Polarization Effects on the Cellulose Dissolution in Ionic Liquids

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Polarization Effects on the Cellulose Dissolution in Ionic Liquids: Molecular Dynamics Simulations With Polarization Model and Integrated Tempering Enhanced Sampling Method Zigui Kan, Qiang Zhu, Lijiang Yang, Zhixiong Huang, Biao-Bing Jin, and Jing Ma J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b12647 • Publication Date (Web): 18 Apr 2017 Downloaded from http://pubs.acs.org on April 23, 2017

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The Journal of Physical Chemistry

Polarization Effects on the Cellulose Dissolution in Ionic Liquids: Molecular Dynamics Simulations with Polarization Model and Integrated Tempering Enhanced Sampling Method Zigui Kana,b, Qiang Zhua, Lijiang Yangc*, Zhixiong Huangd, Biaobing Jind and Jing Maa* a) School of Chemistry and Chemical Engineering, Key Laboratory of Mesoscopic Chemistry of MOE, Nanjing University, Nanjing, 210093, People’s Republic of China b) School of Sciences, China Pharmaceutical University, Nanjing 211198, People’s Republic of China c) Institute of Theoretical and Computational Chemistry, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, People’s Republic of China d) Research Institute of Superconductor Electronics (RISE), School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, People’s Republic of China

1 Environment ACS Paragon Plus


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ABSTRACT Conformation of cellulose with various degree of polymerization of n = 1~12 in ionic liquid 1,3-dimethylimidazolium chloride ([C1mim]Cl) and the intermolecular interaction between them were studied by means of molecular dynamics (MD) simulations with fixed-charge and charge variable polarizable force fields, respectively. The integrated tempering enhanced sampling method was also employed in the simulations in order to improve the sampling efficiency. Cellulose undergoes significant conformational changes from a gaseous right-hand helical twist along the long axis to a flexible conformation in ionic liquid. The intermolecular interactions between cellulose and ionic liquid were studied by both infrared spectrum measurements and theoretical simulations. Designated by their puckering parameters, the pyranose rings of cellulose oligomers are mainly arranged in a chair conformation. With the increase in the degree of polymerization of cellulose, the boat and skew-boat conformations of cellulose appear in the MD simulations, especially in the simulations with polarization model. The number and population of hydrogen bonds between the cellulose and the chloride anions show that chloride anion is prone to form HBs whenever it approaches to the hydroxyl groups of cellulose and, thus, each hydroxyl group is fully hydrogen bonded to the chloride anion. MD simulations with polarization model presented more abundant conformations than that with non-polarization model. The application of the enhanced sampling method further enlarged the conformational spaces could be visited by facilitating the system escaping from the local minima. It was found that the electrostatics interactions between the cellulose and ionic liquid contribute more to the total interaction energies than the van der Waals interactions. Although the interaction energy between the cellulose and anion is about 2.9 times than that between the cellulose and cation, the


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role of cation is non-negligible. In contrast, the interaction energy between the cellulose and water is too weak to dissolve cellulose in water.



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1. INTRODUCTION Cellulose is a natural polysaccharide consisting of linear chains of several hundreds to many ten thousands of β(1→4)-glycosidic units (Figure 1). It is the most abundant in nature and could be found in all plants.1-2 However, cellulose is difficult to be dissolved in water and most of organic solvents, probably due to the extensive network of both inter- and intra- hydrogen-bonds between the cellulose molecules. 3-6 Such an insolubility hinders the industrial applications of cellulose and a large amount of them are untapped every year. 7 Swatloski et al. found that cellulose could be dissolved in the ionic liquids (ILs) such as 1-n-butyl-3-methylimidazolium chloride ([C4mim]Cl) up to 25 wt% by microwave heating. 8 Ionic liquid is a class of ionic compound, composing of ions and therefore exhibiting ionic conductivity, whose glass transition temperature and/or melting point is below 100℃.9 Since then, many attempts on increasing the solubility of cellulose in different ILs have been reported.10-19 The cations of the ILs have many choices

such

as

imidazolium, pyridinium, pyrrolidinium,

ammonium, phosphonium,

piperidinium, pyrazolium, thiazolium, sulfonium and so on. 6,20-21 The paired anions can be either inorganic ions (e.g., Cl-, Br-, OAc-, ) or organic ions (e.g., HCOO-, (C6H5)COO-, SCN-).

6,20-21

Recent progress of cellulose dissolution in ILs and their applications have been presented in a series of reviews. 22-26


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Figure 1. Schematic representation of cellulose, ionic liquid, and simulation model of cellulose dissolution in IL.

It is generally believed that the disrupting and breaking of the inter- and intra-molecular hydrogen bonding of cellulose is the key factor of its dissolution mechanism.3 However, there are conflicting observations about the effects of cation and anion of ILs on the dissolution mechanism. On the one hand, many chemists concluded that the dissolution process of cellulose in ILs is mainly governed by anions of ILs, while cations only play a minor role. In 2002, Swatloski et al. postulated that the ILs containing Cl-, instead of ‘non-coordinating’ anions such as [BF4]- and [PF6]-, appear to be the most effective solvents,8 which was later demonstrated by 13

C and

35/37

Cl NMR measurements by the same group 27. They provided the direct evidence of

chloride anions binds strongly to the hydroxyl groups of cellulose.27 Some other groups also found that the hydrogen bond interactions occur between the cellulose and anions of ILs, but not cations.28-29 On the other hand, many researchers believed that cations are also involved in the dissolution process of cellulose in ILs. Similar to the anions of ILs, cations would also attack on the hydroxyl oxygen atoms of cellulose. Hence, the solubility of cellulose in ILs comes from the



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collective contributions of both their anions and cations.10,30-31 In addition, the acidic proton in the cationic heterocyclic rings, polarizability of the cation and its ion pairing strength with anion would also affect the dissolution of cellulose in the ILs.31 Computational simulation, which is a powerful supplement to discover the nature of sophisticated inter- and intra- molecular interactions, has also been used to study the dissolution mechanism of cellulose in ILs. Youngs et al. carried out molecular dynamics simulations on the solvation of glucose in the [C1mim]Cl and [C4mim][PF6].32-33 The force field of Canongia Lopes et al. was employed to describe the ionic liquid, 34 and glucose parameters were taken from OPLS-AA.35 They found that the predominant contribution is from glucose/chloride electrostatic interactions and the van der Waals interactions between glucose and the cations are small but cannot be neglected,33 Liu et al. used the generalized AMBER force field (GAFF) for the [C4mim][OAc] and the GLYCAM force field for β-D-glucose oligomers (degree of polymerization n = 5, 6, 10, and 20) to study the interactions between cellulose and IL.36 The presence of the strong hydrogen bonds between the hydroxyl groups of the cellulose and the anion acetate was revealed and some of the cations were in close contact with the polysaccharides through hydrophobic interactions.36 Combined effects of cations and anions on the dissolution of the cellulose with the degree of polymerization of 10 in ILs were investigated by Zhao et al. with MD simulations.37,38 Recently, ab initio MD has also been applied to study the dissolution mechanism of the cellobiose in [C2mim][OAc] 39. It was found that anions play a crucial role in the cellulose dissolution39 and a high enthalpy difference with a moderate entropic contribution appears to be vital in determining the solubility of cellulosic biomass in ILs at room temperature.40


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Since the significant role of the intermolecular interaction has been demonstrated in understanding the dissolution mechanism of cellulose in ILs, the accurate description of electrostatics interaction is necessary. Most of previous theoretical studies concentrated on the anions and/or cations of ILs and neglected polarization of the cellulose itself. In most cases, the cellulose was modelled with β-D-glucose or its oligomers using non-polarizable fixed-charge force field. That is, the partial charges on each atom were fixed from the beginning to the end of the simulations. However, the partial charges are indeed fluctuating according to the conformational changes of molecules, which necessitates the application of polarizable force field under specific circumstance, especially in the highly polar systems (e.g. ILs systems) or systems in the external electric field. 41 Various flavors of polarization models have been implemented in the fields of material science and biological system.42-45 The imidazolium-based ionic liquids were studies by Wendler K. et al. from the quantum electronic scale to the classical atomistic scale: generate the suitable partial charges for classical force field using the Blöchl method to reproduce the multipole distribution for bulk systems, leading to a substantial improvement in the description of dynamical properties, such as electric conductivity. 46 Recently, Bernardes el al. designed a novel regression model based on atomic polarizabilities and volumes to predict the molecular volume and polarizability of an unknown ionic liquid as well as its mass density and refractive index.47 Simulation with such a polarizable force field shows a good agreement with quantum-chemical calculations at a MP2/aug-cc-pVDZ level for neutral molecules with reasonable accuracy.47 In the present work, we attempt to gain insight on how polarization take effects on conformation changes and the dissolution of cellulose in ionic liquids by using the polarizable force fields with the partial charges updated from density functional theory (DFT) calculations at each time step. Changes in conformation and



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intermolecular interaction corresponding to the charge variations of cellulose oligomers with degree of polymerization, n = 1, 2, 3, 6, 9, 12 were investigated from the MD trajectories by using fixed-charge and polarization models, respectively. Since the polarization model is more expensive than that using the conventional force fields, the integrated tempering sampling (ITS) approach for enhanced sampling 48-50 was used in this work to improve the visiting efficiency of conformational spaces. We found that the MD simulations with polarization model could explore much larger conformational spaces of cellulose. The contributions of both electrostatics and van der Waals interactions to the interaction energy between the cellulose and ionic liquid were also analyzed. As a comparison, the MD simulation of the mixture of cellulose and water molecules was also performed to detect the interaction between cellulose and water, which is much weaker than that in the cellulose-IL mixture.

2. COMPUTATIONAL METHODS AND EXPERIMENTAL DETAILS DFT Calculations of Species in Vacuum. The geometry optimizations of oligosaccharide with the degree of polymerization n =1, 2, 3, 6, 9, 12 (abbreviated as (GLU)n when n > 1), the cationic ion [C1mim]+, and the ionic liquid [C1mim]Cl were performed by using the density functional theory (DFT) method at B3LYP/6-311G(d,p) levels with the Gaussian 03 program. The frequency calculations were also performed to assign the experimental terahertz (THz) spectrum of cellulose and IR spectra of IL and cellulose. MD Simulations of Cellulose in ILs. All the MD simulations were performed in the AMBER 9.0 simulation package.51 One oligosaccharide with the degree of polymerization n =1, 2, 3, 6, 9, 12 and ionic liquid solvents were placed in the cubic box. The corresponding mole fractions of a serial of oligosaccharides in the MD simulation systems were listed in Table S1 in the supporting


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information (SI). The GAFF force field was used for the cellulose oligomers. After optimization of the oligosaccharides in vacuum using the density functional theory (DFT) method at B3LYP/6-311G(d,p) levels with the Gaussian 03 program 52, the partial charges were derived by fitting the electrostatic potential using the RESP approaches with RED-III.4 software 53. The force field constructed by José N. Canongia Lopes et al. was used for the cation [C1mim]+. 34 The Lennard-Jones (LJ) radius and well depth of the anion Cl- are 2.47 Å, 0.1 Kcal·mol-1, respectively. 54 After the initial minimization and heating procedure at constant volume from 0K to 480 K, the 10 ns MD simulations were then performed with the isothermal-isobaric (NPT) ensemble at 480 K and 1 bar. The simulation temperature of 480 K, which is higher than the melting point of [C1mim]Cl of 398 K, is same as what was used in previous MD studies of the system containing [C1mim]Cl and glucose

32-33

. Periodic boundary conditions were used in all three dimensions.

Coordinate sets were saved every 0.1 ps for subsequent analyses. The temperature was maintained by Langevin dynamics 55 with the collision frequency of 1 ps-1, and the time step was set to 0.5 fs. The direct spatial non-bonded interactions were truncated at 8.0 Å. For the polarizable MD simulations (called polar for short), the initial conformation of oligosaccharides were picked up from the 200 ps non-polarizable MD (non-polar) simulation trajectories. The procedures of polar simulations are almost the same as the non-polar one, except that the DFT based partial charges of oligosaccharides were calculated every 2 ps for βD-glucose and 4 ps for (GLU)n (n = 2, 3, 6) at B3LYP/6-311G(d,p) level, depending on the relative deviation (Dev.) of total energy between the two successive MD steps (Figure 2a).56-57 The partial charges of the oligosaccharides would be renewed if Dev. is below 10%. Table S2 in SI listed the partial charges of β-D-glucose molecule in different simulation systems with or



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without polarizable force field. The selected updating period of partial charges from QM calculation is in fact the ultrafine resolution. It was found that there is no distinct difference in conformational changes of oligomers when the partial charges renewed every 10, 50, and 100 ps, respectively.58

Figure 2. Plot of (a) the polar MD simulation and (b) the ITSMD simulation with and without polarization model. The Dev. is defined as the energy difference between the two successive time steps, called n and n + 1, respectively.

The integrated tempering enhanced sampling method was developed by Gao and coworkers.48-50 Through the test simulations of alanine dipeptide, methyl maltoside as well as 20amino acid protein TrpCage (N20LYIQWLKDGGPSSGRPPPS39) in TIP3P water, Yang et al.


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provided an efficient and reliable sampling methodology for the large and complex systems by enhancing the sampling over the relevant energy/conformation spaces of the interested subsystems (e.g., the solute) while avoiding structural transitions of the reset of the systems (e.g., the solvent).50 Briefly speaking, a modified effective potential U’ at the desired temperature (thus β0) is obtained from the original potential (U) by making use of an energy distribution function over a series of sampled temperatures (βk=1/kBTk, k = 1, N): 50 N

p(U ) = e− β0U ' = ∑ nk e− βkU

(1)

k =1

Thus

U'=−

1

β0

N

ln ∑ nk e− βkU

(2)

k =1

A series of iteration procedures are then applied to calculate the weighting factor nk until the convergence criterion is satisfied. Then the biased potential U’ is obtained. The sampled potential energy range by such a method is largely expanded when compared to standard MD simulations. In the current MD simulations with ITS method (ITSMD), we used 200 discrete temperatures, which are evenly spaced in the range of 450–510 K, around at the desired temperature 480 K, to simulate the system containing the solute (GLU)6 . The initial conformation of (GLU)6 was picked up from the 200 ps non-polar simulation and the other simulation parameters were set up as those employed in the non-polar simulations. Moreover, the polarization effect was also introduced in the ITS simulation and the computational scheme was illustrated in Figure 2b. The total simulation time of the simulations employing ITS, with or without the application of polarizable force field, are 5 ns.



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ITSMD simulations with non-polar were also used for the (GLU)6/water mixture in order to study the interaction between the (GLU)6 and water molecules. One (GLU)6 molecule and 1152 TIP3P59 three-site rigid water model were placed in the rectangular parallelepiped box. A 5 ns MD simulation was performed with the NPT ensemble at 300 K and 1 bar. The other simulation parameters were the same as those mentioned above.

Experimental Details. The ionic liquid [C1mim]Cl was purchased from TOKYO Chemical Industry Co., LTD (Japan) , purity better than 98.0%. The cellulose microcrystalline was obtained from the Sinopharm Chemical Reagent Co., Ltd. (China). The mixture of [C1mim]Cl and cellulose microcrystalline was first prepared in glove box, the corresponding concentration was 1.51 mol%. As we just mentioned, the melting point of [C1mim]Cl is about 398 K, 60 but the boiling point has not been reported yet. The onset temperature for decomposition of [C2mim]Cl was reported to be 576 K, and the increase of the alkyl chain length of imidazolium based ionic liquids with anion Cl- would decrease their thermal stability 61. In the present work, the mixture was heated to about 430 K to molten the IL, in order to dissolve the cellulose microcrystalline without any decomposition. (Figure 1). The slight different between the simulation temperature (480 K) and experimental melting temperature (430 K) will not bring about much difference in the microscopic dissolution picture from the viewpoint of intermolecular interactions. The 1H and

13

C NMR spectra of the IL [C1mim]Cl pre- and post-dissolution were measured on an

AVANCE AV-500 MHz spectrometer (Bruker, Switzerland). The solvent was deionized water (D2O) and the measurement frequencies are: 1H = 500 MHz;

13

C = 125.8 MHz. Infrared

spectrum (IR) of the mixture and pure [C1mim]Cl in solid state (KBr pellet) were carried out on a Shimadzu IRAffinity-1S Fourier transform infrared spectrophotometer. The IR absorbance


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spectrum was taken from 400 to 4000 cm-1 averaged 15 times. All the measurements were done in air at room temperature. In order to understand the solid state properties, a THz time-domain spectroscopy (TDS) is used for the detection of THz spectrum of cellulose microcrystalline.

3. RESULTS AND DISCUSSION 3.1 Gaseous Structures of ILs and Cellulose Oligomers The optimized structures of four possible [C1mim]Cl ion conformations were depicted in Figure 3. According to the relative position of Cl- ion and the imidazolium ring, the different conformations were named as front, side-1, side-2 and side-3, respectively, with the corresponding bond distances around the imidazolium ring and the crystal structure 62 reported in the supporting information (Table S3). The binding energy, Eb, was calculated as the energy difference between the ionic pair and the individual cation and anion. The side-2 conformer of [C1mim]Cl is the most stable conformer with the largest value of binding energy, whose geometry is similar with the conformation observed in crystal. The intermolecular bond length between the hydrogen of C1 of cationic [C1mim]+ and Cl- of side-2 and crystalline structure 62 is 1.99 and 2.66 Å, respectively, which means the strong ion pair interactions in gas phase will be weakened in solid state due to the packing effects.



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Figure 3. Geometry and binding energy, Eb, of (a) [C1mim] + and [C1mim]Cl structures: (b) front, (c) side-1, (d) side-2 and (e) side-3. The crystal structure is taken from Ref. 62. In DFT optimized conformation of cellulose oligomers, the adjacent two –CH2OH groups are all taking an anti-anti state, being distinguished by the torsional state of these two -CH2OH groups (Figure 4a). The torsion angles ω of cellobiose and (GLU)3 is -12.2° and -34.0°(on average), respectively (Figures S1 and S2 of SI). However, the mean torsion angles ω(O5’-C1’C4 -C5) of the optimized geometries of (GLU)6 is 22.0° (ω= 23.0°, 21.5°, 22.3°, 21.2° and 22.0°). Hence, the conformation of oligosaccharide (GLU)6 in vacuum takes a right-hand helical twist down the long axis, similar to a screw (Figure 4b), which was also found in certain cellulose nanocrystals.63-64 Payal et al.40 calculated the conformational free energy profile of cellobiose across ω in the gas phase using the OPLS force field for carbohydrates. The minimum in the free energy of cellobiose also occurred at -25°, corresponding to its anti-anti state. In Figure 4b, the radius of the bottom surface of the cylinder is 2.5 Å. The end-to-end distance (L, Figure 4b)


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between the centers of the terminal pyranose rings shows almost a linear relationship with the polymerization of cellulose (Figure 4c) and the distance L for (GLU)6 molecule is about 26.0 Å. The helical twist topology is also held in the optimized conformations of the longer oligomers, (GLU)9 and (GLU)12 (Figure S5 of SI). There always lies a hydrogen bond between the –O6H of one pyranose ring and the O3’ atom of the adjacent pyranose ring (Figure 4a). The distance of O3’…H(O6) is almost the same: 2.18 Å (2.19 Å) and the mean angle of hydrogen bond, ∠(O3’…H–O6), is 169.6°. Figure 4a also displays two selected vibration modes at 2938 and 3581 cm-1, respectively. The CH-stretching motion in pyranose ring contributes to the band of 2938 cm-1. The vibration at 3581 cm-1 is assigned as the –OH stretching vibrational mode concerning the intramolecular hydrogen bonds. It will be shown that these two vibration modes are affected significantly upon aggregation. Because of the ignorance of anharmonicity effects, incomplete incorporation of electron correlation and dependence on the choice of functions and basis sets in DFT calculations, the computed harmonic vibrational frequencies are usually red-shifted than the fundamentals observed experimentally. In the current work, the frequency scaling factor of 0.9672, which was recommended in literature 65 at B3LYP/6-311G(d,p) level, was used here for the frequency analysis.



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Figure 4. (a) The intra-molecular hydrogen bonds and two selected vibrational modes of (b) helical twist structure of (GLU)6 in vacuum. (c) The relationship of the distance (L) between the centers of terminal pyranose rings and polymerization of cellulose, n. Color scheme: C, cyan; O, red; H, white.

3.2 Cellulose-IL Mixture in Condensed Phase


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The measured 1H and 13C NMR spectra of the IL [C1mim]Cl in pre- and post-dissolution were presented in the Figures S3 and S4 in the SI. After dissolution, 1H NMR (500 MHz, D2O, δ in ppm) signals were observed as 8.66 (s, 1H-C(2) ), 7.44 (s, 2H-C(4-5) ), 3.91 (s, 6H-C(6-7)) and 13C NMR (125.8 MHz, D2O, δ in ppm) signals were 136.61(C(2)), 123.61(C(4-5) ), 35.72 (C(6-7)). Both of 1H and

13

C NMR spectra of post-dissolution are close to those detected for the pure

[C1mim]Cl in this work (Figures S3a and S4a of SI) and other previous works66-68, implying that the IL does not decompose during the dissolution process.

Shown in Figure 5a is the THz spectrum of pure cellulose microcrystalline, in comparison with the vibration spectrum of cellulose oligomer (GLU)6 calculated from the DFT method at B3LYP/6-311G(d,p) levels. From the analysis of vibrational modes within 0.2~2.0 THz of (GLU)6, we found that all of the low-frequency bands are originated from the overall backbone vibration (Figure 5a). In crystalline stacking system, the absence of the distinct peaks in the observed THz spectrum is reasonable, because the intermolecular interactions of cellulose aggregates hinder the low-frequency vibration of cellulose. Going up to the higher frequency region from 2600 cm-1 to 3650 cm-1, the infrared absorbance spectra of cellulose microcrystalline (blue line), IL (red line) and the molten cellulose-IL mixture (black line) are presented in Figure 5b. The original infrared absorbance spectra of cellulose microcrystalline, IL and the mixture of them (pink line) were provided in Figure S6 in the SI. The interaction between the cellulose and IL was qualitatively estimated by using the absorbance difference, which is defined as A = −log(I/Io), where I and I0 are, respectively, the transmitted intensities of the sample [C1mim]Cl-cellulose mixture and the pure [C1mim]Cl in solid state.



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The most stable conformation (Conformation d in Figure 3) of gaseous [C1mim]Cl was used for the vibrational analysis. For ionic liquid [C1mim]Cl, peaks in the range from 2800 to 3000 cm-1 are originated from the methyl attached to the imidazolium ring; peaks at 2859 and 2959 cm-1 are attributed to the modes of the symmetric and anti-symmetric stretch mode of -CH3 groups, respectively (Figure S7 of SI). Peaks at 3102 and 3158 cm-1 were, respectively, from the anti-symmetric and symmetric HC4-C5H vibrational modes of the imidazolium ring (Figure S7 of SI). Since the [C1mim]Cl is a highly hydrophilic material, the experimental band at 3432cm-1 may come from the O-H stretch mode of water. As mentioned above, the typical bands at 2800 ~ 3000 cm-1 in the infrared spectrum of cellulose microcrystalline are originated from the vibrational modes of –CH or –CH2 groups, illustrated in Figure 4a. The calculated peaks of O-H vibrations of hydroxyl groups of (GLU)6 lie near 3581 cm-1 (Figure 4a). From the absorbance difference spectrum of cellulose-IL mixture, we found that both these two bands are depressed remarkably relative to the pure cellulose, suggesting possible intermolecular interactions between the cellulose and IL whenever the cellulose was molten in IL, even when the mixture has been cooled to room temperature. Especially, the absence of CH stretching bands at 2800 ~ 3000 cm-1 in the IR spectra of cellulose-IL mixture indicates the evident deformation of pyranose rings upon the dissolution of cellulose. Thus, the key to understand the dissolution of cellulose in IL lies in the conformation changes of cellulose and the resultant variations of intermolecular interactions.


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Figure 5. (a) The THz (in solid curve) and calculated vibrational spectra (in dotted stick) of cellulose microcrystalline. (b) The experimental IR and calculated vibrational spectra of cellulose microcrystalline (blue), IL (red) and the absorbance difference (A = −log(I/Io ), black curve) between the transmitted intensities of pure IL (I0) and the mixture (I) of IL and cellulose in solid state.

3.3 Glucose Conformation in IL For β-D-glucose molecule there lies 38 possible distinct pyranose conformations69 , shown in Figure 6a, which can be designated by ring puckering coordinates defined by D. Cremer and J.A.



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Pople70. In Figure 6b we displayed the fluctuation of puckering amplitudes over the simulation times from the polar MD simulations. Three β-D-glucose-IL binary solutions (with concentrations of 2.04, 1.03 and 0.52 mol%, respectively) were simulated in the current work. It can be seen that the chair conformation of β-D-glucose is the majority during the entire simulations of different concentrations. In the simulation of 2.04 mol% solution, the maximal population of puckering parameters (Q, θ) is located at the position around (0.54, 11.0) and its population density is about 2.0×10-3. In MD simulations, the boat and skew-boat conformation of pyranose ring of β-D-glucose were only occasionally observed. The puckering parameters’ distributions obtained from the simulations with non-polar model were similar to those from polarizable one, as shown in Figure S8 in the SI. It seems that the IL may stabilize and trap the conformation of the sugar for a long time, thus restrict its transformation to the other geometries. In 2007, Youngs et.al

33

studied the β-D-glucose and [C1mim]Cl system (1:5 and 1:96 sugar:IL

ratio) with the MD simulation performed with DL_POLY. The β-D-glucose molecule was modeled with the OPLS-AA and the [C1mim]+ cation was modeled with the same force field as the present work. In both concentrations they found the conformational changes of the pyranose of β-D-glucose from the chair to the skew-boat conformer.


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Figure 6. (a) Puckering amplitude (Q) of β-D-glucose and (b) its distribution population of puckering parameters (Q, θ) from the polar MD simulation trajectories (right). The ■, □, ▲, ▼ and ☉ indicate the chair, boat, half-chair, skew-boat and envelope conformation of glucose, respectively.

With the increase in the degree of polymerization of the cellulose, there appears some changes of the conformational distribution of the different pyranose rings. When n ≤ 6, the dominant conformation of pyranose ring is chair. As shown in Figure 7, the maximal population of puckering parameter (Q, θ) of (GLU)6 calculated from non-polar MD simulation appears at (0.55 ± 0.02, 11.0 ± 2) and its corresponding population density is (2.3 ± 0.3)×10-3. When the chain is elongated to n > 6, the boat and skew-boat conformations of the different pyranose rings of oligosaccharide come out (Figure S9 of SI). In the simulation of (GLU)12 molecule in



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[C1mim]Cl, expect for the major chair conformation, there are also boat and skew-boat conformations in five pyranose rings. In the simulation with the polarization model for the (GLU)6-IL system, the boat and skewboat conformations which are rarely observed in the non-polar MD simulations start to occur with non-negligible probability (Figure 7). For the G2 pyranose ring of (GLU)6, the maximal population of puckering parameters (Q, θ) is around (0.7, 91.0), which is corresponding to the cross-conformation between the boat and skew-boat conformation. The population of the other five pyranose rings is similar as that in the β-D-glucose monomer. Application of ITS in MD simulations would facilitate the transformation of pyranose conformations of cellulose oligomers. In the ITS non-polar MD simulations of (GLU)6, the chair conformation is also dominant in all the pyrnaose rings of (GLU)6. However, three pyrnaose rings (G1, G4 and G5) of (GLU)6 display obvious cross-conformation between the boat and skew-boat conformation, which is a clear evidence that MD simulations with selective integrated tempering methodology could better sample the conformational changes of central solute 50 although the simulation time is only the half of that without ITS non-polar method (Figure 7). When the oligomer chain is increased to n = 12, the backbone exhibits flexibility to some degree, as characterized by the change in torsion angle Ψ(C5’-C1’-O4-C4) and Φ(C1’-O4-C4-C5). Figure S10 displayed the distribution of (Ψ, Φ) and their corresponding distribution relating to the eleven glycosidic linkages of the oligosaccharide (GLU)12 from the non-polar simulation trajectories. Our MD simulation shows that most of the torsion angles relating to the glycosidic linkages are located at the several typical basin of (Ψ, Φ), i.e., (Ψ = -120 ~ 60°, Φ = -180 ~ -60°). With the AMBER/GLYCAM force field and replica exchange MD sampling method, Tongye Shen et al. shows that there are three main basins for (Ψ , Φ ),71 which are in agreement with the


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present results. Meanwhile, there lie two other pairs of (Ψ, Φ) in addition to the three basins mentioned above. One pair of (Ψ, Φ) is centered at (50°, -170°) with its population density of about 2.6 × 10-3 and the other is centered at (±170°, 100°) with the density of 1.4 × 10-3 (Figure S10 of SI).



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Figure 7. The Puckering amplitude (Q) distribution population of puckering parameters (Q, θ ) of (GLU)6 from the (a) non-polar, (b) polar, (c)ITS non-polar and (d) ITS polar MD simulation trajectories. The ■, □, ▲, ▼ and ☉ indicate the chair, boat, half-chair, skew-boat and envelope conformation of glucose, respectively.

Figure 8 lists the end-to-end C1···C4’distance (L) of (GLU)12 in the simulation. After the initial 1 ns, the end-to-end distance tends to be frozen in the following simulations. The end-toend distance ranges from about 40.7 Å to 53.8 Å, with the mean value of 47.2 Å. In order to illustrate the relationship between the end-to-end distance and the degree of polymerization of the oligosaccharide, we calculated the mean distance ( )̅ of pyranose-to-pyranose for the different oligosaccharides of n = 1, 2, 3, 6, 9, 12 (Figure 8). The deviation of the mean distance is also provided. For the cellobiose molecule, the mean distance of glucose-to-glucose is 7.7 ranging from 6.2 Å to 8.6 Å. While for the (GLU)12 molecule, the mean distance is reduced to 4.3 ± 0.6 Å. We could conclude that with the increase of the number of glucose, the mean distance of glucose-to-glucose becomes smaller. This is probably due to the twist of the flexible glycosidic linkage. In the MD simulation trajectories of cellulose oligomer in IL, the gaseous helical twist structures of cellulose mentioned above did not exist anymore.


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Figure 8. The mean distance of glucose-to-glucose and its deviation for the different oligosaccharides (GLU)n of n = 2, 3, 6, 9, 12 from the non-polar ( ▇ ) and polar (★) MD simulations, and in inset: time evolution of the end-to-end distance L of (GLU)12 from the nonpolar MD simulation with the ITS enhanced sampling method.

3.4 Coordination of Ions around Sugar Sugar-Anion Distribution. The radial distribution functions (RDFs) could display the distribution of solvents around one specific atom. It has been addressed that the anion plays an important role in the dissolution of cellulose.24 Figure S11a of SI provided the RDFs for the anion Cl- around the different oxygen atoms of β-D-glucose molecule, indicating that both MD simulations using the polarizable/non-polarizable force field present similar feature of intermolecular interactions. There lies a sharp peak at about 2.80 ~ 2.90 Å in the RDFs of hydroxyl oxygen atoms. However, the magnitude of gr is smaller for the polarization model than that for the non-polar model. Typically, the peak densities for O3 calculated from the non-polar and polar simulations are equal to 5.2 and 6.7 respectively, although the mean partial charge of O3 is almost the same in the two simulations with and without the polarizable force field.



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Furthermore, from the RDFs we can see that MD simulation with the non-polar model is inclined to overestimate the interaction between the oxygen atoms and the anion Cl-. In the RDFs calculated from the non-polar simulations, there lie two obvious peaks at about 4.9 Å and 6.0 Å for the oxygen atom (O5) in the pyranose ring of β-D-glucose, which almost vanished in the simulations with the polarization model. The same situation occurred among the RDFs for the anion Cl- around the other oxygen atoms of glucose. With the increase in the degree of polymerization of the cellulose, although the RDFs display differences, there are still similarities irrespective of the chain length. Taking the (GLU)6 -IL systems as an example, the overestimation of interaction between the oxygen atoms in the pyranose rings and the Cl- is even obvious (Figure 9a) in the simulations using non-polarizable model. From the simulations using the polarization model, the RDFs of all the six oxygen atoms in the different pyranose rings of (GLU)6 gave the same peak position around 5.9 Å. Sugar-Cation Distribution. The RDFs for the cation [C1mim]+ around the different oxygen atoms of β-D-glucose molecule were displayed in Figure S11b in the SI. In statistical analysis of RDFs, the geometric center (Labelled as X) of pyranose ring was used as the reference point. All the RDFs for the cation around the hydroxyl oxygen atoms have a small peak with the magnitudes of gr are about 1.3 ± 0.1, in accordance with the previous MD works.33 Smaller magnitude of gr means weak interaction between the cation and glucose. In order to further get a global coordinate picture of pyranose ring and glucose, the RDFs for the different carbon atoms of [C1mim]+ around the different oxygen atoms of β-D-glucose molecule were also calculated. Figure S12 presents an example using the polar MD simulations of mixture with molar fraction of glucose 0.52%. The RDFs for the symmetry-equivalent carbons on the imidazolium ring have been averaged. One can see that the cation are mainly coordinated to glucose molecules through


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the methyl carbon atoms (C6/C7), not the carbon atoms (C2, C4/C5), in the pyranose ring. In addition, the coordination capacities of C2 and C4/C5 to glucose are similar to each other. With the increase of pyranose rings in cellulose oligomers (GLU)n (n = 2, 3, 6, 9, 12) in IL [C1mim]Cl, the cation also attacks very loosely to the glucose units, as shown in Figure 9b. There is no obvious differences among the RDFs for [C1mim]+ around different pyranose rings of (GLU)6, either the side or the middle ones. This is partly because that the anion binds tightly with the cellulose and hence dominates in the first solvation shell around cellulose.72

Figure 9. The RDFs for (a) anion Cl- and (b) cation [C1mim]+ around O1/O4, O2 and O5 atoms of different pyranose ring of (GLU)6 from the non-polar and polar simulation, where X denotes the geometric center of imidazolium ring.



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The interaction between the sugar and the chloride anions is further investigated from the view of the hydrogen bonds (HBs). HBs can be defined by geometric or energetic criteria 73 and, in fact, different criteria have been used to estimate the HBs in IL systems.29, 33, 36-38, 71 In the current work, the recommended criteria of International Union of Pure and Applied Chemistry (IUPAC) for the formation of a hydrogen bond was applied: dO···Cl = 3.5 Å, ∠O-H···Cl = 110°.70 Shown in Table S4 of SI are the results concerning hydrogen bonds between the hydroxyl oxygen atoms of different β-D-glucose oligomers and chloride anions. For the β-D-glucose-IL system, the average number of HB is about 4.9, which is consistent with that reported from many other works with experimental

29

or theoretical techniques 29, 32-33. The averaged HB number per

β-D-glucose is gradually decreased with the increase in the degree of polymerization. On average, each pyranose ring of (GLU)6 and (GLU)12 is hydrogen bonded to about 3.2 and 3.0 chloride atoms, respectively. The strong O-H···Cl interaction renders each hydroxyl group to form hydrogen bond about one chloride anion in all the systems. That is, all the hydroxyl groups are fully hydrogen bonded, provided that the chloride anions are approaching to the cellulose. In Figure 10a, we provided the HBs population of chloride anions around the hydroxyl groups in the system of the solute (GLU)6 using the enhanced sampling MD. The cutoff distance is 3.5 Å. The peak of the population centered around 168.0°. About 99.9% of the nearby chloride anions are hydrogen bonded. Moreover, there lies no obvious differences between different pyranose rings whether they are in the terminal or not. As for the ITS simulations with non-polar model of (GLU)6 in water, the averaged HB number per pyranose ring is about 7.3 and the total number of HB is about 43.5 (Table S4 of SI). If the commonly used criteria of HB (dO-H···O = 3.2 Å, ∠O-H···O = 120°) is employed, the average number of HB is reduced to 4.6.


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In either criteria of HB, the averaged HB number per pyranose ring in water is larger than that in ILs. However, the HB formed by chloride anion and hydroxyl group is much stronger than that between the oxygen atom of water and hydroxyl group of oligosaccharide, which would be discussed in the next subsection. What’s more, the water molecules approaching to the sugar are displaying a random and broad distribution of bond angles to some extent (Figure 10b) and only about 49.9% of waters are hydrogen bonded.

Figure 10. The population of hydrogen bonds of the solute (GLU)6 and the solvent IL (a) and water (b) obtained from ITS MD simulation.

3.5 Sugar-IL Interaction Energies Glucose-IL. In Table 1 the calculated interaction energies between β-D-glucose and the IL, Einttot, as well as its cation (labelled as Eint+) and anion (Eint-) are listed. With the non-polar model, the mean value of the interaction energy between glucose and the cation (anion) is -22.8 (-64.0) kcal/mol. The ratio of Eint-/Eint+ is 2.8, in good agreement with Youngs et al.’s results that Eint- is about 3 ~ 4 times than Eint+ in their work.33 The interaction energy has two components: van der Waals (Eint-vdW) and electrostatic energy (Eint-ele). The calculated ratio of Eint-ele/Eint-vdW from the non-polar simulations is about 12.3. In the interaction energy between the



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cations and β-D-glucose, Eint+, the van der Waals interaction is dominant, while in the interaction energy between the anions and glucose, Eint-, the electrostatic interaction is overwhelming (the van der Waals interaction even provides the negative contribution to the total interaction energies). When the glucose-IL mixture was simulated with polarizable force field, the calculated Einttot is lowered to -74.9 kcal/mol (Table 1). In fact, the calculated Eint+ values are almost the same in the simulations with both models (non-polar versus polar). The major difference of Einttot between the simulations with non-polar and polar model lies in the difference of Eint-.

Table 1. The averaged interaction energies in kcal/mol between glucose/(GLU)6 molecules and the IL, IL cation and anion ions from the simulated system with non-polar and polar model employing the ITS method or not. The corresponding concentration of the solvent is 2.04 mol%. Cation effect Solute

Method

Eint

Anion effect

tot

Eint+

Eint-vdW+ Eint-ele+

Eint-

Eint-vdW-

Eint-ele-

Non-polar

-87.8

-22.8

-20.5

-2.3

-64.0

13.3

-77.3

Polar

-74.9

-23.2

-20.5

-2.7

-51.1

9.7

-60.8

Non-polar

-353.7

-90.4

-95.7

5.3

-263.3

49.6

-312.9

Non-polar with ITS

-350.2

-87.9

-96.5

8.6

-262.3

51.7

-314.0

Polar

-308.7

-90.7

-94.0

3.3

-218.0

37.1

-255.1

Polar with ITS

-306.7

-90.1

-94.2

4.1

-216.6

37.8

-254.4

Glucose

(GLU)6

(GLU)6-IL. With the increment of the glucose units, the application of the ITS enhanced sampling scheme is highly desired to sample more conformations. To analyze the interaction


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energies, we take the mixture of (GLU)6 and IL as an example in the following part. The distribution populations of potential energy (Epot) of (GLU)6–IL system obtained from different models were depicted in Figure 11a. With the non-polar model, the simulated Epot of (GLU)6 ranges from 663.2 kcal/mol to 792.4 kcal/mol, with the mean value, , of 724.9 kcal/mol. The simulation with the ITS method enlarges the range of Epot to 668.2 ~ 808.3 Kcal/mol ( = 726.9 kcal/mol). The region of Epot calculated from the polarization model is also enlarged from 595.9 to 922.7 Kcal/mol with mean value of 730.4 kcal/mol. The interaction energies obtained from the MD simulations with or without using the polarization model bare a great difference. In the ITS MD simulations without and with polarization model, the mean interaction energy is -306.7 and -350.2 kcal/mol, respectively. In Figure 11b, two separated ranges of interaction energy are displayed at -398.3 ~ -294.2 kcal/mol (non-polar) and -362.9 ~ -237.0 kcal/mol (polar), respectively. Such a difference in the interaction potential energy is mainly reflected in the interaction energy between (GLU)6 and anion, especially the electrostatic interactions (Table 1). Similar with what was discussed above in the simulation of glucose in the IL, the polarization effect has a larger impact on the Eint-. The calculated Eint+ from the MD simulations with/without the polarizable force field (using ITS method or not) are almost equivalent. The cationic contribution is mainly attributed to the van der Waals interaction. Our simulation results do not fully support the prediction made by Lu et al. who believed that the van der Waals interaction of cations with cellulose is not important in the dissolubility of cellulose in ILs.

31

Another difference in the most popular conformations

between the non-polar (A) and polar (B) models is clearly shown in Figure 11c. The most populated conformation (B) obtained with polarization model is more folded than that (A) found in non-polar simulations.



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The interaction energy between the (GLU)6 and water molecules was also calculated for comparison. In the ITS MD simulation with non-polar model, the mean value of (GLU)6-water interaction is about -283.9 kcal/mol, much smaller than that from the MD simulations of (GLU)6 solvated in IL. The van der Waals and electrostatic contributions to Einttot in the (GLU)6-water simulated system is -28.8 kcal/mol and -255.1 kcal/mol, respectively. It can be seen that the interaction of (GLU)6 and water is mainly from the electrostatic interactions. Although water could form more HBs with (GLU)6 than IL anions, the interaction energy is weaker than cellulose-IL interaction, so that it may be more difficult for water than IL to break the hydrogen bonds formed between cellulose molecules, which partly explained why cellulose can be dissolved in ILs not water. Furthermore, the averaged Epot of (GLU)6 calculated from ITS simulation with non-polar model is 633.4 kcal/mol, ranging from 589.8 kcal/mol to 680.1 kcal/mol. Obviously, the Epot are lower than that in the IL systems, which means (GLU)6 takes more stable conformations in water.


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Figure 11. The potential energy (Epot) of the solute (GLU)6 (a) and the interaction potential energy (Einttot) between the (GLU)6 and the solvent [C1mim]Cl (b) from different MD trajectories with different simulation methods. (c) Typical conformations with the maximal population of Eint were also provided.



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4. CONCLUSIONS DFT calculations predicted that cellulose oligomers have the helical structure, holding by the intramolecular hydrogen bonds between the –O6H of one pyranose ring and the O3’ atom of the adjacent pyranose ring. The adjacent two –CH2OH groups all displays in an anti-anti state. A comparison was made on experimental vibration spectra of cellulose, pure [C1mim]Cl , and the mixture of IL and cellulose in solid state. The CH and OH stretching bands at 2800 ~ 3000cm-1 and 3585 cm-1 of cellulose microcrystalline are greatly depressed in the cellulose-IL mixture, indicating the deformation of

pyranose rings and intermolecular interactions between the

cellulose and IL upon dissolution of cellulose in IL. Conformations of β-D-glucose and its longer oligomers in [C1mim]Cl have been studied by MD simulations with different models (with or without polarization and ITS enhanced sampling model). The pyranose ring of β-D-glucose mainly displayed in a chair conformation with the maximal population of puckering parameters (Q, θ) around (0.54, 11.0). When the degree of polymerization of cellulose is larger than 6, the rarely observed boat and skew-boat conformations of the pyranose ring come to appear. Typically, only one of the pyranose ring of (GLU)6 displayed a cross-conformation. The polar MD simulations without ITS method show that the population of the boat and skew-boat conformations has been increased to a certain extent. The appearance of cross-conformation between the boat and skew-boat conformation of the pyranose rings of (GLU)6 in ITS MD simulations evidently demonstrates that the selective integrated tempering methodology could better sample the conformational changes of central solute within shorter simulation time. From the measured end-to-end distance of (GLU)n, we found that the mean distance of glucose-to-glucose becomes smaller with the increase of polymerization of cellulose.


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The interaction energies between sugar and IL have been analyzed by RDFs and interaction energy. For the β-D-glucose and its longer oligomers, chloride anions is prone to form the HBs whenever it is near the hydroxyl groups and 99.9% of the neighboring chloride anions are hydrogen bonded to the hydroxyl groups of cellulose. The mean HBs per hydroxyl group is about 1. That is, each hydroxyl group is fully hydrogen bonded to the chloride anions. Further analyses of the interaction between the sugar and the anions as well as cations show that the interaction is composed of not only the major contribution from anion ions through the electrostatic interactions, but also the contribution from the cation ions through the van der Waals interactions. Although cellulose oligomers can form more HBs in water solvents, the interaction energy of cellulose-water mixture is much weaker than that of cellulose-IL. This rationalizes why the cellulose is difficult to be dissolved in water. In addition, the employment of polarization model and enhanced sampling technique can effectively yield larger conformational spaces, ensuring not only the efficiency but also the accuracy.

ASSOCIATED CONTENT Supporting Informations: The Supporting Information is available free of charge on the ACS Publications via the Internet at http://pubs.acs.org. Conformations of cellulose oligomers and IL obtained with DFT calculations, computational details of MD simulations, mean partial charges of glucose in the polar MD simulations, the distribution populations of cellulose oligomers (n = 1, 2, 3, 6, 9, 12), and the experimental infrared absorbance spectra of cellulose and ionic liquids.



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AUTHOR INFORMATION Corresponding Author *[email protected], [email protected] ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 21290192, 21273102, 21673111, 21373016, U1430237).

We are grateful to the High

Performance Computing Center of Nanjing University for doing the quantum chemical calculations in this paper on its IBM Blade cluster system.


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TOC Graphic



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TOC Graphic 76x44mm (300 x 300 DPI)

ACS Paragon Plus Environment

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Figure 1. Schematic representation of cellulose, ionic liquid, and simulation model of cellulose dissolution in IL. 175x52mm (300 x 300 DPI)



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Figure 2. Plot of (a) the polar MD simulation and (b) the ITSMD simulation with and without polarization model. The Dev. is defined as the energy difference between the two successive time steps, called n and n + 1, respectively. 105x135mm (300 x 300 DPI)


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Figure 3. Geometry and binding energy, Eb, of (a) [C1mim]+ and [C1mim]Cl structures: (b) front, (c) side-1, (d) side-2 and (e) side-3. The crystal structure is taken from Ref. 62. 177x100mm (300 x 300 DPI)



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Figure 4. (a) The intra-molecular hydrogen bonds and two selected vibrational modes of (b) helical twist structure of (GLU)6 in vacuum. (c) The relationship of the distance (L) between the centers of terminal pyranose rings and polymerization of cellulose, n. Color scheme: C, cyan; O, red; H, white. 163x155mm (300 x 300 DPI)


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Figure 5. (a) The THz (in solid curve) and calculated vibrational spectra (in dotted stick) of cellulose microcrystalline. (b) The experimental IR and calculated vibrational spectra of cellulose microcrystalline (blue), IL (red) and the absorbance difference (A = −log(I/I0 ), black curve) between the transmitted intensities of pure IL (I0) and the mixture (I) of IL and cellulose in solid state. 168x127mm (300 x 300 DPI)



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Figure 6. (a) Puckering amplitude (Q) of β-D-glucose and (b) its distribution population of puckering parameters (Q, θ) from the polar MD simulation trajectories (right). The ■, □, ▲, ▼ and ☉ indicate the chair, boat, half-chair, skew-boat and envelope conformation of glucose, respectively. 176x91mm (300 x 300 DPI)


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Figure 7. The Puckering amplitude (Q) distribution population of puckering parameters (Q, θ ) of (GLU)6 from the (a) non-polar, (b) polar, (c)ITS non-polar and (d) ITS polar MD simulation trajectories. The ■, □, ▲, ▼ and ☉ indicate the chair, boat, half-chair, skew-boat and envelope conformation of glucose, respectively. 166x170mm (300 x 300 DPI)



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Figure 8. The mean distance of glucose-to-glucose and its deviation for the different oligosaccharides (GLU)6 of n = 2, 3, 6, 9, 12 from the non-polar (▇) and polar (★) MD simulations, and in inset: time evolution of the end-to-end distance L of (GLU)12 from the non-polar MD simulation with the ITS enhanced sampling method. 82x61mm (300 x 300 DPI)


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Figure 9. The RDFs for (a) anion Cl- and (b) cation [C1mim]+ around O1/O4, O2 and O5 atoms of different pyranose ring of (GLU)6 from the non-polar and polar simulation, where X denotes the geometric center of imidazolium ring. 169x98mm (300 x 300 DPI)



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Figure 10. The population of hydrogen bonds of the solute (GLU)6 and the solvent IL (a) and water (b) obtained from ITS MD simulation. 82x43mm (300 x 300 DPI)


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Figure 11. The potential energy (Epot) of the solute (GLU)6 (a) and the interaction potential energy (Einttot) between the (GLU)6 and the solvent [C1mim]Cl (b) from different MD trajectories with different simulation methods. (c) Typical conformations with the maximal population of Eint were also provided. 158x170mm (300 x 300 DPI)


Polarization Effects on the Cellulose Dissolution in Ionic Liquids

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