Water vapor adsorption dynamics of plant fibers
Water vapor adsorption experiments were conducted at 25 ℃ and 80 % RH, and the moisture adsorption curves of the experimental samples were shown in Fig. 2a. It exhibited the corresponding increase of MCe with the increasing content of glycerol, evidenced by 14.22 %, 19.64 %, 25.03 %, and 34.41 % for ZZP0, ZZP1, ZZP2 and ZZP3, respectively. The prolonged hygroscopic equilibrium time was another marker by the increased application content of glycerol, evidenced by that 8.38 h for ZZP0, and 18.37 h for ZZP3 (2.19 times). According to Eq. (2), the adjusted R-square of the fitting line correlating Ln(MR) and the time was greater than 0.994 for each sample (Fig. 2b). The calculated Deff values were 1.841×10− 13 m2/s, 1.565×10− 13 m2/s, 1.391×10− 13 m2/s and 1.255×10− 13 m2/s for ZZP0, ZZP1, ZZP2 and ZZP3, respectively, revealing a decrease of Deff upon the increased application of glycerol. The moisture capacity strengthened on the contrary to Deff of the same plant fibers. Fig. S1 displayed the MCe of the samples enhanced with the increase of glycerol especially at higher than 60 % RH. According to the International Union of Pure and Applied Chemistry (IUPAC) classification, as shown in Fig. S1a, the adsorption property of ZZP0 was characterized by the sigmoidal shape (type Ⅱ) isotherm (Hill et al. 2010; Popescu et al. 2014) and an absolute hysteresis was observed around 80 % RH (Hill et al. 2010). In contrast, the adsorption isotherms of ZZP2-3 (Fig. S1c-d) conformed to type Ⅲ (Popescu et al. 2014), and the hysteresis phenomenon barely existed for ZZP2-3. It verified that the added glycerol as a humectant changed the adsorption property of the plant fiber. The variation trend of these adsorption isotherms illustrated the interaction mechanisms between water vapor and the samples were changed, that reversible formation of hydrogen bonding occurred during the moisture adsorption for the plant fiber with glycerol was concluded (Hill et al. 2010).
The water vapor adsorption performance of the plant fibers could be mainly ascribed to the distributed adsorption sites, as amorphism region (Garg et al. 2021), hydrophilic functional groups, etc, and the diffusion resistance associated with the pore characteristics and permeability (Jiang et al. 2017).
As a major component of the plant fibers, cellulose contains a crystalline structure with intra- and inter-molecular hydroxyl groups (Dereka, et al. 2021). Water molecules on the plant fibers are linked to their amorphism region by the hydrogen bond. Figure 3 shows XRD patterns of all the prepared glycerol-loaded samples. The background signal including the sample holder and basal signal should be subtracted from XRD data to optimize the fitting progress, and this measure could ensure a stable baseline fitting, demonstrating the XRD patterns with nearly zero intensity at 30° 2θ (Fig. 3). The XRD curves of these samples presented that Miller indices of (1–10), (110) and (200) peaks located at about 15.04°, 16.04° and 22.49° 2θ respectively, which could be the main contributors of the diffraction intensity and exhibited cellulose Ⅰ (French, 2014; Wang et al. 2016). Additionally, the indices of (011) and (012) for the peaks at about 12.16° and 20.34° 2θ respectively indicated the presence of cellulose Ⅱ (French, 2014). Hence, ZZP0-3 would have the crystal structure of cellulose Ⅰ and Ⅱ. The CrI of the experimental samples should be calculated by the integrated area ratio of deconvoluted crystalline peaks and the whole diffraction curve (French and Cintrón 2013; French 2020). Deconvolution method with Voigt function (Yao et al. 2020) representing the separate peaks associated with crystalline structures and amorphous area was proposed to analyze the CrI of the samples. Figure 3 showed that CrI decreased from 88.02–83.78% upon the increase of glycerol-loading and the R2 of the fitting for every sample was above 0.9978. This was likely deduced to the fact that glycerol enlarged the amorphism region of the material whose crystalline structure was not affected, neither its surface morphology (see Fig. S2). The enlarged amorphism region promoted the WHC of the plant fibers.
The hydroxyl group in glycerol seemed to have contributed to the formation of hydrogen bond between the water molecule and the sample. Both the hydroxyl group and the MC were positively correlated with the glycerol content during the water vapor adsorption. For example, ZZP3 had the highest glycerol content and the MC value. FTIR spectroscopy could reveal the presence of organic functional groups in the plant fibers (see Fig. S3 and Table S1), especially the existence of hydrogen bonding structures which agreed with the literature on this type of materials (Bu et al. 2018). As shown in Fig. 4, the wide band at 3330 cm− 1 could be attributed to -OH vibrational stretching (Bu et al. 2018; Kolbuk et al. 2020), 2900 cm− 1 to the -CH stretching, 997 cm− 1 to methylene (-CH2-), 1051 cm− 1 to methine (≡ CH) and 1025 cm− 1 to the alcoholic hydroxyl vibration (C-OH) (Lorenzo et al. 1999), respectively. The peaks at 2900 cm− 1, 997 cm− 1, 1051 cm− 1 and 1025 cm− 1 were enhanced, and the water adsorption sites (Taniguchi et al. 1978) increased due to the inter-molecular force between the plant fiber and the added glycerol according to the increase of glycerol content (Fig. 4a). ZZP3 presented a dramatic increase of 3330 cm− 1 and 1647 cm− 1 peak patterns after water vapor adsorption (Fig. 4b), the later attributed to the H-O-H stretching of absorbed water (Dereka et al. 2021), and WHC of the plant fibers was promoted due to the interaction in the water-glycerol-plant fiber system, including hydrogen bonds and adsorption site in the amorphous zones of the plant fibers, particularly the hydrogen bonds between the glycerol and water.
These findings showed that loading with glycerol extended the equilibrium time and decreased Deff of the plant fibers (Fig. 2). It could be deduced that outstanding moisture capacity cost longer equilibrium time during the water vapor adsorption, and the added glycerol enhanced the water transport resistance due to the change of pore characteristics (possibly pore structure, pore size or porosity) and the water permeability of the plant fibers.
According to Fig. 5a, the samples demonstrated macroporous structure (25–75 µm, dominantly) by MIP, and the loading of glycerol reduced the porosity of the plant fibers. ZZP1 indicated an obvious decline of macroporous proportion comparing to ZZP0, while the macroporous proportion of ZZP2 and ZZP3 reduced gradually comparing to ZZP1 (Fig. 5a). The porosity values were 80.32 %, 77.83 %, 75.88 % and 67.52 % for ZZP0, ZZP1, ZZP2 and ZZP3, respectively (Fig. 5b). By inference, some glycerol molecules impregnated into the macropore preferentially during the impregnation, and others possibly binded the surface of the plant fibers. Glycerol occupied part of the pore volume, and fiber layers wrapped with glycerol extended the path of water molecules in further during the water vapor adsorption. The permeability of the sample reflected the mass transport capacity of the sample (Liu et al. 2006), and the values were 1.13628×10− 11 m2, 1.1179×10− 11 m2, 1.11382×10− 11 m2 and 1.09365×10− 11 m2 for ZZP0, ZZP1, ZZP2 and ZZP3, respectively.
The porosity of the plant fibers decreased with the increase of glycerol content (Fig. 5b), and the decline of macroporous proportion and porosity (Fig. 5c) caused the permeability reduction. These factors led to the rise in the water diffusion resistance of the plant fibers increased during the water vapor adsorption.
Effect of glycerol on the water state of plant fibers
2D LF-NMR relaxation spectra reflect the change of water state and distribution (Melo et al., 2021) in the plant fibers caused by glycerol during the water vapor adsorption, SR-CPMG sequence was applied to capture 2D T1-T2 maps of the samples. Three main water states in plant fibers could be distinguished clearly, according to T2. T21 in the range of 0.02-10 ms was assigned as bound water (strongly adsorbed water), T22 10–80 ms as immobile water, and T23 > 100 ms as free water (Wang et al. 2020b; Yang et al. 2020).
ZZP0 in dry state presented a small amount of bound water (Fig. 6a), which T21 was 0.21–2.5 ms. As water vapor adsorption time extended, bound water and free water coexisted in ZZP0, the former was given priority. Figure 6b presented that there were bound water mainly (T21 0.21–2.5 ms), free water partially (T23 80–193 ms) when ZZP0 achieved to moisture adsorption equilibrium. This result was similar to other biomass and porous media for water vapor adsorption (Wang et al. 2020b).
ZZP3 showed differences in water state and its distribution compared with ZZP0 at dry state. Glycerol affected the interaction between water molecules with the plant fibers for ZZP3. The proton signal of glycerol and fibrous matrix overlapped partly due to the interaction between them, and merely bound water (T21 0.021-3.5 ms) existed in ZZP3 at the dry state (Fig. 7a). The outstanding hydrophilicity of glycerol was proved according to Fig. 7b, indicating that the proton signals of glycerol and immobile water were indivisible when ZZP3 attained moisture adsorption equilibrium. Even the WHC of the samples improved significantly, there was no free water existing in the plant fibers loaded with glycerol.
Figure 8a declared that there was a small amount of water left in per sample and the MC was proportional to the content of glycerol at the dry state. ZZP0 demonstrated the lowest intensity, and T21 0.21–2.5 ms represented bound water only, while the other samples contained bound water and immobile water mixed with glycerol whose intensity was detected at T22 10–35 ms.
The amount of bound water heightened with the increase of glycerol content, while their immobile water decreased. With the time of moisture adsorption extended at 25 ℃ and 80 % RH, T2 relaxation spectra of the four samples were shown in Figs. 8b-f respectively. The peak area of T22 (immobile water mixed with glycerol) accounted for a major proportion and the amount of immobile water increased significantly, while the corresponding peak area of T21 (bound water) was much less and increased slowly. With the MC of the samples enhanced, the value of relaxation time T22 shifted to the right, especially for the high glycerol content samples ZZP2 and ZZP3. When the immobile water closed to the saturation point, the peak area of T22 moved towards stability, and that of T21 gradually enlarged until the moisture adsorption equilibrium was achieved. The result indicated the adsorption of bound water responded behind immobile water for the plant fibers added with glycerol. When ZZP0 reached its moisture adsorption equilibrium, the free water distribution pattern of T23 could be monitored (Fig. 8f), while no free water signal appeared in ZZP1-3. We postulated that water molecules entered the plant fibers with glycerol as immobile water preferentially during the water vapor adsorption, and when the immobile water was closed to the saturation, part of them became bound water, simultaneously external water molecules supplemented the vacancy of immobile water until the bound and immobile water were saturated. During the whole adsorption process, the bound and immobile water were the main forms of water molecules for the plant fibers loaded with glycerol.
Moisture adsorption mechanisms of plant fiber before and after loading with glycerol
Figure 9 illustrates the moisture adsorption behavior during the transfer of water molecules in ZZP0 and ZZPX loaded with an amount of glycerol. Comparing with ZZP0, glycerol decreased the porosity and CrI of the plant fibers, enlarged the amorphism region and increased the active adsorption sites due to intra- and inter-molecular hydrogen bonds for ZZPX. As a consequence, glycerol enhanced the MCe of the plant fibers. However, it strengthened the moisture transport resistance (Alak et al. 2000), and caused a decrease of Deff.
For ZZP0, as the moisture adsorption time extended, the water molecules entered the surface and amorphous zone (Stevanic and Salmén, 2019) of the plant fibers and transformed into bound water, simultaneously part of the water existed in the macropore or on the surface of ZZP0 as free water. For ZZPX, water molecules entered the plant fibers as the immobile water preferentially, due to the hydrogen bond interaction between glycerol and water molecules. When the immobile water tended to be saturated, part of the immobile water acted with the amorphous zone of the plant fibers and transformed into bound water, simultaneously external water molecules supplemented the vacancy of immobile water until the bound and immobile water were both saturated due to the hydrogen bond interaction between glycerol and water molecules.