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 = kappa(2)([Na+](aq))(gamma) where kappa(2) and gamma 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+], should yield the same value of N-A. This was confirmed, and the values of kappa(2) were determined.