Naphthalene as a fluorescent probe in micellar systems

Time-Resolved Fluorescence in Micellar Systems: A Critical Application to the Partitioning of Naphthalene in Aqueous Sodium Dodecyl Sulfate

“…However, previous lifetime data for naphthalene in CTAB, − or CTAC to which a small fraction of CTAB has been added, have shown that both lifetimes observed are distinctly shorter than that in a pure aqueous solution or those in the SDS or pure CTAC systems. From the theoretical consideration of the effects of such quenching presented here, and in view of the comparable drastic reduction of lifetimes observed in CTAB 3-5 and in CTAC with which only small amounts of CTAB are admixed, it would seem quite possible that a full nanosecond time-resolved examination of the aqueous naphthalene/CTAB system as a function of micelle concentration might indicate by constancy of one lifetime and linear micelle concentration dependence of the inverse of the other that in this heavily quenched system no exchange of excited - state naphthalene takes place, the equilibrium being confined to the ground-state fluorophore only. If this is indeed the case, it is obvious too, without going into specific details, that steady-state fluorescence measurements must yield an incorrect estimate for the ground-state equilibrium constant in such a 100% quenching efficiency system.…”

“…To examine quantitatively some basic characteristics of this behavior, two model systems with the following parameters will be discussed: (I) k m ≃ 0.0167 ns -1 (τ m = 60 ns), k w ≃ 0.0278 ns -1 (τ w = 36 ns), = 2 × 10 10 M -1 s -1 , = 1 × 10 6 s -1 , K = 2 × 10 4 M -1 , ε m /ε w = 1, k fm / k fw = 1 and (II) k m = 0.05 ns -1 (τ m = 20 ns), other parameters as in (I). As seen from the parameters chosen, the first model system should approximately simulate the behavior of naphthalene in aqueous SDS or CTAC micellar solutions, while the second should approximately simulate that in CTAB . The in-rate coefficient is taken to be on the order expected for the mutual diffusion coefficient of naphthalene and SDS micelles in water ( quod vide ), and the out-rate is set to obtain the same equilibrium constant in the excited state as in the ground state, K * = K .…”

“…Steady-state and time-resolved fluorescence spectroscopies have found extensive application in a variety of studies of aqueous micellar systems. , Among these, some considerable interest has focused on the partitioning of neutral arenes into detergent micelles, particularly of naphthalene into aqueous sodium dodecyl sulfate (SDS) and cetyltrimethylammonium chloride (CTAC) or bromide (CTAB). − In the fluorescence studies, since only rather small changes are noted in the emission (as also in absorption) spectral distributions of naphthalene between aqueous and hydrocarbon solvents, use has been made of the much larger changes observed in lifetime 3-5 and quantum yield. , …”

“…It was suggested that the component assigned to the aqueous environment is not resolvable in presence of the intense emission assigned to the micellar environment unless the latter is heavily quenched, as in CTAB or in CTAC or SDS to which additional bromide or cupric ions, respectively, have been added . On general grounds, however, it is desirable to obtain estimates of such parameters in as unperturbed a system as possible, particularly since the quenching of fluorophore emission in micellar systems can lead to rather complex, nonexponential decays under many circumstances ( e .…”