Phase transition of maize starch in aqueous ionic liquids: Effects of water:ionic liquid ratio and cation alkyl chain length

The thermal phase transition behavior of maize starch in water:ionic liquid (IL) mixtures was investigated. With decreasing water:IL molar ratio to 10:1, the endothermic transition shifted to higher temperatures, and then to lower temperatures at 5:1 water:IL ratio. At 2:1 water:IL ratio, an exothermic transition occurred at a lower temperature than gelatinization temperature of starch in pure water. At the same water:IL ratios (35:1 to 5:1), the endothermic transition temperatures of starch increased with decreasing alkyl chain length of the cation, whereas an opposite trend was found for the exothermic transition at 2:1 water:IL ratio. Rheological, H-1 NMR and FTIR analyses of water:IL mixtures showed that with decreasing water:IL ratio, the viscosity of water:IL mixture and the interactions between cation and anion increased, whereas the interactions between IL and water increased and then decreased. The endothermic transition of starch in water:IL mixtures of 35:1 to 5:1 was affected by the water availability for gelatinization. However, the exothermic transition of starch in water:IL mixtures of 2:1 was mainly caused by the interactions between IL and starch and viscosity of water:IL mixtures.