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

The aggregation numbers, NA, 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 NA = κ2([Na+]aq)γ, where κ2 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 NA. This was confirmed, and the values of κ2 were determined.