Article pubs.acs.org/Langmuir
Switchable Pickering Emulsions Stabilized by Silica Nanoparticles Hydrophobized in Situ with a Conventional Cationic Surfactant Yue Zhu,† Jianzhong Jiang,† Kaihong Liu,† Zhenggang Cui,*,† and Bernard P. Binks*,‡ †
The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, 1800 Lihu Road, Wuxi, Jiangsu 214122, People’s Republic of China ‡ Surfactant and Colloid Group, Department of Chemistry, University of Hull, Hull HU6 7RX, U.K. S Supporting Information *
ABSTRACT: A stable oil-in-water Pickering emulsion stabilized by negatively charged silica nanoparticles hydrophobized in situ with a trace amount of a conventional cationic surfactant can be rendered unstable on addition of an equimolar amount of an anionic surfactant. The emulsion can be subsequently restabilized by adding a similar trace amount of cationic surfactant along with rehomogenization. This destabilization−stabilization behavior can be cycled many times, demonstrating that the Pickering emulsion is switchable. The trigger is the stronger electrostatic interaction between the oppositely charged ionic surfactants compared with that between the cationic surfactant and the (initially) negatively charged particle surfaces. The cationic surfactant prefers to form ion pairs with the added anionic surfactant and thus desorbs from particle surfaces rendering them surfaceinactive. This access to switchable Pickering emulsions is easier than those employing switchable surfactants, polymers, or surface-active particles, avoiding both the complicated synthesis and the stringent switching conditions.
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responsive to pH.9 These emulsions, however, are thermodynamically unstable and need in general high concentration (>critical micelle concentration, cmc) of surfactant or polymer as stabilizer, whether they are switchable or not. It is well-known that emulsions can also be stabilized by surface-active colloid particles,26,27 and so-called Pickering emulsions can be very stable due to the presence of a dense particle film at the oil−water interface which provides a mechanical barrier to coalescence. Many attempts have been made recently to prepare switchable or stimuli-responsible surface-active colloid particles such as those which are responsive to pH,8,10,11,28−31 temperature,32−34 redox,35 and light irradiation36,37 magnetic field,38,39 and to dual stimuli such as pH−temperature,6,7,40,41 light−temperature,42 and magnetic field intensity−temperature.43,44 Specifically various particles responsive to CO2/N2, an environmentally benign trigger, have been developed very recently.5,11,41,45−50 Nevertheless these particles are mostly functional polymeric particles, and their preparation is in general complicated. On the other hand, commercially available inert inorganic nanoparticles such as silica and calcium carbonate can also be made surface-active and
INTRODUCTION Emulsions and emulsification are significant in the preparation of many products such as foods, cosmetics, paints, and pesticides and in various technical and industrial processes such as emulsion polymerization, metal cutting and cleaning, and synthesis of nanoparticles.1 On the other hand, the opposite process of demulsification is also crucial in some industrial processes such as industrial extraction and petroleum recovery.1 Moreover, temporarily stabilized emulsions that need only to be stabilized for a specified period and have to be demulsified finally, such as those involved in emulsion polymerization, fuel production, and oil transport, need to be designed. Switchable or stimuli-responsive macro- or microemulsions, which can be transformed between stable and unstable forms are therefore attractive, and attention has been paid to them in recent years.2−11 Since the stability of an emulsion depends mainly on the emulsifiers/stabilizers used, the preparation of switchable or stimuli-responsive emulsions relies on the development of the corresponding switchable or stimuli-responsive stabilizer. Conventionally emulsions are stabilized by surfactants or polymers, and thus various switchable or stimuli-responsive surfactants and polymers have been developed recently, including surfactants responsive to CO2/N2,2 redox,12−15 pH,16−18 temperature,19 and light irradiation20−25 and polymers © 2015 American Chemical Society
Received: October 29, 2014 Revised: February 28, 2015 Published: March 3, 2015 3301
DOI: 10.1021/acs.langmuir.5b00295 Langmuir 2015, 31, 3301−3307
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surfactant concentrations are expressed as weight percent (wt %) and moles per liter (M) relative to the water phase, respectively. The emulsion type was determined using the drop test,51 and micrographs of the emulsion droplets were recorded using a VHX-1000 microscope system (Keyence Co.). The droplet sizes and size distribution of the emulsions were measured using a Malvern Mastersizer 2000 instrument by dispersing a drop of emulsion in an aqueous solution containing the same concentration of CTAB as in the aqueous phase used for preparing the emulsion. Emulsions (7 mL of water + 7 mL of oil) stabilized by 0.5 wt % silica nanoparticles together with 0.01 mM CTAB were rendered unstable by adding 0.07 g (mL) of 1 mM aqueous SDS solution followed by hand shaking or homogenization or sonication. They were restabilized by adding 0.07 g (mL) of 1 mM aqueous CTAB solution followed by homogenization at 7000 rpm for 2 min. Similar cycling was carried out in which SDS was replaced by its homologues of different chain length. The precision of adding concentrated CTAB or SDS solutions was controlled at 0.070 ± 0.002 g, and the head of the homogenizer was cleaned and dried each time before use during the cycling. Measurements. (a). Surface Tension. The surface tension of the surfactant solution with and without particles was measured using the du Noüy ring method at 25 °C using a tensiometer as described elsewere.51 (b). ζ Potential. The ζ potentials of 0.5 wt % silica nanoparticles dispersed in surfactant solutions at 25 °C were measured using a Brookhaven ZetaPLAS instrument 24 h after dispersion. (c). Adsorption of CTAB at Particle−Water Interface. The adsorption isotherm at 25 °C of CTAB at the silica particle−water interface at equilibrium concentrations < cmc was measured by depletion. The equilibrium surfactant concentration in a series of 0.5 wt % silica dispersions after adsorption for 24 h was calculated from the surface tension of the dispersions using the surface tension of CTAB solution without silica particles as calibration. (d). Accumulation of Silica Nanoparticles at the Air−Water Surface. 0.1 g amount of silica nanoparticles was initially dispersed in 20 cm3 of 0.1 mM CTAB solution in a 7.5 cm (h) × 2.5 cm (d) glass bottle, and the dispersion was transferred to a 125 cm3 cylindrical funnel. The funnel was then well-stoppered and shaken up and down vigorously 20 times. After settling for 10 min (to allow water drainage), the dispersion beneath the foam was drained through a valve at the bottom into a beaker and dried at 105 °C for 24 h. The weight of nanoparticles was weighed, and the percentage of nanoparticles adsorbed at the air−water surface of the foam was then calculated.55 All experiments were carried out at room temperature (20−25 °C) unless specified otherwise.
used as excellent Pickering emulsion stabilizers by either ex situ coating26 or in situ hydrophobization4,51,52 to modify their surface wettability. The latter method, based on the adsorption of ionic surfactants on oppositely charged particle surfaces, has advantages that the wettability/surface activity of the particles can be controlled by selection of the concentration and structure of the amphiphiles.51,52 Based on this principle, we have recently found that if a switchable surfactant is used, the switch of the surfactant can be transferred to particles, and thus switchable silica nanoparticles employing the CO2/N2 trigger and switchable Pickering emulsions and foams with the same trigger can be obtained using only a small amount (0.1−0.5 cmc) of switchable surfactant.4,53 Nevertheless the switching on and off of the emulsions or foams stabilized by switchable surfactants or particles with a CO2/N2 trigger (where the CO2 is combined with the alkylamidine molecule to form an alkylamidinium bicarbonate) has to be carried out at low (0−5 °C) and high (65 °C) temperature and takes a long time (1 h), which is practically inconvenient. Also the synthesis of such switchable surfactants is complicated and limited currently to cationic surfactants. Recently, Su et al.54 have described a method for making a conventional anionic surfactant such as sodium dodecyl sulfate (SDS) switchable or responsive to CO2 by reaction with dimethylethanolamine. Also, Liu et al.11 have developed zwitterionic polymer particles responsive to CO2/N2 which can be switched on and off at room temperature, where the trigger is changes in pH induced by bubbling CO2. Herein we report that silica nanoparticles hydrophobized in situ with a conventional cationic surfactant in water can be made switchable between surface-active and surface-inactive forms at room temperature. This is accomplished simply by in situ dehydrophobization of particles via formation of surfactant ion pairs with an equimolar amount of an anionic surfactant, which leads to desorption of the cationic surfactant from particle surfaces. Emulsions stabilized by such particles are thus destabilized.
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EXPERIMENTAL SECTION
Materials. Silica nanoparticles (HL-200; purity > 99.8%; primary particle diameter ≈ 20 nm; Brunauer−Emmett−Teller (BET) surface area = 200 ± 20 m2/g) were provided by Wuxi Jinding Longhua Chemical Co., China. Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the particles are shown in Supporting Information (SI) Figure S1. Cetyltrimethylammonium bromide (CTAB) of 99% purity was purchased from Sinopharm Chemical Reagent Co. Dodecyltrimethylammonium bromide (DTAB, 98%), sodium dodecyl sulfate, sodium decyl sulfate, sodium octyl sulfate (all of 99% purity), and sodium hexyl sulfate were purchased from Sigma. Dodecane (purity ≥ 98%), toluene (analytically pure), and tricaprylin (purity ≥ 99%) were purchased from Aladdin, Sinopharm, and Sigma, respectively, and were columned three times through neutral alumina before use. All other chemicals were analytically pure and purchased from Sinopharm. Ultrapure water with a resistance of 18.2 MΩ cm and a pH of 6.1 at 25 °C was purchased from Wuxi Huawei Co. Ltd., China. Preparation and Characterization of Dispersions and Emulsions. Silica nanoparticles were dispersed in pure water or surfactant solution using an ultrasonic probe (JYD-650, Shanghai) of tip diameter 0.6 cm operating at an output of 50 W for 1 min. For preparing emulsions, equal volumes (7 mL) of aqueous phase with either surfactant or silica nanoparticles or both and oil were put in a glass bottle of volume 25 cm3 (7.5 cm (h) × 2.5 cm (d)) followed by homogenization at 7000 rpm for 2 min using an ultraturrax homogenizer (IKA T18 basic, S18N-10G head). The particle and
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RESULTS AND DISCUSSION (a). Pickering Emulsions by in Situ Functionalization of Silica by CTAB. The bare silica nanoparticles with an isoelectric point at pH ≈ 2.5 (SI Figures S1 and S2) cannot stabilize a dodecane-in-water emulsion alone (Figure 1A) due to them being very hydrophilic.4 Similarly, CTAB, a typical conventional cationic surfactant, cannot stabilize a dodecane-inwater emulsion at concentrations lower than its cmc (0.9 mM, SI Figure S3), as shown in Figure 1B. However, in mixtures of 0.5 wt % silica nanoparticles and CTAB, very stable oil-in-water emulsions to coalescence were obtained as indicated by both the appearance of the vessels (Figure 1C−E) and the micrographs of the droplets (Figure 2A,F) which show no change between 24 h and 6 months after preparation. The high stability is also proved by the unchanged average droplet diameter of ∼60 μm in the presence of 0.06 mM CTAB (Figure 3) which is in good agreement with the sizes of the droplets in the micrograph (Figure 2C). The micrographs of the emulsions (Figure 2) show that the droplets decrease in size with increasing CTAB concentration but are in general much bigger 3302
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Figure 3. Size distribution of the dodecane-in-water (1:1) emulsion stabilized by a mixture of 0.5 wt % silica and 0.06 mM CTAB, 24 h and 1 month after preparation measured by light diffraction.
Figure 1. Photographs of dodecane-in-water (1:1) emulsions stabilized by (A) 0.5 wt % silica particles alone, (B) CTAB alone at different concentrations, and (C−E) mixtures of 0.5 wt % silica and CTAB at different concentrations taken at 24 h (A−C), 1 week (D), and 6 months (E) after preparation. [CTAB] in water from left to right: (B) 0.03, 0.06, 0.1, 0.3, 0.6, 1, 3, 6, and 10 mM; (C−E) 0.003, 0.006, 0.01, 0.03, 0.06, 0.1, 0.3, 0.6, 1, 3, 6, and 10 mM. Due to the high emulsion viscosity in vessels 9 and 10, some emulsion adheres to the glass wall giving the appearance of a larger overall volume.
Figure 4. ζ potentials (filled circles, left-hand ordinate) of 0.5 wt % silica nanoparticles dispersed in aqueous CTAB solutions and in equimolar CTAB + SDS solutions (open circles) as a function of initial CTAB concentration, and adsorption isotherm (triangles, right-hand ordinate) of CTAB at the silica particle−water interface as a function of equilibrium CTAB concentration.
than those stabilized by CTAB alone at 3 mM. This implies that these droplets are stabilized mainly by surfactant-coated silica nanoparticles instead of CTAB molecules at CTAB concentrations lower than its cmc, and the emulsions obtained are thus Pickering emulsions. The silica nanoparticles used are negatively charged in water at pH > 2.5 (SI Figure S2), and a ζ potential of −25.2 mV was measured in pure water of pH = 6.1. When dispersed in aqueous CTAB solutions, however, the ζ potential of the particles increases from negative to positive with increasing CTAB concentration as shown in Figure 4, indicating adsorption of CTAB molecules at the particle−water interface. The adsorption can be quantified further by determining the adsorption isotherm (also in Figure 4) based on depletion of CTAB in solution obtained by measuring the surface tension of
CTAB solutions with and without silica nanoparticles (SI Figure S3). The data show that at an equilibrium CTAB concentration of 0.83 mM which is close to the cmc of 0.90 mM, the adsorbed amount of CTAB increases to 0.44 mmol/g. This equates to a molecular area of 0.75 nm2 for CTAB at the particle surface implying monolayer adsorption. It is believed that the cationic surfactant molecules adsorb at the particle surface via electrostatic interactions forming a monolayer with their hydrophobic tails exposed toward water, which together with the neutralization of the negative charges on the particle surfaces renders particles partially hydrophobic and thus surface-active.51,53
Figure 2. Optical micrographs of dodecane-in-water (1:1) emulsions stabilized by a mixture of 0.5 wt % silica and CTAB at different concentrations (A−D and F) taken 24 h (A−D) and 6 months (F) after preparation and by CTAB alone (E) taken immediately after preparation. [CTAB] (from A to F): 0.01, 0.03, 0.06, 0.3, 3, and 0.01 mM. 3303
DOI: 10.1021/acs.langmuir.5b00295 Langmuir 2015, 31, 3301−3307
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Langmuir (b). Destabilization and Restabilization Cycling of Pickering Emulsions. We have noticed that when an equimolar amount of an anionic surfactant such as SDS is added into an emulsion stabilized by 0.5 wt % silica containing a trace amount of CTAB (0.01 mM), demulsification occurs soon after gentle agitation. If a further trace amount of CTAB is added followed by rehomogenization, a stable emulsion can be prepared again. This means that the Pickering emulsion stabilized by silica hydrophobized in situ with a conventional cationic surfactant can be made switchable. Successive cycling between stable and unstable emulsions is possible. For an emulsion containing 7 mL of water and 7 mL of dodecane stabilized by 0.5 wt % silica and 0.01 mM CTAB, demulsification occurs after adding 0.07 g (mL) of 1 mM aqueous SDS solution (moles of SDS added is equal to that of CTAB in the sample and the volume change of the aqueous phase is negligible) followed by gentle agitation. No stable emulsion can be formed if the mixture is homogenized again (Figure 5, cycle 1). However, once 0.07 g (mL) of 1 mM
Figure 6. Switching between stable and unstable toluene-in-water (1:1) emulsion (A) and tricaprylin-in-water (1:1) emulsion (B) containing 0.5 wt % silica nanoparticles in combination with 0.01 mM CTAB followed by addition of 0.01 M SDS and subsequently 0.01 mM CTAB, taken 24 h (stable) and 30 min (unstable) after addition, respectively.
then rendered bare again, and demulsification occurs. This is evidenced by the change of the ζ potential of the particles dispersed in CTAB solution following addition of SDS and CTAB alternately. It was found that the ζ potential of the particles increases from −25.2 mV in pure water to −15.4 mV in 0.01 mM CTAB solution, then decreases to −25.5 mV, and subsequently increases to −14.8 mV following addition of SDS and then CTAB in the first cycle. The corresponding data are −25.3/−15.3 mV for the fifth cycle and −25.1/−15.1 mV for the tenth cycle, as shown in Figure 5 together with the photographs and micrographs of emulsion samples. In fact the ζ potential of the silica nanoparticles dispersed directly in an equimolar mixture of CTAB + SDS at different concentrations shows no change with the value the same as in pure water, as shown in Figure 4 by the open circles, suggesting no adsorption of CTAB onto particle surfaces. These same dispersions do not form any stable emulsion with dodecane either. This postulated mechanism can further be evidenced by measuring the change of the amount of particles adsorbed at the oil−water interface following a cycle. In fact, we examined the change of the adsorbed amount of particles at the air−water interface by measuring the amount of particles remaining in the water phase after foaming.55 This is because both systems follow the same principle and it is easier and more accurate to collect particles in water from the drained foam than from the emulsion as only a fraction of the water is separated after emulsion creaming. It was found that by foaming 20 mL of a dispersion of 0.5 wt % silica in 0.1 mM aqueous CTAB solution (with 0.1 g particles initially) in a 250 mL column funnel, the mass of silica nanoparticles remaining in the water phase was 0.013 ± 0.001 g, corresponding to an adsorption percentage of 87%. By contrast, after adding an equimolar amount of SDS followed by shaking (resulting in defoaming), the mass of silica nanoparticles remaining in water increased to 0.074 ± 0.001 g, corresponding to an adsorption percentage of only 26%. This means that by adding an equimolar amount of SDS into the system, approximately 60% of the particles originally adsorbed at the air−water interface are returned to the water phase. It has been reported that the adsorption of cationic surfactants at a negatively charged silica surface in water is not affected in general by the presence of anionic surfactants.58 We think that this is probably true for ionic surfactants with relatively short chains so that the ion pairs formed are quite
Figure 5. ζ potentials (points) of 0.5 wt % silica nanoparticles dispersed in pure water, in an aqueous phase containing excess CTAB at 0.01 mM and in an aqueous phase containing CTAB + SDS equimolar mixture at 0.01 (cycle 1), 0.05 (cycle 5) ,and 0.1 mM (cycle 10), together with the photographs and micrographs of the dodecanein-water emulsions following switching on and switching off cycles taken 24 h after preparation.
aqueous CTAB solution is added, a stable emulsion forms again after homogenization. This change between stable and unstable emulsions has been cycled for 10 times by adding CTAB and SDS (each time 0.07 g of 1 mM solution) alternately until the concentration of both CTAB and SDS with respect to the water phase reaches 0.1 mM. Figure 5 shows the photographs of the emulsion and micrographs of the droplets in cycles 1, 5, and 10. It is seen that during the cycling the droplet sizes of the stable emulsion are similar, indicating that the stability and droplet sizes of the emulsion depend only on the presence and concentration of the excess CTAB in the aqueous phase. This switching behavior also occurs for other oil-in-water emulsions, including that for toluene and the triglyceride tricaprylin, as shown in Figure 6. (c). Postulated Mechanism of Destabilization of Emulsions. It is suggested that, in the absence of SDS, silica nanoparticles are hydrophobized in situ in water by the adsorption of CTAB molecules and become surface-active on addition of oil. However, when an equimolar amount of SDS is added into the emulsion, CTAB molecules tend to form ion pairs or complex with SDS molecules56,57 and thus desorb from particle surfaces due to the stronger electrostatic interaction between the anionic and cationic surfactants than that between the cationic surfactant and the negatively charged particle surface. The particles originally coated by cationic surfactant are 3304
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Langmuir soluble in water. Here the total hydrocarbon chain length of CTAB and SDS is so long (28 carbon atoms) that the tendency of CTAB molecules to form ion pairs with SDS molecules is very strong due to the limited solubility of the ion pairs. Kume et al.56 report that the ion pair of dodecylpyridinium chloride (DPCl) and SDS has a solubility product of 2.24 × 10−4 mM2, corresponding to an equimolar concentration of 0.015 mM. Thus, in our systems the product of the concentrations of surface-active cationic ion (CTA+) and surface-active anionic ion (DS−), which is between 1.0 × 10−4 and 1.0 × 10−2 mM2, may have been beyond its solubility product considering the longer hydrocarbon chain in CTAB compared with DPCl. The precipitation of ion pairs will reduce the concentration of CTAB in solution and thus improve desorption of CTAB from particle surfaces. This explanation is supported by experiments aimed at finding the minimum total number of carbon atoms in the surfactant hydrocarbon chains required for effective demulsification of the emulsions. In these experiments, both CTAB (C16) and DTAB (C12) were used as cationic surfactants and sodium alkylsulfates from C6 to C12 were used as anionic surfactants. It was found that at any concentration of DTAB between 0.01 and 0.6 mM, 0.5 wt % silica + DTAB can form a stable dodecane-in-water emulsion, as shown in SI Figure S4. For the emulsion stabilized by 0.5 wt % silica + 0.1 mM DTAB, demulsification only occurs on adding an equimolar amount of sodium decyl sulfate (C10) or SDS (C12), but not on adding sodium hexyl sulfate (C6) or sodium octyl sulfate (C8), as shown in Figure 7A. This suggests a minimum effective total
Figure 8. Photographs of emulsions stabilized by silica nanoparticles + CTAB demulsified by adding an equimolar amount of SDS followed by either shaking or sonication for 5 min, taken 2 h after operation. The concentration of particles and the individual concentrations of CTAB and SDS are shown on the vessels.
in SI Figure S5. Adding two or three times the number of moles of anionic surfactant even hindered the demulsification as shown in SI Figure S6. On the other hand demulsification becomes complete by placing the vessels in an ultrasonic bath for 5 min as shown in Figure 8. It turns out that the middle residual emulsion layer after demulsification is quite different from the starting emulsion and is composed of large drops alongside flocculated particles, as shown by the micrographs in Figure 9. After sonication the particles are dispersed and
Figure 9. Optical micrographs of the residual emulsion in dodecane (7 mL)-in-water (7 mL) emulsion demulsified by adding an equimolar amount of SDS, taken 24 h after addition: (A) after a third cycle with very large oil droplets; (B) after a tenth cycle with flocculated particles. The emulsion was originally stabilized by 0.5 wt % silica nanoparticles + 0.01 mM CTAB.
Figure 7. Demulsification of dodecane-in-water (1:1) Pickering emulsions stabilized by 0.5 wt % silica + 0.1 mM DTAB (A) or by 0.5 wt % silica + 0.01 mM CTAB (B) by adding an equimolar amount of sodium alkylsulfate of different chain lengths (given) followed by hand shaking, taken 2 h after shaking.
returned to the aqueous phase. The percentage of oil separated from a Pickering emulsion (7 mL of water + 7 mL of oil) stabilized by 0.5 wt % silica nanoparticles in combination with 0.01 mM CTAB after adding an equimolar amount of SDS (without soncation) is ca. 95% of that separated from an equal volume of a dodecane−water mixture (without particles or CTAB) after homogenization. Therefore, although about 26% of the particles do not return to the aqueous phase after adding SDS to destabilize the emulsions, the demulsification is nearly complete even without sonication. When the individual concentrations of CTAB or SDS are beyond 0.1 mM, the cycling of the emulsion between stable and unstable can be continued provided that the excess or free CTAB concentration in the systems is neither less than 0.01 mM nor beyond the minimum concentration required to stabilize an emulsion if used alone (Figure 1B), since in the latter case we no longer retain a Pickering emulsion. The cycles in the presence of relatively high concentrations of surfactants are shown in SI Figure S7, which indicate that the stabilization of the emulsions is quite good but the efficiency of demulsification reduces with an increase in the concentration of both surfactants in the system. The accumulation of the surfactants in the system is thus a drawback of the approach for
carbon number of 22. For the emulsion stabilized by 0.5 wt % silica + 0.01 mM CTAB, all of the sulfates from C6 to C12 are effective demulsifiers to some extent, although the C6 sulfate is much less effective, also suggesting a minimum effective total carbon number of 22, Figure 7B. We propose that the alkyl sulfates with shorter hydrocarbon chains are less effective in demulsification because the ion pairs formed with a cationic surfactant have larger solubility products. From the photographs in Figures 5−7, it can be seen that the demulsification by adding an equimolar amount of SDS is not complete. A layer of residual emulsion remains at the bottom of the coalesced oil phase. We have investigated several procedures for improving the demulsification, e.g., by decreasing the particle concentration, by homogenizing instead of hand shaking after adding anionic surfactant, by adding more than an equimolar amount of anionic surfactant, or by sonication. It was found that reducing the particle concentration from 0.5 to 0.25 wt % did not improve demulsification (Figure 8), and replacing hand shaking by homogenizing accelerated demulsification but the final resolved oil volume (2 h later) was similar as illustrated 3305
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a Switchable Surfactant. Angew. Chem., Int. Ed. 2013, 52, 12373− 12376. (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.; Lee, M. F. X.; Zhang, W.; Zhao, B.; Berry, R. M.; Tam, K.C. Dual Responsive Pickering Emulsion Stabilized by Poly[2(dimethylamino)ethyl methacrylate] Grafted Cellulose Nanocrystals. Biomacromolecules 2014, 15, 3052−3062. (7) Yamagami, T.; Kitayama, Y.; Okubo, M. Preparation of StimuliResponsive “Mushroom-Like” Janus Polymer Particles as Particulate Surfactant by Site-Selective Surface-Initiated AGET ATRP in Aqueous Dispersed Systems. Langmuir 2014, 30, 7823−7832. (8) Morse, A. J.; Armes, S. P.; Thompson, K. L.; Dupin, D.; Fielding, L. A.; Mills, P.; Swart, R. Novel Pickering Emulsifiers Based on pHResponsive Poly(2-(diethylamino)ethyl methacrylate) Latexes. Langmuir 2013, 29, 5466−5475. (9) Patel, A. R.; Drost, E.; ten Hoom, J. S. K.; Velikov, K. P. Fabrication and Characterization of Emulsions with pH Responsive Switchable Behavior. Soft Matter 2013, 9, 6747−6751. (10) Liu, H.; Wang, C.-Y.; Zou, S.-W.; Wei, Z.-J.; Tong, Z. Simple, Reversible Emulsion System Switched by pH on the Basis of Chitosan without Any Hydrophobic Modification. Langmuir 2012, 28, 11017− 11024. (11) 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. (12) Saji, T.; Hoshino, K.; Aoyagui, S. Reversible Formation and Disruption of Micelles by Control of the Redox State of the Head Group. J. Am. Chem. Soc. 1985, 107, 6865−6868. (13) Aydogan, N.; Abbott, N. L. Comparison of the Surface Activity and Bulk Aggregation of Ferrocenyl Surfactants with Cationic and Anionic Headgroups. Langmuir 2001, 17, 5703−5706. (14) Datwani, S. S.; Truskett, V. N.; Rosslee, C. A.; Abbott, N. L.; Stebe, K. J. Redox-Dependent Surface Tension and Surface Phase Transitions of a Ferrocenyl Surfactant: Equilibrium and Dynamic Analyses with Fluorescence Images. Langmuir 2003, 19, 8292−8301. (15) Schmittel, M.; Lal, M.; Graf, K.; Jeschke, G.; Suske, I.; Salbeck, J. N,N′-Dimethyl-2,3-Dialkylpyrazinium Salts as Redox-Switchable Surfactants? Redox, Spectral, EPR and Surfactant Properties. Chem. Commun. (Cambridge, U. K.) 2005, 5650−5652. (16) Dexter, A. F.; Malcolm, A. S.; Middelberg, A. P. J. Reversible Active Switching of the Mechanical Properties of a Peptide Film at a Fluid−Fluid Interface. Nat. Mater. 2006, 5, 502−506. (17) Balasuriya, T. S.; Dagastine, R. R. Interaction Forces between Bubbles in the Presence of Novel Responsive Peptide Surfactants. Langmuir 2012, 28, 17230−17237. (18) Malcolm, A. S.; Dexter, A. F.; Middelberg, A. P. J. Foaming Properties of a Peptide Designed to Form Stimuli-Responsive Interfacial Films. Soft Matter 2006, 2, 1057−1066. (19) Fameau, A. L.; Saint-Jalmes, A.; Cousin, F.; Houssou, B. H.; Novales, B.; Navailles, L.; Nallet, F.; Gaillard, C.; Boué, F.; Douliez, J. P. Smart Foams: Switching Reversibly between Ultrastable and Unstable Foams. Angew. Chem., Int. Ed. 2011, 50, 8264−8268. (20) Eastoe, J.; Sánchez-Dominguez, M.; Wyatta, P.; Heenan, R. K. A Photo-Responsive Organogel. Chem. Commun. (Cambridge, U. K.) 2004, 2608−2609. (21) Eastoe, J.; Vesperinas, A. Self-Assembly of Light-Sensitive Surfactants. Soft Matter 2005, 1, 338−347. (22) Ruchmann, J.; Fouilloux, S.; Tribet, C. Light-responsive hydrophobic association of surfactants with azobenzene-modified polymers. Soft Matter 2008, 4, 2098−2108. (23) Kö rdel, C.; Popeney, C. S.; Haag, R. Photoresponsive Amphiphiles Based on Azobenzene-Dendritic Glycerol Conjugates Show Switchable Transport Behavior. Chem. Commun. (Cambridge, U. K.) 2011, 47, 6584−6586. (24) Li, L.; Rosenthal, M.; Zhang, H.; Hernandez, J. J.; Drechsler, M.; Phan, K. H.; Rütten, S.; Zhu, X.; Ivanov, D. A.; Möler, M. Light-
multiple switching cycles compared with that employing a CO2/N2 trigger.
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CONCLUSION We have demonstrated that switchable Pickering oil-in-water emulsions can be prepared using negatively charged silica nanoparticles hydrophobized in situ with a trace amount of a conventional cationic surfactant. Destabilization is achieved by adding an equimolar amount of a conventional anionic surfactant which forms ion pairs with the cationic surfactant and restabilization occurs again on adding cationic surfactant. The trigger is the differential electrostatic interaction between the oppositely charged ionic surfactants and between the cationic surfactant and the negatively charged particle surfaces. This access to switchable Pickering emulsions is easier than that employing switchable surfactants, polymers, surface-active particles, or particles hydrophobized in situ with switchable surfactants, due to avoidance of both the complicated synthesis and the rigorous switching conditions such as changing temperature or pH. The particles and ionic surfactants are all commercially available, and the ionic surfactants used are in trace amount (≪cmc) if only one or a few switching cycles are needed, the case for most practical applications. We anticiapte that this principle is also applicable for initially positively charged particles. Although the accumulation of the surfactants in the systems may be a problem, the surfactant added is usually in trace amounts and is therefore negligible compared with emulsions stabilized solely by surfactants.
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ASSOCIATED CONTENT
S Supporting Information *
Figures S1−S7 showing SEM and TEM images of powdered silica nanoparticles, ζ potentials of silica nanoparticles in different pH solutions, surface tensions of aqueous solutions of CTAB with and without silica nanoparticles, and photographs of dodecane emulsions stabilized by silica nanoparticles. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*(Z.C.) E-mail: [email protected]. *(B.P.B.) E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (Grant No. NSFC 21473080) and from the Fundamental Research Funds for the Central Universities (Grant No. JUSRP51405A) is gratefully acknowledged.
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REFERENCES
(1) Rosen, M. J., Kunjappu, J. T. Surfactants and Interfacial Phenomena, 4th ed.; John Wiley & Son: Hoboken, NJ, 2012; Chapter 8. (2) Liu, Y. X.; Jessop, P. G.; Cunningham, M.; Eckert, C. A.; Liotta, C. L. Switchable Surfactants. Science 2006, 313, 958−960. (3) Eastoe, J.; Wyatt, P.; Sánchez-Dominguez, M.; Vesperinas, A.; Paul, A.; Heenan, R. K.; Grillo, I. Photo-stabilized Microemulsions. Chem. Commun. (Cambridge, U. K.) 2005, 2785−2786. (4) Jiang, J.-Z.; Zhu, Y.; Cui, Z.-G.; Binks, B. P. Switchable Pickering Emulsions Stabilized by Silica Nanoparticles Hydrophobized In Situ by 3306
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(46) Zhang, Q.; Wang, W.-J.; Lu, Y.-Y.; Li, B.-G.; Zhu, S.-P. Reversibly Coagulatable and Redispersible Polystyrene Latex Prepared by Emulsion Polymerization of Styrene Containing Switchable Amidine. Macromolecules 2011, 44, 6539−6545. (47) Su, X.; Jessop, P. G.; Cunningham, M. F. Surfactant-Free Polymerization Forming Switchable Latexes That Can Be Aggregated and Redispersed by CO2 Removal and Then Readdition. Macromolecules 2012, 45, 666−670. (48) Pinaud, J.; Kowal, E.; Cunningham, M.; Jessop, P. 2(Diethyl)aminoethyl Methacrylate as a CO2-Switchable Comonomer for the Preparation of Readily Coagulated and Redispersed Polymer Latexes. ACS Macro Lett. 2012, 1, 1103−1107. (49) Zhang, Q.; Yu, G.-Q.; Wang, W.-J.; Yuan, H.-M; Li, B.-G.; Zhu, S.-P. Switchable Block Copolymer Surfactants for Preparation of Reversibly Coagulatable and Redispersible Poly(methyl methacrylate) Latexes. Macromolecules 2013, 46, 1261−1267. (50) Lin, S. J.; Theato, P. CO2-Responsive Polymers. Macromol. Rapid Commun. 2013, 34, 1118−1133. (51) 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 Cationic Surfactant. Langmuir 2010, 26, 4717−4724. (52) Cui, Z.-G.; Cui, C.-F.; Zhu, Y.; Binks, B. P. Multiple Phase Inversion of Emulsions Stabilized by in Situ Surface Activation of CaCO3 Nanoparticles via Adsorption of Fatty Acids. Langmuir 2012, 28, 314−320. (53) Zhu, Y.; Jiang, J.-Z.; Cui, Z.-G.; Binks, B. P. Responsive Aqueous Foams Stabilized by Silica Nanoparticles Hydrophobized in Situ with a Switchable Surfactant. Soft Matter 2014, 10, 9739−9745. (54) Su, X.; Robert, T.; Mercer, S. M.; Humphries, C.; Cunningham, M. F.; Jessop, P. G. A Conventional Surfactant Becomes CO2Responsive in the Presence of Switchable Water Additives. Chem. Eur. J. 2013, 19, 5595−5601. (55) 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. (56) Kume, G.; Gallotti, M.; Nunes, G. Review on Anionic/Cationic Surfactant Mixtures. J. Surf. Det. 2008, 11, 1−11. (57) Tah, B.; Pal, P.; Mahato, M.; Talapatra, G. B. Aggregation Behavior of SDS/CTAB Catanionic Surfactant Mixture in Aqueous Solution and at the Air/Water Interface. J. Phys. Chem. B 2011, 115, 8493−8499. (58) Jin, Y.-Z.; Wang, C.; Zhang, J.; Bao, Y.-X.; Zhao, Z.-G.; Xiao, J.X. Adsorption of Cationic-Anionic Surfactant Mixtures on Silica Gel/ Water and Silica Gel/Mineralized Water Interfaces. Acta Chim. Sin. 2005, 63, 239−242.
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Switchable Pickering Emulsions Stabilized by Silica Nanoparticles Hydrophobized in Situ with a Conventional Cationic Surfactant Yue Zhu,† Jianzhong Jiang,† Kaihong Liu,† Zhenggang Cui,*,† and Bernard P. Binks*,‡ †
The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, 1800 Lihu Road, Wuxi, Jiangsu 214122, People’s Republic of China ‡ Surfactant and Colloid Group, Department of Chemistry, University of Hull, Hull HU6 7RX, U.K. S Supporting Information *
ABSTRACT: A stable oil-in-water Pickering emulsion stabilized by negatively charged silica nanoparticles hydrophobized in situ with a trace amount of a conventional cationic surfactant can be rendered unstable on addition of an equimolar amount of an anionic surfactant. The emulsion can be subsequently restabilized by adding a similar trace amount of cationic surfactant along with rehomogenization. This destabilization−stabilization behavior can be cycled many times, demonstrating that the Pickering emulsion is switchable. The trigger is the stronger electrostatic interaction between the oppositely charged ionic surfactants compared with that between the cationic surfactant and the (initially) negatively charged particle surfaces. The cationic surfactant prefers to form ion pairs with the added anionic surfactant and thus desorbs from particle surfaces rendering them surfaceinactive. This access to switchable Pickering emulsions is easier than those employing switchable surfactants, polymers, or surface-active particles, avoiding both the complicated synthesis and the stringent switching conditions.
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responsive to pH.9 These emulsions, however, are thermodynamically unstable and need in general high concentration (>critical micelle concentration, cmc) of surfactant or polymer as stabilizer, whether they are switchable or not. It is well-known that emulsions can also be stabilized by surface-active colloid particles,26,27 and so-called Pickering emulsions can be very stable due to the presence of a dense particle film at the oil−water interface which provides a mechanical barrier to coalescence. Many attempts have been made recently to prepare switchable or stimuli-responsible surface-active colloid particles such as those which are responsive to pH,8,10,11,28−31 temperature,32−34 redox,35 and light irradiation36,37 magnetic field,38,39 and to dual stimuli such as pH−temperature,6,7,40,41 light−temperature,42 and magnetic field intensity−temperature.43,44 Specifically various particles responsive to CO2/N2, an environmentally benign trigger, have been developed very recently.5,11,41,45−50 Nevertheless these particles are mostly functional polymeric particles, and their preparation is in general complicated. On the other hand, commercially available inert inorganic nanoparticles such as silica and calcium carbonate can also be made surface-active and
INTRODUCTION Emulsions and emulsification are significant in the preparation of many products such as foods, cosmetics, paints, and pesticides and in various technical and industrial processes such as emulsion polymerization, metal cutting and cleaning, and synthesis of nanoparticles.1 On the other hand, the opposite process of demulsification is also crucial in some industrial processes such as industrial extraction and petroleum recovery.1 Moreover, temporarily stabilized emulsions that need only to be stabilized for a specified period and have to be demulsified finally, such as those involved in emulsion polymerization, fuel production, and oil transport, need to be designed. Switchable or stimuli-responsive macro- or microemulsions, which can be transformed between stable and unstable forms are therefore attractive, and attention has been paid to them in recent years.2−11 Since the stability of an emulsion depends mainly on the emulsifiers/stabilizers used, the preparation of switchable or stimuli-responsive emulsions relies on the development of the corresponding switchable or stimuli-responsive stabilizer. Conventionally emulsions are stabilized by surfactants or polymers, and thus various switchable or stimuli-responsive surfactants and polymers have been developed recently, including surfactants responsive to CO2/N2,2 redox,12−15 pH,16−18 temperature,19 and light irradiation20−25 and polymers © 2015 American Chemical Society
Received: October 29, 2014 Revised: February 28, 2015 Published: March 3, 2015 3301
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surfactant concentrations are expressed as weight percent (wt %) and moles per liter (M) relative to the water phase, respectively. The emulsion type was determined using the drop test,51 and micrographs of the emulsion droplets were recorded using a VHX-1000 microscope system (Keyence Co.). The droplet sizes and size distribution of the emulsions were measured using a Malvern Mastersizer 2000 instrument by dispersing a drop of emulsion in an aqueous solution containing the same concentration of CTAB as in the aqueous phase used for preparing the emulsion. Emulsions (7 mL of water + 7 mL of oil) stabilized by 0.5 wt % silica nanoparticles together with 0.01 mM CTAB were rendered unstable by adding 0.07 g (mL) of 1 mM aqueous SDS solution followed by hand shaking or homogenization or sonication. They were restabilized by adding 0.07 g (mL) of 1 mM aqueous CTAB solution followed by homogenization at 7000 rpm for 2 min. Similar cycling was carried out in which SDS was replaced by its homologues of different chain length. The precision of adding concentrated CTAB or SDS solutions was controlled at 0.070 ± 0.002 g, and the head of the homogenizer was cleaned and dried each time before use during the cycling. Measurements. (a). Surface Tension. The surface tension of the surfactant solution with and without particles was measured using the du Noüy ring method at 25 °C using a tensiometer as described elsewere.51 (b). ζ Potential. The ζ potentials of 0.5 wt % silica nanoparticles dispersed in surfactant solutions at 25 °C were measured using a Brookhaven ZetaPLAS instrument 24 h after dispersion. (c). Adsorption of CTAB at Particle−Water Interface. The adsorption isotherm at 25 °C of CTAB at the silica particle−water interface at equilibrium concentrations < cmc was measured by depletion. The equilibrium surfactant concentration in a series of 0.5 wt % silica dispersions after adsorption for 24 h was calculated from the surface tension of the dispersions using the surface tension of CTAB solution without silica particles as calibration. (d). Accumulation of Silica Nanoparticles at the Air−Water Surface. 0.1 g amount of silica nanoparticles was initially dispersed in 20 cm3 of 0.1 mM CTAB solution in a 7.5 cm (h) × 2.5 cm (d) glass bottle, and the dispersion was transferred to a 125 cm3 cylindrical funnel. The funnel was then well-stoppered and shaken up and down vigorously 20 times. After settling for 10 min (to allow water drainage), the dispersion beneath the foam was drained through a valve at the bottom into a beaker and dried at 105 °C for 24 h. The weight of nanoparticles was weighed, and the percentage of nanoparticles adsorbed at the air−water surface of the foam was then calculated.55 All experiments were carried out at room temperature (20−25 °C) unless specified otherwise.
used as excellent Pickering emulsion stabilizers by either ex situ coating26 or in situ hydrophobization4,51,52 to modify their surface wettability. The latter method, based on the adsorption of ionic surfactants on oppositely charged particle surfaces, has advantages that the wettability/surface activity of the particles can be controlled by selection of the concentration and structure of the amphiphiles.51,52 Based on this principle, we have recently found that if a switchable surfactant is used, the switch of the surfactant can be transferred to particles, and thus switchable silica nanoparticles employing the CO2/N2 trigger and switchable Pickering emulsions and foams with the same trigger can be obtained using only a small amount (0.1−0.5 cmc) of switchable surfactant.4,53 Nevertheless the switching on and off of the emulsions or foams stabilized by switchable surfactants or particles with a CO2/N2 trigger (where the CO2 is combined with the alkylamidine molecule to form an alkylamidinium bicarbonate) has to be carried out at low (0−5 °C) and high (65 °C) temperature and takes a long time (1 h), which is practically inconvenient. Also the synthesis of such switchable surfactants is complicated and limited currently to cationic surfactants. Recently, Su et al.54 have described a method for making a conventional anionic surfactant such as sodium dodecyl sulfate (SDS) switchable or responsive to CO2 by reaction with dimethylethanolamine. Also, Liu et al.11 have developed zwitterionic polymer particles responsive to CO2/N2 which can be switched on and off at room temperature, where the trigger is changes in pH induced by bubbling CO2. Herein we report that silica nanoparticles hydrophobized in situ with a conventional cationic surfactant in water can be made switchable between surface-active and surface-inactive forms at room temperature. This is accomplished simply by in situ dehydrophobization of particles via formation of surfactant ion pairs with an equimolar amount of an anionic surfactant, which leads to desorption of the cationic surfactant from particle surfaces. Emulsions stabilized by such particles are thus destabilized.
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EXPERIMENTAL SECTION
Materials. Silica nanoparticles (HL-200; purity > 99.8%; primary particle diameter ≈ 20 nm; Brunauer−Emmett−Teller (BET) surface area = 200 ± 20 m2/g) were provided by Wuxi Jinding Longhua Chemical Co., China. Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the particles are shown in Supporting Information (SI) Figure S1. Cetyltrimethylammonium bromide (CTAB) of 99% purity was purchased from Sinopharm Chemical Reagent Co. Dodecyltrimethylammonium bromide (DTAB, 98%), sodium dodecyl sulfate, sodium decyl sulfate, sodium octyl sulfate (all of 99% purity), and sodium hexyl sulfate were purchased from Sigma. Dodecane (purity ≥ 98%), toluene (analytically pure), and tricaprylin (purity ≥ 99%) were purchased from Aladdin, Sinopharm, and Sigma, respectively, and were columned three times through neutral alumina before use. All other chemicals were analytically pure and purchased from Sinopharm. Ultrapure water with a resistance of 18.2 MΩ cm and a pH of 6.1 at 25 °C was purchased from Wuxi Huawei Co. Ltd., China. Preparation and Characterization of Dispersions and Emulsions. Silica nanoparticles were dispersed in pure water or surfactant solution using an ultrasonic probe (JYD-650, Shanghai) of tip diameter 0.6 cm operating at an output of 50 W for 1 min. For preparing emulsions, equal volumes (7 mL) of aqueous phase with either surfactant or silica nanoparticles or both and oil were put in a glass bottle of volume 25 cm3 (7.5 cm (h) × 2.5 cm (d)) followed by homogenization at 7000 rpm for 2 min using an ultraturrax homogenizer (IKA T18 basic, S18N-10G head). The particle and
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RESULTS AND DISCUSSION (a). Pickering Emulsions by in Situ Functionalization of Silica by CTAB. The bare silica nanoparticles with an isoelectric point at pH ≈ 2.5 (SI Figures S1 and S2) cannot stabilize a dodecane-in-water emulsion alone (Figure 1A) due to them being very hydrophilic.4 Similarly, CTAB, a typical conventional cationic surfactant, cannot stabilize a dodecane-inwater emulsion at concentrations lower than its cmc (0.9 mM, SI Figure S3), as shown in Figure 1B. However, in mixtures of 0.5 wt % silica nanoparticles and CTAB, very stable oil-in-water emulsions to coalescence were obtained as indicated by both the appearance of the vessels (Figure 1C−E) and the micrographs of the droplets (Figure 2A,F) which show no change between 24 h and 6 months after preparation. The high stability is also proved by the unchanged average droplet diameter of ∼60 μm in the presence of 0.06 mM CTAB (Figure 3) which is in good agreement with the sizes of the droplets in the micrograph (Figure 2C). The micrographs of the emulsions (Figure 2) show that the droplets decrease in size with increasing CTAB concentration but are in general much bigger 3302
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Figure 3. Size distribution of the dodecane-in-water (1:1) emulsion stabilized by a mixture of 0.5 wt % silica and 0.06 mM CTAB, 24 h and 1 month after preparation measured by light diffraction.
Figure 1. Photographs of dodecane-in-water (1:1) emulsions stabilized by (A) 0.5 wt % silica particles alone, (B) CTAB alone at different concentrations, and (C−E) mixtures of 0.5 wt % silica and CTAB at different concentrations taken at 24 h (A−C), 1 week (D), and 6 months (E) after preparation. [CTAB] in water from left to right: (B) 0.03, 0.06, 0.1, 0.3, 0.6, 1, 3, 6, and 10 mM; (C−E) 0.003, 0.006, 0.01, 0.03, 0.06, 0.1, 0.3, 0.6, 1, 3, 6, and 10 mM. Due to the high emulsion viscosity in vessels 9 and 10, some emulsion adheres to the glass wall giving the appearance of a larger overall volume.
Figure 4. ζ potentials (filled circles, left-hand ordinate) of 0.5 wt % silica nanoparticles dispersed in aqueous CTAB solutions and in equimolar CTAB + SDS solutions (open circles) as a function of initial CTAB concentration, and adsorption isotherm (triangles, right-hand ordinate) of CTAB at the silica particle−water interface as a function of equilibrium CTAB concentration.
than those stabilized by CTAB alone at 3 mM. This implies that these droplets are stabilized mainly by surfactant-coated silica nanoparticles instead of CTAB molecules at CTAB concentrations lower than its cmc, and the emulsions obtained are thus Pickering emulsions. The silica nanoparticles used are negatively charged in water at pH > 2.5 (SI Figure S2), and a ζ potential of −25.2 mV was measured in pure water of pH = 6.1. When dispersed in aqueous CTAB solutions, however, the ζ potential of the particles increases from negative to positive with increasing CTAB concentration as shown in Figure 4, indicating adsorption of CTAB molecules at the particle−water interface. The adsorption can be quantified further by determining the adsorption isotherm (also in Figure 4) based on depletion of CTAB in solution obtained by measuring the surface tension of
CTAB solutions with and without silica nanoparticles (SI Figure S3). The data show that at an equilibrium CTAB concentration of 0.83 mM which is close to the cmc of 0.90 mM, the adsorbed amount of CTAB increases to 0.44 mmol/g. This equates to a molecular area of 0.75 nm2 for CTAB at the particle surface implying monolayer adsorption. It is believed that the cationic surfactant molecules adsorb at the particle surface via electrostatic interactions forming a monolayer with their hydrophobic tails exposed toward water, which together with the neutralization of the negative charges on the particle surfaces renders particles partially hydrophobic and thus surface-active.51,53
Figure 2. Optical micrographs of dodecane-in-water (1:1) emulsions stabilized by a mixture of 0.5 wt % silica and CTAB at different concentrations (A−D and F) taken 24 h (A−D) and 6 months (F) after preparation and by CTAB alone (E) taken immediately after preparation. [CTAB] (from A to F): 0.01, 0.03, 0.06, 0.3, 3, and 0.01 mM. 3303
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Langmuir (b). Destabilization and Restabilization Cycling of Pickering Emulsions. We have noticed that when an equimolar amount of an anionic surfactant such as SDS is added into an emulsion stabilized by 0.5 wt % silica containing a trace amount of CTAB (0.01 mM), demulsification occurs soon after gentle agitation. If a further trace amount of CTAB is added followed by rehomogenization, a stable emulsion can be prepared again. This means that the Pickering emulsion stabilized by silica hydrophobized in situ with a conventional cationic surfactant can be made switchable. Successive cycling between stable and unstable emulsions is possible. For an emulsion containing 7 mL of water and 7 mL of dodecane stabilized by 0.5 wt % silica and 0.01 mM CTAB, demulsification occurs after adding 0.07 g (mL) of 1 mM aqueous SDS solution (moles of SDS added is equal to that of CTAB in the sample and the volume change of the aqueous phase is negligible) followed by gentle agitation. No stable emulsion can be formed if the mixture is homogenized again (Figure 5, cycle 1). However, once 0.07 g (mL) of 1 mM
Figure 6. Switching between stable and unstable toluene-in-water (1:1) emulsion (A) and tricaprylin-in-water (1:1) emulsion (B) containing 0.5 wt % silica nanoparticles in combination with 0.01 mM CTAB followed by addition of 0.01 M SDS and subsequently 0.01 mM CTAB, taken 24 h (stable) and 30 min (unstable) after addition, respectively.
then rendered bare again, and demulsification occurs. This is evidenced by the change of the ζ potential of the particles dispersed in CTAB solution following addition of SDS and CTAB alternately. It was found that the ζ potential of the particles increases from −25.2 mV in pure water to −15.4 mV in 0.01 mM CTAB solution, then decreases to −25.5 mV, and subsequently increases to −14.8 mV following addition of SDS and then CTAB in the first cycle. The corresponding data are −25.3/−15.3 mV for the fifth cycle and −25.1/−15.1 mV for the tenth cycle, as shown in Figure 5 together with the photographs and micrographs of emulsion samples. In fact the ζ potential of the silica nanoparticles dispersed directly in an equimolar mixture of CTAB + SDS at different concentrations shows no change with the value the same as in pure water, as shown in Figure 4 by the open circles, suggesting no adsorption of CTAB onto particle surfaces. These same dispersions do not form any stable emulsion with dodecane either. This postulated mechanism can further be evidenced by measuring the change of the amount of particles adsorbed at the oil−water interface following a cycle. In fact, we examined the change of the adsorbed amount of particles at the air−water interface by measuring the amount of particles remaining in the water phase after foaming.55 This is because both systems follow the same principle and it is easier and more accurate to collect particles in water from the drained foam than from the emulsion as only a fraction of the water is separated after emulsion creaming. It was found that by foaming 20 mL of a dispersion of 0.5 wt % silica in 0.1 mM aqueous CTAB solution (with 0.1 g particles initially) in a 250 mL column funnel, the mass of silica nanoparticles remaining in the water phase was 0.013 ± 0.001 g, corresponding to an adsorption percentage of 87%. By contrast, after adding an equimolar amount of SDS followed by shaking (resulting in defoaming), the mass of silica nanoparticles remaining in water increased to 0.074 ± 0.001 g, corresponding to an adsorption percentage of only 26%. This means that by adding an equimolar amount of SDS into the system, approximately 60% of the particles originally adsorbed at the air−water interface are returned to the water phase. It has been reported that the adsorption of cationic surfactants at a negatively charged silica surface in water is not affected in general by the presence of anionic surfactants.58 We think that this is probably true for ionic surfactants with relatively short chains so that the ion pairs formed are quite
Figure 5. ζ potentials (points) of 0.5 wt % silica nanoparticles dispersed in pure water, in an aqueous phase containing excess CTAB at 0.01 mM and in an aqueous phase containing CTAB + SDS equimolar mixture at 0.01 (cycle 1), 0.05 (cycle 5) ,and 0.1 mM (cycle 10), together with the photographs and micrographs of the dodecanein-water emulsions following switching on and switching off cycles taken 24 h after preparation.
aqueous CTAB solution is added, a stable emulsion forms again after homogenization. This change between stable and unstable emulsions has been cycled for 10 times by adding CTAB and SDS (each time 0.07 g of 1 mM solution) alternately until the concentration of both CTAB and SDS with respect to the water phase reaches 0.1 mM. Figure 5 shows the photographs of the emulsion and micrographs of the droplets in cycles 1, 5, and 10. It is seen that during the cycling the droplet sizes of the stable emulsion are similar, indicating that the stability and droplet sizes of the emulsion depend only on the presence and concentration of the excess CTAB in the aqueous phase. This switching behavior also occurs for other oil-in-water emulsions, including that for toluene and the triglyceride tricaprylin, as shown in Figure 6. (c). Postulated Mechanism of Destabilization of Emulsions. It is suggested that, in the absence of SDS, silica nanoparticles are hydrophobized in situ in water by the adsorption of CTAB molecules and become surface-active on addition of oil. However, when an equimolar amount of SDS is added into the emulsion, CTAB molecules tend to form ion pairs or complex with SDS molecules56,57 and thus desorb from particle surfaces due to the stronger electrostatic interaction between the anionic and cationic surfactants than that between the cationic surfactant and the negatively charged particle surface. The particles originally coated by cationic surfactant are 3304
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Langmuir soluble in water. Here the total hydrocarbon chain length of CTAB and SDS is so long (28 carbon atoms) that the tendency of CTAB molecules to form ion pairs with SDS molecules is very strong due to the limited solubility of the ion pairs. Kume et al.56 report that the ion pair of dodecylpyridinium chloride (DPCl) and SDS has a solubility product of 2.24 × 10−4 mM2, corresponding to an equimolar concentration of 0.015 mM. Thus, in our systems the product of the concentrations of surface-active cationic ion (CTA+) and surface-active anionic ion (DS−), which is between 1.0 × 10−4 and 1.0 × 10−2 mM2, may have been beyond its solubility product considering the longer hydrocarbon chain in CTAB compared with DPCl. The precipitation of ion pairs will reduce the concentration of CTAB in solution and thus improve desorption of CTAB from particle surfaces. This explanation is supported by experiments aimed at finding the minimum total number of carbon atoms in the surfactant hydrocarbon chains required for effective demulsification of the emulsions. In these experiments, both CTAB (C16) and DTAB (C12) were used as cationic surfactants and sodium alkylsulfates from C6 to C12 were used as anionic surfactants. It was found that at any concentration of DTAB between 0.01 and 0.6 mM, 0.5 wt % silica + DTAB can form a stable dodecane-in-water emulsion, as shown in SI Figure S4. For the emulsion stabilized by 0.5 wt % silica + 0.1 mM DTAB, demulsification only occurs on adding an equimolar amount of sodium decyl sulfate (C10) or SDS (C12), but not on adding sodium hexyl sulfate (C6) or sodium octyl sulfate (C8), as shown in Figure 7A. This suggests a minimum effective total
Figure 8. Photographs of emulsions stabilized by silica nanoparticles + CTAB demulsified by adding an equimolar amount of SDS followed by either shaking or sonication for 5 min, taken 2 h after operation. The concentration of particles and the individual concentrations of CTAB and SDS are shown on the vessels.
in SI Figure S5. Adding two or three times the number of moles of anionic surfactant even hindered the demulsification as shown in SI Figure S6. On the other hand demulsification becomes complete by placing the vessels in an ultrasonic bath for 5 min as shown in Figure 8. It turns out that the middle residual emulsion layer after demulsification is quite different from the starting emulsion and is composed of large drops alongside flocculated particles, as shown by the micrographs in Figure 9. After sonication the particles are dispersed and
Figure 9. Optical micrographs of the residual emulsion in dodecane (7 mL)-in-water (7 mL) emulsion demulsified by adding an equimolar amount of SDS, taken 24 h after addition: (A) after a third cycle with very large oil droplets; (B) after a tenth cycle with flocculated particles. The emulsion was originally stabilized by 0.5 wt % silica nanoparticles + 0.01 mM CTAB.
Figure 7. Demulsification of dodecane-in-water (1:1) Pickering emulsions stabilized by 0.5 wt % silica + 0.1 mM DTAB (A) or by 0.5 wt % silica + 0.01 mM CTAB (B) by adding an equimolar amount of sodium alkylsulfate of different chain lengths (given) followed by hand shaking, taken 2 h after shaking.
returned to the aqueous phase. The percentage of oil separated from a Pickering emulsion (7 mL of water + 7 mL of oil) stabilized by 0.5 wt % silica nanoparticles in combination with 0.01 mM CTAB after adding an equimolar amount of SDS (without soncation) is ca. 95% of that separated from an equal volume of a dodecane−water mixture (without particles or CTAB) after homogenization. Therefore, although about 26% of the particles do not return to the aqueous phase after adding SDS to destabilize the emulsions, the demulsification is nearly complete even without sonication. When the individual concentrations of CTAB or SDS are beyond 0.1 mM, the cycling of the emulsion between stable and unstable can be continued provided that the excess or free CTAB concentration in the systems is neither less than 0.01 mM nor beyond the minimum concentration required to stabilize an emulsion if used alone (Figure 1B), since in the latter case we no longer retain a Pickering emulsion. The cycles in the presence of relatively high concentrations of surfactants are shown in SI Figure S7, which indicate that the stabilization of the emulsions is quite good but the efficiency of demulsification reduces with an increase in the concentration of both surfactants in the system. The accumulation of the surfactants in the system is thus a drawback of the approach for
carbon number of 22. For the emulsion stabilized by 0.5 wt % silica + 0.01 mM CTAB, all of the sulfates from C6 to C12 are effective demulsifiers to some extent, although the C6 sulfate is much less effective, also suggesting a minimum effective total carbon number of 22, Figure 7B. We propose that the alkyl sulfates with shorter hydrocarbon chains are less effective in demulsification because the ion pairs formed with a cationic surfactant have larger solubility products. From the photographs in Figures 5−7, it can be seen that the demulsification by adding an equimolar amount of SDS is not complete. A layer of residual emulsion remains at the bottom of the coalesced oil phase. We have investigated several procedures for improving the demulsification, e.g., by decreasing the particle concentration, by homogenizing instead of hand shaking after adding anionic surfactant, by adding more than an equimolar amount of anionic surfactant, or by sonication. It was found that reducing the particle concentration from 0.5 to 0.25 wt % did not improve demulsification (Figure 8), and replacing hand shaking by homogenizing accelerated demulsification but the final resolved oil volume (2 h later) was similar as illustrated 3305
DOI: 10.1021/acs.langmuir.5b00295 Langmuir 2015, 31, 3301−3307
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a Switchable Surfactant. Angew. Chem., Int. Ed. 2013, 52, 12373− 12376. (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.; Lee, M. F. X.; Zhang, W.; Zhao, B.; Berry, R. M.; Tam, K.C. Dual Responsive Pickering Emulsion Stabilized by Poly[2(dimethylamino)ethyl methacrylate] Grafted Cellulose Nanocrystals. Biomacromolecules 2014, 15, 3052−3062. (7) Yamagami, T.; Kitayama, Y.; Okubo, M. Preparation of StimuliResponsive “Mushroom-Like” Janus Polymer Particles as Particulate Surfactant by Site-Selective Surface-Initiated AGET ATRP in Aqueous Dispersed Systems. Langmuir 2014, 30, 7823−7832. (8) Morse, A. J.; Armes, S. P.; Thompson, K. L.; Dupin, D.; Fielding, L. A.; Mills, P.; Swart, R. Novel Pickering Emulsifiers Based on pHResponsive Poly(2-(diethylamino)ethyl methacrylate) Latexes. Langmuir 2013, 29, 5466−5475. (9) Patel, A. R.; Drost, E.; ten Hoom, J. S. K.; Velikov, K. P. Fabrication and Characterization of Emulsions with pH Responsive Switchable Behavior. Soft Matter 2013, 9, 6747−6751. (10) Liu, H.; Wang, C.-Y.; Zou, S.-W.; Wei, Z.-J.; Tong, Z. Simple, Reversible Emulsion System Switched by pH on the Basis of Chitosan without Any Hydrophobic Modification. Langmuir 2012, 28, 11017− 11024. (11) 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. (12) Saji, T.; Hoshino, K.; Aoyagui, S. Reversible Formation and Disruption of Micelles by Control of the Redox State of the Head Group. J. Am. Chem. Soc. 1985, 107, 6865−6868. (13) Aydogan, N.; Abbott, N. L. Comparison of the Surface Activity and Bulk Aggregation of Ferrocenyl Surfactants with Cationic and Anionic Headgroups. Langmuir 2001, 17, 5703−5706. (14) Datwani, S. S.; Truskett, V. N.; Rosslee, C. A.; Abbott, N. L.; Stebe, K. J. Redox-Dependent Surface Tension and Surface Phase Transitions of a Ferrocenyl Surfactant: Equilibrium and Dynamic Analyses with Fluorescence Images. Langmuir 2003, 19, 8292−8301. (15) Schmittel, M.; Lal, M.; Graf, K.; Jeschke, G.; Suske, I.; Salbeck, J. N,N′-Dimethyl-2,3-Dialkylpyrazinium Salts as Redox-Switchable Surfactants? Redox, Spectral, EPR and Surfactant Properties. Chem. Commun. (Cambridge, U. K.) 2005, 5650−5652. (16) Dexter, A. F.; Malcolm, A. S.; Middelberg, A. P. J. Reversible Active Switching of the Mechanical Properties of a Peptide Film at a Fluid−Fluid Interface. Nat. Mater. 2006, 5, 502−506. (17) Balasuriya, T. S.; Dagastine, R. R. Interaction Forces between Bubbles in the Presence of Novel Responsive Peptide Surfactants. Langmuir 2012, 28, 17230−17237. (18) Malcolm, A. S.; Dexter, A. F.; Middelberg, A. P. J. Foaming Properties of a Peptide Designed to Form Stimuli-Responsive Interfacial Films. Soft Matter 2006, 2, 1057−1066. (19) Fameau, A. L.; Saint-Jalmes, A.; Cousin, F.; Houssou, B. H.; Novales, B.; Navailles, L.; Nallet, F.; Gaillard, C.; Boué, F.; Douliez, J. P. Smart Foams: Switching Reversibly between Ultrastable and Unstable Foams. Angew. Chem., Int. Ed. 2011, 50, 8264−8268. (20) Eastoe, J.; Sánchez-Dominguez, M.; Wyatta, P.; Heenan, R. K. A Photo-Responsive Organogel. Chem. Commun. (Cambridge, U. K.) 2004, 2608−2609. (21) Eastoe, J.; Vesperinas, A. Self-Assembly of Light-Sensitive Surfactants. Soft Matter 2005, 1, 338−347. (22) Ruchmann, J.; Fouilloux, S.; Tribet, C. Light-responsive hydrophobic association of surfactants with azobenzene-modified polymers. Soft Matter 2008, 4, 2098−2108. (23) Kö rdel, C.; Popeney, C. S.; Haag, R. Photoresponsive Amphiphiles Based on Azobenzene-Dendritic Glycerol Conjugates Show Switchable Transport Behavior. Chem. Commun. (Cambridge, U. K.) 2011, 47, 6584−6586. (24) Li, L.; Rosenthal, M.; Zhang, H.; Hernandez, J. J.; Drechsler, M.; Phan, K. H.; Rütten, S.; Zhu, X.; Ivanov, D. A.; Möler, M. Light-
multiple switching cycles compared with that employing a CO2/N2 trigger.
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CONCLUSION We have demonstrated that switchable Pickering oil-in-water emulsions can be prepared using negatively charged silica nanoparticles hydrophobized in situ with a trace amount of a conventional cationic surfactant. Destabilization is achieved by adding an equimolar amount of a conventional anionic surfactant which forms ion pairs with the cationic surfactant and restabilization occurs again on adding cationic surfactant. The trigger is the differential electrostatic interaction between the oppositely charged ionic surfactants and between the cationic surfactant and the negatively charged particle surfaces. This access to switchable Pickering emulsions is easier than that employing switchable surfactants, polymers, surface-active particles, or particles hydrophobized in situ with switchable surfactants, due to avoidance of both the complicated synthesis and the rigorous switching conditions such as changing temperature or pH. The particles and ionic surfactants are all commercially available, and the ionic surfactants used are in trace amount (≪cmc) if only one or a few switching cycles are needed, the case for most practical applications. We anticiapte that this principle is also applicable for initially positively charged particles. Although the accumulation of the surfactants in the systems may be a problem, the surfactant added is usually in trace amounts and is therefore negligible compared with emulsions stabilized solely by surfactants.
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ASSOCIATED CONTENT
S Supporting Information *
Figures S1−S7 showing SEM and TEM images of powdered silica nanoparticles, ζ potentials of silica nanoparticles in different pH solutions, surface tensions of aqueous solutions of CTAB with and without silica nanoparticles, and photographs of dodecane emulsions stabilized by silica nanoparticles. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*(Z.C.) E-mail: [email protected]. *(B.P.B.) E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (Grant No. NSFC 21473080) and from the Fundamental Research Funds for the Central Universities (Grant No. JUSRP51405A) is gratefully acknowledged.
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REFERENCES
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