A small angle X-ray scattering investigation of the structure of a ternary water-in-oil microemulsion

A ternary water-in-oil microemulsion composed of xylene, water and a surface-active extractant, sodium bis-2-ethylhexyl phosphate, was investigated by small angle X-ray scattering (SAXS) and conductimetry.

In order to decide between two possible interpretations of the SAXS data, i.e. evidence for the occurrence of highly polydisperse, independent droplets or of slightly polydisperse and interacting ones, the effects of temperature, water content, dilution and type of solvent were investigated. The scattering profiles were simulated using a log—normal size distribution and the mean spherical approximation (MSA) for the interactions. This led us to proposes model of slightly polydisperse and aggregated particles with a mean radius of about 1.4 nm. The addition of water swells the micelles while concentration and aggregation number remain constant. This then generates an aggregation phenomenon which was confirmed by conductimetry measurements. This investigation makes it possible to optimize the conditions for producing a microemulsion such as is often used in liquid—liquid extraction of metals.