Understanding the Inhibiting Effect of Small-Molecule Hydrogen Bond

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Understanding the Inhibiting Effect of Small-Molecule Hydrogen Bond Donors on the Solubility of Cellulose in Tetrabutylammonium Acetate/DMSO Jenny Bengtsson, Carina Olsson, Artur Hedlund, Tobias Köhnke, and Erik Bialik J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b08501 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on December 6, 2017

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The Journal of Physical Chemistry

Understanding the Inhibiting Effect of Small-Molecule Hydrogen Bond Donors on the Solubility of Cellulose in Tetrabutylammonium Acetate/DMSO Jenny Bengtssona, Carina Olssona Artur Hedlunda, Tobias Köhnkea*, and Erik Bialikb a

Bio-based fibres, Swerea IVF, P.O. Box 104, SE-431 22 Mölndal, Sweden

b

Molecules in Motion, Åkerbyvägen 390, SE-187 38 Täby, Sweden

* Corresponding author. E-mail: [email protected], Tel: +46 31706 6338

ACS Paragon Plus Environment

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ABSTRACT

Certain ionic liquids are powerful cellulose solvents, but tend to be less effective when smallmolecule hydrogen bond donors are present. This is generally attributed to competition with cellulose for hydrogen bonding opportunities to the anion of the ionic liquid. We show that the solubility of cellulose in dimethyl sulfoxide solutions of tetrabutylammonium acetate is less strongly affected by water than by ethanol on a molar basis, contrary to what can be expected based on hydrogen bond stoichiometry. Molecular dynamics simulations indicate that the higher tolerance to water is due to water-cellulose interactions that improves solvation of cellulose and, thereby, marginally favors dissolution. Through Kirkwood-Buff theory we show that water, but not ethanol, improves the solvent quality of DMSO and partly compensates for the loss of acetate-cellulose hydrogen bonds.


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INTRODUCTION Cellulose in the form of wood pulp is an important renewable resource. Beyond paper, it can be used to manufacture a range of materials, including regenerated cellulose textile fibers. As cellulose cannot melt, re-shaping cellulose into films or fibers, requires it to be dissolved, extruded and precipitated in a coagulation liquid. While a large number of cellulose solvent systems are known,1 only a few are in industrial use. The Viscose process, well over a century old, is globally dominant in terms of production volumes,2 but there are sustainability issues related both to its high energy demand and to the use of hazardous carbon disulfide. The only new method for producing synthetic cellulose fibers to reach industrial scale within living memory is the Lyocell process. It is based on the N-methylmorpholine-N-oxide (NMMO)/water system, which enables a closed loop process with essentially full recovery and recycling of the non-toxic NMMO solvent.3 For this reason, it is considered a green alternative to the viscose process. However, the more expensive Lyocell process can compete with the Viscose process only in the high-end market segment, thanks to favorable fiber properties. During the last two decades, ionic liquids (IL) have emerged as a promising class of cellulose solvents for fiber spinning applications.4,5 ILs are bulky organic salts with low vapor pressure and low melting points, considered as green solvents. Since the seminal work by Youngs and coworkers a little over a decade ago,6,7,8 several applications of molecular simulation methods to carbohydrate dissolution and solvation in IL systems have been reported.9 This has contributed to the fact that the dissolution mechanism in neat ionic liquids is in large part understood: the main driver of carbohydrate dissolution appears to be formation of hydrogen bonds with the anion. Co-solvents such as DMSO are sometimes added to decrease viscosity and increase the dissolution rate.10


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A prerequisite for an IL-based cellulose solvent system to be viable in industrial applications is that it can be recycled. Any economically feasible recycling process can be expected to leave some remnant of the coagulation medium and that such impurities are tolerated in the dissolution and spinning processes is vital. The best solvent systems for such applications are thus not necessarily the best solvents in the thermodynamic sense but solvents that can be turned ’on’ and ’off’ at will, i.e. where the thermodynamic solvent quality can be tuned with a feasible intervention. Water and low molecular weight alcohols inhibit the solubilizing action of IL cellulose solvents and are strong candidates as coagulating agents in such systems. It is therefore important to assess the effect of such compounds as impurities at the dissolution stage. For cellulose solutions in 1-ethyl-3-methylimidazolium acetate (EmimAc)/DMSO mixtures, about half as much ethanol as water on a molar basis is required to induce coagulation.11 This is surprising as both water and ethanol are believed to cause coagulation by competing with cellulose for the opportunity to form hydrogen bonds to acetate. Water has two polar hydrogen atoms that may be donated to form hydrogen bonds while ethanol has only one. A naive application of the hydrogen bond competition concept would thus suggest that water would be the stronger coagulating agent. Tetrabutylammonium acetate (TBAAc) solutions in dimethyl sulfoxide (DMSO) can dissolve cellulose in near stoichiometric manner, such that approximately one formula unit of TBAAc is able to dissolve an amount of cellulose corresponding to one anhydroglucose unit (AGU) over a broad range of TBAAc concentration.12 Here, we demonstrate that the same relationship between the coagulation properties of ethanol and water is obtained in TBAAc/DMSO as in EmimAc/DMSO. To investigate the mechanism, we perform Molecular Dynamics (MD) simulations using a molecular model that has previously been successfully applied to cellulose solutions in TBAAc/DMSO.


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EXPERIMENTAL AND THEORETICAL METHODS Solubility measurements Dissolving pulp ( = 162300 g/mol, = 518009 g/mol) was provided by Södra Cell (Mörrum, Sweden) in dry sheets that were milled and further dried at 40 °C for 12 hours before use. Tetrabutylammonium acetate (97%) was purchased from Aldrich and dimethyl sulfoxide (99.7%) from Acros Organics, both used as received. The amount of solvent used in the experiments was corrected for water content. TBAAc was dissolved in DMSO by stirring at 40°C at a fixed weight ratio of 2:7. A certain amount of non-solvent, deionized water or 99.6% ethanol was added, followed by cellulose. The mixtures were stirred for 1 h in a closed container at 60°C. Any non-dissolved fiber fragments were detected by observation through a Nikon SMZ 1500 light microscopy with crossed polarizers. Molecular dynamics simulations We consider a series of systems composed of a single periodic cellulose chain, 36 TBAAc ion pairs and 486 DMSO molecules as well as between 0 to 108 water or ethanol molecules, i.e., up to 3 non-solvent molecules per TBAAc ion pair. The cellulose molecule was composed of eight unique anhydroglucose units (AGU) and spanned the box in the z−direction such that it was bonded to its periodic images, similar to the system considered in the work by Horinek et al.13 The box length in the z-direction was held constant at 4.2 nm and pressure coupling was applied in the x- and y- directions only. This resulted in an approximately cubic box, sufficiently large that the cellulose molecule did not interact with their periodic neighbors in the x- and ydirections; the system mimics a single infinitely long chain at infinite dilution. All small


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molecules started out randomly distributed in the simulation box. From an initial low-density state in which particles could be added in a non-overlapping way with reasonable insertion probability, the system was energy-minimized and then compressed in a 100 ps simulation at 100 bar pressure. The simulations were run for 500 ns in the NpT ensemble at ambient pressure, using a weak-coupling barostat,14 and 40 ◦C temperature using the Bussi thermostat.15 Longrange electrostatic interactions were handled using particle-mesh Ewald summation.16 The cutoff for short-range interactions was 0.8 nm. All simulations were performed using the GROMACS simulation package.17 We used the GLYCAM force field to represent cellulose,18 and the same model as previously for TBAAc and DMSO.12 Namely, the bonded and Lennard-Jones force field parameters for TBAAc were taken from GLYCAM. Partial charges for acetate were taken from electronic structure calculations by Liu et al.19 and scaled by 0.75 to give effective charges that include the effect of electronic polarization in the condensed phase.20 Effective partial charges of 0, 0.1, 0.05, 0.025 and 0.0125 unit charges were assigned for the TBA+ nitrogen and α-, β-, γ-, and δ-carbon, respectively. For DMSO, the GAFF force field was used with partial charges from Geerke et al.21,22 This force field combination for cellulose/DMSO/TBAAc was shown in Idström et al.12 to give a reasonable quantitative agreement with measured diffusion coefficients and to capture the trend in these quantities with increasing cellulose concentration up to the saturation limit. We used the TIP4P/2005 model for water,23 and for ethanol we used the bonded and non-bonded parameters from GLYCAM with partial charges as described in Benmore et al.24 As the partial charges for DMSO, water, and ethanol were selected on the basis of condensed phase data, they were assumed to reflect the condensed phase polarization environment and no scaling was applied. Note that both GAFF and GLYCAM force fields are based on the AMBER protein force


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field and constructed to be compatible with this force field. Therefore, we expect them to be compatible with each other to an extent such that makes the combination fit for our present purposes. To ensure that the force field model is qualitatively correct with respect to the studied phenomenon, i.e., cellulose is uniformly dissolved in the absence but not the presence of a sufficient amount of non-solvent, we considered a selection of systems with finite cellulose concentration. These were composed of four cellodecaose molecules, 90 TBAAc ion pairs and 1215 DMSO molecules, corresponding to a 5 % (w/w) solution as well as 0, 1, 2, 3, or 6 water molecules or 3 ethanol molecules per TBAAc ion pair. The cellulose molecules were inserted by hand parallel to the principal axes of the box and all other molecules started out randomly distributed. The trajectories were propagated for 1.2 µs. Kirkwood-Buff theory The pair distribution function, , gives the local concentration of species in the vicinity of a particle of species relative to the bulk concentration; the position vector pertains here to a coordinate system fixed on an arbitrary -molecule. The pair distribution function can be connected to the solubility via Kirkwood-Buff (KB) theory,25 wherein concentration derivatives of the chemical potentials are expressed in terms of the spatial correlations in number density. This has found application particularly in the study of the effects of co-solvents, including IL, on proteins.26 The number density correlations are quantified in terms of so-called KB integrals, = − 1.

(1)

The integration is over all space, which implies that the selection of coordinate system for does not affect the value of . The excess coordination number = , where is the number density of species , is the (possibly negative) number of ’extra’ molecules of species in a


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sufficiently large volume containing a molecule of species relative to an equal volume of solution with the average solution composition. The preferential binding parameter, Γ = −

,

(2)

measures the excess of co-solvent (3) relative to that of solvent (1) to a solute (2). It has been shown that for a three-component system,27 the solubility, , of species 2 expressed in molar concentration is related to Γ as ! ln % = Γ * , ! ln $ &,',(

(3)

)

, -. /

where * = + , -. 1 0 2 0

&,'

is the co-solvent ‘activity derivative’, and * and $ are the co-

solvent activity and concentration, respectively, on the molar scale. The ’activity derivative’ * is a measure of the deviation from ideality of the solvent–co-solvent (1–3) mixture and it is strictly positive; were this not the case, the solution would be in the unstable region of the phase diagram beyond the spinodal line. Thus, a positive Γ guarantees that further addition of species 3 improves the solubility of species 2, i.e., species 3 is indeed a co-solvent, and a negative Γ guarantees the converse, species 3 is a non-solvent. The cellulose/TBAAc/DMSO/non-solvent systems considered here are four component systems. While KB theory is valid for any number of components28, the complexity of the resulting expressions increases steeply with the number of chemical species present. In practice, the analytic benefits from KB theory typically evaporate beyond two or three components. We calculated Γ for the infinite dilution system without non-solvent, a three-component system for the purposes of eq. ( 2 ), and for analogous systems with the TBAAc replaced by water or ethanol at the same molar ratios to DMSO. The preferential binding parameter was calculated from29


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1 3 1 3 Γ 3 = −

1 3 − 1 3 , −

(4)

1 3 is the number within a Where is the total number of particles of species i and

cylindrical volume of radius R around the cellulose molecule. Γ 3 approaches Γ for large R.

RESULTS AND DISCUSSION Cellulose solubility The apparent dissolution limit of cellulose for each concentration of water or ethanol nonsolvent was determined through light microscopy. The results are shown in Figure 1. The cellulose solubility decreased seemingly linearly with the concentration of non-solvent. The tolerance for water as non-solvent was found to be approximately twice as high as that for ethanol, on a molar basis. Molecular dynamics simulations Model validation To ensure that the model adequately represents the behavior of the cellulose solvent system, we consider the association of cellulose molecules in finite concentration solutions. The number of contacts between cellulose molecules as function of the simulation time are shown in Figure 2. Even for 0 or 1 water molecule per TBAAc ion pair, where the cellulose is expected to be soluble, there is visible association. However, none of the aggregates observed grows beyond about ten contacts and tend to dissolve after a few hundred ns. For 2, 3 and 6 water molecules and for 3 ethanol molecules per TBAAc ion pair, where the cellulose is expected to be partly and completely insoluble, persistent clusters of two or three chains are formed. The number of contacts between molecules in these clusters tends to increase for a few hundred ns and then reach a plateau.


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In Figure 3, the structures of the complexes at the end of the trajectories are shown as distance matrices, showing the average distances between individual glucose residues, and snapshots. The diagonal bands in the distance matrices correspond to residues on the same molecule. Association between chains can be seen as off-diagonal bands or spots. In the absence of nonsolvent, no association can be seen, confirming that the model solvent is indeed a solvent. For one water molecule per TBAAc ion pair, a single off-diagonal spot is visible in the distance matrix. In each of the systems with precipitated cellulose, there are at least one off-diagonal band in the distance matrices that correspond to the side-by-side association of cellulose chains. Both parallel and anti-parallel modes of aggregation can be seen. Thus, cellulose forms persistent aggregates for solvent compositions where precipitation it is expected from experiments but not for solvent compositions where complete dissolution was observed. While these results should not be construed as a quantitative determination of the solubility of model cellulose in the model solvent, they demonstrate that the behavior of the model is in at least qualitative accord with that of the real system. This indicates that the force field model is fit for purpose. Interactions between solvents and cellulose in the presence of non-solvents The number of hydrogen bonds between cellulose and other species present in the system are shown in Figure 4a. A hydrogen bond is taken to be present if two oxygen atoms are within 0.35 nm and the angle between the O-H bond of the donor hydroxyl group forms an angle of less than 30 degrees with the line between the oxygen atoms. For low concentrations of non-solvent, the number of acetate–cellulose hydrogen bonds is the same for each of the non-solvents. Above non-solvent/acetate ratios of one, a larger number of acetate–cellulose hydrogen bonds remain


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for ethanol than for water.

However, based on the total number of protons available for

hydrogen bond donation in water and in ethanol, the difference is surprisingly small. Figure 4b shows the number of hydrogen bonds per non-solvent molecule with each of the hydrogen-bond forming species. The relative decrease in the number of hydrogen bonds per nonsolvent molecule to acetate with increasing non-solvent concentration is similar to that for acetate-cellulose hydrogen bonds in Figure 4a. This is in line with the notion that the non-solvent acts mainly by competing with cellulose for the acetate; as there are more hydrogen bond opportunities present, the probability that any one of them is realized decreases proportionally. Two facts contribute to explain that the difference in the number of hydrogen bonds to acetate between water and ethanol is not larger: first, and most importantly, water is less selective towards acetate over DMSO than ethanol is, and, second, water hydrogen-bonds also to itself. The first fact is most evident when comparing the interactions of water and ethanol with acetate and DMSO for the lowest non-solvent concentration of 0.5 Nnon-solvent/Nacetate, in Figure 4b. Water interactions are approximately equally divided between acetate and DMSO, whereas only one third of the ethanol hydroxyls bond to DMSO and about two thirds bond to acetate. DMSO does in this context substitute for the acetate, in interacting with the non-solvents, which can be summarized as a buffering effect from DMSO on additional non-solvent. However the lower basicity of DMSO, relative to acetate, induces selectivity for water, whose acidity is higher than that of ethanol.30 This makes the buffering effect of DMSO more prominent for water; an effect previously observed in coagulation experiments with 1-ethyl-3-methylimidazolium acetate.11 The facts that water is less selective towards acetate over DMSO than ethanol, and that water hydrogen-bonds also to itself, are also pronounced when increasing the non-solvent concentration. For ethanol, the number of hydrogen bonds per ethanol molecule to DMSO and


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to other ethanol molecules increases with increasing ethanol concentration such that the total number of hydrogen bonds remains unchanged. This suggests a situation of straightforward competition between possible hydrogen bond acceptors; acetate is the strongest acceptor, but DMSO is able to compete due to its abundance. In contrast, the average number of hydrogen bonds per water molecule to DMSO remains constant as the concentration of water is increased. The decrease in the number of water-acetate hydrogen bonds per water molecule is instead mirrored by an increase in the number of water-water hydrogen bonds. The number of DMSOcellulose hydrogen bonds increases slightly with concentration of non-solvent for both nonsolvents, which suggests that DMSO binds opportunistically to cellulose when acetate leaves. Such substitution could have an additional positive effect on dissolution.

Direct interaction between non-solvent and cellulose The slight difference in the number of acetate-cellulose hydrogen bonds between solutions with water or ethanol has neither the right sign nor the sufficient magnitude to explain the fact that the non-solvent effect of ethanol is roughly 2 times greater than that of water, see Figure 1. In terms of solubility, a water concentration of 3 Nwater/Nacetate corresponds to an ethanol concentration of 1.5 Nethanol/Nacetate. At these concentrations of water or ethanol the corresponding numbers of acetate-cellulose hydrogen bonds per AGU are approximately 1 and 1.5 respectively, as can be noted in Figure 4a. Thus, it can be concluded that the number of acetate-cellulose hydrogen bonds per AGU, or the non-solvents’ relative ability to compete with cellulose for hydrogen bonds to acetate, are not the only reasons behind the observed difference in nonsolvent strength, even if hydrogen bond competition is the main driving force of the non-solvent action. Water forms a dramatically larger number of hydrogen bonds with cellulose than ethanol


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does, see Figure 4a, which suggests that also direct non-solvent—cellulose interactions have an influence on the solubility. KB theory rigorously connects preferential binding to changes in solubility.27 The results from the KB analysis for cellulose in binary mixtures of DMSO and one solvent or non-solvent are shown in Figure 5. As only one of the species TBAAc, water, or ethanol is present in any of these systems, these preferential binding parameters measure the intrinsic effect of each substance on the solubility of cellulose by mechanisms distinct from competition for hydrogen bonds. The preferential binding parameter is positive for both TBAAc and water and but negative for ethanol. Thus, only ethanol is intrinsically a non-solvent in a DMSO environment whereas the non-solvent action of water appears to be purely due to competition for hydrogen bonds to acetate. Water itself improves the solvent quality of DMSO with respect to cellulose, though evidently not to the extent that cellulose is soluble to a measurable extent in DMSO:water mixture of any composition. The fact that water is a weaker non-solvent in the TBAAc/DMSO system can therefore be conceptualized by water improving the solvent quality of DMSO while counteracting the solubilizing effect of the TBAAc acetate ions. This is in line with the observation that a 90:10 DMSO:water mixture by weight is a better solvent for xylans than neat DMSO.31 Xylans are chemically similar to cellulose but more soluble due to branching and other irregularities. Density maps, i.e., isosurfaces for selected values of , for acetate, water, and ethanol are shown in Figure 6. We take to be cellulose, oriented such that the six atoms forming the relatively rigid pyranose ring are fitted to a reference structure, and consider a selection of solvent and non-solvent atoms for . The acetate and water data is taken from the infinite dilution system with three water molecules per acetate ion and the ethanol data is taken from the


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corresponding system with three ethanol molecules per acetate ion. As cellulose is somewhat flexible, aligning the pyranose rings does not cause the whole molecules to overlap; examples of cellulose conformations are overlaid in Figure 6. The oxygen atoms of each species are concentrated close to the cellulose hydroxyl groups, as expected given that hydrogen bonding is the main association mechanism. The region between the hydroxyl groups on carbon atoms 2 and 3 (bottom of each panel; the reducing end of the cellulose molecule would be to the right) constitutes a binding site for oxygen for all solvent and non-solvent species considered. For water and ethanol, a small region of hydrogen enrichment is also present close to the hydroxyl group on carbon 3. This indicates that the non-solvent has a strong propensity to form hydrogen bonds to this hydroxyl group. It thus appears that the non-solvent tends to form a donor hydrogen bond with this atom. As this places the non-solvent oxygen atom in a favorable position to accept a hydrogen bond from the hydroxyl group of carbon 2, it appears that a mode of dual hydrogen bonding not available to the acetate ion plays a role in non-solvent binding. Such binding appears to be qualitatively similar between water and ethanol but is quantitatively more prominent for water. The co-solvent action of water in DMSO appears to be due to direct solvation of the cellulose molecule by water molecules. This suggests that water/DMSO mixtures are better at solvating cellulose than either of the neat solvents. This is consistent with the conclusions about carbohydrate solvation in water/DMSO solution of an earlier study,32 and with our observation that the total number of hydrogen bonds increases with the amount of added non-solvent for water but not for ethanol, see Figure 4a. In common with cellulose-dissolving IL, the DMSO environment is rich in hydrogen bond acceptors but completely lacks hydrogen bond donors. Water provides such donors, which may partly explain the facts that the mixing of DMSO and


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water is exothermic and that the resulting mixtures show significant deviations from ideality.

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The water molecules compete with cellulose for binding DMSO in the same way as they compete for the IL anion, but they also allow modes of solvation of the cellulose molecule that would otherwise not be possible. In the range of water concentration considered here, the DMSO is in large excess over the water. This implies that the competition effect is not decisive for the influence of water on DMSO-cellulose interactions, in contrast to the situation for the less abundant acetate ion.

Structure of the TBAAc/DMSO – non-solvent mixture To quantify the differences in the hydrogen bond networks formed in the presence of water and ethanol, we computed the populations of water molecules in hydrogen bonded clusters containing 5) 6 7865 ) water (ethanol) molecules and 91 : acetate ions. The results are shown in Figure 7, along with example cluster structures. Even for a non-solvent/Ac- molar ratio of one, there is an appreciable proportion of non-solvent molecules present in clusters with more than one non-solvent member. There is a higher population of large clusters with water than with ethanol for 91 : = 0, 1 and for water, but not ethanol, there is an appreciable proportion of molecules in clusters with 91 : = 2. Water molecules tend to associate either directly to acetate by one hydrogen bond or into hydrogen bonded chains terminated by acetate ions. Both these alternatives would leave one hydrogen bonding opportunity for each water molecule to associate to DMSO. Evidently, water molecules in the solvation shells of acetate ions may even form hydrogen bonds to other acetate ions or their solvation shell, as can be seen from the example water cluster in Figure 7. Such association should to some extent be counteracted by the Coulomb repulsion between acetate ions. However, this repulsion is screened in the highly polar


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DMSO environment and hydrogen bonding is evidently able to compete. This ‘bridging’ between acetate ions cannot happen with ethanol as the single hydrogen bonding opportunity is occupied when an ethanol molecule solvates an acetate ion. Thus, there are significant topological differences between the hydrogen bond networks formed in TBAAc/DMSO by the two non-solvents.

SUMMARY AND CONCLUSIONS The impact of protic non-solvents during dissolution of cellulose in TBAAc/DMSO was studied both experimentally and by molecular dynamics simulations. It was found that the tolerance for water as non-solvent during cellulose dissolution is approximately twice as high as that for ethanol, on a molar basis, contrary to what can be expected based on hydrogen bond stoichiometry. The explanation for this was investigated using molecular dynamics simulations and it was found that the relevant difference between water and ethanol for explaining the difference in non-solvent action is not their respective ability to compete with cellulose for hydrogen bonds to acetate. Instead, analysis based on KB-theory suggest that the higher tolerance to water is due to water-cellulose hydrogen bond interactions that improves solvation of cellulose and, thereby, marginally favors dissolution.

ACKNOWLEDGMENTS We acknowledge funding from the Swedish Research Council Formas and the Södra research foundation within the framework of the Avancell program. EB would like to thank Mikael Lund and LUNARC for providing computational resources.


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Figures

Figure 1: Results of cellulose solubility measurements in the presence of water and ethanol non-solvent, as indicated. Red crosses indicate that fibers are visible under the microscope and green circles indicate that no fibers can be seen. The micrographs A and B correspond to the measurement points indicated. The straight lines roughly indicate the cellulose solubility limit.


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Figure 2: Number of contacts between pairs of cellulose molecules averaged over 1 ns windows as a function of simulation time. Contacts are defined as oxygen atoms on different molecules being within 0.4 nm of each other.


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Figure 3: Distance matrices, averaged over the final 100 ns of each simulation, represented such that brighter pixels correspond to smaller distance between residues and snapshots for the finite concentration systems indicated.


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Figure 4: Simulations of cellulose solutions with NAGU/NAc- corresponding to 0.2. Panel a: Number of hydrogen bonds between solvent and non-solvent species and cellulose per AGU for acetate ions (red), DMSO molecules (orange), non-solvent molecules (blue), and other groups on the same cellulose molecule (gray). The total number of hydrogen bonds is shown as black squares. Filled symbols correspond to water and open symbols to ethanol. For species for which both are possible, both donor and acceptor hydrogen bonds are counted. Panel b: Same, but for the average number of hydrogen bonds to a non-solvent molecule.


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Figure 5: Estimate of preferential binding parameter per glucose residue, Γ23, as a function of R, see eq (4), for cellulose as solute (species 2) and TBAAc as co-solvent (species 3) for the non-solvent–free system and for analogous systems with the TBAAc replaced such that species 3 is water or ethanol at the same molar ratio to DMSO, as indicated.


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Figure 6: Density maps for acetate (red: oxygen, cyan: carboxyl carbon), water (blue: oxygen, white: hydrogen), and ethanol (blue: oxygen, white: hydrogen) as indicated. The isosurfaces correspond to ten (transparent) and fifty (opaque) times the bulk concentration.


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Figure 7: Relative population of water molecules in clusters containing 5) 6 water molecules or 7865 ethanol molecules and 91 : acetate ions for systems with one or three non-solvent molecules per acetate ion, as indicated. An acetate-water cluster with 91 : = 2 and 5) 6 = 4 and an acetate-ethanol cluster with 91 : = 1 and =>?@ = 2, as indicated, are shown.


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Understanding the Inhibiting Effect of Small-Molecule Hydrogen Bond

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