Key factors determining surface charge and colloidal stability in phosphorylated cellulose nanocrystals via metaphosphoric acid–urea functionalization

Phosphorylated cellulose nanocrystals (P-CNCs) have emerged as an alternative to sulfuric acid-derived conventional CNC due to their enhanced colloidal stability. In this study, P-CNCs were synthesized from bleached hardwood kraft pulp (BEKP) via a controlled hydrolysis route involving H₃PO₃ pretreatment followed by reaction with metaphosphoric acid (HPO₃) and urea. A full factorial design was applied to evaluate the effects of reaction time (60–90 min) and HPO₃ concentration (3–4 M) on surface charge and zeta potential. Surface charge densities ranged from 757 to 1993 mmol/kg, while zeta potential values ranged from −30.17 to −67.40 mV, confirming high colloidal stability. Statistical analysis revealed that reaction time had a significant main effect on surface charge (p = 0.0022), and the only factor significantly affecting zeta potential (p = 0.0488), whereas HPO₃ concentration showed a secondary but significant effect on surface charge (p = 0.0203). A significant interaction between the two factors (p = 0.0097) indicates that their effects are not independent. These results demonstrate that surface functionalization and colloidal behavior of P-CNCs can be effectively tuned through controlled processing conditions, with reaction time playing a central role. These findings contribute to a better understanding of the surface modification and stability of P-CNCs and support the sustainable production of functional nanocellulose materials.