Facile Fabrication of Cyclodextrin-Modified Magnetic Particles for

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Facile Fabrication of Cyclodextrin-Modified Magnetic Particles for Effective demulsification from Various Types of Emulsions Jianrui Zhang, Yiming Li, Mutai Bao, Xiaolong Yang, and Zhining Wang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b01941 • Publication Date (Web): 26 Jul 2016 Downloaded from http://pubs.acs.org on July 31, 2016

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Environmental Science & Technology

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Facile Fabrication of Cyclodextrin-Modified Magnetic Particles for

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Effective demulsification from Various Types of Emulsions

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Jianrui Zhang, Yiming Li,∗ Mutai Bao, Xiaolong Yang, and Zhining Wang

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Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education,

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Ocean University of China, 238 Songling Road, Qingdao, 266100, Shandong

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Province, China

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ABSTRACT

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Effective oil−water phase separation from various emulsions, especially those

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stabilized by surfactant, is of great importance. Although superhydrophobic and

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superoleophilic materials have attracted considerable attention in recent years, they

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are incapable of directly separating all types of oil−water mixtures. To separate

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various types of emulsions, one of the most important features of particles is that they

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can be dispersed in the continuous phase for delivery and target dispersed phases. In

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this study, cyclodextrin-modified magnetic composite particles (M-CDs) have been

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fabricated for this goal, based on their special interfacial activity and response to an

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external magnetic field. Though M-CDs are hydrophilic, the intelligent M-CDs can

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switch from hydrophilicity to hydrophobicity spontaneously, due to the formation of

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CD-oil inclusion complexes (ICs) at the oil−water interface. Physicochemical

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characterization reveals that M-CDs can adsorb at the oil−water interface and locate at

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the droplet surface as an effective Pickering emulsifier. By applying an external

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magnetic field, M-CDs are removed from the droplet surface and a rapid oil−water

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phase separation occurs. Our M-CDs can demulsify, for the first time, surfactant-free

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or surfactant-stabilized oil-in-water (O/W) and water-in-oil (W/O) emulsions directly, 1

ACS Paragon Plus Environment


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with high separation efficiency. Furthermore, the recycled MNPs still show high

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demulsification efficiency. In view of the sustainability of cyclodextrin and effective

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recycling ability of MNPs, M-CDs provides a new opportunity to develop an

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environmentally friendly interfacial material for practical applications in wastewater

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treatment.

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KEYWORDS: magnetic particles, cyclodextrin, Pickering emulsion, oil−water

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mixture, demulsification, inclusion complexes

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Table of Contents

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INTRODUCTION

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In recent years, the increased industrial oily wastewater and frequent oil spill

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accidents have promoted a research on the demulsification and separation of

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oil−water mixture, to meet the growing economic and environmental demands.1,2

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Facile and efficient process for oil−water separation is greatly desired, especially

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surfactant-stabilized oil−water mixtures. 3,4 In terms of the diameter of liquid droplets

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(d), surfactant-stabilized oil−water mixtures are classified as free oil and water if d >

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150 µm, a dispersion if 20 µm ≤ d ≤ 150 µm, or an emulsion if d < 20 µm.5 Formation

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of surfactant-stabilized oil−water mixtures is a common problem in the petroleum

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industry. The most difficult step in separating oil−water mixture is demulsification.

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Conventional techniques, such as gravity separators and skimming, are not able to

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separate oil−water mixture from dispersion or emulsion.5 Based on the wettability of

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various materials, membrane or mesh technologies have been exploited for

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demulsification.6−12 Although membrane techniques are effective, they are easily

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fouled by oil and unsuitable for the separation of oil−water mixtures where d is

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smaller (d < 150 µm).13 Until now, only a few studies have described the separation of

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oil−water mixture with d ≤ 150 µm.14 Another problem for membrane techniques is

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their incapacity of separating all types of oil–water mixtures. The membranes dealing

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with the successful separation of water-in-oil (W/O) emulsion are incapable of

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separating oil-in-water (O/W) emulsion. To separate O/W type of emulsion, other

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membranes or techniques are needed.15 Therefore, it is still a great challenge to design

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functional materials that are effective for demulsifing both W/O and O/W types of

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emulsions simultaneously. Only very recently, a novel membrane was fabricated,

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which can separate both O/W and W/O emulsions.16 To effectively separate all types 4


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of oil−water emulsion, these membranes are designed to be hydrophilic and

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oleophobic both in air and in water.

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In addition to membrane techniques, nanoparticles show considerable attractive

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features in oil−water demulsification and separation.17,18 Magnetic nanoparticles

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(MNPs) with desired surface properties are a kind of promising nanomaterial, because

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of their special magnetic response and easy separation. The application of MNPs in

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solving oil−water separation problems has been paid more and more attention in

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recent years. For example, magnetic superhydrophobic particles were prepared to

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achieve oil−water separation from water surface as oil-absorbent materials.19−22

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However, these superhydrophobic nanoparticles are incapable of separating oil−water

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mixture from emulsions because of their poor dispersion in water phase. Recently, a

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breakthrough on using superhydrophobic particles to achieve demulsification was

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reported by Duan and co-workers.13 The superhydrophobic iron particles they

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fabricated exhibit oil separation ability from surfactant-free O/W emulsion, but they

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cannot separate oil from W/O emulsion directly.

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In order to achieve the goal of demulsification for various types of emulsion, one of

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the most important features of particles is that they are dispersible in the continuous

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phase and able to target dispersed phase. Particles with this special wettability may

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transfer from continuous phase to the oil−water interface. Since the pioneering work

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of Pickering and Ramsden,23 Pickering emulsion has been shown great interests,

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wherein solid particles are adsorbed at the oil−water interface and provide emulsion

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stability. Based on the peculiar surface characteristics and magnetic responses of

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MNPs, controlled stabilization and breaking of Pickering emulsion may be realized.24

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Hence, MNPs may be a wonderful candidate for the demulsification of oil−water

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emulsion. However, the magnetic separation technique based on the principle of

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Pickering emulsion has seldom been reported.25

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As a Pickering emulsifier, it is important for these particles to have surface

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wettability for desired functions. The surface wettability can be modified by surface

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coating, adsorption, and self-assembly.26 Recently, poly(N-isopropylacrylamide)-

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grafted magnetic particles were fabricated and used as solid-stabilizers to harvest

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oil.27 They found that the harvested oil phase by magnetic particles consisted of

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different sizes of oil droplets and complete oil−water phase separation was achieved

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by controlling the temperature of Pickering emulsion. In a study reported by Peng et

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al., 28 bromoesterified ethyl cellulose was grafted to the magnetic nanoparticle surface.

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The resulting MNPs have been applied to demulsify W/O emulsion under an external

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magnetic field. Brugger et al. synthesized magnetic microgels as stimuli-responsive

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Pickering emulsifiers for the separation of oil droplets from O/W emulsions.29 Li et al.

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used polyether polyol-modified magnetic nanoparticles to separate the oil from O/W

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emulsions by introducing a magnetic field.30 In spite of these important studies, the

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materials they fabricated still cannot demulsify O/W and W/O emulsions

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simultaneously.

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In this work, we proposed a novel and facile preparation of composite MNPs that

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can attach to the dispersed liquid droplets in an emulsion, thereby demulsifying both

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surfactant-free and surfactant-stabilized O/W or W/O emulsions. To synthesize the

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MNPs with tunable wettability, desired chemicals have to be chosen to modify the

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surface of MNPs for effective tagging of the dispersed droplets. Biocompatible and

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nontoxic cyclodextrins (CDs) are attractive chemicals to modify the surface

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wettability of MNPs due to their ability to form host–guest inclusion complex (ICs).31,

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The unique cone structure of CDs makes them possess a hydrophilic exterior and 6


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hydrophobic interior. CDs are hydrophilic in water and cannot change the surface

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tension of water. However, the hydrophobic inner cavity allows CDs to incorporate

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oil molecules to form ICs through hydrophobic interactions. ICs are formed

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spontaneously at the oil−water interface by a self-assembly process.33 These ICs can

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further assemble into microcrystals and decrease the oil−water interfacial tension.34

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Microcrystals of ICs can attach on the emulsion droplet surface and form densely

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packed layers, which leads to an effective Pickering emulsification. With these special

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properties, CDs may be an ideal candidate for preparing magnetically responsive

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particles with interfacial activity. When CD-modified MNPs (M-CDs) are added into

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an oil−water mixture, they can attach to the droplet surface and switch from

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hydrophilicity to hydrophobicity through the formation of ICs (M-ICs), which act as a

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Pickering emulsifier. The concept of corresponding mechanism is illustrated in

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Scheme 1. By applying an external magnetic field, the attached M-ICs may be

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removed from Pickering emulsion droplets, leading to a rapid and obvious

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demulsification. Here, the desired M-CDs are prepared by using a facile opposite

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charges attraction between MNPs and CDs. The positively charged MNPs were

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obtained by preparing amine-functionalized silica-coated MNPs.

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The novel strategy of tuning surface characteristic of MNPs by addition of CDs

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provides a facile and effective oil−water demulsification process, as well as a shorter

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phase separation time. M-CDs can be directly used to demulsify not only O/W

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emulsions, but also W/O emulsions. It is also worthy to note that the prepared M-CDs

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can be applied in various types of oil phase. As far as we know, this is the first report

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on using a facile method to fabricate interfacially active MNPs for demulsifing

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various types of emulsions, even surfactant-stabilized emulsions. As a magnetic

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demulsifier, M-CDs can be recovered and reused. The facile fabrication procedure 7



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and effective performance of M-CDs endow these particles a greatly potential

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application in the demulsification of various oil−water emulsions.

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Scheme 1. Schematic illustration of the interaction between MNPs and β-CD, and the

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wettability modification of M-CD due to the formation of M-ICs.

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EXPERIMENTAL SECTION

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Materials. FeCl3·6H2O, trisodium citrate, sodium acetate (NaAc), methanol, ethanol,

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ethylene glycol, rhodamine B and β-cyclodextrin (β-CD) were obtained from

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Sinopharm Chemical Reagent Co., Ltd (China). Tetraethoxysilane (TEOS), solid

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paraffin, tetradecane and hexadecane were purchased from Aladdin (China). Soybean

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oil was purchased from Wal-Mart and purified by percolating them through a column

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packed with florisil (60-100 mesh) before use. Dimethyl silicone oil (viscosity~1000)

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was acquired from Beijing Solarbio Science & Technology Co., Ltd (China). 3-

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aminopropyltriethoxysilane (APTES) was purchased from Adamas-beta. Sodium

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dodecyl sulfonate (SDS) was obtained from Sigma-Aldrich. The water used was tri-

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distilled water. All reagents were used as received.

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Fabrication of CD-modified Magnetic Particles. The water dispersible Fe3O4

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particles were synthesized according to the method reported in a literature.24 Briefly,

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1.3 g of FeCl3·6H2O, 2.4 g of NaAc and 0.5 g of trisodium citrate were added in 40

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ml of ethylene glycol under magnetic stirring at room temperature. After stirring for 8


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30 min, the mixture was transferred into a Teflon-lined stainless steel autoclave,

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sealed and treated at 200 °C for 10 h. Finally, the autoclave was cooled to room

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temperature and Fe3O4 particles were obtained. After washed by distilled water, the

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prepared Fe3O4 particles were dried under dynamic vacuum at 50 °C.

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A modified Stöber method was used to prepare Fe3O4@SiO2 particles. 0.15 g of the

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Fe3O4 particles were dissolved in a solution, which contained 6 ml of distilled water, 2

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ml of NH3·H2O and 60 ml of ethanol. The above solution was sonicated for 4 h at

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room temperature. Then, 0.6 ml of TEOS was added in 5 ml of ethanol. Finally, 0.15

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ml of TEOS/ethanol mixture was slowly added into Fe3O4 suspension under

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ultrasonication. After 90 min, these particles were separated by using a magnet and

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washed thoroughly with ethanol. 100 mg Fe3O4@SiO2 particles were then allowed to

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react with 20 mL of 2 mmol L-1 APTES in methanol for 30 min. The prepared

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APTES-modified magnetic particles (Fe3O4@SiO2-NH2) were separated by a hand

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magnet, washed with methanol and dried at room temperature.

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CD-modified magnetic particles were prepared by combining a certain amount of

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Fe3O4@SiO2-NH2 particles and different concentrations of β-CD solution, and then

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the mixture was treated by ultrasonication for 10 min at room temperature. Based on

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the electrostatic attractions, a suspension of CD-modified magnetic particles was

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obtained finally.

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Preparation of Emulsions and Droplet Size Determination. To prepare surfactant-

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stabilized O/W and W/O emulsion, SDS aqueous solution was mixed with oil phase at

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an oil-to-water volume ratio of 1:4 for O/W emulsion and 4:1 for W/O emulsion.

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After mixing, they were homogenized for 30s by IKA T10 instrument. For surfactant-

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free emulsions, soybean oil or dimethyl silicone oil was directly mixed with distilled

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water and stirred vigorously until a uniform emulsion was obtained.15 The prepared 9



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emulsions were imaged by optical microscopy (Leica DM1000 LED, Leica,

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Germany). The droplets sizes were estimated by Nano Measurer software.

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Demulsification of Various Types of Emulsion. Surfactant-free or surfactant-

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stabilized O/W (1:4, v/v) and W/O (1:4, v/v) emulsions with a total volume of 15.0

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mL were prepared as mentioned above. A small amount of M-CDs suspension

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(0.3−0.5 mL) was then added to various types of as-formed emulsion slowly. The

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final concentrations of MNPs and β-CD in the aqueous phase were calculated. After

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addition of M-CDs suspension, the mixture was vortexed at 3000 rpm for 30 s, which

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results in the transfer of M-CDs from aqueous phase to the emulsion droplet surface.

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The mixture was then placed on a magnet. Oil−water phase separation was completed

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in 10 min, except highly viscous dimethyl silicone oil.

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Characterization. The interfacial tension between tetradecane and different aqueous

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solution was determined by the pendant drop method through droplet shape profile

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analysis (OCA instrument, Dataphysics ES, Germany). Firstly, tetradecane was

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poured into a cuvette and a volume of ca. 20 µL aqueous solution containing MNPs

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and a certain concentration of β-CD was injected into it by a syringe. Then, the

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droplet shape profile was analyzed to acquire the value of interfacial tension. At least

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three independent measurements were performed. To measure the three-phase contact

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angle of M-CDs at the oil−water interface, a silicon wafer was immersed into M-CDs

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suspension and left to equilibrate for 24 hours. After equilibration, the wafer was

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washed by distilled water to remove excess particles and dried prior to use. The wafer

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was then immersed in tetradecane bath and ca. 2 µL water droplet was placed gently

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on the wafer. The three-phase contact angle was recorded using the same OCA

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instrument. Contact angle was obtained by measuring three different spots on a wafer.

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The size and zeta potential of M-CDs with and without tetradecane were measured

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by using a Zeta Nanosizer instrument (Malvern Instruments, UK), at a fixed scattering

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angle of 90°. All measurements were repeated at least three times. Fourier transform

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infrared (FTIR) spectrometer (Tensor 27, Bruker, Germany) was used to characterize

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the particles synthesized in different stages. Thermogravimetric analysis (TGA)

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experiments were performed on a STA 449 F3 thermogravimetric analyzer

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(NETZSCH instruments). The analysis was carried out in N2 atmosphere at a heating

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rate of 10 °C min-1. The magnetic hysteresis loops were determined with a DTM-151

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Gaussmeter/Teslameter (Lake Shore, Germany). TEM (JEM-2100, JEOL, Japan) and

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SEM (S-4800, Hitachi, Japan) were used to visualize the morphology of bare Fe3O4

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and Fe3O4@SiO2 composite particles. The prepared fresh dispersion was applied to a

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carbon-coated copper grid, and then the grid was air dried prior to imaging.

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Fluorescent images were obtained by a confocal laser scanning microscope (CLSM,

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Fluo View FV1000, Olympus Corporation, Japan). Prior to visualization, Rhodamine

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B (< 1×10−6 mol L-1) was dissolved in the Fe3O4@SiO2-NH2 suspension to

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fluorescently label these MNPs. High performance liquid chromatography (Agilent

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1220 Infinity, USA) was used to quantitatively analyze the concentration of β-CD in

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aqueous solution. The amount of trace oil or oil-CD complex in water phase was

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measured by infrared oil content analyzer (Beijing Chinainvent instrument Co. Ltd.,

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China). The amount of trace water left in oil was measured by Karl Fischer method.

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Conventional SEM technique was used to visualize the adsorption of M-CDs on an

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emulsion droplet surface. Paraffin was used as the oil phase instead of tetradecane,

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due to its low freezing point. The mixture of M-CDs dispersion and paraffin was

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homogenized to prepare an O/W emulsion at higher temperature (ca. 80 °C). Then,

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the formed emulsion was cooled to room temperature for rapid solidification and air 11



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dried. The dried powders were sprayed with Aurum and observed by the S-4800 SEM

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instrument.

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RESULTS AND DISCUSSION

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Figure 1 indicates the SEM and TEM images of bare Fe3O4 and Fe3O4@SiO2 particles.

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After surface coating, Fe3O4@SiO2 particles are much smoother than Fe3O4. The

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TEM image of Fe3O4@SiO2 particles shows a clear silica interface, which confirms

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the formation of core−shell structured magnetic particles.

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Figure 1. SEM and TEM images of Fe3O4 (a, b) and Fe3O4@SiO2 (c, d).

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The zeta potentials of various magnetic particles are shown in Table 1. After silica

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coating, Fe3O4 particles become negatively charged, which shows the successful

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fabrication of Fe3O4@SiO2. After further modification with APTES, positively

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charged Fe3O4@SiO2-NH2 particles are obtained.

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Table 1. Zeta potentials for different particles. Particle type

Zeta potential (mV)

Bare Fe3O4

7.08 ± 1.8

Fe3O4@SiO2

-40.60 ± 1.2

Fe3O4@SiO2-NH2

37.00 ± 1.4

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The XRD patterns of particles match well the data of standard JCPDS file of Fe3O4,

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which shows that Fe3O4 particles are successfully fabricated and the crystallographic

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structure of Fe3O4 is well-retained after coating with a layer of silica (Figure 2a). To

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meet the condition of selective wettability by water and oil, the as-formed magnetic

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particles need to be further modified by β-CD molecules. Several strategies may be

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used to achieve this goal, such as chemical grafting and molecular adsorption.35,36

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Adsorption is a quite easy method. Based on the electrostatic interactions between β-

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CD and positively charged magnetic particles, CD-modified magnetic particles (M-

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CDs) were prepared. To validate the successful binding of β-CD on MNPs, the

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particles obtained at each synthetic stage were characterized using FTIR (Figure 2b).

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When Fe3O4 particles are coated by silica, a strong band at 3487 cm-1 can be observed,

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which originates from Si–OH vibrations. The adsorption peak appearing at 1562 cm-1

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(N−H bending vibration bands) for Fe3O4@SiO2-NH2 particles is an exact proof that

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ATPES has grown from the initiator particles, as observed in the magnification inset

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of Figure 2b. The disappearance of N−H bending vibration band in the spectrum of

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M-CDs is attributed to their interactions with the −OH groups from β-CD molecules.

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These spectrum features confirm the adsorption of β-CD on positively charged MNPs.

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The amount of β-CD adsorbed onto the surface of MNPs was investigated by TGA 13



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(Figure 2c). When M-CDs are heated, a significant mass loss is observed, which is

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caused by the decomposition of β-CD molecules. By measuring the TGA curve of β-

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CD alone, it is demonstrated that the decomposition is almost completed over the

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temperature range up to 800 °C. Therefore, the amount of β-CD adsorbed onto the

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MNPs is calculated to be around 100 mg g-1. In contrast, there is no obvious mass loss

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for bare Fe3O4, Fe3O4@SiO2, and Fe3O4@SiO2-NH2 particles. Compared with bare

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Fe3O4 particles, M-CDs show a decreased saturation magnetization (Figure 2d). Even

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so, the magnetic hysteresis loops of M-CDs show a ferromagnetic behavior, implying

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these particles can be readily manipulated by using an external magnetic field.

Intensity (a.u)

Bare Fe3O4

Fe3O4@SiO2

(b)

Bare Fe3O4 Fe3O4@SiO2 Fe3O4@SiO2-NH2 M-CDs β-CD

Si-OH Transmittance

(a)

Transmittance

286

20

30

40 50 2θ (degree)

295

60

70

Bare Fe3O4 Fe3O4@SiO2 Fe3O4@SiO2-NH2

90

M-CDs

80 80 β-CD

Bare Fe3O4

40

0 0

0

Fe3O4@SiO2 Fe3O4@SiO2-NH2 200

100

400

200

M-CDs

600

800

300

400

500

2500

1600

1550

2000

Si-O-Si

1500

1000

-1

Magnetization (eum/g)

Weight (%)

296

3000

60

(d)

100

60

3500

1650

Wavenumber(cm )

(c)

70

4000

N-H

C=C

1700

600

700

40 M-CDs

20 0 -10000

-5000

0

5000

10000

-20 -40 -60

800

Temperature (°C)

Bare Fe3O4

Magnetization Field Strength B (G)

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Figure 2. (a) XRD patterns. (b) FTIR spectra of bare Fe3O4, Fe3O4@SiO2,

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Fe3O4@SiO2-NH2, M-CDs and β-CD. (c) TGA curves for bare Fe3O4, Fe3O4@SiO2,

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and Fe3O4@SiO2-NH2 and M-CDs. The inset in (c) shows the TGA curve for β-CD

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alone. (d) Magnetization curves of Fe3O4 and M-CDs. The photographs show the

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magnetic response of M-CDs to a hand magnet.

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Based on the formation of ICs and magnetic response characteristic of MNPs, M-

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CDs were used to demulsify surfactant-stabilized O/W and W/O emulsions. Firstly,

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tetradecane-in-water (1:4, v/v) and water-in-tetradecane (1:4, v/v) emulsions were

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prepared with SDS used as the stabilizer. These emulsions were very stable and no

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obvious phase separation was observed in 3 months. After the emulsion was prepared

307

and left 30 min, a certain amount of M-CDs was added into the as-formed O/W or

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W/O emulsions. Then, the mixture was vortexed for 30s. As shown in Figure 3, a

309

rapid macro phase separation of emulsified water or oil droplets from emulsions are

310

observed, by applying an external magnetic field. Optical microscopy images of

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separated oil phase confirm the successful demulsification of O/W and W/O

312

emulsions. The attachment of M-CDs on the droplet surface as a magnetic response

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Pickering emulsifier allows for the control of the stability of O/W or W/O emulsions.

314

The introduction of a magnetic field results in removal of M-ICs from the droplet

315

surface and thus droplet coalescence is initiated without solid-stabilizer present. M-

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CDs can effectively demulsify both O/W and W/O emulsions. In contrast, there is no

317

obvious phase separation observed when CD-unmodified MNPs are used as

318

demulsifier. Without an external magnetic field, phase separation would not occur,

319

either. This demonstrates that the ability of M-CDs to form M-ICs with oil molecules

320

at the droplet surface, in combination with their magnetic response, play an important

321

role in the demulsification of emulsions. Usually, emulsions with droplet size ≤ 20

322

µm are difficult to demulsify. Through the enhancement of homogenization rate and

323

increase of SDS concentration, O/W and W/O emulsions with droplet size ≤ 20 µm

324

were prepared. M-CDs can demulsify these emulsions effectively (see Figure S1 in 15



325

the Supporting Information). However, the demulsification efficiency and time will be

326

influenced remarkably. M-CDs also can effectively demulsify SDS-stabilized

327

hexadecane-in-water and water-in-hexadecane emulsions (see Figure S2 in the

328

Supporting Information). The oil−water separation process is similar to that of

329

tetradecane.

330 331

Figure 3. Photographs showing the demulsification of SDS-stabilized emulsions

332

under an external magnetic field. (a) Tetradecane-in-water emulsion (O/W). (b)

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Water-in-tetradecane emulsion (W/O). Here MNPs means Fe3O4@SiO2-NH2 particles.

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The volume ratio of oil to water is 1:4 in O/W emulsion and 4:1 in W/O emulsion,

335

respectively. M-CDs (0.4 wt% MNPs and 5 mmol L-1 β-CD) are used as demulsifer. 4

336

mmol L-1 SDS is used to form SDS-stabilized emulsions.

337

It is known that surfactant-free emulsions are common in many fields. Therefore, it

338

is important to demonstrate the application of M-CDs in demulsifying surfactant-free

339

emulsions. Here, soybean oil and dimethyl silicon oil are introduced to prepare

340

surfactant-free O/W (1:4, v/v) and W/O (1:4, v/v) emulsions as reported.15 M-CDs are

341

applicable for the demulsification of surfactant-free O/W and W/O emulsions (see 16


Page 16 of 29

Page 17 of 29


342

Figure S3 in the Supporting Information). By adding M-CDs, effective oil−water

343

phase separation is rapidly completed for soybean oil. Because of the high viscosity of

344

dimethyl silicon oil, it takes longer time (24 h) to completely demulsify water-in-

345

dimethyl silicon oil emulsion.

346

After demulsification, we quantitatively analyzed the amount of trace oil or oil-CD

347

complex in water phase by infrared oil content analyzer. The amount of trace water

348

left in oil phase was measured by Karl Fischer method. Four different oil phases were

349

investigated, respectively. The small amount of trace oil left in water and trace water

350

left in oil confirm the effective demulsification of M-CDs (see Figure S4 in the

351

Supporting Information). Additionally, only < 1 wt% of β-CD disassemble from

352

MNPs surface after demulsification, though the electrostatic attraction between β-CD

353

and MNPs is weak (see Figure S5 in the Supporting Information).

354

The effective oil−water demulsification process by M-CDs may be explained in

355

terms of the formation of M-ICs. Several studies have reported the formation of CD-

356

oil ICs at the oil−water interface, the assembly of ICs into microcrystals, and

357

Pickering stabilization by microcrystals.31,34,33,37 However, this is the first report on

358

using CD-modified MNPs to demulsify various types of emulsion via complexation

359

between β-CD and oil molecules. Due to the formation of M-ICs between M-CDs and

360

oil, magnetic M-CDs become interfacially active and are able to attach to the liquid

361

droplets, as observed via the fluorescence images shown in Figure 4a and 4b. The

362

visible clear bright red rings around the liquid droplets and the dark continuous phase

363

region show the presence of fluorescently labeled magnetic particles around the

364

droplets as an effective Pickering emulsifier. Since the as-prepared emulsion is

365

stabilized by SDS molecules, it is speculated that the SDS molecules should be

366

displaced from the interface due to the adsorption of M-CDs. SEM image of the 17



367

paraffin-in-water emulsion shows the shell morphology of the droplet surface (Figure

368

4c). Here solid paraffin is used as oil phase instead of tetradecane, because of its low

369

freezing point. It is a convenient method to observe the surface morphology of

370

emulsion droplets by conventional SEM technique. Figure 4c illustrates that the

371

droplets are covered with M-ICs. These magnetically responsive particles may be

372

easily manipulated by external magnetic field.

373 374

Figure 4. (a, b) Fluorescence images of O/W and W/O Pickering emulsion stabilized

375

by M-CDs, confirming the attachment of M-CDs at the droplet surface. (c) SEM

376

image of paraffin-in-water emulsion stabilized by M-CDs.

377

To further verify the formation of M-ICs, 10 µL of tetradecane was added into 10

378

mL of M-CDs solution. Mathapa et al. reported that ICs could be synthesized upon

379

dispersing tetradecane into CD aqueous solution.34 The average size and zeta potential

380

of M-CDs in the absence and presence of tetradecane are compared, as shown in

381

Figure 5. With β-CD concentration increase, the enhanced average size of M-CDs and

382

corresponding more negative zeta potential are observed. This implies more and more

383

β-CD molecules are adsorbing onto the MNPs via electrostatic attractions. In addition

384

to this, there is a remarkable difference in the average size and zeta potential between

385

the two curves without and with tetradecane present. The average size of M-CDs with

386

tetradecane present is much higher than that without tetradecane. This is attributed to

387

the formation of ICs between CD and tetradecane on the MNPs surface. The 18


Page 18 of 29

Page 19 of 29


formation of ICs is also validated by the slightly reduced negative charge of M-CDs.

389

The variance of surface charge is caused by the changed hydrolysis degree of β-CD in

390

water, as a result of formation of ICs. From the SEM images shown in Figure 6, it

391

clearly indicates that through the complexation of β-CD with oil molecules, MNPs are

392

surrounded by a dense layer of interfacially active ICs. (a) Z-Average (r.nm)

2000 1500 1000 500 0

393

without n-tetradecane with n-tetradecane

(b)

-5

Zeta potential (mV)

388

-10

without n-tetradecane with n-tetradecane

-15 -20 -25 -30 0

2 4 6 8 10 Concentration of CD (mM)

2 4 6 8 10 Concentration of CD (mM)

394

Figure 5. Influence of β-CD concentration on the average size (a) and zeta potential

395

(b) of MNPs without and with tetradecane present.

396 397

Figure 6. SEM images of M-CDs (a) and M-ICs (b).

398

Although β-CDs are not surface active at the air−water interface, they can form

399

pseudo-surfactants with oil molecules as ICs at the oil−water interface and act as a

400

Pickering emulsifier.31 Therefore, the hydrophilic M-CDs are also expected to

401

assemble into interfacially active M-ICs on the emulsion droplet surface, leading to an 19



402

effective Pickering stabilization. The decay of interfacial tension measured by a

403

pendent droplet method clearly indicates that interfacially active M-ICs are indeed

404

formed at the tetradecane−water interface (Figure 7). These M-ICs are formed

405

spontaneously via a self-assembly process at the tetradecane−water interface. With β-

406

CD acting as hydrophilic head groups and the non-included part of tetradecane chain

407

as the hydrophobic tail, a pseudo-surfactant structure on the surface of MNPs is

408

created. Then, M-ICs act as an effective Pickering emulsifier. The wettability of M-

409

ICs situated at the oil−water interface also plays an important role in Pickering

410

emulsification. The wettability of M-ICs is evaluated by measuring their three-phase

411

contact angle, as shown in Figure 7. Without β-CD, the contact angle of MNPs is

412

around 60 °. However, an enhanced contact angle is observed when MNPs are

413

modified with β-CD. This is caused by the formation of ICs. The three-phase contact

414

angle of M-ICs increases from 65 ° at 1 mmol L-1 β-CD to 89 ° at 10 mmol L-1 β-CD,

415

which means more ICs are formed on the surface of MNPs and these ICs change the

416

wettability of magnetic particles. Studies reported by Kaptay et al. showed that the

417

optimum stability for a Pickering emulsion is when the three-phase contact angle of

418

particles approaches 90 °.38 Therefore, a favorable Pickering stabilization is expected

419

at higher β-CD concentration. The adsorption of solid particles at the oil−water

420

interface is driven by the free energy of adsorption, which is related to the size of

421

particle, interfacial tension and contact angle, as described in Eq. (1).39 Considering

422

the desirable size, interfacial tension and contact angle of M-ICs, we may expect that

423

M-CDs are able to rapidly adsorb at the emulsion droplet surface and strongly hold at

424

the oil−water interface as a result of formation of M-ICs.

425

∆ ads F = −π R 2γ o − w (1 − cos θ ) 2

(1) 20


Page 20 of 29


100

50 -1

Interfacial tension (mN m )

Page 21 of 29

45

o

Contact angle( )

80 60 40 40 20 0

35

Contact angle Interfacial tension

0

2

4

6

8

10

30

Concentration of β-CD (mM)

426 427

Figure 7. Three-phase contact angle of M-ICs and interfacial tension between

428

tetradecane and M-CDs aqueous solution. The concentration of MNPs used here is 0.4

429

wt%.

430

The ratio of β-CD to MNPs greatly affects the demulsification ability of M-CDs.

431

Here, the ability of M-CDs to demulsify various types of emulsions is investigated as

432

a function of β-CD and MNPs concentration, respectively (see Figure S6 in the

433

Supporting Information). When the amount of MNPs is fixed, effective oil−water

434

phase separation only occurs under an appropriate β-CD concentration range. Under

435

lower concentrations of β-CD, ineffective phase separation suggests that the amount

436

of β-CD is not enough to modify the surface characteristics of MNPs. At higher β-CD

437

concentration, the demulsifiction ability of M-CDs is not shown, either. We propose

438

that a large amount of β-CD leads to the formation of ICs between free β-CD and

439

oil.28 Though the M-ICs have been removed from droplet surface by an external

440

magnetic field, the ICs formed from free β-CD and oil molecules still act as effective

441

Pickering emulsifiers. In contrast, at a fixed β-CD concentration, the higher

442

concentration of MNPs promotes a more effective demulsification for O/W and W/O

443

emulsions, which is attributed to the increased amount of M-CDs in the system. 21



444

On the basis of these results, the following mechanism is proposed to demonstrate

445

the phase separation process of oil−water emulsion: (1) when composite particles of

446

M-CDs are exposed to the liquid droplets, they spontaneously adsorb at the oil−water

447

interface and are strongly held at the droplet surface as a Pickering emulsifier, due to

448

the formation of M-ICs, (2) the reduced oil−water interfacial tension by M-ICs

449

facilitates the formation of stable Pickering emulsion, (3) by applying an external

450

magnetic field, the attached magnetically responsive M-ICs are removed from the

451

surface of emulsified droplets, leading to a rapid oil−water phase separation.

452

Additionally, MNPs can be recycled due to its magnetic response. By washing with

453

organic solvent, the used MNPs can be easily regenerated. Then, these recycled

454

MNPs were dispersed in new β-CD solution and the mixture was used for the next

455

cycle of demulsification experiment. As shown in Figure S7a and S7b in the

456

Supporting Information, the recycled MNPs still show high demulsification efficiency

457

even after 8 cycles for both O/W and W/O emulsions, which means the satisfied

458

reusability of MNPs. The lower separation efficiency after 8 cycles is mainly caused

459

by the decreased amount of recycled MNPs (Figure S7c). These results show that the

460

strategy of in situ tuning of surface characteristics of magnetic particles using β-CD,

461

together with their response to external magnetic field, is a facile and powerful

462

method to make advanced materials that have potential use in demulsifing various

463

types of emulsions. The well designed M-CDs do well in aspects of convenient

464

preparation, efficient demulsification, and excellent recycling, which will produce

465

great implications for the development of particle-based demulsifier.

466

Compared with the various magnetic particles applied in oil−water demulsification

467

reported previously,13,27,28,36,40−43 for the first time, our CD-modified magnetic

468

particles have exhibited excellent ability to directly demulsify all types of oil−water 22


Page 22 of 29

Page 23 of 29


469

mixtures in the same separation manner, including surfactant-free and surfactant-

470

stabilized O/W and W/O emulsions. In previously reported studies, the magnetic

471

nanomaterials cannot separate O/W and W/O emulsions in the same manner. For

472

example, Duan et al reported that the superhydrophobic/superoleophilic iron particles

473

(SHIPs) they fabricated can separate O/W emulsions.13 However, for separating W/O

474

emulsion, they have to develop a device where a SHIPs-based composite membrane is

475

fixed. Additionally, a recent study by Kumar et al reported the preparation of iron-

476

oxide/β-CD nanocomposite through precipitation method, but their magnetic

477

nanocomposite was only capable of adsorbing oils from surfactant-free water surface

478

as oil-adsorbent materials.36 Based on the electrostatic attractions between β-CD and

479

MNPs, our studies have demonstrated the feasibility of employing a much simpler

480

modification method to obtain an effective demulsifier for various types of oil−water

481

mixtures. This offers a considerable advantage over other Fe3O4-based demulsifier as

482

well. This environmentally friendly and ecologically acceptable MNPs material will

483

have wide applications in the separation of commercially relevant emulsions,

484

industrial wastewater treatment, and cleanup of oil spills.

485

ASSOCIATED CONTENT

486

Supporting Information

487

Photographs showing the demulsification of SDS-stabilized O/W and W/O emulsions

488

with different droplet sizes; photographs of vessels showing the demulsification of

489

SDS-stabilized hexadecane−water mixture after 10 min separation time; photographs

490

of vessels showing the demulsification of surfactant-free emulsions prepared from

491

soybean oil and dimethyl silicon oil; trace oil left in water phase and trace water left

492

in oil phase after demulsification; the mass ratio of disassembled β-CD from MNPs 23



Page 24 of 29

493

after demulsification; influence of β-CD and MNPs concentration on the

494

demulsification

495

photographs showing the demulsification after each recycle and the recovery rate of

496

MNPs. This material is available free of charge via the Internet at http://pubs.acs.org.

497

AUTHOR INFORMATION

498

Corresponding Author

499

∗Y. M. Li. Phone: 86-532-66782509. E-mail: [email protected].

500

Notes

501

The authors declare no competing financial interest.

502

ACKNOWLEDGEMENTS

503

This research is supported by the Natural Science Foundation of Shandong Province

504

(ZR2014DQ026), the Applied Basic Research Programs of Qingdao in China (14-2-4-

505

119-jch), and National Natural Science Foundation of China (41376084). This is

506

MCTL contribution No.120.

507

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