Electrochemical Behavior of Anthraquinone in Reverse Micelles and Microemulsions of Cetyltrimethylammonium Bromide

The electrochemical behavior of an anthraquinone (AQ) was studied in aqueous solutions at a glassy carbon electrode, using the sodium salt of anthraquinone-2-sulfonic acid (AQS), by employing cyclic voltammetry. AQ undergoes a two-electron reduction in aqueous media. The electrochemical behavior of AQ was also investigated in micelles, reverse micelles (CTAB/1-butanol/water), and microemulsions (CTAB/1-butanol/water/cyclohexane) of cetyltrimethylammonium bromide (CTAB). The electrode reactions of AQ in reverse micelles and microemulsions are nearly reversible at low oil (cyclohexane) content. However, at higher oil content, the reversibility is gradually lost. In the case of reverse micelles, the reduction current, as well as the reduction potential, of AQ depend on the transition from a micellar solution to a stable solution of reverse micelles that occurs with added 1-butanol. In microemulsions, the change in cyclohexane content was found to cause a linear increase in the peak current for AQ reduction as well as a linear decrease in the corresponding reduction potential. As the cyclohexane content is increased, the o/w microemulsions dominated by micelles undergo a transition to a w/o microemulsion dominated by reverse micelles, which causes changes in the electrochemical behavior.     

Autoxidized phospholipids in hexane: nano-self-assemblies studied by synchrotron small-angle X-ray scattering

Synchrotron small-angle X-ray scattering (SAXS) was used to analyze the structure of self-assembled autoxidized phospholipids in a very dilute solution of hexane. In addition, it was used to build a self-consistent model of the aggregates, taking into account their inner heterogeneities and polydispersity. The scattering intensity from a dilute mixture of different types of noninteracting components of the phospholipid system was represented as a linear combination of partial intensities from the components weighted by their volume fractions. Applying this approach the final model of the system was described as a mixture of polydisperse reverse micelles and aggregates with spherical and cylindrical shapes. Spherical aggregates were represented as hollow spheres with inner radius 0.7 nm (occupied by water or hexane) and outer radius 1.5 nm. Geometrical parameters of the aggregates did not change much during the oxidation process, while the ratio of reverse micelles and aggregates in solution varied. The amount of the reverse micelles increased from very low to about 80%, whereas the content of other aggregates constantly reduced. The analysis performed in this study helps one to better understand the processes of phospholipid oxidation, which may occur in biological membranes.     

Nanometer particles synthesis in reverse micelles: Influence of the size and the surface on the reactivity

In this paprr we are presenting the synthesss “in situ” of nanoparticles in reverse micelles. In the case of aggregates containing copper ions, it is possible to form metallic particles surrounded or not by an oxide layer. By mixing aggregates containing cadmium and sulphide ions, CdS particles are formed. The size and polydispersity of the particles are controlled. The photoelectron transfer reaction depends mainly on the surface composition. In the range of 1 to 5 nm, the efficiency in the electron transfer does not depend on the size of the particle. The reverse micelles are formed by using either sodium di(2-ethyl hexyl)sulfosuccinate, usually called {AOT} or mixed bivalent and sodium di(2-ethyl hexyl)sulfosuccinate {AOl/M(AOT)₂}.     

Structure and size of spontaneously formed aggregates in Aerosol OT/PEG mixtures: effects of polymer size and composition

Dynamic light scattering and Cryo-TEM measurements have allowed us to obtain the size and structure of spontaneous aggregates formed by mixtures of Aerosol OT, AOT, and ethylene glycol polymers of different molecular mass. The results presented in this work show that small unilamellar vesicles predominate in pure Aerosol OT solutions and in dilute polymer solutions mixed with AOT. In the latter case, elongated micelles coexist with unilamellar vesicles. When polymer concentration increases above a certain concentration, the small vesicles disappear and the size of the elongated micelles decreases to a radius compatible with spherical micelles. For PEG concentrations above the overlapping ones, spherical micelles coexist with very large aggregates probably formed by large rod like micelles or by superstructures of elongated micelles embedded in a polymer network. This behavior is consistent with theoretical models based in molecular mean-field theory [M. Rovira-Bru, D.H. Thompson, I. Szleifer, Biophys. J. 83 (2002) 2419]. The properties of the different types of aggregates are obtained by fluorescence spectroscopy and electrophoretic mobility measurements.     

Synthesis and Stabilization of Gold Nanoparticles in Reverse Micelles of Aerosol OT and Triton X-100

Mechanisms of the formation and stabilization of gold nanoparticles in reverse micelles of micro-emulsions based on Triton X-100 (TX-100) and Aerosol OT (AOT) are studied. The instability of AOT-based microemulsions is shown to be caused by the oxidative degradation of gold nanoparticles in micelle water pools. Methods are proposed for the stabilization of these microemulsions. It is revealed that the mean size of gold nanoparticles synthesized in TX-100 reverse micelles in the presence of sodium sulfite is markedly smaller than that of particles prepared in AOT reverse micelles. This is explained by the fact that gold clusters are formed in the micelle shell rather than in the water pool. In the shell, the clusters are stabilized by oxyethylene groups of TX-100 molecules.__________Translated from Kolloidnyi Zhurnal, Vol. 67, No. 4, 2005, pp. 534–540.Original Russian Text Copyright © 2005 by Spirin, Brichkin, Razumov.     

Structural evolution of a two-component organogel

Dry reverse micelles of AOT in isooctane spontaneously undergo a microstructural transition to an organogel upon the addition of a phenolic dopant, p-chlorophenol. This microstructural evolution has been studied through a combination of light scattering, small-angle neutron scattering (SANS), NMR, and rheology. Several equilibrium stages between the system of dry reverse micelles of AOT and a 1:1 AOT/p-chlorophenol (molar ratio) gel in isooctane have been examined. To achieve this, p-chlorophenol is added progressively to the dilute solutions of AOT in isooctane, and this concentration series is then analyzed. The dry micelles of AOT in isooctane do not undergo any detectable structural change up to a certain p-chlorophenol concentration. Upon a very small increment in the concentration of p-chlorophenol beyond this "threshold" concentration, large strandlike aggregates are observed which then evolve to the three-dimensional gel network.     

Polystyrene-block-polyglycidol micelles cross-linked with titanium tetraisopropoxide. laser light and small-angle X-ray scattering studies on their formation in solution

Hybrid micelles from polystyrene-block-polyglycidol (PS-b-PG) copolymers with chemically cross-linked cores by titanium tetraisopropoxide (Ti(OC(3)H(7))(4)) were prepared in toluene solution. Additionally, micellization of PS-b-PG copolymers with different mass fractions of polyglycidol (x(PG)), was studied by static and dynamic light scattering as well as small-angle X-ray scattering. It was observed that copolymers with x(PG) smaller than 0.5 self-assembled in toluene into spherical core-shell micelles with hydrodynamic radii R(h) between 12 and 23 nm. On the other hand, copolymers with larger PG content formed particles with R(h) = 50-70 nm and aggregation numbers of several thousands. The presence of these aggregates in solution was attributed to the nonequilibrated form of block copolymers upon dissolving, most probably due to hydrogen bonding. In the following, spherical PS-b-PG micelles were loaded in toluene with hydrochloric acid and titanium tetraisopropoxide. Confined hydrolysis of Ti(OC(3)H(7))(4) induced by HCl in the micellar core was confirmed by small-angle X-ray scattering experiments. The subsequent condensation of the precursor with hydroxyl groups of polyglycidol chains led to covalently stabilized hybrid organic-inorganic particles. The presence of cross-linked PS-b-PG micelles was proven in two ways. First, micelles with "frozen" core showed stable hydrodynamic size in time upon dilution below critical micellization concentration while non-cross-linked PS-b-PG micelles underwent disintegration under the same conditions within several hours. Second, light scattering experiments revealed the presence of stable, swollen particles in N,N-dimethylformamide, which is a good solvent for both blocks.     

Physicochemical investigation of the solubilization of ytterbium nitrate in AOT reverse micelles and liquid crystals

A wide investigation of the solubilization of the water-soluble salt Yb(NO₃)₃ in sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles and AOT liquid crystals has been carried out. After saturation of water/AOT/organic solvent w/o microemulsions with pure Yb(NO₃)₃, the Yb(NO₃)₃/AOT composites were prepared by complete evaporation under vacuum of the volatile components (water and organic solvent) of the salt-containing microemulsions. It was observed that these composites can be totally dissolved in pure n-heptane or CCl₄, allowing the solubilization of a noticeable amount of Yb(NO₃)₃ in quite dry apolar media. By UV–vis–NIR, FT-IR, and ¹H NMR spectroscopies, some information on the state of Yb(NO₃)₃ within AOT reverse micelles were acquired, whereas by small angle X-ray scattering (SAXS), it has been ascertained that Yb(NO₃)₃ is quite homogeneously distributed as very small clusters among the reverse micelles. An analysis of SAXS and wide-angle X-ray scattering spectra of Yb(NO₃)₃/AOT composites leads to the hypothesis that, also in these systems, Yb(NO₃)₃ is dispersed in the surfactant matrix as very small clusters.     

Integral physicochemical properties of reverse micelles of sodium bis(2-ethylhexyl) sulfosuccinate (AOT)

The effect the degree of hydration has on optical and electrophysical properties of water/AOT/n-hexane system is studied. It is found that AOT reverse micelles form aggregates whose dimensions grow along with the degree of hydration and temperature. Aggregation enhances their electrical conductivity and shifts the UV spectrum of AOT reverse emulsions to the red region. Four states of water are found in the structure of AOT reverse micelles.     

Light scattering and cryo-transmission electron microscopy investigation of the self-assembling behavior of Di-C12P-nucleosides in solution

Aggregates formed from freshly prepared and annealed samples of dilauroyl-phosphatidyl-adenosine, dilauroyl-phosphatidyl-uridine, and their 1:1 mixture have been investigated by dynamic light scattering, cryo-transmission electron microscopy (cryo-TEM) observations, and circular dichroism. The two surfactants differ only for the nucleoside at the phospholipid polar headgroup and self-assemble in solution to form supramolecular structures that behave dissimilarly. The uridine derivative forms long wormlike aggregates that are invariant with the aging of the solution, while the wormlike aggregate of the adenosine derivative undergoes, as the sample ages, a subsequent self-assembling process forming giant helicoidal aggregates that coexist with the smaller wormlike aggregates. Dynamic light scattering and cryo-TEM show that the large helicoidal structures are formed at the expense of the small wormlike micelles. The 1:1 mixture behaves as the adenosine derivative and evolves to form giant superstructures for all the lipid concentrations investigated. Circular dichroism measurements suggest that the formation of the supramolecular helicoidal structure might not be driven by a purely chiral effect, but rather stacking and hydrogen bonding, present at the phospholipid headgroups of the self-assembled nucleosides, contribute to the final supramolecular structure.     

Shape-controllable synthesis of crystalline Ni complex particles via AOT-based microemulsions

This article presents a simple method for the fabrication of shape-controllable Ni complex particles via an AOT-based single microemulsion. In this approach, Ni(2+)/N2H4/EG solution is used as the dispersed phase, and cyclohexane is used as the continuous phase to obtain a microemulsion by the aid of the anionic surfactant AOT. The primary Ni complex particles with diameters of 20-30 nm were first formed in the reverse micelles and then self-organized into spindle-like, ellipse-like, cuboid, and cubic morphologies, depending on the reaction conditions. When aged at 100 degrees C for 24 h, these Ni complex particles changed into crystalline Ni. A possible evolution mechanism of the Ni complex particles with different morphologies is also discussed.     

Oil-continuous microemulsions mixed with an amphiphilic graft copolymer or with the parent homopolymer Polymer–droplet interactions as revealed by phase behavior and light scattering

Polymer–droplet interactions have been studied in AOT/water/isooctane oil-continuous microemulsions mixed with an amphiphilic graft copolymer, or with the parent homopolymer (AOT = sodium bis(2-ethylhexyl) sulfosuccinate). The graft copolymer has an oil-soluble poly(dodecyl methacrylate) backbone and water-soluble poly(ethylene glycol) side chains. Pseudo-ternary polymer/droplet/isooctane phase diagrams have been established for both the parent homopolymer and the graft copolymer, and the two types of mixture display entirely different phase behavior. The homopolymer–droplet interaction is repulsive, and a segregative phase separation occurs at high droplet concentrations. By contrast, the graft copolymer–droplet interaction is attractive: the polymer is insoluble in the pure oil, but dissolves in the microemulsion. A comparatively high concentration of droplets is required to solubilize even small amounts of polymer. Static and dynamic light scattering has been performed in order to obtain information on structure and dynamics in the two types of mixture. For optically matched microemulsions, with a vanishing excess polarizability of the droplets, the polymer dominates the intensity of scattered light. The absolute intensity of scattered light increases as phase separation is approached owing to large-scale concentration fluctuations. Dynamic light scattering shows two populations of diffusion coefficients; one population originates from “free” microemulsion droplets and the other from the polymer (for homopolymer mixtures) or from polymer–droplet aggregates (for mixtures with the graft copolymer). The graft copolymer forms large polymer–droplet aggregates with a broad size distribution, which coexist with a significant fraction of free droplets.     

Partition Equilibria for Alcohols in Reverse Micellar AOT-Oil-Water Systems Studied by PGSE-FT NMR. A Comparison Between AOT-containing and the Corresponding AOT-free Systems

Microemulsions of the reverse micellar type were investigated by determining the self-diffusion coefficients of the components using the Pulsed Field Gradient Spin Echo – Fourier Transform NMR method (PGSE-FT NMR). The microemulsions were composed of the surfactant AOT, water and an oil (either benzene or cyclohexane), forming a water core in an oil continuum. The primary alcohols ranging from methanol to 1-decanol were added to the microemulsions as a fourth component. The degree of binding, p, of the alcohol to the micelles was determined from the measured self-diffusion coefficients for this component. Partition equilibrium constants were calculated from the values of p. Thermodynamic partition equilibrium constants, K _c , calculated from the partition coefficients are presented and compared with values based on literature data for AOT-free systems. Similarities and differences between these cases are revealed and interpreted.     

Study on the Morphology Feature of 1-Butyl-3-methylimidazolium Tetrafluoroborate Based Ionic Liquid Reverse Microemulsions

The micromorphology of 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF₄)-in-cyclohexane and bmimBF₄-in-triethylamine ionic liquid microemulsions was investigated by two-dimensional NMR and freeze-fracture transmission electron microscopy. The reverse micelles of Triton X-100/cyclohexane were destroyed by adding bmimBF₄ and reverse microemulsions were induced to form by successively adding bmimBF₄. However, no micelles appeared in triethylamine. But if adding bmimBF₄ to a certain extent, a reverse microemulsion was also formed. The driving force of such aggregations could be attributed to the presence of different types of interactions between Triton X-100 and bmimBF₄. A staggered arrangement of surfactant led to the irregular droplet structure and large aggregate size.     

琥珀酸二异辛酯磺酸钠微乳体系中纳米银的制备及其连续相的影响

在琥珀酸二异辛酯磺酸钠(AOT)为表面活性剂、环己烷为连续相形成的微乳体系中, 利用水合肼还原AgNO₃制备了分散性良好的纳米银. 利用紫外-可见(UV-Vis)光谱和透射电镜(TEM)对所得产物进行了表征, TEM显微图像表明形成粒子为球形结构, 平均粒径为5.10 nm, 标准偏差为2.84 nm. 分别利用正己烷、正庚烷、正辛烷、环己烷和十二烷等作连续介质, 研究了微乳液中连续相对纳米银形成的影响. 随着正烷烃碳链长度的增加, 微乳液中胶束之间的交换速率增大, 形成粒子的平均粒径逐渐减小. 十二烷形成的微乳体系制备的纳米银溶胶具有最宽的共振吸收峰, 所得的纳米银粒子平均粒径最小. 环己烷形成的微乳液中反胶束具有特殊的界面强度, 导致纳米银晶核的形成速率过低, 纳米银晶粒的生长不完全.     

Sizing of Reverse Micelles in Microemulsions using NMR Measurements of Diffusion

This paper reports the size of reverse micelles (RMs) in AOT/octane/H(2)O and CTAB/hexanol/H(2)O microemulsions using magnetic resonance (MR) pulsed field gradient (PFG) measurements of diffusion. Diffusion data were measured using the pulsed gradient stimulated echo (PGSTE) experiment for surfactant molecules residing in the RM interface. Inverse Laplace transformation of these data generated diffusion coefficients for the RMs, which were converted into hydrodynamic radii using the Stokes-Einstein relation. This technique is complementary to those previously used to size RMs, such as dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS), but also offers several advantages, which are discussed. RM sizes, determined using the PGSTE method, in the AOT (sodium bis(2-ethylhexyl) sulfosuccinate) and CTAB (cetyltrimethylammonium bromide) microemulsions were compared with previous DLS and SAXS data, showing good agreement. Methods for determining number distributions from the PGSTE data, through the use of scaling factors, were investigated.     

Aggregation Behaviors of Tricarbocyanine Dye in Water and in AOT Reverse Micelles

The photophysical property of the tricarbocyanine dye IR144 has been extensively studied in non‐aqueous solvents. However, as a potential near‐infrared biomedical imaging probe, the photophysical property of IR144 in water is still little known. So, the aggregation behaviors of IR144 in water with steady‐state absorption spectroscopy and integrated polarization dependent femtosecond pump‐probe spectroscopy were investigated. Through comparing the absorption spectral bandshape of IR144 in water and in water pool of AOT reverse micelles, It is found that IR144 form dimer aggregates in water even at very low concentration (<1.0×10^?7 mol·L^?1). And the absorption spectrum of the IR144 aggregates always displays a bimodal feature, which is independent of the dye concentration ranging from 1.0×10^?7 to 1.0×10^?4 mol·L^?1. For better understanding the aggregation behaviors of IR144 in water, we measured the ground state recovery kinetics and the reorientation kinetics of IR144 in water and in water pool of AOT reverse micelles (W₀=[H₂O]/[AOT], W₀=40). It is found that the fluorescence quantum yield of IR144 in water is lower than that in water pool of AOT reverse micelles, and the reorientation time of IR144 in water is slower than that in water pool of AOT reverse micelles. Those kinetic measurements also verify that IR144 exists as dimer aggregates in water.     

反相胶束中辣根过氧化物酶的停流光谱研究

用停流光谱法研究了HRP在AOT、CTAB和SDS反相胶束中的吸收光谱和反应动力学，实验结果显示在AOT反相胶束中，HRP的吸收峰位置与水相中相同；而另外两种反相胶束对HRP的分子结构产生了较大影响，快速反应动力学研究显示在反相胶束中HRP形成化合物Ⅰ的速率常数远远高于化合物Ⅰ形成HRP—Ⅱ的反应速率常数，推测这是反相胶束的特殊性质造成的结果。     

Hydrocarbon peroxidation: protective effect of reverse micelles

Pulse radiolysis technique was used to study radical processes in AOT/n-heptane and AOT/n-heptane/water reverse micellar systems. It was found that reverse micelles, especially socalled 'wet' micelles ([H₂O]/[AOT] > 10), significantly diminished the yield of peroxyl radicals formed when the system contained trace amounts of oxygen. The possible mechanism of such protective effect of micellar aggregates is discussed in terms of scavenging of charges, both electrons and cation radicals, by micellar aggregates where they can eventually form radicals. The latter are however separated from the traces of oxygen which remains mainly in the continuous hydrocarbon phase, where it dissolves much better than in the micellar water core.     

Excited-state proton transfer of 2-(2'-pyridyl)benzimidazole in microemulsions: selective enhancement and slow dynamics in aerosol OT reverse micelles with an aqueous core

Excited-state proton transfer (ESPT) of 2-(2'-pyridyl)benzimidazole (2PBI) in reverse micelles has been studied by steady-state and time-resolved fluorescence spectroscopy. The nanometer sized water pool in the n-heptane/Aerosol OT (AOT)/water microemulsion is found to promote tautomer emission of this probe, as is evident from the emergence of a Stokes shifted band at 450 nm at the expense of the normal emission band on increasing the water content of the system. In the nonaquous microemulsion with a methanol core, the normal emission is quenched but no tautomer emission is obtained. With an acetonitrile core, there is no change in emission properties. Similarly, there is no evidence of ESPT in Triton X-100 reverse micelles. This indicates the requirement of ESPT to occur in microheterogeneous media; the medium should be a ternary system comprised of water and a hydrophobic phase separated by a negatively charged interface. In the microemulsions with an aqueous core, the fluorescence decays of 2PBI at the red end exhibit rise times of 0.8 ns and the time-resolved area-normalized emission spectra (TRANES) exhibit an isoemissive point, indicating slow dynamics of the two-state ESPT of 2PBI in aqueous AOT reverse micelles. The origin of the selective enhancement in AOT microemulsions as well as the slow dynamics is explored using fluorescence spectroscopic techniques, with support from quantum chemical calculation.