The binding of the protonated fourth generation poly(amido amine) dendrimer as a guest polyelectrolyte (GPE) to the anionic homopolyelectrolyte poly(l-glutamic acid) sodium salt as a host polyelectrolyte (HPE) has been investigated by potentiometric and turbidimetric titration in aqueous 0.25 M NaCl solutions. The polyelectrolyte behaviors of interpolyelectrolyte complex (IPEC)-forming mixtures were compared with those of hypothetically noninteracting individual polyelectrolytes (PEs) at different stoichiometric ratios, Φ, of GPE to HPE. From the overall degree of protonation, β, at a given pH and Φ, the degree of conversion, ϑ(pH), of oppositely charged groups to ion pairs with released counterions has been calculated, as well as the thermodynamic dissociation constant, K D, of an ion pair in the IPEC. A high cooperativity of the binding of the ammonium groups of the GPE to the HPE is concluded from the strong linear increase in the pK D with ϑ. As soon as the GPE becomes partly charged below pH 11, both ϑ and the turbidity, which are proportional to the molecular weight, increase linearly with the decreasing pH, showing that the initial increase in ϑ is due to an increased number of bound GPEs. From the increase in ϑ to well above 0.5, it is likely that the linear HPE penetrates partly into the interior shells of the dendrimer GPE.

The size and molecular mass of water-soluble interpolyelectrolyte complex (PEC) formed between poly(l-glutamate) and poly(amido amine) dendrimer in aqueous solution were estimated as a function of the decreasing pH from the turbidities (τ) and their wavelength dependence, by comparison with numerical τ calculations by applying the Rayleigh−Debye−Gans and Mie scattering theories. Close to the pK a of the peripheral amino groups of the dendrimer, PECs with densities comparable to that of fractal clusters from a reaction limited colloid aggregation process collapsed to dense particles with ∼1 μm diameter. At a pH close to the pK a of the amino groups in the interior of the dendrimer, systems with carboxylate excess over amino/ammonium groups formed compact particles, and ion pairing likely extended to the ammonium groups in the dendrimer interior. PECs with the highest polymer density were formed at the stoichiometric ratio:  They appeared to be stable against dissociation at pH 3 in contrast to PECs formed with high dendrimer excess.