Properties of Microemulsions

Abstract

The microemulsion system toluene - brine - sodium dodecyl sulphate - butanol was studied in the region where two - phase/three - phase/two - phase transitions are observed for varying brine salinity. Measurements on the coefficient of ellipticity of the system's microemulsion/oil and microemulsion/brine interfaces show a dramatic change in the nature of the interfaces. From the middle of the three - phase region towards the limiting salinities where one phase disappears the corresponding interface is apparently diffuse. From the middle of the three - phase region towards the two - phase regions a layer of the third phase is present at the interface. An intruding layer of microemulsion is present at the interface between the oil and brine phases. The height of liquid rise in Pyrex glass capillaries at the interfaces was measured. Results show that, in the two - phase region at low salinities σog > σmg, and in the two - phase region at high salinities, σmg > σwg, where σog, σmg and σwg are the surface tensions at the interfaces between the glass and the oil, microemulsion and brine phases, respectively. These differences increase with distance from the three - phase region. In the three - phase region σmg > σwg > σog for freshly etched glass and σwg > σmg > σog for untreated glass surfaces, the differences being small. Estimates of the cosine of the contact angle are consistent with the interfacial tension measurements of Pouchelon et al. (Chem. Phys. Lett. 76, 277 (1980)). Pulsed Field Gradient NMR was used to examine the translational molecular diffusion of the oil, water and surfactant components in the microemulsion. The diffusion of both oil and water shows a smooth transition with brine salinity, and the diffusion is three - dimensional and unrestricted. Dynamic effects are needed to interpret these results. In the three - phase region the surfactant diffusion shows a peak, and the microemulsion has a bicontinuous structure. The electrical conductivity and permittivity of the microemulsion was measured over the frequency range 5 Hz to 13 MHz Results show that in the two – phase region at high salinities, where the microemulsion consists of brine droplets in oil, a percolation transition occurs. This transition is identified by a steep rise in the conductivity and a peak in the permittivity. A comparison between the conductivity and molecular diffusion measurements shows that the "stirred" percolation model proposed by Laguës (J. Phys. Lett. 40, L-331 (1979)) to describe percolation in dynamic systems is not applicable to the system studied here. Grest, Webman, Safran and Bug (Phys Rev. Lett. 33, 2846 (1986)) have attributed the frequency dependence of the conductivity observed in microemulsions to dynamic effects I have compared my conductivity and permittivity measurements with the predictions of the Bruggeman model for percolation in static systems, and I show that the static Bruggeman model more correctly predicts the frequency dependence observed in the microemulsion studied here.