Dielectric relaxation measurements on aerosol-OT/water/cyclohexane-solutions at low water content

The complex relative dieletric permittivity of aerosol-OT(AOT)/water/cyclohexane solutions has been measured within the frequency range 5 kHz-10 GHz. The investigated solutions were of oil-rich type with varying AOT- and water content. A marked dielectric relaxation has been found. The dielectric increment as well as the conductivity steeply increase with the water content, while the relaxation time decreases. Theoretical models of heterogeneous dielectrics consisting of polar, highly conductive inclusions in a nonpolar solvent with low conductivity have been applied to the results, allowing conclusions with regard to the inclusions, shape and conductivity.     

Synthesis,characterization, and electrophoretic concentration of titanium dioxide nanoparticles in AOT microemulsions

Stable organosols of TiO₂ nanoparticles were prepared by hydrolysis of titanium tetraisopropoxide (TTIP) in microemulsions of sodium bis(2‐ethylhexyl)sulfoxynate (АОТ) in n‐decane with increasing the content of aqueous pseudophase from 0.15 to 0.85 vol.%. As the water content increased, the hydrodynamic diameter of nanoparticles grew from 10 to 225 nm, and the  ζ‐potential, from ‐6 to 18 mV (the surface of TiO₂ nanoparticles was recharged when the water content was 0.45 vol.%). Nonaqueous electrophoresis in a capacitor‐type cell made it possible to concentrate nanoparticles with a diameter of 60 to 225 nm (concentration factor was 10), separate 20 nm and 225 nm particles, and decrease the content of АОТ in organosol by an order of magnitude. Preparation of a concentrate of nanoparticles with a low content (0.015 M) of AOT included the following stages: (i) electrophoresis after synthesis; (ii) sampling of the concentrate and its twenty‐fold dilution with pure n‐decane; and (iii) repeated electrophoresis. In situ laser and spectrophotometric scanning of the interelectrode space showed the formation of a sharp boundary between the raffinate and the layer of moving nanoparticles during electrophoresis.     

Estimation of the aggregation number and core radius of microemulsions

Summary A simple working method for the estimation of the aggregation numbers and core radii from weight measurements of sodium bis-2-ethylhexylsulphosuccinate (aerosol-OT orAOT)/decane/water microemulsions of several compositions is proposed.Starting out from the hypotheses that (i) the density of the water inside the droplets is the same as that of free water, (ii) all the surfactant is localized as interface, and (iii) the penetration of the organic phase into the interface is negligible, values for the aggregation number and core radii comparable to those determined using more sophisticated methods were obtained.The results/methodological simplicity ratio may be of great advantage in kinetic work on reaction mechanisms in microemulsions.

---

Abschätzung von Aggregationszahl und Radius von Mikroemulsionen

Zusammenfassung Es wird eine einfache Methode zur Abschätzung von Aggregationszahl und Core-Radius von Mikroemulsionen aus Gewichtsmessungen von Natriumbis-2-ethylhexylsulphosuccinat (aerosol-OT oderAOT)/Decan/Wasser-Mikroemulsionen verschiedener Zusammensetzung vorgeschlagen. Unter den Annahmen, daß (i) die Dichte von Wasser innerhalb der Aggregationen die gleiche als bei freiem Wasser ist, (ii) das ganze Emulsionsmittel in der Grenzfläche vorliegt und (iii) das Eindringen der organischen Phase in die Grenzfläche vernachlässigbar ist, können Werte für Aggregationszahlen und Core-Radien erhalten werden, die durchaus mit entsprechenden Werten aus aufwendigeren Methoden vergleichbar sind. Das Leistungsverhältnis (Ergebnisse/geringer Aufwand) ist sehr vorteilhaft und macht die Methode für kinetische Arbeiten zur Bestimmung von Reaktionsmechanismen in Mikroemulsionen interessant.

     

Study of water solubilized in AOT/n-decane/water microemulsions

The state of water in AOT-n-decane-water microemulsions has been studied by using Fourier Transform Infrared spectroscopy (FT-IR). The existence of three kinds of water, which correspond to water populations with different degrees of organization, has been observed. Densities of these microemulsions have been measured. From these studies (FT-IR and densities), the apparent molar volumes of the different types of water molecules have been estimated. These apparent molar volumes, as well as their variations with respect to the composition of the microemulsion, depend on the type of water. The volume of the water bound to the ionic heads of the surfactant is lower than that for bulk water and shows a slight dependence on the molar ratio W. In contrast, no dependence on W is found for the volume of water trapped between the tails of the surfactant.     

流动注射微乳液化学发光法测定福尔可定

在酸性条件下,福尔可定分子中氮原子被质子化后与阴离子AuCl4-形成离子缔合物,该缔合物被CH2Cl2带入鲁米诺的氯化十六烷基三甲基铵反胶束微乳液中,离解出来的AuCl4-立即与鲁米诺产生化学发光。在一定浓度范围内,发光强度与福尔可定的质量浓度呈线性关系,从而间接测定福尔可定。线性范围为0.001~15μg/mL,检出限(3σ)为0.04 ng/mL,对1.0μg/mL的福尔可定进行11次平行测定,RSD为2.2%。已成功用于片剂与生物体液中福尔可定的测定。     

J-complexes of retinol formed within the nanoparticles prepared from microemulsions

Retinol nanoparticles have been obtained by direct precipitation of retinol in the inner water cores of AOT/heptane/water microemulsions. The retinol dissolved in chloroform was injected into the microemulsion. The diameter of the so-obtained nanoparticles was measured using transmission electron microscope pictures where the revelation was made thanks to adsorbed iodine on the nanoparticles. The size is ca 6.0 nm, and it is not dependent either on the size of the water droplets or the concentration of the retinol molecules. This phenomenon is explained by the thermodynamic stabilization of the nanoparticles at a certain size. UV-visible spectra of the nanoparticles show a new band the maximum of which has a bathochromic shift with respect to the absorption band of the retinol monomers. If the bathochromic shift is plotted as a function of the line width, a linear correlation is obtained, the line width is decreasing with increasing shift. This behavior is interpreted as being due to an excitonic transition of a J-complex. Quantum chemical calculations have been carried out to confirm the presence of J-complexes. Taking into account the various possible geometries, the results confirm the presence of J-complexes composed of three head-to-tail molecules on the average.     

Zirconium phosphate nanoparticles from water-in-oil microemulsions

Non-ionic water-in-oil (w/o) microemulsions of pentaoxyethylenenonylphenyl ether (Igepal-CO520)/cyclohexane/water at 25 °C have been used as reaction vessels to obtain zirconium hydrogen phosphate nanoparticles.     

Impedance spectroscopy investigation of the water-in-oil microemulsions formation

In this work it was investigated the microemulsion formation using impedance spectroscopy analysis. The results indicated that the microemulsion formation is clearly observed on the impedance complex plane. The phase transition related with microemulsion formation is characterized by a time relaxation distribution. In the condition of the microemulsion formation, the impedance spectra are characterized by a single relaxation time.     

Recent developments in the characterisation of microemulsions

Microemulsions are becoming increasingly complex systems by containing more sophisticated surfactants, polymers, biomolecules, inorganic nanoparticles, etc. The detailed understanding of such more complex systems requires increasingly more refined and comprehensive characterisation. This is typically done by the combination of complementary techniques and is aided by the fact that several experimental methods have been improved (such as electron microscopy) in recent times, new ones have become available (such as fluorescence correlation spectroscopy), and the theoretical understanding of structural data is advancing.     

Evaporation from microemulsions

The composition of W/O microemulsions during slow evaporation at room temperature and at 90°C was determined by chemical analysis. One series of microemulsions was stabilized by pentanol combined with sodium dodecyl sulfate, the other one was combined with hexadecyltrimethylammonium bromide.The hydrocarbon was styrene and the evaporation in the microemulsion was compared to that of the corresponding styrene/pentanol solutions.The results showed significant influence on the evaporation by the microemulsion structure; especially for water contents close to the minimum for stability     

Percolation in alkyl polyglycoside microemulsions

Mixtures of oil, water, alkyl polyglycosides and long-chain alcohols form almost-temperature-invariant microemulsions. The phase behaviour depends on the content of cosurfactant, usually long-chain alcohols. We show that the system C_8/10G_1.5/octane/water/octanol exhibits cosurfactant-induced percolation phenomena. The percolation transition from an electrically conducting oil-in-water microemulsion to an electrically non-conducting water-in-oil microemulsion with increasing cosurfactant content is observed by measurements of electrical conductivity and time-resolved electric birefringence. The field-off relaxation time yields information on the internal length scale. The scaling behaviour of field-off relaxation times and Kerr constants with respect to the percolation point leads to insight into the influence of cosurfactant on phase behaviour. Received: 27 July 1999/Accepted: 8 February 2000     

Formation of zinc sulfide and hydroxylapatite nanoparticles in polyelectrolyte-modified microemulsions

The paper is focused on the formation of nanoparticles, i.e., zinc sulfide (ZnS) and hydroxylapatite, in a microemulsion template phase consisting of heptanol, water, and a surfactant with a sulfobetaine head group in the absence and presence of an added polyelectrolyte. In the absence of a polyelectrolyte, beside larger particles, spherical ZnS nanoparticles with a diameter below 10 nm can be redispersed after solvent evaporation. In the presence of the synthetic cationic polyelectrolyte poly(diallyldimethylammonium chloride), a reloading of the particle surface is observed, and cationic charged ZnS nanoparticles, of about 5 nm in size, can be redispersed as a main fraction. When hydroxylapatite is formed in the presence of the more stiff biopolymer chitosan hydroxylapatite, hybrid structures were formed. Transmission electron micrographs show fiber-like aggregate structures, consisting of individual small nanoparticles ordered along the polymer chain.     

Ionic liquids in microemulsions

Ionic liquids (ILs) are being increasingly studied in many different chemical application areas, particularly in green solvent applications that are extending into microemulsion applications. We summarize herein these initial microemulsion formulations and applications where ILs are used as oil substitutes, water substitutes, co-surfactants (additives), and surfactants.     

Investigation of AOT reverse microemulsions in supercritical carbon dioxide

Bis(2-ethylhexyl) sodium sulphosuccinate (AOT) was successfully solubilised in supercritical carbon dioxide (scCO₂), with ethanol or pentanol as co-solvent. Three molecular spectroscopic probes: methyl orange (MO), 8-hydroxy-1,3,6-pyrenetrisulphonic acid trisodium salt (HPTS), and riboflavin (RF) were used to examine the solubilisation characteristics of the water/scCO₂ microemulsions formed with AOT. MO was extracted at various operating conditions, although the wavelength of its solvatochromic absorption maximum was not indicative of bulk water properties. Instead, the spectral results imply that MO may be located at the surfactant/water interface. The highly water-soluble dye HPTS was unable to be extracted into scCO₂/AOT/water systems, suggesting that the water in the reverse micelle core was not as polar under supercritical conditions as those at ambient conditions. Finally, RF was extracted into the supercritical phase (40°C, 175 bar) with pentanol co-solvent, with an apparent enhanced uptake compared with the value at 40°C and ambient pressure in bulk water. This appears to be due to the presence of microcrystals dispersed in the supercritical phase.     

Mechanistic understanding of growth of nanorods in microemulsions

One-dimensional (1D) nanostructures have garnered enormous attention because of applications in plasmonics, optoelectronics, sensors, electronics, spintronics, catalysts, energy conversion and storage, medical diagnostics, drug delivery, therapeutics and also act as key units to construct nano-dimensional electronic and photonic devices. One of the versatile and successful methodologies to obtain such 1D nanostructures is microemulsions based reaction scheme. Microemulsions are thermodynamically stable oil-water systems stabilized by surfactant and cosurfactant and form reverse micelles (RMs) under appropriate concentration of each component. CTAB (Cetyltrimethylammoniumbromide)-based quaternary reverse micelles (RMs), a tunable soft template has turned out to be a successful method to synthesize nanorods with tailored structural characteristics (size and aspect ratio) of a variety of materials. Herein, we present a comprehensive overview of the growth mechanism of the heterogeneous evolution of nanorods inside the polar core of CTAB RMs. Such 1D nanostructure growth requires a symmetry-breaking mechanism to be operative that usually leads to directional characteristics i.e. preferential selection of growth along the longitudinal axis over the lateral axis that finally results in nanorod based anisotropic structures. We show the generality of the proposed microemulsion based heterogeneous growth mechanism by its validation for both slow and relatively fast kinetics for nanorod synthesis.     

The structure of O/W microemulsions formulated with alkylglucosides

By following a method proposed by Kahlweit, an equilibrium diagram was determined in O/W quaternary microemulsions. These systems were formed by octanol (cosurfactant)/water/octane (oil)/alkylglucoside (surfactant). The experiment study was performed at two different temperatures (25°C and 50°C). The objective of this study work was to determine the structure of these lower microemulsions and to study the influence of the cosurfactant. Hence, different experimental techniques were employed: light scattering (static and dynamic), Kerr effect (static and dynamic), viscosity and refractometric measurements. It was concluded that the surfactant volume fraction in equilibrium with micelles is 0.044 and the micelle shapes are revolution ellipsoids.     

Study of dielectric parameters of liquid crystal elastomer

Liquid crystal elastomers exhibit several interesting phenomena like elastic response, shape memory effect and mechanical actuation triggered by external stimuli and have got much importance among all types of actuated materials. In the present study, we synthesised nematic liquid crystal elastomers (NLCEs) by using Finkelmann procedure. The chemical structure of elastomers is confirmed by Fourier transform infrared spectroscopy. The dielectric response of NLCEs and the monomer used in its synthesis has been studied in the frequency range 20 Hz to 1 MHz. The effect of bias on dielectric response of NLCE and its comparison with its liquid crystal analogue is also reported.     

Dielectric approach to polystyrene microcapsule analysis and the application to the capsule permeability to potassium chloride

A method of numerical analysis is proposed to determine the phase parameters from dielectric observations of polystyrene microcapsules by using the conductivity at intermediate frequencies between two dielectric relaxations observed. The method was more feasible for the analysis of the microcapsule observations than the method by which the calculation is performed by use of the permittivity at lower frequencies. The method was applied to the dielectric observations of the KCl-loaded microcapsules to obtain the permittivity and the conductivity of the capsule interior, the thickness of the capsule wall, the volume fraction of the suspended capsules and the conductivity of the surrounding aqueous phase. The conductivity of capsule interior was found to be unchanged with time for as long as 3 months. From the results it is concluded that the microcapsules are impermeable to KCl.     

Polymerisation of vinyl acetate in microemulsions with methanol

 The homogeneity domains for the system vinyl acetate – methanol – monomaleate of nonylphenol ethoxylated with 25 mol ethylene oxide as an aqueous solution (MEMNPEO25) (33%) have been studied; it has been shown that, due to the shorter chain of methanol, these domains are more reduced than in the case of ethanol. The changes in refractive indexes and electrical conductivities have shown the formation of microemulsions with different structures (water-in-oil, bicontinuous or oil-in-water). Ammonium persulphate is soluble only in oil-in-water or in bicontinuous microemulsions. The initial polymerisation rates as well as the decomposition rates for the initiator are apparently affected when the microemulsion structure is modified. The polymerisation rate of vinyl acetate is higher in the presence of methanol than in the presence of ethanol for similar compositions; this fact may be attributed to a higher degree of ionisation of the initiator in the presence of methanol. Also, in polymerisations initiated by benzoyl peroxide, the conversions are influenced by the microstructure; thus, the maximum corresponds to water-in-oil microemulsions. By measuring the amount of unreacted MEMNPEO25 (gel permeation chromatography) we were able to show that the copolymer formed consisted of vinyl acetate and reactive surfactant. Received: 9 March 1999/Accepted: 13 March 2000     

Dielectric properties of ion-exchange beads of sulfonic acid type dispersed in aqueous solutions — effect of fixed charge density

Ion-exchange beads with different densities of fixed charges were prepared by sulfoethylation of dextran gel beads. The relative permittivities and the electrical conductivities of the ion-exchange beads in a sodium form were evaluated by a dielectric technique consisting of the following two procedures proposed in a previous study (2): (a) Dielectric measurements for densely packed sediments of the ion-exchange beads; (b) Analysis of the observed dielectric relaxations by means of a theoretical equation of interfacial polarization for suspensions of spherical particles. The deduced permittivities of the ionexchange beads in equilibrium with water were about 60, which is lower than those of outer aqueous phases. The deduced conductirities were of the order of 1 to 10 mS cm^–1 and were increased reasonably with the fixed charge density. Closer consideration on these deduced values indicates that no specific interaction exists between the counter ions and the fixed charges irrespective of changes in fixed charge density and temperature.