Chitin

Chitin is tough, biodegradeable and relatively inert. It can not be dissolved in water and most ordinary solvents [1]. It can not readily be fabricated into fibers or membranes. Partially deacetylated chitin (chitosan) can be crosslinked by glutaraldehyde to an amber hydrogel [2].

Chitin is soluble in N,N-dimethylacetamide, N-methyl-2-pyrollidone and their mixed solvents in the presence of 5% LiCl [1,4]. Certain N,N-disubstituted amides with LiCl can dissolve chitin without degrading it. NMR and IR experiments suggests, that Li⁺ is coordinated to the carbonyl oxygen. For chitin the solubility parameter (δ) is 12.5, for the chitin-LiCl complex, d~11.2. Two good amide polymer solvents (a polymer solvent is a solvent with the approximately the same solubility parameter as the polymer) are dimethyl-acetamide (δ=10.8) and N-methyl pyrrolidinone (δ=11.3) [2].

Swelling of chitin by water: R_w/R_d: 1.2, S_w/S_d: 1.4, V_w/V_d: 1.7, (R_w: Diameter of fibers at wet state, R_d: Diameter of fibers at dry state, S_w: Cross section of fibers at wet state, S_d: Cross section of fibers at dry state, V_w: Volume of fibers at wet state, V_d: Volume of fibers at dry state) [2].

Chitin will coagulate, forming fibres, when dissolved in trichloroacetic acid (TCA) and a chlorinated hydrocarbon like chloromethane, dichloromethane or 1,1,2-trichloroethane at TCA concentrations 25-75% and chitin concentrations 1-10%. Fibres are formed extruding thru an acetone bath [1]. Another process for fibres is water/dichloroacetic acid with basic CuCO₃-NH₄OH as coagulant [1]. Several other methods on forming chitin fibres revolving around these two methods has been developed and patented. Alternatively a method involving N-Methyl-2-pyrrolidone, N,N-Dimethyacetamide and LiCl has been developed [1]. As TCA is a strong acid, chitin will degrade upon treatment. Generally chlorinated solvents are plastisizers of chitin. The tensile strength and elongations of the fibres is thus dependent on the process [1].

Chitin can be deacetylated to chitosan by treatment with 40% NaOH at 120°C [1].

Chitin-fibers absorbs calcium ions very selectively, forming a chelate. The amide groups involvement is very clear, and the hydroxy groups (either the C(6) or the C(2)) has been suggested to participate too. The selectivity seems to be caused by the chelation mechanism [2].

Chitin, chitosan and the phosphorylates derivatives are known to adsorb uranium [2].

Chitin membranes are selective permeable, making them usable for separating water and solutes [2].

Chitin exhibits basic properties. Derivatives fall within two categories, in each case, the native N-acetyl groups are removed and the exposed amino function reacts either with acyl chlorides or anhydrides to give the group NHCOR or is modified by reductive amination to NHCH₂COOH [2].