Classification of Surfactants
Surfactants are diverse in variety, and their classification methods are also numerous. In practical applications, the most commonly used and instructive classification method is based on the dissociation properties of polar groups. According to this standard, surfactants are mainly divided into four major categories: anionic, cationic, amphoteric, and nonionic. In addition, there is a special category of polymeric surfactants.
I. Anionic Surfactants
1. Characteristics:
After dissolving in water, their hydrophilic groups carry a negative charge.
2. Classification and Common Representatives:
They are mainly divided into two major categories: soaps and sulfates.
(1) Soaps, also known as carboxylates, have the general formula (RCOO)ₙM, with representative varieties including sodium stearate, sodium oleate, and sodium laurate, which are salts of higher fatty acids.
(2) Sulfates, i.e., sulfate ester salts, have the general formula R-O-SO₃⁻M⁺, with representative varieties including sulfated oils and higher fatty alcohol sulfates, with sodium dodecyl sulfate being a typical example.
3. Functions and Characteristics:
This is the most widely used and largest-volume category of surfactants. They are characterized by excellent detergency, foaming, and emulsifying properties, and are relatively inexpensive.
4. Main Application Scenarios:
(1) Detergents: such as the main active ingredients in laundry powders and dishwashing liquids (e.g., sodium dodecylbenzene sulfonate).
(2) Personal care products: common cleansing ingredients in shampoos and body washes (e.g., stearic acid).
(3) Industrial fields: as emulsifiers in emulsion polymerization.
II. Cationic Surfactants
1. Characteristics:
After dissolving in water, their hydrophilic groups carry a positive charge.
2. Common Representatives:
Quaternary ammonium compounds (such as benzalkonium chloride), benzalkonium chloride (Jie'er Mie), benzalkonium bromide (Xin Jie'er Mie), polyvinylpyridine dodecyl bromide quaternary ammonium salt, cetyltrimethylammonium bromide (CTAB), and dodecyldimethylbenzylammonium chloride.
3. Functions and Characteristics:
The main activity of this type of surfactant is reflected in its cations, and they are often referred to as "positive soaps." Their molecular structures mostly contain a pentavalent nitrogen atom, so most belong to quaternary ammonium compounds. They generally possess good bactericidal and disinfecting capabilities and have strong adsorption affinity for solid surfaces (such as fabrics and minerals).
4. Main Application Scenarios:
(1) Disinfection and sterilization: medical disinfectants, food processing equipment disinfection, and bactericidal/algaecidal agents in water treatment.
(2) Textile industry: as softeners to make fabrics feel smoother and softer.
(3) Mining and building materials: mineral flotation agents and asphalt emulsifiers.
III. Amphoteric (Zwitterionic) Surfactants
1. Characteristics:
The molecular structure contains both positively and negatively charged groups, with the overall molecule being electrically neutral.
2. Property Characteristics:
Their properties change with the pH of the environment.
(1) In acidic solutions, they behave as cationic types, with enhanced bactericidal capacity.
(2) In alkaline solutions, they behave as anionic types, with enhanced foaming and detergency.
3. Classification and Common Representatives:
Structurally, one end of the molecule contains both acidic groups (such as hydroxyl, sulfonic acid, or phosphoric acid groups) and basic groups (such as amine or quaternary ammonium groups). Those in which the cationic part is constituted by amine salts are called amino acid types; those in which the cationic part is constituted by quaternary ammonium salts are called betaine types. Common varieties include lecithin, amino acid-type surfactants, and betaine-type surfactants.
4. Main Application Scenarios:
(1) High-end personal care products: infant and child wash products, high-end facial cleansers, and eye care products, due to their extreme mildness and low irritation.
(2) Pharmaceutical preparations: preparation of liposomes and emulsions.
IV. Nonionic Surfactants
1. Characteristics:
They do not ionize in aqueous solutions and carry no charge. Therefore, they have high stability, are less affected by electrolytes and pH, and have good compatibility with other types of surfactants.
2. Main Varieties and Applications:
(1) Fatty acid glycerides: commonly used as food emulsifiers.
(2) Sorbitan fatty acid esters (Span series): with strong lipophilicity, commonly used in the preparation of water-in-oil (W/O) emulsions.
(3) Polysorbates (Tween series): with strong hydrophilicity, commonly used in the preparation of oil-in-water (O/W) emulsions and as solubilizers.
V. Polymeric Surfactants
1. Characteristics:
They have relatively large molecular weights and are typically polymer structures. They combine the thickening and film-forming properties of polymeric materials with the emulsifying and dispersing properties of surfactants.
2. Main Varieties and Applications:
Polymeric surfactants can be classified into nonionic, anionic, and cationic types according to their ionic types, each with different applications.
(1) Polyvinyl alcohol (PVA) is a nonionic polymeric surfactant, mainly used as an emulsifier, dispersant, and protective colloid.
(2) Polyacrylamide (PAM) can be either nonionic or anionic, mainly used as a flocculant and thickener, and in petroleum extraction as a tertiary oil recovery flooding agent.
(3) Sodium polyacrylate is an anionic type, mainly used for thickening, dispersing, and water retention, often applied in systems requiring thickening and moisturizing.
(4) Polyvinylpyridine dodecyl bromide quaternary ammonium salt is a cationic type, mainly used for sterilization and antistatic purposes.
(5) Lignosulfonate is an anionic type, mainly used as a dispersant and concrete water-reducer, with wide applications in the construction and building materials fields.
