The mechanism of dispersion of starch by urea and by guanidinium and lithium salts

The ability of various aqueous salt solutions to disperse high‐amylose corn starch (70 % apparent amylose) was examined. The results were interpreted in light of a proposed theory concerning the structure and properties of hydrated ions and molecules. In those solutions where both cation and anion have negative hydration (possess only B‐regions) such as guanidinium thiocyanate, the solubility of starch continues to increase to the saturation point of the salt. On the other hand, if the cation has positive while the anion has negative hydration, then the solubility of starch goes through a maximum (at 6 M LiBr or 6 M LiSCN). It is proposed that only the negatively hydrated domains of a salt disperse starch. The negatively hydrated domains of an anion or cation disperse starch granules in one of two ways. If the effective dielectric constant of the ion is greater than that of water, then the dispersion occurs by swamping out the electrostatic forces involved in a hydrogen bond. If the effective dielectric constant of the negatively hydrated ion is less than that of water, then the ion can disperse starch granules only when it becomes unhydrated as in the case of 9 M LiCl. In other words, dispersion occurs by hydrating the hydroxyl groups to the ions. Such electrostatic interactions may be involved in some proteinpolysaccharide complexes.