Structure and phase behavior of poly(acrylic acid)–ferric ion complex aqueous solutions

Abstract

In this study, we investigate the conformational evolution and phase behavior of poly(acrylic acid) (PAA) solution upon the introduction of ferric ions through a combination of small angle X-ray scattering (SAXS), turbidity, zeta-potential and pH measurements. Salt-free PAA aqueous solution is a weak polyelectrolyte solution. The introduced ferric ion can coordinate with the carboxylic acid groups, yielding H⁺ ions to lower the pH value. We find two transitions with increasing concentration of the ferric ions: a polyelectrolyte to apparent good solution transition characterized by the disappearance of the polyelectrolyte peak in the X-ray scattering, and a phase separation characterized by a sharp increase of the turbidity. Detailed analyses of pH and zeta-potential reveal the molecular details of the three regions. Namely, (1) the polyelectrolyte region locates at log[H⁺] (= −pH) ≫ log(3[Fe³⁺]), where the H⁺ ions are mainly contributed from the dissociation of carboxylic acid, and the polymer chains are negatively charged, (2) the good solution region locates at log[H⁺] ∼ log(3[Fe³⁺]), where the H⁺ ions are mainly yielded from coordination between COO⁻ and Fe³⁺, and polymer chains are nearly neutralized, and (3) the phase separation locates at log[H⁺] ≪ log(3[Fe³⁺]), where the Fe³⁺ ions are not fully coordinated, and charge inversion occurs. The phase separation occurs when the chains are densely and tightly coordinated with Fe³⁺ ions.