Fractionation of chitosan by supercritical carbon dioxide/acetic acid aqueous solution

Chitosan was extracted by supercritical carbon dioxide (scCO2) containing 1 wt% acetic acid (AC) aqueous solution as a cosolvent at 35, 45, 50 °C and 20, 30, 40 MPa for 1 h. Effects of operating conditions on the extraction yield and on the degree of deacetylation (DD), molecular weight (MW), and polydispersity (PD) of the extract were investigated. Without the AC cosolvent, chitosan could scarcely be extracted by pure scCO2 whereas, with the AC in scCO2, 1.65–3.04% by weight of chitosan could be extracted with the conditions studied. The extraction yield was increased by increasing density of scCO2. The extract had lower MW and PD but higher DD than the untreated chitosan. The DD data revealed that in chitosan the amount of the amino groups (NH2) other than the carbonyl (CO) in the residual acetyl groups (CH3CO) governed the extraction process. The DD, MW, and PD of the extract were higher as extracted by scCO2 (+1 wt% AC solution) of higher densities. By tuning density of scCO2 in the presence of AC via controls of temperature and pressure, chitosan could be fractionated by the method established here.