Modulation of Cyclodextrin Particle Amphiphilic Properties to

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Modulation of cyclodextrins particles amphiphilic properties to stabilize Pickering emulsion Yongkang Xi, Zhigang Luo, Xuanxuan Lu, and Xichun Peng J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b03940 • Publication Date (Web): 18 Dec 2017 Downloaded from http://pubs.acs.org on December 20, 2017

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Journal of Agricultural and Food Chemistry

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Modulation of cyclodextrins particles amphiphilic properties to stabilize

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Pickering emulsion

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Yongkang Xi1, Zhigang Luo1,3*, Xuanxuan Lu2*, Xichun Peng4

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1. School of Food Science and Technology, South China University of Technology,

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Guangzhou 510640, China

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2. Department of Food Science, Rutgers, The State University of New Jersey, 65

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Dudley Rd, New Brunswick, New Jersey 08901, USA

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3.Overseas Expertise Introduction Center for Discipline Innovation of Food Nutrition

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and Human Health (111 Center), Guangzhou 510640, China

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4. Department of Food Science and Engineering, Jinan University, Guangzhou,

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510630, China

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*

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Zhigang Luo, Tel: +86-20-87113845, Fax: +86-20-87113848. E-mail address:

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[email protected]

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Xuanxuan Lu, Tel:

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[email protected]

Corresponding author:

+1 848 565 5791, Fax: +1 732 932 6776, E-mail address:

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ACS Paragon Plus Environment


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ABSTRACT: Cyclodextrins was proved to form complexes with linear oil molecules

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and stabilize emulsions. Amphiphilic properties of cyclodextrins particles were

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modulated through esterification reaction between β-cyclodextrins (β-CD) and

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octadecenyl succinic anhydride (ODSA) under alkaline conditions. ODS-β-CD

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particles with DS of 0.003, 0.011 and 0.019 were obtained. . The introduced

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hydrophobic long chain that was linked within β-CD cavity led to the change of

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ODS-β-CD in terms of morphological structure, surface charge density, size and

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contact angle, upon which the properties and stability of the emulsions stabilized by

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ODS-β-CD were highly dependent. The average diameter of ODS-β-CD particles

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ranged from 449 nm to 1484 nm. With the DS increased from 0.003 to 0.019, the

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contact angle and absolute zeta potential value of these ODS-β-CD particles improved

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from 25.7° to 47.3°, 48.1 to 62.8 mV, respectively. And the cage structure of β-CD

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crystals was transformed to channel structure, then further to amorphous structure

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after the introducing of the octadecenyl succinylation chain. ODS-β-CD particles

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exhibited higher emulsifying ability compared to β-CD. The resulting Pickering

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emulsions formed by ODS-β-CD particles were more stable during storage. This

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study investigates the ability of these ODS-β-CD particles to stabilize oil-in-water

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emulsions with respect to their amphiphilic character and structural properties.

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KEYWORS: β-Cyclodextrin, Octadecenyl succinic anhydride, Pickering emulsion,

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INTRODUCTION

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Particle-stabilized emulsions, usually referred as Pickering emulsions, have unique

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properties due to the almost irreversible adsorption of particles at the oil-water

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interface.1 Until now, a number of different particulate emulsifiers have been reported

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to stabilize Pickering emulsions, such as silica,2 clays,3 microgels4 and latex.5

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Recently, environmentally benign particle emulsifiers have received reasonable

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attention due to their wide applications, especially in food, cosmetic and pharmaceutic

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industry. It has been found that some biomass based particles, such as cellulose

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nanocrystals,6 chitin nanocrystals7 and starch granules8 are able to stabilize Pickering

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emulsions.

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Cyclodextrins (CDs) are cyclic oligosaccharides composed of α-1,4 linked

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D-glucose units.9 The molecular arrangement of the CDs is similar to the truncated

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cone. It is widely used in the fields of activity material transfer, enzyme-like catalysis

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and other fields because of its special non-polar cavity structure, biocompatibility,

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good biodegradability and so on.10,

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researchers have proved that assembled particles of CDs with other hydrophobic

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molecules also possess the potential to stabilize Pickering emulsions. Shimada et al.

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firstly disclosed that pure α-CD and β-CD were not surface active until they formed

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pseudo-surfactants as inclusion complexes with linear oil molecules at the oil–water

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interface.12 Hashizaki et al.13 further clarified that the inclusion complex could attach

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to the oil-water interface to form dense interface film, thus hindering the further

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aggregation of droplets. Meanwhile, part of dissociated inclusion complex in the

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With the in-depth understanding of CDs,

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continuous phase also maintained the distance between droplets due to formation of

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three-dimensional network structures. However, the emulsifying ability of CDs was

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affected by oil phase used. Inclusion complexes with shorter fatty acid chain

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presented relatively low hydrophobicity at the oil–water interface and thus leading to

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the poor emulsifying properties.14

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Taking into account the deficits of traditional CD-stabilized emulsions, we

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proposed that it was necessary to enhance the emulsifying capacity of CDs by

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hydrophobically modifying them into the stable, controllable and surface active

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submicro/nanoparticles. Since CDs possessed a hydrophilic structure due to abundant

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hydroxyl groups, it was able to effectively reduce the solubility of CDs and achieve

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more stable structure in aqueous solution if hydrophobic long chains were introduced.

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Meanwhile, CDs with controllable amphiphilic properties were obtained by adjusting

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the reaction degree with hydrophobic chains, thus making them a good model for

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investigating the emulsifying mechanism of these emulsifiers. Moreover, the resulting

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amphiphilic structure formed through hydrophobic modification might endow CDs

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themselves with surface activity.

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Octadecenyl succinic anhydride (ODSA) is a common food additive, which

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contained a hydrophobic chain of 18 carbon atoms and an acid anhydride ring.

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Octadecenyl succinate starch esters have been successfully synthesized and are widely

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used as emulsifiers in food, pharmaceutical, personal care and biodegradable plastics

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industries.15 Therefore, esterification of CDs with ODSA might provide an efficient

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and simple way to meet our requirements of modifying CDs. 4


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In this study, octadecenyl succinate cyclodextrin esters (ODS-β-CD) with different

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degrees of substitution were prepared. The molecular structures of ODS-β-CDs were

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systematically analyzed by 1D 13C NMR and 2D 1H NMR. Then, we investigated the

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effects of octadecenyl succinate groups on the structure, morphology, size and surface

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charge change of ODS-β-CDs. Moreover, the amphiphilic character of ODS-β-CDs

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on the stabilizing of Pickering emulsion was discussed.

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MATERIALS AND METHODS

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Materials. β-Cyclodextrins (β-CD) was purchased from Boao Biological

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Technology Co., Ltd. (Shanghai, China) and dried in a vacuum for 24 h before use.

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1-Octadecenyl succinic anhydride (ODSA) was donated by TCI Chemical Industry

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Development Co., Ltd. (Shanghai, China). Fish oil was purchased from the Shanxi

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high-tech industrial co., LTD (Xi’an, China). All other chemicals were of analytical

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grade unless specified.

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ODS-β-CD Synthesis. ODS-β-CD was prepared following the method reported

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earlier by Cheng et al. with some modifications.16 Briefly, β-CD (10 g, dry basis) was

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suspended in 90 mL deionized water by gently magnetic stirring at 50 ± 1℃ for 1 h.

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The pH of the suspension was adjusted to 8.5 with 3% (w/v) NaOH solution. Various

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doses of ODSA (1, 5 and 11 wt% based on dry β-CD) were added slowly within 1.5 h.

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The reaction was completed when the pH reached a constant value. After the

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completion of the reaction, the solution was neutralized to pH 6.5 with 3% HCl and

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then freeze-dried. About five times (v/w) of hexane/isopropanol (3:1 v/v) was added 5



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into the freeze-dried powder samples in tubes as cleaning agent and the tubes were

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shaken for 15 min at 160 rpm using a shaking incubator and centrifuged for 20 min at

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8000 rpm.17 The supernatants were collected for further analysis. Furthermore, the

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cleaning procedure was repeated several times until no Cl- and ODSA were detected

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using AgNO3 solutions. ODS-β-CD-1, ODS-β-CD-2 and ODS-β-CD-3 represented

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the final products when 1, 5 and 11 wt% of ODSA was added during the reactions,

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respectively.

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Degree of Substitution (DS). The degree of substitution (DS) was the average

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number of hydroxyl groups substituted per glucose unit with ODSA. The content of

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substituted acid groups could be determined by alkali saponification followed by back

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titration of excess alkali with HCl.18 The octadecenyl succinylation level of the

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modified β-CD was determined using titrimetric method. Briefly, the ODS-β-CD

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sample (5 g, dry weight) was accurately weighed and suspended by stirring for 30 min

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in 25 mL of hclisopropyl-alcohol solution (2.5 M). Then, 100 mL 90% (v/v) aqueous

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isopropyl alcohol solution was added, and the slurry was stirred for additional 10 min.

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The suspension was filtered through a glass filter (1.5-2.5 µm) and the residue was

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washed with 90% isopropyl alcohol solution until no Cl- could be detected (using 0.1

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M AgNO3 solution). The sample was redispersed in 300 mL distilled water, and then

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the dispersion was heated in a boiling water-bath for 20 min with stirring. The sample

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solution was titrated with 0.1 M standard NaOH solution using phenolphthalein as an

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indicator. A blank titration of the unmodified β-CD was also performed as a control

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and the difference between control and modified samples was assumed to be due to

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chemically bound ODSA groups. The DS was calculated by the following equation:

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(1)

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(2)

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(3)

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Where %ODS was the weight percentage of ODS in ODS-β-CD, V1 was the

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titration volume of NaOH (mL) of ODS-β-CD, V2 was the titration volume of NaOH

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(mL) of native β-CD, and W was the dry weight (g) of the ODS-β-CD.

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Fourier Transform Infrared Spectroscopy (FTIR). The FT-IR spectra of the

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samples were recorded using a Perkin Elmer Spectrum RXIFT-IR Spectrometer (USA)

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at room temperature.19 The sample powder was blended with KBr powder and pressed

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into tablets before spectra were obtained over the range of 4000 to 500 cm-1 at a

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resolution of 8 cm-1.

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Nuclear Magnetic Resonance (NMR) Spectroscopy. NMR spectra were recorded

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with a Varian Inova 500 spectrometer (1H, 500 MHz, and 13C, 125 MHz, respectively)

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based on a previous method.20 Briefly, β-CD and its derivatives were dissolved in D2O

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at room temperature. Then, 2D

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enhancement spectroscopy (NOESY) spectrum and

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with a Varian Inova.

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H–1H two-dimensional nuclear overhauser 13

C NMR spectra were obtained

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X-Ray Diffraction Spectra (XRD). XRD data of the samples were obtained with a

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RU200R X-ray diffractometer (Rigaku, Tokyo, Japan) by using Cu Kα radiation at 35 7



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kV and 20 mA, a theta-compensating slit, and a diffracted beam monochromator in a

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range of 2θ = 4−35°.21

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The Igor software package (Wavemetrics, Lake Oswego, Oregon) was used for

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curve fitting. The obtained values from the fitting coefficients are those that minimize

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the value of Chi-squared, which is defined as: (4)

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where γ is a fixed value for a given point, γi is the measured data value for the point,

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and σi is an estimate of the standard deviation for γi . The curve fitting operation is

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carried out iteratively. For each iteration, the fitting coefficients are refined to

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minimize chisquared. The Levenberg-Marquardt algorithm is used to search for the

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coefficient values that minimize chi-squared. This is a form of nonlinear, least squares

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fitting. The iterative fit terminates when the fractional decrease of chi-squared

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between successive iterations is less than 0.001.22

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Morphology Observation. Morphology structures of samples were recorded by a

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scanning electron microscope (LEO 1530-VP, Zeiss, German) with the field emission

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gun operating at 3 kV. The β-CD and ODS-β-CD were coated with around 18 nm

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Au/Pt and examined.

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Determination of ODS-β-CD morphology at oil-water interface: emulsions were

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prepared by homogenizing the mixture of 45% (WOil/Wwater) n-hexane and 3%

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(Wparticle/Wwater) corresponding concentration ODS-β-CD suspensions. The resultant

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mixtures were sheared using an Ultra-Turrax T10 homogenizer (IKA-Works Inc.,

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Germany) at 17000 rpm for 4 min at room temperature to yield Pickering emulsions. 8


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Emulsion samples were placed on culture dish and freeze-dried. The resulting

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powders were placed onto a sample stage with an aluminum tape, coated with Au/Pt

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and the morphology of the powders was measured by scanning electron microscopy.

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Particle Size and Zeta (ζ)-Potential Measurements. The ODS-β-CD particles

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were diluted to 1 mg/mL with Millipore water, and the pH was adjusted to 7.0, then

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the particle size and ζ-potential were measured using a Nano ZS Zetasizer instrument

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(Malvern Instruments, Worcestershire, UK). All measurements were carried out at

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25 °C, and the results were reported as averages of three readings.

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Contact Angle Measurement. The water contact angles of β-CD and ODS-β-CD

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were measured using an OCA 20 AMP (Dataphysics Instruments Gmb H, Germany).

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The sample powders were pressed as pellets of 13 mm in diameter and 2 mm in

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thickness. Next, a drop of pH 3.5 Milli-Q water (5 µL) was deposited on the surface

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of the pellets using a high-precision injector. After 30 seconds for equilibration, the

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drop image was photographed using a high-speed video camera, and the profile of the

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droplet was numerically solved and fitted to the Laplace−Young equation.23 Contact

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angles were measured on each of four pellets per sample, and three measurements

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were performed for each pellet.

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Preparation of Pickering Emulsion. The Pickering emulsions were prepared by

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homogenizing the mixture of 5%-45% (WOil/Wwater) fish oil and 1-5% (Wparticle/Wwater)

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corresponding concentration β-CD or ODS-β-CD with different DS (0−0.019)

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suspensions. Fish oil (6.75 mL) was added into the β-CD or ODS-β-CD suspensions

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(15 mL) in a glass vial, and the resultant mixtures were sheared using an Ultra-Turrax 9



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T10 homogenizer (IKA-Works Inc., Germany) at 17,000 rpm for 4 min at room

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temperature to form Pickering emulsions.

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Emulsion Morphology Observation. Optical characterization of emulsions was

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investigated using an optical microscope (Olympus bx50, japan) which was equipped

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with a video camera. The emulsions were diluted 2 times with glycerol and one drop

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was added on a glass plate with cover glass for microscopic observation using a 200

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magnification lens.

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Confocal Laser Scanning Microscopy (CLSM). CLSM images were recorded on

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a Leica TCS SP5 confocal laser scanning (Leica Microsystems Inc., Heidelberg,

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Germany) equipped with a 20-HC PL APO/0.70NA oil immersion objective lens.24

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The samples were stained with mixture of Nile blue (0.1%) and Nile red (0.1%). The

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stained samples were placed on concave confocal microscope slides and were

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examined using a 100 magnification lens. The argon/krypton laser had a Helium Neon

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laser (He-Ne) with excitation at 488 nm and 633 nm for the mixed dyes.

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Creaming Index (CI). The CI was determined according to the method of

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Moschakis et al.25 with modifications. Emulsion samples were put into 10 mL

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flat-bottom glass tubes immediately after treatment. The tubes were sealed with

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plastic tops to prevent evaporation. The emulsion samples were stored quiescently at

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ambient temperature (25 °C). The extent of creaming was characterized by the

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creaming index (CI,%): CI = (V2 / V1) × 100%, where V2 was the volume of the

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observed emulsion (i.e. the non-clear fraction) and V1 was the volume of all the

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phases, i.e. oil phase, added granule, and continuous phase. 10


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Statistical Analysis. Each test was conducted in triplicate. Analyses of variance for

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all the treatments was calculated by Duncan’s multiple-range test (p < 0.05), using

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SPSS (SPSS Inc., Chicago, IL, USA, version 13.0).

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RESULTS AND DISCUSSION

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Molecular structure of ODS-β-CD. Succinylation leaded to the substitution of

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hydroxyl groups in the β-CD molecules with carbonyl groups of ODSA. The

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introduction of carbonyl groups was confirmed by FT-IR spectroscopy (see

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supporting information Fig S1.).26 Fig.1(I) is the

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ODS-β-CDs, where peaks arising from the glucose in β-CD were assigned according

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to the literature,27 their chemical shifts are also provided in Table 1. According to

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Bai’s theory,28 if the hydroxyl groups were substituted on the O-2,3,6 of glucose units,

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it must affect the chemical shift of its own and adjacent C signals simultaneously,

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based on which the substitution position of glucose units in ODS esterification

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reaction was determined.

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C-NMR spectra of β-CD and

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From C signals chemical shifts of ODS-β-CD-1, we found the δ of C-2 and its

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adjacent C-1 and C-3 were significantly displaced, confirming that ODS-β-CD-1 was

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esterified at the O-2 position. However, as the δ of C-4 (adjacent to C-3) and the δ of

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C-6 were not substantially displaced, it could be concluded that ODS-β-CD-1 did not

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undergo an esterification reaction at the O-3 and O-6 position. With the increasing in

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DS, the δ of C-2 and its adjacent C-1 and C-3 on the ODS-β-CD-2 glucose unit was

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still significantly displaced, meanwhile, the δ of C-6 and its adjacent C-5 presented 11



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corresponding displacement as well (C-6 was connected to only C-5 in the glucose

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unit). This indicated that the substitution positions were on both the O-2 and O-6 in

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ODS-β-CD-2. When it came to ODS-β-CD-3 with the highest DS, we surprisingly

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found that the δ displacement happened in every C on the glucose units, revealing that

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esterification reaction occurred at all O-2,3,6 position. Therefore, it was proposed that

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the hydroxyl reaction activity of the glucose units in the esterification reaction

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followed O-2 > O-6> O-3 sequence.

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2D 1H-1H NOESY spectrum is a useful tool to analyze the spatial relative position

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of atoms, in which the response peaks occur when the distance between the two

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corresponding atoms is less than 5-6 Å. Hence, the structure of ODS-β-CDs with

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different DS could be analyzed using 2D 1H-1H NOESY. In this study, we selected the

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ODS-β-CD-3 as a model since it might contain all kinds of structures. As shown in

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Fig 1(II). The resonance in the diagonal line represented the signals between their

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own atoms (unnecessary to analyze). Meanwhile, there was signal corresponding to

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4.70 ppm since ODS-β-CD-3 was dissolved in the water. Resonance of the methylene

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protons on ODSA hydrophobic chain was close to 1.23 ppm, the positions of H-3 and

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H-5 protons on the glucose units in the β-CD cavity belong to 3.84 and 3.72 ppm.29

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Interestingly, we noted that the H-3 and H-5 in the β-CD cavity had a response peak

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in NOESY spectrum for the methylene H protons in the hydrophobic chain of ODSA.

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Therefore, we concluded that the methylene proton on the hydrophobic chain was

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contained in the β-CD cavity. In addition, since the bond length of the C-C bond was

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0.154 nm and the height of the β-CD cavity was 0.78 nm,30 we calculated that more

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than 5 C atoms in the hydrophobic chain were located in the β-CD cavity.

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On the other hand, the 1, 4, 6 hydrogen protons in the ODS chain had

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corresponding resonance with the hydrogen periphery (1', 4' and 6' on glucose unite)

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of the β-CD (corresponding to a, b, c, d and h in Figure 1 II), indicating that ODS

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hydrophobic chains at positions 1-6 were situated outside the β-CD cavity.

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Simultaneously, we have speculated that the hydrophobic chain passed through the

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β-CD cavity, instead of completely covered. It was due to the methyl of ODS with the

266

hydrogen protons (in the β-CD cavity) were no signal. Finally, the all spatial structure

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of ODS-β-CDs was deduced (Fig 1 IV).

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Crystalline structure and morphology by X-ray diffraction (XRD) and SEM.

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The arrangement of CD molecules within the crystal lattice could be one of two

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principal modes, either cage or channel structure (Fig 2).31 When CD molecules were

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packed in “herringbone” pattern with both ends of the cavity being blocked by

272

adjacent CDs, an isolated cage-type structure would be formed. In the channel pattern,

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two neighboring CDs formed “dimer units” through head-to-head or head-to-tail

274

orientation, thus stacking to form an endless column structure.31

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As shown in Fig 2, the crystalline structures of the β-CD and ODS-β-CDs were

276

investigated by XRD. For β-CD, there were three salient peaks associated with its

277

crystalline structure occurring at 2θ = 10.6°, 12.5°, and 19.5° (Fig 2a), which

278

concluded that the β-CD crystals was a cage structure.31,

279

esterification reaction, a new characteristic peak at 2θ = 6.7°, 7.3°, 10.0° and 20.0° 13


32

However, after


280

appeared in the XRD diagram of ODS-β-CDs, which indicated the ODS-β-CD

281

molecules hosting guest molecules would result in channel-type packing.33

282

34

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fewer reflections since the characteristic peaks at 2θ = 4°-30° showed a slightly

284

decrease.33 Correspondingly, the crystallinity of samples was reduced from 70.4 to

285

19.2% as well. Based on the above phenomenon, we proposed that it was hard for

286

channel-type crystals to establish orderly structure as a result of rapid formation, thus

287

possibly leading to the formation of amorphous regions.33 Meanwhile the disorder of

288

the polymeric chains inevitably happened, since the ODSA backbone was introduced

289

into the β-CD molecules.35

Furthermore, with the increasing of DS, the XRD peaks of ODS-β-CDs showed

290

The morphological structure of β-CDs and ODS-β-CDs was revealed by SEM (Fig

291

3). Normally, the original β-CDs in aqueous solution were the tetragonal packing

292

nanocrystals, and changed into the hexagonal packing nanocrystals as they formed the

293

inclusion complexes with fish oil.30 Most of the β-CDs particles exhibited rod-shape

294

morphology. After hydrophobic modification, noticeable changes in the morphology

295

of the synthesized ODS-β-CDs were observed. With the increasing of DS, the

296

resulting ODS-β-CDs showed less rod-like structure. Some rhombic particles with

297

lamellar structure appeared, which might be resulted from the changing of molecular

298

packing in ODS-β-CDs due to the existing of hydrophobic chains.

299

Particle size, surface charge density and contact angle. As shown in Table 2,

300

after the introduction of ODS group, particle size and zeta potential of β-CD had great

301

changes. It could be seen that electronegativity of ODS-β-CD was decreased from 14


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-22.1 mV to -62.8 mV with the increase of degree of substitution, which was resulted

303

from the ionization of carboxyl groups on the ODS group. However, the zeta potential

304

between ODS-β-CD-3 and ODS-β-CD-2 had not changed much. This is probably

305

because the original ions in the solution inhibited the ionization of the carboxyl

306

group.36

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As shown in Fig.4, there were three peaks in particle size distribution of β-CD. The

308

peak appeared below 1 nm indicated the existing of some free β-CD molecules. And

309

the large particles with size around several micrometers also existed. The large

310

variations in the particle size of β-CD molecules, especially the presence of these free

311

β-CD molecules, decreased the emulsifying ability of β-CD. The peak related to free

312

molecule was disappeared after esterification modification. The particle size

313

distributions of ODS-β-CDs were observed to range from 400 to 1200 nm. The

314

average particle size of ODS-β-CDs were in the order ODS-β-CD-1< ODS-β-CD-2

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Modulation of Cyclodextrin Particle Amphiphilic Properties to

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