Ionization of short weak polyelectrolytes: when size matters

The ionization of short weak polyelectrolytes in dilute solutions is investigated by 1H nuclear magnetic resonance (NMR). The increase of the pH of the medium causes both the ionization of the polyacids and, due to the intra-molecular repulsions, the swelling of the chains. As a result, a noticeable effect of the pH change on the NMR chemical shift δ, and on the self-diffusion coefficient D is observed. The ionization of the polyelectrolyte occurs at higher pH as the length of the chain increases. The variation of the self-diffusion coefficient with pH exhibits the opposite dependence on the length of the chain. However, no detectable effect of the length of the polyelectrolyte chain on the evolution of the chemical shift with pH is observed. These apparent contradictions are examined to clarify the impact of the chain length on the polyelectrolyte properties, and the counterion role in the ionization process.