Two distinct regions in micellar aggregates identified with pyrene-labeled dendrimers.

This study challenges the basic representation that micellar aggregates (MAs) are simple assemblies of ionic surfactants held together by their hydrophobic tails and stabilized by their charged headgroups by demonstrating that they exhibit specific surfactant microdomains. These microdomains are the result of local phase separation between charged and partially neutralized with sodium chloride surfactants inside the same MA of SDS. They were characterized with pyrene-labeled polyamidoamine dendrimers. The charged and partially neutralized surfactants were found to constitute the polar edge and apolar middle regions of MA, respectively, among which the dendrimers would spatially partition themselves. The fluorescence response of the dendrimers was directly related to the volume fraction of the edge region calculated by assuming that the MAs were cylinders whose edges were capped with two identical hemispheres consisting of 35 charged SDS molecules. The fact that merging the two hemispheres generates a spherical SDS micelle led to a general mechanism rationalizing the formation of MA.