Experimental Evidence of a Nonuniform Microemulsion in the Low Surfactant Concentration Range

Abstract

The sodium dodecyl sulfate (or SDS)/ n-butanol/toluene/ brine system has been selected for investigation in the low surfactant concentration range. For a 6.5% NaCl salinity and the following proportions of the other components: brine 65.74%, toluene 31.90%, n-butanol 2.30% and SDS 0.04%, the sample is composed of a transparent upper oily region, an intermediate turbid microemulsion region, and a lower transparent one. The oil-microemulsion interface is sharp; the intermediate and lower regions are separated by a diffuse interface, a few millimeters thick. Index of refraction and gas chromatographic analyses plus quasielastic light scattering (QELS) characterize the sample as composed of a non uniform vertical composition aqueous domain of microemulsion type and an excess oily solution. In the aqueous domain, two regions with constant but different index of refraction (n) and mutual diffusion coefficient (D) correspond approximately to the intermediate and lower regions and are separated by a region with progressive n and D variation. The composition and structure results at a given temperature suggest an alcohol-rich composition of the microemulsion interfacial film, the n-butanol/SDS ratio becoming larger than for usual SDS/n-butanol/toluene/brine systems. Due to this fact, an unusually low value of the rigidity constant of the interfacial film is expected. Preliminary experimental results define the thermal range in which the sample displays a diffuse interface, and show that the surfactant is near its solubilization limit for the given proportions of the other components.