Positron annihilation spectroscopy and micellar systems

Positron annihilation spectroscopy (PAS) has emerged as a powerful technique for investigating structural changes, phase transitions and microenvironmental transformations in a variety of systems. The process of molecular aggregation in micellar systems is known to be cooperative and size limited; it shows features similar to that of a classical phase transition. Similarly, the changes in the concentration of surfactant and the solubilizate bring about several microstructural and conformational transformations in these systems. High sensitivity of positron annihilation parameters to such changes makes it a potential candidate for investigating micellar and microemulsion systems. This paper deals with this aspect of positron annihilation spectroscopy. Applications of this technique to investigate conformational, structural and microenvironmental transformations in micellar and microemulsion systems are discussed. Its superiority over the conventional techniques in such investigations is demonstrated. It is shown that this technique reveals finer details of otherwise considered to be single phase regions in a phase diagram. Its usefulness in delineating phase boundaries and hence in mapping of phase diagrams is also discussed.