Enthalpies of proton dissociation of poly(acrylic acid). Comparison between experiment and theory for a polyelectrolyte system

Abstract

Enthalpies of proton dissociation δH_a of poly(acrylic acid) at three different temperatures have been determined as a function of the degree of dissociation α. Despite the apparent increase in pK_a with increasing α, the enthalpy of proton dissociation decreases with increasing α, reaching an almost constant value around α= 0.3, which possibly reflects that above this value of α ion condensation behaviour applies.

The experimental values are compared with a theoretical calculation of δH_a based on a solution of the Poisson—Boltzmann equation. The model correctly predicts that δH_a decreases with increasing α and the calculated δH_a is of the right order of magnitude and shows the correct α-dependence. The decrease in δH_a with increasing polyion charge is coupled with the anomalous temperature dependence of the dielectric permittivity of water.