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Dec 8, 2011 - Self-propelled oil droplets in a nonequilibrium system have drawn much attention as both a primitive type of inanimate chemical machiner...
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pH-Sensitive Self-Propelled Motion of Oil Droplets in the Presence of Cationic Surfactants Containing Hydrolyzable Ester Linkages Taisuke Banno,† Rie Kuroha,† 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 ‡ 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 oil droplets in a nonequilibrium system have drawn much attention as both a primitive type of inanimate chemical machinery and a dynamic model of the origin of life. Here, to create the pH-sensitive self-propelled motion of oil droplets, we synthesized cationic surfactants containing hydrolyzable ester linkages. We found that n-heptyloxybenzaldehyde oil droplets were selfpropelled in the presence of ester-containing cationic surfactant. In basic solution prepared with sodium hydroxide, oil droplets moved as molecular aggregates formed on their surface. Moreover, the selfpropelled motion in the presence of the hydrolyzable cationic surfactant lasted longer than that in the presence of nonhydrolyzable cationic surfactant. This is probably due to the production of a fatty acid by the hydrolysis of the ester-containing cationic surfactant and the subsequent neutralization of the fatty acid with sodium hydroxide. A complex surfactant was formed in the aqueous solution because of the cation and anion combination. Because such complex formation can induce both a decrease in the interfacial tension of the oil droplet and self-assembly with n-heptyloxybenzaldehyde and lauric acid in the aqueous dispersion, the prolonged movement of the oil droplet may be explained by the increase in heterogeneity of the interfacial tension of the oil droplet triggered by the hydrolysis of the ester-containing surfactant.

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INTRODUCTION The autonomous transformation or actuation of molecular assemblies or polymer networks has drawn much attention as a primitive type of inanimate chemical machinery, that is, the transduction of chemical energy to mechanical energy.1,2 The phase transition of some artificial gels affords mechanical stress to trigger shape transformation and actuation that is associated with chemical reactions.3,4 Micrometer-sized vesicles exhibit autonomous deformation and even self-propelled motion.5,6 In addition, multilamellar lipid tubes exhibit unique morphological transformations, such as winding, branching, elongation, and shrinking.7−9 Moreover, several in vitro models of cellular movement rely upon the assembly and dynamics of encapsulated protein matrixes.10,11 Among examples of autonomous locomotion and molecular self-assembly, self-propelled oil droplets demonstrate exotic dynamics in a nonequilibrium system.12−23 When the driving forces of self-propelled droplets are examined in a surfactant solution, the droplets can propel themselves in the following stages. Stage 1: When the surfactant molecules adsorb on the water−oil surface heterogeneously, they move from the site of low interfacial energy to that of high interfacial energy (Marangoni flow) on the droplet surface. In line with the movement of the surfactant molecules, the molecules in the oil droplet move. Stage 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. Stage 3: As a result, convection accrues in the droplet and it moves commensurate © 2011 American Chemical Society

with the convective flow in the oil droplet. Such motion involves the flow of water surrounding the oil droplet.18,24−27 The more surfactant the moving droplet takes in, the more the droplet continues to move until an equilibrated state is reached. When the flow of molecules and the self-propelled motion of the droplets are coupled with the chemical reaction of its components of the droplet and the surfactant, the heterogeneity of the droplet surface is sustained due to the accumulation and release of the products and, as a result, the droplets swim in the aqueous solution without the assistance of an air−water or solid−liquid interface.28−31 The self-propelled motion is sustained until the precursor in a droplet is consumed. For example, Hanczyc et al. showed that an oleate anhydride droplet containing nitrobenzene swim in an aqueous solution from low to high pH.30 Also, Grzybowski and coworkers reported that a floating dichloromethane droplet containing 2-hexyldecanoic acid solves a maze by selfmovement from high to low pH.32 However, as far as we know, there is no report of a pH-sensitive oil droplet swimming without the chemical degradation of the oil droplet in an aqueous solution. Here, we synthesized cationic surfactants containing ester linkages that are hydrolyzable depending on the pH. Figure 1 shows our novel approach for self-propelled oil droplets that Received: November 17, 2011 Revised: December 5, 2011 Published: December 8, 2011 1190

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Figure 1. Approach for sustaining oil droplet motion caused by the hydrolysis of ester-containing cationic surfactant. (a) The movement of the surfactant molecules at the water−oil interface; (b) the internal convective; and (c) the directional motion of the oil droplet. (16H, m), 0.88 (3H, t, J = 6.5 Hz). MS-ESI in MeOH (m/z): 272 [M + H+]; calcd, 272.44 [M + H+]. The quaternarization of lauroyloxyethylene-N,N-dimethylamine (816 mg: 3.0 mmol) was carried out by methyl bromide (343 mg: 3.6 mmol) in dry tetrahydrofuran (3.0 mL) at room temperature for 30 min with stirring. After the reaction, the solvent and unreacted methyl bromide were removed by evaporation under reduced pressure to obtain the crude product. The product was purified by reprecipitation from chloroform (2.0 mL) and ethyl acetate (5.0 mL) to obtain LOETAB, at an 89% yield (978 mg), as a white crystal. 1 H NMR (270 MHz, CDCl3): δ 4.62−4.50 (2H, m), 4.20−4.08 (2H, m), 3.58 (9H, s), 2.36 (2H, t, J = 7.8 Hz), 1.68−1.52 (2H, m), 1.42−1.18 (16H, m), 0.87 (3H, t, J = 7.5 Hz). MS-ESI in MeOH (m/z): 286 [M+ − Br]; calcd, 286.47 [M+ − Br]. Synthesis of 2-Hydroxyethylene-N,N,N-trimethylammonium Bromide (CB). To evaluate the effect of the primary degradation products of LOETAB on the dynamics of the oil droplets, choline-type alcohol (CB) was chemically prepared. CB was prepared by the quaternarization of N,N-dimethylaminoethanol (267 mg: 3.0 mmol) with methyl bromide (342 mg: 3.6 mmol) in tetrahydrofuran (3.0 mL) under nitrogen at room temperature for 30 min with stirring. The product was purified by reprecipitation from methanol (1.0 mL) and ethyl acetate (3.0 mL) to obtain CB, at a 76% yield (418 mg), as white crystals. 1 H NMR (270 MHz, DMSO-d6): δ 5.33−5.23 (1H, m), 4.89−3.77 (2H, m), 3.42 (2H, t, J = 5.1 Hz) 3.13 (9H, s). MS-ESI in MeOH (m/ z): 104 [M+ − Br]; calcd, 104.17 [M+ − Br]. Time Course Measurement of the Hydrolysis of LOETAB. LOETAB was dissolved in water (200 μL) to a final concentration of 100 mM in the presence or absence of 10 μL n-heptyloxybenzaldehyde (HBA) at room temperature and was lyophilized and dissolved in DMSO-d6. Hydrolytic degradation of LOETAB was analyzed by means of the 1H NMR spectra of the DMSO-d6 solution of LOETAB and its hydrolytic product, and the remaining ester % was calculated from the integration value of the peak of the methylene protons adjacent to the ester linkage at δ 4.51−4.37 ppm by using the methyl protons at δ 0.88 ppm as an internal standard peak. Optical Microscopic Observation of Oil Droplet Dynamics. The observation specimen was prepared as follows. An emulsion of HBA was formed by agitating 200 μL of the surfactant solution (100 mM) with 10 μL of HBA. Immediately after mixing HBA 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), we carried out real-time observations of the dynamics of the micrometer-sized oil droplets at room temperature under a phase contrast microscope (IX71, Olympus, Japan) equipped with a CCD camera (U-LH100, Olympus, Japan).

are pH-sensitive in aqueous solution. When the estercontaining cationic surfactants are partially hydrolyzed to produce the corresponding fatty acids and choline-type alcohols, the fatty acids are successively converted to carboxylate-type anionic surfactants by ionization or neutralization. Then, a cation−anion complex surfactant can form from a cation of an ester-containing surfactant and an anion of carboxylate-type surfactant. This complex is called a catanionic surfactant and exhibits superior properties compared to those of individual surfactants, such as a lower critical micelle concentration, a lower surface tension, a higher adsorption efficiency etc.33−37 Sumino et al. reported that the spontaneous deformation and self-movement of a tetradecanoic droplet dissolving palmitic acid in a cationic surfactant solution and the formation of a gel-like intermediate phase on the edge of the oil droplet, are caused by the production of catanionic surfactant.38−40 We, therefore, hypothesized that a similar increase in the heterogeneity of our system would sustain the movement of the oil droplet from the viewpoint of our proposed mechanism of self-propelled motion. Moreover, the hydrolysis rate of ester linkages can be regulated by changing the pH. Therefore, we expect that the current system will afford the pH-sensitive self-propelled movement of oil droplets in an aqueous solution of ester-containing cationic surfactant.

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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), respectively. They were used without further purification. Water was distilled and deionized before use by using a Milli-Q system from Millipore (Massachusetts, U.S.). Synthesis of Lauroyloxyethylene-N,N,N-trimethylammonium Bromide (LOETAB). Ester-containing cationic surfactant (LOETAB) was synthesized according to the procedure of TehraniBagha et al.41 N,N-Dimethylaminoethanol (1.78 g: 20 mmol) in dichloromethane (5 mL) was added dropwise to a dichloromethane solution (4 mL) of lauroyl chloride (2.19 g: 10 mmol). The reaction mixture was stirred under nitrogen at room temperature for 4 h. The mixture was then concentrated under reduced pressure, and the residue was dissolved in ethyl acetate (30 mL). The organic layer was washed three times with a 5% sodium hydrogen carbonate solution (15 mL), and then dried over anhydrous magnesium sulfate. The solvent was evaporated under reduced pressure to obtain lauroyloxyethylene-N,N-dimethylamine, at a 98% yield (2.66 g), as a pale yellow syrup. 1 H NMR (270 MHz, CDCl3): δ 4.17 (2H, t, J = 5.7 Hz), 2.56 (2H, t, J = 5.8 Hz), 2.45−2.20 (8H, m), 1.68−1.52 (2H, m), 1.40−1.15 1191

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RESULTS AND DISCUSSION Oil Droplet Movement under Different pH Conditions. First, we explored the movement of the oil droplets in the cationic surfactant solution and found that the HBA droplet exhibited reproducible, self-propelled motion in a solution (100 mM) of n-dodecyl-N,N,N-trimethylammonium bromide (DTAB) for approximately two minutes until the HBA dissolved away.42 The speed of the motion was greater than 5 μm s−1, and the maximum speed was 90 μm s−1. In previous studies of self-propelled oil droplets, droplets move autonomously with speeds greater than 5 μm s−1, which is significantly faster than the random walk pace of particles of the same size.29−31 Therefore, we defined the self-propelled motion of these oil droplets as locomotion with a speed above 5 μm s−1. We then evaluated the effect of pH on the self-propelled motion of HBA in the presence of cationic surfactant. We confirmed the self-propelled motion of the HBA oil droplet in the DTAB solution over a pH range of 2−12, prepared by the addition of HCl or NaOH, under the microscope. The length of time for all of the droplets to stop their motion in the sample chamber ranged from 2−6 min (Table 1) and we called this time the lasting time for self-propelled motion. Besides DTAB, we also tried adding hydrolyzable surfactant LOETAB to the oil droplets. As shown in Table 1, no significant differences in the lasting time of oil droplets of ca. 100 μm were observed between LOETAB and DTAB at pH 2. In an acidic solution prepared with HCl (pH 2) and containing HBA, 3% of LOETAB was hydrolyzed after 10 min, producing the corresponding lauric acid (LA) and choline-type alcohol (CB). These results indicate that the self-propelled motion of HBA was not influenced by the hydrolysis of LOETAB at pH 2. However, the lasting time of the self-propelled motion in the presence of LOETAB was longer than that of DTAB under basic conditions (pH 10, 11, and 12) induced with NaOH (Table 1). Moreover, oil droplets moved as molecular aggregates formed on their surface at pH 12 in the presence of LOETAB, as shown in Figure 2 (see also the Supporting Information movie). To evaluate the mechanism of this unique self-propelled motion, we observed the oil droplet dynamics in the presence of LNa, which is produced by the hydrolysis of LOETAB and the subsequent neutralization of LA with NaOH.

The oil droplets self-propelled for 2 min without molecular aggregates forming on their surface. On the basis of these results, the unique self-propelled motion in the presence of LOETAB at pH 12 is probably due to the presence of not only LNa, but also LOETAB, LA, and CB. Time Course of Hydrolysis of LOETAB under Basic Conditions. To verify the relationship between the hydrolysis of LOETAB and oil droplet dynamics, we followed the percentage of the remaining LOETAB in the NaOH solution (pH 12) in the presence and absence of HBA. At room temperature, the hydrolysis of LOETAB (7.3 mg) was examined in 0.01 M NaOH solution (200 μL) in the presence and absence of HBA (10 μL). Figure 3 shows the time course for the hydrolytic degradation of LOETAB at room temperature. We found that 18% of LOETAB hydrolyzed in the aqueous dispersion (pH 12), producing the corresponding lauric acid (LA) and choline-type alcohol (CB). The hydrolysis equilibrated in 2 min. When HBA was added to the LOETAB solution 20 s after the preparation, the hydrolysis of LOETAB was suppressed resulting in 84% of LOETAB remaining unhydrolyzed at the equilibrium. We therefore deduce that the hydrolysis of LOETAB takes place in the water phase rather than the oil phase of HBA and that LOETAB is likely absorbed into the oil phase after the addition of HBA. Next, we estimated the concentration of each component in the basic solution of LOETAB containing HBA at the equilibrium. Six min after the preparation of the LOETAB solution with NaOH (10 mM) and HBA (10 μL), the concentration of the remaining ester was 84 mM (Figure 3). Therefore, the concentration of the produced CB and LA was 16 mM for each at this time. Because LA was neutralized with NaOH (10 mM), the concentration of LNa reached 10 mM. Also, by use of the funnel extraction method we confirmed that the LA was distributed only in the HBA phase and that CB was only in the water phase, at the equilibrium. Therefore, it is indicated that the LA (6 mM) was distributed into the oil phase of HBA, whereas the CB was dissolved in the water phase (16 mM). The concentration and corresponding molar fraction of each compound are summarized in Figure 4. To determine whether the HBA oil droplet movement in this system resulted from the hydrolysis of LOETAB, we observed the dynamics of the HBA oil droplet (10 μL) by dissolving LA (6 mM) in the neutral aqueous solution containing LOETAB (84 mM), CB (16 mM), and LNa (10 mM). In the absence of

Table 1. Self-Propulsion of HBA Droplets of ca. 100 μm under Various pH Conditions at Room Temperature lasting time (min) surfactant

pH 2

pH 10

pH 11

pH 12

LOETAB DTAB

4.6 4.2

6.4 5.7

6.0 3.5

8.2 2.4

Figure 2. Sequential micrographs (time interval = 2 s) of selfpropelled oil droplets of HBA in a solution of LOETAB (100 mM, pH 12). The droplet moved from the bottom to the top of the viewing area as indicated by the black arrow. White arrows indicate molecular aggregates on the surface of the oil droplet. Oil droplets marked by asterisks had already stopped at the observation time. Bar: 50 μm.

Figure 3. Time course of the hydrolytic degradation of LOETAB at room temperature, as measured by 1H NMR, in the presence of HBA (open circles); in the absence of HBA (filled circles). The solid lines are to aid visualization. 1192

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Figure 4. Hydrolysis and distribution equilibrium of LOETAB in 0.01 M NaOH solution at 6 min from the preparation of the LOETAB solution.

NaOH, the lasting time of oil droplets was 15 min and selfpropelled oil droplets produced molecular aggregates on their posterior surface. The lasting time of the self-propelled motion of the oil droplets was 3 min in the absence of molecular aggregate formation in 100 mM LOETAB solution in the absence of NaOH. These results suggest that the sustained selfpropelled motion of the oil droplet when molecular aggregate formation is due to the increase in the heterogeneity of the system triggered by the hydrolysis of LOETAB with NaOH. We also evaluated the percentage of LOETAB that remained in the basic solutions at pH 10 and 11 in the presence and absence of HBA. At room temperature, no significant hydrolytic degradation of LOETAB was observed at pH 10 for 10 min. Also, no significant differences in lasting time were observed between LOETAB and DTAB at pH 10 (Table 1). However, LOETAB was hydrolyzed 5% at pH 11 for 10 min, indicating the presence of 1 mM LNa in the solution, and the lasting time of the HBA droplet in the presence of LOETAB was longer than that of DTAB at pH 11 (Table 1). These results demonstrate that the larger production of LNa caused by hydrolysis of LOETAB in the NaOH solution tends to yield longer lasting times of oil droplets. Mechanism of Sustained Self-Propelled Motion of Oil Droplets. Because the current system of the self-propelled oil droplet is driven by the multiple components afforded by the hydrolysis of LOETAB, it is important to evaluate the effect of each compound produced by the chemical reaction on the dynamics of oil droplets of 10−150 μm. We examined eight different experimental conditions for observing self-propelled oil droplets (summarized in Table 2). Even though the dispersion was prepared without NaOH in all conditions, the lasting time of self-propelled motion was extended beyond 10 min only in the presence of both LOETAB and LNa (conditions 1, 4, 6, 7 in Table 2). We focused on the role of the combination of LOETAB and LNa as a complex surfactant (LOETA+-L−). This cation−anion complex surfactant has a

Table 2. Lasting Time of Self-Propelled Oil Droplets of HBA in Milli-Q Water Containing Each Compound

higher surface activity compared to individual surfactants33−37 and forms a relatively large self-assembly such as a vesicle.43−48 We then examined the surface activity and self-assembly of LOETA+-L−, which was prepared by mixing LOETAB and LNa. First, we measured the surface tensions of the solution (100 mM) of LOETA+-L− and LOETAB by using the Wilhelmy vertical plate method. Figure 5 shows the surface tension of the surfactant solution when the molar ratio of LOETAB/LNa was changed from 10/0 to 7/3. The larger molar ratio of LNa tended to show a lower surface tension at room temperature. This result implies that the complex formation can induce a decrease in the interfacial tension of HBA. Second, we measured the average diameters of the molecular selfassemblies of LOETAB and LOETA+-L− (100 mM) by using dynamic light scattering (DLS). Figure 6 shows the particle size distribution of the LOETA+-L− and LOETAB solutions. The larger molar ratio of LNa tended to show a larger particle size. This result indicates that this complex surfactant can form larger molecular aggregates with HBA and LA compared to LOETAB. 1193

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Figure 5. Relationship between the molar ratio of LNa and the surface tension at the air−water interface as measured by use of the Wilhelmy vertical plate method. The solid line is to aid visualization. Figure 7. Proposed mechanism for the sustained self-propelled motion of HBA droplets involving the formation of molecular aggregates on the surface of the oil droplets in a basic aqueous solution of LOETAB.

the droplet moves. The continuous processing of substrates further feeds the interfacial imbalance as described above and activates the convection inside the self-propelled droplet, likely due to Marangoni instability. This molecular transfer, from the adsorption of LOETA+-L− to the accumulation and release of molecular aggregates, sustains the self-propelled motion of HBA. During the preservation of this asymmetry, the original oil droplet moves unidirectionally.

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CONCLUSIONS We have demonstrated a sustainment of a self-propelled oil droplet system by the hydrolysis of LOETAB under high pH. This system is characterized by the following factors: (i) the hydrolysis of ester-containing cationic surfactant under basic conditions that include sodium hydroxide, (ii) the formation of a cation−anion complex surfactant, and (iii) the chemical Marangoni effect. The cation−anion complex surfactant also forms a molecular self-assembly on the surface of the oil droplet. This complex formation causes a decrease in the interfacial tension of the oil droplet and the formation of a molecular aggregate in the aqueous dispersion that sustains the oil droplet movement. These findings may be useful to construct inanimate chemotaxis machinery that is pH-sensitive.

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Figure 6. Particle size distribution of the LOETA -L solution (100 mM) as measured by use of DLS. LOETAB/LNa = 10/0 (open circles), 9/1 (filled circles), 7/3 (open squares).

On the basis of these results, we interpret that, at pH 12, the unique self-movement of the oil droplet in the presence of the ester-containing LOETAB consists of three stages. Stage 1: In a solution containing LOETAB, the hydrolysis of LOETAB occurs at the basic water phase. Then, the LA produced by the hydrolysis of LOETAB is rapidly converted to LNa by NaOH. Stage 2: The complex surfactant (LOETA+-L−) is formed because of the presence of the cationic LOETAB and the anionic LNa, as shown in Figure 7A. Stage 3: Because LOETA+-L− shows higher surface activity than LOETAB or LNa, the symmetry-breakage of the oil droplet is likely sustained. This causes a large imbalance in the interfacial tension among the LOETA+-L−-adsorbing, LOETAB-adsorbing, and bare sites on the oil surface (Figure 7A). The flow of the molecule due to this imbalance in interfacial tension is maintained by Marangoni instability.12−27 Namely, the interfacial energy of the leading edge of the oil droplet where the LOETA+-L− complex adsorbs on the oil surface becomes lower than that of the trailing edge, which induces an interfacial dynamic fluctuation. In addition, the formation of molecular aggregates comprising surfactant, oil, and water occurs on the interface of the oil droplet. These fluctuations, coupled with the accumulation and directional release of the molecular aggregates from the interface, drive the motion of the oil droplet, as shown in Figure 7B. If the flow resulted in the equilibration of the interface tension around the droplet, then the movement would stop. However, the leading edge of the self-propelled droplets takes up additional LOETA+ and L− as

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ASSOCIATED CONTENT

S Supporting Information *

One video of self-propelled motion of HBA oil droplets in the presence of 100 mM LOETAB under the basic condition (pH 12). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.:+81-3-5465-7634; fax:+81-3-5465-7634; e-mail: [email protected].

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ACKNOWLEDGMENTS We thank Prof. Tadashi Sugawara for fruitful discussions and Prof. Masanori Fujinami for assistance with surface tension measurements by Wilhelmy vertical plate method. 1194

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dx.doi.org/10.1021/la2045338 | Langmuir 2012, 28, 1190−1195