AOT Concentrations Research Articles

Study of self-organization and structural phase transitions in water /AOT / isopropyl myristate biocompatible system

This work reports preparation and characterization of biocompatible sodium bis(2-ethylhexyl) sulfosuccinate (AOT) / water/ isopropyl myristate (IPM) systems. We studied their ability to form various self-organized structures such as microemulsions and liquid crystal templates for producing nanoparticles such as drug delivery vehicles, which play an important role in delivery of various substances. We determined concentration and temperature conditions that allow for forming various structures in this system and characterized the structural phase transition from the microemulsion to liquid crystalline phase, which is important for delivery of bioactive compounds. The parameters of the liquid crystalline phase of the water/AOT/isopropyl myristate system were calculated for the first time in this work. The sizes of microemulsion droplets were determined. To describe molecular motions in the studied microemulsion, self-diffusion coefficients of each component were measured. The values of relative diffusion coefficients demonstrated that reverse microemulsions were observed at AOT concentrations up to 60 % wt. Further increase in surfactant concentration slows down their molecular mobility and results in formation of aggregates with a bimodal size distribution and a subsequent formation of the hexagonal liquid crystalline phase. Geometry of the mesophase and its structure were investigated by X-ray diffraction. The respective phase diagram was plotted and analyzed using the data of polarized light microscopy, dynamic light scattering, and NMR self-diffusion. In this diagram, the areas corresponding to the microemulsion, the liquid crystal phase, and the structural phase transition were identified.The study of the structural phase transition from the microemulsion to liquid crystal will offer a predictive power for characterizing incorporation of drugs and controlling their delivery to target cells. A deeper understanding of the structural changes is necessary to clearly determine the conditions of drug delivery.

X-ray study of structural changes in “di-(2-ethylhexyl) sulfosuccinic acid (AOT) /n-heptane-water” liquid crystalline system depending on the concentration of components

Structural changes in the “sodium salt of di-(2-ethylhexyl)sulfosuccinic acid (AOT) - n-heptane-water” liquid-crystalline system depending on the AOT concentration in the n-heptane-water mixed solvent were studied using the X-ray diffraction method. The studies were carried out in a wide range of the AOT concentrations from 27 to 92% in a mixed solvent n-heptane/water=5.8 ratio.The mesomorphism of a liquid-crystalline system is on the concentration of the system. It is confirmed that at concentrations the AOT above 50%, a liquid crystal structure with an intradomain "flat" liquid crystal layered phase takes place. A “flat” lamellar lyotropic-liquid-crystalline phase with an intradomain lamellar structure is formed in the system. The structural parameters, the compactness of the liquid crystal system, the thickness of the formed layers, the surface area per molecular head on the surface of the layers depending on the concentration and thickness of the bound water layer are revealed. The liquid crystal system under study can serve as a model for studying various problems of biological membranes.

Aqueous solutions of AOT as a dispersion medium for stabilization of SiO2 nanoparticles

SiO2 nanoparticles and surfactant sodium bis-(2-ethylhexyl) sulfosuccinate (AOT) are widely used in modern nanochemistry to obtain novel materials in various processes. However, only a few publications are devoted to investigation of the stabilization mechanisms of SiO2 dispersions in aqueous AOT solutions. In this work, the surface electrical properties of SiO2 nanoparticles and the nature of free charges in aqueous solutions were studied over a wide range of AOT concentrations (2.5∙10–6 ÷ 2.5∙10–2 M). The absolute value of ζ-potential of the particles increased from –42 to –58 mV, while the hydrodynamic radius remained constant and equal to ca. 40 nm. Calculations within the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory revealed that stability of hydrosols is caused by the electrostatic component of disjoining pressure. Due to the repulsion of similarly charged nanoparticles and glass substrate, the hydrosols formed coffee rings during evaporation of sessile droplets. The results obtained make it possible to control surface properties of the particles, thus opening the possibility to use SiO2-AOT hydrosols for the formation of various 2D materials and substrates in photonics, catalysis and biosensorics.

Influence of amphiphilic molecules on the peroxidase-like behavior of nanoparticles in an aqueous solution.

Carbon dots (CDs) have drawn considerable attention in recent decades due to their outstanding biocompatibility and environmental friendliness. In this study, we synthesized ionic liquid (1-aminopropyl-3-methyl-imidazolium bromide)-modified carbon dots (IL-CDs) showing good peroxidase-like activity. Furthermore, we investigated their enzymic behavior in the presence of two different amphiphilic molecules, namely tert-butanol (TBA, a typical hydrotrope) and sodium bis (2-ethylhexyl) sulfosuccinate (AOT, a typical anionic surfactant). In an aqueous solution of TBA, a microscopic heterogeneous structure was formed in a certain concentration range of TBA, which resulted in an anomaly in the reaction process. However, in the AOT aqueous solution, the situation became more complicated. IL-CDs formed vesicles or precipitation at different concentrations of AOT, which led to different enzymic activities of IL-CDs due to the variance in the structure and the surface electronic density.

Spontaneous Self-Assembly of Thermoresponsive Vesicles Using a Zwitterionic and an Anionic Surfactant.

Spontaneous formation of vesicles from the self-assembly of two specific surfactants, one zwitterionic (oleyl amidopropyl betaine, OAPB) and the other anionic (Aerosol-OT, AOT), is explored in water using small-angle scattering techniques. Two factors were found to be critical in the formation of vesicles: surfactant ratio, as AOT concentrations less than equimolar with OAPB result in cylindrical micelles or mixtures of micellar structures, and salt concentration, whereby increasing the amount of NaCl promotes vesicle formation by reducing headgroup repulsions. Small-angle neutron scattering measurements reveal that the vesicles are approximately 30-40 nm in diameter, depending on sample composition. Small-angle X-ray scattering measurements suggest preferential partitioning of OAPB molecules on the vesicle inner layer to support vesicular packing. Heating the vesicles to physiological temperature (37 °C) causes them to collapse into smaller ellipsoidal micelles (2-3 nm), with higher salt concentrations (≥10 mM) inhibiting this transition. These aggregates could serve as responsive carriers for loading or unloading of aqueous cargoes such as drugs and pharmaceuticals, with temperature changes serving as a simple release/uptake mechanism.

Interaction of thymidine with sodium bis(2-ethylhexyl) sulfosuccinate in alcohol and water: Studies with UV-Vis technique

The accurate measurement of thymidine absorbance in the presence of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in ethanol and water systems at room temperature by UV-vis technique has been reported. The AOT concentrations varied from 0.0005 to 0.014 [mol/L] in ethanol whereas from 0.0002 to 0.042 [mol/L] in water. The concentration of thymidine was 0.0073 [mol/L] during UV-vis spectrum registration. The thymidine spectrum was examined in the presence of AOT concentration. Using a statistical method of non-linear regression (NLREG), thymidine binding and thymidine distribution constants were evaluated. A noticeable decrease in thymidine absorbance in the presence of AOT concentration was shown in the water system; whereas, in the ethanol system, just the opposite trend has been found. The results are discussed in the context of binding and distribution constants in the studied systems.

Diesel Emulsion Fuels with Ultralong Stability

Water-in-diesel emulsions have garnered attention in recent years as a potential route to mitigate vehicular pollution. In this work, the formation, flow behavior, and microstructure of water-in-diesel microemulsions using an anionic surfactant, sodium bis(2-ethylhexyl) sulfosuccinate (AOT), is investigated and compared with the water–decane–AOT microemulsion system. Rheological measurements on the microemulsions indicate Newtonian behavior over a moderate range of shear rate. The viscosity of the microemulsions shows a weak dependence on the molar ratio of water-to-surfactant (ω) but increases significantly with the increase in the dispersed phase volume fraction (Φ). Dynamic light scattering (DLS) investigation of the microemulsions indicates a single exponential decay of the intensity autocorrelation functions over a wide range of water and AOT concentrations, indicating that the dispersed phase consists of a monodispersed population of water droplets. Results suggest that optimizing ω can regulate the...

Size-tunable silica nanowires prepared using reverse microemulsion-gel systems

Abstract Self-assembled hybrid gels were formed by adding amounts of tetraethyl orthosilicate (TEOS) dissolved in water to organic solutions of sodium bis(2-ethylhexyl) sulfosuccinate (Aerosol-OT, AOT). Two kinds of hybrid gels were prepared and discussed in this study. In the first, gels were prepared through chemical hydrolysis-condensation reactions of TEOS. The growth of a one-dimensional silica network in the confinement of the AOT reverse microemulsions led to gel formation. In the other, gels were prepared through reverse hexagonal mesophases due to high concentrations of AOT. Rheological studies demonstrated that the elastic modulus, G', is greater than the viscous modulus, G, at all frequencies, and G' did not depend upon the frequency in either system, implying they can be used as gels. The gels obtained through silica networks had smaller G' values and higher transformation strains than those gels prepared through AOT reverse hexagonal mesophases. Small-angle X-ray scattering (SAXS) results showed that both systems exhibited cylindrical structures with short-range order, and the radii of the cylinders were significantly influenced by the change of AOT, TEOS and water concentrations. Transmission electron microscopy (TEM) confirmed that silica nanowires were found in both gel systems after removing the AOT components. The length of the nanowires for the gels prepared through silica networks was larger than that for the gels prepared through AOT reverse hexagonal mesophases. This was because the silica grew into a one-dimensional network (axial direction) due to the confinement of the cylindrical structures.

Influence of Conventional Surfactants on the Self-Assembly of a Bola Type Amphiphilic Peptide.

Structural and morphological regulation is a distinctly important topic in peptide self-assembly, and is also regarded as the fundamental point in peptide-based biomaterials development. In this paper, we showed that adding anionic surfactant SDS to a bola amphiphilic peptide KI4K could result in the reconstruction of β-sheet secondary structure besides the changes in self-assembly morphologies from nanotubes to helical ribbons, nanofibers, or straight nanotapes according to the negatively stained transmission electron microscopy, atomic force microscopy, circular dichroism spectroscopy, and Fourier transform infrared spectroscopy results. The inducing effect of SDS was observed at both above and below its CMC but with different transformation rates. Through comparison to other surfactants, including CTAB, C12EO4, and AOT, we proposed that the transitions of KI4K self-assemblies induced by anionic surfactants could be mainly attributed to the effect of hydrophobic interaction and electrostatic attraction between surfactants and peptide molecules. Rheological property measurement and dye adsorption experiments were also carried out to evaluate the properties of hydrogels formed by the peptide/surfactant hybrids. The samples formed self-supporting hydrogels at proper SDS or AOT concentrations, and the charges of hydrogel could be regulated by peptide to surfactant ratio.

Exploring the effect of formulation parameters on the particle size of carboxymethyl chitosan nanoparticles prepared via reverse micellar crosslinking

Carboxymethyl chitosan (CMCh), as a non-toxic, biocompatible and biodegradable semi-synthetic biopolymer with mucoadhesive properties, is widely investigated for targeting drug or gene delivery applications. Here, fabrication of CMCh nanoparticles will be reported using a reverse (w/o) micellar system to provide particles with tuneable mean sizes. Reverse microemulsions were prepared by dispersing aqueous solution of CMCh in n-hexane using sodium-bis(ethylhexyl)-sulfosuccinate (AOT) and glutaraldehyde (GA) as an emulsifier and a crosslinking agent, respectively. The obtained particles were perfectly spherical in the nanometric size range (40–140 nm) with a narrow size distribution according to the FE-SEM images. To determine the effective parameters on the mean nanosphere size, CMCh, AOT and GA concentrations were varied according to a full-factorial, three-level design-of-experiments. After evaluating the results, it was found that AOT and CMCh concentrations had a significant effect on the mean nanosphere size while GA concentration surprisingly showed non-significant, minor effects on the same response.

Preparation of Nanowire Silica Inside Self-Assembled Sodium Bis(2-ethylhexyl) Sulfosuccinate (AOT) Gels.

In conventional sol-gel methods, gel formation occurs due to aggregation of particles into irregular shapes of larger size. In this study, we conducted hydrolysis-condensation reactions of tetraethyl orthosilicate (TEOS) within water-laden channels inside the space created by self-assembled AOT molecules to prepare regular and nanosized silica in self-assembled sodium bis(2-ethylhexyl) sulfosuccinate (AOT) gels. The AOT gels were obtained by adding small amounts of water to organic solvents containing high concentrations of AOT. Adding silica significantly influenced the rheological properties and microstructures of these AOT/silica gels. Rheological studies showed that the storage modulus G' and loss modulus G″ of the AOT gel systems became very close and even crossed, indicating that the gel is "weak"; however, for the AOT/silica gel systems, the rheological data demonstrated that G' is greater than G″ at all frequencies, indicative of a real gel with a G' of approximately 10(5) pa. Small-angle X-ray scattering (SAXS) results showed that the gels initially had a hexagonal close-packed cylindrical structure with long-range order and transitioned to nonclose-packed cylindrical structures without long-range order as the silica formed. The cylinder is expected to comprise stacks of silica molecules surrounded by AOT molecules, and the radius of the cylinder is close to the sum of the length of one AOT molecule and half the size of one silica molecule. The rheological and SAXS data show that silica in the AOT/silica systems grew in the axial direction due to the confinement of these cylindrical structures, leading to nanowire silica structures. After removal of the AOT components, the nanowire silica was approximately 5-10 nm in diameter, as observed using transmission electron microscopy (TEM).

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.

Study of laccase activity and stability in the presence of ionic and non-ionic surfactants and the bioconversion of indole in laccase-TX-100 system

The aim of this study was to characterize the stability and activity of laccase from Trametes versicolor in the presence of three different surfactants, namely sodium di-2-ethylhexylsulfosuccinate (AOT), Triton X-100 (TX-100), and cetyltrimethylammonium bromide (CTAB). The kinetic parameters (such as Km, kcat, kcat/Km ratio), optimal pH and temperature and the thermostability of the enzyme at different temperatures were determined and compared in the absence and presence of the three surfactants. Results revealed that the catalytic activity of the enzyme was greatly improved in the presence of low concentrations of AOT, whereas the activity declined in the presence of TX-100 and CTAB inactivated it almost completely. Results also depicted that, in general, the presence of the surfactants affected the enzyme optimum pH and temperature. In terms of stability, TX-100-induced stabilization and AOT and CTAB-mediated destabilization of the enzyme were observed. Laccase-mediated bioconversion of indole to 2,2-bis(3′-indolyl)-indoxyl in the presence of TX-100 as the effective stabilizing surfactant and TEMPO as the enzyme mediator was also investigated.

Obtaining electrostatically bound CdS-SiO2 aggregates from electrophoretic concentrates of CdS nanoparticles

Nonaqueous electrophoresis reveals that the electrokinetic potential of CdS nanoparticles increases slightly (85–120 mV) along with the concentration (0–5 × 10−3 M) of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in n-decane, while negatively charged SiO2 particles acquire positive charge (switching from −75 up to +135 mV). The energies of interparticle interactions in CdS-CdS and CdS-SiO2 systems are calculated from these parameters and the literature values of the Hamaker constants according to the Deryaguin-Landau-Verwey-Overbeek (DLVO) theory. It is concluded that the presence of a minimum (2.5 kBT) on the potential dependences of the CdS-SiO2 system indicates the formation of CdS-SiO2 aggregates electrostatically bound by heterocoagulation at low concentrations of AOT. The luminescent properties of the obtained ultrafine CdS-SiO2 powders depend on the CdS content.

A reliable and reproducible method for the lipase assay in an AOT/isooctane reversed micellar system: Modification of the copper-soap colorimetric method

The copper-soap method, which is based on the absorbance of a fatty acid-copper complex at 715nm, is a widely used colorimetric assay to determine the lipase activity in reversed micellar system. However, the absorbance of the bis(2-ethylhexyl) sodium sulfosuccinate (AOT)-copper complex prevents the use of an AOT/isooctane reversed micellar system. An extraction step was added to the original procedure to remove AOT and eliminate interference from the AOT-copper complex. Among the solvents tested, acetonitrile was determined to be the most suitable because it allows for the generation of a reproducible calibration curve with oleic acid that is independent of the AOT concentrations. Based on the validation data, the modified method, which does not experience interference from the AOT-copper complex, could be a useful method with enhanced accuracy and reproducibility for the lipase assay.

Effect of the organic solvent polarity on the structure of the gold nanoparticle adsorption layer as observed by photon-correlation spectroscopy

Spherical gold nanoparticles with the metal core of 5nm in diameter (according to the data of the transmission electron-microscopy) are prepared by the reduction of chloroauric acid with hydrazine monohydrate in reverse micelles of sodium bis-(2-ethylhexyl sulfosuccinate) (AOT). The photon correlation spectroscopy (PCS) method is used to determine the structure of the gold nanoparticle adsorption layer after their concentration and redispersion in AOT solutions in organic solvents with different polarity (in a series of decane, toluene, and chloroform). It is shown that in case of the abovementioned solvents at low AOT concentrations a gold nanoparticle is surrounded by a monolayer consisting of AOT molecules. When the concentration increases up to 1 M, the absorption layer thickness sharply increases; an increase in the organic solvent polarity enhances this increase. An increase in the temperature (from 20°C to 55 °C) leads to a partial and reversible decrease in the absorption layer thickness.

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.

Size and diffusion phenomena of AOT/alcohol/water system in the presence of morin by dynamic light scattering

Presented paper is a continuation of our studies on morin interaction with AOT (sodium bis(2-ethylhexyl) sulfosuccinate) reversed micelles solutions in two solvents: ethanol and n-decanol. Now we focused on morin influence on size and diffusion phenomena in the system morin/solvent/AOT/water. In this paper precise measurements of dynamic light scattering (DLS) of the effects of temperature, solvents (alcohols), water on the size and diffusion of AOT reversed micelles in the morin/AOT/alcohol/water system are reported. The concentrations of AOT were varied from 0.51 to 0.78mol/L. Morin concentration in during auto-correlation function registration was not the same in each solvent because of its different solubility depending on the solvent. Water concentration in the studied systems was defined by R parameter according to relation: R=(H2O)/(AOT) and was equal 0 and 30 in ethanol, and 0 in n-decanol. DLS measurements were done at 298.15 and 308.15K.DLS experiment involved on detection two relaxation modes (fast and slow) in the systems containing AOT reversed micelles, water, morin and solvents (ethanol and n-decanol). The DLS data clearly show the solvent influence as well as morin presence on AOT reversed micelles size and consequently their diffusion coefficients. Contrary to n-decanol strong competition between morin and ethanol molecules in AOT reversed micelles palisade layer has been found. It suggests that morin molecules replaced ethanol in AOT reversed micelles and locate in their palisade layer strongly increasing AOT reversed micelles size. Furthermore, it was found a sharp increase in correlation radii of slow modes of AOT reversed micelles containing morin molecules and their diffusion coefficients diminishing.

Gold nanoparticles tune the activity of laccase in anionic reverse micelles.

The interfacial property of reverse micelles is an important factor affecting the catalytic activity of enzymes hosted in the micelles. In this article, the effect of gold nanoparticles (GNPs) on the catalytic activity of laccase (non-surface-active enzyme) and the related mechanism are reported. It was found that laccase activity was dependent on the size of the particle and its concentration as well as on the water content and the concentration of AOT. It was shown that there existed several types of micelles in the present reverse micellar system in the presence of GNPs. The population of the various micelles depended on the concentrations of both GNPs and AOT. Fluorescence and circular dichroism spectra of laccase at different water contents and GNP concentrations indicated that the conformation of laccase and its activity were tuned by GNPs via changing the structure of the reverse micelles. Analysis showed that changes in the thickness of the water layer (Lw) and in the apparent occupied area of individual AOT molecules (AAOT) caused by GNPs were the main parameters affecting the activity of laccase. The present work extends and deepens the understanding of the tuning mechanism of GNPs on enzymatic performance in reverse micelles and provides guidance for rational design of the optimal microenvironment of laccase.

UV-vis Investigation of β-Carotene in Presence of AOT/n-Heptane, Cyclohexane, Tetrahydrofuran/Water System

The precise measurements of β-carotene absorbance in presence of AOT/solvent/water systems at 25oC by UV-vis technique are reported. The solvents selected were: n-heptane, cyclohexane, and tetrahydrofuran. The concentrations of AOT [Aerosol-OT (sodium bis(2-ethylhexyl) sulfosuccinate)] were varied from 0.01 to 0.2mol/kg.β-carotene concentration in quvette during UV-vis spectrum registration was not the same in each solvents because of different absorption intensity depending on the solvent. Water concentration in the studied systems was defined by R parameter according to relation: R=[H2O]/[AOT] and was equal 0; 5; 10 both in n-heptane and cyclohexane, and 0; 10 and 20 in tetrahydrofuran. Obtained results showed a noticeable decreasing of β-carotene absorbance as a function of AOT concentration. There is evidenced red shift of β-carotene absorbance depending on the solvent in the following order: tetrahydrofuran &gt;cyclohexane&gt;n-heptane. Presented results show that β-carotene absorbance decreased with water concentration in the system. It had been happened because the water molecules located in the core of micelles make its curvature bigger and the hydrocarbon chains forming palisade layer in AOT reversed micelles get more space. As a consequence β-carotene molecules penetrated this layer dipper comparing to systems without water.