Interaction between starburst dendrimers and SDS micelles studied by continuous-wave and pulsed electron spin resonances

A computer aided analysis of both cw-and pulsed-Electron Paramagnetic Resonance (EPR) spectra of 5doxylstearic acid (5DXSA) as a probe was carried out to compare the aggregation process of Sodium Dodecyl Sulfate (SDS) surfactants in the absence and in the presence of Starburst Dendrimers (SBDs), and to provide information on the interactions between SDS and SBDs. Mobility and polarity parameters were extracted from the cw-EPR analysis, whereas the analysis of the Electron Spin Echo Envelope Modulation (ESEEM) signal provided details about the doxyl environment in the SDS micelles. In the absence of SBDs, the formation of SDS micelles was revealed by the decrease in mobility of the probes inserted in the micelles. The high packing of SDS chains in the micelles prevented the water permeability at the doxyl site. In the presence of the dendrimers, the analysis of the EPR spectra suggested the formation of SDS aggregates at the dendrimer surface (cooperative interaction). The larger the size of the dendrimers and the protonation of their surface, the stronger the interactions resulted between the SDS surfactants and the SBD surface. The analysis of the ESEEM pattern indicated that the cooperative interaction of the surfactant with the SBD surface led to a less packed structure of the aggregates. A schematic view was proposed to describe the local structure of the doxyl group and its environment in the absence and in the presence of the dendrimers.