Dissolution of cellulose using a combination of hydroxide bases in aqueous solution
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In order to further understand the role of the cation when dissolving cellulose in aqueous solutions of hydroxide bases, different bases were combined in solution. Up to 5 wt.% of microcrystalline cellulose was dissolved using a combination of NaOH and the organic base tetramethylammonium hydroxide (TMAH) in water at low temperatures. Thermoscans of solutions containing both NaOH(aq) and TMAH(aq) indicated that cellulose interaction with TMAH seems to be favoured over NaOH. Dynamic rheology measurements of the solutions revealed that combining the two bases delayed gelation significantly when compared to cellulose dissolved in NaOH(aq) or TMAH(aq) alone. Intrinsic viscosity of cellulose in the combined NaOH- and TMAH(aq) solutions was slightly higher than that of the single-base solutions, indicating a slight increase in solvent quality. This shows that combining bases may lead to synergies that improve solvent stability without requiring the use of other additives.
KeywordsCellulose Dissolution Solvent NaOH TMAH Aqueous
Cellulose is an excellent raw material for the development of renewable, biodegradable materials that can replace or complement, for example, single-use plastic articles and fossil-based textiles. Many of these applications require processing in order to shape the cellulose into films, membranes, textile fibers, etc. This requires dissolution-based processing, however, since cellulose degrades before it melts. In order to dissolve cellulose, cellulose-solvent interactions are required to overcome the attractive stabilising forces between the cellulose chains: cellulose chains are, namely, stabilised through strong intra- and intermolecular hydrogen bonding as well as hydrophobic interactions and are, as such, organized in semi-crystalline fibrils that are further assembled in complex layered hierarchical morphology. Despite this, numerous solvents have been developed and most of them are complex systems, such as ionic liquids (Wang et al. 2012), specific salt-solvent combinations [e.g. DMAc/LiCl (McCormick et al. 1985), DMSO/TBAF (Liebert and Heinze 2001)], aqueous solutions of bases or acids [e.g. NaOH(aq) (Sobue et al. 1939; Davidson 1934), quaternary ammonium hydroxides(aq) (Powers and Bock 1935), phosphoric acid(aq) (Boerstoel et al. 2001)], hydrated metal amine salts [e.g. Schweizer’s reagent (Schweizer 1857)] as well as those relying on the derivatization of cellulose [e.g. industrially important CS2/NaOH(aq)]. Aqueous solution of NaOH is of particular interest since it is inexpensive, non-toxic, readily available and already in use in the pulp and paper industry. Dissolution of cellulose in NaOH(aq), however, only occurs below +1 °C and in solutions with a NaOH concentration between 7 and 10 wt.% (Budtova and Navard 2016). The use of this solvent system has also been held back, partly due to its inability to dissolve cellulose with a DP over ca 200 and partly due to problems with the instability of the solutions as they gel with increasing time, temperature and/or concentration of cellulose (Roy et al. 2003). Considerable efforts have therefore been made to improve dissolution in the cold NaOH(aq) system and different additives have been identified, including urea (Zhou and Zhang 2000), thiourea (Zhang et al. 2002), ZnO (Yang et al. 2011) and polyethylene glycol (Yan and Gao 2008). Whilst a general stabilisation mechanism for these additives has not been established, emphasis in current research is now being placed on the importance of hydrophobic interactions in solvent systems, since the amphiphilic nature of cellulose has been investigated widely. It has been shown, for example, that increasing concentrations and molecular weights of cellulose can be dissolved by increasing the hydrophobicity of the cation in quaternary ammonium hydroxide bases (Wang et al. 2018).
The aim of this work was to increase understanding of the dissolution of cellulose in aqueous solvents and, more specifically, the role of the cation, by combining different hydroxide bases and investigating whether or not cellulose displays an affinity for different cations. The resulting solutions were investigated using differential scanning calorimetry to identify the hydrate structures of the bases in solution, and how these are affected by each other and by cellulose. Moreover, NMR spectra of selected solvents were analysed to shed additional light on molecular interactions. In order to investigate if these solutions displayed properties different to those of single-base solutions, intrinsic viscosity analysis was used to compare the solvent quality, while dynamic rheology measurements were performed to investigate effects on solution stability.
Microcrystalline cellulose (MCC), Avicel PH-101 purchased from FMC BioPolymer, a purified partially depolymerized cellulose made by acid hydrolysis of specialty wood pulp, with a degree of polymerization of 180 as measured by GPC-MALLS (personal communication with Majid Ghasemi at Södra skogsägarnas ekonomiska förening), was used. Granulated sodium hydroxide (NaOH) known commercially as Emplura, tetrabutylammonium hydroxide (TBAH) 40 wt% in H2O, methyl-α-D-glucopyranoside (99%) and deuterium oxide (D2O, 99.9%) were purchased from Merck (previously Sigma-Aldrich) and used as received. Potassium hydroxide (KOH) pellets (analysis grade) were purchased from Merck. Tetramethylammonium hydroxide (TMAH) aqueous solutions made from either TMAH pentahydrate or 25 wt% TMAH solution in water, were also purchased from Merck and diluted with deionized water.
Dissolution of cellulose
The solvent was prepared by dissolving the desired amount of base in deionized water. MCC was added to the solvent under stirring in an ice bath and left to stir for 5 min or until dispersed. The solution was then stored in a freezer at − 20 °C for 20 min before being stirred in an ice bath for up to 5 min to remove any ice crystals that might have formed, and to ensure a more homogeneous sample.
Determination of the maximum solubility of MCC in solutions
Solutions of cellulose were prepared as described above (see “Dissolution of cellulose”) with increasing concentrations of cellulose, starting from 3 wt% MCC. Immediately after dissolution, a droplet of solution was placed on a glass plate and pressed between it and a glass window before being observed in a microscope (ZEISS SteREO Discovery.V12) using cross-polarized light at room temperature. When several undissolved crystals were observed, the dissolution limit was deemed reached. It is relevant to mention that this does not determine whether the cellulose is molecularly dissolved or not but it is a quick method to estimate a rough dissolution limit.
Cellulose solutions were prepared as described above (see “Dissolution of cellulose”) with cellulose concentrations in the range of 0.1 to 1.1 g/dL. Solutions, with or without cellulose, to be used for determining intrinsic viscosity were placed in a 25 °C water bath directly after dissolution for 30 min; the viscosity was then measured using a capillary viscometer with circulating water (for the purpose of temperature control) at 25 °C. Three measurements were made for each sample and the average was used to calculate the relative viscosity, which was determined with a maximum error of 2%. The intrinsic viscosity was obtained from linear regression with a coefficient of determination of at least 0.97.
Oscillatory dynamic rheology measurements were performed to monitor stability of solutions over time. A TA Discovery Hybrid Rheometer (HR-3), with a sandblasted 40 mm plate-plate geometry with a gap of 1 mm, was employed and the temperature was controlled by a Peltier plate with circulating cooling liquid. Strain (γ) sweeps were conducted to determine the linear viscoelastic region and can be found in the supplementary information in Figures S2–S7. From these an angular frequency of 1 rad/s and a strain of 10% were chosen for samples with 3 wt% MCC to increase measurement sensitivity and an angular frequency of 1 rad/s and a strain of 1% for samples with 5 wt% MCC. Samples were measured directly after dissolution with a water-filled solvent trap and brought to the desired temperature in the rheometer without pre-shearing. The point of gelation was taken when G’=G’’. Even though this is a rough estimation, it is here used for comparative purposes under the same conditions.
Differential scanning calorimetry (DSC)
Aqueous solutions of bases with and without cellulose were prepared as described above (see “Dissolution of cellulose”) and thermoscans of the solutions were performed using a DSC 250 from TA Instruments Discovery series equipped with stainless steel pans. Aqueous solutions of bases were cooled at a cooling rate of 10 °C/min down to − 70 °C and kept at − 70 °C for 5 min; all of the samples were then heated up to 80 °C at a heating rate of 1 °C/min, with the exception of 2.3 M NaOH(aq), which was heated up to 10 °C. The procedure for solutions with dissolved cellulose was the same as for NaOH(aq).
Nuclear magnetic resonance (NMR)
NMR analysis was performed on samples containing methyl-α-D-glucopyranoside (0.4 M) dissolved in D2O with 2.3 M NaOH, 2.3 M TMAH and 2.3 M 50/50 mol% NaOH/TMAH, respectively. The NMR measurements were run on an 800 MHz magnet equipped with a Bruker Avance HDIII console and a TXO cryoprobe. Spectra were recorded with a low-angle radio frequency pulse to minimize relaxation-weighting using a single pulse experiment with 1H decoupling during acquisition and a relaxation delay set to 5 s; 8 scans were collected. A capillary containing D2O with 3-(trimethylsilyl)-1-propanesulfonic acid sodium salt (DSS) was placed inside the tube as an internal reference.
Results and discussion
Solubility of cellulose in aqueous solutions of combined hydroxide bases
The observation that a certain minimum concentration of base is required to dissolve cellulose has also been made for the cold NaOH(aq) system (Sobue et al. 1939), and seems to be applicable to solvents consisting of these types of hydroxide bases in water. When phase diagrams of NaOH(aq) (Egal et al. 2007) or TMAH(aq) (Mootz and Seidel 1990) are viewed it is clear that, for concentrations of base below the minimum required to dissolve cellulose, still the same eutectic hydrate structure is present but diluted in this case by bulk (unbound) water. This indicates that it is not just the hydrated base that is crucial for dissolution: these hydrates need to exceed a certain critical concentration to provide dissolution. Another driving force for cellulose to go into solution could be deprotonation of hydroxyl groups: as this only occurs when a high pH is reached, it therefore requires a high concentration of base.
Maximum solubility of MCC in solution
Solubility tests were continued to determine the dissolution limit of cellulose by using a microscope to identify undissolved fibres. Although this method does not determine whether the cellulose is molecularly dissolved or merely very swollen, it does give a quick and rough estimation of the dissolution capacity of the solvent. It was found that up to and including 5 wt% of MCC could be dissolved in 2.3 M 50/50 mol% NaOH/TMAH(aq), 6 wt% in 2.3 M TMAH(aq) and 3 wt% in 2.3 M NaOH(aq). Although the dissolution capacity of 2.3 M 50/50 mol% NaOH/TMAH(aq) doesn’t reach that of 2.3 M TMAH(aq), it nevertheless indicates that interactions between cellulose and TMAH/NaOH are affected by combining the two bases, which could be related to a change in the hydrate structure of the dissolved bases.
Whilst combining hydroxide bases for dissolution of cellulose in water has been researched only scarcely, it isn’t entirely novel. It is interesting to note that, in a patent from 1924 (which also appears to be one of the first times aqueous solutions of quaternary ammonium hydroxides are reported as being used as solvents for cellulose), it is stated that “the presence of caustic soda enhances the solvent action of the bases”. This shows that observations of some type of improved dissolution upon addition of NaOH to solutions of, for example, TMAH(aq) (Lilienfeld 1924) had also been made.
Investigating the structure of hydrates in solution using differential scanning calorimetry (DSC)
Thermoscans were performed using DSC to identify melting temperatures and enthalpies of different hydrates in order to investigate the structure of the hydrated bases in the water solutions when NaOH and TMAH are combined, and to study their interactions with cellulose.
Aqueous solutions of NaOH and TMAH
NaOH forms different hydrates when dissolved in water and for NaOH concentrations around 2.3 M, both an eutectic hydrate salt with a reported composition of NaOH • 9 H2O (melting temperature of − 34 °C) and free water are present (Roy et al. 2001). Navard et al. characterized the dissolution of MCC in NaOH(aq) and found that the enthalpy of the eutectic hydrate salt decreases with increasing dissolution of cellulose, suggesting that the hydrates interact with the dissolved cellulose chains which prevents them from crystallizing (Egal et al. 2007).
Melting temperature (Tm) (°C) and enthalpy (ΔH) (J/g sample) of hydrates in 2.3 M(aq) base with the specified ratio of NaOH and TMAH, measured using DSC
Ratio of base
Aqueous solutions of NaOH and TMAH combined
Varying ratios of the bases with increasing level of TMAH were measured, as can be seen in Table 1, in order to investigate how the hydrates of NaOH and TMAH would be affected by each other when combined. In a solution of 75/25 mol% NaOH/TMAH(aq), hydrates of NaOH and TMAH are formed with the same structures as in the reference solutions of 100 mol% NaOH(aq) and 100 mol% TMAH(aq), based on the fact that there was no significant shift in their melting temperatures. The enthalpy of the NaOH hydrate, however, decreased significantly, indicating only a modest formation of NaOH hydrates. When the concentration of TMAH was increased further to 50/50 mol% NaOH/TMAH(aq), two hydrate salts were again observed: the melting temperature of the NaOH hydrate had however shifted to − 27.8 °C, which is closer to the melting point reported for NaOH • 7H2O than for NaOH·9H2O (Pickering 1893). Upon increasing the concentration of TMAH even further to 25/75 mol% NaOH/TMAH(aq), only one peak (besides that of ice) was observed, with a melting temperature and enthalpy consistent with a TMAH salt. This could have several explanations: the peak of NaOH hydrate is hidden under the peak of TMAH hydrate; no NaOH hydrate salt could be formed at this high TMAH concentration; together, NaOH, TMAH and water formed an eutectic salt.
These measurements indicate that the presence of TMAH can disturb both the level and structure of the NaOH salt formed in the solution whereas TMAH probably retains its structure, with its melting temperature affected only slightly by a change in its surrounding molecular environment. Another observation that was made is that the enthalpy of the TMAH hydrate in the 2.3 M TMAH(aq) solution is roughly the same as in the 2.3 M 50/50- and 25/75 NaOH/TMAH(aq) solutions, i.e. around 70 J/g. This shows that there is the same amount of hydrate in all three solutions, despite the lower concentration of TMAH in the two solutions that also contain NaOH. It indicates that the addition of NaOH favours the formation of TMAH • 16 hydrate; the additional hydrates probably arise from other hydrates of TMAH, which act as a depot and reform into TMAH • 16 hydrate upon the presence of NaOH.
Cellulose dissolved in aqueous solutions of base
Melting temperature (Tm) (°C) and enthalpy (ΔH) (J/g sample) of hydrates in solutions of 3 wt% MCC dissolved in 2.3 M base with the specified ratio of NaOH and TMAH, measured using DSC
Ratio of base
Upon dissolution of cellulose in 2.3 M TMAH(aq) or 2.3 M 50/50 NaOH/TMAH(aq), the same hydrate structures were formed as in cellulose-free solutions but only the enthalpy of the TMAH hydrate decreased, possibly indicating a preferred cellulose interaction with the TMAH hydrate.
It is interesting to note that although both bases are required for dissolution, the DSC results indicate that only TMAH seems to be interacting with the cellulose. Another significant feature is that, at the observed dissolution limit of 5 wt% MCC, the levels of the eutectic salts do not reach zero (5 wt% MCC corresponds to ca 3 TMAH/AGU). If the linear trend of decreasing TMAH-hydrate would continue, it would reach zero at 14 wt% MCC (as seen in Fig. 3), corresponding to ca 1 TMAH/AGU.
These results depict a scenario where cellulose is dissolved in a solution containing both NaOH and TMAH hydrates but only interacts with TMAH, whilst NaOH hydrates are affected by the presence of both the cellulose and TMAH through a change in the water structure but are not associated to either one. This raises the question of whether the properties of the solution would be similar to cellulose dissolved in either TMAH(aq) or NaOH(aq), or a mixture thereof.
Taking into consideration that there is an error margin of 2% when determining the relative viscosities and a linear regression is made using these values, the differences between the intrinsic viscosities measured for TMAH(aq) and NaOH(aq) are not significant. This implies that the inherent hydrophobicity of a cation such as TMAH, does not improve the solvent’s quality significantly compared to NaOH, at least when measured by intrinsic viscosity. It is reasonable to assume that dissolution in NaOH(aq) and TMAH(aq) occurs through similar mechanisms and that the effects on the conformation and subsequent entanglement of the cellulose will be similar. This is possibly why the intrinsic viscosity of NaOH(aq) and TMAH(aq) are comparable. Combining the two bases did improve the quality slightly; the results from DSC indicate that a change in the hydrate structure could be the cause, but this requires further investigation.
Furthermore, changes in 13C chemical shifts (see Figure S1 in the supporting information), albeit modest, additionally witness of perturbation of electron density experienced by the glucose ring upon addition of TMAH in the NaOH(aq) system. Carbon atoms in positions 2, 4 and 6 show deshielding effects when the amount of TMAH is increased (displacement of the chemical shift downfield corresponding to 0.2 ppm when going from NaOH(aq) to TMAH(aq)), while those in positions 1, 3 and 5 seem to experience a very poor shielding effect (a modest chemical shift displacement upfield).
Interestingly enough, this does not comply with the deprotonation signature commonly observed: an upfield displacement of 1H chemical shifts together with a downfield displacement of the 13C signals originating from the C atoms carrying deprotonable OH-groups. Consequently, the presence of TMAH is probably not associated with enhanced deprotonation of the carbohydrate, it is more likely involved in other interactions responsible for deshielding of the glucose C–H moieties.
Stability of cellulose/base solutions
The instability of cellulose solutions over time might present a problem when processing cellulose dissolved in cold NaOH(aq) and similar solvents, so dynamic oscillatory viscosity measurements were performed to record the stability of solutions over time by monitoring gelation.
The reason for the delayed gelation needs more investigation but the implication from time-resolved rheology is that the two bases have somewhat different stabilisation mechanisms: the more hydrophobic TMAH provides better stabilisation of the dissolved cellulose when the temperature is increased whereas NaOH provides better stabilisation at lower temperatures. One possible explanation here is that attractive hydrophobic interactions between cellulose molecules are less pronounced at lower temperatures since cellulose adapts a conformation that minimises the exposed hydrophobic surfaces and thus minimises hydrophobic cellulose-cellulose interactions (Lindman and Karlström 2009), thereby making stabilisation through hydrophobic interactions less important. At higher temperatures, on the other hand, the inherent hydrophobic properties of TMAH inhibit hydrophobic attractive forces between the cellulose chains and stabilises the solution.
Moreover, the combined solvent does not display the properties of either pure solvents or an average of the two. Based on the DSC results discussed earlier, it could be concluded that the presence of NaOH might have favoured the formation of TMAH • 16 H2O hydrate so that there was the same amount of hydrate in a 2.3 M 50/50 mol% NaOH/TMAH(aq) solution as in a 2.3 M TMAH(aq) solution. This could be an indication as to why the combined solvent displays increased stability: the cellulose gains a more hydrophobic cation to interact with at the same time as NaOH is present in solution.
Up to 5 wt% of MCC can be dissolved using a combination of NaOH and the organic base TMAH in water. These are levels at which each of the bases cannot dissolve cellulose alone, indicating that the two bases can cooperate to do so. The solution of the combined bases exhibits a slightly higher intrinsic viscosity than NaOH(aq) or TMAH(aq) alone, showing that combining the two bases improves the quality of the solvent slightly. DSC measurements revealed that the amount of eutectic salt of TMAH decreases linearly with increasing concentration of cellulose, thereby indicating that cellulose interacts preferably with TMAH rather than NaOH. The combined NaOH and TMAH solvent delayed gelation over time significantly: this is an interesting result, the cause of which needs to be elucidated further.
Open access funding provided by Chalmers University of Technology. Thanks are extended to Anna Ström (Chalmers) for the use of her rheology equipment and to Aleksandar Matic (Chalmers) for the use of his DSC equipment.
This study was funded by FORMAS, the Swedish Research Council for Sustainable Development.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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