Reverse Micelle Media Research Articles

Photophysical properties and rotational dynamics of coumarin 30 dye in reverse micelle systems

In this study, photophysical properties and photodynamics of Coumarin 30 (C30) dye were studied in detail in reverse micelles by using spectroscopy techniques. The reverse micelle systems (RM) were created using three different surfactants: anionic, cationic, and neutral. Photophysical and rotational dynamics of C30 were maped and interpreted as function of W0 by using the fluorescence quantum yields, fluorescence lifetime values, radiation and non-radiation rate constants, anisotropy values and rotational relaxation times. We have found that C30 is an applicable probe for the assignment of microviscosity of different reverse micelle media.

On the design of a versatile ionic liquid, AOBH-DEHP, which can be used as a new molecular probe to investigate supramolecular assemblies

The motivation of our work is to find an exciting new molecular probe, which has the advantages that contains the acridine orange moiety but also has an ample range of solubility in different media. The probe is obtained through the acid-base reaction between acridine orange base (AOB) and bis-(2-ethylhexyl) phosphoric acid (HDEHP) giving AOBH-DEHP. This synthesized dye results to be a protic ionic liquid, since it is a viscous liquid at room temperature and, it is only constituted by ions.The spectroscopic behavior of AOBH-DEHP in homogeneous medium, using water, benzene and n-heptane as solvents and, in benzene or n-heptane/sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT)/water reverse micelles (RMs) has been investigated. Absorption, emission and single photon counting techniques, were used to fully characterize the new molecule in different environments.It is shown that AOBH-DEHP has all the advantages of its chromophore, AOBH+ but, the presence of the hydrophobic anion, DEHP−, gives to the dye, unique solubility in solvents of very different polarities. As such and, in contrast to AOBH+ chloride salts, AOBH-DEHP can be used to determine different properties, such as micropolarity, restricted motion environment, critical micellar concentration in different RMs media.

Visible and fluorescence spectroscopic studies on interaction of water-soluble anionic azo dyes with Triton X-100 in reverse micelle media

A series of Triton X-100 (TX-100) reverse micelles was prepared in isooctane and characterized with respect to size, polarity, and electric conductivity. Three typical azo dyes including methyl orange (MO), Acid Black 234 (AB 234) and Reactive Red 195 (RR 195) were selected and solubilized in the water phase to investigate their interaction with TX-100 reverse micelles. Effect of water content on the spectra of these dyes in the micelles was also examined. It has been revealed from the experimental results that at low water content, MO and AB 234 may be located in the polar core but outside the water pool of the micelles. Increasing water content leads to their gradual movement towards the water pools. AB 234 may bind to the micelles more closely than MO, especially at high water content. Moreover, the stronger interaction between RR 195 and TX-100 result in the complex formation. Higher water content enhances the transition from bound RR 195 to free RR 195. It can be concluded that the spectral characteristics of the three dyes in the micelles are very different owing to the space limitation and the polarity evolution of the microenvironment of the micelles.

On the Investigation of the Droplet–Droplet Interactions of Sodium 1,4‐Bis(2‐ethylhexyl) Sulfosuccinate Reverse Micelles upon Changing the External Solvent Composition and Their Impact on Gold Nanoparticle Synthesis

AbstractThe effect of the composition of the nonpolar organic media on the properties of sodium 1,4‐bis(2‐ethylhexyl) sulfosuccinate (AOT) reverse micelles (RMs) at a fixed temperature were investigated. To monitor interfacial micropolarity and sequestrated water structure in n‐heptane:benzene/AOT/water RMs, the solvatochromic behavior of coumarin 343 (C343) as an absorption and emission probe was studied, and the size of the droplets was measured by dynamic light scattering (DLS). The DLS results confirm the formation of the n‐heptane:benzene/AOT/water RMs at every n‐heptane mole fraction investigated. The data show that as the n‐heptane content increases, the interdroplet attractive interactions and the droplet size both increase. With C343 spectroscopy, we determined the “operational” critical micellar concentration, the interfacial micropolarity of the RMs, the hydrogen‐bond ability of the media, and the sequestrated water structure in every RM system studied. To verify the modulation of the interdroplet interactions by the external solvent, we have used a well‐known synthetic method to create gold nanoparticles in different RM media. The results support the conclusion that the droplet–droplet interactions are favored with n‐heptane as the nonpolar solvent. Thus, the rate of material exchange among micelles increases, and small, monodispersed, and highly concentrated gold nanoparticles are formed. The situation is different for AOT RMs prepared in benzene, in which subnanometer gold clusters are produced. The formation of clusters instead of nanoparticles reflects poor material exchange among the micelles owing to unfavorable droplet–droplet interactions. In this way, our results offer a new appealing way to focus the production of nanoparticles into subnanometer clusters, which is one of the hottest current topics in nanoparticle synthesis. Therefore, it is possible to dramatically affect different properties of the RMs by changing the solvent blend; such effects cannot be obtained with only a single solvent composition and may have a large impact on nanotechnology. We anticipate remarkable effects on nanoparticle sizes and polydispersity with different organic solvent blends.

Inhibited Phenol Ionization in Reverse Micelles: Confinement Effect at the Nanometer Scale

We found that the absorption spectra of 2-acetylphenol (2-HAP), 4-acetylphenol (4-HAP), and p-nitrophenol (p-NPh) in water/sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT)/n-heptane reverse micelles (RMs) at various W(0) (W(0) = [H(2)O]/[surfactant]) values studied changed with time if (-)OH ions were present in the RM water pool. There is an evolution of ionized phenol (phenolate) bands to nonionized phenol absorption bands with time and this process is faster at low W(0) values and with phenols with higher bulk water pK(a) values. That is, in bulk water and at the hydroxide anion concentration used, only phenolate species are observed, whereas in AOT RMs at this fixed hydroxide anion concentration, ionized phenols convert into nonionized phenol species over time. Furthermore, we demonstrate that, independent of the (-)OH concentration used to prepare the AOT RMs, the nonionized phenols are the more stable species in the RM media. We explain our results by considering that strong hydrogen-bonding interactions between phenols and the AOT polar head groups result in the existence of only nonionized phenols at the AOT RM interface. The situation is quite different when the phenols are dissolved in cationic benzyl-n-hexadecyldimethylammonium chloride RMs. Therein, only phenolates species are present at the (-)OH concentrations used. The results clearly demonstrate that the classical definition of pH does not apply in a confined environment, such as in the interior of RMs and challenge the general idea that pH can be determined inside RMs.

Catalytic performance and thermostability of chloroperoxidase in reverse micelle: achievement of a catalytically favorable enzyme conformation

The catalytic performance of chloroperoxidase (CPO) in peroxidation of 2, 2'-azinobis-(-3 ethylbenzothiazoline-6-sulfononic acid) diammonium salt (ABTS) and oxidation of indole in a reverse micelle composed of surfactant-water-isooctane-pentanol was investigated and optimized in this work. Some positive results were obtained as follows: the peroxidation activity of CPO was enhanced 248% and 263%, while oxidation activity was enhanced 215% and 222% in cetyltrimethylammonium bromide (CTABr) reverse micelle medium and dodecyltrimethylammonium bromide (DTABr) medium, respectively. Thermostability was also greatly improved in reverse micelle: at 40 °C, CPO essentially lost all its activity after 5 h incubation, while 58-76% catalytic activity was retained for both reactions in the two reverse micelle media. At 50 °C, about 44-75% catalytic activity remained for both reactions in reverse micelle after 2 h compared with no observed activity in pure buffer under the same conditions. The enhancement of CPO activity was dependent mainly on the surfactant concentration and structure, organic solvent ratio (V(pentanol)/V(isooctane)), and water content in the reverse micelle. The obtained kinetic parameters showed that the catalytic turnover frequency (k(cat)) was increased in reverse micelle. Moreover, the lower K(m) and higher k(cat)/K(m) demonstrated that both the affinity and specificity of CPO to substrates were improved in reverse micelle media. Fluorescence, circular dichroism (CD) and UV-vis spectra assays indicated that a catalytically favorable conformation of enzyme was achieved in reverse micelle, including the strengthening of the protein α-helix structure, and greater exposure of the heme prosthetic group for easy access of the substrate in bulk solution. These results are promising in view of the industrial applications of this versatile biological catalyst.

Comparative Study of the Photophysical Behavior of Fisetin in Homogeneous Media and in Anionic and Cationic Reverse Micelles Media†

The 3,3', 4',7 tetrahydroxiflavone (fisetin) is a natural therapeutically active and fluorescent polyhydroxyflavone, with important spectroscopic and biological behavior. Fisetin shows dual emission, with a normal band (N) from the S1 --> S0 transition and the one generated in the excited state (phototautomer; PT) from the intramolecular proton transfer (ESIPT) process. The influence of different interfaces on the ESIPT process of fisetin was investigated in reverse micelles media (RMs) made of the anionic sodium 1,4-bis (2-ethylhexyl) sulfosuccinate (AOT) and cationic benzyl n-hexadecyl dimethylammonium chloride (BHDC) surfactants, in benzene. The studies were carried out by absorption, emission spectroscopy, steady-state anisotropy and time-resolved fluorescence measurements. Fisetin behavior was also investigated in homogeneous media with special emphasis in water and benzene, which are the polar core and the organic pseudofase in the RMs, respectively. In addition, the effect of concentration in benzene and the variation of the pH in water were studied. Fluorescence lifetime measurements show that in water the ESIPT process is independent on the concentration, while in benzene it was possible to detect fluorescent aggregate species (Nas) formed in the ground state. The effect of the pH in water allowed us to identify the anionic fisetin (A-) emission. The studies in RMs show that fisetin interacts specifically with the head of the surfactants, which always results in diminishing the emission of the PT. Also the formation of A- is detected particularly at W0 > 0. Appreciable high anisotropy values are obtained in RMs, as compared with those in fluid homogeneous media, which are independent of the water content confirming that fisetin molecules are anchored in the anionic as well as in the cationic interfaces.

Catalysis in Micellar Media. Kinetics and Mechanism for the Reaction of 1-Fluoro-2,4-dinitrobenzene with n-Butylamine and Piperidine in n-Hexane and AOT/n-Hexane/Water Reverse Micelles

The nucleophilic aromatic substitution SNAr reaction of 1-fluoro-2,4-dinitrobenzene with n-butylamine and piperidine was studied in n-hexane and AOT/n-hexane/water reverse micelle media. The kinetic profile of the reactions was investigated as a function of variables such as AOT and the amine concentration and the amount of water dispersed in the reverse micelles, W0 = [H2O]/[AOT]. The reactions are faster in the micellar medium than in the pure solvent. The overall reactions are accelerated by increasing the AOT concentration until a value of [AOT] ≅ 0.1 M, where saturation in the micellar interface is reached and no further micellar catalysis is observed. The reactions are base catalyzed in n-hexane but not in a micellar medium. Thus, despite the partition of the reactants in both pseudophases the reactions effectively take place at the interface of the aggregates. The kinetic behavior can be quantitatively explained by taking into account the distribution of the substrate and the nucleophile between the bulk solvent and the micelle interface. The results were used to evaluate the amine distribution constant between the micellar pseudophase and organic solvent and the intrinsic second-order rate coefficient of the SNAr reaction in the interface. A mechanism to rationalize the kinetic results is proposed.

Direct monitoring of formaldehyde vapour and detection of ethanol vapour using dehydrogenase-based biosensors

Biosensors capable of directly detecting low levels of formaldehyde and ethanol vapour were constructed. Both biosensors are based on dehydrogenase enzymes which produce reduced nicotinamide adenine dinucleotide as part of the oxidation of formaldehyde and ethanol. The enzymes were immobilized in a reverse micelle medium which did not dehydrate significantly over time, and allowed direct gas-phase monitoring. A screen-printed electrode was used as transducer. Formaldehyde and ethanol vapour partitioned into the reverse micelle media, where it was acted upon by the relevant enzyme. Reduced nicotinamide adenine dinucleotide was oxidized at the working electrode at a potential of 800 mV versus an Ag/AgCl reference electrode. Formaldehyde could be measured over the concentration range 1 ppb–1.3 ppm and ethanol could be detected over the range 50–250 ppm.