Estimation of Water Content in Cellulose Materials
Many technical issues, such as those involved in developing wastewater remediation methods, concern the interaction between cellulose and water. That is why, it is important to understand how water interacts with cellulose materials. It is known that a cellulose material contains alternating crystalline and amorphous phases, and thus, it is possible to consider it as a biphasic system. We think that it is rational to study the water–cellulose interaction mechanism in two stages. In the first stage, the water quantity that penetrates in the two types of areas is determined. In the second stage, the molecular mechanism of interaction is studied. The ability of water molecules to make hydrogen bonds with cellulose chains lies at the basis of this interaction.
The infiltration of water molecules into the space between the chains leads to a redistribution of the hydrogen bonds in the areas adjacent to these water molecules and changes the structure of these areas. We think that this redistribution is different for crystalline and amorphous areas because of the differences in their initial structure. Is there a possibility for water molecules to penetrate crystalline areas? At the first glimpse, the ordered structure of these areas excludes such a scenario. However, literature data bring evidence in favor of penetration. Warwicker et al. discussed the function of sheets of cellulose chains in the swelling reactions of cellulose. In the study by Papkov et al., it was noted that the transition from cellulose I to cellulose II cannot happen without the intracrystalline swelling stage, when water is present in crystalline areas. More recent works have also reported evidence about the penetration of water molecules into cellulose crystallites or modelled cellulose–water interactions in crystalline areas. As mentioned above, a cellulose material may be considered as a diphasic system because it contains alternating crystalline and amorphous areas.
The latter have large free volumes because of their disorder. These free volumes are partially realized as voids. The existence of voids has as consequence the fact that water molecules fill these voids in the first place, when the cellulose material is in contact with water. At the same time, there is an opinion that the order in the crystalline areas excludes water penetration into these areas. Agarwal et al. show the size of cellulose crystallites depends on the presence of water in cellulose materials of different types. These two different points of view are a good basis for further investigation provided in this work. In this study, we not only concentrate on the questions whether water penetrates cellulose crystallites or not, but also we try to estimate the water quantity in the crystalline and amorphous areas and to propose a method that would allow fulfilling this task using X-ray and thermogravimetric data.
Article Information: S. Tkachov, Taras Shevchenko National University of Kyiv; Cellulose Chemistry and Technology, 2020.