Theoretical Insights into the Role of Water in the Dissolution of

Article pubs.acs.org/JPCB

Theoretical Insights into the Role of Water in the Dissolution of Cellulose Using IL/Water Mixed Solvent Systems Ramakrishnan Parthasarathi,†,‡ Kanagasabai Balamurugan,§ Jian Shi,†,‡,∥ Venkatesan Subramanian,§ Blake A. Simmons,†,‡ and Seema Singh*,†,‡ †

Deconstruction Division, Joint BioEnergy Institute, Emeryville, California 94608, United States Biological and Engineering Sciences Center, Sandia National Laboratories, Livermore, California 94550, United States § Chemical Laboratory, CSIR-Central Leather Research Institute, Adyar, Chennai 600 020, India ∥ Current with Department of Biosystems & Agricultural Engineering, University of Kentucky, Lexington, Kentucky 40546, United States

J. Phys. Chem. B 2015.119:14339-14349. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 08/29/18. For personal use only.

‡

ABSTRACT: The use of certain ionic liquids (ILs) as pretreatment solvents for lignocellulosic biomass has gained great interest in recent years due to the IL’s capacity for efficient cellulose dissolution in aqueous solution as compared to other common pretreatment techniques. A fundamental understanding on how these ILs in aqueous environments act on cellulose, particularly at lower IL concentrations with water as a cosolvent, is essential for optimizing pretreatment efficiency, lowering pretreatment cost, and improving IL recyclability. The IL 1-ethyl-3-methylimidazolium acetate ([C2C1Im][OAc]) is one of the most efficient cellulose solvents known, greatly altering cellulose structure for improved enzymatic saccharification. To understand the role of water as a cosolvent with [C2C1Im][OAc], we investigated the dissolution mechanism of microcrystalline cellulose, type Iβ, in different [C2C1Im][OAc]:water ratios at room (300 K) and pretreatment (433 K) temperatures using all atom molecular dynamics (MD) simulations. These simulations show that 80:20 ratios of [C2C1Im][OAc]:water should be considered as “the tipping point” above which [C2C1Im][OAc]:water mixtures are equally effective on decrystallization of cellulose by disrupting the interchain hydrogen bonding interactions. Simulations also reveal that the resulting decrystallized cellulose from 100% [C2C1Im][OAc] begins to repack in the presence of water but into a less crystalline, or more amorphous, form.

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unit cells, respectively.7,8 The chains of cellulose are held together strongly by hydrogen bonding (H-bonding) and stacking9 of glucose units that need to be disrupted (usually through a pretreatment process) into individual chains in order to increase substrate accessibility to hydrolytic enzymes thus generate high yields of fermentable sugars.10−15 A number of pretreatment processes using thermochemical, physical, biological, and mechanical processes have been investigated over several decades.16−19 In recent years, the pretreatment of biomass with certain ionic liquids (ILs) has received considerable attention due to their superior dissolution capability of lignocellulosic biomass, very low vapor pressure,

INTRODUCTION The continued reliance of the global transportation energy sector on nonrenewable fossil fuels is a major challenge to sustainability, due to concerns related to carbon emissions and dependence on a finite resource. There is growing importance for developing utilization of biobased feedstocks as advanced renewable resources for the production of liquid transportation fuels.1−3 Transformation of polysaccharides present in nonfood feedstocks into fermentable sugars is one of the keys to the biochemical conversion of biomass into renewable fuels and chemicals.4 The critical challenges in converting biomass into drop-in fuels and chemicals are associated with the compact packing of polysaccharides and their interactions with lignins.5,6 Cellulose, as the most abundant plant polysaccharide, exists in nature as microcrystalline cellulose I with two distinct crystalline forms, Iα and Iβ, that possess triclinic and monoclinic © 2015 American Chemical Society

Received: March 19, 2015 Revised: September 22, 2015 Published: September 25, 2015 14339

DOI: 10.1021/acs.jpcb.5b02680 J. Phys. Chem. B 2015, 119, 14339−14349

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The Journal of Physical Chemistry B

Figure 1. Representation of the simulated cellulose Iβ and its solvated structure in the presence of [C2C1Im][OAc].

and relatively low flammablility.20−27 Rogers and co-workers reported that imidazolium-based ILs could dissolve large amounts of crystalline cellulose, and the dissolved cellulose is typically recovered by the addition of water or ethanol, producing an a substrate with significantly reduced crystallinity and increased surface area that is rapidly hydrolyzed into glucose by commercial cellulase mixtures.20 Lately, the potential of 1-butyl-3-methylimidazolium acetate ([C4C1Im][OAc]) and 1-ethyl-3methylimidazolium acetate ([C2C1Im][OAc]) for the pretreatment of biomass has been demostrated.28−30 In addition to experimental research,31−37 theoretical studies using molecular dynamics (MD) simulations have been carried out to capture fundamental understanding of the interactions of ILs with cellulose.38−42 Simulation of the dissolution of cellulose in binary mixture of IL and water has been investigated in the past.41 The effect of chain length of cellulose, pretreatment temperature and the presence of water were studied.43,44 The mechanism for the breakup of small bundles of cellulose Iα and Iβ was illustrated by Ismail and coworkers.44 Wang and co-workers studied the cosolvent/ antisolvent properties of DMSO and water on the cellulose− IL mixture.45 Zhang and co-workers examined the effect of cosolvent on the dissolution of cellulose using the IL system,46 and found that protic solvents decrease cellulose dissolution, whereas aprotic solvents accelerate the dissolution of cellulose. Several studies have reported on the solvation, entropy, and other associated factors on the dissolution of cellulose in water and ILs.47−57 Experimental results show that the temperature and % of water content are influencing the dissolution of cellulose in ILs.53,54 Molecular modeling studies on water interaction with cellulose clearly show the formation of Hbonding networks between water molecules and cellulose chains.55 Ab initio molecular dynamics simulations indicate that the low concentration of water in the IL solution does not considerably alter the nature of the coordination environment of cellobiose unit.56 The molecular origin of the insolubility of cellulose in water and its solubility in 1-butyl-3-methylimidazolium chloride was investigated using replica-exchange molecular dynamics.57 Although the global structural features of the cellulose chains are similar for both IL and water, the local structural properties of cellulose resulted in significant conformational variability in the IL solution than in water due the impact in cellulose intersheet interactions by IL ions.57 The main challenges facing IL pretreatment are the cost of ILs, IL recycle and recovery, and biomass solute separation processes. Pretreatment with IL-water mixture offers decreased IL usage and an aqueous environment that can be more compatible with the enzymatic catalysis and biofuel production. In the process of IL pretreatment, the dissolution of cellulose first takes place followed by the precipitation of cellulose with

water. Even though this mode of IL pretreatment has been extensively studied, the underlying role of IL and water remains elusive. In order to have a comprehensive understanding on the pretreatment process of cellulose using IL:water mixture, a series of molecular dynamics (MD) simulations were performed at various ratios of [C2C1Im][OAc]:water and two temperatures (300 and 433 K). We have recently carried out a systematic experimental investigation on understanding the role of water during IL pretreatment of lignocellulosic biomass, particularly at 160 °C, the temperature typically used in the IL pretreatment process.58 We observed that [C2C1Im] [OAc] at concentrations of 50−80% at 160 °C is effective for cellulose solubilization and provides a substrate with enhanced enzymatic digestibility. Thus, in our present investigation, we chose 300 K (room temperature) and 433 K (pretreatment temperature) to understand the underlying physical phenomenon in terms of pretreatment and solubilization using all atom MD simulations. The process of regeneration of cellulose in water was also studied. This study sought to provide a comprehensive understanding on cellulose dissolution and regeneration in [C2C1Im][OAc]:water mixtures using all atom MD simulations. The specific objectives are to (1) Compare the dissolution process of cellulose in IL, water and IL:water mixtures at different temperatures. (2) Explore the impact of IL:water ratios on the interaction and energetics of cellulose chains. (3) Understand the intermolecular H-bonding interactions in terms of cellulose chain stability. (4) Understand the process of cellulose chain regeneration.

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MATERIALS AND METHODS Molecular dynamics (MD) simulations were carried out to study the effect of [C2C1Im][OAc]:water ratios on the dissolution of cellulose. The cellulose Iβ with nine chains, each chain having a degree of polymerization of six glucose units, was used in the study as the model system for cellulose (Figure 1). The above model system of cellulose was simulated in different [C2C1Im][OAc]:water ratios. It is well known that the degree of polymerization (DP) of cellulose highly influences solvation properties. Cellulose with DP < 6 is quite soluble in water and the solubility of cellulose in water decreases as the chain-length increases.41 However, the results may quantitatively vary with higher DP and concentration of solvent. The IL concentrations were set as 0, 20, 50, 80, and 100 wt % in water. For these concentrations, two sets of simulations were carried out (1) at room temperature (300 K) and (2) at the experimental pretreatment temperature (433 K). The carbohydrate force field parameters (GLYCAM) were used for the cellulose oligomer. The GAFF (General Amber Force Field) parameters were applied for [C2C1Im][OAc] as 14340


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The Journal of Physical Chemistry B described in the previous study40 and water molecules were treated with the standard TIP3P model. Box size and number of molecules in the aforementioned model systems are compiled in Table 1. All simulations were

433 K to simulate the systems at 433 K) followed by another equilibration of 500 ps in NPT ensemble (with respective final temperature). The temperature was retained at 300 or 433 K using the stochastic velocity rescaling method as described by Bussi and pressure was maintained at 1 bar using Nose− Hoover barostat.59 A 2 fs time step was used to integrate the equation of motion. Electrostatic interaction was calculated using Particle Mesh Ewald sums with a nonbonded cutoff distance of 10 Å. Bonds between hydrogen and heavy atoms were constrained at their equilibrium values using the LINCS algorithm.60 Analysis of the energy parameters revealed that the systems were well equilibrated. Subsequently, each system was subjected to production run of 100 ns with trajectories being saved for every 1 ps for further analysis. MD simulations were carried out using GROMACS 4.5 package (http://www. gromacs.org/). The analysis of the trajectories was made using the GROMACS suite of programs61−63 and the results were visualized using VMD package.64

Table 1. Details of the Simulated Systems s. no

system name

no. of ion pairs

1 2 3 4 5

water 20% IL 50% IL 80% IL IL

41 110 176 220

no. of water molecules 2080 1664 1040 416

box size (nm) 3.40 3.40 3.40 3.40 3.40

× × × × ×

4.80 4.80 4.80 4.80 4.80

× × × × ×

5.20 5.20 5.20 5.20 5.20

simulation time (ns) 100 100 100 100 100

carried out with periodic boundary conditions. The minimization of the initial geometries was carried out using the steepest descent algorithm followed by equilibration for 250 ps in NVT (300 K was applied for room temperature simulation and the temperature of the systems was increased from 300−

Figure 2. Final structural snapshots of the simulated system for 100 ns at (A) 300 K and (B) 433 K. 14341


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Figure 3. Change in the number of interchain H-bonds in the cellulose during the simulation at (A) 300 K and (B) 433 K.

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RESULTS AND DISCUSSION Effect of Temperature in the Dissolution of Cellulose at Specific [C2C1Im][OAc]:Water Ratios. In order to examine the effect of temperature on the pretreatment process of cellulose at different [C2C1Im][OAc]:water ratios, we carried out MD simulation of the chosen systems at 300 and 433 K. The final snapshots of the simulated systems at both temperatures are presented in Figure 2. It is found that the cellulose bundle remains almost intact at 300 and 433 K in water. Further examination shows that these systems are relatively intact at 300 K in the presence of [C2C1Im][OAc]. The presence of [C2C1Im][OAc] at 433 K leads to dissolution of cellulose. It is interesting to note that the dissolution of cellulose seems to be more efficient in the [C2C1Im][OAc]:water mixtures, particularly for 80:20 mixture, when compared to 100% [C2C1Im][OAc]. The number of interchain H-bonds between the cellulose units is the most important factor to interpret the organization of cellulose. The efficiency of cellulose dissolution is reflected by the changes of interchain H-bonding interactions over time in the simulated systems at different [C2C1Im][OAc]:water ratios. The variation of H-bonding interaction is displayed in Figure 3. It is observed from the results that the number of interchain H-bonds decreases with increased concentration of [C2C1Im][OAc] at both temperatures. The magnitude of the decrease in the number of H-bonds is significantly higher at 433 K as compared to those present at 300 K. The decrease in the number of H-bonds in the presence of 80% [C2C1Im][OAc] is comparable to that observed for 100% [C2C1Im][OAc]. To provide a robust understanding of these results, the mean number along with standard deviation of the H-bonds was calculated for various systems simulated in the study and is presented in Table 2. Although the results show the overall decreasing trend of the number of H-bonds at both simulated temperatures, there is a profound effect of increasing temperature on the observed decrease in the number of Hbonds present. The number of H-bonds decreased gradually

Table 2. Mean Number of H-Bonds in Cellulose Simulated in Various Solvent Systems along with Their Standard Deviation Values mean no. of H-bonds (standard deviation) s. no

system name

1 2 3 4 5

water 20% IL 50% IL 80% IL IL

@300 K 48 43 43 41 37

± ± ± ± ±

5 5 4 3 4

@433 K 32 24 9 4 5

± ± ± ± ±

6 7 5 5 8

from 48 ± 5 in 100% water to 37 ± 4 in 100% [C2C1Im][OAc] at 300 K, whereas there is a significant decrease from 32 ± 6 in 100% water to 5 ± 8 in 100% [C2C1Im][OAc] at 433 K. The decrease in the number of H-bonds on higher temperature was due to the weakening of inherent H-bonding interactions of cellulose and the enhanced interaction of the IL with cellulose chains at higher temperature. In order to explain the observation from interchain H-bonds simulation, we calculated the radius of gyration (Rg) of the cellulose in different [C2C1Im][OAc]:water mixtures at 300 and 433 K and the results are presented in Figure 4. It can be seen from the simulation of the cellulose chains at 300 K that cellulose is found to be less compact in water followed by the [C2C1Im][OAc] and [C2C1Im][OAc]:water mixtures. From the simulation of the system at 433 K, the compactness of cellulose in different environment is found to vary as [C2C1Im][OAc]:water mixtures > 100% water > 100% [C2C1Im][OAc]. The increase in temperature has a pronounced effect on the association of the cellulose bundle which can be observed from the enhanced Rg values. From these results, it can be inferred that higher temperature enhances the progression of dispersion of cellulose chains in IL:water solutions, which is evident from the reduction in the number of interchain H-bonding interactions and increase in the Rg values at 433 K. Because the dissolution of cellulose 14342


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Figure 4. Radius of gyration (Rg) cellulose during the simulation at (A) 300 K and (B) 433 K.

bundle into individual chains, the interchain interaction energy values help to understand the dissolution process in different solvent environments. Therefore, the interaction energy between the cellulose chains within themselves in various IL/ water concentrations were calculated and presented in the Figure 6. Given the fact that the interaction energy represents the binding strength of cellulose chain, the higher the interaction energy, the stronger the cellulose bundles hold together. The results from Figure 6 show that the interaction energy values of cellulose bundle in 80:20 [C2C1Im][OAc] water mixture is very close to that of pure [C2C1Im][OAc] suggesting that 80:20 [C2C1Im][OAc] water mixture is as effective as 100% [C2C1Im][OAc] in dissolution of the cellulose bundles. Diffusion of Cellulose in Different [C 2 C 1 Im][OAc]:Water Mixtures. On top of cellulose dissolution, the diffusion of cellulose in various solvent mediums may also play important roles in cellulose interaction with [C2C1Im][OAc]:water mixtures. To elicit the mechanistic insight into the dissolution process, the diffusion of cellulose in various solvent mediums has been plotted in Figure 7. Mean square displacements (MSD) of atoms from a set of initial positions of the “whole cellulose bundle” are calculated with respect to the center of mass. The diffusion coefficient is determined by linear regression of the MSD. The MSD values of cellulose in the various solvent systems were calculated using the unit of 10−5 cm2/s with respect to time for the entire simulation. The diffusion constants were fitted for the data collected from 10 to 90 ns of the respective trajectory. In order to have an overview of all the systems, the values of both the MSD and time are converted into log scale and are shown in Figure 7. The computed diffusion trend clearly shows that the diffusion of cellulose in the medium is directly proportional to the amount of water, that is, the cellulose exhibits highest diffusion coefficient in the water medium 433 K. It is also found that the diffusion of cellulose is the lowest in the pure [C2C1Im][OAc]. In the case of cellulose in water, cellulose units move as a bundle due to the intactness of interchain H-bonding and high diffusion rate in water medium and concomitantly swelling takes place. In the case of the pure [C 2C1Im][OAc] solubilization of cellulose, the [C2C1Im][OAc] ions are efficient in disrupting the H-bonds existing between the cellulose. However, the pure [C2C1Im][OAc] does not support the diffusion of cellulose. As a consequence, the separated chains

takes place at higher temperature, results from simulation of model systems at 433 K have been considered for further analysis to gain insight into the efficient dissolution of cellulose by the [C2C1Im][OAc]:water mixtures. Energetics of Cellulose Interactions with Different Ions. The calculated interaction energy between cellulose and other species (anion, cation, and water molecules) is presented in Figure 5. The energy values presented are the overall interaction energies consisting of both electrostatic and van der Waals energies of the specified components such as anions/ cations and water interactions with the simulated cellulose system. As shown in the previous studies,32 the anion plays a predominant role in the disruption of H-bonds between cellulose chains, which can be seen from the total interaction energy of cellulose with [OAc]− (maximum interaction energy is ∼ −10 000 kJ/mol) in pure [C2C1Im][OAc]. The interaction energy of [C2C1Im]+ and water with cellulose is marginally less (max value is ∼ −4500 kJ/mol). The energies are reasonable on considering the highly ionic nature of the system whereby the electrostatic energy dominates the interaction process. Thus, it is found that the [OAc]− plays an important role in the dissolution of cellulose when compared to water. The interaction between the planar aromatic imidazole ring of [C2C1Im]+ with cellulose may enable the penetration of the [OAc]−.50,65,66 Thus, the synergistic effect of these interactions is primarily responsible for the dissolution of cellulose. The thermodynamics and energetics of the dissolution of cellulose in ILs were reported using calorimetric methods.67−69 Recently, Ismail and co-workers investigated the influence of water content on the mechanism of H-bond interactions of cellulose with different ILs.70 They simulated the cellulose with 70−90 mol % water concentration and showed that the water molecules interact with the anions of the ILs. According to observed interaction tendency of IL−water with cellulose that the anions are responsible for breaking the H-bonds exist between the cellulose bundles, at higher loading, water leads to complete solvation of anions, which in turn hinders the interaction between the anion and the cellulose. In our study, we also observed that the [C2C1Im][OAc]:water ratio of 20% (low water loading) appears to be still efficient in the cellulose dissolution, as compared to the conditions at higher water loading (∼80 or 50%), where cellulose dissolution is greatly impacted. Energetics of Cellulose Interaction within Chains. As dissolution of cellulose is the process of disrupting the cellulose 14343


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Figure 5. Interaction energy (I.E) of cellulose with different concentrations of (A) [OAc]−, (B) [C2C1Im]+, and (C) water. 14344


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Correlated Motion of Cellulose Chains. To further understand the dissolution of cellulose in [C 2 C 1 Im][OAc]:water mixtures, the correlated movement of cellulose has been obtained from the atomic covariance matrix (Figure 8). Increases in the anticorrelated motions of cellulose were observed in the presence of [C2C1Im][OAc] as indicated by the off-diagonal values. The cross correlation analysis predicted that many of the chains are undergoing correlated motions in water (Figure 8A). The analysis of structural snapshots of the simulated system (Figure 2B) and change in the number of interchain H-bonds (Figure 3) revealed that all of these cellulose chains in the presence of [C2C1Im][OAc] are more frustrated than the cellulose chains in water. Upon anion/cation interaction, many of the chains retain dynamics and also remain highly frustrated. The large-scale conformational change (Figure 8E) found in the 100% [C2C1Im][OAc], which occurs at the center core unit of cellulose bundle and where transitions between surface chains appear to be concomitant with correlated motions. These transitions show that the correlations that indicate the association cellulose chains in [C2C1Im][OAc]:water mixtures act as a mechanism to propagate cellulose dissolution. Particularly, it can be seen that as the concentration of [C2C1Im][OAc] increases, the self-correlated movements in the cellulose diminishes and anticorrelation movements enhances, which together lead to the dissolution of cellulose bundle. The results of the correlated motion of cellulose chains are consistent with the other analysis discussed in this work of various IL/water systems such as decreasing interaction energy trend and diffusion process. Regeneration of [C2C1Im][OAc] Pretreated Cellulose on Water Environment. In order to ensure the aggregation of cellulose in water, the final coordinates of the cellulose as obtained from the100 ns simulation at 433 K was subjected to further simulation in pure water for 10 ns. The snapshot obtained from this simulation is given in Figure 9. Results reveal the aggregation of cellulose chains into the amorphous assembly. The calculated number of H-bonds during the simulation is presented in Figure 10, which shows that the number of Hbonds increases with time. The present analysis reveals two possibilities: (1) partial dissolution of cellulose by lower level of [C2C1Im][OAc] loading and (2) subsequent addition of water leading to aggregation of cellulose chains. The treatment of enzyme on the aggregated system of regenerated cellulose may considerably favor saccharification. It is clear from the simulations of 50 to 80% content of [C2C1Im][OAc] in the mixture that the presence of beyond 50% of IL is equally effective in the decrystallization of cellulose (the extent of disruption of interchain H-bonding interaction is similar). A similar observation is also reported recently by Rabideau et al.70 in which they show that 70 mol % of water with [C2C1Im][OAc] has closer values to the 0 mol % of water in terms of the H-bond formation of acetate with cellulose. Our 80% [C 2 C 1 Im][OAc] corresponds to 76 mol % of water concentration. Our MD simulations reveal that the resulting decrystallized cellulose from the 100% [C2C1Im][OAc] system begins to repack in water at a lower temperature and form a more amorphous structure.

Figure 6. Interaction energy of cellulose chains with each other in various solvent systems at 433 K.

Figure 7. Diffusion of cellulose at different [C2C1Im][OAc]:water ratios.

are held together. In the case of dissolution of cellulose with [C2C1Im][OAc]:water mixtures, H-bonding interaction between cellulose chains are disrupted by the presence of [C2C1Im][OAc], and the presence of water in the medium supports the diffusion of individual cellulose chains and enables the effective dissolution of cellulose. It matches the observation from the results that [C2C1Im][OAc] is more efficient in breaking the inter chain H-bonds of cellulose. In contrast, the cellulose chains are less correlated motion (see section Correlated Motion of Cellulose Chains) in [C2C1Im][OAc] than in water. It is found from our computational results and experimental observations that the presence of more than 50% [C2C1Im][OAc] is efficient in the dissolution of cellulose when compared to pure [C2C1Im][OAc].55 It is well known that the diffusion of a particular solute in a medium depends on the viscosity of the medium. The results can also be understood in terms of viscosity of the solvent medium. Majer and co-workers reported the change in the viscosity of several pure and water saturated ILs and shown that the temperature and addition of water molecules significantly reduces the viscosity of the medium.71

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CONCLUSIONS Understanding cellulose dissolution and regeneration in aqueous IL provides knowledge on efficient cellulose dissolution, IL recycle and recovery, and biomass solute 14345


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Figure 8. Covariance matrix of the cellulose in different [C2C1Im][OAc]:water ratios of (A) 0%, (B) 20%, (C) 50%, (D) 80%, and (E) 100% [C2C1Im][OAc].

Figure 9. Final structure of cellulose obtained from the 100% [C2C1Im][OAc] system @ 433 K (A) was simulated in water at 300 K for 10 ns (B).

separations which are critical factors to the rational design of a cost-effective IL pretreatment process. This study provides a comprehensive picture on how cellulose dissolute and regenerate in different [C2C1Im][OAc]:water mixtures by conducting all-atom MD simulations on cellulose model systems at two different temperatures, 300 and 433 K. Comparison of the dissolution process of cellulose under different conditions indicates that temperature has a dominant

effect on the process of dissolution of cellulose chains in the presence of [C2C1Im][OAc] with cellulose bundle remains intact at 300 K, whereas it is disrupted at 433 K in pure [C2C1Im][OAc]. Observed trends are consistent with the drastic reduction in the number of interchain H-bonding interactions and increase in the Rg values. The simulations were used to identify specific interactions of [OAc]−, [C2C1Im]+, and water with the cellulose. Preferred solvation of [OAc]− 14346


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Industrial Research, New Delhi, India for financial support through Multiscale Modelling project No CSC 0129.

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Figure 10. Changes in the number of H-bonds within cellulose unit during the regeneration of cellulose in water.

energetically favors their accumulation around the cellulose hydroxyl groups of the glucan chains and disrupts interchain Hbonds. It is found that interaction of [C2C1Im][OAc] and water at certain concentrations plays as physicochemical driving forces assisting the cellulose dissolution process. In contrast, increases in water concentration leads to spontaneous solvation of both [C2C1Im][OAc] and cellulose, which in turn disrupts the overall pretreatment process efficacy. The MD simulations suggest that levels of 50 to 80% [C2C1Im][OAc] can effectively break the H-bonding present in cellulose. On the other hand, the presence of water at certain concentration increases the diffusivity of cellulose in the medium and aids in the dissolution of cellulose. The knowledge gained from this study provides a better understanding of the dual role played by the water (as a cosolvent/antisolvent) in dissolving cellulose. Evidence from this study provides possible clues for the targeted design of IL− water mixtures that are effective for pretreatment of biomass. Furthermore, this work presents a more general computational method for the selective identification of the mixtures of IL:water solvent systems that are necessary for dissolution of cellulose. It is hypothesized that investigating and developing blended salts and related ions as hybrid solvation mediums could result in highly robust pretreatment solvents that have the potential to be more affordable, scalable, and sustainable than traditional pretreatment solvents, such as acids and bases.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

R.P. and K.B.: These authors contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work conducted by the Joint BioEnergy Institute was supported by the Office of Science, Office of Biological and Environmental Research, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. This research used High Performance computing resources of the National Energy Research Scientific Computing Center (NERSC) and EMSL. Authors V.S and K.B wish to thank Council of Scientific and 14347


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