AOT Micelles Research Articles

\u03b1-Tocopherol/AOT/alkane/water system

In this paper the calorimetric studies on α-tocopherol/AOT/alkane/water systems are presented. Structural representations of surfactant AOT (a) and α-tocopherol (b) are shown below. α-Tocopherol (antioxidant) is a form of vitamin E that is preferentially absorbed and accumulated in humans. AOT is an anionic type of surfactant. Its molecule contains two non-polar hydrocarbon tails connected to the polar head. The paper contains new experimental data of heat of mixing (Q) for heterogeneous system α-tocopherol/AOT/n-alkane with and without water measured at 309.2 K. The heat of mixing for AOT (sodium bis (2-ethylhexyl) sulfosuccinate) and α-tocopherol solutions in n-alkane (heptane, decane, hexadecane) as the function of surfactant concentration and parameter R given as the water to surfactant molar ratio R = [H2O]/[AOT] was measured. It can be seen that the Q values grow significantly with temperature increase which indicates on entropic origin of this process. The significant influence of surfactant concentration on Q values was noticed. Unexpectedly, the water presence in the reverse micelles did not have any noticeable influence on the heat of mixing values measured at 309.2 K. There was not noticed the significant hydrocarbon chain length influence on the thermal effect of mixing α-tocopherol and AOT solutions. The thermodynamic parameters: the binding constant (K) and the molar enthalpy of transition (ΔHtr0) of α-tocopherol between solvent and AOT reverse micelles, were calculated from the heat of mixing data as a function of surfactant concentration using the nonlinear regression method. The new calorimetric data measured at 309.2 K were compared to results obtained earlier at 298.2 K, as well as have been discussed in the context of the hydrocarbon chain length and water presence influence on α-tocopherol solubility in AOT reverse micelles. Generally in all investigated systems have been not found a significant influence of water concentration in AOT reverse micelles on α-tocopherol molecules distribution between organic phase and micellar phases. The exothermic values of α-tocopherol molecules transition from organic to micellar phase indicate on spontaneous motion of α-tocopherol towards AOT aggregates. For the systems with n-heptane and n-decane as a solvent, the (ΔHtr0) values are between −51 and −52 kJ mol−1 whereas for n-hexadecane around −43 kJ mol−1. Such tendency can be explained through hydrocarbon chains orientation effect existing in hexadecane which probably is responsible for a α-tocopherol molecules freedom motion limitation. Moreover, the solubility of the α-tocopherol in AOT reverse micelles measured with calorimetric technique has been compared to the literature data obtained, respectively, with UV spectrophotometer for reverse micelles, and for phospholipids bilayer by other techniques. Finally, transferring of α-tocopherol from solvent to AOT reverse micelles, intermolecular interactions between α-tocopherol and AOT micelles were discussed, and a privileged place of α-tocopherol in the palisade layer of AOT reverse micelle has been deduced.

Effect of chain length of oil on location of dye within AOT nanometer-sized droplet microemulsions at constant water content

In this work, effect of chain length of oil from C-10 (n-decane) to C-5(n-pentane) as continuous phase of AOT microemulsion on the droplet size and location of hydrophilic dye of Rhodamine B (RhB) in the water/AOT/oil microemulsions was studied at the water-to-AOT molar ratio of 40 by means of dynamic light scattering (DLS) and fluorescence spectroscopy. The results of dynamical study of AOT micelles by using DLS technique showed that by increasing the chain length of oil in the AOT microemulsion, the droplet size increased and the interactions between nano-droplets were more attractive as a function of the mass fraction of nano-droplets (MFD). It was also observed that hydrophilic dye of Rhodamine B can be located in the continuous phase of microemulsion because of the surfactant-dye electrostatic interactions and the number of formed AOT-RhB ion pair complex in the continuous phase of AOT microemulsion increased as the chain length of oil decreased.

Optical and electrical properties of inverted emulsions based on sodium bis(2-ethylhexyl) sulfosuccinate containing cadmium sulfide particles

The optical and electrical properties of inverted emulsions based on sodium bis(2-ethylhexyl) sulfosuccinate (AOT) containing nanoparticles of cadmium sulfide are studied. The particle size of the synthesized samples is determined from UV spectroscopy data. The state of solubilized water in invert emulsions is found to change in the presence of cadmium sulfide due to the formation of aqua complexes. The shape of AOT micelles and the structures formed by drying the invert AOT emulsions are shown to be affected by the degree of hydration and the solubilization of CdS particles.

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.

Synthesis, spectroscopic properties and photodynamic activity of two cationic BODIPY derivatives with application in the photoinactivation of microorganisms

Two cationic BODIPYs 3 and 4 were synthesized by acid-catalyzed condensation of the corresponding pyrrole and benzaldehyde, followed by complexation with boron and methylation. Compound 3 contains methyl at the 1,3,5 and 7 positions of the s-indacene ring and a N,N,N-trimethylamino group attached to the phenylene unit, while 4 is not substituted by methyl groups and the cationic group is bound by an aliphatic spacer. UV-visible absorption spectra of these BODIPYs show an intense band at ∼500 nm in solvents of different polarities and n-heptane/sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/water reverse micelles. Compound 3 exhibits a higher fluorescence quantum yield (ΦF = 0.29) than 4 (ΦF = 0.030) in N,N-dimethylformamide (DMF) due to sterically hindered rotation of the phenylene ring. BODIPYs 3 and 4 induce photosensitized oxidation of 1,3-diphenylisobenzofuran (DPBF) with yields of singlet molecular oxygen of 0.07 and 0.03, respectively. However, the photodynamic activity increases in a microheterogenic medium formed by AOT micelles. Also, both BODIPYs sensitize the photodecomposition of l-tryptophan (Trp). In presence of diazabicyclo[2.2.2]octane (DABCO) or D-mannitol, a reduction in the photooxidation of Trp was found, indicating a contribution of type I photoprocess. Moreover, the addition of KI produces fluorescence quenching of BODIPYs and reduces the photooxidation of DPBF. In contrast, this inorganic salt increases the photoinduced decomposition of Trp, possibly due to the formation of reactive iodine species. The effect of KI was also observed in the potentiation of the photoinactivation of microorganisms. Therefore, the presence of KI could increase the decomposition of biomolecules induced by these BODIPYs in a biological media, leading to a higher cell photoinactivation.

Dynamic and spectroscopic studies of nano-micelles comprising dye in water/ dioctyl sodium sulfosuccinate /decane droplet microemulsion at constant water content

In the present work, the dynamic and spectroscopic properties of water-in-decane dioctyl sodium sulfosuccinate (AOT) microemulsions comprising dye, Rhodamine B (RB), were studied by varying content of decane at the constant water content (W = 20), by using dynamic light scattering (DLS), UV/visible, and fluorescence techniques. The characterization results of DLS of AOT micelles showed that by decreasing concentration of Rhodamine B in the water/AOT/decane microemulsion, the inter-droplet interactions changed from attractive to repulsive as the mass fraction of nano-droplets (MFD) increased. A deviation in the absorption spectra of Rhodamine B from the Beer's law at the high Rhodamine B concentration (0.001) was observed in the AOT reversed micelles. The Quenching in the emission intensity of AOT droplets comprising Rhodamine B and red shift in λmax of fluorescence of dye was observed as a function of concentration of RB in AOT RMs. The Stokes shift of AOT droplets containing the high concentration of RB, increased with mass fraction of nano-droplet (MFD), whereas at the low Rhodamine B concentration, its variation remained constant up to MFD = 0.07, and then increased.

AOT Micelles/Vesicles for Synthesis of Silver Nanoparticles and Micellar Transitions Affected by Nanoparticles

A facile method for the synthesis of silver nanoparticles (Ag NPs) is reported using sodium para-toluene sulphonate (NaPTS) as the reducing agent and AOT micelles and vesicles as capping agents, leading to the development of a system comprising of AOT micelles/vesicles along with Ag NPs. A single component, Na-PTS, has been used to tune the micellar aggregates as well as for the synthesis of Ag NPs. The aggregation behavior of AOT in aqueous Na-PTS has been studied by the method of surface tension and the changes in the morphology of AOT micelles by small angle neutron scattering (SANS) measurements. The synthesis of Ag NPs have also been carried out using common reducing agents such as sodium borohydride and ascorbic acid. SANS measurements have also been performed in the ternary systems of micelles, vesicles and NPs and reveal the formation of fractal structures of the NPs. The ternary systems have been also tested for their antibacterial activity. A very important observation made in the present study is the large decrease in the concentration of salt required to bring about micellar shape transition in the presence of small amounts of NPs.

Flavonoid-surfactant interactions: A detailed physicochemical study

The aim of this article is to study the interactions between flavonoids and surfactants with attention of finding the probable location of flavonoids in micellar media that can be used for controlling their antioxidant behavior. In present study, the micellar and interfacial behavior of twin tailed anionic surfactants viz. sodium bis(2-ethylhexyl)sulfosuccinate (AOT) and sodium bis(2-ethylhexyl)phosphate (NaDEHP) in the presence of two flavonoids, namely quercetin (QUE) and kaempferol (KFL) have been studied by surface tension measurements. UV–visible, fluorescence and differential pulse voltammetric (DPV) measurements have been employed to predict the probable location of flavonoids (QUE/KFL) within surfactant (AOT/NaDEHP) aggregates. Dynamic light scattering (DLS) measurements further confirmed the solubilization of QUE/KFL in AOT/NaDEHP aggregates deduced from increased hydrodynamic diameter (Dh) of aggregates in the presence of flavonoids. Both radical scavenging activity (RSA) and degradation rate constant (k) of flavonoids are found to be higher in NaDEHP micelles as compared to AOT micelles.

Effect of polymer conformation on polymer-surfactant interaction in salt-free water

The aim of the present paper was to study the interaction between polyvinylpyrrolidone (PVP) and sodium bis(2-ethylhexyl) sulfoccinate (AOT), as an anionic surfactant, over a temperature range of 25–60 °C by viscosity and electrical conductivity measurements. A coil-to-globule transition of PVP in water was observed. The critical micellar concentration (CMC) was determined by conductivity at 25 and 50 °C. The formation of the complex PVP-AOT in water was studied by conductivity and viscometry at 25 and 50 °C, where the polymer chain adopts respectively coil and globule conformations, and the obtained curves show two break points corresponding to the critical aggregation concentration (CAC) and the polymer saturation point (PSP). The viscometric behavior of PVP-AOT system was studied by using three selected AOT concentrations: CAOT,1, CAOT,2, and CAOT,3 with CAOT,1 < CAC < CAOT,2 < PSP < CAOT,3. For CAOT,1, the system behaves as a neutral polymer. A pseudo-polyelectrolyte behavior was observed for the surfactant concentration (CAOT,2). Above the PSP, and for the CAOT,3 concentration, a screening effect appears due to the increase of the free AOT micelles concentrations. In presence of surfactants, the polymer chains swell especially when the PVP is in globular state.

Probing Nanoscale Thermal Transport in Surfactant Solutions.

Surfactant solutions typically feature tunable nanoscale, internal structures. Although rarely utilized, they can be a powerful platform for probing thermal transport in nanoscale domains and across interfaces with nanometer-size radius. Here, we examine the structure and thermal transport in solution of AOT (Dioctyl sodium sulfosuccinate) in n-octane liquids using small-angle neutron scattering, thermal conductivity measurements, and molecular dynamics simulations. We report the first experimental observation of a minimum thermal conductivity occurring at the critical micelle concentration (CMC): the thermal conductivity of the surfactant solution decreases as AOT is added till the onset of micellization but increases as more AOT is added. The decrease of thermal conductivity with AOT loading in solutions in which AOT molecules are dispersed as monomers suggests that even the interfaces between individual oleophobic headgroup of AOT molecules and their surrounding non-polar octane molecules can hinder heat transfer. The increase of thermal conductivity with AOT loading after the onset of micellization indicates that the thermal transport in the core of AOT micelles and across the surfactant-oil interfaces, both of which span only a few nanometers, are efficient.

Influence of oscillatory centrifugal forces on the mechanism of Turing pattern formation.

Constantly acting centrifugal forces on Turing pattern forming systems have been observed to induce orientation and wavelength changes on Turing structures. Here, we will consider a periodic modulation of such centrifugal forces and their effects on pattern formation. Depending on the oscillation period the system exhibits a wide variety of stationary (stripes, H(0), etc.) or nonstationary patterns (black eyes, etc.), as well as transitions and instabilities such as Eckhaus, zigzag, etc. In this paper, a detailed description of the different patterns and patterning mechanisms will be described and understood within the previous context. The system considered is the Belousov-Zhabotinsky reaction encapsulated in AOT micelles modeled by the adapted version of the Oregonator model.

Cross-diffusion-induced convective patterns in microemulsion systems.

Cross-diffusion phenomena are experimentally shown to be able to induce convective fingering around an initially stable stratification of two microemulsions with different compositions. Upon diffusion of a salt that entrains water and AOT micelles by cross-diffusion, the miscible interface deforms into fingers following the build-up of a non-monotonic density profile in the gravitational field. A diffusion model incorporating cross-diffusion effects provides an explanation for the mechanism of the buoyancy-driven hydrodynamic instability and for the properties of the convective fingers.

The size, charge, and electrokinetic potential of gold nanoparticles in solutions of sodium bis(2-ethylhexyl) sulfosuccinate in n-decane-chloroform mixtures

The hydrodynamic diameters and electrokinetic potentials of gold nanoparticles in n-decane, chloroform, and mixtures thereof have been studied as functions of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) concentration. At AOT concentrations in n-decane below 2 mM, gold nanoparticles are covered with AOT monolayer and do not reveal electrophoretic mobility. At a concentration of 0.3 M that, according to the data of photon-correlation spectroscopy, corresponds to the onset of adsorption of AOT micelles on the nanoparticle surface, the ζ-potential reaches a maximum value of 190 mV; then, it decreases with increasing surfactant concentration. A rise in the volume fraction of chloroform in decane-chloroform mixtures drastically increases the ζ-potential even at low AOT concentrations. The obtained results have been analyzed in terms of the Debye-Huckel theory.

Modeling the Self-Aggregation of Small AOT Reverse Micelles from First-Principles.

This Letter is the first attempt at studying the self-aggregation of AOT reverse micelles from first-principles. It focuses on predicting the aggregation number, the radius of gyration, and the hydrodynamic radius of a low water content reverse micelle by theoretical means. We show that molecular dynamics simulation in the μs time range combined with atomistic potentials is capable of reproducing and explaining, to a convenient degree, experimental results on the size and dimensions of reverse micelles of AOT of low water content, [H2O]/[AOT] ≈ 5.

Crystallization of confined water pools with radii greater than 1 nm in AOT reverse micelles.

Freezing of water pools inside aerosol sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles has been investigated. Previous freezing experiments suffer from collision and fusion of AOT micelles and resultant loss of water from the water pool by shedding out during the cooling process. These phenomena have restricted the formation of ice to only when the radius of the water pool (Rw) is below 1 nm, and only amorphous ice has been observed. To overcome the size limitation, a combination of rapid cooling and a custom-made cell allowing thin sample loading is applied for instantaneous and homogeneous freezing. The freezing process is monitored with attenuated total reflection infrared spectroscopy (ATR-IR) measurements. A cooling rate of ca. -100 K/min and a sample thickness of ca. 50 μm overcomes the limitations mentioned above and allows the crystallization of water pools with larger radii (Rw > 1 nm). The corresponding ATR-IR spectra of the frozen water pools with Rw < 2.0 nm show similar features to the spectrum of metastable cubic ice (Ic). Further increase of the radius of the water pool (Rw > 2.0 nm), unfortunately, drastically decreased the integrated area of the ν(OH) band observed just after freezing, indicating the breakup of the micellar structure and shedding out of the water pool. In addition, it was revealed that Ic ice can also be formed in flexible organic self-assembled AOT reverse micelles for at least Rw ≤ ca. 2 nm, as well as in inorganic and solid materials with a pore radius of ca. 2 nm. The dependence of the phase transition temperature on the curvature of the reverse micelles is discussed from the viewpoint of the Gibbs-Thomson effect.

Spectrometric study on the binding of curcumin with AOT: Effect of micelle-to-vesicle transition

In this work, the role of micelle-to-vesicle transition of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in the binding, stability and antioxidant activity of curcumin has been studied using absorption and fluorescence measurements. As AOT molecules aggregate into micelles and vesicles, curcumin bound with AOT often gives higher intensities of absorption and fluorescence than that of free curcumin. The most enhanced absorption and fluorescence of curcumin induced by AOT vesicles, are attributed to the location of curcumin in their lipid bilayer. The measurement of curcumin anisotropy suggests that the bilayer of AOT vesicles provides curcumin with a more hydrophobic microenvironment than the palisade layer of AOT micelles. The binding constant (Kb) of curcumin with AOT vesicles is three times that of curcumin with AOT micelles. Moreover, AOT vesicles are found to be superior to AOT micelles for enhancing the stability and radical scavenging ability of curcumin.

Investigation of influence of nonionic additives on structural changes of water droplets encapsulated in AOT reverse micelles by instrumental methods

The microstructure of sodium bis(2-ethylhexyl) sulfosuccinate reverse micelles was investigated using infrared, nuclear magnetic resonance and ultraviolet–visible spectroscopic methods, also by water-in-oil microemulsion chromatography. The influence of structure-making and structure-breaking nonionic additives on the microenvironment in reverse micelles was studied. The values of chemical shift in the presence of chaotropic urea are different than their values in case of kosmotropic glucose additives according to proton magnetic resonance spectroscopic data. Urea promotes an increasing of free water fraction in the water pockets of the reverse microemulsions, but glucose supports formation of the trapped water on the basis of deconvolution of the OH stretching vibrational absorption spectra in the region of 3000–3800cm−1 into three subpeaks with a Monte Carlo method. The different influence of kosmotropic and chaotropic nonionic additives on both binding constant Kb of o-nitroaniline to the micelles of AOT and retention of the model compounds were also revealed by UV–visible spectroscopic and water-in-oil microemulsion liquid chromatography methods respectively.

β-Cyclodextrin improves the linearity of polyaniline synthesized enzymatically in AOT micellar solution

Abstract The influence of β-cyclodextrin (β-CD) on the laccase-catalyzed aniline polymerization in sodium bis-(2-ethylhexyl)sulfosuccinate (AOT) micellar solution is first presented. Results show that the anionic AOT micelle is compatible with laccase and β-CD also has little negative effect on the expression of the catalytic activity of laccase. β-CD can improve the linearity or conductivity of polyaniline biosynthesized in AOT micellar solution. Component analysis shows that AOT is doped in polyaniline chain while β-CD does not thread on the polyaniline chain. Mechanism analysis indicates that the improvement of the linearity of polyaniline by β-CD is due to steric hindrance close to the ortho-position of aniline, which favors the para-coupled aniline polymerization.

Effect of ethylene glycol on the special counterion binding and microstructures of sodium dioctylsulfosuccinate micelles

Sodium dioctylsulfosuccinate (AOT) micelle has a special counterion binding behavior (SCB), which refers to the abrupt twofold increase in the counterion binding constant (β) at a critical concentration (c*) of added NaCl (in water c*≈0.015molkg−1). In this paper, the SCB of AOT has been studied in a mixture of water and ethylene glycol (EG) by applying surface tension, fluorescence, and small angle neutron scattering (SANS) methods. The SCB exists in water+10% (w/w) EG as well, but disappears when the EG% is⩾20. It has been found out that the SCB of AOT occurs in media having cohesive energy density values in the range of 2.3–2.75Jm−3. SANS data indicate co-existence of vesicles and cylindrical micelles of AOT in water+10% EG when the added NaCl concentration is greater than c* thereby revealing that change in the morphology of aggregated species is the probable cause for the SCB of AOT. From this study it has become clear that the Corrin–Harkins (CH) equation, commonly used for determining β, can be applied only above a limiting concentration ce# of added electrolyte. In aqueous organic or pure organic polar solvents below ce# sharp deviation from the CH equation occurs with reversal of slope rendering this equation inapplicable for the determination of β.

Photophysical properties of pyronin dyes in reverse micelles of AOT

The photophysical properties of pyronin B (PyB) and pyronin Y (PyY) in reverse micelles formed with water/sodium bis (2-ethyl-1-hexyl) sulfosuccinate (AOT)/n-heptane were investigated by UV–vis absorption, steady-state and time-resolved fluorescence spectroscopy techniques. This study was carried out a wide range of reverse micelle sizes, with hydrodynamic radii ranging from 1.85 to 9.38nm. Significant photophysical parameters as band shifts, fluorescence quantum yields and fluorescence lifetimes were determined to understand how photophysical and spectroscopic features of the dye compounds were affected by the variation of reverse micelle sizes. In this regard, control of reverse micelle size by changing W0, the molar ratio of water to surfactant, allowed tuning the photophysical properties of the dyes in organic solvent via reverse micelle. Non-fluorescent H-aggregates of pyronin dyes were observed for the smaller reverse micelles whereas an increase in the reverse micelle size induced an increment in the amount of dye monomers instead of dye aggregates. Thus, the fluorescence intensities of the dyes were improved by increasing W0 due to the predomination of the fluorescent dye monomers. As a result, the fluorescence quantum yields also increased. The fluorescence lifetimes of the dyes in the reverse micelles were determined by the time-resolved fluorescence decay studies. Evaluation of the fluorescence lifetimes calculated for pyronin dyes in the reverse micelles showed that the size of reverse micelle affected the fluorescence lifetimes of pyronin dyes.