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DOI: 10.1002/cssc.201500272
Understanding Cellulose Dissolution: Energetics of Interactions of Ionic Liquids and Cellobiose Revealed by Solution Microcalorimetry Heitor Fernando Nunes de Oliveira and Roberto Rinaldi*[a] In this report, the interactions between fifteen selected ionic liquids (ILs) and cellobiose (CB) are examined by high-precision solution microcalorimetry. The heat of mixing (DmixH) of CB and ILs, or CB and IL/molecular solvent (MS) solutions, provides the first ever-published measure of the affinity of CB with ILs. Most importantly, we found that there is a very good correlation between the nature of the results found for DmixH(CB) and the sol-
ubility behavior of cellulose. This correlation suggests that DmixH(CB) offers a good estimate of the enthalpy of dissolution of cellulose even in solvents in which cellulose is insoluble. Therefore, the current findings open up new horizons for unravelling the intricacies of the thermodynamic factors accounting for the spontaneity of cellulose dissolution in ILs or IL/MS solutions.
Introduction Native cellulose does not always exhibit the desired properties for its application as a high-performance material.[1–6] Accordingly, to tailor the biopolymer properties, it is often necessary to dissolve the biopolymer and controllably recover the processed cellulose from its solutions.[7] Several ionic liquids (ILs) were demonstrated to be excellent solvents for cellulose.[5, 6, 8] In 2011, we reported solutions of cellulose-dissolving ILs in molecular solvents (e.g. DMSO), giving even better performance in the dissolution of cellulose than for the ILs themselves.[9] Perhaps one of the most thought-provoking advantages of these solutions over neat ILs is the fact that only a minor mole-fraction of an IL (cIL) is required for cellulose to dissolve instantaneously at 373 K.[10, 11] Regarding the improvement of dissolution kinetics, several studies reported that the mass transport of the “real” solvent (i.e., cellulose-dissolving ILs) is enhanced when the dissolution is performed in a solution of IL in a molecular solvent (MS).[12–14] However, the dissolution process is not only governed by kinetics, but also by thermodynamics. Concerning the process energetics, most of the information on the interactions between IL-ions and cellulose stems either from molecular dynamics simulations[15–19] or from the analysis of solvatochromic parameters.[20–23] As regards the latter approach, a criticism was made on the unsuitability of solvatochromic probes in reporting preferential solvation of ILs.[24] Moreover, the solvatochromic probes are sensitive to measurement conditions and procedures employed.[25] Therefore, we identified a real need for the development of a calorimetric protocol for the determination of the affinity of a molecular probe related to the cellulose structure, that is, cellobiose, with [a] H. F. N. de Oliveira, Dr. R. Rinaldi Max-Planck-Institut fìr Kohlenforschung Kaiser-Wilhelm-Platz 1, 45470 Mìlheim (Ruhr) (Germany) E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201500272.
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ILs or IL/MS solvent systems, providing in-depth insight into the overall energetics of cellulose in solution. To understand the reason why the energetics of interactions between cellulose and ILs or IL/MS systems is of importance for cellulose solubility, some considerations should be made. Logically, dissolution of cellulose is a spontaneous process only if the interplay of entropic and enthalpic contributions results in a negative value of Gibbs free energy at a given temperature and pressure. In the dissolution of cellulose, the total entropy change is a balance of solvent entropy reduction (caused by the cellulose solvation), and solute entropy gain (due to increased conformational freedom of the polymeric chains in solution). Through molecular dynamic simulations, Chu et al.[26] investigated the entropy of cellulose dissolution in 1-butyl-3methylimidazolium chloride ([C4C1Im]Cl) and water. Interestingly, solvent entropy reduction in [C4C1Im]Cl was found to be smaller than that in water, and therefore, to counteract the solute entropy gain to a lesser extent.[26] They predicted the term TDS to assume a positive value of about 4–6 kJ mol¢1glucan in the cellulose dissolution in [C4C1Im]Cl (at temperatures of 425, 450, 500 K). In this manner, at a given temperature and pressure, should the overall change in enthalpy be highly endothermic, the term TDS may not suffice to counterbalance the resulting total (endothermic) DH, and thus, the process will not be spontaneous. Hildebrand theory for polymer dissolution describes the total change in enthalpy as the enthalpy of mixing (DmixH), which is given by Equation (1): Dmix H ¼ Vmix ðd1 ¢ d2 Þ2 1 2
d¼
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1=2 Dvap H ¢ RT 1=2 E ¼ V V
ð1Þ
ð2Þ
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Full Papers where, Vmix is the mixture volume, f1 and f2 are the volume fractions of the two components, and d is the solubility parameter (or also called Hildebrand parameter). For Equation (2), the term E/V stands for the cohesive energy density, DvapH is the latent heat of vaporization, R is the gas constant, T is the absolute temperature in Kelvin for the solvent vaporization, and V is the molar volume of the component. From Equation (1), the condition d1 d2 is required in order to obtain an enthalpy of mixing near zero, and thus, DG < 0. As defined by Equation (2), the d parameter is directly related to the cohesive energy (E). For a solvent, the physical meaning of this term is the energy needed for a molecule to be removed from its nearest neighbor, forming a cavity in the solvent for insertion of solute molecules. For a polymer, the cohesive energy is the energy required for all intermolecular interactions between the polymeric chains to be disrupted. Noteworthy, Hildebrand parameters do not account for specific interactions (e.g. hydrogen bonding). Although this limitation poses a problem for the prediction of cellulose solubility through the direct comparison of Hildebrand parameters, the rationale of the theory is still valid for describing the general thermodynamic condition required for the spontaneous dissolution of a polymer (i.e. a value of DmixH 0, assuming that DmixS > 0). Herein, we introduce a solution microcalorimetry protocol for the determination of the DmixH(CB) in ILs or IL/MS systems. We demonstrate that there is a direct correlation between DmixH(CB) values and the solubility behavior of cellulose. This finding indicates that cellobiose is a suitable probe molecule to estimate the solvation power of ILs on cellulose. This paper is organized as follows. First, the criteria for the design of solution microcalorimetry experiments are introduced. Thereafter, the effects of molecular solvent and cIL on DmixH(CB) are assessed. Next, the impact of the chemical nature of anions on DmixH(CB) is examined. In addition, the contributions of cation
structures to the process energetics are analyzed. Finally, the effect of water on the interactions between cellobiose and IL/ MS is addressed.
Results and Discussion On the criteria for the experimental design Solution calorimeters are the classical instruments used in order to determine the heat absorbed or released upon dissolution or mixing of a solute into a solvent or medium. Shown in Figure 1 a is a schematic representation of an isoperibolic solution-microcalorimeter setup for the determination of DmixH of CB and ILs, or CB and IL/MS solvent systems. Typically, DmixH(CB) values were measured by crushing a glass ampoule against a sapphire edge in the calorimetric vessel (Figure 1 b). By this procedure, the ampoule content—0.5 g of a solution of CB in DMSO at a molal concentration of 0.325 mol·kg¢1—is released into the medium of interest contained in the vessel (25.0 mL, Figure 1 b). Noteworthy, in this type of experiment, both the initial state and the final state of the system are well defined. Logically, this condition constitutes a core requirement for any thermodynamic determination. Recently, Parviainen et al.[27] reported the use of differential scanning calorimetry (DSC) for the determination of the enthalpy of cellulose dissolution in three ILs. The measurements were performed on cellulose pastes in IL. Notably, in this kind of experiment, the initial state of the system cannot be defined because the biopolymer is already undergoing physical transformations (e.g. swelling and decrystallization) upon mixing cellulose and ILs for the sample preparation for the DSC analysis. A clear indication for this (arte)fact is that the heat values determined by using a DSC were insensitive to changes in the sample crystallinity.[27] Moreover, they reported that partial dissolution of cellulose and remnant structures of the fiber wall
Figure 1. (a) Details of a high-precision solution microcalorimeter (TA instruments), and (b) the calorimetric vessel. For clarity, the calorimetric vessel and glass ampoule were filled with colored solutions, which do not correspond to any of the ILs or solutions studied in this work.
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Full Papers did not contribute to the DH value.[27] Of course, since enthalpy is an extensive thermodynamic property, any measurement of the value for enthalpy of cellulose dissolution must provide results that respond to the amount of cellulose dissolved in the medium. Therefore, in the best-case scenario, DSC appears to provide only a very rough measure of the heat of cellulose dissolution. Regarding our method, the use of cellobiose solutions, instead of cellulose, enables fast mixing of the components. Furthermore, the final cellobiose Figure 2. Typical results from a solution-calorimetry experiment performed on a high-precision solution microcaconcentration, achieved by lorimeter (TA instruments). crushing the ampoule in the calorimetric vessel, was 6·10¢3 mol·kg¢1. For such a dilute solution, the values of the (which is provided with a TAM III calorimeter by TA Instruments). enthalpy of cellobiose dissolution in water at 298.15 K are simiDisplayed in Figure 2 is a typical result obtained from a solular to those obtained by extrapolation at infinite dilution,[28] tion calorimetry experiment. The first and third thermal events and therefore, solute–solute interactions are absent. correspond to the first and second electrical calibrations, reImportantly, since very dilute solutions of cellobiose are obspectively. In these events, a known amount of heat (Q) is protained by crushing the ampoule, we found that cellobiose is vided to the calorimeter vessel system (glass vessel, solvent, still soluble in ILs or IL/MS in which cellulose is insoluble. For stirrer, and filled ampoule) at a given power (P). In our experithe cases which cellulose is insoluble in a medium, the heat ments, the values of Q and P were 6 J and 200 mW, respectivedetermined by a solution calorimeter does not correspond to ly. From the corresponding DT and monitoring of the baseline, the enthalpy of dissolution, but to the heat of wetting, which the parameters of the calorimeter vessel system are deterdoes not provide any insight into the energetics of the dissolumined. tion process. Hence, the present method provides unpreceThe second thermal event displayed in Figure 2 corresponds dented information for developing the understanding of the to the heat generated or absorbed upon crushing the ampoule interactions of cellulose in solution from both perspectives of and mixing its content into the medium in the calorimeter solvents and non-solvents for cellulose. vessel. After data treatment by using the SolCal software, the In an isoperibolic microcalorimeter, the difference of tempermeasured DT is converted into the heat of mixing. ature (DT) between the reaction vessel and the surroundings In this study, the molar enthalpy of mixture for the dilution correlates with the heat (Q) of the chemical or physical process of a 0.325 mol·kg¢1 cellobiose solution (in DMSO) into a molecoccurring in the calorimeter vessel, as given by Equation (3). ular solvent or IL/MS mixtures (DmixH(CB)) was calculated by Q ¼ C p ¡ DT ð3Þ using Equation (4): where: Cp stands for the heat capacity of the calorimetric vessel system (i.e. calorimetric vessel, its content, stirrer, and filled ampoule). Equation 3 is a simplified description of the phenomena taking place in a solution calorimetry experiment. Formally, the data treatment requires the rate of heating exchange with the surroundings and spurious heat (i.e., thermal drift caused by stirring) to be carefully considered. For such a data treatment, the baseline is monitored before and after each thermal event (calibration or ampoule crushing, as shown in Figure 2). Those readers interested in a detailed description of the formalism for the treatment of data from an isoperibolic solution calorimeter are referred to Ref. [29]. In our experiments, the measured DT values were processed by using a software called SolCal ChemSusChem 2015, 8, 1577 – 1584
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Dmix HðCBÞ ¼
¨ ¦ 1 QCBþDMSO ¢ nDMSO Dmix HðDMSOÞ nCB
ð4Þ
where, nCB and nDMSO stand respectively for the quantity of cellobiose (in mole) and the quantity of DMSO (in mole) contained in ca. 500 mg of a 0.325 mol·kg¢1 solution of cellobiose in DMSO; QCB + DMSO corresponds to the heat of mixing found for the experiments performed with the cellobiose solution in the glass ampoule. DmixH(DMSO)(= QDMSO/nDMSO) is the average value of the enthalpy of mixing of DMSO into a molecular solvent or into an IL/MS mixture; this blank experiment was performed in duplicate for each composition of the IL/MS. Hence, by using Equation (4), the values of DmixH(DMSO) into DMSO (¢0.001 0.001 kJ mol¢1), DMF (0.73 0.08 kJ mol¢1) or
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Full Papers DMI (1.077 0.009 kJ mol¢1) was subtracted from the corresponding raw dataset of DmixH(CB). The presented DmixH(CB) values are the average value of two measurements followed by its respective standard deviations. Effect of molecular solvent and IL mole fraction on DmixH In our first attempts to obtain information on the energetics of interactions between cellobiose and [C4C1Im]Cl, we chose to assess the effect of MS in the behavior of the enthalpy of mixing of a cellobiose solution in DMSO with the [C4C1Im]Cl/ MS solvent systems (where MS: dimethyl sulfoxide, DMSO; N,N-dimethylformamide, DMF; or 1,3-dimethyl-2-imidazolidinone, DMI). Crushing an ampoule containing the solution of CB in DMSO into neat DMSO (cIL = 0) leads to a nearly athermic process (¢0.9 0.2 kJ mol¢1). However, the dilution of the CB/ DMSO solution either into DMF or DMI resulted in considerably endothermic processes (in DMF, 11.5 0.5 kJ mol¢1; in DMI, 15.5 0.5 kJ mol¢1). Displayed in Figure 3 are the profiles of DmixH(CB) with regard to the mole fraction of IL (cIL). The different shape of the pro-
sumes exothermic values. It is worth mentioning that a direct calorimetric determination of DmixH(CB) in [C4C1Im]Cl is not possible, since this IL is a solid at 298 K. However, by extrapolation of the data, the value of DmixH(CB) at cIL = 1 is estimated at ¢11.2 kJ mol¢1. The second general profile of the dependence of DmixH(CB) with cIL is found for both DMF- and DMI-based systems. Here, DmixH(CB) values progressively reduce with cIL. Nonetheless, compared with the DMF-based system, the DMI-based system shows a sharp decrease in DmixH(CB) at cIL = 0.05, followed by a slight reduction in DmixH(CB) for cIL > 0.1. By extrapolation of the data for DMF- and DMI-based system, DmixH(CB) at cIL = 1 are estimated at ¢9.6 and ¢10.3 kJ mol¢1, respectively. Altogether, the three estimated values of DmixH(CB) in [C4C1Im]Cl are very consistent among each other (¢10.4 0.8 kJ mol¢1). To examine whether the data from Figure 3 correlates with the solubility behavior of cellulose in solvent systems at varying cIL, solubility tests of microcrystalline cellulose (Avicel) in [C4C1Im]Cl/DMSO and [C4C1Im]Cl/DMI systems at cIL = 0.1 and cIL = 0.3 were carried out. The later solvent composition corresponds to that in which (10 wt %) Avicel becomes soluble in [C4C1Im]Cl/DMI at 373 K for 15 min.[9] Summarized in Table 1
Table 1. DmixH(CB) values found for the selected solvent systems and the solubility behavior of cellulose.
Figure 3. Heat of mixing of a cellobiose solution with MS (cIL = 0), [C4C1Im]Cl or [C4C1Im][AcO] solutions, determined at 298.150 0.001 K. In the values of DmixH(CB), the heat of mixing of DMSO with each medium was subtracted.
files suggests that the molecular solvent should play a secondary, but yet important role in the energetics of cellulose dissolution in solvent systems containing an IL-comprising an anion with a relatively limited H-bond acceptor capability (i.e. Cl¢). Nonetheless, since the calorimetric data account for the overall pattern of interactions broken and (re)established in the system, no conclusion regarding specific interactions among CB-IL-MS can be directly drawn. Figure 3 also reveals two general profiles of the dependence of DmixH(CB) with cIL. The first is found for DMSO-based systems. In the range of 0 < cIL 0.1, DmixH(CB) increases and assumes endothermic values. A maximum value of DmixH(CB) was detected at a cIL of 0.1 (DmixH(CB) = 3.3 0.2 kJ mol¢1). From cIL > 0.1, DmixH(CB) steadily decrease with cIL. From cIL > 0.3, DmixH(CB) asChemSusChem 2015, 8, 1577 – 1584
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Entry
Solvent system
cIL
DmixH(CB) [kJ mol¢1]
Solubility behavior of Avicel
1 2 3 4
[C4C1Im]Cl/DMI [C4C1Im]Cl/DMI [C4C1Im]Cl/DMSO [C4C1Im]Cl/DMSO
0.10 0.30 0.10 0.31
¢7.25 0.01 ¢7.89 0.09 3.3 0.2 0.3 0.5
Insoluble Soluble Insoluble Soluble
are DmixH(CB) values found for the selected solvent systems and the solubility behavior of cellulose. At cIL = 0.1 and even heating the suspension at 373 K for 1 h, cellulose was insoluble in both solvent systems. However, in both solvent systems containing cIL = 0.3, complete dissolution of 1 wt % of Avicel was achieved at 373 K in less than 10 min. This observation agrees with the importance of a nearly athermal—or even better, a highly exothermic contribution—for driving the spontaneity of cellulose solubility. However, the solubility behavior of cellulose in [C4C1Im]Cl/DMI at cIL = 0.1 does not correlate with the exothermic value of DmixH(CB) (Table 1, entry 1), suggesting that there be other factors accounting for solubility behavior of cellulose that are yet unknown. Presented in Figure 3 are also the DmixH(CB) values found for [C4C1Im][AcO]/DMSO systems. In contrast to the [C4C1Im]Cl/ DMSO systems, the values of DmixH(CB) are much more exothermic in the entire range of cIL. Furthermore, the values of DmixH(CB) exponentially decrease with cIL. Although [C4C1Im] [AcO] is a viscous liquid at 298 K, it was possible to determine the value of DmixH(CB) in the IL with high precision. For [C4C1Im] [AcO], DmixH(CB) is about four times more exothermic (¢38.0 0.5 kJ mol¢1) than that estimated for [C4C1Im]Cl (¢10.4 0.8 kJ mol¢1). Interestingly, the current DmixH(CB) in [C4C1Im][AcO]
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Full Papers (¢38.0 0.5 kJ mol¢1) is similar to the value of enthalpy of cellulose dissolution in 1-ethyl-3-methylimidazolium acetate [C2C1Im][AcO] determined at 353.15 K (¢132 8 J g¢1 or ¢42 3 kJ mol¢1-C12H20O10, where C12H20O10 corresponds to an anhydrocellobiose formula).[30] The results from Figure 3 are in line with the observation that [C4C1Im][AcO] is a much better solvent for cellulose than [C4C1Im]Cl.[5, 6] Most importantly, the data from Figure 3 clearly show that only a minor mole fraction of a cellulose-dissolving IL already suffices for DmixH(CB) to assume highly exothermic values, thus showing evidence supporting cellulose dissolution in IL/MS mixtures with a minor cIL.[9] In fact, the results agree well with the observation that cellulose dissolves in [C2C1Im] [AcO]/DMSO at cIL = 0.08.[9] As will be discussed in the next sections, for the acetate 1-alkyl-3-methylimidazolium ILs, the cation plays a secondary role compared with the anion role in the overall energetics of the system. Impact of the chemical nature of anions on DmixH(CB) To broaden the scope of our analysis, DmixH(CB) values in roomtemperature ionic liquids (RTIL) and the IL/DMSO systems at cIL = 0.5 were measured. Compared in Figure 4 are the values of DmixH(CB) in neat ILs and in IL/DMSO systems. The values of DmixH(CB) found for the IL/MS systems are higher than those found for DmixH(CB) in the corresponding neat IL. In spite of this, the overall trend for DmixH(CB) at cIL = 0.5 is identical to that found for DmixH(CB) at cIL = 1. Moreover, it is possible to identify
Figure 5. Correlations between the DmixH(CB) values (at cIL = 0.5 and 1) and the solvatochromic parameter b.[25] The solid black line is the result of a second-order polynomial fit of the data. An open circle was used for [C4C1Im]Cl in order to indicate that this DmixH(CB) value was determined by extrapolation. The ILs presented are [C4C1Im] + -based ILs.
Figure 4. Heat of mixing of a cellobiose solution into [C4C1Im]X/DMSO solvent systems (cIL = 0.5) and into the neat [C4C1Im]-based ILs (cIL = 1), determined at 298.150 0.001 K. In the values of DmixH(CB), the heat of mixing of DMSO with each medium was subtracted.
the best IL candidates for cellulose dissolution through both trends. Indeed, cellulose is well-known to be soluble in [C4C1Im]Cl, [C4C1Im][(MeO)2PO2], and [C4C1Im][AcO], which lead to values of DmixH(CB) < 0; however, the biopolymer is insoluble in [C4C1Im][NTf2] and [C4C1Im][SCN], which give rise to values of DmixH(CB) > 0. Notably, the values DmixH(CB) at cIL = 0.5 appear ChemSusChem 2015, 8, 1577 – 1584
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to provide reliable data for establishing a thermodynamic rank of affinity of cellobiose towards a medium, without the need to perform the expensive task of screening a large range of cIL in order to estimate DmixH(CB) values at cIL = 1. In order to assess whether the values of DmixH(CB) are correlated to the Kamlet–Taft b parameter, which describes the ability of the ILs to accept H-bonds, the values of DmixH(CB) obtained from the systems (MS, IL/MS, and neat IL) and the b parameter were plotted. The parameter b used for neat IL were collected from ref. [25]. In turn, the b parameter of the DMSO, DMI and DMF were taken from Refs. [9, 44]. For IL/MS systems (at cIL = 0.5), we assumed that the b parameter should have a value similar to the b parameter of the neat IL. This assumption is based on the observation that the evolution of b values with cIL in the [C2C1Im][AcO]/DMI solvent system showed that the solvent mixture has a value of b identical to that of this IL even at a low values of cIL (0.18).[9] Presented in Figure 5 are the correlations between DmixH(CB) values and parameter b. Interestingly, there is a correlation between DmixH(CB) values obtained from neat ILs and their respective b parameters, as indicated by the dashed curve in Figure 5. From this correlation, one may find that a solvent with a b parameter equals to 0.8 should provide an athermic DmixH(CB). Indeed, the nearly athermic value of DmixH(CB) in DMSO verifies this hypothesis. However, the values of DmixH(CB) either in DMF or in DMI do not match the trend found for neat ILs.
Overall, the solvatochromic parameters seem to serve as a rough indicator of the strength of specific interactions involved in the systems comprising neat solvents (i.e. IL or MS). Interestingly, the values of DmixH(CB) in IL/MS systems appear to form another correlation pattern with b parameters. We chose not to indicate this correlation with a trend line in
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Full Papers for [C4C1Im]Cl (6),[26] should be accounting for the spontaneity of cellulose dissolution. Although the interplay of entropic and enthalpic contributions defines the extent to which cellulose is soluble in a medium, the current results agree well with the fact that the solubility of cellulose (cotton linters, DP = 1198) in (1) is about 2.5 times lower than in (6).[5] Moreover, the value of DDmixH(CB) found for the pair (1) and (6) corresponds to 10.7 0.4 kJ mol¢1, demonstrating that (IL-cation)C(2)¢H···OH-(cellulose), as reported in spectroscopy studies,[31–34] provides a considerable contribution to the exothermicity of DmixH(CB) in the system [C4C1Im]Cl-cellobiose. Therefore, the current results also help in understanding the role of the IL-cation structures in the ability of an IL to be a solvent for cellulose, from new perspectives. The current results also offer new evidence for strengthening, and sometimes also limiting, general conclusions from molecular dynamics (MD) simulations. For instance, several MD studies predicted the H-bond of (IL-cation)C(2)¢H···OH-(cellulose) to be of secondary importance for the overall energetics, Contributions of cation structures to the process energetics when compared with (IL-anion)Cl¢···HO¢(cellulose).[16, 35–39] Actually, for the chloride-based ILs (1) and (6), the IL-cation role The thermodynamic role of the IL-cation in the cellulose dissoin the energetics cannot be referred to as a secondary role. lution has been intensively studied, but it is still not fully unThe tentative conclusion that IL-cation plays a secondary role derstood.[5, 6] Motivated by the microcalorimetry results, we dein the energetic of cellulose dissolution is only valid when the cided to examine the effect of the structure of the imidazolium counter ion is an excellent H-bond acceptor (e.g. acetate, cation on DmixH(CB). Summarized in Figure 6 are the DmixH(CB) (MeO)2PO2¢ , as shown in Figure 4). Considering acetatebased ILs, the DDmixH(CB) found for the pair [C4C1Im][AcO] and [C4C1Im]Cl is about 28 kJ mol¢1 (at cIL = 1). Of course, in this case, the IL-cation will assume a secondary role because of the large exothermic contribution of IL-anion to the system energetics. Exothermic values were found for the interaction between cellobiose and an IL solution of the homologous series [CnC1Im]Cl (411). Interestingly, DmixH(CB) values slightly decrease (from ¢2.5 0.2 kJ mol¢1 [4] to ¢3.2 0.3 kJ mol¢1 [6]) with the size of Figure 6. Comparison of heat of mixing of a cellobiose solution in IL/DMSO systems (cIL = 0.5) determined in an the saturated alkyl side-chain isoperibolic precision solution microcalorimeter operating at 298.150 0.001 K. In the values of DmixH(CB), the heat (from C2 to C4). The DmixH(CB) of mixing of DMSO into each solvent system was subtracted. value determined for [C4C1Im]Cl/ DMSO solvent system corresponds to the numerical minimum in the homologous series. values found for selected IL (1–11) solutions in DMSO (cIL = However, since the standard deviation of the experiment is ap0.5). Interestingly, endothermic values of DmixH(CB) were found proximately 0.5 kJ mol¢1 (caused by the high viscosity of the for 1 (7.5 0.4 kJ mol¢1), 2 (5.2 0.5 kJ mol¢1), and 3 (3.2 0.5 kJ mol¢1). We found that cellulose (Avicel) is insoluble in (2) medium), the minimum cannot be statistically confirmed. For saturated alkyl chains higher than C5, DmixH(CB) values increase and (3), while the biopolymer is soluble in (1) at concentrations as high as 10 wt %. Therefore, these results reveal that (from ¢2.3 0.4 kJ mol¢1 [8] to ¢1.0 0.5 kJ mol¢1 [9–11]). The a cellulosic solution in (1) constitutes a unique system in which results from the homologous series agree well with the data a greater entropic contribution, compared to those predicted for cellulose solubility,[40] which also shows an abrupt decrease Figure 5 because such a general trend may lack physical meaning when attempting to grasp information concerning the interaction strength between cellobiose and IL/MS systems based on the b values of IL/MS solutions. Owing to preferential solvation, a similar value of b parameter is often found for both IL and IL/MS (cIL = 0.5),[9] indicating that the solvation environment around the probe in the IL/MS solutions should be similar to the solvation environment found for neat ILs. In contrast to the evolution of b parameter with cIL, DmixH(CB) values do continue to respond to a rise in cIL, as shown in Figure 3. Logically, as the concentration of IL increases in the IL/MS systems, progressive changes in the interaction patterns of IL-IL, MS-MS, CB-MS, and CB-IL happen. Detected by microcalorimetry is the overall balance of these changes, which lead to the non-ideal behavior of the trends shown in Figure 3, for instance. Altogether, the values of DmixH(CB) provide a solid and clear measure of the strength of interaction between CB and IL, IL/MS, or MS.
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Full Papers in cellulose solubility in ILs containing a side-chain higher than C5.[40] Importantly, within the series of [CnC1Im]Cl (4–8), we could not reproduce, either by solubility tests or by the microcalorimetry experiments, the previous observations that higher capacities for cellulose dissolution would be found for the ILs which have an even number of carbons in their alkyl chain substituent, compared to those having an odd number.[6, 41] In solubility tests, we found that [C3C1Im]Cl and [C5C1Im]Cl show a capacity for cellulose dissolution similar to [C4C1Im]Cl. Again, this observation agrees with the data presented in Figure 6 that shows the DmixH(CB) in (5), (6) and (8) to have similar values. Interestingly, the unsaturated side chain CH2CH=CH2 gives rise to the lowest value of DmixH(CB) (7, ¢3.7 0.5 kJ mol¢1) shown in Figure 6. It is known that 7 is a better solvent than [C4C1Im]Cl because of its lower viscosity.[5] Current results also suggest that the energetics of cellulose dissolution should be slightly more favorable in 7 than 6. Interactions between water and cellobiose/IL system Finally, to verify whether solution calorimetry can provide new insight into the effect of water on the energetics of cellulose regeneration, DmixH(CB) values for the mixture of cellobiose solution into binary mixtures of [C4C1Im]Cl and water were determined. Presented in Figure 7 is the dependence of DmixH(CB) as a function of the cwater in the [C4C1Im]Cl/H2O system.
the regeneration is not only driven by an unfavorable entropic contribution, as predicted by MD simulations,[26] but also by the endothermic contribution of DmixH to the enthalpic term of the Gibbs free energy. Interestingly, at cwater 0.5, DmixH(CB) assumes a considerable endothermic value of about 10 kJ mol¢1. This finding is in line with the observation that the presence of an alcohol group in the cation structure also results in a considerably endothermic value of DmixH(CB) (3, Figure 6). These observations suggest that the preferential solvation of Cl¢ anion either by water[6] or by an alcohol substituent in the IL-cation structure should weaken the interaction of cellulose hydroxyl groups with the anion. As a result, cellulose may become insoluble in the medium.
Conclusions In summary, we demonstrated a protocol based on solution microcalorimetry to provide unprecedented information on DmixH(CB). The values of DmixH(CB) represent a solid and clear measure of the solvation power of an IL, IL/MS, and MS on cellobiose. Remarkably, there is a very good correlation between the nature of results found for DmixH(CB) and the solubility behavior of cellulose in ILs or IL/MS. This finding suggests that DmixH(CB) offers a good estimate of the enthalpy of dissolution of cellulose. Moreover, these results provide new insight into which type of ions and structures are more conducive to the solvation of cellulose leading to its dissolution. In a larger picture, the current results constitute a unique dataset for the community to optimize and validate the predictions obtained from molecular dynamics studies on cellulose dissolution and regeneration from its solutions in ILs or IL/MS solvent systems.
Experimental Section Chemicals d-(+ +)-cellobiose (for microbiology, 99.0 %, Fluka) was carefully dried under reduced pressure (10¢7 mbar at 323 K) for 7 days. DMSO (anhydrous, 99.9 %, Aldrich), DMF (anhydrous, 99.8 %, Aldrich), and DMI (purum 99 %, Aldrich), were stored in activated molecular sieves (3 æ). Fresh Milli-Q water was used in order to prepare the [C4C1Im]Cl/H2O solutions. Ionic liquids were used as purchased from IoLiTec GmbH (Germany). Table S1 summarizes the purity level of the ionic liquids. Solution preparation and handling of IL prior to the measurements were performed in a glove box. Figure 7. Comparison of heat of mixing of a cellobiose solution into IL/H2O systems determined in an isoperibolic precision solution microcalorimeter operating at 298.150 0.001 K. In the values of DmixH(CB), the heat of mixing of DMSO into each binary mixture was subtracted.
Heat Determination
Surprisingly, DmixH(CB) assumes endothermic values at cwater > 0.35 (i.e. Cwater > 5 wt %). Again, these results agree well with the fact that water is an anti-solvent when its concentration in the solution of cellulose in IL (e.g. [C4C1Im]Cl) is higher than 35 wt %.[18, 42] Moreover, in the light of the data from Figure 7, we arrive at a new conclusion regarding the regeneration of cellulose by addition of water. The current results indicate that ChemSusChem 2015, 8, 1577 – 1584
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A precision solution microcalorimeter (TAM III, TA Instruments, Figure 1) was used for the determination of the heat of mixing of a cellobiose solution in a medium. The solution microcalorimeter vessel was thermostated at 298.150 0.001 K in a Fluke 7008 calibration bath (with a water volume capacity of 42 L). About 0.50000 g ( 0.00001 g) of a 0.325 mol·kg¢1 solution of cellobiose in DMSO was placed in a glass ampoule (Figure 1 b). The ampoule was sealed with a silicon stopper and beeswax. The ampoule was crushed in the calorimeter vessel containing the medium of interest (25 0000 mL, gravimetrically determined with precision of 0.0001 g). The stirrer was set at 300 rpm. The heat determination
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Full Papers for each sample was performed in duplicate. To determine the heat associated with the ampoule crushing and dilution of the medium with DMSO, the glass ampoule was filled with 0.45000 g ( 0.00001 g) of DMSO, sealed and crushed as described above. The heat determined in this blank experiment was subtracted from the value obtained for the samples of interest. Data treatment to account for thermal drift was performed by using the software SolCal, TA Instruments.
Acknowledgements R.R. is thankful to the Alexander von Humboldt Foundation for the funds provided by the Sofja Kovalevskaja Award 2010 endowed by the Federal Ministry of Education and Research. This work was performed as part of the Cluster of Excellence “Tailor-Made Fuels from Biomass”, funded by the German Federal and State governments to promote science and research at German universities. Keywords: calorimetry · cellulose · dissolution · electrolytes · ionic liquids [1] O. A. El Seoud, A. Koschella, L. C. Fidale, S. Dorn, T. Heinze, Biomacromolecules 2007, 8, 2629 – 2647. [2] D. Klemm, B. Heublein, H.-P. Fink, A. Bohn, Angew. Chem. Int. Ed. 2005, 44, 3358 – 3393; Angew. Chem. 2005, 117, 3422 – 3458. [3] X. Qui, S. Hu, Materials 2005, 6, 738 – 781. [4] P. Domnguez de Mara, J. Chem. Technol. Biotechnol. 2014, 89, 11 – 18. [5] H. Wang, G. Gurau, R. D. Rogers, Chem. Soc. Rev. 2012, 41, 1519 – 1537. [6] N. Sun, H. Rodriguez, M. Rahman, R. D. Rogers, Chem. Commun. 2011, 47, 1405 – 1421. [7] A. F. Turbak, R. B. Hammer, R. E. Davies, H. L. Hergert, Chemtech 1980, 10, 51 – 57. [8] A. Pinkert, K. N. Marsh, S. Pang, M. P. Staiger, Chem. Rev. 2009, 109, 6712 – 6728. [9] R. Rinaldi, Chem. Commun. 2011, 47, 511 – 513. [10] R. Rinaldi, J. Chem. Eng. Data 2012, 57, 1341 – 1343. [11] A. Pinkert, J. Chem. Eng. Data 2012, 57, 1338 – 1340. [12] J.-M. Andanson, E. Bordes, J. Devemy, F. Leroux, A. A. H. Padua, M. F. C. Gomes, Green Chem. 2014, 16, 2528 – 2538. [13] Y. Zhao, X. Liu, J. Wang, S. Zhang, J. Phys. Chem. B 2013, 117, 9042 – 9049. [14] B. D. Rabideau, A. Agarwal, A. E. Ismail, J. Phys. Chem. B 2014, 118, 1621 – 1629. [15] H. Liu, K. L. Sale, B. M. Holmes, B. A. Simmons, S. Singh, J. Phys. Chem. B 2010, 114, 4293 – 4301. [16] R. S. Payal, R. Bharath, G. Periyasamy, S. Balasubramanian, J. Phys. Chem. B 2012, 116, 833 – 840. [17] A. Casas, S. Omar, J. Palomar, M. Oliet, M. V. Alonso, F. Rodriguez, RSC Adv. 2013, 3, 3453 – 3460.
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[18] Z.-D. Ding, Z. Chi, W.-X. Gu, S.-M. Gu, J.-H. Liu, H.-J. Wang, Carbohydr. Polym. 2012, 89, 7 – 16. [19] Y. Zhao, X. Liu, J. Wang, S. Zhang, Carbohydr. Polym. 2013, 94, 723 – 730. [20] A. Brandt, J. P. Hallett, D. J. Leak, R. J. Murphy, T. Welton, Green Chem. 2010, 12, 672 – 679. [21] Y. Fukaya, K. Hayashi, M. Wada, H. Ohno, Green Chem. 2008, 10, 44 – 46. [22] L. Crowhurst, P. R. Mawdsley, J. M. Perez-Arlandis, P. A. Salter, T. Welton, Phys. Chem. Chem. Phys. 2003, 5, 2790 – 2794. [23] M. Zavrel, D. Bross, M. Funke, J. Bìchs, A. C. Spiess, Bioresour. Technol. 2009, 100, 2580 – 2587. [24] A. Brandt, J. Grasvik, J. P. Hallett, T. Welton, Green Chem. 2013, 15, 550 – 583. [25] A. F. M. Cludio, L. Swift, J. P. Hallett, T. Welton, J. A. P. Coutinho, M. G. Freire, Phys. Chem. Chem. Phys. 2014, 16, 6593 – 6601. [26] A. S. Gross, A. T. Bell, J.-W. Chu, Phys. Chem. Chem. Phys. 2012, 14, 8425 – 8430. [27] H. Parviainen, A. Parviainen, T. Virtanen, I. Kilpelinen, P. Ahvenainen, R. Serimaa, S. Grçnqvist, T. Maloney, S. L. Maunu, Carbohydr. Polym. 2014, 113, 67 – 76. [28] J. B. Taylor, Trans. Faraday Soc. 1957, 53, 1198 – 1203. [29] S. Sunner in Combustion Calorimetry (Eds.: S. Sunner, M. Mansson), Pergamon, 1979, pp. 13 – 34. [30] J.-M. Andanson, A. A. H. Pdua, M. F. Costa Gomes, Chem. Commun. 2015, 51, 4485 – 4487. [31] R. C. Remsing, I. D. Petrik, Z. Liu, G. Moyna, Phys. Chem. Chem. Phys. 2010, 12, 14827 – 14828. [32] R. C. Remsing, R. P. Swatloski, R. D. Rogers, G. Moyna, Chem. Commun. 2006, 1271 – 1273. [33] J. Zhang, H. Zhang, J. Wu, J. Zhang, J. He, J. Xiang, Phys. Chem. Chem. Phys. 2010, 12, 14829 – 14830. [34] J. Zhang, H. Zhang, J. Wu, J. Zhang, J. He, J. Xiang, Phys. Chem. Chem. Phys. 2010, 12, 1941 – 1947. [35] H. M. Cho, A. S. Gross, J.-W. Chu, J. Am. Chem. Soc. 2011, 133, 14033 – 14041. [36] H. Liu, K. L. Sale, B. A. Simmons, S. Singh, J. Phys. Chem. B 2011, 115, 10251 – 10258. [37] Z. Liu, C. Remsing Richard, B. Moore Preston, G. Moyna in Ionic Liquids IV, Vol. 975, American Chemical Society, 2007, pp. 335 – 350. [38] B. G. Janesko, Phys. Chem. Chem. Phys. 2011, 13, 11393 – 11401. [39] T. G. A. Youngs, C. Hardacre, J. D. Holbrey, J. Phys. Chem. B 2007, 111, 13765 – 13774. [40] R. P. Swatloski, S. K. Spear, J. D. Holbrey, R. D. Rogers, J. Am. Chem. Soc. 2002, 124, 4974 – 4975. [41] T. Erdmenger, C. Haensch, R. Hoogenboom, U. S. Schubert, Macromol. Biosci. 2007, 7, 440 – 445. [42] L. K. J. Hauru, M. Hummel, A. W. T. King, I. Kilpelainen, H. Sixta, Biomacromolecules 2012, 13, 2896 – 2905. [43] J. Vitz, T. Erdmenger, C. Haensch, U. S. Schubert, Green Chem. 2009, 11, 417 – 424. [44] Y. Marcus, Chem. Soc. Rev. 1993, 22, 409 – 416.
Received: February 20, 2015 Published online on April 8, 2015
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DOI: 10.1002/cssc.201500272
Understanding Cellulose Dissolution: Energetics of Interactions of Ionic Liquids and Cellobiose Revealed by Solution Microcalorimetry Heitor Fernando Nunes de Oliveira and Roberto Rinaldi*[a] In this report, the interactions between fifteen selected ionic liquids (ILs) and cellobiose (CB) are examined by high-precision solution microcalorimetry. The heat of mixing (DmixH) of CB and ILs, or CB and IL/molecular solvent (MS) solutions, provides the first ever-published measure of the affinity of CB with ILs. Most importantly, we found that there is a very good correlation between the nature of the results found for DmixH(CB) and the sol-
ubility behavior of cellulose. This correlation suggests that DmixH(CB) offers a good estimate of the enthalpy of dissolution of cellulose even in solvents in which cellulose is insoluble. Therefore, the current findings open up new horizons for unravelling the intricacies of the thermodynamic factors accounting for the spontaneity of cellulose dissolution in ILs or IL/MS solutions.
Introduction Native cellulose does not always exhibit the desired properties for its application as a high-performance material.[1–6] Accordingly, to tailor the biopolymer properties, it is often necessary to dissolve the biopolymer and controllably recover the processed cellulose from its solutions.[7] Several ionic liquids (ILs) were demonstrated to be excellent solvents for cellulose.[5, 6, 8] In 2011, we reported solutions of cellulose-dissolving ILs in molecular solvents (e.g. DMSO), giving even better performance in the dissolution of cellulose than for the ILs themselves.[9] Perhaps one of the most thought-provoking advantages of these solutions over neat ILs is the fact that only a minor mole-fraction of an IL (cIL) is required for cellulose to dissolve instantaneously at 373 K.[10, 11] Regarding the improvement of dissolution kinetics, several studies reported that the mass transport of the “real” solvent (i.e., cellulose-dissolving ILs) is enhanced when the dissolution is performed in a solution of IL in a molecular solvent (MS).[12–14] However, the dissolution process is not only governed by kinetics, but also by thermodynamics. Concerning the process energetics, most of the information on the interactions between IL-ions and cellulose stems either from molecular dynamics simulations[15–19] or from the analysis of solvatochromic parameters.[20–23] As regards the latter approach, a criticism was made on the unsuitability of solvatochromic probes in reporting preferential solvation of ILs.[24] Moreover, the solvatochromic probes are sensitive to measurement conditions and procedures employed.[25] Therefore, we identified a real need for the development of a calorimetric protocol for the determination of the affinity of a molecular probe related to the cellulose structure, that is, cellobiose, with [a] H. F. N. de Oliveira, Dr. R. Rinaldi Max-Planck-Institut fìr Kohlenforschung Kaiser-Wilhelm-Platz 1, 45470 Mìlheim (Ruhr) (Germany) E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201500272.
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ILs or IL/MS solvent systems, providing in-depth insight into the overall energetics of cellulose in solution. To understand the reason why the energetics of interactions between cellulose and ILs or IL/MS systems is of importance for cellulose solubility, some considerations should be made. Logically, dissolution of cellulose is a spontaneous process only if the interplay of entropic and enthalpic contributions results in a negative value of Gibbs free energy at a given temperature and pressure. In the dissolution of cellulose, the total entropy change is a balance of solvent entropy reduction (caused by the cellulose solvation), and solute entropy gain (due to increased conformational freedom of the polymeric chains in solution). Through molecular dynamic simulations, Chu et al.[26] investigated the entropy of cellulose dissolution in 1-butyl-3methylimidazolium chloride ([C4C1Im]Cl) and water. Interestingly, solvent entropy reduction in [C4C1Im]Cl was found to be smaller than that in water, and therefore, to counteract the solute entropy gain to a lesser extent.[26] They predicted the term TDS to assume a positive value of about 4–6 kJ mol¢1glucan in the cellulose dissolution in [C4C1Im]Cl (at temperatures of 425, 450, 500 K). In this manner, at a given temperature and pressure, should the overall change in enthalpy be highly endothermic, the term TDS may not suffice to counterbalance the resulting total (endothermic) DH, and thus, the process will not be spontaneous. Hildebrand theory for polymer dissolution describes the total change in enthalpy as the enthalpy of mixing (DmixH), which is given by Equation (1): Dmix H ¼ Vmix ðd1 ¢ d2 Þ2 1 2
d¼
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1=2 Dvap H ¢ RT 1=2 E ¼ V V
ð1Þ
ð2Þ
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Full Papers where, Vmix is the mixture volume, f1 and f2 are the volume fractions of the two components, and d is the solubility parameter (or also called Hildebrand parameter). For Equation (2), the term E/V stands for the cohesive energy density, DvapH is the latent heat of vaporization, R is the gas constant, T is the absolute temperature in Kelvin for the solvent vaporization, and V is the molar volume of the component. From Equation (1), the condition d1 d2 is required in order to obtain an enthalpy of mixing near zero, and thus, DG < 0. As defined by Equation (2), the d parameter is directly related to the cohesive energy (E). For a solvent, the physical meaning of this term is the energy needed for a molecule to be removed from its nearest neighbor, forming a cavity in the solvent for insertion of solute molecules. For a polymer, the cohesive energy is the energy required for all intermolecular interactions between the polymeric chains to be disrupted. Noteworthy, Hildebrand parameters do not account for specific interactions (e.g. hydrogen bonding). Although this limitation poses a problem for the prediction of cellulose solubility through the direct comparison of Hildebrand parameters, the rationale of the theory is still valid for describing the general thermodynamic condition required for the spontaneous dissolution of a polymer (i.e. a value of DmixH 0, assuming that DmixS > 0). Herein, we introduce a solution microcalorimetry protocol for the determination of the DmixH(CB) in ILs or IL/MS systems. We demonstrate that there is a direct correlation between DmixH(CB) values and the solubility behavior of cellulose. This finding indicates that cellobiose is a suitable probe molecule to estimate the solvation power of ILs on cellulose. This paper is organized as follows. First, the criteria for the design of solution microcalorimetry experiments are introduced. Thereafter, the effects of molecular solvent and cIL on DmixH(CB) are assessed. Next, the impact of the chemical nature of anions on DmixH(CB) is examined. In addition, the contributions of cation
structures to the process energetics are analyzed. Finally, the effect of water on the interactions between cellobiose and IL/ MS is addressed.
Results and Discussion On the criteria for the experimental design Solution calorimeters are the classical instruments used in order to determine the heat absorbed or released upon dissolution or mixing of a solute into a solvent or medium. Shown in Figure 1 a is a schematic representation of an isoperibolic solution-microcalorimeter setup for the determination of DmixH of CB and ILs, or CB and IL/MS solvent systems. Typically, DmixH(CB) values were measured by crushing a glass ampoule against a sapphire edge in the calorimetric vessel (Figure 1 b). By this procedure, the ampoule content—0.5 g of a solution of CB in DMSO at a molal concentration of 0.325 mol·kg¢1—is released into the medium of interest contained in the vessel (25.0 mL, Figure 1 b). Noteworthy, in this type of experiment, both the initial state and the final state of the system are well defined. Logically, this condition constitutes a core requirement for any thermodynamic determination. Recently, Parviainen et al.[27] reported the use of differential scanning calorimetry (DSC) for the determination of the enthalpy of cellulose dissolution in three ILs. The measurements were performed on cellulose pastes in IL. Notably, in this kind of experiment, the initial state of the system cannot be defined because the biopolymer is already undergoing physical transformations (e.g. swelling and decrystallization) upon mixing cellulose and ILs for the sample preparation for the DSC analysis. A clear indication for this (arte)fact is that the heat values determined by using a DSC were insensitive to changes in the sample crystallinity.[27] Moreover, they reported that partial dissolution of cellulose and remnant structures of the fiber wall
Figure 1. (a) Details of a high-precision solution microcalorimeter (TA instruments), and (b) the calorimetric vessel. For clarity, the calorimetric vessel and glass ampoule were filled with colored solutions, which do not correspond to any of the ILs or solutions studied in this work.
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Full Papers did not contribute to the DH value.[27] Of course, since enthalpy is an extensive thermodynamic property, any measurement of the value for enthalpy of cellulose dissolution must provide results that respond to the amount of cellulose dissolved in the medium. Therefore, in the best-case scenario, DSC appears to provide only a very rough measure of the heat of cellulose dissolution. Regarding our method, the use of cellobiose solutions, instead of cellulose, enables fast mixing of the components. Furthermore, the final cellobiose Figure 2. Typical results from a solution-calorimetry experiment performed on a high-precision solution microcaconcentration, achieved by lorimeter (TA instruments). crushing the ampoule in the calorimetric vessel, was 6·10¢3 mol·kg¢1. For such a dilute solution, the values of the (which is provided with a TAM III calorimeter by TA Instruments). enthalpy of cellobiose dissolution in water at 298.15 K are simiDisplayed in Figure 2 is a typical result obtained from a solular to those obtained by extrapolation at infinite dilution,[28] tion calorimetry experiment. The first and third thermal events and therefore, solute–solute interactions are absent. correspond to the first and second electrical calibrations, reImportantly, since very dilute solutions of cellobiose are obspectively. In these events, a known amount of heat (Q) is protained by crushing the ampoule, we found that cellobiose is vided to the calorimeter vessel system (glass vessel, solvent, still soluble in ILs or IL/MS in which cellulose is insoluble. For stirrer, and filled ampoule) at a given power (P). In our experithe cases which cellulose is insoluble in a medium, the heat ments, the values of Q and P were 6 J and 200 mW, respectivedetermined by a solution calorimeter does not correspond to ly. From the corresponding DT and monitoring of the baseline, the enthalpy of dissolution, but to the heat of wetting, which the parameters of the calorimeter vessel system are deterdoes not provide any insight into the energetics of the dissolumined. tion process. Hence, the present method provides unpreceThe second thermal event displayed in Figure 2 corresponds dented information for developing the understanding of the to the heat generated or absorbed upon crushing the ampoule interactions of cellulose in solution from both perspectives of and mixing its content into the medium in the calorimeter solvents and non-solvents for cellulose. vessel. After data treatment by using the SolCal software, the In an isoperibolic microcalorimeter, the difference of tempermeasured DT is converted into the heat of mixing. ature (DT) between the reaction vessel and the surroundings In this study, the molar enthalpy of mixture for the dilution correlates with the heat (Q) of the chemical or physical process of a 0.325 mol·kg¢1 cellobiose solution (in DMSO) into a molecoccurring in the calorimeter vessel, as given by Equation (3). ular solvent or IL/MS mixtures (DmixH(CB)) was calculated by Q ¼ C p ¡ DT ð3Þ using Equation (4): where: Cp stands for the heat capacity of the calorimetric vessel system (i.e. calorimetric vessel, its content, stirrer, and filled ampoule). Equation 3 is a simplified description of the phenomena taking place in a solution calorimetry experiment. Formally, the data treatment requires the rate of heating exchange with the surroundings and spurious heat (i.e., thermal drift caused by stirring) to be carefully considered. For such a data treatment, the baseline is monitored before and after each thermal event (calibration or ampoule crushing, as shown in Figure 2). Those readers interested in a detailed description of the formalism for the treatment of data from an isoperibolic solution calorimeter are referred to Ref. [29]. In our experiments, the measured DT values were processed by using a software called SolCal ChemSusChem 2015, 8, 1577 – 1584
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Dmix HðCBÞ ¼
¨ ¦ 1 QCBþDMSO ¢ nDMSO Dmix HðDMSOÞ nCB
ð4Þ
where, nCB and nDMSO stand respectively for the quantity of cellobiose (in mole) and the quantity of DMSO (in mole) contained in ca. 500 mg of a 0.325 mol·kg¢1 solution of cellobiose in DMSO; QCB + DMSO corresponds to the heat of mixing found for the experiments performed with the cellobiose solution in the glass ampoule. DmixH(DMSO)(= QDMSO/nDMSO) is the average value of the enthalpy of mixing of DMSO into a molecular solvent or into an IL/MS mixture; this blank experiment was performed in duplicate for each composition of the IL/MS. Hence, by using Equation (4), the values of DmixH(DMSO) into DMSO (¢0.001 0.001 kJ mol¢1), DMF (0.73 0.08 kJ mol¢1) or
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Full Papers DMI (1.077 0.009 kJ mol¢1) was subtracted from the corresponding raw dataset of DmixH(CB). The presented DmixH(CB) values are the average value of two measurements followed by its respective standard deviations. Effect of molecular solvent and IL mole fraction on DmixH In our first attempts to obtain information on the energetics of interactions between cellobiose and [C4C1Im]Cl, we chose to assess the effect of MS in the behavior of the enthalpy of mixing of a cellobiose solution in DMSO with the [C4C1Im]Cl/ MS solvent systems (where MS: dimethyl sulfoxide, DMSO; N,N-dimethylformamide, DMF; or 1,3-dimethyl-2-imidazolidinone, DMI). Crushing an ampoule containing the solution of CB in DMSO into neat DMSO (cIL = 0) leads to a nearly athermic process (¢0.9 0.2 kJ mol¢1). However, the dilution of the CB/ DMSO solution either into DMF or DMI resulted in considerably endothermic processes (in DMF, 11.5 0.5 kJ mol¢1; in DMI, 15.5 0.5 kJ mol¢1). Displayed in Figure 3 are the profiles of DmixH(CB) with regard to the mole fraction of IL (cIL). The different shape of the pro-
sumes exothermic values. It is worth mentioning that a direct calorimetric determination of DmixH(CB) in [C4C1Im]Cl is not possible, since this IL is a solid at 298 K. However, by extrapolation of the data, the value of DmixH(CB) at cIL = 1 is estimated at ¢11.2 kJ mol¢1. The second general profile of the dependence of DmixH(CB) with cIL is found for both DMF- and DMI-based systems. Here, DmixH(CB) values progressively reduce with cIL. Nonetheless, compared with the DMF-based system, the DMI-based system shows a sharp decrease in DmixH(CB) at cIL = 0.05, followed by a slight reduction in DmixH(CB) for cIL > 0.1. By extrapolation of the data for DMF- and DMI-based system, DmixH(CB) at cIL = 1 are estimated at ¢9.6 and ¢10.3 kJ mol¢1, respectively. Altogether, the three estimated values of DmixH(CB) in [C4C1Im]Cl are very consistent among each other (¢10.4 0.8 kJ mol¢1). To examine whether the data from Figure 3 correlates with the solubility behavior of cellulose in solvent systems at varying cIL, solubility tests of microcrystalline cellulose (Avicel) in [C4C1Im]Cl/DMSO and [C4C1Im]Cl/DMI systems at cIL = 0.1 and cIL = 0.3 were carried out. The later solvent composition corresponds to that in which (10 wt %) Avicel becomes soluble in [C4C1Im]Cl/DMI at 373 K for 15 min.[9] Summarized in Table 1
Table 1. DmixH(CB) values found for the selected solvent systems and the solubility behavior of cellulose.
Figure 3. Heat of mixing of a cellobiose solution with MS (cIL = 0), [C4C1Im]Cl or [C4C1Im][AcO] solutions, determined at 298.150 0.001 K. In the values of DmixH(CB), the heat of mixing of DMSO with each medium was subtracted.
files suggests that the molecular solvent should play a secondary, but yet important role in the energetics of cellulose dissolution in solvent systems containing an IL-comprising an anion with a relatively limited H-bond acceptor capability (i.e. Cl¢). Nonetheless, since the calorimetric data account for the overall pattern of interactions broken and (re)established in the system, no conclusion regarding specific interactions among CB-IL-MS can be directly drawn. Figure 3 also reveals two general profiles of the dependence of DmixH(CB) with cIL. The first is found for DMSO-based systems. In the range of 0 < cIL 0.1, DmixH(CB) increases and assumes endothermic values. A maximum value of DmixH(CB) was detected at a cIL of 0.1 (DmixH(CB) = 3.3 0.2 kJ mol¢1). From cIL > 0.1, DmixH(CB) steadily decrease with cIL. From cIL > 0.3, DmixH(CB) asChemSusChem 2015, 8, 1577 – 1584
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Entry
Solvent system
cIL
DmixH(CB) [kJ mol¢1]
Solubility behavior of Avicel
1 2 3 4
[C4C1Im]Cl/DMI [C4C1Im]Cl/DMI [C4C1Im]Cl/DMSO [C4C1Im]Cl/DMSO
0.10 0.30 0.10 0.31
¢7.25 0.01 ¢7.89 0.09 3.3 0.2 0.3 0.5
Insoluble Soluble Insoluble Soluble
are DmixH(CB) values found for the selected solvent systems and the solubility behavior of cellulose. At cIL = 0.1 and even heating the suspension at 373 K for 1 h, cellulose was insoluble in both solvent systems. However, in both solvent systems containing cIL = 0.3, complete dissolution of 1 wt % of Avicel was achieved at 373 K in less than 10 min. This observation agrees with the importance of a nearly athermal—or even better, a highly exothermic contribution—for driving the spontaneity of cellulose solubility. However, the solubility behavior of cellulose in [C4C1Im]Cl/DMI at cIL = 0.1 does not correlate with the exothermic value of DmixH(CB) (Table 1, entry 1), suggesting that there be other factors accounting for solubility behavior of cellulose that are yet unknown. Presented in Figure 3 are also the DmixH(CB) values found for [C4C1Im][AcO]/DMSO systems. In contrast to the [C4C1Im]Cl/ DMSO systems, the values of DmixH(CB) are much more exothermic in the entire range of cIL. Furthermore, the values of DmixH(CB) exponentially decrease with cIL. Although [C4C1Im] [AcO] is a viscous liquid at 298 K, it was possible to determine the value of DmixH(CB) in the IL with high precision. For [C4C1Im] [AcO], DmixH(CB) is about four times more exothermic (¢38.0 0.5 kJ mol¢1) than that estimated for [C4C1Im]Cl (¢10.4 0.8 kJ mol¢1). Interestingly, the current DmixH(CB) in [C4C1Im][AcO]
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Full Papers (¢38.0 0.5 kJ mol¢1) is similar to the value of enthalpy of cellulose dissolution in 1-ethyl-3-methylimidazolium acetate [C2C1Im][AcO] determined at 353.15 K (¢132 8 J g¢1 or ¢42 3 kJ mol¢1-C12H20O10, where C12H20O10 corresponds to an anhydrocellobiose formula).[30] The results from Figure 3 are in line with the observation that [C4C1Im][AcO] is a much better solvent for cellulose than [C4C1Im]Cl.[5, 6] Most importantly, the data from Figure 3 clearly show that only a minor mole fraction of a cellulose-dissolving IL already suffices for DmixH(CB) to assume highly exothermic values, thus showing evidence supporting cellulose dissolution in IL/MS mixtures with a minor cIL.[9] In fact, the results agree well with the observation that cellulose dissolves in [C2C1Im] [AcO]/DMSO at cIL = 0.08.[9] As will be discussed in the next sections, for the acetate 1-alkyl-3-methylimidazolium ILs, the cation plays a secondary role compared with the anion role in the overall energetics of the system. Impact of the chemical nature of anions on DmixH(CB) To broaden the scope of our analysis, DmixH(CB) values in roomtemperature ionic liquids (RTIL) and the IL/DMSO systems at cIL = 0.5 were measured. Compared in Figure 4 are the values of DmixH(CB) in neat ILs and in IL/DMSO systems. The values of DmixH(CB) found for the IL/MS systems are higher than those found for DmixH(CB) in the corresponding neat IL. In spite of this, the overall trend for DmixH(CB) at cIL = 0.5 is identical to that found for DmixH(CB) at cIL = 1. Moreover, it is possible to identify
Figure 5. Correlations between the DmixH(CB) values (at cIL = 0.5 and 1) and the solvatochromic parameter b.[25] The solid black line is the result of a second-order polynomial fit of the data. An open circle was used for [C4C1Im]Cl in order to indicate that this DmixH(CB) value was determined by extrapolation. The ILs presented are [C4C1Im] + -based ILs.
Figure 4. Heat of mixing of a cellobiose solution into [C4C1Im]X/DMSO solvent systems (cIL = 0.5) and into the neat [C4C1Im]-based ILs (cIL = 1), determined at 298.150 0.001 K. In the values of DmixH(CB), the heat of mixing of DMSO with each medium was subtracted.
the best IL candidates for cellulose dissolution through both trends. Indeed, cellulose is well-known to be soluble in [C4C1Im]Cl, [C4C1Im][(MeO)2PO2], and [C4C1Im][AcO], which lead to values of DmixH(CB) < 0; however, the biopolymer is insoluble in [C4C1Im][NTf2] and [C4C1Im][SCN], which give rise to values of DmixH(CB) > 0. Notably, the values DmixH(CB) at cIL = 0.5 appear ChemSusChem 2015, 8, 1577 – 1584
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to provide reliable data for establishing a thermodynamic rank of affinity of cellobiose towards a medium, without the need to perform the expensive task of screening a large range of cIL in order to estimate DmixH(CB) values at cIL = 1. In order to assess whether the values of DmixH(CB) are correlated to the Kamlet–Taft b parameter, which describes the ability of the ILs to accept H-bonds, the values of DmixH(CB) obtained from the systems (MS, IL/MS, and neat IL) and the b parameter were plotted. The parameter b used for neat IL were collected from ref. [25]. In turn, the b parameter of the DMSO, DMI and DMF were taken from Refs. [9, 44]. For IL/MS systems (at cIL = 0.5), we assumed that the b parameter should have a value similar to the b parameter of the neat IL. This assumption is based on the observation that the evolution of b values with cIL in the [C2C1Im][AcO]/DMI solvent system showed that the solvent mixture has a value of b identical to that of this IL even at a low values of cIL (0.18).[9] Presented in Figure 5 are the correlations between DmixH(CB) values and parameter b. Interestingly, there is a correlation between DmixH(CB) values obtained from neat ILs and their respective b parameters, as indicated by the dashed curve in Figure 5. From this correlation, one may find that a solvent with a b parameter equals to 0.8 should provide an athermic DmixH(CB). Indeed, the nearly athermic value of DmixH(CB) in DMSO verifies this hypothesis. However, the values of DmixH(CB) either in DMF or in DMI do not match the trend found for neat ILs.
Overall, the solvatochromic parameters seem to serve as a rough indicator of the strength of specific interactions involved in the systems comprising neat solvents (i.e. IL or MS). Interestingly, the values of DmixH(CB) in IL/MS systems appear to form another correlation pattern with b parameters. We chose not to indicate this correlation with a trend line in
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Full Papers for [C4C1Im]Cl (6),[26] should be accounting for the spontaneity of cellulose dissolution. Although the interplay of entropic and enthalpic contributions defines the extent to which cellulose is soluble in a medium, the current results agree well with the fact that the solubility of cellulose (cotton linters, DP = 1198) in (1) is about 2.5 times lower than in (6).[5] Moreover, the value of DDmixH(CB) found for the pair (1) and (6) corresponds to 10.7 0.4 kJ mol¢1, demonstrating that (IL-cation)C(2)¢H···OH-(cellulose), as reported in spectroscopy studies,[31–34] provides a considerable contribution to the exothermicity of DmixH(CB) in the system [C4C1Im]Cl-cellobiose. Therefore, the current results also help in understanding the role of the IL-cation structures in the ability of an IL to be a solvent for cellulose, from new perspectives. The current results also offer new evidence for strengthening, and sometimes also limiting, general conclusions from molecular dynamics (MD) simulations. For instance, several MD studies predicted the H-bond of (IL-cation)C(2)¢H···OH-(cellulose) to be of secondary importance for the overall energetics, Contributions of cation structures to the process energetics when compared with (IL-anion)Cl¢···HO¢(cellulose).[16, 35–39] Actually, for the chloride-based ILs (1) and (6), the IL-cation role The thermodynamic role of the IL-cation in the cellulose dissoin the energetics cannot be referred to as a secondary role. lution has been intensively studied, but it is still not fully unThe tentative conclusion that IL-cation plays a secondary role derstood.[5, 6] Motivated by the microcalorimetry results, we dein the energetic of cellulose dissolution is only valid when the cided to examine the effect of the structure of the imidazolium counter ion is an excellent H-bond acceptor (e.g. acetate, cation on DmixH(CB). Summarized in Figure 6 are the DmixH(CB) (MeO)2PO2¢ , as shown in Figure 4). Considering acetatebased ILs, the DDmixH(CB) found for the pair [C4C1Im][AcO] and [C4C1Im]Cl is about 28 kJ mol¢1 (at cIL = 1). Of course, in this case, the IL-cation will assume a secondary role because of the large exothermic contribution of IL-anion to the system energetics. Exothermic values were found for the interaction between cellobiose and an IL solution of the homologous series [CnC1Im]Cl (411). Interestingly, DmixH(CB) values slightly decrease (from ¢2.5 0.2 kJ mol¢1 [4] to ¢3.2 0.3 kJ mol¢1 [6]) with the size of Figure 6. Comparison of heat of mixing of a cellobiose solution in IL/DMSO systems (cIL = 0.5) determined in an the saturated alkyl side-chain isoperibolic precision solution microcalorimeter operating at 298.150 0.001 K. In the values of DmixH(CB), the heat (from C2 to C4). The DmixH(CB) of mixing of DMSO into each solvent system was subtracted. value determined for [C4C1Im]Cl/ DMSO solvent system corresponds to the numerical minimum in the homologous series. values found for selected IL (1–11) solutions in DMSO (cIL = However, since the standard deviation of the experiment is ap0.5). Interestingly, endothermic values of DmixH(CB) were found proximately 0.5 kJ mol¢1 (caused by the high viscosity of the for 1 (7.5 0.4 kJ mol¢1), 2 (5.2 0.5 kJ mol¢1), and 3 (3.2 0.5 kJ mol¢1). We found that cellulose (Avicel) is insoluble in (2) medium), the minimum cannot be statistically confirmed. For saturated alkyl chains higher than C5, DmixH(CB) values increase and (3), while the biopolymer is soluble in (1) at concentrations as high as 10 wt %. Therefore, these results reveal that (from ¢2.3 0.4 kJ mol¢1 [8] to ¢1.0 0.5 kJ mol¢1 [9–11]). The a cellulosic solution in (1) constitutes a unique system in which results from the homologous series agree well with the data a greater entropic contribution, compared to those predicted for cellulose solubility,[40] which also shows an abrupt decrease Figure 5 because such a general trend may lack physical meaning when attempting to grasp information concerning the interaction strength between cellobiose and IL/MS systems based on the b values of IL/MS solutions. Owing to preferential solvation, a similar value of b parameter is often found for both IL and IL/MS (cIL = 0.5),[9] indicating that the solvation environment around the probe in the IL/MS solutions should be similar to the solvation environment found for neat ILs. In contrast to the evolution of b parameter with cIL, DmixH(CB) values do continue to respond to a rise in cIL, as shown in Figure 3. Logically, as the concentration of IL increases in the IL/MS systems, progressive changes in the interaction patterns of IL-IL, MS-MS, CB-MS, and CB-IL happen. Detected by microcalorimetry is the overall balance of these changes, which lead to the non-ideal behavior of the trends shown in Figure 3, for instance. Altogether, the values of DmixH(CB) provide a solid and clear measure of the strength of interaction between CB and IL, IL/MS, or MS.
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Full Papers in cellulose solubility in ILs containing a side-chain higher than C5.[40] Importantly, within the series of [CnC1Im]Cl (4–8), we could not reproduce, either by solubility tests or by the microcalorimetry experiments, the previous observations that higher capacities for cellulose dissolution would be found for the ILs which have an even number of carbons in their alkyl chain substituent, compared to those having an odd number.[6, 41] In solubility tests, we found that [C3C1Im]Cl and [C5C1Im]Cl show a capacity for cellulose dissolution similar to [C4C1Im]Cl. Again, this observation agrees with the data presented in Figure 6 that shows the DmixH(CB) in (5), (6) and (8) to have similar values. Interestingly, the unsaturated side chain CH2CH=CH2 gives rise to the lowest value of DmixH(CB) (7, ¢3.7 0.5 kJ mol¢1) shown in Figure 6. It is known that 7 is a better solvent than [C4C1Im]Cl because of its lower viscosity.[5] Current results also suggest that the energetics of cellulose dissolution should be slightly more favorable in 7 than 6. Interactions between water and cellobiose/IL system Finally, to verify whether solution calorimetry can provide new insight into the effect of water on the energetics of cellulose regeneration, DmixH(CB) values for the mixture of cellobiose solution into binary mixtures of [C4C1Im]Cl and water were determined. Presented in Figure 7 is the dependence of DmixH(CB) as a function of the cwater in the [C4C1Im]Cl/H2O system.
the regeneration is not only driven by an unfavorable entropic contribution, as predicted by MD simulations,[26] but also by the endothermic contribution of DmixH to the enthalpic term of the Gibbs free energy. Interestingly, at cwater 0.5, DmixH(CB) assumes a considerable endothermic value of about 10 kJ mol¢1. This finding is in line with the observation that the presence of an alcohol group in the cation structure also results in a considerably endothermic value of DmixH(CB) (3, Figure 6). These observations suggest that the preferential solvation of Cl¢ anion either by water[6] or by an alcohol substituent in the IL-cation structure should weaken the interaction of cellulose hydroxyl groups with the anion. As a result, cellulose may become insoluble in the medium.
Conclusions In summary, we demonstrated a protocol based on solution microcalorimetry to provide unprecedented information on DmixH(CB). The values of DmixH(CB) represent a solid and clear measure of the solvation power of an IL, IL/MS, and MS on cellobiose. Remarkably, there is a very good correlation between the nature of results found for DmixH(CB) and the solubility behavior of cellulose in ILs or IL/MS. This finding suggests that DmixH(CB) offers a good estimate of the enthalpy of dissolution of cellulose. Moreover, these results provide new insight into which type of ions and structures are more conducive to the solvation of cellulose leading to its dissolution. In a larger picture, the current results constitute a unique dataset for the community to optimize and validate the predictions obtained from molecular dynamics studies on cellulose dissolution and regeneration from its solutions in ILs or IL/MS solvent systems.
Experimental Section Chemicals d-(+ +)-cellobiose (for microbiology, 99.0 %, Fluka) was carefully dried under reduced pressure (10¢7 mbar at 323 K) for 7 days. DMSO (anhydrous, 99.9 %, Aldrich), DMF (anhydrous, 99.8 %, Aldrich), and DMI (purum 99 %, Aldrich), were stored in activated molecular sieves (3 æ). Fresh Milli-Q water was used in order to prepare the [C4C1Im]Cl/H2O solutions. Ionic liquids were used as purchased from IoLiTec GmbH (Germany). Table S1 summarizes the purity level of the ionic liquids. Solution preparation and handling of IL prior to the measurements were performed in a glove box. Figure 7. Comparison of heat of mixing of a cellobiose solution into IL/H2O systems determined in an isoperibolic precision solution microcalorimeter operating at 298.150 0.001 K. In the values of DmixH(CB), the heat of mixing of DMSO into each binary mixture was subtracted.
Heat Determination
Surprisingly, DmixH(CB) assumes endothermic values at cwater > 0.35 (i.e. Cwater > 5 wt %). Again, these results agree well with the fact that water is an anti-solvent when its concentration in the solution of cellulose in IL (e.g. [C4C1Im]Cl) is higher than 35 wt %.[18, 42] Moreover, in the light of the data from Figure 7, we arrive at a new conclusion regarding the regeneration of cellulose by addition of water. The current results indicate that ChemSusChem 2015, 8, 1577 – 1584
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A precision solution microcalorimeter (TAM III, TA Instruments, Figure 1) was used for the determination of the heat of mixing of a cellobiose solution in a medium. The solution microcalorimeter vessel was thermostated at 298.150 0.001 K in a Fluke 7008 calibration bath (with a water volume capacity of 42 L). About 0.50000 g ( 0.00001 g) of a 0.325 mol·kg¢1 solution of cellobiose in DMSO was placed in a glass ampoule (Figure 1 b). The ampoule was sealed with a silicon stopper and beeswax. The ampoule was crushed in the calorimeter vessel containing the medium of interest (25 0000 mL, gravimetrically determined with precision of 0.0001 g). The stirrer was set at 300 rpm. The heat determination
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Full Papers for each sample was performed in duplicate. To determine the heat associated with the ampoule crushing and dilution of the medium with DMSO, the glass ampoule was filled with 0.45000 g ( 0.00001 g) of DMSO, sealed and crushed as described above. The heat determined in this blank experiment was subtracted from the value obtained for the samples of interest. Data treatment to account for thermal drift was performed by using the software SolCal, TA Instruments.
Acknowledgements R.R. is thankful to the Alexander von Humboldt Foundation for the funds provided by the Sofja Kovalevskaja Award 2010 endowed by the Federal Ministry of Education and Research. This work was performed as part of the Cluster of Excellence “Tailor-Made Fuels from Biomass”, funded by the German Federal and State governments to promote science and research at German universities. Keywords: calorimetry · cellulose · dissolution · electrolytes · ionic liquids [1] O. A. El Seoud, A. Koschella, L. C. Fidale, S. Dorn, T. Heinze, Biomacromolecules 2007, 8, 2629 – 2647. [2] D. Klemm, B. Heublein, H.-P. Fink, A. Bohn, Angew. Chem. Int. Ed. 2005, 44, 3358 – 3393; Angew. Chem. 2005, 117, 3422 – 3458. [3] X. Qui, S. Hu, Materials 2005, 6, 738 – 781. [4] P. Domnguez de Mara, J. Chem. Technol. Biotechnol. 2014, 89, 11 – 18. [5] H. Wang, G. Gurau, R. D. Rogers, Chem. Soc. Rev. 2012, 41, 1519 – 1537. [6] N. Sun, H. Rodriguez, M. Rahman, R. D. Rogers, Chem. Commun. 2011, 47, 1405 – 1421. [7] A. F. Turbak, R. B. Hammer, R. E. Davies, H. L. Hergert, Chemtech 1980, 10, 51 – 57. [8] A. Pinkert, K. N. Marsh, S. Pang, M. P. Staiger, Chem. Rev. 2009, 109, 6712 – 6728. [9] R. Rinaldi, Chem. Commun. 2011, 47, 511 – 513. [10] R. Rinaldi, J. Chem. Eng. Data 2012, 57, 1341 – 1343. [11] A. Pinkert, J. Chem. Eng. Data 2012, 57, 1338 – 1340. [12] J.-M. Andanson, E. Bordes, J. Devemy, F. Leroux, A. A. H. Padua, M. F. C. Gomes, Green Chem. 2014, 16, 2528 – 2538. [13] Y. Zhao, X. Liu, J. Wang, S. Zhang, J. Phys. Chem. B 2013, 117, 9042 – 9049. [14] B. D. Rabideau, A. Agarwal, A. E. Ismail, J. Phys. Chem. B 2014, 118, 1621 – 1629. [15] H. Liu, K. L. Sale, B. M. Holmes, B. A. Simmons, S. Singh, J. Phys. Chem. B 2010, 114, 4293 – 4301. [16] R. S. Payal, R. Bharath, G. Periyasamy, S. Balasubramanian, J. Phys. Chem. B 2012, 116, 833 – 840. [17] A. Casas, S. Omar, J. Palomar, M. Oliet, M. V. Alonso, F. Rodriguez, RSC Adv. 2013, 3, 3453 – 3460.
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[18] Z.-D. Ding, Z. Chi, W.-X. Gu, S.-M. Gu, J.-H. Liu, H.-J. Wang, Carbohydr. Polym. 2012, 89, 7 – 16. [19] Y. Zhao, X. Liu, J. Wang, S. Zhang, Carbohydr. Polym. 2013, 94, 723 – 730. [20] A. Brandt, J. P. Hallett, D. J. Leak, R. J. Murphy, T. Welton, Green Chem. 2010, 12, 672 – 679. [21] Y. Fukaya, K. Hayashi, M. Wada, H. Ohno, Green Chem. 2008, 10, 44 – 46. [22] L. Crowhurst, P. R. Mawdsley, J. M. Perez-Arlandis, P. A. Salter, T. Welton, Phys. Chem. Chem. Phys. 2003, 5, 2790 – 2794. [23] M. Zavrel, D. Bross, M. Funke, J. Bìchs, A. C. Spiess, Bioresour. Technol. 2009, 100, 2580 – 2587. [24] A. Brandt, J. Grasvik, J. P. Hallett, T. Welton, Green Chem. 2013, 15, 550 – 583. [25] A. F. M. Cludio, L. Swift, J. P. Hallett, T. Welton, J. A. P. Coutinho, M. G. Freire, Phys. Chem. Chem. Phys. 2014, 16, 6593 – 6601. [26] A. S. Gross, A. T. Bell, J.-W. Chu, Phys. Chem. Chem. Phys. 2012, 14, 8425 – 8430. [27] H. Parviainen, A. Parviainen, T. Virtanen, I. Kilpelinen, P. Ahvenainen, R. Serimaa, S. Grçnqvist, T. Maloney, S. L. Maunu, Carbohydr. Polym. 2014, 113, 67 – 76. [28] J. B. Taylor, Trans. Faraday Soc. 1957, 53, 1198 – 1203. [29] S. Sunner in Combustion Calorimetry (Eds.: S. Sunner, M. Mansson), Pergamon, 1979, pp. 13 – 34. [30] J.-M. Andanson, A. A. H. Pdua, M. F. Costa Gomes, Chem. Commun. 2015, 51, 4485 – 4487. [31] R. C. Remsing, I. D. Petrik, Z. Liu, G. Moyna, Phys. Chem. Chem. Phys. 2010, 12, 14827 – 14828. [32] R. C. Remsing, R. P. Swatloski, R. D. Rogers, G. Moyna, Chem. Commun. 2006, 1271 – 1273. [33] J. Zhang, H. Zhang, J. Wu, J. Zhang, J. He, J. Xiang, Phys. Chem. Chem. Phys. 2010, 12, 14829 – 14830. [34] J. Zhang, H. Zhang, J. Wu, J. Zhang, J. He, J. Xiang, Phys. Chem. Chem. Phys. 2010, 12, 1941 – 1947. [35] H. M. Cho, A. S. Gross, J.-W. Chu, J. Am. Chem. Soc. 2011, 133, 14033 – 14041. [36] H. Liu, K. L. Sale, B. A. Simmons, S. Singh, J. Phys. Chem. B 2011, 115, 10251 – 10258. [37] Z. Liu, C. Remsing Richard, B. Moore Preston, G. Moyna in Ionic Liquids IV, Vol. 975, American Chemical Society, 2007, pp. 335 – 350. [38] B. G. Janesko, Phys. Chem. Chem. Phys. 2011, 13, 11393 – 11401. [39] T. G. A. Youngs, C. Hardacre, J. D. Holbrey, J. Phys. Chem. B 2007, 111, 13765 – 13774. [40] R. P. Swatloski, S. K. Spear, J. D. Holbrey, R. D. Rogers, J. Am. Chem. Soc. 2002, 124, 4974 – 4975. [41] T. Erdmenger, C. Haensch, R. Hoogenboom, U. S. Schubert, Macromol. Biosci. 2007, 7, 440 – 445. [42] L. K. J. Hauru, M. Hummel, A. W. T. King, I. Kilpelainen, H. Sixta, Biomacromolecules 2012, 13, 2896 – 2905. [43] J. Vitz, T. Erdmenger, C. Haensch, U. S. Schubert, Green Chem. 2009, 11, 417 – 424. [44] Y. Marcus, Chem. Soc. Rev. 1993, 22, 409 – 416.
Received: February 20, 2015 Published online on April 8, 2015
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