Critical dissolution ionic strength of aqueous solution of chitosan hydrochloride salt

Abstract

The formation of aggregates of chitosan hydrochloride in aqueous solution, with and without added salt (i.e. NaCl), and the dissolution thereof have been studied using wide-dynamical range light transmittance measurements. The concentrations of chitosan and NaCl examined were 4 to 20 g/L and 0 to 2 mol/L, respectively. A large hysteresis loop was found for the formation of aggregates during cooling and the dissolution thereof during heating. In spite of the existence of the hysteresis, and regardless of the precise aggregation state and heating rate, the temperature at which the aggregates dissolved (namely the dissolution temperature) was uniquely determined for any given concentration of chitosan and NaCl. Further a critical dissolution ionic strength, below which no aggregation was detected, was established from the variation of dissolution temperature with ionic strength. The value of the Debye screening length, calculated at the critical dissolution ionic strength, corresponds fairly well to the size of the monomeric units comprising the chitosan chain. We therefore conclude that the aggregates of the chitosan are formed when electrostatic repulsive interactions between chitosan polyions are screened out allowing non-electrostatic attractive interactions to dominate.