First, an interface is a boundary surface that exists between two substances with different properties, and interfaces exist between liquids and solids, liquids and liquids, and liquids and gases.
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Surfactants enhance performance by performing functions such as washing, emulsifying, dispersing, wetting, and penetrating at this interface.
Interface = a boundary surface that exists between two substances with different properties
Liquid and solid: cup and coffee, machine and lubricant
Liquid and liquid: water and oil
Liquid and gas: seawater and air, soap bubbles
Examples of roles of surfactants
Cleaning ・・・ Removing dirt
Emulsification ・・・ Dispersion ・・ Making unmixable things easier to mix
Wetting / Penetration ・・・ Makes wetting and soaking easier
Nonionic surfactants are used in the largest quantities among surfactants, and their raw materials, such as ethylene oxide, are stably supplied in large quantities.
Nonionic surfactants are surfactants that have hydroxyl groups (-OH) or ether bonds (-O-) as hydrophilic groups that do not dissociate into ions in water.
However, since hydroxyl groups and ether bonds do not dissociate into ions in water, their hydrophilicity is quite weak, so they alone do not have the power to dissolve large hydrophobic groups in water. Therefore, several of these groups come together in one molecule to exhibit good hydrophilicity. This is very different from anionic and cationic surfactants, where only one hydrophilic group is sufficient to exhibit hydrophilicity.
Polyethylene glycol chains are hydrophilic because water bonds loosely to the oxygen atoms of the ether bonds in the chain.
When an ether bond is hydrogen bonded to a water molecule, the surrounding water molecules see it as a peer, making it easier to dissolve in water. This is why they are hydrophilic.
In aqueous solutions of polyethylene glycol-type nonionic surfactants, water molecules are loosely attached to the ether bond sites by hydrogen bonds. Therefore, as the temperature rises or salts dissolve into the solution, the hydrogen bonds with the water molecules tend to gradually break off.
When an aqueous solution of a polyethylene glycol-type nonionic surfactant is heated and the temperature is gradually increased, the bound water molecules are gradually dislodged accordingly, resulting in a gradual decrease in hydrophilicity, and finally the surfactant is no longer soluble in water and precipitates, turning the initially clear solution into a cloudy emulsion.
Thus, when a clear aqueous solution of polyethylene glycol-type nonionic surfactant is gradually heated, the temperature at which the entire solution suddenly becomes cloudy and the surfactant precipitates as fine droplets is called the cloud point.
If the hydrophobic group materials are the same, the cloud point will also increase as the hydrophilicity increases with an increase in the number of moles of ethylene oxide added, and this cloud point can be used as a value representing the hydrophilicity of the nonionic surfactant.
The cloud point can be understood as an indication of how strong the hydrophilicity of the polyethylene glycol moiety attached to the hydrophobic group is compared to the strength of the hydrophobic group.
The effect of a surfactant is originally derived from the balance between the opposing properties of the hydrophobic and hydrophilic groups, and the cloud point, which indicates the degree of this balance, is the most important value that determines the properties of polyethylene glycol-type nonionic surfactants.
In fact, quality control of this type of surfactant and guidelines for its use are based on the measurement of the cloud point. For example, it is generally accepted that a surfactant with a cloud point near the operating temperature has excellent permeability. However, the presence of salts or alkalis such as sodium hydroxide causes the cloud point to drop dramatically, so in such cases, it is necessary to measure the cloud point under operating conditions to make a judgment.
Among alkylphenol EO adducts, nonylphenol, dodecylphenol, octylphenol, octylcresol, and other EO adducts are known.
Among them, nonylphenol EO adducts have been the mainstay of polyethylene glycol ether-type nonionic surfactants because of their superior detergency, penetration, and emulsifying power.
However, alkylphenols have been found to have endocrine disrupting effects, and the use of alkylphenol EO adducts has been declining as they are being replaced by alternative surfactants.
Natural alcohols have been replaced by synthetic alcohols for some time due to their generally high price volatility, but their use is now increasing due to environmental concerns and other factors. Generally speaking, C12~C14 alcohols are more suitable as surfactant raw materials than C16~C18.
The most typical saturated natural alcohol is palm oil-reduced alcohol (C12~C14), which is obtained by esterifying palm oil with methanol and reducing the resulting methyl palm oil fatty acid.
Unsaturated alcohols include palm oil alcohol and olive oil alcohol, which are obtained in a similar fashion from palm oil and olive oil, respectively. Both are mixtures of oleyl alcohol (CI8 double bond 1) and cetyl alcohol (C16), among others.
Beef fat reducing alcohol (C16~C18) obtained by hydrogenating methyl bovine fatty acid and macko alcohol (C16-C18 double bond 1) obtained by hydrogenating macko whale oil were also used, but are rarely used anymore due to the avoidance of using animal materials and the protection of whale resources. However, it is rarely used anymore due to the avoidance of using animal materials and the protection of whale resources.
It is made through the process of ethylene polymerization by the Ziegler process. It has a chemical structure (linear primary alcohol) identical to that of saturated natural alcohols.
The reaction of olefin with carbon monoxide and hydrogen yields a primary alcohol with one more carbon atom (oxo method). Although there are some special olefins that use branched-chain olefins such as the trimer and tetramer of propylene as the raw material olefin, the most common method uses linear-chain α-olefins, which are mainly linear primary alcohols like natural alcohols, with some branched primary alcohols mixed in.
Since ester bonds are susceptible to hydrolysis, this type of product may decompose into soap when used in strongly alkaline baths. This type of soap is also produced by addition of ethylene oxide as described above, but can also be easily produced by direct esterification of fatty acids with polyethylene glycol.
Polyoxyethylene fatty acid esters are generally inferior to higher alcohols or alkylphenol EO adducts in terms of penetration and detergency. Therefore, they are mainly used as emulsion dispersants, textile oils (for spinning and finishing), or dyeing auxiliaries.
To strengthen its characteristics as an oil-soluble emulsifier, polyethylene glycol is added to fats and oils such as olive oil and an alkali-catalyzed ester exchange reaction is performed to make a mixture of polyethylene glycol monooleate and glycerin monooleate, which is also widely used.
However, most are used as raw materials for blending and are not commercially available.
Ethylene oxide can be added also to higher alkylamines or fatty acid amides in the presence of alkaline catalyst.
Higher alkyl amines react particularly easily with ethylene oxide, so the reaction can be carried out without a catalyst. In such cases, the polyethylene glycol chain grows after the complete addition of two moles of ethylene oxide to the nitrogen atom first. This type of product has properties intermediate between those of nonionic and cationic surfactants and is used as a dyeing aid.
Fatty acid amides are relatively unreactive with ethylene oxide and usually react as in the following equation, but in reality they are a complex mixture of reactants. In the usual synthesis process, exchange reactions occur during the reaction and the ester and amide bonds are interchanged, resulting in the formation of some of the following compounds, which are nonionic surfactants with somewhat cationic properties. This type of surfactant is used for special applications and is used in relatively small quantities.
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A compound similar to ethylene oxide is propylene oxide.
Propylene oxide reacts by addition in the same way as ethylene oxide. However, its polymerization product, polypropylene glycol, has a limited water solubility; it is soluble in water up to a molecular weight of several hundred, but insoluble with molecular weight beyond that range. Therefore, polypropylene glycol with a molecular weight of about 1,000 to 2,500 is suitable as a hydrophobic group raw material.
Nonionic surfactants made by adding ethylene oxide to polypropylene glycol were first marketed by the Wyandotte Company in the United States under the trade name "Pluronic" and are therefore called Pluronic-type nonionic surfactants. and are therefore referred to as pluronic nonionic surfactants.
Pluronic-type nonionic surfactants have an unusual shape with hydrophilic groups at both ends with hydrophobic groups in between, as shown in the following formula. Since this type of surfactant has a molecular weight of several thousand, it is much higher in molecular weight than ordinary surfactants (molecular weight of several hundred), so it is sometimes classified as a polymer type surfactant.
Pluronic-type nonionic surfactants are not very promising as penetrating agents due to their molecular weight or molecular shape, but they are used in special applications due to their recognized characteristics as special low-foaming detergents, emulsifying dispersants, viscose additives, and the like.
Nonionic surfactants with hydrophobic groups attached to amino alcohols (e.g., diethanolamine) having -NH2 or >NH groups in addition to -OH groups or to saccharides (e.g., glucose) having 1CHO groups are similar to the polyhydric alcohol type. Therefore, they are collectively referred to as polyhydric alcohol-type nonionic surfactants in this section.
The main hydrophilic group materials of polyhydric alcohol-type nonionic surfactants are listed in the table below. Of these, glycerin, pentaerythritol, sorbitan, and diethanolamine are particularly important. Fatty acids are the most commonly used hydrophobic raw materials.
As shown in the table below, many polyhydric alcohol-type nonionic surfactants are not soluble in water, and most are only hydrophilic enough to be emulsified and dispersed in water. Therefore, they are rarely used as detergents or penetrating agents.
The appearance of polyhydric alcohol esters is similar to that of fats, oils, or fatty acids, and they are light yellow solids. Both glycerol esters and pentaerythritol esters are widely used as emulsifiers or raw materials for textile oils (spinning oil or softener), but there are differences in their detailed properties.
Fatty acid esters of glycerin
Glycerol monolaurate or glycerol monostearate is widely used as an emulsifier in food and cosmetics because of its high safety, and especially the technology to produce high-purity products has been developed. They are also used as oils for textiles, but their characteristics as fabric softeners are limited to relatively specialized applications.
Sorbitol is a sweet-tasting polyhydric alcohol produced by reducing glucose with hydrogen and has six hydroxyl groups.
Since sorbitol has no aldehyde groups in its molecule, it is more stable to heat and oxygen than glucose, and there is no risk of decomposition or coloration when reacting with fatty acids.
Sorbitol esters are suitable for textile softeners, but do not work well as general W/O emulsifiers.
Sorbitan is a polyhydric alcohol with four hydroxyl groups, but various isomers are formed depending on the position of the hydroxyl group that reacts when sorbitol is dehydrated. Therefore, what is commonly called sorbitan is a mixture of various sorbitans, not a compound with a single composition. Sorbitan is further dehydrated and has only two hydroxyl groups. In fact, when sorbitol is dehydrated, the reactions shown above occur in a complex manner to produce a mixture of many compounds. Therefore, these sorbitol dehydration products are sometimes collectively called anhydrosorbitols.
When the esterification reaction of sorbitol is carried out at 230-250°C, intramolecular dehydration (sorbitanation) of sorbitol also occurs at the same time. If the reaction is stopped after an appropriate time, monopalmitate esters of sorbitan can be obtained in one step.
If the reaction is further continued to proceed with intramolecular dehydration, a product consisting mainly of the sorbitan ester can be obtained.
Sorbitan esters have excellent performance as emulsifiers and textile oils.
Sorbitan ester-type nonionic surfactants are so well known that they are called "spun-type nonionic surfactants" since they were first marketed by Atlas Corporation in the United States under the trade name "Span" (Span) in various varieties.
These sorbitan esters are mainly used as emulsifiers, but since they themselves are almost insoluble in water, they are rarely used alone.
Fatty acid esters of polyhydric alcohols and sugars are susceptible to hydrolysis. Instead of these esters, those linked by amide bonds are surfactants that are also resistant to hydrolysis. Many polyhydric alcohol-type nonionic surfactants with amide bonds have been synthesized by combining fatty acids with compounds that have amino and hydroxyl groups.
The most prominent of these polyhydric nonionic surfactants with amide bonds is fatty acid alkanolamide, which is synthesized by the condensation of alkanolamine and fatty acids.
Fatty acid alkanolamides were first marketed by the U.S.-based Ninol Corporation and were therefore also called "Ninol-type detergents. This is the product of dehydration-condensation of 1 mole of lauric acid or palm oil fatty acid with 2 moles of diethanolamine.
Although this formula may seem to leave an extra mole of diethanolamine, the extra diethanolamine is actually loosely bound to the produced lauric acid diethanolamide, making the resulting fatty acid alkanolamide very water soluble.
It is also called 1:2 fatty acid diethanolamide because it is produced at a ratio of 2 moles of diethanolamine to 1 mole of fatty acid.
The detergency-enhancing and foam-stabilizing effects of 1:2 fatty acid alkanolamides described above are caused by their main component, the fatty acid alkanolamide, and have little to do with the second mole of diethanolamine. Therefore, when added as a foam stabilizer to a highly water-soluble detergent, such as sodium dodecylbenzenesulfonate, the extra diethanolamine is unnecessary, as it is added simply to provide water solubility.
From this perspective, 1:1 type fatty acid diethanolamides without the second mole of diethanolamine were produced for compounding. Lauric acid or coconut oil fatty acid is still used as the fatty acid, but it is usually made into a methyl ester to facilitate the reaction.
This one is widely used as a base for detergent formulations because of its high purity and economic efficiency. A 1:1 type alkanolamide is also made from monoethanolamine and monoisopropanolamine and used for similar purposes.
Of the raw materials shown in this table, ethylene oxide is produced inexpensively due to the development of petrochemistry. In addition to those derived from natural products, a wide variety of synthetic higher alcohols have also appeared on the market.
Furthermore, considering the excellent performance and versatility of polyethylene glycol-type nonionic surfactants, this type of product is likely to become increasingly important in the future.
In addition to those listed in the table above, there are also higher alkyl mercaptans (R-SH) as hydrophobic group materials and dipentaerythritol and polyglycene as hydrophilic group materials, but these are omitted in this section.
Reference: "Introduction to Surfactants" by Takehiko Fujimoto, Sanyo Kasei Kogyo ()
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