Highly Efficient Dissolution of Wool Keratin by Dimethylphosphate

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Highly efficient dissolution of wool keratin by dimethylphosphate ionic liquids shuangshuang zheng, Yi Nie, Suojiang Zhang, Xiangping Zhang, and Lijun Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 27 Sep 2015 Downloaded from http://pubs.acs.org on September 29, 2015

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ACS Sustainable Chemistry & Engineering

Highly efficient dissolution of wool keratin by dimethylphosphate ionic liquids Zheng Shuangshuanga,b, Nie Yib, Zhang Suojiang*,b, Zhang Xiangpingb,and Wang Lijunc a

Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of Multiphase Complex Systems, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China.

b

School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, China.

c

College of Chemistry and Chemical Engineering, Qufu Normal University, Shandong 273165, China.

Keywords: keratin, ionic liquids, dimethylphosphate, dissolution capability

Abstract: Ionic liquids (ILs) are eco-friendly solvents and exhibit excellent performance for dissolution and regeneration of wool keratin. In this work, a series of ILs are designed and synthesized to investigate effects of ILs structures on dissolving wool keratin and properties of regenerated keratin. The results show that both cations and anions have important impacts on dissolution capability for wool, while side chain lengths of imidazole cation have little effect. Structures and properties of regenerated keratin are characterized by various techniques such as FT-IR, NMR, SEM, TGA and XRD. It is found that the dissolution time is shorten to 1.5 h by [Emim]DMP, thermal stability of regenerated keratin from its solution is superior to that from

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[Bmim]OAc solution, and the proportion of α-helix structure is as high as 78.7%. In addition, five cycles of [Emim]DMP suggest that it has a good reusability, and the rheological property of DPILs solutions shows their excellent potential for fiber spinning.

Introduction Keratin is a kind of fibrous protein and potential renewable biopolymer, which can be derived from several source, for instance wool, feathers, hair, animal horns and hooves. Wool, which consists of approximately 95% pure keratin (11-17% cysteine) [1] has been widely used in textile industries because of outstanding mechanical properties. However, significant amounts of short and coarse wool keratin are currently discarded as leftovers from the textile industries every year, which results in waste of keratin and environmental pollution. Therefore, it is valuable to regenerate and convert biodegradable wool keratin into high value-added commercial biomaterials. Efficient keratin dissolution is the foundation of recycling waste wool keratin and fabrication of biomaterials. Secondary structure of wool keratin are α-helix and β-sheet, which is difficult to cleavage due to the strong hydrogen bonding and disulfide bonding between polypeptide chains. Several traditional methods including reduction, oxidation, acid-alkali and sulfitolysis are used to dissolve keratin.[2,3] However, reagents used in these reactions are toxic and non-renewable, most importantly these reagents would result in excessive degradation of keratin and a lower molecule weight of regenerated keratin which decreases the value of regenerated keratin, being not suitable for spinning. As a consequence, it is urgent to develop efficient solvents to improve the dissolution capability for wool keratin and properties of regenerated keratin. Ionic liquids (ILs) are green solvents with attractive properties like non-volatility, nonflammability, thermal stability, easy recycling, and tunable structure. To date, they have been used


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in dissolving biological macromolecules such as cellulose,[4-6] silk,[7-9] wool,[1,10-12] feather[13-15], and show excellent solubility. Several conventional ILs have been used in dissolving keratin of feather or wool. 1-Allyl-3-methylimidazolium chloride [Amim]Cl and 1-butyl-3-methylimidazolium chloride [Bmim]Cl have been found to be effective for dissolving wool by H. Xie.[11] Same ILs were also used to dissolve feather and wool keratin by A. Idris.[1,14] However, influence of structures of cations and anions on dissolution capability for wool keratin and properties of regenerated keratin is still ambiguous, which is very important for designing efficient ILs for dissolving keratin and improving the value of regenerated keratin. In this paper, dissolution capability of ILs with various cations (imidazole, pyridine, quaternary ammonium and phosphonium) and various anions (acetate, phosphate ester and halogen) is thoroughly studied. Meanwhile, side chain lengths of imidazole cation ([Cnmim]+, n=2, 4, 6, 8) on it are also investigated. Structures and properties of regenerated keratin including morphology, supramolecular structure, degree of crystallinity and other physiochemical properties are discussed in detail (Scheme 1). In addition, the recovery of ionic liquid is also investigated. After comprehensive consideration of dissolution capability and properties of regenerated keratin, dimethylphosphate ionic liquids (DPILs) are chosen as the optimal ILs. Therefore, basing on the structure-function relationship between wool keratin and ILs, we can have an insight into the dissolution mechanism of wool keratin. For next work, we will design task-specific ILs to further improve the solubility and properties of regenerated keratin, which is important for industrial application of wool keratin.


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Scheme 1. The research strategy in this study.

Scheme 2. Partial chemical structures of ILs studied.

Experiment Material preparation. Wool keratin was supplied by Henan Dingda Biotechnology Co., LTD. and ground into small pieces. Nitrogen content of the wool keratin was determined by Kjeldahl nitrogen determination apparatus (weight percentage of nitrogen is 15.3% and protein is 95.4%). 1-Alkyl-3-methylimidazolium chloride [Bmim]Cl (98%), [Emim]Cl (98%), [Hmim]Cl (98%), and [Omim]Cl (98%), tetra-butyl-phosphonium chloride [P4444]Cl (98%), tetra-butyl-


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ammonium chloride [N4444]Cl (98%), 1-butyl-pyridinium chloride [BPy]Cl (98%) were provided by Henan Linzhou Keneng Co., LTD. Tri-methyl phosphate (≥98%) was purchased from Tokyo Chemical industry Co., LTD. Tri-ethyl amine (≥99%), iron (III) chloride hexahydrate (≥99%), and lead (II) acetate tri-hydrate (≥99.5%) were supplied from Xi long Chemical industry Co., LTD. Nalkyl-imidozole (≥98%) and sodium thiocyanate (≥98.5%) were supplied from Sinopharm Chemical Reagent Co., LTD. 1-Ethyl-3-methylimidazolium dimethylphosphate ([Emim]DMP) and tri-ethyl-methylammonium dimethylphosphate ([N2221]DMP) were synthesized by mixing tri-ethyl amine (nethyl-imidazole) and tri-methyl phosphate at 423K for 12 h according to the published procedure (Scheme 2).[16,17] The preparation of other ILs was described in the supporting information. All ILs used in this paper were purified before dissolving wool keratin. Water contents of ILs were determined by C20 Coulometric KF Titrator (Mettle Toledo) (supporting information Table S1). Dissolution and regeneration of wool keratin. About 0.4 g wool keratin was added to ILs (5 g) with magnetic stirring in a tube at 403K.[11] The microscope was used to detect the dissolving samples at a certain time via light scattering under 200×by Olympus Microscope. The regenerated keratin was precipitated from the IL solution by addition of ethanol at room temperature, separated by centrifugation for 5 minutes at 10000 rpm, and then dried at 333K for 2 days. Morphological analysis. Morphological analysis of wool fiber and regenerated keratin was studied using a scanning electron microscope (SU8020 Hitachi). The investigation was performed with an acceleration voltage of 1 kV, 10 μA of current probe, and 3.4 mm of working distance. The samples were mounted on aluminum specimen stubs with double-sided adhesive tape.


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Fourier transform infrared spectroscopy (FT-IR). Infrared spectra of samples were collected in a Thermo Nicolet 380 with the wavenumbers range of 400 to 4000 cm −1.The spectra were taken with 32 scans with a resolution of 4 cm−1 and were baseline corrected. Crystallinity analysis (XRD). X-ray diffraction study was carried out on wool fiber and regenerated keratin powder. The data were collected at room temperature using Bruker D8 Focus. Diffraction intensities were recorded at 45 kV and 200 mA with 2θ ranging from 5 to 50o at a scan speed of 0.05 s-1. Crystallinity index (CI), indicating the relative crystallinity degree of fiber, was calculated using L. Segal method as the following eq. 1 (I − I14 ) ⁄I CI = 9

9

（1）

Where, CI is the crystallinity index, I9 is the maximum intensity of crystal lattice diffraction with 2θ at around 9o, and I14 is the minimum diffraction intensity with 2θ at around 14o. In general, a higher value of CI indicates higher crystallinity of samples.[18-20] Herein, CI is the CI of regenerated keratin to raw material. Nuclear magnetic resonance spectroscopy (NMR). 1H NMR and 13C NMR spectra were recorded at 600 MHz on a Bruker spectrometer with samples diluted in deuterated dimethyl sulfoxide. The solid

13

C–NMR spectra of regenerated samples were acquired at 400 MHz WB

Solid-State NMR Spectrometer using a 4 mm rotor with RF-field 62.5 kHz, 90 deg pulse 4.17 µsec 101 Watt. Thermogravimetric analysis (TGA). The TGA curves were obtained with a thermogravimetric analysis (TGA, Q5000 V3.15 Build 263) from 298K to 1173K. The sample mass was ca. 2–10 mg per measurement, and the measurements were carried out under nitrogen atmosphere at a heating rate of 10 K/min.


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Rheological measurement of keratin solution. The rheological property of keratin solutions was measured on a parallel plate’s type rheometer (Discovery HR-2 hybrid rheometer). The measurements were performed under temperature at a range of 313 to 393K, shear rate γ was 0600 s-1. The solutions were degassed under vacuum 24 h before measurement. The non-Newtonian index is the degree of non-Newton fluid deviating from Newton fluid. A useful form of expressing the flow behavior is the power law relationship of the Ostwald de Waele model: σ = K𝑟 𝑛

(2)

Where σ (Pa) denotes the shear stress, γ (s-1) is the shear rate, n and K are constants known as non-Newtonian index and the consistency index, respectively. Non-Newtonian n is the scope of log σ= f(log γ).[21-23]

Results and Discussion The effect of cations on dissolution capability for wool keratin. According to previous reports,[11,14] temperature has a significant effect on solubility of wool keratin in ILs, and better solubility of wool keratin are noted at 403K. In addition, elevated temperature can provide much energy to open hydrogen bonding and disulfide bonding. Therefore, the temperature for dissolving wool keratin is maintained at 403K in order to eliminate the effect of temperature on dissolution capability of ILs. Meanwhile, the percentage of wool to ILs also has an impact on dissolving keratin, so mass ratio of wool keratin to ILs here is 8 wt%. Dissolution time is closely related to dissolution capability under these conditions. The effect of cations on dissolving wool keratin is studied and the results are shown in Table 1. From the table, it can be seen that 8 wt% wool keratin can be completely dissolved in [Bmim]Cl, [N2221]DMP and [Emim]DMP by 5 h, 3 h and 1.5 h, respectively. No dissolution of wool keratinis found in [P4444]Cl, [BPy]Cl or [N4444]Cl in 24 h. Thus, dissolution time of wool keratin in various


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ILs with the same anion Cl- follows the increasing order of [Bmim]+[N2221]DMP>[Cnmim]Cl (n= 2, 4, 6, 8) >[Bmim]SCN>[Bmim]FeCl4, and no keratin is dissolved in [P4444]Cl, [N4444]Cl, [BPy]Cl,


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[EtOHmim]Cl or [EtOHmim]FeCl4. The results show that structures of cations and anions of ILs play important roles in dissolving wool keratin. When the regulation of solubility by cations reaches to limit, anions of ILs show more significant effect on dissolving keratin. Thus, only cations and anions play a synergistic effect perfectly, can ILs achieve highlight dissolving performance. In this work, DPILs are used to dissolve wool keratin and can completely dissolve wool keratin in 1.5 h, which have shown excellent dissolution capability compared with previous reports. Finally, shorter dissolution time is found in [Bmim]OAc and DPILs. However, the optimal ILs for dissolving wool keratin would be determined according to both dissolution capability and properties of regenerated keratin, because properties of regenerated keratin are very important for fabricating keratin into high value-added materials. Therefore, properties of regenerated keratin need to be studied further. Surface topography changes. Fig.1 shows the dissolving process of wool keratin in [Emim]DMP. From these photographs, it can be seen that wool fiber initially are thick and long, with dissolution time increasing, wool fiber becomes thinner and shorter, eventually disappears.

(a)

(b)

Fig.1 Dissolution process of wool keratin in [Emim]DMP (a) 5 min; (b) 1.5 h.


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(A)

(B)

(C)

(D)

Fig.2 SEM of (A) raw material ; regenerated keratin from 8% wool keratin solution of (B) [N2221]DMP, (C) [Bmim]OAc, (D) [Emim]DMP.

In addition, after wool keratin is added to ILs, fiber becomes swelling, resulting in solution viscosity increasing. Finally, solution viscosity is reduced when wool keratin is gradually dissolved. Fig.2 shows SEM micrographs of wool keratin and regenerated keratin from [Bmim]OAc and DPILs solutions. As for raw material, the surface is very smooth. The regenerated keratin materials become rough and amorphous, while morphology of wool fiber is almost destroyed. Therefore, regenerated keratin can be directly applied due to their loose structure, which is greatly easier hydrolysis than raw material. Structures change. The X-ray diffraction of wool keratin and regenerated keratin is shown in Fig.3. These figures show two large peaks at about 9° (most contribution comes from α-helix) and 20° (most contribution comes from β-sheets structure).[1,11] It can be seen that regenerated keratin materials from ILs solutions exhibit similar diffraction patterns with raw material. However, the peaks at about 9°are significantly weaker in regenerated keratin and almost disappear for that form [Bmim]OAc solution, suggesting a lesser content of α-helix structure. Although [Bmim]OAc


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has remarkable dissolution capability, it has severely damaged the structure and properties of keratin. The XRD results are consistent with the 13C-NMR spectra discussed below. It is obvious that regenerated keratin materials have reformed α-helix and β-sheet structures of wool keratin. For quantitative comparison of crystallinity change, CI is used as relative crystallinity degree compared with raw material. Table 3 shows that regenerated keratin has lower crystallinity and could not completely reconstruct the tight arrangement of polypeptides existed in raw wool. The crystallinity structure of regenerated keratin from [Bmim]OAc has been damaged, so the CI cannot be calculated. The results show that CI value of regenerated keratin from [N2221]DMP solution is greater than that of [Emim]DMP, which is contrary to dissolution capability of ILs. Thus, it is suggested that the better dissolution capability of ILs, the more serious degradation of keratin, resulting in lower crystallinity. Therefore, it is very important to choose appropriate ILs to ensure crystallinity and improve the value of regenerated keratin.

Regenerated keratin from [Bmim]OAc solution

Regenerated keratin from [N2221]DMP solution

Regenerated keratin from [Emim]DMP solution

Raw material

5

10

15

20

25

2

30

35

40

45

50

Fig.3 XRD of wool keratin and regenerated keratin from 8 wt% keratin in ILs solutions.


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Table 3 The CIa of regenerated keratin from different ILs solutions

CI a

[N2221]DMP

[Emim]DMP

71.13%

54.31%

The CI of regenerated keratin are calculated by equation (1).

(d)

Amide II Amide I

Amide III

(c)

(b)

(a)

4000

3500

3000

2500

2000 -1

Wavenumbers(cm )

1500

1000

500

Fig.4 IR of (a) wool keratin and regenerated keratin from (b) [N2221]DMP, (c) [Bmim]OAc, (d) [Emim]DMP ILs solutions.

IR spectrums of wool keratin and regenerated keratin are shown in the Fig.4. The absorption bands agree well with the previous reports.[14,24,25] The results show that spectrums of wool keratin and regenerated keratin from different ILs solutions are similar, which indicates the main structure (Amide І, II, III) of regenerated keratin being intact. In order to study subtle change of structure of regenerated keratin, 13C-NMR analysis is followed. The local structures of raw material and regenerated keratin are investigated by solid

13

C-

NMR displayed in Fig.5. Chemical shifts of these keratin are consistent with the literature reports.[1,14,26] The asymmetric peaks between 170 and 180 ppm are ascribed to carbonyl carbons. It is found that these NMR spectra of regenerated keratin are similar to raw material, which


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 -carbon alkyl of side chains

C=O

-carbon

Regenerated keratin from [Bmim]OAc solution

Regenerated keratin from [Emim]DMP solution

Regenerated keratin from

[N2221]DMP solution

Raw material

250

200

150

100

50

0

-50

ppm (a) α-helix

α-helix Raw material

β-sheet

160

165

Regenerated keratin from [N2221]DMP

β-sheet

170

175

180

190 160

185

165

170

175

180

α-helix

165

170

190

α-helix Regenerated keratin from [Emim]DMP

Regenerated keratin from [Bmim]OAc

β-sheet

160

185

ppm

ppm

β-sheet

175

180

185

190160

165

170

175

180

185

190

ppm

ppm

(b) Fig.5 The

13C-NMR

spectra of (a) raw material and regenerated keratin from [N2221]DMP, [Bmim]OAc and

[Emim]DMP solutions respectively; (b) raw material and regenerated keratin from [N2221]DMP, [Bmim]OAc and [Emim]DMP solutions respectively which were fitted with Gaussian fitting functions.


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suggests that main structure of keratin is maintained after dissolving. In addition, secondary structure of keratin consisting of α-helix and β-sheet, results in slightly different chemical shift of C=O in NMR spectrum. Thus, the C=O peaks are shown in Fig.5 (b) individually to estimate the percentage fraction of α-helix and β-sheet structures.[14,26,27] The fitting of C=O peaks results in two peaks, for raw material one peak at 174.7 ppm relates to α-helix, and the other at 171.4 ppm ascribes to β- sheet molecular conformations, respectively. For regenerated keratin from ILs solutions, the fitting results are described in Table 4. The percentage fraction of α-helix of regenerated keratin follows the order of [Emim]DMP>[N2221]DMP>[Bmim]OAc. In the case of [Bmim]OAc, the carbonyl peak breaks into two overlap peaks and fraction of α-helix is decreased

Table 4 The percentage fraction of α-helix and β-sheet of raw material and regenerated keratin Peaks

Chemical shift (ppm)

HW (ppm)

Fraction (%)

α-helix

174.7

5.92

83.8 %

β-sheet

171.4

3.38

16.2 %

Raw material

Keratin regenerated from [N2221]DMP solution α-helix

175.4

5.88

78.7 %

β-sheet

172.4

3.89

21.3 %

Keratin regenerated from [Bmim]OAc solution α-helix

183.3

3.03

45.5 %

β-sheet

174.5

4.84

54.5%

Keratin regenerated from [Emim]DMP solution α-helix

173.7

6.26

90.9%

β-sheet

171.7

3.13

9.1%

It is fitted with Gaussian function. The error in the fitting is ±5%


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to 45.5%. The most likely explanation of this observation is that the dissolution of wool in [Bmim]OAc results in a deeper extent of disruption of the protein.[1,14] The NMR date in Table 4 displaying a lesser fraction of α-helix when compared to β-sheet for all materials, agree with their corresponding XRD spectral which also shows a lower diffraction intensity of α-helix compared to raw material. Thermal stability. To study the thermal stability of wool keratin and regenerated keratin, TGA is investigated (supporting information Fig.S1). Td is the onset temperature at weight loss 5% of dry samples and shown in Table 5. The decomposition temperature is mainly between 503K and 523K, slightly higher than raw material. It is noted that regenerated keratin materials from DPILs solutions show higher thermal stability than that from [Bmim]OAc. The poor thermal stability of regenerated keratin from [Bmim]OAc solution is consistent with the loss of crystallinity observed in XRD. Table 5 Decomposition temperature of raw material and regenerated keratin Regenerated keratin from ILs solution

Td(K)

Raw material

512

[Emim]DMP

521

[N2221]DMP

509

[Bmim]OAc

479

Recycle of ionic liquids. Previous works have demonstrated that [Emim]DMP is an effective solvent for dissolving wool keratin. In addition, thermal decomposition temperature of [Emim]DMP is about 510 K(Fig.S2) , which means it is very stable at dissolving temperature. Therefore, the reusability of [Emim]DMP is investigated under the same condition. The regenerated keratin is immiscible with ethanol, while [Emim]DMP has good solubility in ethanol.


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Therefore, ethanol is added to the ionic liquid solution after wool keratin is completely dissolved. The recovery of ionic liquid is showed in Fig.6. It is found that the recovery of [Emim]DMP is above 96% and dissolution time is consistent with the first time in each recycle. This indicates [Emim]DMP with good reusability. 1H-NMR spectra of recovered ionic liquids are displayed in supporting information Fig.S3. 100

recovery of ionic liquids(%)

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


80

60

1

2

3

Number of recycles

4

5

Fig.6 The reusability of [Emim]DMP for dissolving wool keratin.

Rheological property. Rheological property of keratin solution is an important factor in spinning industry. Therefore, rheological property of DPILs solutions is investigated in this paper. With other factors relatively constant, the higher value of n, the more potential for fiber spinning of solution. Fig.7 shows the viscosity η decreases with γ increasing. The [Emim]DMP solution shows noticeably non-Newtonian flows, which can also be found in [N2221]DMP solution. Meanwhile, the logarithmic curve of shear rate and shear stress presents a good linear relationship.


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3.0

2.6

2.4

2.5

2.2

log/Pa

2.0

(Pa·s)

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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1.5

2.0

1.8

1.6

1.0

1.4

0.5 1.2

0

100

200

-1 300

s 

400

500

600

1.2

1.4

1.6

1.8

2.0

log( /s-1)

2.2

2.4

2.6

2.8

3.0

Fig.7 Dependence of apparent viscosity on shear rate and logarithmic plots of shear stress versus shear rate of [Emim]DMP solution at mass ratio 8 wt% and 333K.

Table 6 shows the non-Newtonian index n of DPILs solutions at different temperature. The values of n can be higher than 0.9 at specific temperature, indicating excellent potential for fiber spinning. Meanwhile, the value of n increases initially and sequentially decreases with temperature increasing. From the Table 6, it can be found that the most likely spinning temperature range of [N2221]DMP solution at 8 wt% is 333-343K, that of [Emim]DMP solution is 313-343K, indicating lower energy consumption of DPILs. From Fig.8, the value of n decreases with increasing of wool keratin content in DPILs solutions. The reason is that true shear viscosity and non-Newtonian behavior increase when concentration is promoted. In order to achieve high spinning performance, the spinning temperature and concentration should be rigidly controlled. Table 6 The non-Newtonian index n with solution mass ratio 8 wt%

ILs solution

313K

333K

343K

353K

363K

373K

393K

[N2221]DMP

0.86

0.88

0.89

0.89

0.88

0.87

0.85

[Emim]DMP 0.85

0.85

0.85

0.84

0.84

0.83

0.81


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0.9

0.8

non-Newton n

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


8wt% 10wt% 12wt% 15wt%

0.7

0.6

40

60

o

80

Temperature( C)

Fig.8 Non-Newton of [N2221]DMP solutions with various concentrations.

Conclusions In the present work, a series of ILs are used to dissolve wool keratin. Effects of cations, anions, cationic side chain lengths and OH group on dissolution capability for wool keratin and properties of regenerated keratin are investigated in detail. Based on this study, the dissolution capability is determined by complex interaction between cations and anions, while side chain lengths of imidazole ring have little effect on that. The dissolution time is shorten to 1.5 h by [Emim]DMP. Thermal stability of regenerated keratin from DPILs solutions is superior to raw material. In addition, despite [Bmim]OAc has outstanding dissolution capability for wool keratin, the structure and thermal stability of regenerated keratin have been damaged. In the view of dissolution capability and properties of regenerated keratin, DPILs are superior to [Bmim]OAc. The DPILs solutions also show excellent potential for fiber spinning. Simple synthesis process, good reusability and low cost make them promising for industry application. Subsequent test will be carried out on the design of task-specific ionic liquids to improve solubility of wool keratin and properties of regenerated keratin. Supporting Information Available


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The preparations of ILs, water content of ILs, TGA curves of raw material and regenerated keratin, 1

H-NMR of ionic liquid recycling .The Supporting Information is available free of charge on the

ACS Publications website at http://pubs.acs.org. Author information Corresponding Author *Email: [email protected]. Acknowledgements This work is supported financially by 973 Program (2015CB251403), National Natural Science Foundation of China (No. 21576262 and NO. 21210006) and External Cooperation Program of BIC, Chinese Academy of Sciences (Grant No. GJHZ201306).

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Highly Efficient Dissolution of Wool Keratin by Dimethylphosphate

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