EFFECTS OF CCL4 ON POSITRONIUM FORMATION IN PURE ISOOCTANE AND IN AOTWATER/ISOOCTANE MICROEMULSIONS/

Previous work, on positron lifetime spectroscopy (LS) measurements on AOT/water/isooctane microemulsions showed that most positronium (Ps) i s formed in the aqueous subphase, and; predicted that a water insolubl e electron scavenger should inhibit much less Ps formation than if Pa was mainly formed in the organic solvent. To test this prediction, Pa inhibition by CCl4 is examined in pure isooctane and in the micellar s ystem. In the former case, from the combined analysis of LS and Dopple r 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 PsCCl(4) bound-state. As expected, CCl4 in the micel lar 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 obtaine d by assuming that, besides Ps inhibition in the organic solvent, CCl4 also promotes some inhibition of that fraction of Ps formed in the aq ueous 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 wi th 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 th e organic solvent. (C) 1997 Elsevier Science B.V.