Investigation of the Microenvironment in Nonionic Reverse Micelles Using Methyl Orange and Methylene Blue as Absorption Probes

Study of structure of nonaqueous reverse micelles with o-nitroaniline and methyl orange as molecular probes: comparison with an aqueous reverse micelles

Instead of water reversed micelles can also be formed with polar organic solvents possessed with high dielectric constant and very low solubility in oil phase. Nonaqueous reverse micelles or microemulsions represent an interesting microreactors for various reactions, especially for reactions, where reagents can react with water. Study of localization places of molecular probes in organic polar pockets of reverse micelles is topical. The solvatochromic behavior of optical probes ortho-nitroaniline (o-NA) and methyl orange (MO) was studied in nonaqueous reverse micelles on the basis of surfactants sodium bis (2-ethylhexyl) sulfosuccinate (AOT) and polyoxyethylene (4) lauryl ether (C12E4) and polar organic solvents (acetonitrile, dimethylformamide, glacial acetic acid, etc.) insoluble in oil phase hexane. The strength of binding of o-NA and MO to AOT and C12E4 reversed micelles was assesssed via binding constant (K b ) and association degree (α) respectively. Donor, acceptor, or dipole-dipole interactions ability of the solvent to the head groups of surfactant was taken into account in order to explain results obtained with UV–visible spectroscopic method. The binding constants of o-NA with reverse AOT micelles in the presence of various solvents in the pockets of reversed micelles increase in the following row water < glacial acetic acid < acetonitrile < dimethylformamide < dimethyl sulfoxide, but this sequence is reversed when o-NA binds to C12E4 reverse micelles. The high value of the proton donor or acidity parameter in the water molecule (x d  = 0.37) determines the weak binding of o-NA to the head AOT groups (K b  = 20.8) in case of aqueous reverse micelles. The high value of the dipole parameter in the dimethylformamide molecule (x n  = 0.40) promotes its strong interaction with nonionic polyoxyethylene groups of C12E4, which results in low value of binding constant (K b  = 26.5) in case of optical probe o-NA and low value of association degree (α = 0.60) using MO as absorption probe. The results of this article will contribute to the improvement of the concept of interfacial processes, viz.: (i) some issues of supramolecular chemistry, (ii) revealing the contribution of parameters of donor, acceptor or dipole-dipole interaction in a polar organic solvent at the surfactant/nonpolar organic solvent interface, and (iii) features of the dissolution of optical probes in non-aqueous reverse micelles.

Photoactive Red Fluorescent SiO2 Nanoparticles Based on Controlled Methylene Blue Aggregation in Reverse Microemulsions

We present a reverse microemulsion synthesis procedure for incorporating methylene blue (MB), a known FDA-approved type-II red-absorbing photosensitizer and ¹O₂ generator, into the matrix of hydrophobic-core/hydrophilic-shell SiO₂ nanoparticles. Different synthesis conditions were explored with the aim of controlling the entrapped-dye aggregation at high dye loadings in the hydrophobic protective core; minimizing dye aggregation ensured highly efficient photoactive nanoentities for ¹O₂ production. Monitoring the synthesis in real time using UV-vis absorption allowed tracking of the dye aggregation process. In particular, silica nanoparticles (MB@SiO₂ NPs) of ∼50 nm diameter size with a high local entrapped-MB concentration (∼10^-2 M, 1000 MB molecules per NP) and a moderate proportion of dye aggregation were obtained. The as-prepared MB@SiO₂ NPs showed a high singlet oxygen photogeneration efficiency (Φ_Δ = 0.30 ± 0.05), and they can be also considered as red fluorescent probes (Φ_F ∼ 0.02, λ_max ∼ 650 nm). The distinctive photophysical and photochemical characteristics of the synthesized NPs reveal that the reverse microemulsion synthesis procedure offers an interesting strategy for the development of complex theranostic nano-objects for photodynamic therapy.

A novel CPE procedure by oil-in-water microemulsion for preconcentrating and analyzing thorium and uranium

A novel cloud point extraction (CPE) procedure was developed to preenrich Th4+ and UO2 2+ by oil-in-water (O/W) microemulsion. Coupling CPE to ICP-MS, the separation and analysis were achieved at a trace level, in which the low detection limits were 0.019 and 0.042 ng mL−1 for Th(IV) and U(VI), respectively. N,N′-diethyl-N,N′-ditolyl-2,9-diamide-1,10-phenanthroline (Et-Tol-DAPhen), as an extremely hydrophobic extractant, was failed to dissolve in single or mixed micelles, but was successfully solubilized to CPE system owing to O/W microemulsion. The extraction efficiency and selectivity for Th4+ and UO2 2+ were excellent under acidic condition of 1.0 mol L−1 HNO3, and the recovery of ultra-trace Th4+ and UO2 2+ was almost 100% even at the presence of large amounts of lanthanides, exhibiting high tolerance limits for lanthanides. The solubilization, extraction and coordination behaviours were studied systematically via DLS, UV–vis, 1H NMR and FT-IR. Moreover, the solubilization of N,N′-dioctyl-N,N′-dioctyl-2,9-diamide-1,10-phenanthroline (Oct-Oct-DAPhen) and efficient extraction for UO2 2+ were also realized by O/W microemulsion, which further proved the feasibility of the method.

Study of Structure of Water Droplets of Nonionic Polyoxyethylene (4) Lauryl Ether Reverse Micelles in the Presence of Sodium Cholate

The microenvironment of water droplets of nonionic polyoxyethylene (4) lauryl ether reverse micelles was investigated with infrared and ultraviolet-visible spectroscopy, also by viscosity and dynamic light scattering measurements in the presence of biological surfactant sodium cholate as additive. Influence of concentrations of polyoxyethylene (4) lauryl ether and sodium cholate on the microstructure of mixed reverse micelles was studied. Influence of sodium cholate as additive on the ratio of free, bound and trapped water fractions was studied via deconvolution of the O-H stretching vibrational absorption spectra in the region of 3 000–3 800 cm−1 into three subpeaks with a Monte Carlo method. Several characteristic parameters of reverse microemulsions were calculated on the basis of measurements of their kinematic viscosity. A different influence of concentration of sodium cholate on the binding constant and association degree of optical probes (o-nitroaniline and methyl orange respectively) to ethylene oxide groups of nonionic surfactant was observed. It was found that the bound water fraction, the binding constants and association degrees, the relative density of water pockets, etc. change their values passing through an extremum under the influence of sodium cholate.

A reverse micellar system with Triton X-100: effect of surfactant polydispersity and preparation of monodisperse silica nanoparticles

Reverse micellar systems possess a characteristic nanoscale water-in-oil (w/o) structure and can offer mild conditions as a unique and versatile reaction medium. Reverse microemulsions containing water/TX-100 + hexanol/hexane are studied in this work through experimental techniques and simulation methods. Surfactant dosages and water amount affect the micellar structure profoundly, and the polydispersity of the surfactant molecules affects the micellar structure remarkably. TX-100 with 9-10 EO units can form micelles in a simply piling way, while TX-100 with 5-10 EO units endows the micelles with a hierarchical micellar interface and a more compact structure, leading to monodisperse micelles with a smaller diameter. Water in the polar cores has three states. In the reverse micellar system using TX-100 with 9-10 EO units, hydrolysis of tetraethoxysilane happens rapidly and the formed silica gels are apt to aggregate, resulting in polydisperse silica nanoparticles. For the micellar system using TX-100 with 5-10 EO units, the micellar hierarchical distributed interface facilitates the material exchange of tetraethoxysilane and limits the hydrolysis of tetraethoxysilane inside the micelles, providing monodisperse silica nanoparticles.

Aggregation behavior of 1-dodecyl-3-methylimidazolium bromide in aqueous solution: effect of ionic liquids with aromatic anions

The effects of ionic liquids (ILs), 1-butyl-3-methylimidazolium methylsulfonate (bmimMsa), 1-butyl-3-methylimidazolium benzenesulfonate (bmimBsa), and 1-butyl-3-methylimidazolium 2-naphthalenesulfonate (bmimNsa), on the aggregation behavior of 1-dodecyl-3-methylimidazolium bromide (C12mimBr) in aqueous solution were investigated by surface tension, dynamic light scattering measurements, and (1)H NMR spectroscopy. The ability to promote the surfactant aggregation is in the order bmimNsa > bmimBsa > bmimMsa. Nevertheless, only bmimNsa distinctly reduces both the CMC value and the surface tension at CMC. Due to the penetration of C10H7SO3(-)anions into the surfactant aggregate, bmimNsa is found to induce a phase transition from micelles to vesicles, whereas the other ILs only slightly increase the sizes of micelles. The combined effect of intermolecular interactions, such as hydrophobic effect, electrostatic attractions, and π-π stacking interactions, is supposed to be responsible for this structural transformation, in which π-π stacking plays an important role.

Role of solubilized water in micelles formed by Triton X-100 in 1-butyl-3-methylimidazolium ionic liquids

We demonstrate here the aggregation behavior of a nonionic surfactant Triton X-100 in two 1-butyl-3-methylimidazolium ionic liquids (the hydrophilic IL [bmim][BF(4)] and the hydrophobic IL [bmim][PF(6)]) by surface tension measurements. The effects of added water on the microstructure of Triton X-100 in ILs micelles are investigated. When small amounts of water are added to Triton X-100 in [bmim][PF(6)] micelles, the water molecules are first bound to the ethylene oxide (EO) units of Triton X-100 and then form the water pool in the core of the microemulsion. When water molecules are added to the Triton X-100 in [bmim][BF(4)] micelles, there is no microemulsion formed; these water molecules are first solubilized in [bmim][BF(4)]. When the solubilization is saturated, the water molecules start to bind to the EO group of Triton X-100; these results are confirmed by UV-vis, FTIR, and (1)H NMR spectra.

Use of reverse micelles for the simultaneous extraction of oil, proteins, and glucosinolates from cruciferous oilseeds

Cruciferous oilseeds are important sources of oil, proteins, and glucosinolates (GLs), potentially available when biorefinery processes are used. The proposed extraction technology is based on the use of reverse micelles (RMs) made with cetyltrimethylammonium bromide (CTAB) dispersed in organic solvent. The physicochemical properties of this extraction system and the good water solubility of many high value compounds, such as GLs and some proteins, permit the simultaneous extraction of oil, and these products from cruciferous oilseed meals. This procedure is based on three main steps: (i) seed conditioning; (ii) solid-liquid extraction by RM solution; and (iii) back-transfer of the RM solution for recovery of the extracted compounds. The method makes it possible to simultaneously extract almost the same amount of oil as with pure organic solvents used in the current extraction plants and more than 90% of soluble proteins and GLs. It is a promising biorefinery technology alternative to traditional oil extraction processes.

Temperature-Dependent Magnetic Field Effect Study on Exciplex Luminescence: Probing the Triton X-100 Reverse Micelle in Cyclohexane

The microenvironment within the reverse micelle of the nonionic surfactant Triton X-100 (TX-100) in cyclohexane has been investigated by studying the magnetic field effect (MFE) on pyrene-dimethylaniline exciplex luminescence. The nature of exciplex fluorescence and its behavior in the presence of a magnetic field have been found to vary significantly with the water content of the medium. Results are discussed in light of multiple exciplex formation within the micelle which is further supported by the fluorescence lifetime measurements. Those exciplexes emitting at longer wavelength are found to be magnetic field sensitive while those emitting toward the blue region of the spectrum are insensitive toward magnetic field. Since the exciplex's emission characteristics and magnetic field sensitivity depend on its immediate surrounding, it has been concluded that the environment within the micelle is nonuniform. With an increase in hydration level, different zones of varying polarity are created within the reverse micelle. It has been pointed out that the magnetic field sensitive components reside inside the polar core of the micelle while those located near the hydrocarbon tail are field insensitive. However it has been presumed that an interconversion between the different types of exciplexes is possible. The environment within the reverse micelle is found to be largely affected by the change in temperature, and this is reflected in the exciplex emission property and the extent of magnetic field effect. Interestingly, the variation of MFE with temperature follows different trends in the dry and the wet reverse micelle. A comparison has been drawn with the reverse micelle of the ionic surfactant to get an insight into the difference between the various types of micellar environment.

On the Possibility That Cyclodextrins' Chiral Cavities Can Be Available on AOT n-Heptane Reverse Micelles. A UV−Visible and Induced Circular Dichroism Study

The formation of reverse micelles (RMs) of sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT) in n-heptane including two different beta-cyclodextrin (beta-CD) derivatives (hydroxypropyl-beta-CD, hp-beta-CD, and decenyl succinyl-beta-CD, Mod-beta-CD) is reported. Both cyclodextrins can be incorporated into AOT RMs in different zones within the aggregate, while beta-CD cannot. Using UV-vis and induced circular dichroism (ICD) spectroscopy and different achiral molecular probes (some azo dyes, p-nitroaniline and ferrocene), it was possible to determine that Mod-beta-CD is located with its cavity at the oil side of the AOT RM interface, while for hp-beta-CD the cavity is inside the RM water pool. Among the molecular probes used, methyl orange (MO) was the only one which gave the ICD signal when dissolved in the AOT RMs with hp-beta-CD, so a detailed study of MO behavior in homogeneous media was also performed to compare with the microheterogeneous media. The solvatochromic behavior of the dye depends not only on the polarity of the media but also on other specific solvent properties. A Kamlet-Taft analysis shows that the MO absorption spectrum shifts to longer wavelength with an increase in the solvent polarity-polarizability (pi*) and the hydrogen donor ability (alpha) of the medium. MO appears to be almost 3 times more sensitive to the pi* parameter than to the alpha parameter. In addition, from the MO absorption spectral changes with the hp-beta-CD concentration, the association equilibrium constants in pure water (K11W) and inside the RMs (K11RM) were computed. The results show that K11W is almost 10 times larger than the value inside the RMs. The latter can be explained considering that MO resides anchored to the RM interface through hydrogen bond interaction with the hydration bound water. This study shows for the first time that the cyclodextrin chiral cavity is 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.

The Effect of Water on the Microstructure of 1-Butyl-3-methylimidazolium Tetrafluoroborate/TX-100/Benzene Ionic Liquid Microemulsions

The ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) forms nonaqueous microemulsions with benzene with the aid of nonionic surfactant TX-100. The phase diagram of the ternary system was prepared, and the microstructures of the microemulsion were recognized. On the basis of the phase diagram, a series of ionic liquid-in-oil (IL/O) microemulsions were chosen and characterized by dynamic light scattering (DLS), which shows a similar swelling behavior to typical water-in-oil (W/O) microemulsions. The existence of IL pools in the IL/O microemulsion was confirmed by UV/Vis spectroscopic analysis with CoCl2 and methylene blue (MB) as the absorption probes. A constant polarity of the IL pool is observed, even if small amounts of water are added to the microemulsion, thus suggesting that the water molecules are solubilized in the polar outer shell of the microemulsion, as confirmed by FTIR spectra. 1H NMR spectroscopic analysis shows that these water molecules interact with the electronegative oxygen atoms of the oxyethylene (OE) units of TX-100 through hydrogen-bonding interactions, and the electronegative oxygen atoms of the water molecules attract the electropositive imidazolium rings of [bmim][BF4]. Hence, the water molecules are like a glue that stick the IL and OE units more tightly together and thus make the microemulsion system more stable. Considering the unique solubilization behavior of added water molecules, the IL/O microemulsion system may be used as a medium to prepare porous or hollow nanomaterials by hydrolysis reactions.

Studies on the micropolarities of bmimBF4/TX-100/toluene ionic liquid microemulsions and their behaviors characterized by UV-visible spectroscopy

Ionic liquids (ILs), 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4), were substituted for polar water and formed nonaqueous microemulsions with toluene by the aid of nonionic surfactant TX-100. The phase behavior of the ternary system was investigated, and microregions of bmimBF4-in-toluene (IL/O), bicontinuous, and toluene-in-bmimBF4 (O/IL) were identified by traditional electrical conductivity measurements. Dynamic light scattering (DLS) revealed the formation of the IL microemulsions. The micropolarities of the IL/O microemulsions were investigated by the UV-vis spectroscopy using the methyl orange (MO) and methylene blue (MB) as absorption probes. The results indicated that the polarity of the IL/O microemulsion increased only before the IL pools were formed, whereas a relatively fixed polar microenvironment was obtained in the IL pools of the microemulsions. Moreover, UV-vis spectroscopy has also shown that ionic salt compounds such as Ni(NO3)2, CoCl2, CuCl2, and biochemical reagent riboflavin could be solubilized into the IL/O microemulsion droplets, indicating that the IL/O microemulsions have potential application in the production of metallic or semiconductor nanomaterials, and in biological extractions or as solvents for enzymatic reactions. The IL/O microemulsions may have some expected effects due to the unique features of ILs and microemulsions.

Photoisomerization of a carbocyanine derivative in the reverse phases of a block copolymer: evidence for the existence of water droplets

In an attempt to understand the nature of water present in the reverse phases of aggregates formed with the triblock copolymer poly(ethylene oxide)(20)-poly(propylene oxide)(70)-poly(ethylene oxide)(20) (P123) and also investigate how these confined environments influence the rates of photoisomerization, fluorescence lifetimes and quantum yields of a carbocyanine derivative--3,3'-diethyloxadicarbocyanine iodide (DODCI)--were measured in these systems over the temperature range of 293-318 K. Three different copolymer-oil-water compositions were chosen such that the mole ratio of water to copolymer (W) spans the range of 50-150. In these systems, butyl acetate was used as the oil or the nonpolar component. It has been noticed that in all three systems the fluorescence decays of DODCI comprise a long component whose contribution is 85-90%, and this has been ascribed to the fraction of solute solubilized in the core region where hydrated poly(ethylene oxide) units are present. A short-decay component is associated with the remaining fraction, and its values match with those measured in water, indicating that the water present in these reverse phases is in the form of droplets. The photoisomerization rate constants of DODCI located in the core regions of the reverse phases are identical in the three systems at a given temperature and similar to the ones obtained in normal phases of P123. The reasons for the observed behavior have been discussed.

Kinetics of dissociation of tris-(2,2′-bipyridyl) iron(II) in water solubilized by Triton X-100 reverse micelles

The dissociation of tris-(2,2'-bipyridyl) iron(II) ([Fe(bipy)3]2+) has been studied in the Triton X-100/hexanol/cyclohexane reverse micellar medium. The reaction obeys simple first-order kinetics with no evidence of autoinhibition. The first-order rate constant (k1) has been determined at different values of W ([H2O]/[Triton X-100]). The rate (k1) decreases with increasing value of W. k1 also increases with increase in Triton X-100 concentration at constant values of W, showing that the reaction takes place at greater speed at the micellar interface. The kinetic results can be interpreted by the monomolecular pseudo-phase model. The effect of W on rate (k1) is more pronounced in the range of W from 1.55 to 4.2 but less pronounced at higher W. The reaction is further accelerated by Cl- and SCN- ions and the kinetic results provide evidence for the formation of ion pairs between the cation [Fe(bipy)3]2+ and each of these anions. The formation of such ion pairs has not been observed in aqueous medium but has been reported earlier in aqueous-alcohol mixtures. This result therefore provides evidence for the lower micropolarity of solubilized water compared to ordinary water.

Vibrational Relaxation of Azide in Formamide Reverse Micelles

Static and ultrafast infrared spectroscopy have been used to measure absorption spectra and vibrational energy relaxation (VER) times for the antisymmetric stretching vibrational band of azide, N(3)(-), in formamide-containing reverse micelles (RMs). RMs were formed in n-heptane using the surfactant AOT, sodium bis(2-ethylhexyl) sulfosuccinate. The VER times were found to be significantly longer than in bulk formamide. The VER times became longer as the molar ratio of formamide to AOT, omega(F), was decreased. Decreasing omega(F) also resulted in substantial blue shifts of the azide static absorption band compared to the frequency in bulk formamide. The omega(F) dependent studies are consistent with expected size trends, where a larger RM results in more bulklike polar solvent and faster VER rates. These results are in contrast to aqueous AOT RMs where VER times were indistinguishable from those in the bulk and the static spectral shifts were much smaller. The differences between the static and dynamic behavior in aqueous and formamide RMs are related to differences in structural changes upon confinement in RMs.

Surfactant Charge Effects on the Location, Vibrational Spectra, and Relaxation Dynamics of Cyanoferrates in Reverse Micelles

Ultrafast infrared spectroscopy has been used to measure vibrational energy relaxation (VER) and reorientation (Tr) times for the high frequency CN stretches of potassium ferrocyanide and ferricyanide and the NO stretch of sodium nitroprusside (SNP) in several reverse micelle (RM) systems using cationic, anionic, and nonionic surfactants. The confinement effects on anion vibrational spectra and dynamics in aqueous RMs depend on the charge of the surfactant that is used to form the RMs. Spectra and VER dynamics of ferrocyanide are not significantly altered in the limited number of RMs in which it could be solubilized. The static spectra of ferricyanide suggest an environment that is most bulklike in anionic RMs and least bulklike in cationic RMs. The dynamics of ferricyanide are slower in cationic RMs and indistinguishable from the bulk in nonionic RMs. The VER dynamics and static spectra of SNP are indistinguishable from the bulk in anionic RMs, but much slower in cationic RMs. This suggests a strong surfactant-solute repulsion in the former and an attraction in the latter. Broad static spectra and probe frequency dependent dynamics are seen for SNP in nonionic RMs, indicating an inhomogeneous distribution of environments. Similar measurements were carried out for SNP in mixtures of water and a model compound containing only the hydrophilic portion of the nonionic surfactants in which RMs are not formed. The results closely resemble those observed for SNP in nonionic RMs and provide evidence that in the latter water penetrates the interface and hydrates the ethylene oxide groups before forming a water pool. The results are consistent with the explanation that Coulombic forces determine the anion location. The anions are repelled to the interior of the water pool, which has a bulklike environment in anionic RMs, and are attracted to the interface in cationic RMs, resulting in a strong interaction with the surfactant. The solute location in the nonionic RMs depends on the hydrophilic nature of the probe, with ferrocyanide and ferricyanide being more hydrophilic than SNP. These results and the dependence on surfactant charge are similar to those reported for azide.