Chakkaphan Pho-Ngernngam, Ekarat Detsri, Piyathida Thaipukdee, Nawasit Chotsaeng, Suparat Rukchonlatee, Arjnarong Mathaweesansurn
Microwave-assisted acid hydrolysis was developed as an intensified strategy for producing cellulose nanocrystals (CNCs) from cotton-derived cellulose. Single-acid H2SO4 and mixed-acid H2SO4/HNO3 hydrolysis systems were systematically investigated under controlled microwave irradiation to elucidate the role of acid composition in governing CNC formation, crystalline evolution, surface functionality, thermal stability, and colloidal behavior. Microwave-induced volumetric heating promoted rapid acid-cellulose interactions and efficient cleavage of amorphous cellulose domains, enabling CNC isolation within markedly shortened reaction times compared with conventional thermal hydrolysis. Under optimized conditions, microwave-assisted H2SO4 hydrolysis produced CNCs with a yield of 49.0%, crystallinity index of 86.4%, aspect ratio of 5.3 ± 2.7, and sulfate half-ester groups (-OSO3 -) content of 0.14 mmol g-1. In comparison, microwave-assisted H2SO4/HNO3 hydrolysis yielded CNCs with a comparable yield of 47.7%, higher crystallinity index of 89.2%, aspect ratio of 7.2 ± 2.7, and -OSO3 - group content of 0.13 mmol g-1. The mixed-acid route further introduced carboxyl/carboxylate (-COOH/-COO-) functionalities through limited HNO3-assisted oxidation, as evidenced by FT-IR, solid-state 13C NMR, XPS, and zeta potential analyses. Consequently, the mixed-acid CNCs exhibited the most negative zeta potential (-42.4 ± 0.9 mV) and the highest thermal stability, with a T 5% of 285.7 °C and a major DTG peak at 355.4 °C. Compared with conventional H2SO4 hydrolysis, microwave-assisted hydrolysis improved CNC recovery while reducing acid concentration and processing time. These findings demonstrate that microwave-assisted mixed-acid hydrolysis provides a controllable and efficient route for producing CNCs with tunable crystallinity, surface functionality, colloidal stability, and thermal performance.