1. Interfacial properties of organosilicon surfactants
The most used silicone surfactants are EO/PO modified silicone surfactants, whose properties are related to various factors such as the ratio of EO/PO and the degree of polymerisation of the surfactant, etc. EO is the hydrophilic portion of the surfactant modified group and PO is the lipophilic portion. When the ratio of EO/PO changes, the performance of the silicone surfactant changes. It was found that if the EO/PO ratio becomes larger, the HLB value of the surfactant will increase, indicating increased hydrophilicity; a smaller EO/PO ratio will make the hydrophilicity of the surfactant weaker. When the length of the EO of the grafted polyether-modified silicone oil is the same, its surface tension increases with the decrease of polysiloxane polymerisation degree. This is due to the fact that the shorter the molecular chain of the polysiloxane, the tighter the stacking at the air/water interface and the more methyl groups on the surface. When PO is introduced, it increases the hydrophobicity of the polysiloxane chain, thus increasing the surface tension of the polyether-modified silicone oil.
2、Super wettability of silicone surfactants
The reason why polydimethylsiloxane chain is easy to spread on the polar surface (such as water, metal, fibre, etc.) is that the oxygen in the siloxane chain can form an oxygen bond with the polar molecule or atomic group, which increases the force between the siloxane chain and the molecules on the polar surface, prompting it to spread into a single molecule layer, thus making the hydrophobic siloxane lying on the polar surface, and forming a unique ‘stretch chain’ configuration, while the hydrophobic groups of ordinary surfactants are upright on the polar surface. When the methyl group in polysiloxane is replaced by other groups (such as large alkyl, alicyclic, aryl, silyl or carbon-functional groups), the polarity or spatial resistance of the substituent group will be changed, which will inevitably affect the hydrophobicity of polysiloxanes and the spreading speed and state of the polar surface. For example, substitution of a methyl group by a larger alkyl or aryl group significantly reduces the spreading ability of polysiloxanes, as well as their ability to orientate on polar surfaces.
3, the ability of silicone surfactants to stabilise the emulsion
Some grafted silicone surfactants can keep the emulsion stabilised in the presence of salt, ethanol and organic solvents, which is not the case with conventional hydrocarbon surfactants. Silicone surfactants were found to have interaction forces at the interface by atomic force microscopy (AFM). Nonionic surfactants lose their surface activity in 25% ethanol solution, whereas organosilicon surfactants are still able to reduce the surface tension at ethanol volume fractions up to 80%. This property of silicone surfactants reflects that polydimethylsiloxane is not only hydrophobic, but that silicone surfactants are insoluble in organic solvents as the polydimethylsiloxane content increases.
4、Organosilicone surfactant and CO2 action
Rocha et al. concluded that poloxamer trisiloxane surfactant can make CO2 and water form emulsion, and by adjusting the EO number and changing the ‘hydrophilic-CO2 balance (HCB) value’ of surfactant, the emulsion can be transformed from CO2-in-water (W/C) to C/W. Sarbu et al. considered the phase behaviour of organosilicon surfactants in supercritical CO2, and found that the phase behaviour was sensitive to CO2-phobic groups and less sensitive to the size of the. produced a series of cationic organosilicon surfactants, and studied their behaviour in behaviour in high density CO2. Changing both the chain length and the counter ion of polydimethylsiloxanes strongly altered their surface activity in critical CO2.