| Chitosan, also called deacetylated chitin, shows poor solubility in water because of the intermolecular or intramolecular hydrogen bonds between amino and hydroxyl groups and the high crystallinity, which results in the limited applications of this polysaccharide. Therefore, it is very important for us to improve the water solubility of chitosan. At present, there have developed a lot of chemical modification methods for the preparation of various chitosan derivatives in order to improve water solubility of chitosan. These ways can enhance solubility of chitosan in water to some extent, but there exists all kinds of problems, such as a lot of poisonous and harmful reagent usage, long reaction time, high cost, the existence of potential safety hazard and environmental pollution and so on. So it is needed to develop a relatively simple, safe and low cost new method of preparing the water solubility of chitosan modified product, and we explored the relationship between structural changes and properties, which has significant economic and social value for the practical applications of chitosan and its derivatives.Firstly, chitosan-citric acid was prepared through acylation reaction between chitosan and citric acid with sodium hypophosphite as catalyst. Secondly, the structural characterizations of chitosan and its derivative were described by fourier transform infrared spectroscopy, scanning electron microscopy, thermogravimetric analysis, powder X-ray diffraction and solid state 13 C nuclear magnetic resonance in order to testify the happening of reaction and this paper explored the relationship between structure and water solubility. Thirdly, according to the above method, we studied that different irradiation dose of chitosan reacted with citric acid, then we discussed the relationship between structures and properties of before and after modification. Finally, it also studied the influences of different conditions on heavy metal cadmium ion adsorption and their ability of eliminating hydroxyl and DPPH free radicals. The main contents and results of this paper were as follows:1. The preparation of chitosan-citric aicdThe research revealed the effects and influences of sodium hypophosphite dosage, reaction time, reaction temperature and citric acid dosage on the yield and weight-average molar mass of chitosan-citric acid. When the preparation conditions were as follows: citric acid and sodium hypophosphite mole ratio is 10:1, reaction temperature is 110 ℃, reaction time is 3 h and citric acid and chitosan mass ratio is 2.50:1, the overall yield and the weight-average molar mass of chitosan-citric acid were 48.62% and 529,000, respectively. Compared with chitosan, chitosan-citric acid can be good solubility in water.2. The structural characterization of chitosan-citric acidCompared with the structure of chitosan, the infrared spectra showed that chitosan-citric acid had obvious amide(1580 cm-1) and carboxyl(1717 cm-1) absorption peaks. Besides, solid 13 C nuclear magnetic spectra appeared a carboxyl carbon(δ=178.48 ppm) absorption peak, and thermogravimetric analysis photos had distinct quality mass peak which was attributed to the side chain of citric acid(177.60 ℃) and the amide(250.10 ℃) decomposition. At the same time, powder X-ray diffraction figures and electronic scanning electron microscopy images demonstrated that the crystallinity of chitosan-citric acid reduced and its molecular size was amaller and its structure was porous, respectively. These structural characterization methods were directly or indirectly proved that the amino of chitosan and the carboxyl of citric acid really reacted, which destroyed the intermolecular and intramolecular hydrogen bonds of chitosan and introduced the hydrophilic groups. These structural changes resulted in water solubility of chitosan-citric acid was better than that of chitosan.3. The structural and functional properties research of different dose irradiated chitosan acylating modified productsThis method, which was used for acylating modification of different dose irradiated chitosan, praparing various chitosan-citric acid products. The lower the irradiation dose, the greater the yield, and with the increase of irradiation dose, the weight-average molar mass and the viscosity of chitosan gradually reduced. Besides, compared with irradiaed chitosan acylating modified products, no irradiaed chitosan(0 kGy) acylating modified product was better water solubility. Moreover, there had little impact on the structures of chitosan and chitosan-citric acid.4. The research on functional properties of chitosan-citric acidThis paper discussed the adsorption effect of chitosan and chitosan-citric acid on heavy metal cadmium ions under the different conditions(pH, adsorption time and adsorbent dosage), the results showed that they all have a certain adsorption effect and the latter was superior to the former. In addition, the free radical scavenging capacity of chitosan-citric acid was better than that of chitosan under the same conditions. |
PDF Full Text Request