J . Phys. Chem. 1989, 93, 6480-6485
6480
or have an isotropic random tilt. The intensity variations of the diffraction peaks may account for only a slight anisotropic chain tilt of a few degrees. A short-range translational and long-range bond orientational order in accordance with the results of Garoff and co-workersI3 is observed. They explained it with a hexatic headgroup arrangement and a disorder of the chain orientation. However, it should be emphasized that a major characteristic of the hexatic phase is the possibility of short translational correlation length together with a long bond orientational ordering. Therefore, the displacements and point imperfections of the hexatic headgroup lattice itself may cause the ordering characteristics. The long-range bond orientational ordering probably reflects large domains similar to those that are observable through electron microscopy27 and in the microfluorescence (own observation) in fatty acid monolayers (pH 6, pressures above the coexistence pressure). These domains have diameters of several tens to hundreds of micrometers. However, domain-like structures have (to our knowledge) not yet been visualized in the cadmium salt monolayers, neither with electron microscopy nor in the microfluorescence. (Our experiments show that the fluorescence of the probe is quenched.) It should be noticed that all studies up to now show that the translational correlation length is restricted to about 100 ,&, an area of only several hundred molecules. This may be due to preparational imperfections, or as proposed earlier,'9,27the lateral cooperativity of the phase transition and ordering is restricted principally, thus confining the domain size. Suitable experiments are necessary to exclude preparation and
substrate surface conditions as factors that restrict the correlation length. A thermotrope phase transition like disorder of the chains at temperatures well below the main melting transition is observed by a pronounced decrease of the diffraction intensity. The pretransitional melting temperature increases with increasing chain length, and the melting is not reversible within several hours. It is explained by an increasing overall random disordering based on the desorption of single molecules accompanied by the loosening of the lattice structure. The layer reaches a stable amorphous-like state at temperatures significantly higher than the onset of the pretransition. Not affected by this disordering is a fundamental hexatic arrangement, because the hexagonal diffraction pattern is preserved over the entire temperature range. This indicates a decoupled transition for the hydrophilic and hydrophobic parts of the molecule^.'^^^^
Acknowledgment. Helpful discussions with Helmuth Mohwald, Ian Peterson, Roland Steitz, and Kyle Vanderlick are gratefully appreciated. Many thanks are also given to members to the Department of Biophysics, E22, at the Technical University Munich, especially Prof. Erich Sackmann and Wolfgang Frey, where the preparation and electron microscopy work were done. The data analysis was accomplished at AT&T Bell Labs, Princeton, NJ, and was supported by the local Thin Film Group, namely, Jane LeGrange and Wes Townsend. Registry No. Cadmium stearate, 2223-93-0; cadmium arachidate, 14923-81-0; cadmium behenate, 34303-23-6.
Ion Transport in a Polypyrrole-Based Ion-Exchange Polymer Huanyu Mao and Peter G. Pickup* Department of Chemistry, Memorial University of Newfoundland, St. John's, Newfoundland, Canada A1 B 3x7 (Received: January 11, 1989)
Rotating disk voltammetry and ionic conductivity measurements have been used to investigate the transport of I-, CI-, and Fe(CN)$- in poly[ l-methyl-3-(pyrrol-l-ylmethyl)pyridinium](polyMPMP') films. The diffusion coefficient for I- in polyMPMP' is strongly dependent on the solvent. In water Diodide,pl is very high (1.5 X IO-' cm2s-') due to significant swelling of the polymer, while in acetonitrile it is over 2 orders of magnitude lower. PolyMPMP' is significantly more permeable than polypyrrole in water, especially to Fe(CN)6". It is over lo00 times more permeable than the reduced form of polypyrrole. However, in acetonitrile polypyrrole (oxidized) is the more permeable of the two polymers. This difference is explained in terms of the degree of solvation and swelling of the polymers in the two solvents. It is concluded that solvation/swelling is much less significant in acetonitrile.
Introduction Many of the potential applications of electronically conducting polymers',2 require rapid transport of ions through the polymer matrix. There have therefore been many attempts to measure ion transport rates in conducting polymers such as polypyrr~le,'~~ polymethylthiophene," and polyaniline.'e~'9 A number of methods for improving ion transport rates through these materials have been d e v e l ~ p e d . ' ~ . ~ ~ ~ " ~ ~ ~ ~ ~ Despite the widespread effort that has been applied, there is a paucity of satisfactory experimental data for ion diffusion coefficients in conducting polymers. This is largely due to the widespread use of large-amplitude potential step techniques, which have recently been shown to be inappropriate for such measurem e n t ~ . ' ~However, ,'~ a number of more appropriate techniques *To whom correspondence should be addressed
0022-3654/89/2093-6480$01.50/0
have recently been used, and it is to be expected that a sound body of data will be compiled rapidly. These techniques include ac (1) Chandler, G. K.; Pletcher,
D. Spec. Period. Rep. Electrochem. 1985,
10, 117-150.
(2) In Handbook of Conducting Polymers; Skotheim, T. A,, Ed.; Marcel Dekker: New York, 1986. (3) Bull, R. A,; Fan, F.-R. F.; Bard, A. J. J . Electrochem. Soc. 1982, 129, 1009-101 5. (4) Genies, E. M.; Bidan, G.; Diaz, A. F. J . Electroanal. Chem. 1983, 149, 101-1 13. (5) Genies, E. M.; Pernaut, J. M. Synth. Met. 1984/85, 10, 117-129. (6) Burgmayer, P.; Murray, R. W. J . Phys. Chem. 1984,88,2515-2521. (7) Pickup, P. G.; Osteryoung, R. A. J . Electroanal. Chem. 1985, 195, 271-288. (8) Tanguy, J.; Mermilliod, N.; Hoclet, M. J. Electrochem. Soc. 1987, 134, 795-802. (9) Shimidzu. T.; Ohtani, A.; Iyoda, T.; Honda, K. J . Electroanal. Chem. 1987, 224, 123-135.
0 1989 American Chemical Society
Ion Transport in a Polypyrrole-Based Polymer impedance ~pectroscopy,3-~*'~-'~ ionic conductivity measurements,6J6 concentration gradient permeation measurements,6 radiotracer techniques," a small-amplitude current-pulse method,I5 and small-amplitude potential step chronoamperometry.16 An additional technique, rotating disk voltammetry, will be described here. In this paper we present the results of an investigation of ion transport in poly[ 1 -methyl-3-(pyrrol-l-ylmethyl)pyridinium] (polyMPMP', I).23 This conducting polymer has a high con-
Me
I
centration (5.6 M, estimated from density and from ion-exchange capacity23)of permanent anion-exchange sites that make it attractive for electrocatalytic and electroanalytical applications. Initial studies23of the electrochemistry of ascorbate and ferrocyanide at electrodes coated with polyMPMP' indicated that it was significantly more permeable than polypyrrole to anions, another attractive feature for the above-mentioned applications. In this work we have therefore quantified the ion transport properties of polyMPMP' and have investigated the factors responsible for its improved permeability over polypyrrole. We have also investigated the influence of the solvent and the ion charge/size on transport rates. The main technique used has been rotating disk voltammetry, which has been used extensively to investigate mass transport though redox and ion-exchange polym e r and ~ has ~ recently ~ ~ ~been applied to a conducting polymer.32
(10) Osaka, T.; Naoi, K.; Ogano, S.; Nakamura, S. J. Electrochem. SOC. 1987, 134,2096-2102. (11) Schlenoff, J. B.; Chien, J. C. W. J. Am. Chem. Soc. 1987, 109, 6269-6274. (12) Tsai, E. W.; Pajkossy, T.; Rajeshwar, K.; Reynolds, J. R. J . Phys. Chem. 1988, 92,3560-3565. (1 3) Reynolds, J. R.; Sundaresan, N. S.; Pomerantz, M.; Basak, S.; Baker, C. K. J. Electroanal. Chem. 1988,250,355-371. (14) Osaka, T.; Naoi, K.; Ogano, S. J. Electrochem. SOC.1988, 135, 107 1-1 077. (15) Penner, R. M.; Van Dyke, L. S.; Martin, C. R. J. Phys. Chem. 1988, 92,5274-5282. (16) Paulse, C. D.; Pickup, P. G.J . Phys. Chem. 1988,92,7002-7006. (17) Marque, P.;Roncali, J.; Gamier, F. J. Electroanal. Chem. 1987,218, 107-118. (18) Glarum, S. M.; Marshall, J. H. J . Electrochem. SOC.1987, 134, 142-1 47. (19) Rubinstein, I.; Sabatani, E.; Rishpon, J. J . Electrochem. SOC.1987, 134,3078-3083. (20) Fan, F.-R. F.; Bard, A. J. Electrochem. SOC.1986, 133,301-304. (21) Penner, R. M.; Martin, C. R. J . Electrochem. SOC.1986, 133, 310-3 15. (22) Nagasubramanian, G.;Di Stefano, S.; Moacanin, J. J. Phys. Chem. 1986,90,4447-445 1. (23) Mao, H.; Pickup, P. G.J. Electroanal. Chem., in press. (24) Gough, D. A.; Leypoldt, J. K. Anal. Chem. 1979,51, 439. (25) Ikeda, T.; Schmehl, R.; Denisevich, P.; Willman, K.; Murray, R. W. J . Am. Chem. SOC.1982, 104, 2683-2691. (26) Anson, F. C.; Saveant, J.-M.; Shigehara, K. J. Am. Chem. SOC.1983, 105. 1096-1106. (27) Doblhofer, K.; Braun, H.; Lange, R. J . Ekctroanal. Chem. 1986,206, 93-100. (28) Marrese, C. A.; Miyawaki, 0.; Wingard, Jr., L. B. Anal. Chem. 1987, 59,248-252. (29) Doblhofer, K.; Lange, R. J . Electroanal. Chem. 1987,229,239-247. (30) Haas, 0.; Sandmeier, B. J. Phys. Chem. 1987,91,5072-5076.
The Journal of Physical Chemistry, Vol. 93, No. 17, 1989 6481 anodic current ( m A C& 0.5
anodic current (mAcm2)
A
0
. .
0.5-
0
W potential ( V vs SSCE 1
Figure 1. Cyclic voltammograms of 0.3-pm polyMPMP+ films in 0.1 M TEAP/CH,CN (A) and 0.1 M NaClO,(aq) (B). In (B), four successive scans are shown. Scan speed = 100 mV s-I.
We have also obtained supplementary data from ionic conductivity measurements.
Experimental Section Electrochemistry. Electrochemical experiments were carried out in conventional three-compartment glass cells under an argon atmosphere at 23 f 2 "C. A 0.458-cm2 Pt rotating disk electrode sealed in PTFE (Pine Instruments), a Pt wire counter electrode, and a saturated sodium chloride calomel electrode (SSCE) reference electrode were used. All potentials are quoted with respect to the SSCE. A 1.88-cm2 Pt flag electrode was used for the Ipartition coefficient determinations. Chemicals. l-Methy1-3-(pyrrol-l-ylmethyl)pyridiniumtetrafluoroborate (MPMPBF,) was prepared as previously described.23 Tetraethylammonium perchlorate (TEAP, Fluka), tetraethylammonium tetrafluoroborate (TEABF4, Fluka), tetrabutylammonium iodide (TBAI, Eastman), tetrabutylammonium chloride (TBAC1, Eastman), acetonitrile (Fisher, HPLC grade), and other chemicals were used as received. Preparation of PolyMPMP+ Films. MPMPBF, was electropolymerized from 0.05 or 0.1 M acetonitrile solutions containing 0.1 M TEABF, at constant current (0.5 or 0.8 mA cm-2) as previously described.23 A charge density of 0.15 C cm-2 produces a 1-pm-thick polymer film.23 Representative voltammograms for polyMPMP+ in acetonitrile and water are shown in Figure 1. The voltammograms in 0.1 M NaClO,(aq) show deactivation of the film at potentials above +0.8 V. Measurement of Partition Coefficients f o r Iodide. PolyMPMP+ was deposited onto a Pt flag in the usual manner and reduced at 0 V in the preparation solution. It was then rinsed with acetone and immersed in the appropriate I--containing solution for 10 min with stirring. After thorough rinsing of the electrode with water and immersion in distilled water for 5 min, the I- was extracted from the film by immersion in stirred 0.2 (31) Van Koppenhagen, J. E.; Majda, M. J. Electroanal. Chem. 1987,236, 113-138. (32) Rault-Berthelot, J.; Orliac, M.-A.; Simonet, J. Electrochim. Acta 1988,33,811-823.
6482 The Journal of Physical Chemistry, Vol. 93, No. 17, 1989
Mao and Pickup
0
A
I
Anodic
L1’....., to5
0
0
+05
POTENTIAL ( V vs SSCE )
Figure 2. Rotating disk voltammetry of tetrabutylammonium iodide (1 .O mM) in 0.1 M LiC104/CH3CN at naked Pt (A; -) and poIyMPMP-coated Pt electrodes. Film thicknesses: (A) 35 (--), 70 (-), I00 (---), and 140 nm (-, lower); (B) 70 nm. Rotation rates: (A) 2000 rpm; (B) (a) 500, (b) 1000, (c) 2000, (d) 3000,and (e) 4000 rpm. Scan speed = 20 m V s-I.
M aqueous NaC104 for 10 min. The I- content of the extract was determined potentiometrically by using a Ag/AgI electrode.33 The equilibration times used here were shown to be adequate by estimation of the time required for the diffusion layer thickness in the film to equal the film thickness. For the lowest diffusion coefficient (in acetonitrile, see below) and the thickest film, this time was just 10 s. Ionic Conductivity Measurements. The dc method and the conductivity cell have been previously described.I6 Briefly, the polymer is sandwiched between two microscope slides (0.9 mm thick) such that it blocks concentric holes (3.5“ diameter) in the slides. A constant current is passed through the film, and the potential difference across it is measured by using two SSCEs. The solution resistance, measured in the absence of a film, is subtracted from all results. Equipment. A Pine Instruments RDE4 potentiostat/galvanostat was used with a BBC MDL780 X-Y recorder and a Pine Instruments ASR electrode rotator. A Tatung TS-7000 microcomputer was interfaced to the potentiostat and electrode rotator via a Data Translation DT2801 ADC/DAC card and was used to collect and analyze rotating disk electrode data. An Orion Research 601 Digital Ionanalyser was used for potentiometric measurements.
Results Rotating Disk Voltammetry at PolyMPMP-Coated Electrodes. ( i ) Iodide in Acetonitrile. Rotating disk voltammograms for 1.0 mM tetrabutylammonium iodide in acetonitrile at naked and polyMPMP+-coated electrodes are shown in Figure 2. At naked Pt, I- exhibits two oxidation waves in the ratio 2:1 at half-wave potentials ( E l / z )of +0.17 and + O S 3 V, respectively. These correspond to the formation of 13- and Iz, respectively. At the polymer-coated electrodes, the first wave is smaller, a new wave appears at E l l z +0.43 V, and the 13- Iz wave is shifted to ca. +0.8 V (not shown in Figure 2). The height of the first wave depends inversely on the thickness of the polymer film, and the combined height of the first two waves is equal to the height of the IIs- wave at the naked Pt electrode. These results can be interpreted as follows. At potentials lower than +0.3 V, the polymer has a very low electronic conductivity (
