Free fatty acids are not used as surfactants, due to their low water solubility. However water-soluble fatty acid salts like alkali and short-chain amine salts (ethanol amine, diethanol amine, triethanol amine) show good water affinity and are widely used. Saturated sodium soaps are extremely soluble in water up to C₈ (these are not yet true surfactants); they become less soluble up to C₁₈ (i.e., the domain of effective surfactants) and insoluble above C₂₀. The fatty acids can be either saturated or unsaturated, starting from C₁₆chain lengths. Unsaturated fatty acids are prone to undergo oxidation and form oxides and peroxides which cause rancidity and yellowing. Potassium soaps and salts of alkanolamines are more fluid and also more soluble than sodium salts.

Fatty acid alkali salts ($\mathrm{R}={\mathrm{C}}_7{\mathrm{H}}_{15}\text{−}{\mathrm{C}}_{17}{\mathrm{H}}_{35}$)

Fatty acid earth alkali salts ($\mathrm{R}={\mathrm{C}}_7{\mathrm{H}}_{15}\text{−}{\mathrm{C}}_{17}{\mathrm{H}}_{35}$)

Fatty acid ethanol amine salts ($\mathrm{R}={\mathrm{C}}_7{\mathrm{H}}_{15}\text{−}{\mathrm{C}}_{17}{\mathrm{H}}_{35}$)

Fatty acid isopropanol amine salts ($\mathrm{R}={\mathrm{C}}_7{\mathrm{H}}_{15}\text{−}{\mathrm{C}}_{17}{\mathrm{H}}_{35}$)

They are monoesters of di- and tricarboxylic acids. These esters are produced by condensation reactions involving different types of molecules: either an alcohol with a polycarboxylic acid (e.g., tartaric or citric acid), or an hydroxyacid (e.g., lactic acid or citric acid) with a carboxylic acid. The alcohol may have been previously ethoxylated to enhance water solubility and surface activity.

Sodium dilaureth-7-citrate

Stearoyl disodium tartrate

These surfactants are formed by the reaction of sodium chloracetate with ethoxylated alcohols. Due to the addition of ethoxylated groups, ether carboxylates are more soluble in water and less sensitive to water hardness compared to conventional soaps. Also, keeping the best properties of nonionic surfactants, they do not exhibit any cloud point and show good wetting and foam stability. Ether carboxylates do not undergo hydrolysis in the presence of alkalis or acids.

Alkyl polyglycol ether carboxylate, sodium salt ($\mathrm{R}={\mathrm{C}}_8{\mathrm{H}}_{17}\text{−}{\mathrm{C}}_{18}{\mathrm{H}}_{37}$)

Alkyl sulfates are organic esters of sulfuric acid; the sulfur atom is bridged to the carbon atom of the hydrocarbon chain via an oxygen atom. Sodium lauryl sulfate (SLS), one of the most common surfactants, belongs to this class.

Sodium alkylsulfate (${\mathrm{R}}_{\text{opt.}}={\mathrm{C}}_{12}{\mathrm{H}}_{25}\text{−}{\mathrm{C}}_{14}{\mathrm{H}}_{29}$)

Ammonium alkylsulfate (${\mathrm{R}}_{\text{opt.}}={\mathrm{C}}_{12}{\mathrm{H}}_{25}\text{−}{\mathrm{C}}_{14}{\mathrm{H}}_{29}$)

Monoethanol amine alkylsulfate (${\mathrm{R}}_{\text{opt.}}={\mathrm{C}}_{12}{\mathrm{H}}_{25}\text{−}{\mathrm{C}}_{14}{\mathrm{H}}_{29}$)

Alkyl ether sulfates (AES), which are also called alcohol ethoxy sulfates (AEOS), result from the sulfation of an ethoxylated alcohol.

Sodium alkyl ether sulfate

Three major types of alkyl sulfonates must be considered: the primary and secondary paraffin sulfonates (PS and SAS) and the $\alpha$-olefin sulfonates (AOS).

Primary sodium alkyl sulfonate

Secondary sodium alkyl sulfonate

Sodium alkene sulfonate

Sodium hydroxy alkane sulfonate

Linear alkylbenzene sulfonates are the most important surfactants used. They are biodegradable. Linear alkylbenzene sulfonates exhibit good chemical and thermal stabilities and can be incorporated in spray-dried slurries.

Sodium linear alkylbenzene sulfonate (LAS) ($\mathrm{R}={\mathrm{C}}_9{\mathrm{H}}_{19}\text{−}{\mathrm{C}}_{15}{\mathrm{H}}_{31}$)

Sulfosuccinates are the sodium salts of alkyl esters of sulfosuccinic acid; monoesters of sulfosuccinic acid based on linear fatty alcohols are only partially water-soluble and hardly dispersible. Those based on fatty alcohol ethoxylates exhibit much better solubility. Dialkyl esters based on alcohols with less than nine carbons, preferably five to eight carbon atoms, as well as those based on fatty acid ethanol amides are water soluble and, therefore, are generally preferred. Disodium salts of monoesters deliver good detergency and foam properties. Due to the ester linkage, all sulfosuccinates are sensitive to hydrolysis, especially under acidic conditions.

Sodium dialkyl sulfosuccinate

Sodium alkyl sulfosuccinamate

Sodium ethoxylated alkylamido sulfosuccinate

Among this group of surfactants the $\alpha$-sulfo fatty acid esters are commonly used on an industrial scale. The $\alpha$-sulfo fatty acid esters contain the sulfonate group statistically distributed along the carboxylate chain.

Alkyl ester of $\alpha$-sulfo fatty acid sodium salt

Taurides (or taurates) are acylamino alkane sulfonates which have chemical structures close to isethionates.

Fatty acid isethionate

Tauride

Acyl glutamtes are based on $\alpha$-aminoglutaric acid.

Sodium acylglutamate

Acyl peptides are formed from hydrolyzed proteins e.g., animal collagen. The average polypeptide molecular weight can vary from about 350 to 2000.

Sodium acyl polypeptide(X = amino acids side groups)

Sarcosinates (or salts of acyl amino acids) are the condensation products of fatty acids with N-methylglycine (CH₃-NH-CH₂-COOH) (or sarcosine).

Acylamino acid sodium salt

The water solubility of quaternaries primarily depends on the nature of R substituents (hydrophobic chain lengths, polarity, etc.). Quaternaries carrying two or more long hydrophobic chains have very poor water solubility. Low-solubility quaternaries can adsorb on various substrates and impart various useful conditioning effects (softening, antistatic, corrosion inhibition, etc.). Quaternaries are generally not compatible with anionics because of the formation of a water-insoluble complex.

Heterocyclic quaternaries are derived from heterocyclic aliphatic or aromatic compounds. They are often based on morpholine, imidazoline, pyridine and isoquinoline.

Alkylethyl morpholinium ethosulfate

Alkylpyridinium chloride

Dialkylmethyl imidazolinium methosulfate

There is a wide range of emulsifiers, wetting agents and solubilizers based on fatty alcohols available. They vary in both: the chain length of the fatty alcohol and the ethoxyl content. The influence of the fatty alcohol chain length on the properties of the compound is small compared to that of the polyoxyethylene chain.

Fatty alcohol polyglycol etherwith $x={\mathrm{C}}_8\text{−}{\mathrm{C}}_{18}$ and $y=\sim 2\text{to}300$

These surfactants consist of a central polypropylene glycol part (PPG) representing the hydrophobic portion of the molecule and two hydrophilic polyethylene glycol chains (PEG). They are also called EO/PO block polymers. Depending on the mol weight and the proportion between PPG and PEG a wide variety of surface active agents can be produced, showing different properties.

They are also called alkylphenol polyglycol ethers and belong to the most important types of washing active substances with excellent wetting properties. Due to ecotoxicological reasons their use however diminishs. The octylphenol and nonylphenol ethers are of special importance.

Octylphenol polyglycol ether

Nonylphenol polyglycol ether

The numerous hydroxyls of glucoside groups ensure the solubility of the whole molecule in water. Alkylpolyglucosides (APGs) show good water solubility and have their cloud points at rather high temperatures (generally above 100℃); they are only slightly sensitive to the presence of electrolytes and are only very rarely influenced by water hardness. The optimal detergency of APGs is found for an average alkyl chain length around 13 and a glucosidic content of about 65 %. Alkylpolyglucosides show good chemical stability at neutral and alkaline pH. Their biodegradability is excellent.

AlkylpolyglucosideR = fatty acid

This class of surfactants covers ethoxylated derivatives of lanolin (wool fat) and castor oil. Lanolin is the generic name of a wax containing a complex mixture of esters and polyesters of high-molecular-weight alcohols (aliphatic, steroid, and triterpenoid) and fatty acids (saturated, unsaturated, hydroxylated, and nonhydroxylated). Castor oil is the natural extract resulting from cold pressing of ricinus seeds, a mixture of a triglyceride of fatty acids (ricinoleic 87.5 %, oleic 5 %, linoleic 4 %, palmitic 1.5 %, linolenic 0.5 %, stearic 0.5 %, dihydroxystearic 0.5 % and arachidic 0.5 %).

Alkanolamides are N-acyl derivatives of monoethanolamine and diethanolamine.

Monoalkanolamide

Dialkanolamide

Esteramide

The reaction of an alkanolamide with ethylene oxide leads to an ethoxylated amide. Their properties are comparable to those of ethoxylated alcohols.

Polyethoxylated monoalkanolamide

Polyethoxylated dialkanolamide

Theoretically POE could react at both ends with one fatty acid. Nevertheless the reaction of a fatty acid with ethylene oxide leads mainly to the monoester with a broad distribution in the molar EO number $\left(n\right)$; the reaction of polyethylene glycol with fatty acids leads to a mixture of mono- and diesters besides free fatty acid.

Fatty acid monoester

Fatty acid diester

The monoesters are much more soluble in water than the diesters. These surfactants are readily hydrolyzed under acidic or alkaline conditions.

The esters of fatty acids and glycol or glycerol are lipopholic surfactants showing a HLB value below 10.

Ethylene glycol ester

Propylene glycol ester

Glycerol monoester(Monoglyceride)

Glycerol-1,3-diester(Diglyceride)

Sorbitol can form two different sorbitans by internal dehydration: the 1,4- and the 1,5-sorbitan. Further dehydration of the 1,4-sorbitan leads to iso-sorbide. In common sorbitan based surfactants the 1,4-sorbitan is dominating. Fatty acid esters of sorbitan are insoluble in water, their HLB value is below 10.

Chemical structure of the basic compounds

Sorbitol

1,4-sorbitan

1,5-sorbitan

iso-sorbide

Chemical structure of sorbitan based emulsifiers

1,4-Sorbitan monoester

1,4-Sorbitan triester

Alkyl carbohydrate esters are also known under the names ”sugar esters” and”sucrose esters”. Normally they ar based on saccharose. Mono- and diesters are produced. The mono esters are water soluble while the dieseters are not. Due to the steric effects, primary hydroxyl groups are almost exclusively subject to esterification. They are of great interest due to their natural origin of their components and good biodegradability.

Saccharose fatty acid monoester

Saccharose fatty acid diester

Ethoxylated glycol monoester

Ethoxylated glycerol monoester

Besides the monoesters diesters are possible mainly in the case of glycerol.

Ethoxylation of sorbitan fatty acid esters leads to an important surfactant class for pharmacy and cosmetics. The fatty acids used are: lauric, myristic, palmitic, stearic and oleic acid.

Ethoxylated 1,4-sorbitan monooleate (as an example) $w+x+y+z\cong 20$

This is the only class of ether bounds containing emulsifiers which do not contain polyoxyethylene groups. The ether linkage is formed from glycerol units.

Polyglycerol monoleate