Studies On The Dissolution Of Chitosan In Ionic Liquids

Chitosan is a renewable and abundant carbohydrate in nature. However, it is a challenge that the application of chitosan is limited in many fields, due to its inefficient dissolution in many common organic solvents. Ionic liquids (ILs) exhibit unique physicochemical properties such as virtually no vapor pressure, non-volatility, non-flammablity, tunable structure and properties. Thus using ILs as new and green media to dissolve chitosan is attracting the attention of a growing number of scientists and engineers in recent years. However, the mechanism for the dissolution of chitosan in ILs and the structure of the regenerated chitosan with different precipitants are poorly undrestand. Therefore, as a part of the research project supported by the National High Technology Research and Development Program (863 program, No.2007AA05Z454), in the present work, we designed and synthesized a series of ILs with different structures of cation and anion, and determined the solubilities of chitosan in these ILs. Then, the influence of structures of the ILs on the solubilities of chitosan was examined, and the possible mechanism for the dissolution of chitosan in ILs was analyzed. At the same time, the appearance and thermal properties of the regenerated chitosan were studied in some details. The major content of the present work is as follows.1. Based on the relationship between structure and properties, a series of ILs have been designed and synthesized. The cations of these ILs include 1-butyl-3-methylimidazolum cation [C4mim]+, 1-allyl-3-methylimidazolium cation [Amim]+ and 1-ethyl-3-methylimidazolum cation [C2mim]+, while the anions are formate [HCOO]-, acetate [CH3COO]-, propionate [CH3CH2COO]-, butyrate [CH3CH2CH2COO-, glycollate [HOCH2COO]-,glycinate [H2NCH2COO]-, lactate [CH3CHOHCOO]-, benzoate [C6H5COO]-, and dicyanamide [N(CN)2]-.These ILs have also been characterized by 1H NMR.2. The solubility of chitosan in these ILs were measured as a function of temperature, and the impact of cationic and anionic structures of the ILs on solubilities of chitosan was studied by means of 1H NMR chemical shift and solvatochromic UV/vis probe. The experimental results demonstrated that the hydrogen bonds accepting ability of ILs anions strongly affects the solubility of chitosan, and ILs with greater hydrogen bonds accepting ability can dissolve chitosan better.3. Take [C4mim][CH3COO] as an example, we determined 13C NMR chemical shift of [C4mim][CH3COO] and [C4mim][CH3COO]/chitosan at 110℃, and analyzed the probable mechanism for the dissolution of chitosan in the ILs. It was shown that the oxygen atom of [CH3COO]- forms hydrogen bond with the proton on the hydroxy or amino groups of chitosan. Meanwhile, the proton in 2-position of the imidazolium ring interacts with hydroxy oxygen or amino nitrogen of chitosan to form hydrogen bonds. Consequently, the intra- or inter-molecular hydrogen-bond network in chitosan was disrupted, and led to the dissolution of chitosan.4. Chitosan was regenerated by adding different precipitants such as water, ethanol, acetone and acetonitrile into ILs/chitosan solutions. The influnence of the precipiant properties on the appearance and thermostability of the regenerated chitosan was studied by FTIR, TGA and XRD. The results suggest that the ILs only affect the appearance and thermostability of the regenerated chitosan, while the nature of precipiant strongly influence the regenerated speed, appearance, molecular structure and thermostability of the regenerated chitosan.