Intercluster exchange rates in AOT water-in-oil microemulsions: percolation, material transport mechanism and activation energy

Abstract

The time-resolved luminescence quenching technique was employed to investigate cluster dynamics in ionic water-in-oil aerosol-OT microemulsions. Using the long living probe Tb(pyridine-2,6-dicarboxylic acid)₃^3–(lifetime 1.9 ms) and the quencher bromophenol blue, we studied the exchange of material between clusters in AOT water-in-oil microemulsions with different alkane oils. The investigations were performed at constant amphiphile concentration and constant water: surfactant molar ratio; under these conditions the percolation temperatures are shifted to lower values with longer-chain alkane oils. In the pre-percolative regime the exchange process must be activated to break the surfactant monolayer with activation energies of 126–188 kJ mol^–1. The longer the alkane-oil chain of the solvent, the higher is the activation energy. This is in contrast with expectations on the basis of literature data on the bending moduli of flat surfactant monolayers and the oil penetration concept. The results are discussed as an effect of molecular oil properties on the surfactant-monolayer compressibility. The rate constant for exchange between clusters at the percolation threshold is 3.8 × 10⁸ dm³ mol^–1 s^–1 for all microemulsions studied. With the percolation transition a change in mechanism occurs from activation-controlled rate limiting to a more complex situation, in which diffusion-controlled cluster aggregation prevails. From the observed dynamics we conclude that in the percolated state the droplet structure is maintained, although the shape-restoring interfacial forces are weak. The decay kinetics are not stretched exponential, implying that the averaged droplet arrangement in a cluster on a millisecond timescale is not fractal-like.