Aqueous Reverse Micelles Research Articles

Study of structure of nonaqueous reverse micelles with o-nitroaniline and methyl orange as molecular probes: comparison with an aqueous reverse micelles

Abstract Instead of water reversed micelles can also be formed with polar organic solvents possessed with high dielectric constant and very low solubility in oil phase. Nonaqueous reverse micelles or microemulsions represent an interesting microreactors for various reactions, especially for reactions, where reagents can react with water. Study of localization places of molecular probes in organic polar pockets of reverse micelles is topical. The solvatochromic behavior of optical probes ortho-nitroaniline (o-NA) and methyl orange (MO) was studied in nonaqueous reverse micelles on the basis of surfactants sodium bis (2-ethylhexyl) sulfosuccinate (AOT) and polyoxyethylene (4) lauryl ether (C12E4) and polar organic solvents (acetonitrile, dimethylformamide, glacial acetic acid, etc.) insoluble in oil phase hexane. The strength of binding of o-NA and MO to AOT and C12E4 reversed micelles was assesssed via binding constant (K b ) and association degree (α) respectively. Donor, acceptor, or dipole-dipole interactions ability of the solvent to the head groups of surfactant was taken into account in order to explain results obtained with UV–visible spectroscopic method. The binding constants of o-NA with reverse AOT micelles in the presence of various solvents in the pockets of reversed micelles increase in the following row water < glacial acetic acid < acetonitrile < dimethylformamide < dimethyl sulfoxide, but this sequence is reversed when o-NA binds to C12E4 reverse micelles. The high value of the proton donor or acidity parameter in the water molecule (x d = 0.37) determines the weak binding of o-NA to the head AOT groups (K b = 20.8) in case of aqueous reverse micelles. The high value of the dipole parameter in the dimethylformamide molecule (x n = 0.40) promotes its strong interaction with nonionic polyoxyethylene groups of C12E4, which results in low value of binding constant (K b = 26.5) in case of optical probe o-NA and low value of association degree (α = 0.60) using MO as absorption probe. The results of this article will contribute to the improvement of the concept of interfacial processes, viz.: (i) some issues of supramolecular chemistry, (ii) revealing the contribution of parameters of donor, acceptor or dipole-dipole interaction in a polar organic solvent at the surfactant/nonpolar organic solvent interface, and (iii) features of the dissolution of optical probes in non-aqueous reverse micelles.

Imim-DEHP reverse micelles investigated with two molecular probes reveals how are the interfacial properties and the coordination behavior of the surfactant

Aqueous reverse micelles (RMs) generated with the ionic liquid (IL)-surfactant 1-methylimidazolium bis-(2-ethylhexyl)phosphate (imim-DEHP) and the biocompatible solvents methyl laurate (ML) and isopropyl myristate (IPM) were studied through the use of two molecular probes: the IL 3,6-bis(dimethylamino) acridinium bis-(2-ethylhexyl) phosphate (AOBH-DEHP) and the complex N,N,N′,N′-tetramethylethylenediamine copper(II)acetylacetonate tetraphenylborate ([Cu(acac)(tmen)][B(C6H5)4]). The techniques used were absorption and emission spectroscopy. Specifically, we explore how the chemical nature of the surfactant alters RMs properties such as micropolarity, specific interactions, and restricted motion environment. Furthermore, we analyzed how the imim-DEHP assembly affects the coordination behavior of the surfactant. The results show that the interfacial properties of the imim-DEHP RMs are dominated by the chemical nature of the surfactant as a consequence of a strong interaction of the imim+ cation with water molecules. Because of this strong interaction imim+-water, the anion DEHP− has an enhanced capacity to coordinate ion Cu+2 in comparison when the surfactant is in a homogeneous medium. Also, our results demonstrate that the strength of the anion DEHP− as a ligand in imim-DEHP RMs can be modulated both through the choice of the external solvent and the water content. In summary, the obtained results are very interesting for the potential use of the imim-DEHP RMs as nanoreactors and in metals extraction.

The ionic liquid-surfactant bmim-AOT and nontoxic lipophilic solvents as components of reverse micelles alternative to the traditional systems. A study by 1H NMR spectroscopy

Abstract Aqueous reverse micelles (RMs) of the ionic liquid-surfactant 1-butyl-3-methylimidazolium 1,4-bis-2-ethylhexylsulfosuccinate (bmim-AOT) in the biocompatible nonpolar solvents isopropyl myristate (IPM) and methyl laurate (ML) were formulated and characterized by DLS and 1H NMR techniques; the goal is to develop less toxic and more environmentally friendly reverse micellar systems than traditional RMs formed by organic solvents such as n-heptane (n-Hp). The interfacial composition of these new micellar systems was compared with water/bmim-AOT/n-Hp RMs, analyzing the effect of changing the conventional external nonpolar solvent n-Hp for the biocompatible solvents IPM and ML. Also, they were compared with the RMs of the traditional surfactant sodium 1,4-bis-2-ethylhexylsulfosuccinate (Na-AOT) in IPM and ML to analyze the role of the bmim+ counterion compared to Na+ in the interfacial properties of the generated RMs. The results indicate that both ML and IPM can be used as nonpolar solvents to prepare RMs using bmim-AOT as surfactant. In comparison to n-Hp, IPM and ML penetrate more to the micellar interface, therefore, the ester group of the biocompatible solvent located at the micellar interface interacts with the bmim+ cation. As a consequence of this fact, the bmim+-AOT− interaction decreases when IPM or ML are used as the external solvent instead of n-Hp. In the Na-AOT/IPM and Na-AOT/ML systems, the dissimilar penetration of the external solvent makes the interfacial properties of both RMs clearly different. Interestingly, in the bmim-AOT/biocompatible solvent systems, the micellar interface properties are strongly governed by the bmim+ counterion, finding similar behavior if the external biocompatible solvent is IPM or ML.

Computer simulation study of fluorocarbon phosphate surfactant based aqueous reverse micelle in supercritical CO2: roles of surfactant functional groups, ionic strength, and phase changes in CO2.

Structural and dynamic properties of an aqueous micelle organized from fluorocarbon phosphate surfactant molecules in supercritical carbon dioxide (CO2) are investigated via molecular dynamics computer simulations. The roles of the functional groups and ionic strength of the surfactants on the formation of reverse micelles in supercritical CO2, and related water dynamics characterized as translational and reorientational dynamics, are systematically demonstrated by employing three different phosphate-based surfactants paired with sodium cations. The strong electrostatic interactions between the phosphate head groups and sodium cations result in formation of an aqueous core inside the surfactant aggregates, where water molecules are bonded together with loss of the tetrahedral hydrogen bonded network found in bulk water. It is found that all the three surfactants with CO2-philic fluorocarbon double tails build up well-stabilized reverse micelles in supercritical CO2, avoiding direct contacts between CO2 and water molecules. Despite this, the surfactant with a carboxylic ester linkage between the phosphate head and fluorocarbon tail group tends to coordinate water molecules toward sustaining the inter-water hydrogen bonds, indicating better efficiency at covering the aqueous core with hydrophobic groups compared to one without a carboxylic ester group. As for water molecules confined in the reverse micelle, their translational and reorientational motions, and fluctuating dynamics of the inter-water hydrogen bonds, significantly slow down compared to bulk water at ambient temperature. The water dynamics become more restricted with an increase in ionic strength of the anionic surfactant; this is attributed to divalent surfactant heads and sodium cations being more tightly bound together with bonding to water compared to monovalent ones. Lastly, the structural and dynamic changes of the reverse micelle caused by a phase change in CO2 are monitored with gradually decreasing temperature and pressure from the supercritical to gaseous state for CO2. The average reverse micelle structure equilibrated in supercritical CO2 is found to remain stable over a time period of 0.2 ms through a depressurization process to gaseous CO2. We note that the diverse pathways of surfactant self-aggregation in gaseous CO2 could be controlled by the preceding solvation procedure in the supercritical regime which governs the final aggregated structures in gaseous CO2.

Electrochemical Methodology as an Useful Tool for the Interfacial Characterization of Aqueous Reverse Micelles

AbstractIn this work, for the first time, the electrochemical behavior of hydroquinone (H2Q) by linear voltammetry in aqueous reverse micelles (RMs) of sodium 1,4‐bis(2‐ethylhexyl) sulfosuccinate (AOT)/n‐heptane, as well in homogenous media at different pH values, were studied. The choice of this molecular probe is because of its electrochemical response in water, strongly depend on the pH value thus, our goal is to know more about an intriguing issue in the soft matter field, what is the pH value for water encapsulated in the water pool of AOT RMs. Our work will demonstrate that the interfacial apparent pH value of AOT RMs is not strongly acidic as some of the literature works claim, but it is around a pH value of 6.7. Thus, through the electrochemistry technique, the present work reinforces the idea that the concept of is strongly dependent on the water‐interface interactions and the molecular probe location.

Photophysics of crystal violet lactone in reverse micelles and its dual behaviour

The photophysics of Crystal Violet Lactone (CVL) in aqueous and non aqueous reverse micelles (RMs) have been studied. It was observed that in RMs, the CTA → CTB transformation is retarded.

Excited state proton transfer dynamics of Topotecan inside biomimicking nanocavity.

The excited state proton transfer (ESPT) dynamics of a potentially important anticancer drug, Topotecan (TPT), has been explored in aqueous reverse micelle (RM) using steady-state and time-resolved fluorescence measurements. Both the time-resolved emission spectrum and time-resolved area normalized emission spectrum infer the generation of excited state zwitterionic form of TPT from the excited state cationic form of TPT, as a result of ESPT process from the -OH group of TPT to the nearby water molecule. The ESPT dynamics were found to be severely retarded inside the nanocavities of RMs, yielding time constants of 250 ps to 1.0 ns, which is significantly slower than the dynamics obtained in bulk water (32 ps). The observed slow ESPT dynamics in RM compared to bulk water is mainly attributed to the sluggish hydrogen-bonded network dynamics of water molecules inside the nanocavity of RM and the screening of the sodium ions present at the interface.

Effect of Confinement on Excited-State Proton Transfer of Firefly’s Chromophore d-Luciferin in AOT Reverse Micelles

Excited-state intermolecular proton transfer of D-luciferin in reverse micelles has been investigated using steady-state and time-resolved fluorescence spectroscopy measurement. The different polar cores have been chosen for the study of proton transfer dynamics in aerosol-OT (AOT) reverse micelles. It is shown that aqueous reverse micelle is the suitable environment for the photoprotolytic reaction of D-luciferin. The neutral form of the chromophore is present both in ground and excited state at W0 = 0. The proton transfer in nanometer size water pool of water/AOT/n-heptane begins at W0 = 8 and increases with increasing W0 values. However, the intermolecular excited-state proton transfer (ESPT) of D-luciferin is inhibited in nonaquous reverse micelles with DMF and DMSO as a polar core. Thus, the requirement of ESPT of D-luciferin to take place in reverse micelles consists of polar protic solvent like water as a polar core.

The photophysics of 7-(diethylamino)coumarin-3-carboxylic acid N-succinimidyl ester in reverse micelle: excitation wavelength dependent dynamics

This paper focuses on the photophysical studies of 7-(diethylamino)coumarin-3-carboxylic acid N-succinimidyl ester (7-DCCAE) in aqueous reverse micelles using steady state absorption, fluorescence emission and picosecond time resolved emission spectroscopy. We used sodium dioctylsulfosuccinate (AOT) as surfactant to prepare reverse micelles. We have observed excitation wavelength dependent photophysics of 7-DCCAE in the reverse micelles. The red edge excitation shift (REES) was found in the reverse micelles, at different w0 values. The REES gradually decreases with increase in size of the water pool. The rotational relaxation time of the dye molecule decreases with increasing the mole ratio of water to surfactant (w0) inside the reverse micelles. The fluorescence anisotropy decays were found biexponential in nature with fast and slow reorientation times, which is explained by two steps and “wobbling-in-cone model”. We have observed the excitation wavelength dependent dynamics of 7-DCCAE in the reverse micelles. We have observed the difference in the photophysics 7-DCCAE with its acid form.

Folding dynamics of flavin adenine dinucleotide (FAD) inside non-aqueous and aqueous reverse micelles

Present Letter describes dynamics of FAD in non-aqueous and aqueous reverse micelles (RMs). FAD in non-aqueous reverse micelles (containing MeOH, glycerol, formamide or DMF) shows a prominent rise of quantum yield with increasing solvent loading, whereas in water RM FAD shows a rise in quantum yield only up to w0=10 and afterwards drops sharply up to bulk. A relative difference in polarity of dry AOT to solvent loaded RMs can be attributed to the opposite trend of changes in emission. This Letter of FAD in different RMs and its preferential orientation might be useful for understanding FAD dynamics inside flavoproteins.

Solute and Solvent Dynamics in Confined Equal-Sized Aqueous Environments of Charged and Neutral Reverse Micelles: A Combined Dynamic Fluorescence and All-Atom Molecular Dynamics Simulation Study

Here a combined dynamic fluorescence and all-atom molecular dynamics simulation study of aqueous pool-size dependent solvation energy and rotational relaxations of a neutral dipolar solute, C153, trapped in AOT (charged) and IGPAL (neutral) reverse micelles (RMs) at 298 K, is described. RMs in simulations have been represented by a reduced model where SPC/E water molecules interact with a trapped C153 that possesses realistic charge distributions for both ground and excited states. In large aqueous pools, measured average solvation and rotation rates are smaller for the neutral RMs than those in charged ones. Interestingly, while the measured average solvation and rotation rates increase with pool size for the charged RMs, the average rotation rates for the neutral RMs exhibit a reverse dependence. Simulations have qualitatively reproduced this experimental trend and suggested interfacial location for the solute for all cases. The origin for the subnanosecond Stokes shift dynamics has been investigated and solute-interface interaction contribution quantified. Simulated layer-wise translational and rotational diffusions of water molecules re-examine the validity of the core-shell model and provide a resolution to a debate regarding the origin of the subnanosecond solvation component in dynamic Stokes shift measurements with aqueous RMs but not detected in ultrafast IR measurements.

Stabilization of a vanadium(v)–catechol complex by compartmentalization and reduced solvation inside reverse micelles

The kinetics of 1 : 1 complex formation and hydrolysis between catechol and 3-substituted catechols with aqueous vanadium(V) at pH ∼ 1 have been investigated in NaAOT (sodium bis(2-ethylhexyl)sulfosuccinate) derived aqueous reverse micelle microemulsions in isooctane. Compared with the reaction in bulk water, the forward rate constant of catechol complexation was modestly accelerated (2 fold) in the reverse micelle microemulsions for even the smallest nanosized water pools (wo = 2). In contrast, the first order reverse (aquation) reaction was significantly suppressed in the water pools below wo = 10. The modest rate acceleration of complex formation within the microemulsions is attributed to changes in water solvation, reaction properties and compartmentalization of the reactants. The dramatic fall off in the rate of catechol dissociation from [VO2(cat)(OH2)2]− is attributed to the reduction in water content as the size of the nanopools is decreased below 200 water molecules. The result is a 10 fold increase in the kinetic formation constant for [VO2(cat)(OH2)2]− under confinement in the RM microemulsion environment. This increase predicts that vanadium catechol complexes will be more stable in vivo and may suggest a general principle for fine-tuning efficacy of metal-based drugs.

Molecular dynamics simulation of self-assembly structure for AOK based reverse micelle in supercritical CO 2

Molecular dynamics simulations were performed to investigate the structural properties of aqueous reverse micelles of AOK (aerosol-octyl-ketone), a non-fluorinated hydrocarbon surfactant with two symmetrical carbonyl groups in the tail, in supercritical CO 2 fluid. The result for the first time demonstrates the atomic-level structural picture for the formation of nanodomain in water-in-CO 2 microemulsion based on the AOK surfactant, which is in good agreement with the recent experiment. The dynamics process shows that the self-assembly of the reverse micelle takes a relatively short period of time (∼4 ns) and remains stable over 50 ns. For comparison, the simulation for the AOK/scCO 2 system has also been performed and an irregular and sparse reverse micelle structure can be formed in the absence of water molecules. The radial density profiles, the pair radial distribution functions and the orientation distribution have been calculated to investigate the interfacial structure properties. The carbonyl groups in the surfactant tails may enhance the solvation of the AOK surfactant, thereby affording to stabilize the reverse micelle.

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 w(0). Correlation of photophysical parameters such as fluorescence quantum yield, fluorescence lifetime, and emission maxima with w(0) indicate distinctly different trends below and above w(0) approximately 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 w(0) approximately 7 for aqueous reverse micelles (and around w'(0) approximately 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 w(0) approximately 7 (or w'(0) approximately 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 D(2)O in reverse micelles with varying w(0). 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 (w(0)=7 and w'(0)=3) in the observed breaks in the two types of reverse micelles is also discussed.

Reverse micelles template assisted fabrication of ZnO hollow nanospheres and hexagonal microtubes by a novel fast microemulsion-based hydrothermal method

ZnO hollow micro/nanostructures were fabricated by a novel fast hydrothermal method based on the microemulsion. The aqueous reverse micelles were used as templates and different amount of Zn2+ colloid was compelled to hydrolyze on its surface. Scanning electron microscopy indicates that the products grown in the solution with colloid volume concentration of 12.3 and 0.5 v.% are hollow nanospheres and hexagonal microtubes, respectively. It is believed that this difference should attribute to the initial shape of hydrolysate and the core/shell state of water/surfactant during hydrothermal treatment.

Fourier transform infrared spectroscopy of azide and cyanate ion pairs in AOT reverse micelles

Evidence for ion pair formation in aqueous bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles (RMs) was obtained from infrared spectra of azide and cyanate with Li(+), Na(+), K(+), and NH(4)(+) counterions. The anions' antisymmetric stretching bands near 2000 cm(-1) are shifted to higher frequency (blueshifted) in LiAOT and to a lesser extent in NaAOT, but they are very similar to those in bulk water with K(+) and NH(4)(+) as the counterions. The shifts are largest for low values of w(o) = [water]/[AOT] and approach the bulk value with increasing w(o). The blueshifts are attributed to ion pairing between the anions and the counterions. This interpretation is reinforced by the similar trend (Li(+)>Na(+)>K(+)) for producing contact ion pairs with the metal cations in bulk dimethyl sulfoxide (DMSO) solutions. We find no evidence of ion pairs being formed in NH(4)AOT RMs, whereas ammonium does form ion pairs with azide and cyanate in bulk DMSO. Studies are also reported for the anions in formamide-containing AOT RMs, in which blueshifts and ion pair formation are observed more than in the aqueous RMs. Ion pairs are preferentially formed in confined RM systems, consistent with the well established ideas that RMs exhibit reduced polarity and a disrupted hydrogen bonding network compared to bulk water and that ion-specific effects are involved in mediating the structure of species at interfaces.

Protons in Non-ionic Aqueous Reverse Micelles

Using molecular dynamics techniques, we investigate the solvation of an excess proton within an aqueous reverse micelle in vacuo, with the neutral surfactant diethylene glycol monodecyl ether [CH3(CH2)11(OC2H4)2OH]. The simulation experiments were performed using a multistate empirical valence bond Hamiltonian model. Our results show that the stable solvation environments for the excess proton are located in the water-surfactant interface and that its first solvation shell is composed exclusively by water molecules. The relative prevalence of Eigen- versus Zundel-like solvation structures is investigated; compared to bulk results, Zundel-like structures in micelles become somewhat more stable. Characteristic times for the proton translocation jumps have been computed using population relaxation time correlation functions. The micellar rate for proton transfer is approximately 40x smaller than that found in bulk water at ambient conditions. Differences in the computed rates are examined in terms of the hydrogen-bond connectivity involving the first solvation shell of the excess charge with the rest of the micellar environment. Simulation results would indicate that proton transfers are correlated with rare episodes during which the HB connectivity between the first and second solvation shells suffers profound modifications.

Tuning the Cooperativity of the Helix−Coil Transition by Aqueous Reverse Micelles

We show in this letter that the thermodynamic properties of helical peptides can be tuned by varying the degrees of backbone hydration. The latter was achieved by solubilizing peptides in the water pool of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles with different water contents or w0 values. Far-UV circular dichroism measurements on a series of alanine-rich peptides indicate that the helicity of shorter peptides is significantly increased in AOT reverse micelles at low w0 values, as compared to the corresponding helical content in buffer. This result therefore corroborates the previous simulation studies suggesting that desolvation of backbone CO and NH groups increases the stability of monomeric helices. In addition, it was found that the thermal unfolding transition of these peptides can either be very noncooperative or very cooperative, depending on w0 and peptide chain length. A simple model, which considers the heterogeneous distribution of the water molecules inside the polar core of AOT reverse micelles as well as the geometric confinement effect exerted on the peptide by the reverse micelles, was used to interpret these results.

Molecular Dynamics Simulation Studies of Polyether and Perfluoropolyether Surfactant Based Reverse Micelles in Supercritical Carbon Dioxide

Molecular dynamics simulations are performed to investigate the structural properties of aqueous reverse micelles in supercritical carbon dioxide. A carboxylate perfluoropolyether is used as the surfactant in the first simulation. The results show that the micelle remains stable over a 5 ns time period. Calculated micellar properties are found to be in good agreement with the available experimental data. Two other simulations are performed on systems where all fluorine atoms of the surfactant molecules are replaced with hydrogen atoms. In the first of these simulations the number of molecules is the same as in the simulation with perfluorinated surfactant. In the second simulation the exposure of an aqueous core to carbon dioxide is similar to the one observed in the simulation with perfluorinated surfactant. We observe that fluorinated chains coordinate more carbon dioxide molecules compared to the pure polyether chains. This has a direct influence on the micellar structure.

Micellar catalysis for partial oxidation of toluene to benzoic acid in supercritical CO 2: effects of fluorinated surfactants

One of the key hindrances on development of solid catalysts containing cobalt species for partial oxidation of organic molecules at mild conditions in conventional liquid phase is the severe metal leaching. The leached soluble Co species with a higher degree of freedom always out-performs those of solid supported Co species in oxidation catalysis. However, the homogeneous Co species concomitantly introduces separation problems. We have recently reported for the first time, a new oxidation catalyst system for the oxidation of organic molecules in supercritical CO 2 using the principle of micellar catalysis. [CF 3(CF 2) 8COO] 2Co· xH 2O (the fluorinated anionic moiety forms aqueous reverse micelles carrying water-soluble Co 2+ cations in scCO 2) was previously shown to be extremely active for the oxidation of toluene in the presence of sodium bromide in water–CO 2 mixture, giving 98% conversion and 99% selectivity to benzoic acid at 120 °C. In this study, we show that the effects of varying the type of surfactant counterions and the length of the surfactant chains on catalysis. It is found that the use of [CF 3(CF 2) 8COO] 2Mg· yH 2O/Co(II) acetate is as effective as the [CF 3(CF 2) 8COO] 2Co· xH 2O and the fluorinated chain length used has a subtle effect on the catalytic rate measured. It is also demonstrated that this new type of micellar catalyst in scCO 2 can be easily separated via CO 2 depressurisation and be reused without noticeable deactivation.