An NMR relaxivity and ESR study of the interaction of the paramagnetic manganese(II) and gadolinium(III) ions with anionic, cationic and neutral water-soluble polymers and their mixtures

Abstract

© 2017 Elsevier B.V.Solutions of the series of polymers (sodium polystyrene sulfonate, polyethyleneimine, and poly-N-vinylpyrrolidone) and certain mixtures thereof were studied by NMR relaxation and ESR spectroscopy using manganese(II) and gadolinium(III) ions as paramagnetic probes. The growth of the NMR-relaxation efficiency (relaxivity, R1,2) is observed in solutions of the anionic polymer, as well as the decrease of R1,2 - in the case of the cationic polymer. For mixtures of polymers, the dependent on the solution composition non-monotonic changes of R1,2 are caused either by a formation of metal-interpolymer complexes or by a competitive cation/polymer substitution. For both ions, the changes in the ESR spectra of their polyelectrolyte solutions were analyzed. In case of sodium polystyrene sulfonate and blends thereof, the ESR spectra parameters are only slightly broadened compared to the pure metal aqua ion's, whereas for the polyethyleneimine solution the ESR spectra lines broaden greatly at pH > 7 up to their disappearance. All these effects were explained by the distinction of ion probes’ binding mode to polyelectrolytes of different nature. Addition of NaCl to manganese(II) solutions containing polystyrene sulfonate, or mixtures thereof with PVP or PEI, lead to recovery of R1,2 values, close to the probes’ aqua ions relaxivities due to the release of the latter from the polymer/medium interface into the bulk water, while no changes were detected for PEI solutions.