Model for the interactions between anionic dendrimers and cationic surfactants by means of the spin probe method

The supramolecular structures formed when anionic polyamidoamine starburst dendrimers, in the presence of low concentrations of spin probe surfactants, are added to aqueous solutions of nonlabeled cationic surfactants have been investigated by EPR spectroscopy. In order to obtain an overview of dendrimer/surfactant systems, a variety of spin probe surfactants, differing from one another in chain length, structure, polarity, charge, solubility, and self-aggregating ability, were employed. Computer simulation of the experimental EPR spectra allowed evaluation of mobility and polarity parameters of the spin probes in the supramolecular structures formed in addition to determination of the degree of partitioning of the probe among various supramolecular structures. Depending on the concentrations of both the surfactant and the dendrimer, the dendrimer size, and temperature, the model envisions two types of structures, i.e., primary structures consisting of probe monomers adsorbed on the dendrimer surface and secondary structures consisting of probe monomers adsorbed in surfactant aggregates bound to the dendrimer surface. Depending on the probe concentration and on the probe solubility into the surfactant aggregates, line broadening was observed, which-was consistent with spin-spin interactions supporting a model in which more than one probe was inserted in the aggregates. This solubility was enhanced by the presence of dendrimers and by increasing temperature.