Reversed micelles effects

Nazario LMM, Hatton TA, Crespo JPSG (1996) Nonionic cosurfactants in AOT reversed micelles Effect on percolation, size, and solubilization site. Langmuir... 

Reverse micelles -effect of supercntical fluids [SUPERCRITICAL FLUIDS] (Vol 23)... 

B.K. Paul and R.K. Mitra 2005 Water solubilization capacity of mixed reverse micelles Effect of surfactant component, the nature of the oil, and electrolyte concentration, J. Colloid Interf. Sci. 288, 261-279. 

Han, D., Rhee, J.S., Lee, S.B. 1987. Lipase reaction in AOT-isooctane reversed micelles Effect of water on equilibria. Biotechnol. Bioeng. 30, 381-388. 

I Lisiecki, M BjOrling, L Motte, B Ninham, MP Pileni. Synthesis of copper nanosize particles in anionic reverse micelles effect of the addition of a cationic surfactant on the size of the crystallites. Langmuir 11 2385-2392, 1995. 

Moreover, taking into account that reversed micelles coexist with surfactant monomers, in principle, further effects due to the aggregation of polar and amphiphilic so-lubihzates with surfactant monomers and the shift of the monomer/reversed micelle equilibrium must be also considered [25,26],... 

The effects of the intramicellar confinement of polar and amphiphilic species in nanoscopic domains dispersed in an apolar solvent on their physicochemical properties (electronic structure, density, dielectric constant, phase diagram, reactivity, etc.) have received considerable attention [51,52]. hi particular, the properties of water confined in reversed micelles have been widely investigated, since it simulates water hydrating enzymes or encapsulated in biological environments [13,23,53-59]. 

Differential scanning calorimetry measurements have shown a marked cooling/heat-ing cycle hysteresis and that water entrapped in AOT-reversed micelles is only partially freezable. Moreover, the freezable fraction displays strong supercooling behavior as an effect of the very small size of the aqueous micellar core. The nonfreezable water fraction has been recognized as the water located at the water/surfactant interface engaged in solvation of the surfactant head groups [97,98]. 

The vibrational dynamics of water solnbilized in lecithin-reversed micelles appears to be practically indistingnishable from those in bulk water i.e., in the micellar core an extensive hydrogen bonded domain is realized, similar, at least from the vibrational point of view, to that occurring in pure water [58], On the other hand, the reorientational dynamics of the water domain are strongly affected, due to water nanoconfmement and interfacial effects [105,106],... 

In recent decades, many investigations have been carried out on the solubilization and on the physicochemical characterization of a wide variety of substances confined in water-containing reversed micelles. Even if these studies have not produced a general theory to predict a priori all the effects accompanying the solubihzation process, some general aspects nonetheless have been underhned. In the following, the results of some of these investigations, selected to show the extent of some peculiar behaviors, will be reported. 

Electrolytes are obviously solubilized only in the aqueous micellar core. Adding electrolytes in water-containing AOT-reversed micelles has an effect that is opposite to that observed for direct micelles, i.e., a decrease in the micellar radius and in the intermicellar attractive interactions is observed. This has been attributed to the stabilization of AOT ions at the water/surfactant interface [128]. 

Effects on the micellar shape are also induced by electrolyte addition. It has been observed that, in decane, the water-containing AOT-reversed micelles become more spherical upon addition of salt (NaCl, CaCli) [6]. 

The different location of polar and amphiphilic molecules within water-containing reversed micelles is depicted in Figure 6. Polar solutes, by increasing the micellar core matter of spherical micelles, induce an increase in the micellar radius, while amphiphilic molecules, being preferentially solubihzed in the water/surfactant interface and consequently increasing the interfacial surface, lead to a decrease in the miceUar radius [49,136,137], These effects can easily be embodied in Eqs. (3) and (4), aUowing a quantitative evaluation of the mean micellar radius and number density of reversed miceUes in the presence of polar and amphiphilic solubilizates. Moreover it must be pointed out that, as a function of the specific distribution law of the solubihzate molecules and on a time scale shorter than that of the material exchange process, the system appears polydisperse and composed of empty and differently occupied reversed miceUes [136],... 

The microenvironment in water-containing AOT-reversed micelles has a marked effect on the spectral properties of flnorescein. The absorption peaks are red-shifted by about 10 nm from the corresponding positions in aqueous solution, the absorption extinction coefficient increases with R, and the fluorescence is more effectively quenched in AOT-reversed micelles than in aqueous solution [149],... 

By flourescence techniques, it was observed that the fluorescence yield and lifetime of 1,8-anilinonaphthalenesulfonate decrease with an increase in the aqueous core of AOT-reversed micelles, while the position of the emission maximum shifts to longer wavelengths [64], These changes in the electronic properties were attributed to the peculiar effective polarity and viscosity of the micellar core and to their evolution with R. 

The fluorescence lifetime of trans-4-[4-(dimethylamino-styryl]-l-methylpyridinium iodide trapped in water-containing AOT-reversed micelles has been found to be markedly influenced by R, implying a significant effect on its excited-state twisting motion [122],... 

The solubilization of amino acids in AOT-reversed micelles has been widely investigated showing the importance of the hydrophobic effect as a driving force in interfacial solubihzation [153-157]. Hydrophilic amino acids are solubilized in the aqueous micellar core through electrostatic interactions. The amino acids with strongly hydrophobic groups are incorporated mainly in the interfacial layer. The partition coefficient for tryptophan and micellar shape are affected by the loading ratio of tryptophan to AOT [158],... 

As a result of the micellar environment, enzymes and proteins acquire novel conformational and/or dynamic properties, which has led to an interesting research perspective from both the biophysical and the biotechnological points of view [173-175], From the comparison of some properties of catalase and horseradish peroxidase solubilized in wa-ter/AOT/n-heptane microemulsions with those in an aqueous solution of AOT it was ascertained that the secondary structure of catalase significantly changes in the presence of an aqueous micellar solution of AOT, whereas in AOT/n-heptane reverse micelles it does not change. On the other hand, AOT has no effect on horseradish peroxidase in aqueous solution, whereas slight changes in the secondary structure of horseradish peroxidase in AOT/n-heptane reverse micelles occur [176],... 

The activity of a-chymotrypsin was found to be insensitive to the R value, i.e., from the size of the reversed micelles. This was taken as an indication that this enzyme is able to create its own micelles in the hydrocarbon rather than occupy empty ones and that the so-called exclusion effect, i.e., protein larger than the empty micelle cannot be solubilized, is incorrect [181,182],... 

In addition, it is of interest to note that investigations of the microscopic processes leading to nucleation, growth, oriented growth by the surfactant monolayer, and growth inhibition of nanoparticles in reversed micelles and of confinement and adsorption effects on such phenomena represent an intriguing and quite unexplored research field [218]. 

Solutions of surfactant-stabilized nanogels share both the advantage of gels (drastic reduction of molecular diffusion and of internal dynamics of solubilizates entrapped in the micellar aggregates) and of nonviscous liquids (nanogel-containing reversed micelles diffuse and are dispersed in a macroscopicaUy nonviscous medium). Effects on the lifetime of excited species and on the catalytic activity and stability of immobilized enzymes can be expected. 

Templates made of surfactants are very effective in order to control the size, shape, and polydispersity of nanosized metal particles. Surfactant micelles may enclose metal ions to form amphiphilic microreactors (Figure 11a). Water-in-oil reverse micelles (Figure 11b) or larger vesicles may function in similar ways. On the addition of reducing agents such as hydrazine nanosized metal particles are formed. The size and the shape of the products are pre-imprinted by the constrained environment in which they are grown. 

Investigation of water motion in AOT reverse micelles determining the solvent correlation function, C i), was first reported by Sarkar et al. [29]. They obtained time-resolved fluorescence measurements of C480 in an AOT reverse micellar solution with time resolution of > 50 ps and observed solvent relaxation rates with time constants ranging from 1.7 to 12 ns. They also attributed these dynamical changes to relaxation processes of water molecules in various environments of the water pool. In a similar study investigating the deuterium isotope effect on solvent motion in AOT reverse micelles. Das et al. [37] reported that the solvation dynamics of D2O is 1.5 times slower than H2O motion. 

Effectiveness of a crude oil demulsifier is correlated with the lowering of shear viscosity and dynamic tension gradient of the oil-water interface. Using the pulsed drop technique, the interfacial dilational modulii with different demulsifiers have been measured. The interfacial tension relaxation occurs faster with an effective demulsifier. Electron spin resonance with labeled demulsifiers indicate that the demulsifiers form reverse micelle like clusters in bulk oil. The slow unclustering of the demulsifier at the interface appears to be the rate determining step in the tension relaxation process. 

Kitchens, C.L., McLeod, M.C. and Roberts, C.B. (2003) Solvent effects on the growth and steric stabilization of copper metallic nanoparticles in AOT reverse micelle systems. Journal of Physical Chemistry B, 107 (41), 11331-11338. 

The reverse micelles stabilized by SDS retard the autoxidation of ethylbenzene [27]. It was proved that the SDS micelles catalyze hydroperoxide decomposition without the formation of free radicals. The introduction of cyclohexanol and cyclohexanone in the system decreases the rate of hydroperoxide decay (ethylbenzene, 363 K, [SDS] = 10 3mol L [cyclohexanol] =0.03 mol L-1, and [cyclohexanone] = 0.01 mol L 1 [27]). Such an effect proves that the decay of MePhCHOOH proceeds in the layer of polar molecules surrounding the micelle. The addition of alcohol or ketone lowers the hydroperoxide concentration in such a layer and, therefore, retards hydroperoxide decomposition. The surfactant AOT apparently creates such a layer around water moleculesthat is very thick and creates difficulties for the penetration of hydroperoxide molecules close to polar water. The phenomenology of micellar catalysis is close to that of heterogeneous catalysis and inhibition (see Chapters 10 and 20). 

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