Multinuclear NMR studies on the effect of electrostatic and hydrophobic interactions on bindings to counterions to weakly acidic and basic polyelectrolytes

Abstract

23Na, 35Cl, 81Br, and 87Rb NMR have been applied to clarify the influence of electrostatic and hydrophobic interactions on the counterion binding equilibria of weakly acidic and basic polyelectrolytes with negatively and positively charged polyions, respectively. Na+ and Rb+ ions capable of binding to weakly acidic polymers (i.e., polyacrylate acid (PAA)), and Cl− and Br− ions capable of binding to weakly basic polymers (i.e., poly(N-vinylimidazole) (PVIm) and polyallylamine (PAAm)) were used as representative examples to measure these interactions. The results revealed that the longitudinal and transverse NMR relaxation rates of the Cl−, Br− and the Rb+ counterions in the PVIm and PAA systems were faster than those of free Cl−, Br− and Rb+ ions when the polyions were completely neutralized. The binding interactions between the Cl−, Br− and the Rb+ ions and PVIm and PAA were found to be hydrophobic in nature because the magnitudes of these effects were found to be dependent on the hydrophobicities of the polyelectrolytes and the counterions. Although a slight hydrophobic interaction was detected between the Rb+ ions and PAA, where the degree of neutralization of the polyacid (α) was 1, Na+ ions also started to bind to PAA at α = 0.65, where the linear charge density corresponded to the Bjerrum length.