Analysis and modelling of positron lifetime spectroscopy data for the characterisation of water-in-oil microemulsion systems stabilised by Aerosol OT Effect of changing the counter-ion

Abstract

In previous work using positron lifetime spectroscopy, a model was proposed leading to the satisfactory description of both positronium (Ps) formation and decay in sodium AOT–water–isooctane reverse micelles. The 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 this lifetime with two fitting parameters: r₀, the aqueous core radius, and h, a factor which describes the ease of passage of Ps at the water/isooctane interface. To assess the validity of the model, the experiments have been extended to the study of the Ca(AOT)₂ and 2:1 mole ratio Na(AOT):Ca(AOT)₂–water–isooctane systems, to examine the effect of the counter-ion on r₀ as a function of the water/AOT ratio, w₀ and (for Ca²⁺) of temperature. Using, for all systems, the value of h previously found for the Na(AOT) microemulsions, the presence of the Ca²⁺ counter-ion appears to result in the formation of larger reverse micelle aggregates, compared to with Na(AOT), with radii in agreement with published data. With the Ps yields, the model implies a semi-empirical parameter, A_trap, related to the probability that the energetic positrons end up in the aqueous subphase before forming Ps. As expected and experimentally verified, A_trap is proportional to r₀². Unexpectedly however, with the above value for h, the variation of A_trap with r₀² is found to depend on the counter-ion. Therefore, 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 r₀², independent of the counter-ion. In this case, however, the radii for the microemulsions containing Ca²⁺ are significantly lower than those found in previous work.