D. Langevin

Light—scattering study of water-oil microemulsions

Microemulsions have attracted much interest since their introduction because of their applications in numerous fields of science and technology. They are apparently homogeneous and transparent systems of low viscosity, containing emulsifier molecules, oil, and water. They usually behave as dispersions of very small spherical droplets of water (or oil) surrounded by emulsifier molecules, in a continuous medium containing the oil (or water). The droplet sizes range from 100 to 1000 A in diameter.

Low interfacial tensions in three-phase systems obtained with oil-water surfactant mixtures

Abstract We have studied the quaternary mixture brine-toluene-SDS-butanol, in the domain where it gives three phases, a microemulsion in contact with an organic and an aqueous phase, and measured three low interfacial tensions γmo, γXXX and γow. We find γow stays equal to the largest of the two others and conclude that the origin of low interracial tensions between the microemulsion and organic or aqueous phases are due to different phenomena.

Origin of low interfacial tensions in systems involving microemulsion phases

We have studied the systems obtained from phase separation of a quaternary mixture: brinetoluene-SDS-butanol, in the domain where they give two phases: a microemulsion in contact with an organic or an aqueous phase. We have measured the resulting low interfacial tensions and we show that they are not affected by microemulsion dilution even until all the micelles are removed. We conclude that in these systems the low interfacial tensions are due to the presence of a surfactant layer at the interface.

Percolation and critical points in microemulsions

Microemulsions are currently considered either as hard-sphere-like systems or as oil and water bicontinuous media. In this paper we will show that most of the time the microemulsion structure is of an intermediate character. This problem is closely related to the existence of critical consolute points in the oil–water–surfactant phase diagram. The study of the dynamical properties of these systems aids a better understanding of structural features. Our discussion is based on recent experimental data obtained on water-in-oil systems from various techniques: light scattering, electrical conductivity, ultrasonic absorption and transient electrical birefringence. A tentative picture of the possible mechanisms leading to a structural inversion to oil-in-water systems is proposed.

Small-angle neutron scattering experiments on microemulsion droplets: relation to the bending elasticity of the amphiphilic film

Abstract Small-angle neutron scattering (SANS) experiments and measurements of interfacial tension have been carried out on droplet type microemulsions in the system nonionic surfactant (CiEj)/hydrocarbon/water. The SANS spectra in the shell contrast can be very well interpreted by a model of polydisperse spherical shell structured aggregates. In addition it has been shown that a more precise description is obtained if one takes into account the diffuseness of the amphiphilic film. However, the outcome for the structural parameters does not depend on the choice of the particular model employed. For microemulsion droplets structural quantities such as radius, polydispersity index, and macroscopic interfacial tension can be related directly to the bending constants (bending modulus κ and Gaussian modulus gk) of the system. The sum 2κ + gk can be deduced independently from interfacial tension and polydispersity index (from SANS data in the shell contrast) and both are in good agreement. Variation of the chain length of the surfactant shows that this sum increases with increasing thickness of the amphiphilic film. In contrast, variation of the chain length of the hydrocarbon does not influence the elastic properties of the surfactant film. A dilution series also allows for a determination of 2κ + gk from the concentration dependence of the deduced radii and polydispersity indices. Again good agreement is observed with the values obtained before and the most appropriate entropy of mixing term to describe such a microemulsion appears to be a random mixing approximation. Experiments both in oil-in-water (O/W) and water-in-oil (W/O) droplets indicate that 2κ + gk decreases with increasing temperature (W/O at higher temperature). In general one can state that the properties of these microemulsions are well described by the elastic theory for the amphiphilic film. Structural parameters such as radius and polydispersity are related to the interfacial tension and if two of these quantities are known the third can be predicted reliably.

Correlation between interfacial tension and microemulsion structure in Winsor equilibria : role of the surfactant film curvature properties

Abstract We present here a correlation between measured interfacial tensions, dispersion sizes and surfactant film curvature properties in two model multiphase

Interfacial Tension: Theory and Experiment

Surfactant molecules spontaneously adsorb at liquid interfaces: either liquid—air in the case of aqueous or organic surfactant solutions or liquid—liquid in the case of both aqueous and organic surfactant solutions. When the solubility of the surfactant in the aqueous and/or organic phases is very low, the surfactant molecules concentrate at the interface. They form a two-dimensional (2D) monolayer [1]. The energy per unit area of the interface is by definition the surface tension. Before adsorption the surface tension is 70, after adsorption.γ. The change in free energy per unit area due to the monolayer is (by analogy with 3D systems) the surface pressure П: n n

Heterodyne spectroscopy at very low frequencies

gamma = {{gamma }_{0}} - Pi