Introduction to Amphoteric 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

Examples of amphoteric surfactants

Anionic surfactants command a major share of the household detergent market, including soaps, and cationic surfactants are well- known as germicides for disinfection and as finishing agents for synthetic fibers such as nylon. Then, what is a familiar example of an amphoteric surfactant? 

It is an important ingredient in cosmetics, especially shampoo. One of the most popular amphoteric surfactants is cocoamidopropyl betaine, derived from coconut fatty acid and dimethyl aminopropylamine.

Classification of Amphoteric Surfactants

Although only limited kinds of amphoteric surfactants are used practically, many kinds are conceivable. More concretely, as amphoteric surfactants are composed of anion-cation combinations, the theoretical number of amphoteric surfactants that could be synthesized should be at least as large as the number of kinds of anionics multiplied by that of cationics.

In many practical cases, however, either amine salts or quaternary ammonium salts are used as a hydrophilic group with a cationic property. Therefore, it is convenient to classify an amphoteric surfactant by the type of its anionic part.

The classification of amphoteric surfactants by type of anionic portion is summarized below.

However, this table represents experimentally developed amphoteric surfactants only. In practice, most amphoteric surfactants currently on the market are carboxylate type. In this page, therefore, discussion is focused on carboxylate type amphoteric surfactants.

A carboxylate type amphoteric surfactant is one that possesses a carboxyl group as its anionic part. Those possessing an amine salt type cationic part are called amino acid type amphoteric surfactants, and those possessing a quaternary ammonium salt type cationic part are called betaine type amphoteric surfactants (As shown in table above).

A compound with the following composition is produced when one mole of methyl acrylate (CH2 = CHCOOCH3) is added during stirring to one mole of higher alkylamine (C12 to C18) such as lauryl amine that has been melted by heating to 60 to 70 °C.

Fig. Synthesis of methyl lauryl amino propionate (secondary amine)

The methyl lauryl amino propionate thus produced is a secondary amine, although it has a slightly complex structure. Therefore, it yields water-soluble cationic surfactants by neutralization with an acid such as hydrochloric acid.

Fig. Neutralization reaction of methyl lauryl amino propionate (cationic surfactant)

However, by focusing on the ester linkage rather than the amine, it becomes apparent that the carboxyl group can be regenerated by saponification with alkali. In this way, an excellent amino acid type surfactant can be obtained.

Fig. Formation reaction of sodium lauryl amino propionate (amino acid type amphoteric surfactant)

The saponification can be carried out easily by adding one mole of aqueous sodium hydroxide solution, slowly, during stirring, to one mole of methyl lauryl amino propionate, in a beaker heated in a boiling water bath. The resultant sodium lauryl amino propionate is easily soluble in water, yielding a transparent solution. Its aqueous solution foams well, and is fairly alkaline because its molecule contains not only sodium carboxylate, as in the case of soap, but also a free secondary amino group.

In an alkaline solution, as shown above, the amino group of an amino acid type amphoteric surfactant performs a relatively minor function as a hydrophilic group because it does not exist as a salt, and the hydrophilic property of the surfactant is attributable mainly to its carboxylate group. Therefore, amphoteric surfactants of this type exhibit properties similar to those of anionic surfactants in alkaline solution.

Behavior of aqueous amino acid-type amphoteric surfactant solutions upon neutralization with hydrochloric acid

Fig. Behavior of alkaline aqueous amino acid-type amphoteric surfactant solutions upon neutralization with hydrochloric acid

Applications and Synthesis of Sodium Lauryl Amino Propionate and Other Amino Acid Type Amphoteric Surfactants

Sodium lauryl amino propionate has fairly good detergency and is used as a specialty detergent. There are a variety of products in this category with different lengths of alkyl groups and different positions and numbers of amino groups and carboxyl groups. On the whole, they have similar general properties to those shown in the above example, but their applications are diverse.

In addition to the process mentioned above, amphoteric surfactants of alkyl amino propionate type can be synthesized by an alternative process using acrylonitrile, which is more economical.

Fig. Synthesis of sodium stearyl propionate (method using acrylnitrile)

Surfactants with a similar structure but with one less methylene group (-CH2-) between the amino and carboxyl groups can be synthesized easily by reacting a higher alkylamine with an aqueous sodium mono chloroacetate solution.

Some amino acid type amphoteric surfactants with this structure are used as germicides with the feature of less toxicity than cationic products.

Fig. Reaction of lauryl amine with sodium monochloroacetate

Betaine and betaine surfactants

Fig. Formation reaction of lauryl dimethyl betaine

Features of lauryl dimethyl betaine

Lauryl dimethyl betaine dissolves in water transparently and foams well. It has excellent detergency and can be considered a representative betaine type amphoteric surfactant.

The largest difference between betaine type and amino acid type amphoteric surfactants is that betaine type surfactants dissolve in water easily, regardless of whether the betaine type surfactant is acidic, neutral or alkaline in nature. Therefore, in the case of betaine type surfactants, there is almost no danger of precipitation even at the isoelectric point, and there is the added advantage of being usable across the entire pH range.

Dimethyl stearyl betaine, dihydroxyethyl lauryl betaine

Amphoteric surfactants (cocoacid amidopropyl betaine, lauryl imidazolinium betaine) used as shampoo base

Cocoamidopropyl betaine and laurylimidazolinium betaine are widely used as shampoo bases. These amphoteric surfactants are manufactured from fattyamide amine and alkylimidazoline, respectively, through the reactions indicated in the following reaction formula. However, alkylimidazolinium betaine is actually a complex mixture of products resulting from a cleavage of the imidazoline ring. 

Fig. Synthetic route of coconaut fatty acid amidopropyl betaine

Fig. Synthetic route of lauryl imidazolinium betaine

A surfactant consisting of an anionic part and a cationic part in its molecule is generally referred to as an amphoteric surfactant. In this class of surfactants, there are various types such as the carboxylate type and the sulfonate type. The most familiar is the carboxylate type, which is subdivided into the amino acid type and the betaine type.

Amino acid type surfactants tend to precipitate at the isoelectric point, but betaine type products dissolve in water relatively easily even at the isoelectric point. In general, the betaine type is better than the amino acid type.


Among these carboxylate-type amphoteric surfactants, the more commonly used forms can be classified based on the raw materials of their hydrophilic and hydrophobic groups, as shown in the table below.

Consumption of amphoteric surfactants is small in comparison to other surfactants. However, if you encounter difficulties with regard to surfactant application, it may be more rewarding than expected to consider using amphoteric surfactants.

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