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

•A reliable method for soft neutralization of chitosan acetate films is presented.•Spontaneous neutralization is controlled by water activity.•Water activity of 0.53 neutralizes the films after only five days of storage.•Chitosan films in the acetate form can be stored at low water activity.•Protona...

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Bibliographic Details
Published in:Vibrational spectroscopy 2018-01, Vol.94, p.1-6
Main Authors: Mauricio-Sánchez, Reina Araceli, Salazar, Ricardo, Luna-Bárcenas, J. Gabriel, Mendoza-Galván, Arturo
Format: Article
Language:English
Subjects:
Acetic acid
Chitosan
Chitosan acetate
Cosmetics
Dehydration
Equilibrium
Fourier transforms
FTIR spectroscopy
Influence
Infrared spectroscopy
Moisture
Moisture absorption
Neutralization
Protonation
Relative humidity
Spectrum analysis
Studies
Water activity
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Summary:•A reliable method for soft neutralization of chitosan acetate films is presented.•Spontaneous neutralization is controlled by water activity.•Water activity of 0.53 neutralizes the films after only five days of storage.•Chitosan films in the acetate form can be stored at low water activity.•Protonation degree can be tuned by adjusting the ambiance relative humidity. 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.
ISSN:0924-2031
1873-3697
DOI:10.1016/j.vibspec.2017.10.005