| Due to the unique physicochemical properties and high structure designability of ionic liquids?ILs?,the usage of ILs as new green starch solvents has become a research frontier and hotspot of starch science,since ILs can regulate phase transitions of starch,promote chemical modifications of starch,and endow starch with better application properties.By reviewing the research progress,development trend and related scientific issues of ILs as starch solvents,we have constructed green and safe choline amino acid?[Cho][AA]?ILs from natural raw materials,and applied these[Cho][AA]ILs into starch based on the requirements of food safety.By a thorough illustration of microstructure evolution of[Cho][AA]IL solvent system,the molecular regulation mechanism of[Cho][AA]IL solvents on the thermal phase transition and corresponding molecular chain aggregation behavior of starch was disclosed.And a systematic and effective method of using AA-anions and water of[Cho][AA]IL solvents to synergistically regulate starch molecular chain aggregation behavior evolution,thus starch thermal phase transition and properties of starch-[Cho][AA]-water solution systems was established.All these investigations provide new research ideas and technical supports of[Cho][AA]ILs as a real green,safe and efficient starch solvent,and of promoting high value utilization of starch.The details and main conclusions are shown in the following.[Cho][AA]ILs with high purity was synthesized by a simple neutral reaction between choline hydroxide and six amino acids,of which glycine?Gly?and serine?Ser?were chosen as neutral anions,arginine?Arg?and lysine?Lys?were chosen as basic anions,aspartic acid?Asp?and glutamic acid?Glu?were chosen as acid anions.By using density functional theory?DFT?methods,the effect of side chain length and functional groups of AA-anions on the microstructure and interaction mode and intensity of[Cho][AA]ILs was revealed.Results showed the[Cho]2[AA]2 dimer interaction models can better explain the properties of[Cho][AA]ILs.The interaction intensity of ion pair,and properties?thermal decomposition temperature,glass transition temperature,and viscosity?of[Cho][AA]ILs with different anions followed the order of[Cho][AA]ILs with acid anions>[Cho][AA]ILs with basic anions?[Cho][AA]ILs with neutral anions.Compared with water,[Cho][AA]ILs with acid anions suppressed thermal phase transition of starch,while[Cho][AA]ILs with neutral and basic anions either enhanced or suppressed thermal phase transition of starch.The regulation effect of[Cho][AA]ILs on thermal phase transition,and molecular chain aggregation behavior of starch,as well as the structure and rheological properties of the starch-[Cho][AA]-water solution systems was not achieved by degradation or derivation of starch,but achieved by breaking hydrogen bonds between starch own molecules,and suppressing the interwinding of starch chains due to the formation of hydrogen bonds between ions and starch molecule.During this regulation process,the structure of AA-anions played a vital role.The high viscosity of[Cho][AA]ILs with acid anions resulted in more starch molecular chain aggregation and winding,thus the lowest correlation length,highest viscosity and strongest pseudo-plasticity of solution systems.While for[Cho][AA]ILs with basic anions,high basicity could further suppress the aggregation and interwinding of starch chains,thus its solution systems had low viscosity and pseudo-plasticity and more homogeneous sub-microstructure.Moreover,the ratios of water to ionic liquid?w:ILs?also significantly affected thermal phase transition and molecular chain aggregation behavior of starch.With the increase of IL ratios of[Cho][AA]ILs with neutral and basic anions,thermal phase transition of starch followed the pattern of being“suppressed-enhanced-suppressed”;yet within whole range of w:ILs,[Cho][AA]ILs with acid anions suppressed thermal phase transition of starch.In the w:IL-9:1 and w:IL-7:3 solvent systems,the interactions between water and starch dominated,and the increasing IL ratio would promote molecular chain aggregation behavior of starch,leading to the increased pseudo-plasticity and elastic gel structure of its solution system.However,in the w:IL-5:5 and w:IL-4:6 solvent systems,IL ions and water synergistically interacted with starch,and the increasing IL ratio would suppress the molecular chain aggregation behavior of starch,enhance the fluidity and favor the“gel-sol”transformation of its solution systems.In the w:IL-2:8 solvent system,the interactions between IL ions and starch dominated,the mode of starch phase transition changed from endothermic gelatinization to exothermic dissolution,starch granules were dissolved slowly without swelling.The microstructure evolution of[Cho][AA]-water solvent systems with different w:ILs,and the interaction mode and intensity of starch-[Cho][AA]-water solution systems were explored by second derivative infrared spectrum and DFT methods.And successively,a model for understanding the regulation of w:IL and anion structure of ILs on thermal phase transition and molecular chain aggregation behavior of starch was established.It was revealed at molecular level that[Cho][AA]microstructure underwent a“three dimensional network structure-ion cluster-intimate ion pair-water separated ion pair-free solvated ions”dynamic process.And it turned out that the formation of intimate ion pair and water separated ion pair microstructure enhanced[Cho][AA]ions interacting with starch thus thermal phase transition of starch.Also,as for different AA-anions,the strong interaction between its own ions of[Cho][AA]ILs with acid anion and its more ordered surrounding water structure suppressed the thermal phase transition of starch.While the strong interaction between ions and starch of[Cho][AA]ILs with neutral and basic anions enhanced the thermal phase transition of starch,and more intense of this interaction,more enhancement effect of[Cho][AA]ILs.In summary,this work firstly involved in-depth investigation of the scientific problem regarding the application of[Cho][AA]ILs as solvents to regulate thermal phase transition and molecular chain aggregation behavior of starch.The results show high innovation,academic value and practical significance,which provides theoretical supports of promoting application of[Cho][AA]ILs as green and safe starch solvents,technical supports and basic data for the design of[Cho][AA]solvent systems with the regulation of starch thermal phase transition to meet different application requirements,and a new way of high value utilization of starch. |
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