Preparation And Characterization Of N,O-Carboxymethyl Chitosan

Carboxymethyl chitosan (CMC) was prepared by a two-step alkalization and refined by the method of isoelectric point. The factors, such as the amounts of alkali and chloroacetic acid, the volumes of isopropyl alcohol and water, reaction temperature, etc., influencing the yield, degree of substitution (DS) and intrinsic viscosity were respectively investigated. The reaction conditions were optimized in terms of the yield and the degree of substitution of products. Under the optimized reaction conditions, the two-step alkalization was validated with different molecular-weight chitosans, and 5-times and 10-times scale-up experiments. The CMC products refined by precipitation at the isoelectric point (IP) were then basified to water-soluble sodium carboxymethyl chitosan (CMCNa). Finally, the CMC products were characterized for DS, molecular weight, isoelectric point (IP), IR spectra, hydrophilicity, and thermal stability by potentiometric titration, viscosimetry, UV spectroscopy, IR spectroscopy and differential scanning calorimetry (DSC), respectively.The optimized reaction conditions were obtained as: m (chitosan) : m (monochloroacetic acid) : m(sodium hydroxide) = 1:4.8:4.8, reaction temperature 30℃, reaction time 4 h, and amounts of isopropyl alcohol and water 50mL and 22mL, respectively. In comparison with the conventional methods, the two-step alkalization showed remarkable increase in the yield and DS values. Under the optimized conditions, the yield and DS reached as high as 95.2% and 1.32 respectively, and decreased slightly on 5-times and 10-times scale-up experiments.Compared with the conventional refining methods, isoelectric point precipitation showed two advantages, including removing the unreacted chitosan and low-substituted carboxymethyl chitosan, consuming only one-eighth of the amount of organic solvents required by the conventional method. The CMC products refined by isoelectric point precipitation became water-soluble after being basified. The water solution of the obtained CMCNa product with 1% (w/v) concentration showed a pH value of 9.15, while its values for IP, DS, molecular weight and water ratio were 5.42, 1.28, 7.529×105 and 5.6%, respectively.The study of factors influencing yield indicated that the amounts of alkali, chloroacetic acid and water, and reaction temperature were the key factors influencing the yield. The yield decreased with the increase in temperature, while increased and then followed by a decrease with increasing amounts of alkali, chloroacetic acid and water.The study of factors influencing intrinsic viscosity showed that the amounts of alkali and chloroacetic acid, and reaction temperature were the key factors influencing the intrinsic viscosity. Intrinsic viscosity decreased with the increase in reaction temperature, while increased and then followed by a decrease with increasing amounts of alkali, chloroacetic acid.The study of factors influencing DS showed that the amounts of alkali and chloroacetic acid, and reaction temperature were the key factors influencing the DS. The DS increased with the increase in the amount of alkali, while increased and then followed by a decrease with increasing amount of monochloroacetic acid and temperature.The IR spectra of CMC indicated that the product was N, O-CMC and the carboxymethylation mainly occurred on the oxygen (O) position, which was in accordance with the result obtained from potentiometric titration. The differential scanning calorimetry (DSC) of samples indicated that the hydrophilicity of CMC increased with the increase of degree of substitution (DS) while its thermal stability decreased with the increasing DS.