Microemulsions

Abstract

Chemical system containing an anionic soap, alcohol, pure hydrocarbon, pure water or brine were studied intensively during recent years. Different experiments on the same systems were carried out in order to obtain detailed information on the thermodynamic behavior and structure of microemulsions. For example, two critical end points were observed in the three-phase region of brine systems. The middle phase composition varies strongly in the function of overall concentration but the composition of oil or water excess phases of these Winsor III type system varies very little. Critical point lines appear also in phase diagrams of rich oil systems with pure water. A classical opalescence phenomenon is observed in light scattering studies of the one-phase systems close to these critical points and correlation lenghts are 500 -1000 å. Near these points the two-phase systems in equilibrium are microemulsions. Two possible origins of very low interfacial tensions are proposed. The tension between the microemulsion and one excess phase in Winsor III systems is zero at the critical end point and remains very low far from this point. But another mechanism can exist in two-phase systems with a particular structure of the interfacial film. The structure of diluted water in oil droplets is now well established. The droplets are spherical with only a very small size dispersion and their radius depends mainly on the water/soap ratio. Neutron and light scaterring measurements yield the measurement of the second virial coefficient B of the osmotic pressure. B is directly connected with the energy potential of interactions between droplets. B values can be positive or negative and correspond to hard sphere or attractive potential. B depends on droplet radius, salinity, hydrocarbon and even more on the alcohol and soap used. A theoretical attractive potential has been proposed which accounts for the experimental scaterring data. In this calculation the water in oil micelles are pictured as spherical droplets with a hard core and a thick and penetrable shell. The largest contribution to the potential comes from van der Waals interactions in the overlapped region of two closed droplets. This potential provides an approach for the understanding of the variations of the interactions. For example, the effect of micellar size and alcohol chain length can be interpreted as changes of the overlapped volume. For a high attractive potential a critical point occurs and percolation begins at very low concentration. Fast exchange between water cores could be possible. The structure of diluted oil in water microemulsions is less well known. Spherical droplets exist with a large size distribution in high salinity systems and probably in pure water. Soap aggregate numbers can now be determined by using flourescence labels such as pyrene. This model of well defined spherical droplets is not satisfying for mixtures with similar amounts of oil and water. Distances between oil or water cores become short and interaction forces increase strongly. Rapid molecular exchange can be possible and particle distortion can occur. A sponge model with random connected oil or water domains is the only description which can be proposed presently for these bicontinuous phases. Lamellar structure can be observed in birefringent oil rich phases with small amount of soap and alcohol. of course, the microemulsion stability depends strongly on dispersion entropy. and the interfacial tension of the soap and alcohol film. In some cases attractive interactions between droplets can also induce phase separation in oil or ??? rich systems. Many important properties of microemulsions such as droplets size or interfacial tension with excess phases depend strongly on soap and alcohol concentration on: 1) the interfacial film; 2) the dispersed; and 3) the continuous phase. A microemulsion model which takes into account the alcohol self-association allows those concentrations in these three ”pseudophases“ to be calculated. Therefore, new stability theories for microemulsions with a more realistic description of these fascinating but quite complicated systems can be proposed.