"Ordered" structure in ionic dilute solutions: Dendrimers with univalent and bivalent counterions - art. no. 051808

As an intermediate sample of ionic solutes between colloidal particles (mac roions) and simple electrolyte ions, we made small-angle x-ray scattering ( SAXS) measurements for aqueous Solutions of poly(amido amine) dendrimers of three generations (G4, G7, and G10). The SAXS curves of univalent acid Sol utions showed a single scattering peak, as observed for synthetic macroions . The peak position was dependent oil the dendrimer concentration but indep endent not only of the acid concentration (degree of protonation) but also of the counterion species. The effective charge density of the dendrimer de termined by conductivity Measurements was found to be insensitive to the ac id concentration and the counterion species, The nearest neighbor interpart icle distance 2D(exp) calculated from the peak position of the structure fa ctor of G7 and G10 was obviously smaller, though slightly, than the average interparticle distance 2D(0) calculated from molecular weights and concent rations of dendrimers, implying that acid solutions of dendrimers formed th e two-state structures by the attractive force. The ultra-small-angle x-ray scattering curve for the hydrochloric acid Solution did not show in upturn , which indicates the existence of large scale structural inhomogeneities s uch as localized ordered structures, probably due to the weak attraction an d hence less clear distinction of the ordered and disordered regions. For s ulfuric acid solutions, clear scattering peaks were not observed. The bival ent counterions were more strongly associated with the dendrimer ions than the univalent ones. The resulting low charge number of the dendrimers with the bivalent counterion was confirmed directly by the conductivity Measurem ents. These observations confirm that the counterion-mediated attraction do es exist even with the univalent counterions and point out that the frequen tly advanced claim that the effective potential is essentially repulsive wi th univalent counterions while attraction appears with bivalent counterions is not necessarily correct, It is noted that the intensity of the counteri on-mediated attraction in dendrimer solutions is dictated by both the effec tive charge density and the effective charge number, in contrast with macro ionic solutions or colloidal dispersions in which only the effective charge density appeared to be important.