Solubility of Cellulose in Supercritical Water Studied by Molecular

Subscriber access provided by SUNY DOWNSTATE

Article

The Solubility of Cellulose in Supercritical Water Studied by Molecular Dynamics Simulations Lasse K. Tolonen, Malin Bergenstråhle-Wohlert, Herbert Sixta, and Jakob Wohlert J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b01121 • Publication Date (Web): 10 Mar 2015 Downloaded from http://pubs.acs.org on March 16, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry B is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

The Solubility of Cellulose in Supercritical Water Studied by Molecular Dynamics Simulations Lasse K. Tolonen,‡ Malin Bergenstråhle-Wohlert,∫ Herbert Sixta, ‡ Jakob Wohlert∫* ‡

Aalto University, Department of Forest Products Technology, P.O. Box 16300, FI-00076

Espoo, Finland. ∫

KTH Royal Institute of Technology, Wallenberg Wood Science Center, Teknikringen 56-58,

SE-10040 Stockholm, Sweden *

Corresponding author: Jakob Wohlert. Email: [email protected]. Phone: +46 (0)8 790 8037

The insolubility of cellulose in ambient water and most aqueous systems presents a major scientific and practical challenge. Intriguingly though, the dissolution of cellulose has been reported to occur in supercritical water. In this study, cellulose solubility in ambient and supercritical water of varying density (0.2, 0.7 and 1.0 g cm-3) was studied by atomistic molecular dynamics simulations using the CHARMM36 force field and TIP3P water. The Gibbs energy of dissolution was determined between a nanocrystal (4x4x20 anhydroglucose residues) and a fully dissociated configuration using the two-phase thermodynamics model. The analysis of Gibbs energy suggested that cellulose is soluble in supercritical water at each of the studied densities, and that cellulose dissolution is typically driven by the entropy gain upon the chain dissociation while simultaneously hindered by the loss of solvent entropy. Chain dissociation

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 44

caused density augmentation around the cellulose chains, which improved water-water bonding in low density supercritical water whereas the opposite occurred in ambient and high density supercritical water.

Introduction Each year biosynthesis captures 1014 kg of carbon into biomass.1 Together with hemicelluloses and lignin, cellulose is one of the main components constituting typically 40-45 % of wood biomass.2 Being globally available in abundant quantities, recyclable and biodegradable, it is one of the key components in replacing inevitably dwindling fossil resources with renewable energy and materials. Chemically, cellulose is a linear condensation polymer consisting of danhydroglucopyranose units (AGU) linked by covalent β-1,4-glycosidic bonds. It has one primary and two secondary hydroxyl groups, all of them in equatorial position. In native cellulose, the degree of polymerization (DP), which is the number of AGUs, can be as high as 14000, although in isolated cellulose it is normally 2500 or less.3 These polymers are packed together forming slender fibrils of a few nanometers in diameter. The fibrils contain ordered, crystalline domains referred to as nanocrystals that are separated from each other by less-ordered paracrystalline or amorphous cellulose.4 Owing to its role in the biosphere where it provides mechanical strength for plants, it is fully understandable that cellulose does not dissolve in water or other naturally occurring solvents. This recalcitrance to dissolve, however, hampers the chemical conversion of cellulose into fuels and platform chemicals. The insolubility of cellulose has historically been attributed almost exclusively to its ability to form a rigid inter and intrachain hydrogen bond network. Cellulose can be rendered water-soluble by disabling hydrogen bonding and tight crystalline packing via chemical derivatization of the hydroxyl groups. Likewise, disruption of the hydrogen bond


2

Page 3 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


network with concentrated sodium hydroxide in aqueous medium can swell or dissolve low DP cellulose. However, the interaction energy between water molecules and the hydroxyl groups in cellulose is very similar to that between the hydroxyl groups in cellulose.5 Therefore the enthalpy of the hydrogen bonds cannot solely explain the insolubility; instead the low configurational entropy of dissolved cellulose chains due to its stiff structure6, together with hydrophobic association7-9, and the intersheet forces10 contribute to cellulose’s insolubility. Intriguingly though, it seems possible to make water a cellulose solvent by altering its properties. In the early 1990s Adschiri et al. reported that pressurized high temperature water in the vicinity of the critical point (647.1 K and 22.1 MPa) dissolves crystalline cellulose.11 By definition no liquid-vapor interface is formed in supercritical water (SCW) and many of its physical properties are different from those of liquid water. The hydrogen bonding of water is reduced, and while some remains, the continuous hydrogen bond network ceases to exist above 573 K.12-14 The self-dissociation of water molecules is reduced15 and the dielectric constant decreases16. These differences alter the solubility of many substances. For instance nonpolar organic solutes dissolve in SCW while salts that are highly soluble in ambient water precipitate17. The compressibility of SCW enables tailoring its solvent properties at a given temperature by changing pressure and association of nonpolar solutes is enhanced with increasing pressure and density whereas the opposite holds for polar solutes. Although offering a potentially simple and green route to dissolve cellulose, the drastic conditions in SCW degrade the polymer in fractions of a second.18-20 Consequently, the experimental determination of the intrinsic solubility of cellulose in SCW is nearly impossible. Molecular dynamics (MD) simulations offer a way to investigate the solubility of cellulose in the absence of the degradation reactions and pressure limits, also providing new insights to the


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 44

thermodynamics of solvation. Earlier MD simulations were used, for example, to reveal a reduced hydrophobic association of benzene in SCW compared to that in ambient water.21 In the 2000s MD simulations have been employed in increasing numbers to investigate the cellulose’s (in)solubility in ambient8, 10, 22, 23 and subcritical and supercritical water24, 25. In this study, the hypothesis that supercritical water is a cellulose solvent was tested using atomistic MD simulations. The Gibbs energy of dissolution was determined as a difference between a nanocrystal and dissociated cellulose chains using the two phase (2PT) thermodynamics model26, 27 for entropy analysis as reported by Gross et al.23 In addition, simple 10 ns simulations without positional restraints, and independent of the Gibbs energy analysis, were conducted in order to test the predictions by the Gibbs energy analysis.

Methods Three sets of simulations were performed: restrained simulations in which the cellulose chains was forced to stay in either 1) a nanocrystal or 2) dissociated state, and 3) unrestrained simulations starting from a nanocrystal configuration. In each set, four conditions were investigated, hereafter referred to as ambient water, and low, medium, and high density SCW as tabulated in Table 1. Each simulation was performed using the GROMACS software package28 (version 4.6.3) employing the CHARMM36 carbohydrate force field29, 30 with 2 fs integration steps and a leap-frog algorithm. A cut-off of 1.0 nm was applied for short-range Lennard-Jones and Coulomb interactions, while long range interactions were handled with the particle-mesh Ewald method.31,

32

Temperature was controlled with a modified Berendsen thermostat using

stochastic velocity rescaling (1 ps time constant).33, 34 The cellulose Iβ nanocrystal used in the


4

Page 5 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


simulations consisted of 16 chains, each of them 20 AGUs in length and arranged in a 4x4 formation as reported earlier (Figure 1).35 In the 10 ns simulations without positional restraints, the nanocrystal was placed in a dodecahedral box (1692.73 nm3) with three-dimensional periodic boundary conditions and solvated with pre-equilibrated TIP3P water36 with densities 0.167, 0.693, 1.00 g cm-3 for SCW at 673 K and 1.00 g cm-3 for ambient water at 298 K. These densities correspond to a pressure of 250, 1000, and 7384 bar at 673 K.37 Energy minimization was carried out until the maximum force was below 1000 kJ mol-1 nm-1 and the systems were equilibrated for 1 ns under constant number of particles, volume and temperature (NVT) conditions with positional restrains on all heavy atoms. Thereafter all restraints were lifted and the simulations were run for 10 ns. For the analysis of Gibbs energy of dissolution under the same conditions as in the unrestrained simulations, the crystalline state was simulated using a similar nanocrystal in a periodic cubic box with initial dimensions of 13.0x13.0x13.0 nm. In order to mimic cellulose solution, the individual DP20 cellulose chains were arranged in a rectangular 4x4 formation with 3 nm intervals in the center of the box (Figure 1). Harmonic position restraints were placed on C1 atoms in the terminal residues of each chain (1000 kJ mol-1 nm-2) to prevent the dissolution of the nanocrystal or the aggregation of the dissociated chains while allowing small movements. The systems were solvated and equilibrated as above for 1 ns under constant number of particles, pressure and temperature (NPT) conditions using a Berendsen barostat with 1 ps time constant, followed by 4 ns equilibration using a slower 10 ps time constant with the same barostat. The NPT conditions during the allowed to adjust the pressures to those in the 10 ns simulations. After the equilibration, the box dimensions were fixed to match the average dimensions from the last part of the equilibration, and the simulations were continued for 5 ns in the NVT ensemble.


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 44

Figure 1. Snapshots of nanocrystal (left) and dissociated chain (right) configurations during a simulation at 673 K. Water molecules are not shown for clarity. Table 1. Conditions applied in the simulations. Simulation

Waters

Box

Pressure

Temperature

nm3

bar

K

Ten nanosecond simulations without positional restraints (a Ambient

54507

1692.7

-386.2 ± 1.00

298.0 ± 0.02

Low density SCW

9643

1692.7

226.7 ± 0.75

672.9 ± 0.06

Medium density SCW

38314

1692.7

1457.1 ± 0.66

673.0 ± 0.04

High density SCW

54507

1692.7

7266.8 ± 1.10

673.0 ± 0.05

Gibbs energy analysis, Nanocrystal configuration (b Ambient

72379

2233.1

-384.4 ± 0.4

298.0 ± 0.01

Low density SCW

13424

2438.6

226.5 ± 0.1

673.0 ± 0.02

Medium density SCW

45370

2009.9

1407.2 ± 0.3

673.0 ± 0.01

High density SCW

67610

2086.2

7262.7 ± 0.8

673.0 ± 0.02

Gibbs energy analysis, Dissociated chain configuration (b Ambient

72379

2232.7

-386.2 ± 0.8

298.0 ± 0.00

Low density SCW

13424

2163.8

226.5 ± 0.3

673.0 ± 0.02


6

Page 7 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Medium density SCW

45370

2007.3

1407.3 ± 0.2

673.2 ± 0.02

High density SCW

67610

2087.3

7263.1 ± 0.3

673.0 ± 0.02

a) Average of 5-10 ns b) Average over 5 ns NVT simulations after NPT equilibration

The computation of the difference in Gibbs energy between the dissociated and the crystalline state requires the determination of differences in both enthalpy and entropy since the definition of Gibbs energy reads: ∆G = ∆U + p∆V − T∆S

[1]

Here, S is entropy, T temperature, U internal energy, p pressure, and V volume of the simulation box. The change in total potential energy corresponds to the change in the internal energy of the system, ∆U, since the kinetic contribution in a classical system will cancel out at a constant temperature (1/2 kBT per degree of freedom). The work done by the environment, W, V2

was calculated as W = ∫

V1

p (V )dV ≈ p∆V , where p is the average pressures of the crystalline

and dissolved states and ∆V is the corresponding difference in the box volume. The entropy of water was analyzed from the density of state function (DoS) using the 2PT model. Three 100 ps velocity trajectories for SCW and five for ambient water, separated by 900 ps intervals between the trajectories, were stored with 2 fs intervals. These trajectories were divided into five 20 ps (low and medium density SCW) or ten 10 ps trajectories (high density SCW and ambient water) for analysis. The DoS was obtained from Fourier transformation of the autocorrelation function of the velocity trajectories38, using the analysis tool g_dos.39 For water, being a polyatomic molecule, motions can be separated into one translational, and one rotational part (the water model used here is completely rigid, so there is no contribution from internal degrees of freedom). The translational part of the total DoS, DoStrn, may be


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 44

calculated using g_dos from a trajectory containing only the center of mass velocities of each water molecule, and the rotational part is then obtained as the difference DoSrot = DoS – DoStrn, where the total DoS is calculated using the fully atomistic trajectory. Next, following the 2PT method of Lin et al.26, 27, we assume that the total DoS of water is a superposition of a solid-like component and a gas-like component: DoS(v) = DoSsolid(v) + DoSgas(v), where the gas-like component is assumed to be that of a hard-sphere gas, DoSgas (ν ) =

DoS(0)  π DoS(0)ν  1+    6 fN 

[2]

Here DoS(0) is the intensity of the density of states at zero frequency, and f is the fluidity, ranging between zero and one, which determines how large portion of the total degrees of freedom that are gas-like. By assuming that f is equal to the ratio between the real diffusivity of the system, and that of a hard sphere system at the same conditions, f, and consequently DoSgas, can be determined uniquely from the simulations (see Lin 2003, ref 26, for details). The values of fluidicity f are tabulated in the supporting information section in Table S1. The solid-like component is now calculated from DoSsolid = DoS – DoSgas, with DoS being the total DoS from simulations, and DoSgas from Eq. 2. The partitioning into respective components is done for translational and rotational contributions separately. Finally, the entropy is calculated by integrating an appropriate weighting function W(v) over the corresponding DoS(v), ∞

S = kB

∫ DoS(ν )W (ν )dν 0

[3]

where kB is Boltzmann’s constant. For the solid-like components, W(v) is given by the entropy of a quantum mechanical harmonic oscillator40,


8

Page 9 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


W (ν) =

βhν + ln[1 − exp(−βhν )] exp(βhν ) −1

[4]

with β = 1/kBT, and h being Planck’s constant. For the gas-like components, Wtrn(v) = SHS/3kB and Wrot(v) = SR/3kB, where SHS and SR is the hard-sphere entropy, and the entropy of a rigid rotor respectively (see Lin et al. ref 27 ). It may seem crude to rely on a hard sphere approximation for the water, but we shall see later that it provides good agreement with reference data, also at supercritical conditions. Cellulose entropy was determined from the same trajectories as the solvent entropy using a harmonic approximation for the internal motions, and analytical expressions for the center-ofmass motions. The internal DoS of the cellulose chains was computed as DoScell(ν) = DoStotcell(ν) - DoStranscell(ν), where DoStranscell(ν) contains the center-of-mass translational modes of the cellulose chains, which were small due to the presence of positional restraints, and ν is the frequency. In this way, DoScell(ν) only contains contributions from internal degrees of freedom: bonds, angles and torsions. The internal cellulose entropy was then computed by integrating Eq. 4 over DoScell(v). This approach thus corresponds to using the solid like component only, i.e. f = 0, in the 2PT expressions above. To compensate for the effect of the positional restrains, translational and rotational entropy was computed using the Sackur-Tetrode equation and a rigid rotor approximation respectively.40 These contributions were added to the internal entropy giving the total entropy of the cellulose chains. The 2PT model for the entropy analysis of supercritical water was validated by simulating neat water under ambient and supercritical conditions and comparing the obtained entropies with tabulated reference data.37 The temperature and pressure of the simulations were adjusted to those in the Gibbs energy simulations but the reference data was obtained based on the temperature and density of water in the simulation trajectories. The equilibration of 5000 water


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 44

molecules was done in the similar manner as for the analysis of Gibbs energy, and the entropy was then analyzed from five subsequent 100 ps trajectories using the 2PT model similarly as described above. Solvent accessible surface area was analyzed with the GROMACS utility g_sas using default settings.41 The confidence of the entropy calculation was estimated as a confidence interval of the difference of population means using Student’s t-distribution at the 95% confidence level. Error estimates for the enthalpy were obtained directly from GROMACS. The total confidence interval was obtained as the sum of the error estimates of entropy and enthalpy contributions. The g_rdf analysis tool was utilized to analyze the average number density ρ(r) of oxygen and hydrogen atoms within a perpendicular distance r to the “surface” of the dissociated cellulose chains, approximating each cellulose chain as an indefinitely long cylinder with a radius of 0.2 r

nm: ρ (r ) = ∫ n(r )(π (r 2 − r02 )) −1 dr , where n(r) is the total number of atoms within a distance r 0

from the nearest atom of the cellulose chain, and r0 is the radius of the cylinder. The number and average lifetime of the hydrogen bonds was analyzed by the g_hbond analysis tool. For the analysis of the number and average lifetime of the cellulose-water hydrogen bonds, additional 200 ps trajectories were simulated and stored with 10 fs intervals using the equilibrated coordinate file from the Gibbs energy simulations as starting point. The average lifetime of a water-water hydrogen bond under ambient and supercritical conditions was analyzed using the method of Luzar et al.42 from 500 ps trajectories of 5000 water molecules employing 10 fs steps in the analysis. For hydrogen bonds, a geometric criterion based on an OO distance of 0.35 nm and an OH-O angle of 30 degrees between the acceptor and the donor were applied. The radial distribution function of hydrogen bonding was approximated by


10

Page 11 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


computing the number of hydrogen bonds for sets of water molecules which were categorized according to their distance from the dissociated cellulose chains.

Results and discussion The results from the Gibbs energy calculations, the enthalpy and entropy contributions, and hydrogen bonding will here be presented separately. Finally, the simulation results are compared with independent 10 ns simulations without positional restraints and what has been observed experimentally in earlier studies.

Gibbs energy of dissolution The Gibbs energy from Eq. 1 (∆G) of dissolution as the difference between a dissociated and an associated state was determined in order to investigate whether dissolution is thermodynamically favored. The two states were constructed by restraining the positions of the chains in an associated or dissociated configuration, and below they are referred to as nanocrystal and dissociated state, respectively, following the approach reported by Gross et al. 23

Figure 2. The Gibbs energy of dissolution and the contributions by enthalpy (white), cellulose entropy (light gray) and solvent entropy (dark gray). Total Gibbs energy marked with the black


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 44

bars in which the height of the bar shows the 95% confidence interval. All values are normalized to kJ/molAGU. The values are tabulated in the supporting information in Table S2. The Gibbs energy (∆G) defined as ∆G = Gdissociated - Gnanocrystal is shown in Figure 2 and the corresponding numerical values are listed in Table S2. The Gibbs energy in ambient water was +0.2 ± 0.5 kJ/molAGU and indicated that the nanocrystal is not soluble under these conditions, although the result is within the uncertainty limits of the analysis. The result was expected because the DP of the chains is above the solubility limit of cellooligosaccharides43 and dissolved DP20 chains are known to precipitate in ambient water44. The results showed a delicate balance between cellulose and solvent entropy in which the gain in cellulose entropy upon dissolution drives dissolution while the loss of water entropy opposes it. In SCW, ∆G was 3.0±0.8, -11.8±0.8, and -12.2±0.7 kJ/molAGU, in low, medium and high density SCW, respectively, showing that cellulose dissolution is thermodynamically favored in SCW. The constituents of ∆G are discussed below.

Enthalpy Here, the enthalpic contribution (∆H=∆U+p∆V) to ∆G, presented in Figure 2 and Table S2, is analyzed. In ambient water ∆H was +0.06 kJ/molAGU disfavoring dissolution. The difference in the box volume was +0.02% and the compression work contributed +0.03 kJ/molAGU. The low value of ∆H is expected because the energy of water-water hydrogen bonds and hydrogen bonds between water and the hydroxyl groups in cellulose are comparable.5 The change in the potential energy of the short-range interactions (Figure 3) showed expectedly that the cellulose-cellulose and water-water bonding decreased (resulting in increased potential energy contributions) while the cellulose-water bonding increased upon chain dissociation, lowering the total energy.


12

Page 13 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The most exceptional enthalpy contribution was found in the low density SCW system where ∆H had a value of -14.5 kJ/molAGU, dominated by the compression work of the simulation box (11.7 kJ/molAGU). The contracting simulation box and the cellulose-water pair correlation function (Figure 4) indicated a high local solvent density around the dissociated chains, often referred to as density augmentation or clustering.45 The potential energy of short range waterwater interactions was lower with the dissociated chains (Figure 3). In the bulk phase of low density SCW the mobility of water molecules is high and the degree of hydrogen bonding low.37, 46

Therefore the enhanced water-water bonding can be explained by a partial immobilization of

the water molecules in the solvation layer. At the same time the cellulose-water Coulomb potential decreased by -18.5 kJ/molAGU as new water-cellulose bonds were formed. Curiously the short-range cellulose-cellulose Coulomb interaction energy was higher with dissociated chains, which must be related to new intrachain interactions because the chains were separated by more than the one nanometer cut-off range used for short-range interactions. In medium density SCW, the enthalpy increased by +11.8 kJ/molAGU, thus working against dissolution. The enthalpy change was mostly due to the interaction energies because the box volume remained basically constant (Table 1) rendering the compression work small (-0.7 kJ/molAGU). Also the cellulose-water pair-correlation function in Figure 4 showed the reduced density augmentation around the chains.


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 44

Figure 3. The difference in short-range non-bonded Lennard-Jones (white) and Coulomb (black) interactions between dissociated and crystalline states. Potential energy is normalized to kJ/molAGU. The difference in the number of hydrogen bonds (AGU-1) presented with circles.


14

Page 15 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. The average number density of oxygen (black) and hydrogen (red) within a distance r perpendicular from the center axis of the dissociated chains. The densities are normalized to the density of ambient water. The radial distribution functions of neat water are provided in the ESI in Figure S2.

Figure 5. Density of state functions of water with an aggregated cellulose nanocrystal (top) and the difference between dissociated and aggregated nanocrystal states (bottom). The total density of state functions marked with solid black line, and its translational and rotational modes with red and black dashed lines, respectively. The low wavenumber regions are presented in the inserts. The curves are normalized to the degree of freedom in each case.


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 44

Table 2. Solvent entropies. Snanocrystal

Sdissociated

∆Swater

(J molH2O-1 K-1)

(J molH2O-1 K-1)

(J molAGU-1 K-1)

STrans

SRot

STrans

SRot

∆STtrans ∆SRot ∆STotal

Ambient

56.1

11.4

55.9

11.4

-32.2

-5.5

Low density SCW

102.6 39.0

101.3

38.7

-52.2

-12.4 -64.6 ± 1.1

-37.7 ± 1.4

Medium density SCW 86.5

33.7

86.5

33.6

-1.1

-6.7

-7.8 ± 1.0

High density SCW

31.6

78.2

31.6

1.0

0.8

1.9 ± 0.7

78.2

In high density SCW, enthalpy increased somewhat more (+16.5 kJ/molAGU) than in medium density SCW. The box expanded and although the contribution to enthalpy was only +1.6 kJ/molAGU, it showed that the density augmentation was diminished, similar to what has been reported for methanol in high density SCW.47 Water-water Coulomb potential energy increased like in ambient water (Figure 3) suggesting a reduced water-water bonding around solutes compared with the bulk solvent phase.

Solvent entropy Here the contribution to ∆G due to changes in solvent entropy is being discussed. Note that the numbers presented in this section are ∆S values only, whereas in Figure 2 and Table S2 they are given as energies, i.e. as –T∆S. The entropy difference of water (∆Swater) between the dissociated and nanocrystal states was computed from the DoS, as described in the Methods section, and is presented in Table 2. The ∆Swater of ambient water agreed with -38 J K-1 molAGU-1 reported by Gross et al.23 who simulated 576 AGUs in 28443 water molecules at 300 K. In low density SCW the decrease in entropy was


16

Page 17 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


substantially larger than in ambient water. This supports the hypothesized loss of translational and rotational degrees of freedom caused by the immobilization in conjunction with the density augmentation, which was pronounced in low density SCW. In medium density SCW this large entropy loss dwindled, lowering ∆G markedly. In high density SCW the entropy of water even increased upon chain dissociation despite the equal densities and the comparable short water-water bonding with ambient water. An explanation for the increasing solvent entropy in high density SCW is that the cavity formation for a large solute (a nanocrystal) induces some order in the system unlike a smaller one (a dissociated chain). This may be related to the reduction of the number of water-water hydrogen bonds upon dissolution (Figure 3 and Figure 6) as will be discussed below. It should be remembered that in high density SCW the absolute entropy is higher (119.37 J K-1 mol-1) than in ambient water (69.95 J K-1 mol-1).37 Also by definition, no liquid surface is formed around the solute in SCW, which may alter the entropy of dissolution. The solvent entropy contributions from translational and rotational motions were considered separately. The DoS(ν) from which solvent entropy was calculated from the velocity autocorrelation functions of individual atoms from the simulation trajectories. The top of Figure 5 shows the DoS of water, which contains translational and rotational modes but no internal vibrations due to the rigid structure of TIP3P water. Ambient water exhibited similar characteristic shape of its DoS spectrum as reported by Lin et al.27 with maxima at 33 and 435 cm-1. The DoS of SCW were dominated by low frequency modes representing high diffusion of water molecules. The maximum of rotational modes occurred at lower wavenumbers compared to ambient water and shifted from 190 to 349 cm-1 with increasing density. Although the modes


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 44

at higher wavenumbers became pronounced with increasing density of SCW, the DoS of high density SCW remained essentially different from that of ambient water. The bottom of Figure 5 shows differential DoS spectra between the dissociated and nanocrystal states (∆DoS) and corresponding solvent entropy values are presented in Table 2. In each case, the low frequency translational modes, representing the diffusional movement of water molecules, were reduced in the dissociated state. The reduction of diffusional modes and the total ∆DoS was substantial with low density supercritical water and decreased as density of water increased. The translational modes at wavenumbers 30-200 cm-1 increased, having maxima at 50-70 cm-1, the rotational modes exhibited a shallow minimum around 170 cm-1. A major difference was found between low and medium density SCW. In medium density SCW, only 14% of ∆Swater was due to translational modes and the rest 86% by rotational modes whereas in low density SCW 81% and 19% of ∆Swater was caused by translational and rotational modes, respectively.

Hydrogen bonding The average number of water-water hydrogen bonds increased upon the chain dissociation in low density SCW while it decreased in medium and high density SCW as well as in ambient water (Figure 3 and Table S3). In most cases, the number of hydrogen bonds followed the trend seen in short-range energetic interactions. This did not, however, entirely explain the changes in them: the medium density SCW showed a decreased number of water-water hydrogen bonds in spite of the decreased short-range water-water Coulomb interaction energy. One must remember though, that hydrogen bonds are not included or treated separately in these simulations, they are rather an effect of strong electrostatic interactions. The definition of a hydrogen bond is based on


18

Page 19 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


strictly geometric criteria, and is thus associated with some arbitrariness. Therefore it is likely that using more complex criteria for hydrogen bonds would alter the results.48 The average lifetime of a water-water hydrogen bond increased with the density of SCW (0.30, 0.48, and 0.57 ps for low, medium and high density SCW, respectively) but remained shorter than the average lifetime in ambient water (1.24 ps). In SCW, the lifetime of a cellulose-water hydrogen bond was shorter than that of a water-water hydrogen bond (0.13, 0.13, and 0.18 ps for low, medium and high density SCW, respectively) while it was longer in ambient water (2.59 ps). The number of cellulose-water hydrogen bonds per one AGU was 1.1, 2.6, 3.9, and 5.5 in low, medium, and high density SCW, and ambient water, respectively. For water-water hydrogen bonds, both the number (each water molecule was involved in 0.7, 1.8, 2.4, and 3.4 hydrogen bonds in neat low, medium, and high density SCW, and ambient water, respectively) and the lifetimes reflect the change in physical properties of SCW. Higher temperature and lower density make the hydrogen bonds fewer and more short-lived. The complete results of the lifetime analysis are tabulated in Table S4 in the ESI.

Figure 6. Radial distribution functions of the number of hydrogen bonds in which a water molecule is involved. Black markers show the water-water hydrogen bonds and white markers include both water-water and water-cellulose hydrogen bonds. The radial distance was approximated based on the data presented in Figure 4. The red diamond presents the bulk hydrogen bonding obtained from the simulation of neat TIP3P water.


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 44

The number hydrogen bonds in which a water molecule was involved depended on the radial distance from a cellulose chain (Figure 6) and the density of SCW. In low density SCW, waterwater hydrogen bonding in the first solvation layer was at the same level than in bulk water. When the cellulose-water hydrogen bonds were included, the number of hydrogen bonds was higher in the vicinity of the cellulose chains than in the bulk water. In medium and high density SCW, the total number of the hydrogen bonds in the solvation layers was close to that in bulk water. When the cellulose-water bonds were excluded, the number of hydrogen bonds was lower in the vicinity of the chains than in bulk water, indicating a reduced density augmentation around a solute.47 The shape of the hydrogen bonding distribution in ambient water resembled that in high density SCW but the number of lost hydrogen bonds was larger. This was in a good agreement with Figure 3 regarding the number of lost hydrogen bonds upon cellulose dissolution. The present data shows that a supercritical temperature and decreasing density reduce the hydrogen bonding of neat water. It is not unreasonable to think that this would enhance solubility since there should be a lower penalty of breaking the water structure to create the space needed to fit the solute. However, our results show that in low density SCW, the water structure is not further disturbed – it is actually enhanced upon cellulose dissolution. This is a consequence of the density augmentation mentioned above, leading to that more favorable water-water hydrogen bonding in the vicinity of the solute, however, at a cost of a decreased entropy (see Figure 2). This effect was mitigated in medium and high density water, which was reflected to a less pronounced change in the solvent entropy.

Cellulose entropy


20

Page 21 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Not only solvent but also cellulose entropy is strongly affected upon dissolution. Here this contribution to ∆G due to changes in cellulose entropy is being discussed. Again, the numbers presented are ∆S values only, whereas Figure 2 and Table S2 contain the entire –T∆S energy term. The total entropy change upon dissolution ∆Scellulose was found to be 37.2, 47.5, 42.8, and 40.8 J K-1 molAGU-1 for ambient, low, medium, and high density SCW, respectively. These values were obtained as a sum of internal entropy and a compensation for the effect of positional restraints. The density of states was calculated for the internal vibrations of cellulose by explicitly removing center-of-mass contributions. The cellulose internal entropy was then calculated by approximating each mode as a quantum-mechanical harmonic oscillator38 (Eqs. 3 and 4). The DoS functions of cellulose’s internal vibrations did not reveal major characteristic differences between dissociated and nanocrystal states and shifts occurred mostly at low wavenumbers. The DoS of internal vibrations are provided as supplementary material (Figure S1). The internal entropy of the dissociated cellulose chains was higher than in the nanocrystal and the difference was the largest in low density SCW (16.9 J K-1 molAGU-1), decreasing with increased density. In ambient water the contribution of the internal vibrations was 7.1 J K-1 molAGU-1. To compensate for the effect of the positional restraints that were used to keep the chains in their initial positions, the translational (~10 J K-1 molAGU-1) and rotational (~20 J K-1 molAGU-1) components of cellulose entropy were calculated from the Sackur-Tetrode equation and from a rigid-rotor approximation respectively. Even though the sensitivity of the solvent entropy to the density of SCW made it pivotal for dissolution, the gain in cellulose entropy from dissociation of the chains (∆Scellulose) played a major role for the overall ∆G. The ∆Scellulose in turn was relatively insensitive toward the density.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 44

Importantly however, ∆Scellulose depends on the DP of chains to be dissolved. Thus cellulose may be insoluble unless the DP is sufficiently low. This can be rationalized, for example, in FloryHuggins theory, which predicts an inverse proportionality between DP and ∆Spolymer.49 The effect is seen also in practice: the dissolution temperature of amylose increased from 57 to 119 °C when the DP increased from 12 to 55.50 Finally, it is important to remember that ∆Scellulose also depends on concentration through the Sackur-Tetrode contribution, which gives higher translational entropy when the volume of the water phase is increased. Dissolution is, just as in any real system, promoted by low concentration. The cellulose concentration in this study was 4% or more, depending on the number of water molecules in the system (Table 1). Although the dissimilar concentrations somewhat obstructed the direct comparison of the Gibbs energies, even the lowest concentration was comparable to the concentrations applied in most experimental studies making the prediction of cellulose dissolution conservative in that respect.

Simulation without positional restraints Ten nanosecond simulations were used to test whether the simulation trajectories supported the Gibbs energy analysis and the predicted dissolution of the nanocrystal. In these simulations the cellulose chains were allowed to move, rotate and dissolve freely. These simulations were also independent of the Gibbs energy analysis and possible deficiencies in the method applied. Snapshots showing the appearance of the nanocrystal at the end of the 10 ns simulations are presented in Figure 7. In ambient water slight twisting took place but the nanocrystal remained intact. A similar twisting was reported earlier for other force fields as well.51-53 In low density SCW, the nanocrystal was transformed to an amorphous-like structure with a considerable twisting and bending of the chains. The chains did not, however, dissociate into the


22

Page 23 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


bulk solvent phase. In contrast, medium density SCW dissolved the nanocrystal completely, forming a homogeneous solution. The dissociated chains took a bent and twisted, yet extended conformation in the solution. In high density SCW the crystalline structure was largely retained after 10 ns, although the ends of the nanocrystal were fringed into the solvent.

Figure 7. The final appearance of the nanocrystals. Water molecules are not shown for clarity. In medium density SCW only some of the chains are shown.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 44

Figure 8. Total (black), hydrophobic (blue) and hydrophilic (red) solvent accessible surface area of cellulose as a function of time. The final solvent accessible surface area (Figure 8) was investigated in order to quantify the degree of dissolution of the nanocrystal. The accessible area was found to match the final appearance of the nanocrystal. In ambient water the solvent accessible surface remained unchanged around 151 nm2; just a minor increase in hydrophilic and a minor decrease in the hydrophobic surface area was noticed in the early stages of the simulation. In low density SCW, the rearrangement of the chains enlarged the solvent accessible area. The enlargement was similar for both the hydrophilic and hydrophobic surfaces and took place mainly during the first three nanoseconds. For the medium density SCW simulation, in which the nanocrystal looked completely dissolved by visual inspection of Figure 7, the solvent accessible surface area increased to around 500 nm2 in three nanoseconds. In high density SCW, the increase in the solvent accessible surface area was slow but retained its ascending trend. The non-bonded shortrange Lennard-Jones and Coulomb interactions corresponded well with appearance of the nanocrystals and the solvent accessible surface area. These contributions are reported in the supporting information section in Figure S4. Although serious cautions must be applied when drawing conclusions from such short simulations (in ten nanoseconds light travels less than three meters in vacuum!), the observed


24

Page 25 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


rate of dissolution in medium density water is thrilling. Noteworthy are the facts that dissolution is thermodynamically more preferred in high than medium density SCW, and cellulose dissolution was thermodynamically favored in each of the SCW simulations. There are certain differences that may be related to the difference in the in the rate of dissolution. For instance, the life-time of the hydrogen bonds increased with density. Also the diffusivity measured as the mean squared displacement of neat supercritical water was found to depend on the density of SCW (197.9, 48.4; 26.0, and 5.8 10-5 cm2 s-1 for low, medium, high density SCW, ambient water, respectively). Still, unfortunately, the present data do not provide final explanation for the observed differences.

Validation of simulation results against literature data An important aspect regarding the validity of the above results is the two-phase thermodynamics model’s ability to reliably reproduce the water entropy under supercritical conditions. The water entropy along the saturation line was thoroughly investigated earlier, and it was found that the 2PT model with TIP3P force sufficiently reproduces water entropy field under ambient conditions.27 Under supercritical conditions, the simulated entropy of neat water followed accurately (R2>0.999) the equation y=0.8586x+7.240 where y is simulated entropy and x is the reference entropy (Figure S3). Although the actual values from simulations were 3.910.5% lower than the reference data,37 the results showed that the 2PT model successfully follows the changes in entropy as a function temperature and density of water under the supercritical conditions relevant to this study. While the insolubility of cellulose in ambient water is generally taken for granted, the simulated cellulose dissolution in SCW agrees well with a wide experimental body of evidence.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 44

Already subcritical water dissolves cellulose as oligosaccharides, which retain a sufficiently high DP to re-precipitate in ambient water; the DP of dissolved oligomers and polymers increases with the temperature. Below 573 K dissolution took place heterogeneously from the surfaces of nanocrystals44,

54

but a crystalline-to-amorphous transformation and dissolution of cellulose

nanocrystals was observed in water at around 593 K. Similar behavior was not observed in ethanol at the same temperature excluding a possibility of mere thermal decomposition.55 Furthermore, the favorable solvent properties of supercritical water were seen as a steeply increasing rate of cellulose dissolution at the critical point under 25 MPa.20, 55 Unfortunately, the number of studies with cellulose in supercritical water under pressures exceeding 250 bar is very limited. The reason is the complexity of the experimental conditions. High pressure experiments required closed reactors that do not allow very short treatment times nor the fast recovery of formed reaction products.56, 57 Ogihara et al.56 reported that the density of 0.85 g cm-3 resulted in the lowest dissolution temperature at 320 °C when cellulose suspension was gradually heated in a diamond cell system with a fixed water density. With the densities 0.67-0.85 g cm-3 swelling of crystallites prior dissolution was reported and under higher densities dissolution was hindered. While in their experiment the dissolution took place under near critical conditions, the density was close to the medium density of this study, which resulted in a very rapid dissolution of the nanocrystal in the unrestrained simulations. Finally, there is also some theoretical evidence that a general polar solute should experience a solubility optimum with respect to water density at supercritical conditions, based on a polar contribution to the hydration free energy that decreases with density, and a cavity formation term that increases.58


26

Page 27 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Conclusions Cellulose solubility in supercritical water was studied and compared with ambient water. It was shown, for the first time, that cellulose dissolution in supercritical water is thermodynamically favored over a range of densities from 0.17 to 1.0 g cm-3, but not in ambient water. The analysis of the Gibbs energy’s components showed a complex balance with several competing contributions of similar importance. The driving force of cellulose dissolution is determined by the entropy gain upon the dissociation of the cellulose chains. This is counterbalanced by the reduction of the solvent entropy and in some cases also by enthalpy. The entropy cost of water upon dissociation of the cellulose chains in low-density supercritical water was large and associated with augmentation of water density and increased water-water bonding around the dissociated chains. In medium and high density supercritical water, the density augmentation did not occur, and the chain dissociation reduced the water-water interactions, similar to what occurs in ambient water. Acknowledgements Wallenberg Wood Science Center funded by the Knut and Alice Wallenberg Foundation, High performance computing center north (SNIC 2013/1-199), Future Biorefinery (FuBio) program within Finnish Bioeconomy Cluster ltd., and Swedish Foundation for Strategic Research (SSF) are acknowledged for supporting this study. Supporting information available: The evolution of potential energies in the 10 ns simulations, DoS of the internal vibrations of cellulose, comparison of the simulated solvent entropies with tabulated literature data, tabulated values of fluidicity parameter f, tabulated constituents of Gibbs energy, tabulated changes in hydrogen bond upon chain dissociation, life-


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 44

time analysis of hydrogen bonds, and energetic contributions in the 10 ns simulations without positional restraints. This material is available free of charge via Internet at http://pubs.acs.org. References (1) Field, C. B.; Behrenfeld, M. J.; Randerson, J. T.; Falkowski, P. Primary Production of the Biosphere: Integrating Terrestrial and Oceanic Components. Science 1998, 281, 237-240. (2) Alén, R. In Structure and Chemical Composition of Wood; Gullichsen, J., Paulapuro, H., Eds.; Forest Product Chemistry; Fabet Oy: Helsinki, Finland, 2000; pp 12-57. (3) Nevell, T. P.; Zeronian, S. H. In Cellulose Chemistry Fundamentals; Nevell, T. P., Zeronian, S. H., Eds.; Cellulose Chemistry and its applications; Ellis Horwood limited: Chichester, England, 1985; pp 15-29. (4) Haigler, C. H. In The Functions and Biogenesis of Native Cellulose; Nevell, T. P., Zeronian, H. S., Eds.; Cellulose Chemistry and its Applications; Ellis Horwood Limited: Chichester, England, 1985; pp 30-83. (5) Lindman, B.; Karlström, G.; Stigsson, L. On the Mechanism of Dissolution of Cellulose. J. Mol. Liq. 2010, 156, 76-81. (6) Shen, T.; Langan, P.; French, A. D.; Johnson, G. P.; Gnanakaran, S. Conformational Flexibility of Soluble Cellulose Oligomers: Chain Length and Temperature Dependence. J. Am. Chem. Soc. 2009, 131, 14786-14794. (7) Matthews, J. F.; Skopec, C. E.; Mason, P. E.; Zuccato, P.; Torget, R. W.; Sugiyama, J.; Himmel, M. E.; Brady, J. W. Computer Simulation Studies of Microcrystalline Cellulose II. Carbohydr. Res. 2006, 341, 138-152.


28

Page 29 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(8) Bergenstråhle, M.; Wohlert, J.; Himmel, M. E.; Brady, J. W. Simulation Studies of the Insolubility of Cellulose. Carbohydr. Res. 2010, 345, 2060-2066. (9) Chandler, D. Interfaces and the Driving Force of Hydrophobic Assembly. Nature 2005, 437, 640-647. (10) Gross, A. S.; Chu, J. On the Molecular Origins of Biomass Recalcitrance: The Interaction Network and Solvation Structures of Cellulose Microfibrils. J. Phys. Chem. B 2010, 114, 1333313341. (11) Adschiri, T. Supercritical Fluid as a Reaction Medium. Koatsuryoku no Kagaku to Gijutsu 1993, 2, 287-294. (12) Gorbaty, Y. E.; Kalinichev, A. Hydrogen Bonding in Supercritical Water. 1. Experimental Results. J. Phys. Chem. 1995, 99, 5336-5340. (13) Bernabei, M.; Botti, A.; Bruni, F.; Ricci, M.; Soper, A. Percolation and ThreeDimensional Structure of Supercritical Water. Phys. Rev. E 2008, 78, 021505. (14) Blumberg, R. L.; Stanley, H. E.; Geiger, A.; Mausbach, P. Connectivity of Hydrogen Bonds in Liquid Water. J. Chem. Phys. 1984, 80, 5230-5241. (15) Marshall, W. L.; Franck, E. U. Ion Product of Water Substance, 0-1000 °C, 1-10,000 Bars. New International Formulation and its Background. J. Phys. Chem. Ref. Data 1981, 10, 295-304. (16) Uematsu, M.; Franck, E. U. Static Dielectric Constant of Water and Steam. J. Phys. Chem. Ref. Data 1980, 9, 1291-1306.


29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 44

(17) Hodes, M.; Marrone, P. A.; Hong, G. T.; Smith, K. A.; Tester, J. W. Salt Precipitation and Scale Control in Supercritical Water Oxidation - Part A: Fundamentals and Research. J. Supercrit. Fluids 2004, 29, 265-288. (18) Tolonen, L. K.; Penttilä, P. A.; Kruse, A.; Serimaa, R.; Sixta, H. The Swelling and Dissolution of Cellulose Crystallites in Subcritical and Supercritical Water. Cellulose 2013, 20, 2731-2744. (19) Sasaki, M.; Adschiri, T.; Arai, K. Kinetics of Cellulose Conversion at 25 MPa in Sub- and Supercritical Water. Am. Ind. Chem. Eng. J. 2004, 50, 192-202. (20) Cantero, D. A.; Bermejo, M. D.; Cocero, M. J. High Glucose Selectivity in Pressurized Water Hydrolysis of Cellulose using Ultra-Fast Reactors. Bioresour. Technol. 2012, 135, 697703. (21) Gao, J. Supercritical Hydration of Organic Compounds. The Potential of Mean Force for Benzene Dimer in Supercritical Water. J. Am. Chem. Soc. 1993, 115, 6893-6895. (22) Gross, A. S.; Bell, A. T.; Chu, J. Thermodynamics of Cellulose Solvation in Water and the Ionic Liquid 1-Butyl-3-Methylimidazolim Chloride. J. Phys. Chem. B 2011, 115, 13433-13440. (23) Gross, A. S.; Bell, A. T.; Chu, J. Entropy of Cellulose Dissolution in Water and in the Ionic Liquid 1-Butyl-3-Methylimidazolim Chloride. Phys. Chem. Chem. Phys. 2012, 14, 84258430. (24) Miyamoto, H.; Abdullah, R.; Tokimura, H.; Hayakawa, D.; Ueda, K.; Saka, S. Molecular Dynamics Simulation of Dissociation Behavior of various Crystalline Celluloses Treated with Hot-Compressed Water. Cellulose 2014, 21, 3203-3215.


30

Page 31 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(25) Ito, T.; Hirata, Y.; Sawa, F.; Shirakawa, N. Hydrogen Bond and Crystal Deformation of Cellulose in Sub/Super-Critical Water. Jpn. J. Appl. Phys. 2002, 41, 5809-5814. (26) Lin, S.; Blanco, M.; Goddard III, W. A. The Two-Phase Model for Calculating Thermodynamic Properties of Liquids from Molecular Dynamics: Validation for the Phase Diagram of Lennard-Jones Fluids. J. Chem. Phys. 2003, 119, 11792-11805. (27) Lin, S.; Maiti, P. K.; Goddard III, W. A. Two-Phase Thermodynamic Model for Efficient and Accurate Absolute Entropy of Water from Molecular Dynamics Simulations. J. Phys. Chem. B 2010, 114, 8191-8198. (28) Pronk, S.; Páll, S.; Schulz, R.; Larsson, P.; Bjelkmar, P.; Apostolov, R.; Shirts, M. R.; Smith, J. C.; Kasson, P. M.; van der Spoel, D. GROMACS 4.5: A High-Throughput and Highly Parallel Open Source Molecular Simulation Toolkit. Bioinformatics 2013, 29, 845-854. (29) O. Guvench; S. N. Greene; G. Kamath; J. W. Brady; R. M. Venable; R. W. Pastor; A. D. MacKerell, J. Additive Empirical Force Field for Hexopyranose Monosaccharides. J. Comput. Chem. 2008, 29, 2543-2564. (30) Guvench, O.; Hatcher, E.; Venable, R. M.; Pastor, R. W.; MacKerell Jr, A. D. CHARMM Additive all-Atom Force Field for Glycosidic Linkages between Hexopyranoses. J. Chem. Theory Comput. 2009, 5, 2353-2370. (31) Darden, T.; York, D.; Pedersen, L. Particle Mesh Ewald: An N⋅ Log (N) Method for Ewald Sums in Large Systems. J. Chem. Phys. 1993, 98, 10089-10092. (32) Essmann, U.; Perera, L.; Berkowitz, M. L.; Darden, T.; Lee, H.; Pedersen, L. G. A Smooth Particle Mesh Ewald Method. J. Chem. Phys. 1995, 103, 8577-8593.


31


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 44

(33) Berendsen, H. J.; Postma, J. P. M.; van Gunsteren, W. F.; DiNola, A.; Haak, J. Molecular Dynamics with Coupling to an External Bath. J. Chem. Phys. 1984, 81, 3684-3690. (34) Bussi, G.; Donadio, D.; Parrinello, M. Canonical Sampling through Velocity Rescaling. J. Chem. Phys. 2007, 126, 014101. (35) Wohlert, J.; Bergenstråhle-Wohlert, M.; Berglund, L. A. Deformation of Cellulose Nanocrystals: Entropy, Internal Energy and Temperature Dependence. Cellulose 2012, 19, 18211836. (36) Durell, S. R.; Brooks, B. R.; Ben-Naim, A. J. Solvent-Induced Forces between Two Hydrophilic Groups. J. Phys. Chem. 1994, 98, 2198-2202. (37) Lemmon, E. W.; McLinden, M. O.; Friend, D. G. Thermophysical Properties of Fluid Systems. http://webbook.nist.gov/chemistry/fluid/ (accessed 06/13, 2013). (38) Berens, P. H.; Mackay, D. H. J.; White, G. M.; Wilson, K. R. Thermodynamics and Quantum Corrections from Molecular Dynamics for Liquid Water. J. Chem. Phys. 1983, 79, 2375-2389. (39) Caleman, C.; Maaren, P. J. v.; Hong, M.; Hub, J. S.; Costa, L. T.; Spoel, D. v. d. Force Field Benchmark of Organic Liquids: Density, Enthalpy of Vaporization, Heat Capacities, Surface Tension, Isothermal Compressibility, Volumetric Expansion Coefficient, and Dielectric Constant. J. Chem. Theory Comput. 2012, 8, 61-74. (40) McQuarrie, D. A. In Statistical Mechanics; University Science Books: Sausalito, CA 94965, 2000.


32

Page 33 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(41) Eisenhaber, F.; Lijnzaad, P.; Argos, P.; Sander, C.; Scharf, M. The Double Cubic Lattice Method: Efficient Approaches to Numerical Integration of Surface Area and Volume and to Dot Surface Contouring of Molecular Assemblies. J. Comput. Chem. 1995, 16, 273-284. (42) Luzar, A. Resolving the Hydrogen Bond Dynamics Conundrum. J. Chem. Phys. 2000, 113, 10663-10675. (43) Wolfrom, M. L.; Dacons, J. C. The Polymer-Homologous Series of Oligosaccharides from Cellulose. J. Am. Chem. Soc. 1952, 74, 5331-5333. (44) Yu, Y.; Wu, H. Characteristics and Precipitation of Glucose Oligomers in the Fresh Liquid Products obtained from the Hydrolysis of Cellulose in Hot-Compressed Water. Ind. Eng. Chem. Res. 2009, 48, 10682-10690. (45) Kajimoto, O. Solvation in Supercritical Fluids: Its Effects on Energy Transfer and Chemical Reactions. Chem. Rev. 1999, 99, 355-390. (46) Kalinichev, A. G. Molecular Simulations of Liquid and Supercritical Water: Thermodynamics, Structure, and Hydrogen Bonding. Rev. Mineral. Geochem. 2001, 42, 83-129. (47) Cochran, H.; Cummings, P.; Karaborni, S. Solvation in Supercritical Water. Fluid Phase Equilib. 1992, 71, 1-16. (48) Kalinichev, A.; Bass, J. Hydrogen Bonding in Supercritical Water. 2. Computer Simulations. J. Phys. Chem. A 1997, 101, 9720-9727. (49) Flory, P. J. In Principles of Polymer Chemistry; Cornell University Press: Ithaca, New York, 1953.


33


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 44

(50) Moates, G. K.; Noel, T. R.; Parker, R.; Ring, S. G. The Effect of Chain Length and Solvent Interactions on the Dissolution of the B-Type Crystalline Polymorph of Amylose in Water. Carbohydr. Res. 1997, 298, 327-333. (51) Hadden, J. A.; French, A. D.; Woods, R. J. Unraveling Cellulose Microfibrils: A Twisted Tale. Biopolymers 2013, 99, 746-756. (52) Paavilainen, S.; Rog, T.; Vattulainen, I. Analysis of Twisting of Cellulose Nanofibrils in Atomistic Molecular Dynamics Simulations. J. Phys. Chem. B 2011, 115, 3747-3755. (53) Matthews, J. F.; Beckham, G. T.; Bergenstråhle-Wohlert, M.; Brady, J. W.; Himmel, M. E.; Crowley, M. F. Comparison of Cellulose Iβ Simulations with Three Carbohydrate Force Fields. J. Chem. Theory Comput. 2012, 8, 735-748. (54) Tolonen, L. K.; Zuckerstatter, G.; Penttilä, P. A.; Milacher, W.; Habicht, W.; Serimaa, R.; Kruse, A.; Sixta, H. Structural Changes in Microcrystalline Cellulose in Subcritical Water Treatment. Biomacromolecules 2011, 12, 2544-2551. (55) Deguchi, S.; Tsujii, K.; Horikoshi, K. Crystalline-to-Amorphous Transformation of Cellulose in Hot and Compressed Water and its Implications for Hydrothermal Conversion. Green Chem. 2008, 10, 191-196. (56) Ogihara, Y.; Jr, R. L. S.; Inomata, H.; Arai, K. Direct Observation of Cellulose Dissolution in Subcritical and Supercritical Water Over a Wide Range of Water Densities (5501000 kg/m3). Cellulose 2005, 12, 595-606. (57) Cantero, D. A.; Bermejo, M. D.; Cocero, M. J. Reaction Engineering for Process Intensification of Supercritical Water Biomass Refining. J. Supercrit. Fluids 2015, 96, 21-35.


34

Page 35 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(58) Wohlert, J.; Tolonen, L. K.; Bergenstråhle-Wohlert, M. A simple model for the solubility of cellulose in supercritical water. Nord. Pulp Paper Res. J. 2015, 30, 14-19.


35


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

40x20mm (600 x 600 DPI)


Page 36 of 44

Page 37 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50x35mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

152x329mm (600 x 600 DPI)


Page 38 of 44

Page 39 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


39x9mm (600 x 600 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

72x31mm (600 x 600 DPI)


Page 40 of 44

Page 41 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


39x9mm (600 x 600 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

150x363mm (600 x 600 DPI)


Page 42 of 44

Page 43 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


38x8mm (600 x 600 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of content graphic 39x19mm (600 x 600 DPI)


Page 44 of 44

Solubility of Cellulose in Supercritical Water Studied by Molecular

Recommend Documents