ACID-BASE PROPERTIES OF POLY(PROPYLENE IMINE) DENDRIMERS

Potentiometric titration experiments of poly(propylene imine) dendrime rs (up to the fifth generation) were carried out at salt concentration s of 0.1, 0.5 and 1.0 M KCl and NaCl. The experiments were performed a t two different locations on different instruments and were converted to titration curves using two different methods, resulting in a consis tent experimental data set for the dendrimers measured. The titration curves feature two distinct steps around pH 6 and 10 with an intermedi ate plateau at 2/3 of the total ionizable groups. This protonation pat tern reflects short-ranged repulsive interactions between ionizable si tes and can be modeled using an Ising model with nearest-neighbour pai r interactions. The intermediate plateau results from the stability of an onion-like structure where all odd shells of the dendrimer are pro tonated, while the even ones remain deprotonated. The Ising model perm its a quantitative analysis of the titration curves. For larger dendri mers, this Ising approach is shown to be superior to the classical ana lysis in terms of successive protonation equilibria. (C) 1998 Elsevier Science Ltd. All rights reserved.