3.1 The effect of HBD in DESs on nanofibrillation of cellulose
In our previous work, polyethylene glycol (PEG) was used to moisten cellulose to break the hydrogen bond network of cellulose, and then through powerful mechanical processing to prepare cellulose microfiber (CMF) and CNF (Wang et al. 2017). CNF or lignocellulose nanofibers (LCNF) can also be prepared by the combination of glycerol and slight acid system swelling and mechanical treatment (Zhang et al. 2022b). In addition, carboxyl-DESs had been shown to be excellent solvent systems for the preparation of CNF (Liu et al. 2021, Wang et al. 2023c). In the preparation process of CNF, cellulose fibers after solvents pretreatment were more easily stripped off to obtain CNF than untreated cellulose fibers. The hydrogen bond network of cellulose is destroyed after solvent pretreatment, which makes the preparation of CNF easy. Solvents seem to play an important role in breaking down hydrogen bond networks of cellulose. Therefore, the key factors for the preparation of CNF in DES system were investigated in detail.
Firstly, the effect of LA and ChCl pretreatment on the morphology of prepared cellulose fibers was investigated (shown in Fig. 1a and 1b). As can be seen from the figure, CMF can be prepared after LA solution pretreatment combined with mechanical treatment, but the diameter of most cellulose fibers was difficult to reach the nanometer level (Fig. 1a). Analogously, the cellulose fibers pretreated with ChCl solution basically maintained the morphology of the cellulose fibers after beating treatment, and the diameter of the cellulose fibers became slightly thin but without significant change (Fig. 1b). It can be seen that the use of LA or ChCl alone is not effective on cellulose nanofibrillation, because the solvent is difficult to resist the dense hydrogen bond network of cellulose and then enter the cellulose interior to promote cellulose nanofibrillators. However, CNF can be successfully prepared by using LA/ChCl DES pretreatement with mechanical grinding (Fig. 1c). After LA/ChCl DES high-temperature swelling and colloid mill grinding, the fiber diameter was reduced to tens of nanometers. It has been reported that the presence of LA/ChCl DES leads to esterification modification and effective swelling of cellulose, and the interaction between LA/ChCl DES and cellulose destroyed a large number of hydrogen bonds between molecular chains (Yang et al. 2019, Hong et al. 2020). LA/ChCl DES molecules can efficiently diffuse into the dense structure of cellulose by hydrogen bonding and alter its internal interactions. Among them, Cl− competed with hydroxyl groups in cellulose to form hydrogen bonds, resulting in a weakening of strong hydrogen bond interactions in cellulose. Therefore, LA/ChCl DES can depolymerize cellulose by breaking hydrogen bonds in molecular chains under mild conditions (Wu et al. 2023). Although few studies have reported that DESs composed of alcohols, urea or amino acids and ChCl were used for the preparation of CNF, but the effect is generally not up to expectations, and it was only applied to the pretreatment of biomass (Tong et al. 2023, Suopajärvi, Sirviö and Liimatainen 2017, Smirnov et al. 2020). The results of this work were roughly the same. As shown in Fig. 1d and 1e, CNF could not be prepared by treating cellulose in Ur/ChCl and EG/ChCl DES systems, and the fiber diameter remained at the micron level. Only, CMFs were obtained, which also highlighted the advantages of LA/ChCl DES in the preparation of nanocellulose.
The esterification modification of cellulose caused by DESs was also shown in Fig. 1f. It can be found that compared to untreated cellulose, the samples treated with LA and LA/ChCl DES show distinct banding at about 1720 cm-1, which was attributed to the characteristic peak of the carbonyl group. It was proved that the hydroxyl group of cellulose could be esterified with the carboxyl group of LA during LA/ChCl DES pretreatment (Zeng et al. 2023, Lim et al. 2021). Moreover, the peak strength of the cellulose samples obtained by LA/ChCl DES treatment was higher than that of LA system, which proved that the presence of Cl- weakened the strong hydrogen bond network of cellulose, making it easier for LA to contact the hydroxyl group of cellulose for esterification modification. After the treatment of cellulose by Ur/ChCl and EG/ChCl DES, the surface groups of cellulose did not change significantly, and no other significant differences were observed in the FT-IR spectrum, which proved that the chemical modification of cellulose was very limited (Xu et al. 2023, Sánchez-Badillo et al. 2022). Therefore, LA can provide more active protons than other systems in the preparation of CNF, thereby improving the proton mobility of LA/ChCl DES, thereby breaking the hydrogen bond network of cellulose.
3.2 The effect of HBA in DESs on nanofibrillation of cellulose
Hydrogen bond was one of the most important intermolecular directed interactions in cellulose. The free hydroxyl group can form a large number of hydrogen bonds, which made its bond energy become huge. In DESs, HBA and HBD can also form a large number of hydrogen bonds, but their bond energy was low. When DES moistened cellulose and reacts with it, HBD and HBA separate and form a new hydrogen bond network with cellulose, weakening the hydrogen bond network of cellulose itself. Among them, the essence of breaking the cellulose hydrogen bond was to provide a large enough energy to the hydrogen bond, such as heating, mechanical handling or by providing more electronegative HBA molecules (Chen et al. 2023a).
In this work, the effects of anions and cations in HBA on cellulose nanofibrillation were discussed in detail. The effects of different anions on cellulose nanofibrillation were shown in Fig. 2a1-a3. Cellulose was treated with DES composed of three different anions (bromine ion (Br-), lactate ions and dihydrogen citrate ions), and CNF with a diameter of less than 100 nm can be prepared successfully after treatment using LA/ChBr DES (Fig. 2a1). It is found that Br- and Cl- are both halogens, both of them play an important role in opening cellulose chains during cellulose dissolution, and larger Br- are more conducive to the opening of cellulose chains (Lara-Serrano et al. 2020), thus making the structure of cellulose become soft during pretreatment, and the nanofibrillation of cellulose can be successfully achieved with mechanical treatment. In summary, both Br- and Cl- can form new hydrogen bonds with cellulose hydroxyl groups, thus weakening the original hydrogen bonds in the cellulose structure, and making the deconstruction of cellulose fibers simple. However, LA/Ch-Ca (Fig. 2a2) and LA/Ch-La (Fig. 2a3) DES system did not contain any halogen ions, high temperature swelling treatment and colloid grinding were difficult to break the hydrogen bond network of cellulose, the diameter of the cellulose fibers can’t be reduced to the nanometer scale after colloid grinding, and there is no significant change compared with cellulose fibril. The reason may be that HBD formed a strong interaction with LA and could not effectively act on the hydrogen bond of cellulose, which strongly limits the role of the DES towards cellulose crystallite (Sánchez-Badillo et al. 2022). In contrast, CNF can be successfully prepared using DESs composed of different cationic systems containing Cl-. As shown in Fig. 2b1-b3, the diameter of the obtained CNF was less than 100 nm, and the length was tens to hundreds of nanometers with deep colloid mill grinding. CNF can be successfully prepared using LA/NH4Cl and LA/DTAC DESs. It was worth noting that the diameter of the fibers is very uniform and the shape of the fiber seems to be very similar to CNC after LA/ZnCl2 DES treatment. Similarly, recycled molten ferric chloride with strong acidity has been reported as an excellent solvent system for the preparation of CNC from hydrolyzed cellulose. It can penetrate into the cellulose molecular chain, destroy the hydrogen bond between cellulose molecules, and promote the disintegration of CNC (Yang et al. 2023). However, molten ferric chloride can cause excessive damage to the cellulose crystal structure and corrode equipment, if the conditions of LA/ZnCl2 DES treatment of cellulose were properly regulated, this might be a very good systems for preparing CNC. Therefore, the key role of anions in cellulose nanofibrillation was confirmed, and the presence of Cl- would disrupt the cellulose hydrogen bond network and contribute to cellulose nanofibrillation.
The nanofibrillation of cellulose in carboxylic DES system is usually accompanied by surface esterification, which also prevents the cellulose from being over-hydrolyzed to obtain a high-yield CNF (Liu et al. 2021). The FT-IR spectra of cellulose samples also showed the esterification modification of cellulose by DESs (Fig. 2c). The main peak of O−H tensile vibration is 3428 cm-1. At 2901 cm-1, 2856 cm-1, and 897 cm-1, a series of vibration peaks appear corresponding to the deformation vibration of C−H in cellulose. Peaks at 1429 cm-1, 1161 cm-1, 1110 cm-1, 1059 cm-1, and 1033 cm-1 are typical for cellulose (Zhang et al. 2022a). In addition, a distinct band appeared at 1720 cm-1, which was the characteristic peak of the carbonyl group. It is proved that zinc ions, aluminum ions and other transition metal elements have the ability of catalyzing the esterification reaction (Hong et al. 2019; Yuan et al. 2023). It can be seen from the figure, the DES systems containing LA had esterification reaction in the high-temperature swelling treatment of cellulose. In addition, the peak strength of ester bond peaks in DESs composed of different cations (LA/ChCl, LA/ZnCl2, LA/NH4Cl and LA/DTAC.) was higher than that in pure LA systems (Fig. 2c1), and the ester bond peak strength of cellulose treated with LA/ZnCl2 DES system was the highest due to the catalytic action of metal zinc ions. Similarly, an obvious characteristic peak appeared at 1720 cm-1 due to the key role of Br- in the LA/ChBr (Fig. 2c2), indicating that the system was also highly accessible to cellulose. Therefore, the key role of anions is confirmed, more hydroxyl groups were exposed after the cellulose chain was opened, and the esterification reaction with LA was more accessible, which led to a significant increase in the carbonyl peak in the FT-IR spectrum. However, the ester bond peak strength after the treatment of LA/Ch−Ca and LA/Ch−La DES systems was very weak, which also reflected the small effect of the system on cellulose, ultimately limited the cellulose nanofibrillation. It was concluded that anions played an important role in destroying the hydrogen bond network of cellulose and increasing the accessibility of solvents to cellulose.
In addition, the amorphous regions of cellulose were easily removed by DESs during pretreatment, which might increase the overall crystallinity of cellulose (Kumar et al. 2015, Loow et al. 2018). In the latest study, some DESs was used to dissolve cellulose, which undergoes a type I to type II transformation in the dissolution process (Zhong et al. 2022). Moreover, it is reported that only internal or external fibrillation occurred with DES treatment without a great extent of modification in terms of chemical composition and crystallinity (Mnasri et al. 2022). In this work, the CNF obtained by all DES systems maintained the natural cellulose type I structure (shown in Fig. 2d1- 2d2), and the crystallinity of cellulose did not change significantly (about 75%), indicating that the crystal structure of cellulose was not broken by different DES treatment. It was also proved that different anions and cations in this study had similar effects on the crystal structure of cellulose, and the obtained CNF still have high crystallinity.
XPS technology can provide qualitative and quantitative analysis of the elements and chemical states on the surface of the sample, and had a strong ability to analyze the surface groups of cellulose materials (Greczynski et al. 2023). The high-resolution XPS scans over C (1s) peaks of the cellulose samples treated by different solvent systems were shown in Fig. 3. Theoretically, pure cellulose exhibits only two carbon signals in C (1s) XPS spectra: C−O (alcohols and ethers) at binding energy of 286.0 eV and O−C−O (acetal) moieties at around 287.8 eV (Fig. 3a1). The binding energies with peaks at 284.8, 286.6, 287.8 and 288.2 eV were attributed to C−C/C=C groups, C−O/C−OH groups, O−C−O groups and O−C=O groups, respectively (Zhang et al. 2020, Wang et al. 2023a).
The binding energy of the cellulose treated with EG/ChCl and Ur/ChCl systems did not change significantly, indicating that the groups on the cellulose surface did not change (Fig. 3a2-a3), and the same phenomenon also appeared in the H2O/ChCl system (Fig. 3b2). However, The O−C=O groups can be detected by esterification on the surface of cellulose at high temperature of LA (Fig. 3b1). The same reaction also occurred in LA/ChCl, LA/ChBr, LA/ZnCl2, LA/NH4Cl, and LA/DTAC DES systems (Fig. 3c1 and d1-d3), indicating that chloride ions can destroy the hydrogen bond network inside and outside the cellulose molecules during the pretreatmnet, thus more hydroxyl groups were exposed on the cellulose surface for esterification, which protected the cellulose from excessive hydrolysis and promotes the further nanofibrillation of cellulose. The degree of substitution (DS) of CNF prepared by LA/ChCl, LA/ChBr, LA/ZnCl2, LA/NH4Cl, and LA/DTAC DES systems were 0.55, 0.56, 0.73, 0.52, 0.66, respectively. The DS of cellulose obtained by LA treatment alone was only 0.25 (seen in Table 2). It has also been shown that anions have an important contribution to the destruction of cellulose hydrogen bond networks, thereby improving the accessibility of cellulose.
On the contrary, in the LA/Ch-Ca and LA/Ch-La DES systems, although the system contained LA, no O−C=O groups was detected in the cellulose sample (consistent with the results of only weak carbonyl peaks in the two systems in the FT-IR spectrum), indicating that the hydrogen bond network of cellulose cannot be destroyed in this system (Fig. 3c2-c3). As a result, LA molecules cannot penetrate into the interior of cellulose for esterification, and can only undergo a very limited esterification reaction with the hydroxyl group on the surface of cellulose, and ultimately the cellulose cannot be nano-dispersed. It can be seen that Cl- played an important role in the process of cellulose nanofibrillation, weakening the hydrogen bond network of cellulose, and LA can modify the hydroxyl group on the surface of cellulose to increase the dispersion stability of CNF, and at the same time realize the nanofibrillation and surface modification of cellulose under DES systems.
3.3 The effect of hydrogen bond alkalinity of DES on cellulose nanofibrillation
There seem to be many indications that the synergistic effects of chemical reactions and non-covalent interactions favor DESs in regulating CNF formation. Some researchers had analyzed the possible mechanism of the formation of nanocellulose regulated by DESs by using molecular dynamics simulation and quantum chemistry calculation techniques, and speculated that the hydrogen bond and van der Waals force interaction between DES and cellulose were the key factors of cellulose nanofibrillation (Bi et al. 2023, Wang et al. 2023b). Kamlet-Abboud-Taft parameter (KAT value) was often used to calculate the solvent polarity of different dissolution solvents, including hydrogen bond acidity (α), hydrogen bond alkalinity (β) and dipolarity/polarizability (π*) (Eyckens et al. 2016, Zhang et al. 2023b). In this case, the β value obtained in the solvate chroma study may be a good indicator of the ability to dissolve cellulose, since high hydrogen bond alkalinity weakened intermolecular and intramolecular hydrogen bonds in cellulose crystals (Abe, Fukaya and Ohno 2010). The theory also applied to DESs of cellulose dissolution. It was very promising to select components with high hydrogen bond alkalinity anions, such as Cl-, OAc-, HCOO-, etc., to form DESs for cellulose dissolution. Cellulose nanofibrillation disrupt the aggregation structure of cellulose through the destruction of its hydrogen bond network, which leads to a more uniform dispersion of cellulose particles in the solvent. Nevertheless, the dissolution of cellulose involves breaking the hydrogen bond network of cellulose in order to change the interactions between cellulose molecules, enabling cellulose to interact with solvents and form a more stable solution. Similar to cellulose dissolution, weakening intermolecular and intramolecular hydrogen bonds in cellulose crystals also appears to be the first step in the cellulose nanofibrillation. Therefore, DESs with high hydrogen bond alkalinity is very promising for cellulose nanofibrillation.
The Kamlet-Taft empirical parameter curves for different DES systems in different dyes were shown in Fig.4. The absorption peaks of different DESs systems in NR (Fig. 4a), NEt2 (Fig. 4b), Ome (Fig. 4c1) and NH2 (Fig. 4c2) were mainly concentrated at 550−670 nm, 400−450 nm and 300−350 nm, respectively (corresponding calculation results were shown in Table 3). It can be calculated that DESs formed by LA and halogen elements had high β values (> 0.5). Among them, the β value of the system containing Cl− was slightly higher than that of the system containing Br−, indicating that the system had a higher hydrogen bond acceptance capacity. The DESs synthesized by LA and chloride had a relatively high α value, indicating that a strong hydrogen bond was formed between the components of DESs, and the strong attraction of Cl− to the hydrogen proton in LA lead to the delocalization of the hydrogen proton, increasing the hydrogen bond acidity of DESs. It can be concluded that DES composed of LA and chloride had a high β value, which indicated that DES have a high hydrogen bond acceptance capacity to strip the hydroxy hydrogen from cellulose, and LA made the system have a high α value, allowing the glycosidic bond of cellulose to be hydrolyzed and broken, thus reducing the size of cellulose. When DES composed of LA and chloride reacting with cellulose, the original hydrogen bond network of cellulose was greatly changed, and a new hydrogen bond structure was formed. DES molecules easily penetrate into cellulose to moisten and esterify the cellulose, cellulose chains were constantly being opened up, thus making the cellulose nanofibrillation process simple (the deconstructed CNF can be clearly seen in the Fig. 2a1 and b1-b3).
It has been reported that DES composed of carboxylic acid and ChCl can successfully prepare CNF, which is an excellent system for preparing CNF, and can modify cellulose during the nanofibrillation of cellulose (Liu et al. 2021, Wang et al. 2023c). In this study, we demonstrated that carboxylic acid (in the case of LA) and Cl- play a key role in the cellulose nanofibrillation process, the hydrogen bond network of cellulose was easily broken by DESs containing LA and Cl-. DES with Cl- had a high β value, which was easy to form competitive hydrogen bonds with the hydroxyl group of cellulose when cellulose was swelled by DESs at a high temperature, and change the original hydrogen bond network of cellulose. LA molecules will randomly esterify with the hydroxyl group on the surface of cellulose, and the ionized hydrogen protons will randomly break the glucoside bond of cellulose, making the cellulose length shorter and the fibers moistened, fluffy and soft. After subsequent mechanical treatment, CNF can be successfully prepared (the schematic presentation and possible mechanism diagram of CNF preparation process from cellulose treated by DESs is shown in Fig. 5).