THE INFLUENCE OF NEAREST-NEIGHBOR AND NEXT-NEAREST-NEIGHBOR INTERACTIONS ON THE POTENTIOMETRIC TITRATION OF LINEAR POLY(ETHYLENIMINE)

Potentiometric titrations of linear poly(ethylenimine) were performed in the presence of, as well as in the absence of, added NaCl. In the NaCl concentration range of 0.1-1.0 M, the titration curves, with an inflexion point at beta = 0.5, have a total pH change of about 6 units and a shape which was independent of the NaCl concentration. The potentiometric behavior of LPEI in an excess of added salt is explained by taking into account the nearest-and next-nearest-neighbor interactions between charges on the LPEI chain. The experimental titration curves of LPEI solutions in 0.1 M NaCl, 0.5 M NaCl, and 1.0 M NaCl could well be described by the set of potentiometric equations derived from a statistical mechanical treatment of the dissociation equilibrium, based on the Ising model with three parameters: the intrinsic dissociation constant K0 and the excess energies epsilon(d) and epsilon(t) determined by the nearest and next-nearest-neighbor interactions, respectively. The intrinsic viscosity of LPEI in 0.2 M NaCl, 0.5 M NaCl, and 1.0 M NaCl showed a strong charge density dependence, which is consistent with the assumption of nearest- and next-nearest-neighbor interactions between the charges on the chain.