Effects of CCl4 on positronium formation in pure isooctane and in AOT/water/isooctane microemulsions Original Research Article

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 CCl4 is examined in pure isooctane and in the micellar system. In the former case, from the combined analysis of LS and Doppler broadening results, CCl4 is found to strongly inhibit Ps formation (k = 36.6 M−1) by the successive capture of e−, to form CCl4−, and of e+, to form the PsCCl4 bound-state. As expected, CCl4 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, CCl4 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 CCl4 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.