J. Phys. Chem. 1984, 88, 2515-2521 (using the value of the P function from the table of ref 57) and / 7" T/ 1 1 / 2 \
2515
standard table in ref 57. However. this is = 1.0 for V. 1 0.1 V. For such potential range, one gets'with the use of eq"C13
since
Ion Gate Electrodes. Polypyrrole as a Switchable Ion Conductor Membrane Paul Burgmayer and Royce W. Murray* Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 2751 4 (Received: September 22, 1983)
m o l e can be electroplymerizedfrom acetonitrilesolutions onto Au minigrid electrodes or Au-coated polycarbonate microfilters to form membranes with electrically switchable ionic permeability. The ionic permeability is switched by control of the polypyrrole oxidation state with potential applied to the membrane-embeddedAu electrode. As shown by ac impedance and dc resistance and direct analytical permeability measurements, the anionic permeability of oxidized (polycationic) polypyrrole is very high whereas that of reduced (more neutral) polypyrrole is lower by 21000-fold. Oxidized polypyrrole is permselective to chloride vs. potassium ion.
Membranes which act as selective barriers find use in many diverse fields such as industrial processing, biomedical engineering, and space science.' Methods have been developed to fabricate membranes with desired, targeted characteristics such as gas permeability for isotopic separations, permselectivity for ion-selective electrodes and ion-exchange membranes, and small uniform pore size for ultrafiltration applications. However, once a membrane has been fabricated, its characteristics are fixed (Le., static) within a particular environment. For example, a membrane made to separate isotopes of UF, cannot be transformed to separate COz from air. Or, in desalinization, an cation permselective exchange membrane cannot be transformed in situ so as to exhibit anion permselectivity. If dynamic control of useful membrane characteristics, e.g., gas permeability, ion permeability, or molecular size selectivity, could be achieved, not only does optimization of membrane performance become possible but presently unknown applications for membranes might appear. For instance, an ion-exchange membrane with a variable, controllable rate of ionic solute permeation could be used, with a detector in a servosystem, to control the rate of drug release into the human body or the rate of reagent release into an automated analyzer or an industrial process. In a previous paper, we presentedZa new method to dynamically control the ionic permeability of an ion-exchange membrane with an electrical signal. An electroactive,polyionic polymer membrane was grown around a porous metal substrate by electrochemical polymerization of pyrrole onto a gold minigrid electrode. By control of the potential of the porous Au electrode, the electroactive polypyrrole membrane was, upon reduction, converted from a polycationic (anion exchanger) membrane to a more neutral membrane. The porous electrode potential could thus be used (1) Hwang, S.; Kammermeyer, K. "Membranes in Separations: Techniques of Chemistry"; Wiley: New York, 1975; Vol. 111. (2) Burgmayer, P.;Murray, R. W. J. Am. Chem. SOC.1982, 104, 6139.
0022-365418412088-2515$01.50/0
to set the ionic permeability of the membrane to that of a polycationic membrane, a neutral membrane, or any state in between, dynamically and reversibly. This paper presents a more detailed proof of this effect, which we called the "ion gate", including full ac impedance curves of the membranes in the oxidized and reduced forms, analytical permeation studies of chloride ion and KC1 electrolyte, a study of the ion-exchange properties of oxidized polypyrrole, and the effect of polypyrrole electropolymerization parameters on its ionic permeability. Besides unequivocally demonstrating that ion flow is responsible for the ion gate response, the data provide the first quantitative ionic permeability results on polypyrrole, a polymer whose electronic conductivity characteristics have been of considerable i n t e r e ~ t . ~ - ~ Experimental Section Materials. Acetonitrile (Burdick and Jackson), gold minigrid (200 lineslin., Buckbee-Mears Co., Minneapolis, MN) and LiClod, KCl, LiC1, and NaCl (all Baker Reagent) were used as received. Water was triply distilled in this lab. Pyrrole was purified as described previously.6 Tetraethylammonium perchlorate and tetraethylammonium chloride were recrystallized three times before use. Membrane Preparation. Ion gate electrodes with two different areas were prepared: 0.24 cmz for conductivity measurements and 0.64 cm2 for permeation studies. The large area electrodes were used to obtain higher concentrations of C1- and KCl in the permeation studies. The smaller area electrodes, which were easier (3) Wegner, G.Angew. Chem., Znt. Ed. Engl. 1981, 29, 361. (4) Salmon, M.; Diaz, A. F.; Logan, A. J.; Krounbi, M.; Bargon, J. Mol. Cryst. Liq. Cryst. 1982, 83, 265. (5) Bull, R.A,; Fan, F. R.; Bard, A. J. J . Electrochem. SOC.1952, 129, 1009. (6) Burgmayer, P.;Murray, R. W. J. Electroanal. Chem. Interfacial Electrochem. 1983, 147, 339.
0 1984 American Chemical Society
2516 The Journal of Physical Chemistry, Vol. 88, No. 12, 1984
CATIONIC FILM NEUTRAL
II
E \-o :l:9 .v
VS
SCE
i
A
C
Figure 1. (A) Experimental setup for ac impedance measurements with electrochemical control of polypyrrole membrane oxidation state: (a) platinized platinum electrodes; (b) constant-voltage power supply; (c) gold minigrid; (d) polypyrrole film; (e) 1.0 M KCl(aq) solution; (f) constant-current ac circoit, which consists of a frequency generator, lock-in amplifier, and a series resistor (50 kohm or 1 Mohm, depending on membrane impedance) to maintain constant ac current to cell. (B) Microscopic schematic of polypyrrole ion gate membrane permeability to Cl- as a function of membrane oxidation. (C) Experimental setup for electrolyte permeation with electrochemical control of membrane oxidation state. Arrows indicate direction of pumped solution flow. Actual cell volume for each side is 0.24 cm3,within which stirring is accomplished with a magnetic micro stir bar.
to fabricate and more durable, were employed in the measurements of impedance and resistance. For both sizes, a piece of gold minigrid was epoxied (3M 5-min epoxy) between predrilled glass microscope slides, leaving a tab of gold minigrid protruding from one edge, where electrical contact to the gold minigrid tab was established with a wire and conductive silver paint. The polymerization was carried out in a large three-compartment cell at +0.8 V vs. SCE in acetonitrile or at +0.65 V vs. SCE in water with a PAR 173 potentiostat. Unless noted, the polymerizations were in 0.1 M Et,NClO,/acetonitrile. Polymerizations in water were performed according to the procedure in ref 6. The concentration of pyrrole in all solutions was 0.57 M. Polymerization was carried out until the holes in the gold minigrid were closed by the polymer as seen by optical microscopy and then continued for an additional 1 min to help close any remaining pinholes. Typically, the total polymerization time was 6-10 rnin in H 2 0solutions and 15-20 min in acetonitrile solutions. Impedance Measurements. The cell described previously2 was used for both dc and ac membrane impedance experiments; a schematic is shown in Figure 1A. Briefly, in the ac impedance experiment, a constant-amplitude ac current of known frequency was passed across the cell solutions (e) and membrane (c d) by using platinized platinum electrodes (a) placed in the two sides of the cell. The resulting voltage developed across the cell between those electrodes, less than 5 mV in all cases, was detected (f') by a PAR HR-8 lock-in amplifier and resolved into in-phase (real) and 90" out-of-phase (imaginary) components which reflect the resistance and capacitance, respectively, generated by ion movement in the cell. Complex plane curves were obtained from 1.5 H z to 150 kHz, the lock-in amplifier limits, calibrating the lock-in amplifier at each frequency. Voltage readings on the lock-in amplifier were converted to resistance and capacitance by using decade resistance and capacitance boxes. For dc resistance experiments, a constant current generated by a PAR 173 potentiostat/galvanostat was impressed across the cell between the two Pt electrodes. The voltage developed across the membrane was measured with an Orion Ionanalyzer set to
+
Burgmayer and Murray millivolts using two Ag/AgCl reference electrodes placed in the solutions (e) on cither side and equidistant from the membrane. The dc resistances were taken from the linear slopes of impressed current vs. measured potential plots by using this four-electrode arrangement. Measurement of the Ion Gate Effect Using ac Impedance and dc Resistance Measurements. After initial mounting of a 0.24-cmZ membrane, N2-purged 1.0 M KCI electrolyte was pumped through both sides of the cell for 30 rnin to deoxygenate the cell and equilibrate the membrane in the electrolyte. Pumping was continued throughout the conductivity experiments to minimize electrolyte concentration polarization and the effects of O2permeation into the cell through the O-ring seals. The oxidized form of the polypyrrole membrane is air stable, but the reduced form is degraded by oxygen. For an initially mounted, oxidized membrane, measured dc resistance and ac impedance did not change after applying a 0.0-V potential vs. SCE counterelectrode to the Au minigrid with a Hewlett-Packard constant-voltagesource (Model 6281A). The potential of the Au minigrid was then stepped from 0.0 to -0.9 V, a voltage sufficient to drive the polypyrrole reduction while the in-phase impedance at 2 Hz was monitored. When the 2-Hz impedance had leveled off, the ac impedance and dc resistance were again measured with and without a voltage applied to the membrane. Disconnecting the Au minigrid with the membrane in the reduced polypyrrole state usually resulted in a very slow decrease in dc resistance and ac impedance values, though two membranes with dc resistances of 11 kohms were found to be stable. After stepping the Au minigrid potential back to 0.45 V, so as to reoxidize the polypyrrole, the ac impedance and dc resistance were remeasured. Permeation Measurements. The permeation cell was similar to cells developed for permeation measurements through more conventional a schematic is shown in Figure 1C. The membrane separates two pools of electrolyte flowing from separate reserviors by means of a peristaltic pump. By placing an appropriate detector in the effluent of solution from one side of the membrane, we can detect permeation of species from the solution on the other side of the membrane. In one experiment, 1 X M KCl was pumped through the detector side and 1.0 M KCl through the other side of the cell. The detector of KC1 electrolyte permeating through the membrane was a small-volume (0.5 mL) flow-through conductivity cell consisting of two parallel Pt/Pt electrodes (area ca. 12 mm2) sealed in glass and connected to the ac impedance system used above, monitoring the in-phase voltage at 2000 Hz. The cell effluent was passed through a constant-temperature bath (25 "C) before entering the conductivity detector, to minimize temperature fluctuations. The detector was calibrated by pumping sequentially 1.01 X lo-,, 1.05 X and 1.10 X M KCl 1.00 X solutions directly through the conductance cell. In the second type of permeation experiment, 1.0 M KNO, electrolyte was pumped through one side of the cell in Figure 1C (the detector side) and 1.O M KCl through the other side. Now, the permeation is exchange of C1- and NO,- ions through the membrane. This was detected potentiometrically, measuring chloride ion with a small-volume (1 mL) flow-through cell with silver/silver chloride and SCE reference (downstream) electrodes and an Orion Ionanalyzer (Model 601A) set to millivolt output. The detector calibration curve (potential vs. log [Cl-1) was linear above lo-, M with 59 mV/decade slope, curved gently below M to a slope of ca. 50 mV/decade at lo4, and became relatively M insensitive to lower concentrations. Concentrations 1 chloride could be reliably measured. The pump used for permeation and impedance experiments, a Manostat Junior cassette pump, was calibrated volumetrically for flow rates of 3.4 X 10-'-9.3 X lo-, mL/s. Measurement of the Ion Gate Effect by Measuring KCI Electrolyte Permeation. After mounting the initially oxidized (7) Olsen, C. L.;Sokoloski, T. D.; Pagay, S. N.; Michaels, D. Anal. Chem. 1969,41, 865. (8) Little, C. M.; Osterhoudt, H. W. Ion Exch. Membr. 1972, 1, 75.
Polypyrrole as a Switchable Ion Conductor Membrane
ionic
I
The Journal of Physical Chemistry, Vol, 88, No. 12, 1984 2517
ion1
'
t Ime
+E
., - E
.
+E
Figure 2. Idealized macroscopic ionic membrane resistance reponse of an ion gate membrane whose oxidation state is switched between cationic (oxidized) and neutral (reduced) membranes by varying the potential applied to the electrode embedded in the membrane. \
polypyrrole membrane, degassed 1.0 X M KC1 was pumped through the detector side of the cell and degassed 1.0 M KCI through the other side, conductometrically detecting KCl permeation as above. When a stable conductance base line was obtained, the potential of the Au minigrid electrode was stepped to -0.9 V so as to reduce the polypyrrole membrane. A decrease in conductance was seen after ca. 5 min. After the new conductance had stabilized, a reading was taken. As with the resistance measurements, disconnecting the potential applied to the Au minigrid electrode caused a very slow increase in the membrane permeation rate (the conductance increased over a 10-20 min time period) to a value between the 0.0- and -0.9-V levels. Reconnecting the Au minigrid electrode at -0.9 V would restore the previous reduced membrane value. Changing the Au electrode potential to +0.45 V, so as to reoxidize the polypyrrole membrane, increased the measured conductance to almost the initial value (see Results Section). Measurement of the Zon Gate Effect by Measuring Chloride Zon Permeation. Chloride permeation as a function of oxidation state of the polypyrrole membrane was studied in a manner similar to KCl permeation except that chloride was detected potentiometrically as above. The two solutions were similarly 1.O M KCI and 1.0 M K N 0 3 (detector side). Zon-Exchange Properties. The cell used for impedance measurements was used as a potentiometric concentration cell, using the oxidized polypyrrole membrane as the cell divider between two SCE's equidistant from the membrane. The cell potential data are corrected for slight differences (9 mV) in the SCE potentials observed when the same electrolyte concentration was present on both sides of the membrane. In a variant of the concentration cell experiment, current was passed between two Pt electrodes in the two sides of the cell while the potential difference between the two SCE electrodes was monitored. The solutions used in this case were 0.1 M and 1.0 X M KCl.
Results and Discussion The qualitative behavior sought for an ion gate membrane, composed of a porous electrode embedded in and controlling the oxidation state of an electroactive polymer membrane, is shown in Figure 2 and microscopically in Figure 1B. For the reduction of the membrane from a polycationic to a neutral polymer, the ion permeability, such as for chloride, drops to some low value or conversely the ionic resistance of the membrane increases. Upon reoxidation of the membrane to a polycationic form, the initial permeability or resistance is restored. Ideally, the cycle of lowhigh (e.g., on-off) ion permeability can be repeated many times by control of the embedded electrode potential. The resistance or permeability of membranes to ions can be monitored in many ways. Our preliminary paper* was confined to illustrating a response like Figure 2 , showing that the in-phase ac impedance of a polypyrrole membrane, measured at constant frequency (2 Hz), increased and decreased in response to reduction or oxidation of the polypyrrole membrane by the Au minigrid electrode embedded in it. Since polypyrrole is better known as an electronically conducting than as an ion-conducting material, we desired to expand the experimental investigation of the polypyrrole ion gate membrane to more convincingly confirm our interpretation of its behavior. The measurements involve (i) the
i
4
6
8
10
12
14
16
1 '8
Figure 3. Complex impedance plane representation from 150 kHz to 2 H z of polypyrrole (pp) membrane with an embedded 2000 lines/in. gold minigrid, separating two pools of 1.0 M KC1 solution: ( 0 )pp in reduced state. Z, is the in-phase impedance in kohms and Z, is the 90' outof-phase impedance. Inset shows from left to right: oxidized pp, a blank (no pp on minigrid), and reduced pp. (+) is dc resistance of reduced polypyrrole membrane.
full frequency/phase ac impedanceg and the dc resistance of the polypyrrole membrane, in which the ion flow is driven by electrical current, and (ii) direct analytical measurement of the permeability of the membrane to chloride and KC1 electrolyte, where the ion flow is driven by concentration gradients. ac Impedance and de Resistance as a Function of Membrane Oxidation State. As described in the Experimental Section, after degassing the cell solution, the first membrane impedance measurement was of a freshly polymerized, oxidized polypyrrole membrane. The complex impedance curve (left-hand curve, inset Figure 3) is very similar to that of the cell with no polypyrrole deposited on the gold minigrid electrode (blank, middle curve in inset), demonstrating that, in its oxidized state, the membrane offers little resistance to ion flow relative to the ionic resistance of the bulk solution in the cell.lo The dc resistance measurements by the four-electrode method (Experimental Section) confirm this, giving values of 80-90 ohms for a series of oxidized polypyrrole membranes and 72 ohms for a Au minigrid electrode blank (no polypyrrole). The difference provides an estimate of 10-20 ohms for the resistance of the polypyrrole plus Au minigrid membrane to ion flow, for a membrane thickness of ca. 1 X cm. Such a low resistance is typical of ion-exchange membranes where special techniques are necessary to measure Ohmic resistance." After 30 min of reduction by applying -0.9 V to the Au minigrid electrode, the observed complex plane curve is altered radically (Figure 3). The magnitudes of both resistance and capacitance, reflected in the change in 2, and Z,, have increased enormously. The in-phase resistance measured at 2 Hz, for instance, has increased 1000-fold. In addition, the shape of the impedance curve has changed to a single, slightly sunken semicircle indicative of parallel resistance and capacitance components in the membrane equivalent circuit. The semicircle is attributed to a bulk geometric process in which ions moving through the polymer generate the (9) The in-phase impedance 2, for the experimental setup described represents the membrane resistive component while the 90° out-of-phase 2, represents the capacitive component. The vector sum of these two describes a point on a plot of 2,vs. ZR. If one plots the two components as a function of frequency, a shape is generated which can be compared to that of the frequency response of an equivalent circuit (such as a series resistance/capacitance or parallel resistance/capacitance). In this way, models of membrane transport, based on equivalent circuits, can in principle be developed. For more information on the various shapes expected for different equivalent circuits, see ref 12. (10) The complex plane plots (Figure 3 inset) generated by the oxidized and blank electrodes probably represent mainly the resistance of the solution in series with the double-layer capacitance of the two Pt electrodes. The tilting over from the expected shape (vertical line from Z , = R for a series RC) as the frequency is lowered is probably due to the roughness of the Pt electrode. (11) Tiravanti, G. J. Membr. Sci. 1981, 9,229.
2518 The Journal of Physical Chemistry, Vol, 88, No. 12, 1984
resistance and capacitance. If the thickness of the polypyrrole membrane is increased by increasing the time of electropolymerization, both Z , and Z , increase in magnitude, but the shape of the impedance curve remains unchanged. Complex plane semicircles with origins sunken below the Z , = 0 axis have been observed previously12when a distribution of membrane thicknesses results in an overlapping distribution of RC time constants for the process, in this case ions moving through the bulk of the polymer. Polypyrrole films grown on gold minigrids are topologically rough and have a wide range of thicknesses (SEM; vide infra), which is perhaps the origin of the sunken semicircles. The average time constant for the bulk ion motion p r m s (the frequency at which the capacitance (Z,) is a maximum and the resistance equals half of the extrapolated intersection with the Z , axis) is 15 Hz from Figure 3. In the absence of any process(es) slower than the bulk migration of ions through the reduced polypyrrole polymer, extrapolating the semicircle in Figure 3 to 0 Hz should correspond to the dc resistance to ion flow through the membrane. A value of 17.2 kohms results. The dc resistance measured for this particular membrane by using the four-electrode constant-current method, 17.2 kohms, is the same within experimental error. Considering that slow oxidation of the neutral polypyrrole by trace oxygen in the cell solution is a potential interference, the comparison is strikingly good. When the potential applied to the Au minigrid electrode is stepped back to +0.45 V, so as to reoxidize the polypyrrole membrane, the impedance curve after 5 min reverted to the shape of the original oxidized form (Figure 3 inset) except that the impedance curve is tilted somewhat further over on the Z , axis. Measurement of Ion Permeation as a Function of Membrane Oxidation State. The most conclusive evidence that the polypyrrole membrane has a different resistance to ion flow in oxidized and reduced states comes from permeation studies. Two experiments were performed. The first measured the flux of C1- from a 1.0 M KC1 solution, through the membrane, to a 1.0 M K N 0 3 solution; the second measured the flux of KC1 from a 1.0 M KCI solution, through the membrane, to a 1.0 X lo4 M KCl solution. The permeation rate of C1- and of KCl salt through the membrane, respectively, depended in both cases on the oxidation state of the polypyrrole membrane. The measured permeation rates are expressed as the product, DP, the diffusion coefficient of the ion in the membrane times its partition coefficient from the solution into the membrane. Calculation of DP in these experiments involves two assumptions. First, the flux of permeant through the membrane is assumed independent of the pumped solution flow rate through the cell, Le., that a depletion layer', is not built up in the bathing solutions next to the membrane. Second, the ion motion is assumed not be affected by an electric potential inside the cell. The second assumption was checked for both C1- and KCl permeants by placing a freshly polymerized membrane in the cell and monitoring the C1- and KCI permeation rates with detectors as in the Experimental Section as a function of voltage applied to the Au electrode embedded in the membrane. Voltages of 0.0 V and then +0.4 V (which do not cause a change in the polymer redox state) were applied while the permeation rates were monitored. No change in the concentrations of C1- or of KC1 moving through the membrane to the detector side of the cell was seen beyond initial instabilities. In addition, the permeation rates for both C1and KCl during the actual experiments did not linearly depend on potential but instead followed the same S-shaped curve in the region 90.4 to -0.9 V we described before: indicating that potential gradients associated with the Au minigrid electrode have a negligible effect on our permeation measurements. (12) Buck, R. P. "Third Symposium on Ion-Selective Electrodes, 1980, Matrafured, Hungary"; Publishing House of the Hungarian Academy of Sciences: 1981; p 27. (b) Buck, R. P.; Ion-Sel. Electrode Rev. 1982, 4 , 3 . (13) This assumption was only approximate in the C1- permeation studies, where the flux of C1- was relatively high. This was a result of our cell design where the top portion of the cell was designed for use with the 0.24-cm2 electrodes which did not give high enough fluxes.
Burgmayer and Murray TABLE I: Ion Flow Driven by Concentration Gradients" for the Cell 1.0 M KCI/IG/l.O M KNO, reoxidoxidized reduced ized 3.4 9.3 3.4 3.4 cell flow rate (x103), mL/s analysis for [Cl-] 1.65 1.33
