There are many types of surfactants. Ranked by production volume: anionic surfactants account for 56 %, non‑ionic surfactants 36 %, amphoteric surfactants 5 %, and cationic surfactants 3 %.
2.Anionic Surfactants
2.1 Sulfonate‑type Anionic Surfactants
Common surfactants of this type include sodium linear‑alkylbenzene sulfonate and sodium α‑olefin sulfonate.
Sodium linear‑alkylbenzene sulfonate, also known as LAS or ABS, is a white or pale‑yellow powdery or flaky solid soluble in water. It exhibits poor water solubility at low temperatures, with an aqueous solubility below 3 at ambient temperature; nevertheless, it dissolves well in compounded surfactant systems. It is relatively stable against alkalis, dilute acids and hard water, with a decomposition temperature of 240 ℃. Its 10 % solution has an irritation index of 5.0, microbial biodegradability of 80 %‑90 %, and an LD₅₀ of 1300‑2500 mg/kg.
Sodium α‑olefin sulfonate, alias AOS, appears as a yellow transparent liquid at an active‑substance content of 38 %‑40 %, and is highly soluble in water. It maintains good stability over a wide pH range; after three days at 30 ℃, the hydrolysis rate is 0 at pH 2, pH 4 and pH 10. It has low skin irritation, 100 % microbial biodegradability, and an LD₅₀ of 1300‑2400 mg/kg.
LAS is generally not used in shampoos and is rarely added to shower gels. It is widely applied in laundry liquid detergents and dish‑washing liquids. In dish‑washing liquids, LAS can make up roughly half of the total surfactant content; in laundry liquid detergents, its proportion can be adjusted across a broad practical range.
LAS shows favourable water solubility mainly at elevated temperatures (e.g. 60 ℃) or when compounded with certain other surfactants. A typical ternary compound system for dish‑washing liquids is “LAS‑AES‑FFA”. For laundry liquid detergents, common compound formulations include “LAS‑soap base‑η·SAA”. It should be noted that direct blending of LAS with the non‑ionic surfactant alkanolamide may not yield satisfactory results; the “LAS‑FFA” system is unstable, features low viscosity and forms a white milky appearance.
LAS is the highest‑volume (290 kt/a) and cheapest synthetic surfactant. Among the top‑five synthetic surfactants by output, LAS has the lowest price, comparable to soap bases (fatty‑acid soaps) among conventional anionic surfactants. Its prominent advantages are good stability, strong detergency and low cost; its major disadvantage is high irritation.
Among sulfonate‑class surfactants, AOS delivers the best performance. It retains or amplifies the merits of general sulfonates while avoiding their drawbacks. AOS is one of the primary surfactants frequently used in shampoos and shower gels. Its application in other liquid detergents will gradually expand as domestic production localises and brings down its price. Its key strengths include excellent stability, good water solubility, favourable compatibility, low irritation and outstanding microbial biodegradability. Its main weakness is its relatively high price among anionic surfactants.
2.2 Sulfate‑type Anionic Surfactants
Typical surfactants in this category are sodium fatty alcohol polyoxyethylene ether sulfate and sodium dodecyl sulfate.
Sodium fatty alcohol polyoxyethylene ether sulfate, also named AES or alcohol ether sulfate, dissolves readily in water. At 70 % active‑substance content, it is a pale‑yellow viscous (semi‑transparent) liquid. Its stability is inferior to that of common sulfonates. It hydrolyses rapidly below pH 4 yet possesses good hydrolytic stability under alkaline conditions. After three‑day storage at 30 ℃, its hydrolysis rates reach 100 %, 50 % and 0 at pH 2, pH 4 and pH 10 respectively. It causes mild irritation, with an irritation index of 2.3 for its 10 % solution, achieves over 90 % biodegradation, and has an LD₅₀ of 1800 mg/kg.
Sodium dodecyl sulfate, also known as AS, K12, coconut alcohol sulfate or sodium lauryl sulfate (foaming agent), is soluble in water with a solubility of approximately 15 at 25 ℃, lower than that of AES. It is insensitive to alkalis and hard water, yet less stable under acidic conditions than general sulfonates (comparable to AES). Long‑term heating should not exceed 95 ℃. It exhibits moderate irritation among surfactants; the irritation index of its 10 % solution is 3.3, higher than AES and lower than LAS, with an LD₅₀ of 1300 mg/kg.
AES can be used in shampoos, shower gels, dish‑washing liquids and laundry liquid detergents. Where product pH specifications permit, the pH should be raised as far as practicable to neutral or weakly‑alkaline levels. When AES must be employed at low pH values (as in shampoos), its ethanolamine salt form is normally adopted. AES has better water‑solubility than AS and can be formulated into transparent aqueous solutions in any proportion at room temperature. Compared with LAS, AES enjoys broader application in liquid detergents and superior compatibility; it can form transparent aqueous solutions via binary or multi‑component blending with many surfactants. Ranking third in production volume among synthetic surfactants, AES costs less than AS; in 2002, the price of 70 % AES stood at 8500 yuan per tonne. AES is valued for low irritation, good water solubility, excellent compatibility and positive performance in preventing dry and rough skin. Its disadvantage is moderate instability in acidic media — pH must be kept well above 4 — and its detergency is weaker than LAS and AS.
When AS is incorporated into liquid detergents, the pH medium must be controlled to avoid excessive acidity. For shampoo and shower‑gel formulations, its ethanolamine or ammonium salts are required. Ethanolamine salts not only enhance acid‑resistance stability but also help reduce irritation; the 10 % triethanolamine salt solution gives an irritation index of 3.0. AS is seldom used in dish‑washing liquids and rarely serves as the primary surfactant (low dosage in formulations), mainly because it raises production costs and such products have little demand for foaming power. AS ranks fifth in output among synthetic surfactants and commands a high price; its powder grade sold at 15000 yuan per tonne in 2002. Apart from high foaming capacity and strong detergency, AS underperforms AES in most functional aspects: its acid stability is slightly poorer, its irritation is relatively high (only lower than LAS), and it carries the highest price among common anionic surfactants.
The main varieties of non‑ionic surfactants include alkanolamide (FFA), fatty alcohol polyoxyethylene ether (AE), and alkylphenol polyoxyethylene ether (APE or OP). Non‑ionic surfactants feature excellent solubilization, detergency, antistatic performance, low irritation, and good lime‑soap dispersing capacity. Their practical applicable pH range is broader than that of common ionic surfactants. Except for detergency and foaming power, their other properties are often superior to ordinary anionic surfactants. Experiments show that adding a small amount of non‑ionic surfactant to an ionic surfactant system can improve the surface activity of the system, when compared at the same active‑substance content.
Alkanolamides are a class of non‑ionic surfactants with outstanding performance, wide application scope and high frequency of use, and are commonly adopted in various liquid detergents. The commonly‑used grades of alkanolamides in liquid detergents are “2:1 amide” and “1.5:1 amide”; “1:1 amide” can also be used. These three grades differ in water‑solubility and thickening performance. Generally speaking, “1.5:1 amide” has moderate properties and is widely used in dishwashing detergents. Normally, “1:1 amide” dissolves readily only when compounded with other water‑soluble surfactants. Alkanolamides are more suitable for alkaline detergents and can also be applied in mildly acidic detergents. Alkanolamide is the lowest‑priced variety among non‑ionic surfactants, with a price of 7,800 RMB per ton in 2002. Alkanolamides are used more frequently than fatty alcohol polyoxyethylene ethers in liquid detergents. Alkanolamides are the non‑ionic surfactants frequently applied in shampoos. Possible reasons are as follows: FFA delivers more comprehensive or superior functions than AE; FFA products are cheaper than AE; FFA has better solubility than AE; FFA outperforms AE in foaming capacity.
Amphoteric surfactants possess both anionic and cationic hydrophilic groups. Accordingly, they behave as cationic substances in acidic solutions, anionic substances in alkaline solutions, and exhibit non‑ionic‑like properties in neutral solutions. Amphoteric surfactants are readily soluble in water, concentrated acid and alkali solutions, and even concentrated inorganic‑salt solutions. They boast good hard‑water resistance, low skin irritation, excellent fabric softening performance, favourable antistatic properties, sound bactericidal effects, and high compatibility with various surfactants.
Such products can be applied over a wide pH range. Nevertheless, judging from their ionic states under different acid‑base conditions, they perform better under acidic and neutral conditions than under alkaline conditions. In general, amphoteric surfactants are more expensive than non‑ionic surfactants.
Important varieties of amphoteric surfactants include dodecyl dimethyl betaine and carboxylate‑type imidazolines. Compared with anionic surfactants, non‑ionic surfactants have more comprehensive properties with fewer drawbacks, only weaker in detergency and foaming power. When set against non‑ionic surfactants, amphoteric surfactants excel in certain properties and match them in others. Amphoteric surfactants have stronger foaming capacity than ordinary non‑ionic surfactants (AE has poor foaming ability), stronger bactericidal power relative to non‑ionic and anionic surfactants, and better conditioning performance. Therefore, in liquid detergents, amphoteric surfactants are mainly used in shampoos, followed by skin cleansers such as body washes.
Common cationic surfactants comprise hexadecyl dimethyl ammonium chloride (1631), octadecyl trimethyl ammonium chloride (1831), cationic guar gum (C‑14S), cationic panthenol, cationic silicone oil, dodecyl dimethyl amine oxide (OB‑2), and so forth. Different from other surfactants, cationic surfactants have poor detergency and foaming power, and exert slight irritant toxicity.
In liquid detergents, cationic surfactants serve as auxiliary surfactants, namely conditioning components with low dosage in formulations. They are generally used in high‑end products, chiefly shampoos. Cationic surfactants cannot be directly combined with anionic surfactants. Though favourable results may occur upon cation‑anionic compounding, there is a high risk of precipitation (crystallisation).
Numerous cationic surfactants are used in shampoos without heavy reliance on one or two specific varieties; they are often formulated into commercial conditioning agents. Cationic surfactants account for a small proportion of total surfactant output and are usually more costly than other categories. Among all types of surfactants, cationic surfactants provide the most prominent conditioning effect and the strongest bactericidal activity. Despite their disadvantages including weak detergency, poor foaming capacity, limited compatibility, relatively high irritation and high cost, they are irreplaceable by other surfactants as conditioning ingredients in high‑end liquid detergents such as shampoos. It is noteworthy that cationic surfactants shall only be utilised as conditioning components or bactericides.
Post time: Aug-06-2026