Introduction to Cationic 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

Hydrophilic Groups of Cationic Surfactants

A sodium ion (Na+) and a potassium ion (K+) are certainly hydrophilic groups with positive charges, but there is no way to combine them with hydrophobic groups. However, among the same inorganic cations, it seems possible to find a way of combining an ammonium ion (NH4+) with hydrophobic groups. For instance, it might be possible to substitute a hydrogen atom of ammonium chloride (NH4+Cl-) with a hydrophobic group such as an alkyl group.

In fact, a hydrogen atom can be substituted with an alkyl group, and furthermore, not only one, but also two, three or four hydrogen atoms can be substituted freely. That is, four kinds of substituted ammonium chlorides, each having a specific name, can be prepared, as shown in the table below.

Synthesis of Cationic Surfactants by Neutralization of Amines

In actual practice they are not manufactured by substitution of hydrogen atoms of ammonium chloride with alkyl groups. Instead, just as ammonium chloride is prepared by neutralization of ammonia with hydrochloric acid, alkylated ammonium chlorides (hydrochloric acid salts of amines) with up to three alkyl groups are manufactured easily by neutralization with hydrochloric acid of appropriate primary amines, secondary amines or tertiary amines, respectively, as shown below.

Thus, higher alkylamines can be converted easily to cationic surfactants by simple neutralization with acids. Accordingly, not only strong inorganic acids such as hydrochloric acid, but also relatively weak lower fatty acids such as formic acid and acetic acid may yield amine salt type cationic surfactants by neutralizing amines with them.

Examples of laurylamine acetate

For example, acetic acid salt of laurylamine is obtained as a result of exothermic neutralization by adding a calculated amount of acetic acid during stirring to laurylamine (C12H25NH2, a white, waxy solid insoluble in water), which has been melted by heating to 60 to 70 °C.

Acetic acid salt of laurylamine thus obtained is an excellent surfactant which is easily soluble in water. 

Fig. Formation reaction of laurylamine acetate

Raw Material Amines for Cationic Surfactants

In this connection, special attention should be paid to the fact that neutralization of higher alkylamines with acids such as higher fatty acids, which are difficult to dissolve in water, results in formation of salts of amines that are insoluble in water.

Then, what kinds of amines can be used as raw materials for the cationic surfactants mentioned above ? To make you familiar with these amines, The table below shows not only higher alkylamines that are convertible into surfactants directly but also lower alkylamines, which are important as intermediates.

Synthesis of quaternary ammonium chlorides with alkylating agents

Incidentally, you may note that mono-, di- or trialkylammonium chloride (hydrochloric acid salt of primary, secondary or tertiary alkylamine) can certainly be prepared by the process mentioned above, but tetraalkylammonium chlorides (quaternary ammonium chlorides) cannot be prepared in the same way In other words, it is impossible to prepare alkylammonium chlorides with four alkyl groups that are linked to the central nitrogen atom by neutralizing of any kind of amine with hydrochloric acid.

The table bellow shows typical examples of alkylating agents which are used frequently to manufacture quaternary ammonium salt type cationic surfactants. These alkylating agents yield quaternary ammonium salts through the following reaction examples.

Fig. Formation reaction of quaternary ammonium salts by alkylating agents

In this page, salts of primary, secondary and tertiary amines are called collectively amine salt type cationic surfactants because there are no sharp distinctions among them, as a result of their similar properties. Furthermore, many commercial products contain primary and secondary amines together.

Since products of this type are classified into surfactants, it is no wonder that each of their hydrophobic groups consists predominantly of alkyl groups with carbon numbers from 12 to 18. However, curiously enough, amine salt type cationic surfactants are rarely used for applications that would require surface activity, in the basic sense of the term. Examples will be introduced in the following pages.

Synthetic method for aliphatic higher amines

Higher alkylamines are synthesized by hydrogenating fatty nitriles, which are prepared by heating fatty acids or esters with ammonia.

Fig. Synthetic route of aliphatic amines

Coconut amine containing mainly laurylamine (C12), and hydrogenated tallow amine containing mainly stearylamine (C18), are manufactured commercially by this process from coconut oil and tallow, respectively Low-priced amines such as rosin amine produced from rosin acid are also available.

As previously discussed, these amines are converted to surfactants easily by neutralization with acids such as hydrochloric acid and acetic acid. Relatively speaking, however, only a small amount of salts of higher alkylamines is consumed commercially.

Synthetic method for higher alkylamine ethylene oxide adducts

Water solubility of higher alkylamine ethylene oxide adducts

Examples of applications of higher alkylamine ethylene oxide adducts

The effect of this structure's nonion-cation combination is directly apparent in its properties, and it is used relatively widely for special applications, for example, as a dyeing auxiliary.

Higher alkylamines are fairly expensive, so there is a demand to manufacture cationic surfactants from raw materials that are cheaper. Whatever the approach, there may be no appropriate way to achieve this other than to use something like fatty acid, which is a cheap source of hydrophobic groups. Then, it will be of great practical value to try to synthesize cationic surfactants by reacting fatty acids such as oleic acid and stearic acid with lower alkylamines. In practice, the reaction proceeds very smoothly, and furthermore, it easily yields products with a variety of structures.

In many cases, this series of products is not only inexpensive, but also excellent in performance. Cationic surfactants used as textile softeners generally belong to this series.

Products in this category in particular are among the less expensive cationic surfactants because they can be manufactured easily with cheap raw materials. They also have high performance, and are still used commercially today. Their disadvantage is the easy hydrolysis of the ester linkage that combines the hydrophobic group with the hydrophilic group. Hydrolysis is prevented to some extent when strong acid is used for neutralization of the amine.

Ahcovel A is a typical product among the Ahcovel series of cationic softeners, first developed by Arnold Hoffman Co. of the USA in the 1940s.

Ahcovel A is manufactured by a series of complicated reactions. An intermediate obtained by condensation between stearic acid and aminoethyl- ethanolamine during heating is reacted with urea, and the resultant amine is neutralized by acetic acid. 

Fig. Synthesis route of urea condensed amine salt type cationic surfactant (Ahcobel A)

Close examination of the structural formula of Ahcovel A reveals that it does not contain any amino group. If so, how is neutralization with acetic acid possible? As the above formula appears in a patent owned by that company, this is the formula that has generally been in use. 

Urea condensed amine salt type cationic surfactant applications

This Ahcovel series of products shows excellent performance as textile softeners, and many companies supply equivalents. Other amines such as diethylenetriamine are also used in some cases. Cationic surfactants sold as textile softeners of this kind are supplied generally in the form of a white paste of aqueous emulsion with a concentration of 20% to 40%. When warm water is added during stirring, a dilute emulsion with fine and uniform particles is easily obtained.

When amines such as aminoethylethanolamine and polyethylenepolyamine are reacted with fatty acids at 160 to 180 °C, amines in the form of amide are obtained, as discussed above. When they are reacted at higher temperatures between 200 and 250 °C, amines of a new type called imidazoline derivatives are obtained. Imidazoline derived from oleic acid and aminoethylethanolamine is shown in the following formula, but it is actually not as simple as the compound shown in this formula. Rather, it is a mixture of products resulting from cleavage of an imidazoline ring.

Fig. Example of formation reaction of imidazoline derivatives

Imidazoline type cationic surfactants derived from oleic acid can be used as emulsion breakers. They are also used as intermediates for further reactions to manufacture quaternary ammonium salt type cationic surfactants or amphoteric surfactants.

Onyxan HSB (Refined-Onyx Div., Milmaster Onyx Corp.) is a famous textile softener in this category, and its nature is very similar to that of Ahcovel G. Imidazoline type cationic surfactants derived from lauric acid are used as intermediates to manufacture amphoteric surfactants. 

Quaternary ammonium salt type cationic surfactants are manufactured by means of the reaction between tertiary amines and alkylating agents. Various combinations of the two reactants yield a great variety of products.

These quaternary ammonium salts, which have particularly strong germicidal properties, are produced by alkylating alkyldimethylamine (tertiary amine) with benzyl chloride. Alkyl groups in the vicinity of lauryl (C12) are preferred.

These compounds are also called benzalkonium chlorides, and are produced in fairly large quantities as germicides. They dissolve in water transparently and generate large amounts of foam. They are used widely in hospitals and similar facilities as their germicidal effect is about 500 times stronger than that of phenol. Furthermore, these compounds have no smell and will not irritate the skin.

Fig. Synthesis route of benzylauryl dimethyl ammonium chloride (benzalkonium chloride)

It is possible to manufacture inexpensive quaternary ammonuim salts starting with lower alkylamines and cheap hydrophobic groups. As this result in a wider choice of structures and properties, there has been a lot of development and diversification of this type of surfactant.

Cationic surfactants are divided broadly into two categories, amine salt type and quaternary ammonium salt type. These two types have different properties and manufacturing processes. It is very important to understand the differences between the two types, as summarized in the table bellow.

On mixing cationic and anionic surfactants

Aqueous cationic surfactant solutions are generally acidic in contrast to those of anionic surfactants, which are generally neutral or alkaline. That is, roughly speaking, cationic surfactants ionize into weaker base and stronger acid, and anionic surfactants ionize into weaker acid and stronger base. The table bellow explains the matter to enable better understanding by comparing equivalent inorganic compounds.

The table bellow shows the usual classification of cationic surfactants according to the combination of hydrophobic and hydrophilic raw materials. 

In this page, discussion of cationic surfactants is limited to only nitrogen compounds. In fact, all commercial products of this type are nitrogen compounds. From the scientific point of view, however, some sulfur compounds, such as sulfonium salts, and some phosphorus compounds, such as phosphonium salts, are also regarded as cationic surfactants.

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