Barrios et al. Biotechnology for Biofuels and Bioproducts
(2025) 18:48
Page 16 of 23
classified based on their thermodynamic response and are separated by a thermodynamic boundary known as the capillary condensation point or fiber saturation point (FSP) in the case of cellulosic fibers [99, 100]. Bound water is absorbed below the capillary condensation point, which is characteristic of porous and hydrophilic materials [20]. Capillary condensation occurs in narrow capillaries at pressures lower than bulk water’s normal saturation vapor pressure [101]. Consequently, the interaction of water with cellulosic surfaces leads to two subcategories of bound water: FBW and NFBW [102]. NFBW is tightly associated through hydrogen bonding with the fiber matrix, particularly within the first hydration layers, preventing water first-order phase transitions, such as crystallization or melting [19]. As the hydration layers across the fiber matrix increase, the interaction strength between water layers is reduced, allowing water molecules to move, accommodate, and condense [103]. On the other hand, the FBW exhibits a depressed melting point response due to the lower capillary pressure in the fiber’s matrix. As the hydration layers extend beyond the fiber saturation point, the cellulose–water interactions decrease, and water exhibits bulk-like behavior [99]. The effect of enzymatic treatments on bleached softwood fibers has previously been reported [45]. However, no significant changes in bound water content were observed with mild treatments using cellulases. The effects of enzymatic treatments have shown to be highly dependent on the type of enzyme and the nature of the substrate employed [104]. The effect of cell-free enzyme treatments on the bound water content of the hardwood fibers is shown in Fig. 6. As expected, refining (Refined 1k) increases the amount of bound water (BW) due to the enhanced exposure of fiber surfaces. The increase in surface area leads to a higher association of water molecules with the fibers [105]. Interestingly, the application of enzymes (0.5E, 0S) and the independent addition of cationic starch (0E, 0.5S) both resulted in a decrease in FBW content, while NFBW remained unchanged. The lack of effect of the enzymes on NFBW can likely be attributed to the nanopore dimensions within the fiber matrix, where NFBW is typically located. These nanopores are too small for the enzymes used in this study to access and modify, explaining why NFBW was unaffected by the treatment [106]. Computational studies have shown that the water layers corresponding to bound water are inside microfibril bundles, highlighting the uniqueness and complexity of structural hierarchy and chirality of natural fibers [107, 108]. On the other hand, a synergistic effect was observed when enzyme and starch treatments were combined (0.5E, 0.5S), leading to a reduction in the overall BW
measurements of different thermal properties, such as HRW and BW, were conducted; the results are shown in Fig. 6. The HRW content of cellulosic fibers is a critical parameter that describes the amount of water strongly bound to fibers, which remains difficult to remove during drying [44]. It is defined as the water content present during the transition from the constant rate drying zone to the falling rate drying zone [46]. This transition point, identified through the second derivative curve during TGA, allows for the precise calculation of HRW by dividing the mass at the transition stage by the mass of the dried fiber [45, 95]. Refining is crucial in increasing the HRW content due to changes in fiber structure, such as internal and external fibrillation, fiber shortening, and fines formation. These structural modifications increase the fiber surface area and expose more hydroxyl groups, which can hold more water molecules. The data presented in Fig. 6 show that refining significantly increases HRW content from 1.48 g water/g dry sample in unrefined fibers to 2.02 g water/g dry sample in fibers without enzyme or starch treatment (0E, 0S). This increase aligns with the expected outcome, as refining exposes more of the fiber surface area to water, enhancing its water retention capacity [21]. The effect of enzymatic pretreatment on HRW content was also significant. For example, when 0.5% enzyme (0.5E) was added without starch (0S), the HRW content decreased slightly to 1.91 g water/g dry sample. However, the combination of 0.5% enzyme and 0.5% cationic starch (0.5 A, 0.5S) resulted in the lowest HRW content at 1.49 g water/g dry sample. This suggests a synergistic effect between the enzyme and starch treatments in reducing water retention, possibly due to the enzyme’s ability to clean fiber surfaces, making them more receptive to cationic starch, which then acts to reduce fiber–water interactions [96, 97]. Bound water Identifying different types of water in cellulose fibers is challenging when relying solely on water retention value experiments or other dynamic dewatering methods. Specific spectroscopic and calorimetric techniques are necessary to accurately distinguish between these water types, which can detect the interactions between water and cellulose. Modulated Differential Scanning Calorimetry (MDSC), compared to conventional DSC, allows for precise quantification of first-order phase transitions, such as melting and crystallization, which are observed in the non-reversing curve of the thermal response. Within the fiber structure, two primary types of water exist: free water and bound water, which result from the strength of interaction between cellulose and water molecules [98]. These distinct types of water are
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