Anion Bridging-Induced Structural Transformation of Cellulose

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Anion Bridging-Induced Structural Transformation of Cellulose Dissolved in Ionic Liquid Takatsugu Endo,*,† Shota Hosomi,† Shunsuke Fujii,† Kazuaki Ninomiya,‡ and Kenji Takahashi*,† †

Faculty of Natural System, Institute of Science and Engineering and ‡Institute for Frontier Science Initiative, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan S Supporting Information *

ABSTRACT: We performed structural investigations of cellulose mixed with 1-ethyl-3methylimidazolium acetate ([Emim][OAc]) in the entire concentration range (0−100 mol %) by wide-angle X-ray scattering with the aid of quantum chemical calculations and 13 C solid-state NMR spectroscopy. We particularly focused on a highly concentrated region (≥30 mol %), which has previously been overlooked. At concentrations of 15−30 mol %, a periodic peak corresponding to cellulose chain alignment emerged; this is associated with a lyotropic cholesteric liquid-crystalline phase. At concentrations of ≥30 mol %, the structure is transformed into ordered layers where OAc anions and Emim cations intercalate. This transformation is found to be driven by a change in the interaction between the IL anions and the OH groups of cellulose. At low concentrations, the anion mainly interacts with the OH group of cellulose in a 1:1 ratio, as previously reported; at high concentrations, the anions bridge the OH groups of two cellulose chains.

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Here, we focus on a highly cellulose concentrated regime in ILs. Microcrystalline cellulose (Avicel PH-101) was mixed with 1-ethyl-3-methylimidazolium acetate ([Emim][OAc]), which is the most representative IL for cellulose dissolution, with various IL mol % ratios, as shown in Figure S1. The definition of mol % here is the number of moles of glucose unit divided by the total number of moles (IL + glucose unit in cellulose), and mol % is approximately equal to wt % in this system. At room temperature, 26−28 mol % is considered the upper limit of complete dissolution, based on previous experimental reports.11,12 This is expected from a stoichiometric standpoint because at 25 mol % the ratio of IL anions and OH groups is 1:1. For these reasons, structural investigations have never been performed at concentrations greater than 30 mol %. In this study, cellulose/IL mixtures within the entire concentration range (0−100 mol %) were subjected to wide-angle X-ray scattering (WAXS) with the aid of computational calculations and 13C solid-state NMR spectroscopy. Figure 1a shows the WAXS patterns of cellulose/[Emim][OAc] mixtures; two intriguing features were observed here. A characteristic peak that does not exist in the pattern of neat cellulose emerged in the q range of 3−8 nm−1 for the mixtures with concentrations of 15−80 mol %. Furthermore, complete destruction of the original cellulose crystalline structure was observed at concentrations as high as 40−45 mol %, which is significantly greater than the concentration previously defined as the upper limit of complete dissolution (26−28 mol %).11,12

he molecular states of solutes and solvents in polymer solutions govern the physical properties of these solutions; consequently, these states are of great importance in polymer chemistry and engineering. Cellulose is a particularly significant polymer because it is a renewable resource and the most abundant biopolymer on Earth. However, cellulose is difficult to dissolve in conventional solvents, and only a few complicated solvent systems are known;1,2 this limits our knowledge of the properties of dissolved cellulose. In 2002, a new class of cellulose solvent, ionic liquids (ILs), was discovered;3 ILs are salts with melting points below ambient temperature. They may be breakthrough solvents for cellulose due to their outstanding properties, for example, high designability (numerous cellulose-soluble ILs have already been reported4) and high thermal/chemical/ electrochemical stability; also, ILs require no cosolvents and possess the practical characteristics of nonflammability and nonvolatility. Many studies have been devoted to understanding the interaction between cellulose and ILs; the most critical finding is that IL anions play a prominent role in cellulose dissolution by interrupting the intra/intermolecular hydrogen bonding of cellulose.4−6 Most of these anions interact with the OH groups of cellulose in a 1:1 ratio,7 while recent molecular dynamics (MD) simulations have revealed that anions partially interact with more than one OH group.8−10 In general, the concentration of a solution drastically changes the states of the solute and solvent, particularly at high concentrations, where solute molecules can self-assemble. However, research on concentrated cellulose/IL systems has been severely lacking to date. © XXXX American Chemical Society

Received: October 27, 2016 Accepted: November 29, 2016 Published: November 29, 2016 5156

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presence of this peak has not been reported for cellulose/IL systems, this is not surprising because a cholesteric liquidcrystal phase has already been observed for cellulose/[Emim][OAc] in solution with concentrations in the range of 10−19 mol %.20 The liquid-crystalline phase has been proven to exist at concentrations as high as 70 mol % by our polarized microscopic observations (Figure S3). However, a significant finding is that two components are buried in this peak, as demonstrated by the second derivative analysis of the 50 mol % mixture (Figure 1a inset). To the best of our knowledge, the presence of this minor component has never been reported in any cellulose-related system. We sought to determine the exact structural origin of these peaks. Figure 1b shows the curve fitting results of the peaks, and Figure 1c summarizes the derived periodic lengths. At low concentrations of 15−30 mol %, only one peak is observed; its periodic length varies with concentration. In contrast, at the 35−70 mol % range, two peaks appear, and the peak positions of both components are independent of cellulose concentration. The average lengths of these peaks are determined to be 1.17 ± 0.02 and 0.94 ± 0.03 nm, respectively. Considering that the cellulose chain alignments expanded in the presence of the IL, the interaction between the OAc anions and the OH groups of cellulose is expected to play a dominant role.5,6 A model structure is calculated (Figure 2a), and the cellulose chain distance is estimated to be 1.21 ± 0.04 nm. The excellent agreement between the observed and calculated lengths demonstrates that this structure is the origin of the major component (blue in Figure 1b,c). On the basis of this model, which forms a sheet (Figure 2a), intersheet interactions may occur; consequently, periodic structures may exist. MD simulations indicate that the imidazolium cations are located above and below the glucose rings via weak (hydrophobic) interactions,5,6 and then, a model reproducing the cellulose− cation−cellulose interaction is evaluated, as shown in Figure 2b. This provides a cellulose distance of 0.80 ± 0.03 nm, which is somewhat smaller than the experimental value of 0.94 nm for the minor component. In pure cellulose, the scattering intensity from the intersheet spacing (q = 15.8 nm−1) gives the strongest peak,21 while the peak from the 0.94 nm spacing is very weak. This suggests that the intersheet periodic structures are disordered and/or fluctuate due to weak interactions between the cation and cellulose, which is relevant to the presence of the cholesteric phase (each layer is twisted with a certain angle). This disorder and/or fluctuation is likely the reason for the underestimation of the d spacing by calculation, in addition to the simplified calculation model (MM method) employed.

Figure 1. (a) WAXS patterns of cellulose/[Emim][OAc] mixtures. The inset shows the second derivative of the 50 mol % mixture pattern (exposure time: 13 h) after pattern smoothing operations (box smoothing). (b) Curve fitting results for the peak in the low q region. (c) Period lengths of the major (blue) and minor (orange) components. The green dashed line represents the periodic length of 0.33 nm, corresponding to the peak emerging at 19 nm−1 (see Figure S2).

This result agrees well with the ones from solid-state NMR spectroscopy (vide infra). Because it has been revealed that the latter feature is strongly associated with the former, we will first address structural assignment of the peak that emerged in the low q range. Actually, some concentrated cellulose/cellulose derivative solutions13−16 and cellulose derivatives17−19 are already known to exhibit these peaks. The peaks have been assigned to the periodicity of the cellulose chain alignments, which is associated with a cholesteric liquid-crystal phase. Although the

Figure 2. (a) Model structure of cellulose chains intercalated by OAc anions, calculated by the ONIOM method. The upper and lower layers represent the ball frame and the wire frame, respectively. (b) Model structure of cellulose chains intercalated by the Emim cation. Two glucose model molecules22 and one Emim cation were used for the MM/UFF calculations. 5157

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interaction is mostly 1:1, which is consistent with previous results.7−10 In contrast, when the ratio of OAc/OH becomes 1:2 (corresponding to 40 mol %), the bridging B1 state is the dominant state (B1: −131.5 kJ mol−1 > N1: −69.7 kJ mol−1). These findings demonstrate that increasing the concentration alters the interaction between the anions and OH groups. The phenomenon of switching between the bridging and nonbridging states by changing the concentration is in good agreement with the 13C NMR chemical shift behavior, as shown in Figure 4a. The 13C peak of the anion COO site moves downfield relative to that of the pure IL when the cellulose concentration increases (Figure 4b). This behavior can be explained by the change in the interaction partners of the anions from the cations to the OH groups of cellulose.23,24 The downfield shift continues even at concentrations greater than 25 mol %, although the anion partners should be completely switched below this concentration. The chemical shift change between 25 and 40 mol % represents a switch from the nonbridging to the bridging state of the anion. This is validated by the calculated results, which show a clear difference in the 13 C chemical shift of COO between the nonbridging and bridging states (Figure 3 bottom). The change in the chemical shift of the COO peak is minimal at concentrations greater than 40 mol %; this also appears to be the case for the other 13C sites (Figure 4b,c). This demonstrates that above this concentration the anions do not change their states; that is, they maintain their bridging form. The results of this study are summarized in Figure 5. Because the cations interact with the glucose rings weakly and there are Coulombic interactions between the cations and the anions, the location of the cations can be anywhere between the sheets in the anion-bridging region. To the best of our knowledge, this is the first report of a cellulose-related system with a layered structure that exhibits intersheet periodicity; therefore, it is worthwhile to mention the unique features of the cellulose/ [Emim][OAc] mixture. Compared to other cellulose solvents, [Emim][OAc] has two notable characteristics. First, this IL provides very strong hydrogen-bonding interactions with OH groups (ca. −70 kJ mol−1, several times stronger than conventional hydrogen bonds), which additionally are not interfered with by the presence of anticosolvents, for example, water (such solvents are frequently used in other cellulose solvents but significantly disrupt hydrogen-bonding network of cellulose solutions). Second, OAc anions, as well as Cl and alkyl phosphonate/phosphate anions (representative anions in ILs for cellulose dissolution), are known to simultaneously interact with multiple OH groups.8−10,27,28 These features may enable the OAc anions to form bridges and, consequently, ordered periodic structures. The complete deconstruction of the crystalline structure of cellulose at IL concentrations of 40−45 mol % (Figure 1a), the second characteristic feature in the WAXS patterns, is easily explained by the bridging phenomenon. The anion bridging enables the IL to deconstruct the cellulose crystals at this high cellulose concentration because OAc/OH = 1:2 at a concentration of 40 mol %. The complete deconstruction at 40 mol % is also confirmed by the 13C solid-state NMR results (Figure 4d). The NMR results indicate that this deconstruction accompanies the conformational transformation of the hydroxymethyl group of cellulose. The loss of crystal structure may have induced chain mobility and twisting,28,29 which disrupted the formation of weak interactions and, consequently, resulted in the disappearance of

Note that a minor component emerges, accompanied by a small broad peak at 19 nm−1 (corresponding to the 0.33 nm distance in real space), which is distinct from the original cellulose peaks (Figures 1a,c and S2). Although the structural assignment is not yet conclusive for this broad peak, it may be due to short-range ordering stemming from the intramolecular d spacing of cellulose, possibly superimposed scatterings from high-Miller-index planes, as seen in the original cellulose structure.21 These results demonstrate that at relatively low concentrations of 15−30 mol % the cellulose chains align with onedimensional periodicity; meanwhile, this structure is transformed into a highly ordered structure with three-dimensional (layered) periodicity at concentrations above 30 mol % (please note that at concentrations greater than 40 mol % dissolved and undissolved regions coexist). The next question that arises is why this transformation occurs. Considering the results from Figures 1 and 2, it has been revealed that the driving force is bridging of the OH groups of two cellulose chains by the anion (anion bridging) through hydrogen bonding. This bridging induces the formation of sheets with alternative cellulose−anion alignments, which allows formation of the layered structure intercalated by cations through hydrophobic interactions. However, this bridging phenomenon appears to contradict previous studies, which reported that OAc anions mainly interact with the OH groups of cellulose in a 1:1 ratio.7−10 This can be simply resolved by considering the concentration difference. Figure 3 shows four model compounds that mimic

Figure 3. Four models for hydrogen bonding between IL anions and the OH groups of the alcohols. B1/B2 and N1/N2 denote the states where the anion bridges two alcohols and where it interacts with one alcohol, respectively. Eint represents the calculated interaction energy between the anion and OH groups, while CS represents the calculated 13 C chemical shifts for COO in the anion.

the anion−OH group hydrogen-bonding interactions: two are in nonbridging states (N1/N2), and the other two are in bridging states (B1/B2). When considering the existence of two OAc/OH sets (corresponding to 25 mol %), the formation of two N1 states is the most energetically favorable (−69.7 × 2 = −139.4 kJ mol−1). This indicates that at a concentration of ≤25 mol %, the ratio of the anions and the OH groups in the 5158

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Figure 4. (a) 13C solid-state NMR spectra obtained at 5 kHz of MAS for the cellulose/[Emim][OAc] mixtures. CP + HPD (black): 30−100 mol %. HPD only (gray): 0−30 mol %. At 30 mol %, the efficiency of CP decreases significantly, while observation of the HPD spectra is facilitated. The C4 site of the glucose unit is split into two peaks, characterized as the C4 conformations in the crystalline (Cr, 89 ppm) and noncrystalline (NCr, 84 ppm) phases.25 The C6 site shows three peaks, assigned to the trans−gauche (tg), gauche−trans (gt), and gauche−gauche (gg) conformations of the hydroxymethyl group.2,26 (b) Chemical shift differences of the IL peaks. Values from the CP and HPD spectra are represented by filled and open symbols, respectively. (c) Magnification of (b). Because the peaks of Im 4 and Im 5 overlap, the average values are displayed. (d) Peak area ratio dependence on the IL concentration for the C4 and C6 sites.

Figure 5. Schematic summary of this study. At concentrations ≤ 25 mol %, the anions mainly interact with the OH groups in a 1:1 ratio (nonbridging), providing cellulose chain alignment. With increasing concentration, the nonbridging state is gradually transformed to the state where the anions interact with multiple OH groups in two different cellulose chains (bridging), which induces a layered structure. At 40 mol %, the transformation is mostly completed, while at the higher concentrations, the intact cellulose region is mixed.

periodic structures with lengths of 0.33 and 0.94 nm at ≤30 mol % (Figure 1c). Around this concentration, the anion interaction shifts from a bridging to a nonbridging state with decreasing concentration. This leads to an increase in the number of anion molecules that intercalate between the cellulose molecules (Figure 5 left); consequently, the d spacing of the cellulose chains widens (Figure 1c). These WAXS results strongly imply that in the dynamic dissolution process the anions initially disrupt the hydrogen bonding of cellulose due to their preference for the bridging form; finally, when the number of anions increases, they transform to the nonbridging form. In summary, we performed structural investigations on cellulose/([Emim][OAc]) mixtures in the entire concentration

range, particularly focusing on high concentrations (≥30 mol %), which have been totally unexplored to date. In the lyotropic cholesteric liquid-crystalline phase regime, we discovered a structural transformation between a cellulose-aligned structure with one-dimensional periodicity and an ordered layer structure at concentrations of approximately 30 mol %. This is the first time that the latter structure has been observed in a celluloserelated system. The driving force for the transformation is a change in the interaction between the anions and the OH groups of cellulose from a nonbridging to a bridging state. The bridging phenomenon and the resulting transformation are observed because the OAc anion can form very strong hydrogen bonds (without interference due to the presence of an anticosolvent) and can undergo multiple interactions. 5159

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Because these features are also expected for Cl and alkyl phosphonate/phosphate anions (representative anions in ILs used for cellulose dissolution), bridging and structural transformation may be general phenomena in cellulose/IL mixtures. It should be also noted that the structures observed at high concentrations may exist as intermediate states during cellulose dissolution in the IL.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b02504. Experimental details, curve fitting procedures and parameters used for NMR, optical micrographs, additional X-ray data, magnification of figures, and sample stability (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: tkendo@staff.kanazawa-u.ac.jp (T.E.). *E-mail: ktkenji@staff.kanazawa-u.ac.jp (K.T.). ORCID

Takatsugu Endo: 0000-0002-7272-0715 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported in part by the Advanced Low Carbon Technology Research and Development Program (ALCA) (Grant Number 2100040), the Cross-ministerial Strategic Innovation Promotion Program (SIP), and the Center of Innovation Program “Construction of next-generation infrastructure using innovative materials: Realization of safe and secure society that can coexist with the Earth for centuries” from the Japan Science and Technology Agency and also by JSPS KAKENHI Grant Number 15H04193.

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REFERENCES

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