Formation Of Reverse Micelles Research Articles

Lipid Enhanced Exfoliation for Production of Graphene Nanosheets

Liquid-phase exfoliation of graphite is a widely used method to obtain graphene nanosheets, and therefore the development of a simple and efficient exfoliation procedure remains challenging. Here, we present a one-step method of graphene exfoliation in lecithin/chloroform solution. The graphene nanosheets produced by the lecithin assisted exfoliation method were analyzed by microscopy techniques, including statistical analysis of atomic force microscopy (AFM) images and Raman spectroscopy, which both indicate the presence of few-layer graphene nanosheets, including substantial content of three-layer sheets. Molecular dynamics simulations on the time scale of 0.5+ μs suggested that stability of the obtained colloid may originate from formation of lecithin reverse hemimicelles and micelles, which prevents the aggregation of exfoliated graphene flakes by entropic repulsion of the lipid hydrophobic chains.

A new reverse worm-like micellar system from a lecithin, multivalent carboxylic acid and oil mixture

We developed new lecithin organogels composed of reverse worm-like micelles with lecithin/multivalent carboxylic acid/oil systems, and discussed their phase behavior and rheological properties. The most important findings in this study are the following. From a screening test of many carboxylic acids for gelation, it was found that the number and position of the carboxyl groups of the multivalent carboxylic acids are the determinants for the formation of reverse worm-like micelles, and appropriate carboxylic acids such as citric acid and 1,2,3-propanetricarboxylic acid can change the lecithin/oil solution into a gel. Furthermore, upon addition of these carboxylic acids, the zero-shear viscosity of solutions increased monotonically until phase separation or cloudiness occurred. For example, when citric acid was used, the maximum zero-shear viscosity of the solution was 70,000,000 times larger than that of n-decane. From studies on the scaling of rheological parameters, it was found that further addition of multivalent carboxylic acids not only induced the formation of linear reverse worm-like micelles but also brought about their branching.

Micellar Catalysis of Chromic Acid Oxidation of Methionine to Industrially Important Methylthiol in Aqueous Media at Room Temperature

Abstract Oxidation of organic molecule by metal is very important. Selective oxidants require non aqueous media, which is toxic and hazardous. L-methionine is oxidized to industrially important methyl thiol in micellar media by chromic acid. The overall reaction follows a first order dependency on substrate and hexavalent chromium and second order dependency on hydrogen ion. Here, reverse micelle formation is observed. TX-100 increases the rate where as SDS retards the rate of oxidation.

Molecular Interactions between Lecithin and Bile Salts/Acids in Oils and Their Effects on Reverse Micellization

It has been known that the addition of bile salts to lecithin organosols induces the formation of reverse wormlike micelles and that the worms are similar to long polymer chains that entangle each other to form viscoelastic solutions. In this study, we further investigated the effects of different bile salts and bile acids on the growth of lecithin reverse worms in cyclohexane and n-decane. We utilized rheological and small-angle scattering techniques to analyze the properties and structures of the reverse micelles. All of the bile salts can transform the originally spherical lecithin reverse micelles into wormlike micelles and their rheological behaviors can be described by the single-relaxation-time Maxwell model. However, their efficiencies to induce the worms are different. In contrast, before phase separation, bile acids can induce only short cylindrical micelles that are not long enough to impart viscoelasticity. We used Fourier transform infrared spectroscopy to investigate the interactions between lecithin and bile salts/acids and found that different bile salts/acids employ different functional groups to form hydrogen bonds with lecithin. Such effects determine the relative positions of the bile salts/acids in the headgroups of lecithin, thus resulting in varying efficiencies to alter the effective critical packing parameter for the formation of wormlike micelles. This work highlights the importance of intermolecular interactions in molecular self-assembly.

JR 400–NaAOT interaction: a detailed thermodynamic study of polymer–surfactant interaction bearing opposite charges

The present study entails interaction between the cationic polymer N,N-dimethylhydroxyethyl cellulose (JR 400) and the double-tailed anionic surfactant Na-bis-2-ethyhexylsulphosuccinate (NaAOT). This oppositely charged polymer and surfactant are expected to cause coacervation and precipitation; hence, we have observed formation of thick solution similar to diluted gel at [JR 400] ~ 0.01 and 0.10 % w/v in aqueous solution. Viscometry, conductometry, tensiometry, and microcalorimetry techniques are used to monitor the interaction process. The results are explained in the light of both intrachain and interchain linking by way of NaAOT reverse micelle formation. Adsorption of NaAOT monomers onto the charged side chains of the polymer shields interchain electrostatic repulsion, leading to the formation of hydrophobic microdomains and microscopic heterogeneity in the solution. The morphologies of the domains depend on the level of addition of NaAOT in the system. The different stages of physiochemical changes that arise in solution have been identified by the use of different techniques, and correlations of the results have been attempted in terms of pragmatic models.

Adsorption and filtration behaviour of non-ionic surfactants during reverse micellar-enhanced ultrafiltration

Micellar-enhanced ultrafiltration is an established method for the separation of oil-in-water micelles from aqueous streams, e.g., in wastewater treatment and catalytic reaction processes. However, its counterpart, the ultrafiltration (UF) of reverse micelles, known as water-in-oil-micelles, has been rarely researched until now. Our study shows that UF of non-ionic reverse micelles using the ternary system 1-dodecene/water/Marlophen NP5 is not technically feasible. The effects of hydrophobic and hydrophilic membrane properties as well as the variation of surfactant concentration on the filtration behaviour were investigated. Two regions could be distinguished in terms of filtration behaviour when surfactant concentration was increased: (a) below the critical micelle concentration (cmc) flux declined sharply, and (b) above the cmc it increased. Below the cmc, the adsorption of surfactants onto the membrane surface seems to be irreversible when oil (long-chain olefin) is the continuous phase. Above the cmc, the reverse micelle formation leads to a high flux recovery. However, due to surface diffusion, retention of micelles was negligible in all cases.

Transport of charged colloids in a nonpolar solvent

In nonpolar solvents, surfactants stabilize charge through the formation of reverse micelles; this enables the dissociation of charge from the surfaces of particles, thereby charge-stabilizing particle suspensions. We investigate the dynamics of such charged particles by directly visualizing their motion across a microfluidic channel in response to an external electric field. The presence of the reverse micelles has a significant effect on particle motion: in a constant field, the particles initially move, then slow down exponentially, and eventually stop. This is due to the accumulation of reverse micelles at the channel walls, which screens the applied field, leading to the subsequent decay of the internal electric field. The time constant of decay depends on the electrical conductivity of the particle suspension and the width of the channel; this behavior is modeled as an equivalent RC circuit.

Locations of methanol in methanol-containing AOT reverse micelles revealed by photophysics of IR125

Methanol has appeared in many publications reporting reverse micelles (RMs) formation, but it remains debate whether it can be effectively encapsulated by surfactant AOT to form RMs. In this work, we investigated the formation of methanol-containing AOT RMs in n-heptane solvents by using IR125 as a probe. We studied the photophysics of IR125 in methanol/AOT/n-heptane RMs at different loading methanol content (w′0: w′0=[methanol]/[AOT]) by means of steady-state absorption and fluorescence spectroscopies as well as time-resolved fluorescence spectroscopy. We obtained the absorption and fluorescence maxima, fluorescence lifetimes, and reorientation times of IR125 in methanol/AOT/n-heptane RMs at different w′0. We found that the methanol can be effectively encapsulated by AOT in n-heptane solvents to form stable methanol/AOT/n-heptane RMs and the size of methanol/AOT/n-heptane RMs maintains constant with the increase of w′0.

Influence of surfactant polydispersity on the structure of polyoxyethylene (5) nonylphenyl ether/cyclohexane/water reverse microemulsions

The distribution of the number of oxyethylene groups in molecules of polyoxyethylene (5) nonylphenyl ether (NP5) is shown to significantly affect the structure of NP5/cyclohexane/water reverse microemulsions. The effect is caused by the difference in solubility of NP5 components. This difference leads to fractionation: surfactant molecules dissolved in the intermicellar nonpolar medium have, on average, shorter polyoxyethylene chains than surfactant molecules forming reverse micelles. A model is proposed which takes into account the influence of this fractionation on reverse micelle size and free surfactant concentration. The validity of this model for NP5/cyclohexane/water reverse microemulsions is experimentally confirmed by dynamic light scattering. It is found that changing the degree of surfactant fractionation in the above mentioned microemulsions can result in the formation of non-spherical reverse micelles.

A Unique Ionic Liquid with Amphiphilic Properties That Can Form Reverse Micelles and Spontaneous Unilamellar Vesicles

Catanionic surfactants: the synthesis of a new surfactant ionic liquid with unique properties is described. The formation of reverse micelles in benzene and large unilamellar vesicles, formed spontaneously without the help of any mechanical of chemistry methods, in water is demonstrated by using dynamic light scattering and small-angle X-ray scattering techniques.

Extension to the Charge Fluctuation Model for the Prediction of the Conductivity of Apolar, Reverse Micellar Systems

This paper presents an extension to current theory regarding charging behavior in apolar, micellar systems. Electrical conductivity in such systems accompanying the formation of neutral reverse micelles is commonly explained by the possibility of intermicellar collisions resulting in a pair of oppositely charged micelles. The sequestration of the resulting charges within the micelles prevents their immediate recombination. The current theory underlying the charging process has thus far been applied in only approximate form, and is only used to validate experimental trends and to abstract values for the fraction of charged micelles. The extended theory proposed here uses knowledge of the solvent and surfactant characteristics, together with water content, to predict solution conductivity in absolute terms. It is verified in experiments with the solvent Isopar-L and surfactants Aerosol OT, OLOA 11000, and Span 80, in which significant differences from the approximate theory are observed.

Novel reverse mixed micelle mediated transport of platinum and palladium through a bulk liquid membrane from real samples

A facilitated uphill transport for the separation and pre-concentration of platinum and palladium through a bulk liquid membrane procedure utilizing reverse mixed micelles as transport vehicles was developed. A mixed micellar solution in a liquid membrane process is a representation of mixed monolayer at the feed/membrane interface, and mixed bilayer aggregate in the membrane/receiving interface. A cationic hydrophobic surfactant along with, a nonionic surfactant was used to form reverse mixed micelles in trichloroethylene, which act as a carrier for extracting the metal ions into bulk liquid membrane. This procedure is based on the formation of hydrophobic ion-associates of platinum and palladium with the reverse mixed micelles in the presence of an acid mixture which are extractable into the membrane phase. The formation of reverse mixed micelles hastens the transport of platinum and palladium into the organic phase making room temperature extraction feasible providing an energy efficient extraction. These ion associates are then stripped into the receiving phase from these reverse mixed micelles. Several factors influencing the extraction efficiency, such as the acid concentration, surfactant concentration, extraction time and temperature were investigated and optimized for providing better recoveries. The formation of reverse micelles and reverse mixed micelles has been analyzed using TEM (transmission electron microscope). The preconcentration factors for both platinum and palladium individually were found to be 10 using this method. The accuracy of the procedure was validated by analyzing PTM-1a nickel–copper sulfide matte. The procedure has been utilized for the recovery of platinum and palladium from spent catalytic converters and environmental samples.

CTAB/Water/Chloroform Reverse Micelles: A Closed or Open Association Model?

The micellization of cetyltrimethylammonium bromide (CTAB) in chloroform in the presence of water was examined. Three scenarios of the reverse micelle formation, the closed, open and Eicke's association models, were considered in the interpretation of the experimental data. The growth of the aggregates was observed through the changes of NMR signals of associated water, probing the microenvironment of the premicellar aggregates and the interior of reverse micelles. This technique if combined with isothermal titration calorimetry (ITC) revealed that hydrated surfactant premicellar aggregates are already present at ∼6 mM CTAB. NMR, ITC and conductometry were used to determine the critical micelle concentration (cmc) to be ∼40 mM CTAB. It is suggested that the variation of the cmc values reflects the fact that the NMR analysis indicated the beginning of the reverse micelle formation, whereas conductometry and ITC measurements provided the upper limit and an average value of a so-called apparent cmc, respectively. The cmc values were found to be unaffected by the water content. The presence of reverse micelles, the existence of multiple equilibria, and high polydispersity of the samples were evidenced by DOSY NMR spectroscopy. As a result, we validated Eicke's association model, according to which cyclic inverse micelles are formed by a structural reorganization of linear associates within a narrow concentration range, called the apparent cmc. New experimental results have also been gained for micellization of cetyltrimethylammonium chloride (CTAC) in chloroform in the presence of water; a similar mechanism of reverse micelle formation has been suggested.

Studies on understanding the mechanism of the enhanced conductivity of the third phase in PUREX process during reprocessing of fast reactor fuel

Towards development of electrical conductivity based sensors for online detection of formation of third phase during reprocessing of Pu rich spent nuclear fuels, laboratory studies were carried out using U4+ solutions in various experimental conditions. Third phases were generated by extracting U4+ from nitric acid medium by 1.09 M TBP at different A/O (A: aqueous, O: organic) ratios. The results of this study indicate that the third phase is nearly 100–300 times more conducting than lean organic phase and saturated phase. The higher conductivity of third phase as compared to that of other two phases is explained based on the principle of reverse micelle formation and charge movement between the micro emulsion globules by percolation phenomenon.

Calorimetric studies of microemulsion systems with lecithin, isooctane and butanol

In the present study, a thermodynamic analysis was performed using an isothermal titration microcalorimeter to measure changes in the enthalpy of mixing (ΔmixH) of water in microemulsion systems formed by soybean lecithin, isooctane and butanol. These energies were important to determine the driving forces associated with the formation of reverse micelles. It was observed that the titration curves of water in pure isooctane and in isooctane with soybean lecithin present values of ΔmixH equal to −9.46×10−3kJ/mol and −5.68×10−3kJ/mol, respectively. This shows that water does not form intermolecular interactions with these components. Exothermic behavior, with values of ΔmixH ranging from +2.17kJ/mol to −1.63×10−2kJ/mol, was observed from the titration curve of water in systems formed by soybean lecithin, isooctane and butanol. These values were associated with the water dissolution process, where the formation of reverse micelles had been observed in the evaluated systems. Differences in behavior of ΔmixH for the water curves were observed in systems formed by butanol and soybean lecithin, only pure butanol and butanol with isooctane. The curve of the system containing soybean lecithin, butanol and isooctane clearly shows that the formation of aggregates occurred in this system (reverse micelles). Finally, it was also noted that soybean lecithin did not suffer any degradation process, since the energy involved in the process of water titration in the systems containing the same substance was considered low. Thus, soybean lecithin may be used without problems in this type of system.

Nonionic reverse micelle formulation and their microstructure transformations in an aromatic solvent ethylbenzene

Formation of reverse micelles at room temperature generally requires inclusion of water or other polar molecules in a mixture of long chain ionic surfactant and an organic solvent. In this paper, we report the formulation of nonionic reverse micelles at room temperature in an aromatic solvent, ethylbenzene using short-chain glycerol-based surfactants, and investigated the microstructure transformations induced by surfactant chain length, concentration, and temperature using SAXS and rheometry. We have also determined water solubility in reverse micelle and investigated water induced microstructure transitions. We found that a short chain nonionic surfactant, diglycerol monocaprate (C10G2), spontaneously self-assembles into an ellipsoidal prolate type micelle. Increasing surfactant chain induces a prolate-to-globular type transition forming globular micelles in diglycerol monomyristate (C14G2), which is expected to be caused by an increase in the lipophilicity of the surfactant. Increases in surfactant concentration favored one-dimensional micellar growth; the micellar size increases ca. 4.5–7.25nm upon increase in the surfactant concentration from 3 to 20% while the cross-sectional radius maintained almost constant ca. 1.10nm in the diglycerol monolaurate (C12G2) system. In contrast to the nonionic micelles in aqueous systems, reverse micelles get smaller with increasing temperature, which is similar to a rod-to-sphere type transition. Addition of trace water promotes a significant micellar growth; both the maximum dimension and radius of micelles increases and the size of the water-incorporated micelles was approximately three times greater than the micelles in the absence of water. Microstructure transitions revealed by SAXS are supported by rheological measurements; the relative zero-shear viscosity decreased with increase in the chain length of surfactant. On the other hand, relative viscosity increased nearly one order of magnitude upon increase in surfactant concentration from 3 to 20%.

Ionic Concentration Effects on Reverse Micelle Size and Stability: Implications for the Synthesis of Nanoparticles

We present a systematic investigation and analysis of the structure and stability of reverse micelle systems with the addition of NH(4)OH, ZrOCl(2), and Al(NO(3))(3) salts. We demonstrate that the reverse micelle size decreases with increasing salt additions until one reaches a critical concentration, which characterizes the onset of system destabilization. The concept of an electrical double layer, as it applies to reverse micelles, is considered for explaining features of destabilization, including the initial decrease in reverse micelle size, the destabilization concentration, and the effect of cation valence. We propose that the reduction in size prior to instability is caused by compression of the reverse micelle electrical double layers, as higher concentrations of salts are present. The reduced thickness of the electrical double layers allows the decaying potentials to move into closer proximity to each other before generating enough repulsion to balance the forces for reverse micelle formation and form a new equilibrium average reverse micelle size. The point of reverse micelle instability has been related to the formation of a two-phase system as a result of the inability to further compress the salt co-ions in the core of the reverse micelles, which would cause an excessive repulsive force between the overlapping potentials. We have extracted a critical potential of -89 nV between the two overlapping potentials for the AOT/water/isooctane (ω(0) = 10) systems studied. All these effects have important implications for the preparation of nanopowders by reverse micelle synthesis. If the reverse micelles are unstable before the precipitates are formed, then the advantage of reverse micelle synthesis is immediately lost.

Superfast Electrophoretic Transport of Carbocyanine Dyes In Situ: An Improved Method in Nerve Pathway Tracing

Carbocyanine dyes are an important tool used to trace the innervations to specific structures, and although the rate of dye transport has improved, faster rates are required to make these dyes effective especially in fixed tissues. We applied a direct electrical current to the sciatic nerve of a cat to electrophoretically transport pastes of DiI and DiD in situ, using a sodium borate (SB) buffer ‐ filled anode apparatus and hypodermic needle as a cathode. Dye labeling was visualized using a Bio‐Rad Radience 2100MP confocal microscope. DiI was transported at an average rate of 60.2 ± 4.1 mm/hr, which was >5,000 X faster than previously reported for crystal diffusion (Lukas et al. 1998, Swift et al. 2005) and 50 X faster than using a DC field in vitro. The average electrophoretic transport rate of DiD (60.9 ± 7.0 mm/hr) was similar to DiI. Lower crystal solubility and reverse micelle formation may be responsible for the slower diffusion rates reported previously. Use of the paste form with a lower alcohol content together with SB buffer solution, could reduce reverse micelle formation, and improve diffusion. Initial counter‐ion distribution at the anode may facilitate development of superfast electrophoresis in a weak field (5mA). This could allow the small ions to facilitate superfast electrophoretic transport of the carbocyanine dyes at low currents.Grant Funding Source : Palmer College of Chiropractic

The effect of different interfaces and confinement on the structure of the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide entrapped in cationic and anionic reverse micelles

The behavior of the ionic liquid (IL) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][Tf(2)N]) entrapped in two reverse micelles (RMs) formed in an aromatic solvent as dispersant pseudophase: [bmim][Tf(2)N]/benzyl-n-hexadecyldimethylammonium chloride (BHDC)/chlorobenzene and [bmim][Tf(2)N]/sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT)/chlorobenzene, was investigated using dynamic light scattering (DLS), FT-IR and (1)H NMR spectroscopies. DLS results reveal the formation of RMs containing [bmim][Tf(2)N] as a polar component since the droplet size values increase as the W(s) (W(s) = [[bmim][Tf(2)N]]/[surfactant]) increases. Furthermore, it shows that the RMs consist of discrete spherical and non-interacting droplets of [bmim][Tf(2)N] stabilized by the surfactants. Important differences in the structure of [bmim][Tf(2)N] entrapped inside BHDC RMs, in comparison with the neat IL, are observed from the FT-IR and (1)H NMR measurements. The electrostatic interactions between anions and cations from [bmim][Tf(2)N] and BHDC determine the solvent structure encapsulated inside the nano-droplets. It seems that the IL structure is disrupted due to the electrostatic interaction between the [Tf(2)N](-) and the cationic BHDC polar head (BHD(+)) giving a high ion pair degree between BHD(+) and [Tf(2)N](-) at a low IL content. On the other hand, for the AOT RMs there is no evidence of strong IL-surfactant interaction. The electrostatic interaction between the SO(3)(-) group and the Na(+) counterion in AOT seems to be stronger than the possible [bmim](+)-SO(3)(-) interaction at the interface. Thus, the structure of [bmim][Tf(2)N] encapsulated is not particularly disrupted by the anionic surfactant at all W(s) studied, in contrast to the BHDC RM results. Nevertheless, there is evidence of confinement in the AOT RMs because the [bmim](+)-[Tf(2)N](-) interaction is stronger than in bulk solution. Thus, the IL is more associated upon confinement. Our results reveal that the [bmim][Tf(2)N] structure can be modified in a different manner inside RMs by varying the kind of surfactant used to create the RMs and the IL content (W(s)). These facts can be very important if these media are used as nanoreactors because unique microenvironments can be easily created by simply changing the RM components and W(s).

Generation of fluorescent silver nanoscale particles in reverse micelles using gamma irradiation

Reverse micelles (RMs) containing aqueous solutions of Ag(+) ions in their core produce fluorescent Ag species, upon exposure to gamma irradiation. A two-phase liquid system is used for RM formation. The RMs can be employed in novel gamma radiation detectors with appearance of fluorescence indicating that radiation was once present.