Switchable Oil-in-Water Emulsions Stabilized by Like-Charged

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

Switchable oil-in-water emulsions stabilized by likecharged surfactant and particles at very low concentration Maodong Xu, Lifei Xu, Qi Lin, Xiaomei Pei, Jianzhong Jiang, Haiyan Zhu, Zhenggang Cui, and Bernard P. Binks Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b04159 • Publication Date (Web): 26 Feb 2019 Downloaded from http://pubs.acs.org on February 27, 2019

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Switchable oil-in-water emulsions stabilized by like-charged surfactant and particles at very low concentration Maodong Xu,1,2 Lifei Xu, 1 Qi Lin, 1 Xiaomei Pei, 1 Jianzhong Jiang,1 Haiyan Zhu, 1 Zhenggang Cui1,* and Bernard P. Binks3,* 1 The

Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, 1800 Lihu Road, Wuxi, Jiangsu 214122, P.R. China

2School

of Biological and Chemical Engineering, Anhui Polytechnic University, Wuhu 241000, P.R. China 3

Department of Chemistry and Biochemistry, University of Hull, Hull. HU6 7RX. U.K.

Submitted to:

Langmuir on 26.2.19

Contains ESI

*Corresponding authors: [email protected]; [email protected]

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Abstract A novel CO2/N2 switchable n-decane-in-water emulsion was prepared which is stabilized by a CO2/N2 switchable surfactant (DDMA) in cationic form in combination with positively charged alumina nanoparticles at concentrations as low as 0.01 mM and 0.001 wt.% respectively. Particles do not adsorb at the oil-water interface but remain dispersed in the aqueous phase between surfactant-coated droplets. A critical zeta potential of the particles of ca. +18 mV is necessary for the stabilization of the novel emulsions, suggesting that the electrical double layer repulsions between particles and between particles and oil droplets are responsible for their stability. By bubbling N2 into the emulsions, demulsification occurs following transformation of DDMA molecules from the surface-active cationic form to the surface-inactive neutral form and desorption from the oil-water interface. Bubbling CO2 into the demulsified mixtures, cationic DDMA molecules are re-formed which adsorb to droplet interfaces ensuring stable emulsions after homogenization. Compared with Pickering emulsions and traditional emulsions, the amount of switchable surfactant and like-charged particles required for stabilisation are significantly reduced, which is economically and environmentally benign for practical applications.

Keywords: CO2/N2 switchable emulsions; alumina; double layer repulsion; Zeta potential

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Introduction In recent years switchable or stimuli-responsive emulsions has become an attractive topic [1-6] since these systems can be made stable or unstable on demand, which is important in the areas of emulsion polymerization, interfacial catalysis, drug delivery, oil transport and fossil fuel production.[3,7-9] In some cases, emulsions need to be temporarily stable even though demulsification is ultimately required. It is well known that traditional emulsions are stabilized by molecular emulsifiers, i.e. surfactants or amphiphilic polymers, which adsorb at the oil-water interface to reduce the interfacial tension and prevent flocculation and coalescence of droplets via either electrostatic or steric repulsion. [10,11] The type and stability of an emulsion formed are mainly dictated by the hydrophile-lipophile balance (HLB) number of the emulsifier, and those with high hydrophilicity (HLB no. > 7) favor oil-in-water (O/W) emulsions and vice versa. [11,12] Emulsions are thermodynamically unstable but can be made kinetically stable on addition of molecular emulsifiers at concentrations near the critical micelle concentration (cmc). [10-12] In contrast to the above emulsions, so-called Pickering emulsions stabilized by colloidal particles are usually much more stable, where colloid particles adsorb or aggregate at the oil-water interface forming a solid shell preventing coalescence of droplets.[10, 13-15] The type and stability of a Pickering emulsion are connected to the wettability of the particles, which can be characterized by the contact angle the particle makes with the oil-water interface measured through water, o/w. Particles with very low (< 20º) or very high (> 160º) o/w are surface-inactive and tend to disperse completely in the water or oil phase, respectively.[10,13] Only those with intermediate o/w values (say 30-150º) can stabilize either O/W (o/w < 90º) or W/O (o/w > 90º) emulsions at equal oil and water volume ratio. The contact angle is thus analogous to the HLB number of a surfactant in dictating emulsion type and stabilization.[10] Both heterogeneously coated particles (Janus) [16-18] and homogeneously coated particles of suitable wettability either from natural sources [14,19,20] or from synthesis [6, 10, 13-15, 21-24] are good particulate emulsifiers. These emulsions, however, are more difficult to demulsify. In order to render an emulsion switchable or responsive to stimuli, the most efficient Page | 3


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method is to design switchable or stimuli-responsive surfactants or particles, which can be transformed between surface-active and surface-inactive forms by certain triggers. [1-3, 6] Up to now, the effect of various single triggers such as pH, [25-27] redox, [28,29] temperature, [30-32] light irradiation, [33-35] CO2/N2, [1-5, 29, 36-38], magnetic field, [39] as well as multi-triggers like pH-temperature, [40], light-temperature, [41] magnetic field-temperature, [42,43] and CO2-redox [29] have been described. An interesting question is what will happen when surfactant molecules meet particles in an emulsion system? Normally surfactant tends to adsorb at the oil-water interface, but they may also adsorb on particle surfaces imparting surface modification which depends on the interactions between surfactant and particles. Particles may adsorb at the oil-water interface or disperse in either liquid phase. The alteration of interfacial properties may lead to complex behavior of the systems. Up to now, systems containing oppositely charged inorganic particles and ionic surfactants have been extensively studied.[44-52] Here, charged particles which are initially hydrophilic and surface-inactive like silica, [44] calcium carbonate,[45,46], alumina[49] etc. can be hydrophobized in situ by adsorbing oppositely charged ionic surfactant via electrostatic attraction rendering them surface-active and capable of stabilizing Pickering emulsions. Depending on the type and concentration of the surfactant used the Pickering emulsion can undergo an O/W to W/O phase inversion or O/W to W/O to O/W double phase inversion.[45, 47,48] Moreover, when the in situ hydrophobization can be made reversible, switchable or stimuli-responsive Pickering emulsions can be prepared. [3, 49-53] In these systems the surfactant concentration required to yield a stable emulsion is typically around 0.1 cmc, much less than that required when they are used alone. [3, 49] In addition, by means of reversible in situ hydrophobization, switchable or stimuli-responsive Pickering emulsions or foams can be prepared using conventional surfactants [49, 50-53] with triggers being CO2/N2, [49] ion pair formation,[50,51] pH [52] and temperature. [53] By contrast, systems involving similarly charged particles and surfactants are much less investigated because no strong interaction between them is expected. Although hydrophilic particles like glass beads and silica have been found to improve the stability of foams stabilized by soap [54] and the adsorption of anionic surfactant sodium dodecyl sulphate (SDS) at the air-water interface,[55-58] few of these mixtures are known for their behavior in Page | 4


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emulsions. Pilapil et al. [59] investigated synergistic stabilization between bare silica nanoparticles and a zwitterionic surfactant of O/W emulsions with the surfactant concentration set above the cmc and the particle concentration at 0.33 wt.%. They observed that particles enhanced emulsion stabilization even they do not adsorb at the oil-water interface. However, they claimed that such enhanced stabilization was not observed at particle concentrations below 0.33 wt.%. Their emulsion system involves no electrostatic repulsion since both particles and oil droplets are nearly neutral with zeta potentials both of -1 mV. In a recent paper, [60] we discovered that significant synergism in stabilizing O/W emulsions exists between similarly charged nanoparticles and ionic surfactants. For example, stable O/W emulsions can be prepared using extremely low (0.001-0.01 cmc) concentrations of cationic surfactant cetyltrimethylammonium bromide (CTAB) in combination with positively charged alumina nanoparticles at concentrations as low as 0.0001-0.01 wt.%. The finding was found to occur for various oils such as alkanes, aromatic hydrocarbons and triglycerides and seems to be universal for positively charged particles with cationic surfactants and negatively charged particles with anionic surfactants. [60] In contrast to Pickering emulsions, in these emulsions particles do not adsorb at the oil-water interface but remain in the continuous phase to increase the thickness of the lamellae and thereby prevent droplets from flocculation and coalescence. In this paper, we combine earlier ideas with this recent finding and report a novel CO2/N2 switchable O/W emulsion synergistically stabilized by a CO2/N2 switchable surfactant (cationic) and positively charged alumina nanoparticles. The concentrations of the surfactant and particles required are extremely low, or 0.001-0.01 cmc and 0.001-0.01 wt.%, respectively, which are both economically and environmentally benign for practical applications. It is suggested that a critical zeta potential is necessary for the charged particles to exhibit synergistic stabilization with similarly charged ionic surfactant.

Experimental Materials Hydrophilic alumina nanoparticles (Al2O3, > 99.8%) were purchased from Sigma which have a primary particle diameter of 13 nm and specific surface area of 85-115 m²/g according to the supplier. Both the TEM and SEM images of the particles are shown in Figure S1. Silica Page | 5


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nanoparticles (HL-200, 99.8%) with a primary particle diameter of 20 nm and a BET surface area of 200  20 m2/g were provided by Wuxi Jinding Longhua Chemical Co., China. Cationic surfactant cetyltrimethylammonium bromide (CTAB, > 99%), anionic surfactant sodium dodecyl benzene sulfonate (SDBS, > 95%), n-decane (> 98% ) and n-hexane (> 98%) were purchased from Shanghai Aladdin Bio-Chem Technology Co. Ltd., China. Anionic surfactant sodium dodecyl sulfate (SDS, > 99%) was purchased from Sigma. Gasoline oil (No. 95) was purchased from a local oil station. N'-dodecyl-N,N-dimethylacetamidine (DDMA, > 98%), a switchable surfactant with CO2/N2 trigger shown in Figure S2 was synthesized in our lab. [1, 3, 49] Ultrapure water was produced from a Simplicity Pure Water System (Merck Millipore, Shanghai) which has a pH of 6.1 and a resistance of 18.2 MΩ cm at 25C. All other chemicals used were analytically pure and were purchased from Sinopharm Chemical Reagent Co, Ltd., China, unless specified otherwise.

Methods Preparation and characterization of emulsions A stock solution of surfactant (DDMA, cationic) at 2 mM was prepared by dissolving the surfactant in pure water followed by bubbling CO2 (40 mL/min) at 0-5 C for 150 min to ensure complete ionization. The stock solution was then diluted to different concentrations using pure water. A certain mass of alumina nanoparticles was weighed into a glass vessel (25 mm (d) 65 mm (h)) of 25 mL. Then 7 mL of pure water or aqueous surfactant solution was added to the vessel, and the mixture was dispersed using an ultrasonic probe (JYD-650, Shanghai) at an output of 50 W for 40 sec. After that, 7 mL oil was added to the dispersion and the mixture was homogenized (IKA T18 basic, S18N-10G head) for 2 min at a speed of 11,000 rpm. The type of emulsion was determined by the drop test, [48] and micrographs of the emulsions were recorded using a microscope system (vhx-1000, Keyence Co.). The morphology of emulsion droplets was also visualised by scanning electron microscopy (SEM, Hitachi S-4800). A drop of emulsion was spread on a dry silicon wafer previously cleaned with ethanol, followed by drying in an incubator at 25 C. The silicon wafer was fixed on the specimen stage of the SEM using conductive adhesive, and the sample was sputter-coated with gold and analyzed at an acceleration voltage of 3 kV. The volume ratio of water to oil in Page | 6


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the emulsions is fixed at 1:1, and the concentration of particles (wt.%) and surfactant (M) in the emulsions are relative to the aqueous phase.

Measurement of Zeta potential and particle size Alumina nanoparticles at a concentration of 0.1 wt.% were dispersed in a series of DDMA solutions of different concentration (bubbled with CO2) using ultrasound (50 W, 40 sec), and the dispersions were allowed to stand at 25  0.5 C for 24 h to reach equilibrium. The zeta potential of the particles was measured using a ZetaPLAS (Brookhaven) instrument at 25C. The size distribution of the particles (0.1 wt.%) dispersed in pure water and 2 mM aqueous DDMA aqueous solution was also measured using this instrument at 25C.

Measurement of surface tension The surface tension of aqueous DDMA solutions with and without dispersed alumina nanoparticles was measured by means of the du Noüy ring method using a home-built instrument [48] at 25C. Before measurement, the solutions were transferred to glass Petri dishes and were allowed to stand at 25C for 24 h to reach adsorption equilibrium.

Determination of concentration of alumina nanoparticles in aqueous phase of an emulsion In a series of vessels (2.7 cm (d)9.5 cm (h)) of 40 mL, 15 mL dispersions of alumina nanoparticles (0.5 wt.%) in aqueous DDMA solutions of different concentration were prepared, followed by the addition of 15 mL n-decane. The mixtures were emulsified by homogenization as mentioned above, and the vessels were sealed and allowed to settle upside down in an air thermostat (25  0.5°C) for creaming to take place. After 24 h the resolved aqueous phase was removed using a syringe through the rubber in the cap and transferred into a clean glass vessel (25 mL) weighed in advance. The mass of the aqueous phase was recorded and the water was evaporated in an oven heated close to 100°C. Finally, the vessels were allowed to dry at 110°C until constant weight. The mass of the particles in each vessel was then obtained by weighing and the concentration of the alumina nanoparticles in the aqueous phase of the emulsion was calculated. Page | 7


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Demulsification/re-stabilization cycle of the novel emulsion An O/W emulsion containing 7 mL aqueous phase and 7 mL n-decane was prepared by homogenization in a glass bubbling device shown in Figure S3. The aqueous phase contained 0.2 mM DDMA (cationic form) dispersed with 0.01 wt.% alumina nanoparticles. The emulsion was bubbled with N2 at 65 C for 50 min at a flow rate of 30 mL/min to induce demulsification. After that, the demulsified sample was bubbled with CO2 in an ice bath for 150 min at a flow rate of 40 mL/min, and a stable emulsion was re-formed by homogenization. In this way, the demulsification/re-stabilization cycle was realized by alternately bubbling with N2 and CO2 followed by homogenization. Unless specified otherwise, all experiments were carried out at room temperature (22-25 °C).

Results and discussion (a) Stabilisation of novel emulsions by alumina nanoparticles and cationic DDMA Alumina nanoparticles are positively charged and highly hydrophilic in pure water as reflected by the zeta potential of +41  5 (pH = 5.85) shown in Figure S4. Although the particles have a primary size of 13 nm according to the supplier, they form aggregates as reflected by a low measured BET surface area of 90 m2/g and an average diameter of 210 nm when ultrasonically dispersed in pure water (Figure S5). Accordingly, partial sedimentation of the particles in water was observed due to their high density (3.95 g/cm3), as shown in Figure 1A. Similar to bare hydrophilic silica nanoparticles, [47,48] bare alumina nanoparticles alone are too hydrophilic to stabilize an emulsion with n-decane as oil phase as shown in Figure 1A, where complete phase separation occurred rapidly with some particles adhering on the glass wall and most of the particles returning to the aqueous phase. When DDMA in cationic form was used as emulsifier alone, no emulsion could be stabilised at DDMA concentrations below 1.0 mM (cmc = 2.3 mM), [3] as shown in Figure 1B. When some alumina nanoparticles (0.5 wt.%) were ultrasonically dispersed in the DDMA solution however, stable O/W emulsions were formed at DDMA concentrations far below the cmc as shown in Figure 1C. After standing for a month at room temperature, no coalescence Page | 8


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of oil droplets was observed suggesting good stability of the emulsions as shown in Figure 1D. The micrographs of the latter emulsions (taken 24 h after preparation) shown in Figure 2 indicate a gradual decrease in the average droplet diameter with increasing DDMA concentration, from around 80 μm at 0.01 mM DDMA to about 30 μm at 1.0 mM DDMA. The droplet sizes are in general smaller than those in Pickering n-decane-in-water emulsions stabilized by alumina nanoparticles hydrophobized in situ by an anionic surfactant SDS (> 100 μm), but larger than those in n-decane-in-water emulsions stabilized by a typical cationic surfactant CTAB alone (15-25 μm) prepared under similar conditions, as shown in Figure S6. The novel emulsions are all fluid of low viscosity, quite different from many Pickering emulsions which are gel-like at high particle concentrations. [60] In Pickering emulsions of alumina plus SDS shown in Figure S7C and D, upon increasing the SDS concentration the resolved water phase after creaming becomes more and more transparent due to an increased surface activity of the particles which transfer from water to droplet interfaces. In contrast in the non-Pickering emulsions of alumina and DDMA (Figure 1C and D), the aqueous phases are turbid and a fraction of the particles sediment to the bottom of the vials on standing. In order to explore emulsion stabilization at low particle concentration, emulsions were prepared with alumina particles at concentrations as low as 0.01 wt.% and 0.001 wt.%. The appearance and optical micrographs of these emulsions are shown in Figures 3-5. Figure 3A and C show that at an alumina concentration as low as 0.01 wt.%, n-decane-in-water emulsions can also be obtained over a wide range of DDMA concentration, and the average droplet size decreases from around 200 m to 30 m between 0.003 mM and 1.0 mM DDMA (Figure 4). However, after one month emulsions containing DDMA below 0.01 mM (droplet diameter > 100 m) were very unstable to coalescence. Thus, at a particle concentration of 0.01 wt.%, the minimum surfactant concentration required to stabilize an n-decane-in-water emulsion to coalescence is about 0.01 mM. Surprisingly, stable emulsions can even be obtained at a lower particle concentration of 0.001 wt.%, where the minimum DDMA concentration required is hardly changed and the droplet diameters seem to be similar, depending only on the surfactant concentration and less affected by particle concentration (Figure 3B and D and Figure 5). A phenomenon difficult to understand is that emulsions of Page | 9


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alumina and DDMA become unstable at DDMA concentrations beyond 0.3 mM as seen in Figure 3C and D. Here, the adsorption of DDMA at the oil-water interface should be increased and emulsion stabilization be improved, even though at such low concentrations DDMA alone cannot stabilize an emulsion. It is thus suggested that other factors are responsible for the emulsion breakdown which will be discussed later. As reported before,[60] it seems that the alumina particles in the novel emulsions (alumina + DDMA) do not adsorb at the oil-water interface. To confirm this prediction, we measured the alumina concentration in the aqueous phase separated after creaming of the emulsions and compared it with that from true Pickering emulsions (alumina + SDS). The results shown in Table 1 show that at an initial alumina concentration of 0.5 wt.%, there is very little loss of particles from water to the emulsion in the case of the novel emulsions at a range of surfactant concentrations. Bearing in mind experimental error, the low percentage loss (4%) of particles from the aqueous phase is negligible. In contrast, for the Pickering emulsions, the alumina concentration in the aqueous phase after creaming decreases significantly with increasing SDS concentration, and the percentage of particles adsorbed at oil-water interface increases concomitantly from 23% at 0.01 mM SDS to 97 % at 1.0 mM SDS. The alumina particles in the novel emulsions therefore tend to stay in the aqueous phase. This is further supported by the SEM images and optical micrographs of the emulsions shown in Figure 6, where after evaporation of the oil phase (either n-decane or more volatile n-hexane) the droplets in the novel emulsions are shown as round holes free of any particles (Figure 6(a) and (b)), suggesting no interfacial particles with the solid particles all distributed in the continuous water phase. In contrast, in Pickering emulsions stabilized by alumina hydrophobized in situ by SDS, the half dried droplets possess wrinkled surfaces (Figure 6 (c)) and the dried droplets show stamps of particles (Figure 6 (d)), all consistent with the presence of interfacial particles. In addition, no particles were observed in the continuous phase.

(b) Demulsification/re-stabilization cycling of novel emulsions To examine the switchable property of the novel emulsions, a n-decane-in-water emulsion stabilized by 0.01 wt.% alumina nanoparticles in combination with 0.2 mM cationic Page | 10


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DDMA was prepared. The appearance and optical micrograph of the emulsion are shown in Figure 7(A) and (a) respectively. The emulsion was then bubbled with N2 at a flow rate of 30 mL/min at 65C for 50 min. (to avoid evaporation of the liquids, the bubbling tube was connected to a condenser cycled with cold water). Droplet coalescence was observed and complete demulsification occurred as shown in Figure 7(B). Then CO2 was bubbled into the separated oil-water mixture at a flow rate of 40 mL/min at 0-5C (ice bath) for 150 min. followed by homogenization (11,000 rpm for 2 min.). A stable emulsion with similar droplet sizes as before was re-formed, as shown in Figure 7(C) and (c). The second cycle was also successful as shown in Figure 7(D) and (E), and the micrograph (e) shows that similar droplet size can be recovered after two cycles. However, with subsequent cycles, emulsion stabilization is reduced probably due to a reduction in the concentration of cationic DDMA caused by incomplete conversion from the neutral form during switching off/on cycles. [3] However, by increasing the initial DDMA concentration or adding extra DDMA, more cycles can be realized.

(c) Stabilization mechanism of the novel emulsions It has been well recognized that the reduction of the oil-water interfacial tension (IFT) is beneficial to the stabilization of surfactant-containing emulsions.[11,12] It is worth examining the effect of particles on the IFT. Since the air-water surface tension of a surfactant solution varies analagously to the oil-water IFT [60] and the determination of surface tension is simpler, we examined the effect of adding alumina particles on the surface tension of aqueous DDMA solutions at 25C. The results are shown in Figure 8, which indicate that with addition of 0.5 wt.% alumina nanoparticles the surface tension is unchanged at very low DDMA concentrations and at relatively high concentrations (close to and beyond cmc). Only at intermediate concentrations does a small reduction in surface tension occur compared with the solutions without particles. The maximum decrease is about 3.5 mN/m at CDDMA = 610-4 M, or 0.26 cmc, where the surface tension of the surfactant solution without particles is 42.05 mN/m. By fitting the measured -logC curves using the Szyszkowski equation, - = 2RT ln(1+KC), where  and  are the surface tensions of pure water and surfactant solution, C is the surfactant concentration, R is the gas constant, T is the absolute temperature and K is a Page | 11


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constant, it is estimated that the saturated adsorption  of DDMA increases from 2.0610-10 mol/cm2 in the absence of particles to 2.54 10-10 mol/cm2 in the presence of 0.5 wt.% alumina particles. A similar effect was observed for the IFT between n-decane and aqueous CTAB solution, where a small extra IFT reduction is observed in the middle of the CTAB concentration range upon addition of alumina nanoparticles,[60] and in other similarly charged particle-ionic surfactant systems.[55, 61] It is thus suggested that the addition of similarly charged particles into ionic surfactant solutions can improve the adsorption of surfactant from bulk to the fluid-fluid interface probably due to electrostatic repulsion.[58, 61] However, such an adsorption improvement is quite limited and negligible at very low particle concentrations, and cannot account for the stabilization of the novel emulsions in this régime. In contrast, at higher surfactant concentration, demulsification occurs which suggests that other factors may dominate stabilization of the emulsions. The zeta potential of alumina nanoparticles dispersed in aqueous cationic DDMA solutions displays a progressive decrease with surfactant concentration as shown in Figure 9. By comparing with the emulsion findings in Figure 3 (and Figure S8 where particle concentration is 0.1 wt.% and coalescence occurs at [DDMA] beyond 0.3 mM), a critical zeta potential of approx. +18 mV is obtained (dashed line). We hypothesise that particles with zeta potential above this value ensure stable emulsions whereas those with zeta potential below it give unstable emulsions even though the surfactant concentration is higher. In the latter case the double layer repulsion between particles is not sufficient to resist the van der Waals attraction, particles flocculate and sediment from the lamellae. Flocculation and coalescence of droplets occurs due to their own lower zeta potential. This however was not observed in alumina + CTAB systems,[60] where the zeta potential decreases only slightly with surfactant concentration (Figure 9). At 1.0 mM CTAB the zeta potential is still high at +35 mV ensuring repulsion between particles and emulsion instability is not observed since CTAB alone can stabilize n-decane-in-water emulsions at higher concentration.[60] It is well known that the zeta potential of particles decreases with increasing electrolyte concentration due to compression of the electrical double layer. The cationic surfactants involved here are also electrolytes, which may account for the small decrease in the zeta potential in aqueous CTAB solutions. However, the large decrease observed in aqueous Page | 12


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DDMA solutions is more than a simple electrolyte effect. Since in preparing DDMA solutions CO2 was bubbled into them, we examined the effect on the zeta potential of bubbling CO2 (40 mL/min for 150 min) into an aqueous alumina dispersion (0.1 wt.%). We found that the zeta potential decreases from 42.6  1.7 mV in pure water (pH = 6.1) to 21.8  0.9 mV after bubbling CO2 (pH = 4.1), a reduction of 50%. Since this decrease in pH can only lead to a small reduction of the zeta potential (5 mV, Figure S4), it is suggested that the HCO3 ions produced by bubbling CO2 in the aqueous phase may bind strongly to particle surfaces leading to the significant reduction of the zeta potential in DDMA solutions. This idea is supported by experimental evidence in the literature where HCO3 as a competing anion can significantly affect the adsorption of fluoride onto alumina [62] and HCO3 can be removed from water by adsorption onto inorganic particles like FeOOH. [63] The presence of a critical zeta potential of charged particles in synergistically stabilizing oil-in-water emulsions with similarly charged ionic surfactant is also applicable to the alumina + CTAB system,[60] where demulsification does occur by addition of electrolyte which results in significant reduction of the zeta potential of the particles. The idea is also supported by observations in negatively charged silica + anionic sodium dodecyl benzene sulfonate (SDBS) mixtures to stabilize gasoline-in-water emulsions, as shown in Figure S9. Here, stable emulsions were only formed using pure water whereas extensive coalescence occurred using tap water containing ions (conductivity = 345 S/cm, pH = 8 at 25C). The zeta potential of the particles was above (-27.6  0.3 mV in pure water) and below (-13.5  0.5 mV in tap water) the critical zeta potential respectively. Based on all the above experimental observations, the stabilization mechanism of the novel emulsions can be outlined. Electrical double layer repulsions between charged particles in water and between charged particles and charged droplets are crucial. In the absence of charged particles the zeta potential of oil droplets is quite low (not measurable) due to the very low surfactant concentration (0.004-0.13 cmc). It is believed that such a low droplet zeta potential is insufficient to prevent flocculation of the droplets once they are formed by homogenization.[11, 12] Since there is no dense charged surfactant film at the oil-water interface, the aqueous lamellae become very thin after flocculation so that coalescence occurs easily leading to emulsion breakdown. However, in the presence of like-charged particles, Page | 13


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Page 14 of 34

although they do not adsorb at the oil-water interface to constitute a solid film like in Pickering emulsions, they disperse in the aqueous phase providing thick lamellae even at very low particle and surfactant concentrations. For example, at a particle concentration of 0.01 wt.%, it is estimated that each droplet (average diameter of 80 m) is surrounded by 2.1106 primary alumina particles or 490 aggregated particles (of diameter 210 nm, Figure S5) after creaming. Since each particle and droplet is surrounded by a thick electrical double layer, the inter-droplet spacing is then increased (relative to the case without particles), which is beneficial for reducing the van der Waals attraction between droplets ensuring stable emulsions. For example, the van der Waals attractive interaction between two droplets of the same diameter with one particle (210 nm) between them (D  600 nm) is only 0.034% of that in the absence of particles (D = 0.2 nm) supposing both the droplet and particle have the same Debye length (96 nm at 0.01 mM electrolyte), according to DLVO theory. The critical zeta potential of alumina nanoparticles required for stabilization of the emulsion (+18 mV) is close to that suggested for colloid stabilization ( 25 mV) [11, 12]. The electrical double layer repulsion between droplets and particles is also crucial for the stabilization of the emulsions. This is supported by the fact that for alumina nanoparticles plus surfactant systems no stable emulsion can be formed when the cationic surfactant is replaced by a nonionic surfactant [60] where there is no electrical double layer surrounding oil droplets. Also, demulsification occurs when equal moles of an anionic surfactant (SDS) is added to the cationic surfactant-containing emulsions which destroys the electrical double layer surrounding both the droplets and particles. On the other hand, at very low surfactant and particle concentrations, increasing the concentration of either surfactant or particles enhances the electrical double layer repulsion between droplets and particles so as to improve emulsion stabilization.[60] In the systems described here, by bubbling N2 into the emulsions the DDMA surfactant is transformed from cationic form (amidinium) to neutral form (amidine) and the electrical double layers surrounding droplets are destroyed leading to demulsification. Once CO2 is bubbled into the system, DDMA is transformed back to cationic form and the electrical double layers are re-established following homogenization leading to recovery of a stable emulsion, as illustrated in Figure 10. It is noticed that although these emulsions are quiescently stable after preparation, Page | 14


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Langmuir

demulsification does occur when they are magnetically stirred (moderate speed) or rotated on a rotation mixer (ca. 60 rpm/min), especially for emulsions containing very low concentrations of particles and surfactant. Since droplets do not have a zeta potential high enough to prevent flocculation and coalescence, the static arrangement shown in Figure 10 can be damaged during stirring so that the probability of the droplets contacting each other increases significantly. This shear-induced coalescence can be significantly weakened and avoided by using relatively high particle and surfactant concentrations (0.01-0.1 wt.% and 0.01-0.1 cmc).

Conclusions Although neither positively charged alumina nanoparticles nor cationic DDMA surfactant at concentrations far below the cmc can stabilize emulsions alone, their mixture enables novel n-decane-in-water emulsions to be stabilised. The effective concentration of the similarly charged particles and surfactant required for emulsion stabilization can be as low as 0.001 wt.% and 0.01 mM (or 0.004 cmc) respectively, much lower than that required to stabilise Pickering emulsions or traditional emulsions. The average droplet diameter in novel emulsions decreases from 80 m to 30 m with increasing surfactant concentration but is unaffected by particle concentration. In these emulsions, particles do not adsorb at the oil-water interface but remain dispersed in the aqueous phase. In the absence of particles, emulsions stabilized solely by low concentrations of cationic surfactant are unstable, due mainly to low adsorption of the surfactant at the oil-water interface such that coalescence is prevalent. However, when similarly charged nanoparticles are added, they disperse evenly in the aqueous phase each being surrounded by an electrical double layer. Double layer repulsion between particles and between particles and charged droplets enable relatively thick lamellae to develop which significantly increases the minimum distance between droplets. Correspondingly, van der Waals attraction between droplets is dramatically reduced. We identify that a critical zeta potential of the particles (ca. 18 mV) is necessary for emulsion stabilisation, close to that required for colloid stabilization. The novel emulsion is also switchable. By bubbling N2 into the stable emulsions, cationic DDMA is transformed to its neutral form which desorbs from the oil-water interface Page | 15


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Page 16 of 34

destroying the electrical double layer repulsion between particles and droplets leading to demulsification. By bubbling CO2 into the demulsified mixtures, cationic DDMA is re-formed which adsorbs again to the oil-water interface upon homogenization yielding stable emulsions. Compared with Pickering emulsions and traditional emulsions, the amounts of switchable surfactant and particles required in the novel emulsions are significantly reduced which is economically and environmentally attractive for practical applications.

Acknowledgment Financial support from the National Natural Science Foundation of China (NSFC 21872064, 21573096, 21473080), from MOE & SAFEA for the 111 Project (B13025) and from Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX18-1809) are gratefully acknowledged.

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References 1.

Liu, Y. X.; Jessop, P. G.; Cunningham, M.; Eckert, C. A.; Liotta, C. L. Switchable Surfactants. Science 2006, 313, 958-960.

2.

Liang, C.; Harjani, J. R.; Robert, T.; Rogel, E.; Kuehne, D.; Ovalles, C.; Sampath, V.; Jessop, P. G. Use of CO2-Triggered Switchable Surfactants for the Stabilization of Oil-in-Water Emulsions. Energy Fuels 2012, 26, 488-494.

3.

Jiang, J. Z.; Zhu, Y.; Cui, Z. G.; Binks, B. P. Switchable Pickering Emulsions Stabilized by Silica Nanoparticles Hydrophobized in situ by a Switchable Surfactant. Angew. Chem. Int. Ed. 2013, 52, 12373-12376.

4.

Liu, P. W.; Lu, W. Q.; Wang, W. J.; Li, B. G.; Zhu, S. P. Highly CO2/N2‑Switchable Zwitterionic Surfactant for Pickering Emulsions at Ambient Temperature. Langmuir 2014, 30, 10248-10255.

5.

Liang, C.; Liu, Q. X.; Xu, Z. H. Surfactant-Free Switchable Emulsions Using CO2-Responsive Particles. ACS Appl. Mater. Interfaces 2014, 6, 6898-6904.

6.

Tang, J. T.; Quinlan, P. J.; Tam, K. C. Stimuli-Responsive Pickering Emulsions: Recent Advances and Potential Applications. Soft Matter 2015, 11, 3512-3529.

7.

Pera-Titus, M.; Leclercq, L.; Clacens, J. M.; De, C. F.; Nardello-Rataj, V. Pickering Interfacial Catalysis for Biphasic Systems: From Emulsion Design to Green Reactions. Angew. Chem. Int. Ed. 2015, 46, 2006-2021.

8.

Fernandes, D. A.; Fernandes, D. D.; Li, Y.; Yan, W.; Zhang, Z.; Rousseau, D.; Gradinaru, C. C.; Kolios, M. C. Synthesis of Stable Multifunctional Perfluorocarbon Nanoemulsions for Cancer Therapy and Imaging. Langmuir 2016, 32, 10870-10880.

9.

Reyes, J.; Cerón-Camacho, R.; Martínez-Palou, R.; Villanueva, D.; Vallejo, A. A.; Aburto, J. Study of the Formation and Breaking of Extra-Heavy-Crude-Oil-in-Water Emulsions. A proposed strategy for transporting extra heavy crude oils. Chemical Engineering and Processing 2015, 98, 112-122.

10. Binks, B. P. Particles as Surfactants—Similarities and Differences. Curr. Opin. Colloid Interface Sci. 2002, 7, 21-41. 11. Becher, P. Encyclopedia of Emulsion Technology; Marcel Dekker: New York and Basel, 1983. Page | 17


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

12.

Page 18 of 34

Rosen, M. J.; Kunjappu, J. T. Surfactants and Interfacial Phenomena, 4th ed.; Wiley: Hoboken, 2012.

13. Aveyard, R.; Binks, B. P.; Clint, J. H. Emulsions Stabilised Solely by Colloidal Particles. Adv. Colloid Interface Sci. 2003, 100, 503-546. 14. Binks, B. P.; Horozov, T. S. (eds.) Colloidal Particles at Liquid Interfaces; Cambridge University Press: Cambridge, 2006. 15.

Chevalier, Y.; Bolzinger. M. A. Emulsions Stabilized with Solid Nanoparticles: Pickering Emulsions. Colloids Surf. A, 2013, 439, 23-34.

16.

Binks, B. P.; Fletcher, P. D. I. Particles Adsorbed at the Oil-Water Interface: A Theoretical Comparison Between Spheres of Uniform Wettability and “Janus” Particles. Langmuir 2001, 17, 4708-4710.

17.

Walther, A.; Hoffmann, M.; Müller, A. H. Emulsion Polymerization Using Janus Particles as Stabilizers. Angew. Chemie 2008, 120, 723–726.

18.

Fujii, S.; Yokoyama, Y.; Miyanari, Y.; Shiono, T.; Ito, M.; Yusa, S.; Nakamura, Y. Micrometer-Sized Gold–Silica Janus Particles as Particulate Emulsifiers. Langmuir 2013, 29, 5457-5465.

19. Binks, B. P.; Clint, J. H.; Mackenzie, G.; Simcock, C.; Whitby, C. P. Natually Occurring Spore Particles at Planar Fluid Interface and in Emulsions. Langmuir 2005, 21, 8161-8167. 20.

Lam, S.; Velikov, K. P.; Velev, O. D. Pickering Stabilization of Foams and Emulsions with Particles of Biological Origin. Curr. Opin. Colloid Interface Sci. 2014, 19, 490-500.

21.

Tambe, D. E.; Sharma, M. M. Factor Controlling the Stability of Colloid-Stabilized Emulsions: I. An Experimental Investigation. J. Colloid Interface Sci. 1993, 157, 244-253.

22. Binks, B. P.; Cui, Z. G.; Fletcher, P. D. I. Optical Microscope Absorbance Imaging of Carbon Black Nanoparticle Films at Solid and Liquid Surfaces. Langmuir 2006, 22, 1664-1670. 23. Tsugita, A.; Takemoto, S.; Mori, K.; Yoneya, T.; Otani, Y. Studies on O/W Emulsions Stabilized with Insoluble Montmorillonite-Organic Complexes. J. Colloid Interface Sci. 1983, 95, 551-560. 24.

Zhai, X.; Efrima, S. Chemical and Physical Aspects of Macroemulsions Stabilized by Interfacial Colloids. J. Phys. Chem. 1996, 100, 11019-11028.

25.

Fujii, S.; Cai, Y.; Weaver, J. V. M.; Armes, S. P. Syntheses of Shell Crosslinked Page | 18


Page 19 of 34 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

Micelles Using Acidic ABC Triblock Copolymers and Their Application as pH-Responsive Particulate Emulsifiers. J. Am. Chem. Soc. 2005, 127, 7304-7305. 26.

Tu, F.; Lee, D. Shape-Changing and Amphiphilicity-Reversing Janus Particles with pH-Responsive Surfactant Properties. J. Am. Chem. Soc. 2014, 136, 9999-10006.

27. Kalliola, S.; Repo, E.; Srivastava, V.; Zhao, F.; heiskanen, J. P.; Sirvio, J.A.; Limatainen, H.; Sillanpaa, M. Carboxymethyl Chitosan and Its Hydrophobically Modified Derivative as pH-Switchable Emulsifiers. Langmuir 2018, 34, 2800-2806. 28.

Quesada, M.; Muniesa, C.; Botella, P. Hybrid PLGA-Organosilica Nanoparticles with Redox-Sensitive Molecular Gates. Chem. Mater. 2013, 25, 2597-2602.

29.

Zhang, Y. M.; Guo, S.; Ren, X. F.; Liu, X. F.; Fang, Y. CO2 and Redox Dual Responsive Pickering Emulsion. Langmuir 2017, 33,12973-12981.

30.

Binks, B. P.; Murakami, R.; Armes, S. P.; Fujii, S. Temperature-Induced Inversion of Nanoparticle-Stabilized Emulsions. Angew. Chem. Int. Ed. 2005, 44, 4759-4798.

31.

Zoppe, J. O.; Venditti, R. A.; Rojas, O. J. Pickering Emulsions Stabilized by Cellulose Nanocrystals Grafted with Thermo-Responsive Polymer Brushes. J. Colloid Interface Sci. 2012, 369, 202-209.

32.

Thompson, K. L.; Fielding L. A.; Mykhaylyk O. O.; Lane, J. A.; Derry, M. J.; Armes, S. P. Vermicous Thermo-Responsive Pickering Emulsifiers. Chem. Sci. 2015, 6, 4207-4214.

33.

Eastoe, J.; Sánchez Dominguez, M.; Wyatt, P.; Heenan, R. K. A Photo-Responsive Organogel. Chem. Commun. 2004, 22, 2608-2609.

34. Kördel, C.; Popeney, C. S.; Haag, R. Photoresponsive Amphiphiles Based on Azobenzene-Dendritic Glycerol Conjugates Show Switchable Transport Behavior. Chem. Commun. 2011, 47, 6584-6586. 35. Anwar, N.; Williams, T.; Grimme, B.; Kuehne, A. J. C. Light-Switchable and Monodisperse Conjugated Polymer Particles. ACS Macro Lett. 2013, 2, 766-769. 36. Zhang, Q.; Yu, G. Q.; Wang, W. J.; Yuan, H. M.; Li, B. G.; Zhu, S. P. Preparation of N2/CO2 Triggered Reversibly Coagulatable and Redispersible Latexes by Emulsion Polymerization of Styrene with a Reactive Switchable Surfactant. Langmuir 2012, 28, 5940-5946. Page | 19


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

37.

Page 20 of 34

Darabi, A.; Jessop, P. G.; Cunnigham, M. F. CO2-Responsive Polymer Materials: Synthesis, Self-Assembly, and Functional Applications. Chem. Soc. Rev. 2016, 45, 4391-4436.

38.

Zhang, Y. M.; Ren, X. F.; Guo, S.; Liu, X. F.; Fang, Y. CO2-Switchable Pickering Emulsion Using Functional Nanoparticles Decorated by Amine Oxide-Based Surfactants. ACS Sustain. Chem. Eng. 2018, 6, 2641-2650.

39.

Lam, S.; Blanco, E.; Smoukov, S. K.; Velikov, K. P.; Velev, O. D. Magnetically Responsive Pickering Foams. J. Am. Chem. Soc. 2011, 133, 13856-13859.

40.

Tang, J. T.; Lee, M. F. X.; Zhang, W.; Zhao, B. X.; Berry, R. M.; Tam, K. C. Dual Responsive

Pickering

Emulsion

Stabilized

by

Poly[2-(dimethylamino)ethyl

Methacrylate] Grafted Cellulose Nanocrystals. Biomacromol. 2014, 15, 3052-3062. 41.

Fameau, A. L.; Lam, S.; Velev, O. D. Multi-Stimuli Responsive Foams Combining Particles and Self-Assembling Fatty Acids. Chem. Sci. 2013, 4, 3874-3881.

42.

Rahman, M. M.; Chehimi, M. M.; Fessi, H.; Elaissari, A. Highly Temperature Responsive Core-Shell Magnetic Particles: Synthesis, Characterization and Colloidal Properties. J. Colloid Interface Sci. 2011, 360, 556-564.

43.

Brugger, B.; Richtering, W. Magnetic, Thermosensitive Microgels as StimuliResponsive Emulsifiers Allowing for Remote Control of Separability and Stability of Oil-in-Water Emulsions. Adv. Mater. 2007, 19, 2973-2978.

44.

Binks, B. P.; Rodrigues, J. A.; Frith, W. J. Synergistic Interaction in Emulsions Stabilized by a Mixture of Silica Nanoparticles and Cationic Surfactant. Langmuir 2007, 23, 3626-3636.

45.

Cui, Z. G.; Shi, K. Z.; Cui, Y. Z.; Binks, B. P. Double Phase Inversion of Emulsions Stabilized by a Mixture of CaCO3 Nanoparticles and Sodium Dodecyl Sulphate. Colloids Surf. A 2008, 329, 67–74.

46. Cui, Z. G.; Cui, Y. Z.; Cui, C. F.; Chen, Z.; Binks, B. P. Aqueous Foams Stabilized by in situ Surface Activation of Calcium Carbonate Nanoparticles via Adsorption of Anionic Surfactant. Langmuir 2010, 26, 12567-12574. 47.

Binks, B. P.; Rodrigues, J. A. Double Inversion of Emulsions by using Nanoparticles and a Di-Chain Surfactant. Angew. Chem. Int. Ed. 2007, 46, 5389-5392.

48.

Cui, Z. G.; Yang, L. L.; Cui, Y. Z.; Binks, B. P. Effects of Surfactant Structure on the Phase Inversion of Emulsions Stabilized by Mixtures of Silica Nanoparticles and Page | 20


Page 21 of 34 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

Cationic Surfactant. Langmuir 2010, 26, 4717-4724. 49.

Xu, M. D.; Zhang, W. Q.; Pei, X. M.; Jiang, J. Z.; Cui, Z. G.; Binks, B. P. CO2/N2 Triggered Switchable Pickering Emulsions Stabilized by Alumina Nanoparticles in Combination with a Conventional Anionic Surfactant. RSC Adv. 2017, 7, 29742-29751.

50.

Zhu, Y.; Pei, X. M.; Jiang, J. Z.; Cui, Z. G.; Binks, B. P. Responsive Aqueous Foams Stabilized by Silica Nanoparticles Hydrophobised in situ with a Conventional Surfactant. Langmuir 2015, 31, 12937-12943.

51.

Zhu, Y.; Jiang, J. Z.; Liu, K. H.; Cui, Z. G.; Binks, B. P. Switchable Pickering Emulsions Stabilized by Silica Nanoparticles Hydrophobized in situ with Conventional Cationic Surfactant. Langmuir 2015, 31, 3301-3307.

52.

Liu, K. H.; Jiang, J. Z.; Cui, Z. G.; Binks. B. P. pH-Responsive Pickering Emulsions Stabilized by Silica Nanoparticles in Combination with a Conventional Zwitterionic Surfactant. Langmuir 2017, 33, 2296-2305.

53.

Zhu, Y.; Fu, T.; Liu, K. H.; Lin, Q.; Pei, X. M.; Jiang, J. Z.; Cui, Z. G.; Binks, B. P. Thermo-Responsive Pickering Emulsions Stabilized by Silica Nanoparticles in Combination with Alkyl Polyoxyethylene Ether Nonionic Surfactant. Langmuir 2017, 33, 5724-5733.

54.

Aveyard, R.; Binks, B. P.; Fletcher, P. D. I.; Rutherford, C. E. Contact Angles in Relation to the Effects of Solids on Film and Foam Stability. J. Disp. Sci. Technol. 1994, 15, 251-271.

55.

Arab, D.; Kantzas, A.; Bryant, S. L. Nanoparticle Stabilized Oil in-Water Emulsions: A Critical Review. J. Petrol. Sci. Eng. 2018,163, 217-242

56.

Ma, H.; Luo, M. X.; Dai, L. L. Influences of Surfactant and Nanoparticles Assembly on Effective Interfacial Tensions. Phys. Chem. Chem. Phys. 2008, 10, 2207-2213.

57.

Sun, Q.; Li, Z. M.; Wang, J. Q.; Li, S. Y.; Li, B. F.; Jiang, L.; Wang, H. Y.; Lü, Q. C.; Zhang, C.; Liu, W. Aqueous Foam Stabilized by Partially Hydrophobic Nanoparticles in the Presence of Surfactant. Colloids Surf. A 2015, 471, 54-64.

58.

Jin, J.; Li, X.; Geng, J.; Jing, D. Insights into the Complex Interaction Between Hydrophilic Nanoparticles and Ionic Surfactants at the Liquid/Air Interface. Phys. Chem. Chem. Phys., 2018, 20, 15223-15235.

59.

Pilapil, B.K.; Jahandideh, H.; Bryant, S.L.; Trifkovic, M. Stabilization of Oil-in-Water Emulsions with Noninterfacially Adsorbed Particles. Langmuir 2016, 32, 7109-7116.

60.

Xu, M. D.; Jiang, J. Z.; Pei, X. M.; Song, B. L.; Cui, Z. G.; Binks, B. P. Novel Oil-in-Water Emulsions Stabilised by Ionic Surfactant and Similarly Charged Nanoparticles at Very Low Concentrations. Angew. Chem. Int. Ed. 2018, 57, Page | 21


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7738-7742. 61.

Ahualli, S.; Iglesias, G. R.; Wachter, W.; Dulle, M.; Minami, D.; Glatter, O. Adsorption of Anionic and Cationic Surfactants on Anionic Colloids: Supercharging and Destabilization. Langmuir 2011, 27, 9182–9192.

62.

Tang, Y. L.; Guan, X. H.; Su, T. Z.; Gao, N. Y.; Wang, J. M. Fluoride adsorption onto activated alumina: Modeling the effects of pH and some competing ions. Colloids Surf. A 2009, 337, 33-38.

63.

Sharrad, M.O.; Fan, M. Adsorption of carbonate and bicarbonate on FeOOH. Int. J. Adv. Technol. Eng. Sci. 2015, 3, 150-169.

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Tables and Figures

Table 1. Percentage of alumina nanoparticles adsorbed at oil-water interfaces in n-decane-in-water emulsions stabilized by 0.5 wt.% alumina nanoparticles in combination with either DDMA (cationic) or SDS (anionic) at different concentrations, obtained by measuring the concentration of particles in the aqueous phase after creaming (Cp).

Alumina + DDMA (novel emulsions)

Alumina + SDS (Pickering emulsions)

CDDMA/mM

Cp/wt.%

Percentage adsorbed/%

CSDS /mM

Cp/wt.%

Percentage adsorbed/%

0.01

0.479  0.001

4.2  0.3

0.01

0.385  0.006

23.0  0.2

0.1

0.477  0.003

4.6  0.3

0.1

0.060  0.002

88.0  3.3

1.0

0.476  0.001

4.8  0.3

1.0

0.016  0.002

96.8  12.5

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Page 24 of 34

Figure 1. Digital photos of n-decane-in-water emulsions (7 mL/7 mL) stabilized by (A) alumina nanoparticles alone, (B) cationic DDMA alone at different concentrations and by (C, D) 0.5 wt.% alumina nanoparticles in combination with DDMA at different concentrations, taken 24 h (A-C) and one month (D) after preparation. Concentration of alumina/wt.% in (A) and [DDMA]/mM in (B-D) are shown above the vessels. 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0%

0.02

0.03

0.06

0.1

0.2

0.3

0.6

1.0mM

0.02

0.03

0.06

0.1

0.2

0.3

0.6

1.0mM

0.02

0.03

0.06

0.1

0.2

0.3

0.6

1.0mM

A

0.01

B

0.01

C

0.01

D

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Figure 2. Micrographs of n-decane-in-water emulsions stabilized by 0.5 wt.% alumina nanoparticles in combination with cationic DDMA at different concentrations given taken 24 h after preparation. (a) 0.01mM

(b) 0.02mM

100m

(d) 0.06mM

(c) 0.03mM

100m

(f) 0.2mM

(e) 0.1mM

100m

(g) 0.3mM

100m

100m

(i) 1.0mM

(h) 0.6mM

100m

100m

100m

100m

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Figure 3. Digital photos of n-decane-in-water emulsions (7 mL/7 mL) stabilized by (A, C) 0.01 wt.% and (B, D) 0.001 wt.% alumina nanoparticles in combination with cationic DDMA at different concentrations, taken 24 h (A and B) and one month (C and D) after preparation. [DDMA]/mM is shown above the vessels. 0.001 0.003

0.006

0.01

0.03

0.06

0.1

0.3

0.6 1.0mM

0.006

0.01

0.03

0.06

0.1

0.3

0.6 1.0mM

0.006

0.01

0.03

0.06

0.1

0.3

0.6 1.0mM

0.006

0.01

0.03

0.06

0.1

0.3

0.6 1.0mM

A

0.001 0.003

B

0.001 0.003

C

0.001 0.003

D

Page | 26


Page 27 of 34 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

Figure 4. Micrographs of n-decane-in-water emulsions stabilized by 0.01 wt.% alumina nanoparticles in combination with cationic DDMA at different concentrations given taken 24 h after preparation. (b) 0.006mM

(a) 0.003mM

100m

(d) 0.03mM

(c) 0.01mM

(e) 0.06mM

100m

(g) 0.3mM

(f) 0.1mM

100m

100m

(i) 1.0mM

(h) 0.6mM

100m

100m

100m

100m

100m

Page | 27


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Page 28 of 34

Figure 5. Micrographs of n-decane-in-water emulsions stabilized by 0.001 wt.% alumina nanoparticles in combination with DDMA at different concentrations taken 24 h after preparation. (a) 0.003mM

(b) 0.006mM

100m

(d) 0.03mM

(c) 0.01mM

100m

(e) 0.06mM

100m

(g) 0.3mM

(f) 0.1mM

100m

100m

(i) 1.0mM

(h) 0.6mM

100m

100m

100m

100m

Page | 28


Page 29 of 34 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

Figure 6. (a, b) SEM images and (c, d) optical micrographs of dried (a, b and d) or half-dried (c) oil-in-water emulsion droplets stabilized by alumina nanoparticles in combination with either DDMA (a, b) or SDS (c, d) taken 24 h after preparation. (a) 0.001 wt.% alumina + 0.01 mM DDMA, n-decane, (b) 0.01 wt.% alumina + 0.01 mM DDMA, n-hexane, (c, d) 0.5 wt.% alumina + 0.1 mM SDS, n-hexane.

(a)

droplets (n-decane)

(b) droplets (n-hexane)

alumina particles

(c)

alumina particles

(d)

half dried droplets (n-hexane)

solid stamps of alumina particles

50 m

50 m

Page | 29


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Page 30 of 34

Figure 7. Appearance and micrographs of n-decane-in-water emulsions stabilized by 0.01 wt.% alumina nanoparticles in combination with 0.2 mM DDMA upon alternately bubbling N2 (30 mL/min at 65C for 50 min) and CO2 (40 mL/min at 0-5C for 150 min) followed by homogenization (H).

A

B

C

E

D

N2

CO2

N2

CO2

65oC

0-5oC (H)

65 oC

0-5oC (H)

(c)

(a)

100m

(e)

100m

100m

Page | 30


Page 31 of 34

Figure 8. Surface tension of aqueous solutions of cationic DDMA with (filled points and solid line) and without (open points and dashed line) 0.5 wt.% alumina nanoparticles as a function of initial DDMA concentration at 25 C. The points are measured and the lines are fits to the Szyszkowski equation.

-1

80

surface tension/mN m

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

70 60 50 40 30 20 10

DDMA alone DDMA+0.5% particles

0 1.0E-06 1.0E-05 1.0E-04 1.0E-03 1.0E-02 1.0E-01 [DDMA]/M

Page | 31


Langmuir

Figure 9. Zeta potential of 0.1 wt.% alumina nanoparticles dispersed in aqueous cationic DDMA solution as function of initial surfactant concentration and comparison with that dispersed in aqueous CTAB solution. The dashed line shows the critical zeta potential which separates stable (above the line) and unstable (below the line) emulsions.

zeta potential/mV

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

Page 32 of 34

50 45 40 35 30 25 20 15 10 5 0 1.0E-05

in DDMA in CTAB

1.0E-04

1.0E-03

1.0E-02

[surfactant]/M

Page | 32


Page 33 of 34

Figure 10. Illustration of the demulsification/re-stabilization cycles of an n-decane-in-water emulsion stabilized by positively charged alumina nanoparticles and cationic DDMA at very low concentration by bubbling N2 or CO2 followed by homogenization, respectively.

n iz a ti o

ng

Ho

o C)

mo

(60 N2

Water

gen

Oil

li bb

Oil

Oil

Oil

Bu

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

Water

Oil Water

Bubbling CO 2(0-5oC) Cationic DDMA

Al2O 3 Nanoparticle

Neutral DDMA

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Langmuir

TOC graphic

n iz a ti o

ng

Ho

o C)

mo

(60 N2

Water

gen

Oil

li bb

Oil

Oil

Oil

Bu

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

Page 34 of 34

Water

Oil Water

Bubbling CO 2(0-5oC) Cationic DDMA

Al2O 3 Nanoparticle

Neutral DDMA

Page | 34


Switchable Oil-in-Water Emulsions Stabilized by Like-Charged

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