Supporting Information

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1 Supporting Information Characterizing Emulsions by Observation of Single Droplet Collisions Attoliter Electrochemical Reactors Byung-Kwon Kim, Aliaksei Boika, Jiyeon Kim, Jeffrey E. Dick, and Allen J. Bard* Center for Electrochemistry, Department of Chemistry, The University of Texas at Austin, Austin, Texas, Experimental Details Reagents Toluene was obtained from Fisher Scientific. Tetrabutylammonium perchlorate (TBAP), trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)amide (IL-PA) ( 95.0%), ferrocenemethanol (97%), ferrocene (Fc) (98%), potassium ferrocyanide and acetonitrile (CH 3 CN) were obtained from Sigma or Aldrich. Au (99.99%) wire (10 µm diameter) was obtained from Goodfellow (Devon, PA). Preparation of ultramicroelectrodes (UME) Au UME was prepared following the general procedure developed in our lab. Briefly, a 10 µm Au wire was sealed in glass after rinsing with ethanol and water. The electrode was then polished with alumina powder water suspension to a mirror finish. The surface area was checked with standard redox electrochemistry of ferrocenemethanol. Before every experiment, the solution was deaerated with Ar and all electrodes were polished prior to use with alumina (0.05 µm) paste on microcloth pads (Buehler, Lake Bluff, IL). Instrumentation The electrochemical experiments were performed using a CHI model 900B potentiostat (CH Instruments, Austin, TX) with a three-electrode cell placed in a Faraday cage. A 0.5 mm diameter Pt wire was used as a counter electrode, and the reference electrode was Ag/Ag+ (0.01 M AgNO 3, 0.1 M TBAP/CH 3 CN). Emulsions were made by Q500 ultrasonic processor (Qsonica, Newtown, CT) with a microtip probe. Dynamic laser scattering (DLS) data was obtained by Zetasizer Nano ZS (Malvern, Westborough, MA). Preparation of toluene (Fc + IL-PA)/water emulsions S1

2 The toluene (Fc + IL-PA)/water emulsions were prepared by dissolving Fc (20 mm) and IL-PA (400 mm) in toluene. The 100 µl of mixture of toluene was added to 5 ml of distilled water. After that, the solution was vortexed vigorously for 20 s, and immediately ultrasonic power (500 watts, amplitude 40%) was applied using pulse mode (3 s on, 7 s off, 20 cycles repeated). The number concentration of toluene (Fc + IL-PA)/water emulsion droplets was approximately calculated from the total toluene (Fc + IL-PA) (100 µl) volume divided by the average single emulsion droplet (dia. 624 nm) volume (127 al). Our system does not have an additional surfactant but was stable for at least 16 h. We can attribute this stability to the ionic liquid, acting as both an emulsifier and supporting electrolyte. Three different types of emulsion solutions were made to confirm that the ionic liquid increased the stability of the emulsions (Figure S3). We made a toluene/water emulsion, a toluene (Fc)/water emulsion, and a toluene (Fc + IL-PA)/water emulsion. Without the ionic liquid, the emulsions coalesced within 1 h, and completely separated within 7 h. As seen in Figure S3, the emulsion containing the ionic liquid was the most stable over time. DLS, or more precisely, ELS (electrophoretic light scattering), data showed the ζ-potential of the emulsion was 15.8 mv, indicating that the droplet is negatively charged (Figure S4B), suggesting that the amide functional group of IL-PA is acting as an emulsifier. Figure S1. Structure of ionic liquid, trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)amide (IL-PA). S2

3 Figure S2. CV of 20 mm Fc and 400 mm IL-PA in toluene at Au UME (dia. 10 µm) at a scan rate of 20 mv/s. Figure S3. Photographs of emulsions as a function of time. (A) Toluene/water emulsion, (B) toluene (20 mm Fc)/water emulsion, (C) toluene (20 mm Fc mm IL-PA)/water emulsion. S3

4 Figure S4. (A) Size and (B) ζ-potential distribution of toluene (Fc + IL-PA)/water emulsion obtained from DLS and ELS measurements. S4

5 Figure S5. (A) i-t plots of 24.8 pm toluene (IL-PA)/water emulsion (red line) and toluene (Fc + IL-PA)/water emulsion (black line) collisions at a Au UME (dia. 10 µm). (B) Enlarged pictures of i-t plots. S5

6 Figure S6. Collision counts (number of collisions) vs. current spike charge plot for 24.8 pm of toluene (Fc + IL-PA)/water emulsion. The peak charge data was collected between 200 and 1000 s. Droplet Diameter (nm) Droplet Volume (al) Fc Concentration in Droplet (mm) Number of Fc in Droplet (amol) Charge in Droplet (pc) Figure S7. Volume, Fc concentration, number of Fc molecules, and a charge determined from various droplet diameters. S6

7 Figure S8. (A) Schematic diagram of EDR method and expected current responses. (B) Enlarged i-t curve and analysis of single current spike. The experimental data were obtained every 50 ms (black dots). The simulated i-t behavior data for a first order, homogeneous electrolysis reaction is shown by red circles. The d drop (dia. of an emulsion droplet) is calculated from the integrated peak charge (C p ). The simulated i-t behavior data were calculated from the following equation. 1 m = 4D Fc πr a i(t) = i p e ma V t (1) (2) where, m is the mass-transfer coefficient for a disk electrode, D Fc is the diffusion coefficient of Fc, r a is the contact radius between UME and the emulsion droplet, i p is the initial peak current, A is the contact area between UME and the emulsion droplet, V is the emulsion droplet volume, and t is the electrolysis time. In this equation, we assumed that the contact radius between UME and the emulsion droplet is 0.3 nm. This 0.3 nm is the radius of a toluene molecule. S7

8 Figure S9. (A) i-t plot of 12.9 pm of toluene (Fc + IL-PA)/water emulsion collisions at a 10 µm Au UME. (B) Enlarged pictures of the i-t plot. S8

9 Figure S10. Collision counts (number of collisions) vs. current spike charge plot of 12.9 pm of toluene (Fc + IL-PA)/water emulsion. The peak charge data was collected between 200 and 1000 s. Figure S11. Overlay of collision counts vs. diameter plot and diameter vs. relative intensity plot. Diameter vs. relative intensity data from DLS data. S9

10 Figure S12. CV of 200 mm K 4 Fe(CN) 6 aqueous solution at a 10 µm Au UME at a scan rate of 20 mv/s. S10

11 Figure S13. (A) i-t plot of 24.8 pm of toluene (Fc + IL-PA)/water emulsion at a 10 µm Au UME. Current steps were observed due to the blocking of oxidation of K 4 Fe(CN) 6 by the emulsion droplets. (B) Enlarged pictures of the i-t plot. References 1 Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications 2 nd ed., 2001, Wiley, NY. S11