Size invariance of polyelectrolyte dendrimers

To quantify the swelling response of polyelectrolyte dendrimers in aqueous solutions as affected by changes in solution conditions, we have studied ei ghth generation, G8, poly(amidoamine) (PAMAM) dendrimers in D2O using small -angle neutron scattering. The charge density is manipulated by varying the pH of the solution in order to protonate and deprotonate the primary and t ertiary amines. The coherent scattered intensity at high q values yields si ze information from the form factor of individual dendrimers. The distance distribution functions of the dendrimers in a variety of experimental situa tions were determined by indirect Fourier transform. For all the conditions studied, we find that the size of G8 dendrimers is essentially insensitive to variations of the pH (4.7 < pH < 10.1) and of ionic strength (up to 3 M of NaCl). In all cases, the scattering curves yield density profiles that correspond well to a uniform sphere with a radius of gyration R-G = 4.0 +/- 0.15 nm. We conclude that the dendrimer size is essentially independent of the charge density or ionic strength of the solvent, contrary to the predi ctions of current models and simulations.