Zinc chloride aqueous solution as a solvent for starch

It is important to obtain starch-based homogeneous systems for starch modification. Regarding this, an important key point is to find cheap, low-cost and low-toxicity solvents to allow complete dissolution of starch and its easy regeneration. This study reveals that a ZnCl₂ aqueous solution is a good non-derivatizing solvent for starch at 50 °C, and can completely dissolve starch granules. The possible formation of a "zinc-starch complex" might account for the dissolution; and the degradation of starch, which was caused by the H⁺ inZnCl₂ aqueous solution, could not contribute to full dissolution. From polarized light microscopic observation combined with the solution turbidity results, it was found that the lowest ZnCl₂ concentration for full dissolution was 29.6 wt.% at 50 °C, with the dissolving time being 4 h. Using Fourier-transform infrared (FTIR), solid state ¹³C nuclear magnetic resonance (NMR), and X-ray diffraction (XRD), it was revealed that ZnCl₂ solution had no chemical reaction with starch glucosides, but only weakened starch hydrogen bonding and converted the crystalline regions to amorphous regions. In addition, as shown by intrinsic viscosity and thermogravimetric analysis (TGA), ZnCl₂ solution caused degradation of starch macromolecules, which was more serious with a higher concentration of ZnCl₂ solution.