CHEMICAL-REACTIONS IN MICROEMULSIONS - PROBING THE LOCAL DIELECTRIC NUMBER OF THE DISPERSED WATER

The experiments described in this paper were performed for answering the question how to distinguish between a microemulsion and a weakly structured solution. The local dielectric number of water dispersed in ternary mixtures of water, oil, and amphiphile is probed by studying two chemical reactions sensitive for this physical property. The kinetics of the decomposition of murexide and the pK(a) value of Thymol blue are used as chemical probes. Two hydrophilic probes are chosen to ensure that the probe is located in the aqueous domains and not at the interface or in the nonpolar domains. If the probe studied in the ternary mixture experiences an environment similar to bulk water, the mixture is classified as a microemulsion. If the dielectric number of th environment of the probe is much lower than that of water, the ternary mixture is classified as a weakly structured solution. A borderline can be drawn at that concentration of water needed to hydrate the head groups of the amphiphile molecules. The presence of microstructure in the weakly structured solution can also be shown by studying a probe reaction. The complexation reaction of Ni2+ with the hydrophobic ligand pyridin-2-azodimethylaniline (PADA) is a sensitive probe for the presence of hydrophobic aggregates in a solution.