Time-resolved fluorescence probing of oil-in-water microemulsions stabilised by non-ionic surfactants

Abstract

Time-resolved fluorescence (TRF) of fluorescent and quencher probe molecules has been used to investigate the dynamics, mean size and polydispersity of oil-in-water (o/w) droplet microemulsions stabilised by a range of alkyloply(oxyethylene glycol ether) non-ionic surfactants. Close to the low-temperature (solubilisation) phase boundary of the microemulsions, there is no significant rate of exchange of probe molecules between the droplets. Exchange occurs on the experimental timescale of a few µs as the temperature is increased away from the phase boundary. An analysis of the measured mean aggregation numbers of the o/w microemulsion droplets indicates that the surfactant tailgroups in the curved monolayers coating the droplets are rather densely packed with areas per surfactant tail in the range 0.2–0.3 nm². The surfactant tall areas found in this work for relatively small microemulsion droplets are singificantly lower than observed previously by dynamic light scattering (DLS). We show here that areas derived using DLS data are mainly sensitive to the tailgroup areas for large droplets whereas the TRF data give accurate values for small droplets. Combination of the TRF and DLS data shows that the tailgroup areas of the surfactants studied here increase with increasing droplet size. The TRF method is more sensitive to the aggregate polydispersity than radiation scattering methods. The relative radius polydispersities for the microemulsion systems tested here are all in the range 0.1–0.2 and decrease with increasing overall surfactant chain length.