Molecular Weight and Degree of Acetylation of Ultrasonicated Chitosan

Chitosan is a glucosamine polymer produced by deacetylation of chitin from crustacean shells. The functional properties of chitosan, such as thickening, film-formation and antimicrobial activity, are related to its molecular weight and degree of acetylation (DA). High intensity ultrasonication has the potential to modify molecular weight of chitosan and thus alter or improve chitosan functional properties. The objective of this research was to determine the DA and molecular weight of chitosan molecules as a function of sonication intensity and treatment time.

High molecular weight shrimp chitosan was purified by alkaline precipitation and dialysis from aqueous solution. A 1 % (w/v) chitosan in 1 % (v/v) aqueous acetic acid was sonicated for 0, 1, 2, 10, 30, and 60 minutes at 25 °C. A Misonix 3000 ultrasonic homogenizer was used to sonicate 50 mL samples at power levels of 16.5, 28, and 35.2 W/cm² with pulsed output (1 s sonication, 1 s break). The DA was determined by high performance liquid chromatography with photodiode array detector (HPLC-PDA), monitoring acetyl groups released after complete hydrolysis and deacetylation of the samples and by Fourier Transform InfraRed Spectroscopy with Attenuated Total Reflection (FTIR-ATR). Molecular weight was determined by measuring the intrinsic viscosity of sonicated solutions.

The DA of purified chitosan was 21.5 %. Results indicated that neither power intensity nor sonication time deacetylated the chitosan molecules. However, intrinsic viscosity of samples decreased exponentially with increasing sonication time. Reduction rates of intrinsic viscosity increased linearly with ultrasonic intensity. A first order kinetic reaction model of molecular weight decay as a function of sonication time was suggested and an Arrhenius-type relationship for the dependence of the reaction rate on the ultrasonic intensity was developed. Our results confirm the hypothesis that high intensity ultrasonication can be utilized to reduce molecular weight of chitosan while not reducing the degree of acetylation.