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Anal. Chem. 1903, 65, 3417-3423
Effects of Carbon Dioxide Fluids on Perfluorinated Ionomer Films: A Study by Voltammetry, DSC, and SEM Eve F. Sullenberger and Adrian C. Michael* Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
The effects of fluid density on the voltammetry of ferrocene in CO2-based fluids have been investigated with microelectrodes coated with Nafion films. Voltammograms have been obtained in water-modified COz over a range of temperatures and pressures spanning the critical point of C02 (2' = 31.4 OC, 1073 psi). Thus, the voltammetry of ferrocene has been examined in both subcritical and supercritical fluids. In order to qualitatively investigate the effect of fluid density on the partition coefficient of ferrocene between the film and fluid, the absorbance due to ferrocene in the fluid was also measured. Each time the fluid density was changed, whether by temperature or pressure programming, the change in absorbance bore an inverse relationship to the change in voltammetric current. This shows the impact of the density-dependent partition coefficient on the voltammetric detection of a fluid solute. Ex situ scanning electron microscopy showed that exposure of Nafion films to high-density fluid caused swelling of the film. Swelling of the film also affected the voltammetry by decreasing the transport rate of ferrocene to the microelectrode. Analysis of Nafion films by DSC after exposure to water-modified C02 showed that the water content of the films was too low to be determined, which confirms that it is the density of the fluid, rather than water content of the film, which causes the observed changes in the voltammetry.
adsorption and interaction of carbon dioxide with polymeric stationary phases has been revealed by trace pulse chromatography'" and by measurements of the swellingof polymeric materials exposed to supercritical CO2.7-9 Even though these investigations provide insight into the processes that affect retention, only a few studies have specifically explored the impact of stationary-phase swelling.6~8J2The amount of work in this area is minimal due to the difficulties in combining analytical techniques within a high-pressure system. A single in situ or on-line analytical method that could provide a more qualitative and quantitative understanding into the effects of COz-based fluids on stationary-phase retention would be ideal. Recently, microelectrodes coated with conductive polymers have been used for voltammetry in modified and unmodified COz.lslQ A microelectrode contained within a polymer film can be used to elucidate the effects of fluid density, including swelling, on the transport and partitioning of a solute. This is useful, since these are the same factors that affect solute retention in chromatography and extraction methods based on carbon dioxide fluids. Polymer-coated microdisk electrodes are useful for characterizing the transport and partitioning of a solute because the shape and amplitude of a voltammogram is a function of the diffusion and concentration of an electroactive compound in both the fluid and the polymer. Diffusion of a fluid-soluble reactant to a polymer-coated microdisk electrode can be regarded as involving a pair of sequential steps: diffusion through the fluid, followed by diffusion through the polymer. Under conditions of steady-state diffusion, the diffusioncontrolled current can be expressed in terms of the diffusion in the fluid and in the polymer in the following manner:
-. l --? +- 1 %at,ea
INTRODUCTION Within the past few years, a number of groups have investigated the interaction of COz-based fluids with polymeric materials as a function of fluid density and composition.'-9 One aim of those studies was to gain a more complete understanding of the physicochemical events that govern solute retention by stationary phases in SFC.10-16 The (1) Yonker, C. R.; Smith, R. D. J. Chromatogr. 1991,550, 775-785. (2) Strubinger, J. R.; Song, H.; Parcher, J. F. Anal. Chem. 1991, 63, 98-103. (3) Strubinger, J. R.; Song, H.; Parcher, J. F. Anal. Chem. 1991, 63, 104-108. (4) Yonker, C. R.; Smith, R. D. J. Chromatogr. 1990, 505, 139-146. (5) Roth, M. J. Phys. Chem. 1990,94,4309-4314. (6) Lochmuller, C. H.; Mink, L. P. J.Chromatogr. 1989,471,357-366. (7) Kamiya, Y.; Hirose, T.; Naito, Y.; Mizoguchi, K. J. Polym. Sci., Polym. Phys. Ed. 1988,26, 159-177. (8) Springston, S. R.;David, P.; Steger, J.; Novotny, M. Anal. Chem. 1986,58,997-1002. (9) Fleming, G. K.; Koros, W. J. Macromolecules 1986,19,2285-2291. (10) Engel, T. M.; Olesik, S. V. Anal. Chem. 1991, 63, 1830-1838. (11) Janseen, H.G.; Schoenmakere,P. J.;Cramers, C. A.J.Chromatogr. 1991.552.527-537. ~ .,.-- - ,- (12) Janseen,-H. G.; Schoenmakers, P. J.; Cramers, C. A. Mikrochim. Acta 1991,2, 337-351. 0003-2700/93/0365-3417$04.00/0
Ef,
(1)
$,ea
where is the total steady-state current, if,= is the contribution from diffusion in the fluid, and i p , is the contribution from diffusionin the polymer. When the current is controlled by diffusion in the fluid, the usual expression for steady-state current a t a microdisk electrodeM-21can be used for if,ea: &fm ' =4nFr$J!*
(2)
(13) Bartle, K. D.; Clifford, A.A.;Jafar, S. A.J. Chem. SOC.,Faraday Trans. 1990,86, 855-860. (14) Luffer, D. R.; Ecknig, W.; Novotny, M. J.Chromatogr. 1990,505, 79-97. (15) Yonker, C. R.; Smith,
R. D. In Supercritical Fluid Extraction and Chromatography: Techniques and Applications; Charpentier, B.
A,,Sevenanta,M.R.,Eds.;ACSSymposiumSeries366;AmericanChemical Society: Washington, DC, 1988; Chapter 9. (16) Sullenberger, E. F.; Michael, A. C. Anal. Chem. 1993,65,23042310. (17) Dressman, S. F.; Garguilo, M. G.; Sullenberger, E. F.; Michael, A. C. J. Am. Chem. SOC.1993,115, 7541-7542. (18) Michael, A. C.; Wightman, R. M. Anal. Chem. 1989,61,270-272. (19) Michael, A. C.; Wightman, R. M. Anal. Chem. 1989, 61, 21932200. (20) Wightman, R. M.; Wipf, D. 0. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; p 15. (21) Michael, A. C.; Wightman, R. M.; Amatore, C. A.J. Electroanal. Chem. 1989,267, 33-45.
0 1993 Amerlcan Chemlcal Society
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 23, DECEMBER 1, 1993
where ro is the radius of the microdisk electrode, Df is the diffusion coefficient of the reactant in the fluid, C* is the concentration of reactant in the bulk fluid and nF has the usual meaning. On the other hand, when the steady-state current is controlled by diffusion of a reactant across the polymer film, the Nernst layer approximation can be used for ip,sa: (3)
where D, is the diffusion coefficient of the reactant in the polymer, K is the partition coefficient of the reactant ( K = C,/Cf, where C, and Cf are the concentration in the polymer and fluid, respectively), and 6 is the thickness of the polymer layer. Equation 3 applies only when diffusion through the polymer is planar and so can only be used when the thickness of the polymer film is much less than the radius of the microdisk electrode. Here, we do not consider the case of radial steady-state diffusion within the polymer film because all the experiments described in this paper involved films that were thin in comparison with the electrode radius. Although eqs 2 and 3 are quantitatively correct under the limiting conditions to which they apply, their combination into eq 1 can only be regarded as semiquantitative. Nevertheless, as discussed below, eqs 1-3 provide a convenient framework for interpretation of the voltammetric results obtained with polymer-coated electrodes in COP-basedfluids. Equations 1-3 are helpful because they provide a catalog of the electrochemical variables that may be influenced by changes in the density of the fluid: C*, Df, D,, K , and 6. If changes in the density and composition of the fluid have little affect on the film, then D, and 6 will remain constant and changes in itot,sscan be related to changes in C*, Df and K . For this reason, we have employed UV-visible spectroscopy to independently inspect the dependence of C* on fluid density. On the other hand, if the polymer film is affected by changes in fluid density, e.g., by swelling, then changes in 6 and D, will also contribute to changes in itot,ss. We have used ex situ scanning electron microscopy to study the swelling of Nafion films by COz-based fluids. Furthermore, as will be shown below, changes in 6 and D, can be distinguished on the basis of the voltammetric behavior under non-steady state diffusion conditions. The purpose of the studies described in this paper was to elucidate how temperature and pressure programming of COZbased fluids influences the voltammetry obtained at microelectrodes coated with thin Nafion films. The electroactive test compound, ferrocene, was chosen for these studies because its electrochemistry is uncomplicated (i.e., few kinetic effects) and it has been previously studied in COz with Nafion-coated microelectrodes.18J9 The temperature and pressure programming used in these studies span a wide range of densities near the critical point of COz, so both subcritical and supercritical fluids have been included. We have found that fluid density alters the voltammetric results in two distinct ways. First, the effect of density on the partition coefficient of ferrocene between the fluid and Nafion film is marked. Second,the Nafion film is swollen by high-density COz,which can be attributed to the unusually high solubility of alkylfluorocarbons in this fluid.22-26 (22) DeSimone, J. M.; Guan, Z.; Elsbernd, C. S. Science 1992, 257, 945-947. (23) Hoefling, T.; Stofesky, D.; Reid, M.; Beckman, E.; Enick, R. M. J . SuDercrit. Fluids 1992. 5. 237-241. (24) Laintz, K. E.; Wai,C.'M.; Yonker, C. R.; Smith, R. D. J . Supercrit. Fluids 1991,4, 194-198. (25) Consani, K. A.; Smith, R. D. J.Supercrit. Fluids 1990,3, 51-65. (26) Iezzi, A.; Bendale, P.; Enick, R. M.; Turberg, M.; Brady, J. Fluid Phase Eqtklib. 1989, 52, 307-317.
EXPERIMENTAL SECTION Reagents. Supercriticalfluid chromatography grade COzwith a dip tube (99.99% purity, Scott Specialty Gas, Plumstead, PA) was used as received. A 0.5 wt % Ndion solution (EW = 1100, proton form) was prepared by dilution of a 5.0 wt % solution (Aldrich Chemical Co., Milwaukee, WI) with isopropyl alcohol dried over 4-A molecular sieves (Fisher Scientific, Pittsburgh, PA). Ferrocene (Aldrich Chemical Co.) was purified by sublimation before use. Ultrapure water (Nanopure) was used for modifying the carbon dioxide-based fluids. High-pressure Microelectrode Probe Fabrication. The microelectrode probe for voltammetry under high pressure is similar to that previously described.16JgA 5-pm-radius platinum disk electrode (Goodfellow, Malvern, PA) and a 26-gauge platinum (Goodfellow)quasi-referenceelectrode (QRE)were heat sealed into the end of a 4-mm-0.d. soft glass tube. The sealed end of the tube was ground flat to expose the pair of coplanar inlaid platinum disks, which were separated by ca. 300 Wm. The assembly was sealed with Torr Seal (Varian Associates,Palo Alto, CA) into a l/le-in. stainless steel NPT fitting (Swagelok) so that the probe could be mounted into the high-pressure vessel described below. Before use, the probe was polished to a mirror finish with 5.0-, LO-,and 0.3-pm alumina (Buehler, Lake Bluff, IL). Nafion films were prepared by depositing 1.0 pL of the 0.5% Nafion solution onto the face of the probe with a syringe. The film was dried at room temperature for 10 min and then at 80 "C for 45 min. High-pressure Cell. The basic design of the high-pressure cell used in this investigation has been previously de~cribed.~eJD The cell bottom and lid, machined from stainless steel, were sealed with six bolts and a neoprene O-ring. The cylindrical cavity in the cell bottom had a total volume of 26 mL. Two quartz optical windows (314-in.diameter and ' 1 Z - h . thickness, Behm Quartz Industries, Dayton, OH) were mounted into opposing sides of the cell bottom and held in place with hollow aluminum bolts and neoprene O-rings. The path length between the quartz windowsis 4.8 cm. Four heating cartridges (Hot Watt, Inc., Danvers, MA) were tapped into the cell bottom and powered by a locally constructed circuit based on a thermocouple (Type K, Omega, Stamford, CT), a temperature controller (Series CN380, Omega),and a continuously operating dc power supply. Four l/16-in. NPT ports were tapped into the cell lid for the electrochemicalprobes, a fluid inlet/outlet, and the thermocouple. Fluid pressure was controlled with a manual syringe pump (High Pressure Equipment,Co., Erie, PA) and monitored witha pressure transducer (305 series, 0-5000 psi, Omega). Safety Considerations. The high-pressure cell is potentially hazardous, especiallywhen fitted with optical windows. Several precautions are used when the cell is pressurized. First, the cell is rated to 10 000 psi, which represents a safety factor of about 10 since our experimentsnever exceed 1300psi. As an additional precaution, a safety head equipped with a 3000 psi rupture disk (HighPressure Equipment) is included in the plumbing between the pump and the cell. While pressurized, the cell is mounted inside a safety box constructed with llz-in.thick Lucite. All valves for sealing or venting the cell are mounted outside the box so that the cell can be vented before the box is opened. All electrical connections are made before the cell is pressurized. Experience shows that quartz windows are likely to fracture under high pressure. Usually,this does not create a problem, but any attempt to view the contents of the cell should always be from outside the safety shield. Voltammetry with Fluid Programming. Ferrocene, ultrapure water, and a stir bar were added to the bottom of the cell before it was sealed. Ferrocene was added by placing 35 pL of a 0.075 M solution in acetonitrile in the cell and allowing the solvent to evaporate. This amount of ferrocene would produce a concentration of 0.1 mM in the cell when completely dissolved. Unless stated otherwise, 40 pL of water was also added to the bottom of the cell. The cell was connected to the COz delivery system and purged with COz three times. The temperature of the cell was monitored and maintained by the controller circuit described above. The pressure was controlled manually with the syringe pump. During fluid programming, the pressure was adjusted by either increasing or decreasing the volume of the
ANALYTICAL CHEMISTRY, VOL. 65, NO. 23, DECEMBER 1, 1993
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m V v s ORE Flgwo 1. Cycilc voltammograms of ferrocene In water-modifled COP: (A) T = 32 O C , p = 850 pal and (B) T = 32 O C , p = 1000 psl; scan rate 1 V I S . syringe pump. Once the cell was stabilized at the desired temperature and pressure, cyclic voltammograms were obtained at scan rates from 0.1 to 100 VIS. Voltammetry was performed with a potentiostat (Model EI400, Ensman Instruments,Blmmington, IN) in the two-electrode mode with both the auxiliary and reference leads connected to the counter electrode. The potentiostat was interfaced to a 80386-based personal computer (Twinhead SS-600/25C)and controlled with software developed in-house. Ultraviolet-Visible Spectroscopy of Ferrocene. Spectra of ferrocene in the fluid were acquired by positioning the highpressureoptical cell in the sample cavity of an IBM 9430 UV-Vis spectrophotometer. The cell was prepared for spectral analysis in the same fashion as described above for the voltammetric experiments. Spectra were collected from 350 to 600 nm at a scan speed of 400 nm/min and a bandwidth of 2.00 nm. The absorbance of ferrocene was recorded at a wavelength of 436 nm. This band is associated with a metal atom d-d transition which is not greatly affected by changes in solvent.ni28 Differential Scanning Calorimetry (DSC) of Nafion Films. DSC traces of Nafion films were obtained with a PerkinElmer DSC7, using extradry nitrogen as the purge and cooling gas. Samples were prepared by casting a 5% Nafion solution directly onto an aluminum sample pan and were air-dried overnight. The Nafion samples weighed between 5 to 6 mg, the sample size needed for DSC. Some samples were spiked with water (0.1 or 0.5 pL) using a syringe. Before analysis, aluminum lids were crimped over the sample pans. A heating rate of 40 OC/min was used over a temperature range of 30-300 OC. Scanning Electron Microscopy (SEM) of Nafion Films. Micrographs of Nafion films were obtained with a JEOL JSM35C scanning electron microscope. Films were prepared by casting 2 pL of a 5 w t 5% Nafion solution onto a polished flat 4-mm heat-sealed glass tubing, which mimics the high-pressure electrochemicalprobe, and then dried in the same manner as the films used in the voltammetric studies. The glass rods containing the polymer films were gold sputtered for 120 s using a Polaron E-5100 sputterer before the micrographs were acquired. Micrographs of the films were obtained with acceleration voltages of 10 and 15 kV and a magnification range of X3000-X5400. Samplesexhibited heating at the higher magnification, limiting the extent of the analysis.
900 11001300 900 1100 1300
Pressure (psi) Flgure 2. Ferrocene current and absorbance In water-modified CO2 at 32 O C : (A) steady-stateanodic (trlangles)and cathodic peak currents (circles)at1 V I Sand (B) absorbanceat 436 nm; filled symbols represent increasing pressure while hollowed symbols represent decreasing pressure.
Voltammetry during Pressure Programming. Voltammogram A in Figure 1shows the voltammetric response of ferrocene obtained in water-modified COZat a temperature of 32 OC and a pressure of 850 psi, i.e., in the gas phase. The interpretation of the shape of the voltammogram has been presented in detail elsewhere.1sJ9~21Briefly, the steady-state oxidativewave demonstrates that the ferrocenediffusionlayer
extends beyond the Nafion film into the bathing fluid. This conclusion is based on the fact that the dimension of the steady-state diffusion layer at a microdisk electrode exceeds the radius of the microdisk itself ( 1 = loro,where 1 is the size of the diffusion layer),18J9p21while the dimension of the films used in this work is less than the radius of the electrodes as revealed by electron microscopy (6 r&3). On the other hand, the peaked reductive wave arises because the ferrocinium cation, being insoluble in the fluid, can diffuse only within the film. Somewhat surprisingly,the electrochemical behavior shown in Figure lA, which was obtained under subcritical conditions (32 "C, 850 psi), is essentially identical to that observed under supercritical conditions a t 80 OC and 1300 psi.'8J@ Even a t these two widely separated points on the C02 phase diagram, the density of the fluid is quite similar (0.16 vs 0.15 g/mL),m which implies that density is a key variable in determining the voltammetric response obtained with polymer-coated microelectrodes in COz-based fluids. Figure 2A shows how the steady-state limiting current for ferrocene oxidation changed as the pressure was increased from 850 to 1300 psi a t a temperature of 32 OC (closed symbols). Throughout this experiment a scan rate of 1V/s was used and all the voltammograms exhibited steady-state oxidative waves, analogous to those in Figure 1. As the pressure was increased, both the anodic and cathodic currents remained stable until ca. 1000 psi and then decreased dramatically until no current could be observed at pressures = 1073 psi). greater than the critical pressure of C02 Voltammogram B in Figure 1, which was obtained a t 32 "C and lo00 psi, shows that the decrease in current during pressurization was not accompanied by ohmic distortion of the voltammogram. Neither the half-wave potential nor the slope of the oxidative wave changed significantly during pressurization. At 32 "C and 1300 psi the voltammogram of ferrocene had completely disappeared, whereas the oxidation and reduction of water as well as a stripping peak due to oxides on the platinum working electrode could still be observed. So, the loss of the ferrocene voltammogram a t high pressure is not caused by a loss of film conductivity. When the pressure was decreased from 1300psi back toward the initial value of 850 psi, voltammograms of ferrocene reappeared a t ca. 1000 psi (Figure 2A, open symbols). At each pressure, however, the current amplitude observed during depressurization was less than that observed during pressurization. These two observations, the loss of the ferrocene voltammogram a t high pressures and the difference
(27)h t r o n g , A. T.;Smith,F.; Elder, E.; McGlynn, S. P. J. Chem. Phys. 1967,46, 4321-4328. (28)Scott, D. R.;Becker, R. S. J. Chem. Phya. 1961,35, 516-531.
(29)International Thermodynamic Tables oftheFluid State, Carbon Dioxide; Angus, S., Armstrong, B., dehuck, K. M., EMS.; Pergamon Press: New York, 1976;p 3.
RESULTS AND DISCUSSION
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 23, DECEMBER 1, 1993
between the current during pressurization and depressurization, will next be discussed in turn. Density-Dependent Partitioningof Ferrocene. All the voltammetric data in Figures 1 and 2 were obtained under conditions that gave rise to a steady-state voltammogram for the oxidation of ferrocene. Hence, eqs 1-3 can be used when the oxidative part of the voltammograms is being considered. According to these equations, several parameters (C*, K , Df, D,, 6) could explain the loss of the ferrocene oxidation current at high fluid pressures. To narrow down the number of possible parameters and to determine whether the decrease in current amplitude is related to a decrease in fluid concentration, C*, ferrocene absorbance was measured as a function of pressure at 32 "C. Figure 2B shows that, in contrast to the voltammetric current in Figure 2A, the absorbance increased with pressure (closed symbols). The absorptivity of ferrocene is reported to be insensitive to the nature of the s o l ~ e n t . ~Assuming ~ ~ ~ * that the absorptivity of ferrocene is also insensitive to fluid pressure, these data show that the fluid concentration of ferrocene increases with pressure. So, the decrease in current cannot be attributed to a decrease in C*. The inverse relationship between absorbance and current can, however, be rationalized by considering the partition coefficient (K) which, based on SFC theory, is expected to depend on pressure. As the pressure of the COz increases so does its solvent ~trength,3C-3~ resulting in extraction of ferrocenefrom the film to a sufficient extent that voltammetric detection at the electrode is prevented at high pressures. Evidence that the extraction is complete a t 1300 psi and 32 "C can be obtained by noting that the ferrocene concentration derived from the absorbance agrees to within 10% with the amount of ferrocene placed in the cell (1X lo-' M). This shows that the amount of ferrocene used is completely soluble in the fluid at 1300 psi and 32 OC. The calculation is based on a ferrocene molar absorptivity of 100 M-l cm-1.27,28 If this interpretation is correct, Le., that the loss of the ferrocene voltammogram at elevated pressures is due to a decrease in the partition coefficient, then the inverse relationship between absorbance and voltammetric current will be a function primwily of fluid d e n ~ i t y l and ~ J ~will ~ ~arise during any density change, whether caused by temperature or pressure programming. This is true because the solvent strength of a supercritical fluid is, to a first approximation a t least, dependent on density.29-32 Figure 3 shows that the inverse relationship between current and absorbance also occurs when the pressure is varied at 50 O C . At this temperature, however, the effect of pressure on current and absorbance is much less dramatic than at 32 "C (Figure 2). This is to be expected because the density of a fluid exhibits less pressure dependence as temperature is elevated above the critical point temperat~re.2~ Also, when Figure 3A is compared to Figure 2A, ferrocene voltammetry is still observed at 50 "C and 1300 psi even though no voltammogram was observed at this same pressure at 32 "C. Furthermore, Figures 3B and 2B show that a t 1300 psi the absorbance of ferrocene at 50 OC is about half that a t 32 "C. So, extraction of ferrocene from Nafion is not complete at this elevated temperature. Figure 4 shows that the inverse relationship between current and absorbance also occurs during an isobaric experiment at 1300 psi in which the temperature is decreased from 80 to 32 (30) Lira, C. T. In Supercritical Fluid Extraction and Chromatography: Techniquesand Applications; Charpentier, B. A., Sevenants, M. R., Eds.; ACS Symposium Series 366; American Chemical Society: Washington, DC, 1988; Chapter 1. (31) Smith, R. D.; Wright, B. W.; Yonker, C. R. Anal. Chem. 1988,60, 1323A-1336A. (32) Giddings, J. C.; Myers, M. N.; McLaren, L.; Keller, R. A. Science 1968, 162, 67-73.
B
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ot 900 1100 1300 900 1100 1300
Pressure (psi) Figure 3. Ferrocene current and absorbance in water-modified COz at 50 "C: (A)steady-stateanodic (triang1es)andcathodic peak currents (circies)at 1 V/sand(B)absorbanceat 436 nm; filledsymbols represent increasingpressure and hollow symbols represent decreasing pressure.
1
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50
70
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,
,
,
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40
50
60
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Temperature ( C ) Figure 4. Voltammetry of ferrocene in water-modified Con at 1300 psi from 80 to 32 "C: (A) steady-state anodic (triangles) and cathodic peak currents (circles) at a scan rate of 1 V I Sand (B) absorbance at 436 nm.
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3 e n s i t y (g/rnL) Figure 5. Amplitude of the steady-state anodic current at 1 V/s vs COz fluid density. Data from Figure 2 (triangles) and Figure 3 (circles) with filled symbols representing increasing pressure and hollowed symbols representingdecreasing pressure;data from Figure 4 (squares) representing decreasing temperature.
"C. Since density decreases with increasing temperature at a given pressure, the results of Figures 3 and 4 further substantiate that the fluid density influences the voltammetric current by affecting the partition coefficient of ferrocene. To summarize the impact of fluid density on the voltammetry of ferrocene, the steady-state anodic current from Figures 2A, 3A, and 4A have been replotted in Figure 5 against density values obtained from the IUPAC equation of state for pure C02.29 (Equations of state for water-saturated COZ do not seem to exist, but since the solubility of water in C02 is low, the effect of water on the fluid density can be presumed negligible for the purposes of this discussion.) There is
ANALYTICAL CHEMISTRY, VOL. 85, NO. 23, DECEMBER 1, 1993
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m V v s QRE Flguro 6. Cycllc voltammograms of ferrocene In modified C02: T = 32 "C and p = 850 psi (A) before and (B) after the pressure cycle; scan rate 25 VIS. considerable spread in the plot because data from different experiments with different Nafion films have been combined. Nevertheless, all the data exhibit a similar relationship between current and fluid density, which bears a striking similarity to the relationship between partition coefficient and fluid density usually noted in chromatography.12,33," In SFC, for example, the dependence of the capacity factor, k', on density often takes the form
lnk'=a-bp where a and b are empirical constants. Since capacity factor is directly related to partition coefficient, the same relationship also applies to partitioning.12 Ferrocene Voltammetry and Absorbance during Depressurization. Although the decrease in ferrocene current upon pressurization of the fluid can largely be attributed to the effect of fluid density on the partition coefficient of ferrocene, this effect alone cannot explain all the features of the voltammetric data. Both Figure 2A and Figure 3A show that the ferrocene current observed at each pressure during depressurization (open symbols) from 1300 to 850 psi was significantly less than that observed during pressurization (closed symbols) from 850 to 1300 psi. This cannot be explained by the partition coefficient alone, since the fluid density and ferrocene absorbance a t any given pressure is the same during pressurization and depressurization (Figures 2B and 3B). Thus, these data reveal that the exposure of the Ndion film to high-density C02 alters the functional properties of the film itself. In the next sections, experiments aimed a t elucidating the nature of the effects of high-density C02 on the Ndion films are discussed. Effect of Fluid Density on Nafion Films by Voltammetric Methods. The voltammetry obtained at a microelectrode allows mass transport rates to be investigated, a t least in a qualitative way, without knowledge of the concentration of the electroreactant. This is due to the nature of the diffusion process a t microelectrodes, which gives rise to voltammograms of different shapes depending upon the relative dimensions of the electrode and the diffusion layer.lsJgt21 If the mass transport rate is fast (i.e., when r,[nFu/DRlrJ1'2 I1)then radial or steady-state diffusion will be observed, but if the mass transport rate is slow (Le., when r [ n F v / D R n 2 1) then planar diffusion will be observed.20.21 We have used this capability to investigate the effect of exposure of Nafion-coated electrodes to high-density COz on the transport rate of ferrocene. Figure 6 compares voltammograms collected at 25 V/s at (33) Villermet, A.; Thiebaut,D.;Caude, M.; Roaset, R. J.Chrornatogr. 1991, 667, 86-97. (34) Hutz, A.; Schimtz,F. P.;Leyendecker,D.; Kleaper, E. J.Supercrit. Fluids 1990,3, 1-7.
I
, 0
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1
2
1
,
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2
Log v Flgure 7. Amplitude of the cathodic peak current vs scan rate In water-modified COP: T = 32 OC, p = 850 psi; (A) before and (B) after the pressure cycle.
32 "Cand 850 psi, before (A) and after (B) the pressure cycle depicted in Figure 2. The oxidative wave of the voltammogram in Figure 6A shows a predominantly steady-state or sigmoidal response for the oxidation of ferrocene, while that in Figure 6B shows a predominantly peak-shaped response. Since both voltammograms were obtained at the same scan rate, this change in the shape of the voltammogram indicates that the pressure cycle causes a decrease in the transport rate of ferrocene to the electrode. Figure 7 shows the scan rate dependence of the peak amplitude of the reductive part of the voltammogram before (A) and after (B) the pressure cycle. Before thegressme cycle the cathodic peak amplitude initially increased with scan rate and then decreased (Figure 7A), whereas after the pressure cycle it remained the same or decreased at all scan rates (Figure 7B). The change in the behavior of the cathodic peak after the pressure cycle is also consistent with a decrease in the transport rate.lg Inspection of eq 3, however,showsthat the slower transport rate observed after exposure of the film to high-density COz could have two possible origins: a decrease in the diffusion coefficient, D,, of ferrocene in the film or an increase in the film thickness, 6. Swelling of Nafion Films by High-Density COZ. Swelling of the Nafion film by the CO2-based fluid is one possible mechanism by which the physical properties and thickness of the film could be altered. Since it is difficult with our high-pressure electrochemical system to obtain a direct in situ measurement of film thickness and swelling, we have turned to an ex situ analysis using scanning electron microscopy (SEM) to examine the films before and after exposure to both low- and high-density COz-based fluids. Figure 8 shows SEM micrographs of Nafion films after exposure to water-modified C02 a t 32 "C and 850 psi (top) and at 32 "C and 1300 psi (bottom). After exposure to lowdensity COZ,the films had an extremely smooth and featureless surface and were indistinguishable from films that had not been exposed to COZ. After exposure to high-density COz, the films no longer had a smooth surface. Rather, many protrusions varying in diameter from 1to 10 pm can be seen in the SEM. Similar surface features were observed when the films were exposed dry COZat these same temperatures and pressures, showingthat the phenomenon does not depend on the addition of water to the fluid. It is important t o note that the films have been exposed to the vacuum of the sputtering chamber (10-2 Torr) before analysis and of the microscope (10-6 Torr) during the analysis. The ability to observe these features even after applying a vacuum to the film suggests that the effect of C02 on Nafion films is likely to be even more robust than revealed by SEM. The humps that appear on the surface of the film after exposure to highdensity COz confirm that swelling of the film contributes to
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 23, DECEMBER 1, 1993
50
100 150 200 250 300
Temperature (C) Flguuro 8. DSC traces of NaRon films: (A) without added water, (B) spiked wkh 0.1 p l of water, (C) spiked with 0.5 pL of water. and (0) after exposure to highdensity watermodiRed CO, (T = 32 O C . p = 1300 psi).
Flpure 8.
SEM micrographs of Nafion films exposed to watermodified
C02: (lop) T = 32 O C and 850 psi and (bottom) T = 32 O C and 1300 psi for 30 min.
the difference in the voltammetric currents observed during the pressurization and depressurization of the fluid. Theswellingof Nafion filmisconsistent with recent reports on the unusually high solubility of perfluorinated alkanes in dense COz.2~z6However, we have never observed complete dissolution of the film by high-density COz-basedfluids. The SEMs, for example, show that the films are intact following exposure to COz. The outright dissolution of Nafion films in high-density COz is unlikely because of the high molecular weight of the polymer which entropically inhibits dissolut i ~ n ~ ~and . " because the ionic sulfonate groups cannot be solvated by COz. Even though the film does not dissolve in COz, the interaction of high-density COz with the fluorinated hackhone of the Nafiion film is sufficient to swell the film. Once the Nafiion film is swollen hy COz, the diffusion coefficientof ferrocenein the film (LIP)is expected to increase due to the decrease in polymer density. For example, when thick films of poly(ethy1eneoxide) (6 > 50 pm) were exposed to high-density COz, the swellingdid in fact cause an increase inthetransportrateofferrocenewhichwasevidentinachange in the shape of oxidative wave.16 The results of Figures 6 and I, however, indicate slower transport of ferrocene to the micrcdisk electrode following exposure of these thin films to high-densityCOZ.Therefore, the dominant factor givingrise to the reduced transport rate of ferrocene to the electrode is the increase in film thickness ( 6 ) induced hy swelling. (35) Barton, A. F. M.CRCHondbaok ofPolymerLipuidlnteroetion Parnmetem and Solubility Pornmetem; CRC Press, Inc.: Boea Raton, FI.. 1..M . .... (36)Flory, P. J. Princplea ofPolymer Chemistry; Cornell University Press: Ithara, NY, 1953; Chapters 12,13.
_.
Effect of Modifier on Voltammetry. To this point, the discussionhasfmusedentirely on the interaction offerrocene and COz with the Nafion film. The impact of the other component of the fluid, the water modifier, must also be considered. Ferrocene is highly water insoluble,so any change in the water content of the film could also contribute to the change in the partition coefficient of ferrocene. To address this issue, the water content of the Nafion films before and after exposure to high-density COz has been assessed ex situ with DSC. Figure 9 shows DSC traces for Nafiion films that have been treated in different ways. Trace A is for a Nafiion film cast onto a DSC sample pan and dried overnight under ambient laboratory conditions. The trace contains small endotherms near 100 and 225 OC, attributable to the water content of the film and the film itself, respectively." Traces B and C in Figure 9 were obtained from Nafiion samples spiked with 0.1 and 0.5 pL of ultrapure water, respectively. As the water content in the film increased,the integrated amplitude of the endotherm near 100 OC increased, showing that DSC can be used to detect ca. 2 w t % water in the Nafion fdm. The 0.1 fiL of water added to the film in trace B corresponds to 0.5 9% of the modifier used in the voltammetric experiments in COz. Trace D in Figure 9 was obtained from a Nafion sample that had been exposed to high-density water-modified fluid. The DSC sample pan containing the Nafiion film was placed in the high-pressure cell with 40 pL of water and pressurized to 1300 psi at 32 "C for 15 min. Finally, the cell was vented and the DSC sample pan was removed and sealed as quickly as possible. The trace shows virtually no evidence of bulk water in the Nafion sample despite the sensitivity of the DSC method demonstrated by traces A-C. A DSC trace obtained after the film was exposed to water-modified COzat only low fluid density (32 "C, 850 psi) was not significantly different from the trace obtained before exposure to COz (Figure 9A). The DSC results in Figure 9 show that the Nafion films contain very little water after exposure to water-modified COz. If anything, the films contain less water after exposure to high-density fluid (trace D) than before (trace A). The low apparent water content of Nafion films following exposure to high-densityfluid is in direct contrast tothe reaultsobtained on films of poly(ethy1ene oxide) containing lithium triflate (PEOILi). following exposuretohigh-densitywater-modified COz, the DSC trace of PEO/Li contained a very large water endothermat 100°C.'6 Thisnotonlyconfirmsthesuitability of DSC for probing the water content of films hut also shows the distinct behavior of Nafiion films and PEO/Li in water(37)Mwre, R.B., III; Martin.C. R.Mocmmoleculea 1988,21,13341339.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 23, DECEMBER 1, 1993
modified COz. The lack of evidence of bulk water in the Ndion film is coneistent with the hydrophobic nature of its fluorinated backbone. Thus, the effects of fluid density on the voltammetric response (Figures 1,2A, 3A,and 4A) are likely to arise from the effects of the C02 itself, rather than the water, on the Nafion film. This is consistent with the SEM results, which showed that swelling of film does not depend upon the use of water-modified fluids. Voltammetry without the Addition of Water. The amount of water (40 pL) used in the experiments described so far was chosen so that the fluid would be saturated with water a t all densities used.38 Our reasoning was that the fluid would then contain a sufficient amount of water to provide solvationof the sulfonategroups in the film, which is necessary for conductivity. The SEM and DSC results,however, suggest that these films swell in dry COZand that only a very small quantity of water is required for establishing conductivity. This latter conclusion is consistent with the recent work of Huang and Dasgupta,99 which showed that Nafion-coated electrodescan be used to detect ppm levels of water in various liquids. This prompted us to inspect the voltammetry of ferrocene at Nafion-coated electrodesin unmodified COZ. Even though no water was added to the fluid, the SFC grade COZwed is rated to contain less than 3 ppm water by the supplier. At thislow water level, no ferrocene voltammogramwas obtained initially a t 32OC and 850 psi, but after takingthe fluid pressure up to 1300 psi and back down to 860 psi, a voltammogram similar to those in Figure 1 was obtained. This experiment waa performed five times, and the system plumbing was flushed with Nz before each time. Upon changing to a different tank of SFC grade C02, however, only voltammograms with severe ohmic distortion could be obtained after exposure to dense COZ. Upon changing to SFE grade COZ, which is rated to contain less than 0.1 ppm water, no voltammogram could be obtained a t all. These results confiim that water is required in order to establish conductivity in Ndion films,but they also show that low levels of water are sufficient. These preliminaryreaults in unmodified SFCgrade
3423
COz show that voltammetry can be performed in fluids that are suitable for SFE and SFC, but that careful control of the water content will be required if Nafion films are employed. This is significant given our long-term goal of developing on-line electrochemical detectors for SFE and SFC which consist of electrochemical probes coated with Nafion films.
CONCLUSION The results presented in this paper show how voltammetry with a polymer-coated microelectrode can be used to investigate the effects of fluid density on the transport and partitioning of a solute as well as on the swellingof the polymer film. This type of information is vital not only to the practitioners of electrochemistry in unconventional media, but also to the practitioners of separation methods based on supercritical fluids. The interpretation of the results presented so far, however, has been qualitative rather than quantitative. There are several reasons for this. First, the theory of diffusion a t polymer-coated microdisk electrodes is not well established, as discussed above. Second, because these experiments have been carried out in a static fluid, accurate subtraction of background currents has not been possible. Finally, although ohmic distortion of the voltammograms a t moderate scan rates is not extremely severe, ohmic distortion was evident at high scan rates, making it difficult to straightforwardly compare data and theory. Further developments in the theory of polymer-coated microdisk electrodesand the incorporationof slightly more sophisticated apparatus (i.e., a flow system)than has been used so far should allow these difficulties to be surmounted and, subsequently, permit quantitative data treatment. ACKNOWLEDGMENT Acknowledgment is made to the University of Pittsburgh and the donors of The Petroleum Research Fund, administered by the ACS, for support of this research.
RECEIVED for review June
14, 1993. Accepted September
8,1993.'
~
(38) Chrastil, J. J. Phys. Chem. 1982,86,30163021. (39) Huang, H.;Daaguptu, P. K. AM^. Chem. 1992,64, 2406-2412.
* Abstract published in Advance ACS Abstracts, October 16, 1993.
