Demulsification of Redox-Active Emulsions by Chemical Oxidation

Jul 11, 2016 - This article reports the influence of redox reactions on emulsions of n-octane and an aqueous solution of a ferrocene-containing surfac...
2 downloads 12 Views 10MB Size
Download PDF
Article pubs.acs.org/Langmuir

Demulsification of Redox-Active Emulsions by Chemical Oxidation Yutaka Takahashi,* Nanami Koizumi, and Yukishige Kondo* Department of Industrial Chemistry, Faculty of Engineering, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan S Supporting Information *

ABSTRACT: This article reports the influence of redox reactions on emulsions of n-octane and an aqueous solution of a ferrocene-containing surfactant (FTMA; (11ferrocenylundecyl)trimethylammonium bromide). Above a certain surfactant concentration, stable O/W emulsions were formed from an aqueous solution of reduced FTMA; in contrast, mixtures of n-octane and an aqueous solution of oxidized FTMA did not form emulsions at any surfactant concentration. Furthermore, adding an oxidant to the stable O/W emulsions of reduced FTMA led to coalescence of the oil (octane) droplets in the emulsions, and subsequently, the oil and water (aqueous FTMA solution) phases fully separated from the emulsions, i.e., demulsification occurred. Equilibrated interfacial tension measurements indicate that oxidation of the ferrocenyl group in FTMA brings about an increase in the interfacial tension between the octane and aqueous surfactant solution phases. From these results, we concluded that the oxidation of reduced FTMA to oxidized FTMA led to the desorption of surfactant molecules adsorbed at the interface of the octane/aqueous surfactant solution, leading to demulsification.

■

INTRODUCTION

Recently, stimuli-responsive surfactants have attracted much attention because their interfacial properties can be spatially and temporally controlled by external stimuli.1,2 Furthermore, the net charge of surfactants containing ferrocene can be reversibly changed through redox reactions. Several investigations have been made into the reversible control of the interfacial properties and aggregates of (11-ferrocenylundecyl)trimethylammonium bromide (FTMA; see Figure 1) in aqueous electrolyte solutions by chemical or electrochemical redox reactions.3,4 FTMA can be reversibly cycled between its reduced form, containing a ferrocenyl group, and its oxidized form, containing a ferrocenium ion, through redox reactions of the ferrocene group located at the end of the alkyl chain. The ferrocenyl group of the reduced state is hydrophobic, whereas the ferrocenium ion of the oxidized state is hydrophilic. Thus, changes in the redox state of the FTMA molecule lead to changes in the hydrophilic−hydrophobic balance of the surfactant; consequently, the interfacial properties of the air/ water interface and aggregates of ferrocenyl surfactants can be reversibly controlled by the redox reactions.5,6 In addition, when redox reactions of compounds containing ferrocene occur at interfaces, macroscopic phenomena are observed.7−11 However, no reports have been made concerning the control over the oil/water interface by redox reactions of ferrocenecontaining surfactants. Demulsification, which is the aggressive destabilization of stable emulsions consisting of oil and water phases, has been used in the process involving “emulsion liquid membrane © XXXX American Chemical Society

Figure 1. Chemical structures of reduced and oxidized FTMA.

(ELM)”, e.g., separation and recovery of chemical species.12,13 When stable emulsions containing demulsifiers are subject to physical external forces (e.g., centrifugal14,15 and microwave Received: May 9, 2016 Revised: July 6, 2016

A

DOI: 10.1021/acs.langmuir.6b01772 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 2. (a) UV−vis spectra of 5-mM aqueous FTMA solutions for reduced FTMA (solid line), oxidized FTMA (dotted line), and rereduced FTMA (dashed line). (b) Change in the absorbance at 626 nm for the aqueous FTMA solutions with reversible redox reactions. Open circles, reduced FTMA; filled circles, oxidized FTMA.

irradiation16−18), the oil and water phases of the emulsions separate. To improve the demulsification process, phase separation and demulsification induced by external stimuli such as pH variation,19−21 light,22 magnetism,23,24 CO2bubbling,25−27 and counterion exchange28 have been used and recently reported. Armes et al. demonstrated that chemical reactions leading to changes in the pH of emulsions containing amino- or carboxylic-acid-containing pH-responsive polymers brought about their demulsification.29−34 Previously, we succeeded in the demulsification of stable O/W (oil in water) emulsions prepared using a photoresponsive surfactant by a photoisomerization reaction using UV light irradiation.35,36 The demulsification occurs due to changes in the interfacial properties of these photoresponsive surfactants, i.e., trans−cis photoisomerization initiated by UV-light irradiation. In this article, we focus on redox reactions to control the stability of emulsions prepared using FTMA; this is a novel application of ferrocene-containing surfactants, and we report the influence of the chemical redox reactions on the stability of the emulsions. Furthermore, we studied the demulsification, induced by chemical oxidation, of stable emulsions of n-octane and an aqueous solution of reduced-FTMA. Redox-responsive emulsions are yet to be reported. The formation and demulsification of emulsions are repeatedly demonstrated by reversible chemical redox reactions. We suggest that the demulsification results from desorption of FTMA molecules at the octane/water interface with the chemical oxidation of the ferrocenyl group in FTMA.

■

10,000 rpm using an AHG-160D ultrafast homogenizer (AS ONE, Osaka, Japan) equipped with an HT1008 shaft generator (AS ONE) to prepare emulsions at 25 °C. Oxidized FTMA was prepared by chemical oxidation of FTMA, achieved by the addition of Fe2(SO4)3 (1.0 mol equiv) to the aqueous solution of reduced-FTMA; throughout the reaction, the solution was stirred. Subsequently, (L)ascorbic acid (1.0 mol equiv) was added to the aqueous surfactant solution with stirring, leading to the chemical reduction of the ferrocenium ion in oxidized FTMA. The aqueous oxidized-FTMA solution was subject to emulsion preparation, as was that of reduced FTMA. The stability of obtained emulsions was evaluated by visual observation of the emulsions after storage for 1 week at 25 °C. Types of emulsions were distinguished by whether emulsions disperse readily in water or oil phases. Liquid droplets dispersed in emulsions were observed with a differential interference contrast (DIC) microscope using a Leica DMI 4000B microscope (Leica Microsystems GmbH, Wetzlar, Germany). Digital images of samples were captured using a Leica DFC300 FX digital camera. The diameters of liquid droplets dispersed in the emulsions were estimated by taking light microscope photographs and measuring the diameters of 500 droplets. UV−Vis Measurement. The aqueous FTMA solutions (5 mM) were placed in quartz cuvettes with an optical path length of 10 mm. Spectra were recorded in the wavelength range of 400−800 nm using a V-570 UV−vis spectrometer (JASCO, Tokyo, Japan) at 25.0 ± 0.1 °C without dilution of the aqueous solution. Interfacial Tension Measurement. Equilibrated interfacial tensions of octane/aqueous FTMA solution were measured at 25 °C using the pendant drop method using a DropMaster DM-300 (Kyowa Interface Science Co., Ltd., Saitama, Japan). A pendant drop of aqueous surfactant solutions was created using a syringe in a quartz cuvette that contained octane. The interfacial tensions were determined using the Young−Laplace equation; the drop shape, which was analyzed using the computer program FAMAS (Kyowa Interface Science Co., Ltd.); and the difference in density between octane and water.

EXPERIMENTAL SECTION

Materials. (11-Ferrocenylundecyl)trimethylammonium bromide (FTMA) was synthesized by the method reported in a previous paper.3,37 n-Octane (reagent grade, 99.8%) and iron(III) sulfate nhydrate (Fe2(SO4)3·nH2O) were purchased from Wako Pure Chemical Industries, Ltd. (Tokyo, Japan). Dodecyltrimethylammonium bromide (DTAB) and (L)-ascorbic acid were obtained from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). All materials were used as purchased without further purification. Preparation and Characterization of Emulsion. Aqueous surfactant solutions were prepared in high-purity H2O (Milli-Q pure water; R = 18 MΩ cm, γ = 72.8 mN m−1 at 20 °C) deoxygenated by bubbling nitrogen gas through the solution for 30 min. Binary mixtures of octane and aqueous FTMA solution were homogenized for 5 min at

■

RESULTS AND DISCUSSION

Characterization of the Reduced and Oxidized States of FTMA on Chemical Redox Reactions. The reduced and oxidized states of ferrocenyl surfactants can be controlled by chemical and/or electrochemical redox reactions. These states of FTMA were characterized by UV−vis measurements. Figure 2a shows the UV−vis absorption spectra of aqueous FTMA solutions (5 mM) in various oxidation states. The spectrum of the aqueous solution of reduced-FTMA has an absorption band B

DOI: 10.1021/acs.langmuir.6b01772 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 3. Photographs illustrating the demulsification of a stable emulsion consisting of n-octane/aqueous FTMA solution at 5 mM (= 80/20, w/w): (a) after homogenization, (b) phase separation, demulsification, caused by adding Fe2(SO4)3 to the emulsion shown in a, and (c) subsequent addition of (L)-ascorbic acid to the emulsion shown in b and then homogenization. (d) Mixture consisting of n-octane and 5 mM of aqueous oxidized-FTMA solution after homogenization.

at 440 nm, corresponding to the ferrocenyl group in reduced FTMA. However, an absorbance band at 626 nm appeared on the addition of an oxidant, iron(III) sulfate (Fe2(SO4)3; 1.0 mol equiv), to the aqueous solution of reduced-FTMA; this absorption band is assigned to the ferrocenium ion in oxidized FTMA. When Fe2(SO4)3 was added to the aqueous solutions at greater than 1.0 mol equiv (1.2, 1.5, 2.0-fold excess molar), the absorbance at 626 nm in these spectra was similar to that for oxidized FTMA prepared using 1.0 mol equiv of Fe2(SO4)3. This result indicates that the ferrocenyl group in reduced FTMA was completely oxidized by the addition of 1.0 mol equiv of Fe2(SO4)3. When the aqueous solution of oxidized FTMA was chemically reduced using (L)-ascorbic acid (1.0 mol equiv), the absorbance at 626 nm decreased, and the absorption band of the ferrocenyl group in reduced FTMA reappeared at 440 nm. This result indicates that chemical reduction brought about the transformation from the ferrocenium ion to neutral ferrocene in FTMA. In addition, oxidation or reduction of FTMA in water occurred immediately on addition of the oxidant or reductant, respectively. Figure 2b shows the change in the absorbance at 626 nm on redox chemical reactions. The repetitive addition of reductant and oxidant led to reversible redox reactions (>6 cycles) between the ferrocenyl group and the ferrocenium ion in FTMA. Influence of Redox Reactions on Emulsions. When binary mixtures of n-octane and a 5-mM aqueous solution of reduced-FTMA (= 80/20, w/w) were homogenized, stable emulsions were obtained (Figure 3a). The emulsions were found to be of the O/W-type because a few droplets of the emulsion dispersed readily in the water phase. Figure 4 shows a phase diagram for the mixtures of octane/aqueous FTMA (= 80/20, w/w) at 25 °C. Emulsions that showed no phase separation after 1 week were deemed “stable”, and we found that at a concentration of reduced FTMA greater than 3 mM, stable emulsions formed. When the stable emulsion consisting of n-octane and an aqueous solution of reduced FTMA (5 mM) (octane/aqueous surfactant solution = 80/20, w/w) was observed by light microscopy, oil droplets with diameters ranging from a few micrometers to ca. 40 μm were seen to be dispersed in the aqueous surfactant solution (Figure 5a). In contrast, the addition of Fe2(SO4)3 to the aqueous solution of reduced FTMA led to the oxidation of FTMA. When the

Figure 4. Phase diagrams for mixtures of aqueous FTMA solution and n-octane at various surfactant concentrations at 25 °C (octane/ aqueous surfactant solution =80/20, w/w). In the case that no phase separation was observed after 1 week, the emulsions were deemed “stable emulsions”. When oil and/or water phases separated within 1 week, the emulsions were recorded as “unstable emulsions”. Emulsions prepared using oxidized FTMA were not form at any surfactant concentration (described as “no emulsions”).

oxidant (1.0 mol equiv) was added to stable emulsions consisting of octane and an aqueous solution of reducedFTMA (5 mM), the octane and aqueous FTMA solution phases gradually separated from the emulsions; that is, demulsification occurred. After the addition of an oxidant, these phases separated immediately on gentle shaking (Figure 3b). The addition of the oxidant caused oxidation of FTMA in the emulsions, changing from the reduced to oxidized state. This change in oxidation state was clearly observed because after the addition of the oxidant, the color of aqueous FTMA solutions changed from yellow to blue, a color corresponding to the oxidized state. When (L)-ascorbic acid (1.0 mol equiv) was added to the separated, demulsified mixture, reduction of oxidized FTMA occurred, and on homogenization of this mixture, a stable emulsion was reformed (Figure 3c). The formation and demulsification of the emulsions were evaluated from the height of the emulsion in a vial (container volume: 10 mL, diameter × height = 1.8 cm × 7.5 cm). The height of emulsions obtained without phase separation (reduced FTMA) was 2.0 cm, whereas that containing no emulsion (oxidized FTMA) was 0 cm. Figure 6 shows the change in the height of emulsions on redox. In Figure 6, the height of the emulsion prepared using reduced FTMA at the start of the experiment was 2.0 cm without phase separation. Although emulsions of C

DOI: 10.1021/acs.langmuir.6b01772 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 6. Change in the height of emulsions consisting of n-octane and aqueous FTMA solution (5 mM, octane/aqueous surfactant solution = 80/20, w/w) in a vial container on reversible redox reaction. Open circles, reduced FTMA; filled circles, oxidized FTMA. Solid lines, oxidation and demulsification; dashed lines, reduction and emulsification. The solid and dashed lines connect the plots of data for the reduced and oxidized states.

Possible Mechanism of Demulsification by Chemical Oxidation. We discuss here why the stability of emulsions prepared using reduced FTMA was drastically decreased by oxidation of the ferrocenyl group in FTMA. The stability of the emulsions of aqueous oxidized-FTMA and octane was investigated. A 5-mM aqueous oxidized-FTMA solution was prepared by chemical oxidation with Fe2(SO4)3 (1.0 mol equiv). When mixtures of octane and aqueous oxidized FTMA (80/20, w/w) were homogenized, no emulsions were obtained (Figure 3d). In addition, emulsions were not formed at high surfactant concentrations (50 mM), as shown in Figure 4. Therefore, the emulsification ability of FTMA for mixtures of octane and water decreased drastically on oxidation of the ferrocenyl group in FTMA. In addition, the stability and formation of emulsions prepared using mixtures of reduced and oxidized FTMA were investigated. The formation of these emulsions was evaluated from the height of the emulsion in a vial container. In this experiment, emulsions were prepared using aqueous solutions of mixtures of reduced and oxidized FTMA at a total surfactant concentration of 5 mM. Figure 7 shows the change in the emulsions with a variation of the molar fraction of oxidized FTMA in mixtures of reduced and oxidized FTMA. Emulsions were formed at mole fractions of oxidized FTMA up to 40 mol %, but the octane and/or aqueous FTMA solution phases separated from emulsions containing oxidized FTMA at 30 to 40 mol %. No emulsions were formed at mole fractions greater than 50 mol %. This result indicates that oxidation of 50% of the ferrocenyl groups leads to complete phase separation, i.e., demulsification. Breaking processes of stable emulsions can be mainly explained by creaming, flocculation, coalescence, Ostwald ripening, or a combination of these processes.38 The diameter distributions of octane droplets in the emulsions consisting of octane and 5 mM of aqueous FTMA solution (octane/aqueous surfactant solution = 80/20, w/w) were estimated from the light microscopy images shown in Figure 5 (see Supporting Information, Figure S1). The number-averaged diameter for the emulsion prepared using reduced FTMA is 11.62 ± 7.29 μm. The diameter of octane droplets in the emulsions increased on oxidation. These

Figure 5. Light microscopy images of emulsions consisting of noctane/aqueous FTMA solution at 5 mM (= 80/20, w/w) at 25 °C: (a) reduced FTMA, (b) 20 mol % of oxidized FTMA, and (c) 30 mol % of oxidized FTMA. All scale bars are 60 μm.

reduced FTMA were obtained without phase separation for five cycles, the volume of the obtained emulsions decreased gradually with increasing cycle number (see open circles in Figure 6). This is because the emulsions adhered to the shaft generator in the homogenizer after homogenization. Thus, the formation and demulsification of stable emulsions were cycled (>5 cycles) by chemical redox of the ferrocenyl group in FTMA. D

DOI: 10.1021/acs.langmuir.6b01772 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 7. Change in the height of emulsions consisting of n-octane/ aqueous FTMA solution at 5 mM (= 80/20, w/w) in a vial container with chemical oxidation of the ferrocenyl group in FTMA.

Figure 8. Interfacial tensions between n-octane and aqueous FTMA solution against surfactant concentration. Open circles, reduced FTMA; filled circles, oxidized FTMA.

diameter distributions of octane droplets in the emulsions indicate that the number of smaller sized droplets decreased with oxidation and that those of larger sizes increased. The droplet size distribution for oxidized FTMA at 30 mol % was broader than that for reduced FTMA. Therefore, these results indicated that the oxidation brought about the coalescence of the octane droplets in the emulsions. When mixtures of octane and an aqueous solution of DTAB, in which the ferrocenyl group in FTMA is replaced by a methyl group, (octane/aqueous surfactant solution = 80/20, w/w) were homogenized at various surfactant concentrations (1.0− 50 mM), stable emulsions were obtained at concentrations greater than 16 mM. The influence of adding an oxidant to the stability of the stable emulsions prepared using DTAB was investigated. When Fe2(SO4)3 (1.0 mol equiv) was added to stable emulsions consisting of octane/aqueous DTAB solution at 20 mM (= 80/20, w/w), no phase separation was observed. Thus, oxidation of the ferrocenyl group in FTMA to the ferrocenium ion brought about demulsification on the addition of an oxidant. The change in the Gibbs free energy on emulsion formation is represented by the interfacial tensions of oil/aqueous solutions and the interfacial area in emulsions.38,39 Thus, the interfacial tension is an important parameter in the formation of emulsions. Figure 8 shows the relationship between the equilibrated interfacial tensions between octane and an aqueous solution of FTMA and surfactant concentration. Although both the interfacial tensions of reduced and oxidized states decreased with increasing surfactant concentration, there is a significant difference in the curves of the interfacial tension vs surfactant concentration between the reduced and oxidized states. The interfacial tension for reduced FTMA reached a constant value (7.4 mN m−1) above a concentration of 1 mM, whereas that for oxidized FTMA did not have a constant value beyond 10 mM surfactant concentration. In addition, the interfacial tensions for oxidized FTMA were higher than those for reduced FTMA at a surfactant concentration greater than 0.01 mM. In other words, oxidation of FTMA in the aqueous solution brought about an increase in the interfacial tension. Oxidation of an aqueous reduced-FTMA solution at 5 mM, for example, is accompanied by an increase in the interfacial tension from 7.4 to 33.8 mN m−1. As a result, the equilibrated interfacial tension values

explain the demulsification by oxidation of FTMA and the inability of oxidized FTMA to form a stable emulsion. In addition, possible conformations of FTMA at the octane/water interface were investigated using interfacial tension measurements. We also measured the interfacial tensions between octane and aqueous DTAB solution (see Supporting Information, Figure S2). The occupied areas per molecule (A) for reduced FTMA and DTAB at octane/water interface were calculated using the Gibbs adsorption isotherm and the slope of the interfacial tension vs surfactant concentration40 because FTMA and DTAB molecules are insoluble in octane. The A values for reduced FTMA and DTAB molecules were 0.28 and 0.29 nm2, respectively. This result indicates that the molecular conformation of reduced FTMA at the octane/water interface is similar to that of DTAB at the interface. Therefore, reduced FTMA molecules were vertically adsorbed at the interface between octane and the aqueous FTMA solution in the same way as DTAB, a conventional surfactant. Because the ferrocenium ion is attached to the end of the hydrophobic chain in FTMA and is hydrophilic in the oxidized state, the oxidized FTMA molecule behaves as a bolamphiphile. Therefore, the estimated conformations are inconsistent with the molecular conformations of reduced FTMA reported in previous papers,41−43 which report that FTMA molecules in both reduced and oxidized states have bolamphiphile conformations at the air/water and liquid crystal/water interfaces. These conformations result from the affinity of the ferrocenyl group in FTMA with oil and water phases. Ferrocene molecules are soluble in octane (approximately 0.15 M) but poorly soluble in water (ca. 35 μM44). Therefore, in the octane/water system, the ferrocenyl group in reduced FTMA associates better with the octane phase than water phase. At the octane/water interface, reduced FTMA behaves not as a bolamphiphile but as a conventional surfactant, oriented vertically to the interface. Considering the results of the equilibrated interfacial tension measurements and distributions of the diameters of the oil droplets in emulsions containing FTMA, we suggest the following mechanism for the demulsification of the stable O/W emulsions prepared using reduced FTMA with oxidation of the ferrocenyl group in FTMA. As shown in Figure 9a, reduced FTMA molecules were adsorbed at the interface between the E

DOI: 10.1021/acs.langmuir.6b01772 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 9. Possible mechanism of demulsification on chemical oxidation. The reduced FTMA molecules are adsorbed over the whole n-octane and aqueous surfactant solution interface formed in the stable O/W-type emulsions. Oxidation of the ferrocenyl group in reduced FTMA causes desorption of FTMA molecules and then direct contact between octane and water phases. The exposure of octane and water interface leads to the coalescence of the octane droplets in the O/W-type emulsions. Finally, the octane and water phases fully separate on a macroscopic scale. The colors of the aqueous FTMA solution indicate the reduced or oxidized states. Yellow (a), green (b), and blue (c) indicate reduced FTMA, a mixture of reduced and oxidized FTMA, and oxidized FTMA, respectively.

demulsification due to coalescence of the oil droplets caused by desorption of FTMA molecules from the O/W interface. Furthermore, the formation and demulsification of O/W emulsions could repeatedly be performed by reversibly chemical redox of the ferrocenyl group in FTMA.

octane and aqueous surfactant solution in the stable emulsions. Oxidation of the ferrocenyl group in reduced FTMA led to increased hydrophilicity of the surfactant and subsequent desorption of these FTMA molecules (Figure 9b). The desorption of FTMA molecules brought about an increase in the interfacial tension between the octane and aqueous surfactant solution (see Figure 8), and the oxidized FTMA is only partially adsorbed at octane/water interface. In other words, the octane and water phases are in direct contact on the transformation of reduced FTMA to oxidized FTMA in the O/ W-type emulsions. The exposure of the octane/water interface leads to the coalescence of octane droplets in the emulsions, followed by complete phase separation, i.e., demulsification (Figure 9c). Observation by light microscopy and the change in the height of emulsions with oxidation confirm the mechanism mentioned above (see Figures 5, 7, and S1).

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b01772. Diameter distributions of the octane droplet in the emulsions consisting of n-octane and 5 mM of aqueous FTMA solution and the interfacial tensions between noctane and aqueous DTAB solutions against surfactant concentration (PDF)

■

■

CONCLUSIONS We have investigated the influence of redox reactions on the emulsification property of a ferrocenyl surfactant, FTMA. Mixtures of octane and a reduced-FTMA aqueous solution formed stable O/W emulsions, and no phase separation was observed after 1 week; in contrast, no emulsions were obtained from mixtures of octane and aqueous oxidized-FTMA solution. Chemical oxidation of the ferrocenyl group in FTMA brought about an increase the hydrophilicity of the surfactant and

AUTHOR INFORMATION

Corresponding Authors

*(Y.T.) Phone/Fax: +81-3-5228-8313. E-mail: ytakahashi@ci. kagu.tus.ac.jp. *(Y.K.) Phone/Fax: +81-3-5228-8313. E-mail: [email protected]. ac.jp. Notes

The authors declare no competing financial interest. F

DOI: 10.1021/acs.langmuir.6b01772 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

■

(20) Sun, J.; Yi, C.; Wei, W.; Zhao, D.; Hu, Q.; Liu, X. Nanohybrids from Direct Chemical Self-Assembly of Poly(styrene-alt-maleic anhydride) as pH-Responsive Particulate Emulsifiers. Langmuir 2014, 30, 14757−14764. (21) Liu, H.; Wang, C.; Zou, S.; Wei, Z.; Tong, Z. Simple, Reversible Emulsion System Switched by pH on the Basis of Chitosan without Any Hydrophobic Modification. Langmuir 2012, 28, 11017−11024. (22) Tabor, R. F.; Oakley, R. J.; Eastoe, J.; Faul, C. F. J.; Grillo, I.; Heenan, R. K. Reversible light-induced critical separation. Soft Matter 2009, 5, 78−80. (23) Liang, J.; Li, H.; Yan, J.; Hou, W. Demulsification of Oleic-AcidCoated Magnetite Nanoparticles for Cyclohexane-in-Water Nanoemulsions. Energy Fuels 2014, 28, 6172−6178. (24) Liang, J.; Du, N.; Song, S.; Hou, W. Magnetic demulsification of diluted crude oil-in-water nanoemulsions using oleic acid-coated magnetite nanoparticles. Colloids Surf., A 2015, 466, 197−202. (25) 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. (26) Liu, P.; Lu, W.; Wang, W.-J.; Li, B.-G.; Zhu, S. Highly CO2/N2Switchable Zwitterionic Surfactant for Pickering Emulsions at Ambient Temperature. Langmuir 2014, 30, 10248−10255. (27) Dong, M.; Gong, H.; Tian, W.; Li, Y. Separation of Water from High pH Water-in-Heavy Oil Emulsions Using Low-Pressure CO2. Ind. Eng. Chem. Res. 2014, 53, 12216−12223. (28) Zhao, C.; Tan, J.; Li, W.; Tong, K.; Xu, J.; Sun, D. Ca2+ IonResponsive Pickering Emulsions Stabilized by PSSMA Nanoaggregates. Langmuir 2013, 29, 14421−14428. (29) Fujii, S.; Cai, Y.; Weaver, J. V. M; Armes, S. P. Syntheses of Shell Cross-Linked Micelles Using Acidic ABC Triblock Copolymers and Their Application as pH-Responsive Particulate Emulsifiers. J. Am. Chem. Soc. 2005, 127, 7304−7305. (30) Read, E. S.; Fujii, S.; Amalvy, J. I.; Randall, D. P.; Armes, S. P. Effect of Varying the Oil Phase on the Behavior of pH-Responsive Latex-Based Emulsifiers: Demulsification versus Transitional Phase Inversion. Langmuir 2004, 20, 7422−7429. (31) Binks, B. P.; Murakami, R.; Armes, S. P.; Fujii, S. Effects of pH and Salt Concentration on Oil-in-Water Emulsions Stabilized Solely by Nanocomposite Microgel Particles. Langmuir 2006, 22, 2050−2057. (32) Morse, A. J.; Dupin, D.; Thompson, K. L.; Armes, S. P.; Ouzineb, K.; Mills, P.; Swart, R. Novel Pickering Emulsifiers based on pH-Responsive Poly(tert-butylaminoethyl methacrylate) Latexes. Langmuir 2012, 28, 11733−11744. (33) Morse, A. J.; Armes, S. P.; Thompson, K. L.; Dupin, D.; Fielding, L. A.; Mills, P.; Swart, R. Novel Pickering Emulsifiers Based on pH-Responsive Poly(2-(diethylamino)ethyl methacrylate) Latexes. Langmuir 2013, 29, 5466−5475. (34) Morse, A. J.; Madsen, J.; Growney, D. J.; Armes, S. P.; Mills, P.; Swart, R. Microgel Colloidosomes Based on pH-Responsive Poly(tertbutylaminoethyl methacrylate) Latexes. Langmuir 2014, 30, 12509− 12519. (35) Takahashi, Y.; Fukuyasu, K.; Horiuchi, T.; Kondo, Y.; Stroeve, P. Photoinduced Demulsification of Emulsions Using a Photoresponsive Gemini Surfactant. Langmuir 2014, 30, 41−47. (36) Takahashi, Y.; Koizumi, N.; Kondo, Y. Active Demulsification of Photoresponsive Emulsions Using Cationic−Anionic Surfactant Mixtures. Langmuir 2016, 32, 683−688. (37) Saji, T.; Hoshino, K.; Ishii, Y.; Goto, M. Formation of Organic Thin Films by Electrolysis of Surfactants with the Ferrocenyl moiety. J. Am. Chem. Soc. 1991, 113, 450−456. (38) Tadros, T. F. Emulsion Formation, Stability, and Rheology. Emulsion Formation and Stability; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2013; pp 1−75. (39) Rosen, M. J. Emulsification by Surfactants. Surfactants and Interfacial Phenomena; John Wiley & Sons, Inc.: New York, 2004; pp 303−331.

ACKNOWLEDGMENTS This research is supported by a JSPS KAKENHI Grant-in-Aid for Young Scientists (B) (No. 15K17847).

■

REFERENCES

(1) Liu, X.; Abbott, N. L. Spatial and Temporal Control of Surfactant Systems. J. Colloid Interface Sci. 2009, 339, 1−18. (2) Brown, P.; Butts, C. P.; Eastoe, J. Stimuli-Responsive Surfactants. Soft Matter 2013, 9, 2365−2374. (3) Saji, T.; Hoshino, K.; Aoyagui, S. Reversible Formation and Disruption of Micelles by Control of the Redox State of the Surfactant Tail Group. J. Chem. Soc., Chem. Commun. 1985, 865−866. (4) Gallardo, B. S.; Hwa, M. J.; Abbott, N. L. In Situ and Reversible Control of the Surface Activity of Ferrocenyl Surfactants in Aqueous Solutions. Langmuir 1995, 11, 4209−4212. (5) Sakai, H.; Imamura, H.; Kondo, Y.; Yoshino, N.; Abe, M. Reversible Control of Vesicle Formation using Electrochemical Reaction. Colloids Surf., A 2004, 232, 221−228. (6) Tsuchiya, K.; Orihara, Y.; Kondo, Y.; Yoshino, N.; Ohkubo, T.; Sakai, H.; Abe, M. Control of Viscoelasticity Using Redox Reaction. J. Am. Chem. Soc. 2004, 126, 12282−12283. (7) Bennett, D. E.; Gallardo, B. S.; Abbott, N. L. Dispensing Surfactants from Electrodes: Marangoni Phenomenon at the Surface of Aqueous Solutions of (11-Ferrocenylundecyl)trimethylammonium Bromide. J. Am. Chem. Soc. 1996, 118, 6499−6505. (8) Gallardo, B. S.; Gupta, V. K.; Eagerton, F. D.; Jong, L. I.; Craig, V. S.; Shah, R. R.; Abbott, N. L. Electrochemical Principles for Active Control of Liquids on Submillimeter Scales. Science 1999, 283, 57−60. (9) Luk, Y.-Y.; Abbott, N. L. Surface-Driven Switching of Liquid Crystals Using Redox-Active Groups on Electrodes. Science 2003, 301, 623−626. (10) Nakahata, M.; Takashima, Y.; Yamaguchi, H.; Harada, A. RedoxResponsive Self-Healing Materials Formed from Host−Guest Polymers. Nat. Commun. 2011, 2, 511. (11) Brake, J. M.; Mezera, A. D.; Abbott, N. L. Active Control of the Anchoring of 4′-Pentyl-4-cyanobiphenyl (5CB) at an Aqueous−Liquid Crystal Interface By Using a Redox-Active Ferrocenyl Surfactant. Langmuir 2003, 19, 8629−8637. (12) Skelland, A. H. P.; Meng, X. A New Solution to Emulsion Liquid Membrane Problems by Non-Newtonian Conversion. AIChE J. 1996, 42, 547−561. (13) Hirai, T.; Hodono, M.; Komasawa, I. The Preparation of Spherical Calcium Phosphate Fine Particles Using an Emulsion Liquid Membrane System. Langmuir 2000, 16, 955−960. (14) Allende, D.; Cambiella, Á .; Benito, J. M.; Pazos, C.; Coca, J. Destabilization-Enhanced Centrifugation of Metalworking Oil-inWater Emulsions: Effect of Demulsifying Agents. Chem. Eng. Technol. 2008, 31, 1007−1014. (15) Huang, Q.; Mao, F.; Han, X.; Yan, J.; Chi, Y. Migration of Emulsified Water Droplets in Petroleum Sludge during Centrifugation. Energy Fuels 2014, 28, 4918−4924. (16) Ferreira, B. M. S.; Ramalho, J. B. V. S.; Lucas, E. F. Demulsification of Water-in-Crude Oil Emulsions by Microwave Radiation: Effect of Aging, Demulsifier Addition, and Selective Heating. Energy Fuels 2013, 27, 615−621. (17) da Silva, E. B.; Santos, D.; de Brito, M. P.; Guimarães, R. C. L.; Ferreira, B. M. S.; Freitas, L. S.; de Campos, M. C. V.; Franceschi, E.; Dariva, C.; Santos, A. F.; Fortuny, M. Microwave Demulsification of Heavy Crude Oil emulsions: Analysis of Acid Species Recovered in the Aqueous Phase. Fuel 2014, 128, 141−147. (18) França, B. B.; Nogueira, B.; Carretoni, C.; Pessoa, F. L. P.; Portilho, M. F.; Pinto, J. C.; Nele, M. Stability and Microwave Demulsification of Water in Castor Oil Biodiesel Emulsions. Energy Fuels 2014, 28, 1053−1058. (19) 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, 3052−3060. G

DOI: 10.1021/acs.langmuir.6b01772 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir (40) Rosen, M. J. Adsorption of Surface-Active Agents at Interfaces: The Electrical Double Layer. Surfactants and Interfacial Phenomena; John Wiley & Sons, Inc.: New York, 2004; pp 34−104. (41) Aydogan, N.; Gallardo, B. S.; Abbott, N. L. A MolecularThermodynamic Model for Gibbs Monolayers Formed from RedoxActive Surfactants at the Surfaces of Aqueous Solutions: RedoxInduced Changes in Surface Tension. Langmuir 1999, 15, 722−730. (42) Brake, J. M.; Mezera, A. D.; Abbott, N. L. Effect of Surfactant Structure on the Orientation of Liquid Crystals at Aqueous−Liquid Crystal Interfaces†. Langmuir 2003, 19, 6436−6442. (43) Gallardo, B. S.; Metcalfe, K. L.; Abbott, N. L. Ferrocenyl Surfactants at the Surface of Water: Principles for Active Control of Interfacial Properties. Langmuir 1996, 12, 4116−4124. (44) Ohsawa, Y.; Aoyagui, S. A Correlation between the Half-Wave Potential and the Micelle-Solubilization Equilibrium of Ferrocene in Cationic Micellar Solutions. J. Electroanal. Chem. Interfacial Electrochem. 1982, 136, 353−360.

H

DOI: 10.1021/acs.langmuir.6b01772 Langmuir XXXX, XXX, XXX−XXX