Self-Propelled Oil Droplets Consuming Fuel Surfactant

Transcription

1 Supporting Information Self-Propelled Oil Droplets Consuming Fuel Surfactant Taro Toyota,, Naoto Maru, Martin M. Hanczyc, Takashi Ikegami and Tadashi Sugawara,* Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, Komaba, Meguro-ku, Tokyo , Japan. Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage, Chiba , Japan. Department of Physics and Chemistry, University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark. Department of General Systems Sciences, Graduate School of Arts and Sciences, The University of Tokyo, Komaba, Meguro-ku, Tokyo , Japan. Materials. 4-octylaniline was purchased from Kanto Chemical Co. (Tokyo, Japan) and used immediately after distillation under reduced pressure (b.p. = 125 o C). Other commercially available reagents and solvents were purchased from Tokyo Chemical Industry Co. (Tokyo, Japan) and were used without further purification. Synthesis of N-(4-[3-[trimetylammonio]ethoxy]benzylidene)-4-octylaniline bromide 1. 2-(4-Formylphenoxy)ethyltrimethylammonium bromide (300mg: 1.04 mmol), 4-octylaniline (258 mg: 1.25 mmol) and triethylamine (52.6 mg: mmol) were dissolved in dehydrated MeOH (5 ml) and heated to reflux (85 C) for 50 h. After cooling the reaction mixture, diethylether was poured into the solution until white solids precipitated. The precipitates were isolated by filtration and dried under reduced pressure to give 1 as a white solid (88 % yield). 1 H NMR (270 MHz, CDCl 3, TMS): δ 8.40 (1H, s, -CH=N-), 7.87 (2H, d, J = 8.8 Hz, Ar-H), (4H, m, Ar-H), 7.01 (2H, d, J = 8.8 Hz, Ar-H) 4.58 (2H, br, -CH 2 -), 4.39 (2H, br, -CH 2 -), 3.62 (9H, s, -CH 3 ), 2.61 (2H, t, J = 7.7 Hz, -CH 2 -), (2H, m, -CH 2 -), (10H, m, -CH 2 -), 0.88 (3H, t, J = 6.8 Hz, -CH 3 ). 13 C NMR (67.8 MHz, CDCl 3, TMS): δ 159.0, 158.0, 149.2, 140.7, 130.7, 130.4, 129.0, 120.6, 114.6, 64.9, 62.5, 54.7, 35.5, 31.9, 31.6, 29.5, 29.4, 29.3, 22.7, Methods. Optical microscopic observation of oil droplet dynamics. An aqueous dispersion of fuel surfactant 1 (20 mm of N-(4-[3-[trimetylammonio]ethoxy] benzylidene)-4-octylaniline bromide) was prepared by an ultrasonication method. Oil droplets were prepared from an emulsion of 4-octylaniline 2 containing fluorescent catalyst 4 [Synthesis of 4 is described elsewhere (T. Toyota et al., Langmuir 2008, 24, )] by agitating 100-µL of miliq water with 1-µL of the oily mixture of 2 and 4 (1:0.05, molar ratio). Immediately after mixing the fuel surfactant solution with an equal volume of the suspension containing oil droplets and encasing the mixture into a thin glass-chamber (spacer thickness = ca. 290 µm, MJ Research Inc., Waltham), we carried out a real-time observation of the dynamics of micrometer-sized oil droplets at 23 ± 1 o C under a phase contrast and fluorescence microscope (IX70, Olympus, Japan), equipped with filters (λex, nm; λem, > 515 nm), a CCD camera (JK-TU52H, TOSHIBA, Japan) and a video recording system (WV-9, Sony, Japan). S1

2 Figure S1. Figure S1. Phase contrast micrograph and fluorescence micrograph [filters: λex, nm; λem, >515 nm] of a self-propelled oil droplet with tiny waste droplets on the posterior surface of the oil droplet. The white arrow in each micrograph shows the direction of the motion. The fluorescence image shows the tiny waste droplets do not contain the fluorescent catalyst. S2

3 Figure S2. Figure S2. (a) Phase contrast micrograph of a self-propelled droplet (indicated by orange arrow) with waste droplets on one side. A non-moving droplet adhered on the glass substrate is also shown (indicated by black arrow). This micrograph is a still image synthesized from the real-time captured of the self-propelled droplet (see Supporting Information video). (b, c, d, e) Superimposed time-course images synthesized from the real-time captured video of the self-propelled droplet. Each image is synthesized from sequential 5 still frames (captured in 1/30 s per frame) of the video. These images show the motion of the self-propelled droplet and its inner convection observed at the same focal plane of the center of the self-propelled droplet. Orange circles correspond to the center of the self-propelled droplet, black filled circle to the center of the non-moving droplet, and black open circles to some inner structures (likely water pockets) observed at the same focal plane of the center of the self-propelled droplet. Gray dotted-line shows the outline of the self-propelled droplet at the beginning and ending stage for each period of time. These pictures indicate that the self-propelled droplets have internal structures that exhibit a convective flow, being commensurate with the direction of the self-propelled motion. S3

4 Figure S Remaining percentage of precursor, % Time / min Figure S3. Consumption of precursor 1 through its hydrolysis in a 10-mM aqueous solution in the presence of 2-µL oil droplet of 2 containing 5 mol % of catalyst 4 at 23 ± 1 o C (open circles, triangles, or squares correspond to independent runs). At first, calibration curves of a methanol solution containing 1, 2 and 3 ([1]/[2]/[3] = 50/0/0, 35/15/15/, 25/25/25, 15/35/35, 0/50/50 (µm)) as representative compositions of the reaction mixture were drawn by measuring the light absorbance at 321 nm, which is assigned to the biphenylazomethine moiety of 1. We prepared samples of the reaction mixture which were then diluted with methanol and this absorbance measured as a function of time and each conversion of the precursor was determined using this calibration curves. This data were then plotted as the remaining percentage of 1 in the aqueous reaction mixture. The fact that the fuel is hydrolyzed with the half life-time of ca. 50 min at 23 ± 1 o C corresponds to the microscopically determined life time of ca. 1 h for the self-motion of the oil droplets. S4

5 Figure S4. Figure S4. Phase contrast micrograph of oil droplets of 2 containing 5 mol % of catalyst 4 in a 20-mM solution of a non-reactive surfactant, n-tetradecyltrimethylammonium bromide (TTAB). TTAB has the same trimethylammonium polar head as fuel surfactant 1 and the length of alkyl chain of TTAB is similar to that of the lipophilic part of 1. Most of the droplets (pale orange structures), being adhered on the surface of the glass chamber, and did not show any self-propelled motion. They gradually dissolved and produced giant multilamellar vesicles. Since an equimolar mixture of TTAB and 2 formed giant multilamellar vesicles in water (10 mm), it is reasonable to suspect that giant multilamellar vesicles, consisting of both TTAB and 2 (and 4), emerged in the dispersion. S5

6 Figure S5. Figure S5. Superimposed time-course micrograph of an internal convective flow of particles (indicated by open circles) inside of 1-µL oil droplet of 2 (with 5 mol % of 4) which was completely adhered on a glass surface; time interval = 2 s. The convective flow inside of the still oil droplet (diameter ~ 2.0 mm) was activated by introduction of 10-mM aqueous solution of 1 through a micropipette (inner diameter = 0.5 mm) located at ca. 0.5 mm away from the oil surface. The convective flow in the vicinity of the micropipette was directed toward its apex, however the oil surface, being away from the apex of the micropipette, did not show any internal convective flow. Since the internal convective flow of a selfpropelled oil droplet with its diameter of ca. 50 µm is commensurate with the movement direction (see the main text and Supporting Information video), a gradient of fuel may be one of the factors determining the direction of such self-propelled oil droplet. S6

7 Figure S Initial velocity / µm s Diameter / µm Figure S6. Dot-plot diagram of initial velocity vs. diameter of self-propelled oil droplet of 2 (containing 4) in a 20-mM solution of fuel surfactant 1, which was observed under the phase contrast microscope. Because it was difficult to measure the precise velocity of oil droplets in the threedimensional space by the micrographs, we defined here the velocity of the oil droplet as the travel distance of the center of the self-propelled oil droplet per second, provided that the movement was restricted in a quasi-2d plane due to the shallow depth of the cell. It apparently indicates that the larger oil droplets are, the faster they swim. Since the surface area difference of the leading edge and the trailing edge of the oil droplets is related to their sizes, the velocity of the self-propelled oil droplets is approximately proportional to the surface area difference. Therefore, one can elucidate that the driving force of self-movement of the oil droplets in the current system is related to the difference of interfacial energy around the oil droplet. On the other hand, if the real-time observation of dynamics of the oil droplets of 2 (4) was conducted in a high concentration of 1 (60 mm), they exhibited random motions (see Supporting Information video). No waste droplets were observed at the trailing edge of selfpropelled oil droplets and the internal convection of the oil droplet was also random. This is because the formed waste droplets were likely dissolved in a high concentration of 1 so quickly that the asymmetrical structure could not last enough to fix the direction of the self-propelled motion. It may be deduced from these results that the direction for the self-propelled motion is related to the structure with one-sided burden of wastes on the oil droplets. S7

8 Real-time captured videos. We have attached five video files (named as sugawara_video_01.mpg, sugawara_video_02.mpg, sugawara_video_03.mpg, sugawara_video_04.mpg and sugawara_video_05.mpg ) of self-propelled oil droplets in 20-mM dispersion of precursor 1, and one video file (named as sugawara_video_06.mpg ) of self-propelled oil droplets in 60-mM dispersion of precursor 1, which were captured by a CCD camera (JK-TU52H, TOSHIBA, Japan) and a recorder (WV-DR9, SONY, Japan) equipped on a phase contrast microscope. One video file named as sugawara_video_07.mpg shows the activation of the internal convective flow inside of the fixed oil droplet under a differential interference contrast microscope (IX71, Olympus, Japan) equipped with the same CCD camera and the same recording system. All these videos were in real-time speed. sugawara_video_01.mpg: Repelling motion of two approaching oil droplets. sugawara_video_02.mpg: Turning-together-motion of two approaching oil droplets. sugawara_video_03.mpg: Dissolution of waste droplets via. transformation into giant vesicles. sugawara_video_04.mpg: Stopping of a self-propelled motion of an oil droplet fully covered by waste droplets. sugawara_video_05.mpg: Synchronization of the convection of the inner structure with the direction of a self-propelled motion. sugawara_video_06.mpg: Random motion of oil droplets in the 60-mM dispersion of precursor 1. sugawara_video_07.mpg: Activation of the internal convective flow inside of the fixed oil droplet. S8