Surfactants are compounds that lower the surface tension or interfacial tension between two liquids, between a gas and a liquid, or between a liquid and a solid. Surfactant molecules have either one tail or two and the "tails" of most surfactants are very similar, consisting of a hydrocarbon chain, which can be branched, linear, or aromatic. Surfactants can be classified according to its polar head groups. Non-ionic surfactants have no charged groups on their heads. The head of the ionic surfactant carries a net positive or negative charge, and if the charge is negative, the surfactant is more specifically called an anion; if the charge is positive, it is called a cation. If the surfactant contains a head with two oppositely charged groups, it is called a zwitterion. Surfactants are generally divided into the following categories:
1 Anionic surfactants
Anionic surfactants fall into four categories: sulfonates, Alkyl Sulfates, carboxylates, and phosphates. Anionic surfactants have good decontamination, foaming, dispersing, emulsifying, and wetting properties and that’s why they are widely used as detergents, foaming agents, wetting agents, emulsifiers, and dispersants. The production of anionic surfactants tops the list of surfactants. Anionic surfactants cannot be used with cationic surfactants, as they will precipitate in aqueous solutions and lose their effectiveness.
Typical of such active agents are linear alkyl benzene sulphonate(LAS) and alpha olefin sulfonate (AOS). linear alkyl benzene sulphonate is white or light yellow powder or flake solid, soluble in water and its water solubility is poor at low temperatures. It is stable in alkali, dilute acid and hard water, and the decomposition temperature is 240°C. When the content of the active substance is 38% to 40% in alpha olefin sulfonate, its appearance is yellow transparent liquid and is easily soluble in water. It has a good stability in a wide range of pH. It is less irritating to the skin and its rate of microbial degradation is 100%. LAS is generally not used for shampooing, it is commonly used in laundry detergents and dishwashing liquids. LAS is the most productive and cheapest synthetic surfactant. The outstanding advantages of LAS are its good stability, good detergency and low price, while its biggest drawback is its irritation.
1.2 Alkyl Sulfates(RO-SO3-M)
Typical of such active agents are fatty alcohol polyoxyethylene ether sulfates(AES) and sodium lauryl sulfate(AS; K12). Fatty alcohol polyoxyethylene ether sulfate is easily soluble in water, and its appearance is light yellow viscous liquid (translucent) with 70% of active content, and its stability is inferior to that of normal sulfonate. It hydrolyzes quickly below pH 4 but is stable in alkaline conditions. Sodium dodecyl sulfate is synonymous with AS, K12, sodium lauryl sulfate. It dissolves in water and dissolves in water and its water solubility is inferior to AES. It is insensitive to alkalis and hard water, but is less stable than normal sulfonates in acidic conditions.
1.3 Fatty acid soap((RCOOˉ)n M)
Stearate, oleic acid, and lauric acid are common fatty acid hydrocarbons. According to the different substances represented by M, they can be divided into alkali metal soap, alkaline earth metal soap and organic amine soap. They all have good emulsifying properties and the ability to disperse oil, but they can be easily decomposed. The alkali metal soap can also be decomposed by calcium and magnesium salts, and the electrolyte can also salt it out. Different fatty acid salts have different properties. As a surfactant, the fatty acid salt is soluble in water, but its solubility and surface activity are affected by pH, gold urine ions such as calcium and magnesium, and temperature factors. It is easily hydrolyzed under acidic conditions and loses its surface activity while its water solubility is reduced. At lower temperatures, the water-solubility of fatty acid salts is reduced and can easily become a solid gel.
1.4 Phosphate salt(RO-PO3-M)
Phosphate surfactant is the representative of phosphorus-containing surfactants. This is a widely-used surfactant with high performance. It has excellent properties such as wetting, washing, solubilizing, emulsifying, antistatic, corrosion inhibition and rust resistance, and it is easily biodegradable. Its thermal stability, alkali resistance, electrolyte resistance and antistatic properties are all superior to those of general anionic surfactants. It is widely used in the fields of chemical fibers, textiles, plastics, paper-making, leather and household chemicals. The main varieties of phosphate ester surfactants are alkyl(aryl) phosphates(salts), alkyl alcohol amide phosphates (salts), imidazolines Phosphates (salts), high molecular polyphosphates (salts), and siloxane phosphates. Alkyl (aryl) phosphates are an important class of anionic surfactants.
2 Cationic surfactants
Since the main part of the molecular structure of the cationic surfactant is a pentavalent nitrogen atom, it is also referred to as a quaternary ammonium compound. Its characteristics are water-soluble, stable in acidic and alkaline solutions, and has a good surface activity and bactericidal effect. Common types of cationic surfactants are cetyldimethylammonium chloride, octadecyltrimethylammonium chloride, cationic guar gum, cationic panthenol. Cationic silicone oil, dodecyldimethylamine oxide (OB-2). Cationic surfactants, unlike other surfactants, have poor detergency and foaming properties and tend to have some irritation and lower toxicity. Cationic surfactants are used in liquid detergents as a co-surfactant and as a formulation component with a small amount of formulation, and they are generally used for higher end products such as shampoos. Cationic surfactants are not directly compatible with anionic surfactants, although the combination of cations and anions may have good results, the risk of precipitation (crystallization) is greater. Cationic surfactants have a small share of surfactant production and are often more expensive than other surfactants. Compared with other surfactants, cationic surfactants have the most outstanding adjustment effect and the strongest bactericidal effect. Despite the disadvantages of poor detergency, poor foaming, poor compatibility, irritation, and high cost, as a regulator ingredient in detergents and shampoos, cationic surfactants cannot be replaced by any other types of surfactants.
3 Non-ionic surfactants
The main varieties of non-ionic surfactants are alkyl alcohol amide(FFA), fatty alcohol-polyoxyethylene ether(AE), Alkylphenol ethoxylates(APE or OP). Non-ionic surfactants have good solubilization, washing, antistatic, low irritation, calcium soap dispersion properties. Its practical applicable pH range is wider than general ionic surfactants. Despite of decontamination and foaming properties, other properties of non-ionic surfactants tend to be superior to anionic surfactants. Experiments have shown that adding a small amount of nonionic surfactant to the ionic surfactant can increase the surface activity of the system.
4 Amphoteric Surfactants
Amphoteric surfactants refer to surfactants that have both anionic and cationic hydrophilic groups. This surfactant is cationic in acidic solutions, anionic in alkaline solutions, and has non-ionic properties in neutral solutions. Amphoteric surfactants are easily soluble in water, soluble in concentrated acid and alkali solutions, and even dissolved in concentrated solutions of inorganic salts. They have good resistance to hard water, little irritation to the skin, good fabric softness, antistatic properties and good bactericidal effect. This type of surfactants can be used in a wide range of pH, but their performance under acidic and neutral conditions should be better than under alkaline conditions. In general, the price of amphoteric surfactants is higher than that of nonionic surfactants. Important amphoteric surfactants are lauryl dimethyl betaine and carboxylate imidazoline. Compared to anionic surfactants, nonionic surfactants have more comprehensive properties and fewer defects. Amphoteric surfactants have better foaming capabilities than nonionic surfactants. Therefore, amphoteric surfactants are mainly used in shampoos, shower cleaners and other skin cleaners.