Coexisting aggregates in mixed aerosol OT and cholesterol microemulsions

Dynamic light scattering and NMR spectroscopic experimental evidence suggest the coexistence of two compositionally different self-assembled particles in solution. The self-assembled particles form in solutions containing water, Aerosol OT (AOT, sodium bis(2-ethylhexyl) sulfosuccinate) surfactant, and cholesterol in cyclohexane. In a similar series of studies carried out in 1-octanol only one aggregate type, that is, reverse micelles, is observed. Dynamic light scattering measurements reveal the presence of two different types of aggregates in the microemulsions formed in cyclohexane, demonstrating the coexistence of two compositionally distinct structures with very similar Gibbs energies. One particle type consists of standard AOT reverse micelles while the second type of particle consists of submicellar aggregates including cholesterol as well as small amounts of AOT and water. In microemulsions employing 1-octanol as the continuous medium, AOT reverse micelles form in a dispersed solution of cholesterol in 1-octanol. Although the size distribution of self-assembled particles is well-known for many different systems, evidence for simultaneous formation of two distinctly sized particles in solution that are chemically different is unprecedented. The ability to form microemulsion solutions that contain coexisting particles may have important applications in drug formulation and administration, particularly as applied to drug delivery using cholesterol as a targeting agent.     

A spectral approach to determine location and orientation of azo dyes within surfactant aggregates

The UV–vis absorption properties of azo dyes are known to exhibit a variation with the polarity and acidity of the dye environment. The spectral properties of a series of anionic azo dyes were characterized to further probe the interaction of these dyes with two types of surfactant aggregates: (1) the spherical micelles formed in aqueous solution by alkyltrimethylammonium bromide (C_nTAB) surfactants with n = 10–16 and (2) the unilamellar vesicles spontaneously formed in water from binary mixtures of the oppositely-charged double-tailed surfactants cationic didodecyldimethylammonium bromide (DDAB) and anionic sodium dioctylsulfosuccinate (Aerosol OT or AOT). The observed dye spectra reflect the solvatochromic behavior of the dyes and suggest the location and orientation of the dye within the surfactant aggregates. Deconvolution of the overall spectra into sums of Gaussian curves more readily displays any contributions of tautomeric forms of the azo dyes resulting from intramolecular hydrogen bonding. The rich variation in UV/vis absorption properties of these anionic azo dyes supports their use as sensitive tools to explore the nanostructures of surfactant aggregates.     

Determination of pH in reversed micelles

The pH in the reversed micellar system of di(ethylhexyl) sodium sulfosuccinate (Aerosol OT, AOT) / phosphate buffer solutions/octane was determined by a P-NMR technique, and pHs in the reversed micelles containing buffer solutions other than the phosphate buffer solution were measured by the spectrophotometric method with the aid of Phenol Red. pHs in reversed micelles were found to be substantially determined by the buffer capacity of buffer solutions solubilized into the systems. By means of both the methods, pKa of Phenol Red in the systems was found to be 7.7, which is almost consistent with that in water. Analysis of Na-NMR spectra indicates that the mobility of the sodium ion of AOT is independent of the molar ratio of water to AOT when the ratio is above 7 and is restricted strongly by the interaction with the sulfonate group of AOT. The relationship between pH and the mobility of the sodium ion was discussed on the basis of the data of Na-NMR spectra.     

Effects of bile salts on percolation and size of AOT reversed micelles

The effects of two trihydroxy bile salts, sodium taurocholate (NaTC) and 3-[(3-cholamidylpropyl)dimethylammonio]-1-propane sulfonate (CHAPS), on the size, shape and percolation temperature of reversed micelles formed by sodium bis(2-ethylhexyl)sulfosuccinate (AOT) in isooctane were studied. The percolation temperature of the reversed micelles decreased upon inclusion of bile salts, indicating increased water uptake. Dynamic light scattering (DLS) measurements showed consistent enlargement of reversed micelles upon addition of the bile salts; the hydrodynamic radius increased sixfold in the presence of 10 mM CHAPS and doubled in the presence of 5 mM NaTC. Inclusion of the enzyme yeast alcohol dehydrogenase (YADH) increased the percolation temperature and distorted the spherical structure of the AOT reversed micelles. The spherical structure was restored upon addition of bile salt. These results may help to explain the increase in activity of YADH in AOT reversed micelles upon addition of bile salts.     

Transport of charged Aerosol OT inverse micelles in nonpolar liquids

Surfactants such as Aerosol OT (AOT) are commonly used to stabilize and electrically charge nonpolar colloids in devices such as electronic ink displays. The electrical behavior of such devices is strongly influenced by the presence of charged inverse micelles, formed by excess surfactant that does not cover the particles. The presence of charged inverse micelles results in increased conductivity of the solution, affecting both the energy consumption of the device and its switching characteristics. In this work, we use transient current measurements to investigate the electrical properties of suspensions of the surfactant Aerosol OT in dodecane. No particles are added, to isolate the effect of excess surfactant. The measured currents upon application of a voltage step are found to be exponentially decaying, and can be described by an analytical model based on an equivalent electric circuit. This behavior is physically interpreted, first by the high generation rate of charged inverse micelles giving the suspension resistor like properties, and second by the buildup of layers of charged inverse micelles at both electrodes, acting as capacitors. The model explains the measurements over a large range of surfactant concentrations, applied voltages, and device thicknesses.     

Structure of AOT reverse micelles under shear

Reverse micelles in the AOT/water/isooctane system, at various water contents (W(0)), were studied using rheometry and small angle X-ray scattering (SAXS) experiments under static conditions and under shear. The SAXS analysis confirmed the spherical shape of the micelles at low water content and revealed a transition into elongated micelles at higher water content. A population of spherical micelles was found to coexist with the cylindrical ones, even above the percolation threshold. The shape transformation was correlated with a viscosity leap observed in the rheometry measurements. Reverse micelles at low water content under shear act as a Newtonian fluid, without any detectable shape changes. In contrast, reverse micelles at high water content behave as a shear thinning fluid. SAXS measurements at high water content under shear force have shown that the shear forces induced alignment of the cylindrical micelles in the flow direction, without any other changes in the micelle dimensions. The anisotropy parameter, a measure of the degree of the spatial order, was found to increase with increasing water content and shear rate.     

Location of solubilizate in water-AOT-isooctane systems using positron lifetime spectroscopy

In the present work, positrons have been used as a probe to locate the interaction site of the additive molecules in a membrane mimetic system similar to reverse micelles. The systems investigated are water-AOT Aerosol OT [Sodium bis(2-ethylhexyl) sulfosuccinate)-isooctane solutions. Positron lifetime measurements have been performed in water-AOT-isooctane solutions containing chloroform and dichloromethane, respectively, as additives. The choice of these additives is based on their physiological importance. A comparison of the behavior of positron annihilation parameters in surfactant solutions containing the additives with that in systems without them has been made. It has provided some information about the location of the interaction site of the additive molecules in these solutions. In chloroform, the additive molecules tend to migrate from the bulk solvent to the aggregates and are adsorbed either at the interface or close to it. The dichloromethane molecules, on the other hand, tend to get distributed near the hydrocarbon tail of the AOT molecules closer to the bulk solvent.     

Vesicle-to-spherical micelle-to-tubular nanostructure transition of monomethoxy-poly(ethylene glycol)-poly(trimethylene carbonate) diblock copolymer

Recently, we reported a temperature-sensitive biodegradable diblock copolymer of monomethoxy-poly(ethylene glycol)-b-poly(trimethylene carbonate) (mPEG-PTMC; Macromolecules 2007, 40, 5519-5525). In this paper, we report the detailed morphological transition of the polymer in water as a function of polymer concentration and temperature, using cryo-transmission electron microscopy (cryo-TEM). At a low polymer concentration (0.05 wt %), the mPEG-PTMC diblock copolymers formed vesicles in water. On the other hand, vesicle-to-micelle transition was observed as the polymer concentration increased. The polymer predominantly formed micelles above 2.0 wt %. In the 2.0 wt % polymer solution, the mPEG-PTMC underwent spherical micelle-to-tubular nanostructure transition as the temperature increased from 10 to 40 degrees C, and the transition accompanied an increase in turbidity of the polymer aqueous solution due to the increase in the apparent size of the polymer aggregates. Here, we report that the morphology of vesicles, spherical micelles, and tubular nanostructures is reversibly controlled by a thermosensitive polymer of mPEG-PTMC and the variation of the morphology can be carefully traced by using cryo-TEM. This paper will not only provide an important method for morphological control of an amphiphilic polymer but also improve our understanding of a temperature-sensitive transition mechanism of the polymer.     

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.     

Recognition-induced transformation of microspheres into vesicles: morphology and size control

Polystyrene functionalized with diamidopyridine (DAP) recognition units self-assembles in nonpolar media to form thermally reversible micrometer-scale spherical aggregates. The size and the thermal stability of these microspheres can be controlled by the molecular weight of the polymer. The addition of thymine-functionalized polymer to these self-assembled microspheres converted them into vesicular aggregates with a controlled size. The morphology change was reversible: the addition of DAP-functionalized polymer converted the vesicles back to microspheres.     

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}.     

Transition from micelle to vesicle in aqueous mixtures of anionic/zwitterionic surfactants studied by fluorescence, conductivity, and turbidity methods

Vesicles form spontaneously in a aqueous mixture of sodium bis(2-ethylhexyl) sulfosuccinate (Aerosol OT) and lauryl sulfonate betaine (LSB). Different from catanionic vesicles, the formation or disaggregation of such zwitterionic/anionic vesicles may be easily controlled by adjusting the relative amount of LSB and salinity. The participation of LSB reduces the polydispersity of the vesicles and even results in the formation of monodispersed vesicles at a certain salinity. But as LSB exceeds a certain proportion, vesicles cannot form at any concentration and salinity, making convenient the study of the structural transitions. We applied pyrene as a fluorescence probe and monitored the transition among the monomer, micelle, and vesicle through the variation of I(1)/I(3), accompanied by conductivity and turbidity measurements. In LSB solution and LSB-rich mixture, an abrupt change of the ratio of I(1)/I(3) was found in the transition from monomer to micelle with increasing concentration, as well as in the transition from micelle to vesicle with increasing salinity, which shows that a difference of the polarity of the microenvironment between the micelle and the vesicle bilayer resulted from the composition change. But in AOT solution and AOT-rich mixture, only a gradual change in the transition is observed due to the existence of intermediate structures, which have different microenvironments from micelles and vesicles. So the formation of vesicle experiences a process of monomer to premicelle to micelle to bilayer segment with increasing concentration by combining the conductivity method. The ratio of I(1)/I(3) is independent of the vesicle size once formed.     

Solubilization of a water-insoluble dye in aqueous NaBr solutions of alkylpyridinium bromides and its relation to micellar size and shape

In order to investigate the effect of added salt on micelle size, shape, and structure the solubilization of Orange OT in aqueous NaBr solutions of decylpyridinium bromide (DePB), dodecylpyridinium bromide (DPB), tetradecylpyridinium bromide (TPB), and hexadecylpyridinium bromide (CPB) has been examined. The solubilization powers of DePB and DPB micelles increase with increasing NaBr concentration up to 2.86 and 3.07 mol dm^–3, respectively, but above these concentrations remain unaltered. This suggests that spherical micelles of DePB and DPB can have a maximum and constant size at NaBr concentrations higher than these threshold concentrations. On the other hand, the solubilization powers of TPB and CPB micelles increase in the whole range of NaBr concentration studied. The dependencies of the solubilization powers of their micelles on the counterion concentration change at 0.10 and 0.03 mol dm^–3 NaBr, respectively, as suggests that TPB and CPB micelles undergo the sphere–rod transition at those concentrations. Orange OT is a more suitable probe for detecting the presence of the maximum- and constant-size spherical micelle than Sudan Red B.     

Partial purification of glutaryl-7-ACA acylase from crude cellular lysate by reverse micelles

Glutaryl-7-ACA acylase was partially purified from the cellular lysate ofPseudomonas sp. NCIMB 40409 by means of reverse micelles-water two-phases extractions. The tetrameric enzyme can be solubilized inside the reverse micelles formed by anionic (Aerosol OT, AOT) and cationic (tetradecyltrimethylammoniumbromide, TDAB) surfactants with retention of the enzymatic activity. With TDAB reverse micelles system, the acylase was partially extracted from the aqueous phase and, after backward transfer into a second water phase, a twofold purification factor was achieved. On the other hand, with the AOT micellar system, in conditions were most of the proteins but acylase, were extracted by the organic micellar solution, a sixfold increase of the specific activity of the acylase remaining in the aqueous phase was obtained.     

The sphere-rod transition of micelles of dodecyldimethylammonium bromide in aqueous NaBr solutions,and the effects of counterion binding on the micelle size,shape and structure

Micelle size and shape of dodecyldimethylammonium bromide have been determined by measurement of light scattering from its aqueous NaBr solutions. In water and in the presence of NaBr up to 0.07 M, the Debye plots give straight lines with positive slopes, and spherical micelles having molecular weight less than 30 000 are formed. At higher NaBr concentrations, the Debye plots decrease with increasing micelle concentration, indicating the aggregation of the primary spherical micelles into larger secondary micelles. The molecular weight and the radius of gyration of the secondary micelles increase with increasing NaBr concentration, and the relation between molecular weight and radius of gyration suggests that they are rodlike and flexible. Linear logarithmic relations between micelle molecular weight and ionic strength hold for spherical and rodlike micelles, respectively, and the threshold concentration of NaBr for the sphere-rod transition is located at 0.07 M. The spherical micelle of dodecyldimethylammonium ions has a size more than 20 surfactant ions larger in NaBr solutions than in NaCl solutions, and their rodlike micelle has a shorter length in NaBr solutions than in NaCl solutions, when compared at an identical aggregation number, indicating 2 more surfactant ions in its cross-section.     

Dynamics of spontaneous vesicle formation in fluorocarbon and hydrocarbon surfactant mixtures

The spontaneous self-assembly of unilamellar vesicles was investigated by means of time-resolved synchrotron small-angle X-ray scattering. The self-assembly process was initiated by rapid mixing of anionic surfactant micelles with either zwitterionic or cationic surfactant micelles in equimolar ratio using a stopped-flow device. For the zwitteranionic systems, transient disklike mixed micelles are observed as structural intermediates prior to the onset of vesiculation. These disklike micelles display an exponential growth law, and above a critical size they close to form unilamellar vesicles. In the catanionic system, the earliest observable structures within the mixing time of 4 ms are unilamellar vesicles. Nevertheless, in both systems a narrow distribution of the vesicle size was observed at the initial stages of their formation. The subsequent evolution of the vesicle size distribution depends on the subtle differences in the bilayer composition and properties.     

Nonstoichiometric polymer-surfactant complexes obtained in a lamellar lyotropic medium

The addition of a polyelectrolyte to lamellar media formed by an oppositely charged surfactant often leads to the coexistence of several phases without macroscopic phase separation, which makes their characterization difficult. Here, the effect of the polydiallyldimethylammonium chloride (PD) on the lamellar liquid crystal formed by the anionic surfactant Aerosol OT (AOT) and water is investigated. Small-angle X-ray scattering results are discussed regarding the changes in the lamellar spacing as a function on the PD or AOT concentrations. In most of the samples, two lamellar phases, without macroscopic phase separation, are detected. One of them is a typical swollen phase, while the other is a collapsed phase, which corresponds to the polymer-surfactant complex. At concentrations of polymer up to 3?wt%, the two lamellar phases coexist; however, at a critical concentration higher than 3?wt%, the swollen phase becomes isotropic, and a macroscopic phase separation takes place. A simple model is proposed to calculate the composition of the phases when macroscopic phase separation does not occur. The results thus calculated show that generally the polymer-surfactant complexes are nonstoichiometric containing a lesser amount of polymer than ideally expected.     

Molecular dynamics study of micellization thermodynamics in AOT/hexane system

The thermodynamics of reverse micelle formation from an ionic surfactant, sodium bis(2-ethylhexyl) sulfosuccinate (Aerosol OT, AOT), in hexane is studied by molecular dynamics simulation. A change in the Gibbs free energy upon the addition of one AOT molecule to a reverse micelle is calculated as depending on aggregation number N by the thermodynamic integration method. This dependence has a minimum at N ≈ 20 and maximum at N ≈ 35 and predetermines the monotonically decreasing character of the standard chemical potential of AOT in a micelle with the increase of the aggregation number. The simulation results predict the formation of reverse AOT micelles with an average aggregation number of ≈30, which is in good agreement with experimental data.     

Unusual formation of small aggregates by mixing giant multilamellar vesicles

The phase behavior and structure of aggregates in a hydrophobic block copolymer (L121)/double-tailed surfactant (AOT)/water system have been studied by phase study, fluorescence spectrometry, dynamic light scattering, transmission electron microscopy, small angle X-ray scattering (SAXS) and conductivity measurements. An isotropic, one-phase region is found between two biphasic regions containing large vesicles, namely, transparent samples are formed by mixing two turbid solutions. Depending on the AOT/L121 ratio, the isotropic region can be quite stable against temperature. The phase transition between the two regions can be detected by the used techniques, and structural transitions in the aggregates are inferred. The experimental evidence indicates that mixed aggregates are formed at very low concentrations, much lower than the critical micellar concentration of AOT. These micelle-like aggregates contain a mixed hydrophobic core, are small (2-4 nm), and seem to be quasi-spherical, which is an unexpected result since the packing parameters of the single amphiphiles do not favor such small quasi-spherical shapes. This behavior might have interesting implications in the release of substances from vesicles when their structure is disrupted.