Amonolayer is a single, closely packed layer of entities, commonlyatoms ormolecules.^[1] Monolayers can also be made out ofcells.Self-assembled monolayers form spontaneously on surfaces.Monolayers of layered crystals likegraphene andmolybdenum disulfide are generally called2D materials.

ALangmuir monolayer orinsoluble monolayer is a one-molecule thick layer of an insoluble organic material spread onto anaqueous subphase in aLangmuir-Blodgett trough. Traditional compounds used to prepareLangmuir monolayers areamphiphilic materials that possess ahydrophilic headgroup and ahydrophobic tail. Since the 1980s a large number of other materials have been employed to produce Langmuir monolayers, some of which are semi-amphiphilic, including polymeric, ceramic or metallicnanoparticles andmacromolecules such aspolymers. Langmuir monolayers are extensively studied for the fabrication ofLangmuir-Blodgett film (LB films), which are formed by transferred monolayers on a solid substrate.

AGibbs monolayer orsoluble monolayer is a monolayer formed by a compound that is soluble in one of the phases separated by theinterface on which the monolayer is formed.

Themonolayer formation time ormonolayer time is the length of time required, on average, for a surface to be covered by an adsorbate, such as oxygen sticking to fresh aluminum. If the adsorbate has a unitysticking coefficient, so that every molecule which reaches the surface sticks to it without re-evaporating, then the monolayer time is very roughly:

${\displaystyle t=\frac{3\times {10}^{-4}\mathrm{P}\mathrm{a}\cdot \mathrm{s}}{P}}$

wheret is the time andP is the pressure. It takes about 1 second for a surface to be covered at a pressure of 300 μPa (2×10⁻⁶ Torr).

A Langmuir monolayer can be compressed or expanded by modifying its area with a moving barrier in a Langmuir film balance. If the surface tension of the interface is measured during the compression, acompression isotherm is obtained. This isotherm shows the variation of surface pressure (${\displaystyle \mathrm{\Pi }={\gamma }^o-\gamma }$, where${\displaystyle {\gamma }^o}$ is the surface tension of the interface before the monolayer is formed) with the area (the inverse of surface concentration${\displaystyle {\mathrm{\Gamma }}^{-1}}$). It is analogous with a 3D process in whichpressure varies withvolume.

A variety of bidimensionalphases can be detected, each separated by aphase transition. During the phase transition, the surface pressure doesn't change, but the area does, just like during normal phase transitions volume changes but pressure doesn't.The 2D phases, in increasing pressure order:

• Bidimensional gas: there are few molecules per area unit, and they have few interactions, therefore, analogous of theequations of state for 3D gases can be used: ideal gas law${\displaystyle \mathrm{\Pi }A=RT}$, where${\displaystyle A}$ is the area per mole. As the surface pressure increases, more complex equations are needed (Van der Waals, virial...)
• Expanded liquid
• Compressed liquid
• Solid

If the area is further reduced once the solid phase has been reached, collapse occurs, the monolayer breaks and soluble aggregates and multilayers are formed

Gibbs monolayers also follow equations of state, which can be deduced fromGibbs isotherm.

• For very dilute solutions${\displaystyle \gamma ={\gamma }_o-mC}$, through Gibbs isotherm another analogous of ideal gas law is reached${\displaystyle \mathrm{\Pi }=\mathrm{\Gamma }RT}$
• For more concentrated solutions and applying Langmuir isotherm${\displaystyle \mathrm{\Gamma }={\mathrm{\Gamma }}_{max}\frac{C}{a+C}}$, thus${\displaystyle \mathrm{\Pi }={\mathrm{\Gamma }}_{max}RT\left(1+\frac{C}{a}\right)}$

Monolayers have a multitude of applications both at the air-water and at air-solid interphases.

Nanoparticle monolayers can be used to create functional surfaces that have for instance anti-reflective or superhydrophobic properties.^[2][3]

Monolayers are frequently encountered inbiology. Amicelle is a monolayer, and thephospholipidlipid bilayer structure ofbiological membranes is technically two monolayers. Langmuir monolayers are commonly used to mimic cell membrane to study the effects of pharmaceuticals or toxins.^[4]

Incell culture, a monolayer refers to a layer of cells in which no cell is growing on top of another, but all are growing side by side and often touching each other on the same growth surface.

• Langmuir-Blodgett Film
• Langmuir-Blodgett Trough
• Self-assembled monolayer
• Evaporation suppressing monolayers

• "Monolayer".IUPAC Compendium of Chemical Terminology.IUPAC. 2009.doi:10.1351/goldbook.M04015.ISBN 978-0-9678550-9-7.{{cite book}}:|work= ignored (help)
• Monolayer
• Irving Langmuir demonstrating the preparation of monolayers on a water surface; 1939 film (Youtube)

• Phases of matter
• Thin films
• Physical chemistry

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• Short description is different from Wikidata
• CS1 errors: periodical ignored