Exchange mechanisms for sodium dodecyl sulfate micelles : high salt concentration

International audienceSolute exchange experiments for the pyrene-labeled triglyceride TG-Py solubilized in sodium dodecyl sulfate (SDS) micelles in the presence and absence of salt show that the “observed” rate constant kobs for solute exchange varies by over 6 orders of magnitude as the free sodium ion concentration [Na+]aq is varied between 10 and 850 mM. There is a sharp break in the log−log plot of kobs versus [Na+]aq in the range of [Na+]aq = 200 mM, with the exchange rate showing a weaker dependence on [Na+]aq above this concentration. Up to 100 mM added NaCl, this exchange takes place essentially exclusively by a micelle fission mechanism in which each submicelle carries off one of the solutes. At higher salt concentrations, a bimolecular process becomes increasingly important. This fusion process, which involves formation of a transient supermicelle followed by fission back to two normal micelles, becomes the dominant process at high salt concentrations. The fission rate appears to level off for salt concentrations above 300−400 mM. These fission and fusion processes are related in an intimate way to the changes in the size and shape of the SDS micelles with increasing salt concentration