Removal of non-stoichiometric aspartic acid from an aqueous dispersion of chitosan aspartate nanoparticles

In the traditional method of removing excess acid used to obtain the salt form of chitosan, not only a reaction with neutralizing bases proceeds, but also partial deprotonation of macrochains. Since charged amino groups are responsible for the biological activity of the aminopolysaccharide, it seems important to develop methods for optimizing the acid–base composition of a chitosan-containing system without proton elimination. The paper presents the results of our study on the removal of non-stoichiometric aspartic acid from an aqueous dispersion of chitosan aspartate nanoparticles obtained by in situ self-assembly during counterion association on charged polymer chains and stabilized by a polysiloxane shell coating using isohydric and isothermal crystallization approaches. It is shown that a temperature decrease in the range of 50–4°C in combination with water distillation by 50–85% leads to supersaturation of the nanodispersion and crystallization of the poorly soluble component of the dispersion medium with precipitation. The influence of the acid/polymer molar ratio (1.3–1.7 mol/mol of NH2) in the target substance of the dispersed phase, the cooling rate (10 and 30°C/h) and the dispersion concentration degree (50–85%), as well as the temperature (22±2 and 4°C) and holding time (1–200 days) on the morphostructure, chemical composition, crystalline ordering and quantitative yield of the solid phase has been estimated. The correspondence of the air-dry form of the isolated precipitate to crystals of the zwitterionic form of aspartic acid has been proven by IR spectroscopy, X-ray structural analysis, dialysis, potentiometry and gravimetry. It has been found that the most optimal option for preparative removal of non-stoichiometric double bipolar ions is the concentration of nanodispersion by 85% followed by supercooling from 50 down to 4°C at a rate of 10°C/h. The developed approach to combining isohydric and isothermal crystallization of non-stoichiometric aspartic acid has been successfully tested on moderately concentrated solutions of the complex salt of chitosan hydrochloride–aspartate.