Size of sodium dodecyl sulfate micelle in concentrated salt solutions

We investigate the effect of high salt concentration on the average aggregation number of sodium dodecyl sulfate (SDS) over the temperature range 30-70 °C. The method is based on the increase of self-quenching of the fluorescence of micelle-solubilized pyrene through excimer formation. Transient fluorescence decay is measured and analyzed. It is shown that an exponent weighted averaged aggregation number 〈n〉_e is obtained by this technique; it is smaller than weight average aggregation number for a polydisperse micelle system. For SDS in NaCl solution, we observe an increase of 〈n〉_e as the temperature is lowered and ionic strength is increased. We use the thermodynamic model developed by Missel et al. to calculate 〈n〉_e. Agreements between theory and our experimental results are quantitative.