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Adsorption can look like a list of definitions: physisorption, chemisorption, isotherms. It makes more sense once you meet the person who turned it into a physical picture. Irving Langmuir worked on light bulbs, and the model he built from that work still sits behind catalysis and coatings. This post explains what he found, works an isotherm problem, and ends with what you can borrow from his way of working.
Who was Irving Langmuir?
According to his Nobel biography, Langmuir was born in Brooklyn, New York, on 31 January 1881. He trained as a metallurgical engineer at Columbia University, graduating in 1903, then earned a PhD in physical chemistry under Walther Nernst at Göttingen in 1906. After teaching at the Stevens Institute of Technology until 1909, he joined the research laboratory of the General Electric Company in Schenectady, where he later became associate director.
The biography notes that his studies "were largely the outgrowth of studies of vacuum phenomena". His work on filaments in gases led to the gas-filled incandescent lamp and to the discovery of atomic hydrogen. He received the 1932 Nobel Prize in Chemistry "for his discoveries and investigations in surface chemistry", and died on 16 August 1957.
What did chemists think adsorption looked like before him?
The 1932 presentation speech describes the older view. Gas near a solid was pictured as a condensed layer whose density fell off gradually with distance, rather like Earth's atmosphere thinning with height.
Langmuir proposed something much sharper. The adsorbed layer, at least in ideal cases, is a single layer of molecules. Atoms at the surface have "residual valences" pointing outwards, and these act at fixed points. The speech gives the image that stuck: think of the surface "as a chess-board, where each square can only be occupied by a single gas particle. When all the squares are occupied adsorption ceases."
What was the evidence?
Langmuir used experiments he could do in a light bulb. The speech describes one: hydrogen at very low pressure is split into atoms by a hot filament, and the atoms stick to the cold glass wall. The maximum amount of hydrogen taken up per square centimetre matched the number of atoms that could fit side by side in one layer.
He tested the same idea on liquids. In his Nobel Lecture, he explains why a fatty acid spreads on water but a pure hydrocarbon stays as a drop. The –COOH group is attracted to water, the long hydrocarbon chain is not, so the molecules spread until each has its head in the water. With enough molecules they stand nearly upright, side by side, and the film's thickness equals the chain length. By pushing on the film with a floating paper strip, he measured its "two-dimensional" pressure, and found films that behaved like 2D solids, liquids or gases. His key papers appeared in the *Journal of the American Chemical Society* in 1916 (solids), 1917 (liquids) and 1918, on gases adsorbed on glass, mica and platinum.
How does the Langmuir isotherm work? A worked example
In the lecture, Langmuir derives the isotherm from two simple assumptions: adsorbed molecules do not exert appreciable forces on each other, and only molecules that land on a bare site stick. Balancing the rate of sticking against the rate of leaving gives the fraction of sites covered, θ:
θ = KP / (1 + KP)
Here P is the gas pressure and K is a constant for that gas, surface and temperature.
Problem. For a gas on a metal surface, K = 0.5 atm⁻¹. Find θ at 0.2, 2, 6 and 20 atm.
P = 0.2 atm: KP = 0.1, so θ = 0.1 / 1.1 ≈ 0.091.
P = 2 atm: KP = 1, so θ = 1 / 2 = 0.50.
P = 6 atm: KP = 3, so θ = 3 / 4 = 0.75.
P = 20 atm: KP = 10, so θ = 10 / 11 ≈ 0.91.
At low pressure, θ ≈ KP, so coverage is roughly proportional to pressure. At high pressure, θ approaches 1 because the chessboard is full: raising the pressure from 6 to 20 atm raises coverage only from 0.75 to 0.91.
Langmuir himself warned in the lecture that, given its simplifying assumptions, the equation "should of course not be looked upon as a general equation for the adsorption isotherm", though it fits a surprisingly large number of cases on plane surfaces.
Common mistakes:
Treating θ as an amount. It is a fraction between 0 and 1; multiply by the number of sites to get molecules.
Confusing it with the Freundlich isotherm, x/m = kP^(1/n), which keeps rising with pressure and has no saturation limit built in.
Assuming the model fits multilayer or physisorbed gases near condensation. It is a monolayer model.
Do you need this for JEE or NEET?
The JEE (Advanced) 2026 syllabus lists Surface Chemistry: physisorption and chemisorption, the Freundlich adsorption isotherm, colloids and micelles. The JEE (Main) 2026 syllabus has no surface chemistry unit, and the current NCERT Class 12 Chemistry Part I (reprint 2026-27) runs from Solutions to Coordination Compounds without one. Check your own year's documents. Langmuir's idea still helps with catalysis in chemical kinetics: a solid catalyst works through the molecules adsorbed on it, which our post on collision theory and catalysts covers from the exam side.
What can a student learn from Langmuir?
Start from the thing in front of you. His model grew out of light bulbs and oil on water, not abstract theory.
Make the simplest assumption first, then test it. He chose the simplest possible rules, derived an equation, and checked where it worked and where it did not.
Count to check. He compared the hydrogen adsorbed with the number of atoms that fit in one layer.
Langmuir also wrote on electrons in atoms; his 1919 paper on the arrangement of electrons built on Gilbert Lewis's electron pairs. If coverage curves are hard to picture, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read, and the Fermi Concept Library has a lesson for each chemistry concept you need along the way.
Try it now: with K = 0.5 atm⁻¹, find the pressure at which the surface is 80% covered (answer: 8 atm).
