Direct ionization and solubility of chitosan in aqueous solutions with acetic acid

Abstract

A better understanding of the physicochemical phenomena underlying the direct solubilization of solid chitosan in an aqueous acetic acid medium without the addition of salt is needed. These phenomena have been studied using an inverse approach in which the chitosan is treated as a poly-acid and not as poly-base, and the macrophenomenon observed represents the aggregation of the macromolecules of chitosan instead of their solubilization. The purpose of this study is to present an overview of the physicochemical phenomena underlying this direct solubilization in an aqueous acetic acid medium without the addition of salt through a mathematical model based on equilibrium equations, mass balances, and correlations between experimental data. This model allowed us to model dilute chitosan/acetic acid/water systems without the addition of salt with an adequate fit (R2 > 0.9951) to the trend of the experimental data, enabling the calculation of the pH, degree of ionization, apparent dissociation constant, and the concentrations of the smaller molecules $$\left( {{\text{CH}}_{3} {\text{COOH}}, {\text{CH}}_{3} {\text{COO}}^{ - } ,{\text{H}}_{3} {\text{O}}^{ + } } \right)$$ , with only a priori knowledge of the initial concentration of acetic acid, initial concentration of amine groups, and the dissociation constant of acetic acid. The main finding of this study was that the diluted systems lacking salt studied here showed a tendency toward an apparent dissociation constant of $$pK_{1/2} \to 6.44$$ as the polymer concentration increased. This value is similar to the values reported in the literature for systems containing salt and a degree of deacetylation $$\left( {f_{\text{D}} \sim 0.7} \right)$$ similar to the chitosan evaluated here.