DYNAMICS OF THE QUENCHING OF PYRENE FLUORESCENCE BY THE THIOSULFATE ION IN CATIONIC MICELLES

Time resolved fluorescence decay measurements with pyrene as probe have been employed to study the dynamics of intramicellar quenching and exit of the thiosulfate counterion (S2O32-) from micelles of hexadecyltrimethylammonium chloride (CTACl) over a wide range of concentration of CTACl and added common counterion salt (NaCl). The rate constant k(q) for intramicellar quenching of an excited pyrene in a micelle containing a single S2O32- quencher counterion is 9 (+/-2) 10(6) s(-1), independent of either [CTACl] or [NaCl]. The rate constant k for micellar exit of S2O32- is governed by the micellar surface potential, the effective free energy barrier for escape of S2O32- from the CTACl micelle being equivalent to about half of the overall electrostatic work for complete transfer of the thiosulfate ion from the micelle surface to the aqueous phase.