Anal. Chem. 1995,67,1322-1325
Computerized System for Dual-Electrode Multisweep Cyclic Voltammetry for Use in Clay-Modified Electrode Studies Jennifer A. Stein and Alanah Fitch* Department of Chemistly, Loyola University of Chicago, 6525 North Sheridan Road, Chicago, Illinois 60626
A computerized system for dual-electrode multisweep cyclic voltammetry has been developed. The system was designed to ensure clay film integrity(no strippingof clay) and to eliminate exposure of the clay film to atmospheric drying or oxidizing conditions during electrolyte switching. Because of the control over electrode history, reproducible single clay film swelling and exposure studies may now be carried out. In addition, the kinetics of change in the film structure may be followed. Clay-modified electrodes (CMEs) have been used to study the diffusionalproperties of negatively charged clay Diffusion for an anion like Fe(CN),j3- is controlled primarily by the pore structure, which is, in turn, controlled by the electrolyte solution used to swell the clay film!,5 The ratio of the current for Fe(CN)63- at the CME to the current at the bare electrode (R = Ipcme/lp~are) correlates with interlayer spacing within the SWy-1 film. This is confirmed through comparison to X-ray diffraction dah45 The swelling of the clay is particularly sensitive to prior handling, e.g., spin coating, moisture content, oxygen content in the air, and prior electrolyte exposure. In our earlier sets of electrochemically monitored clay film swelling studies, the issue of prior treatment was handled by re-forming the clay for each and every experiment. Thus a “swelling curve”, obtained by measuring the ratioed current at the preswollen film electrode, consisted of measurements on 12-30 different clay films. To study the kinetics of swelling of a single clay film as a function of controlled prior electrolyte exposure, instrumentation is required which (a) maintains the integrity of the clay film (thus ruling out chromatographic flow through cells which laterally strip the film), (b) minimizes dehydration of the clay in the laboratory atmosphere during electrolyte switching, (c) has a response time in the subsecond time domain with continuous data acquisition, and (d) is amenable to temperature control. Two additional requirements are imposed: (e) the entire system must be subject to Nz control, since the system will be used in studying reduced clays, and (f) the bare electrode current must be acquired simultaneously with the CME current, since real-time monitoring of the data was desired. A first attempt at this system involved using a wall-jet configuration for the clay-modified e l e ~ t r o d e . ~It, was ~ found that the * E-mail:
[email protected]. Fax: (312)5083086. (1) Fitch, A,; Fausto, C. L. j. Electroanal. Chem. 1988,257, 299-303. (2) Subramanian, P.; Fitch, A. Environ. Sci. Technol. 1992,26,1775-1779. (3) Fitch, A; Song, J.; Stein, J. A. Clays Clay Miner., submitted. (4) Lee, S. A; Fitch, A j.Phys. Chem. 1990,94, 4498-5004. (5) Fitch, A; Du, J. J. Electroanal. Chem. 1991,319, 409-414.
1322 Analytical Chemistry, Vol. 67, No. 8, April 75,7995
pulsation of the system contributed unnecessary noise, that 02 was entrained, and that the volume of solution consumed during a complete swelling curve was large. Furthermore, we were unable to confirm that the pressure of the flow stream did not compress the clay film. This article describesthe successful setup for real-time electrochemical monitoring of clay film swelling at a single clay film. As with several other described systems! a fast scan cyclic voltammetric experiment with continuous sweep monitoring is employed. In addition, this system incorporates monitoring of the reference signal (an unmodified electrode) directly into the instrumentation. The unique feature of this system, however, is related to the requirement that the clay film remain unperturbed and in an inert, moist atmosphere during electrolyte switching. The electrodes are sealed in the electrode compartment ( N z atmosphere) but are raised with an X-Y position controlled by a stepper motor above the level of the solution during electrolyte switching. After the electrolyte is exchanged (either via a manually controlled or via a solenoid controlled valve system), the electrodes are brought back into contact with the solution. When the electrodes come to a rest, automatic data collection is triggered (satisfying the subsecond sampling requirement) . EXPERIMENTAL SECTION
Chemicals and Electrodes. Sodium chloride, potassium chloride (Fisher Scienac, Pittsburgh, PA), potassium ferricyanide (Aldrich, Milwaukee,WI), and sodium ferricyanide (City Chemical Corp., New York, NY) were used as received. Standard Wyoming sodium montmorillonite (SWy-1) was obtained from the Source Clays Repository (University of Missouri, Columbia, MO) and purified according to the manner of Jackson! Approximately 15 g of clay was suspended in 450 mL of deionized water and stirred for 48 h. The suspension was centrifuged in a Sorvall RC-5B refrigerated superspeed centrifuge equipped with a GSA rotorhead Oupont Instruments) for 35 min at 3500 rpm and 20 “C. The supernatant was collected. Sodium-exchanged clays were prepared according to the method of Kamat et a1.l0 The supernatant was diluted to twice its volume in 2 M NaCl. This solution was stirred for 36 h and then centrifuged for 45 min at 7000 rpm and 20 “C. The clays were resuspended in 2 M NaC1, and the above (6) Fitch, A J. Electroanal. Chem. 1992,332, 289-295. (7) Stein, J. A; Fitch. A. Electroanalysis 1994,6,23-28. (8) Kennedy, R T.; Jones, S. R; Wightman, R M. j. Neurochem. 1992,59, 449-55. (9) Jackson, M. L. Soil ChemicalAnalysis: Advanced Course, 2nd ed.; published by the author, Madison, WI, 1979. (10) Kamat, P. V.; Gopidas, K R; Makhejee, T.;Joshi, V.; Kotkar, D.; Pathak, V. S.; Ghosh, P. K J. Phys. Chem. 1991,95,9-18.
0003-2700/95/0367-1322$9.00/0 0 1995 American Chemical Society
c
PAR 175 Universal Programmer
Nicolet 4094A Digital Oscilloscope I
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Figure 1. Block diagram of computerized system. The Gateway triggers the valves to open and fill the cell, and then it triggers the motion controller to bring the electrodes down into contact with the solution, at which point the cyclic voltammetric experiment is triggered and data collection begins. An abbreviated version of cell A is shown, missing the temperature probe, the N2 purge, and the plexiglass cover.
procedure was repeated twice. M e r the third rinse, the clays were resuspended in deionized water. The clay solutions were dialyzed to remove any excess ions in Spectra/Por (Fisher Scientific) dialysis tubing with a MWCO of 6000-8000. The tubing was prepared for use by boiling in a weak EDTA solution for 2 h, in a weak sodium carbonate solution for 1h, and finally, in deionized water for 1 h. Dialysis was performed until no precipitate was found with 0.2 M &NO3. The resultant clay solutions were lyophilized to remove excess water. A Flexidry lyophilizer (FTS Systems, Inc., Stone Ridge, NY) was used. The final clay contained 9%water. Two matched Pt electrodes (both 7.47 x cm2 as determined from the Cottrell equation for 2 mM Fe(CN)63-) were constructed in-house from 0.040 in. diameter Pt wire (D. F. Goldsmith, Evanston, IL) sealed in 0.750 in. diameter soft glass. Electrical connections were made with a copper pin joint soldered to copper wire. The electrodes were manufactured separately, as it is difficult to spin coat the CME leaving the bare electrode free from clay, as might be envisioned using a interdigitated array. Electrodes were polished using 0.05 pm alumina and copious amounts of water. The electrodes were then rinsed and sonicated to remove any remaining alumina. This procedure was performed prior to the addition of clay for the CME and prior to the beginning of any run for the bare electrode. Spin-coated clay-modified electrodes (SPCMEs) were prepared by applying 1 pL of a 35 g/L clay solution to the electrode. The electrode was inserted into an inverted Pine MSR electrode rotator and spun at 800 rpm for 20 min. An SCE and a Pt wire were used as reference and counter electrodes respectively. All experiments were run at 0.8 to -0.2 V vs SCE at a scan rate of 500 mV/s. Computerized System. A Gateway 2000 386DX computer was used to control the electronic system (Figure l), consisting of an EGG PAR 175 universal programmer and 179 potentiostat,
a labbuilt current-to-voltage converter (tied to a common ground with the PAR instruments), a Keithley vaunton, MA) MSTEP-5 dual-stepper motor control board and DASHRES high resolution 16 bit analog-digital interface board, a STA-Step stepper motor driver and screw terminals accessory board, CACG200 and CDAS 2000 connecting cables, and a Model 23D-6102A 5.0 V dc, 1 A STEP-MOT1 stepper motor. Power was supplied to the STA-Step by a TrippLite (3 A, 12V dc) power supply, and the stepper motor was attached to an X-Y positioner (Deadal, Harrison City, PA). The working electrodes were connected to the X-Y positioner through an Lshaped piece of plexiglass containing two universal Teflon adapters (Kontes, Vineland, MD). Three 3-way and two 2-way normally shut Cole-Parmer (Skokie, IL) PTFE solenoid valves (9830G32 and 01367-71) were used to control solution flow. The valves were triggered by Keithley DASCON-1 and STA-U boards powered by 2 Micronta 12 V power supplies. For the temperature studies, the electrolyte reservoirs were placed in a circulating water bath (Fisher Scientific). Water from the bath jacketed the electrochemical cell. The temperature of the solution inside the cell was monitored with a T-type thermocouple (ColeParmer) attached to a linearized thermocouple input module inserted into a STA-1360 screw module (Keithley) ,which was connected to the computer through the DASHES. Two electrochemical cell configurations were used. The jacketed cell A had a valve controlled exit port at the bottom. Sealing of the cell was accomplished simply by the weight of the plexiglass cover, a loose pressure fit of the plexiglass cover into the cell depth, and a constant N2 outgassing. Gromet fitted holes were drilled in the top for the various electrodes, temperature probes, and gas and solution input. Cell B was a simple glass beaker sealed by way of wing nuts connecting the top cover plexiglass plate to an underlying plate. Holes drilled in the top plate were fitted with gromets to allow for an air-tight seal around Analytical Chemistty, Vol. 67, No. 8, April 15, 1995
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the four electrodes, the Nz port, and the solution input/output port, a curved stem to the bottom of the cell. Solution was input under Nz pressure via manual control of valves and removed by a syringe from the curved stem at the bottom of the cell. In both systems, the fit of the two working electrodes through the cover gromets was facile enough to allow the X-Y positioner to easily move the electrodes vertically above the solution. Multisweep cyclic voltammetry at 500 mV/s over a 1 V potential window resulted in peak current measurements every 4 s. The data (generally 600 continuous CV) were acquired and exported to a temporary file. The peak currents and peak potentials were obtained from each individual scan via curve smoothing (sliding boxcar average) and linear regression baseline extrapolation. Final stored data consisted of peak currents and/ or ratio values as a function of time. Real-time monitoring of the experiment was accomplished with a Nicolet 4094A digital storage oscilloscope. The program for data acquisition was written in Microsoft GWBasic version 3.23 incorporating Keithley machine language programs. Data manipulation and output programs were written in Microsoft QuickBasic version 4.5.
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RESULTS AND DISCUSSION
The utility of the system is best seen in the time dependence of the electrochemically monitored measurement of the porosity of the film. Figure 2a shows the time dependence of a dry clay film exposed to 0.1 M NaC1. The normalized current (ratio) rises smoothly from zero (an insulating film) to about 0.4. The smoothness of the expansion curve is important because it suggests that the clay film does not expand in a stepwise fashion through lower hydrational states as might be anticipated from surface force apparatus That is, either expansion does not proceed stepwise, or the point at which we begin to monitor swelling has already exceeded the regime of stepwise expansion. The rate of increase in the porosity was greater at larger temperatures, being essentially complete withii 5 min at room temperature and still rising at 30 min at 5 "C. The data shown represent 10 averaged trials. In the unaveraged trials, the data obtained at the lower temperatures appear to cluster into groups of three separate curves. This could be indicative of the presence of different energy levels associated with different initial hydration levels within the clay film.Recent theory and experiments suggest that swelling occurs in epitaxial layers of water with large energy barriers between the formation of the different water layers. When the clay starts from the "dry" state in the humid chamber, there are different energy levels associated with the structure of the clay sheets and the structure of the water present in the system. In order to further explore these effects, modifications to control the effects of the initial humidity within the chamber are underway. In addition to studying the temperature dependence of swelling, the temperature-dependentfinal normalized current, ratios, (11) Odom, J. W.; Low, P. F. Clays Clay Miner. 1978,26, 345-351. (12) Viani, B. E.; Low,P. F.; Roth, C. B. J. Colloid Intetface Sci. 1983,96,229244. (13) Pashley, R. M.; Isrealachvili, J. N. J. Colloid Interface Sci. 1984,101, 511523. (14) Israelachvili, J. Acc. Chem. Res. 1987,20, 415-421. (15) Israelachvili, J. N.; McGuiggan, P. M.; Homola, A M. Science 1988,240, 189-191. (16) McGuiggan, P. M.; Isrealachvili, J. N. J Muter. Res. 1990,5, 2232-2243.
1324 Analytical Chemistry, Vol. 67, No. 8, April 15, 1995
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Temperature I * C Figure 2. Spun dry clay exposed to 0.1 M NaCl using cell A. (A) Fe(CN)63-current ratios as a function of time for 5, 15, 25, 35, and 45 "C.Curves represent the averages of 10 replicates. (B)Final ratio obtained at 45 min. Results are averages of 10 replicates.
are now measurable (Figure 2b). Double-layer theory states that the doublelayer distance will vary with FI2,l7a trend not observed in our study. These results suggest that double-layer theory fails to explain clay swelliig adequately consistent with other recent studies.16-22 One unique feature of this system is the ability to perform timedependent studies on clay films with controlled prior electrolyte exposure. If a clay film is first exposed to a high electrolyte concentration (4 M NaCl) and then exposed to a more dilute solution, time-dependent swelling curves similar to those shown in the temperature study are obtained. At room temperature, swelling is essentially complete withii 5 min. On the contrary, if a clay film first exposed to 0.1 M NaCl is subsequently exposed to 4.0 M NaC1, no change in the ability to transport F ~ ( C N ) Gis~ -noted, even up to 45 min (Figure 3a) or 120 min in other experiments. These results conjirm earlier walljet studies on the hysteresis of electrolyte-drivenmontmorillonite clay swelling.6 This suggests that the clay film maintains a memory of its most swollen state. (17) Sposito, G. The Surface Chemisty ofSoiki; Oxford University Press: Oxford, England, 1984. (18) Nonish, K. FUYU~UY SOC.Discuss. 1954,18, 120-134. (19) Nomsh, IC; Rausell-Colom, J. A Clays Clay Miner 1963,10, 123-149. (20) Del Pennino, U.; Mazzega, E.; Valeri, S.; Alietti, A; Brigatti, F.; Poppi, L. J. Colloid Interface Sci. 1981,84, 301-309. (21) Low. P. F. Soil Sci. SOC.Am. J. 1981,45, 1074-1078. (22) Quirk, J. P. Isr. J. Chem. 1968,6, 213-234.
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Figure 4. Fe(CN)63-ratio currents at 45 min exposure as a function of sequential electrolyte exposures for a single clay film using cell B.
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Figure 3. Fe(cN)e3-current ratios as a function of time and previous 0.1 M NaCl electrolyte exposure of 45 min using cell A. (A) Exposure to 4 M NaCI. (e) Exposure to 4 M KCI.
The most stringent test of this hypothesis is an experiment in which a clay film is f i s t swollen in 0.1 M NaCl and then exposed to 1 M KCl. Among the common cations, K+ is most likely to cause collapse of the swollen film. Montmorillonite has a high selectivity for K+ over Na+, and a dry clay film exposed to any concentration of KC1 does not swell beyond 3.2 A interlayer distance.I8 This is thought to be related to the low hydration energy of K+,which renders it susceptible to stripping of its water during the intercalation process. X-ray diffraction studies suggest that the favored configuration of a collapsed K+ clay includes some puckering of the crystal lattice and rotations of the clay sheets to accommodate this puckering.23 Data in Figure 3b suggest that K+ can, indeed, cause structural rearrangements within the clay film; however, the time scale is long (300 min). We conclude that the ratelimiting step for collapse of the clay film is likely to be structural rearrangements as opposed to ion exchange. The steady state response of a single clay film to decreasing and then increasing NaCl concentrations is shown in Figure 4, where hysteresis is pronounced. The increase in the normalized peak currents, R, occurring at 0.4 M NaCl is typical of prior studies in which individual dry claymodified electrodes are exposed to a (23) Bolt, G. M.; Bmggenwert, M. G. M.; Kamphorst, A. Adsorption of Cations by Soil. In Soil Chemisfy,Purf A: Basic Elements; Bolt, G. M., Bmggenwert, M. G. M., Eds.; Elsevier Scientific Publishing Co.: Amsterdam, 1978. (24) Ravina, I.; Low, P. F. Clays Cluy Miner. 1972, 20, 109-123. (25) Lerot, L.; Low, P. F. Cluys Clay Miner. 1976, 24, 191-199.
single salt solution.435 The lack of collapse of the film in response to increasing electrolyte is in contradiction to our first report on films transferred through air from low to high electrolyte solutions4 but consistent with later wall-jet studies.6 The wall-jet studies had shown that the cIay film could be recollapsed if the CME was removed from solution and oven dried and then re-exposed to high salt concentration. Those results were also replicated here, although a complete collapse was not obtained. The explanation for hysteresis in the steady state “swelling curve” is similar to the structural rearrangements discussed for the K experiment.24~25When the experiment begins with the dry clay film, clay platelets are in a collapsed turbostatic state (aligned parallel to the electrode surface with random rotation around the z or stack axis). When the clay film is placed in solution, electrolyte enters the interlayer region between the clay platelets, expanding the clay structure. Lateral (xy plane) rearrangement of the platelets occurs relative to one another in order to best accommodate the interlayer water molecules, resulting in z axis rotation. When the salt content of the solution is next increased, collapse to the original structure is prevented by a lack of lateral transformations driven by incoming water. In summary, a dual electrode computerized system has been designed for use in clay-modified electrode studies. The instrument is designed uniquely for claymodfied temporal studies aimed at elucidating the swelling process of films whose entire electrolyte exposure history can be controlled. The system allows the film to avoid dehydration during electrolyte transfer, additionally prevents the electrolyte transfer system from shearing off or compressing the clay film, and allows data acquisition to begin on contact with a new electrolyte. The utility of the system was demonstrated in temperature and hysteresis studies. Received for review October 24, 1994. Accepted January 24, 1995.@ AC9410427 @
Abstract published in Aduunce ACS Abstracts, March 1, 1995.
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