Surfactants are substances that, when added in small amounts, can significantly reduce the surface (or interfacial) tension of a solvent (typically water as the standard solvent) and the interfacial tension between two liquids. They possess a specific molecular structure, unique hydrophilic-lipophilic balance, and special adsorption properties.

Surfactant molecules are all amphiphilic compounds with an asymmetric structure. Each molecule consists of a hydrophilic group, which is water-soluble, and a lipophilic (hydrophobic) group, which is insoluble in water but soluble in oil. A schematic representation is shown below: 

Surfactants are widely classified in this field according to their chemical structure. First, they are divided into ionic and non-ionic types based on the nature of the hydrophilic group—specifically, its electrical properties. Ionic surfactants are further categorized into cationic, anionic, and amphoteric types. In addition, there are recently developed mixed-type surfactants that possess both ionic and non-ionic hydrophilic groups. Surfactants contain both hydrophilic and lipophilic moieties. However, since numerous chemical structures can serve as either hydrophilic or lipophilic groups, the variety of resulting surfactants is extensive. To better understand and effectively utilize surfactants, it is essential to grasp their classification system. 

In practical applications, classification by ionic type and process application is the most common. 
I. Classification by Ionic Type
◆ Anionic Surfactants: In water, surfactants that ionize and generate active anionic groups are known as anionic surfactants. The main types include carboxylates (RCOO⁻M⁺), sulfonates (R—SO₃⁻M⁺), sulfate esters (ROSO₃⁻M⁺), and phosphate esters (ROPO₃⁻M⁺). Common products in this category include detergents, penetrants, wetting agents, refining agents, and dispersants.
◆ Cationic Surfactants: In water, surfactants that ionize and produce active cationic groups are referred to as cationic surfactants. Major types include quaternary ammonium salts (RNR'₃⁺A⁻), alkylpyridinium salts (RC₅H₅N⁺A⁻), and ammonium salts (RₘNHₘ⁺A⁻), where m = 1–3 and n = 1–3; R groups may vary or contain heterocycles. Typical applications include softeners, leveling agents, waterproofing agents, and antistatic agents.
◆ Zwitterionic Surfactants: These surfactants ionize in water and possess both positively and negatively charged functional groups simultaneously, giving them surface-active properties. Their molecules contain two hydrophilic groups—one positive and one negative. The positive group is typically an amine or quaternary ammonium salt, while the negative group is usually a carboxyl or sulfonic acid group. The four primary zwitterionic surfactants are amino acids, imidazolines, betaines, and taurines. Main products include softeners, leveling agents, and antistatic agents.
◆ Nonionic Surfactants: These do not ionize in water; their active groups carry no charge but can undergo hydration. They mainly include polyoxyethylene compounds [RO(C₂H₄O)ₙH], sulfoxides (RSOR'), nitrogen oxides (RNO), and polyol derivatives (such as sorbitol, glycerol, and ethylene glycol). Also included are polyether-type surfactants composed of polyoxyethylene and polyoxypropylene. Key products are emulsifiers, leveling agents, and dispersants.
◆ Special Surfactants:
① Fluorinated Surfactants: In these surfactants, hydrogen atoms in the hydrophobic chain are partially or fully replaced by fluorine atoms. Due to weaker intermolecular forces between fluorocarbon chains compared to hydrocarbon chains, fluorocarbon chains exhibit stronger hydrophobicity and also oil-repellent properties. Their surface activity far exceeds that of conventional surfactants, with aqueous solutions achieving surface tensions below 20 mN/m and very low critical micelle concentrations. The hydrophilic groups in fluorocarbon surfactants resemble those in hydrocarbon-based surfactants, existing in anionic, cationic, zwitterionic, and nonionic forms. Main products include water- and oil-repellent agents.
② Silicone Surfactants: These use siloxane chains as the hydrophobic component and exhibit strong surface activity. The hydrophilic groups may be carboxyl, sulfate ester, ammonium salt, or ethylene oxide, forming anionic, cationic, or nonionic surfactants respectively. Major products include silicone softeners and modified amino silicone oils.
③ Polymeric Surfactants: These are substances with relatively high molecular weights (typically 10³–10⁶) and certain surface-active properties. Besides polyethers, synthetic polymeric surfactants include linear polymers formed by condensation of alkylphenols with formaldehyde, which react with ethylene oxide to form water-soluble nonionic surfactants. Other examples include polyvinyl alcohol, partially hydrolyzed polyacrylamide, and polyacrylate salts. Key products include emulsifiers, dispersants, and oxygen bleach stabilizers.