Transitions in ternary surfactant/alkane/water microemulsions as viewed by fluorescence

Abstract

Fluorescent probes incorporated in ternary surfactant systems have proved valuable in elucidating structure, dynamics and phase behaviour. The intriguing case of microemulsions using non-ionic surfactants, such as alkyloligoethylene oxides, can form phases simultaneously bicontinuous in oil and water. Fluorescence analysis provides both sensitivity and selectivity to monitor these systems. We have used pyrene and rhodamine 6G as probes to enrich our knowledge of the C₁2E₅/alkane/water system, with particular relevance to the bicontinuous phase. Pyrene has been used as a dynamic probe, studying both excimer formation and quenching by molecular oxygen. This provides a useful tool to monitor transitions between microemulsion phases. Both steady state and time-resolved fluorescence measurements indicate a change in localisation on passing from one phase to another. Information was also obtained on microviscosities in these systems using fluorescence depolarisation making use of the well-known laser dye rhodamine 6G. Comparison of the fluorescence characteristics of these systems provides a means to monitor at the microscopic level changes in phase behaviour.

Acknowledgements Financial support from the Fundação para a Ciência e a Tecnologia through the PRAXIS XXI programme and Sapiens (POCTI/35415/QUI/2000) is acknowledged.