Active Demulsification of Photoresponsive Emulsions Using Cationic

Article pubs.acs.org/Langmuir

Active Demulsification of Photoresponsive Emulsions Using Cationic−Anionic Surfactant Mixtures 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: The influence of ultraviolet (UV) light irradiation on the emulsification properties of mixtures of an anionic surfactant, sodium dodecyl sulfate (SDS), and a photoresponsive cationic surfactant, 2-(4-(4-butylphenyl)diazenylphenoxy)ethyltrimethylammonium bromide (C4AzoTAB), containing an azobenzene group has been investigated. When mixtures of n-octane and aqueous SDS/trans-C4AzoTAB solution are homogenized, stable emulsions are obtained in regions of specific surfactant concentrations and molar ratios of the mixed surfactants. The stable emulsions are stable for over a week and found to be of the oil-in-water (O/W) type. UV light irradiation of the stable O/W emulsions leads to the coalescence of smaller oil droplets into larger ones in the emulsions, i.e., demulsification. As a result, the oil and aqueous surfactant solution phases are fully separated by UV light irradiation for 90 min, even shorter than our previous result (6 h; Langmuir 2014, 30, 41−47). The use of a microreactor shortens the time required for the photoinduced demulsification into 3.5 min. When mixtures of octane and aqueous SDS/cis-C4AzoTAB solution are homogenized, no emulsions are obtained. The interfacial tension (IFT) between octane and aqueous SDS/cis-C4AzoTAB solution is higher than that between octane and aqueous SDS/trans-C4AzoTAB solution, indicating that the IFT of SDS/trans-C4AzoTAB mixtures increases with the cis photoisomerization of the trans isomer. These results suggest that cis isomerization of the SDS/trans-C4AzoTAB mixtures due to UV light irradiation causes Ostwald ripening of the octane droplets in the emulsions, thereby reducing the interfacial area between the octane and water phases as the IFT between octane and the aqueous surfactant solution increases. Subsequently, the octane and aqueous solution phases separate.

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Scheme 1. Trans−Cis Photoisomerization of C4AzoTAB

INTRODUCTION

Stimuli-responsive surfactants have interfacial properties that can be controlled by external stimuli.1,2 Light irradiation is particularly attractive as an external stimulus because it is comparatively easy to handle and its use generates no pollution in surfactant solutions. Azobenzene derivatives can be reversibly switched between their trans and cis forms by specific wavelengths of light: the trans isomer is converted to the cis one by ultraviolet (UV) light, whereas visible light irradiation converts the cis isomer to the trans one (Scheme 1).3,4 Studies of the reversible control over interfacial properties of aqueous solutions containing surfactants with azobenzene groups have been reported previously.5−8 Emulsions are metastable systems in which immiscible liquid phases (e.g., water and oil) are mixed by emulsifiers. The preparation of emulsions leads to an increase in the interfacial area between the water and oil phases. Thus, emulsions are thermodynamically unstable and undergo eventual phase separation. Lowering the interfacial tension (IFT) is key to stabilizing emulsions. Emulsions have been utilized in many fields (e.g., cosmetics, food, and paint), and therefore a number of researchers have focused on the stabilization of emulsions by surfactants. Additionally, demulsification, which is the breaking of emulsions into phase separation, also plays an important role © XXXX American Chemical Society

in industry, where it is used for as “emulsion liquid membrane extraction”.9,10 Received: October 22, 2015 Revised: December 3, 2015

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homogenized for 5 min at 10 000 rpm using an AHG-160D ultrafast homogenizer (AS ONE, Osaka, Japan) equipped with a HT1010 shaft generator (AS ONE) to prepare emulsions at 25 °C. Aqueous mixtures of SDS/cis-C4AzoTAB were prepared with UV light irradiation (wavelength: 365 ± 15 nm) using a Handy UV lamp SLUV-16 (AS ONE) in a dark room before homogenization. The aqueous SDS/cisC4AzoTAB solutions were provided for emulsion preparation as well as SDS/trans-C4AzoTAB mixtures. Emulsion Characterization. The stabilities of the emulsions were evaluated by visual observation after storing the emulsions for 1 week at 25 °C. The stable emulsion types were identified by placing a drop of each emulsion on both water and oil phases and observing into which phase the drop was easily dispersed. The stable emulsions were observed by light microscopy. Differential interference contrast (DIC) observations were performed with a Leica DMI 4000B microscope (Leica Microsystems GmbH, Wetzlar, Germany). Digital images of the samples were captured using a Leica DFC300 FX digital camera. Averaged diameters of the droplets dispersed in the emulsions were estimated by counting 300 droplets in the light microscopy images. Light Irradiation Method. An aqueous SDS/C4AzoTAB solution (5 mL) was placed in a vial (container volume: 10 mL, diameter × height = 2 cm × 3 cm) and then irradiated by UV or visible light with stirring in a dark room. The cis isomerization of trans-C4AzoTAB was performed using a Handy UV lamp SLUV-16 (wavelength: 365 ± 15 nm; AS ONE). The lamp was located to the side of the vial at a distance of 3 cm. The intensity of irradiated UV light was 3 mW cm−2. SDS/cis-C4AzoTAB solutions were illuminated with visible light (LAX-Cute; Asahi Spectra Co., Ltd., Tokyo, Japan; 100 W Xe lamp; wavelength: 400−700 nm). The molar ratios of trans- and cisC4AzoTAB in the SDS/C4AzoTAB mixtures were estimated from the integral ratios of signals assigned to trans and cis isomers in the 1H NMR spectra. NMR measurements were conducted at 30 °C on a Bruker Avance DPX-400 spectrometer equipped with a QNP probe operating at 400 MHz for 1H nuclei. All samples were prepared using D2O (Acros Organics; 99.8 atom % D). For macroscopic observations, an emulsion (4 mL) prepared using a mixture of SDS/trans-C4AzoTAB was placed in a vial (container volume: 10 mL; diameter × height = 1.8 cm × 7.5 cm) and subsequently irradiated by UV light (wavelength: 365 ± 15 nm; Handy UV lamp SLUV-16) in a dark room at 25 °C. The lamps were placed to the side of the vial at a distance of 3 cm (Supporting Information, Figure S1a). The intensity of the irradiated UV light was 3 mW cm−2. Changes in the emulsions resulting from UV light irradiation were recorded with a video camera (Everio GZ-HM450, JVC KENWOOD, Kanagawa, Japan). For microscopic observations, several drops of emulsions prepared using SDS/trans-C4AzoTAB mixtures were placed on a glass slide and exposed to UV light irradiation (wavelength: 340−380 nm; LAXCute) from a source located 3 cm above the glass slide (Figure S1b). The light intensity was adjusted to 15 mW cm−2. Changes in the emulsion droplets resulting from UV light irradiation were recorded with a Leica microscope using Leica LAS MultiTime software. Interfacial Tension Measurements. The interfacial tensions between octane and aqueous SDS/C4AzoTAB solution were measured at 30 °C according to the pendant drop method using a DropMaster DM-300 (Kyowa Interface Science Co., Ltd., Saitama, Japan). A pendant drop of the aqueous surfactant solution was created using a syringe in a quartz cuvette (wide × depth × height = 2 cm × 2 cm × 2.8 cm) containing octane (8 mL). The interfacial tensions were determined via Young−Laplace equation using the drop shape determined by FAMAS software (Kyowa Interface Science Co., Ltd.) and the density difference between octane and water. The interfacial tensions between octane and aqueous SDS/cis-C4AzoTAB solution were measured with UV light irradiation to the interface in a dark room, whereas those between octane and aqueous SDS/transC4AzoTAB solution were measured under ambient atmosphere at 30 °C. The UV light (wavelength: 340−380 nm; LAX-Cute) was emitted from 3 cm above the drop, and the source was aligned obliquely relative to the sample. The light intensity was adjusted to 15 mW cm−2.

Destabilization of emulsions, demulsification, results from the flocculation, coalescence, and Ostwald ripening of dispersed droplets. Ostwald ripening has been theoretically described by Lifshitz, Slezov, and Wagner (LSW theory).11,12 The Ostwald ripening rate, ω, is given by the equation ω=

8DC∞Vm 2γ dr 3 = dt 9RT

(1)

where r is the number-averaged radius of the dispersed droplets in a continuous phase, D is the diffusion coefficient of the dispersed droplets, C∞ is the solubility of the dispersed droplets at the planner interface, Vm is the molar volume of the dispersed phase, γ is the IFT between the dispersed and the continuous phases, R is the gas constant, and T is the absolute temperature. According to this equation, the ripening rate will increase as the IFT increases. In other words, increases in the IFT will accelerate demulsification. To date, demulsification has been performed using only physical or chemical methods. Recently, studies investigating changes in emulsification properties caused by external stimuli have been reported.13−16 However, there are very few studies on the effect of light irradiation on the emulsification properties. These studies have succeeded in reversible control of the emulsion types by UV and visible light irradiation.17 One of these studies also reported active breakup of emulsions containing a light-responsive polyelectrolyte by visible light.18 To the best of our knowledge, there are no reports of rapid photoinduced demulsification within a few minutes. We previously achieved UV-induced demulsification of oil-inwater (O/W) emulsions prepared using a photoresponsive gemini surfactant (C7-azo-C7) with an azobenzene skeleton as a spacer.19 In our previous paper, we also reported that the photoinduced demulsification resulted from the UV-induced reduction of the molecular area of C7-azo-C7 at the O/W interface, and therefore the azobenzene group in the C7-azo-C7 molecule plays an important role in photoinduced demulsification. However, complete demulsification on a macroscopic scale required 6 h. This length of time is not appropriate for practical applications. Here, on the basis of LSW theory, we hypothesize that controlling the IFT between the oil and water phases with light will accelerate the photoinduced demulsification. In this article, we describe UV-induced demulsification triggered an increase in IFT between oil and water phases with trans−cis photoisomerization of an azobenzene group in a photoresponsive surfactant.

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EXPERIMENTAL SECTION

Materials. Sodium dodecyl sulfate (SDS) and n-octane (reagent grade, 99.8%) were purchased from Kanto Chemical Co., Inc. (Tokyo, Japan), and Wako Pure Chemical Industries, Ltd. (Tokyo, Japan), respectively. These materials were used without further purification. The photoresponsive surfactant, 2-(4-(4-butylphenyl)diazenylphenoxy)ethyltrimethylammonium bromide (C4AzoTAB; see Scheme 1), was synthesized by a Williamson reaction of 4-(4-butylphenylazo)phenol with 1,2-dibromoethane, followed by quaternization with trimethylamine in methanol. Although the synthesis of C4AzoTAB was reported by Hayashita et al.,20 we modified the synthetic process to prepare C4AzoTAB in this work. This modification increased the total yield of C4AzoTAB from 13% to 60%. Emulsion Preparation. Aqueous surfactant solutions were prepared in high-purity H2O (Milli-Q pure water; R = 18 MΩ cm, γ = 72.0 mN m−1 at 25 °C). Aqueous solutions of SDS/C4AzoTAB mixtures were prepared before homogenization. Binary mixtures of octane and the aqueous SDS/trans-C4AzoTAB solutions were B

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Langmuir Demulsification in a Microreactor. The emulsions were demulsified in a microreactor (YMC Co., Ltd., Kyoto, Japan). The microreactor (channel width: 1.0 mm, total channel length: 915 mm, inner volume: 3 mL) was made of Tempax glass, which is similar to Pyrex (transmittance at 365 nm: ≥ 90%), equipped with a reactor housing. An all-plastic syringe containing the emulsions was connected to the microreactor by a PTFE tube (inner diameter: 1 mm, length: 30 cm). The emulsions (4 mL) were pumped through the microreactor by a syringe pump (SPS-2, AS ONE). UV lamps were positioned on both sides of the microreactor, each at a distance of 1.5 cm (Figure S2). In this device, the emulsions were exposed to UV light with an intensity of 3 mW cm−2.

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RESULTS AND DISCUSSION Influence of UV Light Irradiation on Mixtures of Octane and Aqueous SDS/C4AzoTAB Solution. Figure 1

Figure 2. Phase diagram of mixtures of n-octane and aqueous SDS/ trans-C4AzoTAB solution (octane/aqueous surfactant solution = 80/ 20, w/w) at 25 °C. The diagram shows the relationship between the molar ratio of SDS (XSDS) in aqueous SDS/trans-C4AzoTAB solution and the mixed surfactant concentration. Emulsions that exhibited no phase separation within 1 week are described as “stable emulsions”. If phase separation of the oil and water phases was observed within 1 week, the emulsions are considered “unstable emulsions”.

from the visual field when UV light irradiation was applied for 20 s. Figure 3 shows changes in the stable emulsions after UV

Figure 1. Interfacial tension between n-octane and aqueous SDS/ C4AzoTAB solution plotted versus the mixed surfactant concentrations. Open circles: trans isomer. Filled circles: cis isomer.

shows the IFTs between n-octane and aqueous SDS/ C4AzoTAB (= 1/9, mol/mol) solutions with the mixed surfactant concentrations. The IFT for both the trans and cis isomers decreased with increasing surfactant concentration and reached constant values (5 mN m−1) at specific concentrations. The IFT values of the cis isomer were higher than those of the trans isomer between 0.1 and 15 mM, and above 15 mM, no difference between these values was observed. In the range of 0.1−15 mM, UV light irradiation increased the IFTs. Emulsions (4 mL) were prepared by homogenizing the mixtures of n-octane and aqueous SDS/trans-C4AzoTAB solution (octane/aqueous surfactant solution = 80/20, w/w) at 25 °C. Emulsions that exhibited no phase separation within 1 week of preparation were identified as “stable emulsions”. Stable emulsions were obtained in specific regions in the phase diagram (Figure 2). The molar ratios (XSDS) of SDS in aqueous SDS/trans-C4AzoTAB solution and the total surfactant concentration that resulted in stable emulsions were 0 ≤ XSDS ≤ 0.31, 0.7 ≤ XSDS ≤ 1 and >5 mM, respectively. In addition, when a drop of the stable emulsions was placed onto water, the drop penetrated into the water, indicating that the stable emulsions were O/W-type ones. When the stable O/W emulsions with XSDS = 0.1 and a total surfactant concentration of 10 mM were observed by light microscopy under UV light irradiation, the coalescence of small oil droplets (octane phase) into large ones in the emulsions, i.e., demulsification, was observed (see Movie S1). These octane droplets disappeared

Figure 3. Photographs showing the appearances of mixtures consisting of n-octane and aqueous SDS/C4AzoTAB (= 1 mM/9 mM, total surfactant concentration = 10 mM) solution (octane/aqueous surfactant solution = 80/20, w/w) at 25 °C. A mixture of octane and aqueous SDS/trans-C4AzoTAB solution (a) before and (b) after homogenization. (c) UV light irradiation of (b) led to phase separation. (d) The mixture of octane and aqueous SDS/cisC4AzoTAB solution was homogenized. No emulsion was obtained.

light irradiation in a dark room on the macroscopic scale. When the stable emulsions were irradiated by UV light, n-octane and aqueous SDS/C4AzoTAB solution phases were gradually separated from the emulsions (see Movie S2). These phases were completely separated by UV light irradiation for 90 min, which was even more rapid than our previous result (6 h)19 (Figure 3c). As seen in the lower layer of the vial in Figure 3c, UV light irradiation caused cis isomerization of transC4AzoTAB in the emulsions because the color of the aqueous SDS/C4AzoTAB solutions after UV light irradiation changed from yellow to reddish, corresponding to the formation of the cis isomer. In contrast, no phase separation was observed when the stable emulsion was allowed to stand in a dark room without UV light. Therefore, these results indicate that UV light irradiation of stable O/W emulsions causes cis isomerization of C

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Langmuir trans-C4AzoTAB molecules in the emulsions, leading to demulsification. Possible Mechanisms of Photoinduced Demulsification. Here, we discuss why the stability of O/W emulsions prepared using SDS/trans-C4AzoTAB mixtures drastically decreased with UV light irradiation. First, the stability of the emulsions prepared from octane and aqueous SDS/cisC4AzoTAB (= 1 mM/9 mM, total surfactant concentration = 10 mM) solution in a photostationary state of the cis isomer was investigated. UV light irradiation for more than 90 min produced the photostationary state of cis isomer of C4AzoTAB, where the trans isomer/cis isomer molar ratio was 4/96 (measured by NMR spectroscopy). When the mixtures of octane and aqueous SDS/cis-C4AzoTAB (XSDS = 0.1 at a total concentration of 10 mM) solution (octane/aqueous surfactant solution = 80/20, w/w) were homogenized, emulsions were not obtained (Figure 3d). Thus, demulsification results from changes in the emulsion stability caused by photoisomerization from the trans isomer to the cis isomer of C4AzoTAB in SDS/ C4AzoTAB mixtures. The stabilities of O/W emulsions prepared using ionic surfactants have been found to depend on a number of factors, such as the nature of the liquid−liquid interface, electrical repulsion between dispersed droplets, the viscosity of the continuous phase, and/or the size distribution of the dispersed droplets.21 The number distribution of the diameters of the oil droplets (octane phase) dispersed in emulsions consisting of octane/aqueous SDS/trans-C4AzoTAB (XSDS = 0.1 at 10 mM) solution (= 80/20, w/w) were measured by counting 300 droplets in light microscope images (Figure 4). The number-

Figure 5. Change in the cubic value (r3) of the averaged radius of the octane droplets in emulsions consisting n-octane and aqueous SDS/ C4AzoTAB (= 1 mM/9 mM) solution (octane/aqueous surfactant solution = 80/20, w/w) with UV light irradiation.

light irradiation. As can be seen in the figure, the cubic values of the octane droplet radius increased linearly with the irradiation time. Thus, in this system, photoinduced demulsification is caused by Ostwald ripening. We suggest the following mechanism for the photoinduced demulsification of stable O/W emulsions prepared with the trans-isomer. SDS and trans-C4AzoTAB molecules are absorbed onto the entire octane/aqueous surfactant solution interfaces in the stable emulsions. UV light irradiation leads to trans−cis photoisomerization and increased IFT at the octane/ aqueous surfactant solution interface (see Figure 1). The increase in the IFT with trans−cis photoisomerization causes the coalescence of smaller octane droplets into larger ones (Ostwald ripening) in O/W-type emulsions to reduce the interfacial area of the emulsions. Light microscopy observations confirmed the mechanism described above (see Movie S1). Moreover, Ostwald ripening progressed with the trans−cis photoisomerization until the octane and water phases were fully separated. In addition, we investigaed the influence of the presence or absence of SDS in aqueous SDS/C4AzoTAB solutions on photoinduced demulsification. As seen in Figure 2, stable emulsions were obtained from octane and aqueous C4AzoTAB solutions without SDS. UV light irradiation of the emulsion for 8 h did not result in phase separation. The addition of SDS to aqueous C4AzoTAB solutions led to an increase in the differences in the IFTs of the trans and cis isomers in an octane/aqueous SDS/C4AzoTAB solution (Figure S3). Thus, SDS in aqueous SDS/C4AzoTAB solutions plays an important role in photoinduced demulsification. Photoinduced Demulsification Rate. As seen in eq 1, increases in the IFT increase the Ostwald ripening rate. Figure 6 shows the change in the octane phases separated from stable O/W emulsions consisting of octane and aqueous SDS/transC4AzoTAB (= 1/9, mol/mol) solution (octane/aqueous surfactant solution = 80/20, w/w) with UV light irradiation. The octane phases were evaluated by measuring the height of the phases separated from the stable emulsions (4 mL) in a vial (diameter = 1.8 cm). The height of the octane phase gradually increased with UV light irradiation at total surfactant concentrations of 7 and 10 mM, eventually reaching 1.5 cm,

Figure 4. Number distribution of the diameters of oil (n-octane) droplets in emulsions consisting of octane and aqueous SDS/ C4AzoTAB (= 1/9, mM/mM) solution (octane/aqueous surfactant solution = 80/20, w/w) (a) before and (b) after UV light irradiation for 10 s. UV light irradiation decreases the distribution of smaller droplets and increases the distribution of larger ones.

averaged diameter before UV light irradiation was 14.6 ± 9.2 μm. UV light irradiation suppressed the distribution of smaller oil droplets but strengthened that of the larger droplets. This phenomenon seems to favor typical Ostwald ripening. If emulsions are destabilized by Ostwald ripening, the cubic values (r3) of the number-averaged radius of the dispersed droplets should linearly increase with the observable time, according to eq 1. Figure 5 shows the change in the cubic values of the radius of the octane droplets in the stable emulsions with UV D

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irradiation. Therefore, demulsification was triggered by only UV light in the microreactor. Homogenization and Demulsification Processes with Reversible Trans−Cis Photoisomerization. We also repeated the homogenization and photoinduced demulsification processes of mixtures consisting of octane/aqueous SDS/ C4AzoTAB (XSDS = 0.1 at a total concentration of 10 mM) solution (= 80/20, w/w), as follows: After UV light was irradiated onto emulsions prepared using a mixture of SDS/ trans-C4AzoTAB, the phase-separated aqueous phase was irradiated by visible light to cause the cis isomer to isomerize and form the trans isomer of C4AzoTAB. Subsequently, the separated octane and aqueous surfactant solution phases were homogenized to prepare emulsions again. Thus, the homogenization and photoinduced demulsification processes were observed to be reversible (Figure 7). Repetitive trans−cis photoisomerization led to complete phase separation. Therefore, the SDS/C4AzoTAB mixtures can be repetitively subjected to photoinduced demulsification.

Figure 6. Change in the n-octane phase separated from stable O/W emulsions consisting of n-octane and aqueous SDS/C4AzoTAB (= 1/ 9, mol/mol) solution at different surfactant concentrations (octane/ aqueous surfactant solution = 80/20, w/w) with UV light irradiation. Open circles: [SDS + C4AzoTAB] = 7 mM; open squares: [SDS + C4AzoTAB] = 10 mM; open triangles: [SDS + C4AzoTAB] = 20 mM.

the value that corresponds to the complete phase separation. However, when the total surfactant concentration was 20 mM, the maximum height of the octane phase was 0.7 cm because there was almost no difference in the IFTs between the trans and cis isomers (see Figure 1), indicating incomplete demulsification. UV light irradiation caused complete phase separation in 60 min with a total surfactant concentration of 7 mM and in 90 min at a total surfactant concentration of 10 mM. The time required for demulsification at 7 mM was shorter than that at 10 mM. As seen in Figure 1, the differences in the IFTs between the trans and cis isomers at 7 and 10 mM were 16.6 and 10.2 mN m−1, respectively. Thus, any change in the IFT resulting from UV light irradiation caused a corresponding change in the photoinduced demulsification rate. We investigated the time required for complete phase separation (demulsification) of stable emulsions consisting of octane/aqueous SDS/C4AzoTAB (XSDS = 0.1 at a total concentration of 10 mM) solution (= 80/20 (w/w)). On the macroscale, as can be seen in Figure 6 and Movie S2, it takes 90 min for the emulsions to completely separate under UV light irradiation. When the emulsions are placed in vials, UV light illuminates only the surface of the vial, hardly penetrating into the emulsions in this experimental geometry because the emulsions are turbid (Figure S1a). Thus, if the emulsions can be thinned, UV light irradiation will accelerate the demulsification. Here, a microreactor was used to thin the emulsions. The microreactor was made of Tempax glass, which is similar to Pyrex, and equipped with a reactor housing (Figure S3). An all-plastic syringe containing the emulsions was connected to the microreactor by a PTFE tube. The emulsions were pumped through the microreactor by a syringe pump with UV light irradiation. When the emulsions (4 mL) flowed through the microreactor at 70 mL h−1 with UV light irradiation, the octane and aqueous surfactant solution phases were completely separated within 3.5 min. Thus, the time required for photoinduced demulsification in the microreactor was 26 times shorter than that in the vial. In addition, no phase separation was observed in the microreactor without UV light

Figure 7. Change in the n-octane phase separated from stable O/W emulsions consisting of octane and aqueous SDS/C4AzoTAB (= 1 mM/9 mM) solution (octane/aqueous surfactant solution = 80/20, w/w) with repetitive trans−cis photoisomerization and homogenization. When the octane and surfactant solution phases are completely separated, the height of the octane phase is 1.5 cm. Solid lines: photoisomerization with UV light irradiation; dashed lines: visible light irradiation and homogenization. Open circles: SDS/trans-C4AzoTAB; filled circles: SDS/cis-C4AzoTAB.

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CONCLUSION We studied the influence of light irradiation on emulsions prepared using mixtures of an anionic surfactant, SDS, and a photoresponsive cationic surfactant, C4AzoTAB. Stable O/W emulsions were prepared from the mixtures, whereas no stable emulsions were obtained when C4AzoTAB was a cis isomer. UV light irradiation of the stable emulsions promoted the photoisomerization of C4AzoTAB from the trans isomer to the cis isomer and caused demulsification through Ostwald ripening of the oil droplets. This photoinduced demulsification results from an increase in IFTs at the interface between octane and aqueous SDS/C4AzoTAB solution with the trans−cis photoisomerization. Repetitive trans−cis photoisomerization of aqueous SDS/C4AzoTAB solutions in the emulsions repeatedly resulted in photoinduced demulsification. Therefore, the E

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(11) Lifshitz, I. M.; Slyozov, V. V. The kinetics of precipitation from supersaturated solid solutions. J. Phys. Chem. Solids 1961, 19, 35−50. (12) Wagner, C. Theorie der Alterung von Niederschlägen durch Umlösen (Ostwald-Reifung). Elektrochem. 1961, 65, 581−591. (13) 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. (14) Brown, P.; Butts, C. P.; Cheng, J.; Eastoe, J.; Russell, C. A.; Smith, G. N. Magnetic emulsions with responsive surfactants. Soft Matter 2012, 8, 7545−7546. (15) Zhao, C.; Tan, J.; Li, W.; Tong, K.; Xu, J.; Sun, D. Ca2+ Ion Responsive Pickering Emulsions Stabilized by PSSMA Nanoaggregates. Langmuir 2013, 29, 14421−14428. (16) Chen, Q.; Xu, Y.; Cao, X.; Qin, L.; An, Z. Core cross-linked star (CCS) polymers with temperature and salt dual responsiveness: synthesis, formation of high internal phase emulsions (HIPEs) and triggered demulsification. Polym. Chem. 2014, 5, 175−185. (17) Porcar, I.; Perrin, P.; Tribet, C. UV−Visible Light: A Novel Route to Tune the Type of an Emulsion. Langmuir 2001, 17, 6905− 6909. (18) Khoukh, S.; Perrin, P.; Bes de Berc, F.; Tribet, C. Reversible Light-Triggered Control of Emulsion Type and Stability. ChemPhysChem 2005, 6, 2009−2012. (19) Takahashi, Y.; Fukuyasu, K.; Horiuchi, T.; Kondo, Y.; Stroeve, P. Photoinduced Demulsification of Emulsions Using a Photoresponsive Gemini Surfactant. Langmuir 2014, 30, 41−47. (20) Hayashita, T.; Kurosawa, T.; Miyata, T.; Tanaka, K.; Igawa, M. Effect of structural variation within cationic azo-surfactant upon photoresponsive function in aqueous solution. Colloid Polym. Sci. 1994, 272, 1611−1619. (21) Rosen, M. J. Emulsification by Surfactants. Surfactants and Interfacial Phenomena; John Wiley & Sons, Inc.: New York, 2004; pp 303−331.

photoinduced demulsification demonstrated here will be also useful for applications such as liquid-membrane separation processes and the post-treatment of the products of emulsion polymerization.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b03912. Geometries for UV light irradiation to emulsions (Figures S1 and S2); differences in the interfacial tensions in an n-octane/aqueous SDS/C4AzoTAB solution between the trans and cis isomers with respect to the molar ratio of the aqueous SDS/C4AzoTAB solution (10 mM) (Figure S3); changes in the interfacial tensions between n-octane and an aqueous SDS/ C4AzoTAB solution (1:9, mol/mol) at 10 mM with UV light irradiation (Figure S4) (PDF) Movie of the photoinduced demulsification with UV light irradiation on the microscopic scale (AVI) Movie of the photoinduced demulsification with UV light irradiation on the macroscopic scale (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*(Y.T.) E-mail [email protected]; phone +81-35228-8313. *(Y.K.) E-mail [email protected]; phone +81-3-5228-8313. Notes

The authors declare no competing financial interest.

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

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DOI: 10.1021/acs.langmuir.5b03912 Langmuir XXXX, XXX, XXX−XXX