Surfactants are compounds that lower the surface tension
(or interfacial tension) between two liquids, between a gas and a
liquid, 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 is a blend of surface active agent.
In the United States National Library of Medicine's Medical Subject Headings (MeSH) vocabulary, surfactant is reserved for the meaning pulmonary surfactant. For the more general meaning, surface active agent/s is the heading.
Schematic diagram of a micelle – the lipophilic tails of the surfactant ions remain inside the oil because they interact more strongly with oil than with water. The 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 sometimes is known 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).
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 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 alkylbenzene sulfonates (1700 kton/y), lignin sulfonates (600 kton/y), fatty alcohol ethoxylates (700 ktons/y), and alkylphenol ethoxylates (500 kton/y).
Sodium stearate, the most common component of most soap, which comprises about 50% of commercial surfactants
4-(5-Dodecyl) benzenesulfonate, a linear dodecylbenzenesulfonate, one of the most common surfactants
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 can also be formed, such as spherical or cylindrical
micelles or lipid bilayers.
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 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 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 adsorption 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 or electrostatic repulsions.
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 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 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
The "tails" of most surfactants are fairly similar, consisting of a hydrocarbon chain, which can be branched, 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 (polyethylene oxide-like) 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.
Surfactant classification according to the composition of their head: nonionic, anionic, cationic, amphoteric.
Most commonly, surfactants are classified according to polar head
group. A non-ionic surfactant has no charged groups in its head. The
head of an ionic surfactant carries a net positive, or negative 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 (sodium dodecyl sulfate, SLS, or SDS), and the related alkyl-ether sulfates sodium laureth sulfate (sodium lauryl ether sulfate or SLES), and sodium myreth sulfate.
Carboxylates:
These are the most common surfactants and comprise the carboxylate salts (soaps), such as sodium stearate. More specialized species include sodium lauroyl sarcosinate and carboxylate-based fluorosurfactants such as perfluorononanoate, perfluorooctanoate (PFOA or PFO).
Others include:
- Docusate (dioctyl sodium sulfosuccinate)
- Perfluorooctanesulfonate (PFOS)
- Perfluorobutanesulfonate
- Alkyl-aryl ether phosphates
- Alkyl ether phosphates
Cationic head groups:
pH-dependent primary, secondary, or tertiary amines; primary and secondary amines become positively charged at pH less than 10: octenidine dihydrochloride.
Permanently charged quaternary ammonium salts: cetrimonium bromide (CTAB), cetylpyridinium chloride (CPC), benzalkonium chloride (BAC), benzethonium chloride (BZT), dimethyldioctadecylammonium chloride, and dioctadecyldimethylammonium bromide (DODAB).
Zwitterionic surfactants:
Zwitterionic (amphoteric)
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 the sultaines CHAPS (3-[(3-Cholamidopropyl)dimethylammonio]-1-propanesulfonate) and cocamidopropyl hydroxysultaine. Betaines such as cocamidopropyl betaine
have a carboxylate with the ammonium. The most common biological
zwitterionic surfactants have a phosphate anion with an amine or
ammonium, such as the phospholipids phosphatidylserine, phosphatidylethanolamine, phosphatidylcholine, and sphingomyelins.
Nonionic:
Nonionic
surfactants have covalently bonded oxygen-containing hydrophilic
groups, which are bonded to hydrophobic parent structures. The
water-solubility of the oxygen groups is the result of hydrogen bonding.
Hydrogen bonding decreases with increasing temperature, and the water
solubility of nonionic surfactants therefore decreases with increasing
temperature.
Nonionic surfactants are less sensitive to water hardness than
anionic surfactants, and they foam less strongly. The differences
between the individual types of nonionic surfactants are slight, and the
choice is primarily governed having regard to the costs of special
properties (e.g., effectiveness and efficiency, toxicity, dermatological
compatibility, biodegradability) or permission for use in food.
Ethoxylates:
Fatty alcohol ethoxylates:
- Narrow-range ethoxylate
- Octaethylene glycol monododecyl ether
- Pentaethylene glycol monododecyl ether
Alkylphenol ethoxylates:
Fatty acid ethoxylates
Ethoxylated amines and/or fatty acid amides:
Terminally blocked ethoxylates:
Fatty acid esters of polyhydroxy compounds:
Fatty acid esters of glycerol:
Fatty acid esters of sorbitol:
Alkyl polyglucosides:
Amine oxides:
Sulfoxides:
Phosphine oxides
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 (Cl−), bromide (Br−), iodide (I−)
- Polyatomic / Organic:
- Cations: ammonium, pyridinium, triethanolamine (TEA)
- Anions: tosyls, trifluoromethanesulfonates, methyl sulfate
In pharmacy
A
wetting agent is a surfactant that, when dissolved in water, lowers the
advancing contact angle, aids in displacing an air phase at the
surface, and replaces it with a liquid phase. Examples of application of
wetting to pharmacy and medicine include the displacement of air from
the surface of sulfur, charcoal, and other powders for the purpose of
dispersing these drugs in liquid vehicles; the displacement of air from
the matrix of cotton pads and bandages so that medicinal solutions can
be absorbed for application to various body areas; the displacement of
dirt and debris by the use of detergents in the washing of wounds; and
the application of medicinal lotions and sprays to surface of skin and
mucous membranes.
Pharmaceutical forms
The human body produces different types of surfactant in different parts or organs for different purposes. Pulmonary surfactant is produced in lungs
in order to facilitate breathing by increasing total lung capacity,
TLC, and lung compliance. In respiratory distress syndrome or RDS surfactant replacement
therapy helps patients have normal respiration by using pharmaceutical
forms of the surfactants. One of the most important pharmaceutical
pulmonary surfactants is Survanta (beractant) or its generic form Beraksurf produced by Abbvie and Tekzima respectively.
Current market and forecast
The annual global production of surfactants was 13 million tonnes in 2008.
In 2014, the world market for surfactants reached a volume of more than
33 billion US-dollars. Market researchers expect annual revenues to
increase by 2.5% per year to around 40.4 billion US-dollars until 2022.
The commercially most significant type of surfactants is currently the
anionic surfactant alkyl benzene sulfonate (LAS), which is widely used
in cleaners and detergents.
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 (LAS) and the alkyl phenol ethoxylates (APE). 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 (many standard laundry detergent powders
contain levels of chemicals such as alkali and chelating agents
that can be damaging to plants and should not be applied to soils).
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 is due to their diversity, environmentally
friendly nature, possibility of large-scale production, selectivity,
performance under extreme conditions, and potential applications in
environmental protection. A few of the popular examples of microbial biosurfactants includes Emulsan produced by Acinetobacter calcoaceticus, Sophorolipids produced by several yeasts belonging to candida and the 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 toxicity of quaternary ammonium compounds, which are antibacterial and antifungal, varies. Dialkyldimethylammonium chlorides (DDAC, DSDMAC) used as fabric softeners have low LD50 (5 g/kg) and are essentially non-toxic, while the disinfectant alkylbenzyldimethylammonium chloride has an LD50 of 0.35 g/kg.
Prolonged exposure to surfactants can irritate and damage the skin because surfactants disrupt the lipid membrane
that protects skin and other cells. Skin irritancy generally increases
in the series nonionic, amphoteric, anionic, cationic surfactants.
The biodegradability of a surfactant is determined by its hydrophobic hydrocarbon group.
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 of petroleum products from soil. Other studies
found that surfactants are often more toxic than the oil that is being
dispersed, and the combination of the oil and the surfactant can be more
toxic than either alone. Biosurfactants were not used by BP after the Deepwater Horizon oil spill. However, unprecedented amounts of Corexit
(active ingredient: dioctyl sodium sulfosuccinate (DOSS), sorbitan
monooleate (Span 80), and polyoxyethylenated sorbitan monooleate (Tween-80)),
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.
Biosurfactants produced by microbe or bacteria can be used to
enhance oil production by microbial enhanced oil recovery method (MEOR).
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, soaps, paints, adhesives, inks, anti-fogs, ski waxes, snowboard wax, deinking of recycled papers, in flotation, washing and enzymatic processes, laxatives. Also agrochemical formulations such as Herbicides (some), insecticides, biocides (sanitizers), and spermicides (nonoxynol-9). Personal care products such as cosmetics, shampoos,shower gel, hair conditioners (after shampoo), toothpastes. Surfactants are used in firefighting and pipelines (liquid drag reducing agents). Alkali surfactant polymers are used to mobilize oil in oil wells). Surfactants are used as plasticizer in nanocellulose, Ferrofluids, and leak detectors. Surfactants are used with quantum dots 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.
