pH-Induced Motion Control of Self-Propelled Oil Droplets Using a

Jun 16, 2014 - Shoji Takeuchi,. ‡ and Taro Toyota*. ,†,∥. †. Department of Basic Science, Graduate School of Arts and Sciences, The University...
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pH-Induced Motion Control of Self-Propelled Oil Droplets Using a Hydrolyzable Gemini Cationic Surfactant Shingo Miura,† Taisuke Banno,† Taishi Tonooka,‡ Toshihisa Osaki,‡,§ Shoji Takeuchi,‡ and Taro Toyota*,†,∥ †

Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan ‡ Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan § Kanagawa Academy of Science and Technology, 3-2-1 Sakado, Takatsu-ku, Kanagawa 213-0012, 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: Self-propelled motion of micrometer-sized substances has drawn much attention as an autonomous transportation system. One candidate vehicle is a chemically driven micrometer-sized oil droplet. However, to the best of our knowledge, there has been no report of a chemical reaction system controlling the three-dimensional motion of oil droplets underwater. In this study, we developed a molecular system that controlled the self-propelled motion of 4-heptyloxybenzaldehyde oil droplets by using novel gemini cationic surfactants containing carbonate linkages (2G12C). We found that, in emulsions containing sodium hydroxide, the motion time of the self-propelled oil droplets was longer in the presence of 2G12C than in the presence of gemini cationic surfactants without carbonate linkages. Moreover, in 2G12C solution, oil droplets at rest underwent unidirectional, self-propelled motion in a gradient field toward a higher concentration of sodium hydroxide. Even though they stopped within several seconds, they restarted in the same direction. 2G12C was gradually hydrolyzed under basic conditions to produce a pair of the corresponding monomeric surfactants, which exhibit different interfacial properties from 2G12C. The prolonged and restart motion of the oil droplets were explained by the increase in the heterogeneity of the interfacial tension of the oil droplets.

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INTRODUCTION The development of molecular self-assemblies or polymer networks capable of autonomous locomotion or actuation is of great interest for inanimate chemical machinery that transduces chemical energy to mechanical energy.1−7 Among these, millimeter-sized oil droplets that move autonomously at the interface between air and liquid or liquid and solid because of the Marangoni effect, induced by the heterogeneity of the interfacial tension, have attracted much attention. For example, chemical reactions with a surfactant caused oil droplets to move autonomously on a glass substrate.8 An oil droplet floating at the air−water interface exhibited complex shape transformations,9,10 such as blebbing,11 self-division,12 and fusion.13 Motion control of such oil droplets at the air−water interface was examined by varying the pH or irradiating with light; Grzybowski et al. reported a maze-solving oil droplet in response to a pH gradient in a channel.14 Baigl et al. reported the controlled manipulation of oil droplets at the air−water interface using illumination and a photosensitive surfactant.15 These studies imply the possibility of utilizing oil droplets as transportation devices in soft machinery. Meanwhile, we have reported micrometer-sized oil droplets in a surfactant solution that move autonomously in the aqueous © 2014 American Chemical Society

phase without the assistance of a substrate or an air interface.16−19 The mechanism of the self-propelled oil droplets is as follows. (1) When the surfactant molecules heterogeneously adsorb onto the interface around the oil droplet, they move from the site of low interfacial energy to that of high interfacial energy (Marangoni flow) on the droplet surface. (2) A density gradient of molecules appears in the oil droplet, and the molecules in the droplet move from the high density site to the low density site. (3) Consequently, a convection flow occurs in the droplet and makes the droplet move, which in turn causes surfactant molecules to adsorb onto the foreside of the droplet. (4) Surfactants gradually adsorb onto the oil−water interface homogeneously, and the interfacial tension becomes uniform. The convective flow then becomes weak, and the oil droplet eventually comes to rest. We have previously reported that n-heptyloxybenzaldehyde (HBA) droplets exhibit selfpropelled motion in cationic surfactant solutions.18,19 However, precise control of the self-propelled oil droplets moving underwater has not yet been achieved using a relatively simple Received: May 12, 2014 Revised: June 16, 2014 Published: June 16, 2014 7977

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system, such as a pH gradient field. The ability of the selfpropelled oil droplet to swim underwater affords an additional degree of freedom in its motion in comparison with the motion of oil droplets assisted by surfaces; namely, it can move not only in a horizontal two-dimensional manner, but also up and down in an aqueous solution. The control of such motion of self-propelled oil droplet is thus more challenging than the twodimensional self-propelled motion. Here, we demonstrate a molecular system that enables us to prolong and direct the motion of micrometer-sized oil droplets moving underwater using a pH gradient field, which induces a chemical conversion of the surfactant molecules. We designed and synthesized a novel gemini cationic surfactant containing a carbonate linkage between two hydrophilic moieties (2G12C) (Scheme 1). Gemini surfactants that have two hydrophilic and

Rf = 0.3−0.4) to obtain a mixture of di(2-bromoethyl) carbonate and 2-bromoethyl phenyl carbonate (0.58 g) as a light brown syrup. The molar ratio of di(2-bromoethyl) carbonate and 2-bromoethyl phenyl carbonate was 95/5 (molar ratio), as calculated from the 1H NMR spectrum of the mixture (CDCl3). The simultaneous gemini formation and quaternization were carried out using N,N-dimethyldodecylamine (0.77 g, 3.6 mmol) and a mixture of di(2-bromoethyl) carbonate and 2-bromoethyl phenyl carbonate (0.46 g) in dry acetonitrile (6.0 mL) at 80 °C for 3 days with stirring. After the reaction, the solvent was evaporated under reduced pressure. Purification was carried out by reprecipitation from chloroform (1.0 mL) and ethyl acetate (5.0 mL) to obtain 2G12C (45% yield over two steps) as a white crystal. 1 H NMR (270 MHz, CDCl3): δ 5.00−4.97 (4H, m), 4.35−4.32 (4H, m), 3.90−3.81 (4H, m), 3.55−3.42 (12H, s), 1.85−1.65 (2H, m), 1.48−1.17 (36H, m), 0.88 (6H, t, J = 6.8 Hz). MS-ESI in MeOH (m/ z): 271.50 [M − 2Br]2+; calcd, 271.27 [M − 2Br]2+. Synthesis of Di(ethyl)carbonate-bis(N,N-dimethylhexadecyl Ammonium Bromide) (2G16C). To study the effect of the alkyl chain length of the hydrophobic moiety of the gemini cationic surfactant on the oil droplet dynamics, di(ethyl)carbonate-bis(N,Ndimethylhexadecyl ammonium bromide (2G16C) was synthesized. The simultaneous gemini formation and quaternization were carried out using N,N-dimethylhexadecylamine (0.38 g, 1.4 mmol) and a mixture of di(2-bromoethyl) carbonate and 2-bromoethyl phenyl carbonate (0.18 g) in dry acetonitrile (6.0 mL) at 80 °C for 3 days with stirring. After the reaction, the solvent was evaporated under reduced pressure. Purification was carried out by reprecipitation from chloroform (1.0 mL) and ethyl acetate (2.5 mL) to obtain 2G16C (45% yield over two steps) as a white crystal. 1 H NMR (270 MHz, CDCl3): δ 5.00−4.97 (4H, m), 4.35−4.32 (4H, m), 3.90−3.81 (4H, m), 3.55−3.42 (12H, s), 1.85−1.65 (2H, m), 1.48−1.17 (44H, m), 0.88 (6H, t, J = 6.8 Hz). MS-ESI in MeOH (m/ z): 327.50 [M − 2Br]2+; calcd, 327.33 [M − 2Br]2+. Synthesis of Di(6-hexyl)carbonate-bis(N,N-dimethyldodecyl Ammonium Bromide) (6G12C). To study the effect of the alkyl chain length of the linker moiety of the gemini cationic surfactant on the oil droplet dynamics, di(6-hexyl)carbonate-bis(N,N-dimethyldodecyl ammonium bromide) (6G12C) was synthesized by a two-step reaction. Di(6-bromohexyl) carbonate was synthesized according to the method of Röder et al.24 A mixture of 6-bromohexanol (1.7 g, 9.3 mmol), diphenylcarbonate (0.90 g, 4.2 mmol), and p-toluene sulfonic acid (0.050 g, 0.29 mmol) was stirred in toluene (5 mL) under a nitrogen atmosphere at 100 °C for 18 h. The reaction mixture was dissolved in toluene (15 mL). The organic layer was washed using a saturated NaHCO3 solution (10 mL) and brine (10 mL) and then dried over anhydrous MgSO4. The solvent was evaporated under reduced pressure. Purification was carried out by silica gel column chromatography (n-hexane/ethyl acetate = 5/1 (v/v), Rf = 0.3−0.4) to obtain a mixture of di(6-bromohexyl) carbonate and 6-bromohexyl phenyl carbonate (1.3 g) as a light brown syrup. The molar ratio of di(6-bromohexyl) carbonate and 6-bromohexyl phenyl carbonate was 53/47 (molar ratio), as calculated from the 1H NMR spectrum of the mixture (CDCl3). The simultaneous gemini formation and quaternization were carried out using N,N-dimethyldodecylamine (0.36 g, 1.7 mmol) and a mixture of di(6-bromohexyl) carbonate and 6-bromohexyl phenyl carbonate (0.23 g) in dry acetonitrile (6.0 mL) at 80 °C for 3 days with stirring. After the reaction, the solvent was evaporated under reduced pressure. Purification was carried out by gel permeation chromatography using chloroform (retention time: 50 min) to obtain 6G12C (total yield 5% over two steps) as a light brown syrup. 1 H NMR (270 MHz, CDCl3): δ 4.17−4.12 (4H, t, J = 6.2 Hz), 3.65−3.62 (4H, m), 3.51−3.45 (4H, m), 3.41−3.36 (12H, s), 1.95− 1.65 (16H, m), 1.53−1.49 (4H, m), 1.41−1.15 (36H, m), 0.91−0.85 (6H, t, J = 6.8 Hz). MS-ESI in MeOH (m/z): 327.55 [M − 2Br]2+; calcd, 327.33 [M − 2Br]2+. Synthesis of Hydroxyethyl-N,N-dimethyldodecyl Ammonium Bromide (2H12). To investigate the oil droplet dynamics of the hydrolyzed product of 2G12C, hydroxyethyl-N,N-dimethyldodecyl

Scheme 1. Synthesis of Gemini Surfactant 2G12C and Its Hydrolysis Product 2H12

hydrophobic groups show stronger surface activity than conventional monomeric surfactants that have a single hydrophobic chain.20−22 Because 2G12C has a carbonate linkage, it generates two equivalent 2H12 (hydroxyethyl-N,Ndimethyldodecyl ammonium bromide), which has a different surface activity than 2G12C, upon the irreversible basic hydrolysis of the carbonate linkage.23 The production and adsorption of 2H12 results in the heterogeneity of the surface of the oil droplets. Therefore, the motion of the oil droplets is expected to be prolonged by the conversion of the surfactant and the subsequent change in the oil−water interfacial tension. Moreover, the oil droplet can be moved unidirectionally by changing the interfacial tension at one side of the droplet surface. The pH-induced motion control of oil droplets has been achieved with the consumption of oil or surfactant component.14,17,18 We therefore expect that the conversion system from a gemini surfactant to two monomeric surfactants will be able to precisely control the three-dimensional motion of oil droplets.

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MATERIALS AND METHODS

Reagents. Commercially available reagents and solvents were purchased from Tokyo Chemical Industry Co. (Tokyo, Japan), Wako Chemical Co. (Osaka, Japan), and Kanto Chemical Co. (Tokyo, Japan). They were used without further purification. Water was distilled and deionized before use by a Milli-Q system from Millipore (Billerica, MA). Synthesis of Di(ethyl)carbonate-bis(N,N-dimethyldodecyl Ammonium Bromide) (2G12C). The gemini surfactant di(ethyl)carbonate-bis(N,N-dimethyldodecyl ammonium bromide) (2G12C) was synthesized in two steps as shown in Scheme 1. A mixture of 2bromoethanol (1.4 g, 11.4 mmol), diphenylcarbonate (0.64 g, 3.0 mmol), and K2CO3 (1.3 g, 9.3 mmol) was stirred in acetone (10 mL) under a nitrogen atmosphere at room temperature for 24 h. After the reaction, the K2CO3 was removed by filtration, and the solvent was evaporated under reduced pressure. The residue was then purified by silica gel column chromatography (n-hexane/ethyl acetate = 1/5 (v/v), 7978

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Kayaku Co., Tokyo, Japan) was spin-coated on a silicon wafer with a thickness of 200 μm, and a rectangular shape for the channels was patterned by UV lithography. The poly(dimethylsiloxane) (PDMS) channel was obtained from the SU-8 mold above, and the inlets were punched with trephines. Finally, the PDMS channel was bonded to a cover glass by an oxygen-plasma treatment (FA-1, Samco Inc., Tokyo, Japan). Optical Microscopic Observation of the Oil Droplet Dynamics in the Microchannel. After the injection of 50 μL of the emulsion composed of 10 mM surfactant solution (200 μL) and HBA (10 μL) into the microchannel from the left inlet, the left inlet was sealed with a cover glass (Supporting Information Figure S1b). Then, 25 μL of the HBA emulsion was removed from the right inlet and 25 μL of NaOH solution was gently poured into the right inlet of the microchannel using a micropipette. We carried out real-time observations of the oil droplet dynamics at room temperature under the microscope. The image treatment was performed using customized routines written in visual C++ (Microsoft Co., Redmond, WA).

ammonium bromide (2H12) was prepared by the quaternization of N,N-dimethyldodecylamine (0.77 g, 3.6 mmol) with 2-bromoethanol (0.33 g, 3.0 mmol) in acetonitrile (6.0 mL) under nitrogen at 80 °C for 1 day with stirring. After the reaction, the solvent was evaporated under reduced pressure. The product was purified by reprecipitation from chloroform (1.0 mL) and ethyl acetate (3.0 mL) to obtain 2H12 in 81% yield as white crystals. 1 H NMR (270 MHz, CDCl3): δ 4.19−4.15 (2H, m), 3.77−3.74 (2H, m), 3.57−3.51 (4H, m), 3.38 (6H, s), 1.80−1.65 (2H, m), 1.48− 1.25 (18H, m), 0.88 (3H, t, J = 6.8 Hz). MS-ESI in MeOH (m/z): 258.45 [M − Br]+; calcd, 258.28 [M − Br]+. Optical Microscopic Observation of the Oil Droplet Dynamics in a Microchamber. An emulsion of 4-heptyloxybenzaldehyde (HBA) was formed by agitating 180 μL of the surfactant solution with 10 μL of HBA at room temperature. After the addition of 20 μL of NaOH solution into the emulsion, the emulsion was agitated again. Then, we encased 70 μL of the emulsion in a thin glass chamber (15 × 15 × 0.28 mm3; Frame Seal Chamber, MJ Research Inc., Waltham, MA) (Supporting Information Figure S1a) and carried out real-time observations of the micrometer-sized oil droplets at room temperature with a phase contrast microscope (IX71, Olympus, Co., Tokyo, Japan) equipped with a CCD camera (DP-72, Olympus, Co., Tokyo, Japan). Time Course Measurements of the Hydrolysis of 2G12C. An emulsion of HBA was formed by agitating 180 μL of 2G12C solution with 10 μL of HBA at room temperature. After the addition of 20 μL of NaOH solution (10, 100, and 1000 mM) to the emulsion to produce final concentrations of NaOH of 1, 10, and 100 mM, respectively, the emulsion was agitated again and left to sit at room temperature; it was then lyophilized and dissolved in CDCl3. The hydrolytic degradation of 2G12C was analyzed by means of the 1H NMR spectra of 2G12C and its hydrolytic product (2H12) in CDCl3, and the rate of the hydrolysis reaction (%) was calculated from the integration value of the peak of the methylene protons adjacent to the hydroxyl group of 2H12 at δ 4.19−4.15 ppm using the methyl protons at δ 0.88 ppm as an internal standard. Equilibrium Surface Tension Measurements. The equilibrium surface tensions of the surfactant solutions were measured using the Wilhelmy vertical plate technique with a sandblasted glass plate and a digital Kyowa Precise Surface Tensiometer CBVP-A3 (Kyowa Kagaku Co. Ltd., Tokyo, Japan) at 25 °C. The test solutions were allowed to stand at 25 °C for at least 1 h prior to any measurements. The surface area that a molecule occupies (Amin) was calculated according to the Gibbs adsorption equation. The surface excess concentration (Γ) in mol m−2 and the corresponding Amin (in nm2) at the liquid−air interface were calculated using eqs 1 and 2,

Γ=

− 1 ⎛ dγ ⎞ ⎜ ⎟ 2.30nRT ⎝ d log C ⎠

A min =

1018 NA Γ

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RESULTS AND DISCUSSION Oil Droplet Motion in the Gemini 2G12C Solution. In recent reports from our group,18,19 micrometer-sized oil droplets of HBA exhibited self-propelled motion in an emulsion system containing cationic surfactants. The selfpropelled movement begins when an uneven formation of the interface of the oil droplet generates a symmetry breakdown of the internal stress state. The driving force is thought to be the Marangoni effect and the transfer of molecules over the surface of the oil droplet. The synthesized cationic surfactant, 2G12C, is a hydrolyzable surfactant that can generate heterogeneous interfacial tension around the oil droplet, resulting in the selfpropelled motion of the oil droplet. To evaluate the motion of the self-propelled oil droplets, we used a thin observation specimen (an observation microchamber) for keeping them moving underwater in a pseudotwo-dimensional manner (see Supporting Information Figure S1a). We observed the dynamics of the oil droplets in an emulsion prepared with HBA (10 μL) and 30 mM 2G12C solution (200 μL). When we changed the concentration of 2G12C, we observed self-propelled motion of the oil droplets at concentrations over 30 mM. It was observed that many oil droplets in the observation microchamber exhibited autonomous motion, as shown in Figure 1. Convection inside all the moving oil droplets was also observed (see Supporting Information Movie S1). Because the speed of the oil droplets was greater than 10 μm/s, which is significantly faster than the random movement of micrometer-sized particles in a pseudotwo-dimensional space, it could be clearly distinguished

(1)

(2)

where n is a constant that depends on the individual ions comprising the surfactant. The values n = 2 and 3 are used for the single and gemini ionic surfactants, respectively.20,22 The term dγ/d log C is the slope of the surface tension versus concentration curve below the critical micelle concentration (CMC) at a constant temperature, γ is the surface tension in mN m−1, T is the absolute temperature, and R and NA are the ideal gas constant and Avogadro’s number, respectively. Dynamic Surface Tension Measurements. The dynamic surface tensions of the solutions were measured using a SITA online t60 (SITA Messtechnik GmbH, Dresden, Germany) bubble pressure tensiometer, employing a technique that involves the measurement of the maximum pressure necessary to blow a bubble in a liquid from the tip of a capillary. The measurements were conducted with effective surface times ranging from 30 ms to 20 s at 25 °C. Fabrication of Microchannel. A microchannel (Supporting Information Figure S1b) was fabricated using a common soft lithography process.25,26 Briefly, an SU-8 negative photoresist (Nippon

Figure 1. Phase contrast micrographs of HBA oil droplets exhibiting self-propelled motion in 30 mM 2G12C solution in the microchamber; red arrows in (a) represent the direction of a set of selfpropelled oil droplets, and those in (b) represent the sequential direction of a self-propelled oil droplet with the dots plotted at 1 s intervals. Bar: 200 μm. 7979

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from Brownian motion in the observation microchamber. Therefore, we considered oil droplets moving at speeds greater than 10 μm/s as self-propelled motion. A larger droplet tended to be faster speed in all tested surfactant solutions, indicating that the speed of self-propelled oil droplets strongly depends on their size. Over the course of the observations, all the selfpropelled oil droplets gradually decreased their speed and eventually stopped. We assigned the motion time of the oil droplets as the time during which the self-propelled motion of any droplets was sustained in the observation microchamber. The effect of hydrolyzable 2G12C on the self-propelled motion of the oil droplets was investigated by monitoring the motion time of the droplet in 10 mM NaOH solution containing the different surfactants listed in Scheme 2. The

Hydrolysis of Gemini Surfactant 2G12C. We traced the hydrolysis reaction of 2G12C in NaOH solution (1−100 mM). Figure 2 shows the progress of the hydrolysis of 2G12C at

Scheme 2. Molecular Structures of Various Surfactants Used in This Study

Figure 2. Hydrolysis of 2G12C (30 mM) in an emulsion containing NaOH at room temperature as a function of time, as measured by 1H NMR: 100 mM NaOH (triangle), 10 mM NaOH (circle), 1 mM NaOH (square).

room temperature at different concentrations of NaOH, as measured by 1H NMR spectroscopy. 2G12C was gradually hydrolyzed in 10 mM NaOH, and the rate of hydrolysis reached 40% after 10 min. On the other hand, no significant hydrolysis was observed in 1 mM NaOH, while hydrolysis to 2H12 was complete within 0.5 min in 100 mM NaOH. This result supports the notion that the partial hydrolysis of 2G12C into 2H12 in the emulsion is important for the sustainment of the self-propelled motion of the oil droplets. We have previously reported that higher ion strength of surfactant solution tended to be a longer duration time of self-propelled oil droplets due to the counteranion-exchange reaction of cationic surfactants.19 However, the concentration of NaOH (10 mM) is lower than that of 2G12C (30 mM), implying that the longer motion time of self-propelled oil droplets is not strongly influenced by the reversible counteranion-exchange reaction. We therefore deduce that the sustainment of selfpropelled motion is mainly caused by the hydrolysis of 2G12C. Surface Activities of 2G12C and 2H12. It is well-known that the surface activity of gemini surfactants is superior to that of their corresponding single surfactants; that is, gemini surfactants exhibit a lower critical micelle concentration (CMC) and surface tension.20−22 This is due to the relatively strong intra- and intermolecular interactions between surfactant molecules. The relationship between the oil droplet dynamics and the interfacial activity of the surfactants was investigated using the equilibrium and dynamic surface tensions of 2G12C and 2H12 solutions at room temperature. Because 2G12C was gradually hydrolyzed in 10 mM NaOH solution, we used 10 mM NaBr solution instead. Figure 3 shows the plots of the surface tension versus concentration of the surfactant solution. From these plots, we calculated the interfacial parameters for each surfactant. Moreover, the variation of free energy in micellization and adsorption was evaluated using eqs 1 and 2, where pC20 is the negative logarithm of C20 and C20 is the surfactant molar concentration required to reduce the surface tension by 20 mN/m.27,28

average motion times and standard deviations are summarized in Supporting Information Table S1. Gemini surfactant mG12 was synthesized according to our previous report.19 In the solution of 2G12C, which contains a carbonate linkage and two methylene chains in its linker moiety, the oil droplets moved for 18, 27, 30, and 15 min, respectively, four times. Thus, the average of motion time was 23 min and its standard deviation was 6 min. On the other hand, the oil droplets exhibited no self-propelled motion in the solution of 6G12C, containing a carbonate linkage and six methylene chains in the linker moiety. Self-propelled motion in 2G16C solution was not feasible, as it was insoluble in water because of the longer alkyl chains of the molecule. The oil droplets were self-propelled for only 5 min in 8G12 solution, whereas they stayed still in 12G12 solution. We also examined monomeric surfactants n-dodecyltrimethylammonium bromide (DTAB) and 2H12, which is the hydrolysis product of 2G12C (Scheme 1). Because gemini surfactants are composed of two hydrophobic and hydrophilic groups, the concentration of monomeric surfactants was set at 60 mM. Even though self-propelled motion was observed in both cases, the motion times were less than 10 min. These results suggest that the motion time of the oil droplets in a basic solution can be controlled by the surfactant molecules. Among the tested surfactants, the longest self-propelled motion was observed in the presence of gemini surfactant 2G12C, which consists of relatively short linker moieties and n-dodecyl chains. To investigate the effect of the NaOH concentration on the dynamics of the oil droplets, we measured the motion time of the oil droplets in 2G12C solution with varying NaOH concentrations from 1 to 100 mM. The longest motion time (23 ± 6 min) was observed in 10 mM NaOH. In the 1 and 100 mM solutions, the average motion time with standard deviation of oil droplets was 6 ± 0.8 min and 7 ± 2 min, respectively. These results suggest that the sustainment of self-propelled motion in the 2G12C solution is due to the partial hydrolysis of 2G12C to produce 2H12. 7980

° = 2.30nRT log CCMC ΔGmic

(1)

° = −2.30nRT pC20 ΔGads

(2)

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2H12 in a 10 mM NaOH solution after 20 min, as shown in Figure 2. We found that, in the range of 0.03−0.1 s, the surface tension of the mixed solution decreased from 41 to 38 mN/m. This tendency is different from that of the 30 mM 2G12C solution (from 48 to 38 mN/m). This is probably due to the rapid adsorption of 2H12 to the interface. On the other hand, in the range of 0.3−20 s, the surface tension of the mixed solution gradually decreased from 37 to 34 mN/m; this tendency is different from that of the 60 mM 2H12 solution (from 38 to 37 mN/m). This is because 2G12C slowly adsorbs to the interface. These results imply that, in the mixed solution, 2H12 adsorbs rapidly, and then 2G12C adsorbs gradually to the oil−water interface. This is also supported by the ΔG°ads values of 2G12C and 2H12 as shown in Table 1. Proposed Mechanism for the Long Motion Time of the Oil Droplets in 2G12C Solution. The self-propelled motion of HBA droplets in cationic surfactant solutions is considered to occur in three stages:18,19 (1) an imbalance in the interfacial tension arises between sites with adsorbed surfactant molecules and bare sites on the droplet surface, (2) a molecular flow arises around the oil droplet because of the heterogeneity of the interfacial tension and is maintained by the Marangoni instability, and (3) the interfacial energy of the droplet’s leading edge decreases below that of its trailing edge, inducing a dynamic interfacial fluctuation. As a result, the oil droplet exhibits self-propelled motion. The more surfactant the moving droplet takes on, the more oil droplet continues to move because of the flux balance between the oil droplet and the bulk. Because the interfacial tension around the oil droplet equilibrates in a solution of nonreactive surfactants, it slows down and becomes immobile within several minutes. However, in 2G12C solution containing 10 mM NaOH, the oil droplets were self-propelled for over 20 min. On the basis of these results, we propose the following mechanism for the sustainment of the self-propelled oil droplets in 2G12C solution in the presence of 10 mM NaOH (Figure 5). Stage 1: 2G12C adsorbs heterogeneously to the oil−water interface, and the oil droplets begin moving as mentioned above (Figure 5a). Stage 2: Because 2G12C is hydrolyzed under basic conditions, 2H12 is gradually produced and rapidly adsorbs to the oil−water interface (Figure 5b). Stage 3: Because the free energy variation of the adsorption of 2G12C is lower than that of 2H12, 2G12C adsorbs not rapidly but gradually, concomitantly with the desorption of 2H12 (Figure 5c). As this induces the heterogeneity of the interfacial tension at the oil−water interface, the self-propelled motion of the oil droplet is sustained until the interfacial tension becomes homogeneous (Figure 5d). Control of the Oil Droplet’s Motion in a Microchannel in 2G12C Solution by the Addition of NaOH Solution. The oil droplet dynamics under a one-dimensional NaOH concentration gradient were examined in a microchannel composed of poly(dimethylsiloxane) (PDMS) and a cover glass substrate (see Supporting Information Figure S1b).25,26 The penetration of the NaOH gradient was confirmed using

Figure 3. Equilibrium surface tension versus concentration of gemini cationic surfactants, 2G12C (circle) and 2H12 (square), in 10 mM NaBr solution at room temperature. The solid lines are used to aid visualization.

n is a constant that depends on the individual ions comprising the surfactant. The values n = 2 and 3 are used for the monomeric and gemini ionic surfactants, respectively.20,22 T is the absolute temperature, and R is the ideal gas constant. These parameters are summarized in Table 1. The CMC of 2G12C was approximately 100 times lower than that of 2H12. The surface tension of 2G12C at the CMC (γCMC) was lower than that of 2H12, and the pC20 value of 2G12C was higher than that of 2H12. As for the free energy variation of adsorption, the absolute value of 2G12C was larger than that of 2H12. Namely, the use of 2G12C is thermodynamically favorable for adsorption at the interface in the current system. It is also important to clarify the kinetics of adsorption of 2G12C and 2H12 to the interface; thus, dynamic surface tension measurements were recorded using the maximum bubble pressure technique. Figure 4a and 4b show the relationship between the surface age and surface tension of 2G12C and 2H12, respectively. The surface age is defined as the time span associated with bubble growth (from the beginning of the bubble formation to the point at which a hemispherical shape is formed), and the surface tension is calculated from the pressure exerted on the bubble at each surface age over the range of 0.03−20 s. At surface ages in the range of 0.03−0.1 s, the surface tension of the 2G12C solution gradually decreased, while that of the 2H12 solution did not change significantly. For example, the surface tension in 10 mM 2G12C solution (44 times higher than the CMC) decreased from 49 to 38 mN/m. On the contrary, the surface tension of 10 mM 2H12 solution (6 times higher than the CMC) decreased from 45 to 40 mN/m. However, the surface tension of 10 mM NaBr solution was 72 mN/m. Because 2H12 rapidly adsorbs at the air−water interface, we consider that a relatively large amount of 2H12 was already adsorbed onto the interface at 0.03 s. However, 2G12C slowly adsorbed to the interface because of its relatively bulky structure in this range. These results clearly indicate that the adsorption of 2H12 occurs more rapidly than that of 2G12C under these conditions. We also measured the dynamic surface tension of a mixed solution of 2G12C (18 mM) and 2H12 (24 mM) (Figure 4c). This molar composition corresponds to the ratio of 2G12C and

Table 1. Surface Activities of 2G12C and 2H12 in 10 mM NaBr Solution at Room Temperature surfactant 2G12C 2H12 a

CMC (mM) −2

2.3 × 10 1.5

γCMC (mN/m) 34 39

Γ (mol/m2)

Amin (×102 nm2)

pC20

ΔG°ads (kJ/mol)

ΔGmic ° (kJ/mol)

1.5 × 102a 46b

5.6 3.2

−95a −36b

−79a −32b

−6a

1.1 × 10 3.5 × 10−6b

The value is calculated using n = 3. bThe value is calculated using n = 2. 7981

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Figure 4. Variations in the dynamic surface tension with surface age for (a) 2G12C at concentrations of 1 mM (diamond), 10 mM (circle), and 30 mM (triangle); (b) 2H12 at concentrations of 10 mM (square), 30 mM (circle), and 60 mM (triangle); (c) 30 mM 2G12C (diamond), 60 mM 2H12 (triangle), and a mixed solution of 18 mM 2G12C and 24 mM 2H12 (circle) at room temperature.

Figure 5. Schematic representation of the proposed mechanism for the sustained self-propelled motion of oil droplets caused by the partial hydrolysis of 2G12C to produce 2H12 under basic conditions (10 mM NaOH). γ represents the interfacial tension of the surface of the oil droplets.

bromothymol blue (BTB) as a pH indicator (Figure 6a). When we dropped 200 mM NaOH solution into the right inlet, the blue area gradually migrated to the left (Figure 6b). The velocity was approximately constant at 3−5 μm/s at 10 min after the addition. We therefore deduce that the hydroxide ions were gradually dispersed from the right side to the left side. First, we observed the motion of oil droplets in the microchannel without NaOH, and confirmed the random movement of droplets with a diameter of 100−200 μm. The absolute value of the velocity of the oil droplets was below 50 μm/s (Supporting Information Figure S2 and Movie S2). Next, to distinguish the direction of motion more clearly, we prepared immobile oil droplets with 10 mM 2G12C solution and initiated their self-propelled motion by adding NaOH solution.

Figure 6. (a) Schematic representation of oil droplet observation using a microchannel. (b) Migration of the higher pH solution containing hydroxide ion from the larger right inlet toward the smaller left inlet of the microchannel containing a surfactant solution with BTB (0.05 wt %). Upper, 0 min after the addition of NaOH solution; lower, 3 min after the addition of NaOH solution. (c) Phase contrast microscopy images of the convective flow inside the oil droplet before and after motion begins. Exposure time, 16 ms; scale bar, 100 μm. (d) Phase contrast microscopy image of the directed motion of the oil droplet in the microchannel. Red arrows correspond to the direction of the selfpropelled oil droplet. Exposure time, 33 ms; scale bar, 200 μm.

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Figure 7. (a-1) Typical trajectory of a self-propelled oil droplet and (a-2) its velocity after the addition of a 200 mM NaOH solution into the microchannel in the presence of 10 mM 2G12C solution at room temperature (Movie S3). The oil droplet initiates and restarts its self-propelled motion toward the area with a higher concentration of NaOH. (b-1) Typical trajectory of a self-propelled oil droplet and (b-2) its velocity after the addition of a 200 mM NaOH solution into the microchannel in the presence of 10 mM 8G12 solution at room temperature (Movie S5). The oil droplet initiates its self-propelled motion toward the area with a higher concentration of NaOH. Red arrows correspond to the direction of the selfpropelled oil droplet. The dots are plotted at 1 s intervals.

After putting the emulsion into the microchannel, 25 μL of 200 mM NaOH solution was dropped into the right inlet at time t = 0. We determined the start point of the droplet motion as that at which the droplet velocity exceeded 10 μm/s (Figure 6c). It was found that the immobile oil droplets on the right side in the microchannel, where the NaOH concentration was higher, began their motion first (Figure 6d). Following them, the oil droplets on the left side started moving. This initiation motion was observed for almost all the oil droplets in the microchannel. We also found that a convection flow occurred inside the immobile oil droplets, and it gradually became stronger before the motion was initiated (Figure 6c). Figure 7a-1 and a-2 shows the typical locomotion of a selfpropelled oil droplet after the addition of a 200 mM NaOH solution in the presence of 10 mM 2G12C at room temperature. The immobile oil droplet suddenly began to move toward the right side, with a speed over 300 μm/s, 650 s after the addition of NaOH solution. The oil droplet stopped moving within 20 s and flowed ∼500 μm toward the left side for 100 s. The speed of the droplet was equal to the penetration of NaOH (3−5 μm/s); therefore, the oil droplet was considered to move in response to the addition of the NaOH solution. However, the oil droplet exhibited motion again to the right side at a speed of 75 μm/s from 760 to 800 s. We termed this unique motion as the restart motion (Supporting Information Movie S3). The restart motion was also observed in the solution of 10 mM 2G12C containing BTB as a pH indicator (Supporting Information Movie S4). We also observed the oil droplet motion in 8G12 (10 mM) solution instead of in 2G12C solution. As shown in Figure 7b-1 and b-2 and in Supporting Information Movie S5, the oil droplets showed movement for 9 s, from 315 to 324 s, and then stopped. Notably, no restart motion was observed. A similar motion was also observed in the solution of 10 mM 8G12 containing 0.05 wt % BTB as a pH indicator (Supporting Information Movie S6). The restart motion was observed for almost all the oil droplets in the solution of 2G12C containing hydrolyzable

linkage; however, this phenomenon was not observed with nonhydrolyzable 8G12. These results rationalize that the restart motion is linked with the hydrolysis of the surfactant. Proposed Mechanism for the Initiation and Restart Motions of Oil Droplets in 2G12C Solution. Based on these results, we can propose a mechanism of the oil droplet motion observed in the microchannel containing gemini cationic surfactant solution. Even though the immobile oil droplets did not demonstrate any initial motion after the addition of dilute NaOH solution (20 mM), motion was initiated by the addition of a more concentrated NaOH solution (200 mM). Thus, ion-exchanged surfactants produced by the reaction between a counteranion (bromide ion) of the surfactants and a hydroxide ion play a role in initiating the selfpropelled motion of the oil droplets in the microchannel. It is known that the surface activity of the surfactants is strongly influenced by the ionic strength of the solution.29,30 When the concentrated NaOH solution was added into the right inlet of the microchannel, hydroxide ions migrated toward the left inlet. Thus, the concentration of NaOH gradually becomes higher at the right side of the oil droplet. Then, the oil−water interfacial tension locally changes because of the ion exchange reaction between the hydroxide ions and the bromide anions of the surfactants.31 Therefore, heterogeneity of the interfacial tension is established around the oil droplet, and this induces its motion. Because the area where the interfacial tension is significantly changed is always on the right side of the oil droplet, and the convection flow within the oil droplet occurs from the point at which the interfacial tension is relatively low, the oil droplet begins to move to the right. The oil droplet moves until the oil−water interface is completely surrounded by the ion-exchanged surfactants. Therefore, the oil droplets exhibit motion in 8G12 solution. On the other hand, the restart motion was uniquely observed in 2G12C solution. 2G12C not only undergoes ion-exchange, but is also hydrolyzed to produce 2H12 (and ion-exchanged 2H12). Both reactions produce surfactants that exhibit different interfacial properties from 7983

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(4) Wang, W.; Duan, W.; Ahmed, S.; Mallouk, T. E.; Sen, A. Small Power: Autonomous Nano- and Micromotors Propelled by SelfGenerated Gradients. Nano Today 2013, 8, 531−554. (5) Kapral, R. Perspective: Nanomotors without Moving Parts that Propel Themselves in Solution. J. Chem. Phys. 2013, 138, 020901. (6) Magdanz, V.; Stoychev, G.; Ionov, L.; Sanchez, S.; Schmidt, O. G. Stimuli-Responsive Microjets with Reconfigurable Shape. Angew. Chem., Int. Ed. 2014, 53, 2673−2677. (7) Zhang, H.; Duan, W.; Liu, L.; Sen, A. Depolymerization-Powered Autonomous Motors Using Biocompatible Fuel. J. Am. Chem. Soc. 2013, 135, 15734−15737. (8) Bain, C. D.; Burnett-Hall, G. D.; Montgomerie, R. R. Rapid Motion of Liquid Drops. Nature 1994, 372, 414−415. (9) 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. (10) Nagai, K.; Sumino, Y.; Kitahata, H.; Yoshikawa, K. Mode Selection in the Spontaneous Motion of an Alcohol Droplet. Phys. Rev. E 2005, 71, 065301. (11) Sumino, Y.; Kitahata, H.; Seto, H.; Yoshikawa, K. Dynamical Blebbing at a Droplet Interface Driven by Instability in Elastic Stress: A Novel Self-Motile System. Soft Matter 2011, 7, 3204−3212. (12) 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. (13) Caschera, F.; Rasmussen, S.; Hanczyc, M. M. An Oil Droplet Division−Fusion Cycle. ChemPlusChem 2013, 78, 52−54. (14) 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. (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) 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. (17) 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. (18) 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. (19) Banno, T.; Miura, S.; Kuroha, R.; Toyota, T. Mode Changes Associated with Oil Droplet Movement in Solutions of Gemini Cationic Surfactants. Langmuir 2013, 29, 7689−7696. (20) Alami, E.; Beinert, G.; Marie, P.; Zana, R. Alkanediyl-α,ωbis(dimethylalkylammonium Bromide) Surfactants. 3. Behavior at the Air-Water Interface. Langmuir 1993, 9, 1465−1467. (21) Menger, F. M.; Keiper, J. S. Gemini Surfactants. Angew. Chem., Int. Ed. Engl. 2000, 39, 1906−1920. (22) Esumi, K.; Taguma, K.; Koide, Y. Aqueous Properties of Multichain Quaternary Cationic Surfactants. Langmuir 1996, 12, 4039−4041. (23) Dittert, L. W.; Higuchi, T. Rates of Hydrolysis of Carbamate and Carbamate Esters in Alkaline Solution. J. Pharm. Sci. 1963, 52, 852−857. (24) Röder, T.; Kramer, T. Esters of ω-Bromo Alcohols and Their Corresponding Quarternary Ammonium Salts. Synth. Commun. 2004, 34, 3645−3651. (25) Mcdonald, J. C.; Whitesides, G. M. Poly(dimethylsiloxane) as a Material for Fabricating Microfluidic Devices. Acc. Chem. Res. 2002, 35, 491−499. (26) Osaki, T.; Yoshizawa, S.; Kawano, R.; Sasaki, H.; Takeuchi, S. Lipid-Coated Microdroplet Array for in Vitro Protein Synthesis. Anal. Chem. 2011, 83, 3186−3191.

2G12C. Therefore, the restart motion of the oil droplet occurs after the initiation motion ceases. As discussed above, we found that the oil droplets are selfpropelled toward the area with a higher NaOH concentration; moreover, the stopped oil droplets exhibited restart motion in the same direction upon the hydrolysis of 2G12C. This implies a new system for controlling the direction of self-propelled oil droplets using two chemical reactions of surfactant caused by the addition of NaOH solution.

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CONCLUSIONS We have demonstrated control over the underwater selfpropelled motion of micrometer-sized oil droplets using chemical reactions. HBA droplets exhibited self-propelled motion for an extended period of time in 2G12C solution under basic conditions compared to the motion in 8G12 solution, which has no hydrolyzable linkage. In a pH gradient in the microchannel, the oil droplets were self-propelled in the direction of higher pH and they restarted their motion in the same direction in the presence of 2G12C. This is the first experimental proof of chemical-based soft machinery that moves underwater and is controlled and redirected by a surfactant conversion system. This system, where a surfactant generates other surfactants having different surface activity from the original in an aqueous phase, is a prototype for the motion control of oil droplets, which could be underwater transporters or microreactors in microchannels.32,33

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ASSOCIATED CONTENT

S Supporting Information *

Schematic representation of microchamber and microchannel, trajectory, time course of the velocity, motion time, and six videos (mpg format) of the self-propelled oil droplets in the microchamber or microchannel. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. TEL/FAX: 81-35465-7634. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Mr. Yuki Kazayama (The University of Tokyo) is acknowledged for the trajectory analysis of the self-propelled oil droplets and fruitful discussion on the construction of the microchannel. T. Tonooka was supported by a Grant-in-Aid for JSPS Fellowship, JSPS, Japan. This research was supported by a Grant-in-Aid for Scientific Research (No. 25790033 for T. Banno, No. 25620105 for T. Toyota) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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REFERENCES

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