Three-component Microemulsion System Research Articles

Percolation transition in water–AOT–decane microemulsion investigated by transient grating measurement

We report results from a heterodyne-detected transient grating experiment on a three-component microemulsion system, consisting of water–AOT–decane. We studied the microemulsion characterized by a water-to-AOT ratio of X=40.8 and a volume fraction φ =0.50, in a temperature range around the percolation threshold. The measurements have been performed with two exchanged momenta q≈ 1 and 2μ m−1, corresponding to sound frequencies around 200 and 420 MHz. The measured sound velocities show small but significant evidence of the percolation transition in the temperature domain.

High Pressure Cell for Small-Angle Neutron and Light Scattering Studies of Phase Transitions in Complex Liquids

High pressure cells for both small-angle neutron scattering and light scattering studies have been designed and constructed to investigate phase transitions of complex liquids. The cells can be used for polymer blends, polymer solutions, liquid mixtures of small-molecules, etc. Pressure up to 200 MPa can be applied to the cells through a pressurizing liquid. The sample is separated from the pressurizing liquid by a free piston or an O-ring. In addition, time-resolved measurements of phase transition processes induced by a pressure drop can be carried out using the cells. The pressure is stable within 0.5 MPa for the duration of several hours. Temperature can be controlled between 273 and 523 K with an accuracy of ±0.005 K during the measurement time of 30 minutes. In this paper, we show two preliminary experimental results as examples of applications of the cells: (i) a pressure effect on the miscibility and order-disorder transition temperature of a diblock copolymer and its microphase separation process by a pressure jump, and (ii) a pressure induced critical phenomenon in a three-component microemulsion system.

Experimental studies on phase separation in critical microemulsion and micellar systems

We have studied the phase separation process in two systems containing surfactant molecules, i.e. a three-component microemulsion system made by a Aerosol OT-water-decane surfactant and a water-Butoxyethanol (C 4E 1) micellar-like mixture. The analyses were performed by using the time resolved light scattering intensity technique. All the three stages of the process were studied. The time evolution of intensities for the initial stage follows closely the linearized theory. Time evolution of the characteristic wave vector and the intensity distribution in the intermediate and late stages are in good agreement with scaling models. A comparison of the obtained data in the late stage with the findings of a recent Molecular Dynamic simulation, carried out in mixtures containing surfactant, gives a clear indication that in the investigated systems the phase separation process occurs through the formation of different structural coalesing domains. Whereas in the AOT microemulsion system the separation occurs through the formation of irregular bicontinuous-like domains, in the water-C 4E 1 mixture the separation is dominated by micellar domains which depend by thermal fluctuations.

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

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.

Crossover from mean field to three-dimensional Ising critical behavior in a three-component microemulsion system.

Critical density fluctuations of water droplets in an oil-rich three-component microemulsion system have been studied by small-angle neutron scattering as a function of temperature near and far from the boundary of phase decomposition. The observed data in the one-phase region are well described in terms of the asymptotic crossover expression calculated by Belyakov et al. The data are found exclusively in the crossover region between the universality class of three-dimensional Ising and mean field regimes. The Ginzburg number is found to be between one and two orders of magnitude less than that for low molecular weight liquids. \textcopyright{} 1996 The American Physical Society.

Critical Behavior of a Pseudobinary System for a Three-Component Microemulsion

A three-component microemulsion system can be regarded as a pseudobinary mixture at certain conditions. The coexistence curves of (T, n), (T, φ), and (T, Ψ) (n and φ are refractive index and volume fraction, respectively; Ψ is defined as Ψ = φ/[φ + φc(1 − φ)/(1 − φc)]) for a ternary microemulsion (AOT−water−decane) at constant pressure and a constant ratio of water to AOT have been determined within about 10 K from the critical temperature by measurements of refractive index. The critical exponent β was deduced precisely from (T, n), (T, φ), and (T, Ψ) coexistence curves within 1.5 K above Tc. These values were 0.327, 0.324, and 0.325, respectively, and were consistent with the 3D Ising exponent. The volume fraction φ is a better choice of the concentration variable than Ψ used for construction of the order parameter to fit the Ising behavior.

Spinodal decomposition of a three-component water-in-oil microemulsion system.

We have performed a series of spinodal decomposition measurements of a three-component microemulsion system made of a surfactant Aerosol OT, water, and decane. The measurements were made by a temperature jump from a one-phase droplet microemulsion to a two-phase droplet microemulsion along the critical isovolume fraction line (10%) using a time resolved light scattering intensity measurement technique. All three stages of the evolution were studied. Time evolution of intensities for the initial stage follows closely the linearized theory. On the other hand, time evolution of the characteristic wave vector, the maximum scattering intensity, and the intensity distributions in the intermediate and late stages are in good agreement with recent dynamic scaling theories.

Spinodal Decomposition of the Three-Component Microemulsion System: Aot/Water/Decane

ABSTRACTWe have performed a series of spinodal decomposition measurements of a three-component microemulsion system made of a surfactant AOT, water, and decane. The measurements were made by a temperature jump from a one-phase droplet microemulsion to a two-phase droplet microemulsion along the critical iso-volume fraction line (10%) using the time resolved light scattering intensity technique. All three stages of the evolution were studied. The typical scattering intensity distribution is in good agreement with recent dynamic scaling theories.

Visualization of 3D microstructure of bicontinuous microemulsions by combined SANS experiments and simulations

SANS studies were made on a three-component microemulsion system, with equal volume fractions of water and oil, in the one-phase region near the three-phase body, at the hydrophile—lipophile balance temperature, as a function of surfactant volume fraction φs. A prominent peak was found in the scattering intensity distribution, the position of which (Qmax) scaled linearly with φs. This scaling relation can be derived from a model of bicontinuous microemulsions proposed by Teubner and Strey (J. Chem. Phys., 87 (1987) 3195). It can be shown that the generic microstructure of these microemulsions is bicontinuous, characterized by two length scales d and ξ, where d is the average inter-domain distance between two water or oil domains and d/ξ the disorder parameter measuring polydispersity of the domain sizes. Based on this idea, we propose a simulation scheme, which is an extension of an idea of Berk (Phys. Rev. Lett., 58 (1987) 2718-2721; Phys. Rev. A, 44 (1991) 5069-5079), which allows us to generate both the 3D microstructure and the associated Debye correlation function, which can be Fourier transformed to obtain the theoretical scattering intensity distribution function. The main input to this simulation is a spectral function which is an inverse of a sixth order polynomial, containing three length scales. 3D micro-structures are then generated which are fully consistent with the scattering data. Physical meaning of the three length scales so obtained is discussed and compared with the two length scales d and ξ obtained from fitting the Teubner—Strey model to the same data.

High-frequency dielectric dispersions in dilute water-in-oil microemulsions

A set of measurements of the dielectric constant of a three-component microemulsion system sodium bis(2-ethylhexyl) sulfosuccinate-water-decane as a function of frequency (10 6−10 9 Hz) and temperature (5–35°C) and for two values of the volume fraction of the dispersed phase (0.025 and 0.050) has been made. The frequency dependence of the dielectric constant clearly shows a low-frequency dispersion and a high-frequency one, both describable in terms of a Cole-Cole type relaxation. The high-frequency dispersion is characterized by a relaxation time of the order of 10 −9 s and a dielectric increment of a few units, both largely independent of temperature and composition. In this paper, an attempt is made to relate this dispersion to the single-droplet dynamics.

Structural evolution within the one-phase region of a three-component microemulsion system: Water–n-decane–sodium-bis-ethylhexylsulfosuccinate (AOT)

The continuous inversion from a water-in-oil (w/o) microemulsion at low temperatures to an oil-in-water (o/w) microemulsion at higher temperatures within the one-phase channel of water (0.6% NaCl)–n-decane–AOT microemulsion system is investigated by small angle neutron scattering (SANS). At constant AOT (surfactant) weight fraction γ of 12%, the structural evolution as a function of temperature takes place in different forms as the oil-to-water weight fraction α is varied from 15 to 90 %. At low o-w weight fractions (α=15 and 20 %) the microemulsions transform from a water-internal, oil-continuous structure at lower temperatures to an oil-internal, water-continuous droplet structure at higher temperatures jumping across an intermediate region of a lamellar phase (Lα). However, at higher o-w weight fractions (α=80 and 90 %) the evolution goes through a stage of percolation of the water droplets first into extended water clusters, then the structural inversion takes place probably through a transition of these water clusters into an entangled tubular structure. At equal oil-to-water volume ration (α=40%), the structure can be described as bicontinuous at both low and high temperatures. In this case we are able to extract two lengths characterizing the structure from SANS data using different models for the scattering length density fluctuation correlation function of a bicontinuous microemulsion.

Theory and experiment of electrical conductivity and percolation locus in water-in-oil microemulsions.

An extensive set of measurements of low-frequency conductivity \ensuremath{\sigma} of a three-component microemulsion system, sodium-di-2-ethyl-hexylsulfosuccinate (AOT)-water-decane, as a function of temperature (T from 10 to 50 \ifmmode^\circ\else\textdegree\fi{}C) and volume fraction of the dispersed phase (\ensuremath{\varphi} from 0.098 to 0.65) has been made. The measurements were made at a fixed value of the water-to-surfactant molar ratio equal to 40.8, where the microemulsion consists of AOT-coated water droplets, 85 \AA{} in radius, dispersed in oil. \ensuremath{\sigma} can be quantitatively calculated at low \ensuremath{\varphi} by means of a charge fluctuation model of Eicke et al., while it can be interpreted for higher vlaues of \ensuremath{\varphi} in terms of power-law behavior with indices related to the dynamic percolation picture below the threshold and the static one above it. The percolation locus in \ensuremath{\varphi}-T plane has been determined starting from the vicinity of the lower consolute point up to \ensuremath{\varphi}=0.65. This line can be successfully interpreted in terms of a modified version of the analytical theory of percolation given by Xu and Stell.

Static and dynamic light scattering studies of water-in-oil microemulsions in the critical region. Evidence of a crossover effect

Extensive light scattering measurements, including the intensity, turbidity, and linewidth, on a three-component microemulsion system consisting of mixtures of water, decane, and a surfactant sodium di-2 ethylhexylsulfosuccinate (AOT) (WDA), have been made. The critical and several off-critical mixtures have been studied along constant microemulsion droplet volume fraction lines in the one-phase region over a very large temperature range. In the vicinity of the lower phase separation temperature Tp the intensity data are very well accounted for by the standard theory of critical binary fluids using a single value for the short range correlation length ξ0=(13.5±1.5) Å. By combining a mode-coupling theory including the background effects and a linear model equation of state applicable in the critical region, we have been able to fit the dynamic light scattering data using a Debye cutoff length q−1D which is equal to the constant average diameter of microemulsion droplets. Furthermore, we find clear evidence for a crossover from critical to single particle behavior in both static and dynamic light scattering data. A crossover temperature Tx has been identified at which qDξ(Tx)=1. Analyses of the dynamic light scattering data show that qD, which can only be measured far away from Tp, in fact plays a decisive role in controlling the critical dynamics in the whole temperature range.

SANS studies of the microstructure of a three-componnent microemulsion

Small-angle neutron scattering measurements have been performed on a three-component microemulsion system consisting of water, hexane and didodecyl-dimethylammonium bromide. For a water-to-surfactant weight ratio of 0.95 and less than 5 wt% surfactant, these measurements are best fit by a model with a Gaussian size distribution of spherical water droplets in hexane. The average water core radius is determined to be about 2.9 nm. On addition of water while the same water-to-surfactant ratio is maintained, there is some evidence that the basic unit of the microstructure remains a sphere with the same radius. The interaction between the spheres is found to be attractive, and the spheres may be connected for volume fractions of water larger than 5%.