Polyelectrolyte behavior of an interpolyelectrolyte complex formed in aqueous solution of a charged dendrimer and sodium poly(L-glutamate)

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, Phi, of GPE to HPE. From the overall degree of protonation, beta, at a given pH and Phi, the degree of conversion, Theta(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 Theta. As soon as the GPE becomes partly charged below pH 11, both 0 and the turbidity, which are proportional to the molecular weight, increase linearly with the decreasing pH, showing that the initial increase in Theta is due to an increased number of bound GPEs. From the increase in Theta to well above 0.5, it is likely that the linear HPE penetrates partly into the interior shells of the dendrimer GPE.