Introduction to Nonionic Surfactant

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.

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

-Surfactants have different structures in their molecules with different properties: lipophilic groups (oil-loving parts) and hydrophilic groups (water-loving parts).

-Surfactants are broadly classified into four types according to the structure of the hydrophilic group: nonionic, anionic, cationic, and amphoteric (having both anionic and cationic groups).

It consists of seven short movies for each function. 
0:00　Introduction of surfactants functions　
0:16　Part1　Washability
1:00　Part2　Permeability
2:10　Part3　Dispersion
2:55　Part4　Foaming properties
3:25　Part5　Defoaming properties
3:39　Part6　Smoothness
4:20　Part7　Antibacterial properties

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.

Nonionic surfactants can be classified by the type of hydrophilic group into polyethylene glycol type and polyhydric alcohol type.
Polyethylene glycol-type nonionic surfactants are nonionic surfactants made by adding ethylene oxide as a hydrophilic group to a hydrophobic raw material.

Fig. Example of polyethylene glycol-type nonionic surfactant

Polyhydric alcohol-type nonionic surfactants are nonionic surfactants in which a hydrophobic group such as a higher fatty acid is bonded to a polyhydric alcohol such as glycerol, pentaerythritol, sorbitol, etc. The hydrophobic group has multiple hydroxyl groups bonded to it, which gives it hydrophilic properties.

Fig. Example of polyhydric alcohol-type nonionic surfactant

A more detailed classification of polyethylene glycol-type nonionic surfactants is made according to the type of hydrophobic group, whereas in the case of polyhydric alcohol-type nonionic surfactants, it is made according to the type of polyhydric alcohol, the hydrophilic group. This classification is shown in the table below.

Polyethylene glycol-type nonionic surfactants are surfactants made by adding ethylene oxide (EO: ethylene oxide) to hydrophobic raw materials containing reactive hydrogen atoms.

 Reactive hydrogen atoms are specifically hydrogen atoms such as hydroxyl group (-OH), carboxyl group (-COOH), amino group (-NH2), or amide group (-CONH2). Hydrophobic groups bonded to the above atomic groups can react with ethylene oxide to form polyethylene glycol-type nonionic surfactants. For example, a higher alcohol with a hydroxyl group reacts to an EO adduct as follows

Fig. Reaction of higher alcohols with ethylene oxide

The following hydrophobic raw materials with reactive hydrogen atoms are currently used in relatively large quantities. Among these, higher alcohols are particularly important. However, alkylphenols are no longer used since it was found that they have endocrine disrupting effects.

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.

Nonionic surfactants are obtained by adding ethylene oxide to the phenolic hydroxyl group of alkyl phenols or the alcoholic hydroxyl group of higher alcohols. These are also called polyethylene glycol ether type nonionic surfactants because the hydrophobic and hydrophilic groups are linked by an ether bond (-O-). This type of surfactant is rarely degraded by acids or alkalis.

The majority of polyethylene glycol-type nonionic surfactants currently on the market are made by adding ethylene oxide to higher alcohols. Higher alcohols, which are used as raw materials for hydrophobic groups, can be broadly classified into two types: natural alcohols obtained from animal and vegetable oils and waxes, and synthetic alcohols made from petroleum.
Higher alcohols are often used as mixtures of various carbon numbers.

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.

Typical example of saturated natural alcohol: Coconut oil-reduced alcohol

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.

Typical examples of unsaturated natural alcohols: palm oil alcohol, olive oil alcohol

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.

About Natural Alcohols of Animal Origin

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.

Synthetic alcohols include Ziegler alcohol, which has the same composition as natural alcohols, oxo alcohol, which contains a methyl-branched component, and secondary alcohol, which has a hydroxyl group in the middle of the molecule. All are widely used because they are inexpensive and in stable supply.

Secondary alcohol

It is made by air oxidation of paraffin. It has hydroxyl groups randomly attached to the ends of the carbon chain (linear secondary alcohol). It is usually sold as a 3-mol ethylene oxide adduct. This is because ethylene oxide does not uniformly add under normal reaction conditions without the addition of ethylene oxide.

A comparison of polyethylene glycol ether type nonionic surfactants (higher alcohol EO adducts and alkylphenol EO adducts) with anionic surfactants is shown in the table below.

Ethylene oxide can also be added to fatty acids with an alkali catalyst. This type of product is sometimes called a polyethylene glycol ester-type nonionic surfactant because, as the reaction formula shows, the hydrophobic and hydrophilic groups are linked by an ester bond (-COO-).

Fig. EO adducts of fatty acids

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.

Fig. Polyethylene glycol laurate

Ethylene oxide can be added also to higher alkylamines or fatty acid amides in the presence of alkaline catalyst.

Higher alkyl amine ethylene oxide adduct

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.

Fig. Higher alkyl amine ethylene oxide adduct

Fatty acid amide ethylene oxide adduct

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.

Fig. Fatty acid amide ethylene oxide adduct

Pluronic type nonionic surfactant

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.

Fig. Structure of Pluronic Nonionic Surfactant

What is a polyhydric alcohol?

Polyhydric alcohols are organic compounds with many alcoholic hydroxyl groups per molecule, such as glycerin, pentaerythritol, and sorbitol.

Fig. Example of polyhydric alcohol

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.

Fig. Glycerol monolaurate

Fatty acid esters of pentaerythritol

Pentaerythritol stearate, for example, is also used as an emulsifier, and is widely used as a base for textile oil formulations due to its excellent softening properties for human silk, staple fibers, and cotton.

Fig. Example of Pentaerythritol Fatty Acid Esters

Fatty acid esters of sorbitol

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.

Fig. Example of fatty acid esters of sorbitol

Dehydration-condensation reaction of sorbitol

Sorbitol, when treated under appropriate conditions (e.g., acidic and heated), becomes sorbitan, which is dehydrated one molecule of water, followed by sorbide, through further dehydration of another one molecule of water.

Fig. Dehydration-condensation reaction of sorbitol

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.

Synthesis of sorbitan esters

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.

Fig. Example of synthesis of sorbitan ester

Sorbitan Ester Performance and Applications

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.

Ethylene oxide adduct of fatty acid esters of sorbitan

A variety of water-soluble emulsifiers are used in sorbitan esters, including a nonionic surfactant with the trade name Tween, which is made by adding ethylene oxide to sorbitan esters.

Fig. Ethylene oxide adducts of fatty acid esters of sorbitan

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.

Fig. Fatty acid alkanolamides

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.

1:2 type fatty acid alkanolamide

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.

Fig. 1:2 fatty acid alkanolamide (Ninol detergent)

1:1 type fatty acid alkanolamide

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.

Fig. 1:1 Fatty Acid Diethanolamide

Two very different types of nonionic surfactants with very different properties

Many polyethylene glycol-type nonionic surfactants are well soluble in water and are mainly used as detergents, dyeing aids, and emulsifiers,

Many polyhydric alcohol-type nonionic surfactants are insoluble in water and are mainly used as fabric softeners and emulsifiers. Nonionic surfactants are classified by hydrophobic and hydrophilic group materials as shown in the table below.

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 (2014)

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