Abstract:In this study，cellulose nanocrystals（CNCs）containing carboxyl groups were prepared by ultrasonicassisted citric acid hydrolysis using microcrystalline cellulose（MCC）asraw material，and the preparation process was optimized by single factor test and response surface test.The performance of the prepared CNCs was characterized.The results showed that the optimal acid hydrolysis conditions obtained by response surface optimization were 51∶1（mL/g）of liquid-solid ratio，80 ℃ of reaction temperature，and 3.75 h of reaction time，under these conditions，the predicted carboxyl content of CNCs was 1.19 mmol/g.After three times of verification experiments on the optimum acid hydrolysis conditions，the measured average carboxyl content of CNCs was 1.18 mmol/g，and the measured value was close to the predicted value.The results showed that the C=O group appeared at 1 735 cm-1of CNCs prepared under this condition，indicating that citric acid was successfully grafted onto the surface of MCC；CNCs belonged to cellulose type I，and its crystallinity （74.06%）was much higher than that of MCC（69.07%）；the CNCs were short rod-shaped with a diameter of about 10 nm-30 nm and a length of about 80 nm-200 nm.The average particle size and Zeta potential were 358 nm and-39.8 mV，respectively.The carboxyl group on citric acid reacted with the hydroxyl group on the C6 atom of MCC.The results of this study can provide a simple way to prepare carboxylated CNCs.