INTERDROPLET EXCHANGE-RATES OF WATER-IN-OIL AND OIL-IN-WATER MICROEMULSION DROPLETS STABILIZED BY C12E5

We have used a time-resolved fluorescence method to investigate the structure and dynamics of water-in-oil (W/O) and oil-in-water (O/W) microemulsions stabilized by the nonionic surfactant C12E5. The measured aggregation numbers are consistent with the formation of spherical droplets with a low degree of polydispersity. The droplet sizes are in agreement with light-scattering values determined for systems close to the solubilization phase boundaries and at low droplet concentrations. The results of the present study also show the droplet sizes are reasonably constant throughout the temperature range over which the one-phase microemulsion is stable. Droplet sizes are independent of droplet concentration. The intradroplet fluorescence-quenching rates indicate that the fluorescor and quencher probe molecules experience apparent microviscosities within the microemulsion droplets which are 10-100-fold larger than the viscosities of the corresponding bulk oils or water. Exchange of the probe molecules between the droplets occurs on the experimental time scale of a few microseconds for some systems. This exchange is thought to occur via a mechanism of droplet coalescence and reseparation. For both O/W and W/O microemulsions, the rate of this process is slowest at the solubilization phase boundary where the microemulsion phase coexists with an excess phase of the dispersed component. The dynamics of the droplet coalescence process are discussed in terms of the interdroplet interactions and the energies required to bend the surfactant monolayer and to desorb surfactant from the oil-water interface. © 1990, American Chemical Society. All rights reserved.