Introduction

Betaine type surfactants are increasingly becoming a research hotspot, but the product varieties are mainly concentrated in alkyl betaines. At the same time, due to its high price, it is subject to many limitations in application. Cocamidopropyl betaine (CAPB) prepared from low-cost coconut oil is a mild and efficient multifunctional surfactant. This product is a widely used basic ingredient in today's internationally popular shampoo and hair care products As early as the 1960s, the German company GOLD SCHMIDT-AG developed this variety, namely Tego betaine L7.[1]

Cocamidopropyl betaine has mildness, excellent cleaning effect, efficient foaming performance, and superior conditioning performance. When it is combined with some anionic surfactants to prepare shampoo, the thickening effect is excellent. For this reason, many companies and enterprises around the world have conducted extensive and in-depth research on it. The application range of cocamidopropyl betaine is becoming increasingly wide, and its production continues to grow, occupying a dominant position in the European market and gradually becoming an important product among amphoteric surfactants.

1 Structural characteristics[2]

The chemical name of CAPB is cocoyl amide propyl dimethylamine acetic acid, and the structural formula is shown in Figure 1.

In cocamidopropyl betaine molecules, the positive charge is generated by a fixed bond on the nitrogen atom, independent of pH value, and there is a negatively charged carboxylic acid substituent attached to the quaternary nitrogen atom. Due to the strong electron induction effect of quaternary ammonium groups, carboxyl groups exist as salt groups almost throughout the entire pH range, and significant protonation can only occur under extremely acidic conditions (such as pH=1). Therefore, it belongs to zwitterionic species. Cocamidopropyl betaine is a amphoteric surfactant that exhibits different ionic properties under different pH conditions:

(1) Under neutral conditions, it exists in the form of internal salts, and its aqueous solution has the highest surface tension, with properties such as solubilization, emulsification, wetting, and decontamination.

(2) Under acidic conditions, the following reaction equilibrium exists in the solution (Figure 2):

Exhibiting the characteristics of cationic surfactants, with hydrophilic groups of -N⁺-.

(3) Under alkaline conditions, the following equilibrium exists in the solution (Figure 3):

It exhibits the characteristics of anionic surfactants, with a hydrophilic group of -COO^-. And the hydrophilicity of -COO^- is greater than that of -N⁺-. Meanwhile, under alkaline conditions, the hydrophilic group -COO^- is located at the end of the carbon chain, which has a stronger effect on reducing surface tension than when it is in the middle. Therefore, betaine can lower the surface tension of the solution more effectively when alkaline.

Cocamidopropyl betaine has the following three structural characteristics:

(1) The amide group (-CONH-) changes the performance of the molecule to make it have better stability and mildness of foam. In the formula, it also plays a role in increasing viscosity. In addition, it also has better foaming properties, even in the presence of soap and a small amount of oil, it can still maintain foaming properties. Scher believes that betaine contains amide groups, which can greatly improve its compatibility and solubility, expanding its application range.

(2) Quaternary ammonium groups can provide direct antibacterial and bactericidal effects, and this effect is minimally affected by changes in pH value;

(3) Carboxyl group (-COO^-) is a hydrophilic group, while R-N⁺(CH₃)₂-CH₂COO^- is a large hydrophilic group.

The main factor that has a significant impact on performance is the chain length of hydrophobic alkyl groups in betaine type surfactants. When it contains 12-16 carbon atoms, the surfactant has excellent foam stability, tackifying effect, and good moisturizing and conditioning properties The main components of coconut oil are C12 laurel and C14 cardamom. Therefore, due to its unique molecular structure, cocamidopropyl betaine has unique and excellent properties that other types of surfactants do not have.

2. Functional characteristics

Cocamidopropyl betaine not only has the characteristics of odorless, light color, low salt content, and gentle skin, but also has excellent properties such as wetting, dispersing, cleaning, penetrating, anti-static, sterilization, conditioning, thickening, calcium soap dispersion, and biodegradation.

2.1 Water solubility

Cocamidopropyl betaine is a colorless to pale yellow liquid that is stable over a wide pH range, fully compatible with anionic, cationic, and nonionic surfactants, while has good solubilization properties. It can also be used as the cloud point inhibitor of other surfactants, avoiding the use of cloud point inhibitors such as ethylene glycol and ethanol that will reduce the viscosity of the system and reduce foam.

2.2 Decontamination and foaming properties

Cocamidopropyl betaine has remarkable resistance to hard water. It can produce rich foam in both soft and hard water, has good washing effect in cold water, and has excellent decontamination and foaming ability in electrolyte or sea water. It is especially characterized by fast foaming, rich, delicate and very stable foam. Its foam performance is better when it is mixed with anionic surfactants such as alcohol ether sulfate.

2.3 Thickening and conditioning properties

Cocamidopropyl betaine, when combined with other surfactants in shampoo formulations, exhibits a synergistic effect, showing significant thickening and conditioning effects. It is mixed with anionic surfactants, with a mass fraction of 10% to 30%, to obtain high viscosity products, and the viscosity decreases with increasing pH value. Adding weak acid during preparation can increase viscosity. Its superior conditioning performance enhances the softness of hair and skin, and can effectively reduce the friction coefficient between fibers, producing excellent softness effects. It not only makes the fabric hydrophobic, but also has a significant anti-static effect.

2.4 Moderation

A prominent feature of cocamidopropyl betaine is its high safety and no allergic reactions to the human body. Betaine without amide groups is toxic to marine or aquatic organisms, and if the carboxyl group in betaine is replaced by sulfonic acid groups, it also has toxicity. Only when the amide group and carboxyl group coexist can low toxicity or non toxicity be achieved. Cocamidopropyl betaine is a quaternary ammonium compound, similar to amino acid zwitterions, with a structure similar to proteins. It has minimal irritation to the skin, hair, and especially the eyes (cornea), and has relatively low toxicity. It can effectively reduce the irritant effects of alkyl sulfates and alkyl alcohol ether sulfates in cosmetics and toiletries such as baby cosmetics. The Leberco laboratory in the United States measured the irritancy of Tego Betaine L7. The results showed that Tego Betaine L7 had no effect on the cornea and iris of rabbit eyes, only causing slight transient irritation to the binding membrane. Some amide propyl betaines of coconut oil mixed fatty acids, if coconut oil fatty acids or their esters can be fractionated into C8-10 fatty acids or their methyl esters and then acylated to produce betaine, their irritability will be lower.

2.5 Other Performance

An important application of cocamidopropyl betaine is its ability to chelate and dissolve cations such as quaternary ammonium compounds in anionic systems. If the quaternary ammonium compound is dissolved in the amide betaine beforehand, it can be easily added to the anionic formula to achieve true two in one.

The low salt content of cocamidopropyl betaine expands the adaptability of the product and improves its low-temperature stability.

Cocamidopropyl betaine has a certain dispersing effect and is used in the dye industry to treat dyes that are less prone to clumping and achieve better dispersion results. Currently, environmental pollution is attracting widespread attention from various countries, and CAPB has been favored by users for its excellent biodegradability.

References

[1] Cowen F M,Bellis G.Betaines[P].US 3360550,1967.

[2] Tu Ha, Man R, Chen W. Multi-functional amphoteric surfactant cocamidopropyl betaine [J]. Daily Chemical Science.2001,24 (1): 44-46.