I. Structure

In hydrocarbon surfactants, when the hydrogen (H) atoms in the C-H chain are replaced by fluorine (F) atoms, the result is fluorocarbon surfactants. When all hydrogen atoms in the hydrocarbon chain are substituted with fluorine, they are called perfluorinated surfactants, whereas partial substitution results in partially fluorinated surfactants. Currently, most fluorinated surfactants in use are perfluorinated surfactants.

As the hydrocarbon chain transitions to a fluorocarbon chain, significant changes in physical and chemical properties are observed. For instance, fluorinated surfactants are more challenging to synthesize and exhibit the best activity among various types of surfactants.

II. Properties and Classification

1. Properties

Due to their unique structure, fluorinated surfactants exhibit characteristics not found in other surfactants, commonly summarized as “three highs” and “two repellencies”:

• High surface activity

• High thermal stability

• High chemical stability
  Additionally, the fluorocarbon group in fluorinated surfactants is both hydrophobic and oleophobic.

The effectiveness of surfactants is closely related to surface properties and the surfactant structure. Fluorinated surfactants resist the effects of strong oxidants, acids, and alkalis, demonstrating exceptionally high chemical stability.

2. Classification

Fluorinated surfactants are divided into ionic and nonionic types based on their polar group structures. Ionic fluorinated surfactants are further categorized into anionic, cationic, and zwitterionic types. Each type can be further diversified into various series based on their structural groups, as shown in Table 1.

III. Synthesis Pathways

The synthesis of fluorinated surfactants typically involves three steps:

1. Synthesizing fluorinated alkyl compounds containing 6–10 carbon atoms.

2. Preparing fluorinated intermediates that readily introduce various hydrophilic groups.

3. Introducing specific hydrophilic groups.

The synthesis of fluorinated alkyl compounds is the critical step. Current industrial methods for producing fluorinated alkyl compounds include electrochemical fluorination, fluorinated olefin telomerization, and fluorinated olefin oligomerization.

1. Electrochemical Fluorination (ECF)

Developed in the 1940s by J.H. Simons and first applied industrially by 3M, this method involves substituting hydrogen and chlorine atoms in the raw materials with fluorine atoms using highly reactive fluorine generated during electrolysis. Examples include the production of perfluoroalkyl acyl fluoride and perfluoroalkyl sulfonyl fluoride.

Although the ECF process is straightforward and completed in a single step, its drawbacks include high cost, significant power consumption, specialized equipment requirements, and issues such as cracking, cyclization, and rearrangement of reactants, resulting in low yields and numerous byproducts.

2. Fluorinated Olefin Telomerization

(1) Telomerization with Perfluoroiodoalkanes

Example reactions are as follows:

(2) Telomerization of Tetrafluoroethylene and Methanol

3. Fluorinated Olefin Oligomerization

This method involves oligomerizing fluorinated olefins in aprotic solvents to produce highly branched, low-polymerization-degree perfluorinated oligomers. Common approaches include:

• Tetrafluoroethylene oligomerization

• Hexafluoropropylene oligomerization

• Hexafluoropropylene oxide oligomerization

IV. Main Applications

Fluorinated surfactants have broad applications across numerous industries, including chemicals, biology, medicine, machinery, textiles, electronics, papermaking, pigments, coatings, inks, ceramics, metallurgy, fuels, leather, photosensitive materials, construction, petroleum, and firefighting.

1. Applications in Petroleum

In oil extraction, chemical methods such as surfactant flooding and microemulsion flooding can significantly enhance oil recovery (up to 80%–85%). These methods rely on surfactants to lower interfacial tension and improve oil-washing efficiency. Despite the higher costs of fluorinated surfactants compared to hydrocarbon surfactants, their enhanced oil recovery efficiency justifies their use.

2. Applications in Firefighting

Fluorinated surfactants play an irreplaceable role in modern fire extinguishing agents due to their unique properties. For instance, adding 0.005%–0.05% of anionic or nonionic fluorinated surfactants to protein foam extinguishing agents significantly improves fire-extinguishing speed, achieving 3–4 times faster results compared to traditional agents.

3. Applications in Coatings

3M has developed products like FC-4430 and FC-4432, which cater to industrial formulations for coatings, inks, adhesives, and resins. These fluorinated surfactants improve wetting, spreading, leveling, and surface defect prevention at low concentrations, resolving issues such as floating colors and poor adhesion.

4. Applications in Papermaking

In the papermaking industry, fluorinated surfactants serve as dispersants for paper coatings, improving pigment dispersion, flowability, and adhesive compatibility. They also act as effective defoamers, enhancing paper coating processes and performance.

5. Applications in Biomedicine

Fluorinated surfactants are being studied for their potential in forming vesicles, which are ordered molecular assemblies with bilayer structures in aqueous solutions. These vesicles have applications in biomembrane simulation, drug delivery, and catalytic reactions.

6. Other Applications

Fluorinated surfactants have diverse roles across other fields due to their exceptional properties (Table 2).

V. Environmental Policies

1. United States

In September 2022, the U.S. Environmental Protection Agency (EPA) proposed designating perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) as hazardous substances under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA). This designation aims to aid in cleaning contaminated sites and reduce human exposure to these "forever chemicals."

2. China

China’s "List of Strictly Restricted Toxic Chemicals" includes PFOS substances, with regulations effective from January 1, 2020.

VI. Domestic Status and Future Directions

1. Domestic Status

China’s fluorinated surfactant industry is still in the development phase, with limited independent innovation and original technology. Current challenges include:

• A focus on raw material production with limited downstream processing.

• A narrow range of product varieties.

• A lack of reagent-grade products.

• Redundant low-level production technologies.

• Lagging application research and development.

2. Future Directions

To address environmental concerns and replace PFOS, research efforts are focusing on:

• Reducing PFOS usage concentrations by blending with hydrocarbon surfactants.

• Developing short-chain fluorocarbon products to improve biodegradability.

• Using non-perfluorinated segments to replace perfluorinated chains, reducing environmental persistence.