Ion Reactivity in Micro‐Emulsion Systems

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Abstract
We have studied various elementary reaction processes from a kinetic viewpoint in reversed micellar (W/O micro‐emulsion) systems (e.g. Aerosol‐OT, water, heptane). The dispersions formed have been characterised by a variety of physical methods including viscosity, ultracentrifugation, and light scattering, and water droplet diameters have been varied in the region 10‐50·10−10m.Extensive measurements (using the stopped‐flow method) have been made on the metal‐ligand complexation reaction involving divalent metal ions (e.g. Ni2+) and the chromophoric ligand murexide, with both reactants initially contained in separate pools prior to mixing. (These reagents are particularly suitable for study since no partitioning of reactants out of the water pools takes place.Useful information can then be obtained on the following processes.The collision process between the micro‐emulsion droplets and the communication/exchange of reagents between pools.The energetics of the reaction in the water‐pool environment compared with bulk water. The reaction can therefore be used to probe the nature of the heterogeneous water in which the complexation reaction occurs.Our conclusions so far are as follows.Reactions in micro‐emulsion droplet systems exhibit simple kinetic behaviour, and the rate dependence on droplet size and concentration can be explained quantitatively.When Aerosol‐OT is used as a dispersant, there is virtually no energy barrier to communication of reagents between water pools, at least 1 in 104 of the collisions resulting in transfer. Activation energies of −1 can be predicted for this process.As far as ligand substitution on divalent metal ions in heterogeneous water is concerned, the rates for loss of water and ligand are very similar to those measured in bulk water. This is in agreement with previous results obtained for the loss of water from Ni2+ when the metal ion is located close to the surface of an anionic micelle.

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