Editing Monolayer

{{Short description|Single closely packed layer of entities such as atoms}}
A '''monolayer''' is a single, closely packed layer of entities, commonly [[atom]]s or [[molecule]]s.<ref>{{cite journal |last1=Ter Minassian-Saraga |first1=L. |date=1994 |title=Thin films including layers: terminology in relation to their preparation and characterization (IUPAC Recommendations 1994) |journal=[[Pure and Applied Chemistry]] |volume=66 |issue=8 |pages=1667–1738 (1672) |doi=10.1351/pac199466081667 |s2cid=95035065 |url=http://pac.iupac.org/publications/pac/pdf/1994/pdf/6608x1667.pdf }}</ref> 
Monolayers can also be made out of [[#Cell culture|cells]].
''[[Self-assembled monolayer]]s'' form spontaneously on surfaces.
Monolayers of layered crystals like [[graphene]] and [[molybdenum disulfide]] are generally called ''[[2D materials]]''.

== Types ==

[[File:Surfactant.jpg|thumb|Diagram of ambiphilic molecules floating on a water surface.]]

A '''Langmuir monolayer''' or ''insoluble monolayer'' is a one-molecule thick layer of an insoluble organic material spread onto an [[aqueous]] sub[[phase (matter)|phase]] in a [[Langmuir–Blodgett trough|Langmuir-Blodgett trough]]. Traditional compounds used to prepare [[Langmuir (unit)|Langmuir]] monolayers are [[amphiphilic]] materials that possess a [[hydrophilic]] headgroup and a [[hydrophobic]] 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 metallic [[nanoparticle]]s and [[macromolecule]]s such as [[polymer]]s. Langmuir monolayers are extensively studied for the fabrication of [[Langmuir-Blodgett film]] (LB films), which are formed by transferred monolayers on a solid substrate.

A '''[[Josiah Willard Gibbs|Gibbs]] monolayer''' or ''soluble monolayer'' is a monolayer formed by a compound that is soluble in one of the phases separated by the [[interface (chemistry)|interface]] on which the monolayer is formed.

== Properties ==
=== Formation time ===
The '''monolayer formation time''' or '''monolayer 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 unity [[sticking coefficient]], so that every molecule which reaches the surface sticks to it without re-evaporating, then the monolayer time is very roughly:
:<math>t = \frac{3 \times 10^{-4} \, \mathrm{Pa} \cdot \mathrm{s}}{P}</math>
where ''t'' is the time and ''P'' is the pressure. It takes about 1 second for a surface to be covered at a pressure of 300&nbsp;μPa (2×10<sup>−6</sup>&nbsp;Torr).

=== Monolayer phases and equations of state ===

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, a ''compression isotherm'' is obtained. This isotherm shows the variation of surface pressure (<math>\Pi = \gamma^o - \gamma </math>, where <math>\gamma^o</math> is the surface tension of the interface before the monolayer is formed) with the area (the inverse of surface concentration <math>\Gamma^{-1}</math>). It is analogous with a 3D process in which [[pressure]] varies with [[volume]].

A variety of bidimensional [[phase (matter)|phases]] can be detected, each separated by a [[phase 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 the [[equation of state|equations of state]] for 3D gases can be used: ideal gas law <math>\Pi A = RT</math>, where <math>A</math> 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 from [[Gibbs isotherm]].
* For very dilute solutions <math>\gamma = \gamma_o - mC</math>, through Gibbs isotherm another analogous of ideal gas law is reached <math>\Pi = \Gamma R T</math>
* For more concentrated solutions and applying Langmuir isotherm <math>\Gamma = \Gamma_{\max} \frac{C}{a+C}</math>, thus <math>\Pi = \Gamma_{\max}RT \left(1+\frac{C}{a}\right)</math>

== Applications ==
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.<ref>{{Cite news|url=http://www.biolinscientific.com/application/functional-nanoscale-nanoparticle-coatings/|title=Functional Nanoscale and Nanoparticle Coatings - Biolin Scientific|work=Biolin Scientific|access-date=2017-08-03|language=en-US}}</ref><ref>{{Cite news|title=Influence of Thermal Separation of Oleic Acid on the Properties of Quantum Dots Solutions and Optoelectronic of Their Langmuir Monolayers - BioNanoScience|work=BioNanoScience|language=en-US|doi=10.1007/s12668-017-0412-4}}</ref>

Monolayers are frequently encountered in [[biology]]. A [[micelle]] is a monolayer, and the [[phospholipid]] [[lipid bilayer]] structure of [[cell membrane|biological membrane]]s is technically two monolayers. Langmuir monolayers are commonly used to mimic cell membrane to study the effects of pharmaceuticals or toxins.<ref>{{Cite web|url=http://www.biolinscientific.com/zafepress.php?url=%2Fpdf%2FKSV%20NIMA%2FApplication%20Notes%2FKN-AN-01-Biomolecular-interactions-cell-membrane-models.pdf|title=Interactions of biomolecules in cell membrane models|access-date=2017-08-03|archive-url=https://web.archive.org/web/20170803170643/http://www.biolinscientific.com/zafepress.php?url=%2Fpdf%2FKSV%20NIMA%2FApplication%20Notes%2FKN-AN-01-Biomolecular-interactions-cell-membrane-models.pdf|archive-date=2017-08-03|url-status=dead}}</ref>

== Cell culture ==
In [[cell 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.

== See also ==
* [[Langmuir–Blodgett film|Langmuir-Blodgett Film]]
* [[Langmuir–Blodgett trough|Langmuir-Blodgett Trough]]
* [[Self-assembled monolayer]]
* [[Evaporation suppressing monolayers]]

==References==
{{Reflist}}

==External links==
* {{cite book |chapter-url=http://goldbook.iupac.org/M04015.html |work=Goldbook |publisher=[[IUPAC]]|doi=10.1351/goldbook.M04015 |title=IUPAC Compendium of Chemical Terminology |year=2009 |isbn=978-0-9678550-9-7 |chapter=Monolayer }}
* [https://web.archive.org/web/20170803131116/http://www.biolinscientific.com/application/monolayer/ Monolayer]
* [https://www.youtube.com/watch?v=yn4fuWM007c Irving Langmuir demonstrating the preparation of monolayers on a water surface; 1939 film (Youtube)]

{{Authority control}}

[[Category:Phases of matter]]
[[Category:Thin films]]
[[Category:Physical chemistry]]