Analysis of phase diagram and microstructural transitions in an ethyl oleate/water/Tween 80/Span 20 microemulsion system using high-resolution ultrasonic spectroscopy

High-resolution ultrasonic spectroscopy was applied to analyse a pseudo-ternary phase diagram for a mixture consisting of water/ethyl oleate/Tween 80 and Span 20 at 25 °C. The measured changes in the ultrasonic velocity and attenuation with concentration of water in oil/surfactants mixtures showed several, well defined stages and transitions between them, which allowed construction of an ‘ultrasonic’ phase diagram. Quantitative analysis of the ultrasonic parameters enabled characterisation of various phases (microemulsion, liquid crystals and pseudo-bicontinuous) as well as evaluation of the state of the water and particle size in microemulsion phase.