Subscriber access provided by READING UNIV
Article
CO2 and Redox Dual Responsive Pickering Emulsion Yongmin Zhang, Shuang Guo, Xiaofei Ren, Xuefeng Liu, and Yun Fang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02976 • Publication Date (Web): 26 Oct 2017 Downloaded from http://pubs.acs.org on October 30, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 11
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
CO2 and Redox Dual Responsive Pickering Emulsion Yongmin Zhang,†* Shuang Guo,† Xiaofei Ren,† Xuefeng Liu† and Yun Fang† †
Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School of Chemical & Materials Engineering, Jiangnan University, Wuxi 214122, P. R. China.
Corresponding Author *E-mail:
[email protected] (Y. Zhang)
ABSTRACT: Herein, we described for the first time a CO2 and redox dual responsive paraffin oil-in-water Pickering emulsion stabilized by the modified silica nanoparticles with Se-containing tertiary amine, SeTA, in which the tertiary amine serves as a CO2-sensitive group, and the Se atom serves as a redox-sensitive center. The Pickering emulsion can be reversibly switched between stable and unstable with bubbling CO2 and N2 at reduced state, or with the addition of H2O2 and Na2SO3 in the absence of CO2, because of the adsorption and desorption of SeTA on the silica surface. The former is mainly attributed to a CO2-controllable electrostatic attraction, resulting from a transition of molecule between cationic and nonionic states; while the latter is ascribed to a redox-tunable hydrogen-bonding, originating from a transition of molecule between selenide and selenoxide. Whereas, in the presence of CO2, redox can only induce a change in the droplet size, not demulsification. This interesting and unique multi-responsive behaviors endows the Pickering emulsion with a capacity for intelligent control of emulsification and demulsification, as well as the droplet size, which may be an asset for a myriad of technological applications in biomedicine, microfluidics, drug delivery, and cosmetics.
INTRODUCTION 1
to give a wider controllable range or higher precision. To date, combinations including temperature-magnetic field,25,26 pH-magnetic field,27,28 pH-temperature,29-31 pHglucose32 and CO2-light,33 have been used to adjust an emulsion’s characteristics. Nevertheless, to the best of our knowledge, Pickering emulsions responding to individual redox or multi-stimuli including redox have been less well documented. As is well known, redox processes constantly occur in all organisms, such as the formation and destruction of inflamed cells,23,34-36 in which a higher local redox potential may be exploited to selectively influence the emulsion’s characteristics. On the other hand, as a product of metabolism, biocompatible and renewable CO2 gas has been documented as a green trigger because it contaminates neither the products nor the media where they were introduced to.37-39 Therefore, utilizing a particularly intriguing pair of stimuli (redox and CO2) to construct a dual responsive Pickering emulsion is highly desirable, and may exploit their promising applications in physiological environments. Herein, we reported a smart Pickering emulsion whose stability can be switched on and off through the bubbling and removal of CO2 or a simple redox reaction. The droplet size was also controllable. The Pickering emulsifier was constructed based on the silica nanoparticles (SiO2) functionalized in situ with a selenium-containing tertiary amine, 11-(benzylselanyl)-N,N-dimethylundecan-1-amine (SeTA, Scheme 1).
Since they were discovered over one century ago, Pickering emulsions stabilized by colloidal particles have attracted considerable amount of attention both from theoreticians and engineers, because they not only remain most of the basic properties of conventional emulsions, but also dramatically decrease the dosage of surfactant, even to zero. The dense particle film at the oil/water interface formed by the colloidal particles provides a mechanical barrier to coalescence,2 making Pickering emulsions more stable than conventional emulsions. This long-term emulsion stability is critical for food storage or cosmetic formulations.3-5 However, in other cases the high stability may also lead to some inevitable problems, because only temporary stability is desirable in many applications including emulsion polymerization,5 interfacial catalysis,6 and drug delivery.7 Therefore, how to balance the long-term stability and rapid demulsification has become a key focus in the area of Pickering emulsions. The emergence of stimuli-responsive Pickering emulsions has heralded a new dawn. A stable emulsion can existed without stimuli, and then be quickly destroyed upon stimuli, simplifying the demulsification process and enabling the remote control of an emulsion’s characteristics. To date, switching on and off Pickering emulsions has been extensively studied using single stimuli such as pH,8-10 temperature,11-13 light,14 magnetic fields,15,16 and CO2.17-22 Most life processes depend on the response to a combination of environmental changes, rather than a single stimEXPERIMENTAL SECTION ulus.23,24 Thus, a large number of efforts has been devoted Materials recently to construct multi-responsive Pickering emulsions ACS Paragon Plus Environment
Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Silica nanoparticles (HL-200; purity >99.8%; primary particle diameter in 60-150 nm determined by light scattering method (Figure S1); Brunauer−Emmett−Teller (BET) surface area =200 ± 20 m2/g; isoelectric point at pH = 2.94 (Figure S2)) was purchased from Aladdin, and used without further treatment. 11-(benzylselanyl)-N,Ndimethylundecan-1-amine (SeTA) was synthesized following a previously reported procedure.35,36 Paraffin oil (purity ≥ 98%), 30 wt% H2O2, Na2SO3 and Nile red (purity ≥ 99%) were purchased from Sinopharm and Sigma, respectively. Triply distilled water with a resistance of 18.2 MΩ·cm and a pH of 6.2 at 25 °C by a quartz water purification system was used in all the measurements. Preparation of Pickering Emulsions Emulsions were prepared by homogenizing the mixture of 3 mL aqueous phase with either surfactant or silica nanoparticles or both and 4 mL paraffin oil in a glass bottle at 12000 rpm for 90 s using an IKA Ultra-Turrax T-18 homogenizer. The particles and surfactant concentrations are expressed as weight percent (wt%) and moles per liter relative to the water phase, respectively. All the resulting emulsions were stored for more than one month at 25 °C to observe their stability. CO2 or N2 was bubbled using a stainless-steel needle with a fixed rate of 40 mL·min-1 at room temperature. It must be pointed out that CO2 was bubbled after mixing SiO2 and SeTA other than bubbling CO2 to SeTA and then mixing them with SiO2. Characterization of Pickering Emulsions The type of emulsion was judged based on the drop test and staining method with oil-soluble Nile red as probe. The micrographs of the emulsion droplets were recorded using a VHX-1000 microscope system (Keyence Co.). Particle size distributions were measured using a Malvern Mastersizer 2000 instrument by dispersing a drop of emulsion in an aqueous solution containing the same surfactant concentration as in the aqueous phase used for preparing the emulsion. Measurements (a) Zeta potential. The zeta potential of 0.1 wt% silica nanoparticles dispersion using surfactant solution as solvent was determined using a ZetaPLAS instrument (Brookhaven, USA). The samples were equilibrated at 25 o C for no less than 12 h. (b) Contact angle. A glass slide was cut into strips of 8 mm×26 mm, and used as model silica substrates for contact angle measurements. These glass strips were dipped in 30% aqueous sodium hydroxide for 24 h and chromic acid successively, and then washed with deionized water and dried. The clean glass strip was placed inside a glass bottle, leaning against the wall, and then a fixed volume surfactant solution was added to immerse the glass strip. After 24 h (for adsorption), the glass strip was taken out and freezedried. The contact angle of pure water on the glass strop was measured using optical contact angle measuring device (Dataphysics OCA 40, Germany). (c) Adsorption isotherm. The adsorption isotherm of surfactant at the particle-water interface was determined by the depletion method. The equilibrium concentration of
Page 2 of 11
surfactant in a series of 0.5 wt% silica nanoparticles dispersion after adsorption for one day was calculated based on the surface tension of the dispersion. The surface tension of surfactant solution without silica nanoparticles is used as calibration. The surface tension was measured with a Krüss K100 tensiometer by the automatic du Noüy ring model at 25 ± 0.1 oC. (d) Adsorbed amount of SiO2 on the droplets. 15 mL emulsion was prepared as described above. The aqueous phase after creaming was collected in a dried and weighted watch glass using a syringe, and recorded its mass. The samples were freeze-dried to constant weight. The adsorbed amount of SiO2 on the droplets can be estimated following equation:
− × 100% wt% adsorbed =
in which mtotal is the total SiO2 mass in the emulsion, maqueous is the residual SiO2 mass in the aqueous phase. (e) Interfacial tension. The oil-water interfacial tension between the aqueous solution and oil (including fresh and separated paraffin oil after demulsification) was determined at 30 oC by spinning drop technology on SVT20N spinning drop interfacial tensiometer (Germany, Dataphysics). (f) Critical micelle concentration (cmc). The cmc of surfactant was determined by fluorescent spectrometry as described elsewere.38 A Varian Cary Eclipse spectrometer (Varian Inc. USA) with a Neslab circulating water bath was used, and pyrene was used as fluorescent probe. The fluorescence emission spectra were recorded from 350 to 500 nm. The excitation wavelength was set at 335 nm, and the excitation and emission slit widths were set to 10 and 2.5 nm, respectively. Plotting I1/I3 (I1 and I3 are the intensity of the first and third peak in the fluorescent spectra of pyrene) as a function of surfactant concentration, and the cmc was taken as the peak of the first derivative. (g) pH/Conductivity. The pH and conductivity of solutions was monitored by a Sartorius basic pH-meter PB-10 and an FE30 conductometer (Mettler Toledo, USA) at 25 °C, respectively, and the average values were calculated from three repeats. (h) NMR and ESI MS. 1H NMR and 13C NMR spectra were recorded on a Bruker Avance 400 spectrometer at 400 MHz at room temperature. 77Se NMR spectra was recorded on an Agilent DD2 600 spectrometer at 600 MHz at room temperature. Chemical shifts are expressed in ppm downfield from TMS as internal standard. ESI-MS spectra were obtained with the Bruker Daltonics Data Analysis 3.2 system. For NMR measurement, a mixture of CD3OD and D2O (volume ratio =1:2) was used as the solvent. CO2saturated sample was prepared through bubbling CO2 into the initial sample for more than 30 min, and the oxidized sample was prepared through the addition of equimolar H2O2 into the initial sample and equilibrated for 24 h. The presence of a little water originated from 30 wt% H2O2 does not interfere the characterization. RESULTS AND DISCUSSION Redox and CO2-responsive behaviors of SeTA Molecule As is well known, tertiary amine is a typical CO2-sensitive group,18,38-41 while selenium atom has been recently
ACS Paragon Plus Environment
2
Page 3 of 11
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
demonstrated to be very sensitive to reactive oxygen.23,34-36 Thus, 11-(benzylselanyl)-N,N-dimethylundecan-1-amine (SeTA), featuring tertiary amine group and selenium atom together, was expected to be responsive for CO2 and redox. To this end, the evidence at the molecular level was firstly sought using spectroscopic techniques and the conductivity measurement. Before exposure to CO2, the conductivity of an aqueous solution of SeTA (5×10-3 mol·L-1) was as low as 3.95 µS·cm-1 (Figure S3). That is to say, SeTA was in a nonionic state at this point. Upon bubbling CO2 at 25 oC with a fixed rate of 40 mL·min-1, the conductivity rapidly increased and achieved an equilibrium value (ca. 97.7 µS·cm1 , Figure S3) in less than 5 min, indicating the production of charged species, i.e., the protonation of the tertiary amine in SeTA. When CO2 was expelled by bubbling N2 through the mixture, the conductivity decreased accordingly. By comparing the 1H NMR spectra (Figure 1) of SeTA before and after bubbling CO2, one can easily find that the signals of those protons (-CH2-N-(CH3)2) neighboring an N atom evidently shifted down field, further reflecting the protonation of the terminal tertiary amine in SeTA. Furthermore, the product of the protonation of SeTA was confirmed to be ammonium hydrogen carbonate38,42 based on 13 C NMR spectroscopy (Figure S4). Such a bicarbonate was so unstable that its solid sample cannot be obtained, even freeze-dried. This implies that it is more readily deprotonated. In a word, SeTA can be switched to its surface-active form (cationic ammonium, SeTA-CO2, Scheme 1) by exposure to water and CO2 at 25 oC, and then switched back to its non-surface active form (neutral tertiary amine, SeTA, Scheme 1) by bubbling N2 through the mixture at the same temperature, just like other tertiary amines.16,38,39
Figure 1 1H NMR spectra of SeTA, SeTA-CO2, and SeTA-Ox.
On the other side, as exhibited in the 1H NMR spectra (Figure 1), when an equimolar amount of H2O2 was added and equilibrated, four groups of proton signals neighboring the selenium atom all underwent a shift towards the low field region, reflecting a more polar microenvironment. Specifically, the peaks assigned to Ph−CH2-Se- and Ph−CH2−Se-CH2 split into two symmetrical doublets, which is generally ascribed to the formation of a selenoxide group (PhCH2Se=O).23,34-36 Moreover, the selenoxide struc-
ture was also revealed from the changes in the 77Se NMR (Figure S5) and the ESI-MS spectra (Figure S6). The insert of one oxygen atom provides a hydrogen bond site, increasing the hydrophilicity, and leads to a local polar environment. As a result, the signals of the protons next to the Se atom shift down field. Therefore, SeTA is also redox-sensitive, switching between non-polar selenide (SeTA) and polar selenoxide (SeTA-Ox) forms upon alternately adding an oxidant and a reductant (Scheme 1). This variation in molecular polarity can also be confirmed by an increase in the critical micelles concentration (cmc, Figure S7) after oxidization. In addition, it is noteworthy that the redox-responsive behavior of SeTA is closely related with the dosage of oxidant or reductant and the local environment. For an aqueous solution of SeTA, H2O2 can easily interact with selenium atom, and then oxidize it. Upon increasing the dosage of H2O2 from 1.5 to 10 molar equivalents, the time of oxidization can be greatly shorted from 4 h to 20 min (Table S1). However, when SeTA is located on the oil/water interface, the oxidization procedure may be slow. This is mainly because H2O2 and selenium atom are distributed in two distinct phases, respectively. Thereby, their collision gets more difficult, leading to a low reaction rate.36 All the above results implied that SeTA can reversibly respond to redox and CO2. The question then raises to whether smart responsive behavior of SeTA can be transferred to emulsion? Preparation and CO2-responsive behaviors of Pickering emulsion As shown in Figure 2A, in the absence of CO2, the mixture of SiO2 (0.5 wt%) and SeTA (1×10-4 mol·L-1) did not emulsify paraffin oil after homogenization, as well as that happened with only SeTA or bare SiO2 (Figure S8), implying that there are no synergistic effects between them. However, when bubbling CO2 into the forementioned sample, a very stable emulsion stabilized by SiO2-SeTA-CO2 was formed after homogenization (Figure 2B), while neither single SeTA-CO2 nor bare SiO2 could stabilize the emulsion (Figure S8). On the basis of a staining method using oil-soluble Nile red as a fluorescence probe, the emulsion was confirmed to be an oil-in-water type (Figure 2I), and the drop test also done (Figure 2K, L). When the emulsion was freshly prepared via homogenization, it creamed and thus a little water occurred on the bottom of the bottle (Figure 2B). Nevertheless, the creaming quickly stopped within 5 min, and the volume of the emulsion almost remained constant over 3 months (Figure S9), indicative of high stability. In other words, the emulsion did not coalesce, which is different from common emulsions stabilized by a surfactant at high concentration (Figure S10). For example, 1×10-2 mol·L-1 SeTA-CO2 (far above the cmc, 4.08×10-4 mol·L-1, Figure S7) only stabilized the emulsion for less than 5 min, and after oxidization with H2O2 only a transient emulsion (demulsification occurs within several seconds) was formed. This suggests that a high concentration of surfactant without the silica nanoparticles can only be used to prepare a conventional emulsion. The high stability can also be demonstrated by the
ACS Paragon Plus Environment
3
Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
constant droplet size of the emulsion. The light diffraction data showed that three months after preparation, the average droplet size of the emulsion, ca. 73 µm (Figure S11), only showed a slight increase and a narrower size distribution with increasing time due to Ostwald ripening.43 Furthermore, the micrographs of the droplets (Figure 3B, D) between 24 h and 3 months after preparation also show no pronounced differences, and the droplet size was close to that obtained by light diffraction. Such an emulsion with high stability may be stabilized mainly by the surfactantmodified silica nanoparticles rather than by a single ammonium surfactant,2 i.e., Pickering emulsion.
Figure 2 The reversible responsive behaviors of oil-in-water Pickering emulsion stabilized by SiO2 (0.5 wt%) and SeTA (1×10-4 mol·L-1). (A, B) CO2/N2 cycles with initial oil; (C-G) CO2/N2 cycles with new oil; (A, H) redox cycles with H2O2 and Na2SO3; (I-L) emulsion type.
Figure 3 Micrographs of the droplets of oil-in-water emulsion stabilized by 0.5 wt% silica nanoparticles and SeTA-CO2. The concentration are (A) 5×10-5 mol·L-1, (B, D) 1×10-4 mol·L-1, and (C) 1×10-3 mol·L-1, respectively. (A-C) were taken 24 h after preparation and (D) was taken 3 months after preparation.
This comparable stable oil-in-water Pickering emulsion could be obtained in a very wide concentration range of SeTA-CO2 from 5×10-6 mol·L-1 to 1×10-3 mol·L-1 (Figure S9), nearly three orders of magnitude. The lowest value was no more than 1/80 cmc of SeTA-CO2, while the highest concentration in the present experiment was about 2.5 times cmc. On the view of appearance, the stability of emulsion at high concentration is obviously better than that at low concentration. Moreover, the light diffraction data (Figure S11) and micrographs of the droplets (Figure 3)
Page 4 of 11
both revealed that the droplet size decreased with increasing the concentration of surfactant, reflecting an increase in the stability. Interestingly, when CO2 was removed upon bubbling N2 through the mixture under the same conditions, the forementioned CO2-induced oil-in-water Pickering emulsion was quickly demulsified and separated into two phases within 10 min (Figure 2A, C). A stable emulsion did not appear again even if re-homogenization. However, when re-exposed to CO2 followed by re-homogenization, the paraffin oil was emulsified once again, reforming the Pickering emulsion. With alternately bubbling CO2 and N2 at 25 o C, the emulsion can be reversibly switched “on (stable)” and “off (unstable)” several times, without any obvious deterioration in its macroscopic appearance and microscopic droplet size (Figure 4).
Figure 4 Size distribution of oil-in-water emulsion stabilized by 0.5 wt% silica nanoparticles and 1×10-4 mol·L-1 SeTA-CO2 undergoing three CO2/N2 cycles. (A) oil was not renewed, and (B) removing the original oil and adding fresh oil.
More importantly, after demulsification with N2, bubbling CO2 without homogenization could make a clearer oil/water interface because of the sedimentation of white floccules comprised of silica nanoparticles (Figure 2D). Such a weak flocculation was believed to be favorable for stabilizing the emulsion.44 When the upper oil was completely removed, the residual water phase could be reused in the preparation of the Pickering emulsion with fresh paraffin oil (Figure 2E-G). The droplet size remained essentially unchanged after such two cycles (Figure 4), exhibiting excellent repeatable switchability. Furthermore, the interfacial tension between the separated oil after demulsification and pure water was very close to that between fresh oil and pure water (Table 1). This implies that the surfactant was almost completely transferred into the lower water phase after demulsification. Removal of the upper oil phase did not lead to a decrease in the surfactant content in the lower water phase. Consequently, the residual water phase after demulsification can theoretically be repeatedly used to emulsify new oil many times. This facilitates recycling of the Pickering emulsifier composed of silica nanoparticles and CO2-switchable surfactant, making it a green, and energy-saving process. Table 1. Interfacial tension between fresh oil or separated oil after demulsification and pure water at 25 oC. Oil Interfacial tension (mNm-1) Fresh paraffin oil 38.48 1st separated paraffin oil 38.53
ACS Paragon Plus Environment
4
Page 5 of 11
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
2nd separated paraffin oil 3rd separated paraffin oil
Langmuir 38.55 38.33
Redox-responsive behaviors of Pickering emulsion Besides the CO2-responsive behavior, a redoxresponsive of Pickering emulsion was performed. When SeTA was in its reduced state, it was poorly soluble in water because of its strong hydrophobicity and behaved as an oily paste, floating on the water surface. Thereby, SeTA did not emulsify a mixture of paraffin oil−water alone (Figure S8). After the addition of H2O2, the hydrophilicity of SeTA increased due to the formation of selenoxide, but it still did not stabilize the emulsion, even at a high concentration, such as 1×10-2 mol·L-1 (Figure S10), because of the high oil/water interface tension (Figure S12).
and the droplet size remained unchanged essentially. Namely, the Pickering emulsion without CO2 can be also switched “on (stable)” and “off (unstable)” via a redox reaction with the addition of a trace amount of H2O2 or Na2SO3. Note that the processes of oxidization or reduction usually need several hours. A higher concentration of H2O2 or Na2SO3 can speed up the processes.
Figure 6 Effects of (A) oxidization time and (B) H2O2 dosage on the droplet size of emulsion stabilized by SiO2-SeTA-CO2, and the micrographs of emulsion after addition of 2 equimolar H2O2 for (C) 1 h, (D) 5 h and (E) 24 h.
Figure 5 Photographs and the corresponding micrographs of paraffin oil-in-water emulsion stabilized by 0.5 wt% silica nanoparticles and SeTA-Ox taken one month after preparation. (A) 1×10-5 mol·L-1, (B) 1×10-4 mol·L-1, and (C) 1×10-3 mol·L-1.
Nevertheless, when a trace amount of H2O2 (0.0008 wt% of total mass, 5 equimolar) was added to the SiO2-SeTA system (0.5 wt% SiO2 and 1×10-4 mol·L-1 SeTA) and equilibrated for 3 h, a very stable Pickering emulsion was fabricated after homogenization (Figure 2H). The drop test demonstrated that the emulsion is still oil-in-water type (Figure S13). Similarly, the H2O2-induced Pickering emulsion could be formed in a wide concentration window of SeTA-Ox from 1×10-5 to 1×10-2 mol·L-1 (Figure 5). From the micrograph observation and light diffraction data (Figure S14), one can see that the droplet size decreased from 250 to 45 µm upon increasing the concentration of H2O2, just like that observed in the SiO2-SeTA-CO2 system. In a word, the Pickering emulsion can still be formed in the absence of CO2. It must be pointed out that the pH did not show any obvious variation after the addition of H2O2 (Table S2). This indicates that H2O2 does not result in the protonation of SeTA. Namely, there are no cationic species in the dispersion. Thus, the mechanism of stabilization in the presented emulsion may be different from that of the CO2induced Pickering emulsion. As a trace amount of Na2SO3 (0.03 wt% of total mass, 5 equimolar) was added and equilibrated for 12 h, the emulsion demulsified and some white flocculate was suspended in the dispersion. A stable Pickering emulsion does not form after re-homogenization. On the contrary, the Pickering emulsion can be reformed when H2O2 was added again,
Additionally, when a trace amount of H2O2 (
