TIME-RESOLVED FLUORESCENCE PROBING OF OIL-IN-WATER MICROEMULSIONS STABILIZED BY NONIONIC SURFACTANTS

Time-resolved fluorescence (TRF) of fluorescent and quencher probe mol ecules has been used to investigate the dynamics, mean size and polydi spersity of oil-in-water (o/w) droplet microemulsions stabilised by a range of alkylpoly(oxyethylene glycol ether) non-ionic surfactants. Cl ose to the low-temperature (solubilisation) phase boundary of the micr oemulsions, there is no significant rate of exchange of probe molecule s between the droplets. Exchange occurs on the experimental timescale of a few mu s as the temperature is increased away from the phase boun dary. 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 nm(2). The surfactant tail areas found in this work for relatively small microemulsion dropl ets are significantly lower than observed previously by dynamic light scattering (DLS). We show here that areas derived using DLS data are m ainly sensitive to the tailgroup areas for large droplets whereas the TRF data give accurate values for small droplets. Combination of the T RF and DLS data shows that the tailgroup areas of the surfactants stud ied here increase with increasing droplet size. The TRF method is more sensitive to the aggregate polydispersity than radiation scattering m ethods. The relative radius polydispersities for the microemulsion sys tems tested here are all in the range 0.1-0.2 and decrease with increa sing overall surfactant chain length.