Structural properties of methanol-polyamidoamine dendrimer solutions

We used small angle x-ray scattering to study interparticle structural properties of polyamidoamine starburst dendrimers (PAMAM, with an ethylenediamine central core) in methanol solutions. We have explored the intensity profiles for solutions of starbursts of different generations $(g=2,3.5,4)$ at various concentrations. We find from the measured spectra that the half-integer generation, $g=3.5,$ is characterized by pronounced interparticle correlations also in the very dilute regime. By means of a simple statistical-mechanical model, widely used in charged complex fluids (proteins and ionic micellar solutions), we calculate in this latter case the dendrimer-dendrimer structure factor $S\left(q\right).$ The results obtained give clear indication of a partial ionization process which involves the carboxylate $\left({\mathrm{COO}}^{\mathrm{-}}{\mathrm{Na}}^{+}\right)$ terminal groups of the dendrimer. The calculated macroion charge number, independent of the solution concentration, is $Z_0\simeq 6.$