shows that the CNF30, CNF45, and CNF60 suspensions displayed the highest zeta potentials at a concentration of 0.075 g/L (i.e., the original solution was diluted 4-fold, in which the -NH2 concentration was 8.5 105 mol/L), and the zeta potential decreased when the concentration was increased or decreased. This phenomenon may be due to the CNF solvent being ultrapure water, which had a low concentration of Hþ (i.e., approximately 107 to 106 mol/L) and was thus unable to fully protonate -NH2 under higher CNF concentrations dispersions. For CNF concentrations lower than 0.075 g/L, the CNF underwent full protonation, increasing the degree of protonation and zeta potential theoretically. However, because the OH concentration did not change at lower CNFeHþ concentrations, the OH concentration was higher
than the CNF-H+ concentration, which neutralized electric charges and lowered the zeta potential of the CNF suspensions. To eliminate the concentration effect on the zeta potential, only the original CNF/NaCl solutions (i.e., 0.30 g/L), and that diluted 2-fold (i.e., 0.15 g/L) and 4-fold (i.e., 0.075 g/L), were employed in the present study.