Acidic Deep Eutectic Solvents As Hydrolytic Media ... - ACS Publications

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Acidic deep eutectic solvents as hydrolytic media for cellulose nanocrystal production Juho Antti Sirviö, Miikka Visanko, and Henrikki Liimatainen Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.6b00910 • Publication Date (Web): 01 Aug 2016 Downloaded from http://pubs.acs.org on August 8, 2016

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Acidic deep eutectic solvents as hydrolytic media for cellulose nanocrystal production Juho Antti Sirviö*, Miikka Visanko, and Henrikki Liimatainen Fibre and Particle Engineering Research Unit, University of Oulu, P.O. Box 4300, FI-90014, Finland. KEYWORDS: Cellulose nanocrystals, deep eutectic solvent, nanoscience, green chemistry

ABSTRACT

In this study, a new method to fabricate cellulose nanocrystals (CNCs) based on DES pretreatment of wood cellulose fibers with choline chloride and organic acids are reported. Oxalic acid (anhydrous and dihydrate), p-toluenesulfonic acid monohydrate, and levulinic acid were studied as acid components of DESs. DESs were formed at elevated temperatures (60–100 ºC) by combining choline chloride with organic acids and were then used to hydrolyze less ordered amorphous regions of cellulose. All the DES treatments resulted in degradation of wood fibers into micro-sized fibers and after mechanically disintegrating, CNCs were successfully obtained from choline chloride/oxalic acid dihydrate-treated fibers, whereas no liberation of CNCs was observed with other DESs. The DES-produced CNCs had a width and length of 9–17 nm and 310–410 nm, respectively. The crystallinity indexes (CrIs) and carboxylic acid content of

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the CNCs were 66–71% and 0.20–0.28 mmol/g, respectively. CNCs exhibited good thermal stabilities (the onset thermal degradation temperatures ranged from 275–293 ºC). The demonstrated acidic DES method exhibits certain advantages over previously reported CNC productions, namely milder processing conditions and easily obtainable and relatively inexpensive biodegradable solvents with low toxicity (compared, e.g., to ILs).

Introduction Ionic liquids (ILs) can be utilized as an efficient reaction and treatment media for various biomasses.1,2 ILs contain large organic cations and inorganic or organic anions having melting point lower than 100 ºC.3 Compared with traditional molecular solvents, they have several advantages, including high solvent capacity towards both organic and inorganic materials and negligible vapor pressure, which reduces the volatile organic compound (VOC) emissions. Despite their good properties, ILs have several drawbacks, including synthesis mainly from oilbased chemicals; toxicity; low biodegradability; low moisture tolerance; and high cost.4,5 Deep eutectic solvents (DESs) are chemicals that are almost equivalent to ILs (DESs are even regarded as a subcategory of ILs according to some sources6) and exhibit similar properties, such as good solvent capacity and low vapor pressure.7 However, compared with ILs, DESs can be obtained by less demanding methods by simply heating and mixing two or more components at an elevated temperature (mainly around 100 ºC). On a larger scale, DES can be efficiently synthesized using the twin screw extrusion process.8 DESs typically consist of a hydrogen bond donor and acceptor pair. The formation of strong hydrogen bonding and complexation between the components is assumed to prevent


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crystallization, consequently causing the drop in the melting point and resulting in an eutectic mixture.6 A significant amount of DESs can be attained directly from low toxic natural components, such as choline chloride, urea, and glycerol.9 However, the toxicity of DES can be higher compared to its individual components,10 but the toxicity greatly depends on the studied organism.11,12 There is even evidence that the formation of DESs can lower the toxic effect of individual components (e.g., organic acids). DES systems consisting of choline chloride and organic acids have been shown to exhibit certain anti-microbial properties, yet they are well biocompatible, and their toxicity is lower compared to some ILs based on imidazolium- and pyridinium.12 Choline chloride-based DESs are also readily biodegradable.13 Consequently, these favorable properties have encouraged many researchers to switch their interest from traditional molecular solvents and ILs to DESs. Cellulose nanocrystals (CNCs) are stiff, rod-like nanosized (length generally below a micrometer and width from a few nanometers to tens of nanometers) cellulosic materials isolated from natural biomasses, such as plant fibers.14 CNCs have several unique properties such as lightweight, biodegradability, stiffness, a large surface area and are produced from cellulose which is renewable and the most abundant biopolymer.15 Due to the high reinforcement capability, CNCs are widely used in polymer nanocomposites to improve their properties.16 CNCs can also be used as a as replacement of toxic oil-based chemicals in mineral flotation17, as a stabilizer of oil-in-water emulsions18,19 and in biomedicine.20 Acidic hydrolysis using aqueous mineral acids, such as sulfuric21, hydrochloric22, hydrobromic23, and phosphoric acids,24 or polyoxometalates25 has been the most common method to dissolve the amorphous regions of cellulose to yield CNCs. Several other methods, mostly based on different oxidation reactions, have also been applied in CNC production.26–30 Recently, ILs were shown to act as an efficient


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swelling and hydrolysis medium to fabricate CNCs.31–34 DESs have so far been reported to have been used in pretreatment media for cellulose saccharification35,36 and in the fabrication of cellulose nanofibers (CNFs)37 (by retaining amorphous cellulose regions intact, allowing the individualization of flexible and long nanosized filaments). In this study, we report the results of DESs obtained from choline chloride and organic acids as potential acidic hydrolytic solvents to fabricate CNCs from dissolving pulp. For this purpose, oxalic (anhydrous and dihydrate), p-toluenesulfonic monohydrate and levulinic acids were used as the acidic components of DESs under different hydrolytic conditions (temperature and time). The CNCs obtained after DES pretreatment and mechanical microfluidization were characterized by transmission electron microscopy (TEM), wide-angle X-ray diffraction (WAXD), diffuse reflectance infrared Fourier transform spectrum (DRIFT), thermogravimetric analysis (TGA), and conductive titration. Experiments Materials Dissolving cellulose pulp (softwood) was obtained as dry sheets and used as a cellulose material after disintegration in deionized water. The disintegrated pulp was filtered, washed with ethanol, and dried at 60 ºC for 24 h. The properties of cellulose pulp are discussed elsewhere.38 Choline chloride and p-toluenesulfonic acid monohydrate were obtained from TCI (Germany) and oxalic acid (anhydrous and dihydrate) and levulinic acid were purchased from Sigma Aldrich (Germany). All the chemicals were used as received, without further purification. Hydrolysis of cellulose pulp using deep eutectic solvent


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Molar ratios between choline chloride and acids were 1:2 for anhydrous oxalic acid39 and levulinic acid40 and 1:1 for oxalic acid dihydrate41 and p-toluenesulfonic acid monohydrate.42 All the DESs were produced by heating the mixtures at 100 ºC, expect in the case of ptoluenesulfonic acid monohydrate, where a temperature of 60 ºC was used. DES-pretreatments of cellulose were performed by adding 1.2 g of dried pulp into 120 g of DES during mixing for the desired time (2–4 h) at a predetermined temperature (60–120 ºC). The reaction conditions used are presented in Table 1. After mixing, the reaction mixture was removed from the heating source (oil bath) and 100 ml of deionized water was added. The pretreated pulp suspensions were filtered and washed with 400 ml of deionized water. Fabrication of cellulose nanocrystals from deep eutectic solvent-pretreated cellulose fibers DES-pretreated aqueous cellulose fiber dispersions (0.5%) were first mixed at 11 000 rpm with an Ultra-Turrax mixer (IKA T25, Germany) at a pH of 7 (the pH was adjusted using dilute NaOH solution) for 1 min. The fibers were then disintegrated using a microfluidizer (Microfluidics M-110EH-30, USA) to individualize CNCs. Three passes through 400 µm and 200 µm chambers at a pressure of 1300 bar and then three passes through 200 µm and 87 µm chambers at a pressure of 2000 bar were used. Scanning Electron Microscopy Scanning electron microscopy (SEM, Zeiss Zigma HD VP, Germany) images of the freeze-dried (liquid nitrogen and vacuum drying) samples filtered on a polycarbonate membrane with a pore size of 0.2 µm were obtained. The accelerating voltage during imaging was 0.5 kV. Transmission electron microscopy


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The morphological features of the fabricated CNCs were analyzed with a Tecnai G2 Spirit transmission electron microscope (FEI Europe, Eindhoven, the Netherlands). Samples were prepared by diluting each sample with ultrapure water. A small droplet of the dilution was placed on top of a carbon-coated copper grid. The grid was first coated with polylysine by applying a small droplet of 0.1% solution of polylysine on the top of the grid and allowing it to stand for 3 min. The excess polylysine was removed from the grid by touching the droplet with a corner of a filter paper. The small droplet of the sample was then placed on the top of the grid. The excess amount of the sample was removed from the grid by touching the droplet with a corner of a filter paper. Negative staining of the samples was performed by placing a droplet of uranyl acetate (2% w/v) on top of each specimen. The excess uranyl acetate was removed with filter paper, as described above. The grids were dried at room temperature and analyzed at 100 kV under standard conditions. Images were captured using a Quemesa CCD camera, and iTEM image analysis software (Olympus Soft Imaging Solutions GMBH, Munster, Germany) was used to measure the width of the individual nanocrystals. In total, 60 CNCs of each sample were measured. The final results were averaged and standard errors were calculated. X-ray diffraction The crystalline structure of the original pulp and CNCs was investigated using wide-angle X-ray diffraction (WAXD). Measurements were conducted on a Rigaku SmartLab 9kW rotating anode diffractometer (Japan) using a Co Kα radiation (40 kV, 135 mA) (λ = 1.79030 nm). Samples were prepared by pressing tablets of freeze-dried celluloses to a thickness of 1 mm. Scans were taken over a 2θ (Bragg angle) range from 5°–50° at a scanning speed of 10°/s, using a step of 0.5°. The degree of crystallinity in terms of the CrI was calculated from the peak intensity of the


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main crystalline plane (200) diffraction (I200) at 26.2° and from the peak intensity at 22.0° associated with the amorphous fraction of cellulose (Iam), according to Eq. (1)43: ூ

‫ = ܫݎܥ‬ቀ మబబூ

ିூೌ೘

మబబ

ቁ ∙ 100%.

(1)

It should be noted that due to the Co Kα radiation source, the cellulose peaks have different diffraction angles compared to results obtained using the Cu Kα radiation source. Diffuse reflectance infrared Fourier transform spectroscopy Chemical characterization of raw cellulose and DES-pretreated cellulose was performed using DRIFT. Spectra were collected with a Bruker Vertex 80v spectrometer (USA) from freeze-dried samples. Spectra were obtained in the 600–4000 cm−1 range, and 40 scans were taken at a resolution of 2 cm−1 from each sample. Limiting viscosity The effect of the DES pretreatment on the degree of polymerization was estimated using the limiting viscosity, measured in cupriethylenediamine solution according to the ISO 5351 standard. Samples were freeze-dried prior to the measurements. Thermogravimetric analysis TGA measurements were carried out with a thermal analyzer Netzsch STA 409 PC (Germany) apparatus in two different atmospheres: under nitrogen flow and under air flow (dynamic air), both with a constant rate of 60 mL min-1. Each measurement was made using approximately 5 mg of the freeze-dried sample, which was heated from 25–600 °C at a scanning rate of 2 °C/min-


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. The temperature of polymer degradation, Td, was taken as the temperature at the onset point of

the weight loss in the TGA curve obtained. Results and discussion The fabrication of CNCs is commonly based on the hydrolysis and dissolution of amorphous regions of cellulose using strong acids, ILs, or oxidative treatments. Here, four different DES systems containing choline chloride as a hydrogen bond donor and organic acids as a hydrogen bond acceptor were studied as potential new green media for CNC individualization. Schematic illustration of the CNC production process is presented in Figure 1. Table 1 presents the reaction conditions used and the yields after DES pretreatments. All DES pretreatments notably decreased the cellulose mass (yield of 66–88%), and the lowest yield was obtained after 2 and 4 h reaction time with oxalic acid dihydrate and p-toluenesulfonic acid-based DESs, respectively. However, the yield was seen to increase slightly with an oxalic acid dihydrate-based system when reaction time increased. This may be due to the esterification of the hydroxyl groups of cellulose with oxalic acid, either by the formation of monoester or cross-linked diester. In particular, the formation of diester can increase the yield because it can prevent the dissolution of the hydrolysis product of cellulose. However, when p-toluenesulfonic acid was used, the yield decreased when reaction time was increased from 2–4 h, suggesting that DES systems do not possess significant reactivity with cellulose.


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Figure 1. A schematic flow diagram of CNC production using DES. 1st step: Hydrolysis of wood cellulose fibers in DES at elevated temperature; 2nd step: Washing and filtrating the DES treated fibers using water and vacuum filtration; 3rd step: Mechanical disintegration of DES treated and washed microfibers in water with microfluidizer. Table 1. Composition of different DES systems, reaction conditions, yields, limiting viscosity, and carboxylic acid content before and after cellulose treatments.

Sample

Acid

Dissolving pulp DES1 DES2 DES3 DES4 DES5

DES6

DES7 DES8 a

Anhydrous oxalic acid Oxalic acid dihydrate Oxalic acid dihydrate Oxalic acid dihydrate Oxalic acid dihydrate ptoluenesulfonic acid monohydrate ptoluenesulfonic acid monohydrate levulinic acid b

Reaction Molar Reaction temperature ratioa time (h) (ºC)

Carboxylic Limiting Yield acid viscosity d (%) content (dm3/kg) (mmol/g)

-

-

-

-

503

-b

1:2

2

100

72

104

0.25±0.02

1:1

2

100

68

113

0.20±0.04

1:1

4

100

76

104

0.27±0.13

1:1

6

100

78

104

0.23±0.07

1:1

2

120

73

100

0.23±0.03

1:1

2

60

70

108

-c

1:1

4

60

66

104

-c

1:2

2

100

88

267

-c

c

Choline chloride:acid. No carboxylic acid groups detected. Carboxylic acid content not measured. dCalculated from the mass of fibers after the DES treatment versus the mass of the original fibers.


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The fiber structure of the original dissolving pulp was partially degraded during DES pretreatment (Figure 2a), as noticed upon visual observation, and powder-like material was obtained (expect in the case of DES8, which remained a fluffy-like material similar to raw cellulose) after pretreatment and drying. Some of the products appeared as slightly greyish (DES5), whereas the product obtained by DES1 was brown, indicating the formation of chromophoric cellulose degradation products that could not be removed during the washing step. The degradation of the fiber structure of cellulose was confirmed by SEM (Figure 2b). Oxalic acid and p-toluenesulfonic acid-based DES pretreatments resulted in the formation of fragmented cellulose fiber particles, whereas DES8 did not significantly alter the fiber structure of cellulose.


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Figure 2. a) Dry cellulose pulp before and after DES treatments and washing and b) examples of SEM images of cellulose pulp and fibers after DES treatments. Limiting viscosity was used to indicate the effect of the DES pretreatment on the degree of polymerization of cellulose fibers. The original limiting viscosity of the dissolving pulp was decreased from 503 dm3/kg to approximately 100 dm3/kg with DES1–7 (Table 1). These results indicate strong cellulose hydrolysis during the DES pretreatment, which is typical for the mineral acid hydrolysis of cellulose during CNC production.44 No significant differences between DES pretreatments existed, except in the case of DES8, where limiting viscosity was almost three times higher compared to other DES-pretreated fibers. However, the limiting viscosity of DES8 was still almost half that of the original pulp. The differences in the limiting viscosities between DES1–7 and DES8 are most likely caused by the strong acid characteristic of oxalic acid (pKa of 1.25 and 4.14 in water45) and p-toluenesulfonic acid (pKa of -2.8 in water46) compared to levulinic acid (pKa of 4.59 in water47). The esterification of cellulose can take place in the presence of acids, such as carboxylic acids. The attachment of oxalic acid to cellulose after DES pretreatment can be seen in the DRIFT spectra in Figure 3. Compared with the cellulose pulp spectrum, all the oxalic acid dihydrate DES-pretreated fibers (DES2–5) exhibited a new band around 1740 cm-1, which is a superimposed peak of the carbonyl vibrations of ester and carboxylic acid. The overlapping of the ester and carboxylic acid carbonyl vibrations is typical of the half-esters of dicarboxylic acids of cellulose.48 The appearance of carbonyl stretching can also be seen in the spectrum of anhydrous oxalic acid DES-pretreated fibers (DES1), where no difference between original fibers and p-toluenesulfonic (DES6–7) and levulinic acids (DES8) was observed.


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DES1 DES2

Reflectance (Off scale)

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DES3 DES4 DES5

DES6 DES7 DES8 Cellulose 4000

3500

3000

2500

2000

1500

1000

Wavenumber cm-1

Figure 3. DRIFT spectra of cellulose before and after DES pretreatments. The superimposed peak of the carbonyl vibrations of ester and carboxylic acid of oxalic acid containing DESpretreated fibers is represented by a dashed line. The quantity of carboxylic acid groups on the DES-pretreated fibers was determined using conductometric titration. Titration was performed only for the oxalic acid-treated fibers, as based on the DRIFT spectra no significant amount of chemical modification occurred during the DES6-8 treatments. All the oxalic acid (anhydrous and dihydrate) DES-pretreated fibers exhibited similar carboxylic acid content (0.20–0.27 mmol/g) (Table 1) (no detectable amount of the acid groups was observed in the original cellulose pulp). Therefore, the results suggest the carboxylic acid content of DES-pretreated fibers cannot be significantly adjusted by increasing the temperature or the reaction time. This might be due to the over-esterification of diesters. All the DES-pretreated fibers were mechanically disintegrated using a microfluidizer to liberate CNCs. All the samples underwent similar mechanical treatment expect for DES1, which blocked the smallest interaction chamber (IXC) (87 µm), and DES8, which blocked even the largest IXC (200 µm). Further processing of both specimens was therefore discontinued. Other samples


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passed through the microfluidizer without any blocking. However, it was noted that instead of forming a stable suspension, p-toluenesulfonic acid DES-pretreated fibers (DES6–7) formed a milky white suspension in which particle sedimentation occurred almost immediately. All oxalic acid dihydrate DES-pretreated fibers formed slightly gel-like suspensions (Figure4) (DES2–5). These suspensions remained stable at room temperature for several weeks without any notable sedimentation, excluding DES5, for which minor sedimentation was observed after two months of free settling at room temperature.

Figure 4. Aqueous solutions (0.4%) of DES treated celluloses directly after mechanical disintegration (top row) and after free settling for two months at room temperature (bottom row). Due to the sedimentation of DES6 and 7 after disintegration, only the stable specimens (DES2– 5) were studied further. Based on the TEM images (Figure 5, for higher resolution images, see Supporting Information), all the oxalic acid dihydrate DES-pretreated samples (DES2–5) exhibited individually occurring short CNCs. Compared with CNCs obtained at 100 ºC, DES5 had a thin, needle-like structure (average width around 10 nm), whereas DES2–3 had slightly thicker crystals (average width around 15 nm) (Table 2). There was no significant difference


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within the specimens in the length of CNCs; therefore, DES5 had the highest aspect ratio (36) compared with the others (the aspect ratios of DES2–4 ranged from 21–28). The width of CNCs obtained after DES pretreatments were around twofold higher than those of the CNCs typically obtained from sulfuric acid hydrolysis of wood pulp, but the lengths of CNCs obtained here were significantly longer, resulting in similar or even higher aspect ratio values compared with the traditional acid hydrolysis method.49,50 It should be noted that no sample fractionation steps were utilized after mechanical disintegration and therefore a minor amount of longer cellulose nanofibrils was also observed, especially with samples (DES2–3) treated in the mildest conditions. In addition, few thicker (around 200 nm) rod-like particles were present in each sample. Therefore, it would be beneficial to perform some final purification (e.g., using centrifuge) to obtain CNC solutions with higher uniformity. Table 2. Average dimensions and CrIs for CNCs obtained after DES pretreatment and mechanical disintegration (error represents standard error).

Sample Width

Length

DES2 DES3 DES4 DES5

390±25 337±23 364±22 353±16

13.6±1.1 15.7±1.3 13.8±0.7 9.9±0.7

Aspect ratio 28 21 26 36

CrI 69 69 66 71


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Figure 5. TEM images of CNCs obtained from DES-pretreated cellulose fibers. a) DES2, b) DES3, c) DES4, and d) DES5 (ground circles are due to the sample preparation, most likely due to the air bubbles trapped in the cellulose solution). The X-ray diffraction patterns of the CNCs and original cellulose pulp are presented in Figure 6. The diffraction patterns show the characteristic peaks for cellulose I, indicating no rearrangement to other cellulose allomorphs occurred. The presence of the diffraction patterns of cellulose I suggest that no dissolution of cellulose fibers took place during the DES pretreatment and that hydrolysis was purely heterogeneous in nature. The CrIs of DES2–4 were 66–71% (Table), which are slightly higher compared with original cellulose pulp (66%). A small increase in the CrI is similar to the hydrochloride acid hydrolytic treatments reported previously.51 However, the obtained CrIs are lower compared with CNCs obtained by acid hydrolysis but are in line with or higher than those produced by oxidative treatments28,52–54 (it should be noted that the Segal


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method used for CrI calculations is not suitable for the comparison of different types of samples but rather quantifies changes within a single sample set55). It is plausible that crystallinities of DES-pretreated fibers are decreased during the microfluidizer treatment, as it is well known that mechanical treatment can decrease the crystallinity of cellulose due to strong shear forces during homogenization, which may loosen the crystalline structure and result in peeling of the cellulose chains on the crystallites.

Intensity (Off scale)

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DES5 DES4 DES3 DES2 Dissolving pulp 0

10

20

30

40

50

2θ

Figure 6. XRD diffraction patterns of original cellulose pulp and CNCs obtained from DESpretreated cellulose fibers. The thermal stability of CNCs was studied using TGA measurements at air and nitrogen atmosphere. It can be seen from Figure 7 that the onset temperatures of CNCs ranged from 275– 293 ºC. Even though this is significantly lower compared to original cellulose pulp (the high thermal stability of dissolving pulp is most likely due to the high molecular weight and the absence of a significant amount of hemicelluloses), it is still in the range of thermally stable CNCs obtained by phosphoric acid hydrolysis24 and significantly higher than those obtained after sequential periodate oxidation and reductive amination.53 The highest thermal stability was found


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for DES2, whereas there were no significant differences between the other CNCs. In a nitrogen atmosphere, the onset of the main degradation temperature was slightly higher (around 300 ºC for all CNCs) compared to the measurement in air. However, minor degradation (around 10% w) was observed after 200 ºC. The residual mass at 600 ºC for CNCs was over two times higher compared with original cellulose pulp. The formation of residual char is most likely due to the dehydration of cellulose by acidic oxalic acid groups.

Cellulose DES2 DES3 DES4 DES5

a) 100

Weigth (%)

80

60

40

20

0 0

100

200

300

400

500

600

Temperature (oC)

Cellulose DES2 DES3 DES4 DES5

b) 100

80

Weigth (%)

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

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60

40

20

0 0

100

200

300

400

500

600

Temperature (oC)

Figure 7. TGA curves of original pulp and CNCs prepared after DES pretreatment a) in air and b) in nitrogen.


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The CNC solutions (0.1% in water) exhibited transmittances ranging from 45%–75% at a wavelength of 800 nm (Figure 8). The transmittance increased when a longer reaction time or a higher temperature was used in the pretreatment stage, indicating the higher uniformity of CNCs (presumably due to the presence of fewer fiber aggregates after more intensive DES pretreatments). The transmittances in the UV-Vis results are similar to those of the aminomodified CNCs previously reported by sequential oxidation and reduction treatment28 but are lower than those of CNCs obtained by periodate oxidation followed by chlorite oxidation56 or reductive amination with bisphosphonate containing amine.53 Relatively low transmittance, especially at the lower wavelength, indicates that some aggregated cellulose particles (particles that might not be fully disintegrated during the mechanical treatment or reaggregated CNCs) remained in the solution.

70

DES5 DES4

65

DES3

75

60

Transmittance (%)

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

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55 50

DES2

45 40 35 30 25 20 15 10 5 0 400

500

600

700

800

Wavelength (nm)

Figure 8. UV-Vis spectra of DES-pretreated and mechanically disintegrated CNCs (0.1%) The CNCs are highly promising green nanomaterials for the emerging bioeconomy to be used in numerous applications. CNCs themselves have low toxicity at dilute solutions57,58 and are biodegradable.59 Therefore, in respect of the whole material production chain, it is important that


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CNCs can be produced via environmentally sustainable methods and with few working hazards. Oxalic acid itself is a moderately toxic chemical and even though DESs containing oxalic acid has some cytotoxicity properties,13 low toxicity still indicates that choline chloride and oxalic acid-based DES is biocompatible and readily biodegradable.12 Therefore, the methodology introduced here can be seen as one of the most feasible ways to obtain CNCs. However, there is still room for improvement, and other even less toxic and cheaper DESs are waiting to be discovered, which increases the feasibility of producing CNCs. Conclusions DES based on choline chloride and oxalic acid dihydrate was found to be an effective hydrolytic solvent for the production of CNCs. DESs were used only as a pretreatment media, solvent was removed by simple filtration and washing steps, and CNCs were produced mechanically from DES-free fibers. The use of DES as a pretreatment media thus eliminates cumbersome separation of CNCs from the reaction mixture. It was found that even 2 h pretreatment time at 100 °C was suitable for the fabrication of CNCs with a high aspect ratio, and the width could be adjusted by increasing the reaction temperature. Due to the low toxicity, biocompatibility, and biodegradability, DES pretreatment can be seen as an environmentally friendly method for CNC production. ASSOCIATED CONTENT Supporting Information. TEM images of CNCs with different magnitutes. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION


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Corresponding Author *Tel. +358294482424; fax: +358 855 323 27. E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT The following people are gratefully acknowledged for their contribution to this work: Ms. Vilma Haarala for her contribution in the experimental part of the study, Dr. Ilkka Miinalainen for his help with the TEM and SEM images, Mr. Tommi Kokkonen for TG analysis, and Mr. Sami Saukko for his guidance with WAXD analysis. The facilities at the Center of Microscopy and Nanotechnology at the University of Oulu were utilized in this research. REFERENCES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)

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Acidic Deep Eutectic Solvents As Hydrolytic Media ... - ACS Publications

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