Figure 7 shows photographs illustrating the visual appearance of the aqueous suspension of CNCH2SO4 and CNCCr(NO3)3 samples with a white light source. For comparison, all nanocellulose samples were dispersed in water at the same concentration. After the ultrasonication process, the electronic photos of CNCH2SO4 and CNCCr(NO3)3 suspensions in the sample bottle are taken at 0 h, 24 h and 168 h. As can be seen in Figure 7a, two milk-like turbid suspensions were produced, suggesting that CNCH2SO4 and CNCCr(NO3)3 both formed stable dispersions in water, with the latter being whiter than the former. After 24 h standing, the turbid phenomena continued for CNCH2SO4 colloidal suspensions, as shown in Figure 7b. As the time prolonged, however, some sedimentation from CNCCr(NO3)3 were flocculated, whereas the good stability of CNCH2SO4 suspension could be kept at ambient temperature condition. For CNCCr(NO3)3, the original suspension had separated into a two-layer mixture, which consists of a clear layer and a turbid layer that can be visibly observed. This could be attributed to the fact that no surface groups have been attached to CNCCr(NO3)3 when prepared from Cr(NO3)3 metal salt catalyst, and thus the colloidal dispersion was expected to be less stable than CNCH2SO4 hydrolyzed by sulfuric acid. The high stability of sulfated CNCH2SO4 suspension can be achieved due to the sulfate negative charges (SO42−) on the CNCH2SO4 surface being able to create a strong electrostatic repulsion between the cellulose surfaces, and these charged groups limit their ability to flocculate . For this reason, CNCCr(NO3)3 showed a higher risk for agglomerate in a water medium.