Reversible formation of iron(III) ion clusters in the poly(acrylic acid)–Fe3+ complex gel with changes in the water content

Abstract

The hydrogel made by adding poly(acrylic acid)(PAA) to iron(III) chloride solution has been investigated as a function of the water content of the gel by means of EPR, visible and infrared spectroscopies. The red–brown PAA–Fe³⁺ complex gel formed is homogeneous and transparent as is the solution, while the xerogel obtained by drying the gel was yellow. Iron(III) ions in the gel aggregate [as iron(III) ion clusters] and disperse (as mononuclear species) reversibly with repeated wetting with water and drying of the gel. In the hydrated gel PAA carboxyl groups bond strongly to the cluster iron(III) ions, thus networks of intermolecularly cross-linked PAA cause gelation, whereas in the dry gel the networks are loosened.