Dissolution of Maize Starch in Aqueous Ionic Liquids: The Role of

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Dissolution of maize starch in aqueous ionic liquid: the role of alkyl chain length of cation and ratio of water:ionic liquid Fei Ren, Jinwei Wang, Jinglin Yu, Fengjuan Xiang, Shuo Wang, Shujun Wang, and Les Copeland ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06432 • Publication Date (Web): 12 Mar 2019 Downloaded from http://pubs.acs.org on March 17, 2019

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ACS Sustainable Chemistry & Engineering

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Dissolution of maize starch in aqueous ionic liquids: the role of alkyl chain length

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of cation and water:ionic liquid ratio

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Fei Ren†‡, Jinwei Wang†‡, Jinglin Yu†, Fengjuan Xiang†‡, Shuo Wang†§*, Shujun

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Wang†‡*, Les Copeland¶

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† State

Key Laboratory of Food Nutrition and Safety, Tianjin University of Science &

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Technology, 300457, China ‡

School of Food Engineering and Biotechnology, Tianjin University of Science &

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Technology, 300457, China §Tianjin

Key Laboratory of Food Science and Health, School of Medicine, Nankai

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University, Tianjin, 300071, China ¶Sydney

Institute of Agriculture, School of Life and Environmental Sciences, The

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University of Sydney, Sydney, NSW 2006, Australia

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* Corresponding authors

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Dr. Shujun Wang

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E-mail address: [email protected], phone: 86-22-60912486

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Dr. Shuo Wang

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Email address: [email protected], phone: 86-22-85358445

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Mailing address: No 29, 13th Street, TEDA, Tianjin 300457, China.

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ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 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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ABSTRACT: The dissolution behavior of maize starch in water:ionic liquid (IL)

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mixtures at ambient temperature (22-23 oC) was studied. The ionic liquids used were

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1-butyl-3-methylimidazolium chloride ([C4mim][Cl]), 1-propyl-3-methylimidazolium

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chloride ([C3mim][Cl]), and 1-ethyl-3-methylimidazolium chloride ([C2mim][Cl]).

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Structural analyses indicated that long- and short-range molecular order in the starch

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decreased with decreasing water:IL ratio. At water:IL ratios of 10:1 and 5:1, the

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extent of disruption of starch structure followed the order [C4mim][Cl] >

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[C3mim][Cl] > [C2mim][Cl]. At lower water:IL ratio (2:1), the complete disruption of

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starch granule morphology and ordered structures in water:[C3mim][Cl] and

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water:[C2mim][Cl] mixtures indicated these mixtures were more effective in

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dissolving starch than water:[C4mim]Cl mixture. Results from rheological, FTIR and

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1H-NMR

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viscosity of solutions increased, the interaction between IL and water decreased, and

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the interaction between the cation and the anion increased. Stronger interaction

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between the IL and water, and higher viscosity of water:IL mixtures were noted for

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cations with longer alkyl chains. Our results clearly showed that both the alkyl chain

39

length of cations and water:IL ratio played key roles in the dissolution of starch,

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predominantly by affecting the interaction between ILs and water and viscosity of

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water:IL mixtures.

analyses of water:IL mixtures showed that as water:IL ratio decreased, the

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KEYWORDS: starch dissolution; methylimidazolium; ionic liquid-water mixture;

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multi-scale structure; length of alkyl chain; interaction. 2


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INTRODUCTION

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Currently, global expectations for promoting environmental sustainability and

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reducing carbon footprint are creating demands for advanced production technologies

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using more eco-friendly materials and having lower energy input.1 Starch is a

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renewable, biodegradable and inexpensive biopolymer obtained from agricultural

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sources,2 and used extensively in food, paper and pharmaceutical industries.3 Starch is

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considered an important alternative to petroleum-based polymers for fabricating

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eco-friendly materials. Native starch is composed of amylose and amylopectin, which

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are packed into semi-crystalline granules with multi-scale structures ranging in scale

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from nano- to micrometer.4 The multi-scale structural order in starch granules plays a

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key role in determining functionality and suitability for different applications of

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starch.5 However, the use of starch is often limited by its low solubility in

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conventional solvents, due to strong hydrogen bonding between starch chains within

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semi-crystalline granules.6-8 Although the starch structure can be disrupted to some

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extent in several conventional solvents (e.g., dimethyl sulfoxide, N-methyl

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morpholine N-oxide and N, N-dimethylacetamide), the complete dissolution of starch

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still requires high temperatures (typically above 100 oC). Moreover, these solvents

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have disadvantages of volatility and flammability.1, 9

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More recently, ionic liquids (ILs) have been used widely as “green solvents” in the

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fabrication of biologically degradable materials because of their unique properties 3



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such as negligible vapor pressure, non-flammability, high chemical and thermal

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stability.10 ILs belong to a class of salts made up of an organic cation and an organic

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or inorganic anion and having a melting point below 100 °C.11 Imidazolium-based ILs

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have attracted much interest due to their ability to disrupt inter- and intramolecular

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hydrogen bonds present in biopolymers.12-14 Imidazolium-based ILs have been used as

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an effective medium for starch dissolution, plasticization and modification, facilitating

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the development of advanced biomaterials, such as conducting polymers, solid

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polymer electrolytes and modified starches with a high degree of substitution.15

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Nonetheless, significant energy is still required to achieve complete starch dissolution

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due to the high viscosity of ILs.16

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The presence of water in ILs affects many of their properties such as polarity,

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viscosity, conductivity, reactivity and solvating ability.17 Recently, water:IL mixtures

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were reported to have better dissolution ability for starch than pure ILs,3, 18 which was

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attributed to the lower viscosity of water:IL mixtures.3 A mixture of

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1-ethyl-3-methylimidazolium acetate:water (mole ratio 0.15:1) was reported to

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dissolve normal maize starch at 28 oC,9 although in another study, waxy maize starch

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was not dissolved completely in aqueous 1-allyl-3-methylimidazolium chloride at 25

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oC.19

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mainly affected by the interaction between the IL and water,3,

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experimental evidence was provided for this.

The dissolution behaviors of starch in water:IL mixtures were assumed to be

88 4


5

but no direct

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The dissolution mechanism of starch in water:IL mixtures at ambient temperature is

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still under debate. There have been few, if any, studies on the effect of the cationic

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moiety of an IL on its ability to dissolve starch, even though the nature of the cation

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greatly affects the interaction between an IL and water17 and between ILs and

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biopolymers20, 21. Hence, the aim of the present study is to investigate the effect of the

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alkyl chain of the imidazolium cation and different water:IL ratios on the dissolution

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behavior of maize starch by characterizing the structural changes of starch after

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dissolution. To better understand the mechanisms of starch dissolution, the properties

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of water:IL mixtures were investigated by rheology, attenuated total reflectance

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(ATR)-FTIR spectroscopy and nuclear magnetic resonance (NMR) spectroscopy. This

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study increases our mechanistic understanding of the dissolution of starch in ILs, to

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underpin the design new “green chemistry” applications for this important natural

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biopolymer.

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EXPERIMENTAL SECTION

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Materials

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Normal maize starch (NMS, 10.1% moisture and 22.4% amylose content) was

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purchased from Sigma Chemical Co. (St. Louis, MO, USA). The ILs

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(1-butyl-3-methylimidazolium chloride, [C4mim]Cl; 1-propyl-3-methylimidazolium

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chloride, [C3mim]Cl; 1-ethyl-3-methylimidazolium chloride, [C2mim]Cl) were

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supplied by Nuowei Chemistry Co., Ltd. (Wuhu, Anhui, China) and used without

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further purification. According to information supplied by Nuowei Chemistry Co., 5



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Ltd, the purities of the used ILs were ≥ 95 wt% (water content < 0.5 wt%). Figure 1

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depicts the chemical structures of the imidazolium-based ILs. Milli-Q water was used

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in all instances.

114 115

Sample preparation

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The water:IL mixtures of different molar ratios (10:1, 5:1 and 2:1) were prepared.

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Maize starch (0.5 g, dry basis) was dispersed in 4.5 g of the respective water:IL

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mixtures to prepare 10% starch suspensions. The mixtures were stirred on a magnetic

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stirrer for 12 h at room temperature (22-23 °C), after which 25 mL of absolute ethanol

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was added with stirring. The precipitated starch was collected by centrifugation at

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6800 g for 15 min and the IL was removed with the supernatant. The ethanol-washing

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process was repeated three times, and the residual ethanol in the starch after the final

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wash was evaporated in the fume hood overnight. The resulting starch samples were

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dried under a gentle air stream, ground into a powder, passed through a 150 μm sieve,

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and stored in a desiccator prior to analysis. The abbreviation (water:IL-m:n-MS) is

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used subsequently to indicate maize starch that was treated with mixtures of molar

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ratios of m water:n IL.

128 129

X-ray diffraction (XRD)

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The crystallinity of starch samples was determined using an X-ray diffractometer (D8

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Advance, Bruker, Germany) operating at 40 KV and 40 mA. The samples were

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equilibrated over a saturated NaCl solution at room temperature for 3 days before 6


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measurement. The moisture-equilibrated samples were packed tightly in round glass

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cells and examined over the range of 5o to 35o (2θ) at a scanning rate of 2 o/min and a

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step size of 0.02°, as described by Xiang, et al.5. The relative crystallinity was

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calculated as the ratio of the crystalline area to the total area between 5° and 35° (2θ)

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using the DIFFRAC EVA software (Version 3.0, Bruker, Germany).

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Differential scanning calorimeter (DSC)

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Thermal properties of starch samples were measured using a differential scanning

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calorimeter (200 F3, Netzsch, Germany) equipped with a thermal analysis data

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station. As described by Wang, et al.22, approximately 3 mg of starch samples were

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weighed into the aluminum pans and Milli-Q water was added to obtain a starch:water

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ratio of 1:3 (w/v). The pans were sealed and allowed to stand for 6 h at room

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temperature before analysis. The samples were heated from 20 to 100 °C at a heating

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rate of 10 °C min−1. An empty aluminum pan was used as the reference. The onset

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(To), peak (Tp), conclusion (Tc) temperatures and enthalpy of gelatinization (ΔH) were

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obtained through data recording software.

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Laser confocal micro-Raman (LCM-Raman) spectroscopy

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The LCM-Raman spectra of starch samples were obtained using a Renishaw Invia

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Raman microscope system (Renishaw, Gloucestershire, United Kingdom) equipped

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with a Leica microscope (Leica Biosystems, Wetzlar, Germany). A 785 nm green

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diode laser source was used. Spectra in the range of 3200-100 cm-1 were acquired and 7



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the full width at half maximum (FWHM) of the band at 480 cm-1 was obtained using

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the software of Wire 2.0.22

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Light microscopy (LM)

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A light microscope (DM-4000M-LED, Leica, Germany) was used to observe the

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morphology of starch samples. One drop of 0.5 % starch suspensions was applied

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onto a glass slide, and covered with a coverslip.18 Both normal and polarized light

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modes were used to image starch samples at room temperature.

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Scanning electron microscopy (SEM)

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Starch samples were mounted on a stub with double-sided adhesive tape,

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sputter-coated with gold before imaging using a field-emission scanning electron

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microscope (SU-1510, Hitachi, Japan). An accelerating voltage of 5 kV was used

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during imaging.

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Rheology

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Rheological measurements of water:IL mixtures were performed on an Anton Paar

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MCR302 rheometer (Anton Paar GmbH., Austria) with a Peltier temperature control

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system. Measuring system was a cone-plate geometry with 4o angle and 40 mm

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diameter. For each solution, the steady state viscosity was recorded as a function of

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shear rate from 10 s-1 to 500 s-1 at a constant temperature (23 oC).3 Silicone oil was

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placed around the edge of the measuring cell to prevent water vapor absorption and 8


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evaporation.

178 179

Attenuated total reflectance (ATR)-FTIR spectroscopy

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The ATR-FTIR spectra of water:IL mixtures were obtained using a Thermo Scientific

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Nicolet IS50 spectrometer (Thermo Fisher Scientific, USA). Approximately 50 μL

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water:IL mixtures was scanned using the ATR-FTIR from 4000 to 400 cm-1 at

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ambient temperature.17 The spectra were obtained at a resolution of 4 cm-1 with an

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accumulation of 32 scans against air as the background.

185 186

Nuclear magnetic resonance (NMR) spectroscopy

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The mixtures of D2O and IL at different molar ratios (D2O:IL 10:1, 5:1 and 2:1) were

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prepared for NMR measurement. The 1H NMR spectra of D2O:IL mixtures were

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obtained using a DMX 300 NMR spectrometer (300 MHz) (Bruker, Germany) at

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ambient temperature.23

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Statistical analysis

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All analyses were performed at least in triplicate and the results are reported as the

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mean values and standard deviations. In the case of XRD, ATR-FTIR and 1H NMR,

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only one measurement was performed. One way analysis of variance (ANOVA)

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followed by post-hoc Duncan's multiple range tests (p < 0.05) was conducted to

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determine the significant differences between mean values using the SPSS 17.0

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Statistical Software Program (SPSS Inc. Chicago, IL, USA). 9



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RESULTS AND DISCUSSION

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Crystalline structure of starch

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The X-ray diffraction patterns and relative crystallinity of native starch and starch

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samples after treatment with water:IL mixtures are shown in Figure 2 and Table 1,

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respectively. NMS showed a typical A-type diffraction pattern with peaks at 15.0,

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17.0, 18.0 and 23.0° (2θ). After treatment with the water:IL mixtures, the main

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diffraction peaks of starch crystallites gradually became weaker and then disappeared

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with decreasing water:IL ratio (Figure 2). The corresponding values for the relative

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crystallinity of the starch decreased to zero after treatment with water:[C3mim][Cl]

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and water:[C2mim][Cl] mixtures of 2:1 (Table 1). These observations indicated that

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the extent of disruption of starch crystallinity in the water:IL mixtures increased as the

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proportion of water in the water:IL mixture decreased. At water:IL ratios of 10:1 and

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5:1, the extent of disruption followed the order [C4mim][Cl] ＞ [C3mim][Cl] ＞

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[C2mim][Cl]. At the lowest water:IL ratio (2:1), no crystallinity was detected in the

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starch samples treated with water:[C2mim][Cl] and water:[C3mim][Cl] mixtures, but

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some crystallinity was still present in the starch treated with water:[C4mim][Cl]

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mixture (Figure 2). These results indicated that the extent of disruption of starch

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crystallinity increased with increasing alkyl chain length of IL cation at high water:IL

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ratios, but the opposite was observed at a low water:IL ratio.

219 220

Thermal properties of starch 10


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Maize starch presented a typical endothermic transition of gelatinization in the

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temperature range of 65.0 to 76.3 oC, with a ΔH value of 12.7 J/g (Figure. 3, Table

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1). After treatment with water:IL mixtures of 10:1, the endothermic transition shifted

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to lower temperatures. With decreasing water:IL ratio, the endotherm shifted to higher

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temperatures

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water:[C2mim][Cl] and water:[C3mim][Cl] mixtures of 2:1. The starch samples

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treated with water:IL mixtures presented smaller enthalpy changes compared with

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native maize starch. In agreement with the loss of crystallinity observed with XRD

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analysis, the ΔH values of the starch samples decreased with decreasing water:IL ratio

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used, and became zero for the 2:1 mixtures of water:[C2mim][Cl] and

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water:[C3mim]]Cl] (Table 1). At high water:IL ratios (10:1 and 5:1), the ΔH values of

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followed the order [C2mim][Cl] ＞ [C3mim][Cl] ＞ [C4mim][Cl]. At water:IL ratio of

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2:1, no enthalpy change was detected for starch samples treated with

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water:[C2mim][Cl] and water/[C3mim]Cl mixtures, whereas a small enthalpy change

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of 2.3 J/g was still observed for starch treated with water:[C4mim][Cl] mixture.

and

no

endotherm

was

observed

for

starch

treated

with

236 237

Short-range ordered structure of starch

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The LCM-Raman spectra were obtained to characterize the short-range molecular

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order of NMS and starch samples after treatment with water:IL mixtures (Figure 4).

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The values of full width at half maximum (FWHM) of the band at 480 cm-1, which are

241

listed in Table 1, were used to characterize the degree of short-range ordered structure

242

in starch. The FWHM values generally increase with increasing disruption of 11



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short-range molecular order of starch.5, 24-26 NMS had the smallest FWHM value of

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16.05, whereas the FWHM values of pre-treated starch samples increased with

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decreasing water:IL ratio, indicating the gradual disruption of short-range molecular

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order in starch. At high water:IL ratios (10:1 and 5:1), increasing FWHM values

247

indicated that disruption of short-range molecular order was favored by longer alkyl

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chain length of the IL cation. In contrast, at a low water:IL molar ratio (2:1), starch

249

treated with water:[C2mim][Cl] and water:[C3mim][Cl] mixtures presented higher

250

FWHM values than sample treated with water:[C4mim][Cl] mixture, showing that

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mixtures containing IL with a cation of shorter alkyl chain caused greater disruption

252

to the short-range structural order.

253 254

Granular morphology of starch

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The LM and SEM images of native starch and starch samples after treatment with

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water:IL mixtures are shown in Figure 5. Under bright-field illumination and with

257

SEM, NMS showed angular and spherical-shaped granules (Figs 5a-1 and 5a-3). After

258

treatment with water:IL mixtures of 10:1 and 5:1, the granular morphology of starch

259

was partially disrupted, with the extent of disruption being increased with increasing

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alkyl chain length of the IL cation (Figs 5b-1, b-3, c-1, c-3, e-1, e-3, f-1, f-3, h-1, h-3,

261

i-1, i-3). With the most concentrated IL solution (water:IL ratio 2:1), no intact

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granules were observed when mixtures with [C3mim][Cl] and [C2mim][Cl] were used

263

(Figs 5g-1, g-3, j-1, j-3), whereas a few starch granules were still observed after

264

treatment with the [C4mim][Cl] mixture (Figs 5d-1, d-3). 12


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Under polarized light, native maize starch granules displayed clear birefringent

267

“Maltese cross” patterns (Fig. 5a-2). With the more dilute water:IL mixtures (10:1 and

268

5:1 IL), the birefringent patterns became increasingly blurred with increasing alkyl

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chain length of the IL cation. For the most concentrated IL mixture (water:IL 2:1), the

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birefringent patterns disappeared for starch treated with water:[C3mim][Cl] and

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water:[C2mim][Cl] mixtures, but not with water:[C4mim][Cl] mixture.

272 273

Rheological properties of water:IL mixtures

274

The steady state viscosities of pure water and water:IL mixtures are compared in

275

Table 2. Viscosity increased with decreasing water content in the water:IL mixtures,

276

and with increasing alkyl chain length of the IL cation. This was in general agreement

277

with the previous results for the mixtures of water and imidazolium acetate.27 The

278

viscosity of 2:1 water:IL mixtures was 3-4 times greater than the 5:1 mixtures and 6-9

279

times that of the 10:1 mixtures (Table 2). The greater viscosity of mixtures of

280

water:IL with longer cation alkyl chains was attributed to the higher hydrophobicity

281

of longer cation alkyl chain, increasing aggregation of ion pairs.17, 28

282 283

ATR-FTIR analyses of water:IL mixtures

284

ATR-FTIR spectroscopy is a useful tool for investigating molecular interactions

285

between water and ILs.17,

286

mixtures show (Fig. 6), the band in the region 3000-3800 cm-1 of water, which is

23

As the ATR-FTIR spectra of pure water and water:IL

13



287

assigned to the -OH stretching modes,29 shifted to higher wavenumbers after mixing

288

with the IL (Table 2). This blue shift in the spectrum indicated disruption of

289

hydrogen-bonding network of water by the interactions between ILs and water

290

molecules.30 At the same water:IL ratio, the -OH stretching band shifted to higher

291

wavenumbers with increasing alkyl chain length of the IL cation, with the greater shift

292

observed as the alkyl chain length increased from C3 to C4.

293 294

1H

295

The 1H-NMR spectra of water:IL mixtures are shown in Fig. 7 and the chemical shifts

296

of hydrogens tethered at carbons C(2), C(4) and C(5) on imidazolium ring (designated

297

as δH(2), δH(4) and δH(5), respectively) are listed in Table 2. The changes in the chemical

298

shift of these hydrogens are indicative of interactions between cations and anions or

299

between cations and water molecules.31 The δH(2), δH(4) and δH(5) increased dramatically

300

with decreasing water:IL ratio, which was attributed to increased aggregation of IL

301

ion pairs enhancing the cation-anion interaction.23 At the same water:IL ratio, the

302

δH(2), δH(4) and δH(5) of water:IL mixtures increased with increasing alkyl chain length

303

of the IL cation, indicating that the interactions between ILs and water increased, and

304

those between cations and anions decreased. The results from 1H NMR were

305

consistent with the results from ATR-FTIR.

NMR analyses of water:IL mixtures

306 307

GENERAL DISCUSSION

308

In the present study, the dissolution behaviour of maize starch in water:IL mixtures at 14


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309

ambient temperature was investigated. Both the water:IL ratio and the alkyl chain

310

length of the IL cations affected the structural disruption of starch granules. Long- and

311

short-range ordered structures of starch were disrupted increasingly as the proportion

312

of IL in the water:IL mixtures increased. At a ratio of 2:1, water:[C3mim][Cl] and

313

water:[C2mim][Cl] mixtures caused complete disruption of granule morphology and

314

starch structural order, indicating the good dissolution of starch in these systems. In

315

contrast, the water:[C4mim][Cl] mixture did not disrupt completely the ordered

316

structure of starch, indicating only partial dissolution of starch occurred in this

317

mixture. In the more dilute IL solutions (water:IL ratio of 10:1 and 5:1),

318

water:[C4mim][Cl] caused the greatest disruption of granular morphology and starch

319

structure, suggesting that IL mixtures containing cations with longer alkyl chains had

320

better starch-dissolving capability than those with shorter alkyl chain cations.

321 322

Interactions between water and ILs and the viscosity of the water:IL mixtures clearly

323

affected the dissolution behaviour of starch in the solvents used, as illustrated in the

324

following discussion. At a water:IL ratio of 10:1, the blue spectral shift of the -OH

325

stretching band of water molecules was greater than in water:IL mixtures of 5:1 and

326

2:1. Similarly, the δH(2), δH(4) and δH(5) decreased greatly with increasing water:IL ratio.

327

The FTIR blue shift indicated that strong interactions occurred between ILs and water

328

at high water:IL ratio, whereas the dissociation of ILs into separate cations and anions

329

was demonstrated by the NMR chemical shift. With decreasing water:IL ratio, the

330

interaction between ILs and water decreased due to the association of the cations and 15



331

anions of the IL.

332 333

Effects due to the concentration of the IL on the interactions between ILs and water

334

occurred concomitantly with changes in viscosity of water:IL mixtures. The low

335

viscosity of water:IL mixtures of 10:1 would have favored the penetration of the IL

336

into starch granules. However, as the amount of IL was limited, only some of the

337

hydrogen bonds of starch were disrupted, resulting in incomplete dissolution of

338

granules. As the amount of IL in the mixtures increased, the viscosity also increased.

339

The extent of starch dissolution would then be determined by the greater availability

340

of IL to disrupt hydrogen bonds in the starch being balanced against reduced mobility

341

of the IL due to higher viscosity.

342 343

At the same water:IL ratio, the extent of the blue spectral shift of the –OH stretching

344

band and the δH(2), δH(4) and δH(5) increased with increasing alkyl chain length of the

345

cations, indicative of increasing dissociation of the IL. At the higher water:IL ratios of

346

10:1 and 5:1, the interaction between IL and water was stronger for cations with

347

longer alkyl chain length, thus favouring the IL to interact more freely with starch.

348

At a low water:IL ratio of 2:1, the much higher viscosity of the water:[C4mim][Cl]

349

mixtures would have resulted in much slower penetration of the IL into starch

350

granules and a lower extent of structural disruption. In contrast, the lower viscosity of

351

the water:[C3mim] and water:[C2mim][Cl] in 2:1 mixtures would have allowed them

352

to penetrate more easily into starch granules and disrupt the structure. 16


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353 354

CONCLUSIONS

355

The dissolution behavior of starch in water:IL mixtures was affected by both the

356

amount of IL in the mixture and the alkyl chain length of the cation. Maize starch

357

could be completely dissolved in 2:1 water:[C3mim][Cl] and water:[C2mim][Cl]

358

mixtures at ambient temperature, which is much lower than the temperatures

359

commonly used in the polymer dissolution (typically over 100 °C). In the more dilute

360

water:IL mixtures (10:1 and 5:1), water:[C4mim][Cl] mixtures caused greater

361

disruption of starch structure. The interaction between water and the IL and the

362

viscosity of water:IL mixtures were proposed to account for the differences in

363

dissolution behavior of starch observed. Our findings contribute new knowledge on

364

starch dissolution technology to assist in the development of cost effective and lower

365

energy uses for natural biopolymers.

366 367

AUTHOR INFORMATION

368

* Corresponding author:

369

Shujun Wang; Email: [email protected]

370

Shuo Wang; Email: [email protected]

371 372

Author Contributions

373

Shujun W. conceived and designed the study. F. R. conducted the experiments and

374

data analysis. The manuscript was written through contributions of all authors. All 17



375

authors have given approval to the final version of the manuscript.

376 377

Notes

378

The authors declare no competing financial interest.

379 380

ACKNOWLEDGMENT

381

The authors gratefully acknowledge the financial support from the National Natural

382

Science Foundation of China (31430068, 31871796) and Natural Science Foundation

383

of Tianjin City (17JCJQJC45600, 18ZYPTJC00020).

384 385 386 387 388 389 390 391 392 393 394 395 396 18


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397 398

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Page 24 of 33

Table 1. Short- and long-range ordered structures of native starch and starch samples after treatment with different water:IL mixtures. Samples

T0 o ( C)

TP o ( C)

TC (oC)

ΔH (J/g)

Relative crystallinity (%)

NMS

64.97 ± 0.12d

70.90 ± 0.10e

76.33 ± 0.12d

12.7 ± 1.0e

28.4

FWHM of the band at 480 cm-1 16.05 ± 0.07a

Water:[C4mim][Cl]-10:1-MS Water:[C4mim][Cl]-5:1-MS Water:[C4mim][Cl]-2:1-MS

50.99 ± 0.22a 56.73 ± 0.73c 66.83 ± 0.68e

60.20 ± 0.36a 67.20 ± 0.87d 72.33 ± 0.25f

71.43 ± 0.60b 77.40 ± 0.36e 78.80 ± 0.36f

4.3 ± 0.1bc 3.6 ± 0.1b 2.3 ± 0.3a

13.9 11.2 8.5

18.88 ± 0.05e 19.50 ± 0.05f 20.00 ± 0.21g

Water:[C3mim][Cl]-10:1-MS Water:[C3mim][Cl]-5:1-MS Water:[C3mim][Cl]-2:1-MS

51.83 ± 0.75a 54.17 ± 0.70b N.D.

60.67 ± 0.31a 63.12 ± 0.70b N.D.

69.20 ± 0.30a 73.27 ± 0.58c N.D.

6.0 ± 0.7d 4.5 ± 0.1c N.D.

18.8 12.0 0.0

18.45 ± 0.10d 19.07 ± 0.03e 20.69 ± 0.07h

54.13 ± 0.60b 63.17 ± 0.70b 69.23 ± 0.35a 7.9 ± 0.2d 24.6 Water:[C2mim][Cl]-10:1-MS 55.90 ± 0.26c 65.83 ± 0.31c 75.87 ± 0.60d 6.0 ± 0.0c 20.4 Water:[C2mim][Cl]-5:1-MS N.D. N.D. N.D. N.D. 0.0 Water:[C2mim][Cl]-2:1-MS Values are means ± SD. Values with the same lowercase letters in the same column are not significantly different (p < 0.05). N.D. not determined.

24


17.31 ± 0.10b 17.78 ± 0.10c 20.38 ± 0.09h

Page 25 of 33 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

507 508

509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531


Table 2. The viscosity, wavenumbers (-OH of water) and chemical shift (δH(2), δH(4) and δH(5) of cation imidazolium ring) of water:IL mixtures. Wavenumbers Chemical shift -1 Samples Viscosity (cm ) (ppm) (mPa·s) -OH δH(2) δH(4) δH(5) 3389.42 N.D. N.D. N.D. Pure water 0.82 ± 0.00a Water:[C4mim][Cl] 10:1 Water:[C4mim][Cl] 5:1 Water:[C4mim][Cl] 2:1

3.45 ± 0.01d 8.42 ± 0.01g 32.85 ± 0.02j

3404.52 3401.42 3400.91

9.14 9.45 9.64

7.82 8.04 8.18

7.75 7.97 8.11

Water:[C3mim][Cl] 10:1 Water:[C3mim][Cl] 5:1 Water:[C3mim][Cl] 2:1

3.11 ± 0.00c 6.11 ± 0.01f 18.54 ± 0.00i

3403.90 3400.15 3399.52

9.05 9.26 9.43

7.76 7.92 8.01

7.69 7.84 7.93

Water:[C2mim][Cl] 10:1 Water:[C2mim][Cl] 5:1 Water:[C2mim][Cl] 2:1

2.63 ± 0.00b 5.86 ± 0.01e 17.04 ± 0.01h

3401.14 3397.42 3397.08

9.03 9.24 9.41

7.75 7.90 8.00

7.68 7.83 7.92

Values are means ± SD. Values with the same lowercase letters in the same column are not significantly different (p < 0.05). N.D. not determined.

25



532

533 534

Figure 1. Structure of ionic liquids used.

535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 26


Page 26 of 33

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552 553

Figure 2. The XRD diffraction patterns of native starch and starch samples after

554

treatment with different water:IL mixtures.

555 556 557 558 559 560 561 562 563 564 27



565 566

Figure 3. The DSC thermograms of native starch and starch samples after treatment

567

with different water:IL mixtures.

568 569 570 571 572 573 574 575 576 577 28


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578 579

Figure 4. The LCM-Raman spectra of native starch and starch samples after

580

treatment with different water:IL mixtures.

581 582 583 584 585 586 587 588 589

29



590 591

Figure 5. The normal and polarized light microscopy and SEM images of native

592

starch and starch samples after treatment with water:IL mixtures.

593

(a) NMS, (b) Water:[C4mim]Cl-10:1-MS, (c) Water:[C4mim]Cl-5:1-MS,

594

(d) Water:[C4mim]Cl-2:1-MS, (e) Water:[C3mim]Cl-10:1-MS,

595

(f) Water:[C3mim]Cl-5:1-MS, (g) Water:[C3mim]Cl-2:1-MS,

596

(h) Water:[C2mim]Cl-10:1-MS, (i) Water:[C2mim]Cl-5:1-MS,

597

(j) Water:[C2mim]Cl-2:1-MS. NLM, normal light microscopy; PLM, polarized light

598

microscopy. The scale of NLM, PLM and SEM was 50μm, 50μm and 20μm,

599

respectively.

600 601 30


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602 603

Figure 6. The ATR-FTIR spectra of pure water and water:IL mixtures at different

604

molar ratios.

605 606 607 608 609 610 611 612 613 614 31



615

H(2)

H(4), H(5)

616 617

Figure 7. The 1H-NMR spectra of water:IL mixtures at different molar ratios.

618 619 620 621 622 623 624 32


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625


For Table of Contents Use Only

626

627 628 629

Synopsis: Maize starch is dissolved in aqueous ionic liquid at ambient temperature by

630

regulating cation alkyl chain length and water:IL ratio.

631

33


Dissolution of Maize Starch in Aqueous Ionic Liquids: The Role of

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