968
Cellulose (2019) 26:959–970
kraft pulps; i.e. the CNCs consisted of a hydrophilic and a hydrophobic particle population. It was shown that the hydrophilic particles in the CNCs had a similar hydrophobicity value as polyelectrolyte complexes consisting of pure hydrated polysaccharide chains. It was also shown that the hydrophobic particles in the CNC had a similar hydrophobicity value as colloidal wood pitch droplets. It was determined that the increased hydrophobicity of the CNCs was an effect of lipophilic extractives, especially sterols, present in the kraft pulps. These lipophilic components were able to withstand the harsh acidic hydrolysis step of CNC production, and remained with the CNCs through the rest of the production steps. The use of a selective staining agent in combination with FCM analysis enabled detection of the hydropho- bic particle populations within the CNC suspensions, even though the concentrations of lipophilic extrac- tives in the raw materials were quite low. The difference in particle hydrophobicity was not detected from light scattering alone and would go completely undetected without the use of FCM. Furthermore, the FCM technique enables the user to gate certain particle populations and plot/calculate particle populations separate from each other. These facts indicate that FCM is a useful and powerful analysis technique also for CNCs. It should also be concluded that the content of lipophilic extractives in different raw materials need to be determined and taken into account prior to CNC production, if no pre-extraction step is employed. Acknowledgments Open access funding provided by Abo Akademi University (ABO). This work is a part of the project ExtBioNet, appointed by the Academy of Finland, and part of the activities of the Process Chemistry Centre (PCC) at A˚ bo Akademi University, Turku, Finland. Special thanks to Ph.D. student Weihua Zhang and technology student Richard Sundberg at the Laboratory of Wood and paper Chemistry, A˚ bo Akademi University. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http:// creativecommons.org/licenses/by/4.0/), which permits unre- stricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Com- mons license, and indicate if changes were made. Open Access
ASE-extracted with acetone:water:acetic acid (94:5:1 v/v) prior to CNC production. After pre-extraction of the birch kraft pulp, no separate hydrophobic popula- tion was seen in the CNCs, which indicated that the hydrophobic particles most likely originated from lipophilic extractives in the starting material (Fig. 4). These results indicate that the extractives content of different raw materials for CNC production needs to be determined and taken into account. Even low quantities of lipophilic extractives will alter the properties of CNC surfaces significantly, and lead to misinterpretation of data. The raw material containing the lowest amount of lipophilic components, espe- cially sterols, is recommended for CNC production, unless a pre-extraction of is performed to remove these components. Fig. 4 A logarithmic density plot of fluorescence intensity in the red spectrum (red) versus the light scattering in the forward direction (FSC) of CNC made from pre-extracted birch kraft pulp
Conclusions
Cellulose nanocrystals (CNCs) were prepared from kraft pulps of eucalyptus, birch, and softwood. The different kraft pulps were hydrolyzed using strong sulfuric acid in order to obtain colloidally stable CNCs. The CNCs were studied using flow cytometry (FCM). The light scattering properties of the CNCs in side direction and forward direction were documented, as well as the hydrophobicity of the CNCs. Polysaccharide surfaces, such as cellulose, are known to be very hydrophilic in aqueous media. However, the presence of hydrophobic particles were seen clearly in the CNCs from the eucalyptus and birch
Appendix
See Fig. 5.
123
Made with FlippingBook - Online catalogs