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Surfactant

Surfactants are compounds that lower the surface tension of a liquid, the interfacial tension between two liquids, or

that between a liquid and a solid. Surfactants may act as detergents, wetting agents, emulsifiers, foaming agents, and

dispersants.

Micelle in aqueous solution

Etymology and definition

The term surfactant is a blend of surface active agents.[1]

In Index Medicus and the United States National Library of Medicine,

surfactant is reserved for the meaning pulmonary surfactant. For the

more general meaning, surface active agent is the heading.

A micellethe lipophilic tails of the surfactant

ions remain on the inside of the micelle due to

unfavourable interactions. The polar "heads" of

the micelle, due to favourable interactions with

water, form a hydrophilic outer layer that in effect

protects the hydrophobic core of the micelle. The

compounds that make up a micelle are typicallyamphiphilic in nature, meaning that micelles are

soluble not only in protic solvents such as water

but also in aprotic solvents as a reverse micelle.

Composition and structure

Surfactants are usually organic compounds that are amphiphilic,

meaning they contain both hydrophobic groups (their tails) and

hydrophilic groups (their heads)[2]. Therefore, a surfactant contains

both a water insoluble (or oil soluble) component and a water soluble

component. Surfactants will diffuse in water and adsorb at interfaces

between air and water or at the interface between oil and water, in the

case where water is mixed with oil. The insoluble hydrophobic group

may extend out of the bulk water phase, into the air or into the oil

phase, while the water soluble head group remains in the water phase.

This alignment of surfactants at the surface modifies the surface

properties of water at the water/air or water/oil interface.

World production of surfactants is estimated at 15 Mton/y, of whichabout half are soaps. Other surfactants produced on a particularly large

scale are linear alkylbenzenesulfonates (1700 Mton/y), lignin

sulfonates 600, fatty alcohol ethoxylates (700 Mtons/y), alkylphenol

ethoxylates (500 Mton/y).[3]

Structure of surfactant phases in water

In the bulk aqueous phase, surfactants form aggregates, such as micelles, where the hydrophobic tails form the core

of the aggregate and the hydrophilic heads are in contact with the surrounding liquid. Other types of aggregates such

as spherical or cylindrical micelles or bilayers can be formed. The shape of the aggregates depends on the chemical

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structure of the surfactants, depending on the balance of the sizes of the hydrophobic tail and hydrophilic head. This

is known as the HLB, Hydrophilic-lipophilic balance. Surfactants reduce the surface tension of water by adsorbing at

the liquid-gas interface. The relation that links the surface tension and the surface excess is known as the Gibbs

isotherm.

Dynamics of surfactants at interfacesThe dynamics of adsorption of surfactants is of great importance for practical applications such as foaming,

emulsifying or coating processes, where bubbles or drops are rapidly generated and need to be stabilized. The

dynamics of adsorption depends on the diffusion coefficient of the surfactants. Indeed, as the interface is created, the

adsorption is limited by the diffusion of the surfactants to the interface. In some cases, there exists a barrier of energy

for the adsorption or the desorption of the surfactants, then the adsorption dynamics is known as kinetically limited'.

Such energy barrier can be due to steric or electrostatic repulsions. The surface rheology of surfactant layers,

including the elasticity and viscosity of the surfactant layers plays a very important role in foam or emulsion

stability.

Characterization of interfaces and surfactant layersInterfacial and surface tension can be characterized by classical methods such as the -pendant or spinning drop

method Dynamic surface tensions, i.e. surface tension as a function of time, can be obtained by the Maximum

Bubble Pressure apparatus

The structure of surfactant layers can be studied by ellipsometry or X-Ray reflectivity.

Surface rheology can be characterized by the oscillating drop method or shear surface rheometers such as

double-cone, double-ring or magnetic rod shear surface rheometer.

Detergents in biochemistry and biotechnology

In solution, detergents help solubilize a variety of chemical species by dissociating aggregates and unfolding

proteins. Popular surfactants in the biochemistry laboratory are SDS and CTAB. Detergents are key reagents to

extract protein by lysis of the cells and tissues: They disorganize the membrane's lipidic bilayer (SDS, Triton X-100,

X-114, CHAPS, DOC, and NP-40), and solubilize proteins. Milder detergents such as (OctylThioGlucosides) are

used to solubilize sensible proteins (enzymes, receptors). Non-solubilized material is harvested by centrifugation or

other means. For electrophoresis, for example, proteins are classically treated with SDS to denature the native

tertiary and quaternary structures, allowing the separation of proteins according to their molecular weight.

Detergents have also been used to decellularise organs. This process maintains a matrix of proteins that preserves the

structure of the organ and often the microvascular network. The process has been successfully used to prepare organs

such as the liver and heart for transplant in rats.[4]

Pulmonary surfactants are also naturally secreted by type II cells

of the lung alveoli in mammals.

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Classification of surfactants

According to the composition of their tail

The tail of surfactants can be:

A hydrocarbon chain: aromatic hydrocarbons (arenes), alkanes (alkyl), alkenes, cycloalkanes, alkyne-based

An alkyl ether chain:

Ethoxylated surfactants: polyethylene oxides are inserted to increase the hydrophilic character of a surfactant

Propoxylated surfactants: polypropylene oxides are inserted to increase the lipophilic character of a surfactant

A fluorocarbon chain: fluorosurfactants

A siloxane chain: siloxane surfactants.

A surfactant can have one or two tails, these are called double-chained.

According to the composition of their head

Surfactant classification according to the composition of their head: nonionic, anionic,

cationic, amphoteric.

A surfactant can be classified by the

presence of formally charged groups in

its head. A non-ionic surfactant has no

charge groups in its head. The head of

an ionic surfactant carries a net charge.

If the charge is negative, the surfactant

is more specifically called anionic; if

the charge is positive, it is called

cationic. If a surfactant contains a head

with two oppositely charged groups, it

is termed zwitterionic. Commonly

encountered surfactants of each type

include:

Anionic

Sulfate esters

Anionic surfactants contain anionic functional groups at their head, such as sulfate, sulfonate, phosphate, and

carboxylates. Prominent alkyl sulfates include ammonium lauryl sulfate, sodium lauryl sulfate (SDS, sodium

dodecyl sulfate, another name for the compound) and the related alkyl-ether sulfates sodium laureth sulfate, also

known as sodium lauryl ether sulfate (SLES), and sodium myreth sulfate.

Sulfonates

Docusates: dioctyl sodium sulfosuccinate, perfluorooctanesulfonate (PFOS), perfluorobutanesulfonate, linear

alkylbenzene sulfonates (LABs).

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Phosphates

These include alkyl-aryl ether phosphates and the alkyl ether phosphate

Carboxylates

These are the most common surfactants and comprise the alkyl carboxylates (soaps), such as sodium stearate. More

specialized species include sodium lauroyl sarcosinateand carboxylate-based fluorosurfactants such as perfluorononanoate, perfluorooctanoate (PFOA or PFO).

Cationic

pH-dependent primary, secondary, or tertiary amines: Primary amines become positively charged at pH