*3.2. E*ff*ect of Preparation Method on the Suspension Stability of NC Prepared from MCC and MD-MCC*

The suspension states of NC prepared from MCC and MD-MCC by acid hydrolysis, high-pressure homogenization, and high-intensity ultrasonication are shown in Figure 1. All NC samples were well-dispersed in water and their suspensions were stable and uniform without any stratification when fresh. After 3 days at 20 ◦C, flocculation and precipitation occurred in the HH-NC and US-NC samples prepared from MCC whereas the other NC samples remained homogeneous, which indicates that the NC particles prepared by MD-MCC were relatively stable in water.

**Figure 1.** The suspension stability of NC prepared from MCC (AH-1, HH-1, US-1) and MD-MCC (AH-2, HH-2, US-2) t = 0 day and t = 3 days.

Net charge is a critical characteristic that affects the stability of NC particles. In general, higher absolute values of zeta potential correspond to better dispersion and stability [2]. The results from this study reveal that the introduction of trimethyl quaternary ammonium groups by cationic modification of MCC (confirmed by the FTIR results) helped to increase the NC zeta potential and further improved the stability and dispersion of the NC particles. Based on the measured yield and application properties, the NC samples prepared from MD-MCC were selected for further analysis.
