Self-Propelled Oil Droplets and Their Morphological Change to Giant

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Self-Propelled Oil Droplets and Their Morphological Change to Giant Vesicles Induced by a Surfactant Solution at Low pH Taisuke Banno,*,† Yuki Tanaka,‡ Kouichi Asakura,† and Taro Toyota*,‡,§ †

Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan ‡ Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan § Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan S Supporting Information *

ABSTRACT: Unique dynamics using inanimate molecular assemblies based on soft matter have drawn much attention for demonstrating far-from-equilibrium chemical systems. However, there are no soft matter systems that exhibit a possible pathway linking the self-propelled oil droplets to formation of giant vesicles stimulated by low pH. In this study, we conceived an experimental oil-in-water emulsion system in which flocculated particles composed of a imine-containing oil transformed to spherical oil droplets that self-propelled and, after coming to rest, formed membranous figures. Finally, these figures became giant vesicles. From NMR, pH curves, and surface tension measurements, we determined that this far-from-equilibrium phenomenon was due to the acidic hydrolysis of the oil, which produced a benzaldehyde derivative as an oil component and a primary amine as a surfactant precursor, and the dynamic behavior of the hydrolytic products in the emulsion system. These findings afforded us a potential linkage between mobile droplet-based protocells and vesicle-based protocells stimulated by low pH.

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INTRODUCTION Oil and surfactants dispersed or dissolved in water or buffered aqueous solutions form a variety of phases and morphologies that have both unique organization and molecular orientations. Current research into colloidal systems, such as emulsions, liquid crystals, and gels, addresses the fundamental questions regarding self-assembly in soft matter physics, colloidal chemistry, and biology, including the issues on the origins of life.1,2 In the far-from-equilibrium state of a colloidal system, the locomotion of colloidal particles and their transformations have potential applications in pharmaceuticals and industry.3 Among these colloidal particles, self-propelled oil droplets have attracted considerable attention.4−6 The mechanism of their self-propelled motion in bulk water is based on the differences in interfacial tension around the oil droplet and mass transfer as a result of the Marangoni instability.7 In recent reports, oil droplets have been seen to be driven by chemical reactions,8 solubilization,9,10 and phase separation.11 Moreover, some selfpropelled oil droplets have been shown to be controllable by external stimuli, such as gradient fields of chemical compounds12−14 and light irradiation.15,16 These oil-in-water systems can provide a chemical platform for an evolutionary system that will assist in the investigation of the origins of life.17−19 © XXXX American Chemical Society

In this paper, we report an oil-in-water system as an experimental model comprising micrometer-sized oil droplets with the ability to move via the chemical conversion of “inactive” components, producing surfactants and “active” oil components at low pH (Figure 1). Such chemical conversion is one of several effective strategies to induce or alter the motion of underwater oil droplet.8,20,21 Oil droplets composed of nheptyloxybenzaldehyde (HBA), which is called an active oil component here, have frequently been reported to be selfpropelled in N,N,N-n-hexadecyltrimethylammonium bromide (C16TAB) solutions, and the dynamics of these oil droplets are influenced strongly by the surfactant concentration.22,23 Therefore, in the present study, N-(4-(heptyloxy)benzylidene)decyl-1-amine (HBDA), HBA and an amine alkyl chain linked by an imine bond, was designed so that the inactive oil component would be forced to be active. Under acidic conditions, HBDA is hydrolyzed to produce both active HBA and decylamine (DA), and DA can be converted to a cationic surfactant (DAH+) by protonation at low pH. Therefore, in the current system, the oil-in-water system would be activated at Received: July 2, 2016 Revised: August 31, 2016

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a yellow syrup. The crude product was washed three times with chloroform (10 mL) to obtain HBBA as a yellow syrup in 82% yield (0.972 g). 1H NMR (500 MHz, MeOD-d4): δ 8.19 (1H, s), 7.64 (2H, d, J = 8.8 Hz), 6.90 (2H, d, J = 8.8 Hz) 3.98 (2H, t, J = 6.6 Hz), 3.57 (2H, td, J = 7.1, 1.0 Hz), 1.82−1.75 (2H, m), 1.71−1.63 (2H, m), 1.52−1.24 (10H, m), 0.96−0.80 (6H, m). ESI-MS in MeOH (m/z): 276.15 [M + H+]; calcd 276.43 [M + H+]. Optical Microscopy Observations of the Oil Droplet Dynamics. The observation specimen was prepared as follows. An emulsion was formed by agitating 200 μL of a 10 mM C16TAB solution with liquid HBDA (125 mM) at room temperature (23−25 °C). Because HBDA was partially solid after having been stored in a refrigerator, it was warmed to around 40 °C immediately before emulsification. The melted HBDA was liquid at room temperature (23−25 °C). Warming the HBDA did not cause decomposition, determined from the 1H NMR spectra of HBDA obtained before and after warming. Immediately after mixing HBDA in the surfactant solution and encasing the emulsion sample in a thin glass chamber (15 × 15 × 0.28 mm; Frame Seal Chamber, MJ Research Inc., Waltham, MA), real-time observations were conducted at room temperature (23−25 °C) using a phase contrast microscope (IX71, Olympus, Japan) equipped with a CCD camera (DP72, Olympus, Japan). To visualize the flow fields around the aggregates and droplets, fluorescent microscopy observations were carried out using hydrophilic polystyrene fluorescent beads (Fluoresbrite YG Carboxylate Microspheres 1.00 μm, Polyscience Inc., , Warrington, PA) with a concentration of 2.67 × 10−3 wt %. Time Course Measurement of HBDA Hydrolysis. HBDA (125 mM) was added to an aqueous solution in the presence of C16TAB (10 mM) containing 0.1 M HCl (200 μL) at room temperature. Then, the solution was lyophilized and dissolved in MeOD-d4. The hydrolysis of HBDA was analyzed by 1H NMR measurements of a MeOD-d4 solution of C16TAB and products, and the percentage hydrolysis of HBDA was calculated by integration of the decrease in the signals for the imine methine protons at δ 8.22 ppm using the signal for trimethylammonium from the C16TAB protons at δ 3.18 ppm as an internal standard peak (see Figure S3). Measurements of pH in the Dispersion. A dispersion was prepared by adding HBDA (125 mM) to an aqueous solution in the presence of 10 mM C16TAB containing 0.1 M HCl (200 μL) at room temperature. The pH was measured by using an ion sensitive field effect transistor pH electrode (Horiba Ltd., Kyoto, Japan) for 70 min. The pH of the C16TAB dispersion containing 0−125 mM DA was also measured using the same procedure. Equilibrium Surface Tension Measurements. The equilibrium surface tensions of the solutions were measured using a digital Kyowa precise surface tensiometer applying the CBVP method (Kyowa Kagaku Co. Ltd., Tokyo, Japan) at 25 °C. The measurements were taken using the Wilhelmy vertical plate technique with a sandblasted glass plate. The test solutions were allowed to stand at 25 °C for at least 1 h before the measurements. Infrared Spectroscopy Measurements. To estimate the intermolecular interactions between HBA and DA, Fourier transform infrared (IR) spectroscopy of HBDA, HBA, DA, and the equivalent molar mixture of HBA and DA was recorded using the attenuated total reflection (ATR) method on a FT/IR4100 (JASCO Co., Tokyo, Japan).

Figure 1. Conceptual scheme for the formation of an oil-in-water system with self-propelled oil droplets via the hydrolysis of HBDA to produce the surfactant precursor, DA, and the active oil component, HBA, in a C16TAB solution containing HCl.

low pH, and the oil droplets would gain locomotion function because such chemical conversion induces an increase in the surfactant concentration and heterogeneity in the interfacial tension around the oil droplets (Figure 1).

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MATERIALS AND METHODS

Reagents. Commercially available reagents and solvents were purchased from Tokyo Chemical Industry Co. (Tokyo, Japan) and Kanto Chemical Co. (Tokyo, Japan) and used without further purification. Water was distilled and deionized prior to use, using a Milli-Q system from Millipore (Bedford, MA). Synthesis of N-(4-(Heptyloxy)benzylidene)decyl-1-amine (HBDA). HBDA was synthesized using the methodology reported by Yeap et al. (Figure S1).24 A mixture of HBA (1.81 g, 8.2 mmol) and DA (1.29 g, 8.2 mmol) was stirred in anhydrous ethanol (35 mL) for 14 h at 90 °C under a nitrogen atmosphere. After the reaction, the solvent was evaporated under reduced pressure to obtain the crude product as a colorless syrup. The obtained syrup was solidified using liquid nitrogen and washed with methanol. Purification was carried out by recrystallization in methanol (10 mL) to obtain HBDA as pale yellow crystals in 55% yield (1.63 g). 1H NMR (500 MHz, MeOD-d4): δ 8.22 (1H, s), 7.65 (2H, d, J = 6.8 Hz), 6.95 (2H, d, J = 6.8 Hz) 4.00 (2H, t, J = 6.3 Hz), 3.53 (2H, td, J = 7.0, 1.0 Hz), 1.82−1.71 (2H, m), 1.71−1.60 (2H, m), 1.52−1.20 (22H, m), 0.96−0.80 (6H, m). Electrospray ionization mass spectroscopy (ESI-MS) in MeOH (m/z): 360.59 [M + H+]; calcd 360.55 [M + H+]. Synthesis of N-(4-(Heptyloxy)benzylidene)butyl-1-amine (HBBA). As a reference compound, HBBA, which has a different hydrophobic alkyl chain length from HBDA, was designed and synthesized. A mixture of HBA (0.946 g, 4.3 mmol) and butylamine (0.329 g, 4.5 mmol) was stirred in anhydrous ethanol (35 mL) for 14 h at 90 °C under a nitrogen atmosphere. After the reaction, the solvent was evaporated under reduced pressure to obtain the crude product as

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RESULTS AND DISCUSSION Time Evolution of the Dynamics of the HBDA Emulsion. An HBDA emulsion was prepared with a 10 mM C16TAB solution containing 0.1 M HCl and HBDA (9.0 mg per 200 μL, 125 mM) at room temperature (23−25 °C) stirred for 5 s. Immediately after stirring, the sample was observed using a glass chamber (thickness: 0.28 mm) and a phase contrast microscopy (Movie S1). At the initial stages of the emulsion, flocculated particles with a diameter of several hundreds of micrometers had formed and floated close to the B

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Langmuir upper glass surface. These flocculates slowly but gradually transformed to spherical oil droplets, the diameters of which ranged from 10 to 250 μm, over a 24 min period (Figure 2A).

persed in the aqueous phase of the emulsion system showed that the transformation of flocculated particles to spherical oil droplets did not involve water advection around the particulate matter and that oil droplet locomotion was associated with the surrounding water advection to the opposite direction (see Figure S3). These results suggest that the transformation of flocculated particles was not induced by the fluidic kinetics while the droplet locomotion was driven by a mechanism similar to the previous system of self-propelled HBA droplets.21−23 Influences of Hydrolysis of HBDA on the Dynamics of the Particles in the Emulsion. To clarify the dynamics of the flocculated particles and oil droplets regarding the hydrolysis of HBDA, a range of variables, including the surfactant concentration and the pH of the dispersion at room temperature, were examined (Table S1). When the pH was altered by 0.01, 0.1, and 1 M HCl (entries 1−4 in Table S1), the morphological changes of flocculated particles and the subsequent locomotion of spherical droplets were observed only in the presence of 0.1 M HCl. Self-propelled motion of droplets was not observed when the HCl concentrations were varied either below or above 0.1 M. In the case of 0.01 M HCl, the flocculated particles transformed into spherical droplets, but no further motion occurred (entry 3 in Table S1). In the presence of 1 M HCl, only flocculated particles were observed, whereas, in the absence of HCl, only spherical droplets were observed (entries 1 and 4 in Table S1). Therefore, we consider that 0.1 M HCl is the optimal condition for activating the oilin-water system. As a reference, spherical oil droplets were observed in the emulsion with 0.1 M NaCl instead of 0.1 M HCl, and they just floated with no self-propelled motion (entry 5 in Table S1). Therefore, flocculated particles only formed when the HCl concentration was relatively high. Based on these results, the locomotion function in the emulsion required a suitable morphology, i.e., spherical oil droplets, because the flocculated particles did not move until their morphology had been similar to spherical droplets. Therefore, the hydrolytic conversion of HBDA (125 mM) was traced to the production of DA and HBA in a 10 mM C16TAB solution containing 0.1 M HCl for 60 min by 1H NMR spectroscopy (Figure 3A and Figure S4). HBDA was hydrolyzed gradually, and its hydrolytic ratio reached 50% approximately 15 min after the specimen preparation; this corresponds to the time when the morphological change from flocculated particles to spherical droplets was observed frequently. After that the hydrolysis occurred gradually, which exceeded 65% after 30 min. This suggests that the transformation of flocculated particles and the subsequent droplet locomotion are induced by the hydrolysis of HBDA. The effects of the surfactant concentration on the dynamics of the emulsion are significant. In the absence of C16TAB, the flocculated particles composed of HBDA dispersed in 0.1 M HCl changed into motionless spherical droplets that adhered to the glass surface. In addition, when a 50 mM C16TAB solution (including 0.1 M HCl) was used, flocculated particles that were similar to those in a 10 mM C16TAB solution (including 0.1 M HCl) transformed gradually to spherical droplets, and again, these particles displayed no movement (entry 6 in Table S1). This suggests that the spherical droplets required not high surfactant concentrations but an increase in surfactant concentration during locomotion. Under acidic conditions, DA produced by the hydrolysis of HBDA is expected to be protonated, yielding the surfactant, DAH+, as shown in Figure

Figure 2. Typical time evolution dynamics of the HBDA emulsion, which was prepared in an acidic solution (0.1 M HCl) of C16TAB (0.01 M), observed by phase contrast microscopy (A−C) and polarized microscopy (D). (A) Morphological changes from flocculated particles to spherical droplets. (B) Start of the locomotion of droplets. (C) Formation of micrometer-sized giant vesicles around the immobile droplets, as represented by the black arrow. (D) Typical micrographs of the morphological change and locomotion of droplets, which had a bright texture in a part of the aggregate (indicated by white arrows). Bar: 100 μm.

They then started locomotion three-dimensionally before their morphology was not completely spherical, and their motion was sustained for ∼30 min (Figure 2B). Larger self-propelled oil droplets moved faster in all tested surfactant solutions, indicating that the speed of self-propelled oil droplets is strongly size-dependent (Figure S2A). Even though the flocculated particles with a polydisperse size distribution, ranging from several micrometers to several hundred micrometers, were observed in the sample in the initial stage, the dynamic evolution of the particles occurred reproducibly over similar time scales (15−25 min) (Figure S2B). After locomotion had ceased and, after about 10 h, membranous figures had formed around the immobile oil droplets. Finally, these figures changed into giant vesicles (spherical membrane structures exhibiting thermal fluctuation19) as shown in Figure 2C. We also observed this dynamic behavior under a polarized microscope, and liquid crystal phases were found to be present only during the morphology transformation from flocculated particles to spherical droplets (Figure 2D). Fluorescence microscopy observation using fluorescent microspheres disC

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transformed to spherical droplets 12 min after the specimen was prepared (Figure S5). Subsequently, all generated spherical droplets exhibited self-propelled motion under such conditions. On the other hand, in the case of HBA/DA = 3/7 or 7/3 (mol/ mol), spherical oil droplets that were not self-propelled were observed immediately after preparation (Figure S5). This suggests that the mixed emulsions of HBA and DA are different from the mixture of the decomposed HBDA emulsion. Thus, to clarify this hypothesis experimentally, two measurements for HBA, DA, HBDA, and the mixture of HBA and DA (5/5 molar ratio) were carried out: infrared (IR) absorbance spectroscopy and differential scanning calorimetry (DSC). The IR spectra of HBDA and an equimolar mixture of HBA and DA were almost same (Figure 4A). For example, even though the carbonyl

Figure 3. (A) Time course of hydrolysis of HBDA in a 10 mM C16TAB solution containing 0.1 M HCl at room temperature (25 °C) for 60 min. The hydrolytic ratio of HBDA was determined by 1H NMR spectroscopy of the lyophilized residue of the dispersion. (B) Time course of the pH variations in 10 mM C16TAB solution containing 0.1 M HCl. (C) pH of a 10 mM C16TAB solution containing 0.1 M HCl titrated by the various concentrations of DA. (D) Surface tension of a 10 mM C16TAB solution containing 0.1 M HCl with various concentrations of DA.

1. Therefore, pH measurements were taken in an HBDA dispersion containing C16TAB and HCl (Figure 3B). Immediately after the specimens were prepared, the pH of the dispersions was 1.1, subsequently increasing to a pH of 2. Furthermore, the pH suddenly rose to 10.7 after 55 min, thereafter remaining constant. To understand this sudden pH increase, the pH of a 10 mM C16TAB solution (containing 0.1 M HCl) titrated with DA was measured. The pH increased slightly but steadily with increasing amount of DA amount, reaching 8 when the DA concentration exceeded 0.1 M. Even though the dispersion contained 0.1 M HCl, the acid was neutralized completely by the excess of DA (125 mM) to produce DAH+, and the pH rose to 8 due to the presence of soluble DA (Figure 3C). In addition, the protonated DA, DAH+, behaved as a surfactant molecule because the surface tension of the C16TAB solution containing 0.1 M HCl decreased with increasing DA concentration (Figure 3D). Taking these results into account, because the flocculated particles transformation and the subsequent droplet locomotion were observed when the HBDA concentration was at 125 mM, HBDA was partially hydrolyzed to generate HBA and DA, and the latter then converted to surface active DAH+ by protonation, which diffused to the bulk solution and adsorbed to the surface of the droplet, thereby increasing the heterogeneity of the interfacial tension. Influences of Components on the Dynamics in the Emulsion. As reference experiments for the morphological change from flocculated particles to spherical oil droplets, emulsion specimens composed of HBA and DA at molar ratios of 3/7, 5/5, and 7/3 in a 10 mM C16TAB solution containing 0.1 M HCl were observed. In the case of HBA/DA = 5/5 (mol/mol), the flocculated particles, which were similar to those of the HBDA emulsion, formed initially and then

Figure 4. (A) Infrared absorbance spectra and (B) DSC curves of HBDA (green), HBA (red), DA (blue), and a mixture of HBA and DA at a 5/5 molar ratio (orange).

group in HBA was detected at 1687 cm−1, that in the mixture and the imine group of HBDA were detected at 1644 cm−1. The similarity of the spectra of the HBDA and the mixture arises not only from the reverse reaction between HBA and DA (i.e., HBDA is produced by mixing both HBA and DA in neat conditions, as confirmed by 1H NMR) but also the formation of hydrogen bonding between the carbonyl group in HBA and the amine group in DA, which manifested by the appearance of a broad peak at 3600−3100 cm−1. This suggests that the 5/5 mixture dispersion of HBA and DA exhibited the similar behavior at the molecular level in the emulsion system, inducing the formation of flocculated particles composed of an equimolar mixture of HBA and DA which was similar to those of HBDA. The DSC measurements also revealed a strong interaction between HBA and DA (Figure 4B). A single endothermic peak was observed in the DSC profiles of both HBA and DA, and their Tm was −5.1 and 16.0 °C, respectively. On the other hand, their oil mixture exhibited a different Tm from those of the single oil component and showed a large D

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Langmuir endothermic peak, suggesting that some complexed phases had formed. Moreover, polarized microscopy observations of the mixture composed of HBA and DA showed morphological changes in the liquid crystal phases from flocculated particles to spherical droplets (Figure S5). This suggests that the C16TAB, HBA, DA, and DAH+ (and a small amount of water) molecules likely orientated each other in the emulsion. Therefore, the parallel molecular orientation induced the formation of flocculated particles in the oil mixture composed of an equimolar mixture of HBA and DA. Furthermore, the subsequent morphological changes were caused by the variation of the molar composition in HBA and DA due to the production and diffusion of DAH+ into the bulk solution. As a further reference experiment, an emulsion of N-(4(heptyloxy)benzylidene)butyl-1-amine (HBBA), which was used as an oil component instead of HBDA, was prepared in a 10 mM C16TAB solution including 0.1 M HCl. This emulsion exhibited neither the flocculated particle transformation nor locomotion of the oil droplets under the microscope. Spherical oil droplets formed at the initial stages, and the membranous figures then grew gradually from the motionless droplets (Figure 5). The membranous figures were

Figure 6. Schematic diagram of the interpretation on the mechanism of flocculated particle transformation and droplet locomotion in the current emulsion system. The molecules within the dashed windows of (B) are depicted oriented parallel to each other.

Figure 5. Representative dynamics of the HBBA emulsion composed of 0.1 M HCl and 0.01 M C16TAB observed under a phase contrast microscope 6 min after preparation. Bar: 100 μm.

and the produced HBA and DA form a liquid crystal phase owing to the alignment of molecules (Figure 6B). Furthermore, the diffused DAH+ can adsorb to the oil−water interface from the bulk solution, leading to nonuniformity of the surfactant concentration and orientations of surfactant molecules at the interface. This causes heterogeneity in the interfacial tension at the interface, resulting in dynamic interfacial fluctuations evoking the mass flux around the oil droplet due to Marangoni instability. Thus, the spherical droplet exhibits self-propelled motion (Figure 6C). In addition, the pH of the bulk solution varies according to the production and diffusion of DAH+, indicating that the membranous figures form by the combination of C16TAB, HBA, and DAH+ (with DA in some amount), and finally leading to the formation of giant vesicles (Figure S7). Finally, we remark the significance of our findings. In the discussion of possible places for the beginning of life, the thermal vents located under the sea are the likeliest so far.26,27 Because the pH of these circumstances is assumed to be low due to high concentrations of salt,28,29 the oil-in-water system as a chemical “soup” for the beginning of life would be expected to be activated at low pH. Furthermore, a change in the locomotion mode of self-propelled oil droplets under such acidic conditions has been reported.22,23 On the other hand, although some research groups have reported oil-in-water systems activated by high pH,20,21,30 there have been no reports on the activation of oil-in-water system at low pH. Imine bonds are one of the so-called dynamic covalent bonds that can be formed and decomposed under mild conditions31 and have

unstable and dissolved within 10 min. This observation suggests that the unique dynamics of the HBDA emulsion system was induced by the production and diffusion of DAH+, and the membranous figures were stable due to the complexation of HBA and DAH+ (and DA in some amount). Proposed Mechanism of Flocculated Particle Transformation and Droplet Locomotion in the Current Emulsion System. Although the precise mechanism is not completely understood, the phenomena observed in the HBDA emulsion with C16TAB and HCl can be interpreted as follows (Figure 6). Immediately after starting the observations, flocculated particles of HBDA formed in the C16TAB solution with a high concentration of HCl. This results from complexation of the surfactant and oil molecules at the oil− water interface because of the relatively strong interactions of C16TAB to HBDA. Sumino et al. reported that such interactions can induce the formation of a gel, which has an undefined shape.25 Then, water containing an acidic catalyst, H+, is absorbed into immobile flocculated particles from bulk solution due only to their diffusion by the adsorption of C16TAB molecules onto the interface (Figure 6A).22 At the interface, the specific reaction field in which the hydrolysis of HBDA is catalyzed is formed. HBDA is slowly but steadily hydrolyzed to produce HBA and DA, and the latter is then converted to DAH+, which adsorbs to the interface and diffuses into the bulk solution. Such molecular conversion in the flocculated particles induces the morphological change to spherical droplets. During these dynamics, HBDA, C16TAB, E

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(3) Paxton, W. F.; Sundararajan, S.; Mallouk, T. E.; Sen, A. Chemical Locomotion. Angew. Chem., Int. Ed. 2006, 45, 5420−5429. (4) Sumino, Y.; Kitahata, H.; Shinohara, Y.; Yamada, N. L.; Seto, H. Formation of a Multiscale Aggregate Structure through Spontaneous Blebbing of an Interface. Langmuir 2012, 28, 3378−3384. (5) Browne, K. P.; Walker, D. A.; Bishop, K. J. M.; Grzybowski, B. A. Self-Division of Macroscopic Droplets: Partitioning of Nanosized Cargo into Nanoscale Micelles. Angew. Chem., Int. Ed. 2010, 49, 6756− 6759. (6) Caschera, F.; Rasmussen, S.; Hanczyc, M. M. An Oil Droplet Division−Fusion Cycle. ChemPlusChem. 2013, 78, 52−54. (7) Maass, C. C.; Krüger, C.; Herminghaus, S.; Bahr, C. Swimming Droplets. Annu. Rev. Condens. Matter Phys. 2016, 7, 171. (8) Toyota, T.; Maru, N.; Hanczyc, M. M.; Ikegami, T.; Sugawara, T. Self-Propelled Oil Droplets Consuming “Fuel” Surfactant. J. Am. Chem. Soc. 2009, 131, 5012−5013. (9) Nagai, K.; Sumino, Y.; Kitahata, H.; Yoshikawa, K. Mode Selection in the Spontaneous Motion of an Alcohol Droplet. Phys. Rev. E 2005, 71, 065301. (10) Pimienta, V.; Brost, M.; Kovalchuk, N.; Bresch, S.; Steinbock, O. Complex Shapes and Dynamics of Dissolving Drops of Dichloromethane. Angew. Chem., Int. Ed. 2011, 50, 10728−10731. (11) Ban, T.; Yamada, T.; Aoyama, A.; Takagi, Y.; Okano, Y. Composition-Dependent Shape Changes of Self-Propelled Droplets in a Phase-Separating System. Soft Matter 2012, 8, 3908−3916. (12) Lagzi, I.; Soh, S.; Wesson, P. J.; Browne, K. P.; Grzybowski, B. A. Maze Solving by Chemotactic Droplets. J. Am. Chem. Soc. 2010, 132, 1198−1199. (13) Č ejková, J.; Novák, M.; Štěpánek, F.; Hanczyc, M. M. Dynamics of Chemotactic Droplets in Salt Concentration Gradients. Langmuir 2014, 30, 11937−11944. (14) Ban, T.; Nakata, S. Metal-Ion-Dependent Motion of SelfPropelled Droplets Due to the Marangoni Effect. J. Phys. Chem. B 2015, 119, 7100−7105. (15) Diguet, A.; Guillermic, R.; Magome, N.; Saint-Jalmes, A.; Chen, Y.; Yoshikawa, K.; Baigl, D. Photomanipulation of a Droplet by the Chromocapillary Effect. Angew. Chem., Int. Ed. 2009, 48, 9281−9284. (16) Florea, L.; Wagner, K.; Wagner, P.; Wallace, G. G.; BenitoLopez, F.; Officer, D. L.; Diamond, D. Photo-Chemopropulsion − Light-Stimulated Movement of Microdroplets. Adv. Mater. 2014, 26, 7339−7345. (17) Gutierrez, J. M. P.; Hinkley, T.; Taylor, J. W.; Yanev, K.; Cronin, L. Evolution of Oil Droplets in a Chemorobotic Platform. Nat. Commun. 2014, 5, 5571. (18) Hanczyc, M. M. Droplets: Unconventional Protocell Model with Life-like Dynamics and Room to Grow. Life 2014, 4, 1038−1049. (19) Sheng, L.; Kurihara, K. Chem. Commun. 2016, 52, 7786. (20) Hanczyc, M. M.; Toyota, T.; Ikegami, T.; Packard, N.; Sugawara, T. Fatty Acid Chemistry at the Oil-Water Interface: SelfPropelled Oil Droplets. J. Am. Chem. Soc. 2007, 129, 9386−9391. (21) Banno, T.; Kuroha, R.; Toyota, T. pH-Sensitive Self-Propelled Motion of Oil Droplets in the Presence of Cationic Surfactants Containing Hydrolyzable Ester Linkages. Langmuir 2012, 28, 1190− 1195. (22) Banno, T.; Kuroha, R.; Miura, S.; Toyota, T. Multiple-Division of Self-Propelled Oil Droplets through Acetal Formation. Soft Matter 2015, 11, 1459−1463. (23) Banno, T.; Toyota, T. Molecular System for the Division of SelfPropelled Oil Droplets by Component Feeding. Langmuir 2015, 31, 6943−6947. (24) Yeap, G-.Y.; Mohammad, A-.T.; Osman, H. Synthesis and Anisotropic Properties of Novel Asymmetric Diones Fused with 1, 3Oxazepine and Oxazepane Rings. Mol. Cryst. Liq. Cryst. 2012, 552, 177−193. (25) Sumino, Y.; Kitahata, H.; Seto, H.; Nakata, S.; Yoshikawa, K. Spontaneous Deformation of an Oil Droplet Induced by the Cooperative Transport of Cationic and Anionic Surfactants through the Interface. J. Phys. Chem. B 2009, 113, 15709−15714.

recently drawn attention as ligation products of prebiotic molecules at the beginning of life.32 Therefore, we suggest that the current design for the self-propelled oil droplet motion can be linked to mobile droplet-based protocells and vesicle-based protocells whose formation is induced by low pH.

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CONCLUSIONS This paper reported a novel time-evolving emulsion system involving three stepstransformation of flocculated particles to spherical droplets, locomotion of the droplet, and transformation of the droplet to giant vesiclesunder low-pH conditions using the oil HBDA containing an imine bond. This phenomenon was derived by the hydrolysis of HBDA and the protonation of DA, which was revealed by NMR spectroscopy, pH curves, and surface tension measurements. This is the first example of the activation of an oil-in-water emulsion affording the sequential dynamics of self-propelled oil droplets and the subsequent formation of giant vesicles at low pH in a far-fromequilibrium system. These results are relevant to nonenzymatic metabolic systems directed toward the linking the dropletbased protocell and vesicle-based models.18,19,33

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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.6b02449. 1 H NMR spectra of compounds and optical microscopic observation of dynamics in the emulsion (Figure S1−S7 and Table S1) (PDF) Dynamics of the HBDA emulsion (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*(T.B.) E-mail [email protected], Tel +81-45-566-1553, Fax +81-45-566-1560. *(T.T.) E-mail [email protected], Tel & Fax +813-5465-7634. Author Contributions

T.B. and Y.T. contributed equally to this work. T.B. and T.T. designed the experiments. Y.T. and T.B. performed the experiments. Y.T., T.B., and K.A. analyzed and interpreted the data. Y.T., T.B., K.A., and T.T. discussed the results. T.B. and T.T. wrote the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Prof. Hiroyuki Kitahata and Ms. Yuki Koyano (Chiba University) are acknowledged for the trajectory analysis of the self-propelled oil droplets and fruitful discussions. This research was supported by a Grant-in-Aid for Scientific Research (No. 25790033 for T. Banno) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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