Fluorine-Containing Quaternary Ammonium Salts
Fluorine-containing quaternary ammonium salts (QACs) are a new class of surfactants with high surface activity, excellent thermal stability, high chemical stability, and unique hydrophobic and oleophobic properties. By incorporating fluorine atoms into quaternary ammonium salt molecules, these surfactants acquire valuable functional attributes, such as exceptional antimicrobial activity and enhanced surface activity.
Structural Characteristics of Fluorine-Containing QACs
Typically, fluorine-containing QACs consist of a quaternary ammonium cation (with four alkyl groups attached to a nitrogen atom) and a fluorinated anion. The four alkyl groups on the ammonium cation can be identical or varied, while the anion is commonly a halide ion (such as F-, Cl-, Br-, I-) or other anionic groups (like HSO4- or RCOO-).
Properties of Fluorine-Containing QACs
High Surface Activity: These surfactants demonstrate high surface activity, making them effective in reducing surface tension for applications such as cleaning, emulsifying, and solubilizing.
Thermal and Chemical Stability: These compounds typically offer excellent thermal and chemical stability, retaining their performance in high-temperature and various chemical environments.
Hydrophobic and Oleophobic Properties: The hydrophobic and oleophobic nature of fluorine-containing QACs enables their use in waterproofing, oil-proofing, and anti-fouling applications.
Antimicrobial Properties: Perfluoroalkyl quaternary ammonium salt-based antimicrobial agents exhibit strong activity, particularly against Gram-positive bacteria (e.g., Staphylococcus aureus), and have broad applications in textile finishing, imparting multifunctional properties such as antimicrobial, waterproof, oil-proof, and anti-fouling characteristics.
Electrochemical Properties: New fluorine-containing QACs, such as tetrabutylammonium tetrafluoroaluminate, display favorable electrochemical properties, including high conductivity and thermal stability.
Synthesis of Fluorine-Containing QACs
There are multiple synthesis methods for these compounds. One example involves reacting tetrabutylammonium hydroxide aqueous solution with ammonium tetrafluoroaluminate under nitrogen protection. After the reaction, dichloromethane extraction and azeotropic distillation with isopropanol are used to purify the product, yielding a high-purity fluorine-containing quaternary ammonium salt, tetrabutylammonium tetrafluoroaluminate. This method allows not only effective synthesis but also structural and performance characterization through physical and electrochemical properties.
Another synthesis method uses N-methyldiethanolamine (MDEA) and perfluorohexylethyl iodide as primary raw materials. Single-factor and orthogonal experiments help optimize solvent selection, reaction temperature, and reactant molar ratios to successfully synthesize a new fluorine-containing quaternary ammonium salt surfactant. These methods illustrate the diversity and complexity of fluorine-containing quaternary ammonium salts, with different raw materials and reaction conditions yielding products with varying properties.
Applications of Fluorine-Containing QACs
Fluorine-containing QACs have a wide range of applications across multiple fields. Some specific products and applications are listed below:
Phase Transfer Catalyst
Fluorinated solvents are widely used in catalytic reactions but are costly and environmentally hazardous. Perfluoroalkyl quaternary ammonium salts, with their excellent water solubility and surfactant properties, exhibit micelle catalytic effects and phase transfer catalytic activity. They can dissolve in water, replacing fluorinated solvents to act as both phase transfer catalysts and surfactants. U.S. Patent 6054615 reports new fluorinated quaternary ammonium salts that are highly stable in acidic and alkaline environments, making them effective as phase transfer catalysts, especially in acidic media.
Supercritical CO₂ Additives
Supercritical CO₂ is widely applied in extraction, reactions, coatings, materials processing, and microelectronics. However, its low dielectric constant and low polarizability limit its ability to dissolve polar molecules or ionic compounds. By introducing fluorine-containing surfactants into supercritical CO₂ to form an effective microemulsion, polar substances can be dissolved within the microemulsion's polar core, thereby expanding CO₂'s application scope. Tanaka et al. synthesized a new fluorine-containing quaternary ammonium salt, adding it to environmentally friendly supercritical CO₂, with greater fluorination enhancing solubility.
Metal Corrosion Inhibitors
Metal corrosion poses significant economic challenges. For surface treatments before processing or for etching and polishing, the inclusion of fluorine-containing quaternary ammonium salt surfactants as metal corrosion inhibitors imparts water, oil, and dirt resistance to metal surfaces. Gorbunova et al. reported a new fluorine-containing quaternary ammonium salt surfactant with strong potential to inhibit hydrochloric acid corrosion of low-carbon steel.
Multifunctional Textile Finishing Agent
All hospital textiles must minimize the risk of disease transmission, especially surgical gowns, which require resistance to penetration and wetting. Fluorine-containing quaternary ammonium salt surfactants, with their hydrophobic and oleophobic properties, provide excellent water, oil, and stain resistance, making them suitable as multifunctional textile finishing agents. Early textile treatments used zirconium or chromium salts of perfluoroalkyl carboxylic acids, perfluoroalkyl pyridine salts, or perfluoroalkyl quaternary ammonium salts. Shao Hui et al. utilized perfluorooctyl quaternary ammonium salts as multifunctional textile finishing agents, and they synthesized a new perfluoroalkyl quaternary ammonium silane coupling agent, which, as a surface modifier, provides treated textiles with durable antimicrobial, water-repellent, oil-repellent, and blood-resistant properties.
Antimicrobial Agents
In 1935, Domagk synthesized a series of essential quaternary ammonium salts as antimicrobial agents, demonstrating strong antimicrobial efficacy against various bacteria, fungi, and viruses. The introduction of perfluoroalkyl groups or fluorine chains enhances the antimicrobial activity of quaternary ammonium salts, resulting in highly active agents with bactericidal capabilities that are widely used in plastics, ceramics, building materials, furniture, appliances, toys, leather, pesticides, and more.