Quantification of hydrolytic charge loss of DMAEA-Q-based polyelectrolytes by proton NMR spectroscopy and implications for colloid titration

Copolymers of acrylamide and quaternised dimethylaminoethyl acrylate (DMAEA-Q) constitute an economically important range of cationic polyelectrolytes used in sludge conditioning. The latter treatment involves charge neutralisation and bridging induced by these polymers. Since both of these phenomena rely on charge-driven sorption onto the negatively charged colloidal particles, the accurate assessment of their charge density is of primary importance in polyelectrolyte characterisation. The experimental determination of this characteristic generally relies on colloidal charge titration, in which the cationic polymer is titrated against an anionic polymer. Hereby, one of the requirements to have a stoichiometric reaction between the oppositely charged polymers is a sufficiently low polymer concentration. In this study, it is shown that such a low polymer concentration may entail a pronounced hydrolysis effect for DMAEA-Q-based polymers, which leads to a release of the cationic side groups and hence causes considerable errors on the charge titration results. Proton nuclear magnetic resonance spectroscopy was applied to investigate the fast hydrolysis kinetics of DMAEA-Q polymers together with time-dependent charge titration measurements. Diffusion NMR spectroscopy was used to assist in establishing the nature of the hydrolysis compounds. The results from both techniques indicate that a high degree of hydrolysis is reached within minutes after dilution of a concentrated polymer stock solution into aqueous solutions of slightly acidic to neutral pH values. Therefore, a modification to the classic colloid titration procedure is proposed, using a buffered dilution liquid to avoid polymer hydrolysis. It is shown that a buffer pH value of 4.5 avoids not only polymer hydrolysis effects but also possible protonation of the anionic titrant, thereby avoiding overestimation of the charge density. By means of this procedure, reproducible and time-independent charge titration results are obtained.