This study evaluated the physical-chemical properties and structure of cellulose nanocrystals (CNC) produced from pine cellulose pulp, and dried by two different methods: Spray-Drying (SD) or Freeze-Drying (FD). After drying, CNC were characterized in terms of size, z-potential, elemental analysis, conductometric titration, crystallinity and thermogravimetry. Complementary, analysis by force microscopy, dynamic sorption and infrared spectroscopy were carried out. Results showed that different drying methods did not produce significant differences in zeta potential, crystallinity and degradation temperature. However, CNC dried by FD showed smaller size but higher aspect ratio, and higher sulfur content, which would explain the significant higher sorption observed in CNC-FD at relative humidity higher than 80%. Drying method does not influence the formation or modification of cellulose bonds as FTIR suggested. Our study suggests that how CNC is dried will influence the CNC's structure and will modify some of their physico-chemical properties. This information is relevant since it can be used as an input for the scale-up processing of CNC and would define the performance in some of their technological applications.
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Authors would like to thank the financial support received from FONDECYT Grant N3220289 and Fondo de Apoyo a la Investigación ( FAI UANDES - Postdoctoral Grant).
The sorption isotherms obtained from CNC-FD and CNC-SD (Fig. 3) presented sigmoid shape like those described for hydrophilic materials (Belbekhouche et al., 2011). These curves were well fitted with the GAB model (r2 = 0.99, data not shown), which has been reported before for cellulosic materials (Meriçer, Minelli, Giacinti Baschetti, & Lindström, 2017). The GAB model predicts the moisture content of the monolayer (mo) or minimum hydration layer (Lechuga-Ballesteros, Miller, & Zhang, 2002; Porras-Saavedra et al., 2019), which can be considered useful to determine the optimal conditions in terms of relative humidity, in which the matrix material maintains its structure (Bell & y Labuza, 1984; Arslan & y Toğrul, 2006). Our results showed that mo for CNC-FD was lower than for CNC-SD. On the other hand, the parameter C in GAB model has been related with adsorption energies of the monolayer. Thus, a decreasing C value would suggest water molecules are less strongly bound to polar sites available at the material's surface (Enrione, Hill, & Mitchell, 2007). Interestingly, C value in CNC-FD was higher that the C value obtained for CNC-SD, supporting the fact that nanocrystal structure and sulfur content (higher in CNC-FD) would promote higher water sorption. Regarding K values, an increase in this GAB constant towards a value of 1 (0.9 in CNC-SD and 1.0 in CNC-FD) would suggest a smaller difference between the energy associated with the heat of sorption of the multilayer and the heat of condensation of pure water. This agrees with the lower mo value obtained for CNC-FD, but not with the higher value of the C parameter obtained for CNC-FD. This suggests some limitations related with the interpretation and physical meaning of these constants in relation to our experimental data.Authors would like to thank the financial support received from FONDECYT Grant N3220289 and Fondo de Apoyo a la Investigación (FAI UANDES - Postdoctoral Grant).
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- Cellulose nanocrystals