Reverse Micelle System Research Articles

Flavodoxin in a binary surfactant system consisting of the nonionic 1-decanoyl-rac-glycerol and the zwitterionic lauryldimethylamine-N-oxide: Molecular dynamics simulation approach

Due to the short time constant of the spin-spin relaxation process, there is a limitation in the preparation of NMR sample solution for large proteins. To overcome this problem, reverse micelle systems are used. Here, molecular dynamics simulation was used to study the structure of flavodoxin in a quaternary mixture of 1-decanoyl-rac-glycerol, lauryldimethylamine-N-oxide, pentane and hexanol. Hexanol was used as co-solvent. Simulations were performed at three different co-solvent concentrations. The proportion of components in the mixture was selected according to experimental conditions. For comparison, simulation of flavodoxin in water was also performed. The simulation results show that the C$\alpha$-RMSD for the protein in water is less than for the surfactant mixture. Also, the radius of gyration of flavodoxin increased in the presence of surfactants. The distance between the two residues trp-57 and phe-94, as a measure of protein activity, was obtained from the simulations. The results showed that in the surfactant mixtures this distance increases. Analysis of the secondary structure of the protein shows that the N-terminal part of the flavodoxin is more affected by surfactants. The flavodoxin diffusion coefficient in the surfactant mixture decreased in relation to its diffusion coefficient in water.

Reverse micelle chromatography for evaluation of partition of organic solutes to micellar pseudophases

The evaluation of solute partition from an organic solvent to a reverse micelle (RM) is critical for designing effective reaction systems and for synthesizing functional materials using the nano water-phase in the RM. Although spectroscopic methods have been extensively employed for determining the partition constants of solutes in the RM systems, their applications are limited to the case, in which the background absorption is low and the spectroscopic features of the solute are effectively varied by its partitioning to RMs. This paper proposes a novel chromatographic method to overcome this limitation of conventional methods. In the present system, a size-exclusion chromatographic column is used with RM solutions as mobile phases. The RM is excluded from the stationary phase and, therefore, is eluted first. A solute is eluted with the retention volume determined by its affinities to the stationary phase and the RM. The elution volumes of a solute measured by varying the concentrations of RM in the mobile phase allow us to determine its partition constant to the RM. The partition of phenols in hexadecyltrimethylammonium chloride and bromide RMs in chloroform is successfully evaluated without interferences from the UV-absorption of the media. The larger partition constant is confirmed for a smaller water/surfactant molar ratio (w) and for chloride than bromide as the RM counterion. This suggests that the nature of water is more strongly influenced by confinement in the RM cores for smaller w and the chloride counterion because water molecules are strongly imbibed by the interface.

Recovery and purification of Aspergillus niger phytase from crude extract using AOT / isooctane reversed micelles

This work describes the successful extraction of Aspergillus niger phytase from a crude extract (CE) obtained from solid-state fermentation by reversed micelle system using anionic surfactant sodium bis (2-ethylhexyl) sulfosuccinate (AOT) in isooctane achieved in two simple steps: forward and backward extractions. The effects of potassium chloride (KCl) concentration, pH of the aqueous solution, and AOT concentration that affect the system were examined. The best result for the forward extraction was obtained with the CE solution at pH 4.0, 50 mM KCl, and 100 mM AOT, while for the backward extraction the best result was achieved with a stripping aqueous solution at pH 5.5 containing 200 mM KCl, achieving a purification factor of 4.03, 1.15 times higher than that reported for the conventional purification process. Phytase purity was demonstrated by SDS-PAGE (89 kDa) and its activity by zymogram, confirming the efficiency of the process with low time consumption (∼40 min).

Enzymatic Synthesis of β-Sitosterol Laurate by Candida rugosa Lipase AY30 in the Water/AOT/Isooctane Reverse Micelle.

Phytosterols are regarded as compounds able to reduce total and low-density lipoprotein cholesterol in the blood, and their esterified derivatives could help to improve the effectiveness of this function. In the present study, the water/sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT)/isooctane reverse micelle (RM) system was set up as a reaction medium for Candida rugosa lipase AY30 (CRL AY30) to synthesize β-sitosterol laurate (β-SLE). The product was identified by TLC, FT-IR, and HPLC-APCI-QqQ-MS/MS and quantified by HPLC. Through stepwise optimization, it was found that CRL AY30 had the highest activity in the water/AOT/isooctane RM system where 50mM PBS with a pH of 7.5 was adopted as water core to carry CRL AY30, and the proportion of [CRL AY30] (mg/mL), [water] (mM), and [AOT] (mM) was set in 3:375:25, respectively, in isooctane. After screened with single-factor experiments, the esterification reaction conditions in the CRL AY30-water/AOT/isooctane RM system were further optimized by the response surface method as follows: the mole ratio of β-sitosterol to lauric acid of 1:3.5 (25mM β-sitosterol), the enzyme load of 18% (w/w total reactants), the reaction temperature of 47°C, and the reaction time of 48h. As a result, the maximum esterification rate was up to 88.12 ± 0.79%.

Extraction Separation of Rhamnolipids by n‐Hexane via Forming Reverse Micelles

AbstractRhamnolipids (RL) have been regarded to be insoluble in n‐hexane. Unexpectedly, we have noticed that RL could be extracted together with vegetable oil by n‐hexane at analyzing oil content of fermentation broth. This paradoxic phenomenon was assumed to be due to the formation of reverse micelles. As found in this paper, the micelle size as well as conductivity increased due to water solubilization, illustrating the formation of reverse micelles of RL in n‐hexane. In this reverse micellar system, the maximal water solubility was detected to be 6.26 mol H2O/mol RL while the reverse critical micelle concentration of RL was around 3% (w/w). Hence, it seems that the presence of water could increase the dissolution of RL in n‐hexane via forming the reverse micelles of RL/n‐hexane/water. Lastly, the formation of reverse micelles was further applied for extraction of RL by n‐hexane. Using this method, over 99.0% of RL was extracted under the appropriate pH of 4.5. This extraction separation using n‐hexane instead of chloroform proposed a much cleaner and safer strategy in RL manufacture. Moreover, the capability of RL in forming reverse micelle system could benefit the future application on improving enzymatic reactions, stabilizing nanoparticles, environmental treatment, and protein folding, etc.

Removal of methylene blue from aqueous solution using sodium bis (2-ethylhexyl) sulfosuccinate-hexanol reverse micellar system

Textile manufacturing is one of the core industries which discharges a heavy load of chemicals during the dying process. As a result, the release of large contents of dyes through aqueous effluents leads to both environmental and economic concerns. The present study investigates the removal of a model cationic dye, i.e., methylene blue using a reverse micelles system of 1-hexanol as an organic solvent and sodium bis(2-ethylhexyl)sulfosuccinate (AOT) as an anionic surfactant. The influence of different parameters including surfactant concentration, reaction time and dye concentration on extraction performance, was studied. Extraction efficiency was increased with increasing time and surfactant concentration, while it was decreased with the increase in dye concentration. For this system, the optimum extraction condition correspond to surfactant concentration around 0.05M, equilibrium time 20min and dye concentration around 500 ppm, resulting in extraction efficiencies around 99%. The microdomains of water within the reverse micelle system are the driving force for the clarification of methylene blue dye from aqueous solution.

Enhancing enrichment ability of ZIF-8 mixed matrix membrane microextraction by reverse micelle strategy for analysis of multiple ionizable bioactive components in biological samples

Recent research aimed at the design of mixed-matrix membrane (MMM) to be used for microextraction emphasized on membrane extraction phase with high surface area and porosity. This study explored the influence that surfactants have on MMM extraction efficiency for the first time. The zeolitic imidazolate framework 8-based MMM (ZIF-8-MMM) was synthesized by in situ self-assembly of ZIF-8 on the inner wall of a hollow fiber membrane with the aim of fabricating a microextraction device. By prompting the encapsulation of ionizable analytes in the polar core of reverse micelles, the presence of surfactants in extraction solvent assisted the dissolution of analytes in the fiber membrane lumen and enhanced their adsorption onto ZIF-8. Notably, hereby a microextraction method based on the novel ZIF-8-MMM–reverse micelle (ZIF-8-MMM-RM) system was developed and employed for the extraction and quantitation of two alkaloids (berberine and jatrorrhizine) and two flavonoids (wogonin and wogonoside) in biological samples. The main factors affecting microextraction performance, identity of the extraction solvent, surfactant concentration, sample solution pH and extraction time, were investigated in detail. The method showed good linearity (r2 > 0.99) and repeatability (RSD < 10%), low limits of detection (0.10–0.31 ng mL−1) and high relative recoveries (90.03–98.84%). The enrichment factor values ranged between 48.47 and 54.96. Reverse micelle formation prompted by surfactant addition was demonstrated to effectively assist the extraction of multiple ionizable analytes from biological samples, resulting in a marked improvement of ZIF-8-MMM extraction performance.

Overhauser Dynamic Nuclear Polarization: A Tool for Building Maps of Hydration Water

Coating the surface of every macromolecule or macromolecular assembly, one finds a hydration layer composed of water molecules that move typically between 3× and 10× slower than water molecules in bulk water. The interaction between the water molecules in the hydration layer and the macromolecules contributes to the structural stability and sometimes the function of, e.g., proteins and lipid bilayers. Overhauser Dynamic Nuclear Polarization (ODNP) is an emerging electron-spin nuclear-spin (EPR-NMR) double-resonance tool that has demonstrated a capability of measuring the translational dynamics of water in the hydration layer. Here we discuss our efforts on two fronts: First, we design a scheme for measuring the thickness of the hydration layer and the effect of confinement on translational dynamics, as measured by ODNP, with controlled, appropriately labeled reverse micelle systems. Second, we describe the development of an a priori technique for converting ODNP measurements into a 3D “map” of hydration layer properties in dynamic room temperature samples that explore an ensemble of structures. This latter effort focuses on transmembrane model systems and utilizes the modern structure-prediction tool Rosetta in a fashion analogous to successful efforts to predict NMR order parameters. Particular focus is given to improving the quality and automation of the ODNP measurement, as well as validating predicted ensemble structures against both continuous wave EPR and NMR Paramagnetic Relaxation Enhancement (PRE) data.

Response surface optimization of the forward extraction of jacalin from jackfruit seeds using AOT/isooctane reverse micellar system

Jacalin is the major protein contained in the crude extract of jackfruit (Artocarpus heterophyllus) seed that specifically recognizes and binds reversibly to galactose. Conventionally, purification of jacalin is carried out using the tedious and costly chromatographic techniques. In this study, extraction of jacalin from jackfruit seed crude extract were done using the sodium bis(2-ethylhexyl) sulfosuccinate (AOT)-based reverse micellar system. Reverse micellar extraction is an attractive alternative for downstream processing of various proteins. A successful reverse micellar extraction consists of two basic steps: forward and backward extraction. Forward extraction transfers a target protein from an aqueous solution into the reverse micellar solution, while backward extraction releases the protein from the reverse micelles structure into a new aqueous solution. The effects of the aqueous phase pH, NaCl concentration and AOT concentration on the forward extraction efficiency (FEE) are investigated using the response surface methodology (Box-Behnken Design). The main effects and interactions of the parameters are analyzed through the 3D surface plots. The optimum conditions for forward extraction were determined as follows: aqueous phase pH 4.58, 125 mM NaCl and 40 mM AOT. Under the optimal conditions, the FEE reached 88.04±1.30%, closer to 87.99% predicted by the model. The results indicated that AOT/isooctane reverse micelle system is effective in extracting jacalin from the jackfruit seed crude extract and verified the practicability of the BBD model for optimizing the main parameters in the forward extraction of jacalin.

The kinetics of cellulase in reverse micelles using an isothermal titration microcalorimetry technique

We report the activities of cellulase in reverse micelles was investigated via microcrystalline cellulose as substrates using isothermal titration microcalorimetry (ITC). The ITC can record the heat evolved in enzyme catalysis reactions, and the resulting thermal power can be further used to calculate the apparent Michaelis constant (KM) and the maximum rate of the reaction (Vm) of enzymes. Importantly, ITC in enzyme kinetics is not limited to soluble systems—it is equally effective in monitoring reaction heats in suspensions. The reverse micelles studied here were formed by polyoxyethylene lauryl ether (Brij35), n-hexanol, and cyclohexane. The effects of ωo, the molar ratio of water to surfactant, pH, temperature, and ionic effects on the kinetics of cellulase in Brij-35/n-hexanol/cyclohexane reverse micelles were investigated, and the optimal conditions of ωo (the molar ratio of water to surfactant), pH, and temperature were obtained. Furthermore, K+, Na+, Cl−, and NO3− enhanced the cellulase activity in Brij-35/n-hexanol/cyclohexane reverse micelles; Mg2+, Ba2+, SO42-, and SO32- inhibited entrapped cellulase. These results are of interest in understanding the kinetics of cellulase in reverse micelles. The findings might have industrial applications for reverse micelles system that degrades cellulose into sugar.

Degradation of several polycyclic aromatic hydrocarbons by laccase in reverse micelle system

Remediation of polycyclic aromatic hydrocarbons (PAHs) in oily sludge has become the focus of attention. UV spectrophotometer analysis showed that four types of PAHs were found in sample, which including phenanthrene, anthracene, benzo(a)anthracene and benzo(b)fluoranthene. In order to degrade PAH effectively, the laccase reverse micelles system was proposed. The system protects laccase from being affected by organic phase. Reverse micelles were prepared by using isooctane to simulate oil. The optimum water content W0 was 10 by measuring the electrical conductivity of the system. Under this condition, the effects of pH, temperature and ionic strength on the degradation rate of PAHs were investigated. Also, compared with that of non-immobilized laccase, the ratio between the secondary structures of laccase under different conditions was studied. The results showed that the highest laccase activity was obtained at pH 4.2 and 30 °C with 60 mmol/L KCl. Meanwhile, the structure of α-helix accounts for the largest proportion, and the ratio of α-helix in the laccase secondary structure in the laccase-reverse micelle system was higher than that of the non-immobilized one under this condition. Finally, predicting the reactive site of the degradation of polycyclic aromatic hydrocarbons was simulated by ORCA (Version 4.2.0). The application in oily sludge was further conducted. This study provides an effective method and basis for the degradation of PAHs in oily sludge.

The photophysics of a hydrophilic molecule in the presence of graphene oxide

The photophysics of fluorophore molecule was highly modulated when it was confined inside the nanopool of reverse micelle. By adding some foreign substance such as nanoparticles (in our case graphene oxide) it may interact with the surface active part of the surfactants, polar solvent molecules, and fluorophore present in the reverse micelle. This interaction may modulate photophysics of the fluorophore molecule in the reverse micelles compared to that in reverse micelle without adding foreign substance. The effect of foreign materials (graphene oxide) on the photophysics of 7-(n,n׀-diethylamino)coumarin-3-carboxylic acid (DACCA) in the water/AOT/isooctane reverse micelle was investigated via UV–vis absorption, steady-state and time-resolved fluorescence emission spectroscopy. The stability of the reverse micelle in presence of graphene oxide was investigated via dynamic light scattering (DLS), fluorescence lifetime imaging microscopy (FLIM), field emission scanning electron microscopy (FESEM). In the presence of GO the absorption maxima of DACCA become blue shifted and emission maxima of DACCA become red shifted compared to normal reverse micelle system. The average lifetime of DACCA in GO containing reverse micelle is always less than the normal reverse micelle system (particularly at lower w0 values). The rotational relaxation time in GO containing reverse micelle is faster and solvent relaxation time, particularly at lower w0 values is always slower than the normal reverse micelle. In GO containing reverse micelle, quantum yield, decay time, rotational relaxation time, solvent relaxation time at λex = 375 nm is always higher than λex = 405 nm, shows the excitation wavelength dependent photophysics of DACCA in GO containing reverse micelles. DLS, FLIM and FESEM study revealed that GO does not affect the stability and structure of the reverse micelle.

Non-linear dielectric effect in reverse micelles system

Reverse micelles are macromolecular systems consisting of highly polar and non-polar components. External electric field should significantly affect their structure and properties. The non-linear dielectric effect (NDE) measurements of the isooctane/water/AOT ternary system were carried out. Extremely large positive response was noted. A mechanism describing the response of the system on a strong electric field is connected with a movement of ions inside the micelles. A pronounced maximum of the NDE effect was also found in a region of stability of reverse micelles. It was explained by precritical phenomena.

Reverse Micellar Dyeing of Cotton Fiber with Reactive Dyes: A Study of the Effect of Water pH and Hardness.

Effects of hardness and the pH value of water in a water pool on PEG-based nonionic surfactant dyeing of cotton fiber with reactive dyes in a heptane reverse micelle system were investigated in terms of the color yield, reflectance, CIE L*a*b* value, and unlevelness. Results reveal that the effect of the water-pool pH value on the color yield and reflectance are more significant than the effect of hardness in the water pool. The dyed fabrics under an alkaline water-pool condition obtain a lower color yield and higher reflectance percentage than those under acidic and nearly neutral conditions. The increase of hardness in a water pool has higher influence on unlevelness of dyed samples than the increase or decrease of the pH value in a water pool. The changes in hardness and the pH value in a water pool did not result in a significant change in CIE L*a*b* values of dyed specimens, and no chromatic change was found in dyed fabrics. Excellent washing fastness results of the dyed fabrics, guaranteeing adequate removal of unfixed dyes, assure accuracy of the results in a spectrophotometric measurement.

Mechanism of enhanced oxidation ability of dilute nitric acid and dissolution of pure gold in seawater with nitric acid

It has been discovered that dilute nitric acid in reversed micelle systems can oxidize the Br- ion to Br2 and we have proposed that the nitryl (or nitronium) ion NO2+ should be the active species in the oxidation process. Nitration of phenol in reversed micelle systems with dilute nitric acid, CHCl3/CTAC/H2O (2.0 mol dm-3 HNO3 in the 1.0% (v/v) H2O phase), has been performed at 35 oC to obtain 2- and 4-nitrophenols, where CTAC represents cetyltrimethylammonium chloride. In aqueous 2.0 mol dm-3 HNO3 solution accompanied by 4.0 mol dm-3 LiCl (and a small amount of LiBr as the bromide resource), trans-1,4-dibromo-2-butene was successfully brominated to 1,2,3,4-tetrabromobutane. This result is good evidence that the Br- ion can be oxidized to Br2 in dilute nitric acid (2.0 mol dm-3) providing it contains concentrated salts. For chloride salts, the cation effects increased as Et4N+ << Na+ < Li+ < Ca2+ < Mg2+. Even the evolution of Cl2 has been demonstrated from < 2.0 mol dm-3 HNO3 solution containing concentrated LiCl, MgCl2, and CaCl2 as well as AlCl3. The dissolution of precious metals (Au, Pt, and Pd), especially, of gold has been demonstrated in 0.1 - 2 mol dm-3 HNO3 accompanied by alkali metal, alkaline earth metal, and aluminum chlorides. The complete dissolution time of pure gold plate (20±2 mg, 0.1 mm thickness) in 2.0 mol dm-3 HNO3 accompanied by 1.0 mol dm-3 AlCl3 has been shortened remarkably with temperature increase from 15 to 80 oC. The dissolution rate constants, log (k /s-1), of a piece of gold wire (19.7±0.5 mg) in 20 mL of 2.0 mol dm-3 HNO3 accompanied by the metal chlorides, in general, increase with increasing salt concentrations at 40 and 60 oC. The gold can be dissolved in the solution of <1.0 mol dm-3 HNO3 and <1.0 mol dm-3 HCl, i.e. a “dilute aqua regia. We have achieved a total dissolution of five pieces of the gold wire (totally 0.10 g) in 100 mL of the 1:1 mixture between seawater and 2.0 mol dm-3 HNO3 at ca. 100 oC.

Optimal aggregation number of reverse micelles in supercritical carbon dioxide: a theoretical perspective.

The aggregation number is one of the most fundamental and important structural parameters for the micelle or reverse micelle (RM) system. In this work, a simple, reliable method for the determination of the aggregation number of RMs in supercritical CO2 (scCO2) was presented through a molecular dynamics simulation. The process of pulling surfactants out of the RMs one by one was performed to calculate the aggregation number. The free energies of RMs with different numbers of surfactants were calculated through this process. We found an RM with the lowest free energy, which was considered to have the optimal number of surfactants. Therefore, the optimal aggregation number of RMs was acquired. In order to explain the existence of an optimal aggregation number, detailed analyses of surfactant accumulation were conducted by combining molecular dynamics with quantum chemistry methods. The results indicated that in the RMs with the lowest free energy, the head-group and tail-terminal of the surfactants accumulated on an equipotential surface. In this case, the surfactant film could effectively separate water and CO2; thus, the lowest free energy was expected. This method determined the aggregation number of RMs by theoretical calculations that did not depend on experimental measurements. This presented approach facilitates the evaluation of the characteristics of RMs in scCO2 and can be further applied in the RM system of organic solvents or even in the micellar system.

Online monitoring and mass transfer modelling of the growth of Ni‐B nanoparticles in a reverse micelle system

AbstractThe growth of Ni‐B nanoparticles by chemical reduction of nickel acetate tetrahydrate (0.3M) using sodium borohydride (B/Ni = 2.5 molar ratio) in CTAB/n‐hexanol/water ternary reverse micelles was investigated. SEM studies showed that the synthesized nanoparticles within the reverse micelle range are more favourable in terms of microstructure and morphology. Nanoparticle growth has been controlled in the range of 0.83–5.59 nm · h−1 by the precise adjustment of mass fraction of surfactant in the oil phase (0.22–0.47 wt%) and the overall mass fraction of the aqueous phase (0.1–0.3 wt%) values. Particle growth was measured in situ using time resolved UV‐vis absorbance spectroscopy. In addition, a new correlation based on characteristic absorbance peaks of Ni‐B nanoparticles at wavelengths of 285 and 230 nm has been derived for this purpose. An accuracy of about 9 % on the nanoparticle average sizes with respect to sizes measured by dynamic light scattering (DLS) was found (R2 = 0.98). A diffusion controlled growth model involving effective diffusivity of reverse micelles (Deff − RM = 9.60 × 10−11 − 1.17 × 10−8 m2 · s−1) andripening parameter (K = 4.57 × 10−21 − 1.54 × 10−20 m2 · s−1) was developed to describe the growth behaviour of the nanoparticles. The experimental results were very close to the experimental results with slight errors (7–10 %). Finally, TEM micrographs of Ni‐B nanoparticles showed that particle growth as well as agglomeration can be effectively controlled by precise adjustment of each component in the reverse micelle technique.

Rapid microwave-assisted refluxing synthesis of hierarchical mulberry-shaped Na3V2(PO4)2O2F@C as high performance cathode for sodium &amp; lithium-ion batteries

Unique hierarchical mulberry-shaped Na3V2(PO4)2O2F@C nanocomposite was fabricated by a rapid microwave-assisted low-temperature refluxing strategy. The V(acac)3 reverse micelle systems in the water-in-oil microemulsions played key roles in forming the self-assembly architectures. The prepared Na3V2(PO4)2O2F@C nanoparticles with the anisotropic growth along the [002] direction were in - situ encapsulated in carbon shells, which greatly contribute to fast Na+/e− transfer in electrodes. And the self-assemblies with high structure stability help to improve the cycle performance and mitigate voltage fading. The initial discharge capacity of Na3V2(PO4)2O2F@C as cathode for sodium ion batteries is about 127.9 mA h g−1 at 0.1 C. Besides, a high rate performance with a capacity of 88.1 mA h g−1 at 20 C has been achieved, and the capacity retains 82.1% after 2,000 cycles. In addition, the reaction kinetics and Na+ transportation mechanism of Na3V2(PO4)2O2F@C were preliminarily investigated by the ex situ X-ray diffraction, X-ray photoelectron spectroscopy and galvanostatic intermittent titration technique. More interestingly, when coupled with Li, the fabricated hybrid Li/Na-ion batteries also exhibit excellent rate and cycling performances. The proposed rapid refluxing strategy to synthesize mulberry-shaped Na3V2(PO4)2O2F@C opens up a new opportunity to develop high-performance electrode materials for the energy storage systems.

Construction of Surfactin--Polysorbate-80 reversed micelle systems and its application on extraction of cellulasee

This work studied a novel method to construct a mixed reverse micelle system and its application in extracting and purifying enzymes. Mixed reversed micellar extraction formed by biosurfactant surfactin and ionic surfactant polysorbate-80 was applied for extracting cellulase. 40% surfactin and 60% polysorbate-80 were found to be the best system. The activity and protein recovery were obtained with the best conditions: in forward extraction, surfactant concentration was 3.0 mM; pH 7.0; temperature 40°C; KCl 0.05 M. In backward extraction, pH 7.5; temperature 40°C; KCl 0.1 M.

Excited-state prototropism of 7-hydroxy-4-methylcoumarin in [Cnmim][BF4] series of ionic liquid-water mixtures: insights on reverse micelle-like water nanocluster formation.

This study explores the excited state prototropic behavior of the fluorophore, 7-hydroxy-4-methylcoumarin (7H4MC), in the [Cnmim][BF4] (n = 2, 4, 6, 8, 10) series of ionic liquid (IL)-water mixtures at low water contents. In pure IL media, 7H4MC exists in the neutral form in both ground and excited states. However, on addition of water to the ILs, the excited neutral form of the dye is gradually converted to the anionic and the tautomeric species, leading to characteristic changes in the emission spectra. The similarity in the spectral features of 7H4MC in the IL-water system with that in a conventional reverse micelle system rather than with organic solvent-water mixtures, suggests that in the presence of water, the ILs are organized into reverse micelle-like structures with the consequent formation of confined water pockets. The results further suggest that formation of water nanoclusters and the ensuing changes in excited state prototropic behavior of the dye, is facilitated by increase in the alkyl chain length of the IL cation. These propositions are supported by time-resolved fluorescence studies. To the best of our knowledge this is the first report on proton transfer reaction in IL-water mixtures at low water contents. Considering that ILs are useful as solvents and surfactants, and IL-water mixtures in particular have applications in chemical extractions and biocatalysis, an understanding of the structural organization and water pool formation in these systems is quite important. The insights obtained from the prototropic transformations of 7H4MC are significant not only for fundamental self-assembly studies, but also for the development of ILs as chemical reaction media.