What are Silicone surfactants？

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Silicone surfactants are a category of surfactants featuring polydimethylsiloxane as the hydrophobic core, complemented by one or multiple organosilicon polar groups linked to their intermediate or terminal positions. The bond energy of the Si-O in the hydrophobic segment surpasses that of traditional carbon-based surfactants (C-C and C-O), resulting in enhanced hydrophobicity and stability. Furthermore, their substantial molecular weight combined with a branched architecture imparts excellent low-temperature functionality and compatibility, establishing them as highly effective surfactants.

 

The structure can generally be represented by the following formula:

 

By modifying the parameters m, n, and R within the formula, silicone surfactants of varying molar masses (viscosity) and hydrophilic-lipophilic balance (HLB) values can be synthesized for diverse applications. These include foam leveling agents in polyurethane foams, emulsifiers, personal care ingredients, leveling agents for coatings, defoamers, antistatic agents, plastic modifiers, wetting agents, fuel additives, fabric finishing agents, water-soluble lubricants, mold release agents, pesticide additives, and agricultural adjuvants.

01 Characteristics of Silicone Surfactants

1. Exceptional surface tension reduction capability.

2. Outstanding wetting characteristics.

3. Foam stabilization along with antifoaming properties.

4. Generally regarded as physiologically inert.

5. Remarkable emulsifying properties with favorable compatibility.

02 Assessing the Effectiveness of Silicone Surfactants

Critical micelle concentration (CMC) indicates the minimum concentration required to achieve maximum reduction in water surface tension. The micelles formed beyond this concentration trap hydrophobic contaminants within their cores, facilitating reintroduction into the aqueous phase. This CMC value is vital for evaluating the efficiency of biosurfactants and is essential for comparative analyses.

03 Types of Silicone Surfactants

Silicone surfactants can be categorized into four types based on the chemical properties of the hydrophilic R group in their structures: nonionic, anionic, cationic, and zwitterionic. Among these, nonionic surfactants are the most extensively researched and used.

1. Cationic Silicone Surfactants

When the R group contains structural units such as alkyl quaternary ammonium compounds, amido quaternary ammonium compounds, or imidazoline derivative quaternary ammonium compounds, they are classified as cationic organosilicon surfactants. Cationic polysiloxane quaternary ammonium salt surfactants are particularly common, featuring large molar masses that allow compatibility with anionic surfactants. These are non-irritating and exhibit a degree of antibacterial efficacy. The polymeric chains contribute to enhanced smoothness and softness.

2. Anionic Silicone Surfactants

If the R group contains structural units such as phosphate ester salts, sulfate salts, carboxylate salts, sulfonate salts, or sulfosuccinamide esters, they are termed anionic silicone surfactants. The structural configuration shown indicates a polysiloxane phosphate salt surfactant.

 

 

 

If the R' incorporates a fatty acid functional group, it constitutes a polysiloxane phosphobetaine amphoteric surfactant. This surfactant exhibits characteristics of both phosphobetaine and polysiloxane. The properties of the silicone polymer vary with the choice of low or high molar mass polysilane, thus highlighting its compatibility and antibacterial properties.

3. Nonionic Silicone Surfactants

When the R group includes polyether, alkanolamide, ester, and glycoside units, they are classified as nonionic surfactants. Among these, polyether silicone surfactants are the most prevalent, featuring combinations of polysiloxane segments (A) and polyether segments (B). Common configurations include AB type, ABA-type, BAB-type, and branched and side-chain types. The linkage can occur via either Si-O-C or Si-C types, with the former being prone to hydrolysis and the latter being more stable.

4. Amphoteric Silicone Surfactants

Amphoteric polysiloxane surfactants are formed when the R group encompasses structures such as phosphate betaine or betaine.

04 Synthesis of Silicone Surfactants

1. Cationic Silicone Surfactants Synthesis

Synthesis is carried out in inert solvents such as benzene, acetone, xylene, or toluene.

 

2. Synthesis of Anionic Silicone Surfactants

 

3. Nonionic Silicone Surfactants Synthesis

This can be categorized into two primary synthesis methods based on copolymerization and chemical stability. They consist of copolymers linked by Si-O-C chains and Si-C chains. The synthesis typically involves two key steps: initiating with polysiloxane synthesis followed by formulation with polyoxoalkane blocks.

05 Performance of Silicone Surfactants

1. Interfacial Properties of Organosilicon Surfactants

Due to the soft Si-O backbone of silicone surfactants, they possess both hydrophilic and lipophilic characteristics, allowing their use in both aqueous and non-aqueous media—often where conventional hydrocarbon surfactants fall short. Arranging at the interface with methylene groups enables silicone surfactants to lower surface tension to approximately 20 mN/m, whereas traditional hydrocarbon surfactants achieve only around 30 mN/m.

 

The EO/PO modified silicone surfactants are largely utilized, with their performance influenced by factors such as EO/PO ratios and the degree of surfactant polymerization. The hydrophilic EO contrasts with the lipophilic PO, altering surfactant attributes as their ratio changes.

 

When the EO/PO ratio increases, the surfactant's HLB value escalates, indicating heightened hydrophilicity. Conversely, a reduced EO/PO ratio suggests diminished hydrophilicity. Investigations into grafted polyether-modified silicone oils suggest that at equivalent lengths of EO, reduced polysiloxane polymerization correlates with increased surface tension due to tighter aggregation at the air/water interface.

2. Superwettability of Silicone Surfactants

Trisiloxane surfactants can enhance the interfacial tension at the oil/water boundary and expand wettability on low-energy hydrophobic surfaces—an ability dubbed "super-wettability" or "super-spreadability". This is attributable to specific aggregate formations in the surfactant solution, leading to their use as wetting agents.

 

The ease with which polydimethylsiloxane chains spread on polar surfaces (e.g., water, metals, fibers) stems from the oxygen in silicone chains forming bonds with polar molecules, thus enhancing interchain forces and enabling a monolayer spread. In contrast, ordinary surfactants tend to orient upright.

 

When methyl groups in polysiloxane chains are substituted with larger functional groups (alkyl, alicyclic, aryl), significant impacts on hydrophobicity, spreading rates, and surface configurations occur due to differences in polarity and resistance from the substituents. Changes in the number and spatial distribution of substituents also influence these properties.

3. Emulsion Stability of Silicone Surfactants

Some grafted silicone surfactants manage to maintain emulsion stability amidst the presence of salts, ethanol, and organic solvents, a capability cumbersome for traditional hydrocarbon surfactants. Atomic force microscopy (AFM) detected interaction forces at the silicone surfactant interface—demonstrating that nonionic surfactants lose surface activity in 25% ethanol solutions, whereas silicone surfactants retain tension-lowering abilities up to 80% ethanol fractions. This reveals that polydimethylsiloxane is not only hydrophobic but also increasingly insoluble with higher polydimethylsiloxane concentrations in organic solvents.

4. Silicone Surfactants and CO2 Dynamics

Research by Rocha et al. indicated that polyoxyethylene ether trisiloxane surfactants can produce emulsions of CO2 and water. By adjusting the number of EO groups, surfactants can transition emulsions from water-in-CO2 to CO2-in-water formats. Further, works by Sarbu et al. validated the suitability of polysiloxane surfactants for supercritical CO2 environments, while Fink et al. identified small sensitivity variances to CO2-phobic groups and lesser dependency on siloxane dimensions resulting in a random liquid crystal phase. Studies conducted by Folk et al. elaborated on a series of cationic silicone surfactants under high-density CO2 conditions, noting that variations in chain lengths and counterion choices dramatically affect its surface activity.

06 Applications of Silicone Surfactants

1. In Personal Care and Cosmetic Products

Silicone surfactants are recognized for various attributes such as low toxicity, lack of skin irritation, antioxidant properties, UV protection, excellent biocompatibility, and water resistance. These surfactants now find extensive application in cosmetics and personal care items like shampoos, hair care products, and creams. Their low surface tension combined with appropriate viscosity enables effortless spreading on skin and hair, enhancing moisture retention and maintaining skin ventilation. Consequently, hair appears shiny, manageable, and non-static.

For additional information, check out Silicone Surfactant Examples.

 

 

2. In the Textile Sector

Silicone surfactants confer antistatic properties, improved softness, and notable sanitization capabilities, rendering fibers softer. Cationic silicone surfactants primarily function as antistatic agents and softeners, particularly quaternary ammonium salt types utilized for hygienic fiber finishing, blessed for their sterilizing and mildew-resistant properties that ensure safety and durability.

 

3. In Pesticide Formulations

The inclusion of 0.05% - 0.1% (by weight) of organosilicon surfactant in pesticide formulations can significantly optimize physical properties, chemical stability, and diverse application scopes. Pesticides frequently confront barriers during application due to the hydrophobic nature of plant surfaces; incorporating silicone surfactants improves adherence, coverage, and penetration, thereby amplifying efficacy. Additionally, they can diminish surface tension effectively, allowing water to penetrate insect tracheas, thereby elevating lethality.

 

• XJY-703 Bis(trimethylsiloxy)methylsiloxane/Heptamethyltrisiloxane, characterized by a reactive silicon-hydrogen bond, serves as a fundamental element for synthetic polyaleneoxide modified heptamethyltrisiloxane.

 

• XJY-207 Polyalkyleneoxide modified heptamethyltrisiloxane stands out for its structural modification, displaying unmatched wetting, spreading, and stomatal penetration abilities as traditional surfactants struggle to replicate. This variant enhances pesticide effectiveness while reducing applied volumes, impacting herbicides, insecticides, acaricides, fungicides, and growth regulators.

 

4. In Food and Pharmaceuticals

Silicone defoamers, engineered with modified polysiloxane, incorporate various non-ionic surfactants, showcasing excellent foam control capabilities while being non-toxic and non-corrosive. Their efficiency extends to manufacturing processes for monosodium glutamate, soy products, and antibiotics, minimizing foam generation.

• XJY-706 1,1,3,3-Tetramethyldisiloxane is utilized as an intermediary in pharmaceuticals.

 

5. In Leather Treatments

Due to their lubrication and water-repellent properties, silicone surfactants serve effectively as fatliquors and softeners within the leather industry. The synthetically produced organosilicon fatliquoring agents address the challenges of outward migration and allow for cost reduction without sacrificing performance. The silicone-treated leather fibers exhibit ample lubrication and dispersion, delivering premium softness—ideal for garments and footwear and useful in fur production. Aminopolyether co-modified silicone surfactants maintain the desired softness and hydrophilicity.

 

6. In Machining Processes

Metal products necessitate careful cleaning from various contaminants (including metal chips, lubricants, abrasives, dust, and electrolytes). Silicone surfactants are key to delivering robust cleaning capabilities, effectively eliminating oil stains and residues while being non-flammable, non-toxic, and safe for use. They additionally exhibit corrosion inhibition and anti-corrosive properties, supporting energy conservation and diminishing environmental hurt; thus, they are well-suited for automated cleaning procedures.

 

7. In Plastic Manufacturing

Within the polyurethane foam production process, silicone surfactants play a pivotal role, enhancing system dispersion, bubble generation, and stability. Their use is critical in formulating soft, rigid, semi-rigid, and high resilience foams. The flame-resistant silicone foam stabilizers are increasingly relevant in this area.

 

07 Conclusion

Silicone surfactants bring forth economical and accessible raw materials, straightforward manufacturing techniques, extensive applicability, and significant demand. They exhibit remarkable performance, burgeoning growth, and promising market opportunities. Research endeavors within domestic domains are advancing rapidly to bridge the international gap, yielding noteworthy progress in structural research, as well as in cleaning and sustainability advancements of the products.

 

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