Study of bicontinuous phase in (TTAB+pentanol)/water/n-octane reverse micellar system using positron lifetime spectroscopy

A phase diagram of (TTAB+pentanol)/water/n-octane has been mapped by using optical method. It exhibits a reverse micellar (L₂) phase extending over a wide range of concentrations of the constituents. To investigate the fine structure of the L₂ phase, a series of (TTAB+pentanol)/n-octane ternary mixtures having initial concentrations of (TTAB+pentanol) (1:1) in n-octane as 35%, 50% and 65% by weight were prepared. In each of these mixtures, positron lifetime measurements were performed as a function of the concentration of water, using a standard lifetime spectrometer. At water concentrations of 11.8%, 8.5% and 8.4% by weight respectively for the above systems, the o-Ps pick-off lifetime τ₃ shows an oscillatory behaviour while I₃ representing the Ps formation exhibits an abrupt change. These changes in the positron annihilation parameters have been explained on the basis of onset of bicontinuity in the microemulsion phase. The positron annihilation technique thus suggests the existence of droplet-like and bicontinuous structures in the L₂ phase which is otherwise considered optically to be a single phase as the system remains clear and isotropic throughout this phase. Supporting evidence has been provided by the electrical conductivity measurements performed in these systems. These results are presented in this paper.