Because of the dual affinity of an amphiphilic molecule, it does not feel “at ease” in any solvent, be it polar or non-polar, since there is always one of the groups which “does not like” the solvent environment. Therefore, amphiphilic molecules exhibit a very strong tendency to migrate to interfaces or surfaces and to orientate so that the polar group lies in water and the non-polar group is placed out of it, and eventually in oil. In English, the term surfactant (surface-active-agent) designates a substance, which exhibits some superficial or interfacial activity. Only the amphiphiles with equilibrated hydrophilic and lipophilic tendencies are likely to migrate to the surface or interface. It does not happen if the amphiphilic molecule is too hydrophilic or too hydrophobic, in which case it stays in one of the phases.
In other languages, such as French, German, or Spanish the word “surfactant” does not exist, and the actual term used to describe these substances is based on their properties to lower the surface or interface tension, for example, tension active (French), tenside (German), tension active (Spanish). This would imply that surface activity is strictly equivalent to tension lowering, which is not general, although it is true in many cases. Amphiphiles exhibit other properties than tension lowering and therefore they are often labeled as per their main use such as soap, detergent, wetting agent dispersant, emulsifier, foaming agent, bactericide, corrosion inhibitor, antistatic agent, etc. In some cases, they are known from the name of the structure they can build, i.e. membrane, microemulsion, liquid crystal, liposome, vesicle, or gel.
Classification of Surfactants:
From the commercial point of view, surfactants are often classified as per their use. The most accepted and scientifically sound classification of surfactants is based on their dissociation in water.
Anionic Surfactants are dissociated in water in an amphiphilic anion, and a cation, which is, in general, an alkaline metal (Na+, K+ ) or quaternary ammonium. They are the most commonly used surfactants. They include alkyl benzene sulfonates (detergents), (fatty acid) soaps, lauryl sulfate (foaming agent), di-alkyl sulfosuccinate (wetting agent), lignosulfonates (dispersants), etc.
Nonionic Surfactants do not ionize in an aqueous solution, because their hydrophilic group is of a non-dissociable type, such as alcohol, phenol, ether, ester, or amide. A large proportion of these nonionic surfactants are made of a hydrophilic portion (by the presence of a polyethylene glycol chain) and lipophilic portion (alkyl or alkylbenzene).
Cationic Surfactants are dissociated in water into an amphiphilic cation and an anion, most often of the halogen type. A very large proportion of this class corresponds to nitrogen compounds such as fatty amine salts and quaternary ammoniums, with one or several long chains of the alkyl type, often coming from natural fatty acids. They are used as a bactericide and as positively charged substances, which can adsorb on negatively charged substrates to produce antistatic and hydrophobic effects. When surfactant molecules exhibit both anionic and cationic dissociations it is called amphoteric or zwitterionic, for example, betaines or sulfobetaine, and natural substances such as amino acids and phospholipids.
Polymeric surfactants are often not accounted as surfactants. Their importance is growing, however; because they enter in many formulated products as dispersants, emulsifiers, foam boosters, viscosity modifiers, etc. Some of the commonly used are polyEO-PolyPO block copolymers, ethoxylated or sulfonated resins, carboxymethyl cellulose and other polysaccharide derivatives, polyacrylates, xanthene, etc.
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