What are Silicone surfactants？

Recent searches reveal that silicone surfactants are specialized agents designed to reduce surface tension between materials, enhancing their ability to wet and spread. Primarily composed of a polydimethylsiloxane backbone, silicone surfactants are employed across various industries due to their unique properties and versatility.

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Silicone surfactants belong to a specific category of surfactants characterized by polydimethylsiloxane as their hydrophobic base, featuring one or more organosilicon polar groups in their structure. The Si-O bond's higher energy in silicone surfactants compared to C-C and C-O bonds found in traditional carbon-chain surfactants provides enhanced hydrophobic properties and stability. Their large molecular weight and multi-branched arrangement contribute to their outstanding performance at low temperatures and compatibility, making them efficient surfactants.

The general formulation is defined as follows:

By altering the m, n, and R groups in the formula, it’s possible to create silicone surfactants with varying molar masses (viscosity) and hydrophilic-lipophilic balance (HLB values) tailored to various applications. These include foam leveling agents for polyurethane foams, emulsifiers, personal care products, leveling agents for coatings, defoamers, antistatic agents, plastic modifiers, wetting agents, fuel additives, fabric finishing agents, water-soluble lubricants, mold release agents, and pesticide additives, agricultural adjuvants, among others.

01 Silicone surfactant features

1. Excellent performance in lowering surface tension.

2. Superior wetting capabilities.

3. Antifoaming and foam stabilization properties.

4. Non-toxic and physiologically inert.

5. Efficient emulsifying effect and compatibility.

02 Evaluating the effect of silicone surfactants

Critical micelle concentration (CMC) serves as a key parameter to assess the minimum concentration required to reduce the maximum surface tension of water. Micelles are formed above the CMC that encapsulate hydrophobic contaminants, allowing for their removal from surfaces. Thus, the CMC value is crucial for evaluating the effectiveness of any biosurfactant, and is essential in product comparisons.

03 Classification of silicone surfactants

Silicone surfactants can be categorized into four main types based on the chemical nature of the hydrophilic group R in their structures: nonionic, anionic, cationic, and zwitterionic. Nonionic surfactants, in particular, are the most extensively studied and applied.

1. Cationic silicone surfactant

When the R group contains structural components such as alkyl quaternary ammonium derivatives, it's classified as a cationic organosilicon surfactant. Cationic polysiloxane quaternary ammonium salts are particularly notable among cationic surfactants due to their higher compatibility with anionic surfactants, low irritability towards human skin and eyes, and inherent antibacterial properties. This function enables stabilization and enhances softness via their long-chain polysiloxane structures.

2. Anionic silicone surfactant

Anionic silicone surfactants feature structural units such as phosphate esters, sulfate salts, and carboxylates within their R group. For instance, if R contains fatty acid units, it results in a polysiloxane phosphobetaine amphoteric surfactant, combining characteristics of both phosphobetaine and polysiloxane. This category includes products recognized for their low toxicity, anti-bacterial properties, resistance to hard water, and excellent compatibility with various surfactants.

3. Nonionic silicone surfactants

Components within the R group, including polyether, alkanolamide, ester, and glycoside groups, define nonionic surfactants. Polyether silicone surfactants are particularly prevalent, composed of polysiloxane segments (A) and polyether segments (B), which can combine through various methods such as AB, ABA, and side-chain configurations. The connection types include Si-O-C (unstable, hydrolysis type) and Si-C (stable, non-hydrolysis type).

4. Amphoteric silicone surfactants

Amphoteric polysiloxane surfactants have R groups with structures like phosphate betaine or other similar entities.

04 Methods for synthesizing silicone surfactants

1. Cationic silicone surfactants synthesis

This synthesis operation occurs within inert solvents such as benzene, acetone, carbon tetrachloride, xylene, or toluene.

2. Anionic silicone surfactants synthesis

Synthesis methods are diverse but often revolve around creating linkage structures and ensuring the desired surfactant properties.

3. Nonionic silicone surfactants synthesis

Two methods of creation exist: copolymers linked by Si-O-C or Si-C chains, requiring two synthesis steps. The first step synthesizes polysiloxanes, while the second step forms block copolymers with polyoxoalkanes in the configurations of SiO or SiC.

05 Silicone surfactants performance

1. Interfacial properties

The soft Si-O bond characteristic of silicone surfactants allows them to function in both aqueous and non-aqueous media where traditional hydrocarbons fall short. They can reduce surface tension significantly, outperforming conventional surfactants.

The EO/PO modified silicone surfactants are the most commonly utilized, with efficacy influenced by the ratio of EO to PO and their respective polymerization degree; here, EO serves as the hydrophilic segment while PO provides lipophilicity. Adjustments in these ratios directly impact the surfactant's attributes.

2. Superwettability

Trisiloxane surfactants excel at minimizing interfacial tension across oil/water interfaces, exhibiting remarkable "super-wettability" on low-energy hydrophobic surfaces. Such a capability arises from specific surfactant aggregates within the solution.

3. Emulsion stabilization

Grafted silicone surfactants maintain emulsion stability even in the presence of salts and organic solvents, showcasing properties that traditional surfactants lack. According to atomic force microscopy studies, silicone surfactants can still reduce surface tension in environments with high solvent concentrations.

4. Interaction with CO2

Research proposes that polyoxyethylene ether trisiloxane surfactants encourage CO2-water emulsions. Adjusting the EO content affects the "hydrophilic and CO2-philic balance" for effective emulsions. Investigations highlight how silicone surfactants behave uniquely within supercritical CO2 environments.

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06 Silicone surfactants applications

1. Personal care and cosmetics

Due to their non-toxicity, skin compatibility, and additional benefits like oxidation resistance and UV protection, silicone surfactants are prominent in cosmetics, shampoos, and personal care items, enhancing moisturizing effects and improving surface qualities.

2. Textile industry

Their unique properties make silicone surfactants ideal as antistatic agents and softeners, contributing to softness and effective sterilization in textile processing.

3. Pesticides

In pesticide formulations, integrating organosilicon surfactants optimizes stability and chemical performance, enhancing application efficacy and reducing overall usage.

4. Food and medicine

Silicone defoamers are widely used in food production, owing to their exceptional defoaming properties and non-toxic nature, adding value in various culinary and medicinal applications.

5. Leather chemicals

Due to their lubricating and water-repellent properties, silicone surfactants serve as effective fatliquors and softeners in leather production.

6. Machining

Silicone surfactants effectively clean and protect metal surfaces during manufacturing and maintenance, exhibiting excellent cleaning capabilities coupled with corrosion resistance.

7. Plastic industry

In foam production processes, silicone surfactants play vital roles in stabilization and creating different foam types, including rigid and semi-rigid formats, with growing relevance in flame-retardant formulations.

07 Conclusion

Silicone surfactants boast cost-effective raw materials and expansive applicability. They continue gaining traction in various fields, aligning research and innovation with global industrial demands.

References:

"Sweet Silicones": Biocatalytic Reactions to Form Organosilicon Carbohydrate Macromers. Org. Lett. ; 7 :'.

Henkensmeier D et al. (Synthesis and characterization of terminal carbohydrate modified poly(dimethylsiloxane)s.)

Wang G et al. (Adsorption and aggregation behavior of tetrasiloxane-tailed surfactants containing oligo(ethylene oxide) methyl ether and a sugar moiety.)

Kickelbick G et al. (Spontaneous vesicle formation of short-chain amphiphilic polysiloxane-b-poly(ethylene oxide) block copolymers.)

Soni SS et al. (Dynamic light scattering and viscosity studies on the association behavior of silicone surfactants in aqueous solutions.)