Elsevier

Thermochimica Acta

Volume 444, Issue 2, 15 May 2006, Pages 128-133
Thermochimica Acta

The use of DSC curves to determine the acetylation degree of chitin/chitosan samples

https://doi.org/10.1016/j.tca.2006.03.003Get rights and content

Abstract

The use of DSC curves is proposed as an alternative method to determine the degree of N-acetylation (DA) in chitin/chitosan samples, based in both peak area and height of the decomposition signal. Samples with DA from 74 to 16% were prepared from a chitin commercial sample and the DA was determined by 1H NMR, 13C CP/MAS NMR and IR spectra. The effect of water content, heating rate, sample mass and gas flow on the DSC peaks were evaluated and optimized. Using optimized conditions a linear relationship between peak area and height with the DA could be achieved with linear correlation coefficients of −0.998 and −0.999 (n = 7), respectively. The calibration graphs were used to determine the DA of a commercial chitosan sample with relative errors ranging from 2 to 3% for both peak area and peak height, when compared with the DA determined by 1H NMR method.

Introduction

Chitin is a natural, biodegradable and non-toxic polysaccharide widely spread among marine, land invertebrates and fungi. It is usually obtained from waste materials of the sea food-processing industry, mainly crab, shrimp, lobster and krill shells [1]. Chitosan is much less widespread in biomass being produced mainly by thermochemical alkaline deacetylation of acetamide group at the C-2 position in the 2-acetamido-2-deoxy-d-glucopyranose unit [2].

Chemically, chitin and chitosan are closely related since both are linear polysaccharides containing 2-acetamido-2-deoxy-d-glucopyranose (GlcNAc) and 2-amino-2-deoxy-d-glucopyranose (GlcN) units joined by β(1  4) glycosidic bonds. They can be distinguished by their contents of the above-mentioned units [1], [3] as represented in Fig. 1.

The proportion of GlcNAc in relation to the GlcN units is defined as the degree of N-acetylation (DA) of the biopolymer, an important parameter which provides not only its physical–chemical properties [4], [5], [6] but also its biological [7], biomedical [8] and food [9], [10] applications, among others. These applications are remarkably dependent on the physical and chemical properties of the chitin and/or chitosan, such as solubility, strongly dependent of the DA. In this sense the accurate determination of the DA is a very important issue in order to define the applications of such materials.

For some authors the name chitosan means that the DA is around (or lower) than 0.5 with the predominance of 2-amino-2-deoxy-d-glucopyranose residues whose solubility in acidic medium is controlled by these units and the distribution of the acetyl groups remaining along the chain [11], [12] while chitin relates to compound with DA > 0.5.

Usually, a single technique cannot be adopted to cover the full range of DA, i.e. for chitin as well as for chitosan. For chitin, due to the lack of solubility, 13C CP/MAS NMR [3], [4], [13], [14] and infrared spectroscopy [15], [16], [17], [18], [19] can be used. For chitosan, soluble in aqueous acidic media, other methods are available such as: potentiometric and conductometric titrations [4], [13], [20], 1H NMR [1], [3], [4], [11], [21], gel-permeation chromatography [22], ultraviolet spectrometry [23], [24] and infrared spectroscopy [15], [16], [17], [18], [19] among others. All these methods present advantages and difficulties to be performed.

Thermal methods, such as themogravimetry (TG), differential scanning calorimetry (DSC) and differential thermal analysis (DTA) have emerged as powerful thermoanalytical techniques to monitor characteristic physical and chemical changes in biopolymers, including DA determination [25], [26], [27], [28], [29].

Kittur et al. [29] proposed the use of DSC data in order to determine the DA of the chitin and chitosan samples. The method is based on the exothermic degradation peak observed for chitin and chitosan samples, which changes in temperature, area and intensity depending on the DA. Although the authors described properly how the water behaves in these samples during the heating process, they did not issued the influence of thermoanalytical parameters and water content of the samples in the DA determination results. The water content is highly dependent on the sample treatment and conservation. In addition, they have not evaluated the reliability of their results in relation to a reference technique, such as 1H NMR.

The main purpose of the present work is to investigate the dependence of the exothermic peak area and height regarding the amine (GlcN) groups decomposition in DSC curves in order to determine the DA in chitin/chitosan samples comparing the results with reference techniques. However, parameters such as heating rate, sample mass and water content should be optimized to reach this objective.

Section snippets

Chitosan purification

The chitosan from crab shells used in this work was a commercial product of medium molecular weight (Aldrich, USA) of technical grade. The purification was attained by the dissolution of the crude commercial product (approximately 1 g) in 300 mL of dilute 0.5 mol L−1 acetic acid solution. The dissolution of the polysaccharide was assured by stirring the initial suspension during 18 h and precipitated in the hydrogel form by carefully adding concentrated NH4OH. The chitosan hydrogel was washed with

Preparation of chitin/chitosan samples with known DA values

The first step in the present work was the preparation of a set of samples with different, but well-known, DA values ranging from chitin to chitosan.

The packing structure of α-chitin is strongly stabilized by intra-chain, intra-sheet and inter-sheet hydrogen bonds in the three unit cell directions [3]. This structure requires high concentration of alkali and prolonged reaction time for thermochemical deacetylation leading to sparsely as well as non-uniformly accessible block copolymers of

Acknowledgements

The authors acknowledge the Brazilian agency FAPESP for LSG post-doctoral fellowship (Proc. 03/09224-7) and financial support (Proc. 02/03448-8).

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