Chemistry

A Langmuir monolayer or insoluble monolayer is a one-molecule thick layer of an insoluble organic material spread onto an aqueous subphase

Phase (matter)

In the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, and chemical composition...

. Traditional compounds used to prepare 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 macromolecule

Macromolecule

A macromolecule is a very large molecule commonly created by some form of polymerization. In biochemistry, the term is applied to the four conventional biopolymers , as well as non-polymeric molecules with large molecular mass such as macrocycles...

s such as polymer

Polymer

A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

s. Langmuir monolayers are extensively studied for the fabrication of Langmuir-Blodgett film

Langmuir-Blodgett film

A Langmuir–Blodgett film contains one or more monolayers of an organic material, deposited from the surface of a liquid onto a solid by immersing the solid substrate into the liquid. A monolayer is adsorbed homogeneously with each immersion or emersion step, thus films with very accurate...

 (LB films), which are formed by transferred monolayers on a solid substrate. A Gibbs

Josiah Willard Gibbs

Josiah Willard Gibbs was an American theoretical physicist, chemist, and mathematician. He devised much of the theoretical foundation for chemical thermodynamics as well as physical chemistry. As a mathematician, he invented vector analysis . Yale University awarded Gibbs the first American Ph.D...

 monolayer or soluble monolayer is a monolayer formed by a compound that is soluble in one of the phases separated by the interface

Interface (chemistry)

An interface is a surface forming a common boundary among two different phases, such as an insoluble solid and a liquid, two immiscible liquids or a liquid and an insoluble gas. The importance of the interface depends on which type of system is being treated: the bigger the quotient area/volume,...

 on which the monolayer is formed.

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

Sticking coefficient

Sticking coefficient is the term used in surface physics to describe the ratio of the number of adsorbate atoms that do adsorb, or "stick", to a surface to the total number of atoms that impinge upon that surface during the same period of time...

, so that every molecule which reaches the surface sticks to it without re-evaporating, then the monolayer time is very roughly:
where t is in seconds and P is the pressure in pascal

Pascal (unit)

The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...

s. It takes about 1 second for a surface to be covered at a pressure of 300 µPa (2×10^-6 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 (, where is the surface tension of the interface before the monolayer is formed) with the area (the inverse of surface concentration ). It is analogous with a 3D process in which pressure

Pressure

Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

 varies with volume

Volume

Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....

.

A variety of bidimensional phases

Phase (matter)

In the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, and chemical composition...

 can be detected, each separated by a phase transition

Phase transition

A phase transition is the transformation of a thermodynamic system from one phase or state of matter to another.A phase of a thermodynamic system and the states of matter have uniform physical properties....

. 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 equations of state
  Equation of state
  In physics and thermodynamics, an equation of state is a relation between state variables. More specifically, an equation of state is a thermodynamic equation describing the state of matter under a given set of physical conditions...
  
   for 3D gases can be used: ideal gas law , where 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

Gibbs isotherm

The Gibbs adsorption isotherm for multicomponent systems is an equation used to relate the changes in concentration of a component in contact with a surface with changes in the surface tension...

.

• For very dilute solutions , through Gibbs isotherm another analogous of ideal gas law is reached
• For more concentrated solutions and applying Langmuir isotherm , thus

Biology

Monolayers are frequently encountered in biology

Biology

Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...

. A micelle

Micelle

A micelle is an aggregate of surfactant molecules dispersed in a liquid colloid. A typical micelle in aqueous solution forms an aggregate with the hydrophilic "head" regions in contact with surrounding solvent, sequestering the hydrophobic single tail regions in the micelle centre. This phase is...

 is a monolayer, and the phospholipid

Phospholipid

Phospholipids are a class of lipids that are a major component of all cell membranes as they can form lipid bilayers. Most phospholipids contain a diglyceride, a phosphate group, and a simple organic molecule such as choline; one exception to this rule is sphingomyelin, which is derived from...

 lipid bilayer

Lipid bilayer

The lipid bilayer is a thin membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around cells. The cell membrane of almost all living organisms and many viruses are made of a lipid bilayer, as are the membranes surrounding the cell nucleus...

 structure of biological membrane

Cell membrane

The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...

s is technically two monolayers.

Cell culture

In cell culture

Cell culture

Cell culture is the complex process by which cells are grown under controlled conditions. In practice, the term "cell culture" has come to refer to the culturing of cells derived from singlecellular eukaryotes, especially animal cells. However, there are also cultures of plants, fungi and microbes,...

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.