TIME-RESOLVED FLUORESCENCE IN MICELLAR SYSTEMS - A CRITICAL APPLICATION TO THE PARTITIONING OF NAPHTHALENE IN AQUEOUS SODIUM DODECYL-SULFATE

Static and dynamic aspects of the partitioning of naphthalene into the hydrocarbon phase in aqueous sodium dodecyl sulfate micelles have bee n critically examined by nanosecond time-resolved pulse fluorometry. I t is shown that, in such a system, a properly detailed analysis of the excited-state decay behavior as a function of progressive partitionin g of the solute into increasing concentrations of micelles is capable of revealing and directly quantitating the underlying kinetics of the reversible transfer of naphthalene in its excited state between aqueou s and micellar phases. The time-resolved data indicate that, in the aq ueous sodium dodecyl sulfate system, the partition coefficient for exc ited-state naphthalene can differ only marginally from that for the gr ound state. From the estimate obtained for the rate coefficient for en try of excited-state naphthalene into micelles, it would appear that a ny barrier to the crossing of excited naphthalene into the micelle, gi ven that a collision has taken place, is likely to be rather small. Th e contrasting time-resolved behavior of a system in which the basis li fetime of the fluorescence probe in the micellar environment is shorte r than that in the bulk environment rather than longer, as in the case examined experimentally, is also modeled. The effects of quenching of the fluorescence at the interface between the micellar and aqueous ph ases are also examined and discussed in the context of the aqueous cet yltrimethylammonium bromide system.