Intermolecular Interactions and 3D Structure in Cellulose–NaOH–Urea

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Intermolecular Interactions and 3D Structure in Cellulose−NaOH− Urea Aqueous System Zhiwei Jiang,†,‡ Yan Fang,‡ Junfeng Xiang,† Yanping Ma,† Ang Lu,‡ Hongliang Kang,† Yong Huang,† Hongxia Guo,† Ruigang Liu,*,† and Lina Zhang*,‡ †

State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory of Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ Department of Chemistry, Wuhan University, Wuhan 430072, China S Supporting Information *

ABSTRACT: The dissolution of cellulose in NaOH/urea aqueous solution at low temperature is a key finding in cellulose science and technology. In this paper, 15N and 23Na NMR experiments were carried out to clarify the intermolecular interactions in cellulose/NaOH/urea aqueous solution. It was found that there are direct interactions between OH− anions and amino groups of urea through hydrogen bonds and no direct interaction between urea and cellulose. Moreover, Na+ ions can interact with both cellulose and urea in an aqueous system. These interactions lead to the formation of cellulose−NaOH−urea−H2O inclusion complexes (ICs). 23Na relaxation results confirmed that the formation of urea−OH− clusters can effectively enhance the stability of Na+ ions that attracted to cellulose chains. Low temperature can enhance the hydrogen bonding interaction between OH− ions and urea and improve the binding ability of the NaOH/urea/H2O clusters that attached to cellulose chains. Cryo-TEM observation confirmed the formation of cellulose− NaOH−urea−H2O ICs, which is in extended conformation with mean diameter of about 3.6 nm and mean length of about 300 nm. Possible 3D structure of the ICs was proposed by the M06-2X/6-31+G(d) theoretical calculation, revealing the O3H···O5 intramolecular hydrogen bonds could remain in the ICs. This work clarified the interactions in cellulose/NaOH/urea aqueous solution and the 3D structure of the cellulose chain in dilute cellulose/NaOH/urea aqueous solution.

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INTRODUCTION Cellulose is the most abundant renewable biopolymer on the earth and the potential resource of biofuels1 and new functional materials.2−4 The stereoregularity of the linear poly(1 → 4)-βD-glucan chain and the abundant hydroxyl groups of cellulose result in the strong intra- and intermolecular hydrogen-bonding network in cellulose (Scheme 1). Therefore, native cellulose is resistant to chemical accessibility and enzymatic degradation.5,6 On the other hand, due to the strong hydrogen-bonding network, cellulose cannot be melted or be dissolved in common

solvents. The traditional approach for producing regenerated cellulose products is mainly based on viscose process, which is still dominated nowadays. In the viscose process, the usage of CS2 and the byproducts cause serious environmental pollution. Therefore, the finding of new environmental friendly cellulose solvents is always the key issue in both producing regenerated cellulose materials and synthesis new cellulose derivatives. Several cellulose solvent systems, such as N-methylmorpholineN-oxide/water (NMMO/H2O),7,8 ionic liquids (ILs),9−11 lithium chloride/N,N-dimethylacetamide (LiCl/DMAc),12−14 alkali aqueous solutions,15 and alkali/urea aqueous solution,16 have been discovered for dissolving cellulose, in which the NMMO/H2O system has been commercialized for producing regenerated cellulose fibers with the category name of Lyocell.7 NaOH/urea aqueous solution is a cheap, nonvolatile, nontoxic, and environmentally friendly cellulose solvent with quick dissolving power and has been acknowledged to be a milestone in the history of cellulose processing technology.17 Based on this solvent system, regenerated cellulose materials including fiber,18 film,19 particle,20 aerogel,21 and hydrogel22,23 have been prepared. However, the origin of the rapid

Scheme 1. Schematic Representation of Cellulose and Its Hydrogen Bond Network

Received: February 9, 2014 Revised: August 8, 2014 Published: August 11, 2014 © 2014 American Chemical Society

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TEM Observation. The cryo-TEM samples were prepared in a controlled environment vitrification system (CEVS) at 277 K. The cellulose/NaOH/urea aqueous solution with the cellulose mass fraction of 1 wt % was first filtered through a 0.22 μm filter. A micropipet was utilized to load 2.8 μL of the resulting solution onto a carbon-coated copper grid, which was blotted with two pieces of filter paper, resulting in the formation of thin films suspended on the mesh holes. After waiting for about 20 s, the samples were quickly plunged into a reservoir of liquid ethane (cooled by nitrogen) at 108 K. The vitrified samples were stored in liquid nitrogen until they were transferred to a cryogenic sample holder (Gatan 626) and then were observed with a FEI Tecnai 20 TEM (200 kV) at about 99 K. The images were recorded with a Gatan Ultrascan 894 CCD. TEM images were obtained by using a JEM-2010 (HT) transmission electron microscope (JEOL TEM, Japan). The samples were prepared by casting the diluted cellulose solution with the cellulose mass fraction of 0.10% onto a holey carbon film supported on copper grid. To form extremely thin liquid layer for TEM observation, the specimen was tilted during dried at room temperature.

dissolution of cellulose in NaOH/urea aqueous solution at low temperature and the intermolecular interactions in the cellulose/NaOH/urea aqueous solution are still unclear. Hydrogen bonding is a critical factor in condensed structures and dissolution of cellulose. Among the methods for the investigation of the interactions, nuclear magnetic resonance (NMR) spectroscopy is a powerful tool for the investigation of hydrogen bonding interactions, which has been widely applied to investigate structures as well as the dissolution behaviors and mechanism of cellulose,24−29 while the intermolecular interactions between cellulose and solvents were somewhat insufficiently discussed. In present work, NMR experiments, including temperature-dependent 1H NMR, 13C NMR, 23Na, 15 N NMR, and 23Na relaxation, and transmission electron microscope (TEM) were used to study the intermolecular interactions in the cellulose−NaOH−urea aqueous system. The effect of temperature on the intermolecular interactions was also discussed. Moreover, the intermolecular interactions and 3D structure of cellulose in the solutions were further clarified by density functional theory (DFT), a powerful method for simulating the intermolecular interactions and the structure,30,31 based on the experimental data. Our findings would reveal the nature of cellulose dissolution in NaOH/urea solvent at low temperature and provide significant information for the chain structure of complicated macromolecule in solution.

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RESULTS AND DISCUSSION Interactions between Urea and NaOH Revealed by 15 N NMR. Cellulose has a low solubility in NaOH aqueous solution without urea, and the resultant solutions are less stable.16 However, cellulose can be readily dissolved in NaOH/ urea aqueous solution at relatively high solubility, and the cellulose solutions are more stable than those in NaOH aqueous system. To clarify the role of urea on the dissolution of cellulose in NaOH/urea aqueous solution, 15N NMR spectra of the samples Ur12, Na7Ur12, and Na7Ur12-Cell were measured at the temperature of 261, 273, and 298 K (Figure 1). Urea and H2O were replaced by 15N-urea and D2O, respectively, in all the samples. Table 1 summarizes the chemical shifts of 15N NMR spectra of all samples at different temperatures. In the 15N NMR spectra of Ur12, the signal of urea-15N is a multiplet at 298 and 273 K, which is due to the 15N−2D (quintet) and 15 N−1H (triplet) spin−spin coupling. It is difficult to obtain the exact H/D ratio. The 15N NMR signal in the 15N NMR spectra of urea-15N in H2O is a triplet (Figure S1), which confirmed the rapid exchange of the N−H (N−D) proton in the present of base. At 261 K, the NMR signal of 15N becomes a singlet (Figure 1a), which is due to that the solution becoming frozen at this temperature and the resolution of the NMR signal of the frozen solution would be decreased.32 Interestingly, the NMR signal of 15N is a singlet in the 15N NMR spectra of all the samples in the presence of NaOH at all experimental temperatures (Figure 1b,c). This could be explained that the exchange rate of N−H (or N−D) proton in the presence of base is typically rapid.33 A similar phenomenon was observed in the 1H NMR spectra of urea/D2O and NaOH/urea/D2O (Figure S2). Moreover, the 15N resonance of urea with NaOH shifted to low field by 0.49 ppm at 298 K and 0.60 ppm at 273 K compared to that in urea-15N/D2O solution. The low-field shift of the 15N NMR signal is attributed to the formation of hydrogen bonds between amino groups of urea and OH− ions of NaOH in the system. As a result, the C−N bonds are in a more double bond character as shown in Scheme 2.34 Because of the cellulose cannot dissolve in urea aqueous solution, cellobiose, a constitutional unit of cellulose, was used as a model compound of cellulose to investigate the

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EXPERIMENTAL SECTION Materials. Microcrystalline cellulose was purchased from Alfa Aesar. NaOH, NaCl, and urea were of analytical reagent (Shanghai Chemical Reagent Co. Ltd., China) and were used without further purification. The cellulose (cotton linter pulp) with Mw = 9.6 × 104 g/mol was provided by Hubei Chemical Fiber Group Ltd. (Xiangfan, China) and was only used in cryoTEM and TEM experiments. D2O of 99.9% purity for NMR was purchased from Cambridge Isotope Laboratories, Inc. Urea-15N with 98 atom % 15N for NMR was purchased from Sigma-Aldrich. Deionized water was used in all experiments. Sample Preparation. NaOH and urea were dissolved in D2O to obtain 12 wt % urea, 7 wt % NaOH, and 7 wt % NaOH/12 wt % urea D2O solutions, denoted as Ur12, Na7, and Na7Ur12 for short, respectively. Cellulose was added to Na7 and Na7U12 solutions, and the mixtures were stirred for 2 h at room temperature. The mixtures were then transferred into a freezer to become solid for 30 min. The resultant samples were placed at room temperature and thus obtained clear cellulose solutions, denoted as Na7-Cell and Na7Ur12-Cell, respectively. The content of cellulose was kept at 4 wt % for NMR experiments. NMR Measurement. 15N NMR spectra were recorded on a Bruker AVANCE 600 NMR spectrometer with a TBO probe at a settled temperature range from 261 to 298 K (−12 to 25 °C). 23 Na and 13C NMR measurements were carried out on a Bruker AVIII 500WB spectrometer with a TBO probe at certain temperature. 1H NMR measurements were carried out on a Bruker AVANCE 400 at 298 K. Deuterium oxide (D2O) was used as lock signal. Longitudinal relaxation times (T1) were measured by using the inversion recovery sequence. 13C NMR chemical shifts were externally referenced to CDCl3 at 75.38 ppm. Nitromethane (CH3NO2) was used as the external chemical-shift reference at 382 ppm for 15N NMR experiments. 23 Na NMR chemical shifts were externally referenced to methanol solution of 0.7 M NaBPh4 at 25 °C at −3.087 ppm. 10251

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Scheme 2. Structure of OH−−Urea Complex

urea-15N/D2O solutions are shown in Figure S3. The results show that the triplet and quintet NMR signals of 15N still exist in 15N NMR spectra of cellobiose/urea-15N/D2O solution, although the peaks are not clearly separated from each other. Furthermore, there is small variation in chemical shift when cellobiose was added to urea/D2O solution. The results confirmed that there are weak interactions between cellulose and urea. Generally, electrons in p-orbitals of the double bond can give rise to a very large paramagnetic effect, resulting in a downfield shift. Increasing in hydrogen bonding interaction can cause the nitrogen lone pairs to a more delocalized state and makes the nitrogen atoms more positive. However, positive nitrogen atoms reduce the second-order paramagnetic effect at the 15N nucleus and lead to an upfield shift of the NMR signal. With decreasing temperature, the nitrogen resonance of urea aqueous solutions with and without NaOH shifted to upfield, which indicates the enhancement of the intermolecular hydrogen bonding interactions for urea alone and those associated with OH− ions of NaOH. Our results are similar to the association interaction between urea and acetate at different temperatures.34 Moreover, the 15N chemical shift in the NMR spectrum of Na7Ur12 is similar to that of Na7Ur12-Cell at the temperature investigated. In addition, the 13C NMR chemical shifts of cellulose in Na7Ur12-Cell remained fundamentally unchanged compared with those in cellulose/NaOH/D2O solution (Figure S4 and Table S1), which also confirmed that there is no strong interaction between urea and cellulose. Combined with the above results, the role of urea in cellulose−NaOH−urea aqueous system can be summarized as follows. The amino groups of urea bound to OH− ions of NaOH through hydrogen bonds to form complex in aqueous solution. There is almost no direct interaction between urea and cellulose. Low temperature can enhance the hydrogen bonding interactions between OH− ions of NaOH and urea. Cluster Formation Revealed by 23Na NMR. For the clarification of the role of Na+ ions on the dissolution of cellulose, 23Na NMR measurements were carried out for Na7, Na7Ur12, Na7-Cell, and Na7Ur12-Cell solutions, and the results are shown in Figure 2. Only the singlet peak, assigned to 23Na characteristic peak, can be observed on the 23Na NMR spectra of all the samples at 298 K (Figure 2). Adding urea or cellulose in NaOH aqueous solution caused the upfield shift of 23Na NMR signal. Moreover, the presence of both urea and cellulose in NaOH/D2O further resulted in the upfield shift of 23Na resonance signal. The results indicate that Na+ ion can interact with both urea and cellulose in the system. In general, the position of the NMR signal is determined by the total shielding that the nucleus under investigation receives from various sources.35 The shielding can be expressed by the screening constant, σ. On a practical basis, the screening constant can be divided into three independent parts by36 σ = σd + σo + σp (1)

Figure 1. 15N NMR spectra of (a) Ur12, (b) Na7Ur12, and (c) Na7Ur12Cell at the temperature of 261, 273, and 298 K. 15N-urea and D2O were used instead of urea and H2O in all the samples.

Table 1. Chemical Shift of 15N NMR Spectra of Several Solutions chemical shift (ppm) sample

261 K

273 K

298 K

NaOH/urea-15N cellulose/NaOH/urea-15N urea-15N

75.90 75.92

75.99 76.02 75.42

76.12 76.12 75.63

where σd and σo correspond to the contribution from local diamagnetic and other atoms, respectively, which are generally

interactions between urea and cellulose in aqueous solution. The 15N NMR spectra of urea-15N/D2O and cellobiose/ 10252

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shift decreases linearly with the increase in temperature, which suggested that low temperature could enhance the interactions between Na+ ions and surround solvent or ions (water, urea, or OH− ions). The slopes of the lines obtained by linear fitting experimental data are denoted in Figure 3. The slope for Na7 is nearly equal to that for Na7Ur12, which confirmed that the Na+−urea interaction is independent of the temperature. It has been reported that the interaction of Na+−Z− ion pairs (Z− represents the anions such as OH−, Cl−, NO3−, etc.) has no effect on 23Na shift at various temperature.37 Therefore, the shift of 23Na NMR signal as a function of temperature is mainly caused by the Na+−H2O interaction. Combined with 23Na and 15N NMR results, it can be concluded that NaOH can directly interact with urea and cellulose and there was no direct association interaction between urea and cellulose. Based on the above results and discussion, the interactions in the cellulose−urea−NaOH− H2O system can be depicted as follows. Inclusion complexes (ICs) associated with NaOH, urea, cellulose chains, and water clusters were formed in the system and urea hydrates located at shell of the ICs, by which to prevent the self-association of the separated cellulose chains. Thereafter, cellulose molecules can be dispersed at the molecular level to form a stable solution in the NaOH/urea aqueous system. Low temperature can promote the hydrogen-bonding interactions in the system and mainly enhance the Na+−H2O interactions as well as the hydrogen-bonding interactions between OH− ions and urea. Intermolecular Interaction Revealed by 23Na Relaxation. The 23Na nucleus has a spin quantum number I = 3/2 and an electric quadrupole moment Q. The interaction between the Q and the electric field gradient (dE/dr) at the nucleus will give rise to the nuclear magnetic relaxation mechanism.38 Sodium is monatomic, and the electric field gradient originates from intermolecular interactions. Therefore, the relaxation rate of 23Na was very sensitive to its environment. To further investigate the role of Na+ ion in different cellulose solutions and solvent systems, the longitudinal relaxation time T1 of 23Na in Na7, Na7Ur12, Na7-Cell, and Na7Ur12-Cell was determined at different temperature (263−298 K). For all experiments, the 23Na resonance longitudinal relaxation times were monoexponential. The temperature dependence of the relaxation rate (1/T1) of 23Na for Na7, Na7Ur12, Na7-Cell, and Na7Ur12-Cell is shown in Figure 4, and the corresponding data are listed in Table S2. The results indicated that the addition of

Figure 2. 23Na NMR spectra of Na7, Na7Ur12, Na7-Cell, and Na7Ur12Cell at 298 K.

minor on nuclei other than protons. This is a result of the anisotropy in the magnetic susceptibilities of the distant atoms from the resonant nucleus. σp is the paramagnetic term and is usually the dominant shielding term. This contribution is due to the net orbital angular momentum of electrons in the valence orbitals of the nucleus under investigation and makes a negative contribution to the screening. The most important factor affecting these chemical shifts is the orbital angular momentum of electrons occupying the n = 3 shell of sodium and its effect upon σp. This arises from the overlapping of the outer p orbitals of the central sodium ion with the outer s and p orbitals of the neighboring solvent molecules or ions.35 The 23 Na shift values increase with increasing the σp term. As a result, the presence of urea or cellulose in NaOH aqueous solution led to the upfield shift of 23Na resonance signal, indicating that the overlap between the orbitals of Na+ and H2O is larger than that between Na+ and urea or cellulose. This also meant that the Na+−H2O interaction is stronger than the Na+−cellulose or Na+−urea interaction.37 Because urea can dramatically improve the dissolution of cellulose in NaOH aqueous solution, especially at low temperature,16 the temperature-dependent 23Na NMR for Na7 and Na7Ur12 solutions were also used to explore the effect of temperature on the Na+−urea interaction. The 23Na chemical shift for Na7 and Na7Ur12 as a function of temperature is shown in Figure 3. The results show that the 23Na chemical

Figure 3. Temperature dependences of the 23Na NMR chemical shift for Na7 and Na7Ur12.

Figure 4. Temperature dependences of the Na7, Na7Ur12, Na7-Cell, and Na7Ur12-Cell. 10253

23

Na relaxation rates for

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urea and/or cellulose to the NaOH aqueous solutions or the decrease in the temperature of the samples caused a significant increase in 1/T1. Generally, a large 1/T1 corresponds to a low mobility. Therefore, the complex structure formed by attaching the NaOH−urea−H2O cluster on cellulose chains is relatively more stable than that cellulose−NaOH−H2O. These results confirmed that urea can effectively enhance the stability of NaOH attached to the cellulose chains and thus favor to the breakdown of intermolecular hydrogen bonds among cellulose chains. Low temperature favors for the stabilization of the complex structure in the system. Arrhenius plots for the dependence of the relaxation rate (1/T1) of Na7, Na7Ur12, Na7Cell, and Na7Ur12-Cell on temperature are shown in Figure 5.

Figure 5. Arrhenius plot for the dependence of the relaxation rate (1/ T1) of Na7, Na7Ur12, Na7-Cell, and Na7Ur12-Cell on temperature. The number represents the slope of the line.

Figure 6. Typical images of cryo-TEM (a) and HR TEM (b) of cellulose in NaOH/urea aqueous solution and the model of the cellulose ICs (c).

Based on the slopes of the relevant fitting lines, the relative activation energies of the Na7, Na7Ur12, Na7-Cell, and Na7Ur12Cell were calculated to obtain 21.0, 22.3, 20.1, and 20.5 kJ/mol, respectively. The results further confirmed that Na+ ions in Na7, Na7Ur12, Na7-Cell, and Na7Ur12-Cell possess different interactions with the surrounding species in solutions. Intermolecular Hydrogen Bonds in Cellulose/NaOH/ Urea Aqueous Solution. It is known that the partial dissociation of the C6-OH is the crucial factor for cellulose dissolved in NaOH aqueous solution.39 The binding of NaOH to hydroxyl groups of cellulose breaks the intermolecular hydrogen bonds in cellulose and leading to the dissolution of cellulose. In previous work, cellulose chains were proposed in the extended wormlike conformation in NaOH/urea aqueous solution.16 In order to clarify the morphology of cellulose chains in cellulose/NaOH/urea aqueous solutions, cryo-TEM observation was carried out. The samples for cryo-TEM observation were prepared by a rapid deep freezing procedure in liquid nitrogen at 99 K, by which the cellulose−NaOH− urea−H2O ICs in the solution can be successfully maintained in their original morphology and the formation of salt crystals can be prevented. Figure 6 shows the typical images of cryo-TEM and TEM of cellulose/NaOH/urea aqueous solution. As expected, extended wormlike structure was observed in both cryo-TEM and TEM images, which proved that ICs of cellulose−NaOH−urea exist in the solution. The extended wormlike ICs are about 250−350 nm in length and about 3.6 ± 0.4 nm in diameter evaluated by cryo-TEM. The length extended wormlike ICs is comparable with the contour length

(Lc = 300 nm) of cellulose calculated from the length of a projection of the repeating unit (Lunit = 0.5 nm) and the molecular weight of the repeating unit (Munit = 162 g/mol). The diameter of the ICs estimated by TEM is about 4.7 ± 0.6 nm, which is larger than that of cryo-TEM observation due to the collapse of the ICs during drying procedure for preparing TEM samples. Moreover, the cellulose ICs are well dispersed in the solution, indicating that the ICs are very stable in the solution system. It can be explained that urea was as shell of the ICs to prevent the cellulose chains to aggregate each other. On the basis of the above results and discussion, the steric structure of the ICs could be estimated by the M06-2X/631+G(d) level of theory, which is due to that M06-2X has been proven to have better performance than B3LYP for main-group noncovalent interactions.40−42 The M06-2X/6-31+G(d) method is implemented in the Gaussian 03 package.43 Previous work proved that the Na+ or OH− ions are generally in their hydrated forms, and the OH− ions could directly bind to the amino groups of urea molecules as confirmed by 15N NMR and 23Na NMR results. To further reveal the microscopic interaction among the solvent small molecules in NaOH/urea aqueous solutions, several kinds of clusters, such as NaOH(H2O)5·urea, NaOH(H2O)6·urea, NaOH(H2O)7·urea, NaOH(H2O)8·urea, and NaOH(H2O)9·urea, were designed and optimized (Figure S5). The successive binding energies were calculated to be 49.5, 64.0, 41.3, and 30.1 kJ/mol for NaOH(H2O)6·urea, NaOH(H2O)7·urea, NaOH(H2O)8·urea, and NaOH(H2O)9·urea, respectively. The results indicated that the optimized structure 10254

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of NaOH(H2O)7·urea cluster appears as a perfect cage and is more stable than the others, which is due to that the NaOH(H2O)7·urea cluster possesses the biggest successive binding energy (Table S3). In this cage structure, the proton on the amino group of urea is easily transferred to the OH− ion, nearly forming a water molecule, and the C−N bond turns into partly double bond character, which is consistent with the 15N NMR results in this work. To explore the interaction between the hydroxyl groups in cellulose and the NaOH/urea/water clusters, cellobiose with tg conformation (shown in Figure S4) was used as the candidate of cellulose to suppose the possible structures. As for the conformation of cellobiose, there have been a great number of reports to investigate. French et al. have extensively investigated the structures and energies of cellobiose.44 However, in the case of conformations of cellulose in solvated environment, it is complex to confirm all the possible compound structures. In this work, the tg conformation was chose referred to the structure proposed by Nishiyama et al.5 Many researchers from various aspects have indicated that the hydroxyl groups in cellulose tend to interact with OH− ions in the NaOH solutions.39 On the basis of this point, we constructed two possible structures for NaOH/urea/H2O interacting with the hydroxyl groups in C-6 and C2, C3 positions of cellobiose. The optimized structures are shown in Figure 7. As depicted in Figure 7a, the distance between the

Cluster-O2,3 is weaker than that with water in NaOH(H2O)7· urea, which is consistent with the 23Na NMR results in this work. In Cell-Cluster-O2,3 cluster, the intramolecular hydrogen bond between H atom of hydroxyl groups in the C3 position and O5 atom of adjacent AHG unit is about 1.909 Å that are not disrupted when interacting with NaOH/urea/H2O cluster. It should be noted that the conformations shown in Figure 7 are just the typical representatives and not the only way by which the NaOH/urea/H2O cluster interacts with cellulose. This is due to that the minimization in the gas phase may not reflect accurately the complicated conformations of the cellulose in aqueous conditions.45,46 However, due to the complex situation in cellulose/NaOH/urea aqueous solutions and the considerable consuming of the computational time, our theoretical motivation in this work is only primarily for proposing possible interaction models for NaOH/urea/H2O with the hydroxyl groups at C6 and C2, C3 positions by using cellobiose in the tg conformation proposed by Nishiyama et al.5 The primary results are good agreement with the experimental data in this work. Therefore, no more attempts have been tried for determining the global minimized structures of cellulose/ NaOH/urea aqueous solution at the present state. Furthermore, the binding energy of NaOH(H2O)6·urea cluster with cellobiose to generate Cell-Cluster-O6 in Figure 7b is 120.1 kJ/ mol, which is higher than those of cellobiose with cellobiose (109.2 kJ/mol, in Figure S7) and NaOH(H2O)6·urea with water (81.4 kJ/mol, in Figure S5c). This result suggested that the NaOH/urea/water clusters prefer to attach to cellulose chains. Moreover, the diameter of the ICs estimated by quantum chemistry calculation is about 2.8 nm (Figure S8), which is comparable with the result obtained by the cryo-TEM. The strong hydrogen bonding in cellulose was mainly referred for the condensed structure and the dissolution of cellulose in the literature and so did in this work. However, on the basis of reviewing basic physicochemical aspects, Lindman et al. hypothesized that cellulose is significantly amphiphilic and that the low aqueous solubility must have a marked contribution from hydrophobic interactions,47 which was debated by cellulose scientists with a wide range of experiences representing a variety of scientific disciplines.48 Lindman’s hypothesis provided the general perspective of cellulose as a polymer in which intermolecular stress transfer involves more than hydrogen bonds. Hydrophobic and amphiphilic behaviors have been acknowledged for some time but may have been underestimated in conventional considerations of structure, solubility, etc.48 Lindman’s hypothesis and the following debates provided a new viewpoint for clarifying the fundamental problems of cellulose. The effects of the interactions besides the hydrogen in cellulose−NaOH−urea aqueous system will be investigated and discussed based on experimental results in the future.

Figure 7. Optimized structures of (a) Cell-Cluster-O2,3 and (b) CellCluster-O6 by the M06-2X/6-31+G(d) theoretical method.

hydroxyl groups at C2 and C3 is too close to bind two NaOH/ urea/H2O clusters simultaneously. Therefore, two H2O molecules in NaOH(H2O)7·urea cluster were replaced by the hydroxyl groups at C2 and C3 to form the complex structure, denoted as Cell-Cluster-O2,3. For the hydroxyl group at C-6 of cellulose, one H2O molecule in NaOH(H2O)7·urea cluster was replaced by the hydroxyl group at C-6 to result the complex structure, defined as Cell-Cluster-O6 (Figure 7b). In both cases, the cage structure of the primary NaOH(H2O)7·urea cluster are preserved. It is noticed that the conformation of cellulose moiety in the structure of Cell-Cluster-O6 still has the tg conformation with the χ (O6−C6−C5−O5 torsion angle) value of 167.61°. In addition, the distance of Na···O6 in CellCluster-O6 is about 3.929 Å, which is larger than that (2.378 Å) between Na+ and O atom of the corresponding H2O in NaOH(H2O)7·urea. In Cell-Cluster-O2,3, the distances of Na··· O2 and Na···O3 increase to 2.448 and 3.446 Å from 2.378 and 3.227 Å, respectively. These results indicate that the interaction of Na+ ions with hydroxyl groups in Cell-Cluster-O6 and Cell-

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CONCLUSIONS The intermolecular interactions among NaOH, urea, and cellulose in aqueous system were investigated in detail and discussed. Temperature-dependent 23Na and 15N NMR results revealed that OH− ions of NaOH interacted directly with amino groups of urea through hydrogen bonds to form a cluster, and there was no direct interaction between urea and cellulose. Adding urea can effectively enhance the stability of the NaOH−cellulose complex as a result of the strong interaction between NaOH and urea, leading to low mobility of the NaOH−urea−cellulose complex. Low temperature can 10255

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accelerate the breakdown of the intermolecular hydrogen bonds among cellulose and thus prevent the aggregation of the separated cellulose ICs. The stability of the ICs at the lower temperature is a key to the cellulose dissolution. TEM observation provided a direct evidence of the stiffness of cellulose chains and the formation of ICs in the solution. The ICs of cellulose in the solution have an extended wormlike morphology with a diameter of 3.6 ± 0.4 nm and a length about 300 nm estimated by cryo-TEM observation. The model theoretic calculation confirmed that the structure of the NaOH−urea−water cluster maintained when cellulose interacted with the cluster. Moreover, the O3H···O5 intramolecular hydrogen bonds of cellulose remained in the cellulose ICs. The diameter of the ICs estimated theoretically is about 2.8 nm, which was comparable with the data obtained by cryo-TEM. The combination of NMR and quantum chemistry calculation revealed that the key point of cellulose dissolution was cleavage of the intermolecular hydrogen bonds of cellulose as well as that the stabilizing of the NaOH−cellulose complex by the surrounding urea. Low temperature improved the formation and stability of small molecular cluster as well as complex associated with small molecules and cellulose chains through the hydrogen bonding. This work clarified the intermolecular interactions in cellulose/NaOH/urea aqueous solution, which is particularly important for promoting the applications of this green cellulose solvent system in commercial producing regenerated cellulose products.

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ASSOCIATED CONTENT

* Supporting Information S

NMR spectra and data of cellulose solution, the detailed Cartesian coordinates of the optimized structures of NaOH/ urea/H2O clusters and the cellulose IC, total binding energies and successive binding energies of NaOH/urea/H2O clusters, and the electronic energies of the optimized geometries. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (R.L.). *E-mail [email protected] (L.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Basic Research Program of China (973 Program, 2010CB32203), the Major Program of National Natural Science Foundation of China (21334005), the National Natural Science Foundation of China (21274154, 51203122, and 51373191), and the Fundamental Research Fund for the Central Universities (201220302020205). We also thank Dr. Xiaojun Huang and Dr. Gang Ji from Institute of Biophysics, Chinese Academy of Science, for technical support in cryo-TEM samples preparation and data collection.

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