Temperature and time dependence of cellulose solution stability in combined aqueous hydroxide bases

Temperature and time dependence of cellulose solution stability in combined aqueous hydroxide bases | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Temperature and time dependence of cellulose solution stability in combined aqueous hydroxide bases Beatrice Swensson, Alexander Idström, Anette Larsson, Merima Hasani This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6828754/v1 This work is licensed under a CC BY 4.0 License Status: Under Revision Version 1 posted 4 You are reading this latest preprint version Abstract Cellulose solutions in aqueous hydroxide bases often display an intriguing and limited stability over time and within a limited temperature range. Our previous studies pointed out a synergistic effect of combining two hydroxide bases – NaOH and tetramethylammonium hydroxide (TMAH) – on stability over time and on dissolution capacity of cellulose. Here, we hypothesise that the delayed gelation of cellulose in this system is related to the ability of the base combination to disturb the formation of a stable monobase-cellulose salt during aggregation. We combine studies addressing time and temperature dependency of the solution stability with X-ray diffraction and solid-state NMR studies of swollen cellulose to address the hypothesis. The results showed that the dissolution window at low temperatures turned out to be similar for the individual and combined bases. However, increased stability over time was observed in the 50/50 NaOH/TMAH(aq) in the semi-dilute region compared to the individual base solutions, and could be related to different grades of order observed in swollen model samples. Cellulose dissolution NaOH swelling hydroxide gelation X-ray NMR Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Since cellulose cannot melt, dissolution of cellulose is a very useful tool for processing and analysing cellulose. It is, for example, used for producing textiles from pulp and for analytical methods when determining the molecular weight. Aqueous solutions of hydroxide bases, such as NaOH(aq), tetramethylammonium hydroxide(aq) (TMAH), benzyltrimethylammonium hydroxide and several additional quaternary ammonium hydroxides have been known to dissolve cellulose for a long time, with early reports dating back to the 1930s (Brownsett & Clibbens, 1941 ; Budtova & Navard, 2016 ; Davidson, 1936 ; Kostag, Jedvert, Achtel, Heinze, & El Seoud, 2018 ). An interesting feature is that for some of these bases, dissolution only occurs at around 0°C and below. A similar temperature dependency was also observed for the recently reported solvent N,N-dimethylmorpholinium hydroxide, another quaternary ammonium hydroxide that can be used to dissolve cellulose (Naserifar, Swensson, Bernin, & Hasani, 2021 ). Besides the effect on the temperature at which dissolution occurs, the hydroxide base also affects the temperature stability after dissolution. Solutions of cellulose in NaOH(aq) are known to be unstable over time and with increasing temperature (Roy, Budtova, & Navard, 2003 ). In a study by Lu et al., a series of quaternary ammonium hydroxides was investigated. It showed that increasingly hydrophobic cations gave more stable solutions at increasing temperatures, measured as a gelation over repeated temperature cycles (Yang Wang et al., 2018 ). Additives such as urea or ZnO have also been shown to increase the stability of cellulose dissolved in NaOH(aq) (Budtova & Navard, 2016 ). There is no consensus on the reason for the instability over time, but it has been shown that upon initiation of aggregation, the van der Waals-associated molecular sheets are those that assemble first upon crystallization of cellulose II (Isobe, Kimura, Wada, & Kuga, 2012 ; Miyamoto et al., 2009 ). One suggestion, brought forward as an explanation for the instability, is that the solutions are unstable because cellulose II has a lower solubility than cellulose I. This was based on the observation that when a solution is gelled by heat formation of cellulose II can be observed (Pereira et al., 2018 ). The hypothesis that different crystal forms have different solubility is contradicted by an earlier study investigating the effect of crystal structure on solubility. In that study, cotton linters before and after mercerization were equally soluble. The same study reported that dissolution and regeneration of cotton linters in Cuen, which also results in the formation of cellulose II, even improved solubility (Isogai & Atalla, 1998 ). Here, one also needs to consider that besides the crystal structure, the ultrastructure of the fibre also changes upon dissolution and coagulation. Therefore, it is not entirely simple to conclude the impact of crystal structure on solubility. In our previous work, we could report that combining NaOH with TMAH improved the solution properties (i.e. the average size of the cellulose was smaller, indicating better dissolution and gelation was delayed) compared to pure NaOH(aq) as a solvent. A synergistic effect of the two bases on the solution stability over time (through dynamic rheology measurements) could also be observed in a 50/50 NaOH/TMAH aqueous solution (Swensson, Lages, Berke, Larsson, & Hasani, 2021 ; Swensson, Larsson, & Hasani, 2020a ; Swensson et al., 2020b ). We have also hypothesized that the delay in gelation might be due to the presence of two bases disrupting the formation of a stable monobase-cellulose salt (i.e. whether Na-cellulose or TMA-cellulose) preceding formation of ordered aggregated structures (cellulose II), favoring in that way the dissolved state. In this work, we explore further this hypothesis through X-ray diffraction and solid-state NMR studies of swollen cellulose, undertaking also the necessary complementary investigations of how combining the bases affects the temperature required for dissolution and the subsequent temperature stability of the obtained solutions. If the improved stability over time is mainly a consequence of the base combination disturbing the formation of regular monobase-cellulose salt, then the temperature required for dissolution should not be affected by combining the bases. On the other hand, cellulose aggregation, whether induced by temperature or time, should be affected. Material and methods 2.1. Materials Microcrystalline cellulose (MCC), Avicel PH-101 purchased from FMC BioPolymer, a purified partially depolymerized cellulose made by acid hydrolysis of speciality wood pulp was used throughout the study. Granulated sodium hydroxide (NaOH), known commercially as Emplura, and tetramethylammonium hydroxide (TMAH) 25 wt.% (aq) were purchased from Merck (previously Sigma-Aldrich) and used as received. Hydrochloric acid (1 M, reagent grade) was purchased from VWR. 2.2. Dissolution of cellulose Two different protocols were used for the dissolution studies of cellulose. At cellulose concentrations of 10 g/L, the solvent temperature was adjusted before adding the cellulose. For cellulose concentrations of 30 g/L, the cellulose was added to the solvent at room temperature, and then the solution was brought to the desired temperature. This was applied because the formation of a viscous, gel-like shell could be observed when trying to dissolve 30 g/L (above the overlap concentration) in the pre-cooled solvent, hindering and slowing down dissolution. The formation of the gel-like layer was suppressed when the cellulose was first mixed with the solvent at room temperature. The temperature of the solutions was controlled using a Julabo F25 circulating oil bath, with a temperature probe measuring the temperature of the solvent/solutions in situ. A mechanical stirrer (IKA Eurostar, power control visc) was used at a speed of 350 rpm for a total volume of 100 ml solution in a 250 ml beaker. The cellulose solution was always allowed to stir for 30 minutes at the desired dissolution temperature before sampling. 2.3. Time and temperature stability of the solutions For measurements of the stability of the solutions, cellulose was first dissolved as detailed in section 2.2, in 2 M base(aq) at – 5 °C. For studies of the temperature stability, the solution was kept in the dissolution vessel and the temperature of the solution was raised under stirring in 10 °C steps, to a maximum of 75 °C. The transmission was measured at each step as soon as the solution had reached the desired temperature. The entire procedure, starting from when the cellulose had been dissolved at – 5 °C under stirring for 30 minutes, took ca 2.5 h. When measuring the stability over time, the cellulose was transferred to a beaker after the dissolution step and stored at the desired temperature (8 °C in a fridge, 20 °C at room temperature or 35 °C in an oven) without stirring. The transmission of the solutions was measured with a SPECORD 205 UV‐VIS spectrophotometer from Analytik Jena at room temperature. Plastic disposable cuvettes with a light path length of 10 mm were used throughout the measurements, and water was used as the background. A scan was performed over a range of 250 to 1100 nm, and the transmission at 800 nm was used as an indication of whether the cellulose had dissolved or not, as the cellulose free solvents showed a 100% transmission at this wavelength and any undissolved fibres present in the solutions will be larger than 800 nm and detectable by a decrease in the transmission. Transmission as a function of temperature was measured (single measurements) in the interval -10 to 50°C, and the obtained trends studied (Figure 1 and 2). Transmission dependency on time was measured in the interval 0-100 h (at least two measurements per time point) and investigated in Figure 3. 2.4. X-ray diffraction of undissolved but swollen cellulose immersed in solvent Samples were prepared by immersing cellulose in the solvent at a cellulose concentration of 0.167 g/ml in 1.75 M base(aq) solvent, the conditions where (based on the previous reports and experimental experience) only swelling, no dissolution was expected. The swollen samples were then transferred to capillaries or clamped between two Kapton windows in a “sandwich cell”. They were then stored at room temperature for at least 2 h, + 3 °C for 22 h or – 20 °C for 3 h before being measured. Wide-angle X-ray measurements were conducted at room temperature and performed using a Mat:Nordic from SAXSLAB with a Rigaku 003+ high brilliance microfocus Cu-radiation source and a Pilatus 300K detector. The measured q-range was 4 × 10 -3 - 2.3 Å -1 . The two-dimensional scattering pattern was radially averaged using the SAXSGui software and the sample data were corrected by subtracting the solvent and sample holder data, unless otherwise stated. 2.5. Solid-state NMR spectroscopy of cellulose immersed in solvent Solid-state NMR measurements were performed on the same set of samples swollen at – 20 °C as for X-ray diffraction. The measurements were conducted on a Bruker Avance III 500, operating at 500.13 MHz 1 H and 125.76 MHz 13 C, equipped with a 4 mm HX CP-MAS probe. Measurements were conducted at 298 K with a MAS spinning rate of 10 kHz. A cross-polarization magic angle spinning ( 13 C CP/MAS) pulse sequence with a SPINAL-64 decoupling sequence was used with acquisition parameters including a 3.0 ms 1 H pulse, a 1500 ms CP-contact time, 7-23 ms acquisition time, 20000 scans, and 3 s recycle delay. The chemical shifts were referenced to adamantane with CH 2 -signal set to 38.48 ppm. 2.6. Molar mass analysis The samples were prepared as detailed in section 2.2 by dissolving MCC in 2 M base(aq) solutions at – 5 °C at a concentration of 30 g/L for 30 minutes. The cellulose was then coagulated using HCl(aq) and washed until the pH was neutral and the filtrate showed a conductivity below 10 µS. The molecular weight distributions were analyzed by MoRe Research AB, Örnsköldsvik, Sweden, using a PL-GPC 220 with an RI detector (Polymer Laboratories). It included a set of Guard column Mixed-A 20 µm (7.5 * 50 mm) and 2 Mixed-A 20 μm (300 * 7.5 mm), also from Polymer Laboratories, connected in series. The mobile phase contained 0.5 w/v% LiCl /DMAc with flow rate of 1 ml/min operating at 70 °C. Pullulan polysaccharides obtained from Polymer Lab were used as calibration standards. Prior to analysis, 25 mg of the sample was solvent-exchanged 3 times with 5 ml of methanol for 30 minutes, followed by a further 3 times solvent exchange step using DMAc for 30 minutes. The excess of DMAc was then removed and 5 ml of 8% (w/v) LiCl/DMAc was added and left overnight at ambient temperature, with mild magnetic stirring. The samples were thereafter diluted with 20 ml of DMAc. Results and Discussion 3.1. Temperature window for dissolution To be able to compare the solvents at their optimum concentration, and to see if there was a synergistic effect of combining NaOH with TMAH, the temperature window for dissolution at different base concentrations was investigated. It was found that for the three solvents (NaOH(aq), TMAH(aq) and 50/50 NaOH/TMAH(aq)), the window was quite similar (see Figure S1 in the supporting information). The transmission never reached 100% for cellulose in NaOH(aq), which is in line with our previous findings that even at low concentrations of MCC, undissolved material remained in solution (Swensson et al., 2021). The temperature window in which the transmission was close to 100% (and dissolution to some degree occurred) expanded for all three solvents with increasing base concentration up to approximately 2 M, which was the optimum. When going above 2 M, the temperature window shrank again. No synergistic effect of the two bases on the temperature required for dissolution could be observed. 3.2. Stability of the solutions over time and during an increase in temperature The temperature stability after dissolution was measured through monitoring the transmission as a function of increasing solvent temperature at two different concentrations, a more dilute at 10 g/L (Figure 1a) and more concentrated 30 g/L (Figure 1b). The concentrations were chosen based on our previous studies, showing that the overlap concentration (approximated by the inverse of the intrinsic viscosity) was ca. 10 g/L for all three solvents at 25 °C (Swensson et al., 2020a). The result was that at 10 g/L, cellulose dissolved in TMAH(aq) remained stable over the entire temperature interval measured, while in NaOH(aq) the solution became unstable already above 35 °C and the cellulose started to aggregate and precipitate. The cellulose in the 50/50 NaOH/TMAH(aq) solution showed an intermediate behaviour in relation to the two bases (aggregation could be observed above 55 °C), thus, revealing no synergistic effect of the bases. Interestingly, though, a synergistic effect could be observed at 30 g/L: the 50/50 NaOH/TMAH(aq) solution gradually deteriorated when the temperature was raised, but showed a significantly better resistance towards increasing temperature than NaOH(aq) or even TMAH(aq). In the diluted region, stability of the dissolved state is likely governed primarily by the conformation and hydration of individual cellulose chains. The deterioration of these properties due to increasing temperature is most effectively counteracted by the organic hydroxide, TMAH, which is better equipped to stabilize the individual amphiphilic cellulose chains than either NaOH(aq) or the 50/50 combination. On the other hand, in the concentrated regime, above the overlap concentration, where interaction between the chains becomes significant, the combination of the two bases seems to be superior in preventing chain-chain association, possibly by preventing regular packing, as will be further discussed below. It should be noted that during the stability measurements, an increasing colour change in cellulose-containing solutions was observed with temperature or storage time. The molecular weight of the cellulose before and after dissolution was therefore measured, but no significant degradation was measured (see more in the supporting information). Interestingly, when measuring the stability of the solutions over time, similar results to those observed for the temperature stability above the overlap concentration were found (Figure 2). The NaOH(aq) solution was less stable over time at all temperatures, and the stability of the TMAH(aq) solution also decreased with increasing temperature. The 50/50 NaOH/TMAH(aq) solution was more stable over prolonged storage times at all investigated temperatures (Figure 2). Stability over time is probably related to formation of the thermodynamically most stable state and is, according to these results, likely favoured by the combination of the two bases preventing chain-chain association (by providing a favourable hydration/structure of the dissolved state and/or preventing regular packing during coagulation). On the other hand, a raise in temperature might drive the solution into a kinetically arrested (gelled) state primarily due to deteriorated stabilization of the individual polymers. In this state, the presence of the two bases does not show any synergy, unless the chain-chain interactions are significant (i.e. unless in concentrated region). Thus, the coagulation tendency of these solutions could be seen as an interplay between kinetic and thermodynamic driving forces, each of them affected by dissolution conditions, properties of the dissolved state and changes it is subjected to. 3.3. The crystal structure of the cellulose and changes occurring during intracrystalline swelling and dissolution 3.3.1. The crystal structure of the cellulose before and after dissolution The transformation from cellulose I to cellulose II in all three solvents after dissolution, coagulation, washing and drying of the MCC was confirmed through X-ray diffraction (see Figure 3). The identified peaks are listed in Table 1. Table 1: Identified peaks of the MCC before (CI) and after coagulation in the respective solvents (CII). CI CII NaOH CII TMAH CII 50/50 Peak 1 15.2° 12.4° 12.8° 12.8° Peak 2 16.7° 20.2° 20.1° 20.2° Peak 3 20.8° 21.7° 21.5° 21.5° Peak 4 22.9° 28.3° 28.3° 28.5° Peak 5 34.6° 35.0° 35.4° 34.8° 3.3.2. Intracrystalline swelling observed through X-ray diffraction To investigate changes in the crystalline structure indicative of crystalline rearrangement during dissolution and a subsequent coagulation (i.e. the typical conversion from cellulose I to cellulose II) in the studied bases, the cellulose was soaked in the respective solvents and measured with X-ray diffraction. Soaking at room temperature (Figure S3 in the supporting information), or even at + 3 °C for 22 h (Figure 4a) did not change the crystalline structure, as it remained as cellulose I. After storage at – 20 °C for 3 h, upon which the samples froze, the base had penetrated the crystal structure and altered it (see Figure 4b). Similar observations, that low temperatures are required before intracrystalline swelling occurs, have been recently reported for cellulose immersed in sulfuric acid (Li et al., 2021). The diffraction angles for the identifiable peaks in the swollen samples can be found in Table 2, along with the main peaks of cellulose I (measured) and cellulose II (Nam et al., 2016). All three samples showed different patterns (Figure 4b), none of which corresponded completely to cellulose I or cellulose II. For cellulose soaked in TMAH(aq), there were however some similarities to cellulose II, as the peaks at 20.7° and 21.7° are similar to those usually found at 20.5° and 21.9°, attributed to the (110) and (020) planes when named as by Nam et al. (Nam et al., 2016). In addition, a shoulder peak not found in cellulose II could be observed at 5.4° (Figure 6b). A similar observation was made by Sisson and Saner, who measured the swelling of ramie fibers in TMAH(aq) at room temperature. They reported for TMAH interplanar distances corresponding to peaks at 6.6° and 22.2° (λ = 1.54 Å), similar to the peaks observed here (Sisson & Saner, 1939). Relatively strong signals observed for the TMAH sample indicate a comparably strong tendency of the TMAH-treated cellulose to rearrange into an ordered structure. Table 2: Peaks obtained from X-ray diffraction (XRD) of the starting material (CI) and of MCC soaked in 1.75 M(aq) base, stored at - 20 °C for 3 h. Peaks of cellulose II (CII) as reported by Nam et al. (Nam et al., 2016). CI CII MCC NaOH Freezer MCC TMAH Freezer MCC 50/50 Freezer - - - 5.4° - 15.2° - - - - 16.7° 12.3° - - - 20.8° 20.5° - 20.7° 21.1° - 21.9° 21.5° 21.7° 22.3° 22.9° - - - - For MCC soaked in NaOH(aq), only one peak at 21.5° could be observed, although based on its appearance it might be deconvoluted into two peaks (Figure 4b). It can be noted that the dip in the intensity obtained after background subtraction is due to that the pure solvent and the sample with cellulose did not entirely match, indicating that the structure of the NaOH might change upon the addition of cellulose. However, the background subtraction did not alter the position of the cellulose peak, and the original data can be viewed in the supporting information (Figures S4 and S5). The diffraction pattern is similar to the one observed by Crawshaw et al., (Crawshaw, Bras, Mant, & Cameron, 2002) reporting a single crystalline peak at 20° identified as the (200) reflection for ramie fibres immersed in approximately 4 M NaOH(aq) at room temperature, and the spectrum was assigned to Na-cellulose I. For the same type of sample, Kobayashi et al. instead reported an obscure pattern, with two strong equatorial peaks at 4.45 and 4.23 Å (ca 20.0° and 20.9°), and a weaker one at 12.5 Å (7.05°). They refer to this as Na-Cellulose I and attribute the peak at 7° to the swelling of the crystalline lattice by the NaOH solution (Kobayashi et al., 2011). Taken together, this observation points towards an incomplete crystalline rearrangement of cellulose in the NaOH(aq) model system. Interestingly, for the 50/50 combination, two weak peaks could be identified (Figure 4b). The presence of diffraction peaks shows that the cellulose can form a reoccurring structure in the presence of two bases, but their weak appearance points out that there is not much crystalline material in the sample. 3.3.3. Intracrystalline swelling observed through solid-state NMR To further investigate the intracrystalline swelling, the samples that were swollen at – 20 °C were also measured with solid-state NMR spectroscopy. The 13 C CP/MAS spectra can be seen in Figure 5. Similar to what was seen from the diffraction results, neither cellulose I nor cellulose II signals can be observed in the swollen samples. For the cellulose sample swollen in NaOH(aq), the spectrum is similar to that obtained from the similarly treated wood pulp (Porro, Bédué, Chanzy, & Heux, 2007), assigned to contain a mixture of Na-cellulose I and Na-cellulose Q. This lends support to also assigning the X-ray spectra of the NaOH(aq) swollen cellulose to Na-cellulose I. The spectrum of cellulose swollen with TMAH(aq) shows several signals of relatively high order. Although the pattern bears a resemblance to that seen for cellulose II, it does not represent its exact structure. As was described in the previous section, the XRD measurements also indicate that the structure is similar, but not identical, to that of cellulose II. The crystalline conversions in the swollen samples can be seen as condensed consecutive steps representative of the crystalline conversions taking place during dissolution and the subsequent aggregation from the dissolved state. The ordered structure present here in the TMAH-swollen state is thus representative of the ordered structure being formed as a pre-step during aggregation of the well-dissolved cellulose prior to its final conversion into cellulose II, as an intermediate. This is in line with the stability measurements (presented in Section 3.2), which show a surprisingly rapid aggregation of MCC in TMAH(aq), indicating the formation of a relatively ordered structure. Noticeably, the MCC swollen in the 50/50 NaOH/TMAH(aq) solution showed peaks with a similar appearance to the sample swollen only with NaOH, also indicating a lower structural order. The results from both the temperature and the time stability investigations show that the NaOH/TMAH mixture gives improved solution stability over time and in concentrated region compared to only NaOH or only TMAH. It is clear that at low temperatures, the base interacts with the cellulose, penetrates the cellulose crystal and that the cellulose changes the crystalline structure from Cellulose I to Cellulose II after dissolution and washing. We hypothesize that the crystallization is governed by stabilizing forces in the dissolved state as well as driving forces behind the formation of aggregated structures. In light of that, the superior stability of the 50/50 NaOH/TMAH(aq) could be understood as a result of either favourable thermodynamics of the dissolved state or less favoured formation of aggregates. Having in mind the lack of any observable synergistic effect on the required dissolution conditions (see Section 3.1), the tendency to form aggregated states might be of importance. Likewise, an explanation for the rather drastic and unexpected decrease in temperature stability of cellulose in TMAH(aq) at 30 g/L, should be sought for in the same phenomenon, as the more hydrophobic TMAH cation was expected to contribute to increased stability of the dissolved state when the temperature increased. If the formation of a crystalline complex between TMAH and cellulose can explain the poor stability of the TMAH(aq) solutions, a similar explanation can be proposed for the increased stability of the 50/50 combination. Atacticity, or irregular side groups, can have a detrimental effect on the crystallization of a polymer. Analogously, it can be envisioned that a random distribution of two bases, closely interacting with the cellulose chains, could have a similar effect on the crystallisation of cellulose II from a dissolved state. However, the bases are not covalently bonded side groups, and since they can move away from the chain, this could also explain why the combination delays but does not prevent aggregation. In other words, this could be important for the stability of the solutions if the presence of two bases prevents the formation of an ordered precursor for cellulose II formation. Conclusions Aqueous solutions of NaOH or TMAH show very similar temperature dependency for dissolution of cellulose within the range of 1.5 to 2.25 M base. Combining NaOH with TMAH did not significantly affect the temperature at which dissolution occurred. However, it increased the stability of more concentrated solutions over time and against increasing temperatures. Given the X-ray diffraction and solid-state NMR measurements, this might be due to the presence of two bases disturbing the crystallization of cellulose II upon aggregation from the dissolved state. Similarly, the formation of a crystalline complex between TMAH and cellulose, indicated by prominent NMR- and XRD-signals, is proposed to act as a precursor for cellulose II formation, possibly causing poor stability of cellulose in TMAH(aq). Declarations 5. Acknowledgements and funding This study was funded by FORMAS, the Swedish Research Council for Sustainable Development; VINNOVA, Sweden’s innovation agency; Chalmers Area of Advance Materials Science. This work was performed in part at the Chalmers Material Analysis Laboratory, CMAL and the solid-state NMR measurements were conducted at the Swedish NMR Centre in Umeå, Sweden. 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Biomacromolecules , 8 (8), 2586–2593. https://doi.org/10.1021/bm0702657 Roy, C., Budtova, T., & Navard, P. (2003). Rheological Properties and Gelation of Aqueous Cellulose−NaOH Solutions. Biomacromolecules , 4 (2), 259–264. https://doi.org/10.1021/bm020100s Sisson, W. A., & Saner, W. R. (1939). An X-ray Diffraction Study of the Swelling Action of Several Quaternary Ammonium Hydroxides on Cellulose Fibers. The Journal of Physical Chemistry , 43 (6), 687–699. https://doi.org/10.1021/j150393a003 Swensson, B., Lages, S., Berke, B., Larsson, A., & Hasani, M. (2021). Scattering studies of the size and structure of cellulose dissolved in aqueous hydroxide base solvents. Carbohydrate Polymers , 274 , 118634. https://doi.org//10.1016/j.carbpol.2021.118634 Swensson, B., Larsson, A., & Hasani, M. (2020a). Dissolution of cellulose using a combination of hydroxide bases in aqueous solution. Cellulose , 27 (1), 101–112. https://doi.org/10.1007/s10570-019-02780-8 Swensson, B., Larsson, A., & Hasani, M. (2020b). Probing Interactions in Combined Hydroxide Base Solvents for Improving Dissolution of Cellulose. Polymers , 12 (6), 1310. https://doi.org/10.3390/polym12061310 Wang, Yang, Liu, L., Chen, P., Zhang, L., & Lu, A. (2018). Cationic hydrophobicity promotes dissolution of cellulose in aqueous basic solution by freezing–thawing. Physical Chemistry Chemical Physics , 20 (20), 14223–14233. https://doi.org/10.1039/C8CP01268G Zhong, C., Cheng, F., Zhu, Y., Gao, Z., Jia, H., & Wei, P. (2017). Dissolution mechanism of cellulose in quaternary ammonium hydroxide: Revisiting through molecular interactions. Carbohydrate Polymers , 174 , 400–408. https://doi.org/10.1016/j.carbpol.2017.06.078 Additional Declarations No competing interests reported. Supplementary Files SupportinginfotoTemperatureandtimedependenceofcellulosedissolvedincombinedaqueoushydroxidebasesolutionsandtheaffinityofcellulose.docx Cite Share Download PDF Status: Under Revision Version 1 posted Editorial decision: Revision requested 13 Jun, 2025 Editor assigned by journal 13 Jun, 2025 Submission checks completed at journal 06 Jun, 2025 First submitted to journal 05 Jun, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6828754","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":471079440,"identity":"c46bcbd8-8521-4544-a57e-e35cf8d4b242","order_by":0,"name":"Beatrice Swensson","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAqElEQVRIiWNgGAWjYHACNiC2MWCQIFFLmgEPqVoOk6BFfkbysQcfas4b20s3MH/4QIwWgxtp6YYzjt0245E5wCY5gygt0jlm0jxst214JBLYmHmIctjs/G/Sf/6dA2lh/vyHGC0Mt3PYpBnbDpgBtTBIE6XD4P4zc8PevmRjnhuJbZI9RDms5/CzBz++2Rm2z0g+/OEHUdYgAGMDiRpGwSgYBaNgFOAEABnXLyYLIDDFAAAAAElFTkSuQmCC","orcid":"","institution":"Chalmers University of Technology","correspondingAuthor":true,"prefix":"","firstName":"Beatrice","middleName":"","lastName":"Swensson","suffix":""},{"id":471079442,"identity":"f966bc39-03bf-4cf1-a28c-c4dd621b476f","order_by":1,"name":"Alexander Idström","email":"","orcid":"","institution":"Chalmers University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Alexander","middleName":"","lastName":"Idström","suffix":""},{"id":471079444,"identity":"ed4491ab-f48e-4fbf-923d-11794cfbf3d2","order_by":2,"name":"Anette Larsson","email":"","orcid":"","institution":"Chalmers University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Anette","middleName":"","lastName":"Larsson","suffix":""},{"id":471079445,"identity":"28bc5a37-3cd1-42ef-9953-0da79c0e5163","order_by":3,"name":"Merima Hasani","email":"","orcid":"","institution":"Chalmers University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Merima","middleName":"","lastName":"Hasani","suffix":""}],"badges":[],"createdAt":"2025-06-05 11:38:17","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6828754/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6828754/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":86401584,"identity":"65cba155-98c6-4804-9396-1767857a152c","added_by":"auto","created_at":"2025-07-10 08:59:22","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":15755,"visible":true,"origin":"","legend":"\u003cp\u003eTransmission of a) 10 g cellulose/L and b) 30 g cellulose/L dissolved at – 5 °C in 2 M (aq) base solution upon increasing temperature. The measurements were at least duplicates.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-6828754/v1/77a78d393c7db8b1a40fad12.png"},{"id":86402914,"identity":"b352d39c-1120-40c4-afe2-2918385757f2","added_by":"auto","created_at":"2025-07-10 09:15:22","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":24328,"visible":true,"origin":"","legend":"\u003cp\u003eTransmission over time of cellulose (10 g/L) dissolved at – 5 °C in 2 M (aq) base solutions when stored at a) 8 °C, b) room temperature (RT) 20 °C and c) 35 °C.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-6828754/v1/9f001eb3cba635c7e3c5c869.png"},{"id":86401585,"identity":"472b5b23-7906-48a7-965f-286f0e5d733a","added_by":"auto","created_at":"2025-07-10 08:59:22","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":9702,"visible":true,"origin":"","legend":"\u003cp\u003e1-D X-ray curves of the reference starting material MCC, and MCC after dissolution, coagulation, and washing in the respective solvents. The data is background subtracted and the curves have been baseline shifted for improved visibility.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-6828754/v1/62ff692418efd5ccbee95897.png"},{"id":86401590,"identity":"80e68be5-8e54-49b4-92f7-c6045b8330d6","added_by":"auto","created_at":"2025-07-10 08:59:22","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":17546,"visible":true,"origin":"","legend":"\u003cp\u003eX-ray diffraction of MCC soaked in 1.75 M(aq) base, stored at a) + 3 °C for 22 h, b) – 20 °C for 3 h. The background of solvent and capillary has been subtracted, and the curves have been baseline shifted for improved visibility.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-6828754/v1/5f2b598d7454c9377c8289b7.png"},{"id":86401826,"identity":"7072c27a-7af3-450e-92ac-e2ceffa5d818","added_by":"auto","created_at":"2025-07-10 09:07:22","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":42880,"visible":true,"origin":"","legend":"\u003cp\u003e\u003csup\u003e13\u003c/sup\u003eC CP/MAS NMR spectra of MCC soaked in NaOH(aq)(bottom), MCC in TMAH(aq) (middle), and MCC in 50/50 NaOH/TMAH(aq) (upper).\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-6828754/v1/e76d9ca1f7d199a2fe8dab58.png"},{"id":86403417,"identity":"febf39b9-10a8-42b9-9d4d-63d4c88304e2","added_by":"auto","created_at":"2025-07-10 09:23:33","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":763963,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6828754/v1/9c663535-4772-4305-b10e-4ec5a56184cf.pdf"},{"id":86401589,"identity":"4415ebc9-23b9-4180-a7e8-5e503cb6880f","added_by":"auto","created_at":"2025-07-10 08:59:22","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":371195,"visible":true,"origin":"","legend":"","description":"","filename":"SupportinginfotoTemperatureandtimedependenceofcellulosedissolvedincombinedaqueoushydroxidebasesolutionsandtheaffinityofcellulose.docx","url":"https://assets-eu.researchsquare.com/files/rs-6828754/v1/367e2ca26bbb1128a6836d9b.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Temperature and time dependence of cellulose solution stability in combined aqueous hydroxide bases","fulltext":[{"header":"Introduction","content":"\u003cp\u003eSince cellulose cannot melt, dissolution of cellulose is a very useful tool for processing and analysing cellulose. It is, for example, used for producing textiles from pulp and for analytical methods when determining the molecular weight. Aqueous solutions of hydroxide bases, such as NaOH(aq), tetramethylammonium hydroxide(aq) (TMAH), benzyltrimethylammonium hydroxide and several additional quaternary ammonium hydroxides have been known to dissolve cellulose for a long time, with early reports dating back to the 1930s (Brownsett \u0026amp; Clibbens, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1941\u003c/span\u003e; Budtova \u0026amp; Navard, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Davidson, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e1936\u003c/span\u003e; Kostag, Jedvert, Achtel, Heinze, \u0026amp; El Seoud, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). An interesting feature is that for some of these bases, dissolution only occurs at around 0\u0026deg;C and below. A similar temperature dependency was also observed for the recently reported solvent N,N-dimethylmorpholinium hydroxide, another quaternary ammonium hydroxide that can be used to dissolve cellulose (Naserifar, Swensson, Bernin, \u0026amp; Hasani, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eBesides the effect on the temperature at which dissolution occurs, the hydroxide base also affects the temperature stability after dissolution. Solutions of cellulose in NaOH(aq) are known to be unstable over time and with increasing temperature (Roy, Budtova, \u0026amp; Navard, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2003\u003c/span\u003e). In a study by Lu et al., a series of quaternary ammonium hydroxides was investigated. It showed that increasingly hydrophobic cations gave more stable solutions at increasing temperatures, measured as a gelation over repeated temperature cycles (Yang Wang et al., \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Additives such as urea or ZnO have also been shown to increase the stability of cellulose dissolved in NaOH(aq) (Budtova \u0026amp; Navard, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). There is no consensus on the reason for the instability over time, but it has been shown that upon initiation of aggregation, the van der Waals-associated molecular sheets are those that assemble first upon crystallization of cellulose II (Isobe, Kimura, Wada, \u0026amp; Kuga, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Miyamoto et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). One suggestion, brought forward as an explanation for the instability, is that the solutions are unstable because cellulose II has a lower solubility than cellulose I. This was based on the observation that when a solution is gelled by heat formation of cellulose II can be observed (Pereira et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). The hypothesis that different crystal forms have different solubility is contradicted by an earlier study investigating the effect of crystal structure on solubility. In that study, cotton linters before and after mercerization were equally soluble. The same study reported that dissolution and regeneration of cotton linters in Cuen, which also results in the formation of cellulose II, even improved solubility (Isogai \u0026amp; Atalla, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e1998\u003c/span\u003e). Here, one also needs to consider that besides the crystal structure, the ultrastructure of the fibre also changes upon dissolution and coagulation. Therefore, it is not entirely simple to conclude the impact of crystal structure on solubility.\u003c/p\u003e\u003cp\u003eIn our previous work, we could report that combining NaOH with TMAH improved the solution properties (i.e. the average size of the cellulose was smaller, indicating better dissolution and gelation was delayed) compared to pure NaOH(aq) as a solvent. A synergistic effect of the two bases on the solution stability over time (through dynamic rheology measurements) could also be observed in a 50/50 NaOH/TMAH aqueous solution (Swensson, Lages, Berke, Larsson, \u0026amp; Hasani, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Swensson, Larsson, \u0026amp; Hasani, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2020a\u003c/span\u003e; Swensson et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2020b\u003c/span\u003e). We have also hypothesized that the delay in gelation might be due to the presence of two bases disrupting the formation of a stable monobase-cellulose salt (i.e. whether Na-cellulose or TMA-cellulose) preceding formation of ordered aggregated structures (cellulose II), favoring in that way the dissolved state. In this work, we explore further this hypothesis through X-ray diffraction and solid-state NMR studies of swollen cellulose, undertaking also the necessary complementary investigations of how combining the bases affects the temperature required for dissolution and the subsequent temperature stability of the obtained solutions. If the improved stability over time is mainly a consequence of the base combination disturbing the formation of regular monobase-cellulose salt, then the temperature required for dissolution should not be affected by combining the bases. On the other hand, cellulose aggregation, whether induced by temperature or time, should be affected.\u003c/p\u003e"},{"header":"Material and methods","content":"\u003cp\u003e\u003cstrong\u003e2.1.\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eMaterials\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eMicrocrystalline cellulose (MCC), Avicel PH-101 purchased from FMC BioPolymer, a purified partially depolymerized cellulose made by acid hydrolysis of speciality wood pulp was used throughout the study. Granulated sodium hydroxide (NaOH), known commercially as Emplura, and tetramethylammonium hydroxide (TMAH) 25\u0026nbsp;wt.% (aq) were purchased from Merck (previously Sigma-Aldrich) and used as received. Hydrochloric acid (1 M, reagent grade) was purchased from VWR.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.2.\u0026nbsp;Dissolution of cellulose\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTwo different protocols were used for the dissolution studies of cellulose. At cellulose concentrations of 10 g/L, the solvent temperature was adjusted before adding the cellulose. For cellulose concentrations of 30 g/L, the cellulose was added to the solvent at room temperature, and then the solution was brought to the desired temperature.\u0026nbsp;This was applied because the formation of a viscous, gel-like shell could be observed when trying to dissolve 30 g/L (above the overlap concentration) in the pre-cooled solvent, hindering and slowing down dissolution. The formation of the gel-like layer was suppressed when the cellulose was first mixed with the solvent at room temperature. The temperature of the solutions was controlled using a Julabo F25 circulating oil bath, with a temperature probe measuring the temperature of the solvent/solutions in situ. A mechanical stirrer (IKA Eurostar, power control visc) was used at a speed of 350 rpm for a total volume of 100 ml solution in a 250 ml beaker. The cellulose solution was always allowed to stir for 30 minutes at the desired dissolution temperature before sampling.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.3.\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eTime and temperature stability of the solutions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFor measurements of the stability of the solutions, cellulose was first dissolved as detailed in section 2.2, in 2 M base(aq) at – 5 °C. For studies of the temperature stability, the solution was kept in the dissolution vessel and the temperature of the solution was raised under stirring in 10 °C steps, to a maximum of 75 °C. The transmission was measured at each step as soon as the solution had reached the desired temperature. The entire procedure, starting from when the cellulose had been dissolved at – 5 °C under stirring for 30 minutes, took ca 2.5 h. When measuring the stability over time, the cellulose was transferred to a beaker after the dissolution step and stored at the desired temperature (8 °C in a fridge, 20 °C at room temperature or 35 °C in an oven) without stirring.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe transmission of the solutions was measured\u0026nbsp;with a SPECORD 205 UV‐VIS spectrophotometer from Analytik Jena at room temperature. Plastic disposable cuvettes with a light path length of 10 mm were used throughout the measurements, and water was used as the background. A scan was performed over a range of 250 to 1100 nm, and the transmission at 800 nm was used as an indication of whether the cellulose had dissolved or not,\u0026nbsp;as the cellulose free solvents showed a 100% transmission at this wavelength and any undissolved fibres present in the solutions will be larger than 800 nm and detectable by a decrease in the transmission. Transmission as a function of temperature was measured (single measurements) in the interval -10 to 50°C, and the obtained trends studied (Figure 1 and 2). Transmission dependency on time was measured in the interval 0-100 h (at least two measurements per time point) and investigated in Figure 3.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.4.\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eX-ray diffraction of undissolved but swollen cellulose immersed in solvent\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSamples were prepared by immersing cellulose in the solvent at a cellulose concentration of 0.167 g/ml in 1.75 M base(aq) solvent, the conditions where (based on the previous reports and experimental experience) only swelling, no dissolution was expected. The swollen samples were then transferred to capillaries or clamped between two Kapton windows in a “sandwich cell”. They were then stored at room temperature for at least 2 h, + 3 °C for 22 h or – 20 °C for 3 h before being measured. Wide-angle X-ray measurements were conducted at room temperature and performed using a Mat:Nordic from SAXSLAB with a Rigaku 003+ high brilliance microfocus\u0026nbsp;Cu-radiation source and a Pilatus 300K detector. The measured q-range was 4 × 10\u003csup\u003e-3\u003c/sup\u003e - 2.3 Å\u003csup\u003e-1\u003c/sup\u003e. The two-dimensional scattering pattern was radially averaged using the SAXSGui software and the sample data were corrected by subtracting the solvent and sample holder data, unless otherwise stated.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.5.\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eSolid-state NMR spectroscopy of cellulose immersed in solvent\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSolid-state NMR measurements were performed on the same set of samples swollen at – 20 °C as for X-ray diffraction. The measurements were conducted on a Bruker Avance III 500, operating at 500.13 MHz \u003csup\u003e1\u003c/sup\u003eH and 125.76 MHz \u003csup\u003e13\u003c/sup\u003eC, equipped with a 4 mm HX CP-MAS probe. Measurements were conducted at 298 K with a MAS spinning rate of 10 kHz. A cross-polarization magic angle spinning (\u003csup\u003e13\u003c/sup\u003eC CP/MAS) pulse sequence with a\u0026nbsp;SPINAL-64 decoupling sequence was used with acquisition parameters including a 3.0\u0026nbsp;ms \u003csup\u003e1\u003c/sup\u003eH pulse, a 1500\u0026nbsp;ms CP-contact time, 7-23 ms acquisition time, 20000 scans, and 3 s recycle delay. The chemical shifts were referenced to adamantane with CH\u003csub\u003e2\u003c/sub\u003e-signal set to 38.48 ppm.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.6.\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eMolar mass analysis\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe samples were prepared as detailed in section 2.2 by dissolving MCC in 2 M base(aq) solutions at – 5 °C at a concentration of 30 g/L for 30 minutes. The cellulose was then coagulated using HCl(aq) and washed until the pH was neutral and the filtrate showed a conductivity below 10 µS. The molecular weight distributions were analyzed by MoRe Research AB, Örnsköldsvik, Sweden, using a PL-GPC 220 with an RI detector (Polymer Laboratories). It included a set of Guard column Mixed-A 20 µm (7.5 * 50 mm) and 2 Mixed-A 20 μm (300 * 7.5 mm), also from Polymer Laboratories, connected in series. The mobile phase contained 0.5 w/v% LiCl /DMAc with flow rate of 1 ml/min operating at 70\u0026nbsp;°C. Pullulan polysaccharides obtained from Polymer Lab were used as calibration standards. Prior to analysis, 25 mg of the sample was solvent-exchanged 3 times with 5 ml of methanol for 30 minutes, followed by a further 3 times solvent exchange step using DMAc for 30 minutes. The excess of DMAc was then removed and 5 ml of 8% (w/v) LiCl/DMAc was added and left overnight at ambient temperature, with mild magnetic stirring. The samples were thereafter diluted with 20 ml of DMAc.\u0026nbsp;\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cp\u003e\u003cstrong\u003e3.1.\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eTemperature window for dissolution\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTo be able to compare the solvents at their optimum concentration, and to see if there was a synergistic effect of combining NaOH with TMAH, the temperature window for dissolution at different base concentrations was investigated. It was found that for the three solvents (NaOH(aq), TMAH(aq) and 50/50 NaOH/TMAH(aq)), the window was quite similar (see Figure S1 in the supporting information). The transmission never reached 100% for cellulose in NaOH(aq), which is in line with our previous findings that even at low concentrations of MCC, undissolved material remained in solution (Swensson et al., 2021). The temperature window in which the transmission was close to 100% (and dissolution to some degree occurred) expanded for all three solvents with increasing base concentration up to approximately 2 M, which was the optimum. When going above 2 M, the temperature window shrank again. No synergistic effect of the two bases on the temperature required for dissolution could be observed.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.2.\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eStability of the solutions over time and during an increase in temperature\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe temperature stability after dissolution was measured through monitoring the transmission as a function of increasing solvent temperature at two different concentrations, a more dilute at 10 g/L (Figure 1a) and more concentrated 30 g/L (Figure 1b). The concentrations were chosen based on our previous studies, showing that the overlap concentration (approximated by the inverse of the intrinsic viscosity) was ca. 10 g/L for all three solvents at 25 \u0026deg;C (Swensson et al., 2020a). The result was that at 10 g/L, cellulose dissolved in TMAH(aq) remained stable over the entire temperature interval measured, while in NaOH(aq) the solution became unstable already above 35 \u0026deg;C and the cellulose started to aggregate and precipitate. The cellulose in the 50/50 NaOH/TMAH(aq) solution showed an intermediate behaviour in relation to the two bases (aggregation could be observed above 55 \u0026deg;C), thus, revealing no synergistic effect of the bases. Interestingly, though, a synergistic effect could be observed at 30 g/L: the 50/50 NaOH/TMAH(aq) solution gradually deteriorated when the temperature was raised, but showed a significantly better resistance towards increasing temperature than NaOH(aq) or even TMAH(aq). In the diluted region, stability of the dissolved state is likely governed primarily by the conformation and hydration of individual cellulose chains. The deterioration of these properties due to increasing temperature is most effectively counteracted by the organic hydroxide, TMAH, which is better equipped to stabilize the individual amphiphilic cellulose chains than either NaOH(aq) or the 50/50 combination. On the other hand, in the concentrated regime, above the overlap concentration, where interaction between the chains becomes significant, the combination of the two bases seems to be superior in preventing chain-chain association, possibly by preventing regular packing, as will be further discussed below. It should be noted that during the stability measurements, an increasing colour change in cellulose-containing solutions was observed with temperature or storage time. The molecular weight of the cellulose before and after dissolution was therefore measured, but no significant degradation was measured (see more in the supporting information).\u003c/p\u003e\n\u003cp\u003eInterestingly, when measuring the stability of the solutions over time, similar results to those observed for the temperature stability above the overlap concentration were found (Figure 2). The NaOH(aq) solution was less stable over time at all temperatures, and the stability of the TMAH(aq) solution also decreased with increasing temperature. The 50/50 NaOH/TMAH(aq) solution was more stable over prolonged storage times at all investigated temperatures (Figure 2). Stability over time is probably related to formation of the thermodynamically most stable state and is, according to these results, likely favoured by the combination of the two bases preventing chain-chain association (by providing a favourable hydration/structure of the dissolved state and/or preventing regular packing during coagulation). On the other hand, a raise in temperature might drive the solution into a kinetically arrested (gelled) state primarily due to deteriorated stabilization of the individual polymers. In this state, the presence of the two bases does not show any synergy, unless the chain-chain interactions are significant (i.e. unless in concentrated region). Thus, the coagulation tendency of these solutions could be seen as an interplay between kinetic and thermodynamic driving forces, each of them affected by dissolution conditions, properties of the dissolved state and changes it is subjected to.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.3.\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eThe crystal structure of the cellulose and changes occurring during intracrystalline swelling and dissolution\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.3.1.\u0026nbsp; \u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eThe crystal structure of the cellulose before and after dissolution\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe transformation from cellulose I to cellulose II in all three solvents after dissolution, coagulation, washing and drying of the MCC was confirmed through X-ray diffraction (see Figure 3). The identified peaks are listed in Table 1.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eTable 1: Identified peaks of the MCC before (CI) and after coagulation in the respective solvents (CII).\u0026nbsp;\u003c/p\u003e\n\u003ctable border=\"0\" cellspacing=\"0\" cellpadding=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 94px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 56px;\"\u003e\n \u003cp\u003eCI\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 56px;\"\u003e\n \u003cp\u003eCII NaOH\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 62px;\"\u003e\n \u003cp\u003eCII TMAH\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 82px;\"\u003e\n \u003cp\u003eCII\u003c/p\u003e\n \u003cp\u003e50/50\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 94px;\"\u003e\n \u003cp\u003ePeak 1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 56px;\"\u003e\n \u003cp\u003e15.2\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 56px;\"\u003e\n \u003cp\u003e12.4\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 62px;\"\u003e\n \u003cp\u003e12.8\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 82px;\"\u003e\n \u003cp\u003e12.8\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 94px;\"\u003e\n \u003cp\u003ePeak 2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 56px;\"\u003e\n \u003cp\u003e16.7\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 56px;\"\u003e\n \u003cp\u003e20.2\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 62px;\"\u003e\n \u003cp\u003e20.1\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 82px;\"\u003e\n \u003cp\u003e20.2\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 94px;\"\u003e\n \u003cp\u003ePeak 3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 56px;\"\u003e\n \u003cp\u003e20.8\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 56px;\"\u003e\n \u003cp\u003e21.7\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 62px;\"\u003e\n \u003cp\u003e21.5\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 82px;\"\u003e\n \u003cp\u003e21.5\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 94px;\"\u003e\n \u003cp\u003ePeak 4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 56px;\"\u003e\n \u003cp\u003e22.9\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 56px;\"\u003e\n \u003cp\u003e28.3\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 62px;\"\u003e\n \u003cp\u003e28.3\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 82px;\"\u003e\n \u003cp\u003e28.5\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 94px;\"\u003e\n \u003cp\u003ePeak 5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 56px;\"\u003e\n \u003cp\u003e34.6\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 56px;\"\u003e\n \u003cp\u003e35.0\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 62px;\"\u003e\n \u003cp\u003e35.4\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 82px;\"\u003e\n \u003cp\u003e34.8\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003e\u0026nbsp;\u003cstrong\u003e3.3.2. \u0026nbsp;\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eIntracrystalline swelling observed through X-ray diffraction\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTo investigate changes in the crystalline structure indicative of crystalline rearrangement during dissolution and a subsequent coagulation (i.e. the typical conversion from cellulose I to cellulose II) in the studied bases, the cellulose was soaked in the respective solvents and measured with X-ray diffraction. Soaking at room temperature (Figure S3 in the supporting information), or even at + 3 \u0026deg;C for 22 h (Figure 4a) did not change the crystalline structure, as it remained as cellulose I. After storage at \u0026ndash; 20 \u0026deg;C for 3 h, upon which the samples froze, the base had penetrated the crystal structure and altered it (see Figure 4b). Similar observations, that low temperatures are required before intracrystalline swelling occurs, have been recently reported for cellulose immersed in sulfuric acid (Li et al., 2021). The diffraction angles for the identifiable peaks in the swollen samples can be found in Table 2, along with the main peaks of cellulose I (measured) and cellulose II (Nam et al., 2016).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAll three samples showed different patterns (Figure 4b), none of which corresponded completely to cellulose I or cellulose II. For cellulose soaked in TMAH(aq), there were however some similarities to cellulose II, as the peaks at 20.7\u0026deg; and 21.7\u0026deg; are similar to those usually found at 20.5\u0026deg; and 21.9\u0026deg;, attributed to the (110) and (020) planes when named as by Nam et al. (Nam et al., 2016). In addition, a shoulder peak not found in cellulose II could be observed at 5.4\u0026deg; (Figure 6b). A similar observation was made by Sisson and Saner, who measured the swelling of ramie fibers in TMAH(aq) at room temperature. They reported for TMAH interplanar distances corresponding to peaks at \u0026nbsp;6.6\u0026deg; and 22.2\u0026deg; (\u0026lambda; = 1.54 \u0026Aring;), similar to the peaks observed here (Sisson \u0026amp; Saner, 1939). Relatively strong signals observed for the TMAH sample indicate a comparably strong tendency of the TMAH-treated cellulose to rearrange into an ordered structure.\u003c/p\u003e\n\u003cp\u003eTable 2: Peaks obtained from X-ray diffraction (XRD) of the starting material (CI) and of MCC soaked in 1.75 M(aq) base, stored at - 20 \u0026deg;C for 3 h. Peaks of cellulose II (CII) as reported by Nam et al. (Nam et al., 2016).\u003c/p\u003e\n\u003ctable border=\"0\" cellspacing=\"0\" cellpadding=\"0\" width=\"360\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 64px;\"\u003e\n \u003cp\u003eCI\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 57px;\"\u003e\n \u003cp\u003eCII\u003c/p\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 72px;\"\u003e\n \u003cp\u003eMCC NaOH\u003c/p\u003e\n \u003cp\u003eFreezer\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 72px;\"\u003e\n \u003cp\u003eMCC TMAH\u003c/p\u003e\n \u003cp\u003eFreezer\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 95px;\"\u003e\n \u003cp\u003eMCC 50/50\u003c/p\u003e\n \u003cp\u003eFreezer\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 64px;\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 57px;\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 72px;\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 72px;\"\u003e\n \u003cp\u003e5.4\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 95px;\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 64px;\"\u003e\n \u003cp\u003e15.2\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 57px;\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 72px;\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 72px;\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 95px;\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 64px;\"\u003e\n \u003cp\u003e16.7\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 57px;\"\u003e\n \u003cp\u003e12.3\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 72px;\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 72px;\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 95px;\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 64px;\"\u003e\n \u003cp\u003e20.8\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 57px;\"\u003e\n \u003cp\u003e20.5\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 72px;\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 72px;\"\u003e\n \u003cp\u003e20.7\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 95px;\"\u003e\n \u003cp\u003e21.1\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 64px;\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 57px;\"\u003e\n \u003cp\u003e21.9\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 72px;\"\u003e\n \u003cp\u003e21.5\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 72px;\"\u003e\n \u003cp\u003e21.7\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 95px;\"\u003e\n \u003cp\u003e22.3\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 64px;\"\u003e\n \u003cp\u003e22.9\u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 57px;\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 72px;\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 72px;\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 95px;\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003eFor MCC soaked in NaOH(aq), only one peak at 21.5\u0026deg; could be observed, although based on its appearance it might be deconvoluted into two peaks (Figure 4b). It can be noted that the dip in the intensity obtained after background subtraction is due to that the pure solvent and the sample with cellulose did not entirely match, indicating that the structure of the NaOH might change upon the addition of cellulose. However, the background subtraction did not alter the position of the cellulose peak, and the original data can be viewed in the supporting information (Figures S4 and S5). The diffraction pattern is similar to the one observed by Crawshaw et al., (Crawshaw, Bras, Mant, \u0026amp; Cameron, 2002) reporting a single crystalline peak at 20\u0026deg; identified as the (200) reflection for ramie fibres immersed in approximately 4 M NaOH(aq) at room temperature, and the spectrum was assigned to Na-cellulose I. \u0026nbsp;For the same type of sample, Kobayashi et al. instead reported an obscure pattern, with two strong equatorial peaks at 4.45 and 4.23 \u0026Aring; (ca 20.0\u0026deg; and 20.9\u0026deg;), and a weaker one at 12.5 \u0026Aring; (7.05\u0026deg;). They refer to this as Na-Cellulose I and attribute the peak at 7\u0026deg; to the swelling of the crystalline lattice by the NaOH solution (Kobayashi et al., 2011). Taken together, this observation points towards an incomplete crystalline rearrangement of cellulose in the NaOH(aq) model system.\u003c/p\u003e\n\u003cp\u003eInterestingly, for the 50/50 combination, two weak peaks could be identified (Figure 4b). The presence of diffraction peaks shows that the cellulose can form a reoccurring structure in the presence of two bases, but their weak appearance points out that there is not much crystalline material in the sample.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.3.3. \u0026nbsp;\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eIntracrystalline swelling observed through\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003esolid-state NMR\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTo further investigate the intracrystalline swelling, the samples that were swollen at \u0026ndash; 20 \u0026deg;C were also measured with solid-state NMR spectroscopy. The \u003csup\u003e13\u003c/sup\u003eC CP/MAS spectra can be seen in Figure 5. Similar to what was seen from the diffraction results, neither cellulose I nor cellulose II signals can be observed in the swollen samples. For the cellulose sample swollen in NaOH(aq), the spectrum is similar to that obtained from the similarly treated wood pulp (Porro, B\u0026eacute;du\u0026eacute;, Chanzy, \u0026amp; Heux, 2007), assigned to contain a mixture of Na-cellulose I and Na-cellulose Q. This lends support to also assigning the X-ray spectra of the NaOH(aq) swollen cellulose to Na-cellulose I. The spectrum of cellulose swollen with TMAH(aq) shows several signals of relatively high order. Although the pattern bears a resemblance to that seen for cellulose II, it does not represent its exact structure. As was described in the previous section, the XRD measurements also indicate that the structure is similar, but not identical, to that of cellulose II. The crystalline conversions in the swollen samples can be seen as condensed consecutive steps representative of the crystalline conversions taking place during dissolution and the subsequent aggregation from the dissolved state. The ordered structure present here in the TMAH-swollen state is thus representative of the ordered structure being formed as a pre-step during aggregation of the well-dissolved cellulose prior to its final conversion into cellulose II, as an intermediate. This is in line with the stability measurements (presented in Section 3.2), which show a surprisingly rapid aggregation of MCC in TMAH(aq), indicating the formation of a relatively ordered structure. \u0026nbsp;Noticeably, the MCC swollen in the 50/50 NaOH/TMAH(aq) solution showed peaks with a similar appearance to the sample swollen only with NaOH, also indicating a lower structural order.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe results from both the temperature and the time stability investigations show that the NaOH/TMAH mixture gives improved solution stability over time and in concentrated region compared to only NaOH or only TMAH. It is clear that at low temperatures, the base interacts with the cellulose, penetrates the cellulose crystal and that the cellulose changes the crystalline structure from Cellulose I to Cellulose II after dissolution and washing. We hypothesize that the crystallization is governed by stabilizing forces in the dissolved state as well as driving forces behind the formation of aggregated structures. In light of that, the superior stability of the 50/50 NaOH/TMAH(aq) could be understood as a result of either favourable thermodynamics of the dissolved state or less favoured formation of aggregates. Having in mind the lack of any observable synergistic effect on the required dissolution conditions (see Section 3.1), the tendency to form aggregated states might be of importance. Likewise, an explanation for the rather drastic and unexpected decrease in temperature stability of cellulose in TMAH(aq) at 30 g/L, should be sought for in the same phenomenon, as the more hydrophobic TMAH cation was expected to contribute to increased stability of the dissolved state when the temperature increased.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eIf the formation of a crystalline complex between TMAH and cellulose can explain the poor stability of the TMAH(aq) solutions, a similar explanation can be proposed for the increased stability of the 50/50 combination. Atacticity, or irregular side groups, can have a detrimental effect on the crystallization of a polymer. Analogously, it can be envisioned that a random distribution of two bases, closely interacting with the cellulose chains, could have a similar effect on the crystallisation of cellulose II from a dissolved state. However, the bases are not covalently bonded side groups, and since they can move away from the chain, this could also explain why the combination delays but does not prevent aggregation. In other words, this could be important for the stability of the solutions if the presence of two bases prevents the formation of an ordered precursor for cellulose II formation.\u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eAqueous solutions of NaOH or TMAH show very similar temperature dependency for dissolution of cellulose within the range of 1.5 to 2.25 M base. Combining NaOH with TMAH did not significantly affect the temperature at which dissolution occurred. However, it increased the stability of more concentrated solutions over time and against increasing temperatures. Given the X-ray diffraction and solid-state NMR measurements, this might be due to the presence of two bases disturbing the crystallization of cellulose II upon aggregation from the dissolved state. Similarly, the formation of a crystalline complex between TMAH and cellulose, indicated by prominent NMR- and XRD-signals, is proposed to act as a precursor for cellulose II formation, possibly causing poor stability of cellulose in TMAH(aq).\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003e5.\u0026nbsp; \u0026nbsp;Acknowledgements and funding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study was funded by FORMAS, the Swedish Research Council for Sustainable Development; VINNOVA, Sweden’s innovation agency; Chalmers Area of Advance Materials Science. This work was performed in part at the Chalmers Material Analysis Laboratory, CMAL and the solid-state NMR measurements were conducted at the Swedish NMR Centre in Umeå, Sweden.\u0026nbsp;\u003c/p\u003e\n\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eB.S. drafted the main manuscript text and performed all the lab work, except for the NMR-part which was performed and drafted by A.I. All authors reviewed the manuscript.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eProcessed data is provided within the manuscript or supplementary information files. The raw data is stored with the authors and can be provided upon request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eBrownsett, T., \u0026amp; Clibbens, D. A. (1941). 4\u0026mdash;The dissolution of chemically modified cotton cellulose in alkaline solutions. Part VII. The solvent action of solutions of trimethylbenzyl-and dimethyldibenzyl-ammonium hydroxides (tritons B and F). \u003cem\u003eJournal of the Textile Institute Transactions\u003c/em\u003e, \u003cem\u003e32\u003c/em\u003e(2), T32\u0026ndash;T44. https://doi.org/10.1080/19447024108659357\u003c/li\u003e\n \u003cli\u003eBudtova, T., \u0026amp; Navard, P. (2016). Cellulose in NaOH\u0026ndash;water based solvents: a review. \u003cem\u003eCellulose\u003c/em\u003e, \u003cem\u003e23\u003c/em\u003e(1), 5\u0026ndash;55. https://doi.org/10.1007/s10570-015-0779-8\u003c/li\u003e\n \u003cli\u003eCrawshaw, J., Bras, W., Mant, G. R., \u0026amp; Cameron, R. E. (2002). Simultaneous SAXS and WAXS investigations of changes in native cellulose fiber microstructure on swelling in aqueous sodium hydroxide. \u003cem\u003eJournal of Applied Polymer Science\u003c/em\u003e, \u003cem\u003e83\u003c/em\u003e(6), 1209\u0026ndash;1218. https://doi.org//10.1002/app.2287\u003c/li\u003e\n \u003cli\u003eDavidson, G. F. (1936). 10-The dissolution of chemically modified cotton cellulose in alkaline solutions. Part II. 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Dissolution mechanism of cellulose in quaternary ammonium hydroxide: Revisiting through molecular interactions. \u003cem\u003eCarbohydrate Polymers\u003c/em\u003e, \u003cem\u003e174\u003c/em\u003e, 400\u0026ndash;408. https://doi.org/10.1016/j.carbpol.2017.06.078\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"cellulose","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"cels","sideBox":"Learn more about [Cellulose](https://www.springer.com/journal/10570)","snPcode":"10570","submissionUrl":"https://submission.nature.com/new-submission/10570/3","title":"Cellulose","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Cellulose, dissolution, NaOH, swelling, hydroxide, gelation, X-ray, NMR","lastPublishedDoi":"10.21203/rs.3.rs-6828754/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6828754/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eCellulose solutions in aqueous hydroxide bases often display an intriguing and limited stability over time and within a limited temperature range. Our previous studies pointed out a synergistic effect of combining two hydroxide bases \u0026ndash; NaOH and tetramethylammonium hydroxide (TMAH) \u0026ndash; on stability over time and on dissolution capacity of cellulose. Here, we hypothesise that the delayed gelation of cellulose in this system is related to the ability of the base combination to disturb the formation of a stable monobase-cellulose salt during aggregation. We combine studies addressing time and temperature dependency of the solution stability with X-ray diffraction and solid-state NMR studies of swollen cellulose to address the hypothesis. The results showed that the dissolution window at low temperatures turned out to be similar for the individual and combined bases. However, increased stability over time was observed in the 50/50 NaOH/TMAH(aq) in the semi-dilute region compared to the individual base solutions, and could be related to different grades of order observed in swollen model samples.\u003c/p\u003e","manuscriptTitle":"Temperature and time dependence of cellulose solution stability in combined aqueous hydroxide bases","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-07-10 08:59:17","doi":"10.21203/rs.3.rs-6828754/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-06-13T20:41:12+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-06-13T20:38:01+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-06-06T06:01:46+00:00","index":"","fulltext":""},{"type":"submitted","content":"Cellulose","date":"2025-06-05T11:26:17+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"cellulose","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"cels","sideBox":"Learn more about [Cellulose](https://www.springer.com/journal/10570)","snPcode":"10570","submissionUrl":"https://submission.nature.com/new-submission/10570/3","title":"Cellulose","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"1f64c17e-4957-466e-a13f-530320c2ba27","owner":[],"postedDate":"July 10th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"in-revision","subjectAreas":[],"tags":[],"updatedAt":"2025-09-30T13:23:33+00:00","versionOfRecord":[],"versionCreatedAt":"2025-07-10 08:59:17","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6828754","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6828754","identity":"rs-6828754","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}