The aggregation numbers of the surfactant aggregates in surfactant-containing systems can be measured using the steady-state fluorescence quenching (SSFQ) and the time-resolved fluorescence quenching (TRFQ) methods. From a theoretical point of view the SSFQ method involves much more restrictive assumptions than the TRFQ method. As a result, the range of aggregation numbers that can be measured with SSFQ does not extend to values as high as with TRFQ. Nevertheless, SSFQ is much used because the measurements are easy to perform and only require readily available spectrofluorometers. The purpose of this study was to compare as completely as possible the two methods. For this purpose we have performed systematic determinations of the values of the surfactant aggregation number in micellar solutions in pure water and in water + polymer and oil-in-water microemulsions, using SSFQ and TRFQ. A good agreement between the two methods is observed when the quenching rate constant of the fluorescent probe (here pyrene) is at least 5 times larger than the decay rate constant of the probe in the presence of air. For lower values of this ratio, the discrepancy between the two values increases very rapidly and examples are given where SSFQ yields values up to 4 times smaller than TRFQ. The discrepancy between the two methods also increases with the microviscosity of the aggregate. The results call for caution when using SSFQ, particularly for systems where the microviscosity is expected to be large and the aggregation numbers are not very small.

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