Reverse Micelles Interface Research Articles

Mechanism of discoloration of Antarctic krill oil upon storage: A study based on model systems

The reddish-orange color of Antarctic krill oil fades during storage, and the mechanism remains unclear. Model systems containing different combinations of astaxanthin (ASTA), phosphatidylethanolamine (PE), and tocopherol were subjected to accelerated storage. Among all groups containing ASTA, only the ones with added PE showed significant fading. Meanwhile, the specific UV–visible absorption (A470 and A495) showed a similar trend. Peroxide value and thiobarbituric acid reactive substances increased during storage, while ASTA and PE contents decreased. Correlation analysis suggested that oxidized PE promoted fading by accelerating the transformation of ASTA. PE content exceeded the critical micelle concentration (1μg/g) indicating the formation of reverse micelles. Molecular docking analysis indicated that PE also interacted with ASTA in an anchor-like manner. Therefore, it is speculated that amphiphilic ASTA is more readily distributed at the oil-water interface of reverse micelles and captured by oxidized PE, which facilitates oxidation transfer, leading to ASTA oxidation and color fading.

Hidden Beneath the Layers: Extending the Core/Shell Model of Reverse Micelles.

Reverse micelles (RMs) provide a unique and highly tunable model system to study water in confined environments. The complex properties of water within RMs arise from the disruption of extended hydrogen bond (H-bond) networks that mediate local and long-range dynamics in bulk aqueous systems. Modulating the water pool size influences its H-bond dynamics, with smaller RMs increasingly restricting the H-bond network rearrangements leading to slower dynamics; however, within small confined systems, the dynamics of the surfactants also influence the water dynamics. Using ultrafast two-dimensional infrared spectroscopy, we investigate the effects of RM size on the surfactant headgroup rotamer populations and picosecond interfacial H-bond dynamics of aerosol-OT surfactants. We find that the increased water penetration accelerates H-bond dynamics, with larger RMs showing faster dynamics. These results imply that the changes in the RM structure alter the physical structure of the RM interface and thus alter the solvation dynamics. The findings in this study can be used for developing models for structure-specific solvation dynamics that account for the surfactant packing and hydration at the interface.

Green AOT reverse micelles as nanoreactors for alkaline phosphatase. The hydrogen bond "dances" between water and the enzyme, the reaction product, and the reverse micelles interface.

In this work, we present an investigation of the influence of water encapsulated in 1,4-bis-2-ethylhexylsulfosuccinate/methyl laurate and 1,4-bis-2-ethylhexylsulfosuccinate/isopropyl myristate reverse micelles on the enzymatic hydrolysis of 1-naphthyl phosphate by alkaline phosphatase. Our results show that the enzyme is active in the biocompatible reverse micelles studied and that the Michaelis-Menten kinetic model is valid in all systems. We found that both micellar systems studied have a particular behavior toward pH and that the penetration of external solvents into the interfaces is crucial to understanding the effect. Methyl laurate does not disrupt the interface and is not necessary to control the pH value since alkaline phosphatase in the center of the micelles is always solvated similarly. In contrast, isopropyl myristate disrupts the interfaces so that the water and 1-naphthol molecules cannot form hydrogen bond interactions with the polar head of the surfactant. Then, when the water is at pH = 7, the 1-naphthol moves away to the interfaces inhibiting alkaline phosphatase which is not observable when the water is at pH = 10. Our study shows that the concept of pH cannot be used directly in a confined environment. In addition, our research is of great importance in the field of reactions that occur in reverse micelles, catalyzed by enzymes.

Effect of Interfacial Activity of Eugenol and Eugenol Esters with Different Alkyl Chain Lengths on Inhibiting Sunflower Oil Oxidation

AbstractEffects of eugenol esters with different alkyl chain lengths and combinations of eugenol + eugenol esters on sunflower oil oxidation at 338, 358, and 378 K are evaluated by determining different kinetic parameters. Also, water content and reverse micelles size of sunflower oil samples are monitored during oxidation. Eugenyl acetate shows higher antioxidant activity than eugenol in sunflower oil, while eugenyl butyrate, eugenyl hexanoate, and eugenyl decanoate shows lower antioxidant activity than eugenol. Antioxidant activity of eugenol esters decreases by increasing their alkyl chain length. Eugenol shows a synergistic effect with eugenyl acetate and eugenyl butyrate on increasing the induction period and antioxidant activity values, while it shows an antagonistic effect with eugenyl hexanoate and eugenyl decanoate. Also, combinations of eugenol + eugenyl acetate, as well as, eugenol + eugenyl butyrate stabilize the reverse micelles at longer period of time (3263 min for eugenol + eugenyl acetate and 2900 min for eugenol + eugenyl butyrate) than eugenol (2750 min). In addition, eugenol + eugenyl acetate and eugenol + eugenyl butyrate decrease the magnitude of temperature‐related effects on sunflower oil oxidation to the higher extent than eugenol did.Practical applications: Recent studies have suggested that the antioxidants with ability to locate at the active sites of lipid oxidation can efficiently reduce lipid oxidation in bulk oil. Despite a remarkable focus on antioxidant activity of eugenol, there is no information regarding improving its interfacial performance through esterification method. Also, there are few published data regarding the effects of esterified antioxidants on physicochemical changes during the oxidation of bulk oil. In this study, eugenol is esterified with anhydrides with different alkyl chain lengths to increase its accessibility to the active site of lipid oxidation, that is, water–oil interface of reverse micelles in sunflower oil. Effects of eugenol esters on stability of reverse micelles and critical reverse micelles concentration of lipid hydroperoxides have been investigated for the first time.

Hydration of bromide at reverse micelle interfaces studied by X-ray absorption fine structure

Nanoconfined water exhibits various interesting properties, which are not only of fundamental importance but also of practical use. Because reverse micelles (RMs) provide versatile ways to prepare nanoconfined water, the understanding of their physicochemical properties is essential for developing efficient applications. Although the water properties in the RMs could be affected by its interaction with the RM interface, the details have not been well understood. This study focuses on the local structures of Br− in hexadecyltrimethylammonium bromide (HTAB) RMs formed in chloroform and 10% hexanol/heptane. The dependence in Br− hydration on the molar ratio of water to HTAB (w) is investigated using X-ray absorption fine structure (XAFS). These systems cover a wide range of w values (0–30) and allow us to study the impact of this parameter on the local structure of Br− at the RM interface, which comprises water, surfactant headgroups, and organic solvent components. The presence of multiple scattering paths complicates the XAFS spectra and makes it difficult to analyze them using standard fitting methods. The linear combination of the spectra corresponding to the individual scattering paths captures the molecular processes that occur at the RM interface upon increasing w. The maximum hydration number of Br− is found to be 4.5 at w > 15, suggesting that although most of the ions remain at the interface as partly hydrated ions, some of them dissociate as completely hydrated ones.

Nanoconfinement Raises the Energy Barrier to Hydrogen Atom Exchange between Water and Glucose.

In bulk aqueous environments, the exchange of protons between labile hydroxyl groups typically occurs easily and quickly. Nanoconfinement can dramatically change this normally facile process. Through exchange spectroscopy (EXSY) NMR measurements, we observe that nanoconfinement of glucose and water within AOT (sodium bis(2-ethylhexyl) sulfosuccinate) reverse micelles raises the energy barrier to labile hydrogen exchange, which suggests a disruption of the hydrogen bond network. Near room temperature, we measure barriers high enough to slow the process by as much as 2 orders of magnitude. Although exchange rates slow with decreasing temperatures in these nanoconfined environments, the barrier we measure below ∼285 K is 3-5 times lower than the barrier measured at room temperature, indicating a change in mechanism for the process. These findings suggest the possibility of hydrogen tunneling at a surprisingly high-temperature threshold. Furthermore, differences in exchange rates depend on the hydroxyl group position on the glucose pyranose ring and suggest a net orientation of glucose at the reverse micelle interface.

How the external solvent in biocompatible reverse micelles can improve the alkaline phosphatase behavior.

In the last decade, the nature of the nonpolar solvents that can be part of reverse micelles (RMs) has been the topic of several investigations to improve their applications. In this sense, the hydrolysis of 1-naphthyl phosphate catalyzed by the enzyme alkaline phosphatase (AP) was used as a probe to investigate the effect of the change of the external solvent on RMs formulated with the anionic surfactant sodium diethylhexyl sulfosuccinate (AOT). As external nonpolar solvents, two biocompatible lipophilic esters, isopropyl myristate and methyl laurate, and the traditional nonpolar solvents, n-heptane and benzene, were used. The results were compared among the RMs investigated and with the reaction in homogeneous media. Thus, the effect of the nanoconfinement as well as the impact of the replacement of a conventional external nonpolar solvent by biocompatible solvents were analyzed. The results indicate that the catalytic efficiency in the AOT RMs is larger than that in homogeneous media, denoting a different hydration level over the AP enzyme, which is directly related to the different degrees of nonpolar solvent penetration to the RM interface. Our findings demonstrated that toxic solvents such as n-heptane and benzene can be replaced by nontoxic ones (isopropyl myristate or methyl laurate) in AOT RMs without affecting the performance of micellar systems as nanoreactors, making them a green and promising alternative toward efficient and sustainable chemistry.

Is it Necessary for the Use of Fluorinated Compounds to Formulate Reverse Micelles in a Supercritical Fluid? Searching the Best Cosurfactant to Create "Green" AOT Reverse Micelle Media.

Herein, we report the effect of employing two different alcohols, such as n-pentanol and 2,2,3,3,4,4,5,5-octafluoro pentanol (from now on F-pentanol), into 1,4-bis-2-ethylhexylsulfosuccinate (AOT) reverse micelles (RMs), to determine the interfacial activity and establish the best candidate to act as a cosurfactant in supercritical RMs. Dynamic light scattering (DLS), Fourier transform infrared (FT-IR), and fluorescence emission spectroscopy allowed us to determine and understand the behavior of alkanols in RMs. As a result, we found interesting displacements of alkanol molecules within the RMs, suggesting that the electrostatic interaction between SO3- and Na+ weakens because of new interactions of n-pentanol with SO3- through H-bonds, changing the curvature of the micellar interface. According to FT-IR and DLS studies, F-pentanol forms a RM polar core interacting through intermolecular H-bonds, suggesting no perturbations of the AOT RM interface. Hence, n-pentanol was selected as a cosurfactant to form supercritical RMs, which is confirmed by red edge excitation shift studies, using C343 as a molecular probe. Herein, we were able to create RMs under supercritical conditions without the presence of modified surfactants, fluorinated or multitailed compounds, which, to the best of our knowledge, was not shown before.

Characterization of Reverse Micelles Formulated with the Ionic-Liquid-like Surfactant Bmim-AOT and Comparison with the Traditional Na-AOT: Dynamic Light Scattering, 1H NMR Spectroscopy, and Hydrolysis Reaction of Carbonate as a Probe.

The present study investigated how the presence of butylmethylimidazolium cation (bmim+) alters the interfacial properties of reverse micelles (RMs) created with the ionicliquid-like surfactant 1-butyl-3-methylimidazolium 1,4-bis-2-ethylhexylsulfosuccinate (bmim-AOT), in comparison to sodium 1,4-bis-2-ethylhexylsulfosuccinate (Na-AOT) RMs, employing dynamic light scattering (DLS) and 1H NMR techniques. Moreover, through the hydrolysis reaction of bis(4-nitrophenyl)carbonate inside both RMs as reaction probe, interfacial properties changes were explored in more detail. The kinetic solvent isotope effect was also analyzed. Micellar systems were formed using n-heptane as external nonpolar solvent and water as the polar component. According to the DLS studies, water is encapsulated inside the organized media; however, a different tendency is observed depending on the cationic component of the surfactant. For Na-AOT system, the results suggest that the micellar shapes are probably spherical, while in the case of bmim-AOT, a transition from ellipsoidal to spherical micelles could be occurring when water is added. 1H NMR data show that water is structured differently when Na+ cation is replaced by bmim+; in bmim-AOT RMs, the interaction of water with the surfactant is weaker and the water hydrogen-bonding network is less disturbed than in Na-AOT RMs. Kinetic studies reveal that the hydrolysis reaction in bmim-AOT RMs was much more favorable in comparison to Na-AOT RMs. In addition, when water content decreases in bmim-AOT RMs, the hydrolysis reaction rate increases and the solvent isotope effect remains constant, while for Na-AOT solutions, both the reaction rate and the solvent isotope effect decrease. Our results indicate that bmim+ cation would be located in the surfactant layer in such a way the negative charge density in the interface is less than that in Na-AOT RMs, and the reaction is more favorable. Additionally, as 1H NMR studies reveal, the interfacial water molecules would be more available in bmim-AOT RMs to participate in the nucleophilic attack. Therefore, the present study evidences how the replacement of Na+ counterion by bmim+ alters the composition of the interface of AOT RMs.

Structural Characterization of Biocompatible Reverse Micelles Using Small-Angle X-ray Scattering, 31P Nuclear Magnetic Resonance, and Fluorescence Spectroscopy

The most critical problem regarding the use of reverse micelles (RMs) in several fields is the toxicity of their partial components. In this sense, many efforts have been made to characterize nontoxic RM formulations on the basis of biological amphiphiles and/or different oils. In this contribution, the microstructure of biocompatible mixed RMs formulated by sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT) and tri- n-octylphosphine oxide (TOPO) surfactants dispersed in the friendly solvent methyl laurate was studied by using SAXS and 31P NMR and by following the solvatochromic behavior of the molecular probe 4-aminophthalimide (4-AP). The results indicated the presence of RM aggregates upon TOPO incorporation with a droplet size reduction and an increase in the interfacial fluidity in comparison with pure AOT RMs. When confined inside the mixed systems, 4-AP showed a red-edge excitation shift and confirmed the increment of interfacial fluidity upon TOPO addition. Also, the partition between the external nonpolar solvent and the RM interface and an increase in both the local micropolarity and the capability to form a hydrogen bond interaction between 4-AP and a mixed interface were observed. The findings have been explained in terms of the nonionic surfactant structure and its complexing nature expressed at the interfacial level. Notably, we show how two different approaches, i.e., SAXS and the solvatochromism of the probe 4-AP, can be used in a complementary way to enhance our understanding of the interfacial fluidity of RMs, a parameter that is difficult to measure directly.

Effect of sugars on the dynamics of hydrophilic fluorophores confined inside the water pool of anionic reverse micelle: A spectroscopic approach

In this article, we have investigated the structural and chemical effects of two sugar molecules, namely sucrose and sucralose on the structure of anionic reverse micelle (RM) AOT (aerosol-OT; sodium bis(2-ethylhexyl)sulfosuccinate)/n-heptane. The artificial sugar alternative sucralose shows very unique physical properties unlike other sugar molecules including sucrose and it is the reason behind choosing these two sugars in our study. The variation of shapes and sizes of the water pool of RM in presence of the two sugar molecules are investigated using dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS) measurements. It is observed that addition of both the sugar molecules swell the water pool of the RM and this enhancement is greater for sucralose. Moreover, we have employed time-correlated single photon counting (TCSPC) and fluorescence correlation spectroscopy (FCS) techniques for hydrophilic probes Coumarin 343(C343) and Rhodamine 6G (R6G) respectively inside the RM and we have observed that these two sugar molecules strongly retard the solvation and rotational dynamics of C343 and the translational diffusion of R6G inside the RM. Replacement of the smaller sized water molecules by the larger sized sugar molecules at the interface of RM through stronger hydrogen bonding with the interfacial sulfonate moiety of AOT increases the microviscosity and this may be the probable reason behind the slow dynamics of the probes in presence of sucrose. Along with this factor, strong electrostatic interaction of highly polar molecule sucralose with RM interface and with the dye molecules further retards the dynamics of fluorophores. Therefore, sucralose molecule exhibits its distinct feature by swelling the water pool inside the RM and slowing down the dynamics of the hydrophilic probes to a greater extent compared to sucrose which provides a new aspect of RM and sugar interaction.

Subtleties of catanionic surfactant reverse micelle assemblies revealed by a fluorescent molecular probe

In this work, the absorption and emission behavior of the cationic hemicyanine trans-4-[4-(dimethylamino)styryl]-N-methylpyridinium iodide (HC) in reverse micelles (RMs) formed by the catanionic surfactants benzyl-n-hexadecyldimethylammonium-1,4-bis-2-ethylhexylsulfosuccinate (AOT-BHD) and cetyltrimethylammonium-1,4-bis-2-ethylhexylsulfosuccinate (AOT-CTA) have been investigated. Our results show that the spectroscopic behavior of HC changes when the dye is dissolved in AOT-BHD or in AOT-CTA RMs. While HC undergoes an intramolecular charge-transfer process upon excitation in AOT-CTA RMs, in AOT-BHD RMs this process is inhibited due to a specific interaction between HC and the polar head group of the BHD+ cation. This implies that the chemical structure of CTA+ and BHD+ cations has a large impact on the excited stated from which HC emission occurs. Additionally, the structural difference between the two cations impacts on the water–RM interface interaction, which provides a way of controlling the solvation process in these RMs. Furthermore, differences in the interfacial fluidity between the two catanionic RMs is observed, a result that is particularly interesting with regard to these systems being used as nanoreactors.

Hydrophobically Tailored Carbon Dots toward Modulating Microstructure of Reverse Micelle and Amplification of Lipase Catalytic Response.

This article delineates the modulation of microstructure of cationic reverse micelle utilizing hydrophobically modified carbon dots (CDs) with varying surface functionalizations. Citric acid was used as the source of the carbon core, and Na-salt of glycine, glycine, Na-salt of 11-aminoundecanoic acid, 11-aminoundecanoic acid, and n-hexadecylamine were used for the surface fabrication of CDs to produce CD 1s, CD 1a, CD 2s, CD 2a, and CD 3, respectively. All these CDs having dimension of 5-7 nm were characterized by spectroscopic and microscopic techniques. The hydrodynamic diameter of cetyltrimethylammonium bromide (CTAB) reverse micelle (CTAB/isooctane/n-hexanol/water) at z ([cosurfactant]/[surfactant]) = 6.4 and W0 ([water]/[surfactant]) = 44 is around 15-20 nm. Interestingly, the size of the water-in-oil (w/o) microemulsions dramatically increased up to 120-200 nm upon doping hydrophobic surface functionalized CD 2a and CD 3. This is possibly due to change in the micellar exchange dynamics and reorganization of the micellar aggregates via hydrophobic interaction between surfactant (CTAB) tail and hydrophobic surface modifier of the carbon dots. However, no alteration in the size of reverse micelles was noted in the presence of carbon dots CD 1s, CD 1a, and CD 2s. Spectroscopic and microscopic investigations confirmed that the hydrophobic CD 2a and CD 3 were localized at the interface of reverse micelles whereas CD 1s, CD 1a, and CD 2s were possibly located in the water pool (away from interface). The activity of Chromobacterium viscosum lipase encapsulated within CD 3 and CD 2a doped significantly large CTAB reverse micelles showed remarkable improvement (3.7-fold and 3.4-fold) in its catalytic response. However, hydrophilic carbon dots CD 1s and CD 2s as well as moderately hydrophobic CD 1a had no significant effect on the microstructure of reverse micelles as well as on the lipase activity.

How the Type of Cosurfactant Impacts Strongly on the Size and Interfacial Composition in Gemini 12-2-12 RMs Explored by DLS, SLS, and FTIR Techniques.

The limited amount of information about reverse micelles (RMs) made with gemini surfactants, the effect of the n-alcohols in their interface, and the water-entrapped structure in the polar core motivated us to perform this work. Thus, in the present contribution, we use dynamic light scattering (DLS), static light scattering (SLS), and FT-IR techniques to obtain information on RMs structure created, with the gemini dimethylene-1,2-bis(dodecyldimethylammonium) bromide (G12-2-12) surfactant and compare the results with its monomer: dodecyltrimethylammonium bromide (DTAB). In this way, the size of the aggregates formed in different nonpolar organic solvents, the effect of the chain length of n-alcohols used as cosurfactants, and the water-entrapped structure were explored. The data show that the structure of the cosurfactant needed to stabilize the RMs plays a fundamental role, affecting the size and behavior of the aggregates. In contrast to what happens with the RMs formed with the monomer DTAB, water entrapped inside G12-2-12 RMs displays different interaction with the interface depending on the hydrocarbon chain length of the n-alcohol used as cosurfactant. Thus, n-pentanol and n-octanol molecules are located in different regions in the RMs interfaces formed with the gemini surfactant. n-Octanol locates at the RMs interface among the surfactant hydrocarbon tails increasing the water-surfactant polar headgroup interaction. On the other hand, n-pentanol locates at the RMs interface near the polar core, limiting the interaction of water with the micellar inner interface and favoring the water-water interaction in the polar core.

How the cation 1-butyl-3-methylimidazolium impacts the interaction between the entrapped water and the reverse micelle interface created with an ionic liquid-like surfactant

The behavior of the interfacial water entrapped in reverse micelles (RMs) formed by the ionic liquid-like surfactant 1-butyl-3-methylimidazolium 1,4-bis-2-ethylhexylsulfosuccinate (bmim-AOT) dissolved in benzene (or chlorobenzene) was investigated using noninvasive techniques such as dynamic light scattering (DLS), static light scattering (SLS), FT-IR and (1)H NMR. The DLS and SLS results reveal the formation of discrete spherical and non-interacting water droplets stabilized by the bmim-AOT surfactant. Moreover, since the droplet size increases as the W0 (W0 = [water]/[surfactant]) value increases, water interacts with the RM interface. From FT-IR and (1)H NMR data, a weaker water-surfactant interaction in bmim-AOT RMs in comparison with the RMs created by sodium 1,4-bis-2-ethylhexylsulfosuccinate (Na-AOT) is detected. Consequently, there are less water molecules interacting with the interface in bmim-AOT RMs, and their hydrogen bond network is not completely disrupted as they are in Na-AOT RMs. The results show how the nature of the new cation impacts the interaction between the entrapped water and the RM interface, modifying the interfacial water structure in comparison with the results known for Na-AOT.

Wet Interface of Benzylhexadecyldimethylammonium Chloride Reverse Micelle Revealed by Excited State Proton Transfer of a Localized Probe

Excited state proton transfer (ESPT) of an anionic photoacid 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS or pyranine) has been studied inside a cationic reverse micelle (RM), water/benzylhexadecyldimethylammonium chloride (BHDC)/benzene, using steady-state and time-resolved fluorescence spectroscopy. The observed ESPT behavior is found to be remarkably different from the known ESPT trend of HPTS inside anionic AOT and cationic CTAB RMs; the ESPT dynamics approaches that of bulk water at higher w0 (≥10) inside AOT RM while no ESPT was observed for CTAB reverse micelle [ Sedgwick J. Am. Chem. Soc. 2012 , 134 , 11904 - 11907 ]. The ESPT dynamics inside BHDC RM is remarkably slower compared to that of water at all w0 (= [water]/[surfactant]) values and relatively much less sensitive to w0 variation compared to AOT RM. 2D NOESY and fluorescence anisotropy measurements reveal that the probe (HPTS) is embedded inside the positive interface of BHDC RM. Despite its trapped location, HPTS is able to undergo ESPT due to significant penetration of water molecules into the interface. Furthermore, facile ESPT at higher w0 is consistent with higher degree of interface hydration as predicted by a recent MD simulation [ Agazzi Langmuir 2014 , 30 , 9643 - 9653 ]. The study shows that ESPT dynamics inside RM varies not only with the interface charge but also on the nature of the headgroup and solvation.

Insights into the Partitioning Behavior of Secondary Surfactants in a Microemulsion-Based Synthesis of Metal Nanoparticles: A DLS and 2D NMR Spectroscopic Investigation.

Diffusion-ordered NMR spectroscopy (DOSY) and nuclear Overhauser effect spectroscopy (NOESY) have been used to explore the diffusion and partitioning behavior of secondary surfactants added to suspensions of reverse micelles (RMs) containing either silver or gold nanoparticles (NPs), with an aim of advancing our understanding of the mechanism of metal NP extraction and/or surface functionalization with specific capping agents as performed during a microemulsion-based synthesis. We have coupled these NMR techniques with corresponding dynamic light scattering (DLS) measurements of RMs, with and without encapsulated metal NPs, upon addition of secondary surfactants. Using oleylamine (OAm), oleic acid (OA), dodecylamine (DDAm), and dodecanethiol (DDT), we show that all four secondary surfactants can rapidly diffuse into/out of the RM environment with their head groups in close proximity to the RM interior and encapsulated water molecules; however, surfactant molecules containing a terminal -NH2 or -COOH group undergo a persistent association with the molecules of the RMs, thus solubilizing and partially sequestering a portion of the total concentration of these secondary agents within the RM interface for a lengthened period of time (in relation to the time frame of the DOSY experiments) and slowing their rate of exchange with freely diffusing molecules in the bulk solvent. The extraction of Ag or Au NPs from RMs into organic phase was determined to be critically dependent on the type and concentration of secondary surfactant added to the system, with DDT proving to be most efficient for the extraction of Ag NPs, while OA was shown to be most efficient for Au NPs. Consideration of the results obtained from this particular combination of techniques has provided new knowledge with respect to dynamic metal NP-containing microemulsion systems.

Ionic liquids entrapped in reverse micelles as nanoreactors for bimolecular nucleophilic substitution reaction. Effect of the confinement on the chloride ion availability.

In this work was explored how the confinement of two ionic liquids (ILs), 1-butyl-3-methylimidazolium chloride (bmimCl) and 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4), inside toluene/benzyl-n-hexadecyldimethylammonium chloride (BHDC) reverse micelles (RMs) affects the Cl(-) nucleophilicity on the bimolecular nucleophilic substitution (SN2) reaction between this anion and dimethyl-4-nitrophenylsulfonium trifluoromethanesulfonate. The results obtained show that, upon confinement, the ionic interactions between the ILs with the cationic surfactant polar head group and the surfactant counterion modify substantially the performance of both ILs as solvents. In toluene/BHDC/bmimCl RMs, the Cl(-) interacts strongly with bmim(+) (and/or BHD(+)) in such a way that its nucleophilicity is reduced in comparison with neat IL. In toluene/BHDC/bmimBF4 RMs, an ionic exchange equilibrium produces segregation of bmim(+) and BF4(-) ions, changing the composition of the RMs interface and affecting dramatically the Cl(-) availability. These results show the versatility of this kind of organized system to alter the ionic organization and influence on reaction rate when used as nanoreactors.

Effect of the cationic surfactant moiety on the structure of water entrapped in two catanionic reverse micelles created from ionic liquid-like surfactants.

The behavior of water entrapped in reverse micelles (RMs) formed by two catanionic ionic liquid-like surfactants, benzyl-n-hexadecyldimethylammonium 1,4-bis-2-ethylhexylsulfosuccinate (AOT-BHD) and cetyltrimethylammonium 1,4-bis-2-ethylhexylsulfosuccinate (AOT-CTA), was investigated by using dynamic (DLS) and static (SLS) light scattering, FTIR, and (1)H NMR spectroscopy techniques. To the best of our knowledge, this is the first report in which AOT-CTA has been used to create RMs and encapsulate water. DLS and SLS results revealed the formation of RMs in benzene and the interaction of water with the RM interface. From FTIR and (1)H NMR spectroscopy data, a difference in the magnitude of the water-catanionic surfactant interaction at the interface is observed. For the AOT-BHD RMs, a strong water-surfactant interaction can be invoked whereas for AOT-CTA this interaction seems to be weaker. Consequently, more water molecules interact with the interface in AOT-BHD RMs with a completely disrupted hydrogen-bond network, than in AOT-CTA RMs in which the water structure is partially preserved. We suggest that the benzyl group present in the BHD(+) moiety in AOT-BHD is responsible for the behavior of the catanionic interface in comparison with the interface created in AOT-CTA. These results show that a simple change in the cationic component in the catanionic surfactant promotes remarkable changes in the RMs interface with interesting consequences, in particular when using the systems as nanoreactors.

Supramolecular Assemblies Obtained by Mixing Different Cyclodextrins and AOT or BHDC Reverse Micelles

In this contribution we show the effect of the surfactant polar head and the external solvent on the incorporation of different cyclodextrins (CDs) {α-CD, β-CD, γ-CD, decenylsuccinyl-β-CD (Mod-β-CD), and hydroxypropyl-β-CD (hp-β-CD)} in different reverse micelles (RMs) {benzene/sodium 1,4-bis(2-ethylhexyl) sulfosuccinate(AOT)/water, and benzene/benzyl-n-hexadecyldimethylammonium chloride (BHDC)/water} and compare them with previous results obtained in n-heptane/AOT/water RMs. To investigate the different systems, we have used UV-vis spectrophotometry, induced circular dichroism spectroscopy (ICD), and the achiral molecular probe methyl orange (MO). The results show dramatic differences changing the external solvent and the surfactant, which are explained by considering the differences in the RMs interface composition, the water-surfactant interaction, and the CDs' location in the different media investigated. None of the CDs were incorporated into the benzene/AOT/water RMs at any [H2O]/[surfactant] ratio studied (W0) whereas it was previously shown that Mod-β-CD and hp-β-CD could be included in n-heptane/AOT/water RMs. However, all of the CDs are incorporated in benzene/BHDC/water RMs at W0> 10 and hp-β-CD is dissolved even at W0 = 0. Different from what was found in n-heptane/AOT RMs, in BHDC RMs MO showed ICD signals with two different CDs: Mod-β-CD and hp-β-CD. The results are explained by considering the known difference in the interfacial water structure for AOT and BHDC RMs and the electron-rich region on the secondary hydroxyl (wider side of the CDs), which helps to solubilize all CDs in BHDC. This study shows that chiral cyclodextrin could be available for a guest in an organic medium such as the RMs. Therefore we have created a potentially powerful nanoreactor with two different confined regions in the same aggregate: the polar core of the RMs and the chiral hydrophobic cavity of cyclodextrin.