Cosolvent or Antisolvent? A Molecular View of the Interface between

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Cosolvent or Antisolvent? A Molecular View of the Interface between Ionic Liquids and Cellulose upon Addition of Another Molecular Solvent Feng Huo, Zhiping Liu,* and Wenchuan Wang* Division of Molecular and Materials Simulation, State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *

ABSTRACT: Ionic liquids (ILs) are promising nonderivatizing solvents for the dissolution of cellulose and lignin in biomass pretreatment processes, which are, however, retarded by sluggish dynamics. Recent investigations showed that cosolvents such as dimethyl sulfoxide (DMSO) can accelerate the dissolution dramatically. On the other hand, water is used as a common antisolvent to regenerate cellulose from solutions. To understand the co-/antisolvent effects in dissolving cellulose by ILs, we performed molecular dynamics simulations of the interfaces between an Iβ cellulose crystal and different solvent systems, including ILs, DMSO, water, and mixed solvent systems. The density profiles and pair energy distributions (PEDs) show that the anions interact much more strongly with the cellulose surface than the cations, which is responsible for the dissolution of cellulose. It was found that the number of chloride ions in contact with cellulose does not cause the co-/antisolvent effect. In contrast, the cellulose−chloride PEDs are sensitive to the addition of molecular solvents, such as DMSO and water. Detailed analyses show that multiple hydrogen-bond (HB) patterns are formed between chloride and the hydroxyl groups of cellulose that are noticeably changed in the presence of DMSO or water. A combined analyses of both the PEDs and HB patterns can provide valuable information about the enhancement of cellulose dissolution. The simulation results in this work present useful knowledge for the design of solvent systems for dissolving cellulose or other types of biomass.

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INTRODUCTION Lignocellulosic biomass is one of the most abundant resources on Earth. As estimated in a 2011 report, more than 1 billion dry tons biomass can be supplied as potential feedstock for the bioenergy and bioproducts industry by 2030 in the United States.1 Thus, lignocellulosic biomass is a promising renewable and sustainable resource for reducing the heavy dependence of the world economy on exhausting fossil fuels. However, because of its inherent recalcitrance, one of the significant challenges is to develop cost-competitive and environmentally friendly pretreatment technologies prior to the utilization of lignocellulosic biomass at an industrial scale.2 In 2002, Swatloski et al.3 reported that a certain class of ionic liquids (ILs) can be used as nonderivatizing solvents for cellulose under relatively mild conditions, which inspired great interest in understanding the dissolution mechanism for the design of effective solvent systems.4−6 Most of those ILs contain anions with strong basicity, that is, high hydrogen-bond acceptor (HBA) strength, as indicated by the β values in terms of Kamlet−Taft solvatochromic parameters,7,8 such as chloride,3 acetate,9,10 formate,11 and phosphonate,12 combined with methylimidazolium3 or methylpyridinium13 cations with allyl,14 ethyl, or butyl side chains. Some recent efforts have also contributed to the use of more biocompatible ILs, such as amino acid anions with ammonium cations.15,16 © XXXX American Chemical Society

Although good solubilities of cellulose in ILs as high as 25 wt % have been observed,17 a major drawback of ILs is their high viscosities, which hinder not only cellulose dissolution, but also any postprocesses, such as electrospinning. To overcome this obstacle, some aprotic organic solvents, such as dimethyl sulfoxide (DMSO), can be added as a cosolvent to effectively decrease the viscosity without the precipitation of cellulose.18,19 In addition, Rinaldi20 found that cellulose can also be dissolved remarkably rapidly, requiring 2 orders of magnitude less time than for neat ILs in which, for example, some type of IL/cosolvent system and a minor molar fraction of IL (xIL < 0.2) is required. The cellulose dissolution power is also in line with the HBA ability (β values) of the mixture systems.20 Recently, Hauru et al. used a combination of the values of β and β − α, the difference between the basicity and acidity, to explain the ability of mixtures to dissolve cellulose.21 Explanations based on ab intio quantum chemistry calculations have also been discussed.22,23 In addition, Xu et al. found24 that this type of solvent system with an optimized ratio can dissolve cellulose even at ambient temperature, reaching solubilities as high as 15 wt %. On the basis of conductivity measurements, they proposed that the concentration of Received: July 27, 2013 Revised: September 4, 2013

A

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Figure 1. Typical simulation box used in this work. There are 8 × 8 cellulose strands parallel to the z axis (vertical with respect to the plane of the paper), with eight anhydroglucose units (AGUs) for each strand. Two 110̅ surfaces of the Iβ cellulose crystal are exposed that are parallel to xz plane and in contact with solvents. Periodic boundary conditions are applied in all three dimensions. For details, see text.

“free” anions would increase owing to the preferential solvation of cations by the cosolvents.24 In contrast, it is well-known that the presence of water or alcohol in an IL significantly decreases the solubility of cellulose, so the former can be used as antisolvents to regenerate cellulose from IL solutions.3,14 However, to understand the interplay of the co-/antisolvents, cations, anions, and cellulose, a clear picture on the molecular level is needed. Molecular dynamics (MD) simulations are an indispensable tool for investigating the interplay of various interactions and providing insight into the structural and dynamic properties of systems in atomistic detail. As an important theoretical and computational method, MD simulations have been successfully used in a wide range of systems. For IL−cellulose systems, several MD studies have been reported to understand the mechanism of dissolution of glucose,25−27 cellulose oligomers,28−30 and cellulose microfibrils31−35 in ILs of 1-butyl-3-methylimidazolim chloride (BmimCl) and 1-ethyl-3-methylimidazolium acetate (EmimAc). However, only a few simulation studies have focused on the effects of the structures of the cations36,37 and anions.38 All of these simulations indicate that the hydroxyl groups in glucose and cellulose have a strong tendency to form hydrogen bonds (HBs) with anions, which significantly disrupts the intrachain and interchain HBs formed within cellulose itself. This energy-favorable process is believed to be a major driving force for the dissolution of cellulose. Gross et al.34 analyzed the energetic and entropic changes between solvated crystalline microfibrils of 36 glucose chains and fully detached glucose chains in an IL and water. They found34 that both driving forces favor the dissolution of cellulose in BmimCl, whereas the energy contribution is not favorable when dissolving cellulose in water. Therefore, they suggested34 that the internal energy, which is commonly calculated in simulations, is a good indicator reflecting the free energy of dissolution. Liu et al. simulated a cellulose microfibril of nine glucose strands dissolved in EmimAc within 100 ns.32 They concluded that strong HBs between the anions and cellulose contribute mainly to dissolution and that the cations also play an important role in disrupting the crystal structure. Rabideau et al. found35 that small crystalline bundles are stable in water but can be dissolved in three types of ILs within 100 ns by MD simulations. In addition, they presented an underlying mechanism of the initial breakup of cellulose strands. The anions are first bound strongly to the hydroxyl groups of the exterior strands of the bundle, forming negatively charged complexes. Then, the cations intercalate between the individual strands, pushing them apart and initiating the separation.

To the best of our knowledge, very limited simulations have investigated how the addition of an anti- or cosolvent affects the interactions between cellulose and ILs.29,39,40 Liu et al. simulated the solvation of a 20-mer cellulose oligomer in binary mixtures of EmimAc and water,29 focusing on the mechanism of regeneration of cellulose upon the addition of water. They proposed a key intermediate step of water diffusing into the anion−sugar network and weakening the HBs of anions because of the strong anion−water interaction. Gupta et al. performed simulations of 15 10-mer cellulose oligomers in EmimAc with different water concentrations.39 They analyzed the numbers of cellulose−cellulose and cellulose−anion HBs formed, which depend on the concentration of water. A similar mechanism was proposed, attributing the regeneration to the destruction of cellulose− anion HBs. Zhao et al. simulated a mixture of BmimAc and one cellulose chain (degree of polymerization = 10) with different cosolvents. They observed preferential solvation of anions by protic solvents with a reduction of the interactions between the anions and cellulose. In contrast, the aprotic solvents form weak hydrogen bonds with the cations, thus enhancing the anion−cellulose interactions.40 Recently, we proposed an improved force field for ILs,41,42 by which both the thermodynamic and dynamic properties are well described not only for neat ILs but also for the mixtures of ILs, water, and glucose.43,44 In this work, MD simulations were performed to investigate the interfaces between an infinite crystal of Iβ cellulose and ILs of 1-ethyl-3-methyl-imdazolium chloride (EmimCl), 1butyl-3-methyl-imdazolium chloride (BmimCl), and 1-octyl-3methyl-imdazolium chloride (OmimCl). Various amounts of DMSO and water were added to BmimCl as co-/antisolvents to understand their different effects. Then, we analyzed the density profiles of various components in the solvent systems; the pair energy distributions (PEDs) between cellulose and cations, anions, and solvent molecules near the interface; and the hydrogen-bond (HB) patterns formed between cellulose and these same components. The simulation results revealed that the PEDs between anions and cellulose are sensitive to the addition of co-/ antisolvents, as well as the type of anions, which could be used as a promising indicator of the ability of the solvent systems to dissolve cellulose. Combined with the analyses of HB patterns, we also present a clear picture in atomistic detail of the different effects of DMSO and water on the dissolution of cellulose by ionic liquids. All of this information would be useful for the design of solvent systems for use in biomass pretreatment processes. B

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Table 1. Simulated Systems, Contact Numbers, and Average Interaction Energiesa between Anions and the Cellulose Surface number of

solvent fraction

system

ion pairs

solvent molecules

EmimCl BmimCl OmimCl

500 500 500

0 0 0

DM20b DM40 DM50 DM80 DM90 DM100 W33b W50 W67 W80 W100

480 450 450 300 200

120 300 450 1200 1800 2000 225 450 900 1200 3600

xs

interaction energy (kcal/mol)

wt %

contact number

Einter

Eele

ELJ

−29.6 −33.4 −31.5

−31.8 −35.6 −33.7

2.1 2.2 2.2

−31.1 −32.8 −32.9 −36.4 −36.7 −9.8 −26.4 −27.3 −26.6 −25.9 −7.6

−33.5 −35.4 −35.4 −39.0 −39.5 −5.1 −28.0 −29.1 −28.3 −27.7 −6.3

2.4 2.6 2.5 2.6 2.8 −4.7 1.6 1.8 1.8 1.8 −1.4

Neat Ionic Liquid Systems

450 450 450 300

22.2 18.4 17.0 Mixed BmimCl + Co-/Antisolvent Systems 0.2 10.1 16.9 0.4 23.0 16.5 0.5 30.9 15.5 0.8 64.1 11.3 0.9 80.1 6.3 1 100 59.8 0.33 4.9 18.8 0.5 9.3 18.4 0.67 17.1 17.7 0.8 29.2 16.3 1 100 98.1

A molecule/ion in the solvent systems is regarded as “being in contact” with the cellulose surface when a short distance is found between any atoms in the molecule/ion and those in cellulose. Einter is the average interaction energy between cellulose and the anions. Eele and ELJ are the contributions from electrostatic and Lennard-Jones interactions, respectively. bDM and W are abbreviations for DMSO and water, respectively, and the following numbers are their mole fractions in percentages. DM100 and W100 refer to neat DMSO and neat water systems, respectively. Thus, the contact numbers and interaction energies are also for DMSO and water molecules. a

Figure 2. Chemical structures and atom notation of the repeat anhydroglucose unit (AGU) in cellulose and the ionic liquids simulated in this work.

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METHODS Force Fields. The ILs were described by our recently developed united-atom force field, with which the simulated

liquid densities, enthalpies of vaporization, self-diffusion coefficients, and shear viscosities can be predicted in good agreement with experimental values.41,42 Combined with the C

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Figure 3. (a) Mass and (b) number density profiles of neat BmimCl, DMSO, and water in contact with cellulose. In panel a, the mass density of cellulose is also shown to demonstrate the zero point defined by the center of mass (COM) of the outer-layer sheet of cellulose. In panel c, the number densities are illustrated in an accumulated way. Thus, the numbers of anions and cations can be represented by the areas shown in different colors. In the first solvation shell of 2−5.2 Å, there are about 17.8 Cl− ions and 7.6 cations. In the region from 5.2 to 6.4 Å, 1.8 Cl− ions and 11 cations are found. In panel d, the number densities of O atoms in DMSO and water are compared with the number densities of the corresponding centers of mass.

SPC/E model for water45 and an optimized united-atom model for DMSO,46 the OPLS (optimized potential for liquid simulations) force field for carbohydrates47 was employed. These models are all consistent with our AMBER-based force field. The conventional Lorentz−Berthelot mixing rules were used to obtain Lennard-Jones (LJ) parameters of cross interactions. It is also noted that the viscosities of BmimCl− water−glucose systems were well predicted previously by this combination of force fields.43 Cellulose Crystal and Surface. Using the Iβ cellulose crystal data recently solved with X-ray and neutron fiber diffraction,48 a crystal unit was first constructed here containing 8 × 8 cellulose strands with eight anhydroglucose units (AGUs) for each strand. The strand was parallel to the z axis (Figure 1). Thus, every eight strands in the xz plane formed one layer of cellulose sheet, which was made infinitely large by applying periodic boundary conditions (PBCs) in the x and z dimensions. Consequently, eight layers of cellulose sheets were stacked in the y dimension, leaving two outer-layer sheets to form the 11̅0 surfaces of cellulose, which are hydrophilic because of the exposed hydroxyl groups. Then, a pre-equilibrated solvent box with the same size in the x and z dimensions was contacted with the cellulose box on the 11̅0 surface to construct a new simulation box (Figure 1).

Finally, PBCs were applied in the y dimension. Thus, there are two statistically equivalent interfaces in the box. To improve the statistics in the simulations, we used the average properties on the two interfaces in our analyses. It is noted that a sufficient number of solvent molecules must be added to the solvent box to guarantee that the solvent can expand into the bulk-like region (between the two interfaces in Figure 1). Typically, the length in the y dimension of the solvent systems is around 80−160 Å, which is more than twice that of cellulose in the y dimension. Simulation Systems. To evaluate the effects of cations, we first simulated three types of neat ILs in contact with the 11̅0 surface of Iβ cellulose, including 1-ethyl-3-methyl-imdazolium chloride (EmimCl), 1-butyl-3-methyl-imdazolium chloride (BmimCl), and 1-octyl-3-methyl-imdazolium chloride (OmimCl). Then, two types of mixture solvent systems of BmimCl−water and BmimCl−DMSO were investigated in different concentrations of co-/antisolvent, as well as the neat DMSO and water systems. The numbers of molecules and mole fractions for each simulation are listed in Table 1. Because the molecular weights of DMSO and water are quite different, their mass fractions were also calculated for reference. For simplicity, we refer to water and DMSO as the solvent and to BmimCl as the D

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Table 2. Hydrogen-Bond Patternsa Formed by the Chloride Ions in Contact with Cellulose in Different Solvent Systems solvent systems [Bmim]Cl

W33

W50

W67

W80

DM20

DM40

DM50

DM80

DM90

16.1 3.4 7.8 5.0 33.9

13.4 2.0 6.9 4.4 29.1

11.6 3.0 5.5 3.0 23.2

6.2 0.9 3.3 2.0 13.4

−

total 1-HB 2-HB 3-HB HBtotal total 1-HB 2-HB 3-HB 4-HB HBtotal

18.2 4.6 8.3 5.3 37.3

18.4 7.9 6.6 4.0 32.9 6.7 5.1 1.6 0 0 8.5

17.5 16.7 5.8 5.6 8.6 7.3 3.1 3.8 32.3 31.5 Cl−−Water 12.3 12.2 8.1 4.6 3.5 5.0 0.7 2.1 0 0.5 15.8 22.7

15.5 4.7 6.9 3.8 30.1

Cl −Cellulose 15.5 2.5 7.0 6.0 34.5

14.1 3.9 3.9 3.3 3.0 31.2

Total represents the sum of the number of Cl− ions for all HB patterns. n-HB (n = 1−4) refers to the number of Cl− ions forming hydrogen bonds (HBs) with n H atoms in the hydroxyl groups. HBtotal is the total number of H atoms forming HBs with Cl− ions.

a

IL hereinafter. The chemical structures and atom notations of all of the ILs, solvents, and AGUs in cellulose are shown in Figure 2. Simulation Details. The simulations were carried out using LAMMPS49 at a constant temperature of 373 K and pressure of 1 atm with a Nosé−Hoover temperature thermostat and a pressure barostat. The SHAKE algorithm was used to constrain the hydrogen atoms. The cutoff radius was set to 12 Å. The longrange electrostatic interactions were calculated with a particle− particle particle-mesh (PPPM) solver.50 At the beginning of each simulation, the NVE ensemble was used to relax the stress of unphysical configurations. Then, all of the systems were equilibrated for at least 15 ns with a time step of 2 fs. After equilibration, production runs were performed for another 15 ns to analyze the results. The system trajectories were recorded every 200 fs from the production runs, and postanalyses were applied. In this work, we focused on the structure and energy near the cellulose surface. A molecule/ion in the solvent systems is regarded as “being in contact” with the cellulose surface when a short distance is found between any atom in the molecule/ion and an atom in cellulose. The distance is defined by the first valley of the site−site pair radial distribution functions (RDFs). To calculate the density profiles, we set the zero point to the position of the center of mass (COM) of the outer-layer sheet of cellulose (Figure 3a) to avoid a possible COM shift in simulations. The pair energy distributions (PEDs) were obtained from histograms of the interaction energies between the overall cellulose structure and every molecule/ion in contact with the surface. In the HB analyses, we focus on the case of a solvent system defined as the hydrogen-bond acceptor (HBA) and cellulose defined as the hydrogen-bond donor (HBD). As mentioned above, there is strong evidence from both experiments and simulations that the HBA ability is highly correlated with the dissolution power of the solvent systems. Here, Cl− anions and O atoms in DMSO and water can act as HBAs, whereas H atoms in the hydroxyl groups on the cellulose surface can act as HBDs (Figure 2). A HB of O−H···Cl/O was defined by conventional distance and angle criteria.51 Here, the O−Cl/O−O distance was chosen to 3.8/3.5 Å, and the Cl/O···H−O angle was set to be larger than 150° (Figure 2).

Figure 4. Normalized pair energy distributions (PEDs) between different moieties in neat solvent systems and the cellulose surface.

cellulose surface for neat BmimCl. As mentioned above, the COM of the outer layer of cellulose was set to zero to represent the surface of cellulose. The atoms in cellulose were within the region from 0 to 4 Å, intersecting with the region of solvent starting at 2.0 Å. In all simulations in this work, no evidence of cellulose swelling was observed, as indicated by the nearly solvent-independent mass density profiles of the cellulose. This is understandable because we used a surface that was infinite in size without any defects and a limited simulation time of several nanoseconds. However, there are strong interactions in the surface area beyond 2.0 Å extending to the first solvation layer, which is addressed in the following discussion. The solvent structure, HB patterns, and interaction energies in this region could provide valuable information on the distinct features of different solvent systems. The cations and anions present two clear layers expanded to about 15 Å, beyond which a bulk-like oscillation was observed. In addition, the density profile peaks of cations and anions occurred in an interchanged way, indicating the strong correlation between cations and anions (Figure 3a). The first peak of smaller-sized Cl− ions appeared at around 3.7 Å, which is about 2 Å closer to the cellulose surface than that of cations. The first solvation layer of anions, which would play a major role on the cellulose dissolution, can be defined in the region from 2.0 to 5.2 Å (Figure 3b). The peak value gives a number density for Cl− of about 13 mol/L,

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RESULTS AND DISCUSSION Neat BmimCl. Panels a and b of Figure 3 illustrate the mass and number (COM) density profiles, respectively, near the E

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Figure 5. Typical snapshots of BmimCl in contact with the cellulose surface. (a) Side view to show different density profiles of cations and anions around the surface. (b) Top view to show the hydrogen-bond patterns formed between Cl− ions and the cellulose surface (see blue sticks linked Cl− ions and H atoms). Only Cl− ions and H atoms in the exposed hydroxyl groups are represented as space-filling spheres. Atom colors: C, cyan; H, white; N, blue; O, red; Cl−, ochre.

of anions/cations can be represented by the area in different colors. For example, about 17.8 Cl− ions in the first solvation shell are represented by the blue area from 2 to 5.2 Å. In contrast, there are only about 7.6 cations in the same region, represented by the red area. Therefore, a well-defined ‘gap’ region for anions can be found from 5.2 to 6.4 Å, where fewer than 2 Cl− ions were found and crowded by about 9.1 cations.

more than twice the bulk density of BmimCl. Integration of the density profile gives about 17.8 Cl− ions in the first solvation layer, a value that is in good agreement with the contact number, 18.4 (Table 1), and the number of Cl− ions forming HBs with cellulose, 18.2 (Table 2). To demonstrate the number densities of different moieties in the solvents intuitively, we illustrated them in an accumulated way. As shown in Figure 3c, the number F

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As shown in Table 1, the average interaction between Cl− ions and cellulose is about −33.4 kcal/mol, with a further breakdown of the total energy to −35.5 for electrostatic and 2.1 kcal/mol for Lennard-Jones (LJ) contribution. It is noticeable that the positive value of the LJ energy indicates that most of the anions are so close to the surface that their distances are smaller than the corresponding average van der Waals diameters, because of the strong electrostatic interactions. Aside from the average values, pair energy distributions (PEDs) provide more detailed information on the interactions between cellulose and different moieties in the solvent systems. Figure 4 presents the normalized PEDs of the cations and anions in neat BmimCl. Clearly, the interactions of Cl− and cellulose are much stronger than those of the cations. Most of the interaction energies between the cations and cellulose are weaker than −30 kcal/mol, leading to an average value of −12.4 kcal/mol, of which only −3.7 kcal/mol comes from electrostatic interactions. Therefore, the electrostatic part of the cation−cellulose interactions is only about 1/10 that of anion−cellulose interactions. Similar results were also reported for recent simulations of cellulose oligmers in BmimCl.36 Although about 21 H4 and 8 H5 atoms (Figure 2) can be regarded to form C−H···O-type HBs with the cellulose hydroxyl groups, which are favorably consistent with other simulation36 and NMR52 results, the cations should not play a major role in the first step of dissolving cellulose35 because of the much weaker interactions with cellulose. Typical snapshots of BmimCl on the cellulose surface are shown in Figure 5. For clarity, only the surface hydrogen atoms of the hydroxyl groups and Cl− ions are given as space-filling spheres. In the top view on the surface, the cations are also hidden and the HBs are illustrated explicitly. As shown in Figure 5a, the anions are bound to the hydroxyl groups and much closer to the cellulose surface than the cations, which is consistent with the density profiles in Figure 3d, where the first peaks for anions and cations are at 3.7 and 5.5 Å, respectively. In the top view of Figure 5b, it can be seen that there are many types of HB patterns between Cl− ions and the hydroxyl groups on the surface. In most cases, the anions sit in the middle of two cellulose strands and act as a bridge. For example, the H2 and H3 atoms from one AGU are bridged by a Cl− ion with H2 from another AGU in the adjacent strands. In this work, we categorized the HB patterns by the number HBs formed and did not distinguish the atom details for their O−H···Cl− links. Thus, as shown in Table 2, there are 5.3 Cl− ions forming HBs with three hydroxyl groups (3-HB pattern) on the cellulose on average. Similarly, the numbers of 2-HB and 1-HB patterns are 8.3 and 4.6 Cl−, respectively. It would be noted that there are only half of the hydroxyl groups would be exposed to form HBs with Cl−, i.e., 96 hydroxyl groups for a surface covering 8 × 8 AGUs. Thus, about 39% of the surface hydroxyl groups form HBs with Cl− ions. Neat EmimCl and OmimCl. It is well-known that the HBA capability of the anions are important for ILs dissolving cellulose, but the effect of cations is still under debate. In this work, we simulated the interface between cellulose and the ILs of EmimCl and OmimCl to provide insight into the effect of the length of alkyl side chain in cations on the dissolution of cellulose. The density profiles are shown in panels a and b of Figure 6 for the anions and cations, respectively. It was found that different cations do not induce obvious changes in the first peak positions of the anions, whereas the peak values decrease with the alkyl chain length of the cation. As a result, the number of Cl− ions in

Figure 6. Number density profiles of (a) anions and (b) cations of four different neat ionic liquids in contact with cellulose.

Figure 7. Normalized pair energy distributions (PEDs) between anions and cellulose surface in four different neat ionic liquids systems.

contact with the cellulose surface (Table 1) decrease in the order EmimCl > BmimCl > OmimCl. However, the available experimental measurements showed that the solubility of cellulose in BmimCl is the highest among the three ILs,17,53,54 which cannot be interpreted merely by the number of anions near the surface. It is noticed that the interaction energy between the Cl− ions and the cellulose is the strongest in BmimCl and decreases in both EmimCl and OmimCl (Table 1). The PEDs in G

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Figure 8. Concentration (in mole fraction) dependent properties for BmimCl/DMSO and BmimCl/water by simulations in this work. (a) Interaction energies (in absolute values) between cellulose and Cl− ions; (b) Total number of Cl− ions and hydroxyl groups formed hydrogen bonds (HBs) between cellulose and anions; (c) Number of different hydrogen-bond patterns. 1-HB refers to the number of Cl− ions formed 1 HB with cellulose, etc. The numbers of 2-HB and 3-HB are shifted by 10 and 20, respectively. The figures plotted on a mass-fraction scale are also given in the Supporting Information for reference.

Figure 7 provide more detailed information about the interactions between the anions and the cellulose surface. The major peak obviously shifts from about −37 kcal/mol in BmimCl to −30 kcal/mol in OmimCl, which would be caused by the steric effect of the longer alkyl side chains in the cations. When the alkyl chain is longer than butyl, the cations will occupy too much room near the cellulose surface, leading to a decrease in both the contact number and interaction energy of anion−cellulose. In the case of EmimCl, there is sufficient room to accommodate more anions to interact with the cellulose (Table 1). However, the Cl− ions would be too crowded to form strong enough HBs with the hydroxyl groups of the cellulose surface. As a result, a large proportion of the anions cannot efficiently interact with the cellulose, given by energy values weaker than −30 kcal/mol, leading to the decrease of the average interaction energy (Table 1). In summary, Bmim would be the optimal cation for chloridebased ILs in dissolving cellulose. A reasonable explanation for the available experimental observations can be obtained by

combination analyses of the contact numbers and anion− cellulose PEDs. Pure DMSO and Water. Before addition of DMSO or water as the co-/antisolvent, we briefly show their different interactions with cellulose as a pure solvent. The mass and number density profiles are shown in panels a and b, respectively, of Figure 3 for DMSO and water near the cellulose surface. It is noticed that the molar volume of DMSO in the bulk phase is about 5 times larger than that of water. We also plotted the profiles of O atoms in both solvents because of the importance of their roles as HB acceptors. As shown in Figure 3d, the density profiles of COMs and O atoms are nearly the same for the water molecules because of the small contribution of H atoms to the COM. In contrast, the O atoms of DMSO are much closer to the surface than the COMs because of their strong interactions with the hydroxyl groups of the cellulose. In addition, the O atoms in DMSO are also closer to cellulose than those in water (2−3 Å region, Figure 3d). These results validate the fact that DMSO is more efficient than water as H

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a solvent for cellulose swelling. Unlike the case for DMSO, the most energy-favorable positions of water molecules indicate that they could not be placed as close as possible to the cellulose surface, because of their dual behaviors as HB acceptors and donors. As shown in Table 1, about 59.8 DMSO molecules were in contact with cellulose, whereas only 20.5, about one-third of them, were found to form HBs with hydroxyl groups. On average, a DMSO molecule in contact with cellulose leads to an interaction energy of −9.8 kcal/mol, with an electrostatic contribution of −5.1 kcal/mol. In contrast, there are about 98.1 water molecules in contact with cellulose, with an average interaction energy of about −7.6 kcal/mol, a value still lower than that for DMSO. However, the electrostatic contribution is −6.3 kcal/mol and slightly stronger than that for DMSO. In summary, the average interactions are comparable for DMSO, water, and Bmim cations in contact with cellulose, but much weaker than those of Cl− anions. This can be more clearly demonstrated by the PEDs in Figure 4. Mixed Solvent Systems. As mentioned above, DMSO can be used as a cosolvent to improve the process of dissolution by ILs, whereas water behaves as an antisolvent to regenerate cellulose. Because of the importance of anions in the dissolution, we focused on how the DMSO and water molecules influence the interactions between Cl− ions and the cellulose surface. Table 1 lists the simulation results for the mixed BmimCl/DMSO and BmimCl/water systems at different concentrations. The average interactions between cellulose and Cl− ions decreased significantly when even a small faction of water (5 wt %) was added. In contrast, adding DMSO did not lead to a considerable decrease in the cellulose−Cl− interactions. When the mass fraction was higher than about 35%, the interaction was even enhanced by DMSO (Figure 8a). Unlike for the above interaction energies, the contact number of Cl− ions (Table 1) does not present the well-accepted co-/antisolvent effects for DMSO and water. As can be seen in Figure 8b, the number of Cl− ions involved in HBs for the BmimCl/DMSO systems was also generally less than that for BmimCl/water systems. To elucidate the above conflict between the contact number and the interaction energy, we analyzed the HB patterns (Figure 5b) formed between Cl− ions and cellulose in more detail. As shown in Figure 8c and Table 2, the number of the 3-HB pattern in BmimCl/DMSO systems was generally larger than that in BmimCl/water systems, compared with the 1-HB pattern. Consequently, even though the number of Cl− ions in contact with cellulose was lower in BmimCl/DMSO than that in BmimCl/water, there were more hydroxyl groups bound to the Cl− ions when the mole fraction of solvent was less than 0.5 (Table 2). Thus, when DMSO or water was added to BmimCl, it caused a shift between the 3-HB and 1-HB patterns of Cl− ions, leading to the change of interaction energies. However, the subtle difference in HB patterns in the presence of DMSO or water is not sufficient to quantitatively explain the distinction between the interaction energies (Figure 8a). It is more clearly illustrated by the PEDs (Figure 9) between cellulose and the Cl− ions. Unlike that in neat BmimCl, the PED splits into two distinguishable bands upon the addition of DMSO or water (Figure 9a). In the case of water, the higher-value band is shifted from about −37 to −33 kcal/mol. In addition, a broad band around −15 kcal/mol appears, suggesting that the interaction between the Cl− ions and the cellulose is significantly weakened in the presence of water. In contrast, the higher-value band is shifted to about −40 kcal/mol, and only a minor band around

Figure 9. Normalized pair energy distributions (PEDs) between Cl− ions and cellulose surface in mixed solvent systems. For notations, see Table 1.

−20 kcal/mol appears upon the addition of DMSO. The evolutions of the two bands with concentration are also different for DMSO and water (Figure 9b). When more water is added, the two bands become more distinct, and about 60% of the interactions belong to the lower-value band around −17 kcal/mol. It is noticed that the average energies between Cl− ions and cellulose are nearly independent of the water concentration (Table 1), but the PEDs here demonstrate that the interactions are quite different at various concentrations. When more DMSO is added to BmimCl, the lower-value band disappears, and the higher-value band is shifted to about −45 kcal/mol, which is the reason for the enhancement of the average interaction at higher concentrations of DMSO (Table 1). As shown in Figure 10, the density profiles of various moieties are plotted in an accumulated way. The number densities of the anion and cation are represented by the blue bottom and red top areas, respectively. Thus, the green ribbons in the middle demonstrate intuitively the effect of the addition of DMSO of water. Because of its smaller size and its capability to form HBs with both Cl− ions and the hydroxyl groups of cellulose, a water molecule can behave as a linker of cellulose and Cl− ions in the region from 2 to 5.2 Å, that is, the first solvation layer of anions. Thus, the Cl− ions adjust their positions to obtain the most energetically favorable states, in which they simultaneously form two types of HBs, namely, Cl−−cellulose and Cl−−water. I

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Figure 10. Number density profiles of various moieties in mixed solvent systems in contact with cellulose illustrated in an accumulated way (see Figure 3c and context for reference). The anions and cations are represented by the blue bottom and red top areas, respectively. The DMSO/water are demonstrated by the green ribbons in the middle. (a) BmimCl/DMSO; (b) BmimCl/water.

For example, there are about 18.2 Cl− ions forming HBs with 37.3 hydroxyl groups of the cellulose in the neat IL system of BmimCl (Table 2). After a small amount of water is added (W33, nw/nCl− = 1:2, 4.9 wt %), more than one-third of the Cl− ions form HBs with about 8.5 water molecules, although the total number of Cl− ions increases slightly to about 18.4. As a result, the number of cellulose hydroxyl groups forming HBs with the Cl− ions decreases from 37.3 to 32.9 (Table 2), leading to a weakening of the interaction energy from −33.4 to −26.4 kcal/mol (Table 1). When the mole fraction increases to 0.8 (W80, nw/nCl− = 4:1, 29.2 wt %), nearly all of the Cl− ions (about 91%) form HBs with water. On average, each Cl− ion is coordinated by about two water molecules (Table 2). Figure 11a presents a typical snapshot of the cellulose surface for the W67 system, showing that some of the Cl− ions lose HBs with cellulose as a result of coordination by water molecules. In comparison to water, DMSO can not form HBs with the Cl− ions. Thus, there are far fewer DMSO molecules in the first solvation layer of the anions (Figure 10a). For example, the nDMSO/nCl− ratio reaches about 1:3 in the region from 2 to 5.2 Å, when an equal number of moles of DMSO is added into BmimCl. Consequently, the DMSO molecules tend to appear in the region

Figure 11. Typical snapshots of mixed solvent systems in contact with the cellulose surface (see Figure 5 for reference): (a) BmimCl/water = 1:2 (W67); (b) BmimCl/DMSO = 1:4 (DM80). The hydrogen bonds between Cl− ions and cellulose are given by blue sticks, whereas those between Cl− ions and water are given by opaque red sticks.

from 5 to 7 Å (Figure 10a), where they could behave as HB acceptors with H4/H5 in the cations (Figure 2), which would be one of the factors enhancing the Cl−−cellulose interactions (Figure 8a). The different trends for DMSO and water also have an effect on the density profiles of Cl− ions, especially at higher molar fractions of molecular solvents. As shown in Figure 12a, the position of the first peak is shifted by about 0.1 Å toward the cellulose surface in the DM80 system, whereas it is shifted by about 0.2 Å outward from the surface in the W80 system. The number of Cl− ions near the cellulose can be obtained by J

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than the number of Cl− ions in the first solvation layer for the BmimCl/water systems. For the W50 system, there were about 17.0 Cl− ions in the first solvation layer, compared with a contact number of 18.4. This suggests that about 1.4 Cl− ions are in contact with some atoms of cellulose, even though they are not in the first solvation layer. In fact, there are more Cl− ions for BmimCl/water systems in the “gap” region (5.2−6.4 Å, Figure 10a). For example, the number is 1.8 for neat BmimCl, compared with 0.5 for the DM50 system and 3.2 for the W50 system. As shown in Figure 10, the DMSO molecules are more likely to be in the gap region, expelling the anions to the first solvation shell and enhancing the anion−cellulose interactions. In the case of BmimCl/water, some of the Cl− ions in the gap region are “linked” to the hydroxyl groups by water molecules in the first solvation layer, contributing to the weaker interactions between the anions and cellulose.

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CONCLUSIONS To understand the co-/antisolvent effects in dissolving cellulose by ionic liquids (ILs), we performed molecular dynamics simulations of the interfaces between an Iβ cellulose crystal and different ILs including BmimCl, EmimCl, and OmimCl, as well as mixed solvent systems of BmimCl/water and BmimCl/DMSO. We validated that the interactions between the anions and cellulose are significantly stronger than the cellulose−cation and cellulose− solvent interactions. We found that the pair energy distribution (PED) between the anions and cellulose is a good indicator of the ability of the system to dissolve cellulose, as it is sensitive to not only the type of anions but also the addition of co-/antisolvents. In contrast, the number of anions forming hydrogen bonds (HBs) with cellulose does not represent the co- and antisolvent effects of DMSO and water. Detailed analyses show that multi-HB patterns are formed between Cl− ions and hydroxyl groups on the cellulose surface. In the presence of DMSO, an increase in the proportion of the 3-HB pattern leads to a shift of the PED to a stronger interaction energy, indicating the enhancement of the interaction between Cl− ions and cellulose, even though the number of Cl− ions decreases because of the excluded-volume effect. On the contrary, the proportion of the 1-HB pattern increases upon the addition of water, resulting in the appearance of a significant band in the PED at weaker interaction energy. In the presence of water, the Cl− ions have to adjust their positions, because of the strong tendency for HBs to form between the water molecules and the anions. As a result, water shows an antisolvent effect by decreasing the anion−cellulose interaction energy by about 20%. The results obtained in this work suggest the crucial importance of the interactions between cellulose and the nearsurface anions, which can be well described by the PED in simulations. By a combined analysis of both the PED and HB patterns, valuable information can be provided for the design of solvent systems to dissolve cellulose or other types of biomass.

Figure 12. (a) Number density profiles and (b) coordination numbers of chloride ions in mixed BmimCl/DMSO and BmimCl/water systems in contact with cellulose. For concentration notations, see Table 1.

integrating the density profile as a function of the distance to the surface (Figure 12b). In comparison to the neat BmimCl system, the number of Cl− ions within 5.2 Å (first solvation shell) decreases in the presence of either DMSO or water, because of the excluded-volume effect. However, when compared at similar mass fractions (see Table 1), the number of Cl− ions is generally larger in BmimCl/DMSO systems than in BmimCl/water systems, especially in the region closer to the cellulose surface. For example, at distances of less than 4.0 Å, there are 11.5 Cl− ions for the DM20 system (10.1 wt %), compared with 8.8 for the W50 system (9.3 wt %). Obviously, the Cl− ions closer to the cellulose surface contribute more to the interaction energy, leading to a leftward shift of the PEDs (Figure 9) in the presence of DMSO. Upon comparison of the values in Figure 12b and Table 1, it is found that there are noticeable differences between the contact number and the number of Cl− ions in the first solvation layer, namely, the region from 2 to 5.2 Å. By our definitions, the contact number refers to anions “in contact with at least one of the atoms in cellulose”, whereas the first solvation layer represents the region where the anions are “in contact with the cellulose surface”. As mentioned in the section on neat IL systems, the contact number is nearly the same as the number of Cl− ions in the first solvation layer. This remains true in the presence of DMSO. However, we found that the contact number was generally larger

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ASSOCIATED CONTENT

S Supporting Information *

Concentration-dependent hydrogen-bond patterns plotted on a mass-fraction scale and the radial distribution functions of Cl− ions and O6 atoms of cellulose. See the figure captions for details. This material is available free of charge via the Internet at http:// pubs.acs.org. K

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AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support of the Natural Science Foundation in China (Nos. 20976004 and 21176009) and the Ministry of Education in China (NCET-11-0559). We also acknowledge the computational resources provided by the Supercomputing Center of Chinese Academy of Sciences (SCCAS) and the Chemical Engineering Grid Computing Center of Beijing University of Chemical Technology.

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