Transition from Micelles to Microemulsions in Sugar-Based Surfactant Systems Probed by Fluorescence Spectroscopy and Atomic Force Microscopy

The systems investigated by fluorescence spectroscopy and atomic force microscopy were water/sucrose laurate/oil + ethanol. The oils were R (+)-limonene and isopropylmyristate. The mixing ratio (w/w) of ethanol/oil equals unity. The fluorescent probes auramine-O and 8-anilino-1-naphthalenesulfonic acid were used to determine the minimum ω′ value for the transition of reverse micelles to microemulsions in the systems based on the two oils, as well as at different surfactant contents. The fluorescence quenching of Safranine-T (3, 6-diamino-2,7-dimethyl-5 phenyl phenazinium chloride) by the inorganic ions Fe²⁺, Fe³⁺, and Cu²⁺ was studied in reverse micelles and microemulsions. The Stern-Volmer quenching constants at different water/surfactant molar ratios (ω values) were calculated from the data of the quenching process. Atomic force microscopy was used to image the systems based on the two oils for different water to surfactant molar ratios below and above the minimum ω′ value.     

Growth of silica nanoparticles in methylmethacrylate-based water-in-oil microemulsions

The mechanism of silica particle formation in monomer microemulsions is studied using dynamic light scattering (DLS), atomic force microscopy, small-angle X-ray scattering (SAXS), and conductivity measurements. The hydrolysis of tetraethylorthosilicate (TEOS) in methylmethacrylate (MMA) microemulsions (MMA = methylmethacrylate) is compared with the formation of SiO₂ particles in heptane microemulsions. Stable microemulsions without cosurfactant were found for MMA, the nonionic surfactant Marlophen NP10, and aqueous ammonia (0.75 wt%). In the one-phase region of the ternary phase diagram, the water/surfactant ratio (R _w) could be varied from 6 to 18. The DLS and SAXS measurements show that reverse micelles form in these water-in-oil (w/o) microemulsions. The minimum water-to-surfactant molar ratio required for micelle formation was determined. Particle formation is achieved from the base-catalyzed hydrolysis of TEOS. According to atomic force microscopy measurements of particles isolated from the emulsion, the particle size can be effectively tailored in between 20 and 60 nm by varying R _w from 2–6 in heptane w/o microemulsions. For MMA-based microemulsions, the particle diameter ranges from 25 to 50 nm, but the polydispersity is higher. Tailoring of the particle size is not achieved with R _w, but adjusting the particle growth period produces particles between 10 and 70 nm.     

Water Solubilization in Mixed Nonionic Surfactants Microemulsions

The systems investigated were water/sucrose laurate/ethoxylated mono-di-glyceride/oleic phase. The oleic phase used first was the pure oils R (+)-limonene, isopropylmyristate, and caprylic-capric triglyceride; these oils were then mixed with ethanol at different mixing ratios (w/w). The total area of the one phase microemulsion region is dependent on the mixing ratios (w/w) of the mixed surfactants and that of the ethanol/oil. The largest microemulsion phase area formed with a surfactants mixing ratio (w/w) equals unity. For the systems where the oleic phase was a mixture of oil and ethanol, the total area of the monophasic microemulsion increases with the increase in the ethanol/oil mixing ratio (w/w). The Gibbs free energy of solubilization was estimated. It increases as the mixing ratio (w/w) of ethoxylated mono-di-glyceride/sucrose laurate increases and with the increase in the ethanol/oil mixing ratio (w/w). The Gibbs free energy of solubilization decreases with the increase in the water content in the water-in-oil microemulsions. The values of the Gibbs free energy of solubilization are higher for oil-in-water microemulsions compared to those of the water-in-oil microemulsions.     

Small-angle neutron scattering and theoretical investigation of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) stabilized oil-in-water microemulsions

The aim of this study is to determine the effects of oil solutes and alcohol cosolvents on the structure of oil-in-water microemulsions stabilized by poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers. The systems investigated involved the solubilization of 1,3,5-trimethylbenzene or 1,2-dichlorobenzene by P123 (EO(20)-PO(70)-EO(20)) pluronic surfactant micelles in water and water + ethanol solvents. The structures of these swollen micelles were determined by small-angle neutron scattering (SANS). A thermodynamic model was employed to interpret the characterization data. The results of the thermodynamic model for micellization agreed well with the SANS data from samples of micelles swollen by both oils. The model predicted the size of the micelles within 5% accuracy using only one fitting parameter, the micelle polydispersity. Ethanol had significantly different effects on the polymer micelles that contained solubilized oil compared to pure polymer micelles. For pure polymer micelles, the addition of ethanol increased the solubility of the polymer and, therefore, decreased the total volume fraction of micelles, while for polymer-oil aggregates, ethanol tended to have a positive effect on the volume fraction of micelles. SANS results showed that the greatest divergence from pure aqueous solvent results occurred at oil concentrations above the microemulsion stability limit.     

Solubilized states of water and formation of reversed micelles in polyoxyethylated nonylphenyl ethers in cyclohexane media

The limiting amounts of solubilization of water in the concentration range 0–200 mmol kg^–1 polyoxyethylene (6 and 10) nonylphenyl ethers (NP-6 and NP-10)/cyclohexane solutions were measured by the Karl-Fischer method at 25°C. Utilizing NMR, ESR, and near infrared spectroscopic techniques, the states of water in the solubilization region obtained were examined as a function of the molar ratio of water to surfactants at various surfactant concentrations. In NP-6 system, three solubilized states of water, i.e., water interacted directly with the oxyethylene moiety of surfactant, bound water next to the hydrated oxyethylene moiety, and bulk-like water were built up. However, in NP-10 system only directly interacted water was present. It was found that the directly interacted water is distributed between monomeric surfactants and reversed micelles, and others are distributed to swollen micelles and W/O microemulsions. In addition, the minimum amounts of water required to form reversed and swollen micelles were calculated.     

Surfactant Chain Length Effect on the Structural Parameters of Nonionic Microemulsions

In this study we estimated the structural parameters of (water+propylene glycol)/sucrose esters/(benzaldhyde+ethanol) systems. The weight ratios of water/propylene glycol and that of benzaldhyde/ethanol equal 2 and 1, respectively. The sucrose esters were sucrose laurates (L595, L1695, and SM1200), sucrose myristate (M1695), sucrose palmitate (P1670), sucrose oleate (O1570), and sucrose stearate (S1570). The pseudoternary phase behavior at 37°C was explored to determine the extension of the microemulsion phase regions. A one‐phase microemulsion region extending from the oil rich region to the water rich corner was observed in these systems. It was found that minor changes in the surfactant chain length, structure, and composition suffice to provoke a considerable change in the aggregation number, core radius and interfacial area per surfactant and cosurfactant molecules head groups in the formed microemulsions. The interfacial area per surfactant head group decreases with the increase in the surfactant chain length. For a sucrose ester with a given chain length the interfacial area per surfactant head group decreases with the increase in the surfactant monoester content. The interfacial area per surfactant head group increases with the increase in the surfactant concentration and the water core volume in the formulated microemulsions.     

Endogenous growth of the population of reverse micelles

The coupling reaction between cetylbromide (CB) and trimethylamine (TMA) to yield the surfactant cetyltrimethylammonium bromide (CTAB) is studied in the system chloroform/isooctane (2/1,v/v)/water in which CTAB forms reverse micelles. This system affords an endogenous micelle population growth, i.e., an increase of the concentration of the micelles due to appearance of the surfactant in situ. The reaction is studied in the presence of preexisting CTAB reverse micelles. The rate of CTAB formation is measured by NMR spectroscopy, and the endogenous micelle population growth is directly monitored by time-resolved fluorescence quenching analysis. Under our experimental conditions, a 100% yield of the chemical reaction brings about a fourfold increase in the population of the reverse micelles. Since the water concentration is constant during chemical reaction, the newly formed water pools are formed at the expense of the initial ones, which brings about a decrease of the average water pool radius during micellar growth. The implication of the endogenous micelle population growth as a model for biological systems is briefly discussed.     

Characterization of the fluorescence emission properties of prodan in different reverse micellar environments

We have examined the steady state and time resolved fluorescence emission properties of the hydrophobic fluorescence probe, prodan, in three representative reverse micellar systems formed by the surfactants poly(oxyethylene) (tetramethylbutyl) phenylether (Triton X-100, neutral), cetyl trimethylammonium bromide (CTAB, cationic) and sodium bis-(2-ethylhexyl) sulfosuccinate (AOT, anionic) in organic solvent media containing different concentrations of water. The results obtained from the experiments indicate conspicuous dependence of the emission behaviour of prodan on the type of surfactant used and the water/surfactant molar ratio (w0). The nature of the emission profiles, along with relevant parameters namely emission maximum (lambda(em)max), anisotropy (r) and lifetime (tau) data are used to infer the distribution and microenvironments of the prodan molecules in the reverse micelles at different w0 values. Furthermore, quantitative estimates have been obtained for the polarities (in terms of the empirical polarity parameter E(T)(30)) of the sites of solubilization of the fluorophore in different reverse micellar systems.     

Structural properties of palladium nanoparticles embedded in inverse microemulsions

Pd nanoparticles were synthesized by reduction of palladium acetate by ethanol in systems containing tetrahydrofuran (THF) as dispersion medium and tetradodecylammonium bromide (TDABr) surfactant as stabilizer. The polar phase (ethanol) acts at the same time as reducing agent. THF/TDABr/H₂O inverse microemulsions containing micelles of various sizes were also prepared, and the structure of complex liquids was studied by density measurements. Sols containing nanosize Pd⁰ particles were synthesized within the water droplets of this micellar system. The stabilized Pd⁰/surfactant system was characterized by density measurements, absorption spectroscopy, and transmission electron microscopy. The stabilizing surfactant layer adsorbed on the liquid/liquid interface and on the surface of the nanoparticles (i.e., the liquid/solid interface) significantly reduced the excess volume for the palladium nanodispersion in organic solvent. Received: 17 July 2000 Accepted: 5 October 2000     

Formation of water-in-CO(2) microemulsions with non-fluorous surfactant Ls-54 and solubilization of biomacromolecules

The solubility of Ls-54 surfactant in supercritical CO(2) was determined. It was found that the surfactant was highly soluble in SC CO(2) and the water-in-CO(2) microemulsions could be formed, despite it being a non-fluorous and non-siloxane nonionic surfactant. The main reasons for the high solubility and formation of the microemulsions may be that the surfactant has four CO(2)-philic groups (propylene oxide) and five hydrophilic groups (ethylene oxide) and its molecular weight are relatively low. The results of this work provide useful information for designing CO(2)-soluble non-fluorous and non-siloxane surfactants. The phase behavior of the CO(2)/Ls-54/H(2)O system, solvatochromic probe study, and the UV spectrum of lysozyme proved the existence of water domains in the SC CO(2) microemulsions. The method of synchrotron radiation small-angle X-ray scattering was used to obtain the structural information on the Ls-54 based water-in-CO(2) reverse micelles. By using the Guinier plot (ln I(q) versus q (2)) on the data sets in a defined small q range (0.022-0.040 A(-1)), the radii of the reverse micelles were obtained at different pressures and molar ratio of water to surfactant, W(0), which were in the range of 20.4-25.2 A.     

混合反胶束的电导与微结构研究

反胶束是两亲分子在非极性溶剂中形成的一种有序组合体，在医药、化工、采油、胶束催化及酶催化等领域中有重要应用．与胶束溶液相比，人们对反胶束的形成与结构的了解至今仍不充分．特别是对于由混合表面活性剂形成的反胶束的研究几乎无人涉及．本文采用动态光散射、电导及荧光光谱等手段对阴离子表面活性剂AOT与非离子表面活性剂形成的混合反胶束进行了研究，旨在探讨利用表面活性剂的复配来调节和控制反胶束的结构和性能．亚实验部分二异辛基磺化琉璃酸钠（AOT，Sigma公司）；Brij30为含4个氧乙烯基（EO基）的十二碳醇（AcrosOrgani…     

Properties of microemulsions with sugar surfactants and peppermint oil

In this study, we report on the properties of water + propylene glycol/sugar surfactant/peppermint oil + ethanol. The sugar surfactants were sucrose monolaurate and sucrose dilaurate. The mixing ratios (w/w) of water/propylene glycol and that of ethanol/oil equal 2 and 1, respectively. U-type microemulsions were observed in the sucrose monolaurate while S-type microemulsions were observed in the dilaurate-based systems. Temperature-insensitive microemulsions were formulated using the two surfactants. Water volume fraction percolation thresholds were determined by the study of electrical conductivity and dynamic viscosity. The structural parameters that include the periodicity and the correlation length were estimated using small angle X-ray scattering. The periodicity increases linearly with the increase in the water content whereas the correlation length increases with the increase in the water volume fraction to a certain value then decreases. The diffusion properties investigated by nuclear magnetic resonance confirm a progressive transformation of the microemulsions from water-in-oil to bicontinuous and inversion to oil-in-water upon dilution with water. The hydrodynamic radius of diluted microemulsions measured by dynamic light scattering increases with the increase in temperature. The area per polar head group decreases with the increase in temperature. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Lecithin/propanol-based microemulsions used as media for a cholesterol oxidase-catalyzed reaction

Reverse micelles, Winsor III and IV systems were examined as reaction media for the enzymatic conversion of cholesterol to cholestenone by cholesterol oxidase at 298.2 K. The micelles and the microemulsions, stabilized by soybean lecithin and ethanol or 1-propanol as cosolvent, were characterized with respect to phase behavior and distribution of 1-propanol between the phases of the Winsor III systems. The used oils were dodecane, tetradecane, and hexadecane. The Winsor IV systems and the surfactant-rich phase in the Winsor III systems exhibit bicontinuous structures. The reaction yield for the enzymatic conversion performed in a Winsor IV system was much higher than in a Winsor III system or in reverse micelles.     

Exciplex-type behavior and partition of 3-substituted indole derivatives in reverse micelles made with benzylhexadecyldimethylammonium chloride, water and benzene

The fluorescence properties of 3-methylindole (MI), 3-indoleacetic acid (IAA), 3-indoleethyltrimethylammonium bromide (IETA), L-tryptophan (Trp) and tryptamine hydrochloride (TA) were studied in reverse micelles solutions made with the cationic surfactant benzylhexadecyldimethylammonium chloride (BHDC) in benzene as a function of the molar ratio water/surfactant R (= [H2O]/[BHDC]). The fluorescence quenching of the model compound MI by benzene in cyclohexane solutions and by BHDC in benzene solutions were also studied in detail. The fluorescence of MI in benzene is characteristic of a charge-transfer exciplex. The exciplex is quenched by the presence of BHDC, due to the interactions of the surfactant ion pairs with the polar exciplex. In reverse micelle solutions at low R values, all the indoles show exciplex-type fluorescence. As R increases, the fluorescence behavior strongly depends on the nature of the indole derivative. The anionic IAA remains anchored to the cationic interface and its fluorescence is quenched upon water addition due to the increases of interface's micropolarity. For IETA, TA and Trp an initial fluorescence quenching is observed at increasing R, but a fluorescence recovery is observed at R > 5, indicating a probe partition between the micellar interface and the water pool. For the neutral MI, the fluorescence changes with R indicate the partition of the probe between the micellar interface and the bulk benzene pseudophase. A simple two-site model is proposed for the calculation of the partition constants K as a function of R. In all cases, the calculation showed that even at the highest R value, about 90% of the indole molecules remain associated at the micellar interface.     

Electrochemical Behavior of Anthraquinone in Reverse Micelles and Microemulsions of Cetyltrimethylammonium Bromide

The electrochemical behavior of an anthraquinone (AQ) was studied in aqueous solutions at a glassy carbon electrode, using the sodium salt of anthraquinone-2-sulfonic acid (AQS), by employing cyclic voltammetry. AQ undergoes a two-electron reduction in aqueous media. The electrochemical behavior of AQ was also investigated in micelles, reverse micelles (CTAB/1-butanol/water), and microemulsions (CTAB/1-butanol/water/cyclohexane) of cetyltrimethylammonium bromide (CTAB). The electrode reactions of AQ in reverse micelles and microemulsions are nearly reversible at low oil (cyclohexane) content. However, at higher oil content, the reversibility is gradually lost. In the case of reverse micelles, the reduction current, as well as the reduction potential, of AQ depend on the transition from a micellar solution to a stable solution of reverse micelles that occurs with added 1-butanol. In microemulsions, the change in cyclohexane content was found to cause a linear increase in the peak current for AQ reduction as well as a linear decrease in the corresponding reduction potential. As the cyclohexane content is increased, the o/w microemulsions dominated by micelles undergo a transition to a w/o microemulsion dominated by reverse micelles, which causes changes in the electrochemical behavior.     

Carbon dioxide-in-water microemulsions

Liquid and supercritical carbon dioxide swell potassium carboxylate perfluoropolyether (PFPE-K) cylindrical micelles in water to produce novel CO(2)-in-water (C/W) microemulsions. The swelling elongates the micelles significantly from 20 to 80 nm as the molar ratio of CO(2) in the micelles to surfactant (R(CO2)) reaches approximately 8. As the micelles swell to form microemulsions, the solubility of pyrene increases by a factor of ca. 10. Fluorescence spectra suggest that pyrene resides primarily in the low-polarity micelle core rather than in the palisade region. The results illustrate the ability of C/W microemulsions to solubilize both lipophilic and fluorophilic substances simultaneously.     

Viscous Flow Parameters of Mixed Nonionic Surfactants Microemulsions

The systems investigated were water/sucrose laurate/ethoxylated mono-di-glyceride/oil + ethanol. The oils were R (+)-limonene, isopropylmyristate and caprylic-capric triglyceride. The dynamic viscosity of the systems where the mixing ratio (w/w) of ethanol/oil and that of ethoxylated mono-di-glyceride/sucrose laurate equal unity were measured. Dynamic viscosity was measured as function of temperature at different water volume fractions. The measured viscosities for the samples in all of the systems decrease as the temperature increases. The thermodynamic parameters of viscous flow that include enthalpy, entropy, and Gibbs free energy were estimated. In all of the systems studied, the enthalpy of viscous flow remains constant as function of temperature and varies as the water content in the microemulsions vary. The entropy and the Gibbs free energy vary with both the temperature and the composition of the microemulsions. Linear relationships were observed between the enthalpy and entropy of viscous flow for the systems based on the three oils. The enthalpy-entropy compensation temperatures were determined and found to be 265, 349, and 322 K for the microemulsion systems based on R (+)-limonene, isopropylmyristate and caprylic-capric triglyceride, respectively.     

Solution behaviour of Aerosol OT in non-polar solvents

The aggregational behaviour of Aerosol OT in non-polar solvents and the applications of these systems in various areas have been reviewed. Aerosol OT forms reverse micelles in oils without using any cosurfactant and the reverse micellar solution can dissolve large amounts of water with the formation of discrete droplet microemulsion or bicontinuous microemulsions. Due to their simplicity in the sense of the smallest number of components in the reverse micellar systems, these systems have been widely studied and have also been divergently applied. This review gives a detailed account of various aspects of Aerosol OT reverse micelles namely their aggregational, structural, conformational, dynamic and applications reported in the literature to date.     

Effects of surfactant/water ratio and dye amount on the fluorescent silica nanoparticles

Effects of surfactant/water volume ratios and dye amounts on the properties of micelles and fluorescence silica nanoparticles were studied in microemulsions containing nonionic surfactant Triton X-100, hexanol as co-surfactant, cyclohexane as organic solvent, and metal organic dye (tris(2,2′-bipyridyl)dichlororuthenium) via fluorescence probe technique, TEM, and XPS. Fluorescence probe measurements show that the micelle microenvironment becomes stable at the surfactant/water volume ratio higher than 3.5 and the incubation time longer than 10 h. The data suggest that the silica shell, which is formed at the surfactant/water ratio of 3.5, yields an efficient protection of dye molecules against the e-beam irradiation and result in high photostability of fluorescent silica. We pioneered the localization of dye molecules on the surface of dye-doped silica and found that an increase of dye amounts, beyond a threshold, in the microemulsion cannot enhance the fluorescence intensity of dye-doped nanoparticles. These results are of significant importance for optimizing the synthesis of fluorescent nanoparticles with high photostability and low cost.     

Physicochemical investigation of the solubilization of ytterbium nitrate in AOT reverse micelles and liquid crystals

A wide investigation of the solubilization of the water-soluble salt Yb(NO₃)₃ in sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles and AOT liquid crystals has been carried out. After saturation of water/AOT/organic solvent w/o microemulsions with pure Yb(NO₃)₃, the Yb(NO₃)₃/AOT composites were prepared by complete evaporation under vacuum of the volatile components (water and organic solvent) of the salt-containing microemulsions. It was observed that these composites can be totally dissolved in pure n-heptane or CCl₄, allowing the solubilization of a noticeable amount of Yb(NO₃)₃ in quite dry apolar media. By UV–vis–NIR, FT-IR, and ¹H NMR spectroscopies, some information on the state of Yb(NO₃)₃ within AOT reverse micelles were acquired, whereas by small angle X-ray scattering (SAXS), it has been ascertained that Yb(NO₃)₃ is quite homogeneously distributed as very small clusters among the reverse micelles. An analysis of SAXS and wide-angle X-ray scattering spectra of Yb(NO₃)₃/AOT composites leads to the hypothesis that, also in these systems, Yb(NO₃)₃ is dispersed in the surfactant matrix as very small clusters.