Abstract
A phase diagram investigation, by visual observation, has been performed at different temperatures on the system brine/toluene/1-butanol/SDS for a 6.5% NaCl salinity and for a very low SDS concentration (0.01-0.3% w/w). In the range 16-26/sup 0/C a new behavior is detected, characterized by the appearance of a diffuse interface, a few millimeters thick. The composition and the structure of a typical sample have been investigated in that domain, as a function of temperature, by index of refraction, gas chromatographic, turbidity, and quasi-elastic light scattering measurements. The sample is composed of an upper transparent region of the oily type and a lower turbid region of the oil/water (o/w) microemulsion type, separated by a sharp interface; the diffuse interface appears between two regions of different turbidity and composition of the lower aqueous domain. To interpret the nonuniformity of the aqueous domain a discussion is provided in terms of incomplete phase separation, critical type regime, sedimentation of polydisperse globules due to gravity, and globules aggregation. A detailed analysis of the composition profile allows to pinpoint a peculiar composition of the interfacial film with a much higher alcohol/SDS ratio than usual which could be the origin of the new behavior.
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