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The formation of thermalized positronium atoms is greatly reduced if increasing amounts of water become solubilized in reversed micelles formed by sodium bis(2-ethylhexyl) sulfosuccinate in apolar solvents. Similar observations have been made if the surfactant is Triton X-100. The application of the positron annihilation technique to the study of microemulsions consisting of potassium oleate-alcohol-oil-water mixtures indicates, consistent with previous results, that microemulsion formation requires a certain water/oil ratio if the oil is a long-chain aliphatic hydrocarbon such as hexadecane. This ratio is 0.4 in the case of a 1-pentanol-and 0.2 for a 1-hexanol-containing mixture. This minimum water content is strongly reduced if the oil is an aromatic hydrocarbon. The positron annihilation data also sensitively reflect structural rearrangements in these solutions occurring upon further addition of water, such as the transition of spherical aggregates to a disk-like lamellae structure.

The positron annihilation technique was applied to the study of the micelle formation process in micellar systems Aerosol OT and dodecylammonium propionate (DAP) in apolar solvents, such as benzene and isooctane. At certain surfactant concentrations abrupt changes in the number of thermal 0-Ps atoms formed can be observed. The surfactant concentration at which this effect can be seen is widely temperature independent in Aerosol OT solutions while a significant shift to higher surfactant concentrations can be found with increasing temperature in DAP solutions. The effect of the nature of the counterion on the mechanism of micelle formation is further investigated by substituting the counterion Na' in Aerosol OT by tetraalkylammonium ions, NtM4, N+(CH3)*, and N+(CZH5)4. The results are discussed in terms of the pseudophase model and multiple equilibrium model for micelle formation.

The formation of positronium in benzene solutions containing a variety of halogenated compounds, such as CC14, CH2C12, C2HC13, 1,1,1-C2H3C13, o-C6H4CH3Br, o-C6H4Cl2, o-C6H4CH3C1, C6H5C1, and other mono-and dihalobenzene, was studied in the presence and absence of C6F6 additives. The observed /2 values, which are indicative of the number of thermalized positronium atoms formed, showed in each system a good correlation with dissociative electron attachment parameters of the solute species. C8F8 additives increased the number of Ps atoms reaching thermal energies. The experimental results are discussed in terms of a reaction scheme in which Ps is formed as a result of electron abstraction from the surrounding molecules by energetic positrons. These Ps atoms can either react rapidly with solute species if their excess kinetic energy favors such a reaction, or with species generated in the reactions of free electrons produced in the positron spur with solute molecules via dissociative electron attachment. The number of these reactive species is mainly determined by the number of free electrons having sufficient energies to undergo dissociative electron attachment. A compound such as C6F6, which has a high cross section for nondissociative electron attachment, competes for free electrons with the solute, and protects the latter from radiolysis. Thus in the presence of CeF8 fewer reactive species are formed in the spur with which the Ps can rapidly react and more Ps atoms will reach thermal energies. An alternative to this reaction scheme is the previously reported spur reaction model which assumes positronium formation via free positron and free electron combination in the positron spur. The results will be also discussed within the framework of this latter model.

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