MICELLE AGGREGATION NUMBERS OF SURFACTANTS IN AQUEOUS-SOLUTIONS - A COMPARISON BETWEEN THE RESULTS FROM STEADY-STATE AND TIME-RESOLVED FLUORESCENCE QUENCHING

The aggregation numbers of the surfactant aggregates in surfactant-con taining systems can be measured using the steady-state fluorescence qu enching (SSFQ) and the time-resolved fluorescence quenching (TRFQ) met hods. From a theoretical point of view the SSFQ method involves much m ore restrictive assumptions than the TRFQ method. As a result, the ran ge of aggregation numbers that can be measured with SSFQ does not exte nd to values as high as with TRFQ. Nevertheless, SSFQ is much used bec ause the measurements are easy to perform and only require readily ava ilable spectrofluorometers. The purpose of this study was to compare a s completely as possible the two methods. For this purpose we have per formed systematic determinations of the values of the surfactant aggre gation number in micellar solutions in pure water and in water + polym er and oil-in-water microemulsions, using SSFQ and TRFQ. A good agreem ent between the two methods is observed when the quenching rate consta nt of the fluorescent probe (here pyrene) is at least 5 times larger t han the decay rate constant of the probe in the presence of air. For l ower values of this ratio, the discrepancy between the two values incr eases 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 c all for caution when using SSFQ, particularly for systems where the mi croviscosity is expected to he large and the aggregation numbers are n ot very small.