Modulation of Cyclodextrin Particle Amphiphilic Properties to Stabilize

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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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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

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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

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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

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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

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

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

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

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

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However, after

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

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

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34

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

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

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

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

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

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

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into the β-CD molecules.35

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

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

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

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

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

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morphology. After hydrophobic modification, noticeable changes in the morphology

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

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

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

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

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

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

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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

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

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

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

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group.36

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

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

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

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

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

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molecule was disappeared after esterification modification. The particle size

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

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

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