Characterization of Starburst Dendrimers by EPR. 3. Aggregational Processes of a Positively Charged Nitroxide Surfactant

Abstract

The aggregation characteristics of aqueous solutions of a positively charged nitroxide surfactant (CAT16) in the presence and absence of half-generation polyamidoamine starburst dendrimers (n.5-SBDs) have been investigated by electron paramagnetic resonance (EPR). Computer simulation of the EPR spectra allowed the convenient extraction of several parameters that were related to the supramolecular structure of the aggregates formed by CAT16 and SBDs. From examination of the EPR spectra as a function of variation of the concentration of CAT16, the concentration of SBDs, and the ionic strength and application of the EPR parameters available from simulation of the spectra, a paradigm for the structure and dynamics of the aggregates formed by CAT16 in the presence and absence of SBDs under various conditions is deduced. A study of the fluorescence quenching of pyrene in the presence of CAT16 is compared to a previous investigation of the interaction of SBDs with cationic surfactants. It is concluded that at low SBD concentration, for earlier generation SBDs (G < 3.5), whose size is smaller than or comparable to the size of the CAT16 micellar aggregates, the SBDs act as “guests” that bind to the micelles that serve as “hosts”. In contrast, at low SBD concentration of the later generation SBD (G > 3.5), the size of the SBD is now larger than that of the micelles so that the latter can serve as “guests” for the former. A bilayer aggregate of the surfactant on the SBD is proposed. Finally, at high concentration of the later generation SBD, it is proposed that because of the large number of sites compared to the number of surfactants, an aggregate in which two or more SBDs are bridged by bilayers is formed by the surfactant and coexists with CAT16 micelles