Non-spherical micelles in the sodium dodecylsulfate–brine system. A fluorescence quenching and nuclear magnetic resonance study

Abstract

The size of elongated micelles in a system containing 2 wt.% sodium dodecylsulfate (SDS), 3.5 wt.% NaCl and 94.5 wt.% water has been studied by means of two different nuclear magnetic resonance (NMR) methods and time-resolved fluorescence quenching with special emphasis on a comparison between the results from the three methods. The two NMR methods were the determination of self-diffusion coefficients by means of the pulsed-gradient spin–echo method and the determination of relaxation rates for specifically deuterium labelled SDS. In the analysis of the former data, the obstruction effect due to the finite micellar concentration was taken into account. In the analysis of the relaxation data, the combined effect of lateral surfactant diffusion over the micellar surface and micellar rotational tumbling was taken into account in an exact manner for the case of prolates and in an approximate way for the case of hemisphere-capped rods. The results obtained from the three methods were in reasonable agreement. If the micelles are modelled as prolates, the axial ratios obtained was ca. 6 while if they are described as hemisphere-capped rods, the axial ratio is ca. 5.