Cluster description of water-in-oil microemulsions near the critical and percolation points

The three-component ionic microemulsion system consisting of AOT/water/decane shows an unusual phase behavior in the vicinity of room temperature. The phase diagram in the temperature-volume fraction (of the dispersed phase) plane exhibits a lower consolute critical point at about 40 degrees centigrades and 10% volume fraction. A percolation line, starting from the vicinity of the critical point, cuts across the plane, extending to high volume fraction side at progressively lower temperatures. In this paper we review the evidence that allows to interpret the phase behavior of our system in terms of interacting spherical droplets. We also investigate the dynamics of droplets, below and approaching the critical point by dynamic light scattering. The first cumulant and time evolution of the droplet density correlation function can be quantitatively calculated by assuming the existence of polydispersed fractal clusters formed by the microemulsion droplets due to attraction. The relaxation phenomena observed in an extensive set of measurements of electrical conductivity and permittivity close to percolation is also reviewed and interpreted through the same cluster-forming mechanism, which reproduces the most relevant features of the frequency-dependent complex dielectric constant of this system. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Broadband dielectric spectroscopy of inhomogeneous and composite weak conductors

ABSTRACT

In this paper, we discuss broadband dielectric spectroscopy from mHz up to the infrared range mainly for materials with inhomogeneous weak conductivity, including conductor-dielectric nanocomposites. Our discussion is based on the effective medium approximation (EMA) and experiments modeled by this approach are reviewed. We discuss core–shell composites modeled by coated-spheres (Hashin–Shtrikman model) and normal composites with a possible percolation of the conductor component resulting in sharp or smeared percolation threshold of the DC conductivity and diverging static permittivity in the former case. The sharp percolation threshold is modeled by the Bruggeman EMA or by general EMA with arbitrary percolation threshold and arbitrary critical exponents of the DC conductivity and static permittivity. For the case of smeared percolation threshold in the case of complex topologies, we use the Lichtenecker model allowing for partial percolation of both the components. Finally, numerous papers reporting negative permittivity in weakly conducting materials are criticized and concluded to be due to spurious effects.     

Small-angle light scattering in dense microemulsions,transition from droplet to bicontinuous phase

Summary We have performed extensive small-angle light scattering (SALS) measurements on a three-component microemulsion (AOT/decane/water) as a function of the dispersed phase concentration and the temperature. All samples have a water/AOT molar fractionw=40.8. Such a system presents a very complex phase diagram with many structural configurations. With the SALS technique, we have been able to observe all the phase separation lines. In particular we give details on the system structure on the percolation phenomenon and on the bicontinuous phase recently observed. In particular we show that the percolation is driven by a long-scale aggregation between microemulsion droplets. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Size dependent ultrafast cooling of water droplets in microemulsions by picosecond infrared spectroscopy

The ultrafast thermal relaxation of reversed micelles in n-octane/AOT/water (where AOT denotes sodium di-2-ethylhexyl sulfosuccinate) microemulsions was investigated by time-resolved infrared pump-probe spectroscopy. This picosecond cooling process can be described in terms of heat diffusion, demonstrating a new method to determine the nanometer radii of the water droplets. The reverse micelles are stable against transient temperatures far above the equilibrium stability range. The amphiphilic interface layer (AOT) seems to provide an efficient heat contact between the water and the nonpolar solvent.     

Small-angle light scattering studies of dense AOT-water-decane microemulsions

Summary We have performed extensive studies of a three-component microemulsion system composed of AOT-water-decane (AOT=sodium-bis-ethylhexyl-sulfosuccinate is an ionic surfactant) using small-angle light scattering (SALS). The small-angle scattering intensities are measured in the angular interval 0.001–0.1 radians, corresponding to a Bragg wave number range of 0.14 μm⁻¹<Q<<1.4 μm⁻¹. The measurements were made by changing temperature and volume fraction ϕ of the dispersed phase (water + AOT) in the range 0.05<ϕ<0.75. All samples have a fixed water-to-AOT molar ratio,w=[water]/[AOT]=40.8, in order to keep the same average droplet size in the stable one-phase region. With the SALS technique, we have been able to observe all the phase boundaries of a very complex phase diagram with a percolation line and many structural organizations within it. We observe at the percolation transition threshold, a scaling behavior of the intensity data. This behavior is a consequence of a clustering among microemulsion droplets near the percolation threshold. In addition, we describe in detail a structural transition from a droplet microemulsion to a bicontinuous one as suggested by a recent small-angle neutron scattering experiment. The loci of this transition are located several degrees above the percolation temperatures and are coincident with the maxima previously observed in shear viscosity. From the data analysis, we show that both the percolation phenomenon and this novel structural transition are derived from a large-scale aggregation between microemulsion droplets.     

Investigation of W/O microemulsion droplets by contrast variation light scattering

Dynamic and static light scattering experiments have been performed at various molar ratios (γ) of water to AOT and temperatures on water-in-oil (W/O) microemulsions dispersed in n-heptane, n-octane, and n-nonane. Size and shape fluctuations of microemulsion droplets are determined with very high precision because polydispersity influences the characteristic features of scattering data as well as the hydrodynamic radius withγ. Self-consistent interpretation of dynamic and static light scattering data using optical properties and packing consideration on the basis of the layered sphere model are obtained. The estimated extent of polydispersity index of 17% is found, whereas the polydispersity is independent of the alkane types. The geometrical parameters, e.g., hydrodynamic radius, area per head group of the surfactant molecule and thickness of the surfactant layer of microemulsion droplets are also estimated and compared in three different n-alkane types. The best interpretation of the temperature dependence of data has shown a transition from spherical droplets to ellipsoid aggregates with increasing temperature. Axial ratio increases with increase of temperature and the longer the alkane the larger is the axial ratio. The parameters describing the polydispersity and shape change are in agreement with theoretical and experimental results found in the literature     

Preparation of AgBr Nanoparticles in Microemulsions Via Reaction of AgNO<Subscript>3</Subscript> with CTAB Counterion

Nanoparticles of AgBr were prepared by precipitating AgBr in the water pools of microemulsions consisting of CTAB, n-butanol, isooctane and water. An aqueous solution of AgNO₃ added to the microemulsion was the source of Ag⁺ ions. The formation of AgBr nanoparticles in microemulsions through direct reaction with the surfactant counterion is a novel approach aimed at decreasing the role of intermicellar nucleation on nanoparticle formation for rapid reactions. The availability of the surfactant counterion in every reverse micelle and the rapidity of the reaction with the counterion trigger nucleation within individual reverse micelles. The effect of the following variables on the particle size and size distribution was investigated: the surfactant and cosurfactant concentrations, moles of AgNO₃ added, and water to surfactant mole ratio, R. High concentration of the surfactant or cosurfactant, or high water content of the microemulsion favored intermicellar nucleation and resulted in the formation of large particles with broad size distribution, while high amounts of AgNO₃ favored nucleation within individual micelles and resulted in small nanoparticles with narrow size distribution. A blue shift in the UV absorption threshold corresponding to a decrease in the particle size was generally observed. Notably, the variation of the absorption peak size with the nanoparticle size was opposite to those reported by us in previous studies using different surfactants.     

Electronic structure and polarizability of quantum metallic wires

The electronic structure and the linear response to an external electric field of simple metal wires with a quantum-size cross-section have been studied within the density-functional theory and the “jellium” model. It is found that an increase in the wire radius leads to a nonmonotonic change in the work function and static polarizability of the wire. The photoabsorption spectra of Na wires with different cross-sections are obtained. The effect of a dielectric environment on the properties of metallic wires is investigated. An increase in the permittivity of a medium brings about a decrease in the static polarizability of metallic wires. It is demonstrated that the surface plasma resonance in the photoabsorption cross-section for Na wires placed in a dielectric matrix is shifted from the continuous spectrum toward the range somewhat below the ionization threshold.     

Effect of a magnetic field on the permittivity of 80%La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript>/20%GeO<Subscript>2</Subscript> composite

The dielectric properties of a magnetoresistive conducting two-phase 80%La_0.7Sr_0.3MnO₃/20%GeO₂ (wt %) composite have been studied near the percolation threshold in magnetic fields from 0 to 15 kOe at frequencies of the measurement field from 5 kHz to 1 MHz. The samples have inductive impedances; i.e., their permittivities can be considered negative due to a high conductivity in this frequency range. The permittivity increases in magnitude in magnetic field, and the values of the magnetodielectric coefficient reach 23% at room temperature. The reasons for the effect of magnetic field on the dielectric permittivity of samples are discussed.     

Temperature and pressure effects on the bending modulus of monolayers in a ternary microemulsion

We performed small-angle neutron scattering and neutron spin echo experiments on a ternary microemulsion composed of ionic surfactant AOT, water, and decane. Thermal fluctuations of monolayers have been investigated as a function of temperature and pressure. The amphiphilic monolayers become more flexible with increasing temperature and more rigid with increasing pressure. These results are consistent with the microscopic picture that the head-head repulsion of the AOT molecules is enhanced at high temperature while an attractive interaction between the hydrophobic tails of the AOT molecules increases at high pressure.     

Permittivity of diluted pentanol as a function of temperature

The static dielectric constant of pentanol diluted in decane, cyclohexane and carbon tetrachloride was measured as a function of temperature, at different concentrations. The experimental points were adjusted using the Onsager equation to which a term reducing the molecular interaction was added. Such an empirical equation has already been published for decane solutions. Good agreement between experimental and fitted values of the permittivity was found. The results obtained for the different solvents used are compared and analysed, together with the parameters resulting from the fittings.     

Solvatochromic and nonlinear optical properties of Eosin B in solvents and AOT/water/Heptane

The third-order susceptibility and second-order hyperpolarizability and the two-photon absorption coefficient of Eosin-B in AOT/water/Heptane were investigated by using the Z-Scan technique. The droplets were prepared with an AOT/water droplet in a continuous phase of Heptane. The droplets size changes with the amount of water and the droplet concentration decreases with the increase of Heptane concentration. To study the nonlinear optical properties of Eosin-B, the Z-scan measurements were performed by means of a laser at 532 nm and 80 mW power. The nonlinear refractive and the nonlinear absorption coefficient indices were found to be in the order of 10^?12 (cm² W^?1) and 10^?7 (cm W^?1), respectively. The change of nonlinear optical properties of Eosin-B by droplet size and concentration is due to the change of dye aggregation and thermal conductivity and thermo-optic coefficient of samples. The absorption of Eosin-B changes with the polarity of the medium (dielectric constant and refractive index). It has been shown that the intensity of emission spectra of Eosin-B in AOT/water/Heptane is enhanced compared to that of aqueous solution. By the Bilot and Kawski theory, the ratio between the excited state and the ground state of the dipole moments (μ_e/μ_g) of Eosin-B both in water and in droplets is extracted.     

Effects of protein solubilization on the structure of the surfactant shell of reverse micelles

The influence of protein encapsulation in the water pool of the reverse micelles has been studied by the electron spin resonance (ESR) spin-label technique and electrical conductivity measurements. For this purpose water-sodium-bis(2-ethylhexyl)-sulfosuccinate (AOT)-decane water-in-oil microemulsions with solubilized trypsin were used. The ESR data, obtained with the help of 4-(2-n-undecyl-3-oxyl-4,4,-dimethyloxazolidin-2-yl) butyric acid as a label, show that the protein molecule encapsulated in the water core of a reverse micelle forces a portion of water out from the aqueous core into the hydrophobic shell of a micelle. As a result, changes in the packing of AOT hydrocarbon chains and in the polarity of their microenvironment are induced. The effect of the encapsulated protein depends on the difference in the sizes of the protein molecule and the micelle aqueous core. The perturbations in the dynamic structure of the surfactant shell turn out to be most pronounced when the dimensions of the water cavity are close to the size of the protein molecule. The solubilization of protein in the reverse micelles results in remarkable variations in mass exchange between micelles.     

Dynamic percolation in water in oil microemulsions revisited

Microemulsions (dispersions of water droplets, typical radius about 10 nm, in oil) show a particular percolation pattern, a so-called dynamical percolation. Predictions of scaling theory and Monte Carlo simulations were compared with experimental static and frequency dependent conductivity data. The latter gives evidence of two different time scales of charge transport.Dedicated to Professor Harry Thomas on the occasion of his 60th birthday     

A study of the properties of mixed nonionic surfactants microemulsions by NMR, SAXS, viscosity and conductivity

The pseudoternary phase behavior of the water/sucrose laurate/ethoxylated mono-di-glyceride/R (+)-limonene systems was investigated for different surfactants mixing ratios (w/w) at 25 °C. The microemulsion boundaries were determined and the surfactants content at the interface of water- R (+)-limonene was estimated. For surfactants mixing ratio (w/w) equals unity, the area of the one phase microemulsion region reaches its maximum. The system with the maximum microemulsion area was investigated using electrical conductivity, dynamic viscosity, small angle X-ray scattering, and nuclear magnetic resonance. Electrical conductivity increases as the water volume fraction increases and a percolation threshold was observed. Dynamic Viscosity varies as function of the water volume fraction in a non-monotonic way giving two-peaked plot. The characteristics of the domain size of the microemulsions called periodicity measured by small angle X-ray scattering increases with the increase in the water volume fraction. The correlation length of the domain size reaches a maximum when plotted against the water volume fraction in the microemulsions. Relative diffusion coefficients of water increase and those of oil decrease with increasing the water volume fractions in the microemulsions indicating structural transitions.     

Carbon materials with quasi-graphene layers: the dielectric, percolation properties and the electronic transport mechanism

We investigate the dielectric properties of multi-walled carbon nanotubes (MWCNTs) and graphite filling in SiO₂ with the filling concentration of 2-20 wt.% in the frequency range of 10²-10⁷ Hz. MWCNTs and graphite have general electrical properties and percolation phenomena owing to their quasi-structure made up of graphene layers. Both permittivity ε and conductivity σ exhibit jumps around the percolation threshold. Variations of dielectric properties of the composites are in agreement with the percolation theory. All the percolation phenomena are determined by hopping and migrating electrons, which are attributed to the special electronic transport mechanism of the fillers in the composites. However, the twin-percolation phenomenon exists when the concentration of MWCNTs is between 5-10 wt.% and 15-20 wt.% in the MWCNTs/SiO₂ composites, while in the graphite/SiO₂ composites, there is only one percolation phenomenon in the graphite concentration of 10-15 wt.%. The unique twin-percolation phenomenon of MWCNTs/SiO₂ is described and attributed to the electronic transfer mechanism, especially the network effect of MWCNTs in the composites. The formation of network plays an essential role in determining the second percolation threshold of MWCNTs/SiO₂.     

On the dependence of circular Bragg phenomenon of noble metals helicoidally periodic sculptured thin films on visible and IR wavelengths

The concept of local homogenization in the visible and infra-red frequencies is used by estimating the permittivity dyadics of noble metals (Cu, Ag, and Au) in the form of thin film helicoidal bianisotroptic media (TFHBM). Despite the fact that the absorption transitions of dielectric to metal (percolation threshold) in metallic TFHBMs occur at long wavelengths at lower volumetric fraction of metallic particles, at these wavelengths the transition from dielectric to metal in composite relative permittivity scalar occurs at higher volumetric fraction of metallic particles. The latter is responsible for the increase of circular Bragg phenomenon.     

Percolation phenomenon in barium samarium titanate-silver composite

Ba₄Sm_9.33Ti₁₈O₅₄-Ag (BST-Ag) composites were prepared by a solid-state ceramic route and its dielectric properties were investigated in the vicinity of percolation threshold. The structure and microstructure of the composites were analyzed by X-ray diffraction along with optical and scanning electron microscopy observations. The effects of silver content and frequency on the dielectric properties of BST-Ag composites were studied using a LCR meter. The relative permittivity (ε_r) of the composite increases with silver content below the percolation limit and is in agreement with power law. A 0.14 volume fraction of silver loading increases the relative permittivity of the composite from 50 to 450 at 10 kHz. Addition of 0.15 volume fraction of silver increases the relative permittivity of the composite in the order of 10⁵. It is found that the giant relative permittivity is almost constant for frequencies from 1 kHz to 1 MHz. This high ε_r composite offers the perspectives for application in electromechanical devices.     

Dielectric behaviour of aqueous CsCl solutions

Complex dielectric permittivity spectra, in the frequency range 10 MHz to 20 GHz are reported for aqueous cesium chloride (CsCl) solutions at 250C using time domain reflectometry technique. The static dielectric constant, relaxation time and conductivity have been determined using nonlinear least squares fit method. From the static dielectric constant, hydration numbers were determined by using measured solutions density at different concentrations.     

Dielectric parameters of mesoporous sieves filled with NaNO<Subscript>2</Subscript>

This paper reports on a dielectric study of MCM-41 molecular sieves with cellular channels of different sizes filled with the NaNO₂ ferroelectric. The temperature dependences of the permittivity and electrical conductivity of sodium nitrite in cellular channels are calculated from experimental data on the permittivity and electrical conductivity of the composite. The calculations are performed using the relationships obtained for the hexagonal matrix with parallel cylindrical inclusions within pores. The observed increase in the conductivity of sodium nitrite in confined geometry at high temperatures is attributed to partial melting. It is shown that the increase in the permittivity of the composite is caused by Maxwell-Wagner relaxation processes.