FTIR spectroscopy studies on the spontaneous neutralization of chitosan acetate films by moisture conditioning

The influence of moisture ambiance on the spontaneous neutralization of amorphous chitosan acetate films during storage at 30 °C and water activities (aw) in the range of 0.11 to 0.84 is studied by Fourier transform infrared (FTIR) spectroscopy. Three ranges of relative humidity affecting the chitosan acetate films are identified: i) 0.11 ≤ aw ≤ 0.32, produces dehydration and partial neutralization; ii) aw = 0.44 and 0.55, spontaneous neutralization and rearrangement of OH bonds; iii) 0.64 ≤ aw ≤ 0.85, prevalence of films in the acetate form and moisture absorption. Acetate content of films as function of water activity and storage time is assessed from the band accounting for the electrostatic interaction of protonated amine groups (-NH3+) in chitosan and carboxylate ions (-COO−) of acetic acid. FTIR spectroscopy results are correlated with the equilibrium properties determined from moisture sorption isotherm. The simple method presented may help to produce chitosan acetate films with tuned protonation degree that are relevant in many applications such as biomedical, food, cosmetics, and pharmaceutical.