AOT Reverse Micelles Research Articles

Transient absorption studies of vibrational relaxation and photophysics of Prussian blue and ruthenium purple nanoparticles

Transient infrared and visible absorption studies have been used to characterize vibrational and electronic dynamics of Prussian blue (PB) and ruthenium purple (RP) nanoparticles produced and characterized in AOT reverse micelles. Studies include excitation and probing with both infrared (near 2000 cm(-1)) and visible (800 nm) pulses. From IR pump-IR probe measurements of the CN stretching bands, vibrational population lifetimes are determined to be 32 ± 4 ps for PB and 44 ± 14 ps for RP. These times are longer than those for ferrocyanide (4 ps) and ruthenocyanide (4 ps) in normal water and are closer to the times for these species in heavy water (25-30 ps) and for ferrocyanide in formamide (43 ps). The PB and RP lifetimes are also longer than those (<15 ps) previously measured for CN stretching bands following intervalence excitation and back-electron transfer (BET) for dinuclear mixed-valence compounds containing Fe, Ru, and Os in heavy water and formamide [A. V. Tivansky, C. F. Wang, and G. C. Walker, J. Phys. Chem. A 107, 9051 (2003)]. In 800 nm pump-IR probe experiments on RP and PB, transient IR spectra and decay curves are similar to those with IR excitation; a ground state bleach and a red shifted (by ~40 cm(-1)) excited state decay are observed. These results for the visible pumping are consistent with rapid (<1 ps) BET resulting in population in the ground electronic state with vibrational excitation in the CN mode. In addition, transient absorption studies were performed for PB and RP probing with visible light using both visible and IR excitation. The early time response for the 800 nm pump-800 nm probe of PB exhibits an instrument-limited, subpicosecond bleach followed by an absorption, which is consistent with the previously reported results [D. C. Arnett, P. Vohringer, and N. F. Scherer, J. Am. Chem. Soc. 117, 12262 (1995)]. The absorption exhibits a biexponential decay with decay times of 9 and 185 ps, which could have been attributed to the CN band excitation indicated from 800 pump-IR probe results. However, IR pump-800 nm probe studies reveal that excitation of the CN band directly results in a decreased visible absorption that decays in 18 ps rather than an induced absorption at 800 nm. Characteristics of the IR pump-800 nm probe, especially that it induces a bleach instead of an absorption, unambiguously indicate that the transient absorption from the 800 nm pump-800 nm probe is dominated by states other than the CN bands and is attributed to population in other, probably lower frequency, vibrational modes excited following visible excitation and rapid BET.

Spectroscopic Properties of Quercetin-3-O-rhamnoside and Quercetin-3-O-rutinoside in Aerosol-OT Reverse Micelles

The anomalous spectroscopic properties of quercetin-3-O-rhamnoside (QCRM) and quercetin-3-O-rutinoside (QCRT) in AOT reverse micelle were studied. The excited state intramolecular proton transfer (ESIPT) occurs through the strong hydrogen bond between the -OH at position 5 and the carbonyl oxygen. Because the ESIPT can only happens in the <TEX>$S_1$</TEX> state and the Franck-Condon factor involved in the <TEX>$S_2\;{\rightarrow}\;S_1$</TEX> internal conversion is small, the <TEX>$S_2\;{\rightarrow}\;S_o$</TEX> emission alone appears. Because the molecular planarity is improved at the interior of the micelle, the excited state intramolecular charge transfer in the <TEX>$S_1$</TEX> state is extended, and the excited state is more tolerable for any quenching effects in the micelle. Therefore, an <TEX>$S_1\;{\rightarrow}\;S_o$</TEX> emission was newly discovered under this micelle microenvironment. For the <TEX>$S_2\;{\rightarrow}\;S_o$</TEX> emission, the quantum yields increase but the quantum yield of the <TEX>$S_1\;{\rightarrow}\;S_o$</TEX> emission approximately decreases as the water concentration in the micelle increases.

Associated electron and proton transfer between Acridine and Triethylamine in AOT reverse micelles probed by laser flash photolysis with magnetic field

Laser flash photolysis with magnetic field (MF ∼0.08 T) has been used to study interaction between Acridine (Acr) and Triethylamine (TEA) in reverse micelles with w 0 = 2.5–40. Dynamic protonation equilibrium exists between 3Acr and 3AcrH +. The intermediates indicate excited-state proton transfer (PT) between 3AcrH + and TEA. However, application of MF highlights the formation of geminate radical ion pairs (RIPs) with triplet spin-correlation, a signature of latent photoinduced electron transfer between 3AcrH + and TEA co-exists with PT. Magnetic field effect (MFE) is prominent for smaller w 0 showing importance of optimum separation between RIP to maximize MFE, whereas PT remains unaltered.

Aggregation Behaviors of Tricarbocyanine Dye in Water and in AOT Reverse Micelles

AbstractThe photophysical property of the tricarbocyanine dye IR144 has been extensively studied in non‐aqueous solvents. However, as a potential near‐infrared biomedical imaging probe, the photophysical property of IR144 in water is still little known. So, the aggregation behaviors of IR144 in water with steady‐state absorption spectroscopy and integrated polarization dependent femtosecond pump‐probe spectroscopy were investigated. Through comparing the absorption spectral bandshape of IR144 in water and in water pool of AOT reverse micelles, It is found that IR144 form dimer aggregates in water even at very low concentration (&lt;1.0×10−7 mol·L−1). And the absorption spectrum of the IR144 aggregates always displays a bimodal feature, which is independent of the dye concentration ranging from 1.0×10−7 to 1.0×10−4 mol·L−1. For better understanding the aggregation behaviors of IR144 in water, we measured the ground state recovery kinetics and the reorientation kinetics of IR144 in water and in water pool of AOT reverse micelles (W0=[H2O]/[AOT], W0=40). It is found that the fluorescence quantum yield of IR144 in water is lower than that in water pool of AOT reverse micelles, and the reorientation time of IR144 in water is slower than that in water pool of AOT reverse micelles. Those kinetic measurements also verify that IR144 exists as dimer aggregates in water.

Organized media effect on the photochemical deoxygenation of resazurin in the presence of triethanolamine

The photodeoxygenation of the synthetic dye resazurin in the presence of triethanolamine was investigated in water, CTAC and SDS direct micelles, AOT and BHDC reverse micelles, and soybean lecithin microemulsions (LEC). In all cases the only product observed was the deoxygenated dye resorufin. Triplet and reaction quantum yields were determined in all media. The photoreaction proceeds more efficiently in the microheterogenous systems with positive interface, CTAC and BHDC, while the lower yield is observed in AOT, SDS and LEC. The initial step in the mechanism is the interaction of the triplet state of the dye with the amine, and the effect of the interface is interpreted by a decrease of the recombination rate of the radicals formed in the initial electron transfer step. Negative and zwitterionic interfaces have no effect on the quantum yield.

Photophysics and locations of IR125 and C152 in AOT reverse micelles

Many fluorescent chromophores have been employed to investigate the nature and dynamics of the water confined in reverse micelles (RMs). However, some questions remain as to the location of a probe in a RM and the diameter of the RM at which the physical characteristic of the water inside RMs becomes similar to that of bulk water. In this work, we systematically studied the photophysics of IR125 and C152 in AOT RMs at different w(0) by means of static absorption and fluorescence spectroscopy as well as time-resolved fluorescence spectroscopy. We obtained the absorption maxima, fluorescence emission maxima, fluorescence lifetime, and reorientation time of IR125 and C152 in AOT RMs at corresponding w(0). We found that all obtained photophysical parameters of IR125 and C152 in AOT RMs as a function of w(0) have a distinct changeover point around w(0) = 8, indicating that there is a dramatic change in the nature of the water confined in AOT RMs around w(0) = 8. The observed changeover point around w(0) = 8 is well in agreement with the Satpati's report (ChemPhysChem, 2009, 10, 2966). In addition, we observed that the measured reorientation time of IR125 in AOT RMs increases with the increase of w(0), which is opposite to the trend of change in the measured reorientation time of C152 in AOT RMs with the increase of w(0). We found that IR125 prefers to reside in the water pool of AOT RMs and that C152 prefers to reside in the outer side of the interfacial region or the nonpolar n-heptane phase of AOT RMs. Furthermore, we found that the time-resolved fluorescence anisotropy of IR125 in smaller w(0) AOT RMs primarily measures the reorientation of RMs and the time-resolved fluorescence anisotropy of IR125 in larger w(0) AOT RMs measures the reorientation of IR125 in the water pool confined in RMs. This work demonstrated that IR125 is an excellent probe to study the nature and dynamics of the water confined in AOT RMs.

Reduced coupling of water molecules near the surface of reverse micelles

We report on vibrational dynamics of water near the surface of AOT reverse micelles studied by narrow-band excitation, mid-IR pump-probe spectroscopy. Evidence of OH-stretch frequency splitting into the symmetric and asymmetric modes is clearly observed for the interfacial H(2)O molecules. The polarization memory of interfacial waters is preserved over an exceptionally extended >10 ps timescale which is a factor of 100 longer than in bulk water. These observations point towards negligibly small intermolecular vibrational coupling between the water molecules as well as strongly reduced water rotational mobility within the interfacial water layer.

Dynamics of water confined to reverse AOT micelles

We use quasi-elastic neutron scattering (QENS) to study the dynamics of water confined inside reverse micelles. As a model system we use a water-in-oil droplet microemulsion based on the anionic surfactant AOT (sodium bis[2-ethylhexyl] sulfosuccinate), that forms spherical water droplets coated by a monolayer of AOT dispersed in the continuous oil matrix. Combining neutron time-of-flight (TOF) and backscattering (BS) spectroscopy, we access the dynamical behaviour of water over three decades in time from pico- to nanoseconds. We investigate the influence of reverse micelle size on the water dynamics by comparing two sample systems with bigger and smaller water core radii of about Rc ≈ 12 A and 7 A. The temperature is varied over a range where both microemulsion systems are stable, from room temperature down to the region where the confined water is supercooled: 260 K ≤ T ≤ 300 K. Taking explicitly into account the previously measured diffusion of entire reverse micelles in the oil matrix we find the average mobility of the confined water to be considerably slowed with respect to bulk water. The translational diffusion decreases with decreasing reverse micelle size. Dependent on the reverse micelle size we can interpret our data by assuming two dynamically separated water fractions. We identify the faster one with bulk-like water in the middle of the core while the slower one seems to be surfactant bound water. We find that 4 molecules of water per AOT molecule are immobilized on the timescale of QENS, i.e. shorter than nanoseconds.

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.

Disruption of reverse micelles and release of trapped ribonuclease A photochemically induced by Malachite Green leuconitrile derivative

Photoinduced disruption of a sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelle is triggered by a Malachite Green leuconitrile derivative (MGL). UV irradiation of MGL solubilized in an AOT-water-chloroform mixture creates a cationic surfactant that interacts electrostatically with the anionic AOT. We investigated the disruption of the reverse micelle by using proton nuclear magnetic resonance spectroscopy and found that UV irradiation of MGL decreases the number of water molecules solubilized in the interior of the AOT reverse micelles. Furthermore, the photoinduced disruption of the reverse micelle is shown to release ribonuclease A, which is trapped in the water in the interior of the AOT reverse micelle. This photoinduced release may offer a desirable transport system of biopolymers.

Interactions of anionic surfactants with methemoglobin

Interactions of two anionic surfactants, sodium dodecyl sulphate (SDS) and sodium bis(2-ethylhexyl) sulfosuccinate (AOT) at concentrations below and above critical micelle concentration with methemoglobin (metHb) have been investigated by conventional as well as by stopped-flow absorption and fluorescence spectroscopy. The absorption spectra of metHb in AOT reverse micelles have been also analyzed. Both surfactants in their monomeric form convert metHb to reversible hemichrome. This is connected with a diminution of peroxidase-like activity of metHb and with an increase of the susceptibility of heme for a damage by H2O2. In micellar solutions of AOT and SDS as well as in AOT reverse micelles pentacoordinated ferric species seems to be the predominant form of this protein. It has been concluded, basing on a kinetic analysis, that conformational changes in the heme environment of metHb as induced by both surfactants occur independently of the alterations in the tertiary structure of this protein.

Magnetic Resonance Studies of a Redox Probe in a Reverse Sodium Bis(2-ethylhexyl)sulfosuccinate/Octane/Water Microemulsion

The location and dynamics of the [Ru(bpy)(3)](2+) complex inside sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/octane/water microemulsions were studied, over a range of droplet sizes, using magnetic resonance spectroscopy, dynamic light scattering, and molecular modeling. The T(1) magnetic resonance relaxation times of water inside the AOT reverse micelles (RMs) were measured in both the presence and the absence of the [Ru(bpy)(3)](2+) complex. Large size droplet RMs (ω(0) > 20) were found to be sensitive to the presence of the [Ru(bpy)(3)](2+) complex, which was detected through a decrease in the T(1) relaxation time of the water inside the RM core, as compared to RMs containing no [Ru(bpy)(3)](2+). However, no difference in T(1) relaxation time was observed for water in small RMs (ω(0) < 20). Two-dimensional (1)H-(1)H NOESY spectroscopy was performed to probe the location of the [Ru(bpy)(3)](2+) complex in both small (ω(0) = 9.2) and large droplets (ω(0) = 34.9). Cross-peaks between protons in the AOT tail groups and bipyridyl ligands were observed, showing that the [Ru(bpy)(3)](2+) complex resided in the RM interface. Finally, molecular modeling simulations were performed to probe the location of the [Ru(bpy)(3)](2+) complex and the structure of the RM. Molecular dynamics simulations confirmed the location of the [Ru(bpy)(3)](2+) complex in the RM interface and detected differences in the surfactant layer and the amount of water penetration into this layer with changing droplet size.

Effect of Micellar and Reverse Micellar Interface on Solute Location: 2,6-Pyridinedicarboxylate in CTAB Micelles and CTAB and AOT Reverse Micelles

The interface-solute interactions, including solute location, surfactant charge, and geometry of solute interactions were studied in CTAB micelles and reverse micelles and were found to be similar as measured using (1)H NMR spectroscopy and a pH-sensitive probe. (1)H NMR spectra were recorded in the presence and absence of 2,6-pyridinedicarboxylate probe at CTAB concentrations above and below the critical micelle concentration showing interaction between dipic-probe and the micellar self-assembled structure. Downfield chemical shifts are observed for the CTAB surfactant signals upon aggregation and micelle formation. The effect of micelle formation on CTAB chemical shifts was quantitated, and simple ion pairing was ruled out. No significant change in CTAB surfactant signals are observed in the presence of monoanionic probe, whereas significant shifts are observed in the presence of the dianionic probe. The (1)H NMR spectra of the dipic-probe are diagnostic of the protonation state and isomeric form of the dipic-probe. The (1)H NMR chemical shifts in micelles are sensitive to the location of the dipic-probe, and the downfield chemical shift suggests location of part of the molecule in the Stern layer near the charged interface. Other parts of the probe show an upfield chemical shifts consistent with a deeper penetration of the dipic-probe into the surfactant layer. Probe location was confirmed using the 2D ROESY. Spectra recorded of the dipic-probe at various pH values demonstrate that both CTAB micellar and CTAB/pentanol/cyclohexane reverse micellar interfaces are different than those reported in aqueous solution and in AOT/isooctane reverse micelles (Crans et al. J. Org. Chem. 2008, 73, 9633-9640) and suggest interface penetration by dipic(2-). Together these observations and comparisons provide guidelines for future interpretation of chemical shift changes in both micelles and reverse micelles and point to headgroup charge as being a key factor determining the direction of chemical shift change and the depth of solute penetration.

Molecular dynamics simulations of cytochrome c unfolding in AOT reverse micelles: The first steps

This paper explores the reduced form of horse cytochrome c confined in reverse micelles (RM) of sodium bis-(2-ethylhexyl) sulfosuccinate (AOT) in isooctane by molecular dynamics simulation. RMs of two sizes were constructed at a water content of W (o) = [ H₂O ]/[AOT] = 5.5 and 9.1. Our results show that the protein secondary structure and the heme conformation both depend on micellar hydration. At low hydration, the protein structure and the heme moiety remain stable, whereas at high water content the protein becomes unstable and starts to unfold. At W (o) = 9.1 , according to the X-ray structure, conformational changes are mainly localized on protein loops and around the heme moiety, where we observe a partial opening of the heme crevice. These findings suggest that within our time window (10ns), the structural changes observed at the heme level are the first steps of the protein denaturation process, previously described experimentally in micellar solutions. In addition, a specific binding of AOT molecules to a few lysine residues of the protein was found only in the small-sized RM.

ChemInform Abstract: Optical Properties of Amorphous ZnO, CdO, and PbO Nanoclusters in Solution.

The authors prepared ZnO, CdO, and PbO rock salt-coordinated and ZnO quartzite-coordinated nanoclusters with a microemulsion method in the AOT reverse micelle. X-ray and electron beam diffraction techniques indicate that all have amorphous structures. The similarity of the UV-visible absorption and photoluminescence spectra at room temperature for ZnO, CdO, and PbO rock salt-coordinated nanoclusters indicates that they are all similar in electronic structure. All prepared nanosystems show strong emission in the visible region, and the emission features depend on the wavelength of the excitation. This phenomenon is considered as a result of localization of the electronic state due to the amorphous structure of nanocluster and surface capping. Time-resolved photoluminescence studies on ZnO, CdO, and PbO nanoclusters were performed. The carrier lifetimes near the band-tail are in the sub-and nanosecond time ranges with different characteristics from that of bulk amorphous semiconductors and nanocrystals. The observed properties can be explained in terms of quantum confinement, structural disorder, and surface polarization.

3,3′-Diethyloxacarbocyanine iodide: A new microviscosity probe for micelles and microemulsions

Fluorescence studies of a carbocyanine dye 3,3′-diethyloxacarbocyanine iodide (DOCI) in a series of monohydroxy alcohols reveal a monotonic increasing trend for both the fluorescence quantum yield and fluorescence lifetime with solvent viscosity. The results were interpreted in terms of retardation of the photoisomerization rate of DOCI with increasing solvent viscosity, and a definite inverse relationship between the two parameters was established. This relationship was utilized to estimate the microviscosity experienced by DOCI in supramolecular assemblies like micelles and microemulsions of several well-known surfactants and amphiphilic block copolymer. In most of the assemblies, the local environment around DOCI was characterized by rather high microviscosity, comparable to those of high alcohols between butanol and decanol. Our observations suggest that the rod-like DOCI molecule is largely embedded among the hydrophobic tails of the surfactant molecules, which exert a viscous drag strong enough to hinder its photoisomerization. The fluorescence properties of DOCI were also utilized to determine the critical micellization concentration of several surfactants in water. Interestingly, the microviscosity reported by DOCI in water-free AOT reverse micelles is significantly high, and can be explained by considering enhanced electrostatic attraction between the DOC + cation and the anionic AOT head-groups.

Investigation of the activity and stability of papain in different micellar systems

Hydrolysis of PyrPheAlaPNa, catalyzed by papain in reverse micelles of ionogenic surfactant of Aerosol OT, and in octane, was studied in the presence and in the absence of nonionogenic surfactants of a different nature. As found, the dependence of the catalytic activity of papain on the hydration degree of the surfactant in reverse micelles of AOT has two maxima, corresponding to the activity of monomeric and oligomeric forms of the enzyme. It was established that the addition of nonionogenic surfactant of a different nature to the Aerosol OT in octane leads to changes of the catalytic properties of the enzyme, in particular, to its activation in the presence of Pluronic L61. Nanoemulsions of nontoxic nonionogenic surfactant and organic solvents, lecithin and Tween 20 in eucalyptus oil were created and characterized; with the use of the photon-correlation spectroscopy technique it was shown that the size of such particles lie in the range from 34 to 266 nm, depending on the water content, nature, and the concentration of system components, and factors regarding its shape lie in the interval 20–30, suggesting the presence of particles with a rod-like shape. It is important to note that in such systems, suitable for use in medicine and cosmetology, the activity of papain was revealed.

Measuring properties of interfacial and bulk water regions in a reverse micelle with IR spectroscopy: A volumetric analysis of the inhomogeneously broadened OH band

The water OH stretching band (3000–3600 cm −1) was analyzed for absorption contributions from the respective bulk and interfacial water regions of a reverse micelle. This analysis was performed by correlating volume changes of these regions to changes in the OH band absorption as the micelle radius grows. The volumetric analysis is based on the well established expanding core–shell model for AOT reverse micelles and yields the dimensions of the water regions and their individual spectral responses in the OH band. The interfacial shell thickness was determined to be 0.45 nm for AOT reverse micelles in i-octane. It was found that each water region absorbs at most frequencies in the OH band; however, absorption on the red side of the OH band is dominated by bulk water, while absorption on the blue side is dominated by interfacial water. The bulk spectral response was found to be more similar to pure water, while the interfacial spectrum is strongly blue-shifted reflecting the weaker hydrogen bonding in this region. AOT reverse micelles with radii in the range 2–4 nm conformed well to the volumetric model. However, it was found that determination of the bulk water spectral response is particularly sensitive to uncertainty in the micelle radius.

Walnuts and almonds as model systems of foods constituted by oxidisable, pro-oxidant and antioxidant factors

Walnuts and almonds were selected as complex model systems in order to study possible interactions among oxidisable substances, pro-oxidant species and antioxidants. Lipoxygenase activities in a direct-micelle system were determined for these nuts according their globulin contents in their soluble protein fraction. Walnut lipoxygenase activity was 1.5 times higher than that of almonds. Among antioxidant compounds, hydrophilic contents were analysed in these nuts, hence, extractable, hydrolysable and condensed phenolic compound fractions were determined as well as lipophilic phenolic concentration corresponding to tocopherol fraction. Extracts of brown skins and whole walnuts have significantly higher phenolic contents than those of almonds. Radical scavenging activities of phenolic extracts were measured. Besides, the experiments demonstrated the ability of all nut extracts to inhibit the coupled oxidation of linoleic acid with β-carotene induced by lipoxygenase. The antioxidant substances were mainly found in nut brown skins. Walnut extracts showed higher antioxidant activity than those of almonds. Lipoxygenase pro-oxidant activity was also measured in AOT reverse micelles in conditions that model these nuts.

Biochemical synthesis of gold and zinc nanoparticles in reverse micelles

Gold and zinc nanoparticles were obtained in AOT reverse micelles in isooctane by reduction of the corresponding metal ions by the natural pigment quercetin (the biochemical synthesis technique). Gold and zinc ions were introduced into the micellar solution of quercetin in the form of aqueous solutions, HAuCl4 and [Zn(NH3)4]SO4, to the water to AOT molar ratios 1–3 and 3–4, respectively. The process of nanoparticle formation was investigated by spectrophotometry. Nanoparticle size and shape were determined by transmission electron microscopy. The data obtained allow to conclude that there are two steps in metal ion-quercetin interaction: (1) complex formation, and (2) complex dissociation with subsequent formation of nanoparticles and a second product, presumably oxidized quercetin. Gold nanoparticles were found to be of various shapes (spheres, hexahedrons, triangles, and cylinders) and sizes, mainly in the 10–20 nm range; zinc nanoparticles are chiefly spherical and ∼5 nm in size. In both cases, the nanoparticles are stable in the air in micellar solution over long periods of time (from a several months to a several years).