Fluorinated surfactants Surface tension

Fluorination of the hydrophobe decreases markedly the surfactant concentration needed for a substantial surface tension reduction. At 1 g/L concentration, sodium octanesulfonate, C8H 7S03Na, lowers the surface tension of water at 20°C to 65 nM/m, but sodium perfluorooctanoate, CgHivSOsNa, at the same concentration lowers surface tension to 32 mN/m [49]. Conventional surfactants with a hydrocarbon-type hydrophobe can lower surface tension to 30-35 mN/m at 0.1% concentration. With fluorinated surfactants, surface tensions below 30 mN/m have been achieved at concentrations as low as 10-100 ppm. With some fluorinated surfactants, only 100-200 ppm of the surfactant is needed to lower surface tension below 20 mN/m [48,50]. 

Rosen [11 ] has suggested that fluorination of the hydrophobe has apparently only a small effect on r in contrast to the large effeet fluorination has on surface tension. If fluorination does not affect the surface excess concentration F, significantly the large effeet of fluorination on surface tension must be related to the chemical potential of the fluorinated surfactant [Eq. (3)]. The chemical potential depends on the nature of the hydrophobe, as well as the hydrophile. The main factors in reducing surface tension are the decrease in the work needed to transport a surfactant molecule to the surface and the low surface energy of the monomolec-ular film formed by adsorbed surfactant molecules. 

The low surface tension of highly fluorinated organic compounds is commercially important for their appHcation in surfactants, antisoiling textile treatments, lubricants, and specialty wetting agents. 

Carboxjiates with a fiuorinated alkyl chain ate marketed by the 3M/Industrial Chemical Products Company under the trade name Fluotad surfactants. They also include other functional derivatives of fiuorinated and perfluorinated alkyl chains. Replacement of hydrogens on the hydrophobe by fluorine atoms leads to surfactant molecules of unusually low surface tension. This property imparts excellent leveling effectiveness. 

Fluorinated surfactants lower the surface tension of water more strongly than their nonfluorinated analogues. Fluorinated surfactants reduce the superficial pressure of water from 72 to 15-20 dyn/cm while a nonfluorinated agent only decreases the value to 25-35 dyn/cm (Table 1.6). 

Commercially, the production and use of surfactants is dominated by modified hydrocarbon-based chemicals. In a number of instances, however, a hydrocarbon-type surfactant will not provide the desired product attributes or performance and, in such cases, two options are presented. One involves reformulation of the product to accommodate a hydrocarbon-type surfactant and the other is the use of a fluorosurfactant. Fluorosurfactants behave typically as would a hydrocarbon type except that properties such as surface tension reduction are larger in magnitude. Furthermore, the presence of fluorine in the hydrophobic portion of the molecule causes them to differ from their hydrocarbon counterparts in more subtle ways that have commercial importance. An example of a difference would be the reduced dielectric constant or index of refraction of a fluorosurfactant compared to its hydrocarbon analog. While this maybe of no consequence when formulating cleaners, it most certainly exists in a number of electronics applications. 

The aqueous films stabilized with fluorine containing surfactants on organic substrates are an exception since they have very low surface tension (- 10 mM m 1). However, there are no literature data about them. 

Surfactants with hydrocarbon-chain hydrophobic groups generally do not lower the surface tension of alkanes, since any orientation of adsorbed surfactant of this type at the air-alkane surface would not reduce the surface free energy. However, fluorinated surfactants can adsorb and orient at the hydrocarbon-air surface to reduce the free energy there. Fluorinated surfactants of the type C6H5CF(CF3) 0[CF2CF(CF3)0]mC3F7 have been observed to reduce the surface tension of m-xylene (28 mN/in) to 10 mN/in (Abe, 1992). 

Thus, for example, in the presence of some highly fluorinated carboxylic acids and their salts, the value yc for polyethylene is decreased from its usual value of almost 31 mN/m to about 20 mN/m (Bernett, 1959) by adsorption of the fluorinated surfactants onto the polyethylene surface, with the result that solutions of these surfactants having surface tensions less than the normal yc for polyethylene do not spread on it. The requirement that the surface tension of the wetting liquid be reduced by the surfactant to some critical value characteristic of the substrate is thus a necessary, but not sufficient, condition for complete spreading wetting. A surfactant solution whose surface tension is above the critical tension for the substrate does not produce complete wetting, but a solution whose surface tension is below the critical tension for the substrate may or may not produce complete wetting (Schwarz, 1964). 

Table 1. Aqueous Phase Critical Micelle Concentrations (erne s), Limiting Surface Tensions yeme s and Microemulsion Stability Pressures for Fluorinated Surfactants.

At low concentrations fluorinated surfactants are capable of lowering effectively the surface tension of aqueous solutions and non-aqueous liquids and work well in acidic, alkaline and electrochemical media as well as at elevated temperatures. Their destruction may be bound up just with the decomposition of polar groups, e.g. polyoxyethylene chain. Adsorbing with the... 

There is large body of data on the surface and interfacial tensions of aqueous surfactant solutions. This data show that the structure of the surfactant molecule has a pronounced effect on its ability to reduce these tensions. As the length of the alkyl or fluorinated alkyl chain increases, the CMC decreases and the surface excess concentration increases, causing a drop in the interfacial tension at a fixed surfactant concentration. At low surfactant concentrations the reduction in surface tension (or increase in surface pressure O = yo - y) is linear with the molar bulk solute concentration c (in the case of the dilute solution)... 

Another interesting feature of fluorinated surfactants is their ability to greatly reduce the surface tension of water, even at very small concentrations. Some surfactants, such as sodium perfluorooctanoate, can lower the surface tension at 25 °C down to 15 mN m" . It should be pointed out that such low tensions can be achieved with a rather short pefluoalkyl chain, because the perfluoalkyl chains are significantly more hydrophobic than the hydrocarbon alkyl chain with the same number of carbon atoms [46]. A remarkable example is sodium fluorobu-tanoate, with a very short chain length, but still able to form self-aggregates in water [47]. 

Surfactants with hydrophobic chains different from conventional hydrocarbon alkyl chains possess interesting properties as structure-directing agents in self-assembly processes. Both silicone and fluorinated chains are less soluble in many organic solvents than their hydrocarbon counterparts. Moreover, sihcone and fluorinated surfactants reduce the surface tension of water more effectively than... 

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