Solubility and acid-base properties and activity coefficients of chitosan in different ionic media and at different ionic strengths, at T=25 °C

Studies on the acid-base properties and solubility of a polyammonium polyelectrolyte (chitosan) with different molecular weights (MW 310 and 50 kDa), were performed at T = 25 degrees C, in the pH range 2.5-7. The protonation of chitosan was investigated by potentiometry ([H+]-glass electrode) in NaCl, NaNO3 and mixed NaNO3 + Na2SO4 ionic media, at different ionic strengths. Protonation constants were calculated as a function of dissociation degree a by means of two different models, namely, a simple linear model and the modified Henderson-Hasselbalch equation. Experimental data were also fitted using a model independent of alpha (Diprotic-like model), according to which the acid-base properties can be simply described by two protonation constants in all the acidic pH range. The dependence on ionic strength of protonation constants in NaCl aqueous solution was modelled by Specific ion Interaction Theory (SIT). The ion pair formation between protonated chitosan and Cl-, NO3- and SO42- was also considered, and the relative formation constants are reported. Solubility investigations were performed in NaCl aqueous solutions in a wide range of ionic strength (0.1 < l/mol L-1 < 3.0), with the aim to determine the activity coefficients of neutral species and the Setschenow coefficient of chitosan 310 kDa. (C) 2009 Elsevier B.V. All rights reserved.