Dry reversed micelles

Calandra, P., Giordano, C., Longo, A. and Turco Liveri, V. (2006) Physicochemical investigation of surfactant-coated gold nanoparticles synthesized in the confined space of dry reversed micelles. Mater. Chem. Phys., 98, 494-499. 

SANS curves obtained after subtraction of the cell+C02 background are shown in Figure 4b. Data fitting with the model in ref. 37 was consistent with "dry" reversed micelles, as observed with these surfactants in n-heptane solvent (86). UV-vis dye solubilisation measurements shown in Figure 4c also provide evidence for aggregation. If reversed micelles were present the positively charged chromophore, dimidium bromide (A 550 nm), should be... 

Kotlarchyk et al. [67] clearly showed that purified AOT contained some water so that the value of the molar ratio ([water]/[surfactant]) became -0.7 0.2 even when no water was added. The importance of this parameter has been discussed from different angles in Chapters 3 and 5. Beeby et al. [77] concluded that the so-called dry micelles of AOT in hydrocarbon solvents, which are spherical aggregates composed of about 15 molecules, do indeed contain molecules of water of crystallization . In case of NaAOT this is nearly 1 per molecule. For NaAOT and substantially ion-exchanged AOT s in cyclohexane, the water contents were measured by Eastoe et al. [78], as presented in Table 2.4. Table 2.4 indicates that a dry reverse micelle, at least in case of AOT, might be a very rare situation. Further, this may also be true for various other surfactants one example is provided below. 

The critical role of H2O in the formation of dry reverse micelles, as advocated by Eicke and others, did not however match the findings of Yu et al [79] who reported the formation of large reverse micelles when the system was dry and dissolution of the micelles in presence of water. In spite of this, the effect of even trace water on micelle formation in non-polar media cannot be ignored. Some examples of CMC of surfactants in non-aqueous solvents are provided in Table 2.5. 

Dodecylpyridinium iodide-reversed micelles trapping chlorophyll a have been suggested as interesting photochemical model systems [22] and water/AOT/chloroethylene systems as peculiar dry-cleaning solvents [64]. 

FIGURE 1.2. Formation of nanoparticles of metal oxide by reverse micelle method. A solution of inverse micelles is first formed by adding a long-chain alkylamine to a toluene solution. A small amount of water is trapped in the reverse micelle core. Mixing the reverse micelle solution with an aluminum alkoxy amine adduct results in hydrolysis of the aluminum alkoxide adduct and formation of nano-sized particles of aluminum oxyhydroxide after drying. These particles are shown in the SEM picture above. 

After the mixture had been dried, the remaining residue (CD-capped gold nanoparticles + compound 3) was found to express insolubility in dry CHCfi but the solubility was restored when water was used in the equilibration of the chloroform. In our judgment, this finding clearly makes it apparent that there must be some water necessary for the efficient phase transfer of the nanoparticles into CHCfi. This finding leads to the conclusion that the idealized structure that has been proposed for the nanoparticles after they have been transferred to the chloroform phase (Scheme 2) has some aspects that must be similar to the structure of reverse micelles. We come to the conclusion that these nanoparticle-centered assemblies are similar in a conceptual way to gold-filled reverse micelles. (124 words)... 

Use of reverse micelles in synthetic chemistry to improve the rate and the yield of reactions seems likely to be a fruitful area of research in the future. In addition to catalysis, several other applications of reverse micelles can be cited. Just as nonpolar dirt is solubilized in aqueous micelles, so, too, polar dirt that would be unaffected by nonpolar solvents may be solubilized into reverse micelles. This plays an important role in the dry cleaning of clothing. Motor oils are also formulated to contain reverse micelles to solubilize oxidation products in the oil that might be corrosive to engine parts. 

Soft-core reverse micelles are spherical or ellipsoidal aggregates consisting of a water core separated from a continuous apolar phase by a surfactant shell. It is well known that in the absence of water, some surfactants such as sodium bis(2-ethylhexyl) sulfosuccinate (AOT) can form dry aggregates, while others such as sodium dodecyl sulfate (SDS) or hexadecyl-trimethylamonium bromide (CTAB) need a cosurfactant, e.g., a short chain alcohol, to form micelles. 

ZnTPP (Hambright, Washington D.C.), duroquinone (DQ) (Aldrich) and methyl viologen (MV+2) (BDH CHEMICALS) were used without further purification. The sodium salt of anthraquinone 2-sulphonate (AQS) (BDH CHEMICALS) was recrystalized from methanol. Benzyldimethyl-n-hexadecylammonium chloride (BHDC) (BDH CHEMICALS) was dried over P205 and stored under vacuum. The solvent, benzene (pro-analysis) (BDH CHEMICALS) was dried and distilled over sodium wire. Stock solutions of reversed micelles were prepared by adding dropwise the calculated amount of water to the appropriate solution of BHDC. The surfactant concentration used throughout the experiments was 0.1 mol dm 3. 

An examination of solvent effects on reverse micelles without added water provides even greater insight into the results for C11-14 EO5. The solvent effect on the aggregation number of dry alkali dinonylnaphthalenesulfonates was studied in a classic paper by Little and Singleterry (2). For sodium as the counterion, the... 

Sodium di(ethylhexyl)sulfosuccinate (Aerosol-OT, sodium docusate) [577-11-7] M 444.6. Dissolve it in MeOH and the inorganic salts which precipitate are filtered off. Water is added and the solution is extracted several times with hexane. The residue is evaporated to one-fifth its original volume, benzene is added and azeotropic distillation is continued until no water remains. The solvent is evaporated. The white residual soHd is crushed and dried in vacuo over P2O5 for 48hours [El Seoud Fendler J Chem Soc, Fcrcukiy Trans 1 71 452 1975]. [Beilstein 4 IV 114.] It solubilises major myelin trans membrane proteolipids, and forms reverse micelles in hydrocarbon solvents. 

Formation of microemulsion was carried out using a small quantity of ionic surfactant, CTAB (cetyltrimethylammonium bromide), and de-ionized water in excess dried toluene. Above critical micelle concentration (cmc) these three components form reversed micelles. As the size of such micelle system is related to the ratio of water/surfactant (W) added, tailored amount of water to surfactant ratios in order to control the sizes of the nano-composites were investigated. 

In case of Triton X-100, Zhu and Schelly [61,74] recorded the presence of up to about 0.28 % w/w H2O (measured by Karl Fischer titration), so that even a so-called dry Triton X-100 / cyclohexane reverse micelle could have a water / surfactant concentration ratio of - 0.1. 

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