ANALYSIS AND MODELING OF POSITRON LIFETIME SPECTROSCOPY DATA FOR THE CHARACTERIZATION OF WATER-IN-OIL MICROEMULSION SYSTEMS STABILIZED BY AEROSOL OT - EFFECT OF CHANGING THE COUNTERION

In previous work using positron lifetime spectroscopy, a model was pro posed leading to the satisfactory description of both positronium (Ps) formation and decay in sodium AOT-water-isooctane reverse micelles. T he shorter lifetime found for the Ps triplet state in the aqueous core of the micelles, compared to that found in pure water, was attributed to Ps outdiffusion from these cores, enabling the quantification of t his lifetime with two fitting parameters: r(0), the aqueous core radiu s, and it, a factor which describes the ease of passage of Ps at the w ater/isooctane interface. To assess the validity of the model, the exp eriments have been extended to the study of the Ca(AOT)(2) and 2 : 1 m ole ratio Na(AOT) : Ca(AOT)(2)-water-isooctane systems, to examine the effect of the counter-ion on r(0) as a function of the water/AOT rati o, w(0) and (for Ca2+) of temperature. Using, for all systems, the val ue of h previously found for the Na(AOT) microemulsions, the presence of the Ca2+ counter-ion appears to result in the formation of larger r everse micelle aggregates, compared to with Na(AOT), with radii in agr eement with published data With the Ps yields, the model implies a sem i-empirical parameter, A(trap), related to the probability that the en ergetic positrons end up in the aqueous subphase before forming Ps. As expected and experimentally verified, A(trap) is proportional to r(0) (2). Unexpectedly however, with the above value for h, the variation o f A(trap) with r(0)(2) is found to depend on the counter-ion. Therefor e, the possible variation of h was considered. Using different values of h for each system, it is possible to obtain a unique variation of A (trap) with r(0)(2), independent of the counter-ion. In this case, how ever, the radii for the microemulsions containing Ca2+ are significant ly lower than those found in previous work.