Reverse Micellar Core Research Articles

Super-Hydrophilic Co- and Mo-Based Mix-Metallomicellar Film as an Electrocatalyst toward Hydrogen Evolution Reaction

A rational design and facile synthesis that is based upon inexpensive and earth-abundant elements is an effective strategy for electrochemical hydrogen evolution reaction (HER). Herein, we have utilized single-chain metallosurfactants (CoDDA(I) and MoDDA(I)) to develop a mix-metallomicellar film (Co-MoDDA(I)) that is used as an electrocatalyst toward HER. To our knowledge, there have been no reports on mix-metallomicellar films toward HER. Experimental results reveal that the HER activity of Co-MoDDA(I) was comparable with commercial Pt/C (20 wt %), a low Tafel slope (29.2 mV dec–1), and long-term stability. The dramatically improved HER performance of Co-MoDDA(I) was favored by the combination of the following factors: formation of reverse micelles, super-hydrophilic characteristic, and presence of Co(II) and Mo(III) ions in the micellar core. Organic media pushed the Co of CoDDA(I) toward the core of the reverse micelles, which further attracts the Mo ions of MoDDA(I) to form a mixed micellar film. The cooperative effect of Co and Mo in the reverse micellar core generates the catalytic center for efficient hydrogen production. In situ reduction of Co and the synergic effect between 3d (Co) and non 3d (Mo) metal ions in the core of micelles accelerate the HER reaction rate effectively. Several chemical changes were also observed in the material, i.e., removal of chloride ions from the surface and reduction of Co inside the core of reverse micelles as authenticated by X-ray photoelectron spectroscopy (XPS). However, surface characteristics were verified by field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). Metallosurfactants are available to provide controllable structure, orientation, and structural stability and assist to detach the bubbles from the surface.

Modulation of probe-genomic DNA interaction within the confined interior of a reverse micelle: Is the bulk-like properties of water truly achieved in large reverse micelles?

The prime motivation of the present study is to explore the effect of reverse micellar confinement on the binding interaction of an anthracene-based probe 9-methyl anthroate with herring-sperm DNA. The structural modification of the genomic DNA from its native B-form to the non-native C-form and subsequently to the condensed Ψ-form as a function of the level of hydration (W0, defined as [water] / [surfactant]) of the reverse micellar core is found to reveal a remarkable regulatory role on the stability of the stacking interaction (intercalation) of the probe within the DNA helix; the interaction being progressively stabilized at higher W0. Particularly, a close perusal of the dynamical aspects of the interaction is found to be counter-intuitive to the popular notion of the properties of the confined water within the reverse micelles typically approaching bulk-like properties at sufficiently high hydration levels (W0 > 10).

How Does Nanoconfinement within a Reverse Micelle Influence the Interaction of Phenazinium-Based Photosensitizers with DNA?

The major focus of the present work lies in exploring the influence of nanoconfinement within aerosol-OT (AOT) reverse micelles on the binding interaction of two phenazinium-based photosensitizers, namely, phenosafranin (PSF) and safranin-O (SO), with the DNA duplex. Circular dichroism and dynamic light-scattering studies reveal the condensation of DNA within the reverse micellar interior (transformation of the B-form of native DNA to ψ-form). Our results unveil a remarkable effect of the degree of hydration of the reverse micellar core on the stability of the stacking interaction (intercalation) of the drugs (PSF and SO) into DNA; increasing size of the water nanopool (that is, w0) accompanies decreasing curvature of the DNA duplex structure with the consequent effect of increasing stabilization of the drug:DNA intercalation. The marked differences in the dynamical aspects of the interaction scenario following encapsulation within the reverse micellar core and the subsequent dependence on the size of the water nanopool are also meticulously explored. The differential degrees of steric interactions offered by the drug molecules (presence of methyl substitutions on the planar phenazinium ring in SO) are also found to affect the extent of intercalation of the drugs to DNA. In this context, it is imperative to state that the water pool of the reverse micellar core is often argued to approach bulk-like properties of water with increasing micellar size (typically w0 ≥ 10), so that deviation from the bulk water properties is likely to be minimized in large reverse micelles (w0 ≥ 10). On the contrary, our results (particularly quantitative elucidation of micropolarity and dynamical aspects of the interaction) explicitly demonstrate that the bulk-like behavior of the nanoconfined water is not truly achieved even in large reverse micelles.

Prototropism and dynamics of an anticancer drug in reverse micelles: Focus on the variation of pH in reverse micelles having w0 ≥ 10

The modulations of the photophysics and dynamics of an archetypal anticancer β-carboline drug, namely, harmane (HM) within AOT reverse micelles (RMs) have been explored. To this end, particular focus is given on a pH-dependent study commensurate with the effect of increasing water content within the RM interior. Our results reveal markedly different photophysical behavior of the drug encapsulated within the RM compared to that in bulk aqueous medium. For example, our results show the presence (or absence) of the cationic (or neutral) emission of HM in RMs in pH values apparently much higher (or lower) than that of pure aqueous medium. The characteristic zwitterionic emission in strongly alkaline bulk aqueous solutions (e.g., pH12.2 and 13.5) is also found to be lacking within the reverse micellar core, even in RMs with w0≥10. These data are further substantiated from the study of the dynamical aspects of the interaction scenario, that is, modulation of the fluorescence decay and rotational relaxation behaviors of HM entrapped within the RMs. Cumulatively, our results indicate the presence of a proton gradient across the reverse micellar water pool in which the interfacial regime appears to be more acidic in comparison to the central core. The results with alkaline solutions (e.g., pH10.0, 12.2 and 13.5) suggest selective compartmentalization of the hydroxide ions leaving the effective pH of the water pool lower than that of the bulk aqueous solution.

Glycine molecules in ionic liquid based reverse micelles: Investigation of structure and dynamics using molecular dynamics simulations

Abstract Amino acids, the building blocks of proteins, are probed in reverse micellar encapsulation processes of biological molecules. To achieve a theoretical picture of the effect of confinement of amino acids on the structure and dynamics of reverse micellar aqueous core, a water/[C 1 C 10 Im][Br] reverse micelles containing variable number of glycine molecules (0 to 8), dispersed in pure nonane has been studied using atomistic molecular dynamics simulations. The size of the central water pool was found to become stable with increase in the number of glycine molecules. Glycine molecules associate together through H-bonding interaction within the aqueous core of the reverse micelles. The radial density profile of glycine molecules from the center of mass of the reverse micellar aqueous core shows the preferential presence of glycine molecules near the center. Confinement in nano-space region stabilizes the H-bonds formed between glycine molecules compared to bulk aqueous solution. Increase in the number of glycine molecules within the reverse micellar aqueous core increases the survival time of H-bonds among glycine molecules as well as among water molecules. Translational and reorientational motion of glycine and water molecules are found to be directly related to the survival probability of intermolecular glycine–glycine and water–water H-bonds suggesting the correlation between the mobility of molecules with the rearrangement of the H-bonding network.

Effects of Reverse Micellar Structure on the Particle Charging Capabilities of the Span Surfactant Series.

This paper investigates the effects of reverse micellar core size on the particle charging behavior of a series of acidic surfactants in apolar media. A series of Span surfactants was dissolved in deuterated decane at concentrations above the critical micelle concentration. The structures of the reverse micelles were measured using small-angle neutron scattering. It was determined that as the tail length of the surfactant increased, the size of the polar reverse micellar core decreased. Tritailed surfactants formed reverse micelles with the smallest polar cores, with radii of ∼4 Å. The sizes of the polar cores were correlated with the particle charging behavior of the Span surfactant series, as measured in a previous study. It was found that reverse micelles with intermediate core sizes imparted the largest electrophoretic mobilities to the particles. Reverse micelles with very small cores did not offer a large enough polar environment to favor charge stabilization, while very large polar cores favored disproportionation reactions in the bulk, resulting in increased electrostatic screening.

Probing the location of methanol in methanol/AOT/n-heptane system: true microemulsion or bi-continuous medium?

Whether methanol can be effectively encapsulated within a reverse micellar core to form a true microemulsion is still a matter of debate. Thus, in this contribution, we have reported the modified photophysics of a potential chloride channel blocker 9-methyl anthroate (9-MA) in methanol/AOT/n-heptane reverse micelles and methanol/n-heptane binary mixture in order to assess the location of methanol by means of steady-state and time-resolved fluorescence spectroscopic techniques. By comparing the striking similarity between the photophysics of the probe in the methanol/AOT/n-heptane system and in the methanol/n-heptane binary mixture, we conclude that methanol does not form a true microemulsion but rather remains dissolved in the bulk nonpolar phase forming a bi-continuous medium owing to an appreciable mutual solubility between n-heptane and methanol.

Extraction of tea polysaccharides (TPS) using anionic reverse micellar system

Reverse micelles extraction technology are efficient and economical alternative for extraction of bioactive molecules due to its high selectivity, fast mass transfer and relatively low cost. In this study, AOT can form wide range of reverse micellar core sizes in organic phase, which has been used to extract tea polysaccharides (TPS) in AOT/heptane reverse micellar system. Process conditions were optimized by pH, types and concentration of salts, types and concentration of alcohols. Chaotropes (GuHCl and urea) were used to improve extraction yield of tea polysaccharides in the system. In forward extraction, low concentration of GuHCl in this AOT/heptane system may influence the TPS structure and suppress its aggregation formation and promote its solubilization into the water cores. Under optimal conditions (pH 4.6, 0.05molL−1 NaCl, 7% [v/v] methanol and 0.06molL−1 GuHCl), the forward extraction recovery reached 34%. In backward extraction, 100% of backward recovery was achieved by adding GuHCl or urea.

Dynamics of pyrenesemicarbazide and pyrenethiosemicarbazide in reverse micelle of AOT in n-heptane: Probing critical penetration of water molecules toward the palisade

In the present work, we have introduced two similar probe molecules with pyrene tags that can reside in different positions in the reverse micelles and show opposite dynamism on excitation. Herein, we have changed the concentration of water in the reverse micellar core gradually and monitored the spectrochemical changes in the dynamism of the two designed probe molecules. The results indicate that water molecules penetrate through the interface of the reverse micelles up a certain critical amount.

Dynamics of Solvation and Rotational Relaxation of Coumarin 480 in Pure Aqueous-AOT Reverse Micelle and Reverse Micelle Containing Different-Sized Silver Nanoparticles Inside Its Core: A Comparative Study

In this work, we have synthesized different-sized silver nanoparticles in an aqueous-AOT reverse micellar system under the same condition by choosing different reduction processes. We chose two different reducing agents, glucose (mild) and sodium borohydride (strong). In the glucose reduction process, we obtained smaller size nanoparticles in comparison to the nanoparticles obtained in the borohydride reduction process under the same condition. Solvation dynamics study showed that reverse micellar aggregated structures were present after the nanoparticles' formation in a perturbed state. Nanoparticles inside the reverse micellar core were responsible for this perturbation. Larger size nanoparticles were triggering larger perturbation than the smaller size nanoparticles. These changes in perturbation were also reflected clearly in solvation dynamics and rotational relaxation measurements.

In vitro and In vivo Optical Imaging Using Water-Dispersible, Noncytotoxic, Luminescent, Silica-Coated Quantum Rods

A facile method for synthesis of monodisperse, biocompatible, silica coated CdSe/CdS/ZnS quantum rods (QRs) and their use as probes for in vitro and in vivo optical bioimaging is presented. The silica coating procedure involves condensation-polymerization of the precursor tetraethylorthosilicate (TEOS) over QRs encapsulated within the reverse micellar core of Igepal CO-520/cyclohexane microemulsion. The silica-coated QRs have been characterized by TEM, DLS, and photoluminescence spectroscopy. The silica shell thickness has also been shown to be tuned by varying the ratio of TEOS and QRs. Confocal bioimaging of different cell lines (Panc 1 and RAW), treated with silica coated QRs, has shown a robust uptake of the nanoparticles, without the need of specific targeting molecules. To evaluate the cytotoxicity of the nanoparticles, MTS assay has been carried out over a period of 48 h. In vivo imaging of subcutaneously xenografted tumor-bearing mice, following direct intratumoral injection of the nanoparticles, ...

Photophysics and rotational relaxation dynamics of cationic phenazinium dyes in anionic reverse micelles: Effect of methyl substitution

We present here, a detailed photophysical and rotational relaxation dynamical study of three structurally analogous cationic dyes, namely, phenosafranin (PSF), safranin-T (ST), and safranin-O (SO), carried out in well characterized, monodispersed biomimicking anionic reverse micellar nanocavities composed of sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/heptane with increasing water contents. The dyes belong to the phenazinium family and they differ in terms of methyl substitution on the planar phenazinium skeleton. The objective of the present study is to investigate the modification in the photophysical and dynamical behavior of the dyes with the change in the size of the water pool of the reverse micelle and thereby to explore the role of methyl substitution. Steady state and time resolved emission and anisotropy studies have been exploited for the purpose. The dyes are found to exhibit a marked decrease in the fluorescence anisotropy with increasing water/surfactant mole ratio (w), i.e., the water pool size in the reverse micellar core, implying that overall motional restriction experienced by the molecules are decreased with increasing hydration. Some of the depth dependent fluorescence parameters such as fluorescence maximum, fluorescence anisotropy (r) have been monitored for exploring the microenvironment around the probes in the reverse micelles. Fluorescence studies suggest that at low w values, the probes do not penetrate into the reverse micellar core; rather it binds at the interfacial region. Estimates of the micropolarity at the binding sites of the probe molecule have been determined as a function of w. Finally, dynamic studies reveal that both the lifetime and rotational relaxation time decrease with an increase in w for all the three probes, the extent of the decrease being more for PSF than ST and SO. This indicates a stronger binding of the reverse micelle with ST and SO compared to that with PSF which is rationalized in terms of an increase in the hydrophobicity of the former two dyes because of the methyl substitution on the phenazinium moiety.

Modulation of Dynamics and Reactivity of Water in Reverse Micelles of Mixed Surfactants

In this contribution, we attempt to correlate the change in water dynamics in a reverse micellar (RM) core caused by the modification of the interface by mixing an anionic surfactant, sodium bis(2-ethylhexyl) sulfosuccinate (AOT), and a nonionic surfactant, tetraethylene glycol monododecyl ether (Brij-30), at different proportions, and its consequent effect on the reactivity of water, measured by monitoring the solvolysis reaction of benzoyl chloride (BzCl). The dimension of the RM droplets at different mixing ratios of AOT and Brij-30 (X(Brij-30)) has been measured using dynamic light scattering (DLS) technique. The physical properties of the RM water have been determined using Fourier transform infrared spectroscopy (FTIR) and compressibility studies, which show that with increasing X(Brij-30), the water properties tend toward that of bulk-like water. The solvation dynamics, probed by coumarin 500 dye, gets faster with X(Brij-30). The rotational anisotropy studies along with a wobbling-in-cone analysis show that the probe experiences less restriction at higher X(Brij-30). The kinetics of the water-mediated solvolysis also gets faster with X(Brij-30). The increased rate of solvolysis has been correlated with the accelerated solvation dynamics, which is another consequence of surfactant headgroup-water interaction.

Photophysical Study of 3-Acetyl-4-oxo-6,7-dihydro-12H-indolo[2,3-a]quinolizine in Biomimetic Reverse Micellar Nanocavities: A Spectroscopic Approach

Photophysical properties of 3-acetyl-4-oxo-6,7-dihydro-12H-indolo[2,3-a]quinolizine (AODIQ), a bioactive molecule, has been investigated in well-characterized, monodispersed biomimicking nanocavities formed by sodium bis(2-ethylhexyl)sulfosuccinate (AOT) in heptane using steady-state and picosecond time resolved fluorescence and fluorescence anisotropy. The emission behavior of AODIQ is very much dependent upon the water/surfactant mole ratio (W), i.e., on the water pool size of the reverse micellar core. AODIQ exhibits a sharp decrease in fluorescence anisotropy with increasing W, implying that the overall motional restriction experienced by the molecule is decreased with increased hydration. Some of the depth-dependent relevant fluorescence parameters, namely, fluorescence maxima and fluorescence anisotropy (r), have been monitored for exploiting the distribution and microenvironment around the probe in the reverse micelles. Fluorescence spectral position and fluorescence quenching studies suggest that the probe does not penetrate into the reverse micellar core; rather it binds at the interfacial region. Quantitaive estimates of the micropolarity and microviscosity at the binding sites of the probe molecule have been determined as a function of W.

Spectroscopic study of some photographic developing agents in reverse micelles of AOT in heptane

p-aminophenol, N-methyl p-aminophenol sulfate, p-phenylene diamine, 2,4-diaminophenol hydrochloride and pyrogallol are used as photographic developer. These compounds are easily oxidized in the presence of air, in solution as well as in pure solid form. The effects of reverse micelle on these compounds have been studied to investigate their stability and interactions such as H-bond formation in the reverse micellar core. Preferential interaction of –NH 2 group over –OH group with the interface of reverse micelle has also been observed. Effects of pH and salt on these compounds in the reverse micellar core have been investigated.

Modelling of CaCO3 Nanoparticle Formation During Overbasing of Lubricating Oil Additives

A framework of population balance equations has been developed to model formation of CaCO3 nanoparticles during overbasing of lubricating oil. The process involves carbonation of a reverse micellar solution containing lime, present both in the micelles and as a suspension of lime particles in the oil. The mechanism leading to CaCO3 nanoparticles in this setup consists of a number of elementary events such as CO2 transport from gas to reverse micelles through the organic phase, reaction in the reverse micellar core, nucleation of CaCO3, particle growth, and Brownian collisions leading to material exchange, both among reverse micellar drops and between drops and lime particles. A time scale analysis of these steps permits simplification and enables us to divide the whole process into two stages. The first consists of reaction of existing lime in micelles and a burst of nucleation of very short duration, wherein some reverse micelles beget a single nucleus each. The number of such nucleated reverse micelles depends on the relative rates of mass transfer, nucleation, and growth by intermicellar Brownian collisions. This is followed by a slow growth phase of these initial particles through Brownian collisions between nucleated reverse micelles and lime particles. The model predicts the data of Roman et al. (J. Colloid Interface Sci. 1991, 144, 324.), where on average only 10 initial reverse micelles contribute to form a CaCO3 nanoparticle. A simplified version of the model, obtained in the limit of instantaneous gas transfer, is also able to approximately predict the results of Kandori et al. (J. Colloid Interface Sci. 1988, 122, 78.) where, in contrast, a huge number of 108 reverse micelles contribute to form one particle. The model is quite general and can be used for other gas−liquid micellar precipitation systems wherein similar relative orders of time scales are involved.

Synthesis and Characterization of the Surfactant Terbium 3-[[1,2-Bis-[[(2-Ethylhexyl)Oxy]Carbonyl]Ethyl]Thio]Succinate as a Reagent for Determining Organic Analytes

The surfactant terbium 3-[[1,2-bis[[(2-ethylhexyl)oxy]carbonyl]ethyl]thio]succinate has been synthesized and characterized by use of its absorption, luminescence, and microviscosity properties. In the presence of small amounts of water, this surfactant aggregates in cyclohexane to form reversed micelles containing Tb(III) counterions. The critical reverse micelle concentration has been determined to be 5.7 × 10−5 M with the use of an optical probe. Organic analytes solubilized in reverse micelles have been detected indirectly with the use of the luminescence characteristics of Tb(III) counterions. The detection scheme is based on energy transfer from the solubilized organic donor to acceptor Tb(III) counterions. Analytical figures of merit for the micellar system in the presence of organic analytes are presented. The microviscosity of the reverse micellar core has been estimated with the use of a viscosity-sensitive luminescent probe.