Liquid-like Ordering of Negatively Charged Poly(amidoamine) (PAMAM) Dendrimers in Solution

A structural investigation in water solution of the sodium carboxylate-terminated (generation G3.5) Tomalia-type poly(amidoamine) dendrimers has been performed by means of the small angle X-ray scattering (SAXS) technique. A long-range intermolecular interaction, revealed by the presence of sharp peaks in SAXS spectra, gives evidence of a considerable structural order in the system, even at low concentration of the dispersed phase. The experimental interdendrimer structure factor S(q) was analyzed in the framework of the Ornstein-Zernike integral equation by using the hypernetted chain approximation (HNCA) as closure relation. The effective interdendrimer interaction, modeled as a screened Coulombic plus hard-sphere repulsion potential, allows the estimation of the dendrimers' effective surface charge Zeff. The present analysis strongly supports the findings that the effective intra- and interdendrimer charge interactions, as well as the dendrimer solution environment conditions, are crucial parameters for themodulation of the degree of structural organization in solution, suitable for a number of potential applications.