Vestnik MGTU (Sep 2017)
Hydration mechanism of heterogeneous alkaline deacetylation of chitin
Abstract
In the work the hypothesis has been proposed explaining the features of the kinetics of chitin/chitosan alkali deacetylation reaction in highly concentrated solutions of sodium hydroxide, which almost completely stops after 30–60 min after the start of the reaction and does not provide a fully deacetylated product – chitosan. Analysis of the known publications explaining the observed decline in the deacetylation reaction rate has been executed, and some new experimental results confirming the authors' conclusions have been presented. It has been shown that water present in the reaction mixture renders the greatest influence on the kinetics of chitin deacetylation, and can lead to hydration as chitin molecules and ions alkalis as well. The hypothesis has been offered according to which in the reaction mixture there is the dynamic balance between hydrated alkali ions, molecules of chitin and formed acetate ion, which can shift depending on the concentration of the reacting particles. It has been suggested that in concentrated alkali solutions water is present almost entirely or partly in the form of hydrate shells of alkali ions, and to a lesser extent in the form of "free" water. Apparently, the limiting stage of the deacetylation reaction is the hydration of chitin macromolecules by water molecules, which are released during nucleophile replacing hydroxyl ions by acetyl radicals binding with chitin amino groups. Acetate ion is a product of the deacetylation reaction. Acetate ion hydration energy is less than hydration energy of hydroxyl ion, so the acetate ion is less hydrated, as a result some water is released. In these conditions, hydration of chitin macromolecules occurs in the local area around the chitin molecules, where the high water concentration is created after deacetylation. The alkali concentration in all reaction volume remains practically constant.
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