Novel hydrogels composed of malic acid and zinc: their synthesis and characterization

Zinc/malic acid (Zn/MA) carboxylate gels were synthesized by adding a NaOH solution to solutions containing ZnCl2 and MA at room temperature, without adding a NH4OH solution. Glassy hydrogels with the transparency of 20.0–88.2% and with high viscosity were produced by a polymerization reaction in which the carboxyl and hydroxyl groups of MA were directly chelated by zinc ions. The gelation occurred in a narrow pH range as well as a narrow zinc concentrations range at 0.5 mol/l MA concentration. The Raman spectra showed that a new Raman line appeared at 881 cm−1 and we assigned it to the Zn–O–Zn vibration, and a red shift of the carboxyl and hydroxyl groups of MA. Rheological measurement showed the highest values of the storage and loss modulus when pH of the solution was raised to 7.0, and that the increase in the zinc ion concentration increased the storage and loss modulus increased due to the greater density of chelation. The linear correlation of log (G∗) and log (ω) indicated that the glassy hydrogels had a gel-like three-dimensional network.