J. Rouch

Formation and properties of miniemulsions formed by microemulsions dilution

Mini-emulsions have been formed in quaternary systems water/hexanol/sodium dodecyl sulfate/decane by dilution of a microemulsion with an excess of water. We have investigated systematically the effect of composition variables in the droplet size and Ostwald Ripening rate. This droplet size has been investigated by using dynamic light scattering of samples submitted to further dilution in water. According to the dynamic light scattering results, the initial droplet size depends on the initial microemulsion water content, the larger the initial water concentration, the smaller the initial droplet size. This is probably related to the structure of the initial phase. The rate of Ostwald Ripening depends on the final surfactant concentration as expected for differences in interfacial tension. At high surfactant and dispersed phase contents, instability due to flocculation has been observed. Other experiments in which a fifth component (a more hydrophobic oil) was added to slow down Ostwald Ripening showed an initial droplet size increasing comparable to the systems without additive but after a relatively long time of approximately 2 h, a decrease was observed.

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) A. 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.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) A. 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...

Thermodynamics of the 2D-Heisenberg classical square lattice: Part III. Study of the static susceptibility behaviours

Abstract In a previous article labelled II a closed-form expression of the dynamic and static susceptibilities has been derived (J. Curely, Physica B 254 (1998) 277, previous article in this issue). In the present one, we mainly focus on the behaviours of the static susceptibility χ . In the low-temperature domain we point out the mechanisms involved in the construction of the 2D-spin arrangement; a very simple model allows one to give a good description of the χT behaviours. For a 2D-compensated antiferromagnet, we derive a linear variation between the temperature of the maximum of the susceptibility curve T ( χ max ) and the exchange energy J / k B . In addition, above the Neel temperature, there is a remarkable agreement between the experimental and theoretical susceptibilities, for the compounds BaMnF 4 and (CH 3 NH 3 ) 2 MnCl 4 . Finally, when T decreases from the high-temperature (HT) domain, we verify on these experimental results that the HT theoretical expansion of χ leads to a value lower than the one derived from Rushbrookes HT series. In addition, it allows to fit all the experimental points belonging to the characteristic maximum of the susceptibility curve whereas the Pade method just permits to reach it before breaking down.

The even–odd effect in liquid crystals: A collective or intrinsic molecular property

We have made the first observation that nematogens and smectogens exhibit physical properties characterized by even–odd alternation with the number n of carbon atoms in their alkyl tails, even when molecules are isolated. For the cyano‐biphenyl series, when n?5, the alkyl tail is found to be all in the trans configuration. When n≳5, gauche isomers appear and an important even–odd effect is experimentally evidenced and discussed.We have made the first observation that nematogens and smectogens exhibit physical properties characterized by even–odd alternation with the number n of carbon atoms in their alkyl tails, even when molecules are isolated. For the cyano‐biphenyl series, when n?5, the alkyl tail is found to be all in the trans configuration. When n≳5, gauche isomers appear and an important even–odd effect is experimentally evidenced and discussed.

The principle of corresponding states and the dynamic properties of simple liquids

We present a corresponding states analysis of existing data on sound velocity, transport coefficients and intermolecular interaction times of a number of simple fluids. With appropriate choices of parameters for the intermolecular pair potential, such properties as the adiabatic sound velocity, the shear and bulk viscosities, the thermal conductivity and the intermolecular interaction time can be reduced along the liquid-vapor equilibrium curve. Deviations from the simple law of corresponding states are most pronounced for the transport properties, indicating a stronger sensitivity of the dynamical properties to the interaction potential and the molecular structure.

Relaxation phenomena in AOT-water-decane critical and dense microemulsions

We report on extensive measurements of the low and high frequencies sound velocity and sound absorption in AOT-water-decane microemulsions deduced from ultrasonic and, for the first time as far as the absorption is concerned, from Brillouin scattering experiments. New experimental results on dielectric relaxation are also reported. Our results, which include data taken for critical as well as dense microemulsions, show new interesting relaxation phenomena. The relaxation frequencies deduced from very high frequency acoustical measurements are in good agreement with new high frequency dielectric relaxation measurements. We show that along the critical isochore, sound dispersion, relaxation frequency, and static dielectric permittivity can be accurately fitted to power laws. The absolute values of the new exponents we derived from experimental data are nearly equal, and they are very close to β=0.33 characterising the shape of the coexistence curve. The exponent characterising the infinite frequency permittivity is very close to 0.04 relevant to the diverging shear viscosity. For dense microemulsions, two well defined relaxation domains have been identified and the temperature variations of the sound absorption and the zero frequency dielectric permittivity bear striking similarities. We also show that the relaxation frequency of the slow relaxation process is almost independent of temperature and volume fraction and so cannot be attributed to percolation phenomena, whereas it can more likely be attributed to an intrinsic relaxation process probably connected to membrane fluctuations.

Low frequency depolarized light scattering in the VH geometry from quinoline

Low frequency depolarized light scattering spectra in the VH geometry are measured in liquid and supercooled quinoline from 37 to −35 °C and compared with the theory of Gershon and Oppenheim. The spectra are separated into contributions of different modes which can be dissipative and/or vibrating according to Gershon and Oppenheim’s results, and it is shown that a central dip does not necessarily mean dissipative modes, e.g., at 2 °C. Shear viscosities deduced from the spectra are close to values measured macroscopically. Orientational relaxation times obtained are exponential vs 1/T over the temperature range measured, and the coupling parameter between the orientational and the linear momentum densities is about proportional to the viscosity width.

The static electrical conductivity of water-in-oil microemulsions below percolation threshold

We study the static electric conductivity in water-in-oil microemulsion systems both in the droplet phase and in the vicinity of a percolation transition in the non-percolating region. We discuss the mechanisms of conduction in the two regimes. In particular, we interpret the behavior of conductivity far from percolation in terms of charge fluctuations and close to percolation in terms of collection of relaxation times connected to the presence of a polydisperse set of independent fractal clusters.

PHASE SEPARATION IN LIVING MICELLAR NETWORKS

We present a lattice model based on two n→0 spin vectors, capable of treating the thermodynamics of living networks in micellar solutions at any surfactant concentration. We establish an isomorphism between the coupling constants in the two spin vector Hamiltonian and the surfactant energies involved in the micellar situation. Solving this Hamiltonian in the mean-field approximation allows one to calculate osmotic pressure, aggregation number, free end and cross-link densities at any surfactant concentration. We derive a phase diagram, including changes in topology such as the transition between spheres and rods and between saturated and unsaturated networks. A phase separation can be found between a saturated network and a dilute solution composed of long flexible micelles or a saturated network and a solution of spherical micelles.

Cluster formation in water-in-oil microemulsions at percolation: evaluation of the electrical properties

We study water-in-oil microemulsion systems in the droplet phase and in the vicinity of a percolation transition in the non-percolating region. We focus on the electrical conductivity and permittivity, quantities that show large variations when approaching the percolation threshold. The accepted model for the interpretation of the increasing conductivity - very large compared to that of the bathing oil phase - is related to clustering of the microemulsion droplets and migration of charges within the aggregates. Power laws have been used to interpret the behaviour of the static dielectric properties and scaling functions proposed for the frequency-dependent conductivity and permittivity. We review some relevant experiments in this field and the proposed interpretations, and formulate a phenomenological model of conduction. It is based on the physical picture of cluster formation due to attractive interactions among the constituent water droplets, anomalous diffusion in the bulk of fractal aggregates and polydispersity of the clusters. The model gives quantitative expressions for both conductivity and permittivity over the entire frequency range of the percolative relaxation phenomena, including the static behaviour. A closed expression is derived for the scaling function of a scaling variable which involves frequency, the cut-off cluster size and the parameters of the bulk components. The results are also expressed in the time domain in terms of the polarization time correlation function. The latter exhibits a rather interesting behaviour, since it gradually evolves from an exponential decay to a power-law decay and to a stretched exponential as time increases. The time-scales of the different stages are obtained from the typical decay times of the single droplet and the largest cluster. We have analysed many different sets of data obtained for different microemulsion systems as functions of the composition of the dispersed phase, the temperature and the frequency of the applied field, with a very good agreement with the model in all cases.