Ternary Microemulsion System Research Articles

Temperature- and Pressure-dependences of Shape Fluctuations in a Ternary Microemulsion System

Shape fluctuations in a microemulsion system composed of Aerosol-OT (AOT), D 2 O and deuterated decane were investigated by means of neutron spin echo. In this system, a water-in-oil droplet structure at ambient temperature and pressure decomposes into two-phases, both with increasing temperature and pressure. However, its dynamical behavior was different. The observed effective diffusion constant of the droplet shape fluctuation increased with increasing temperature, while it decreased with increasing pressure. Temperature- and pressure-dependences of a bending modulus of AOT monolayers were estimated, and it was confirmed that AOT monolayers become floppy with increasing temperature and stiff with increasing pressure.

Effects of temperature and pressure on phase transitions in a ternary microemulsion system

Temperature variation experiments of small angle neutron scattering (SANS) and neutron spin echo (NSE) were carried out in order to compare effects of temperature and pressure on a structural formation in a ternary microemulsion system composed of AOT (Aerosol-OT; dioctyl sulfosuccinate sodium salt), D2O, and n-decane. From SANS measurements, a phase transition from one-phase dense water-in-oil droplet to two-phase coexistence with a lamellar and a disordered structure was observed with increasing temperature, similar to the case of pressure variation. Another phase transition was observed at a higher temperature above the lamellar phase, although such a subsequent phase transition has not been observed at higher pressure. The characteristic features of structural phase transitions by temperature and by pressure were compared by introducing a reduced temperature and pressure. The dynamical property observed from the NSE measurement was different between the high-temperature phase and the high-pressure phase. These results indicate that the mechanism of the phase transition induced by temperature is different from that by pressure.

三元系マイクロエマルションの圧力誘起構造相転移

The effects of temperature and pressure on the structural formations in a ternary microemulsion system were reviewed. From the static measurement by means of a small angle x-ray and neutron scattering, similar phase transitions with increasing temperature or pressure were observed. Introducing a normalized temperature and pressure, the similarity and dissimilarity of those two phase transitions were clarified. From the dynamic measurement utilizing neutron spin echo, it was demonstrated that the membrane dynamics at the high-pressure phase were completely different from the high temperature phase. These differences suggest differing mechanisms for the pressure-induced phase transition and the temperature-induced transition.

Temperature- and pressure-induced phase transition in a ternary microemulsion system

Abstract Small-angle neutron scattering (SANS) experiments were carried out in order to investigate an origin of a pressure-induced phase transition in a ternary microemulsion system. The observed SANS profiles from pressure- and temperature-induced lamellar structures were essentially the same, however, the concentration dependences of the lamellar spacing and the degree of the concentration fluctuation of amphiphilic molecules were different. An origin of the anisotropy of the quasi-Bragg scattering observed from the lamellar structure was also investigated.

Cationic Gemini Surfactants with Oligo(oxyethylene) Spacer Groups and Their Use in the Polymerization of Styrene in Ternary Microemulsion

The aggregation and phase behavior, micelle and microemulsion structure, and the polymerization behavior of various binary and ternary systems containing water, (oligooxa)alkanediyl-α,ω-bis(dimethyldodecylammonium bromide) surfactants (12-EOx-12), and styrene are reported. The hydrophilic spacer chain length x in the cationic gemini surfactants was varied between 0 and 5. With increasing spacer chain length the surface area per molecule at the air−water interface increases, whereas the single-phase region of the ternary microemulsion systems is largest for x being 1 and decreases with increasing x in the investigated concentration range. Small-angle neutron scattering (SANS) and cryogenic temperature transmission electron microscopy (cryo-TEM) measurements indicate that the micelle and microdroplet dimensions decrease with increasing x. Polymerization was carried out at 25 and 60 °C using 60Co γ-rays. Spherical polystyrene particles in the nanometer size range were obtained, the particle size being dependent on the spacer length of the gemini surfactants and on the polymerization temperature. At 25 °C, particle size and molecular weight of polystyrene were maximum at medium spacer length, while at 60 °C, the particle size decreased with increasing x, in correspondence to the microstructural properties of the microemulsions observed by SANS. The experimental results suggest that the chemical structure of the surfactant molecules used in the microemulsion formulation has a strong influence on the particle properties after polymerization. This influence depends on the curvature of the interfacial film induced by the surfactant molecules.

Relaxation phenomena in critical microemulsion systems

We discuss extensive sets of experimental data including static and dynamic light scattering, ultrasonic velocity measurements and high and low frequency dielectric relaxation, taken in the vicinity the critical point of a ternary microemulsion system. Upon approaching the critical point we observed a slowing down of the dielectric relaxation time and of the first cumulant of the time-dependent droplet density correlation function C 2( t) which shows a non-exponential behavior at long time. These features can be well acccounted for by assuming that the microemulsion system is made of polydispersed transient fractal aggregates having a fractal dimension d f=2.5 and a polydispersity index τ=2.2.

A small-angle neutron-scattering study of the effect of pressure on structures in a ternary microemulsion system

Small-angle neutron-scattering experiments were carried out in order to clarify pressure effects on structures of an AOT(dioctyl sulfosuccinate sodium salt)/water/n-decane ternary microemulsion system. The applied pressure range was from 0.1 up to 120 MPa. The volume fraction of water and n-decane was selected to be equal and φ s, the volume fraction of AOT against whole volume, was changed from 0.209 to 0.230. A pressure-induced phase transition from a dense water-in-oil droplet structure to an ordered lamellar phase was confirmed for 0.209 ⩽ φ s ⩽ 0.230. Pressure dependence of the characteristic repeat distance both in the low-pressure disordered phase and in the high-pressure lamellar phase was obtained.

Lattice-gas simulations of domain growth, saturation, and self-assembly in immiscible fluidsand microemulsions

We investigate the dynamical behavior of both binary fluid and ternary microemulsion systems in two dimensions using a recently introduced hydrodynamic lattice-gas model of microemulsions. We find that the presence of amphiphile in our simulations reduces the usual oil-water interfacial tension in accord with experiment and consequently affects the non-equilibrium growth of oil and water domains. As the density of surfactant is increased we observe a crossover from the usual two-dimensional binary fluid scaling laws to a growth that is {\it slow}, and we find that this slow growth can be characterized by a logarithmic time scale. With sufficient surfactant in the system we observe that the domains cease to grow beyond a certain point and we find that this final characteristic domain size is inversely proportional to the interfacial surfactant concentration in the system.

Critical dynamics of supramolecular mixtures. Application to microemulsion systems and to nonionic amphiphile-water micellar solutions

We discuss the static and dynamic critical behavior of binary solutions of water with some alkyl-oxyethylene-glycol monoether (C iE j) and of di 2 ethylhexyl sodium sulfosuccinate (AOT)-water-decane ternary microemulsion systems. Extensive sets of measurements of the scattered light intensity, of the order-parameter fluctuations time correlation function, and of the shear viscosity, have been performed for different systems. In most of the cases our data sets show systematic deviations from the behavior usually observed for molecular binary fluids. We show that generalized hydrodynamics cannot account for these deviations. A modified version of the Sorensen et al. (1976) dynamical droplet model has been worked out. This model which assumes that the physical clusters formed close to the critical point can be treated like percolating aggregates, and which includes the finite size of the monomers, very well accounts for all our experimental results.

Shape Similarities of Ultra‐Low Interfacial Tension Curves in Ternary Microemulsion Systems of the Water‐Alkane‐CiEj Type

AbstractThe interfacial tensions between water‐ and oil‐rich phases in microemulsion systems have been measured for a series of ternary water‐alkane‐CiEj systems. It is found that interfacial tensions vary over orders of magnitude with temperature displaying a pronounced minimum for each surfactant. The center of the three‐phase temperature interval, Tu‐T1, i.e. the phase inversion temperature (PIT), corresponds to the temperature Tm of the minimum in interfacial tension. Increasing the surfactant chain length the minimum value decreases by 3 orders of magnitude for a change of i from 6 to 12. Interestingly, all interfacial tension curves have a similar shape. Centering the curves around Tm and reducing the temperature scale by Tu‐T1 permits determining the factors, by which the individual interfacial tension curves differ. The factors are found to be proportional, respectively, to square of the maximum length scale ζ or the inverse surfactant volume fraction Φc,i in the interface. Accordingly, plotting σab/Φc, j2 vs. 2(T‐Tm)/(Tu‐T1) an almost perfect superposition of all interfacial tension curves is observed.

Ternary microemulsions as model disordered media

AbstractThe ternary microemulsion systems alkane/water/DDAB (didodecyldimethylammonium bromide) form ideal model‐disordered media. The static microstructure is described by a simple parameter‐free model that can be predetermined and agrees with SAXS and SANS scattering experiments. The component volume fraction can be varied to exhibit bicontinuous random structures with a predicted percolation transition to disconnected water‐in‐oil droplets. Structural transitions are analyzed in the context of theories of percolative phenomena. Experimental transport properties agree well with model predictions based on an effective medium approximation. Critical exponents that describe the scaling of the transport properties near percolation are consistent with theoretical expectations near a static percolation transition. Through variation of component volume fractions a medium of known microstructure can be prescribed, so that independent measurement of transport and mechanical properties is possible.

Local geometry of surfactant monolayers in a ternary microemulsion system.

Small angle neutron scattering was used to measure a series of water-octane-${\mathrm{C}}_{10}$${\mathit{E}}_{4}$ mixtures in the disordered microemulsion and lamellar phases. Varying the hydrogen-deuterium contrast in oil as well as water enabled us to isolate and determine the scattering contributions from the surfactant monolayers and the water-surfactant and oil-surfactant interfaces. Analyses of the resulting scattering patterns that include the effects of water and oil peneration allow us to demonstrate and quantify the inversion of the surfactant film's small mean curvature as a function of temperature and its relation to the overall phase behavior.

Critical behavior of a ternary microemulsion studied by turbidity, density, and refractive index.

A detailed study of turbidity, density, and refractive index in the critical region of two samples of a water in an oil ternary microemulsion system, water plus benzene plus benzyl hexadecyl dimethylammonium chloride (BHDC), is presented. Special care is taken to determine the criticality of the samples. The critical point is approached from the homogeneous phase by varying the temperature at fixed concentrations of the components. Along this path Fisher-renormalized Ising exponents are expected. The critical exponents for the correlation length (\ensuremath{\nu}) and osmotic susceptibility (\ensuremath{\gamma}) are determined through the turbidity. The critical exponent \ensuremath{\alpha} can be deduced from the (1-\ensuremath{\alpha}) behavior of the density and the refractive index. From mere numerical analysis alone turbidity behavior cannot be distinguished between Ising and Fisher-renormalized Ising exponents. However, physical arguments suggest the latter values for the exponents \ensuremath{\nu} and \ensuremath{\gamma}. We do not see any anomaly in the density within a resolution of \ifmmode\pm\else\textpm\fi{}3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}6}$. This is consistent with the renormalization of \ensuremath{\alpha} [${\mathrm{\ensuremath{\alpha}}}^{\mathrm{*}}$=-\ensuremath{\alpha}/(1-\ensuremath{\alpha})]. A very weak anomaly in the refractive index is detected which is better described by a function of the exponential type, which has been discussed for a quaternary microemulsion near the critical end point, than the (1-\ensuremath{\alpha}) power behavior.

Effect of the macrobicyclic ligand Kryptofix 222 on AOT/water/cyclohexane microemulsions

Aerosol OT (AOT) exhibits a pronounced tendency toward reverse curvature, forming water in oil (w/o) droplets in the L{sub 2} region of the phase diagram in the three-component AOT/water/cyclohexane system. AOT's sodium counterion can be complexed by the addition of the macrobicyclic ligand 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo(8.8.8)hexacosane (Kryptofix 222), which leads to significant changes in the phase behavior of the ternary microemulsion system. This contribution focuses on the structural changes occurring in the L{sub 2} phase upon addition of 25 mol % Kryptofix 222 with respect to AOT. The combination of static SANS, QELS, conductivity, viscosity, and PGSE FTNMR measurements lead to the following conclusions: (1) the droplet morphology is maintained; (2) a significant increase in the droplet-droplet attractions is observed; (3) the aggregates decrease in size at constant w{sub o}. The appearance of a critical point in the oil-rich corner of the pseudoternary phase diagram, where the surfactant system separated into dense and dilute microemulsion phases, confirmed the existence of strong interdroplet attractions. The AOT/25 mol % Kr222/water/cyclohexane microemulsion system resembles the AOT/water/decane droplet system in this respect.

Paracrystal model of the high-temperature lamellar phase of a ternary microemulsion system

Small-angle neutron scattering was used to investigate the microstructure of the high-temperature Lα phase in the AOT/water/decane system [AOT = sodium bis(2-ethylhexyl) sulfosuccinate]. The system was modeled as randomly oriented lamellar stacks with a one-dimensional paracrystalline distortion. For samples injected into cells at high temperature, the two-dimensional Q maps show anisotropic scattering consistent with partial alignment of the phase, while samples injected near room temperature do not exhibit this feature. From the latter samples, it was found that a stack typically contains about 15 lamellae, about 80 Å apart. Each layer is coated by a diffuse surfactant interface with a characteristic thickness of approximately 9 Å. The fact that the scattering spectra do not exhibit maxima beyond the first order is explained by the observation that the Hosemann g factor gives about a 20% variation in the interlamellar spacing. The mean spacing is inversely proportional to the surfactant concentration, as expected for a lamellar phase. Over-estimation of the zero-order scattering predicted by the paracrystal model is attributed primarily to improper treatment of the contrast between each layer and the space-filling lamellar matrix.

Dielectric study of percolation in an oil–continuous microemulsion

Measurements of the electrical conductivity and the complex permittivity near a percolation threshold in the ternary microemulsion system composed of water, iso-octane, and AOT are reported. It is shown that the electrical conductivity, which implies charge transfer process, is well described by a dynamic percolation model. The frequency dependence of the dielectric constant and the behavior of the relaxation frequency are also found to be in close agreement with the scaling power laws of the dynamic percolation model.

Phase electric birefrigence measurements in a ternary microemulsion system I. Results for small volume fractions

Phase electric birefringence measurements (frequency spectra of the modulus β B(2ω)β and phase angle ϕ(2ω) of the alternating Kerr constant B (2ω) have been carried out, over the range, 0.1 kHz-10 MHz, on ternary water in oil microemulsion at small volume fraction ø (≤0.04). The measurements give evidence on the on the presence of droplets dimers (relaxation times 0.3-0.5 μs) easily observed at very low ø, and higher order anisotropic droplets aggregates. A semi-quantitative agreement with theory is obtained.

Light scattering study of the critical behaviour in a ternary microemulsion system

Le diagramme de phases composition-temperature du systeme AOT-eau-decane presente une ligne de points critiques. Nous avons examine le comportement critique de ce systeme en fonction de deux parametres qui controlent les interactions entre gouttelettes E/H, a savoir la temperature et le rapport eau sur savon. Le comportement critique est etudie a l'aide de la diffusion de la lumiere. Nos resultats indiquent un comportement de type liquide-gaz. Ils sont en bon accord avec les theories du groupe de renormalisation pour les phenomenes critiques