DYNAMICS AND ENERGETICS OF DROPLET AGGREGATION IN PERCOLATING AOT WATER-IN-OIL MICROEMULSIONS

This work reports on the dynamics of L-2-microemulsions stabilized by Aerosol OT. Time-resolved luminescence quenching measurements using th e probe Tb(pda)3(3-) show the existence of clusters in water-in-oil (w /o) microemulsions. The fast exchange appearing over several microesec onds is attributed to intracluster quenching, whereas the slow exchang e on the millisecond time scale corresponds to intercluster exchange, The fast exchange is decelerated when the temperature is increased and is related to a temperature-induced cluster growth. The slow exchange , conversely, is strongly accelerated within the one-phase region. Bel ow the percolation threshold, the corresponding rate constant obeys an Arrhenius relation. The activation energies increase with the droplet size. III the percolation domain, strong deviations from linearity in the Arrhenius plots occur, which are interpreted by a kinetic scheme considering the limited quenching reaction rate and aggregate collisio ns disturbing the exchange transition state. Enthalpy-entropy compensa tion is established from the intercepts of the Arrhenius plots. The ac tivation entropy is discussed in terms of the clustering entropy due t o an aggregation equilibrium prior to exchange, yielding Delta S-cl in very good agreement with results from other methods. Delta S-cl incre ases with the droplet size and the solvent hydrocarbon length. The per colation phenomenon is discussed as an entropy driven droplet aggregat ion accompanied by a facilitated exchange.