Surfactant


A surfactant is a chemical compound that decreases the surface tension or interfacial tension between two liquids, a liquid and a gas, or a liquid and a solid. The word surfactant is a blend of "surface-active agent", coined in 1950. As they consist of a water-repellent and a water-attracting part, they are emulsifiers, enabling water and oil to mix. They can also form foam, and facilitate the detachment of dirt.
Surfactants are among the most widespread and commercially important chemicals. Private households as well as many industries use them in large quantities as detergents and cleaning agents, but also as emulsifiers, wetting agents, foaming agents, antistatic additives, and dispersants.
Surfactants occur naturally in traditional plant-based detergents, e.g. horse chestnuts or soap nuts; they can also be found in the secretions of some caterpillars. Some of the most commonly used anionic surfactants, linear alkylbenzene sulfates, are produced from petroleum products. However, surfactants are increasingly produced in whole or in part from renewable biomass, like sugar, fatty alcohol from vegetable oils, by-products of biofuel production, and other biogenic material.

Classification

Surfactants are compounds with hydrophilic "heads" and hydrophobic "tails." The "heads" of surfactants are polar and may or may not carry an electrical charge. The "tails" of most surfactants are fairly similar, often consisting of a hydrocarbon chain and may comprise aromatic units. Most commonly, surfactants are classified according to the polarity of their 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, or amphoteric.
However, surfactants may also be classified based on chemical structure or based on their properties / their application.

Classification according to charge / polarity

Anionic: sulfate, sulfonate, and phosphate, carboxylate derivatives

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, and sodium myreth sulfate.
Others include:
Carboxylates are the most common surfactants and comprise the carboxylate salts, such as sodium stearate. More specialized species include sodium lauroyl sarcosinate and carboxylate-based fluorosurfactants such as perfluorononanoate, perfluorooctanoate.

Cationic head groups

Cationic surfactants are extensively described in this review.
pH-dependent primary, secondary, or tertiary amines; primary and secondary amines become positively charged at pH < 10: octenidine dihydrochloride.
Permanently charged quaternary ammonium salts: cetrimonium bromide, cetylpyridinium chloride, benzalkonium chloride, benzethonium chloride, dimethyldioctadecylammonium chloride, and dioctadecyldimethylammonium bromide.

Zwitterionic surfactants

ic 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 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.
Lauryldimethylamine oxide and myristamine oxide are two commonly used zwitterionic surfactants of the tertiary amine oxides structural type.

Non-ionic

Non-ionic 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 non-ionic surfactants therefore decreases with increasing temperature.
Non-ionic surfactants are less sensitive to water hardness than anionic surfactants, and they foam less strongly. The differences between the individual types of non-ionic surfactants are slight, and the choice is primarily governed having regard to the costs of special properties or permission for use in food.
Ethoxylates
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, see also poloxamers.
Fatty alcohol ethoxylates
Fatty acid ethoxylates are a class of very versatile surfactants, which combine in a single molecule the characteristic of a weakly anionic, pH-responsive head group with the presence of stabilizing and temperature responsive ethyleneoxide units.
Special ethoxylated fatty esters and oils
Ethoxylated amines and/or fatty acid amides
Fatty acid esters of glycerol
Tweens:
Alkyl polyglucosides
are a class of non-ionic surfactants made from a sugar and a fatty alcohol. They are produced from renewable resources, possess a high biodegradability and mildness. For these reasons, they are widely used in detergents, cosmetics, and other applications.
Most surfactants comprise "tails" based on saturated or unsaturated hydrocarbons. Fluorosurfactants have fluorocarbon chains. Siloxane surfactants have siloxane chains.
Surfactant molecules have either one tail or two; those with two tails are said to be double-chained.
Amino acid-based surfactants are surfactants derived from an amino acid. Their properties vary and can be either anionic, cationic, or zwitterionic, depending on the amino acid used and which part of the amino acid is condensed with the alkyl/aryl chain.
Gemini surfactants consist of two surfactant molecules linked together at or near their head groups. Compared to monomeric surfactants, they have much lower critical micelle concentrations.

Classification according to properties / application

Composition and structure

Surfactants are compounds that are amphiphilic, which means that this molecule each contains a hydrophilic "water-seeking" group, and a hydrophobic "water-avoiding" group. As a result, a surfactant contains both a water-soluble component and a water-insoluble component. Surfactants diffuse in water and get adsorbed 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 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 lipophilic or lipophobic depending on its chemistry. Hydrocarbon groups are usually lipophilic, for use in soaps and detergents, while fluorocarbon groups are lipophobic, for use in repelling stains or reducing surface tension.
World production of surfactants is estimated at 15 million tons per year, of which about half are soaps. Other surfactants produced on a particularly large scale are linear alkylbenzene sulfonates, 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 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 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 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 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.