4-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 (C-AOT), water to AOT mole ratio (W-0) and temperature (T), from 294 to 333 K, using positron annihilation l ifetime spectroscopy (LS). By four-component analysis of the spectra, it is possible to extract the LS parameters (intensities, I-i, and lif etimes, tau(i)) of the triplet positronium (o-Ps) present in the aqueo us (I-3, tau(3)) and organic (I-4, tau(4)) phases. The latter lifetime is constant and corresponds with the value measured in pure isooctane , while tau(3) 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 tw o fitting parameters, the radius of the water aggregates (r(0)) and a transmission factor (h). Fixation of r(0) = 3.6 nm for C-AOT = 0.1 M, W-0 = 20, and T = 294 K, as known from previous work, allows the quant itative derivation of the r(0) values for all other conditions. The wa ter spheres appear to present some permeability to o-Ps, with a transm ission factor h = 0.12 nm(-1). The sphere radius increases smoothly wi th C-AOT and w(0) and, more importantly, with T. The changes with w(0) give r(0) = 0.181w(0) and 0.186w(0) nm at 294 and 298 K, respectively , and are in excellent agreement with previous proposals. The sum of t he intensities, I-tot = I-3 + I-4, is much lower than the o-Ps intensi ty in pure isooctane. In particular, at C-AOT > 0.04 M, I-tot appears very close to the value found for pure water. The possibility of a str ong inhibition of Ps formation due to the micelles, as proposed in pre vious work, is ruled out because of the negligible electron or positro n scavenging ability of alkyl sulfonates. It is thus concluded that Ps formation occurs primarily in the aqueous part of the micelles, the w ater aggregates representing efficient traps for the positrons while t hey are slowing down in the solutions.