Time-Resolved Fluorescence Quenching Measurements of the Aggregation Numbers of Normal Sodium Alkyl Sulfate Micelles Well above the Critical Micelle Concentrations

The aggregation numbers, N A, of normal sodium alkyl sulfate micelles were measured by time-resolved fluorescence quenching in order to test two aspects of a recent proposal by Quina et al. ( J. Phys. Chem. 1995, 99, 17028) that these micelles grow as a power law of the total counterion concentration, [Na + ] aq, in the aqueous phase according to N A = κ 2 ([Na + ] aq ) γ, where κ 2 and γ are constants for a given detergent. Both the surfactant and any added salt contribute to [Na + ] aq; thus the power law predicts that different combinations of surfactant and salt concentrations leading to the same value of [Na + ] aq should yield the same value of N A . This was confirmed, and the values of κ 2 were determined.