Chitosan dissolution with sulfopropyl imidazolium Brönsted acidic ionic liquids

Journal of Molecular Liquids 293 (2019) 111533

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Chitosan dissolution with sulfopropyl imidazolium Brönsted acidic ionic liquids Yu Sun a, Mengyi Qing a, Luxin Chen a, Jun Liu a, Fei Zhong b, Ping Jiang a, Guowei Wang a,⁎,1, Linghua Zhuang b,⁎,1 a b

College of Food Science and Light Industry, Nanjing Tech University, Nanjing 211800, China College of Chemistry and Molecular Engineering, Nanjing Tech University, Nanjing 211800, China

a r t i c l e

i n f o

Article history: Received 2 May 2019 Received in revised form 20 June 2019 Accepted 7 August 2019 Available online 08 August 2019 Keywords: Brönsted acidic ionic liquid (BAILs) Sulfopropyl imidazolium Chitosan Dissolution Density functional theory

a b s t r a c t Two Brönsted acidic ionic liquids (1-methy-3-(3-sulfopropyl) imidazolium acetate, as mPSAc, and 1-butyl-3-(3sulfopropyl) imidazolium acetate, as bPSAc) were synthesized for chitosan dissolution. Physical properties, including density, pH value, viscosity, Hammett acidity (H0), and Kamlet-Taft parameters (β and π*) of Brönsted acidic ionic liquids (BAILs), were studied and analyzed. The dissolution performance and regeneration properties of chitosan in BAILs were studied and compared with other ordinary ionic liquids ([Emim]Cl, [Bmim]Cl, [Amim] Cl, [Emim]Ac, [Bmim]Ac, [Amim]Ac). 1% chitosan was dissolved in two BAIL aqueous solutions (5% w/w) at 30 °C (35 min and 37 min, respectively). The regenerated chitosan was characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and X-ray diffraction (XRD). Molecular weight and deacetylation degree results showed that mPSAc endowed slight hydrolysis upon chitosan, only dissolution of chitosan at lower temperature. Density functional theory (DFT) simulations were performed to study the interactions between sulfopropyl imidazolium acidic ionic liquids and chitobiose. Four types of hydrogen bonds (CH…O, O-H…O, N-H…S, N-H…O) and many strong hydrogen bonds were found in BAIL-chitobiose, suggesting strong interactions between BAILs and chitobiose. DFT simulation results indicated that the active hydrogen atom of imidazole ring, sulfonate ion and acetate ion played important roles in the chitosan dissolution process by the disruption of native hydrogen bonds of chitobiose. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Chitosan, the total or partial deacetylation of chitin, is an abundant natural biopolymer. Due to its excellent biocompatibility, biodegradability, antibacterial property, and low toxicity, chitosan has been applied in diverse fields, such as food, agriculture, medicine, environmental protection and biotechnology etc. [1–6]. Despite of its favorable advantages, the application of chitosan is limited due to its poor solubility in normal solvents. Some solvents were used for chitosan Abbreviations: ILs, ionic liquids; BAILs, Brönsted acidic ionic liquids; mPSO3, 1-methy3-(3-sulfopropyl) imidazolium; bPSO3, 1-butyl-3-(3-sulfopropyl) imidazolium; mPSAc, 1methy-3-(3-sulfopropyl) imidazolium acetate; bPSAc, 1-butyl-3-(3-sulfopropyl) imidazolium acetate; [Emim]Cl, 1-ethyl-3-methylimidazolium chloride; [Bmim]Cl, 1butyl-3-methylimidazolium chloride; [Amim]Cl, 1-allyl-3-methylimidazolium chloride; [Emim]Ac, 1-ethyl-3-methylimidazolium acetate; [Bmim]Ac, 1-butyl-3methylimidazolium acetate; [Amim]Ac, 1-allyl-3-methylimidazolium acetate; 1HNMR, H-nuclear magnetic resonance; β and π*, Kamlet-Taft parameter; H0, Hammett acidity; FTIR, Fourier transform infrared spectroscopy; TGA, thermogravimetric analysis; XRD, Xray diffraction; DFT, density functional theory. ⁎ Corresponding authors. E-mail addresses: [email protected] (G. Wang), [email protected] (L. Zhuang). 1 Contributed equally to this work.

https://doi.org/10.1016/j.molliq.2019.111533 0167-7322/© 2019 Elsevier B.V. All rights reserved.

dissolution process, such as dilute acetic acid aqueous solution, trifluoroacetic acid aqueous solution and hexafluoro-2-propanol [7,8]. Qian et al. prepared different molecular weight chitosan solutions (1 wt%) by dissolving 0.2 g chitosan into 20 mL water containing 120 μL acetic acid [8]. However, these solvents have the disadvantages of high toxicity, high cost, high corrosivity, and serious environmental burden. Therefore, the green, high-efficiency chitosan dissolution system and the provision of a stable chitosan homogeneous system will play an important role in the effective utilization of chitosan materials. Numerous articles have focused on diverse application of ionic liquids (ILs), for example, chemical catalysis-solvents [9–11], pretreatment and processing of biopolymers or biomass [12–15], extraction or separation processes [16–20], and other different applications [21–24]. Swatloski et al. pioneered extensive studies on cellulose dissolution using common imidazolium ILs. [25–29]. Then some ionic liquids were applied for effective chitosan dissolution. The pioneer work of Xie et al. showed that 10% chitosan was dissolved in [Bmim]Cl at 110 °C [30]. Many imidazolium ionic liquids, such as [Amim]Cl, [Bmim]Cl, [Etmim]Cl, [Emim]Ac, and [Bmim]Ac were applied for chitosan dissolution [31–37]. However, chitosan could only be dissolved in these ionic liquids at relative high temperatures (N100 °C) and the dissolution mechanism of chitosan in ILs is still unclear.

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Y. Sun et al. / Journal of Molecular Liquids 293 (2019) 111533

Acidic ionic liquids (AILs) can be defined as subgroup of functionalized ionic liquids with acidic characteristics. Acidic ionic liquids show classification as Lewis acidic ionic liquids (LAILs) and Brönsted acidic ionic liquids (BAILs), which found comprehensive applications in different fields. Using sultone method (propylsultone or butylsultone as materials), typical BAILs with -SO3H group, 1-(3-propylsulfonic)-3methylimidazolium hydrogensulfate [(HSO3)3C3C1im][HSO4], 1-(4butylsulfonic)-3-methylimidazolium hydrogensulfate [(HSO3)4C4C1im][HSO4], 1-(3-propylsulfonic)-3-methylimidazolium chloride [(HSO3)3C3C1im][Cl], and 1-(4-butylsulfonic)-3-methylimidazolium chloride [(HSO3)4C4C1im][Cl], were synthesized and applied for different applications, including ionogel, dual synthesis media and catalyst, CO2 fixation, fuel-cell, membrane, biomass processing, and metal processing et al. [38–40]. Da Costa Lopes et al. comprehended the state of the art and perspectives of the hydrolysis and conversion of cellulose and lignocellulosic biomass using acidic ionic liquids without additional catalyst [41]. Acidic ionic liquids were used for cellulose hydrolysis and show a higher catalytic activity than that of sulfuric acid [42–44]. The rapid and efficient hydrolysis of glucose and xylose using an acidic ionic liquid ([(HSO3)4C4C1im]HSO4) at mild conditions was reported by Heri Satria [45]. However, normally strong acids, such as sulfuric acid, methyl sulfate, HCl were incorporated to produce Brönsted acidic ionic liquids, which showed higher hydrolysis effect upon materials (cellulose, lignocellulosic biomass, et al.). Until now, few reports were carried out to elucidate the effect of Brönsted acidic ionic liquids on sole chitosan dissolution. In this work, two Brönsted acidic ionic liquids (1-methy-3-(3sulfopropyl) imidazolium acetate, as mPSAc, and 1-butyl-3-(3sulfopropyl) imidazolium acetate, as bPSAc) were synthesized for chitosan dissolution. Physical properties, such as density, pH value, viscosity, Hammett acidity (H0), and Kamlet-Taft parameters (β and π*) of Brönsted acidic ionic liquids (BAILs) were determined. The chitosan dissolution performance using mPSAc, bPSAc and other ionic liquids ([Emim]Cl, [Bmim]Cl, [Amim]Cl, [Emim]Ac, [Bmim]Ac, [Amim]Ac) was compared. The regenerated chitosan from mPSAc was characterized by Fourier transform infrared spectroscopy (FTIR), thermo- gravimetric analysis (TGA), and X-ray diffraction (XRD). Molecular weight and deacetylation degree of the regenerated chitosan was determined. Density functional theory (DFT) simulations were carried out to elucidate the interactions between BAILs and chitobiose. The paper aimed to provide a stable chitosan solution homogeneous system using BAILs and will promote the industrial application of chitosan.

2. Experimental 2.1. Materials Chitosan (Huantai Shell Products Co., Ltd. China) was dried to remove water content at 85 °C for 10 h. Common ionic liquids used in this work ([Emim]Cl, [Bmim]Cl, [Amim]Cl, [Emim]Ac, [Bmim]Ac, and

N

[Amim]Ac) with purity N98%, were purchased from Lanzhou Institute of Chemical Physics (Chinese Academy of Sciences, China). These ionic liquids were dried in vacuo for 24 h at 80 °C before use, and the water content of the ionic liquids were determined by the Karl Fischer titration (b1000 ppm). 1-methylimidazole, 1-butylimidazole, 1,3-propanesultone and dichloromethane (with purity 99%), were from Aladdin Biochemical Technology Co., Ltd. Acetic acid (purity 99.5%) was from Shanghai Shenbo Chemical Co., Ltd. Millipore water was used in this study. 2.2. Synthesis and characterization of BAILs Two BAILs, as shown in Fig. 1, were synthesized using 1methylimidazole, 1-butylimidazole, 1,3-propanesultone, and acetic acid according to the method previously reported with minor modification [46–48]. Synthesis of mPSAc followed the procedure: 1,3-propanesultone (0.105 mol) and dichloromethane (80 mL) was added to the fournecked flask. The mixture was cooled with an ice bath under constant stirring. Then 1-methylimidazole (0.1 mol) was slowly added. The temperature was slowly raised after the addition is completed. And the system stirred 2 h at 25 °C. After that, the precipitate was filtered, and the solid was washed three times with dichloromethane (20 mL each time). The obtained white solid (mPSO3) was dried in vacuo for 2 h at 80 °C. The first step product (mPSO3) and acetic acid were mixed in a fournecked flask at a molar ratio of 1:3 and stirred 4 h at 25 °C. The obtained Brönsted acidic ionic liquid (mPSAc) was dried in vacuo for 24 h at 80 °C for further study. 1-Butyl-3-(3-sulfopropyl) imidazolium acetate (bPSAc) was synthesized using 1,3-propanesultone, 1-butylimidazole, and acetic acid following the above-mentioned procedure. 1 HNMR was recorded on the AVANCE AV-500 (Bruker Co. Ltd., Germany) operating at 400 MHz and chemical shifts were given in ppm units relative to tetramethyl silane (TMS). All products were dissolved using D2O. 1HNMR data was analyzed using MestReNova version 6.1. The splitting patterns are designated as follows: s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet. 2.3. Physical properties of BAILs and BAILs-chitosan solutions Viscosity of BAILs (mPSAc and bPSAc) were determined by SNB-3 digital viscometer (Shanghai Precision Instrument Co., Ltd., China) at 25 °C. Density of BAILs were measured using a density bottle at 25 °C. The pH values of acetate acid (5 wt% aqueous solution), intermediate of BAIL (mPSO3 and bPSO3, 5 wt% aqueous solution) and BAILs (5 wt% aqueous solution) were determined by PHS-3E pH meter (INESA Scientific Instrument Co., Ltd., China) at 25 °C. Kamlet-Taft parameters (β and π*) of BAILs were measured from the absorption peaks of two BAIL-dye solutions according to Kamlet-Taft solvatochromic method [33,35]. 4-nitroaniline (NA) and N,N-diethyl4-nitro-aniline (DENA) were used in the measurement. Firstly, NA and DENA were dissolved in methanol, then BAIL was added into dye-

N

N

SO3H

O

N

SO3H

O

O

O

mPSAc

bPSAc Fig. 1. Chemical structure of two BAILs.

Y. Sun et al. / Journal of Molecular Liquids 293 (2019) 111533

methanol solution and mixed homogeneously. The methanol was removed under vacuum at 40 °C for over 12 h, then BAIL-dye solution was applied for λmax measurement using Cary 60 Ultraviolet visible (UV–vis) spectrophotometer (Agilent Technologies Inc., USA) at 25 °C. β and π* parameters of BAILs were determined according to following Eqs. (1)–(4) β¼

2:64−νðNAÞ þ 1:035  ν ðDENAÞ 2:80

π ¼ 0:314  27:52−ν ðDENAÞ ν ðNAÞ ¼ 10000







 λ maxðNAÞ

ν ðDENAÞ ¼ 10000

 λ maxðDENAÞ

ð1Þ ð2Þ ð3Þ

molecular weight (Mw) of chitosan was calculated by Eq. (6): ½η ¼ KMαw

ð6Þ

where [η] is the intrinsic viscosity, K = 1.81 × 10−3 and α = 0.93, respectively. Deacetylation degree (DD) of chitosan was determined using alkaline titration method. Chitosan (0.25 g) was dissolved in 40 mL of 0.1 mol/L hydrochloric acid (HCl) by magnetic stirring. Then chitosan aqueous solution was filtered to remove impurities, methyl orange was added as the indicator. After that, 0.1 mol/L aqueous sodium hydroxide (NaOH) was slowly added to adjust the solution pH. The volume of NaOH was recorded when the solution turned yellow. Deacetylation degree of chitosan was calculated by Eqs. (7), (8):

ð4Þ

Hammett acidity of BAIL was conducted using Cary 60 UV–vis spectrophotometer (Agilent Technologies Inc.) with 4-nitroaniline (NA) as basic indicator. The method was described previously [49,50]. Briefly, 5 mmol/L BAILs and 10 mg/L 4-nitroanliline were prepared using deionized water. Then same volume of BAILs and 4-nitroanliline aqueous solution (all 5 mL) was mixed and ultrasonic vibration for 8 h. The absorbance of BAIL-NA aqueous solution was determined, and Hammett acidity of BAIL (H0), was calculated using the following Eq. (5).  
H 0 ¼ pK ½Iaq þ log ½I= IH þ

3

ð5Þ

pK[I]aq = 0.99, [I] refers to concentration of free NA (unprotonated form), which can be calculated from absorbance of aqueous NA solution. [IH+] refers to concentration of protonated form of NA. 2.4. Dissolution performance of chitosan in ionic liquids 2 g dried BAILs (mPSAc or bPSAc) was added to a four-necked flask and formulated the 5 wt% aqueous solution in a water bath (HH-4, Changzhou Guohua Electric Appliance Co., Ltd., China) with initial temperature of 30 °C. After that, chitosan (1 wt% of the solution) was added. Then, the complete dissolution time was determined. The dissolution of chitosan in other common ionic liquids ([Emim]Cl, [Bmim]Cl, [Amim]Cl, [Emim]Ac, [Bmim]Ac, and [Amim]Ac) followed same procedure as mentioned above. Density of BAIL-chitosan solution was measured using a density bottle at 25 °C. Viscosity of BAIL-chitosan solution was determined by SNB-3 digital viscometer at 25 °C. 2.5. Regeneration of chitosan To achieve the regenerated chitosan, BAIL-chitosan aqueous solution (40 g) was neutralized to pH =7.0 using 80 mL NaOH aqueous solution (0.2 mol/L) under vigorous agitation. The precipitated chitosan was filtered and washed five times (deionized water) to ensure complete neutralization. The regenerated chitosan was dried under vacuum for 8 h at 80 °C and used for further characterization. The weight of regenerated chitosan was about 0.37 g, the regeneration ratio was 92.5%. 2.6. Characterization of regenerated chitosan The molecular weight of chitosan was determined using Ubbelohde viscometer (diameter = 0.8–0.9 mm) according to Mark-Houwink equation [33,35]. Chitosan was dissolved in 0.2 mol/L sodium chloride-0.1 mol/L acetic acid aqueous solution to obtain four different chitosan concentrations (0.05 mg/mL, 0.25 mg/mL, 0.50 mg/mL, and 0.75 mg/mL). The measurements were performed at 25 °C. The

ðNH2 Þ% ¼

DD ¼

ðc1 v1 −c2 v2 Þ  0:016  100% G

ðNH2 Þ%  100% 9:94%

ð7Þ

ð8Þ

where c1 and c2 stand for the concentrations of HCl and NaOH respectively and v1 and v2 represent the volumes of HCl and NaOH respectively, and G is the chitosan weight. FTIR spectra of different chitosan samples were recorded using a Nicolet 5700 FTIR spectrometer (Thermo Electron Corporation, USA) equipped with a Nicolet Smart Orbit attenuated total reflectance (ATR) accessory incorporating a diamond internal reflection element. For each spectrum, 64 scans were recorded over the range of 4000–500 cm−1 at 25 °C at a resolution of 4 cm−1. The background spectrum was recorded on air and subtracted from the sample spectrum. Mettler Toledo TGA machine (Mettler-Toledo Ltd., Port Melbourne, Australia) was used with 40 μL aluminum crucibles for thermogravimetric analysis (TGA) under nitrogen. A sample mass of about 5 mg was used for each run. The samples were heated from 40 °C to 500 °C and measured in the dynamic heating regime, using a constant heating ramp of 10 K/min. XRD patterns were collected on a SmartLab-3kw XRD diffractometer with Cu-Kα radiation (λ = 0.154 nm) over the range 5–60 degrees (2θ) at a scan speed of 2 degrees (2θ) per minute. 2.7. DFT simulation DFT simulations were carried out with Gaussian 16 program (Version G16 B.01, Gaussian Inc.) [51]. In order to simplify the calculations, chitobiose was chosen as the model unit of chitosan, as shown in Fig. S1 [33,35,52]. The geometries of BAIL intermediates (mPSO3 or bPSO3), BAILs (mPSAc or bPSAc), and BAIL-chitobiose were fully optimized using B3LYP/6-31G(d,p) set [33,35,52]. 3. Result and discussion 3.1. 1HNMR characterization of BAILs 1 HNMR spectrum and the related 1HNMR characterization of BAILs were shown in Supplementary Materials. 1 HNMR spectrum of mPSO3 and mPSAc were shown in Figs. S2 and S3. The related 1HNMR characterization was listed in Table S1. 1 HNMR spectrum of bPSO3 and bPSAc were shown in Figs. S4 and S5. The related 1HNMR characterization was listed in Tables S2 and S3. 1 HNMR spectrum of BAILs agreed with the references [46–48]. The difference was the hydrogen of CH3COO– group in mPSAc or bPSAc as the molar ratio between mPSO3 (bPSO3) and acetic acid was 1:3.

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Y. Sun et al. / Journal of Molecular Liquids 293 (2019) 111533

Table 1 Density of BAILs and BAILs-chitosan.

Table 3 Viscosity of BAILs, BAIL aqueous solutions and BAIL-chitosan aqueous solutions. Density (g/cm3) T = 25 °C

BAILs BAILs-chitosan (1% w/w)

mPSAc bPSAc mPSAc-chitosan bPSAc-chitosan

1.2636 1.1794 1.0232 1.0123

SNB-3 digital viscometer

Torque (%)

Viscosity (mPa·s)

mPSAc mPSAc (5 wt%) aqueous solution mPSAc-chitosan bPSAc bPSAc (5 wt%) aqueous solution bPSAc-chitosan

79.6 ___ 54.4 67.7 ___ 66.9

398.0 ___ 272.0 338.5 ___ 334.5

3.2. Physical properties of BAILs and BAILs-chitosan solutions Density of BAILs (mPSAc or bPSAc) and BAILs-chitosan solutions was measured in a density bottle at 25 °C. The data was shown in Table 1. As shown in Table 1, density of mPSAc (1.2636 g/cm3) was higher than that of bPSAc (1.1794 g/cm3), which revealed that density decreased with longer carbon chain length (from methyl group to butyl group). When 1% chitosan was dissolved in BAILs aqueous solution, BAILs-chitosan solutions showed different density. Density of mPSAcchitosan aqueous solution (1.0232 g/cm3) was higher than that of bPSAc-chitosan (1.0123 g/cm3). The pH values of acetate acid, BAIL intermediates (mPSO3 and bPSO3), and BAILs (mPSAc and bPSAc) (all with 5 wt% aqueous solution) were determined by PHS-3E pH meter (INESA Scientific Instrument Co., Ltd., China) at 25 °C, which were listed in Table 2. As shown in Table 2, pH value of mPSO3 (5% w/w aqueous solution) was 2.33, which was higher than that of mPSAc (pH = 2.03). The pH value of bPSO3 (5% w/w aqueous solution) was 2.50, which was slightly lower than that of bPSAc (pH = 2.56). The pH value of mPSO3 (pH = 2.33) was lower than that of bPSO3 (pH = 2.50). The pH value of mPSAc (pH = 2.03) was much lower than that of acetate acid (pH = 2.39). The pH value of mPSAc, acetate acid, and bPSAc showed the following decreasing trend: mPSAcbacetate acidbbPSAc. The pH value is closely related to the acidity of BAILs. The stronger the acidity of BAIL is, the higher hydrolysis ability of BAIL shows to chitosan. Viscosity of BAILs (pure mPSAc and bPSAc acidic ionic liquids), BAIL aqueous solutions (5% w/w) and BAIL-chitosan aqueous solutions (1% w/w chitosan) was determined by SNB-3 digital viscometer. All measurement was conducted at 25 °C using Rotator SP21 at 100 r/min. As shown in Table 3, viscosity of pure mPSAc was 398.0 mPa·s, which was higher than that of pure bPSAc (338.5 mPa·s). Viscosity of BAIL aqueous solutions (5% w/w), including mPSAc and bPSAc, were not detected (b6 mPa·s). Viscosity of mPSAc-chitosan aqueous solution (1% w/w chitosan) was 272.0 mPa·s, which was much lower than that of bPSAc-chitosan aqueous solution (334.5 mPa·s). The obvious viscosity increases of BAIL-chitosan aqueous solution (mPSAc or bPSAc) revealed that chitosan was dissolved in BAIL aqueous solution. Kamlet-Taft parameter β is obtained by measuring the relative difference of solvatochromism between 4-nitroaniline (NA) and N,Ndiethyl-4-nitroaniline (DENA). Parameter β reveals hydrogen bond accepting basicity (HBA) of ILs. Kamlet-Taft parameter π* is obtained by measuring the wavelength of maximum absorbance (λmax) of N,N-diethyl-4-nitroaniline (DENA). β and π* parameters of BAILs were determined and summarized in Table 4. Table 2 pH of acetate acid, BAIL intermediates, and BAILs. Type

BAILs

pH

BAILs intermediate

Acetate acid mPSO3 bPSO3 mPSAc bPSAc

2.39 2.33 2.50 2.03 2.56

BAILs Derivative = 0.02.

As we known, the hydrogen bond accepting (HBA) ability of BAILs has a positive correlation with anions (type and functional group) in BAILs. In mPSAc or bPSAc, there exists two anions, one is acetic acid, the other is sulfonate (-SO3). As shown in Table 4, β of mPSAc and bPSAc were 1.603 and 1.532, higher than those β values of [Bmim]Ac and [Emim]Ac with same anion acetate acid (1.156 and 1.154, respectively) [33,35]. Comparing the different cationic groups of BAILs and other common acetate based ionic liquids ([Bmim]Ac and [Emim]Ac), mPSAc or bPSAc exists two anions (acetic acid and sulfonate group). The sulfonate group (-SO3) in BAILs showed more electronegative and stronger electronic cloud density than alkyl group (ethyl or butyl). BAILs (mPSAc or bPSAc) show higher β values, which is easier to destroy the inter- or intra-molecular hydrogen bonds in chitosan, so as to facilitate the dissolution of chitosan in BAILs. Kamlet-Taft parameters (β and π*) revealed the interaction between BAILs and chitosan. Chitosan was dissolved in BAILs (mPSAc or bPSAc), while could not be dissolved in common ionic liquids ([Emim]Cl, [Bmim]Cl, [Amim]Cl, [Emim]Ac, [Bmim]Ac, and [Amim]Ac) at same dissolution temperature. Hammett acidity (H0) is an important index to measure the acidity of BAILs. Hammett acidity of BAIL was determined with 4-nitroaniline (NA) as basic indicator. When the ability of BAILs to release hydrogen ion increases, its acidity increases. With the increase of the acidic scale of BAILs, absorbance of the unprotonated form of the basic indicator ([I]) decreased, whereas the protonated form of the indicator ([IH+]) could not be detected because of its small molar absorbance. The molar concentration of the unprotonated form of the indicator ([I]) is directly associated with absorbance of 4-nitroaniline (λ = 378 nm) in BAILs. Hammett acidity of mPSAc and bPSAc was summarized in Table 5. As shown in Table 5, H0 value of mPSAc was 1.6192, lower than that in reference with same cation group and acetate acid (IL6, H0 = 1.7916) [49]. The reason lies on that mPSAc is prepared with mPSO3 and acetic acid (mole ratio = 1:3). H0 value of mPSAc was higher than those of IL-3 and IL-4 (1.54 and 1.53, respectively), with same cationic group (as mPSO3) and different anionic groups (HSO4 and CH3SO4) [50]. H0 value of mPSAc was lower than that of bPSAc (1.6547), which revealed that mPSAc showed higher acidity than bPSAc. This decreasing trend with longer alkyl chain in cationic group was contrary with BAILs in reference (IL7 and IL8, IL19 and IL20) [49].

3.3. Dissolution of chitosan in BAILs and common ILs As shown in Table 6, mPSAc aqueous solution (5% w/w) can quickly dissolve chitosan (1% w/w) in 37 min at 30 °C, while other ordinary ILs ([Emim]Cl, [Bmim]Cl, [Amim]Cl, [Emim]Ac, [Bmim]Ac, and [Amim]Ac) cannot dissolve chitosan in N12 h. Dissolution time of chitosan in Table 4 Kamlet-Taft parameters of BAILs. ILs

λ(NA)

λ(DENA)

β

π*

mPSAc bPSAc

405 415

390 402

1.603 1.532

0.591 0.842

Y. Sun et al. / Journal of Molecular Liquids 293 (2019) 111533 Table 5 Calculation of H0 values of BAILs at 298 K. BAILs

Amax

[I] (%)

[IH+] (%)

H0

No BAILs

0.911(10 mg/L) 0.489 (5 mg/L) 0.396 0.402

100 100 80.98 82.21

0 0 19.02 17.79

/ / 1.6192 1.6547

mPSAc bPSAc

(BAILs concentration: 5 mmol/L, here Amax = 0.489 (5 mg/L) was used as reference absorbance)

Table 6 Dissolution time of chitosan in BAILs and common ILs (min). Aqueous solution

30 °C

50 °C

70 °C

90 °C

mPSAc bPSAc [Emim]Cl [Bmim]Cl [Amim]Cl [Emim]Ac [Bmim]Ac [Amim]Ac

35 37 × × × × × ×

17 20 × × × × × ×

13 15 × × × × × ×

8 11 × × × × × ×

“×” means chitosan cannot be dissolved in the ILs at this temperature (N12 h).

mPSAc aqueous solution reduced with temperature increasing from 30 °C to 90 °C. Chitosan dissolved faster in mPSAc aqueous solution than in bPSAc aqueous solution at same temperature. The difference of the length alkyl chain in cationic group of two BAILs (mPSAc and bPSAc) leads to difference chitosan solubility. Molecular simulation results (in Section 3.5) further clarify the different effects of length alkyl chain in cationic group of BAILs. 3.4. Characterization of chitosan FTIR spectra of native chitosan (a) and regenerated chitosan from mPSAc aqueous solution (b) were shown in Fig. 2. FTIR spectra of native

5

chitosan (Fig. 2a) and regenerated chitosan (Fig. 2b) were nearly identical except absorption peak around 1560–1650 cm−1. Typical absorption peaks of raw chitosan at 893 cm−1, 1026 cm−1, 1151 cm−1, 1377 cm−1, 1417 cm−1, 1591 cm−1, 1643 cm−1, 2920 cm−1, and 3420 cm−1 were highlighted in Fig. 2a. The results showed that no chemical reactions occurred during dissolution and regeneration process. In other words, mPSAc served as solvent media during chitosan dissolution process. TGA curves of native chitosan (a) and regenerated chitosan from mPSAc aqueous solution (b) were shown in Fig. 3. Two obvious weight losses could be observed in Fig. 3a and b. Minor weight loss of raw chitosan occurred at around 100 °C, corresponding to 2.1% weight loss, which was higher than that of regenerated chitosan (80 °C and 1.3% weight loss). The minor weight loss may be attributed to the loss of adsorbed and bound water [35,53]. Significant weight loss of raw chitosan (Fig. 3a) started at 273 °C and stabilized at 330 °C. The onset temperature was 300 °C, the weight loss was 34.88%. Significant weight loss of regenerated chitosan (Fig. 3b) started at 238 °C and stabilized at 306 °C. The onset temperature was 264 °C, the weight loss was 29.6%. This significant weight loss could be attributed to the melting, deacetylation, and decomposition of chitosan [35,53]. Thermal stability of regenerated chitosan was lower than that of raw chitosan, which indicated that the dissolution and regeneration processes could destroy the hydrogen bonds in chitosan but could not disrupt the main chains of chitosan. XRD profiles of native chitosan and regenerated chitosan from the mPSAc aqueous solution were shown in Fig. 4. Two main strong diffraction peaks at 2θ = 10.5° and 19.8° (shown in Fig. 4a), assigned to (020) and (100) crystallographic planes, represent typical crystalline domains of raw chitosan [35,53]. The regenerated chitosan (Fig. 4b) showed medium diffraction peaks at about 2θ = 22.6o. The crystallization intensity of the chitosan decreased sharply during dissolution and regeneration process (shown in Fig. 4, from 30,000 to 10,000 a.u.), which was possibly due to the destruction of the hydrogen bonds and the native crystalline form of chitosan [35,53]. Molecular weight of raw chitosan and the regenerated chitosan were determined using Ubbelohde viscometer (diameter =

Fig. 2. FTIR spectra of the native chitosan (a) and the regenerated chitosan from mPSAc (b).

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Y. Sun et al. / Journal of Molecular Liquids 293 (2019) 111533

Fig. 3. TGA curves of the native chitosan (a) and the regenerated chitosan from mPSAc (b).

0.8–0.9 mm) according to Mark-Houwink equation. Deacetylation degree (DD) of raw chitosan and the regenerated chitosan were determined using alkaline titration method. The results were summarized in Table 7.

As shown in Table 7, molecular weight of the regenerated chitosan decreased 1.01% compared with the native chitosan. Deacetylation degree of the regenerated chitosan increased 0.23% compared with the native chitosan. Molecular weight and deacetylation degree results showed that mPSAc endowed slight hydrolysis upon chitosan, only dissolution of chitosan at lower temperature. 3.5. Possible interaction between BAIL and BAIL-chitosan DFT (density functional theory) was performed using Gaussian 16 program (Version G16 B.01, Gaussian Inc.) at B3LYP/6-31G(d,p) level. Chitobiose is the representative structure of the chitosan. BAIL intermediates (mPSO3 or bPSO3), BAILs (mPSAc or bPSAc), and BAIL-chitobiose were fully optimized. Hydrogen bonds of mPSO3 or mPSAc were shown in Fig. 5 and Table S4. Interaction between mPSAc and chitobiose were summarized in Fig. 6 and Table S5. As shown in Fig. 5 and Table S4, two types of hydrogen bonds (C-H… O, C-H…S) and nine strong hydrogen bonds were found in mPSO3, as C1-H5…O24 (1.874 Å), C1-H5…O25 (2.468 Å), C1-H5…S22 (2.621 Å), C13-H15…O24 (2.484 Å), C16-H18…O25 (2.809 Å), C19-H20…O23 (2.918 Å), C19-H20…O24 (2.716 Å), C19-H21…O23 (2.719 Å) and C19-H21…O25(2.996 Å). Sulfonate ion (SO− 3 ) in mPSO3 showed higher electron absorption properties, which formed hydrogen bonds with active hydrogen atoms of imidazole ring (H5) and hydrogen atoms of methylene group (C13, C16, and C19) in propyl group. Three kinds of hydrogen bonds (C-H…O, O-H…S, O-H…O) and five hydrogen bonds were found in mPSAc, as C1-H5…O27 (1.874 Å), C13Table 7 Molecular weight and deacetylation degree of the native and regenerated chitosan.

Fig. 4. XRD profiles of the native chitosan (a) and the regenerated chitosan from mPSAc (b).

Molecular weight (g/mol) NH2 content (%) Deacetylation degree (%)

Native chitosan

Regenerated chitosan

Difference percent (%)

6.04 × 106

5.98 × 106

1.01

8.58 86.32

8.60 86.52

0.23 0.23

HCl concentration = 0.1086 mol/L，NaOH concentration = 0.0956 mol/L.

Y. Sun et al. / Journal of Molecular Liquids 293 (2019) 111533

7

Fig. 5. Hydrogen bonds of mPSO3 or mPSAc.

H15…O27 (2.622 Å), C16-H18…O27 (2.638 Å), O28-H29…S22 (2.584 Å) and O28-H29…O24 (1.402 Å). The average bond lengths of typical hydrogen bonds in mPSO3 and mPSAc were 2.623 Å and 2.224 Å. Acetate ion (CH3COO−) in mPSAc showed higher electron absorption properties, which formed hydrogen bonds with active hydrogen atoms of imidazole ring (H5) and hydrogen atoms of methylene group (C13 and C16) in propyl group. Sulfonate ion (SO− 3 ) in mPSAc showed higher adsorption effect with hydrogen of acetic acid, revealed by strong bond length of O28-H29… O24 (1.402 Å). There simulation results founded strong interaction be− tween sulfonate ion (SO− 3 ) and acetate ion (CH3COO ) in mPSAc. As shown in Fig. 6 and Table S5, cationic group of mPSAc, including oxygen atoms of sulfonate ion (SO− 3 ), hydrogen atoms of methyl group (C9), and active hydrogen atom (H5) of imidazole ring in, anionic group of mPSAc (mainly CH3COO−) formed hydrogen bonds with chitobiose. The simulation results revealed that cationic group and anionic group of mPSAc both played important roles in hydrogen bond formation with chitobiose. Three kinds of hydrogen bonds (C-H…O, O-H…O, N-H…O) and nine strong hydrogen bonds were found in mPSAc-chitobiose. mPSO3 formed six hydrogen bonds with chitobiose, as C1-H5…O50 (2.954 Å), C1-H5…O79 (2.141 Å), C9-H12…O50 (2.255 Å), O50-H51…O23 (1.689 Å), N52-H54…O24 (2.286 Å) and C67-H68…O23 (2.493 Å). Acetate ion (CH3COO−) formed three hydrogen bonds with chitobiose, as O28-H29…O44 (1.634 Å), C60-H61…O27 (2.630 Å), N74-H75…O27

Fig. 6. Interaction between mPSAc and chitobiose.

(2.243 Å). The average bond lengths of typical hydrogen bonds in mPSO3- chitobiose and acetate ion-chitobiose were 2.303 Å and 2.169 Å. Possible hydrogen bonds of bPSO3, bPSAc and bPSAc-chitobiose were shown in Figs. S6, S7, Tables S6, and S7. As shown in Fig. S6 and Table S6, two types of hydrogen bonds (CH…O, C-H…S) and nine hydrogen bonds were found in bPSO3, as C1H5…O20 (1.865 Å), C1-H5…O21 (2.436 Å), C1-H5…S18 (2.596 Å), C9-H11…O20 (2.473 Å), C12-H14…O21 (2.790 Å), C15-H16…O19 (2.912 Å), C15-H16…O20 (2.726 Å), C15-H17…O19 (2.721 Å) and C15-H17…O21 (2.989 Å). Sulfonate ion (SO− 3 ) in bPSO3 showed higher electron absorption properties, which formed hydrogen bonds with active hydrogen atoms of imidazole ring (H5) and hydrogen atoms of methylene group (C9, C12, and C15) in propyl group. Three types of hydrogen bonds (C-H…O, O-H…S, O-H…O) and five hydrogen bonds were found in bPSAc, as C1-H5…O36 (1.886 Å), C12H13…O36 (2.647 Å), C15-H16…O36 (2.616 Å), O37-H38…S21 (2.590 Å) and O37-H38…O22 (1.406 Å). The average bond length of typical hydrogen bonds in bPSAc was 2.229 Å. Acetate ion (CH3COO−) in bPSAc showed higher electron absorption properties, which formed hydrogen bonds with active hydrogen atoms of imidazole ring (H5) and hydrogen atoms of methylene group (C12 and C15) in propyl group. Sulfonate ion (SO− 3 ) in bPSAc showed higher adsorption effect with hydrogen of acetic acid, revealed by strong bond length of O37-H38… O22 (1.406 Å). The simulation results founded strong interaction be− tween sulfonate ion (SO− 3 ) and acetate ion (CH3COO ) in bPSAc. As shown in Fig. S7 and Table S7, cationic group in bPSAc, including oxygen atoms of sulfonate ion (SO− 3 ), hydrogen atoms of methylene group (C12 and C18) in propyl group, and active hydrogen atom (H5) of imidazole ring, anionic group of bPSAc (mainly CH3COO−) formed hydrogen bonds with chitobiose. The simulation results further revealed that cationic group and anionic group of bPSAc both played important roles in hydrogen bond formation with chitobiose. Four kinds of hydrogen bonds (C-H…O, O-H…O, N-H…S, N-H… O) and twelve strong hydrogen bonds were found in bPSAcchitobiose. The cationic group (bPSO3) formed ten hydrogen bonds with chitobiose, as C1-H5…O88 (1.877 Å), C9-H11…O86 (2.314 Å)，C15-H17…O88 (2.692 Å), C26-H31…O86 (2.621 Å), C57H58…O24 (2.393 Å), O59-H60…O24 (2.957 Å), N61-H63…S21 (2.773 Å), N61-H63…O23 (2.017 Å), N61-H63…O24 (2.758 Å), C76-H77…O24 (2.155 Å). Acetate ion (CH3COO−) formed two hydrogen bonds with chitobiose, as C39-H42…O53 (2.232 Å), N83-H84…O36 (2.140 Å). The average bond lengths of typical hydrogen bonds in mPSO3- chitobiose and acetate ion-chitobiose were 2.456 Å and 2.186 Å. DFT simulation results indicated that the active hydrogen atom of imidazole ring, sulfate ion and acetate ion played important roles in

8

Y. Sun et al. / Journal of Molecular Liquids 293 (2019) 111533

the chitosan dissolution process, by the disruption of native hydrogen bonds of chitobiose. DFT simulation results (energy, dipole moment, and polarizability) of BAIL intermediates (mPSO3 or bPSO3), BAILs (mPSAc or bPSAc), and BAIL-chitobiose were summarized in Table S8. As shown in Table S8, simulation energy of BAIL intermediates (mPSO3 or bPSO3), BAILs (mPSAc or bPSAc), and BAIL-chitobiose decreased. Polarizability (α) of BAIL intermediates (mPSO3 or bPSO3), BAILs (mPSAc or bPSAc), and BAIL-chitobiose increased. Polarizability (α) of mPSAc-chitobiose was 307.3160 (a.u.), which was lower than that of bPSAc-chitobiose (347.6110 a.u.) 4. Conclusion In this paper, two Brönsted acidic ionic liquids (mPSAc and bPSAc) were synthesized for chitosan dissolution. Physical properties, including density, pH value, viscosity, Hammett acidity, and Kamlet-Taft parameters (β and π*) of mPSAc and bPSAc, were studied and analyzed. The dissolution performance and regeneration properties of chitosan in BAILs were studied. The dissolution performance of chitosan was compared with other ordinary ionic liquids ([Emim]Cl, [Bmim]Cl, [Amim]Cl, [Emim]Ac, [Bmim]Ac, [Amim]Ac). 1% chitosan was dissolved in mPSAc and bPSAc aqueous solutions (5% w/w) at 30 °C (35 min and 37 min respectively), while other ordinary ionic liquids cannot dissolve chitosan in N12 h. The regenerated chitosan was characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and X-ray diffraction (XRD). Molecular weight and deacetylation degree results showed that mPSAc endowed slight hydrolysis upon chitosan, only dissolution of chitosan at lower temperature. Density functional theory (DFT) simulations were performed to study the interactions between BAILs and chitobiose. Four types of hydrogen bonds (C-H…O, O-H…O, N-H…S, N-H…O) and many strong hydrogen bonds were found in BAIL-chitobiose, suggesting strong interactions between BAILs and chitobiose. The molecular simulation results indicated that the active hydrogen atom of imidazole ring, sulfate ion and acetate ion played important roles in the chitosan dissolution process by the disruption of native hydrogen bonds of chitobiose. The information obtained here would provide a guide for the design and syntheses of novel solvent systems for the dissolution of chitosan (no hydrolysis process). In addition, it can provide a stable chitosan solution homogeneous system and promote the industrialization of chitosan greatly. Acknowledgements This work was supported by National Natural Science Foundation of China (Grant No. 21706127). The authors also gratefully appreciated the support from Postgraduate Research & Practice Innovation Program of Jiangsu Province (SJCX18-0345). Financial support from Students Innovation and Entrepreneurship Training Program of Nanjing Tech University was also gratefully appreciated (2019DC0768, 2019DC0769, 2019DC0770). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.molliq.2019.111533. References [1] K. Kalantari, A.M. Afifi, H. Jahangirian, T.J. Webster, Biomedical applications of chitosan electrospun nanofibers as a green polymer-review, Carbohydr. Polym. 207 (2019) 588–600. [2] A. El Kadib, Chitosan as a sustainable organocatalyst: a concise overview, ChemSusChem 8 (2015) 217–244.

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