Cationic microemulsion formation

A great variety of surfactants have been used in microemulsion formation. These include common soaps, other anionic and cationic surfactants, nonionic surfactants of the polyethylene oxide type, and other structures. The hydrophobic part contains one or two linear or branched hydrocarbon chains containing about 8-18 carbon atoms. Quite often microemulsions require, in addition to oil, water, and surfactant, the presence of simple electrolytes, alcohols, and/or other weakly surface-active substances. 

It should be pointed out that although the preceding discussion was concerned with the use of alcohols as cosurfactants in microemulsion formation, many other types of material can also be used to the same end. Especially important are primary amines (commonly used with cationic surfactants) and thiols. 

Cationic surfactants may be used [94] and the effect of salinity and valence of electrolyte on charged systems has been investigated [95-98]. The phospholipid lecithin can also produce microemulsions when combined with an alcohol cosolvent [99]. Microemulsions formed with a double-tailed surfactant such as Aerosol OT (AOT) do not require a cosurfactant for stability (see, for instance. Refs. 100, 101). Morphological hysteresis has been observed in the inversion process and the formation of stable mixtures of microemulsion indicated [102]. 

In abroad sense, the model developed for the cobaloxime(II)-catalyzed reactions seems to be valid also for the autoxidation of the alkyl mercaptan to disulfides in the presence of cobalt(II) phthalocyanine tetra-sodium sulfonate in reverse micelles (142). It was assumed that the rate-determining electron transfer within the catalyst-substrate-dioxygen complex leads to the formation of the final products via the RS and O - radicals. The yield of the disulfide product was higher in water-oil microemulsions prepared from a cationic surfactant than in the presence of an anionic surfactant. This difference is probably due to the stabilization of the monomeric form of the catalyst in the former environment. 

For Cu(ll) in water adsorbed on silica gels it was reported 5 that in gels with small pores isolated hydrated ions are detected at 77 K5 in pores larger than -4 nm a broad signal is superimposed on the spectrum of isolated ions. The appearance of the broad signal indicates aggregation of cations and the presence of bulk or freezable water. In a recent publication the Cu(ll) probe was used to test the possibility of ice formation in microemulsions, jce formation was detected in one of the microemulsions studied for very slow cooling rates from -300 K to 77 K. 

This method involves formation of reverse micelles in the presence of surfactants at a water-oil interface. A clear homogeneous solution obtained by the addition of another amine or alcohol-based cosurfactant is termed a Microemulsion. To a reverse micelle solution containing a dissolved metal salt, a second reverse micelle solution containing a suitable reducing agent is added reducing the metal cations to metals. The synthesis of oxides from reverse micelles depends on the coprecipitation of one or more metal ions from... 

One advantage of using a cleavable acetal surfactant instead of a conventional amphiphile has been elegantly demonstrated in a work by Bieniecki and WUk [51]. A cationic 1,3-dioxolane derivative was used as surfactant in a microemulsion formulation that was employed as a reaction medium for an organic synthesis. When the reaction was complete, the surfactant was decomposed by addition of acid and the reaction product easily recovered from the resulting two phase system. Through this procedure the problems of foaming and emulsion formation, frequently encountered with conventional surfactants, could be avoided. 

Many reports are available where the cationic surfactant CTAB has been used to prepare gold nanoparticles [127-129]. Giustini et al. [130] have characterized the quaternary w/o micro emulsion of CTAB/n-pentanol/ n-hexane/water. Some salient features of CTAB/co-surfactant/alkane/water system are (1) formation of nearly spherical droplets in the L2 region (a liquid isotropic phase formed by disconnected aqueous domains dispersed in a continuous organic bulk) stabilized by a surfactant/co-surfactant interfacial film. (2) With an increase in water content, L2 is followed up to the water solubilization failure, without any transition to bicontinuous structure, and (3) at low Wo, the droplet radius is smaller than R° (spontaneous radius of curvature of the interfacial film) but when the droplet radius tends to become larger than R° (i.e., increasing Wo), the microemulsion phase separates into a Winsor II system. 

It has been shown that the addition of a small amount of the anionic surfactant sodium dodecyl sulfate (SDS) to a microemulsion based on nonionic surfactant increased the rate of decyl sulfonate formation from decyl bromide and sodium sulfite (Scheme 1 of Fig. 2) [59,60]. Addition of minor amounts of the cationic surfactant tetradecyltrimethylammonium gave either a rate increase or a rate decrease depending on the surfactant counterion. A poorly polarizable counterion, such as acetate, accelerated the reaction. A large, polarizable counterion, such as bromide, on the other hand, gave a slight decrease in reaction rate. The reaction profiles for the different systems are shown in Fig. 12. More recent studies indicate that when chloride is used as surfactant counterion the reaction may at least partly proceed in two steps, first chloride substitutes bromide to give decyl chloride, which reacts with the sulfite ion to give the final product [61]. 

When a single chain anionic surfactant (such as sodium dodecyl sulfate, SDS) is used, it generally requires a cosurfactant for the formation of a microemulsion. A cosurfactant may not be needed to form a microemulsion if nonionic surfactant(s), certain types of cationic surfactants, or double-chain surfactants such as sodium l,4-bis(2-ethylhexyl)sulfosuccinate (Aerosol OT or simply AOT) are used. 

Lee CT, Psathas PA, Zielger KJ, Johnston KP, Daib HJ, Cochran HD, Melnichenko YB, Wignall GD. Formation of water-in-carbon dioxide microemulsions with a cationic surfactant a small-angle neutron scattering study. J Phys Chem B 2000 104 11094-11102. 

Mixing anionic and cationic surfactants results in the formation of an equimolar catanionic species, which is likely to precipitate even at very low concentration, because it is more hydrophobic (two tails) and less ionic (the charges cancel out at least partially). It was shown, however, that this equimolar catanionic surfactant tends to behave as a hydrophobic amphoteric, i.e. ionic surfactant, which is able to exhibit a linear mixing rule with either of the ionic species provided its proportion remains small, say, less than 20% [57]. For instance, if 5 wt.% of a cationic surfactant is added to 95 wt.% of anionic surfactant, the actual mixture behaves as if it were a mixture of 90 wt.% anionic and 10 wt.% catanionic surfactant. In practice, the pure catanionic species precipitates and hence does not exist as a soluble substance in the microemulsion. Hence, its characteristic parameter has to be estimated by extrapolating the linear trends of the 1 1 mixture, as seen in Fig. 3.10(c). 

Figure 18 [99] shows the optimum formulation for three-phase behavior (as the optimum salinity) as a function of the composition of a mixture of anionic (sodium dodec)4 sulfate) and cationic (tetradecyltrimethylammonium bromide) species loaded with a considerable amount of alcohol to avoid the formation of liquid crystals. The surfactant pair was selected so that both individual surfactants produced a three-phase microemulsion-oil-water behavior at about the same salinity, i.e., 5-10% NaCl. As some cationic surfactant is added to the anionic one, the shaded region that indicates the three-phase behavior goes down (from left to right). This downward displacement and the fact that three-phase behavior is still exhibited means that the addition of a small amount of the cationic surfactant to the anionic one results in a less hydrophilic surfactant mixture. 

Specific roles of the so-called co-surfactants (commonly, but not necessarily alcohols) have been examined by various workers [122, 126, 136] some points are discussed here. For example, a critical thermodynamic analysis in conjunction with experimentations led Eicke [ 136] to the conclusion that a co-surfactant should decrease the interfacial free energy under isothermal conditions, while causing an uptake of water into the microemulsion and extension of its domain. The anionic surfactant AOT assists the formation of large reverse microemulsion domains (high water uptake) in different ternary systems without help from a co-surfactant (Section 2.2), but cationic surfactants do generally need this fourth component. In spite of this, enhanced solubilization by the addition of (small quantities of) a co-surfactant has been observed by various workers in AOT systems. Eicke [136] used cyclohexane, benzene, carbon tetrachloride and nitrobenzene in the system AOT/ isooctane/water and found considerable water uptake (the fraction of the oil phase, i.e. isooctane was 0.8 or more). With increasing polarizability or polarity of the CO-surfactant, the water uptake decreased. 

Fig. 19 (a) Formation of a complex coacervate core microemulsion (C3 — pE) droplet. The core comprises anionic homopolymer green, hp ), cationic homopolymer purple, hp ), and cationic block red, dp ) of diblock copolymer. The corona consists of the electroneutial hydrophilic blocks blue) of the diblock copolymer, (b) Formation of C3-(iE containing coordination polymers. Adapted from [81]. Reprinted with permission from the Royal Chemical Society... 

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