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Visualization of Charge-Carrier Propagation in Water Andrey Klimov† and Gerald H. Pollack* Department of Bioengineering, Box 355061, UniVersity of Washington, Seattle, Washington 98195 ReceiVed June 12, 2007. In Final Form: August 7, 2007 The electrical properties of water in the region between parallel electrodes were investigated using pH indicator dyes. Different pH values corresponded to different colors, which could be registered by a video camera. Imposition of electrical current was able to produce zones of constant pH around, and well beyond each electrode: extremely low pH around the positive electrode and extremely high pH around the negative electrode. The border between alkaline and acid zones was jagged and separated by only a narrow layer of water with neutral pH. When the water was replaced by various salt solutions, similar zones were observed. Again, passage of current produced large zones of extreme pH values near and beyond each electrode. Alkaline zones appeared to propagate from the negative to the positive electrode in narrow channels through the neutral solution. When the power supply was disconnected from the electrodes and replaced by a resistive load, a potential difference was registered, and current flowed through the resistor for some period of time. Hence, the acid and alkaline zones appear to carry opposite charges throughout their volume.
Introduction Previous work from this laboratory revealed an unexpected observation: solutes were profoundly excluded from aqueous zones in the vicinity of various charged surfaces. These surfaces included hydrogels, ion-exchange resins, and polymers as well as biological entities, and the exclusion zones could extend up to hundreds of micrometers from the respective surfaces.1-2 The water in this zone was physically different from bulk water, and appeared to be charged.3 The question arose whether similarly charged aqueous zones might be found when nucleating surfaces were replaced by charged electrodes. In this case, the charge would be continuously supplied electrolytically, rather than being statically lodged within the surface, and could conceivably be more ample. We found indeed, that next to each electrode, the injection of charge produced unexpectedly vast zones with extreme pH values. Methods The following components were used to study electrolytic generation of charge carriers: (1) A universal pH Indicator (Sigma no. 36803), made from a mixture of different dyes whose color depends on pH, added in concentrations of 0.2-2 mM, to produce optimum color, depending on thickness of the water layer. (2) Carboxylated polysterene microspheres, 1 µm diameter, the surfaces of which are covered with hydroxyl groups which are able to assume negative charge in aqueous solutions at pH > 3. (3) In one experiment a physiological salt solution was used, containing universal pH indicator, 0.5 mM EGTA, 100 mM KCl, 20 mM potassium phosphate, 0.2 mM MgCl2, 2 mM DTT. Initial pH ) 7.0. In general, two electrodes, made from platinum-wire welding rods, diameter 0.32 mm, were situated parallel to one another in a * To whom correspondence should be addressed. E-mail: ghp@ u.washington.edu. † Current address: Institute of Theoretical and Experimental Biophysics, Puschino, Russia 142290. (1) Zheng, J. M.; Pollack, G. H. Phys. ReV. E: Stat., Nonlinear, Soft Matter Phys. 2003, 68, 031408. (2) Zheng, J.-M.; Pollack, G. H. In Water and the Cell; Pollack, G. H., Cameron, I. L., Wheatley, D. N., Eds.; Springer: New York, 2006; pp 165-174. (3) Zheng, J.-M.; Chin, W.-C; Khijniak, E.; Khijniak, E., Jr.; Pollack, G. H. AdV. Colloid Interface Sci. 2006, 127, 19-27.
Figure 1. Experimental apparatus. See text for detailed explanation. small chamber with a transparent bottom. The chamber was situated on a table with transparent glass surface, illuminated from below with light from tungsten bulb, scattered by white paper to produce uniform illumination. The electrodes were connected to a power supply, which could deliver the desired voltage and current. The pH indicator dye color, as well as the motion of negatively charged microspheres, were captured by a color video camera (Logitech QuickCam), equipped with a lens whose working distance was 4-8 cm. The camera was connected through USB port to a computer. The chamber, whose floor was made of glass, had 7-mm high Plexiglas walls covered by polymerized silicone made from Silicone Elastomer Kit Sylgard-184 (Figure 1A,B). The platinum elctrodes were mounted in the Plexiglas and silicone walls. The chamber had two compartments. In the A compartment the electrodes had a length of 26 mm and were 13 mm horizontally apart from one another. Their distances from the floor and the top of the solution were 2 mm, and from the nearest wall 2-3 mm. The electrodes were passed horizontally through compartment A, turned vertically upward, turned horizontally, and then turned vertically downward to compartment B separated by 10 mm from one from another. One of the compartments, either A or B, was filled with liquid for experiments. Electrical current drawn from a regulated power supply was divided 10 times by external resistors. In this way, external voltage could be regulated with 10 times better accuracy than with the power supply regulator. Current from the divider was sent to a Fluke 8020 multimeter, which was able to measure current with a precision of 10-6 A. After the ammeter, the current was sent to switch SW1, 2, which could change the polarity of current, and then to the electrodes
10.1021/la701742v CCC: $37.00 © 2007 American Chemical Society Published on Web 10/16/2007
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Figure 2. pH color scale for universal pH indicator (Sigma Aldrich).
Figure 3. Time course of color change in images captured in chamber “A” (Figure 1), with horizontal electrodes and water containing a pH indicator.
in the chamber. The voltage applied to the electrodes was measured with the help of a second high impedance multimeter. When switch SW 3 was OFF, the power supply was disconnected, and the same multimeter could measure the voltage produced in the chamber. Before each experiment electrodes were cleaned mechanically and by passing electrical current in water containing 1 M KOH with direct and reversed polarity of current. When microspheres were used, a transparent plastic film with text was placed under the transparent glass floor of the chamber. This allowed us to check solution opacity. The video camera was positioned so that a sharp image of the electrodes was visible along their entire length. pH was determined by comparing the color on the video image with the color on the color scale that came with the pH indicator (cf. Figure 2). For video recording, the program Microsoft VidCap.exe was used in the Capture/Frames mode. When the cursor was positioned in the VidCap window on the command button “OK,” the left-hand mouse button could be actuated not only manually but also on command from a program, written for selecting arbitrary intervals between frames at any moment of time during the experiment. When the program activated the command button “OK,” electrical impulses were supplied through the serial port of the computer, resistor, and diode on the base of the n-p-n transistor 2N2206, hooked up in parallel to the left mouse button. When the cursor was placed in one of the program’s text boxes, we were able to change time between captured frames or the number of frames to be captured. For storage, the files were saved in the *.avi format. For review of videos on a Mac OS, files were converted to the format *.mov using QuickTime.
Results Figure 3 shows the results of applying a potential difference between electrodes for some time. Five volts were applied. Initial current was ∼20 µA. The first frame was captured 10 s after current was applied to water whose initial pH was ∼5. One may see a very narrow strip of gray-green coloring near the negative electrode and red color near the positive electrode. With continued flow of current these zones grew progressively larger, and within 3 min the zones with changed color (red, pH ≈ 4 and gray-green, pH ≈ 7-8) became substantial; between them, the zone with the initial color (pH ≈ 5) remained. The borders between these three zones were rarely straight; generally they appeared curved or convoluted. After some time, the acid (red, pH < 4) and alkaline (violet, pH ≈ 10) zones met one another, and only a very narrow layer with the initial color remained between them (right panel). This
layer persisted as long as current continued to pass through the electrodessat least up to 3 h of observation in some experiments. The curvature changed with time, the dynamic somewhat resembling that of a forest fire. In some experiments we added microspheres to examine opacity. Results are illustrated in Figure 4. Initial color corresponds to pH 6. The presence of microspheres increases opacity, which blurs the visibility of characters on a film placed beneath the chamber. When 5 V were applied, the pH around the electrodes began to change as described above. Meanwhile, the negatively charged microspheres moved from the negative electrode toward the positive. After 21 min, text could be seen under the bottom of the chamber, as microspheres in solution migrated from the negative electrode, leaving the wide zone transparent. This zone resembled the microsphere-free “exlusion zone” reported earlier.1,3 By contrast, the concentration of microspheres in the red zone (pH < 4) close to the positive electrode grew, as did the opacity. The letters on a paper are not at all visible. A notable feature is that the negative electrode zone looks “cleaner” than the positive electrode zone; i.e., the green-violet color (pH > 8) gets progressively fainter with time. Apparently, dye molecules get progressively excluded from this region. Earlier, we found that several different dyes were excluded from the zone near various surfaces such as Nafion.3 This result looks much the same. The right panel of Figure 4 shows the effect of polarity reversal. Just after capturing the 21-min video on the left, electrode polarity was reversed, and the results are shown in the right panel of Figure 4. Immediately, the microspheres began moving in the opposite direction. The frame taken 7 min after the polarity change is shown at the top. The yellow color in the middle zone corresponds to pH ≈ 6. By 27 min, most of the microspheres were concentrated close to the new positive electrode, and the zone near the new negative electrode became cleaner than the zone near the positive. That is, dye molecules were progressively excluded from that zone. In the next series of experiments we explored the pH behavior using vertically oriented electrodes (Figure 1B). Representative results are shown in Figure 5. Initially, low voltages were studied. For small incremental increases of voltage with time, 220-1300 mV (n ) 9 experiments), current remained less than 1 µA, and only small changes in solution features were seen (Figure 5a). When the highest-level voltage (1.3 V) was switched off, there
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Figure 4. Effect of current flow on solution opacity. On the left panels, the positive electrode is at the bottom. On the right side, the polarity was reversed.
was no measurable voltage or current flow through the voltmeter resistance. After 9 min without external power, the color everywhere in the chamber solution was the same, indicating pH 7. At higher voltages, progressive color changes could be observed, and they were largely similar to those observed with the horizontal electrodes. Figure 5b shows the time course of color change at an applied voltage of 4.3 V. At the negative electrode (left), the color changed from yellow-green (pH 7) to violet (pH 10). At the positive electrode (right), the color changed from yellow-green (pH 7) to orange (pH 4) and red (pH < 4). These zones widened progressively until they met (∼300 s); from then on they remained relatively stable, with only a narrow zone of neutral pH situated in between. Meanwhile, the current did not remain constant. As can be seen from the panels, the current increased progressively over
time from 11 µA up to 14 µA, presumably because of the increase of number of charge carriers. This increase was seen consistently (n ) 9). The last two frames in Figure 5b show what happens during the several minutes following cessation of current flow. When the power supply was switched OFF and no external voltage was applied to the electrodes, the color pattern remained for some time, and disappeared only rather slowly. Lingering pH difference implies long-lasting charge separation. In some experiments we measured the ability of separated charge to drive current. When the external power supply is in the OFF position, the output resistance of the power supply (112 Ω) remains connected to electrodes, and it is possible to measure current flow through this resistance. With time, voltage and current decreased respectively from 4 to 6 mV and 4-6 µA initially, to near zero in 5-10 min (n ) 5 experiments).
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Figure 5. (a) Vertically oriented electrodes, running perpendicular to the plane of each image. Voltage stepped up progressively, as indicated. (b) Time course of pH change near vertically oriented electrodes at 4300 mV. Last panel shows color change following turnoff of input power at 900 s. (c) Continuation of previous experiment (Figure 5b) with V ) 4.3 V and electrical polarity reversed.
The effect of voltage reversal was explored in other experiments. In Figure 5c, voltage with reversed polarity was applied following the 1100 s sequence shown in Figure 5b. In the figure it is possible to see new zones growing around each electrode. Their colors are complementary to the originals. During the first few moments the shapes were typically cylindrical, but very soon they took on various odd shapes and evolved unpredictably. The only consistent feature was that when two zones with different colors collided, they produced narrow curved zones with neutral pH (gray/green), where negative OH- and positive H+ or H3O+ ions apparently annihilated one another and produced neutral water. Ultimately, the new pattern was the inverse of the one obtained prior to reversal, indicating that the process is reversible, and dye composition was not appreciably changed by prolonged flow of currents. From Figure 5, perhaps the most interesting finding is that in solutions of pure water with dye, two distinct zones exist with different charge carriers: one with an excess of H+, H3O+, or larger positively charged clusters of “pure water”; the other with
an excess of OH- or larger negatively charged clusters. With pH < 4 and pH ≈ 10 in the respective zones, the ratio of charge carriers is more than one million. Hence, different charge carriers carry current in the respective zones. The effect of replacing distilled water with physiological salt solution is shown in Figure 6. The salt solution contained the Universal pH indicator, 0.5 mM EGTA, 100 mM KCl, 20 mM potassium phosphate, 0.2 mM MgCl2, 2 mM DTT. Initial pH ) 7.0. When a potential difference of 4.3 V was applied between the electrodes, the pH around the positive electrode changed very quickly to 10. The relative rapidity of the change presumably arose because of the relatively higher conductivity of the salt solution, compared to distilled water. The pH pattern was more uniform close to the positive electrode than to the negative electrode (see panels 40110 s), where current passed through a relatively narrow channel resembling a finger. With time, channel width increased, and finally the alkaline solution occupied 70-80% of the space between electrodes, the remainder filled with acid solution.
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Figure 6. Images of pH changes seen in ordinary physiological buffer during the imposition of current. Electrodes run horizontally. V ) 4.3 V. (Top) Positive. (Bottom) Negative.
Discussion Electrolytic imposition of current into a chamber with water is able to produce zones with low and high pHsextremely low around the positive electrode and extremely high around the negative electrode. The result was seen using pH-sensitive dyes, both in distilled water and in salt solution. The surprising aspect was the extent of these two zones. Soon after onset of current flow, the acid and alkaline zones began to grow away from the respective electrodes. Distinct borders could be seen between each growing zone and the neutral zone in between. Boundaries between zones were initially straight and ran more or less parallel to the horizontally oriented electrodes (Figure 3: 10 s, 180 s). The zones continued to grow and finally met one anothersseparated only by a narrow zone of neutral water (Figure 3, 240 s). At this stage the border between zones became more jagged, constantly moving about. This is not the first report in which solutions of vastly different pH could coexist beside one another. For example, pH gradients were electrochemically formed in microfluidic channels and optically quantified using acid-base indicators, for isoelectric focusing of sample biological analytes.4-5 In these studies, the border between zones of different pH appears straight, not jagged as in our experiments. In another study6 proton migratory fronts were visualized using the acid-base indicator bromocresol green, pH range 3.8-5.4. (4) Macounova, K.; Cabrera, C. R.; Holl, M. R.; Yager, P. Anal. Chem. 2000, 72, 3745-3751. (5) Cabrera, C. R.; Finlayson, B.; Yager, P. Anal. Chem. 2001, 73, 658-666. (6) Gonzalez, G.; Marshall, G.; Molina, F. V.; Dengra, S.; Rossoc, M. J. Electrochem. Soc. 2001, 148, 479-487.
The dye was added to an electrolyte solution containing 100 mM ZnSO4 + 1 mM H2SO4. With constant electrical current through the zinc wire electrodes, it was shown that an increase in viscosity through glycerol additions yielded a more uniform deposit of zinc with smaller separation between branches of metal aggregates, i.e., a change in morphology from more separated compact trees having branches of metal aggregates, to a more dense, fractal-like structure. The border between acid and more neutral zones generally appeared straight. Thus, others have reported extensive zones of differing pH adjacent to one another, although the emphasis of these studies was considerably different from the emphasis here. In addition to noting extensive pH separation, we examined the behavior of microspheres. When current was applied, charged microspheres could move. Negatively charged microspheres translated away from the negative electrode, leaving a clear zone through which writing on a film beneath the chamber could be clearly discerned (Figure 4). The microspheres gathered in the positively charged zone, creating an opacity through which writing could not be seen. The boundary between opaque and transparent zones was jagged, much like the boundary between high-pH and low-pH solutions. This experiment also provided a clue for linking another interesting phenomenon. Negatively charged microspheres are consistently expelled from a broad zone next to many hydrophilic surfaces.3 We found recently that pH-sensitive dyes are also excluded.7 Results of the current experiments are similar: Both the microspheres, and also the pH-sensitive dye, were progres(7) Yoo, H.; Pollack, G. H. Unpublished results.
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sively excluded from the high-pH zone (Figure 4, lower panels). Hence the high-pH zone, which is negatively charged, may be similar in character to the negatively charged solute-exclusion zonesthe latter being physically different from ordinary bulk water.3 When the current was applied through vertically rather than horizontally oriented electrodes, the behavior was largely similar in that the established zones did not mix. They remained separated by a small layer of neutral-pH water (Figure 5). Here, the dye formed jagged boundaries with triangular emanations from the start, moving progressively closer toward one another with time. When physiological solutions containing various salts were substituted for distilled water, application current produced qualitatively similar results, but with some notable differences (Figure 6). Whereas the low-pH zone that formed around the positive electrode was similar to those formed in the absence of salts, the events around the negative electrode were unexpected. The high-pH region first grew more or less parallel to the electrode, but it soon gave rise to a finger-like protuberance extending toward the positive electrode, a structure that implied a more conductive path. Such protuberances were stable in their position but grew in girth. Their presence indicated that grossly nonlinear
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features might be characteristic, particularly in physiological solutions. Although such patterns were noted, they were not studied in detail as they were not the main focus of the study. The experimental results also showed that excess positive charge in the low-pH zone, and excess negative charge in the high pH zone, could drive current through a resistive load. Current flow could persist for minutes. In the absence of the resistive load, however, the zones were maintained next to one another for an extended period, in spite of the huge pH and charge gradients. Almost no mixing occurs. The thin, neutral zone between compartments remains, and the zones themselves remain of uniform color for an extended period. The key question emerging from these results is why these regions with different pH do not immediately neutralize one another. Either the charges must somehow be neutralized by counterionsswhich begs the question of how current could then be driven through the resistor, or the charges must somehow be stabilized within a matrix, as they are for example, in p- and n-type semiconductor materials. Future experiments will need to address these important electrochemical questions. LA701742V
