Dynamics and Energetics of Droplet Aggregation in Percolating AOT Water-in-Oil Microemulsions

Abstract

This work reports on the dynamics of L2-microemulsions stabilized by Aerosol OT. Time-resolved luminescence quenching measurements using the probe Tb(pda)33- show the existence of clusters in water-in-oil (w/o) microemulsions. The fast exchange appearing over several microeseconds is attributed to intracluster quenching, whereas the slow exchange 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. Below the percolation threshold, the corresponding rate constant obeys an Arrhenius relation. The activation energies increase with the droplet size. In 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 collisions disturbing the exchange transition state. Enthalpy−entropy compensation is established from the intercepts of the Arrhenius plots. The activation entropy is discussed in terms of the clustering entropy due to an aggregation equilibrium prior to exchange, yielding ΔScl in very good agreement with results from other methods. ΔScl increases with the droplet size and the solvent hydrocarbon length. The percolation phenomenon is discussed as an entropy-driven droplet aggregation accompanied by a facilitated exchange.