Dispersion Characteristics of Cellulose Nanocrystals in Water and Ethanol Using Small-Angle X-Ray Scattering and Scanning Probe Microscopy

Abstract

The dispersibility of cellulose nanocrystals (CNCs) of differing ionic functional group content was investigated in both water and ethanol. The CNC samples were prepared by hydrolyzing pulp with sulfuric acid (sulfate group content: 0.35 mmol/g) and included a 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized CNC material (carboxyl group content: 2.0 mmol/g). To evaluate dispersibility of the CNCs in each medium, measurement of the turbidity of the dispersed media and small-angle X-ray scattering (SAXS) were used for direct evaluation, and scanning probe microscopy (SPM) was used as an indirect method. The aqueous dispersant was transparent, while the ethanol dispersant was cloudy. Transmittance of the ethanol dispersant was lower in the TEMPO-oxidized CNCs, which contain many ionic functional groups. The sizes of the CNC aggregates in each medium were estimated by preparing Guinier plots for the SAXS profiles. The sizes of the CNC aggregates in ethanol were larger than those in water, and the rate of increase in the size of the aggregates was greater for the TEMPO-oxidized CNCs. This tendency also was seen in the SPM data, possibly due to dependence on both the numbers and types of surface functional groups.