Four-Component Analysis and Modeling of Positron Annihilation Lifetime Parameters in AOT/Water/Isooctane Microemulsions

Reverse micelles are studied in sodium AOT/water/isooctane mixtures as functions of AOT concentration (CAOT), water to AOT mole ratio (w0) and temperature (T), from 294 to 333 K, using positron annihilation lifetime spectroscopy (LS). By four-component analysis of the spectra, it is possible to extract the LS parameters (intensities, Ii, and lifetimes, τi) of the triplet positronium (o-Ps) present in the aqueous (I3, τ3) and organic (I4, τ4) phases. The latter lifetime is constant and corresponds with the value measured in pure isooctane, while τ3 is remarkably lower than the value for pure water. This difference is attributed to the out-diffusion of o-Ps from the water cores to isooctane. The relevant rigorous diffusion equations imply two fitting parameters, the radius of the water aggregates (r0) and a transmission factor (h). Fixation of r0 = 3.6 nm for CAOT = 0.1 M, w0 = 20, and T = 294 K, as known from previous work, allows the quantitative derivation of the r0 values for all other conditions. The water spheres appear to present some permeability to o-Ps, with a transmission factor h = 0.12 nm-1. The sphere radius increases smoothly with CAOT and w0 and, more importantly, with T. The changes with w0 give r0 = 0.181w0 and 0.186w0 nm at 294 and 298 K, respectively, and are in excellent agreement with previous proposals. The sum of the intensities, Itot = I3 + I4, is much lower than the o-Ps intensity in pure isooctane. In particular, at CAOT > 0.04 M, Itot appears very close to the value found for pure water. The possibility of a strong inhibition of Ps formation due to the micelles, as proposed in previous work, is ruled out because of the negligible electron or positron scavenging ability of alkyl sulfonates. It is thus concluded that Ps formation occurs primarily in the aqueous part of the micelles, the water aggregates representing efficient traps for the positrons while they are slowing down in the solutions.http://dx.doi.org/10.1021/jp952838