Amino Acids for Surfactants

The value of amino acids as raw materials for the preparation of surfactants was first recognized in the 20th century. Initially, they were primarily used as preservatives in cosmetics and pharmaceuticals. Subsequently, researchers discovered their potential to combat various pathogenic tumors, bacteria, and viruses, leading to extensive studies. Among the surfactants currently under active research, N-acyl amino acid-based surfactants and N-alkyl amino acid-based surfactants are the most prominent. Of these, N-acyl amino acid surfactants are the most widely developed and have now entered the stage of industrialized series production.

Surfactant Definition

Surfactants are chemical substances capable of significantly reducing the surface tension of liquids or the interfacial tension between two phases. These molecules typically consist of a unique amphiphilic structure, featuring both hydrophilic and hydrophobic parts. This distinctive molecular configuration allows surfactants to form monolayers at water-oil interfaces, altering the surface properties of liquids. Surfactants play a critical role in applications such as detergents, emulsifiers, dispersants, wetting agents, foaming agents, and solubilizers. In response to the growing demand for green chemistry and sustainable development, amino acid-based surfactants have garnered significant attention as eco-friendly surfactants due to their excellent biodegradability, low toxicity, and renewable resource origins. These surfactants are synthesized by combining amino acids or their derivatives with hydrophobic chains, offering diverse structures and functional characteristics.

Fig. 1. Types of surfactants.

Amino Acid Surfactant

Amino acids are organic compounds characterized by the presence of both basic amino groups and acidic carboxyl groups. Amino acid-based surfactants use amino acids as the core structure. Typically, amino acids or short peptide chains serve as the hydrophilic head group, while fatty chains are attached via ester, amide, or acyl linkages. Depending on the type of amino acid used and the production process, various types of surfactants can be obtained. As a new generation of eco-friendly surfactants derived from renewable biomass, amino acid surfactants represent an upgrade to traditional surfactants. They are sourced from abundant biomass materials, exhibit mild performance with low toxicity and irritation, and are highly biodegradable. Furthermore, their green production processes and excellent properties—such as emulsification, wetting, solubilization, dispersion, and foaming—have drawn significant attention. These surfactants are gradually being applied in industries such as detergents, personal care products, and the food industry. Amino acid-based surfactants can be classified into four types based on the ionic characteristics of the amino acid in aqueous solutions: anionic, cationic, amphoteric, and non-ionic surfactants.

• Non-Ionic Surfactant

Non-ionic surfactants are surfactants that do not carry a charge. Their hydrophilic groups are typically hydroxyl or amide groups derived from amino acids, while their hydrophobic chains are sourced from fatty acids or other long-chain hydrocarbons. Since non-ionic surfactants do not dissociate in aqueous solutions, their performance is not affected by pH or ionic strength, making them suitable for a wide range of applications. These surfactants are known for their mildness and low irritation, along with excellent emulsification properties. They remain stable under both acidic and alkaline conditions. Common examples include N-fatty acyl glutamic acid esters and fatty alcohol polyoxyethylene ethers.

• Cationic Surfactant

Cationic surfactants carry a positive charge, with hydrophilic groups typically consisting of amino or quaternary ammonium groups derived from amino acid derivatives. These surfactants are widely used in personal care products and disinfectants due to their excellent antibacterial and sterilization properties. They also exhibit strong adsorption and wetting capabilities, as well as antibacterial and anti-static properties. N-fatty acyl lysine and hydroxypropyl trimethyl ammonium chloride are representative cationic surfactants.

• Anionic Surfactant

Anionic surfactants carry a negative charge, with hydrophilic groups typically consisting of carboxylic or sulfonic acid groups. These surfactants readily dissociate in aqueous solutions to form negatively charged ions. Anionic surfactants derived from amino acids are well-known for their exceptional detergency and foaming properties, making them widely used in detergents, cosmetics, and industrial cleaners. Characteristics of these surfactants include excellent emulsifying and dispersing capabilities, along with superior stability in hard water conditions. Representative compounds include sodium N-fatty acyl glycinate and sodium N-fatty acyl glutamate.

• Zwitterionic Surfactant

Zwitterionic surfactants possess both positive and negative charges within the same molecule, exhibiting either cationic or anionic behavior depending on the pH conditions. The natural zwitterionic properties of amino acids make them ideal materials for developing this type of surfactant. These surfactants are known for their excellent mildness and high efficiency and are widely used in detergents, cosmetics, and pharmaceutical formulations. Their features include mildness, high biocompatibility, exceptional resistance to hard water, and stability across a broad pH range. Representative compounds include N-fatty acyl glycine and phosphatidylserine.

Amino Acid Surfactant List

Common amino acids used in amino acid-based surfactants include sarcosine, glycine, alanine, glutamic acid, and methyl taurine sodium/potassium salts, while the fatty acid groups (R1) connected to them are typically lauryl, cocoyl, oleoyl, palmityl, or stearyl groups. The performance of surfactants varies with different amino acid types. Structurally, glycine has the smallest hydrophilic head, whereas sarcosine and alanine have an additional methyl group, resulting in larger hydrophilic heads than glycine. Glutamic acid possesses an additional carboxyl group chain, giving it the largest hydrophilic head. In terms of performance, larger hydrophilic heads tend to stay at the oil-water interface, enhancing interfacial activity and detergency. Methyl taurine, which also has a relatively large hydrophilic head, connects via a sulfonic group instead of a carboxylic group. The bonding strength of sulfonic groups with water is higher than that of carboxylic groups, giving methyl taurine-based surfactants superior surface activity compared to other amino acid-based surfactants.

What are the Benefits of Amino Acid Surfactant?

Amino acids can be classified into three categories based on their acidity or basicity: basic, acidic, and neutral. Derived amino acid surfactants include anionic, cationic, non-ionic, and zwitterionic types. Among these, N-acyl amino acid surfactants, synthesized through the condensation of fatty acyl groups and α-amino acids, are the most common and widely researched.

• Excellent Surface Activity

Amino acid surfactants exhibit strong emulsification and calcium soap dispersion abilities, along with excellent hard water resistance. Studies have shown that coconut oil-based methyl taurine sodium (at 0.5% concentration) demonstrates consistent foam height (~1.7 cm) in both hard water (150 mg/kg) and pure water. Furthermore, as the carbon chain length of amino acid surfactants increases, the equilibrium surface tension and critical micelle concentration decrease, enhancing their performance.

• Superior Biodegradability

Amino acid surfactants are highly biodegradable and environmentally compatible. Research indicates that N-acyl amino acid surfactants readily degrade through the breakdown of fatty acids and amino acids. For example, sodium lauroyl glutamate shows better biodegradability than linear alkylbenzene sulfonate and is comparable to sodium dodecyl sulfate. In tests synthesizing various amino acid surfactants from amino acids and fatty acids, their biodegradability ranged from 57% to 73% after 14 days, highlighting their environmental advantages.

• Strong Antibacterial Properties

Due to the presence of unsaturated bonds or hydroxyl groups in the acyl chain, amino acid-based surfactants exhibit antibacterial properties, which are enhanced as the degree of unsaturation and the number of hydroxyl groups increase. According to relevant studies, arginine-based surfactants demonstrate more effective antibacterial activity against Gram-positive bacteria compared to Gram-negative bacteria. Researchers have investigated the specific performance of N-acyl amino acid surfactants against Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa, while also analyzing the impact of pH on antibacterial activity. The results indicate that these surfactants exhibit strong resistance against the three bacteria mentioned, although their antibacterial activity decreases when the pH exceeds 6.

What is Surfactant Used For?

Amino acid surfactants possess excellent properties such as wettability, foaming ability, antibacterial action (enhanced by the presence of hydroxyl groups or unsaturated bonds in the acyl chain), anti-corrosion effects, and antistatic capabilities. These surfactants are non-toxic, harmless, gentle on the skin, and environmentally friendly as their degradation products are amino acids and fatty acids. Additionally, they exhibit good compatibility with other surfactants, making them widely applicable in personal care products and other fields.

• Laundry Detergents

Amino acid surfactants provide ideal foaming performance and enhance the softness of knitted fabrics without damaging textile fibers, playing a crucial role in laundry liquids. N-acyl amino acid surfactants, derived from serine, glutamic acid, alanine, and glycine, exhibit excellent low-temperature antifreeze properties. These surfactants prevent laundry liquids from gelling under low temperatures, ensuring normal usage. Studies on detergents based on N-acyl alanine surfactants indicate good cleaning power and stability. Additionally, detergents incorporating N-acyl-1-hydroxyalanine sodium exhibit strong resistance to hard water and effective cleaning ability without causing significant skin irritation.

• Oral Care

N-acyl amino acid surfactants produce abundant, fine foam, are gentle, and possess antibacterial properties, making them suitable for oral care applications. Examples such as sodium N-lauroyl alaninate, sodium N-fatty acyl sarcosinate, and sodium N-lauroyl glutamate are used in toothpaste formulations. These surfactants inhibit the conversion of glucose to lactic acid in the oral cavity, refresh breath, and act as foaming agents. Research demonstrates that toothpaste containing sodium N-acyl glycinate resists gelling at low temperatures, is easy to dispense, and contributes to oral health and disease prevention. Additionally, adding sodium N-lauroyl sarcosinate to medicated candies extends drug efficacy.

• Personal Care Products

Amino acid surfactants are gentle on the skin and eyes, have mild degreasing properties, and leave skin feeling moisturized without damage to the skin or hair, even with prolonged use. They can also condition the skin and hair. For example, researchers formulated mild facial cleansers using amino acid surfactants combined with sodium cocoyl isethionate. These cleansers produce rich, fine foam, leaving the skin feeling soft and refreshed. Cleansing creams using N-acyl glutamate exhibit superior solubility, cleansing power, and foaming ability compared to other market products, with a pH close to that of the skin. In shampoo formulations, amino acid surfactants, such as sodium N-lauroyl alaninate, blended with sodium cocoyl methyl taurinate, create silicone-free shampoos with low irritation, strong cleansing power, and good compatibility.

• Leather Industry

In the leather industry, amino acid surfactants serve as fatliquors, emulsifiers, and dyeing aids, enhancing fatliquor filling, water resistance, and dye affinity while improving leather fullness and softness. Studies show that amphoteric amino acid surfactants in fatliquoring processes distribute oils evenly around leather fibers and improve collagen-oil integration, reducing fatliquor usage by about 10% while maintaining excellent fatliquoring effects.

• Pharmaceuticals and Biotechnology

Amino acid surfactants have seen rapid development in pharmaceuticals and biotechnology. N-acyl amino acid surfactants enhance the water solubility of vitamin E, improve absorption, and are gentle with low irritation. They are also used for the efficient and safe extraction of RNA and DNA from human blood, microorganisms, and cells. In immunology, these surfactants find applications in preparing functional lipid vesicles for drug delivery. Anionic amino acid surfactants increase skin hydration and enhance transdermal flux, showing potential as topical therapeutic agents. Gemini amino acid surfactants are being explored as reagents for gene delivery.

• Food Industry

The biocompatibility and safety of amino acid surfactants meet the stringent requirements of the food industry. Replacing amino acids with amino acid esters as food additives enhances flavor, extends shelf life, and improves nutritional content. Small amounts of N-acyl amino acid surfactants in food additives prevent unpleasant odors in plant oils, candies, and cocoa powder while offering preservative effects. They also inhibit acid fermentation and prevent dental erosion. Additionally, amino acid surfactants exhibit antistatic and antifogging effects, making them suitable for use in food packaging films.

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