J . Phys. Chem. 1992, 96, 3556-3560
3556
Donnan Phenomena in the Proton Doping of Emeraldhe Pierre Chartier, Laboratoire d’Electrochimie et de Chimie-Physique du Corps Solide. Universite Louis Pasteur, URA au CNRS No. 405, 4 Rue BIaise Pascal, 67000 Strasbourg, France
Benjamin Mattes, and Howard Reiss* Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90024 (Received: October 4, 1991; In Final Form: December 19, 1991)
The effect of NaCl on proton uptake by emeraldine base from HCI solutions was investigated by means of conductivity measurements of emeraldine films which were immersed in HCI + NaCl solutions at various pH’s and concentrations. A theory, based on the thermodynamics of phase equilibria in the absence and in the presence of NaCI, leads to the right order of magnitude of the observed effects and supports the existence of a modified Donnan phenomenon.
Introduction
The absorption of protons by emeraldine base has been a fairly well-studied phenomen~n.’-~Absorbed protons are responsible for converting the polymer into an excellent electronic conductor. Indeed the emeraldine form of polyaniline, when doped, exhibits the highest electronic conductivity of three principal species (oxidation states) of polyaniline. Not surprisingly, there has been considerable inquiry into the physical aspects of the phenomenon.6-’ The protons seem to be absorbed primarily by the imine groups in the polymer (see Figure 1). Doping is frequently carried out by immersion in aqueous HCl, but other strong acids may be used. The chloride ion (anion) follows the proton into the polymer phase to complete the formation of an ion pair, e.g. H+WCl-, the wiggle indicating the polymer chain. A variety of physical studies, including X-ray, magnetic resonance, spectroscopy, etc., show that the polymer undergoes substantial structural modification during the process of doping, including changes in its degree of crystallization. Furthermore, the conductivity exhibits a semiconductor to metal transition as doping is increased. The protons can be removed from the polymer by immersion in aqueous base, e.g. in either NaOH or N H 4 0 H . Early studies8of doping and dedoping cycles indicated an apparent hysteresis. At a given pH on the dedoping leg of the cycle, the polymer retained more protons than at the same pH on the doping leg. Such hysteresis is not surprising in view of the structural changes known to occur during doping (although structural changes can also be thermodynamically reversible), but even if such effects were not operative, the cycle should show hysteresis when dedoping is achieved by the addition of NaOH. In this case the solution in contact with the polymer at a given pH will not have the same composition as the solution at the same pH on the doping leg. It will contain NaCl and possess a higher ionic strength than the corresponding solution (which does not contain NaC1) on the doping leg. In other words, although the pH’s are the same, ( 1 ) MacDiarmid, A. G.; Chiang, J. C.; Richter, A. F.; Epstein, A. Synth. Met. 1987, 18. 285. (2) Huang, W. S.;Humphrey, B. D.; MacDiarmid, A. G. J . Chem. Soc., Faraday Trans. I 1986, 82, 2385. (3) Chiang, J. C.; MacDiarmid, A. G.Synth. Met. 1986, 13, 193. (4) MacDiamid, A. G.; Chiang, J. C.; Halpern, M.; Huang, W. S.; Mu, S . L.; Somasiri, N. L. D.; Wu, W.; Yaniger, S.L. Mol. Cryst. Liq. Cryst. 1985, 121, 173. ( 5 ) MacDiarmid, A. G.;Chiang, J. C.; Richter, A. F.; Epstein, A. J. Synth. Mer. 1987, 18, 285. (6) Ray, A.; Richter, A. F.; MacDiarmid, A. G.; Epstein, A. J. Synth. Met. 1989, 29, E I5 I . (7) Ginder. J . M.; Epstein, A. J.; MacDiarmid, A. J. Synth. Met. 1989, 29, E395. J.
(8) Nechtschein, M.; Genoud, F.; Menardo, C.; Mizoguchi, K.; Travers. 1989, 29, E211.
P.;Zilleret, B. Synth. Met.
the two systems are not thermodynamically identical, and there is no reason why they should exhibit the same uptake of protons. The difference is most easily considered in terms of the potential at the interface between the solution and the polymer phase. If the protons were bound infinitely strongly to the imine groups, we would be dealing with a conventional Donnan membrane phenomenon. However, although the protons are strongly bound, they are still dissociable, and this leads to the consideration of an extremely interesting modified Donnan membrane phenomenon in which the focus is on the stability of thefixed ions themselves, rather than on the selection of counterions and d o n s . As a result we performed experiments and developed a theory dealing with Donnan phenomena in emeraldine. The present paper reports some of the results of these studies. Before describing this work it is convenient to call attention to the work of MacDiarmid, Chiang, Richter, and Epsteid on the uptake of protons by emeraldine. These authors performed careful measurements of both percent absorption (ratio of absorbed protons to emeraldine nitrogens) and conductivity as a function of pH. Inaccuracies in earlier workg were largely eliminated in these measurements, and we have used them in the interpretation of our own measurements. Figure 2 and Figure 3 are plots reproduced from MacDiarmid et al.,5 of both the percent uptake and the conductivity versus pH. Experiment
Emeraldine base (EB) films were prepared by solution casting the EB powder in N-methylpyrrolidinone on glass plates. The EB films were cured at 125 OC for 3 h’O and used for further studies as formed. The density of the EB film was measured by the BET method (N2 gas absorption) as 1.25 g ~ m - ~Uniform . sample sizes were obtained (2 mm X 3 mm) with a precision punch. The thickness of each sample was measured with a precision caliper (0.1 mm). Two replicate samples were processed and analyzed for each doping condition. These films were immersed in solutions without stirring (2 mg of EB film/( 100 mL of solution)) at 25 OC. These solutions contained only HCl at various concentrations or HCI + NaCl at various pH’s but always at a total ionic strength of 1 M. The solutions were 0.01 M HCl, 0.1 M HCI in the absence of NaCI, or 0.01 M HCI + 0.99 M NaC1,O.l M HCI + 0.9 M NaC1. After various immersion times the films were removed from the solutions and allowed to dry, and their room temperature conductivities u were measured. dc conductivity was measured by a four point pressure probe with four parallel Pt wires arranged in the vander Pauw configuration.” (9) Chaing, J. C.; MacDiarmid, A. G.Synth. Met. 1986, 13, 193. (IO) Anderson, M. R.; Mattes, B. R.; Reiss, H.; Kaner, R. B. Science 1991, 252, I4 12.
0022-3654/92/2096-3556$03.00/00 1992 American Chemical Society
Donnan Phenomena in the Proton Doping of Emeraldine
The Journal of Physical Chemistry, Vol. 96, No. 8,1992 3557
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TI Figure 1. Protonation equilibrium of emeraldine base (C24HIsN4r MW = 362) into emeraldine salt (C24H20N4r M W = 364).
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3 4 1 t itours days Figure 4. Effect of NaCl on the protonation of emeraldine base films in aqueous HCI solutions of pH LV 1: (0)0.1 M HCI (pH 1.06); ( 0 )0.1 M HCI 0.9 M NaCl (pH 0.84). Room temperature.
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'
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7. 6.0 5.0 4.0 3.0 2.0 1.0 0.0 -1,O EQUILIBRIUM pH OF AQUEOUS HCL
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Figure 2. Equilibrium doping percentage of emeraldine as a function of the pH of aqueous HCI. Reprinted with permission from ref 5. Copyright 1987 Elsevier Sequoia.
Figure 5. Effect of NaCl on the protonation of emeraldine base films in aqueous HCI solutions of pH = 2: (0)0.01 M HCI (pH 2.05); ( 0 )0.01 M HCI + 0.99 M NaCl (pH 1.9). Room temperature. TABLE 1: Equilibrium Values of tbe Conductivities, u, and Redox Potentials, E , against the Silver Chloride Reference Electrode (AglAC113 M NaCI), of Emeraldine Base Films in Various Electrolytes
4
-I
60
Figure 3. Equilibrium conductivity (room temperature) of emeraldine as a function of the doping percentage. Reprinted with permission from ref 5. Copyright 1987 Elsevier Sequoia.
Ohmic resistance was determined and measured with a Hewlett-Packard 3478A multimeter. It should be noted that conductivity measurements were not made under dynamic vacuum as in the case of ref 5 . After long enough immersions the films appeared to reach an equilibrium level of uptake, but the times required for this were dramatically different for the different solutions. Figure 4 and Figure 5 exhibit the measured dependences of log u upon the time of immersion. Each point pertains to a different sample. It is evident that when NaCl is present in such large exthat the ionic strength is 1 M (Figure 4), proton uptake proceeds at a dramatically higher rate. With HCl at 0.01 M (Figure S ) , u remains lower than 10" S cm-' after an immersion time of 20 h, and even after 3 days, u has only increased to 3 X S cm-I. This explains why (as we found in other experiments) reliable measurements of proton uptake kinetics could not be obtained by measurements of electrode potential in pure HCI at 0.01 M. Three days are required to reach a constant conductivity in 0.01 M HC1 + 0.99 M NaCl solutions but only 2 h in 0.1 M HCI + 0.9 M NaCl. Measurements of u as a function of time were not performed using 1 M HC1, but it is reasonable to assume that in 1 M HCI and, a fortiori, in 10 M HCl, the conductivities obtained after 3 (11) Vander Pauw, L. J. Phillips Tech. Reu. 1958, 20, 220.
electrolyte IO M HCI 1 M HC1 0.1 M HCI 0.01 M HCI 0.001 M HCI 0.1 M HCI 0.9 M NaCl 0.01 M HCI 0.99 M NaCl 0.001 M HC1 0.999 M NaCl
series pH u / ( S cm-I) -1.3 1.0 0.29 1.2 1.06 0.5 2.05 4 X IO4 3.08
