Solutions of fluorinated surfactants

Kunieda, H., and Sbinoda, K. (1976) Krafit points, critical micelle concentrations, surfece tension, and solubilizing power of aqueous solutions of fluorinated surfactant. J. Phys. Chem.,... 

Kausch CM et al (2002) Synthesis, characterization, and unusual surface activity of a series of novel architecture, water-dispersible poly(fluorooxetane)s. Langmuir 18(15) 5933-5938 Dams R (1993) Fluorochemical surfactants for hostile environments. Spec Chem 13(l) 4—6 Shinoda K, Hato M, Hayashi T (1972) The physicochemical properties of aqueous solutions of fluorinated surfactants. J Phys Chem 76(6) 909-914... 

Stability of Droplets Formed by Various Nonpolar Liquids in Aqueous Solutions of Fluorinated Surfactant Perfluorodiisononylene-Polyethylene Glycol, (< )-PEG) (5 x 10- wt %)... 

Fig. 2.3 Surface activity of aqueous solutions of fluorinated surfactants derived from perfluorocarbonyl fluoride. (From Ref. 50.)...

The effect of the hydrophilic group on surface tension depends on the structure of the hydrophile and, for ionic surfactants, also on the counterion. For a constant chain length of C7F15, the surface tension of 0.1% solutions of fluorinated surfactants varies between 17 and 47 mN/m, depending on the nature of the hydrophile [56] (Table 4.5). Nonionic fluorinated surfactants usually have lower surface tensions than their ionic counterparts. Nonionic surfactants derived from... 

Table 4.7 Surface Tension of Aqueous Solution of Fluorinated Surfactants C7Fi5CON(R)CH2CH2CH2 CH2S03Na at 30°C...

The unequal adsorption to the liquid-vapor and solid-liquid interfaces has been the subject of several studies [34,36,38]. In contrast to Bemett and Zisman [33], Pyter et al. [34] explained the different wetting characteristics of hydrocarbon surfactants and fluorinated surfactants by low adsorption of fluorinated surfactants on nonpolar solids. The higher contact angles exhibited by solutions of fluorinated surfactants on polyethylene were explained by poor adsorption of fluorinated surfactants instead of the strong adsorption proposed by Bemett and Zisman [33]. 

Solutions of fluorinated surfactants have been investigated and their micellar nature has been confirmed [58,59]. The substitution of the larger and highly electronegative fluorine atom for the smaller hydrogen increases the amphiphilic nature of the surfactant and lowers the surface tension and cmc. The alkali and ammonium salts of perfluoroalkanoic acids exhibit surfactant properties and form micelles for a chain length of four carbon atoms, whereas eight carbon atoms are needed for the nonfluorinated alkanoates. 

Chapter 1 presents an overview of fluorinated surfactants. The synthesis of fluorinated surfactants is discussed in Chapter 2. Since the space limitations precluded a detailed description of processes, patent citations are augmented by references to Chemical Abstracts. Physical and chemical properties are reviewed in Chapter 3. Chapters 4-7 are devoted to the theory of fluorinated surfactants liquid-vapor and liquid-liquid interface (Chapter 4), solid-liquid interface (Chapter 5), solutions of fluorinated surfactants (Chapter 6), and the structure of micelles and mesophases, including mixed surfactant systems, in Chapter 7. The practical application of fluorinated surfactants is the subject of Chapter 8. Various applications are listed in alphabetical order for easy access to information. Chapter 9 reviews the analytical and physical methods for the investigation of fluorinated surfactants. Chapter 10 examines the environmental and toxicological aspects, including the use of fluorinated surfactants in biological systems. 

Chabaud E, Barthelemy P, Mora N, Popot JL, Pucci B. Stabilization of integral membrane proteins in aqueous solution using fluorinated surfactants. Biochemie 1998 80 515-530. 

Microemulsions. Systems comprising microwater droplets suspended in an scCO T oil phase can be achieved with the use of appropriate surfactants, of which the best appear to be fluorinated. Microemulsions in supercritical hydrofluoro carbons are also possible. Potential may also exist for speciality coatings via low concentration solutions of fluorinated products in supercritical fluid for, e.g., thin-fitm deposition, conformal coatings, and release coatings. Supercritical CO2 will dissolve in formulated systems to improve flow and plasticize melt-processable materials to improve melt-flow characteristics and lower the glass transition temperature. 

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). 

Fluorinated surfactants are both hydrophobic and lipophobic. For example, potassium per-fluorooctanesulfonate—an industrially important surfactant —forms a third phase with octanol and water, and it is impossible to determine its octanol-water partition coefficient. " Similar to fluorocarbon-hydrocarbon bulk solvent mixtures, mixed binary systems containing a perfluorocarbon surfactant and a structurally related hydrocarbon surfactant are known to behave nonideally, that is, exhibit phase separation in insoluble monolayers at the air-water interface or form two types of micelles simnltaneously in solution—one type is fluorocarbon-rich and the other is hydrocarbon-rich. This nonideal behavior of fluorocarbon-hydrocarbon surfactant mixtures is used in firefighting foams and powders—an important technical application of fluorinated surfactants. " ... 

The opposite ( mirrorlike ) behavior can be expected in the case of fluorinated surfactants. Indeed, the results obtained for (j)-PEG solutions indicate that the resistance to coalescence was higher for heptane (fluorocarbon surfactant/hydrocarbon liquid, FS/HL system) than for PFD (fluorocarbon surfactant/fluorocarbon liquid, FS/FL) / is 1.2 dyn for heptane and only 0.1 dyn for PFD (Table 4.5). 

The minimum surface tension achievable is also much lower for fluorinated surfactants than for nonfluorinated surfactants. The minima of the surface tensions of surfactants with a hydrocarbon hydrophobe are in the range 25-35 mN/m [51-53], whereas those of fluorinated surfactants are as low as 15-20 nM/m or even lower [54]. The surface tension of aqueous solutions above cmc varies only slightly with surfactant concentration. Surface tension above cmc decreases with increasing fluorocarbon chain length and depends on the counterion (Table 4.1). 

In aqueous solutions of nonfluorinated surfactants, the lowest surface tensions are attained by covering the surface with a close-packed monolayer of vertically oriented hydrocarbon chains forming a continuous layer of —CH3 groups exposed to air [89]. By analogy, the surface tension of a solution of a fluorinated... 

Clayfield et al. [48] attempted to prepare monolayers of fluorinated surfactants FC134 and FX161 on glass by adsorption from acetone solutions. Stable and reproducible contact angles were obtained with n-alkanes, but polar liquids such as water caused desorption of the surfactant from the solid-liquid interface. 

The adsorption of fluorinated surfactants at the electrode-solution boundary is of considerable practical interest for the application in electrochemical systems [60-64) (see Chapter 8). The electrochemical behavior of Zonyl FSN (nonionic), Zonyl FSD (cationic), Zonyl FSA (anionic), Fluorad FC-99 (anionic), and Fluorad FC-135 (cationic) at Hg and Pt electrodes has been investigated by using cyclic voltammetry and interfacial differential capacitance measurements. When the electrode is relatively hydrophobic, such as Hg, and the surface charge density is relatively low, the fluorinated surfactants, as well as hydrocarbon surfactants, are adsorbed with their hydrophobic segments oriented toward the electrode. The interaction of fluorinated surfactants with the Hg electrode is weaker and the adsorbed layer is less compact than those of hydrocarbon surfactants. When the electrode is more hydrophilic, such as Pt, or the surface charge density is high, the surfactants adsorb with their hydrophilic end group toward the electrode surface. 

A micellar solution containing a fluorinated surfactant and a selected hydrocarbon surfactant can exceed the surface activity predicted for an ideal solution in which the components do not interact. This synergism increases the effectiveness of the surfactant and, because of the lower cost of hydrocarbon-type surfactants, results in considerable savings. Because perfluorinated surfactants are not biodegradable (Chapter 10), the use of fluorinated surfactants at lower concentrations is also advantageous from the environmental point of view. 

Analytical techniques are employed to determine the purity or the concentration of a fluorinated surfactant and to characterize a fluorinated surfactant and its solutions. Because most fluorinated surfactants are mixtures of homologs, the term purity has to be redefined for each particular case. In most cases, the determination of purity begins with the analysis of intermediates used to synthesize the surfactant. Usually, the intermediates can be readily analyzed by chromatography and the homolog distribution determined. Gas chromatography has only a limited value for the analysis of fluorinated surfactants proper because most fluorinated surfactants are not sufficiently volatile for gas chromatography. 

In general, the concentration of a fluorinated surfactant in solution can be determined by conventional volumetric or spectroscopic methods used for hydrocarbon-type surfactants [1-5], In addition to the functional groups utilized for the analysis of hydrocarbon-type surfactants, the fluorine content is a unique feature useful for the determination of fluorinated surfactants. If the fluorinated surfactant is the only fluorine-containing species in a solution or a substrate, then the fluorine content indicates the concentration of the fluorinated surfactant. 

For NMR studies of fluorinated surfactants, the most useful nucleus is F, in addition to and H nuclei. Changes ip the F chemical shift at cmc are larger than changes in the proton chemical shifts and, therefore, provide more information on fluorinated surfactants and their micellar structures. F-NMR spectra have been recorded for structural characterization of perfluorononanoic acid [125] and perfluoropolyether surfactants [126]. Micelle formation in solutions of... 

Muto et al. [252] measured pyrene fluorescence lifetime Tq and the ratio IiHt, of the intensities of the first vibronic and the third vibronic band of the monomeric pyrene. The pyrene fluorescence data revealed the existence of a single type of mixed micelle in solutions of LiDS-LiFOS, LiFOS-hexaoxyethylene glycol do-decyl ether, or LiFOS-octaoxyethylene glycol dodecyl ether mixtures. The lifetime and the intensity ratio of vibronic peaks have been used to determine the cmc of fluorinated surfactant micelles [253]. However, the solubility of pyrene in micelles of fluorinated surfactants is not adequate for determining the micelle aggregation number [253,254]. 

Finally a similar study was performed with droplets of fluorinated surfactant. We found that contrary to the SDS case, their permeability is independent of the surfactant concentration and always remains equal to that of pure water. In this case, micelles cannot solubilize heptane (probably because heptane does not wet the fluorinated chains) and thus do not take part in the permeability process. Hence the leak of the fluorinated bubbles, described in the first paragraph, is simply understood in view of the (low) solubility of heptane in these solutions. 

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