CO2-Switchable Pickering Emulsion Using Functionalized Silica

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CO2-Switchable Pickering Emulsion Using Functionalized Silica Nanoparticles Decorated by Amine Oxide-based Surfactants Yongmin Zhang, Xiaofei Ren, Shuang Guo, Xuefeng Liu, and Yun Fang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04162 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 6, 2018

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ACS Sustainable Chemistry & Engineering

CO2-Switchable Pickering Emulsion Using Functionalized Silica Nanoparticles Decorated by Amine Oxide-based Surfactants Yongmin Zhang,* Xiaofei Ren, Shuang Guo, Xuefeng Liu and Yun Fang Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School of Chemical & Materials Engineering, Jiangnan University, No.1800 Lihu Avenue, Wuxi 214122, P. R. China.

E-mail: [email protected] Abstract Herein, we describe a novel CO2-switchable oil-in-water Pickering emulsion stabilized by functionalized silica nanoparticles with a trace amount of myristylamidopropyl amine oxide (C14PAO), which is commercially available, and readily biodegradable. C14PAO in the current system has been demonstrated to be CO2-responsive. Upon alternately bubbling CO2 and N2 under mild conditions (30 oC, 40 mL·min-1), C14PAO is reversibly switched between cationic and nonionic forms, and is thereby adsorbed on or desorbed from the surface of the particles. In this way, interfacially active particles are formed and adsorbed on the surface of oil droplets, stabilizing the emulsion (CO2), or disrupted and desorbed from the surface of oil droplets, breaking the emulsion (N2). Compared with the traditional acid/base cycle, switching the current system with CO2/N2 multiple times does not lead to any evident changes in either macroscopic appearance or microscopic size. Moreover, this CO2-responsive Pickering emulsifier can be recycled when fresh oil was added after removing the original oil, and theoretically the cycling can be maintained, conforming to the principle of green and energy-saving processing. It offers a green, efficient, and recyclable container for oil product transportation, especially in high temperature area. Such a strategy is also suitable for other amine oxide-based surfactants, and does not require complicated organic synthesis. Keywords: CO2-switchable, Pickering emulsion, amine oxide-based surfactant, recyclable, silica nanoparticles, pH-responsive

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Introduction Emulsions, consisting of immiscible liquid phases, play an important role in a number of industrial processes and commercial products.1,2 Conventionally, emulsions are stabilized by surfactants or amphiphilic polymers through their adsorption at the oil/water interface. However, the cost of surfactants or polymers is typically high because of the high dosage, and the recovery of such emulsifiers is not practical in most circumstances.3 Moreover, the potential tissue irritation and even cell damage of surfactants restrict their use in biomedical applications. Pickering emulsions stabilized by colloidal particles retain most of the basic properties of conventional emulsions, while allowing a dramatic decrease in the dosage of surfactant, even to zero. In addition, the dense particle film at the oil/water interface formed by colloidal particles provides a strong mechanical barrier to coalescence,4 making Pickering emulsions more stable than that of conventional emulsions. Long-term emulsion stability is very critical for food storage and cosmetic formulations.1,3,5 However, in other cases, high stability may be problematic, because only temporary stability is desired in some applications, including emulsion polymerization,5 interfacial catalysis,6 drug delivery,7 and so on. Therefore, how to strike a balance between long-term stability and rapid demulsification of an emulsion has become a key focus with regard to Pickering emulsions. The emergence of stimuli-responsive Pickering emulsions has heralded a new dawn. On the one hand, long-term stability of the emulsion can be retained without a stimulus, and then the system can be quickly destroyed upon exposure to a stimulus, simplifying the demulsification process and enabling remote control of the emulsion characteristics. On the other hand, the emulsifiers can be reused, thus favoring more sustainable operation, conforming to the principles of Green Chemistry.8 The switching on and off of Pickering emulsions has hitherto been extensively studied using single stimuli, such as pH,9-12 temperature,13-16 light,17,18 magnetic fields,19,20 redox,21 as well as their combinations, such as temperature-magnetic field,22,23 pH-magnetic fields,24,25 or pH-temperature.26-28 Over the past decades, CO2 as an alternative means of controlling pH has attracted considerable attention in the fields of surfactants and their aggregates,29-40 polymers,41-43 solvents,44 as well as microemulsions,45,46 because of its renewability, low cost, and good biocompatibility. Essentially, CO2 acts as a pH control with advantageous features.37,47 Recently, focus has shifted to the utilization of CO2 as a trigger for switching Pickering emulsions between “on” (stable) or “off” (unstable) states.24,48-53 To the best of our knowledge, CO2-sensitive groups are confined to amidine and tertiary amine moieties for Pickering emulsions stabilized by both inorganic nanoparticles and polymer particles. Before bubbling CO2, they are electroneutral, and then change into cations with positive charge. It is these cations that generally lead to a potential risk of tissue irritation and cell damage.5 Thus, constructing CO2-responsive Pickering emulsions using novel and less potentially irritating CO2-sensitive groups is highly desirabled. Amine oxides with long hydrophobic tails are a particularly interesting class of zwitterionic surfactants, largely because of their small but highly polar headgroups. Various research groups have demonstrated that amine oxide surfactants are of low aquatic toxicity, show low irritation to human skin, and are readily biodegradable,54-57 and thus they are widely used in detergents, and toiletries. More importantly, the hydrophilic heads of amine oxide surfactants can be switched


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between nonionic and cationic states upon adjustment of pH, indicating that they may be sensitive to CO2 stimulus. However, few efforts to date have been devoted to studying the CO2-responsive behavior of amine oxide surfactants. Last but not least, according to Garcìa54 and Yamane,58 the introduction of a soft amide group in the molecular structure contributes to better biodegradability and lower eco-toxicity. Herein, we report the fabrication of a CO2-switchable Pickering emulsifier based on silica nanoparticles and a novel CO2-responsive surfactant, myristylamidopropyl amine oxide (C14PAO, Scheme 1), which can reversibly change between cationic and nonionic forms upon alternately bubbling CO2 and N2 at room temperature. As a result, the emulsifier is switched “on” (active) and “off” (inactive), leading to emulsification and demulsification of the Pickering emulsion.

Scheme 1. Reversible transformation between myristylamidopropyl amine oxide (C14PAO).

non-ionized

and

ionized

forms

of

Experimental section Materials Silica nanoparticles dispersion (SiO2, LUDOX AS-40) was purchased from Sigma-Aldrich and used without further treatment. According to the information from the supplier, it contains 40 wt% of silica as an aqueous dispersion at pH 9.1 of density 1.295 gcm-3 at 25 oC; the particle size is approximately 30 nm obtained from the data of dynamic light scattering; the specific area of the silica is 135 m2g-1.59 N-decane (purity ≥ 98%) and all other chemicals were analytically pure and purchased from Sinopharm. Triply distilled water with a resistance of 18.2 MΩ·cm-1 and a pH of 6.88 by a quartz water purification system was used in all the measurements. Amine oxide-based surfactants were synthesized and recrystallization with acetone following a previously reported procedure.56 Briefly, 50 mmol N-myristamidopropyl-N,N-dimethylamine was dissolved using 80 mL ethanol in a 250-mL three-neck flask, and then 80 mmol H2O2 (30 wt%) was added dropwise at 55-60 oC over thirty minutes under stirring. The reactants were refluxed at 79 °C for about 12 h, then cooled to room temperature and desired Na2SO3 was added to consume the unreacted H2O2, followed by filtering to remove the salt. The filtrate was evaporated off under reduced pressure, then was re-crystalized with acetone for three times. C14PAO was obtained as white power. FT-IR (Figure S1): ν% (cm-1) = 3309 (st, N-H), 1558 (δ, N-H), 1639 (st, C=O), 2918 (st, -CH3), 2848 (st, -CH2), 955 (st, N-O). 1H NMR (400 MHz, D2O, Figure S2), ppm: 0.74-0.77 (t, J=6Hz, 3H), 1.18 (s, 20H), 1.48 (s, 2H), 1.91-1.95 (t, J=8Hz, 2H), 2.11-2.14 (t, J=6Hz, 2H), 3.09 (s, 6H), 3.16 (s, 2H), 3.22-3.26 (t, J=8Hz, 2H). ESI-MS (Figure S3): calcd: 328.31. Found: 329.4 (M+H+). Preparation of Pickering Emulsions Emulsions were prepared by homogenizing the mixture of aqueous phase with either surfactant or SiO2 nanoparticles or both (3 mL) and n-decane (4.5 mL) 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 mmoles per liter (mM) relative to the water phase, respectively. The concentration of SiO2 is fixed at 0.5 wt%. All the resulting emulsions were



stored for more than one month at 30 °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 30 oC. It must be pointed out that CO2 was bubbled after mixing SiO2 and C14PAO other than bubbling CO2 to C14PAO and then mixing them with SiO2. Characterization of Pickering Emulsions The type of emulsion was judged on the basis of the drop test,49 and staining method21 using oil-soluble Nile red as fluorescence probe on a fluorescence microscope (Nikon 80i, excitation wavelength 540 nm), respectively. 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 water. Measurements (a) Zeta potential. The zeta potential of 0.5 wt% SiO2 nanoparticles dispersion using surfactant solution as solvent was determined using a ZetaPLAS instrument (Brookhaven, USA). The samples were equilibrated at 30 oC no less than 12 h. (b) Contact angle. The dispersion of 0.5 wt% SiO2 nanoparticles in surfactant solution was equilibrated at 30 oC for 24 h, and then was dropped on the surface of glass slide, forming a coating. After freeze-drying, the contact angle of pure water on the coating of SiO2 nanoparticles 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 at equilibrium concentrations less than the cmc was determined by the depletion method. The equilibrium concentration of surfactant in a series of 0.5 wt% silica nanoparticles dispersion after adsorption for one day was calculated on the basis of 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 30 ± 0.1 oC. The average area per molecular on the silica surface (a) can be calculated using the following equation:

a = ⁄Γ

(1)

where a0 is the specific surface area of silica nanoparticles, N is Avogadro’s number, and Γ is the adsorbed amount of surfactant on the surface of silica. (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:

wt% adsorbed =

× 100%

(2)

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 n-decane after demulsification) was determined at 30 oC by spinning drop technology on SVT20N spinning drop interfacial tensiometer (Germany,


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Dataphysics). (f) Critical micellar concentration (cmc). The cmc of surfactant was determined by fluorescent spectrometry as described elsewhere.36 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/conductivity of surfactant solutions with bubbling CO2was monitored by a Sartorius basic pH-meter PB-10 and a FE30 conductometer (Mettler Toledo, USA) at 30 °C, respectively. The average values were calculated from three repeats. The CO2 or N2 gas flow rate was fixed at 40 mL⋅min-1. The conductivity of pure water under bubbling CO2 at 30 °C was also determined as a reference. Results and discussion CO2-responsive behavior of amine oxide-based surfactant CnPAO As is well known, the headgroups of an amine oxide-based surfactant can be switched between nonionic and cationic states upon changing the pH. Although essentially CO2 acts as a controller of pH, it offers advantageous features as a trigger, in that it successfully avoids contamination and accumulation of solvents and does not produce any by-product in the system. The question then raises to whether amine oxide-based surfactants are CO2-responsive? Exhibited in Figure 1 is the variations of the conductivity and pH of a 10 mM aqueous solution of C14PAO with bubbling CO2. Before bubbling CO2, the conductivity of the C14PAO solution was about 12 µS·cm-1, very close to that of pure water, indicating that the C14PAO was scarcely ionized. Accordingly, the pH of the C14PAO solution (7.0) was the same as that of pure water, reflecting its neutral state. Based on the pKa of C14PAO (4.99, obtained by potentiometric titration, Figure S4) and pH of solution, the fraction of C14PAOH+ was estimated to be about 0.98% according to Henderson−Hasselbalch equation60, in line with the conductivity. However, when CO2 was bubbled into the solution through a stainless steel needle at a fixed rate of 40 mL·min-1 at room temperature, both the conductivity and pH of the C14PAO solution showed evident changes. The conductivity showed a rapid increase in the initial few minutes and then levelled off at an equilibrium value of 90 µS·cm-1, well above that for pure water (30 µS·cm-1), the pH showed a dramatic decrease and then tended to a minimum of ca. 5.5. Whereas, the change in the conductivity after bubbling CO2 is relatively smaller than those of 46,49 + N,N-dimethyldodecylamine. At this moment, the fraction of C14PAOH was about 30.90%. This suggests that about 30.90% C14PAO has been transformed from a nonionic state to an ionic state after exposing to CO2. The results above implied CO2-responsiveness of the amine oxide-based surfactant. It can reversibly change between cationic and non-ionic forms upon the bubbling or removal of CO2 (Scheme 1), but its surface activity persists, in contrast to the transformation of previously reported CO2-responsive surfactants between active and inactive forms.29,36 In other words, the



amine oxide-based surfactant is CO2-responsive, not CO2-switchable. Note that bubbling CO2 did not cause all the C14PAO molecules to convert into C14PAOH+.

Figure 1. The variations of conductivity and pH values with bubbling CO2 for 10 mM C14PAO. Pickering emulsion stabilized by a mixture of silica nanoparticles and CnPAO Although the amine oxide-based surfactant has been demonstrated to be an excellent emulsifier with low toxicity and low irritation, a high surfactant concentration (>critical micelle concentration, cmc) is generally needed to forming a stable emulsion. In contrast, a Pickering emulsion stabilized by colloidal particles requires only a trace amount of surfactant. Constructing Pickering emulsion employing amine oxide-based surfactant and physiologically inert SiO2 nanoparticles is highly desirabled for some biomedical applications. As shown in Figure 2, before exposing to CO2, neither 0.5 wt% SiO2 nanoparticles, 0.2 mM C14PAO (nearly twice the cmc, Figure S5), nor their mixture could emulsify a mixture of n-decane and water with a volume ratio of 3:2 (Figure 2a-c). After saturation with CO2 at room temperature, however, neither 0.5 wt% SiO2 nor 0.2 mM C14PAO could emulsify a mixture of n-decane and water after homogenization (Figure 2d,e). A very stable emulsion was formed after homogenization of a mixture of 0.5 wt% SiO2, 0.2 mM C14PAO, n-decane, and water (Figure 2f). It should be noted that CO2 was bubbled after mixing SiO2 and C14PAO, rather than bubbling it into C14PAO prior to mixing with SiO2. A staining method using oil-soluble Nile red as a fluorescent probe and a drop test confirmed that the formed emulsion was of the oil-in-water type (Figure S6). When the emulsion was just produced through homogenization, it creamed and thus a little water appeared at the bottom of the bottle (Figure 2f). Nevertheless, the creaming ceased within 5 min, and the creamed water can’t be mixed again with the emulsion phase after simple shaking (Video S1). As a result, the volume of the emulsion remained constant over 3 months (Figure 3), indicative of high stability. In other words, the emulsion did not coalesce, in contrast to common emulsions stabilized by a surfactant. Moreover, the constant droplet size also confirmed the high stability. Light diffraction data clearly indicated that the average droplet size of the oil-in-water emulsion, 73 µm (Figure 2g), did not exhibit any significant change with time, except for a narrowing of the size distribution. Moreover,


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micrographs of the emulsion droplets (Figure S7) likewise showed no obvious difference between 24 h and 3 months after preparation, and the droplet size was very close to that obtained by the light diffraction method. In contrast, an emulsion stabilized by a high concentration of C14PAOH+ or C14PAO (10 mM, Figure S8) alone showed smaller mean droplet size. These observations consistently suggested that the described oil-in-water emulsion was stabilized mainly by surfactant-functionalized silica nanoparticles, rather than by the surfactant alone. In short, it was a Pickering emulsion, not a general emulsion.

Figure 2. Photographs of n-decane-in-water (3:2) emulsions stabilized by (a) 0.5 wt% SiO2 nanoparticles alone, (b) 0.2 mM C14PAO alone, (c) their mixture without CO2, and (d-f) the corresponding systems after bubbling CO2 taken at 24 h after preparation, and (g) size distribution of sample f. This CO2-induced and relatively stable oil-in-water Pickering emulsion could be formed over a very wide concentration range. As shown in Figure 3, in the absence of CO2, emulsion could not be formed after homogenization when the C14PAO concentration was below 2 mM, but could be formed at 5 mM. The micrographs revealed that no dispersed droplets appeared at low concentration (Figure S9), whereas numerous small spherical droplets appeared at 5 mM (Figure 4a). Note that the droplet size at 5 mM was very close to that of an emulsion stabilized by 10 mM C14PAOH+ or C14PAO alone (Figure S8), indicating that it may be a conventional emulsion or mixed system of general emulsion and Pickering emulsion at high surfactant concentration. When CO2 was bubbled in after mixing SiO2 and C14PAO, stable Pickering emulsions were formed in the concentration range of 0.01-0.5 mM after homogenization. Even after shortage for 3 months at room temperature, the volume of the emulsion remained almost constant (Figure 3c). The droplet size decreased with increasing C14PAO concentration, as verified by micrographs (Figure 4b-d). At concentrations in the range 1-2 mM, the emulsions formed after homogenization were unstable.



They underwent coarsening, coalescence, and finally complete phase separation, leaving white floccules at the oil/water interface (Figure 4e). On further increasing the C14PAO concentration to 5 mM, a stable emulsion was formed once more, but its droplet size (Figure 4f) was as small as that before bubbling CO2. Thus, it may be a conventional emulsion. Additionally, replacing C14PAO with one of its homologues, C12PAO or C16PAO, Pickering emulsions could still be formed after bubbling CO2 (Figures S10, S11).

Figure 3. Photographs of n-decane-in-water emulsions stabilized by 0.5 wt % silica nanoparticles and C14PAO. (a) before bubbling CO2, (b,c) after bubbling CO2, that were taken (a) 24 h, (b) one week and (c) three month after preparation, respectively.

Figure 4. Optical micrographs of n-decane-in-water emulsions stabilized by 0.5 wt % silica nanoparticles and C14PAO. (a) 5 mM C14PAO without CO2, (b) 0.02 mM C14PAO with CO2, (c)


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0.1 mM C14PAO with CO2, (d) 0.5 mM C14PAO with CO2, (e) 1 mM C14PAO with CO2, (f) 5 mM C14PAO with CO2, that were taken three month after preparation. The bars are 100 µm for b~e, and 50 µm for a and f.

Figure 5. Size distribution and photographs of n-decane-in-water emulsions stabilized by 0.5 wt % silica nanoparticles and 0.2 mM C14PAO with CO2/N2 cycles with original oil. Interestingly, when N2 was bubbled into the emulsion (0.2 mM surfactant, 0.5 wt% SiO2, 9 mL n-decane, and 6 mL water) to replace CO2 at 30 oC with a fixed flow rate of 40 mL·min-1, the aforementioned CO2-induced decane-in-water Pickering emulsion was quickly demulsified, and thereafter phase separation occurred in less than 30 min (Figure 5). At this point, a stable Pickering emulsion could not be prepared after homogenization. However, when N2 was displaced with CO2 under the same conditions, followed by re-homogenization, a stable Pickering emulsion was formed once more. Upon alternately bubbling CO2 and N2 at 30 oC, the current Pickering emulsion could be reversibly switched between “on” (stable) and “off” (unstable) more than three times without any discernible differences in both macroscopic appearance and microscopic size (Figure 5, S12). This fully reflects the advantages of using CO2 as a pH control, since it does not change the composition after one cycle. Of course, other common pH controls, such as HCl/NaOH, can also induce the transition between emulsification and demulsification for the presented system, and the results shown in Figure S13 support this conclusion. Nevertheless, after several cycles, the accumulation of inorganic salts gives rise to an increase in the droplet size, inferior stability, and even demulsification. Although the emulsion can remain relatively stable in wide range of NaCl, the size of droplets showed notable increase with the addition of NaCl (Figure S14), indicating that the emulsion becomes more unstable in the presence of electrolyte. It may be one main reason of the destabilization of emulsion after HCl/NaOH cycle. More importantly, the current Pickering emulsion can not only be reversibly switched on and off in situ with original oil, but also be triggered with new oil. As shown in Figure 6, when stable Pickering emulsion was demulsified by bubbling of N2, CO2 was bubbled but not homogenized. As a consequent, the system still remained oil/water biphasic. The separated upper oil phase could be completely removed using a syringe. The residual water phase could be further used in



emulsifying new oil under homogenization, forming a new Pickering emulsion. The new oil could be n-decane, or another alkane. After such two cycles, all of the macroscopic appearances features (Figure 6), the microscopic droplet size (Figure S15), and the morphology (Figure S12) of the emulsions were essentially unchanged, exhibiting excellent repeatable switchability. Furthermore, from the data listed in Table 1, it can be seen that the interfacial tension between the separated oil after demulsification and pure water was very close to that between fresh oil and pure water. 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 an evidently 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.

Figure 6. Photographs of n-decane-in-water emulsions stabilized by 0.5 wt % silica nanoparticles and 0.2 mM C14PAO with CO2/N2 cycles, in which original oil was removed and new oil was added. Table 1. Interfacial tension between fresh oil or separated oil after demulsification and pure water at 30 oC. Oil-water interfacial tension (mNm-1)

Oil Fresh n-decane 1st separated n-decane 2nd separated n-decane 3rd separated n-decane

47.63 47.79 47.64 47.60

Mechanism Analysis Next, we consider the underlying reasons for the interesting smart responsive behaviors shown by the present emulsion. To gain insight into the mechanisms, the zeta potential, pH, contact angle, and adsorption were further monitored. Initially, the SiO2 used here, with an isoelectric point of 2.64 (Figure S16), possesses a zeta potential of -29.32 mV in pure water (0.5 wt%), such that


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water can spread well on its surface, indicative of strong hydrophlicity. When C14PAO was added, the pH values of the dispersions were invariably above 9.0 because of the strong basicity of the SiO2 dispersion used here. Therefore, C14PAO was mainly non-ionized (99.99%). With increasing C14PAO content, the zeta potential did not show any obvious change over the whole concentration range (Figure 7), and the contact angle of water on the SiO2 surface showed only a slightly increase (by less than 5o, Figure 8). These results suggest that the addition of C14PAO did not change the surface characteristics of SiO2, including surface charge and hydrophobicity. In other words, the adsorption of C14PAO on the surface of SiO2 particles barely occurred, although hydrogen bonding theoretically existed between C14PAO and SiO2.56,61 Thereby, before bubbling CO2, the emulsion formed at high concentration (5 mM, Figure 3a) was stabilized by mainly surfactant itself other than nanoparticles.

Figure 7. Zeta potential of 0.5 wt% silica nanoparticles dispersed in aqueous C14PAO solutions as a function of C14PAO concentration. The inserted graph is the variation of zeta potential of 0.5 wt% silica nanoparticles dispersed in 0.2 mM C14PAO solutions with alternately bubbling CO2 and N2.

Figure 8. Contact angle of water on the surface of silica nanoparticles modified by C14PAO as a



function of C14PAO concentration, together with the selected photographs. However, after CO2 saturation, the pH values of the dispersions decreased to 5.55~5.72, very close to that of CO2-saturated 10 mM C14PAO solution (Figure 1). This implies that 27.54~18.62% C14PAO molecules have been converted into ionized C14PAOH+. With increasing the concentration of total surfactant (C14PAO+C14PAOH+), the zeta potential of SiO2 and surfactant mixture showed an evident increase accompanied by a reversion from negative to positive (Figure 7). Moreover, the contact angle of water on the SiO2 surface initially increased from ca. 9.6o at 0.01 mM to a maximum ca. 50o at 2 mM (Figure 8), and then decreased. From the combination of zeta potential and contact angle data, it can readily be concluded that cationic C14PAOH+ was adsorbed on the negatively charged SiO2 surface through electrostatic attraction, thereby changing the surface charge and hydrophobicity of the SiO2. When the concentration was below 1 mM, the charge on the SiO2 surface was not completely neutralized by C14PAOH+, preserving partial hydrophilicity and electrostatic repulsion between particles. Meanwhile, the adsorbed C14PAOH+ cations further increased the hydrophobicity of SiO2 nanoparticles through the hydrophobic tails. That is to say, the SiO2 modified by C14PAOH+ was rendered amphipathic, behaving like a solid surfactant. These particles irreversibly absorb on the surface of dispersed oil droplets, imparting the droplets with the charges, and thus the coalescence was hold back due to the short range electrostatic repulsion between droplets. As a result, a stable Pickering emulsion could be formed. At 1-2 mM, the surface charge of SiO2 was completely neutralized by C14PAOH+, leading to the disappearance of electrostatic repulsion between particles. Meantime, the hydrophobic tails of C14PAOH+ adsorbed on the surface of particles exposed to outside, forming a relatively hydrophobic film. At this moment, the interaction between particles was mainly dominated by the attraction originated from hydrophobic tails other electrostatic repulsion. Thus, the particles readily flocculated and emulsion stabilized by the particles was very unstable. Upon further increasing the concentration, the zeta potential passed through zero and continued to increase, and the contact angle started to decrease, implying increasing hydrophilicity. This indicated that a bilayer of surfactant started to form on the particle surface, coinciding with the inversion of zeta potential. The repulsion prevailed over the attraction once again, and stable emulsion occurred again. If the C14PAO concentration was fixed at 0.2 mM, with alternately bubbling CO2 and N2, the zeta potential of SiO2 could be reversibly switched between -11.8 mV and -27.5 mV (Figure 8), implying a transition of its surface between amphipathic and completely hydrophilicity. That is to say, the adsorption of surfactant on the SiO2 surface is CO2-controllable and reversible. However, it must be point out that the contact angle obtained by this current method is not the actual solid contact angle at the oil/water interface, but only reflects a variation of hydrophilicity-hydrophobicity. Additionally, this smart adsorption behaviors could be further confirmed by oil/water interface tension measurements (Figure 9, S17). In the absence of SiO2, the interfacial tension between n-decane and 0.05 mM C14PAO solution showed only a small change upon cyclically bubbling CO2 and N2. It increased from an initial value of 13.37 mNm-1 to 15.60 mNm-1 after saturation with CO2, and then reverted to 13.37 mNm-1 when CO2 was removed with N2. The amplitude of variation was very small for this one-component C14PAO solution, implying that C14PAO maintains good interface activity in the presence and absence of CO2. When 0.5 wt% SiO2 was added, the interfacial tension was about 13.51 mNm-1 before bubbling CO2, very close to that of


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the solution without SiO2. This suggests that the addition of SiO2 does not influence the interfacial tension. In other words, SiO2 does not change the effective concentration of surfactant in the absence of CO2, and thus there is no adsorption. Conversely, the interfacial tension increased rapidly with bubbling CO2, reaching a maximum of 28.88 mNm-1 after saturation with CO2, far higher than that obtained before bubbling CO2 or after bubbling CO2 without SiO2. This indicated a clear decrease in the effective concentration of surfactant in the system, which is mainly ascribed to the adsorption of cationic surfactant on the SiO2 surface. When CO2 was removed, the interfacial tension re-decreased to 13.51 mNm-1, reflecting an increase in effective surfactant concentration. The variation in interfacial tension confirmed that SiO2 can adsorb the protonated C14PAO in the presence of CO2, and then release it after bubbling N2, because of the loss of electrostatic attraction.

Figure 9. Variations of interfacial tension between n-decane and 0.05 mM C14PAO solution with alternately bubbling CO2/N2. Similarly, differences in surface tension between one-component C14PAO solution and a mixture with 0.5 wt% SiO2 also revealed the adsorption of protonated C14PAO on the SiO2 surface (Figure S18). The adsorption isotherm of the surfactant at the particle/water interface at equilibrium concentrations lower than the cmc could be determined on the basis of the depletion method, utilizing the same data. The results showed that the adsorbed amount increased with increasing surfactant concentration. For example, an adsorbed amount of 0.099 mmol·g-1 at 0.1 mM corresponded to a molecular area of 2.26 nm2/per molecule, much larger than the minimum area per molecule (Amin) at the air/water interface (Amin = 0.49 nm2/per molecule) estimated using the Gibbs adsorption isotherm.2 If we assuming the area per molecule at the air/water interface equals to that at the surface of silica, at this point, the coverage degree of surfactant on the surface of silica is ca. 21.65%. Consequently, it is believed that the surfactant molecules adsorbed on the SiO2 surface may be a monolayer, with the hydrocarbon chains protruding towards the water.4 In this way, the SiO2 nanoparticles were rendered partially hydrophobicity, making them interfacially activity. As a result, with increasing surfactant concentration, the hydrophobicity accordingly increased as verified by the change of contact angle (Figure 8), and the zeta potential increased (Figure 7) due to the neutralization of the surface charge of silica. Increasing concentration above cmc (for 0.2 mM), the adsorbed amount on the surface of the silica could not



be determined by the depletion method. However, from the reversion of zeta potential (Figure 7) and the occurrence of the maximum value in contact angle (Figure 8), it is indisputable that the adsorption of surfactant on the SiO2 surface has been a maximum for monolayer at 1-2 mM. At this point, SiO2 possesses relatively large hydrophobicity and electric neutrality, which did not stabilize the Pickering emulsion because of the disappearance of electrostatic repulsion, as verified by the macroscopic appearance (Figure 3). Further increasing concentration, a second layer or other aggregates of surfactants62 probably starts to be formed through hydrophobic interaction between tails, and the data also confirmed it (Figure 7, 8). Nevertheless, high surfactant concentration was not the focus of this work, and thus no more discussion. Generally, the formation of a Pickering emulsion mainly depends on the adsorption of interfacially active particles on the oil/water interface. In the initial stage of emulsion formation, creaming is usually inevitable. The adsorbed amount of SiO2 at the oil/water interface can be simply estimated through measuring the residual SiO2 mass in the aqueous phase derived from creaming. As listed in Table 2, before bubbling CO2, the adsorbed amount of SiO2 on the oil droplets surface was less than 3 wt% over the whole experimental concentration range. The surface coverage of Pickering emulsions (φ) can be simply estimated through the equation

φ=

! ×" #

× 100%

(3)

where M is the total mass of SiO2 in the emulsion, x is the mass fraction of SiO2 that adsorbed on the surface of oil droplets, s is largest area per bare SiO2 particle on surface of droplets (which is considered equivalent to the largest sectional area of spherical SiO2 particle), m is the mass per SiO2 particle, and St is the total interface area of oil droplets (assuming all the droplets in the emulsion to be sphere, and the radius is the central value obtained from light diffraction method). Therefore, in the absence of CO2, φ was 75%), indicating a compact interface film with high mechanical strength, and thus the emulsion showed high stability. In the presence of CO2, with increasing surfactant concentration, the adsorbed amount of SiO2 on the oil droplets surface and φ initially increased, reached a maximum at 0.2 mM, and then decreased, consistent with the change in stability of the macroscopic emulsion (Figure 3). Thus, the greater the adsorbed amount of SiO2 on the oil droplets surface, the more stable the emulsion. Essentially, the φ is difficult to approach 1 because of the electrostatic repulsion between particles. When the repulsion disappeared from the particles, emulsion would be demulsified as occurred for 1~2 mM C14PAO systems. Table 2. Percentage of SiO2 adsorbed at oil−water interfaces in n-decane-in-water emulsions stabilized by 0.5 wt % SiO2 in combination with C14PAO with or without CO2, obtained by measuring the mass of particles remaining in the aqueous phase after emulsification. Concentration (mM) 0.01 0.05

wt% adsorbed N2

CO2

1.33 2.67

53.14 65.17


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0.10 0.20 0.50

0.33 2.01 1.50

79.17 93.83 79.83

Combining all of the above results, a possible mechanism is proposed (Scheme 2). Regardless of the presence or absence of CO2, the 0.5 wt% SiO2 used here possess a lot of negative charge (-19.41 mV or -9.06 mV), and could be well dispersed in water (Figure 2a,d), showing excellent hydrophilicity. Thus, single SiO2 nanoparticles could not stabilize the oil-in-water emulsion. On the basis of pH and conductivity data, it has been confirmed that the amine oxide-based surfactant, C14PAO, can switch between cationic (C14PAOH+) and non-ionic (C14PAO) forms with the bubbling and removal of CO2, although the percent conversion is limited. As a surfactant, C14PAO alone can emulsify an oil/water mixture, forming a general emulsion, but the concentration must be far higher than its cmc. For example, 0.2 mM C14PAO or C14PAOH+ (nearly twice as much as its cmc) cannot stabilize an oil-in-water emulsion for more than 24 h (Figure 2b,e). Conversely, 10 mM C14PAO or C14PAOH+ (nearly 100 times of the cmc) can stabilize an oil-in-water emulsion for more than 24 h (Figure S8). However, such a high dosage may bring some other concerns including cost, environmental problems, and so on. Before bubbling CO2, when 0.2 mM C14PAO was mixed with 0.5 wt% SiO2, the pH was as high as 9.0, and thus non-ionic C14PAO (99.99%) prevailed the system. Theoretically, there was only hydrogen-bonding61, not electrostatic attraction, between SiO2 with negative charge and electroneutral non-ionic C14PAO. This hydrogen-bonding was too weak to absorb C14PAO molecules on the SiO2 surface. Thus, the contact angle and interfacial tension both showed very small changes (Figure 8, 9). In other words, the hydrophilicity of SiO2 remained extremely strong when it mixed with C14PAO (Figure 8), and SiO2 particles tended to be dispersed in the water phase rather than the oil/water interface (Scheme 2, Figure 2c). At this point, the adsorbed amount of SiO2 on the droplets was very small (Table 2), and the formed emulsion was unstable because of the lack of firm interfacial film formation. After bubbling CO2, the pH of the mixture (0.5 wt% SiO2 and 0.2 mM C14PAO) decreased to 5.72, accompanied by an increase in conductivity. This implies that 18.62% C14PAO was converted into cationic ammonium C14PAOH+. The cationic surfactant was first adsorbed on the SiO2 surface with negative charge through electrostatic attraction, as verified by the increase in zeta potential (Figure 7), oil/water interfacial tension (Figure 8) and the adsorbed amount of surfactant (Figure S18). More importantly, the electrostatic attraction caused C14PAOH+ to be adsorbed on the SiO2 surface in a given orientation: the hydrophilic cationic headgroup was anchored on the SiO2 surface, and the hydrophobic chain was stretched out. Thereby, the hydrophlicity of the particle was decreased by charge neutralization, and the hydrophobicity was increased by installing a hydrophobic hydrocarbon chain, as verified by contact angle measurements (Figure 8). With the aid of a trace amount of C14PAOH+ (ca. 75%) and remaining a relatively strong electrostatic repulsion, preventing coarsening and coalescence of the droplets. A stable Pickering



emulsion could be formed after homogenization. When CO2 was removed by bubbling N2, the C14PAOH+ with positive charge reverted to its non-ionic form, C14PAO, leading to loss of electrostatic attraction, and thus desorption from the SiO2 surface. The partially hydrophilic and partially hydrophobic interfacially active particles thereby disrupted, and the SiO2 particles reverted to their initial state with strong hydrophilicity. As a result, they moved away from the oil/water interface, resulting in demulsification. That is to say, the CO2-responsive behaviors of the amine oxide-based surfactant could be transferred to the SiO2 particles, constructing a CO2-swicthable Pickering emulsifier, akin to CO2-triggerrd amidine- or tertiary amine-based surfactants.48,49

Scheme 2. Illustration of CO2-switchable Pickering emulsion stabilized by silica nanoparticles modified by amine oxide-based surfactant. Conclusions In summary, we have reported for the first time the CO2-responsive behavior of a conventional amine oxide-based surfactant, myristylamidopropyl amine oxide (C14PAO). C14PAO has been used to modify hydrophilic SiO2 nanoparticles through CO2-controllable electrostatic attraction, affording a novel kind of CO2-switchable Pickering emulsifier. A Pickering emulsion based on C14 PAO-based emulsifier could be easily switched between “on” (stable) and “off”(unstable) by simply bubbling CO2 (5 min) or N2 (< 30 min) under the same conditions (30 oC, 40 mL·min-1). The conditions for switching were similar to those for an emulsion stabilized by a tertiary amine-based emulsifier,48 but milder than those for a system stabilized by an amidine-based emulsifier.49 However, due to the limited percent conversion of C14PAO after bubbling CO2, the optimal surfactant concentration in the current system was slightly higher than cmc, or those required in common CO2-switchable Pickering emulsions stabilized by ionic surfactant and nanoparticle. Compared with common pH regulation using acid/base, CO2/N2 triggering avoids accumulation of the products of neutralization, which is unfavorable for the stabilization of an emulsion.63 Indeed, experimental results have confirmed that the stability of an emulsion steadily deteriorates upon alternately adding HCl/NaOH (Figure S13), but not with CO2/N2. Moreover, recycling of a Pickering emulsifier composed of silica nanoparticles and CO2-responsive surfactant has been achieved in the current system for the first time. In addition, besides C14PAO, other amine oxide-based surfactants (for example, C12PAO, C16PAO, and alkyldimethylamine oxides, Figures S10, S11, S19) have also been confirmed to be applicable as modifiers for SiO2 nanoparticles, and analogous


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CO2-switchable Pickering emulsions can be formed. These amine oxide-based surfactants are commercially available, obviating the need for complicated organic synthesis, or expensive reagents. However, the low percent conversion of C14PAO after bubbling CO2 may be a flaw, and increases the difficulty of mechanism explanation. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (grant No. 21503094), the Natural Science Foundation (BK20150128) of Jiangsu Province, PR China. Supporting information Characterization of molecule: FT-IR, 1H NMR and ESI-MS, pH titration curve, critical micelle concentration, photographs, micrographs, size distribution of droplets, HCl/NaOH or NaCl induced changed in emulsion, adsorption isotherm. Notes and references 1.

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TOC

Recyclable Pickering emulsifier composed of low toxic, low irritation skin, readily biodegradable and green CO2-responsive surfactant and silica nanoparticles.


CO2-Switchable Pickering Emulsion Using Functionalized Silica

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