Editing Surfactant (section)

==Composition and structure==

[[Image:Micelle scheme-en.svg|thumb|upright=1.35|[[Schematic]] diagram of a [[micelle]]&nbsp;– the [[lipophilic]] tails of the surfactant ions remain inside the oil because they interact more strongly with oil than with water. The [[Chemical polarity|polar]] "heads" of the surfactant molecules coating the micelle interact more strongly with water, so they form a [[hydrophilic]] outer layer that forms a barrier between micelles. This inhibits the oil droplets, the hydrophobic cores of micelles, from merging into fewer, larger droplets ("emulsion breaking") of the micelle. The compounds that coat a micelle are typically [[amphiphilic]] in nature, meaning that micelles may be stable either as droplets of [[aprotic]] solvents such as oil in water, or as protic solvents such as water in oil. When the droplet is aprotic it is sometimes{{when|date= February 2019}} known as a reverse micelle.]]

Surfactants are usually [[organic compound]]s that are akin to [[amphiphilic]], which means that this molecule, being as double-agent, each contains a [[hydrophilic]] "water-seeking" group (the ''head''), and a [[hydrophobic]] "water-avoiding" group (the ''tail'').<ref name="The Lipid Chronicles">{{cite web|title=Bubbles, Bubbles, Everywhere, But Not a Drop to Drink|url=http://www.samuelfurse.com/2011/11/bubbles-bubbles-everywhere-but-not-a-drop-to-drink/|work=The Lipid Chronicles|access-date=1 August 2012|url-status=live|archive-url=https://web.archive.org/web/20120426082602/http://www.samuelfurse.com/2011/11/bubbles-bubbles-everywhere-but-not-a-drop-to-drink/|archive-date=26 April 2012|df=dmy-all|date=2011-11-11}}</ref>  As a result, a surfactant contains both a water-soluble component and a water-insoluble component. Surfactants diffuse in water and get [[Adsorption|adsorb]]ed at [[Interface (chemistry)|interfaces]] between air and water, or at the interface between oil and water in the case where water is mixed with oil. The water-insoluble hydrophobic group may extend out of the bulk water phase into a non-water phase such as air or oil phase, while the water-soluble head group remains bound in the water phase.

The hydrophobic tail may be either [[lipophilicity|lipophilic]] ("oil-seeking") or [[lipophobicity|lipophobic]] ("oil-avoiding") depending on its chemistry. [[Hydrocarbon]] groups are usually lipophilic, for use in soaps and detergents, while [[fluorocarbon]] groups are lipophobic, for use in [[Stain repellent|repelling stains]] or reducing surface tension.

World production of surfactants is estimated at 15 million tons per year, of which about half are [[soap]]s.  Other surfactants produced on a particularly large scale are linear [[alkylbenzene sulfonates]] (1.7 million tons/y), [[lignin sulfonate]]s (600,000 tons/y), [[fatty alcohol]] [[ethoxylate]]s (700,000 tons/y), and [[alkylphenol]] [[ethoxylate]]s (500,000 tons/y).<ref name=Ullmann>Kurt Kosswig "Surfactants" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, 2005, Weinheim. {{doi|10.1002/14356007.a25_747}}</ref>
[[File:sodium stearate.svg|thumb|Sodium stearate, the most common component of most soap, which comprises about 50% of commercial surfactants]]
[[File:Sodium dodecylbenzenesulfonate skeletal.svg|thumb|alt=Sodium dodecylbenzenesulfonate|4-(5-Dodecyl) benzenesulfonate, a linear dodecylbenzenesulfonate, one of the most common surfactants]]

===Structure of surfactant phases in water===
{{main|Wetting solution}}
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 can also be formed, such as spherical or cylindrical micelles or [[lipid bilayer]]s. The shape of the aggregates depends on the chemical structure of the surfactants, namely the balance in size between the hydrophilic head and hydrophobic tail. A measure of this is the [[hydrophilic-lipophilic balance]] (HLB). Surfactants reduce the [[surface tension]] of water by [[Adsorption|adsorbing]] at the liquid-air interface. The relation that links the surface tension and the surface excess is known as the [[Gibbs isotherm]].

===Dynamics of surfactants at interfaces===
The dynamics of surfactant adsorption is of great importance for practical applications such as in foaming, emulsifying or coating processes, where bubbles or drops are rapidly generated and need to be stabilized. The dynamics of absorption depend on the [[diffusion coefficient]] of the surfactant. As the interface is created, the adsorption is limited by the diffusion of the surfactant to the interface. In some cases, there can exist an energetic barrier to adsorption or desorption of the surfactant. If such a barrier limits the adsorption rate, the dynamics are said to be ‘kinetically limited'. Such energy barriers can be due to [[Steric repulsion|steric]] or [[electrostatic repulsion]]s.
The [[surface rheology]] of surfactant layers, including the elasticity and viscosity of the layer, play an important role in the stability of foams and emulsions.

===Characterization of interfaces and surfactant layers===
Interfacial 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 method|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.