Mechanism of Heterogeneous Alkaline Deacetylation of Chitin: A Review

Abstract

This review provides an analysis of experimental results on the study of alkaline heterogeneous deacetylation of chitin obtained by the authors and also published in the literature. A detailed analysis of the reaction kinetics was carried out considering the influence of numerous factors: reaction reversibility, crystallinity and porosity of chitin, changes in chitin morphology during washing, alkali concentration, diffusion of hydroxide ions, and hydration of reacting particles. A mechanism for the chitin deacetylation reaction is proposed, taking into account its kinetic features in which the decisive role is assigned to the effects of hydration. It has been shown that the rate of chitin deacetylation increases with a decrease in the degree of hydration of hydroxide ions in a concentrated alkali solution. When the alkali concentration is less than the limit of complete hydration, the reaction practically does not occur. Hypotheses have been put forward to explain the decrease in the rate of the reaction in the second flat portion of the kinetic curve. The first hypothesis is the formation of “free” water, leading to the hydration of chitin molecules and a decrease in the reaction rate. The second hypothesis postulates the formation of a stable amide anion of chitosan, which prevents the nucleophilic attack of the chitin macromolecule by hydroxide ions.

MOLECULAR DYNAMICS SIMULATIONS OF THE AFFINITY OF CHITIN AND CHITOSAN FOR COLLAGEN: THE EFFECT OF pH AND THE PRESENCE OF SODIUM AND CALCIUM CATIONS

“…Molecular dynamics simulations of the affinity of chitin and chitosan for collagen: the effect of pH and the presence of sodium and calcium cations slow process at room temperature [61,62] and thus the reaction is often carried out at elevated temperatures [11]. Therefore, it is reasonable to investigate potential chitin-collagen complexes at the temperature used in the MD simulations (i.e., 37°C).…”

Chitosan and Its Derivatives: Preparation and Antibacterial Properties

“…While this method is highly effective and can yield chitosan with a high degree of deacetylation, it often requires large quantities of alkali and high energy input due to the need for elevated temperatures, thus raising environmental concerns. Additionally, harsh alkaline conditions can cause depolymerization, leading to chitosan with lower molecular weight [ 25 ].…”

“…Temperature: The rate of deacetylation increases with temperature, resulting in a higher degree of deacetylation. However, excessive heat can lead to the degradation of chitosan [ 25 ];…”

“…Time: Longer reaction times can lead to a higher degree of deacetylation. However, prolonged exposure to harsh conditions can also cause depolymerization, resulting in low-molecular-weight chitosan [ 25 ];…”

“…Concentration: Using higher concentrations of alkali can result in a higher degree of deacetylation. However, higher concentrations can also cause more significant depolymerization, leading to lower-molecular-weight chitosan [ 25 ].…”