Fluorinated hydrocarbons, hydrophobic nature

Highly fluorinated amino acids, such as hexafluoroleucine and hexafluorovaline, enhance the stability of several helical proteins. This enhancement has been attributed to the higher hydrophobicity of the fluorocarbon side chain compared to that of the natural hydrocarbon side chain. 

Two classes of materials that cannot easily be fitted into the known schemes for conventional hydrocarbons are the silicone-based surfactants and those in which hydrogen has been replaced by fluorine atoms. The hydrophobic unit of the silicone-based surfactants consists of low-molecular-weight polyal-kylsiloxane derivatives, usually polydimethylsiloxane. Possibly because of their nonclassic nature they have received little attention in the general scientific literature, although their unique surface characteristics have proved useful in many technological apphcations, especially in nonaqueous solvent systems. 

Whatever the specific type, a valid question for all ordinary emulsions with or without surfactants is what is the maximum amount of the dispersed phase in the continuous phase when the former will still remain dispersed In other words, at what volume ratio does an inversion (i.e. OAV to W/O and the reverse) take place Emulsions for particle preparation are known to have been prepared where the volume ratio of the two phases can go up to near 1 1 [18]. In addition and contrast to this general idea about the relative contents of the two phases, one must also refer to the highly concentrated water-in-oil emulsions which can be prepared with a fluorinated surfactant and a fluorocarbon/hydrogenated surfactant (pronouncedly hydrophobic) and a hydrocarbon [19]. In these W/O emulsions, up to 98% w/w water is added, but inversion is never achieved. Highly concentrated W/O emulsions have also been described recently by Hakansson etal. [20] where the surfactant is of the alcohol ethoxylated type, the dispersed phase is aqueous in nature and the continuous phase, an aliphatic hydrocarbon. It has been indicated that such emulsions may contain more than 99% of the dispersed phase. These are, however, very special cases and do not demand further discussion here. Without going into specificities, let us look at the general factors that may influence inversion [3, 21, 22] ... 

The cmc of a fluorinated surfactant also depends on the nature of the hydrophile but to a lesser effect than on the hydrophobe structure (Table 6.9). The carboxy-lates have higher cmc values than sulfonates (Table 6.11). This is in accord with the order of decreasing cmc values—carboxylates > sulfonates > sulfates—observed by Klevens [145] for hydrocarbon-type surfactants. 

The amount of a solubilizate which can be solubilized depends on several factors. The dominant variables are the structures of the surfactant and the solubilizate. Both the structure of the hydrophobic chain and the nature of the counterion can affect solubilization. Although the relation between solubilization and surfactant structure is complex, it is clear that the interactions between a solubilizate molecule and the lipophobic hydrophobe of a fluorinated surfactant must be different from interactions between the solubilizate and the lipophilic hydrophobe of a hydrocarbon surfactant. Solubilization by fluorinated surfactants is therefore of great theoretical as well as practical interest. The published information on solubilization by fluorinated surfactants is, however, sparse. 

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