Chitosan carboxylic acid salts in solution and in the solid state

Abstract

When chitosan is associated to simple carboxylic acids, such as formic, acetic, etc., electrostatic interactions occur, resulting in a salt formation, to a greater or lesser degree. Fourier transform infra-red (FTir) spectroscopy and potentiometric techniques were used to characterize the nature of such interactions, in aqueous solutions and in films. The evolution of solid state samples during storage and upon dehydration was studied, by FTir spectroscopy and transmission X-ray diffraction. Results showed that the carboxylic acid content of the films progressively decreased in relation with the physicochemical parameters of the acids (p K a, solubility, etc.). These results were compared with those obtained with a strong acid such as HCl or a complex carboxylic acid (lactic). In X-ray diffraction experiments, three crystalline structures of chitosan were found: a hydrated form, independent of the chemical nature of the salt, and two dehydrated forms depending on the chemical structure of chitosan.