What is the performance of silicone surfactants?
1. Interface properties of silicone surfactants
Since the main chain of the silicone surfactant is a soft Si-O bond, neither hydrophilic nor oleophilic, it can be used as a non-aqueous medium in which aqueous solutions and ordinary hydrocarbon surfactants cannot be used. On the other hand, the silicone surfactant is arranged at the interface with a methyl group to reduce the surface tension to about 20 mN/m, while the ordinary hydrocarbon surfactant is arranged at the interface with a methylene group, which only causes the surface tension to drop. Up to 30mN/m.
The most widely used silicone surfactants are EO/PO modified silicone surfactants, and their properties are related to various factors such as the ratio of EO/PO and the degree of polymerization of the surfactant. EO is the hydrophilic portion of the surfactant modifying group and PO is the lipophilic portion. When the ratio of EO/PO changes, the properties of the silicone surfactant change. It has been found that if the EO/PO ratio becomes larger, the HLB value of the surfactant increases, indicating that the hydrophilicity is enhanced; and the EO/PO ratio becomes smaller, which lowers the hydrophilicity of the surfactant. When the length of EO of the graft type polyether-modified silicone oil is the same, the surface tension thereof increases as the degree of polymerization of the polysiloxane decreases. This is due to the shorter the molecular chain of the polysiloxane, the tighter the packing at the air/water interface, and the more methyl groups on the surface. When PO is introduced, the hydrophobicity of the polyether chain is increased, thereby increasing the surface tension of the polyether-modified silicone oil.
2. Super wettability of silicone surfactants
Trisiloxane surfactants not only reduce the interfacial tension at the oil/water interface, but may also wet-extend on low-energy hydrophobic surfaces. This ability is known as "super wettability" or "super-spreadability." This phenomenon is believed to be the presence of specific surfactant aggregates in solution.
The reason why polydimethylsiloxane chains are easily spread on polar surfaces (such as water, metals, fibers, etc.) is that oxygen in the silicon oxide chain forms oxygen bonds with polar molecules or radicals, increasing the silicon oxide chain and the pole. The interaction between the surface molecules causes them to spread into a monolayer, so that the hydrophobic siloxane lies on the polar surface, forming a unique "extension chain" configuration, while the common surfactant is hydrophobic. The base is erected on a polar surface. When a methyl group in a polysiloxane is substituted with another group such as a large alkyl group, an alicyclic group, an aryl group, a silicon functional group or a carbon functional group, it is bound to change the polarity or steric hindrance of the substituent. It will affect the hydrophobicity of the polysiloxane and the spreading speed and state on the polar surface. The same effect on the number and distribution of substituents on the siloxane chain. For example, substitution of a methyl group with a larger alkyl or aryl group can significantly reduce the ability of the polysiloxane to spread while reducing its ability to orient on polar surfaces.
3. The ability of silicone surfactants to stabilize emulsions
Some grafted silicone surfactants allow the emulsion to remain in the presence of salts, ethanol, and organic solvents that are not available with conventional hydrocarbon surfactants. It was found by atomic microscopy (AFM) that the silicon surfactant had an interaction force at the interface. Nonionic surfactants lose surface activity in 25% ethanol solution, while silicone surfactants reduce surface tension at 80% ethanol volume fraction. This property of the silicone surfactant reflects that the polydimethylsiloxane is not only hydrophobic, but also the silicone surfactant is insoluble in the organic solvent as the content of the polydimethylsiloxane increases.
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