Cellulose (2019) 26:959–970
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65%H 2 SO 4 gave CNCs with a length of 100–200 nm, and a width of about 5 nm (Revol et al. 1992, 1994). The measured side scattering of the particles by FCM are in the same size range as previously reported values of the lengths of CNCs made from cotton fibers (Dong et al. 1998). It has been reported that the length of CNCs from black spruce sulfite pulp and eucalyptus pulp were 147 nm and 141 nm respectively after treatment with 64% H 2 SO 4 at45 C for 25 min (Beck- Candanedo et al. 2005). The average length of CNCs produced from eucalyptus kraft pulp has been reported as between 130 and 281 nm, depending on the conditions during hydrolysis with H 2 SO 4 (Chen et al. 2015). It was reported that the length of CNCs produced from microcrystalline cellulose originating from Norway spruce was 200–400 nm, with a width of less than 10 nm (Bondeson et al. 2006). In the current study, the determined SSC values of the different CNCs ranged between 170 and 270 nm. This indicated that the size-estimation from SSC actually represented the length of the CNCs, while the width was far below the detection limit of the FCM apparatus. From the normalized side scattering-curves it was also seen that the size of the CNCs from the birch and eucalyptus kraft pulps resulted in slightly larger particles than the softwood CNCs. The hydrophobicity of the CNCs were measured by FCM analyses with the added hydrophobic staining agent nile red. A small volume of nile red dissolved in methanol was added to the CNCs prior to injection into the FCM apparatus. Nile red is an environment- sensitive fluorophore that exhibits a blue-shift propor- tional to the hydrophobicity of its environment (Greenspan and Fowler 1985). The fluorescence intensity from nile red in the red spectrum has previously been used to investigate the hydrophobicity of various particles in combination with FCM analysis (Va¨ha¨salo et al. 2003; Va¨ha¨salo and Holmbom 2005; Strand et al. 2013, Strand et al. 2018). The nile red is believed to mainly adsorb onto the surfaces of the particles, since its solubility in water is negligible and migration into the CNCs is unlikely (Greenspan and Fowler 1985; Jose and Burgess 2006). The FSC intensities of the CNCs were plotted against their fluorescence intensity in the red spectrum (red) as logarithmic density plots (Fig. 2). Plotting FSC versus particle hydrophobicity revealed that the CNCs in fact consisted of two different particle populations, which was not detectable when only the light-scattering
properties were plotted (Fig. 1). The FCM measure- ments showed a dominating particle population with low hydrophobicity, and a smaller population of particles with higher hydrophobicity in the eucalyptus CNCs and in the birch CNCs (Fig. 2). However, only traces of the hydrophobic population were seen in the CNCs from softwood. The fluorescent intensity in the red spectrum of the CNCs prepared from birch kraft pulp was compared with previously encountered particle populations in FCM. As an example of very hydrophilic particles, polyelectrolyte complexes (PECs) from a commercial cationic starch mixed with carboxymethyl cellulose was chosen (Fig. 3). It is known that these PECs are very hydrophilic particles, since they consist of hydrated polysaccharide chains locked in contact by the attraction of oppositely charged groups (Strand et al. 2018). Calculations showed that the average red fluorescence intensity of this PEC population was 0.35. As an example of very hydrophobic particles, colloidal wood pitch droplets in suspension was chosen (Fig. 3). It is known that these particles are hydrophobic, since they consist of lipophilic extrac- tives dispersed in water. The colloidal wood pitch has been reported to consist of a core of triglycerides and steryl esters, with an outer shell of surface active resin and fatty acids (Qin et al. 2003). Calculations showed that the average red fluorescence intensity of the colloidal wood pitch population was 8.33. When reexamining the two particle populations in the CNCs from birch kraft pulp alongside the PEC population and the colloidal wood pitch population, it is clearly visible that there are traces of these two populations within the CNC sample (Fig. 3). By gating the populations separately, it was possible to calculate the average red fluorescence intensity of the hydrophilic CNC population and the more hydropho- bic population. The hydrophilic part of the birch kraft CNC had an average hydrophobicity value quite close to that of hydrated polysaccharide surfaces, i.e. 0.41. The hydrophobic part of the birch kraft CNC had an average hydrophobicity closer to the average hydrophobicity of colloidal wood pitch, i.e. 5.75. The hydrophilic part of the CNC was slightly more hydrophobic than clean hydrated polysaccharide sur- faces of the polyelectrolyte complexes (Fig. 3a). The hydrophobic part of the CNC was less hydrophobic than colloidal wood pitch on its own (Fig. 3c). This indicated that the two populations were in fact a
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