From Chitin to Chitosan - Chitosan Based Hydrogels: Functions and Applications - page 12

1.3.6 Water Content

“Water” here shall be defined as free water (adsorbed water) and part of the crystal water of
chitin or chitosan. All crystal water cannot be removed by drying under normal pressure.
Water content is measured by the following steps: accurately weighing 1-2 g of chitosan
sample, drying it at 105°C for 4 h until the weight is constant, and calculating weight loss
in order to determine water content according to the following formula, where W is the
weight of chitosan and Wt denotes the weight of chitosan after drying.

Water content = ( W − Wt)/ W × 100%

1.4 Physical Properties
1.4.1 Structure Characteristic

Despite the alteration due to deacetylation, chitosan from crab tendon possesses a crystal
structure showing an orthorhombic unit cell with dimensions a = 0.828, b = 0.862, and
c = 1.043 nm (fiber axis). The unit cell comprises four glucosamine units; two chains pass
through the unit cell with an antiparallel packing arrangement. The main hydrogen bonds
are O3 … O5 (intramolecular) and N2 … O6 (intermolecular) [54]. The crystal structures of
salts and derivatives have also been determined, for instance, for chitosan ascorbate and
salicylate, among others. The structural unit is presented in Figure 1.4 [55].

O 1 ′

C 1 ′

O 5

N 2 ′

C 2 ′

C 5 ′

O 6 ′

C 3 ′

C 4 ′

C 6 ′

O 3 ′

ψ

O 1

ϕ

C 1

O 5

N 2

C 2

C 5

O 6

C 3 C 4

χ

C 6

O 3

Figure 1.4
Chemical structure of a disaccharide segment of chitosan, showing position numbering. Two angles, psi and
ø, defining the chain conformation and the angle chi defining the O6 orientation are shown. Dashed lines
denote the O3-O5 hydrogen bonds. Hydrogen bonds connecting various positions of adjacent chains
are omitted.