Highly Efficient and Reversible Inversion of a Pickering Emulsion

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Highly Efficient and Reversible Inversion of Pickering Emulsion Triggered by CO2/N2 at Ambient Conditions Yunlei Shi, Dazhen Xiong, Zhiyong Li, Huiyong Wang, Yuanchao Pei, Yongkui Chen, and Jianji Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03808 • Publication Date (Web): 10 Oct 2018 Downloaded from http://pubs.acs.org on October 16, 2018

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ACS Sustainable Chemistry & Engineering

Highly Efficient and Reversible Inversion of Pickering Emulsion Triggered by CO2/N2 at Ambient Conditions Yunlei Shi†, Dazhen Xiong‡, Zhiyong Li‡, Huiyong Wang ‡, Yuanchao Pei‡, Yongkui Chen‡, Jianji Wang*,‡ †State

Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical

Engineering, Lanzhou University, Tianshui Road, Lanzhou 730000, P. R. China ‡Collaborative

Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, Key

Laboratory of Green Chemical Media and Reactions, Ministry of Education, School of Chemistry and Chemical Engineering, Henan Normal University, Jianshe Road, Xinxiang, Henan 453007, P. R. China *Corresponding

author: Wang JJ, [email protected]

ABSTRACT Emulsion inversion has great potential in applications such as material science, chemical reactions, and drug delivery. Therefore, developing a simple and green approach to control

the

emulsion

inversion

bis(2-hydroxyethyl)-3-amino

is

highly

bifunctionalized

desirable. silica

Herein,

microsphere

an

octyl

(SM-O-BIS)

and is

designed, prepared and used to fabricate Pickering emulsion. It is found for the first time that this Pickering emulsion can be easily and reversibly inverted from water-in-oil (w/o) to oil-in-water (o/w) by alternative bubbling of CO2 and N2 at room temperature and atmospheric

pressure.

The

molar

ratio

of

trimethoxyoctylsilane

to

bis(2-hydroxyethyl)-3-aminopropyltriethoxysilane on the surface of silica is important for the emulsion inversion. This unique inversion can be recycled multiple times without any deterioration. By utilizing the emulsion inversion strategy, encapsulation and release of curcumin molecules have been actualized on demand. The possible mechanism of 1

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emulsion inversion is also investigated by zeta potential, water contact angle,

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13C

NMR

spectroscopy and FT-IR spectroscopy. It is shown that the significant and reversible change in hydrophilicity/hydrophobicity of SM-O-BIS is the driving force for such a CO2/N2 triggered inversion. KEYWORDS: Pickering emulsion, emulsion inversion, CO2, curcumin, encapsulation and release

INTRODUCTION Pickering emulsion, which is stabilized individually by surface active colloid particles, was first reported by Ramsden1 and Pickering2 in the early 20th century. Since the particles are absorbed at the interface of two immiscible liquids such as oil and water, where a dense particle film is formed to provide a mechanical barrier to prevent the coalescence of droplets, thus Pickering emulsion is more stable compared with traditional emulsion only stabilized by surfactant or polymer.3-4 Moreover, the excellent properties of Pickering emulsion, such as large oil/water interface and low toxicity, have attracted attention of more and more researchers in the past few decades. Thus, the research on Pickering emulsion is significant both in understanding the basic principles and in developing potential applications. Usually, emulsion needs to be temporarily stabilized, then demulsified or reversed as needed in some practical applications such as interfacial catalysis,5-7 drug delivery,8 crude oil transport,9 emulsion polymerization,10 and so on. Therefore, switchable Pickering emulsions11-13 stabilized by stimulus-response surface active particles, which response to any environmental trigger,14 have attracted enormous attention during recent years. On the one hand, the stable emulsion can be retained a long time without any deterioration, and then the system can be quickly demulsified or reversed6,

15

upon

exposure to a stimulus. In this case, the control of emulsion characteristics can be achieved on demand. On the other hand, the emulsifiers can be recycled to favor more sustainable operation, thus confirming to the principles of green chemistry. 2


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In recent years, many efforts have been made to develop intelligent Pickering emulsion via using specific stimulus such as pH,16-19 temperature,20-22 magnetic fields,23-24 light,25-26 redox,27-28 electrolytes29-30 and their combinations.31-32 Among the stimulus used, CO2 is an excellent trigger with renewability, abundance, low cost, non-flammability and good biocompatibility.33 In particular, even if CO2 is introduced into a system, it contaminates neither the product nor the media. Moreover, it can be easily removed by gas-sparging with N2 or air, and accumulation of salts resulting from the acid-base reaction was not found in system. Because of these advantages, CO2 has been used as an external trigger to formulate switchable Pickering emulsions in light of actual needs. Up to now, there are two kinds of strategies for creating a CO2-responsive Pickering emulsion. One approach is using switchable surfactant to functionalize nanoparticles by physical interaction. For example, Binks and co-workers34 reported a CO2 switchable octane-in-water Pickering emulsion for the first time, where negatively charged silica combined with N-dodecyl-N,N-dimethylacetamidine was used as stabilizer to realize the emulsification and disruption of emulsion. Zhang et al.35-36 used amine oxide-based surfactants and tertiary amine, respectively, to functionalize the surface of nanoparticles, and the resultant surface-active nanoparticles were used to fabricate CO2-switchable Pickering emulsions, which could be switched between emulsification and demulsification. Chemical grafting represents another effective approach to prepare switchable Pickering emulsions. In this context, Xu et al.37 synthesized silica nanoparticles with surface grafted CO2 responsive functional groups. By tailoring the surface chemistry, the nanoparticles with different hydrophilicity/hydrophobicity were used to achieve the transformation of emulsion from emulsification to demulsification. Zhu and co-workers38 prepared oil-in-water emulsion stabilized by modified lignin, and the emulsion could be demulsified and re-emulsified by sparging of CO2/N2, alternatively. Cunningham et al.39 reported graft-modified cellulose nanocrystals stabilized Pickering emulsions, which were able to switch between emulsification and 3



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demulsification by alternatively bubbling of N2 and CO2. However, to the best of the authors’ knowledge, studies on CO2/N2-triggered inversion of Pickering emulsion have not been reported so far, although inversion of an emulsion from one style to another is of great importance from both scientific and practical points of view.40-41 Herein, a series of functionalized silica microspheres were designed and synthesized (Scheme 1), where hydrophobic trimethoxyoctylsilane ((MeO)3Si(CH2)7CH3) or/and relatively

hydrophilic

bis(2-hydroxyethyl)-3-

aminopropyltriethoxysilane

((MeO)3Si(CH2)3N(CH2CH2)2(OH)2) which responds to CO2 were used to modify the silica microspheres (SMs) by covalent linkage. Interestingly, the Pickering emulsion stabilized by octyl and bis(2-hydroxyethyl)-3-amino bifunctionalized silica microspheres (SM-O-BIS) could be reversibly switched from w/o to o/w emulsion and then from o/w to w/o emulsion again by bubbling and removal of CO2 alternatively at room temperature and atmospheric pressure. In particular, the applicability of CO2/N2-triggered Pickering emulsion inversion was evaluated through encapsulation and release of curcumin molecules. Thus, a simple synthetic procedure, low cost reactants and highly efficient controllability have been actualized for the development of reversible inversion of Pickering emulsion. The possible mechanism of emulsion inversion was also investigated by various techniques. Scheme 1. Schematic Illustration for the Synthesis of Functionalized Silica Microspheres

4


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EXPRIMENTAL SECTION Materials Tetraethyl orthosilicate (TEOS, 99%) was purchased from Aladdin Co. Ltd. Trimethoxyoctylsilane (98%) and bis(2-hydroxyethyl)-3-aminopropyltriethoxysilane (62% in ethanol) were provided by J&K Scientific Ltd. Toluene (99.5%) was acquired from Aldrich, and the trace of water was eliminated with molecular sieve. Ammonium hydroxide (NH3·H2O, 25 wt %), ethyl acetate (99%), cyclohexane (99%), n-octanol (99%), n-decane (99%), and ethanol (95%) were provided by Alfa Aesar and used as received. Deionized water was used in this study. Synthesis of SiO2 Microspheres SiO2 microspheres were synthesized based on the procedures previously reported.42 Briefly, ethanol (95%, 200 mL), deionized water (50 mL), and NH3·H2O (25 wt %, 11.2 mL) were mixed and stirred for 30 min to form a uniform solution at ambient conditions, and then 8.32 g of TEOS was added into the above solution. After further stirring for 12 5



Page 6 of 26

h, the sediment was collected via filtering of the suspension solution, then washed with deionized water, and dried under vacuum at 60 °C to obtain SiO2 microspheres (SM). Synthesis of bis(2-hydroxyethyl)-3-amino functionalized SiO2 microspheres 0.5 g of the synthesized SMs was put into toluene (5 ml) solution, and 0.75 mmol of bis(2-hydroxyethyl)-3-aminopropyltriethoxysilane (62 wt % in ethanol) was added. The mixture reacted at 90 °C for 2 h and then at 110 °C for 2 h, the whole process was performed under N2 atmosphere. The formed precipitation was separated through centrifugation, and washed with toluene. After being dried under vacuum, bis(2-hydroxyethyl)-3-amino functionalized SiO2 microspheres (SM-BIS) were obtained. Synthesis of octyl functionalized SiO2 microspheres 0.5 g of the synthesized SMs was dispersed into toluene (5 ml) solution, and 0.75 mmol of trimethoxyoctylsilane was added. After refluxing at 110 °C for 4 h under N2 atmosphere, the resultant precipitation was separated through centrifugation, washed three or four times with toluene, then dried under vacuum, octyl functionalized SiO2 microspheres (SM-O) were prepared. Synthesis of bifunctionalized SiO2 microspheres 0.5 g of the SM was dispersed into toluene (5 ml), then 0.41 mmol of trimethoxyoctylsilane

and

0.34

mmol

of

bis(2-hydroxyethyl)-3-

aminopropyltriethoxysilane (62 wt % in ethanol) were added. The reaction was performed at 90 °C for 2 h and then at 110 °C for 2 h again under N2 atmosphere. The product was separated through centrifugation, and washed with toluene three times. After being dried under vacuum, octyl and bis(2-hydroxyethyl)-3-amino bisfunctionalized SiO2 microspheres (SM-O-BIS) were obtained. Preparation of Pickering Emulsions 0.030 g of the functionalized SiO2 microspheres was added into a vial containing the mixture of 3 mL of water and 2 mL of ethyl acetate (toluene, cyclohexane, n-octanol or n-decane). After stirring for 1 min at 1500 rpm with a B11-2 stirrer (Shanghai Sile 6


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Instrument Co. LTD, china), functionalized SiO2 microsphere stabilized emulsion was formulated. All conditions of emulsification were the same unless otherwise specified. Characterization Transmission electron microscope (TEM) images were obtained on a Hitachi UHR FE-SEM (JEM-2100, JEOL, Japan). Thermal gravimetric analysis (TGA) was recorded on a NETZSCH TG analyzer (STA449C, Germany) under nitrogen atmosphere from 25 °C to 900 °C with a heating rate of 10 °C/min. X-ray photoelectron spectra (XPS) were determined with a Thermo Scientific ESCALAB (250Xi, America). The zeta potential of the particles was measured using a ZetaPLAS instrument (Malvern, United Kingdom) at 25 °C. The pH measurements were performed by a PHSJ-4F pH-meter (Rex Electric Chemical, China). Water contact angles on the surface of the functionalized SiO2 microspheres were measured by an optical contact angle measuring device (Kruss DSA 25, Germany). The type of emulsions was determined by conductivity with a conductivity meter (DDS-308A, China). Photographs of emulsions were captured with a digital camera (Canon Inc., Japan), and micrographs of emulsions were taken under a DYP-990 microscope system (China). Fluorescence microscopic images were obtained with an Olympus confocal multiphoton microscope (FV1200MOE, Japan).

RESULTS AND DISCUSSION Structure and morphology of the functionalized Silica Microspheres A series of functionalized silica microspheres were designed and synthesized by grafting hydrophobic


CO2-responsive

((MeO)3Si(CH2)7CH3)

or/and

hydrophilic

bis(2-hydroxyethyl)-3-aminopropyltriethoxysilane

((MeO)3Si(CH2)3N(CH2 CH2)2(OH)2) onto the surface of silica microspheres (SMs). The functionalized

silica

microspheres

were

denoted,

respectively,

as

SM-BIS

((MeO)3Si(CH2)3N(CH2CH2)2(OH)2), SM-O ((MeO)3Si(CH2)7CH3), and SM-O-BIS ((MeO)3Si(CH2)7CH3 and (MeO)3Si(CH2)3N(CH2CH2)2(OH)2) (Scheme 1). Taking SM-O-BIS as an example, hydrophobic molecules endow the material with a certain 7



Page 8 of 26

degree of hydrophobicity, while CO2-responsive groups make the material a switchable property. Changes in hydrophilicity and hydrophobicity may be achieved by bubbling and removal of CO2. Transmission electron microscopy (TEM) and Fourier transform infrared (FTIR) spectroscopy were used to characterize the structure and morphology of the functionalized SMs. It was found from the TEM images that those particles were spherical and their diameters were within the scope of 150−250 nm (Figure 1A). The peak at 1190 cm−1 in the infrared spectra of SM-BIS and SM-O-BIS (Figure S1) was attributed to the typical carbon-nitrogen bond vibration, which confirms that bis(2-hydroxyethyl)-3-amino molecules were grafted onto the surface of SM-BIS and SM-O-BIS. The vibration of carbon-hydrogen bond at 1460 cm−1 was found in the spectra of SM-O and SM-O-BIS microspheres (Figure S1), indicating that octyl was also successfully grafted onto the surface of SM-O and SM-O-BIS. In addition, XPS and TGA were used to quantify the degree of functionalization of silica microsphere surface. According to TGA measurements (Figure S2), the mass loss of the three types of functionalized silica microspheres were, respectively, 2.87, 3.78, and 1.74 wt%, which represent the mass percent of the grafted molecules on the surface of SMs. For SM-O and SM-BIS, the mass loss represents the mass percent of the grafted octyl and bis(2hydroxyethyl)-3-amino, respectively, on the surface of silica microspheres. However, for SM-O-BIS, the loading value of octyl and bis(2-hydroxyethyl)-3-amino on the surface of SM-O-BIS microspheres could be calculated from combination of the TGA and XPS results (Figure 1B, Figure S3-S4). The loading values for the functionalized materials were listed in Figure 1C.

8


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Figure 1. (A), TEM image of SM-O-BIS; (B), XPS spectrum of SM-O-BIS; (C), Functionalized silica microspheres with different loadings of octyl and bis(2-hydroxyethyl)-3-amino groups.

Formation and Inversion of Pickering Emulsion We examined the interfacial activity and inversion ability of the CO2-responsive emulsion composed of ethyl acetate, functionalized SMs and water. Typically, after the functionalized SMs (1wt%), ethyl acetate (2 ml), and water (3 ml) were mixed and stirred for 1 min at 1500 rpm and room temperature, diverse phenomena could be seen for four different materials in Figure 2. SM and SM-BIS were well dispersed in water phase, and no emulsion droplets were found under the view of microscope. Interestingly, SM-O and SM-O-BIS led to the formation of Pickering emulsions, as confirmed by optical microscope. These emulsions could be stable for at least one month at ambient conditions. Conductivity and drop tests indicate that they were water-in-oil (w/o) emulsions (Figure S5-S6). After bubbling of CO2 for 20 min at room temperature and followed by stirring, no change was observed in those samples with SM and SM-O, while SM-BIS was better dispersed in water layer than before. However, SM-O-BIS was 9



Page 10 of 26

transferred to ethyl acetate phase where oil-in-water (o/w) emulsion was obtained (Figure S5-S6), and the (o/w) emulsion could be stable at least for two weeks at ambient conditions. This stability was lower than the w/o emulsion, which may be caused probably by the escape of CO2 from the (o/w) emulsion system. As can be seen from the microphotographs shown in Figure 2, the droplet size of w/o and o/w emulsions was 180-350 μm and 180-400 μm, respectively. The distribution of the emulsions droplet size was relatively uniform. In addition, we noticed that several droplets were non-spherical in Figure 2B, the non-spherical droplet was slightly worse in stability than the spherical droplet. However, the number of this particular shape of droplet was so small that it did not affect the stability of emulsion. Interestingly, the o/w emulsion could be transformed into w/o emulsion after bubbling of N2 at 25 °C for 20 min.

Figure 2. Photographs and selected optical micrographs of ethyl acetate–water emulsions stabilized with different functional silica microspheres (photographs and micrographs were taken at 24 h of standing). (A), before CO2 bubbling; (B), after CO2 bubbling. Scale bar is 100 μm.

Surface chemistry plays an important role in emulsion inversion, the ratio of trimethoxyoctylsilane

to

bis(2-hydroxyethyl)-3-aminopropyltriethoxysilane

on

the

surface of silica was studied to obtain the optimal results (Table S1). It was found that 10


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emulsion inversion did not occur at the molar ratio less than 1.12 or greater than 1.30. Only in the molar ratio range from 1.12 to 1.30, emulsion inversion could be realized by bubbling CO2. This indicates that the molar ratio of trimethoxyoctylsilane to bis(2-hydroxyethyl)-3-aminopropyltriethoxysilane was a very important parameter. Additionally, the effect of mass percent of SM-O-BIS on the inversion of emulsion was also studied. Taking ethyl acetate as the oil phase, microscopic images of Pickering emulsions stabilized by different mass percent of emulsifier were shown in Figure S7. It is clear that all emulsions could be inversed in the mass percent range of the emulsifier from 0.6 wt % to 1.5 wt %. And the average droplet size decreased from 645 to 170 μm with the increase of SM-O-BIS mass percent. Moreover, an excess oil (or water) phase appeared on the top (or bottom) of the vial in all emulsions, and after removal of the redundant oil (or water) phase, all emulsions still showed a good stability without any coalescence or Ostwald ripening. In addition, a homogenizer (T-50, IKA, Germany) was used to study the effect of stirring speed on the droplet size of the emulsion, and the emulsion micrographs at different stirring speeds (1500, 3000, and 6000 rpm) were shown in Figure S8. It can be seen that the droplet size of emulsion became significantly smaller as the stirring speed increased, and the average droplet size was 269 μm, 170 μm and 66 μm, respectively, at different stirring speeds. Thus, the stirring speed had a great influence on the droplet size of the emulsion. Besides the water/ethyl acetate system, different polar organic solvents (cyclohexane, n-decane, toluene, and n-octanol) were selected as oil phase to form emulsions (Figures S9−S12). The results showed that the system composed of any kind of these oil phases (2 ml) and water (3 ml) could form w/o emulsion in the presence of SM-O-BIS (1 wt %). Importantly, all water-in-oil emulsions could be easily inverted to oil-in-water emulsions after bubbling of CO2. This indicates that the functionalized material has a good universality for oil phases with different polarity. This may help to expand its application in different systems. 11



Page 12 of 26

The possible Mechanism of CO2/N2-Triggered Emulsion Inversion The inversion of emulsion is largely dependent on the CO2-responsive surface chemistry of the particles. Here, zeta potential of the functionalized SMs in aqueous solution was measured to understand the variation of particle surface charge. As shown in Figure S13, the value of pH and zeta potential of SM-O-BIS in water was 8.0 and 4.76 mv, respectively, in the absence of CO2. However, the pH value dropped from 8.0 to 5.6, and the value of zeta potential became enormously positive (51.6 mV) under bubbling of CO2 at 25 °C. This result suggests that bis(2-hydroxyethyl)-3-amino on the surface of SM-O-BIS might be protonated. In addition, as CO2 was removed by bubbling of N2, the pH value returned to 7.9 and the zeta potential value decreased to 5.0 mv, which are very close to the pH value and zeta potential before bubbling of CO2. This phenomenon may be

understood

by

a

nearly

complete

deprotonation

of

the

protonated

bis(2-hydroxyethyl)-3-amino. It is known that for a given material, stronger hydrophobicity occurs near the point of zero charge, and the presence of surface charge renders it a certain degree of hydrophilicity.43 Therefore, the protonation of bis(2-hydroxyethyl)-3-amino by CO2 bubbling makes the SM-O-BIS surface hydrophilic, whereas deprotonation of the protonated bis(2-hydroxyethyl)-3-amino by N2 bubbling endows the surface of SM-O-BIS hydrophobic again. The ability of solid particle to stabilize emulsion of two immiscible liquids depends on its wettability at liquid-liquid interface, which can be quantified through the three-phase contact angle θ of water. For a mixture of oil and water, hydrophobic particles of θ > 90° may form w/o emulsion, whereas hydrophilic particles of θ < 90° is prone to stabilize o/w emulsion.4,

44

Therefore, the variation in hydrophilicity/

hydrophobicity switched by CO2 can be confirmed by measurements of the water contact angle. In this work, functionalized SiO2 microspheres were deposited on coverslip surface to form a smooth and uniform coat, and a drop of water (1 µL) was dripping onto the surface of the sample. The value of the water contact angle was then determined by 12


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photogoniometric method, and the results were shown in Figure 3. It can be seen that initially, the contact angle of SM-O-BIS was 105.9° in the absence of CO2, suggesting that the particles have a strong hydrophobicity. This also explains why the functionalized particles can stabilize water-in-ethyl acetate emulsion. After CO2 was bubbled into this system, the value of the water contact angle decreased to 73.4°. Thus, w/o emulsion was destroyed and transformed into o/w emulsion after stirring. In addition, when the hydrophilic SM-O-BIS was further treated with N2, the contact angle (104.7°) was nearly restored to the initial value. As far as SM and SM-O were concerned, no such a variation was found due to the absence of a CO2-responsive moiety. As we expected, water contact angle of SM-BIS really decreased with bubbling of CO2, and it is impossible to achieve emulsion inversion because of the strong hydrophilicity of SM-BIS.

Figure 3. The variation of water contact angles of SM-O-BIS with alternative bubbling of CO2 and N2 at 25 °C.

To obtain a deeper insight into the mechanism of emulsion inversion between w/o and o/w, 13C NMR spectroscopy was determined for SM-O-BIS before and after reaction with CO2 in the mixture of D2O and DMSO (Figure 4). It is clear that a new signal at 159.4 ppm was observed and attributed to HCO3− anion.45-46 This is similar to the change 13



in

13C

Page 14 of 26

NMR spectroscopy after the protonation of diethanolamine (Figure S14), thus

confirming the formation of hydrophilic ammonium bicarbonates. The signal at 124.7 ppm corresponded to

13C

chemical shift of physically absorbed CO2 previously

reported.47-48 FI-IR spectroscopy was also determined for SM-O-BIS before and after reaction with CO2 in aqueous solution, and the results were shown in Figure S15. The peaks at 826 cm−1 and 1048 cm−1 were attributed to the carbonyl group vibration, which indicates the formation of hydrophilic ammonium bicarbonates. This is consistent with the results obtained by 13C NMR spectroscopy. Therefore, combining the zeta potential, water contact angle,

13C

NMR results and FT-IR spectroscopy, the mechanism of

CO2-triggered emulsion inversion may be explained as follows. Initially, SM-O-BIS was so hydrophobic that it can stabilize water-in-oil emulsion. After CO2 was bubbled into the system and reacted with bis(2-hydroxyethyl)-3-amino of SM-O-BIS, hydrophilic ammonium bicarbonates were formed (Figure 5), which made the emulsion transform from w/o to o/w. Therefore, CO2-triggered emulsion inversion involves the acid-base reaction between CO2 and bis(2-hydroxyethyl)-3-amino of SM-O-BIS and the formation of hydrophilic ammonium salts, this may be the main driving force for emulsion inversion.

Figure 4. 13C NMR spectra of SM-O-BIS in the mixture of D2O and DMSO (the volume ratio of D2O 14


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to DMSO is 1:3, DMSO was added into the aqueous solution to increase the solubility of SM-O-BIS) solution.

Figure 5. The possible reaction of SM-O-BIS in aqueous solution by bubbling and removal of CO2, alternatively.

The cycle of Pickering Emulsion Inversion The cycle of the CO2-triggered Pickering emulsion inversion was investigated in some details. It was shown that the signals at 159.4 ppm and 124.7 ppm in the

13C

NMR

spectrum of SM-O-BIS all disappeared after CO2 was removed with N2 bubbling (Figure 4). In addition, Figure S16 showed ten cycles of CO2/N2-triggered processes of the emulsion inversion. When the mixture of ethyl acetate (2 mL), water (3 mL), and SM-O-BIS (0.030 mg) was stirred for 1 min at ambient conditions, a stable w/o Pickering emulsion was easily obtained. Upon CO2 bubbling for 20 min with a fixed rate of 40 mL·min−1, w/o emulsion was transformed into o/w emulsion after homogenization. As CO2 was removed by bubbling of N2 for 20 min with the defined rate of 80 mL·min−1, the stable w/o emulsion was formed again by homogenization. Such a transfer of emulsion between w/o and o/w by alternatively bubbling of CO2 and N2 could be readily cycled for more than ten times without any obvious deterioration (Figure S16). It was also found that the droplet diameter of the emulsion did not have significantly change during the cycles. These results indicate that the CO2/N2-triggered inversion of emulsion stabilized by functionalized silica microspheres have a good recyclability. This conclusion was further supported by the variation in zeta potential (Figure S17) and pH value (Figure 15



Page 16 of 26

S18) of SM-O-BIS functionalized silica microspheres in aqueous solution with alternately bubbling and removal of CO2. Encapsulation and Release of Curcumin molecules Curcumin is an orange-yellow crystalline powder extracted from the rhizomes of some plants in zingiberaceae and araceae. This compound contains two ferulic acid residues covalently linked through a methylene bridge, and has the functions of lowering blood fat, anti-tumor, anti-inflammatory, anti-oxidation and so on.49 In addition, it has been discovered that curcumin helps to treat drug-resistant tuberculosis,50 but it is not soluble in water. Emulsion inversion is a good delivery system to accelerate water solubility of curcumin in the process of encapsulation and release. This can greatly increase efficacy and reduce adverse effects. Therefore, curcumin was selected as a cargo to evaluate the practicability of CO2-triggered emulsion inversion. From fluorescence emission of curcumin itself shown in Figure 6A, it is clear that initially, curcumin distributed in the continuous phase of the droplets of w/o emulsion, indicating that curcumin was not encapsulated into emulsion at this time. After bubbling of CO2, SM-O-BIS became relatively hydrophilic, and o/w emulsion was formed after homogenization. And curcumin was encapsulated as a cargo within the oil phase of o/w emulsion and exhibited green fluorescence (Figure 6B). However, once N2 was bubbled into the system, the emulsion inversed from o/w to w/o, and curcumin was released from o/w emulsion (Figure 6B). Accordingly, encapsulation and release of curcumin could be successfully achieved in the process of emulsion inversion. Therefore, CO2/N2 triggered Pickering emulsion inversion showed promising potential in cargo encapsulation and release.

16


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Figure 6. (A), Photographs of curcumin encapsulation and release by SM-O-BIS stabilized Pickering emulsion inversion: w/o (left, release) and o/w (right, encapsulation); (B), Fluorescence microscopic images of the corresponding Pickering emulsions shown of (A). Scale bar: 200 μm.

CONCLUSIONS In summary, by chemical grafting of CO2-reponsive molecules on the surface of silica microspheres, we successfully prepared functionalized silica microspheres (SM-O-BIS) stabilized Pickering emulsion. The emulsion could be switched between w/o and o/w by alternatively bubbling of CO2 and N2. Interestingly, the process of emulsion inversion was

highly

reversible.

The

molar

ratio

of


and

bis(2-hydroxyethyl)-3-aminopropyltriethoxysilane played an important role in emulsion inversion, and only in the molar ratio range from 1.12 to 1.30, the material could be used to achieve emulsion inversion. Investigations from zeta potential, water contact angle, 13C NMR and FT-IR spectra suggested that reversible acid-base reaction of the bis(2-hydroxyethyl)-3-amino terminated SM-O-BIS with CO2 and thus reversible 17



Page 18 of 26

formation of hydrophilic ammonium bicarbonate led to the significant variation in hydrophilicity/hydrophobicity of the SM-O-BIS surface, which was the main driving force for the CO2/N2-triggered emulsion inversion. Additionally, the emulsion inversion protocol could be applied for the encapsulation and release of curcumin molecules. To the best of our knowledge, this is the first work to show CO2/N2-triggered Pickering emulsion inversion at room temperature and atmospheric pressure and related mechanism. The low cost starting materials, simple synthesis produces of the functionalized silica microspheres and their switchable hydrophilicity/hydrophobicity, together with clean trigger and mild emulsion inversion condition make them an important addition to the family of stimulus-responsive Pickering emulsions. It is expected that the CO2/N2-triggered emulsion inversion may have great potential in drug delivery, interfacial catalysis and among others. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. FTIR, TG, XPS spectra of the functionalized silica microsphere, conductivity and photograph of Pickering emulsions stabilized by SM-O-BIS particles before and after bubbling of CO2, microscopy images of emulsions with different mass fractions of SM-O-BIS and stirring speeds in ethyl acetate–water mixture, appearance of oil-water

emulsion

stabilized

by

SM-O-BIS,

13C

NMR

spectrum

of

N-methyldiethanolamine, photograph of CO2/N2 triggered emulsion inversion process, zeta potential and pH of SM-O-BIS (1 wt%) dispersed in aqueous solution with alternately bubbling of CO2 and N2, the relationship between the molar ratio of octyl/bis(2-hydroxyethyl)-3-amin and the emulsion inversion (PDF) AUTHOR INFORMATION Corresponding Author 18


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*E-mail: [email protected] (J.W.). Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported financially by the National Natural Science Foundation of China (No.21603063, U1704251 and 21733011) and the Program for Innovative Research Team in Science and Technology in University of Henan Province (No. 16A150014). REFERENCES 1.

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Page 26 of 26

TOC

For the first time, highly efficient and reversible Pickering emulsion inversion has been achieved from alternative bubbling of CO2 and N2 at ambient conditions.

26


Highly Efficient and Reversible Inversion of a Pickering Emulsion

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