Physicochemical investigation of the solubilization of cobalt nitrate in sodium bis(2-ethylhexyl)sulfosuccinate reversed micelles

A wide investigation of the solubilization of the water-soluble salt Co(NO₃)₂ in water/sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/n-heptane microemulsions and of some physicochemical properties of the Co(NO₃)₂/AOT/n-heptane and Co(NO₃)₂/AOT systems has been carried out. After saturation of water/AOT/n-heptane microemulsions with pure Co(NO₃)₂, the Co(NO₃)₂/AOT composites were prepared by complete evaporation under vacuum of the volatile components (water and apolar solvent) of the salt-containing microemulsions. It was observed that these composites could be totally dissolved in pure n-heptane, allowing the solubilization of a noticeable amount of Co(NO₃)₂ in a dry apolar organic solvent. By UV-vis-near-IR spectrophotometry, some information on the state of Co(NO₃)₂ within water-containing or dry AOT reversed micelles was acquired, whereas by small-angle X-ray scattering it the occurrence of small nanoparticles in the salt-containing dry AOT reversed micelles was ascertained. Surprisingly, the analysis of the X-ray diffraction spectra corroborated by UV-vis and X-ray photoelectron spectroscopy data of Co(NO₃)₂/AOT composites led to the hypothesis that nanoparticles are mainly constituted of sodium nitrate resulting from the metathesis reaction between AOT and Co(NO₃)₂. By transmission electron microscopy, information on the size and the size distribution of the nanoparticles in salt/AOT composites was gained.     

A simple method to prepare solid nanoparticles of water-soluble salts using water-in-oil microemulsions

 A new and simple method to prepare solid nanoparticles of water-soluble salts using water-in-oil microemulsions is described. In particular, starting from water/sodium bis(2-ethylhexyl)sulfosuccinate/n-heptane microemulsions carrying inside the aqueous core of the reversed micelles some water-soluble salts [CaCl₂, Na₂HPO₄, Cu(NO₃)₂], after evaporation of the volatile components (water and apolar organic solvent), the resulting inorganic salt/surfactant composites were found to be totally dispersible in pure n-heptane. The presence of nanoparticles in these resuspended composites was ascertained by transmission electron microscopy observation of samples obtained by gentle evaporation of the organic solvent. Received: 8 July 1999/Accepted in revised form: 5 October 1999     

Physicochemical investigation of cobalt–iron cyanide nanoparticles synthesized by a novel solid–solid reaction in confined space

Cobalt–iron cyanide (Co_x[Fe(CN)₆]) nanoparticles have been synthesized by a novel solid–solid reaction in the confined space of dry sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reversed micelles dispersed in n-heptane. The reaction has been carried out by mixing two dry AOT/n-heptane solutions containing CoCl₂ and K₄Fe(CN)₆ or K₃Fe(CN)₆ nanoparticles in the micellar core, respectively. By UV-Vis spectroscopy it was ascertained that, after the mixing process, the formation of stable nanoparticles is fast and complete. Microcalorimetric measurements of the thermal effect due to the Co_x[Fe(CN)₆] nanoparticle formation allowed the determination of the stoichiometric ratio (x) and of the molar enthalpy of reaction in the core of AOT reversed micelles. The observed behavior suggests the occurrence of confinement effects and surfactant adsorption on the nanoparticle surface. Further structural information was achieved by small-angle X-ray scattering (SAXS) measurements. From all liquid samples, interesting salt/AOT composites were prepared by simple evaporation of the apolar solvent. Size, crystal structure, and electronic properties of Co_x[Fe(CN)₆] nanoparticles containing composites were obtained by wide-angle X-ray scattering (WAXS) and X-ray photoelectron spectroscopy (XPS).     

Growth of silica nanoparticles in methylmethacrylate-based water-in-oil microemulsions

The mechanism of silica particle formation in monomer microemulsions is studied using dynamic light scattering (DLS), atomic force microscopy, small-angle X-ray scattering (SAXS), and conductivity measurements. The hydrolysis of tetraethylorthosilicate (TEOS) in methylmethacrylate (MMA) microemulsions (MMA = methylmethacrylate) is compared with the formation of SiO₂ particles in heptane microemulsions. Stable microemulsions without cosurfactant were found for MMA, the nonionic surfactant Marlophen NP10, and aqueous ammonia (0.75 wt%). In the one-phase region of the ternary phase diagram, the water/surfactant ratio (R _w) could be varied from 6 to 18. The DLS and SAXS measurements show that reverse micelles form in these water-in-oil (w/o) microemulsions. The minimum water-to-surfactant molar ratio required for micelle formation was determined. Particle formation is achieved from the base-catalyzed hydrolysis of TEOS. According to atomic force microscopy measurements of particles isolated from the emulsion, the particle size can be effectively tailored in between 20 and 60 nm by varying R _w from 2–6 in heptane w/o microemulsions. For MMA-based microemulsions, the particle diameter ranges from 25 to 50 nm, but the polydispersity is higher. Tailoring of the particle size is not achieved with R _w, but adjusting the particle growth period produces particles between 10 and 70 nm.     

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.     

Influence of Confined Water on the Photophysics of Dissolved Solutes in Reverse Micelles

The photophysical parameters of two probes with largely different hydrophobic character, namely, coumarin 1 and coumarin 343, are investigated in sodium bis‐(2‐ethylhexyl)sulfosuccinate (AOT)/hexane/water reverse micelles at various water/AOT molar ratio w₀. Correlation of photophysical parameters such as fluorescence quantum yield, fluorescence lifetime, and emission maxima with w₀ indicate distinctly different trends below and above w₀≈7 for both probes. The variation of the average rotational correlation times obtained from fluorescence anisotropy decays for both probes in reverse micelles further corroborate the above observation. Similar studies were also performed in nonaqueous reverse micelles with acetonitrile as polar solvent. Similar to aqueous reverse micelles, breaks in the photophysical parameters with increasing acetonitrile/AOT molar ratios w′₀ were also observed in these cases, although at a much lower w′₀ value of 3. The present results indicate that around w₀≈7 for aqueous reverse micelles (and around w′₀≈3 for nonaqueous reverse micelles) a distinct change occurs in the probe microenvironment, which is rationalized on the basis of the relative populations of interfacial and core water. We propose that until the ionic head groups and counterions are fully solvated by polar solvents, that is, up to w₀≈7 (or w′₀≈3), the interfacial water population dominates. Above these molar ratios coalescence of excess water molecules with each other to form truncated H‐bonded water clusters leads to a sizable population of core water. This is further substantiated by changes in the IR absorption spectra for the O? D stretching mode of diluted D₂O in reverse micelles with varying w₀. Critical comparison of the present results with relevant literature reports provide clear support for the proposals made on water structure in reverse micelles. The role of relative size of the probe and the reverse micelles for differences in polar solvent to AOT ratios (w₀=7 and w′₀=3) in the observed breaks in the two types of reverse micelles is also discussed.     

Effect of the Media on the Quantum Yield of Singlet Oxygen (O2(1Δg)) Production by 9H‐Fluoren‐9‐one: Microheterogeneous Systems

The quantum yield (Φ_Δ) of singlet oxygen (O₂(¹Δ_g) production by 9H‐fluoren‐9‐one (FLU) is very sensitive to the nature of the solvent (0.02 in a highly polar and protic solvent, such as MeOH, to 1.0 in apolar solvents). This high sensitivity has been used for probing the interaction of FLU with micellar media and microemulsions based on anionic (sodium dodecyl sulfate, SDS; bis‐(2‐ethylhexyl)sodium sulfosuccinate, AOT), cationic (cetyltrimethylammonium chloride, CTAC) and nonionic (Triton X‐100, TX) surfactants. Values of Φ_Δ of FLU vary in a wide range (0.05–1.0) in both microheterogeneous media and neat solvent, and provide information on the microenvironment of FLU, i.e., on its localization within organized media. In ionic and nonionic micellar media, as well as in four‐component microemulsions, FLU is, to various extents, exposed to solvation by the polar and protic components of the microheterogeneous systems (water and/or butan‐1‐ol) in the micellar interfacial region (Φ_Δ=0.05–0.30). In contrast, in AOT reverse micelles (consisting of AOT as surfactant, cyclohexane as hydrophobic component, and water), FLU is located in the hydrophobic continuous pseudophase, and is totally separated from the micellar water pools (Φ_Δ≈1.0).     

非水反相微乳的加溶与电导性质研究

研究了若干非水极性溶剂（甲酰胺、二甲基亚砜和乙腈）及其与水的混合物/AOT/正庚烷反相微乳体系的加溶性质及其电导行为.结果表明，在AOT反相微乳中，非水极性溶剂的最大加溶量均远小于水的加溶量.其最大加溶量顺序为二甲基亚砜< 甲酰胺< 乙腈< 水.甲酰胺和乙腈与水的加溶相互抵制，而水在一定范围内可促进二甲基亚砜的加溶.非水反相微乳的电导率随加溶量的变化规律与含水反相微乳体系类同，但到达电导率极大值和出现渗滤时的加溶量明显比含水反相微乳 体系的要小.     

Improvement in enzyme activity and stability by addition of low molecular weight polyethylene glycol to sodium bis(2-ethyl-L-hexyl)sulfosuccinate/isooctane reverse micellar system

The activity and stability of Chromobacterium viscosum lipase (glycerolester hydrolase, EC 3.1.1.3)-catalyzed olive oil hydrolysis in sodium bis (2-ethyl-1-hexyl)sulfosuccinate (AOT)/isooctane reverse micelles is increased appreciably when low molecular weight polyethylene glycol (PEG 400) is added to the reverse micelles. To understand the effect of PEG 400 on the phase behavior of the reverse micellar system, the phase diagram of AOT/PEG 400/water/isooctane system was studied. The influences of relevant parameters on the catalytic activity in AOT/PEG 400 reverse micelles were investigated and compared with the results in the simple AOT reverse micelles. In the presence of PEG 400, the linear decreasing trend of the lipase activity with AOT concentration, which is observed in the simple AOT reverse micelles, disappeared. Enzyme entrapped in AOT/PEG reverse micelles was very stable, retaining>75% of its initial activity after 60 d, whereas the half-life in simple AOT reverse micelles was 38 d. The kinetics parameter maximum velocity (V _max)exhibiting the temperature dependence and the activation energy obtained by Arrhenius plot was suppressed significantly by the addition of PEG 400.     

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.     

Effects of Nonpolar Solvents on the Solubilization of Pepsin into Bis(2-Ethylhexyl) Sodium Sulfosuccinate and Cetyltrimethylammonium Bromide Reverse Micelles

Solubilization of pepsin by bis(2-ethylhexyl) sodium sulfosuccinate (AOT) and cetyltrimethylammonium bromide (CTAB) reverse micelles has been studied at 20C. Isooctane, cyclohexane and hexane were used as solvents, and n-butanol, amyl alcohol and hexanol were used as cosurfactants for CTAB. AOT concentrations were varied from 50 to 500 mM and pepsin concentrations were varied from 2 to 10 mg-mL^–1. At 250 mM, AOT can solubilize more than 85% of the Pepsin in each solvent. The effect of aqueous-phase pH on the solubilization of Pepsin has been studied from pH 1 to 8. The maximum solubilization of pepsin was observed below the isoelectric point (pI = 1.5) of the protein at pH 1.0 with 300 mM of AOT. The CTAB solutions were prepared by dissolving CTAB in isooctane with varying concentrations (0–100% v/v) of n-butanol, amyl alcohol or hexanol cosurfactants. It was found that 5% cosurfactant with 100 mM of CTAB was sufficient to solubilize more than 90% of the total pepsin. Pepsin solubilization by AOT reverse micelles increases with increasing polarizability and molar volume of the solvents.     

Formation of 1-Butyl-3-methylimidazolium Bis(2-ethyl-1-hexyl)sulfosuccinate Stabilized Water-in-1-Butyl-3-methylimidazolium Bis(trifluoromethanesulfonyl)imide Microemulsion and the Effects of Additives

A new strategy is proposed here to formulate a bis(2-ethyl-1-hexyl)sulfosuccinate (AOT^?) stabilized water-in-ionic liquid microemulsion without any additives. Replacing the inorganic counter ion Na⁺ by the organic 1-butyl-3-methylimidazolium ([Bmim]⁺) ion greatly improves the solubility of AOT^? in hydrophobic 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([Bmim]Tf₂N) (IL) and favors the formation of water-in-IL (W/IL) microdroplets. The existence of the W/IL microdroplets has been confirmed by dynamic light scattering, Fourier transform infrared absorption spectroscopy and ultraviolet–visible absorption spectroscopy. Also, presented for the first time are the effects of salts and alcohols on the microstructure and water solubilization capacity of the ternary H₂O/[Bmim]AOT/[Bmim]Tf₂N system. For inorganic salts, larger concentrations of the salt and higher charge density of the cation result in smaller microdroplet size and weak water solubilization capacity. For 1-hexanol, a high concentration of this alcohol results in small microdroplet size but high water solubilization capacity. Analyses indicate that the salts compress the electric double layers of W/IL microemulsions, decrease the size of the microdroplets and consequently reduce the water solubilization capacity; the alcohol, however, facilitates the aggregation of AOT^?, increases the number of W/IL microdroplets, and therefore improves the water solubilization capacity of the system.     

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.     

Determination of the Water/AOT/Isooctane Reverse Micelles Size Parameters from Their Refractive Index Data

The present paper aims to study the proprieties of water confined in water/AOT/isooctane reverse micelles from their refractive indices at 298.5 K. The refractive indices of the microemulsions were investigated at increasing water concentration and at different micellar volume fractions, Φ _d. The refractive index of micellar water was deduced for a large water to AOT molar ratio, W ₀. The refractive index of interfacial water was also calculated. Then, the molar faction of interfacial water, α, was deduced for different W ₀ values and compared with those measured by NMR as reported in the literature. The results show qualitative agreement. Finally, the average aggregation number, n _agg, the area per surfactant headgroup, σ, as well as the interfacial thickness, d _i, of the reverse micelles were determined for the W ₀ values studied. The values of these structural parameters also show good qualitative agreement with NMR and small angle X-ray scattering data reported in the literature. Thus, the refractive index method can be a cheap and fast alternative for these two methods.     

Study of confined 5-aza[5]helicene in ytterbium(III) bis(2-ethylhexyl) sulfosuccinate reversed micelles

Some relevant physicochemical properties of 5-aza[5]helicene (H5) in solutions of ytterbium bis(2-ethylhexyl) sulfosuccinate (Yb(DEHSS)3) reversed micelles have been investigated by UV-vis-NIR, photoluminescence, and FT-IR techniques with the aim of emphasizing the role played by specific Yb(III)/H5 interactions and confinement effects as driving forces of its binding to reversed micelles, preferential solubilization site, and local photophysical properties. It has been found that the binding strength of 5-aza[5]helicene to reversed micelles, triggered by steric and orientational constrains as well as the water content, is mainly regulated by its interaction with the Yb(III) counterion. Moreover, when H5 is entrapped in Yb(DEHSS)3 reversed micelles, the combined action of this interaction and of confinement effects leads to marked changes of its photophysical properties with respect to those of H5 molecularly dispersed in apolar medium. The influence of the entrapment of finite amounts of H5 on the reversed micelle structure was investigated by SAXS. The analysis of experimental results brings to the hypothesis that H5 is preferentially solubilized and opportunely oriented in the micellar palisade layer and that its insertion causes an unidimensional growth of reversed micelles. From an analysis of WAXS spectra of H5/Yb(DEHSS)3 composites, obtained by complete evaporation of the volatile components of the H5/water/Yb(DEHSS)3/n-heptane solutions, it was ascertained that also on these systems H5 is dispersed molecularly or in a quite amorphous state in the surfactant liquid crystals without forming a separate crystalline nanophase.     

Phase behavior of the orange essential oil/sodium bis(2-ethylhexyl)sulfosuccinate/water system

The ternary phase diagram for the orange essential oil (OEO)/sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/water system was constructed at 25 °C. It indicates a large single phase region, comprising an isotropic water-in-oil (W/O) microemulsion (ME) phase (L₂), a liquid crystal (LC) (lamellar or hexagonal) and a large unstable emulsion phase that separates in two phases of normal and reverse micelles (L₁ and L₂). In this communication the properties of the ME are investigated by viscosity, electric conductivity and small angle X-ray scattering (SAXS) indicating that the isotropic ME phase exhibits different behaviors depending on composition. At low water content low viscous “dry” surfactant structures are formed, whereas at higher water content higher viscous water droplets are formed. The experimental data allow the determination of the transition from “dry” to the water droplet structures within the L₂ phase. SAXS analyses have also been performed for selected LC samples.     

The release of cephanone in CTAB/n-C5H11OH/H2O system

Controlled release of cephanone from hexadecyltrimethylammonium bromide (CTAB) micelles and CTAB/n-C₅H₁₁OH/H₂O microemulsions was studied. The results showed that the release rate of cephanone was reduced in CTAB micelles and CTAB/n-C₅H₁₁OH/H₂O microemulsions, because of the solubilization of cephanone in micelles and microemulsions. The release of cephanone from CTAB micelles and CTAB/n-C₅H₁₁OH/H₂O microemulsions was characterized by Fickian diffusion and non-Fickian diffusion.     

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.     

Temperature structural transformations of reverse micelles of oxyethylated surfactants

The dependence of the hydrodynamic radius of reverse micelles of Tergitol NP-4 on temperature (20–50°C), solubilization capacity (0–7 vol %), electrolyte composition (NH₄NO₃, KNO₃, HNO₃, NH₄OH, and KOH), and electrolyte concentration (0–8 mol/l) in an aqueous pseudophase was studied by photon-correlation spectroscopy. The hydrodynamic radius of micelles was shown to decrease with increasing temperature regardless of the concentration and type of electrolyte, and spherical micelles were formed in the process. It was ascertained that temperature aligns the features of interaction between solubilisate and micelles due to dehydration of the surfactant molecules: the radius of spherical micelles at 50°C depended only on solubilization capacity.     

FT-IR Investigation of the State of Iron(III) Chloride Clusters Confined in AOT Reverse Micelles Dispersed in Carbon Tetrachloride

The state of the water-soluble salt iron(III) chloride in AOT reverse micelles dispersed in carbon tetrachloride has been investigated by FT-IR spectroscopy. Interestingly, while the entrapment of a lot of water-soluble inorganic salts in AOT reverse micelles requires preliminarily the presence of significant amounts of water within the micellar core, solubilization of FeCl₃ occurs without the need to add water in the micellar solution reaching the very high solubility value, expressed as the maximum salt-to-surfactant molar ratio, of 1.30. The analysis of the spectral features of the investigated samples leads to hypothesize that iron(III) chloride is confined within the reverse micellar core as small size melted clusters of ionic species arising from the reactions