Silicon-Containing Fluorinated Surfactants

The surface tensions of silicone surfactants [126-131] are, in general, below those of hydrocarbon surfactants. For example, the surface tension of dimethicones (polydimethylsiloxanes) in water is about 20-21 niN/m. Because perfluoroalkyl substituents usually decrease the surface tension of the parent compound, fluori-nation of silicone surfactants [131-158] can lower the surface tension of silicone surfactants as well. The reduction of this concept to practice is not straightforward, however. 

Fluorinated substituents in the a and p position to silicon weaken the Si—C bond and make it vulnerable especially to a nucleophilic attack [126]. Fluorination of a silane such as CH SiCla or (CH3)2SiCl2 yields compounds of considerable thermal stability, but the exceptionally high electronegativity of fluorine and the consequential strong inductive effect make the Si—C bond hydrolytically unstable [132,133]. Hydrolytic cleavage of the Si— F bond in 

To reduce the inductive effect on the Si—C bond, the perfluorinated group has to be isolated from the silicon atom. Fluorination of an alkylsilane even in the P position yields hydrolytically unstable compounds which undergo thermal rearrangements and liberate Cp2=CR2- Fluorination of an alkyl group in the 7 position, CF3CH2CH2—, does not affect the Si— F bond significantly and the hydrolytic stability is adequate for practical use [132]. The dichlorosilane CF3CH2CH2Si(CH3)Cl2 hydrolyzes to form a polysiloxane. However, the ethylene link increases the bulkiness of the side chains, increases the hydrocarbon/flu-orocarbon ratio, and causes fluorocarbon-hydrocarbon and fluorine-silicone interactions and orientation effects which may reduce the surface activity of the fluorinated compound. 

The surface tension of fluorosiloxanes in the liquid state is measured directly and usually the values obtained are not affected by the measurement technique. The equilibrium surface tension in water is related to the liquid surface tension value. Siloxanes with trifluoropropyl groups are less effective in lowering the surface tension of water than the nonfluorinated poly(dimethyl) siloxanes. The surface tension of poly(3,3,3, trifluoropropylmethylsiloxane) is higher than that of the lower-cost dimethicones [poly(dimethylsiloxane)] [134,135]. Because of the high affinity of fluorine to silicon [136], fluorine atoms may be inclined to coordinate with silicone atoms. The distorted orientation may partially expose the hydrocarbon link of the pendant side chain. 

To increase the surface activity, Owen and Groh [135] increased the fluorocarbon content of the side chains while maintaining the ethylene link between the fluorocarbon group and the silicon atom. Nonafluorohexyl (3,3,4,4,5,5,6,6,6-nonafluorohexyl) disiloxane and trisiloxane surfactants have equilibrium surface tensions in water at the cmc of 20 niN/m [142]. However, the response of their surface tension to dynamic changes is impeded by bulkiness of the flouroalkyl group [142]. The surface tension of the fluorosilicones is shown in Table 1.8. 

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Silicone-containing polymeric surfactants have been described in Section 1.7 and Chapter 12. Polymeric fluorinated surfactants with a carbon containing backbone are discussed below. 

Effective surfactants in CO2 are amphipatic macromolecules containing an anchoring segment (the so-called lipophilic anchor) that became adsorbed on the surface of the polymer phase by grafting and/or physical adsorption, and a stabilizing portion that is soluble in the continuous medium. The stabilizing moiety is usually constituted by a fluorinated or silicone chain. 

Chem. Descrip. Fluorine containing silicone wax Ionic Nature Nonionic Uses Surfactant lubricant Features Patented... 

Foamable hot-melt adhesives containing polyamides and fluorinated or silicone surfactants are useful on coarse surfaces [10]. 

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