Preparation with fluorinated surfactants

The reaction of potassium fluoride with hexafluoroacetone has been employed to prepare nonionic fluorinated surfactants (see Section 2.9). Because hexafluoroacetone has been found to be highly toxic and a teratogen, it is no longer used for industrial preparation of fluorinated surfactants. 

Riess and co-workers [252-263] have prepared nonionic fluorinated surfactants with a polyhydroxy hydrophile by perfluoroalkylating natural products such as monosaccharides and disaccharides, pentitols, hexitols, and so forth. Per-6>-acetylglycopyranosyl bromides were reacted with 1 l-(F-alkyl)-10-undecenols,... 

De-icing compositions for the removal of ice or for prevention of its reformation on automobile windshields are prepared from 1 part polyol, 1 part alcohol, and 0.05 part fluorinated surfactant, e.g., ammonium salts of mono- and bisfluoroalkyl phosphates and their complexes with aliphatic quaternary methyl sulfates [287]. 

Micro emulsions can be formulated with carbon dioxide in supercritical state instead of a hydrocarbon as nonaqueous solvent. Fluorinated surfactants are commonly used to prepare such microemulsions. Water-in-carbon dioxide microemulsions can be made and the droplet size has been found to be similar to the size of the droplets of water-in-hydrocarbon micro emulsions with similar composition [21]. Such a microemulsion was used for conversion of benzyl chloride to benzyl bromide using KBr as hydrophilic nucleophile. The yield was an order of magnitude higher in the carbon dioxide microemulsion than in a conventional microemulsion at similar conditions, a fact that has been ascribed to low interfacial viscosity [22]. The big advantage with these micro emulsions is the environmental friendliness and the ease of work-up associated with carbon dioxide as solvent. 

Fluorinated surfactants can be very intereshng for the preparation of mesoporous materials, due to their good self-aggregation properties, allowing a more precise control of the mesostructure and properties of materials. In this context, the use of fluorocarbon surfactants allows the formahon of ordered mesoporous materials with a long-range organizahon [52, 53],... 

The use of fluorinated cationic surfactants has been studied by Rankin and coworkers [62, 63]. They described the preparation of hexagonally ordered mesoporous silica with rather small pore sizes (pore diameters approximately from 2.0nm to 2.6nm), and therefore they have similar properties to silica prepared with conventional hydrocarbon cationic surfactants (such as MCM-41). Thin pore walls could be a disadvantage, since it may lead to low mechanical and hydrothermal stability. 

Figure 11.15 Electron microscopy images, (a) SEM and (b) TEM, of hollow particles with mesopores in the shell. The particles were prepared using a cationic fluorinated surfactant (Reproduced from Ref [69], with permission).

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] ... 

Microemulsions of surfactants in SCFs have emerged as effective supercritical solvent systems for the preparation and processing of materials. Microemulsions are particularly useful in the case of SCFs such as CO2 that are environmentally benign but of relatively low solvent strength. However, because most conventional surfactants contain no C02-philic moieties, microemulsions in CO2 require special surfactants such as fluorinated amphiphilic molecules (229-235). Other commonly used SCFs are simple hydrocarbons, which are compatible with conventional surfactants such as AOT. A unique advantage of SCF-based microemulsions is that properties of the microemulsions may be varied via changes in the pressure and temperature of the SCF (Figure 24) (236-244). 

Johnston and coworkers reported the preparation of nanoscale CdS particles in a PFPE-based microemulsion in supercritical CO2 for evaluating the effects of the water/surfactant ratio (ITo) of the microemulsion on the nanoparticle properties (249). They found that the particle size increased significantly with an increase in the Wo value, from which a correlation between average nanocrystal radius and water-core radius was established (Figure 26). Recently, Johnston and coworkers also prepared silver nanocrystals coated with fluorinated ligands (240). These coated nanocrystals could be dispersed in CO2 at moderate pressure and temperature. 

Surfactant assemblies are generally utilized as a template to form mesoporous shell. Vesicle-like hollow sihca particles can be prepared by sol-gel reactions in the presence of surfactants. Although most of the materials reported so far have multilamellar structures, hollow mesoporous silica particles with a single-walled vesicle structure have been prepared by the reaction of TEOS in the presence of a fluorinated surfactant (lH,lH,2//,2//-perfluorodecylpyridinium chloride). With... 

Cationic fluorinated surfactants can be prepared by reacting the pentamer with phenol and, subsequently, with chlorosulfonic acid. The sulfonyl chloride reacts with A,A-dimethylpropanediamine to form a tertiary amine sulfonamide, which can be quaternized to form a cationic surfactant ... 

Fluorinated surfactants with an oligo(hexafluoropropene oxide) hydrophobe have been prepared by sulfonating (HFPO) Ar where (HFPO) is an oligo(hexafluoropropene oxide) group and Ar is an aryl group [102]. 

The fluorinated surfactants (CF3)2CF0(CH2)60S03Na (Na" or NH4) have been prepared by sulfation of (CF3)2CF0(CH2)60H, which is obtained by a reaction of hexafluoroacetone, CF3COCF3, with KF and C1(CH2)60H [153]. The toxicity of hexafluoroacetone limits the usefulness of this process. 

In a similar fashion, cationic fluorinated surfactants are prepared from telomer iodides by converting perfluoroalkylethyl iodides to sulfonyl chlorides and reacting these with A, A -dimethyl-l,3-diaminopropane [161]. 

Cationic fluorinated surfactants have been prepared from perfluoroalkyl esters, obtained by converting an acid fluoride into an ester. Reaction of the ester with a diamine and alkylation with a halide or sulfonate gives a cationic surfactant, for example [166] ... 

Using 3-(perfluoro-1,1-dimethyl-butyl)-1-propene as the starting material, cationic fluorinated surfactants have been prepared via the corresponding epoxypropane reacted with a secondary amine [168-170] ... 

The synthetic methods utilized for the preparation of amphoteric fluorinated surfactants are similar to those used for cationic surfactants, except for the alkylation step. A tertiary amine can be prepared by reaction of a fluorinated ester or acid halide with a diamine which contains both a primary or a secondary amine and a tertiary amine [195]. The resulting amine is then treated with chloroacetic acid ... 

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