Surfactants are compounds that lower the surface tension between two liquids or between a liquid
and a solid.
Surfactants may act as detergents, wetting agents, emulsifiers, foaming agents, and dispersants.
Etymology and definition The term surfactant/surfactants is a blend
of surface active agents.
In Index Medicus and the United States National Library of Medicine, surfactant/surfactants
is reserved for the meaning pulmonary surfactant.
For the more general meaning, surface active agent/s is the heading.
Composition and structure Surfactants are usually organic compounds
that are amphiphilic, meaning they contain both hydrophobic groups and hydrophilic groups.
Therefore, a surfactant contains both a water insoluble 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 water-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.
World production of surfactants is estimated at 15 Mton/y, of which about half are soaps.
Other surfactants produced on a particularly large scale are linear alkylbenzenesulfonates,
lignin sulfonates, fatty alcohol ethoxylates, and alkylphenol ethoxylates.
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 structure of the surfactants, namely
the balance in size between hydrophilic head and hydrophobic tail.
A measure of this is the HLB, Hydrophilic-lipophilic balance.
Surfactants reduce the surface tension of water by 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 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 an energetic barrier to adsorption or desorption of the surfactants.
If such a barrier limits adsorption rate, the dynamics is said to be ‘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 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 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, and solubilize proteins.
Milder detergents such as octyl thioglucoside, octyl glucoside or dodecyl maltoside are used
to solubilize membrane proteins such as enzymes and receptors without denaturing them.
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.
Pulmonary surfactants are also naturally secreted by type II cells of the lung alveoli in mammals.
Classification of surfactants The "tail" of most surfactants are fairly
similar, consisting of a hydrocarbon chain, which can be branch, linear, or aromatic.
Fluorosurfactants have fluorocarbon chains.
Siloxane surfactants have siloxane chains.
Many important surfactants include a polyether chain terminating in a highly polar anionic
group.
The polyether groups often comprise ethoxylated sequences inserted to increase the hydrophilic
character of a surfactant.
Polypropylene oxides conversely, may be inserted to increase the lipophilic character of a
surfactant.
Surfactant molecules have either one tail or two; those with two tails are said to be
double-chained.
Most commonly, surfactants are classified according to polar head group.
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, sulfonate, and phosphate 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 and
the related alkyl-ether sulfates sodium laureth sulfate, also known as sodium lauryl ether
sulfate, and sodium myreth sulfate.
Docusates: dioctyl sodium sulfosuccinate, perfluorooctanesulfonate, perfluorobutanesulfonate,
linear alkylbenzene sulfonates.
These include alkyl-aryl ether phosphates and the alkyl ether phosphate
Carboxylates These are the most common surfactants and
comprise the alkyl carboxylates, such as sodium stearate.
More specialized species include sodium lauroyl sarcosinate and carboxylate-based fluorosurfactants
such as perfluorononanoate, perfluorooctanoate.
Cationic head groups pH-dependent primary, secondary, or tertiary
amines: Primary and secondary amines become positively charged at pH < 10:"Bordwell pKa
Table".
Retrieved 11 May 2014.
Octenidine dihydrochloride;
Permanently charged quaternary ammonium cation: Alkyltrimethylammonium salts: cetyl trimethylammonium
bromide a.k.a.
hexadecyl trimethyl ammonium bromide, cetyl trimethylammonium chloride
Cetylpyridinium chloride Benzalkonium chloride
Benzethonium chloride 5-Bromo-5-nitro-1,3-dioxane
Dimethyldioctadecylammonium chloride Cetrimonium bromide
Dioctadecyldimethylammonium bromide
Zwitterionic surfactants Zwitterionic surfactants have both cationic
and anionic centers attached to the same molecule.
The cationic part is based on primary, secondary, or tertiary amines or quaternary ammonium
cations.
The anionic part can be more variable and include sulfonates, as in CHAPSdimethylammonio]-1-propanesulfonate).
Other anionic groups are sultaines illustrated by cocamidopropyl hydroxysultaine.
Betaines, e.g., cocamidopropyl betaine.
Phosphates: lecithin Nonionic surfactant
Many long chain alcohols exhibit some surfactant properties.
Prominent among these are the fatty alcohols, cetyl alcohol, stearyl alcohol, and cetostearyl
alcohol, and oleyl alcohol.
Polyoxyethylene glycol alkyl ethers: CH3–(CH2)10–16–(O-C2H4)1–25–OH: Octaethylene glycol monododecyl ether
Pentaethylene glycol monododecyl ether
Polyoxypropylene glycol alkyl ethers: CH3–(CH2)10–16–(O-C3H6)1–25–OH Glucoside alkyl ethers: CH3–(CH2)10–16–(O-Glucoside)1–3–OH:
Decyl glucoside, Lauryl glucoside
Octyl glucoside
Polyoxyethylene glycol octylphenol ethers: C8H17–(C6H4)–(O-C2H4)1–25–OH:
Triton X-100
Polyoxyethylene glycol alkylphenol ethers: C9H19–(C6H4)–(O-C2H4)1–25–OH:
Nonoxynol-9
Glycerol alkyl esters: Glyceryl laurate
Polyoxyethylene glycol sorbitan alkyl esters: Polysorbate
Sorbitan alkyl esters: Spans Cocamide MEA, cocamide DEA
Dodecyldimethylamine oxide Block copolymers of polyethylene glycol and
polypropylene glycol: Poloxamers Polyethoxylated tallow amine.
According to the composition of their counter-ion In the case of ionic surfactants, the counter-ion
can be: Monatomic / Inorganic:
Cations: metals : alkali metal, alkaline earth metal, transition metal
Anions: halides: chloride, bromide, iodide
Polyatomic / Organic: Cations: ammonium, pyridinium, triethanolamine
Anions: tosyls, trifluoromethanesulfonates, methylsulfate
Current market and forecast The annual global production of surfactants
was 13 million tonnes in 2008, and the annual turnover reached US$24.33 billion in 2009,
nearly 2% up from the previous year.
The market is expected to experience growth in the order of 2.8% annually to 2012 and
by 3.5–4% thereafter.
Specialists expect the global surfactant market to generate revenues of more than US$41 billion
in 2018 – translating to an average annual growth of 4.5%
Health and environmental controversy Surfactants are routinely deposited in numerous
ways on land and into water systems, whether as part of an intended process or as industrial
and household waste.
Some of them are known to be toxic to animals, ecosystems, and humans, and can increase the
diffusion of other environmental contaminants.
As a result, there are proposed or voluntary restrictions on the use of some surfactants.
For example, PFOS is a persistent organic pollutant as judged by the Stockholm Convention.
Additionally, PFOA has been subject to a voluntary agreement by the U.S. Environmental Protection
Agency and eight chemical companies to reduce and eliminate emissions of the chemical and
its precursors.
The two major surfactants used in the year 2000 were linear alkylbenzene sulfonates and
the alkyl phenol ethoxylates.
They break down in the aerobic conditions found in sewage treatment plants and in soil
to the metabolite nonylphenol, which is thought to be an endocrine disruptor.
Ordinary dishwashing detergent, for example, will promote water penetration in soil, but
the effect would last only a few days.
Commercial soil wetting agents will continue to work for a considerable period, but they
will eventually be degraded by soil micro-organisms.
Some can, however, interfere with the life-cycles of some aquatic organisms, so care should
be taken to prevent run-off of these products into streams, and excess product should not
be washed down.
Anionic surfactants can be found in soils as the result of sludge application, wastewater
irrigation, and remediation processes.
Relatively high concentrations of surfactants together with multimetals can represent an
environmental risk.
At low concentrations, surfactant application is unlikely to have a significant effect on
trace metal mobility.
Biosurfactants Biosurfactants are surface-active substances
synthesised by living cells.
Interest in microbial surfactants has been steadily increasing in recent years due to
their diversity, environmentally friendly nature, possibility of large-scale production,
selectivity, performance under extreme conditions, and potential applications in environmental
protection.
Few of the popular examples of microbial biosurfactants includes Emulsan produced by Acinetobacter
calcoaceticus, Sophorolipids produced by several yeasts belonging to candida and starmerella
clade, and Rhamnolipid produced by Pseudomonas aeruginosa etc.
Biosurfactants enhance the emulsification of hydrocarbons, have the potential to solubilise
hydrocarbon contaminants and increase their availability for microbial degradation.
The use of chemicals for the treatment of a hydrocarbon polluted site may contaminate
the environment with their by-products, whereas biological treatment may efficiently destroy
pollutants, while being biodegradable themselves.
Hence, biosurfactant-producing microorganisms may play an important role in the accelerated
bioremediation of hydrocarbon-contaminated sites.
These compounds can also be used in enhanced oil recovery and may be considered for other
potential applications in environmental protection.
Other applications include herbicides and pesticides formulations, detergents, healthcare
and cosmetics, pulp and paper, coal, textiles, ceramic processing and food industries, uranium
ore-processing, and mechanical dewatering of peat.
Several microorganisms are known to synthesise surface-active agents; most of them are bacteria
and yeasts.
When grown on hydrocarbon substrate as the carbon source, these microorganisms synthesise
a wide range of chemicals with surface activity, such as glycolipid, phospholipid, and others.
These chemicals are synthesised to emulsify the hydrocarbon substrate and facilitate its
transport into the cells.
In some bacterial species such as Pseudomonas aeruginosa, biosurfactants are also involved
in a group motility behavior called swarming motility.
Safety and environmental risks Most anionic and nonionic surfactants are
nontoxic, having LD50 comparable to sodium chloride.
The situation for cationic surfactants is more diverse.
Dialkyldimethylammonium chlorides have very low LD50's but alkylbenzyldimethylammonium
chloride has an LD50 of 0.35 g/kg.
Prolonged exposure of skin to surfactants can cause chafing because surfactants disrupts
the lipid coating that protects skin cells.
Biosurfactants and Deepwater Horizon The use of biosurfactants as a way to remove
petroleum from contaminated sites has been studied and found to be safe and effective
in the removal petroleum products from soil.
Biosurfactants were not used by BP after the Deepwater Horizon oil spill.
However, unprecedented amounts of Corexit, were sprayed directly into the ocean at the
leak and on the sea-water's surface, the theory being that the surfactants isolate droplets
of oil, making it easier for petroleum-consuming microbes to digest the oil.
Applications Surfactants play an important role as cleaning,
wetting, dispersing, emulsifying, foaming and anti-foaming agents in many practical
applications and products, including: Detergents
Fabric softeners Emulsions
Paints Adhesives
Inks Anti-fogs
Ski waxes, snowboard wax Deinking of recycled papers, in flotation,
washing and enzymatic processes Laxatives
Agrochemical formulations Herbicides
Insecticides
Quantum dot in order to manipulate growth and assembly of the dots, reactions on their
surface, electrical properties, etc, it is important to understand how surfactants arrange
on the surface of the quantum dots Biocides
Cosmetics: Shampoos
Hair conditioners Toothpastes
Spermicides Firefighting
Pipelines, liquid drag reducing agent Alkali Surfactant Polymers
Ferrofluids Leak Detectors
See also
Anti-fog Cleavable detergent
Emulsion MBAS assay, an assay that indicates anionic
surfactants in water with a bluing reaction.
Niosome Oil dispersants
Pulmonary surfactant Surfactants in paint
References
External links Surfactant films on ocean surface implicated
in global warming