Dynamic Covalent Silica Nanoparticles for pH Switchable Pickering

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Dynamic Covalent Silica Nanoparticles for pH Switchable Pickering Emulsions Gaihuan Ren, Maoxin Wang, Lei Wang, Zengzi Wang, Qianqian Chen, Zhenghe Xu, and Dejun Sun Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00757 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on May 3, 2018

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Dynamic Covalent Silica Nanoparticles for pH Switchable Pickering Emulsions Gaihuan Ren,a MaoXin Wang,a Lei Wang,b Zengzi Wang,a Qianqian Chen,a Zhenghe Xu,cd and Dejun Sun*a a

Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, Shandong University, Jinan, Shandong, 250100, P. R. China

b

College of Chemistry and Molecular Engineering, Qingdao University of Science & Technology, Qingdao 266042, China

c

Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta Canada T6G 2V4 d

Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing, P. R. China 1000084

*Corresponding author. Tel. +86-531-88364749, Fax. +86-531-88364750, E-mail: [email protected].

ACS Paragon Plus Environment

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Abstract: Dynamic covalent surfactants have been recently reported for preparation of pH switchable emulsions [Sun, D. Langmuir, 2017, 33, 3040]. In this study, dynamic covalent silica (SiO2-B) nanoparticles of switchable wettability were fabricated by a pH-responsive dynamic (covalent) imine bond between hydrophilic amino silica (SiO2-NH2) nanoparticles and hydrophobic benzaldehyde molecules. The properties of SiO2-B were characterized by Fourier Transform Infrared (FTIR) Spectroscopy, Elemental Analysis, contact angle measurement and zeta potential measurement. The hydrophilicity and hydrophobicity of SiO2-B were shown to be readily switchable by adjusting pH between 7.8 and 3.5. At pH 7.8, SiO2-B was partially hydrophobic and adsorbed at oil-water interface to stabilize O/W Pickering emulsions, which were characterized by electrical conductivity meter, optical microscopy and confocal laser scanning microscopy. Upon lowering the pH to 3.5, the dynamic covalent bond is dissociated to convert partially hydrophobic SiO2-B into highly hydrophilic SiO2-NH2 and surface inactive benzaldehyde. Both of them desorb from oil-water interface, resulting in a rapid oil-water separation of the Pickering emulsions. Alternating stabilization and phase separation of the Pickering emulsions over three cycles were demonstrated by adjusting the pH. The pH switchable Pickering emulsions show great potential in application to effective oil-water separation of emulsions.

KEY WORDS: dynamic imine bond, switchable wettability, silica nanoparticles, Pickering emulsions


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INTRODUCTION Emulsions are widely used in numerous industrial applications, including emulsion polymerization,1 biphasic reactions,2-5 oil recovery,6 heavy oil transportation,1 and removal of oily makeup cosmetics.7 In these applications, additional physical or chemical disruption methods are needed to destabilize the emulsions after their service. The current methods in general lead to large energy consumption and/or secondary waste production.8 To resolve this issue, many studies have been devoted to the development of switchable emulsions with on/off properties in response to external stimuli, such as temperature,9 light irradiation,10 CO2,1,11,12 and pH.13-21 Switchable emulsions are usually prepared using switchable surfactants or particles. Emulsions stabilized by particles are known as “Pickering emulsions”.22-26 Compared with conventional surfactants, Pickering particles used in Pickering emulsions are of low cost and low toxicity.8,20,27,28 In the view of these significant advantages, there is a growing interest in developing switchable Pickering particles. A distinct advantage of switchable Pickering emulsions is the reuse of Pickering particles, which helps to achieve a sustainable operation. For Pickering particles, surface properties of the particles play important roles.25,29 The surface properties of Pickering particles can be tuned by introducing functional moieties that possess responsive properties.16 With external triggers, functional moieties of the Pickering particles undergo a desired physical or chemical transformation, altering the hydrophilicity and hydrophobicity of the Pickering particles.


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At present, two strategies are usually employed to formulate switchable Pickering particles. One route is to covalently synthesize switchable polymeric particles.12,30 Such approach requires complex and time-consuming synthesis procedures. Another route is to introduce functionalized groups onto the particles through noncovalent adsorption of switchable molecules.31 The noncovalent interactions however are usually weak. Surprisingly, the dynamic covalent bonds have been shown to offer both the strength of covalent bonds and the reversibility of noncovalent interactions. Therefore, introducing dynamic covalent bonds into fabrication of nanoparticles would be a promising candidate to obtain switchable Pickering particles. Dynamic covalent bonds can form and break easily by simply controlling the bonding environment.32 Among the dynamic covalent bonds, dynamic imine bond is widely used because of its pH sensitive Schiff base moieties,33 allowing formation and decomposition of dynamic imine bond by simply changing solution pH. With reversible switching on/off dynamic imine bond, dynamic imine chemistry is increasingly becoming a powerful strategy to prepare pH switchable surfactants. These switchable surfactants are widely used in controlling the assembly and disassembly of micelles,34-36 vesicles,34,37 and microcapsules38 or in controlling the stabilization and destabilization of emulsions.39 However, as far as we know, there are few reports on pH switchable inorganic Pickering particles that utilize dynamic imine bond. To address the complex synthesis of covalent bond chemistry and overcome the weakness of noncovalent interactions, we developed a simple protocol to prepare


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dynamic imine bond-based switchable Pickering nanoparticles, which led to pH switchable Pickering emulsions. Dynamic imine bond was introduced between amino silica (SiO2-NH2) and benzaldehyde to prepare dynamic covalent silica (SiO2-B) with pH switchable wettability. The hydrophilicity and hydrophobicity of SiO2-B can be reversible controlled by the formation/decomposition of dynamic imine bond. At pH 7.8, SiO2-B was partially hydrophobic, and was able to produce stable O/W Pickering emulsions. Upon lowering the pH to 3.5, the partially hydrophobic SiO2-B was hydrolyzed into highly hydrophilic SiO2-NH2 and surface inactive benzaldehyde, leading to a rapid phase separation of Pickering emulsions. By tuning the hydrophilicity and hydrophobicity of SiO2-B, stabilization and destabilization of Pickering emulsions can be achieved as desired.

EXPERIMENTAL SECTION Materials. Fumed silica (200–300 nm) was obtained from Sigma-Aldrich. Benzaldehyde (AR), trifluoroacetic acid (TFA, CR), sodium hydroxide (AR), and hydrogen chloride (36.5 wt%) were purchased from Sinopharm Chemical Reagent Co. Ltd., China. (γ-aminopropyl) triethoxysilane (APTES, 98 %) was supplied by Aladdin Reagents of China. Rhodamine B. was purchased from Shanghai chemical technology co., LTD. Paraffin oil was received from Exxon Mobil. All the chemicals were used as received without further purification. The p-decanoxybenzaldehyde was synthesized following the reported procedure (see Supporting Information).40 Deionized water was used in all the experiments. Synthesis of SiO2-B Pickering Nanoparticles. Fumed silica (1.0 g) and APTES


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(1.0 g) were dispersed in toluene (30 mL) to prepare amino silica (SiO2-NH2). The mixture was refluxed at 110 °C with continuous stirring for 12 hours. The dispersed particles were collected via centrifugation and then rinsed with ethanol for four times. The SiO2-NH2 was obtained after drying these particles in a vacuum oven at 40 °C for 24 hours. In order to prepare switchable dynamic covalent silica (SiO2-B), a mixture of the SiO2-NH2 (0.50 g) and benzaldehyde (0.055 g, at a NH2/CHO molar ratio of 1:1) was made in methanol (10 mL). The mixture was kept at 25 °C with a magnetic stirrer for about 1 hour. The product was collected via centrifugation and washed with ethanol for four times. The product was then dried in a vacuum oven at 40 °C for 24 hours to obtain SiO2-B Pickering nanoparticles. Unless specifically stated, the mentioned SiO2-B Pickering nanoparticles were prepared at a NH2/CHO molar ratio of 1:1. FTIR Characterization. The structure of pristine SiO2, SiO2-NH2, and SiO2-B was investigated using a Fourier transform infrared (FTIR) spectrometer (NICOLET5700, USA) with 32 scans at a resolution of 4 cm-1. FTIR spectra with baseline correction were acquired. Elemental Analysis. A Vario EL CUBE-elemental analyzer (Elementar, Germany) was used to perform elemental analysis for pristine SiO2, SiO2-NH2, and SiO2-B. Contact Angle. The powders of pristine SiO2, SiO2-NH2, and SiO2-B were compressed into circular film using a table press (Shimaduz Press) for contact angle measurement. All the samples were compressed at the pressure of 300 kgf/cm2 and the thickness of the films was maintained at about 2 mm. Contact angle of deionized


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water or acidic water (pH 3.5) on circular film was measured with sessile drop method by a commercial contact angle analyzer (Tracker, France). A droplet (2 µL) of deionized water or acidic water (pH 3.5) was deposited on the film in air using a precision syringe. The image was captured immediately after droplet deposited on the sample. Unless specifically stated, the image at 0 s was captured and used in our experiments. Profile of the droplet was fitted to the Laplace-Young equation. 1

H NMR Characterization. In this experiment, APTES was used to react with

benzaldehyde to form dynamic imine APTES-B. Details about the synthesis of APTES-B are provided in Supporting Information. The pH-responsive behavior of dynamic imine bond in APTES-B was characterized by 1H NMR. This experiment was conducted to illustrate the pH-responsive behavior of dynamic imine bond presented in the SiO2-B. For 1H NMR characterization, a BRUKER AVANCE 400 spectrometer was used. The APTES-B was dissolved in CD3OD, and the acid environment was provided by TFA addition. Zeta Potential. The zeta potential of nanoparticles was measured using a ZetaPals zeta potential analyzer (Brookhaven Instrument, USA). Particle samples (5 mg) were well-dispersed into 10 mL of deionized water under ultrasonication (200 w, 5 s on followed by 5 s off, 40 times) using a SCIENTZ JTY92-IIN sonicator (the probe size d = 6 mm). Particle dispersions were treated with either 1 M NaOH or 1 M HCl to obtain desired pH range (1.5–8.5). The zeta potentials were determined by measuring the velocity and direction of the moving particles. The values of the ζ-potential (zeta potential) were calculated using the Smoluchowski equation:


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ζ =

4πη

ν

ε U/L

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(1)

where η is the viscosity of water, ε is the dielectric constant of water, ν is the mobile velocity of particles in the electric field, U is the voltage, and L is the distance between the electrodes. Preparation of Pickering Emulsions. To prepare Pickering emulsions, paraffin oil and SiO2-B aqueous dispersions (10 mL, pH 7.8) were mixed at a given volume ratio. The concentration of SiO2-B in water was varied from 0.05 to 0.75 wt%. The mixture was homogenized by ultrasonication using a SCIENTZ JTY92-IIN sonicator with 6 mm probe at 400 W for 40 times (5 s on followed by 5 s off). The emulsion type was determined by conductivity measurement. Optical Microscopy. Pickering emulsion drop size and morphology were investigated with an optical microscope. The stability of Pickering emulsions was assessed by monitoring the change in droplet size with time by the optical microscope. A drop of emulsion sample was placed on a microscope slide and observed on an A1Pol optical microscope (ZEISS, Germany). The mean droplet diameter was estimated using Nano Measurer software. Confocal Laser Scanning Microscopy. Rhodamine B-labeled SiO2-B aqueous dispersion and paraffin oil were sonicated to obtain the Pickering emulsion. Pickering emulsion was observed using a confocal fluorescence microscope (Olympus Fluoview 500, Japan). Emulsification and Demulsification. All the Pickering emulsions were formed at pH 7.8. To investigate the emulsification and demulsification behavior of Pickering


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emulsions, HCl (1 M) or NaOH (1 M) aqueous solutions were used to adjust the pH of the continuous aqueous phase. After complete phase separation of the Pickering emulsion, the content of benzaldehyde that transferred into the upper oil phase was determined by measuring the absorbance at 247 nm using a TU-1810 type UV-vis spectrophotometer (Beijing Purkinje General Instrument Co., Ltd.). The mean droplet size of the re-formed stable Pickering emulsion was estimated from the optical micrographs after each demulsification/re-emulsification cycle.

RESULTS AND DISSCUSSION Preparation and Characterization of SiO2-B Pickering Nanoparticles. It is well established that the emulsification ability of Pickering particles depends on the hydrophilic and hydrophobic properties (wettability) of the particles.41,42 Accordingly, we chose the pH-sensitive dynamic imine bond to reversibly tune the hydrophilicity and hydrophobicity of the dynamic covalent silica (SiO2-B). The fabrication process of pH switchable SiO2-B consists of two steps (Scheme 1). First, APTES was used to modify the pristine SiO2 to obtain amino silica (SiO2-NH2) (Scheme 1a). Second, hydrophobic modifier benzaldehyde was linked with hydrophilic SiO2-NH2 through dynamic imine bond to form partially hydrophobic SiO2-B (Scheme 1b). The obtained SiO2-B was characterized by FTIR spectroscopy and elemental analysis. Scheme 1. Principle of Fabricating (a) Amino Silica (SiO2-NH2) and (b) Switchable Dynamic Covalent Silica (SiO2-B)

FTIR spectra of pristine SiO2, SiO2-NH2, and SiO2-B are shown in Figure 1. The


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FTIR spectrum of pristine SiO2 is used as a reference. Just like the peak presented in the pristine SiO2, the strong and wide Si-O-Si absorption peak (over 1000-1250 cm-1)43 also appears in the spectra of SiO2-NH2 and SiO2-B, indicating the presence of SiO2 in all these particles. The FTIR spectrum of SiO2-NH2 shows sp3 C-H asymmetric and symmetric stretching vibration at 2922 and 2847 cm-1, indicating the presence of aminopropyl groups in the SiO2-NH2. For the spectrum of SiO2-B, the presence of aromatic stretching modes at 3063 cm-1, 3036 cm-1, 1442 cm-1 and 693 cm-1 as well as the imine bond (C=N) stretching mode at 1650 cm-1 confirm incorporation of aromatic rings into SiO2-NH2 via imine bond formation in the SiO2-B.44,45

Figure 1. FTIR spectra of pristine SiO2 (blue), SiO2-NH2 (black), and SiO2-B (red). To further confirm the functionalization of SiO2-NH2 with benzaldehyde, elemental analysis was performed. The elemental composition of pristine SiO2, SiO2-NH2 and SiO2-B are given in Table 1. For the pristine SiO2, no N was observed as expected. In the case of SiO2-NH2, 1.46 wt% N was detected. The results indicate incorporation of amino in the SiO2-NH2. It is interesting to note a much higher C/N mass ratio in the


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SiO2-B than in the SiO2-NH2 as anticipated due to binding of benzaldehyde with SiO2-NH2 by dynamic covalent bond in the SiO2-B. Table 1. Elemental Analysis Data for Pristine SiO2, SiO2-NH2 and SiO2-B N (%)

C (%)

H (%)

C/N (calcd., %/%)

SiO2

0

0

0.60

SiO2-NH2

1.46

4.40

1.03

3.01

SiO2-B

0.89

7.52

0.72

8.45

To confirm that SiO2-B was partially hydrophobic (amphiphilic), and suitable for preparing O/W Pickering emulsion, contact angle of deionized water drop on the surface of particle film was measured. Pristine SiO2 is highly hydrophilic and easily wetted by water to show a contact angle of 10 ± 3° (Figure 2a), similar results were also reported by other researchers.31 The SiO2-NH2 also exhibits a highly hydrophilic character with a contact angle of 20 ± 3° (Figure 2b). In contrast, SiO2-B nanoparticles are partially hydrophobic with a contact angle of 75 ± 3° (Figure 2c). In fact, for the benzaldehyde functionalized dynamic covalent silica, contact angles can increase from 35 ± 3° to 75 ± 3° with increasing benzaldehyde contents (not shown). Moreover,

by

replacing

benzaldehyde

with

more

hydrophobic

p-decanoxybenzaldehyde (pDB), the synthesized dynamic covalent silica (SiO2-pDB, NH2/CHO molar ratio = 1:1) has a contact angle as high as 130 ± 3° (Figure 2d). These results suggest that changing the content of benzaldehyde or the type of aldehyde can effectively tailor the wettability of the synthesized dynamic covalent silica over a wide range for producing desired Pickering emulsions. Particles with contact angle less than 90° are desirable to prepare O/W Pickering emulsions, while


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larger than 90° are desirable to prepare W/O Pickering emulsions.46,47 Hence, SiO2-B (NH2/CHO molar ratio = 1:1) particles with contact angle of 75 ± 3° was selected to prepare pH switchable O/W Pickering emulsions.

Figure 2. Contact angle of SiO2 (a), SiO2-NH2 (b), SiO2-B (c) and SiO2-pDB (d). The SiO2-B in (c) and the SiO2-pDB in (d) were prepared at a NH2/CHO molar ratio of 1:1. Switchability

of

SiO2-B

Pickering

Nanoparticles.

To

prove

the

pH

responsiveness of dynamic imine bond in the SiO2-B, 1H NMR measurement was performed using APTES-B to represent SiO2-B. An obvious 1H signal at 10.06 ppm is assigned to aldehyde of the benzaldehyde (Figure S1a). For dynamic covalent APTES-B, the aldehyde signal at 10.06 ppm disappeared while a characteristic imine signal appeared at 8.32 ppm (Figure S1b), indicating the formation of dynamic imine bond between APTES and benzaldehyde. After adjusting the above APTES-B CD3OD solution to acidic environment by adding trifluoroacetic acid (TFA), the imine signal


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at 8.32 ppm disappeared, accompanied by the reappearance of the aldehyde signal at 10.02 ppm (Figure S1c). The results of

1

H NMR measurement proved the

decomposition of dynamic imine bond in acidic environment, decomposing APTES-B into their two building blocks of APTES and benzaldehyde. Similarly, dynamic imine bond in SiO2-B can break up in acidic environment into their building blocks of SiO2-NH2 and benzaldehyde, and hence effectively tune the formation/decomposition of SiO2-B Pickering nanoparticles. In order to determine at which pH dynamic imine bond decomposes, zeta potential of synthesized particle suspensions was measured at different pH. The isoelectric point of SiO2 is determined to be at pH about 2.5 (Figure S2), which is similar to the typical isoelectric point of fused silica reported.48 It is evident that SiO2 nanoparticles are negatively charged over the pH range from 2.5 to 8.5. After successful amination, the SiO2-NH2 becomes positively charged in the pH range of 2.5 to 8.5 due to the attached -NH2 groups (Figure 3). For SiO2-B, in the pH range of 4.0 to 8.5, the zeta potential values are lower than those of SiO2-NH2 (Figure 3). This is attributed to the fewer amount of amine groups on SiO2-B surfaces, as most amine groups have reacted with the aldehyde groups of benzaldehyde. In contrast, at pH 3.5 and below, the zeta potential values of SiO2-B and SiO2-NH2 are almost identical (Figure 3). This result indicates that at pH 3.5 and below the SiO2-B can be considered to be completely decomposed into SiO2-NH2 and benzaldehyde, which resulted in the identical zeta potential values of SiO2-B and SiO2-NH2. Noting that the uncharged benzaldehyde molecules show a negligible influence on the zeta potential of the


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particles. In the following section, experiments were conducted at pH 7.8 (C=N bond formation) and pH 3.5 (C=N bond dissociation) to investigate the pH switchable hydrophilicity/hydrophobicity of SiO2-B Pickering nanoparticles.

Figure 3. Zeta potential of SiO2-NH2 (block square) and SiO2-B (red circle) dispersions as a function of pH. The pH-triggered switch of the SiO2-B in particle hydrophobicity/hydrophilicity was confirmed by the partition behavior of the SiO2-B in oil and water two phases at the aqueous phase pH of 7.8 and 3.5. The partition of SiO2-NH2 was used as a reference. The SiO2-NH2 or SiO2-B dry nanoparticles were added to a 1:1 mixture of paraffin oil and water under quiescent environment. At pH 7.8, SiO2-NH2 settled in the aqueous phase, and formed a homogeneous aqueous dispersion after shaking (Figure S3, left). This result clearly demonstrates the hydrophilic nature of SiO2-NH2 as anticipated. Even the partially deprotonation of the amine at this pH does not make SiO2-NH2 surface sufficiently hydrophobic.29 After lowering the pH of the aqueous phase to 3.5, the SiO2-NH2 remained dispersed in the aqueous phase (Figure S3, right), suggesting the hydrophilic nature of SiO2-NH2. Furthermore, we infer that SiO2-NH2 surface is more hydrophilic at pH 3.8 than 7.8 due to a higher protonation degree of


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the amine at pH 3.5 than 7.8.29 In contrast, the SiO2-B at pH 7.8 still straddle the oil-water interface even after shaking (Figure 4a and 4c), indicating the partially hydrophobic nature of SiO2-B,29, 49 consistent with the contact angle measurement. The partially hydrophobic nature (interfacial activity) of SiO2-B was attributed to the aromatic rings introduced in the SiO2-B. This partially hydrophobic nature of SiO2-B at this pH is beneficial to the formation of Pickering emulsions. However, after lowering the aqueous phase pH to 3.5, the nanoparticles entered the aqueous phase (Figure 4b and 4d), demonstrating a hydrophilic nature of the nanoparticles. Contact angle of acidic water droplet (pH 3.5) on the SiO2-B film shows a rapid decrease from 75 ± 3° to 20 ± 3° within 4.0 s (Figure S4), further confirming the transition of wettability of the surface from partially hydrophobic to hydrophilic in acidic environment. These results demonstrated that, at pH 3.5, partially hydrophobic SiO2-B decomposed into hydrophilic SiO2-NH2 and benzaldehyde, as confirmed by 1

H NMR and zeta potential measurement. Therefore, at pH 3.5, the partition behavior

of the nanoparticles is the same as SiO2-NH2 nanoparticles. The hydrophilic nature of SiO2-B (actually, in the form of SiO2-NH2) at pH 3.5 makes it difficult to stabilize Pickering emulsions. Moreover, raising the aqueous phase pH back to 7.8, the benzaldehyde in the system can in situ react with hydrophilic SiO2-NH2 to form partially hydrophobic SiO2-B again. Williams el al. have reported that such Schiff base reaction can be completely conducted in situ between 1-undecanal in the oil phase and SiO2/PEI in the aqueous phase.50 Therefore, we infer that almost all the SiO2-B were re-formed by the in situ heterogeneous system. As a consequence, the


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re-formed SiO2-B migrated to the oil-water interface again (Figure 4a and 4c). Such a reversible partitioning of the SiO2-B at the oil-water interface boundary and in the aqueous phase is observed for at least three cycles of pH switching between pH 7.8 and 3.5 (Figure 4), illustrating that the surface of the synthesized SiO2-B switches between relatively hydrophobic state and highly hydrophilic state in response to the change of pH. The pH switchable property of SiO2-B is highly desirable for designing switchable Pickering emulsions.

Figure 4. Photographs and schematic illustration of SiO2-B partitioning at oil-water interface at pH 7.8 (a, c) and inside aqueous phase at pH 3.5 (b, d). The SiO2-B concentration is 0.5 wt% with respect to water, and water volume fraction is 0.5. Emulsions Stabilized by SiO2-B Pickering Nanoparticles. Knowing that SiO2-B was partially hydrophobic at pH 7.8, we started to examine the stability of Pickering emulsions prepared using SiO2-B Pickering nanoparticles. The ability of SiO2-B to stabilize the Pickering emulsion was investigated. At a SiO2-B concentration of 0.05 wt%, no homogeneous emulsion was obtained (Figure S5). This result is attributed to the insufficient amount of SiO2-B Pickering nanoparticles to stabilize the droplets.39


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Surprisingly, when the SiO2-B concentrations were higher (0.10–0.75 wt%), homogeneous emulsions were obtained (Figure S5). Conductivity measurement of Pickering emulsions indicate that all the emulsions prepared were O/W type. Analysis of the optical micrographs using Nano Measurer software shows a decrease in the mean droplet diameter from 57.9 µm to 19.3 µm with increasing SiO2-B concentrations from 0.10 wt% to 0.75 wt% (Figure S6). The SiO2-B concentration determines the overall interfacial area and therefore the droplet size.51 As a result, desired emulsion droplet size could be easily obtained by selecting suitable SiO2-B concentration. Besides, according to Equation (S1) in the Supporting Information, the surface coverage is estimated. It shows that the surface coverage increases from 0.34 to 0.79 with increasing SiO2-B concentration from 0.10 wt% to 0.75 wt%. A similar trend was also reported by Li et al. and Thompson et al.52,53 It appears that the SiO2-B require about 0.34 coverage to form stable emulsions. Additional control studies were carried out to see if SiO2 (0.75 wt%), SiO2-NH2 (0.75 wt%) or a mixture of SiO2 and benzaldehyde (0.75 wt%) would allow to stabilize Pickering emulsions at pH 7.8. The results show that no homogeneous emulsions were obtained after sonication, and complete phase separation were observed after 10 hours of storage (Figure S7a-7c), confirming that SiO2, SiO2-NH2 or a mixture of SiO2 and benzaldehyde do not have the ability to form stable emulsions. Both SiO2 and SiO2-NH2 is highly hydrophilic, and hydrophilic particle prefers to stay in the aqueous phase rather than adsorb at the oil-water interface. Thereby, both SiO2 and SiO2-NH2 are not suitable for preparing stable Pickering emulsions. As for the mixture of SiO2 and benzaldehyde, only a small


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quantity of benzaldehyde can be incorporated onto the surface of SiO2 (see Supporting Information), so surface hydrophobization of SiO2 cannot be effectively achieved by benzaldehyde. This explained why Pickering emulsions cannot be prepared by the mixture of SiO2 and benzaldehyde. Therefore, the dynamic covalent modification of SiO2-NH2 with benzaldehyde is necessary for the Pickering emulsion formation. Furthermore, the long-term stability of SiO2-B-stabilized Pickering emulsions was studied, and 0.5 wt% SiO2-B-stabilized Pickering emulsion was used as a representation. Gratifyingly, at pH 7.8, the droplet size (~28 µm) remained almost unchanged throughout one month of storage (Figure 5). This result confirms that, at pH 7.8, the Pickering emulsion prepared using SiO2-B has long term stability against coalescence. To gain insight into the coalescence stability of SiO2-B-stabilized Pickering emulsion, the emulsion was investigated with confocal laser scanning microscopy. The presence of a halo surrounding of each emulsion droplet in Figure 6, implies that SiO2-B nanoparticles adsorbed at the paraffin oil-water interface and formed a densely packed particle layer on the droplet interface. The SiO2-B Pickering nanoparticles at the interface provide a steric barrier to prevent the droplets from coalescence.54 Our results illustrate that the partially hydrophobic SiO2-B is effective for preparation of stable Pickering emulsions at pH 7.8.


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Figure 5. Optical micrographs of the Pickering emulsion taken immediately after emulsification (a) and after 30 days of storage (b). The Pickering emulsion was prepared with 0.5 wt% SiO2-B at pH 7.8 and a water volume fraction of 0.5.

Figure 6. Confocal laser scanning images of the Pickering emulsion. The Pickering emulsion was formed with 0.5 wt% SiO2-B at pH 7.8 and a water volume fraction of 0.5.


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pH Switchable Pickering Emulsions. Since the hydrophilicity/hydrophobicity of SiO2-B can be controlled by tuning pH, such SiO2-B is anticipated to achieve the emulsification/demulsification of Pickering emulsions by changing the pH of the continuous aqueous phase. The Paraffin oil-in-water Pickering emulsion prepared with 0.5 wt% SiO2-B was selected to study the effect of pH on the stability of the Pickering emulsion. At pH 7.8, stable Pickering emulsion was obtained as discussed earlier (section 3.3 and inset of Figure 7a, top). Upon lowering the pH from 7.8 to 3.5, a complete phase separation was achieved within 5 min with gently shaking (inset of Figure 7a, bottom). When the pH was raised to 7.8 again, stable emulsion was obtained upon sonication (inset of Figure 7a, top). More importantly, pH induced reversible process of emulsification/demulsification can be repeated for at least three cycles (Figure 7a). Additionally, after three emulsification/demulsification cycles, the droplet size (~28 µm) of newly formed Pickering emulsion showed a negligible increase as compared with the size of the initially formed Pickering emulsion (Figure 7b and 7c). Alternately adding HCl and NaOH leads to the accumulation of NaCl. Thereby, control experiments were performed to estimate the influence of NaCl on the stability and switchability of the Pickering emulsion. For one pH tuning process, based on the added HCl and NaOH, the accumulated NaCl amount was calculated. After adding the same amount of NaCl to the Pickering emulsion, no phase separation was observed (Figure S9), meaning that NaCl accumulation do not contribute to the demulsification. Besides, alternately adding HCl and NaOH to the above Pickering emulsion, the Pickering emulsion can still be effectively switched between phase


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separated state and stable state for at least three time by tuning pH, which confirms that NaCl accumulation do not affect the emulsion switchability either. The outstanding pH switchability of Pickering emulsions also indicate that the ability of dynamic covalent silica nanoparticles to form stable Pickering emulsions is not affected by the accumulated NaCl.

Figure 7. (a) Three cycles of pH triggered emulsification/demulsification processes with the inset being digital photographs of the SiO2-B-stabilized Pickering emulsion at pH 7.8 (top) and pH 3.5 (bottom). Optical micrographs of initial Pickering emulsion and after 3 cycles are shown in (b) and (c), respectively. Pickering emulsion was prepared with 0.5 wt% SiO2-B at pH 7.8 and a water volume fraction of 0.5. The effect of paraffin oil to water volume ratios on the pH switchable behavior of 0.5 wt% SiO2-B-stabilized emulsions were also studied. At pH 7.8, homogenous and stable O/W Pickering emulsions were obtained with oil to water ratios varying from 3:2 to 1:4 (Figure S10a). The droplet size was found to decrease with decreasing oil to water ratios (Figure S11). Such change is not unexpected as more particles are


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available to stabilize larger interface area at lower oil to water ratios.42 The fast response of the emulsions to the change of pH was also observed: lowering the pH from 7.80 to 3.50 made the homogeneous and stable emulsions to be phase separated within 5 min (Figure S10b). The results demonstrate the outstanding pH switchable behavior of the Pickering emulsion even at oil to water ratio of 1:4. The results described above led us to propose a mechanism of the pH induced emulsification/demulsification switching. At pH 7.8, SiO2-B-stabilized Pickering emulsion is stable (Figure 8a). The formation of stable emulsion is mainly due to the introduced hydrophobic aromatic rings in the SiO2-B Pickering nanoparticles. Hence partially hydrophobic SiO2-B Pickering nanoparticles can effectively adsorb at the oil-water interface and form a densely packed SiO2-B particle layer to prevent droplets from coalescence. Upon lowering the pH to 3.5, Pickering emulsion became readily demulsified (Figure 8b). Under such acidic environment, the dynamic imine bond in the SiO2-B decomposes, resulting in decomposition of the SiO2-B into SiO2-NH2 and benzaldehyde. The surface of SiO2-NH2 is too hydrophilic to stabilize Pickering emulsion as shown in Figure S7b. The highly hydrophilic SiO2-NH2 is anticipated to desorb from oil-water interface and migrate into the aqueous phase. Meanwhile, benzaldehyde is surface inactive and cannot stabilize emulsions effectively either, as reported in our previous study.39 From the result of UV-vis adsorption, we calculate that about 55 wt% benzaldehyde migrate into the oil phase. Desorption of SiO2-NH2 and benzaldehyde from the emulsion droplet interface led to a dramatic coalescence, and finally resulted in a complete phase separation of the


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Pickering emulsion. From the discussion, we clearly see that the emulsification and demulsification are directly related to the switchable characteristic between hydrophilic and hydrophobic nature of the SiO2-B nanoparticles.

Figure 8. Proposed emulsification and demulsification mechanisms of Pickering emulsion. (a) SiO2-B-stabilized emulsion at pH 7.80 and (b) phase separation state after lowering the pH from 7.8 to 3.50. Finally, an attempt was made to generalize the pH-responsive Pickering emulsions based on the dynamic covalent silica nanoparticles. Hence, at pH 7.8, the more hydrophobic SiO2-pDB with contact angle of 130 ± 3° was mixed with paraffin oil, sonicated to emulsify the system, and was found to be effective for W/O Pickering emulsion stabilization. Upon lowering the pH to 3.5, phase separation occurred with gentle stirring. Clearly, the utilization of the more hydrophobic SiO2-pDB enables preparation of pH responsive W/O Pickering emulsions. More details about the responsive W/O Pickering emulsions will be reported in our future work. CONCLUSIONS In conclusion, we demonstrated a simple method to modify the hydrophilic SiO2-NH2 nanoparticles through a pH-sensitive dynamic imine bond to hydrophobic dynamic covalent silica nanoparticles. The wettability (hydrophilicity and


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hydrophobicity) of the obtained dynamic covalent silica nanoparticles-SiO2-B can be controlled

by

the

change

of

pH.

Owing

to

the

pH

switchable

hydrophobicity/hydrophilicity of SiO2-B, Pickering emulsions prepared by SiO2-B Pickering nanoparticles show reversible stabilization/destabilization upon changing the pH of the continuous aqueous phase. At pH 7.8, the SiO2-B is partially hydrophobic, and it can be used as an effective Pickering emulsifier to prepare stable O/W Pickering emulsion. When the pH is lowered to 3.5, dynamic imine bond decomposes so that the partially hydrophobic SiO2-B decomposes into hydrophilic SiO2-NH2 and surface inactive benzaldehyde, leading to a complete phase separation of the Pickering emulsion. The emulsification/demulsification process was repeatable. More inspiringly, even after three emulsification/demulsification cycles, the droplet size of the newly formed Pickering emulsion remained comparable with the initially formed Pickering emulsion. This pH switchable Pickering emulsion provides promising potential applications in oil-water separation, oil recovery and material synthesis.


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ASSOCIATED CONTENT Supporting Information Synthesis of APTES-B and p-decanoxybenzaldehyde; 1H NMR spectra; zeta potential of SiO2 at different pH; partitioning behavior of SiO2-NH2 nanoparticles; contact angle of acidic water droplet (pH 3.5) on the surface of SiO2-B; macroscopic photographs and optical micrographs photographs of emulsions with different SiO2-B concentrations; estimation of surface coverage; emulsification ability of SiO2, SiO2-NH2 or a mixture of SiO2 and benzaldehyde; content of benzaldehyde incorporated onto the surface of SiO2 nanoparticles; macroscopic photographs of Pickering emulsions before and after NaCl addition; macroscopic photographs of emulsions with different oil to water ratios; optical micrographs of emulsions with different oil to water ratios.

AUTHOR INFORMATION Corresponding Author *E-mail (D.S.): [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported by Natural Science Foundation of China (NSFC, 21333005).


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Graphical abstract


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Dynamic Covalent Silica Nanoparticles for pH Switchable Pickering

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