Adsorption behavior of bis (2-ethylhexyl) sodium sulfosuccinate (AOT) at the mercury-electrolyte solution interface as a function of electrode potential and time

The dependence of the differential capacitance (C) of the electrode double layer of a hanging mercury drop electrode in bis (2-ethylhexyl) sodium sulfosuccinate (AOT) solutions on electrode potential (E) and time is measured using three-dimensional phase sensitive ac voltammetry. This methodology, possessing a very wide time window that permits a detailed study of the adsorption phenomena, is based on the reconstruction of C vs E curves, sampled after many phase-sensitive ac chronoamperometric experiments. The shape of these curves allows an estimation of the structure of the layer of AOT molecules absorbed at the electrode surface. AOT molecules form micelles in bulk solutions and they also associate in the charged interface under the strong influence of the electric field into surface aggregates which depend on their concentration and applied potential. The presence of AOT micelles in the bulk solution can be linked with the appearance of a surface film at potentials more negative than those corresponding to a condensed film linked with a capacitance value slightly higher than that normally attributed to a compact layer. The whole phenomenon is proved to be very dependant upon time.     

Temperature-dependent solvation dynamics of water in sodium bis(2-ethylhexyl)sulfosuccinate/isooctane reverse micelles

In this paper, for the first time, we report a detailed study of the temperature-dependent solvation dynamics of a probe fluorophore, coumarin-500, in AOT/isooctane reverse micelles (RMs) with varying degrees of hydration (w0) of 5, 10, and 20 at four different temperatures, 293, 313, 328, and 343 K. The average solvation time constant becomes faster with the increase in w0 values at a particular temperature. The solvation dynamics of a RM with a fixed w0 value also becomes faster with the increase in temperature. The observed temperature-induced faster solvation dynamics is associated with a transition of bound- to free-type water molecules, and the corresponding activation energy value for the w0 = 5 system has been found to be 3.4 kcal mol-1, whereas for the latter two systems, it is approximately 5 kcal mol-1. Dynamic light scattering measurements indicate an insignificant change in size with temperature for RMs with w0 = 5 and 10, whereas for a w0 = 20 system, the hydrodynamic diameter increases with temperature. Time-resolved fluorescence anisotropy studies reveal a decrease in the rotational restriction on the probe with increasing temperature for all systems. Wobbling-in-cone analysis of the anisotropy data also supports this finding.     

Micelles and reverse micelles in the nickel bis(2-ethylhexyl) sulfosuccinate/water/isooctane microemulsion

The ternary system Ni(2+)(AOT)(2) (nickel 2-bis[2-ethylhexyl] sulfosuccinate)/water/isooctane presents w/o and o/w microemulsions with a Winsor progression (2Phi-3Phi-2Phi), without the addition of salt; the "fish diagram" was obtained for alpha=0.5 and gamma=0.02-0.22. Using static and dynamic light scattering the micellar size, the ratio of water to surfactant, and the density of micelles for this system were estimated. In addition, the mean interfacial curvature as a function of temperature was obtained.     

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

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

Ion pairs of crystal violet in sodium bis(2-ethylhexyl)sulfosuccinate reverse micelles

The interfacial localization and the ion pair formation of the positively charged dye crystal violet (CV) in sodium bis(2-ethylhexyl)sulfosuccinate reverse micelles (AOT RMs) were studied by several structural and spectroscopic techniques and by quantum chemical calculations. The size and shape of the AOT RMs in the presence of CV were investigated by small-angle X-ray scattering, showing that CV does not significantly change the RM structure. CV localization as a function of the water to surfactant molar ratio (w(0)) was characterized by H(1) and (13)C NMR, indicating the close proximity of CV to the sulfosuccinate group of AOT at small and large w(0) values. These results were confirmed by calculation of magnetic shielding constants using the gauge-independent atomic orbital method with the HF/6-31G(d) basis set. Two different types of ion pairs between AOT and CV, i.e., contact ion pair (CIPs) and solvent-separated ion pair (SSIPs), were characterized by UV-vis spectroscopy and quantum chemical calculations using the semiempirical ZINDO-CI method. In nonpolar isotropic solvents CIPs are formed with an association constant (K(ASSOC)) of 2 x 10(4) mol(-1) L in isooctane and 750 mol(-1) L in chloroform. In AOT RMs at low w(0), CV-AOT CIPs are also formed. By increasing w(0), there is a sharp decrease in the CIP association free energy, and SSIPs are formed. (CV(+))(H(2)O)(AOT(-)) SSIPs are stable in the AOT RM up to the largest w(0) tested (w(0) = 33).     

Comparative studies on the micellization of sodium bis(4-phenylbutyl) sulfosuccinate and sodium bis(2-ethylhexyl) sulfosuccinate and their interaction with hydrophobically modified poly(acrylamide)

The micellization process of sodium bis(4-phenylbutyl) sulfosuccinate (SBPBS) has been studied compared to that of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) by surface tension, steady-state fluorescence, microcalorimetry, dynamic light scattering (DLS), and transmission electron microscopy (TEM) measurements. Meanwhile, the interaction of these two surfactants with hydrophobically modified poly(acrylamide) (HMPAM) was investigated. The results show that the surface tension at the critical micelle concentration (cmc) of SBPBS and the micropolarity probed by pyrene in SBPBS aggregates are both larger than those of AOT. The enthalpy change of micellization (DeltaH(mic)) of AOT is endothermic, while it is exothermic for SBPBS. Strong pi-pi interaction among the adjacent phenyl groups of SBPBS molecules is likely the cause for the above properties of SBPBS. Moreover, vesicles are observed for AOT and SBPBS by DLS and TEM, especially for AOT, whose micelle-vesicle transition has been first confirmed by its calorimetric curve. In the surfactant-HMPAM systems, the critical aggregation concentration (cac), the saturation concentration of aggregation (C(2)), and the thermodynamic parameters of binding have also been determined. The conclusion may be drawn that the binding strength of SBPBS onto HMPAM is stronger than that of AOT.     

Gas phase charged aggregates of bis(2-ethylhexyl)sulfosuccinate (AOT) and divalent metal ions: first evidence of AOT solvated aggregates

Assembling and chelating properties of sodium bis(2-ethylhexyl)sulfosuccinate (AOTNa) towards divalent metal ions have been investigated in the gas phase by electrospray ionization mass spectrometry. A variety of positively charged monometallated and mixed metal aggregates are formed. Interestingly, several ions contain solvent (MeOH, H(2)O) molecules and constitute the most abundant AOT cationic aggregates not containing sodium. These species are the first example of solvated AOT-metal ion aggregates in the gas phase. By increasing the surfactant aggregation number, the abundance of solvated species becomes lower than that of unsolvated ones. Decompositions of ionic species have been studied by tandem mass spectrometry, and their stability has been determined through energy resolved mass spectrometry. In contrast with positively charged AOT-alkaline metal ion aggregates, whose decompositions are dominated by the loss of individual surfactant molecules, AOTNa-divalent ion aggregates mainly dissociate through the cleavage of the AOT H(2)C-O bond followed by further intramolecular fragmentations. This finding, that is consistent with an enhanced chelation of divalent ions with AOT(-) head groups, has been taken as an indication that such aggregates are characterized by a reverse micelle-like organization with a ionic core formed by the metal cations interacting with the negatively charged surfactant polar heads, whereas the surfactant alkyl chains point outside.     

Microscope measurements for the transient formation of W/O emulsions of sodium bis(2-ethylhexyl) sulfosuccinate in the dodecane/water interfacial region

The present study investigated the transient formation of water-in-oil (W/O) emulsions of sodium bis(2-ethylhexyl) sulfosuccinate (aerosol OT, AOT) in a dodecane/water interfacial region and the anomalous uptake of water in the dodecane phase by in situ bright-field optical microscopy and water concentration measurements in detail. The hydrodynamic radius of the individual W/O emulsions in the dodecane phase was determined to be 0.1-1.2 μm from the analysis of their diffusion behavior; they are much larger than common W/O microemulsions (a few nanometers in radius). At first, they were formed spontaneously in the dodecane/water interfacial region without shaking, and they diffused away into the dodecane phase. Then, almost all of them vanished at the interface by fusion. Their number and the water concentration in the dodecane phase increased first and then decreased gradually. The formation mechanism was discussed with estimated concentration profiles of AOT and water molecules, which suggests that larger W/O emulsions of 0.01-0.44 μm in radius can be formed in the dodecane phase near the interface (within 2 μm) because the concentration of AOT becomes lower than that of water there.     

Micellar electrokinetic chromatography with bis(2-ethylhexyl)sodium sulfosuccinate vesicles determination of synthetic food antioxidants

Capillary electrokinetic chromatography is suitable for the separation of mixtures of uncharged and charged solutes. In the present work the behavior of six synthetic food antioxidants--2[3]-tert.-butyl-4-hydroxyanisole, 2,6-di-tert.-butyl-p-cresol, tercbutylhydroquinone, 3,4,5-trihydroxybenzoic acid propyl ester, 3,4,5-trihydroxybenzoic acid octyl ester and 3,4,5-trihydroxybenzoic acid dodecyl ester--was studied in a capillary electrophoresis system using capillary electrokinetic chromatography with vesicles of the surfactant bis(2-ethylhexyl)sodium sulfosuccinate (AOT). Several studies aimed at calculating the critical aggregation concentration of the surfactant were conducted to check that under the conditions used the AOT was in a state of aggregation. Having checked the association shown by the surfactant, we then explored the greater or lesser capacity of the antioxidants to interact with this compound. We followed the evolution of the molecular absorption spectra of each of the antioxidants in the presence of the surfactant at different concentrations and the retention factors were calculated at different pH values. Additionally, in order to determine which species--anionic or neutral--was present at the pH of the buffer used (boric/borate), the pKa values in acetonitrile-water (20:80) were obtained. Resolution and quantification of the antioxidants demand optimization of the variables involved in the system, such as the percentage of acetonitrile, the concentration of AOT and boric/borate buffer, pH, voltage, etc. When this part of the study had been completed, calibrations were obtained for each of the antioxidants, obtaining good linear correlation coefficients in all cases. Finally, we propose a method that allows the resolution of the six most employed antioxidants in a capillary electrophoretic system in 15 min, using electrokinetic chromatography with AOT as the pseudostationary phase.     

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.     

Micropolarity of sodium bis(2-ethylhexyl) sulfosuccinate reverse micelles prepared in supercritical ethane and near-critical propane

The micropolarity of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles prepared in supercritical ethane and near-critical propane has been determined in terms of a solvent polarity parameter, E _T(30) values, by using absorption probes, 1-ethyl-4-methoxycarbonyl pyridinium iodide and 2,6-diphenyl-4-(2,4,6-triphenylpyridinio)-phenolate as a functions of pressure and the molar ratio of water to AOT, W ₀, at a constant temperature of 310 K. The micropolarity comparable to that of ethanol was observed for reverse micelles containing water of W ₀ = 2. The micropolarity increased with the water content and became independent of pressure after the system changed to a one-phase reverse micelle solution. For a given W ₀ value, no difference in the micropolarity was noticed in the micelles prepared in ethane and propane. Phase behaviour investigations have revealed that complete dissolution of 50 mM AOT occurred at 20 MPa in supercritical ethane, while a much lower pressure of 1 MPa was required in near-critical propane. The amount of water solubilized in reverse micelles formed in supercritical ethane was relatively low, reaching a W ₀ value of 7 at 36 MPa. In contrast, the amount of water solubilized in near-critical propane reverse micelles was W ₀ = 11 at a much lower pressure of 6 MPa. A higher pressure was required to solubilize larger amount of water in reverse micelles prepared in both ethane and propane. Received: 9 October 1998 Accepted in revised form: 12 February 1999     

Phase behavior, microstructure transition, and antiradical activity of sucrose laurate/propylene glycol/the essential oil of Melaleuca alternifolia/water microemulsions

Nonionic sucrose ester microemulsions composed of sucrose laurate (SL), propylene glycol (PG) and water were prepared with the essential oil of Melaleuca alternifolia, commonly known as tea tree oil (TTO), as oil phase to investigate the phase behavior, microstructure, and antiradical activity. The pseudo-ternary phase diagrams were constructed to elucidate the phase behavior of the microemulsion formations at different weight ratios of surfactant and cosurfactant (S_m = SL/PG) of 1:1, 2:1, and 3:1. The extension of the microemulsion zone was found to be strongly dependent on the S_m ratios. The single phase microemulsion domain, especially o/w microemulsion region increased when S_m ratio is increased from 1:1 to 3:1 and no liquid crystalline structure was observed for all formulations studied. Microstructural aspects were studied by electrical conductivity and pulsed gradient spin echo (PGSE) NMR measurements along water titration line L28 (R_o = 2:8). The results from these combined techniques were in good agreement in regard to the microstructure transition points. The microstructural inversion of w/o to bicontinuous microemulsions occurred at 30 wt.% water while the transition from bicontinuous to o/w structure occurred at 55 wt.% water. The physical stability on storage temperature and time was examined by dynamic light scattering after the centrifuge test and freeze–thaw cycles. The droplet size was kept almost the same without any phase separation, providing less temperature-sensitivity up to 70 °C and good stability for 3 months at room temperature. The chemical profile and radical scavenging activity of TTO in o/w microemulsions was evaluated by means of gas chromatography–mass spectroscopy (GC–MS) and 2,2′-diphenyl-1-picrylhydrazyl free radical (DPPH) scavenging method, respectively. The major abundant constituents of crude TTO, monoterpene alcohols (terpinen-4-ol (41.65%), α-terpineol (3.18%)) and hydrocarbons (γ-terpinene (22.95%), α-terpinene (10.16%)) were identified and the composition percentage of each constituent was calculated form the GC peak areas by normalization method. The DPPH scavenging activity of TTO microemulsion was lower than pure TTO because the SL surfactant may obstruct the interaction between the TTO and DPPH, reducing the number of effective collisions.     

A study of sodium bis(2-ethylhexyl) sulfosuccinate adsorption on silica from silver and gold organosols

Adsorption of sodium bis(2-ethylhexyl) sulfosuccinate (Aerosol OT) and silver and gold nanoparticles on SiO₂ from heptane solutions has been investigated by spectophotometery and CHN analysis. The adsorption isotherm of Aerosol OT is described by the Langmuir equation. Sorption capacity (2.4 × 10^–4 mol/g), sorption constant (9.0 m³/mol), and area per Aerosol OT molecule in an adsorption layer (0.83 nm²) have been determined. It has been shown that, at Aerosol OT concentrations lower than 6 × 10^–4 M, gold and silver ((0.3–6) × 10^–4 M) are simultaneously extracted by 98%; however, they have no effect on the adsorption and determination of the surfactant.     

Application of factorial design to the optimization of peroxidase activity in reverse micelles of bis(2-ethylhexyl)sodium sulfosuccinate/isooctane

Two cationic peroxidases isolated from Vaccinium myrtillus were encapsulated in reverse micelles of bis(2-ethylhexyl)sodium sulfosuccinate/isooctane. By using a central composite design, some relevant parameters for the enzymatic activity, such as surfactant and water concentration, pH, and buffer molarity, were analyzed. With the results obtained from this experimental planning, the response surface curves were established. The maximum specific activity obtained (0.19 mM/min · mM of enzyme) was approximately the same for both peroxidases, but the experimental conditions under which this value was attained differed considerably.     

Characterizing interfacial friction in bis(2-ethylhexyl) sodium sulfosuccinate reverse micelles from photoisomerization studies of carbocyanine derivatives

Photoisomerization of two carbocyanine derivatives has been examined in bis(2-ethylhexyl) sodium sulfosuccinate (AOT) reverse micelles to understand the factors that govern this process in the interfacial region of organized assemblies. To this effect, fluorescence lifetimes and quantum yields of 3,3(')-diethyloxadicarbocyanine iodide and merocyanine 540 have been measured in AOT∕isooctane∕water and AOT∕cyclohexane∕water reverse micellar systems as a function of the mole ratio of water to the surfactant, W. The nonradiative rate constants, which have been identified as the rates of photoisomerization for these solutes, were obtained from the experimentally measured parameters. The steady rise and subsequent saturation observed in the nonradiative rate constants upon increasing W has been rationalized in terms of micellar packing. An inverse correlation has been obtained between the nonradiative rate constants and the critical packing parameter, indicating that the interfacial friction experienced by the solute molecule is essentially described by this parameter.     

Phase diagram of the ternary system NMMO/water/cellulose

The phase diagram of the system N-methylmorpholine-N-oxide(NMMO)/H₂O/cellulose has been measured at 80 °C by establishing a solubility map (observation of the mixtures under the microscope), by the analysis of coexisting phases and determining the critical point. These experiments manifest a continuous reduction of the two phase area existing for the subsystem H₂O/cellulose upon the addition of NMMO, where a weight fraction of NMMO in the mixed solvent exceeding 75 wt% is required for Solucell 400 to reach the critical composition. The critical cellulose concentration is only 0.34 wt%, i.e., more than an order of magnitude lower than for the solutions of typical vinyl polymers in mixed solvents. All experimental observations can be well modeled on the basis of composition dependent binary interaction parameters by means of recently established mixing rules. For the subsystems H₂O/cellulose and NMMO/water the corresponding data are known from independent earlier measurements. The adjustment of two parameters to the ternary phase diagram was required to obtain this information for NMMO/cellulose, the third binary subsystem.     

Phase equilibria in the CdI2-Ag2Se system

The phase diagram of the system CdI₂-Ag₂Se is studied by means of X-ray diffraction, differential thermal analysis and measurements of the density of the material. The unit cell parameters of the intermediate phase 2CdI₂·3Ag₂Se were determined a = 0.6387 Å, b = 4.311 Å, c = 4.044 Å; α = 113.72°, β = 90.27° and γ = 94.85°. The intermediate phase 2CdI₂·3Ag₂Se has a polymorphic transition at 125 °C. It melts incongruently at 660 °C.     

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.     

Phase behavior of water-in-supercritical carbon dioxide microemulsion with sodium salt of bis(2,2,3,3,4,4,5,5-octafluoro-1-pentanol) sulfosuccinate

The fluorinated analogues of AOT surfactant, sodium salt of bis(2,2,3,3,4,4,5,5-octafluoro-1-pentanol) sulfosuccinate, which has CO₂-philic chains and a hydrophilic head group, was synthesized for forming water-in-CO₂ microemulsion. The cloud point of this surfactant was measured and the supercritical fluid-phase behavior of water-in-CO₂ microemulsion was investigated by using a variable-volume view cell apparatus. It was found that the phase behavior of microemulsion is affected by the concentrations of water and surfactant in CO₂, molar ratio of water to surfactant (W_o = [water]/[surfactant]), and the amount of CO₂. From this experiment, we found out another cloudy point which exists above the homogeneous microemulsion region. We defined this point as “upper cloud point” and general cloud point as “lower cloud point. The region which exists between these two cloud points was defined as “stable region of microemulsion”. Conditions for the formation of water-in-CO₂ microemulsion were obtained at temperatures up to 370.15 K.     

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.