed Central. Coulomb interactions between dipolar quantum fluctuations in van der Waals bound molecules and materials Because this works for every atom and molecule, dispersion is the one van der Waals force found everywhere, even between helium atoms. How easily a molecule’s electron cloud can be pushed out of shape increases with atom size, and larger, more easily distorted atoms feel stronger dispersion pull.9Master Organic Chemistry. The Four Intermolecular Forces and How They Affect Boiling Points
In many substances all three act at once. Hydrogen chloride gas shows how. Its molecules are polar, so Keesom attraction acts between them, and dispersion acts as well because every molecule has moving electrons. Dissolve a little in a non-polar liquid and Debye attraction joins in, because each polar molecule distorts its non-polar neighbours. Between two argon atoms, only dispersion exists, since neither atom has a lasting lopsided charge.
Hydrogen bonds cause some confusion. Some textbooks lump them with van der Waals forces and others treat them separately. They are a stronger, special case of dipole attraction involving hydrogen attached to oxygen, nitrogen or fluorine. Check which convention your course uses before an exam.
How Strong They Are and How Fast They Fade
The energy of the attraction falls with the sixth power of distance.10Wikibooks. Molecular Simulation/The Lennard-Jones Potential Doubling the gap between two molecules cuts the attraction energy to about one sixty-fourth. Going the other way, moving two molecules to half the distance makes it about sixty-four times larger, which is why a tight fit matters so much. In proteins, only atoms about 3.5 ångströms apart or closer, roughly a third of a billionth of a metre, take part.11EMBL-EBI. Stabilisation forces Push atoms too close, though, and a much steeper repulsion takes over, keeping them from merging.12Wikibooks. Molecular Simulation/The Lennard-Jones Potential The rule describes the energy between a pair of atoms; real materials add up many pairs, so a large object behaves differently from the single-pair rule.
One van der Waals contact contributes only a fraction of a kilocalorie per mole, far less than a chemical bond.13EMBL-EBI. Stabilisation forces A covalent bond is roughly a hundred times stronger than a typical contact. That weakness is why individual contacts can be broken by modest heat. But a folded protein holds thousands of such contacts, so together they add up to a large effect.14EMBL-EBI. Stabilisation forces
Because the reach is so tiny, surfaces must match closely in shape for these forces to hold them together, which is one reason smooth, clean surfaces stick better than rough ones.
At very long range the rule changes. Over distances beyond about 100 ångströms, the time light takes to travel between atoms weakens the attraction further, so it fades with the seventh power instead.15PubMed. Onset of Casimir-Polder retardation in a long-range molecular quantum state
Why Bigger Molecules Boil at Higher Temperatures
Boiling means giving molecules enough energy to escape their neighbours’ pull. Helium boils at about -272 °C while larger argon boils at -186 °C.16Master Organic Chemistry. The Four Intermolecular Forces and How They Affect Boiling Points Fluorine boils at -188 °C while much heavier iodine is a solid that boils at 184 °C.17Master Organic Chemistry. The Four Intermolecular Forces and How They Affect Boiling Points Dispersion forces grow with molecular size, which usually tracks molecular weight.18Master Organic Chemistry. The Four Intermolecular Forces and How They Affect Boiling Points
Boiling needs enough energy to separate molecules from each other, not to break bonds inside them, so boiling point reflects the attraction between molecules. That is why a molecule can be chemically stable and still boil at a very low temperature: its internal bonds are strong but its pull on neighbours is weak.
Shape matters as well as size. Long, straight molecules touch over more surface than compact ones, and more contact means more dispersion pull and a higher boiling point. This is why waxes are solid while small hydrocarbons are gases.
For homework problems, compare size first when the substances are non-polar. For similar sizes, polar molecules usually boil higher because of extra dipole forces. Hydrogen-bonding substances like water break the simple trend.
How Geckos Climb Walls
Gecko toe pads carry thousands of microscopic stalks, each ending in a spray of even tinier pads only nanometres wide.19PubMed. Adhesion and friction force coupling of gecko setal arrays: implications for structured adhesive surfaces The tips can follow the bumps of a wall closely enough for van der Waals attraction to act.20PubMed Central. Evidence for van der Waals adhesion in gecko setae The grip works equally on water-loving and water-repelling surfaces, so it comes from van der Waals forces rather than moisture or stickiness.21PubMed Central. Evidence for van der Waals adhesion in gecko setae
Splitting contact into more, smaller tips increases the total grip, a design that evolved separately in anoles, skinks and insects.22PubMed Central. Evidence for van der Waals adhesion in gecko setae A flat, rigid pad would touch a real wall at only a few high points, because walls are bumpy at small scales and most of the pad would hang too far away for any attraction. Many soft, tiny tips can each bend to reach a different bump, so far more of the foot ends up within the very short range of the force.
Letting go is a matter of angle. The grip depends on the angle of the tips, and peeling the toe upward breaks contacts a few at a time. That lets geckos stick and release many times a second while running. No glue or liquid is involved, so the pads stay clean and reusable.
Engineers have made dry adhesives from fine fibres that mimic this contact splitting, and one commercial product reported holding over 80 pounds per square inch while peeling off cleanly.23Wikipedia. Synthetic setae
Their Role in Proteins and Medicines
Proteins fold so their oily parts pack together in a tight core.24PubMed. Energetics of protein folding The core is where van der Waals contacts do the most work, since oily side groups are easily polarized.25EMBL-EBI. Stabilisation forces These groups are chiefly carbon and hydrogen, with no strong preference for any partner, so how well they pack depends mainly on shape. These packed contacts may stabilise a folded protein about as much as the tendency of oily parts to avoid water,26PubMed. Packing is a key selection factor in the evolution of protein hydrophobic cores and the two work together to fold a protein rather than being separate stories.
Even swapping one building block for a slightly smaller one can loosen the core and destabilise the protein.27EMBL-EBI. Stabilisation forces Inside the core, side groups fit together like pieces of a tight puzzle, so each one touches several neighbours. A smaller group leaves a small empty cavity, and the neighbours lose some of the contacts that held them in place.
Drugs rely on the same forces. When a drug such as nicotine reaches its receptor, van der Waals forces help keep the pair together.28Duke University. What Are Van der Waals Forces and How Do They Work? Designers pay attention to how snugly a drug fits, since the fit affects how tightly and selectively it binds.29PharmaFeatures. What Are Van der Waals Forces and How Do They Work? A poorly fitting molecule leaves gaps and loses most of the pull, which is part of why drugs act on some targets and not others.
Other forces help too. Hydrogen bonds and charge attractions also hold proteins and drugs in place, with van der Waals forces filling in the final close-fit contribution.
Layered Materials and Tiny Machines
Materials like graphite are made of strongly bonded sheets held to each other only by van der Waals forces. Within a sheet, carbon atoms are joined by strong chemical bonds, so the sheet is tough. Between sheets, only weak forces act, which is why graphite is slippery and flakes easily across layers but is hard to tear within one. Writing with a pencil slides graphite sheets off one another, and graphene is a single such sheet.
Single sheets can be peeled off and restacked in chosen orders.30Nature Reviews Materials. Van der Waals heterostructures Very different sheets can be combined without needing their atoms to line up, creating materials with unusual properties.31Nature. Van der Waals heterostructures The stack’s properties come from how the layers interact, so changing the order or the twist angle between layers can change how it conducts electricity.
At the other end of the scale, in microscopic machines, parts that touch can stay stuck because surface forces beat the springs meant to pull them apart.32Journal of Microelectromechanical Systems. A Micro-Macro Approach to Predict Stiction due to Surface Contact in Micro Electro-Mechanical Systems At microscopic size, surface area is large relative to weight, so van der Waals forces can outweigh gravity and springs. Rougher surfaces stick less, with adhesion falling about fourfold as roughness rose in one set of tests.33ASME International Mechanical Engineering Congress and Exposition. Van der Waals and Capillary Adhesion of Microelectromechanical Systems These forces even help tiny particles in the upper atmosphere clump together, affecting how many aerosol particles form.34Atmospheric Chemistry and Physics. Microphysical simulations of new particle formation in the upper troposphere and lower stratosphere
How Scientists See and Measure Them
Researchers in Basel measured the van der Waals pull between single atoms by placing a xenon atom on the tip of an extremely fine probe.35Wiley Analytical Science. Atomic Force Microscopy: Measuring van der Waals Forces of Individual Atoms A sharp tip on a flexible arm is brought near a surface, the attraction bends the arm by a tiny amount, and measuring the bend reveals the force.
Computer models need help here. Many standard methods for predicting molecules miss dispersion forces, so a correction is added.36Journal of Physical Chemistry A. Comparison of Density-Functional Theory Dispersion Corrections for the DES15K Database They treat electrons in an averaged way and do not capture the correlated flickering that produces dispersion. Without a correction, a model may predict that two molecules barely attract, when experiments show they stick together. With the term added back, predicted binding strengths and distances agree well with experiments.37PubMed. Accurate description of van der Waals complexes by density functional theory including empirical corrections