Structure of microemulsions of AOT and water in dodecane

Abstract

Abstract:The dielectric properties of the system water/AOT/dodecane are studied as a function of volume fraction ~ of the dispersed phase and molar ratio n (water/surfactant). Data shows that the spherical model is valid only at low n values or low ~ values. At high concentrations of dispersed phase, one has to consider micellar aggregation or deformation.Key words: Microemulsions, dielectric relaxation, permittivity.Solubility of water in hydrocarbon may be obtained using one or more interface agents. One then obtains a low viscosity, isotropic liquid, usually called "microemulsion". In the case of two interface agents, an ionic surfactant is mostly used with an alcohol partly soluble in water and oil. The use of alcohol may be avoided with non ionic surfactants (in that case microemulsions are obtained in a limited range of temperature) or with two-chain ionic surfactants like sodium-di-2-ethylhexylsulfosuccinate (AOT) or sodium-di-2-pentylsulfosuccinate (AY). In the later case, microemulsions may be obtained in a wide range of temperature.Microemulsions have been widely studied in the last few years because of practical interest in the fields of oil recovery, biotechnology and pharmacochemistry. A wide range of techniques have been used, and are becoming more and more sophisticated: light scattering, neutron scattering, conductimetry, magnetic and electric birefringence, positron anhihilation... Depending on the nature of components, composition and temperature, the structure of microemulsions may be very different. Following some conditions, they are dispersions of very small droplets of water in oil (w/o) or oil in water (o/w), the stability beingensured by a mixed layer of surfactant and co-surfactant. It has already been demonstrated that more complex structures may be found. For example, a bicontinuous structure [1] may be detected from a percolation-type W 977 conductive behaviour in some four-component systems [2][3][4]. In the same way, in the vicinity of the demixion curve, the behaviour of static permittivity [5] has been qualitatively attributed -either to highly anisotropic dispersed elements -or to spherical globule aggregation.The present work deals with the electric properties of w/o microemulsions obtained from a mixing of water, AOT and dodecane. In this particular case of ternary system, the volume fraction of dispersion (AOT + water) may be measured. When using alcohol, this would be possible only along the demixion curve [6], because of the alcohol distribution occurring between the continuous and micellar phases. Then a quantitative discussion of data reported in [5] is not possible. The main feature of the samples is their n ratio, number of water molecules over the number of AOT molecules. We may derive the volume fraction ~0 of dispersed components, assuming that all surfactant is located at the interface, and neglecting oil penetration within the layer. These microemulsions behave like ideal solutions in.the way that volumes of water, AOT, and oil are additive [7,8],The structure of th...

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

“…(i) For small values of Q where QR g < 1, one has a Guinier region and S(Q)=N'exp[ -Q2R~/3], and the radius of gyration is given by (8) This small Q range is beyond our reach in this experiment.…”