Dissolution of Maize Starch in Aqueous Ionic Liquids: The Role of Alkyl Chain Length of Cation and Water:Ionic Liquid Ratio

The dissolution behavior of maize starch in water:ionic liquid (IL) mixtures at ambient temperature (22–23 °C) was studied. The ionic liquids used were 1-butyl-3-methylimidazolium chloride ([C 4 mim][Cl]), 1-propyl-3-methylimidazolium chloride ([C 3 mim][Cl]), and 1-ethyl-3-methylimidazolium chloride ([C 2 mim][Cl]). Structural analyses indicated that long- and short-range molecular order in the starch decreased with decreasing water:IL ratio. At water:IL ratios of 10:1 and 5:1, the extent of disruption of starch structure followed the order [C 4 mim][Cl] > [C 3 mim][Cl] > [C 2 mim][Cl]. At lower water:IL ratio (2:1), the complete disruption of starch granule morphology and ordered structures in water:[C 3 mim][Cl] and water:[C 2 mim][Cl] mixtures indicated these mixtures were more effective in dissolving starch than water:[C 4 mim]Cl mixture. Results from rheological, FTIR, and 1 H NMR analyses of water:IL mixtures showed that as water:IL ratio decreased, the viscosity of solutions increased, the interaction between IL and water decreased, and the interaction between the cation and the anion increased. Stronger interaction between the IL and water and higher viscosity of water:IL mixtures were noted for cations with longer alkyl chains. Our results clearly showed that both the alkyl chain length of cations and water:IL ratio played key roles in the dissolution of starch, predominantly by affecting the interaction between ILs and water and viscosity of water:IL mixtures.