Effects of CCl 4 on positronium formation in pure isooctane and in AOT/water/isooctane microemulsions

Abstract

Previous work, on positron lifetime spectroscopy (LS) measurements on AOT/water/isooctane microemulsions showed that most positronium (Ps) is formed in the aqueous subphase, and predicted that a water insoluble electron scavenger should inhibit much less Ps formation than if Ps was mainly formed in the organic solvent. To test this prediction, Ps inhibition by CCl 4 is examined in pure isooctane and in the micellar system. In the former case, from the combined analysis of LS and Doppler broadening results, CCl 4 is found to strongly inhibit Ps formation ( k = 36.6 M −1) by the successive capture of e −, to form CCl 4 −, and of e +, to form the PsCCl 4 bound-state. As expected, CCl 4 in the micellar system displays a much lower ability to inhibit Ps formation ( k ca. 9 M −1). However, a very good agreement with the data is only obtained by assuming that, besides Ps inhibition in the organic solvent, CCl 4 also promotes some inhibition of that fraction of Ps formed in the aqueous subphase. Quantitatively, the low inhibition constant found for this process (5.1 M −1) is quite consistent with what is expected when considering the fraction of the aqueous positron spurs that overlap with the organic subphase. Another hypothesis is discussed, by which CCl 4 would decrease the probability that the positrons become trapped in the aqueous subphase, due to an increase in the electron density of the organic solvent.