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CO2-Triggered Pickering Emulsion Based on Silica Nanoparticles and Tertiary Amine with Long Hydrophobic Tails Yongmin Zhang, Shuang Guo, Wentao Wu, Zhirong Qin, and Xuefeng Liu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03034 • Publication Date (Web): 26 Oct 2016 Downloaded from http://pubs.acs.org on November 3, 2016
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CO2-Triggered Pickering Emulsion Based on Silica Nanoparticles and Tertiary Amine with Long Hydrophobic Tails Yongmin Zhang,† Shuang Guo,† Wentao Wu,† Zhirong Qin‡ and Xuefeng Liu†* †
School of Chemical & Materials Engineering, Key Laboratory of Food Colloids and Biotechnology Ministry of Education, Jiangnan University, Wuxi 214122, P. R. China. ‡ 6 Zhejiang Zanyu Technology Co. Ltd., Hangzhou 310009, P. R. China. 4
ABSTRACT: Herein, we describe a strategy of fabricating CO2-triggered oil-in-water Pickering emulsion based on silica nanoparticles functionalized in situ by a trace amount of conventional CO2-switchable surfactant, N-(3(dimethylamino)propyl)alkyl amide (CnPMA). By alternately bubbling CO2 and N2 at a moderate conditions (30 oC, 80 -1 10 mL·min ), silica nanoparticles reversibly switch between amphipathic and hydrophilic as a result of the adsorption of ammonium (CO2) and the desorption of tertiary amine (N2). The emulsion can then be smart switched “on (stable)” and “off 12 (unstable),” along with homogenization, without needing cooling and heating. The switching of the current tertiary-based system is simple, moderate, and environmentally friendly, without contamination and the restriction of rigorous conditions. 14 The surfactant concentration window of the Pickering emulsion is closely related to the length of hydrophobic tail, and the upper limit is no more than 0.20 cmc of that of the corresponding ammonium surfactant. Such a strategy is also suitable for 16 commercial alkyl tertiary amines, without needing complicated organic synthesis. 8
INTRODUCTION
netic fields30,31 and redox reactions,32 have been extensively 50 used to trigger the stability of Pickering emulsions.
Because of its important role in numerous industrial processes (emulsion polymerization, metal cutting and cleanVery recently, much attention has been shifted to the 20 ing, synthesis of nanoparticles, etc.) and commercial prod52 utilization of CO2 as a trigger for inducing the transforucts (foods, cosmetics, paints, pesticides, etc.), emulsions mation of emulsions between “on” (stable) or “off” (unsta22 have been a focus both for theoreticians and experimental54 ble) states in light of actual needs.5,13-16,33-35 Although the ists.1 Nevertheless, in some practical applications such as function of CO2 as a trigger in an aqueous environment is 24 industrial extraction, or crude oil transport, only temporary 56 essentially the same as that of pH, the former shows more stability is usually needed. To achieve the transformation advantages over the latter because it is biocompatible and 26 of macro- or micro emulsions between stable and unstable 58 renewable; in particular, it contaminates neither the prodstates, many attempts have been made to develop emulsifiucts nor the media where they are introduced.36-38 In addi28 ers that activate and deactivate in response to external 60 tion, there is no accumulation of inorganic salt derived stimuli in recent years,2-6 including surfactants,6-9 polyfrom neutralization reaction when the uptake CO2 is dis10,11 30 mers and colloid particles.12-18 62 placed by an inert gas or air. Conventionally, surfactants and amphiphilic polymers 32 can adsorb at the oil/water interface and thus stabilize the From what has been reported so far, two strategies are emulsions. However, these emulsions, whether they are 64 generally employed to formulate CO2-triggered Pickering 34 responsive or not, are in fact thermodynamically metastable emulsions. One directly uses covalently synthetic CO214-16 systems and separate with time. More importantly, only 66 sensitive colloid particles. These particles are mostly 36 high concentration of surfactants (>critical micelle concenfunctional polymeric particles whose preparation is genertration, cmc) or polymers can efficiently stabilize the emul68 ally complicated, and emulsions that contain these particles 38 sions, making such emulsions expensive. are CO2-induced demulsification rather than CO2-induced 70 emulsification. Another route is to adsorb a CO2-sensitive As compared to emulsions stabilized by surfactants or molecule on the surfaces of commercially available inert 40 polymers, Pickering emulsions stabilized by surface active 72 inorganic nanoparticles (silica or calcium carbonate) by colloid particles are generally more stable because of the non-covalent interactions. Because the wettability/surface 42 formation of a dense particle film at the oil−water inter74 activity of the particles can be controlled by selecting the face, which provides a mechanical barrier to coales19,20 concentration and structure of the amphiphiles, the latter is 44 cence. Over the past few decades, stimuli-responsive 76 considered to be simpler and to have advantages over the Pickering emulsions have aroused considerable attention former. Cui and his coworkers fabricated a CO2-switchable 46 owing to their long-term stability and smart tunability. Up 13 78 Pickering emulsion based on SiO2 hydrophobized in situ to now, common stimuli such as pH,20-24 temperature,25,26 27,28 12,29 with a CO2-responsive surfactant, N’-dodecyl-N,N48 ultraviolet/visible (UV/vis) light, electrolytes, magACS Paragon Plus Environment 18
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dimethylacetamidinium bicarbonate, completing the transformation of the trigger from a surfactant to colloidal parti82 cles. However, the switching on and off of the emulsion is practically inconvenient because the CO2 has to be bubbled o 84 at 0~5 C for 50 min, and then removed out by bubbling N2 at high temperature (65 oC) for 80 min. Moreover, the syn86 thesis of such a switchable surfactant is complicated and expensive. These issues seriously limit the application of 88 such a system. Therefore, developing novel CO2-triggered Pickering emulsions that can be switched at room tempera90 ture in a short period of time utilizing conventional CO2responsive surfactants is highly desirable. 80
As commercially available industrial materials, tertiary amines with long hydrophobic tails have been demonstrat94 ed to be CO2-responsive, and their hydrogen carbonate can revert back to an initial non-protonated state under more 96 moderate condition in a shorter time as compared to amidine-based surfactants.38-41 Thus, herein we report that 98 silica nanoparticles functionalized in situ with a conventional tertiary amine-based CO2-switchable surfactant, N100 (3-(dimethylamino)propyl)alkyl amide (CnPMA, n=12~16, Scheme 1), in water can be reversibly controlled between 102 surface-active (cationic ammonium) and non-surface-active (neutral tertiary amine) forms by sequentially bubbling 104 CO2 and N2 at room temperature. As a result, the emulsions stabilized by such particles are switched “on” and “off”.
Emulsions were prepared by homogenizing the mixture of equal volumes (7 mL) of aqueous phase with either surfac134 tant or silica nanoparticles or both and n-heptane in a glass bottle at 5000 rpm for 90 s using an IKA Ultra-Turrax T-18 136 homogenizer. The particles and surfactant concentrations are expressed as weight percent (wt %) and moles per liter 138 (M) relative to the water phase, respectively. All the resulting emulsions were stored for more than one month at 140 25 °C to observe their stability. CO2 or N2 was bubbled using a stainless steel needle with a fixed rate of 80 -1 142 mL·min at room temperature. The pH and conductivity were monitored by a Sartorius basic pH-meter PB-10 and a 144 FE30 conductometer (Mettler Toledo, USA), respectively. 132
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Scheme 1. Reversible transformation between non-surfaceactive and surface active forms of N-(3(dimethylamino)propyl)alkyl amide (CnPMA).
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EXPERIMENTAL SECTION Materials
Silica nanoparticles dispersion containing 30 wt% of solid content was synthesized following the method reported in 114 reference [42], in which the primary particle diameter ≈ 100 nm, and its specific surface area is 19.79 ± 0.21 m2·g-1. 116 Scanning electron microscopy (SEM) images of the particles (Figure S1 in Supporting Information (SI)) confirm 118 the uniform morphology and particle size. Hydrophilic silica nanoparticle dispersions containing 0.3 wt% of solid 120 content were obtained by dilution of the stock silica nanoparticle dispersions (30 wt%) with pure water or surfactant 122 solution. 112
N-(3-(dimethylamino)propyl)alkyl amide (CnPMA, 124 n=12~16, HPLC purity ≥ 99%) was synthesized following a previously reported procedure,40,41 respectively. n126 heptane (purity ≥ 98%) was purchased from Admas. All other chemicals were analytically pure and purchased from 128 Sinopharm. Triply distilled water with a resistance of 18.2 MΩ·cm and a pH of 6.2 at 25 °C by a quartz water purifi130 cation system was used in all the measurements. Preparation of Pickering Emulsions
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Characterization of Pickering Emulsions The type of emulsion was judged on the basis of the drop test,5,6 and micrographs of the emulsion droplets were rec148 orded using a VHX-1000 microscope system (Keyence Co.). Particle size distributions were measured using a 150 Malvern Mastersizer 2000 instrument by dispersing a drop of emulsion in an aqueous solution containing the same 152 surfactant concentration as in the aqueous phase used for preparing the emulsion. 146
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Measurements
(a) Zeta potential. The zeta potential of 0.3 wt% silica nanoparticles dispersion using surfactant solution as solvent was determined using a ZetaPLAS instrument (Brookhaven, o 158 USA). The samples were equilibrated at 25 C no less than 12 h. 160 (b) Contact angle. The dispersion of 0.3 wt% silica nanoparticles in surfactant solution was equilibrated at 25 oC for 162 24 h, and then was dropped on the surface of glass slide, forming a coating. After freeze-drying, the contact angle of 164 pure water on the coating of silica nanoparticles was measured using optical contact angle measuring device 166 (Dataphysics OCA 40, Germany). (c) Adsorption isotherm. The adsorption isotherm of sur168 factant at the particle-water interface at equilibrium concentrations less than the cmc was determined by the deple170 tion method. The equilibrium concentration of surfactant in a series of 0.3 wt% silica nanoparticles dispersion after 172 adsorption for one day was calculated on the basis of the surface tension of the dispersion. The surface tension of 174 surfactant solution without silica nanoparticles is used as calibration. The surface tension was measured with a Krüss 176 K100 tensiometer by the automatic du Noüy ring model at 25 ± 0.1 oC. The average area per molecular on the silica 178 surface (a) can be calculated using the following equation: 156
a ⁄Γ where a0 is the specific surface area of silica nanoparticles, N is is Avogadro’s number, and Γ is the adsorbed amount of surfactant on the surface of silica. 182 (d) Critical micellar concentration (cmc). The cmc of surfactant was determined by fluorescent spectrometry as de40 184 scribed elsewere. (e) pKa of CnPMA. CnPMA dispersion was titrated with 10 186 mM hydrochloric acid and the pH continuously monitored with a Sartorius basic pH-meter PB-10. The pKa values 188 were obtained by taking the pH values at the mid-point 180
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between two pH jumps. 190
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RESULTS AND DISCUSSION
Pickering Emulsions by in Situ Functionalization of 192 Silica by Tertiary Amine-Based Surfactant As a long-chain tertiary amine with an amide group, CnPMA is a switchable surfactant39 that can be switched to its surface-active form (cationic ammonium) by exposure to o 196 water and CO2 at ambient temperature (30 C), and switched back to a non-surface active form (neutral tertiary 198 amine) by bubbling N2 or air at the same temperature (Scheme 1). Such transformations can be experimentally 200 confirmed by monitoring the variations in the conductivity of the CnPMA aqueous solution (Figure S2) and the chem202 ical shifts of hydrogen protons neighbouring to the tertiary amine group (Figure S3). 194
Without CO2, C14PMA (8×10-5 mol·L-1) cannot emulsify a mixture of heptane-water alone because of its strong lipo206 philicity and hydrophobicity (Figure 1a). Similarly, neither the bare silica nanoparticles (0.3 wt%) with an isoelectric 208 point at pH = 3.17 (Figure S4), which are typical commercially available inert inorganic nanoparticles, nor a 210 mixture of silica nanoparticles and C14PMA can emulsify a mixture of heptane-water after homogenization (Figure 1b 212 and 1c). After bubbling CO2 using a stainless steel needle with a fixed rate of 80 mL·min-1 for about 5 min at room 214 temperature, the pH values of these systems decreased from 10.24±0.1, 7.81±0.1, and 9.73±0.1 to 4.69±0.1, 216 4.99±0.1, and 5.43±0.3, respectively. According to Henderson–Hasselbalch equation,39 218 pK ⁄ , 99.96% and 99.79% of C14PMA molecules have been transformed into + 220 quaternary ammonium salt, [C14PMAH] HCO3 at this point. The formation of the cationic ammonium surfactant 222 could not stabilize a heptane-in-water emulsion on its own (Figure 1d) because of its low surfactant concentration -5 224 (8×10 mol·L-1, far lower than its critical micellar concentration, cmc ≈2.35×10-3 mol·L-1, Figure S5)，just like 1,6 226 conventional surfactants. Even when the concentration was more than the cmc (for example, 5×10-3 mol·L-1), the + 228 emulsion emulsified by [C14PMAH] HCO3 was still very unstable, and it achieved complete oil-water phase separa230 tion in less than 36 h, accompanied by the disappearance of emulsion droplets from the micrographs (Figure S6). Also, 232 for a silica nanoparticles-only system, the addition of CO2 only led to an increase of the zeta potential, but not to a 234 stable emulsion (Figure 1e). 204
without CO2, and (d-f) the corresponding systems after bubbling CO2 taken at 24 h after preparation.
However, in the CO2-saturated mixture, a very stable emulsion (Figure 1f) was obtained following homogenization, 244 and the type of emulsion was confirmed to be oil-in-water on the basis of the drop test (Figure S7). This emulsion 246 creamed, but did not coalesce. With increasing time, the average droplet diameter of the oil-in-water emulsion, 90 248 µm (Figure 2a) did not show any change except for the size distribution, indicating high stability. These droplets 250 were generally much bigger and more stable than those stabilized by [C14PMAH]+HCO3- alone at 5×10-3 mol·L-1 252 (Figure S6). This indicates that they may be stabilized mainly by surfactant-modified silica nanoparticles rather 20 254 than by single ammonium surfactant. The high stability was also proved by the comparing the micrographs of the 256 droplets (Figure 2b,c) between 24 h and 3 months after preparation. The droplets were similar in size to the aver258 age size obtained by light diffraction. Thus, the emulsion stabilized by CO2-saturated mixture was a Pickering emul260 sion, not a conventional emulsion. Note that an excess water phase occurred on the bottom of the bottle (Figure 1f), 262 and the volume of water did not vary over 3 months (Figure 3b), which may have been caused by the oil/water ratio. 264 When the oil/water ratio was fixed at 5:4, no excess water phase was observed (Figure S8). 242
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Figure 2. (a) Size distribution of the heptane-in-water (1:1) emulsion stabilized by a mixture of 0.3 wt % silica and -5 270 8×10 mol·L-1 ammonium surfactant [C14PMAH]+HCO3-, and (b, c) the selected micrographs of the droplets taken 24 272 h and 3 month after preparation. 268
Figure 1. Photographs of heptane-in-water (1:1) emulsions stabilized by (a) 8×10-5 mol·L-1 N-(3238 (dimethylamino)propyl)myristyl amide (C14PMA) alone, (b) 0.3 wt% silica nanoparticles alone, (c) their mixture 236
Figure 3 exhibits the influence of ammonium surfactant concentration on the Pickering emulsions. When the ammonium surfactant concentration was very low (below -6 276 5×10 mol·L-1, comparable to ~0.0020cmc of 274
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[C14PMAH]+HCO3-), or higher than 5×10-4 mol·L-1 (close + 278 to ~0.20 cmc of [C14PMAH] HCO3 ), stable Pickering emulsions were not obtained, even when the system was 280 saturated by CO2. On the other hand, in the concentration range of 0.002cmc-0.085cmc (a couple orders of magni282 tude), the CO2-saturated mixture stabilized the heptane-inwater emulsion for a very long time. In other words, the 284 concentration window of a Pickering emulsion stabilized by a CO2-saturated mixture of silica nanoparticles and + 286 [C14PMAH] HCO3 is wide, and the dosage is very low. Moreover, with increasing surfactant concentration, the 288 droplets decreased in size (Figure S9) and the emulsion increased in volume. Additionally, if replacing + 290 [C14PMAH] HCO3 with one of its homologues + [C12PMAH] HCO3- or [C16PMAH]+HCO3-, stable Picker292 ing emulsions can still be obtained in an appropriate concentration range. As shown in Figure 4, the concentration 294 window of the Pickering emulsion stabilized by 0.3 wt% silica nanoparticles and [C12PMAH]+HCO3- was the widest, -6 296 from 5×10 mol·L-1 (~0.00066cmc of [C12PMAH]+HCO3-, cmc≈7.63×10-3 mol·L-1, Figure S10) to 1×10-3 mol·L-1 298 (~0.13cmc); whereas the concentration window for [C16PMAH]+HCO3- was the narrowest, from 4×10-5 mol·L1 300 (~0.068cmc of [C16PMAH]+HCO3-, cmc ≈ 5.84×10-4 mol·L-1, Figure S11) to 1×10-4 mol·L-1 (~0.17cmc). Note 302 that all the upper limits of the concentration window were very close for [C12PMAH]+HCO3-, [C14PMAH]+HCO3- and + 304 [C16PMAH] HCO3 , as low as 0.20cmc of the ammonium surfactant. Of special interest is the fact that the concentra+ 306 tion of [C12PMAH] HCO3 needed to stabilize the emulsion was far smaller than that reported for the amidinium surfac-4 -3 308 tant with a C12 hydrophobic tail (0.05-3cmc, 1×10 -6×10 -1 13 mol·L ).
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ble and viscous emulsion was transformed into oil-water two phases after about 40 min, and a stable Pickering emulsion could not be prepared after homogenization. 330 However, when CO2 was further bubbled followed by rehomogenization, the Pickering emulsion formed once 332 again. Such changes of emulsion between stable and unstable by alternately bubbling CO2 and N2 could be easily 334 cycled more than eight times with no deterioration. This suggests that the Pickering emulsion stabilized by the mix336 ture of silica nanoparticles and CnPMA can be made CO2switchable. 328
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Figure 4. Photographs and selected optical micrographs of n-heptane-in-water emulsions stabilized by 0.3 wt % silica and ammonium surfactant (a) 342 nanoparticles [C12PMAH]+HCO3- or (b) [C16PMAH]+HCO3-, that were 344 taken three month after preparation. The surfactant concentration in water from (1) to (9) are 1×10-6, 5×10-6, 1×10-5, -5 -5 -4 -4 -3 -3 -1 346 4×10 , 8×10 , 1×10 , 2×10 , 1×10 , 9×10 mol·L . 340
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Figure 3. Photographs of n-heptane-in-water emulsions stabilized by 0.3 wt % silica nanoparticles and ammonium surfactant [C14PMAH]+HCO3-, that were taken (a) one 314 week and (b) three month after preparation. The surfactant concentration in water from (1) to (9) are 1×10-6, 5×10-6, -5 -5 -5 -4 -4 -4 -3 316 1×10 , 4×10 , 8×10 , 1×10 , 2×10 , 5×10 , 1×10 -1 mol·L . 312
More interesting, the Pickering emulsion stabilized by ammonium surfactant [CnPMAH]+HCO3- showed reversible 320 CO2-responsiveness. When the mixture of n-heptane (7 mL), water (7 mL), C14PMA (8×10-5 mol·L-1) and silica 322 nanoparticles (0.3 wt%) was exposed to CO2 for 5 min using a stainless steel needle with a fixed rate of 80 mL·min-1 324 at room temperature, a very stable Pickering emulsion was easily obtained after homogenization (Figure 5). When 326 displacing CO2 with N2 under the same conditions, the sta318
Figure 5. Variation of pH and zeta potential of 0.3 wt% silica nanoparticles dispersed in 8×10-5 mol·L-1 C14PMA 350 dispersion with alternately bubbling CO2 and N2, together with the photographs of the heptane-in-water emulsions 352 following switching on and switching off cycles taken 24 h after preparation. 348
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Note that with the emulsion cyclically switching between unstable and stable, the pH value of the emulsion or the 356 lower water phase accordingly decreased from 9.81±0.1 (before bubbling CO2) to 5.12±0.3 (after bubbling CO2). It 358 then increased to 9.84±0.1 after bubbling N2, and subsequently decreased to 5.18±0.2 following exposure to CO2 360 again, as shown in Figure 5. On the other hand, the zeta potential increased from an initial -45.89 mV to -18.75 mV 362 in the presence of CO2. It then decreased to -45.01 mV after removing CO2, and re-increased to -17.5 mV as CO2 364 was bubbled again. Both the pH and zeta potential indicate that the hydrophilicity-hydrophobicity of silica nanoparti366 cles may change with bubbling and removing of CO2. 354
Mechanism Analysis To gain insight into the mechanisms, the zeta potential, pH, contact angle, and adsorption were further monitored. Ini370 tially, the silica nanoparticles used here were negatively charged in water at pH > 3.17 (Figure S4), and a zeta po372 tential of -33.32 mV was measured in pure water of pH=6.34 (0.3 wt% silica nanoparticles). As exhibited in 374 Figure 6, however, when ammonium surfactant [C14PMAH]+HCO3- (>99%) was produced in situ in a 0.3 376 wt% silica nanoparticles solution via bubbling CO2, the zeta potential of the silica nanoparticles first showed a slow -4 378 increase below a critical value (2×10 mol·L-1), and then quickly increased, accompanied by a reversion from nega380 tive to positive. By contrast, the zeta potential remained almost unchanged with increasing C14PMA concentration 382 in the absence of CO2. This indicates that the surface charge of the silica nanoparticles was changed by adsorp384 tion of the cationic ammonium surfactant, but not nonionized C14PMA. Moreover, from the photographs shown 386 in Figure 6, one can see that the silica nanoparticles were dispersed well in the solution when the zeta potential was 388 much higher or lower than zero no matter whether they were negatively or positively charged; otherwise it separat390 ed out from the solution. In other words, the hydrophilicityhydrophobicity of the silica nanoparticles changed accord392 ing to the adsorption of the ammonium surfactant. 368
Figure 6. Zeta potential of 0.3 wt% silica nanoparticles dispersed in aqueous C14PMA solutions as a function of 396 C14PMA concentration, together the selected photographs. 394
The contact angle experiment further confirmed this evolution of the hydrophilicity-hydrophobicity (Figure 7). Initially, the contact angle of water on silica nanoparticles was 400 too small to be measured, implying its strong hydrophilicity. With increasing [C14PMAH]+HCO3- concentration that 402 was used to modify the silica nanoparticles, the contact angle of water on the particle surface initially increased o 404 from 33 at 1×10-5 mol·L-1 to a maximum 72o at 5×10-4 -1 mol·L , and then decreased. This means that the presence + 406 of [C14PMAH] HCO3 significantly changed the hydrophobicity of the particle surface, which is agreement with the 408 deduction made from the variation of zeta potential. When the [C14PMAH]+HCO3- concentration was below 5×10-4 -1 410 mol·L , the hydrophobicity of the silica nanoparticles increased, but they still retained some hydrophilicity. That is 412 to say, the silica nanoparticles were amphipathic. At a [C14PMAH]+HCO3- concentration of 5×10-4 mol·L-1, the 414 hydrophobicity of the silica nanoparticles was the strongest, and thus the silica nanoparticles separated out from the + 416 dispersion. Upon further increasing the [C14PMAH] HCO3 concentration, the contact angle started to decrease, imply418 ing the increasing of hydrophilicity of the silica nanoparticles surface. In other words, a bilayer of surfactant started 420 to form on the particle surface, coinciding with the inversion of zeta potential. Nevertheless, without treatment of 422 CO2 the contact angle of water on the particle surface did not show any obvious change with increasing C14PMA o 424 concentration, and was less than 18 over all the experimental concentration, exhibiting similar hydrophilicity to 426 silica nanoparticles. Combining the results of zeta-potential and contact angle in the absence of CO2, one can easily 428 deduce that there is no adsorption of non-ionized C14PMA on the silica surface and therefore no modification of the 430 properties of silica. 398
Figure 7. Contact angle of water on the surface of silica nanoparticles modified by C14PMA as a function of 434 C14PMA concentration. 432
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posed to water, endowing the silica nanoparticles with partial hydrophobicity, making them surface-active.
Combining all the results above, a possible mechanism is proposed (Scheme 2). In the absence of CO2, silica na484 noparticles or CnPMA alone cannot stabilize the oil-inwater emulsion because of their strong hydrophilicity or 486 hydrophobicity (Figure 1a,b), respectively. They dispersed in the water-phase and oil-phase (Scheme 2), respectively, 488 and there was no cationic ammonium surfactant in the system. When CO2 was bubbled at 30 oC at a fixed rate of -1 490 80 mL·min , more than 99% the non-ionized CnPMA was protonated after 5 min, and converted into cationic ammo+ 492 nium [CnPMAH] HCO3 . At this moment, ammonium surfactants would adsorb first on the silica nanoparticle sur494 face owing to the presence of electrostatic attraction, forming a monolayer film. Under the aid of a trace amount of + 496 [CnPMAH] HCO3 , strong hydrophilic silica nanoparticles with negative charge shifted to a partially hydrophobic 498 amphipathic solid surfactant, which was verified by the zeta potential, and contact angle. Such a partially hydro500 philic and partially hydrophobic amphipathic solid surfactant can efficiently adsorb at the oil-water interface. It thus 502 stabilized the oil-in-water emulsion in the form of a Pickering emulsion after homogenization. When N2 was bubbled o -1 504 instead of CO2 at 30 C with a fixed rate of 80 mL·min , the ammonium surfactants converted back to non-ionized 506 CnPMA and thus desorbed from the silica nanoparticle surface owing to the loss of electrostatic attraction. As a result, 508 the amphipathic silica nanoparticles reverted to strong hydrophilicity and a more negative charge, and thus they 510 moved away from the oil-water interface, resulting in demulsification. When CO2 was bubbled again, partially 512 hydrophobic silica nanoparticles were obtained once again, and a stable Pickering emulsion was formed 514 again after re-homogenization. In other words, the CO2-responsive behavior of tertiary amine can be 516 shifted to silica nanoparticles, thus forming a CO2 triggered Pickering emulsion. 482
Figure 8. Surface tension of aqueous [C14PMAH]+HCO3solutions (■) and 0.3 wt% silica nanoparticles dispersed in + 438 aqueous [C14PMAH] HCO3 solutions (□) as a function of + [C14PMAH] HCO3 concentration, and adsorption isotherm + 440 of [C14PMAH] HCO3 at the silica nanoparticle-water interface (▲) as a function of equilibrium [C14PMAH]+HCO3442 concentration. 436
Figure 8 shows the variation of surface tension and the adsorption amount of [C14PMAH]+HCO3- at the surface of the silica nanoparticles with increasing concentration. The 446 adsorption was quantified using the adsorption isotherm based on the depletion of [C14PMAH]+HCO3- in the solu448 tion by measuring the surface tension of [C14PMAH]+HCO3- in the absence or presence of silica When the concentration of 450 nanoparticles. [C14PMAH]+HCO3- was lower than 8×10-5 mol·L-1, the -2 452 adsorbed amount was very small (≤ 2.67×10 mmol·g-1), and the change of adsorbed amount with increasing con+ 454 centration was also very slow. As the [C14PMAH] HCO3 -5 -1 concentration was increased above 8×10 mol·L , the ad456 sorbed amount rapidly increased in a narrow concentration range. The adsorbed amount of 6.67×10-1 mmol·g-1 at -3 458 2×10 mol·L-1 corresponded to a molecular area of 0.49 2 nm /per molecule, which is a little less than that the mini460 mum area per molecule (Amin) at the air–water interface (Amin = 0.51 nm2/per molecule) estimated using the Gibbs 1 462 adsorption isotherm. At this point, the surface of the silica particle are completely dominated by the surfac464 tants monolayer or a partial bilayer. However, when the [C14PMAH]+HCO3- concentration was 8×10-5 mol·L-1, + 466 the average molecular area of [C14PMAH] HCO3 at the silica nanoparticle surface was approximately 1.23 nm2/per 468 molecule, which is much bigger than the Amin . Therefore, it is believed that the adsorption of the cationic ammo470 nium surfactant molecules at the silica nanoparticle surface forms a monolayer, with the hydrocarbon 20 472 chains protruding towards water. Because of the electrostatic attraction, positively charged ammonium ions 474 neutralized and linked with the negatively charged groups of silica nanoparticles via non-covalent bonds, 476 and thus adsorbed at the particle surface, forming a monolayer film comprising of [C14PMAH]+HCO3- at less -3 478 than 2×10 mol·L-1. The long hydrophobic tails of the ammonium surfactant adsorbed onto particle surfaces ex444
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520
Scheme 2. Illustration of CO2-triggered Pickering emulsion stabilized by silica nanoparticles modified by tertiary amine with long hydrophobic tail.
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CONCLUSION
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In summary, we developed a CO2-triggered Pickering oilin-water emulsion stabilized by silica nanoparticles in situ modified by a trace amount of a conventional CO2-
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responsive tertiary amine bearing long hydrophobic chains. Stabilization and destabilization of the current Pickering 528 emulsion can be easily achieved by simple bubbling CO2 (5 min) or N2 (40 min) under the same conditions (30 oC, 80 -1 530 mL·min ), which controlled the hydrophilicityhydrophobicity equilibrium and the adsorption of solid 532 surfactant at the oil-water interface. This condition of switching “on” and “off” was simple, moderate, and envi534 ronmentally benign, without worrying about changing the components, as is the case with other stimuli-responsive 11,19-22,30 536 systems, and the rigorous switching conditions in amidine-based CO2-triggered Pickering emulsion.13 More538 over, the concentration window was closely related to the length of the hydrophobic chain. The shorter the hydropho540 bic tail, the wider the concentration window. The upper limit of concentration was no more than 0.20 cmc for all 542 the tertiary amines, and the lower limit was far lower than the corresponding cmc. Therefore, the potential harm to the 544 environment is negligible as compared to emulsions stabilized solely by surfactants. Additionally, the tertiary amine 546 was not only confined on the current CnPMA, but also on the more common alkyl tertiary amine (for example N,N548 dimethyl dodecyl amine, Figure S12). These tertiary amines are easier to obtain than polymers,14–16 or amidine,13 550 which generally need complicated organic synthesis, or expensive reagents. However, similar to other CO25,13,33 552 responsive emulsions, after demulsification these precursor of CO2-switchable surfactant were mainly soluble in 554 the oil phase, so that the water phase did not emulsify the new oil phase.
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Supporting Information
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The additional results. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION 626 562
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Corresponding Author * E-mail:
[email protected] (Y. Zhang)
[email protected] (X. Liu)
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ACKNOWLEDGMENT 566
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This work was financially supported by the National Natural Science Foundation of China (grant No. 21503094, 21673103), the Natural Science Foundation (BK20150128) and the Qinglan Project of Jiangsu Province, PR China.
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