J. Phys. Chem. 1987, 91, 5813-5818 tributions to conductivity from cations in high-energy sites. The random site model of nearly equivalent cation sites probably gives a better description of the behavior of the majority of Rb and Cs glasses studied in this work. Thus, it appears that there is not a uniform picture for the cationsite interaction, for all five alkali metal borate systems. The distribution of the anionic charge over the glass network, and thus the site charge density, is the important factor determining such cationsite interactions. It is obvious that the site charge density qA is a function of the various borate groups present at each composition x. The compositional dependence of such borate groups is usually taken as being independent of the ~ a t i o n .However, ~ this investigation has shown pronounced differences, between the various cations, in the compositional dependence of their cation-motion bands. A better understanding of such differences requires a more detailed knowledge of the possible preference of each cation to specific boron-oxygen arrangements which in fact dictates the distribution of the anionic charge. Such useful information can be obtained from a comparative Raman and mid-infrared study of the five alkali metal borate systems. This work is in progress and results will be published later.
Conclusions Binary alkali metal borate glasses are of principal interest in view of their importance for fast ionic conducting glass systems. Understanding ionic transport phenomena in such glasses requires a good knowledge of the interaction forces between the charge carrying cations and the borate lattice. Far-infrared spectroscopy provides a direct probe of.these cation-network interactions and their compositional dependence. We report in this study far-infrared spectra of glasses covering all five alkali metal borate glass-forming regions. The cation-motion bands have been observed and analyzed to provide better understanding of their compositional and cation
5813
dependence. Band deconvolution, preformed on all measured spectra, revealed the existence of two kinds of distributions of cation sites in Li, Na, and K glasses. This was also observed for Rb and Cs glasses of high alkali metal oxide content (x > 0.25). However, for lower x values, the latter glasses appeared to be well described by one distribution of cation sites. The squares of the frequencies of cation vibrations, located in high and low potential energy sites, were found to vary linearly with composition, with a characteristic change in slope a t x N 0 . 2 0 . 2 5 . This kink was mostly pronounced for Li, Na, and K glasses and was observed to disappear for Cs glasses. The origin of this effect was attributed to a number of combined structural 0.20. Such changes in the borate network occurring at x changes result in an abrupt increase of the anionic charge density, which in turn is reflected by the compositional dependence of the cation-motion frequency squared. The relationship to the borate anomaly was also discussed. The results of this far-IR study, concerning the kinds of distributions of cation sites, were also considered in terms of the existing models for ionic transport in fast ionic conducting glasses. It was demonstrated that it is difficult to use a single model to describe the behavior of cations in all five alkali metal borate glass systems. ,
Acknowledgment. We express our appreciation to Professor C. A. Nicolaides, of N H R F , for support and encouragement throughout this work. Helpful discussions with Professor P. J. Bray are also gratefully acknowledged. Professor P. P. Schmidt is thanked for providing reprints of his work on far-IR studies of the solvation of alkali metal cations. G.D.C. thanks Professor C. A. Nicolaides and Professor W. M. Risen for making his collaboration possible. This project has been financially supported by NHRF. Registry No. Cs20, 20281-00-9; Rb20, 18088-1 1-4.
Influence of Oxldation State, pH, and Counterion on the Conductlvity of Polyaniline Walter W. Focke,l Gary E. Wnek,*2 and Yen Wei3 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 021 39 (Received: February 9, 1987; In Final Form: May 7, 1987)
The resistivity of electrochemicallysynthesized polyaniline films was measured with the films submerged in electrolyte. The resistivity was found to depend on the redox state of the film, the pH of the solution and, to a lesser extent, on the type of anion present. The resistivity at a given pH is low but only inside a narrow potential window. The width of this window decreases with increasing pH and vanishes at pH 6. The walls of the potential window correlate roughly with the formal potentials of the redox processes as determined by cyclic voltammetry. It has been shown that the resistivity of polyaniline depends on its moisture content. For the wet polymer a small degree of protonation is apparently sufficient to cause a decrease in resistivity of more than 6 orders of magnitude. This behavior may be rationalized by assuming that, in the presence of water, the charge transport mechanism involves proton exchange reactions as well as intermolecular electron transport.
Introduction Polyaniline (PAn) is the electroactive polymer obtained by the chemical4or electrochemical5oxidation of aniline in acidic aqueous media. It is now well established that the structure of PAn, in the base form,is that of a para-linked phenyleneaminimine!.6 As (1) Present address: NIMR, CSIR, Box 395, Pretoria OOO1, South Africa. (2) To whom cOrreSpOndence and requests for reprints should be addressed at the Department of Chemistry, Rensselaer Polytechnic Institute, Troy, N Y 12180-3590. (3) Present address: Department of Chemistry, Drexel University, Philadelphia, PA 19104. (4) (a) Willstfitter, R.; Dorogi, S. Chem. Ber. 1909, 42, 2143. (b) Ibid. 1909,42,4118. (c) Green, A. G.;'Wocdhead, A. E. J. Chem. SOC.1910, 97, 2388. (d) Ibid. 1912, 101, 1117. (5) Diaz, A. F.;Logan, J. A. J . Electroanal. Chem. 1980, I I I, 1 1 1.
such the polymer can exist in various oxidation states characterized by the ratio of imine to amine nitrogens. The conductive form of polyaniline is associated with intermediate oxidation states, e.g., emeraldine which features an equal number of imine and amine nitrogens in the free-base form.7 In the neutralized free-base form, the polymer is an insulator. The conductive salt form is obtained upon treatment with simple Brernsted acids.7 A polyradical cation nature has been postulated (6) (a) Lu, F.-L.; Wudl, F.;Nowak, M.; Heeger, A. J. J . Am. Chem. SOC. 1986, 108, 8311. (b) Vachon, D. J.; Angus, R. 0.; Lu, F.-L.; Liu, Z. X.; Shaefer, H.; Wudl, F.;Heeger, A. J. Synfh. Met. 1987, 18, 297. (7) (a) MacDiarmid, A. G.; Chiang, J.-C.; Halpern, M.; Huang, W.-S.; Mu, S.-L.;Somasiri, N . L. D.; Wu, W.; Yangier, S. I. Mol. Cryst. Liq. Cryst. 1985, 121, 173. (b) MacDiarmid, A. G.; Chiang, J.-C.; Huang, W.; Humphrey, B. D.; Somasiri, N.L.D. Mol. Crysf. Liq. Cryst. 1985, 125, 309.
0022-3654/87/2091-5813$01.50/00 1987 American Chemical Society
5814
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987
Focke et al.
for the conductive salt.s The electrical resistance shows unique dependences on pH7v9and oxidation statelo as well as moisture content." In addition, the polymer exhibits electrochromic properties.]* These interesting aspects of polyaniline, combined with its environmental stability, are responsible for the continuing interest in this polymer. In the present work we present resistivity data obtained with the polymer submerged in electrolyte. This is a relevant environment since anticipated applications include low cost batteries. Paul, Ricco, and Wrighton'O have also studied the resistivity of PAn in contact with electrolyte but other studies have concentrated on the properties of polyaniline in the dry
Experimental Section Instrumentation. Electrochemical measurements were performed using an EG&G PAR Model 273 potentiostat/galvanostat. Resistivities were measured by using four point probe techniques and employed a home-built constant current source. Solution pH was measured with a Fisher Acumet 230A pH/ion meter. Materials. Aniline was purified by vacuum distillation. Commercial buffer solutions were used except for intermediate pH values for which citric acid/NaOH and HCl/KCl buffers were made up from fresh solutions. Chemical Synthesis. Polyaniline powder was synthesized by chemical oxidation with ammonium persulfate following a modification of the Willstiittefl procedure: 5.0 g (0.054 mol) of aniline was dissolved in 500 mL of 1 M HCl and the mixture cooled to below 5 OC by using an ice bath. A few drops of FeS04 solution were added as a catalyst. A precooled solution of 16.73 g (0.073 mol) (NH4)2S208in 200 mL of 1 M HCI was then added dropwise under vigorous stirring over a period of 20 min. The reaction mixture was then left in a freezer for 2 h. After this time the solution was filtered to recover the precipitated polymer. The filter cake was washed with copious amounts of 1 M HCl until the filtrate was clear. The cake was then dried under vacuum. The dried polymer was ground into a fine powder. Pellets were pressed with a standard IR pellet press. The pellets were left exposed to the laboratory atmosphere. Electrochemical Synthesis. Polyaniline films were synthesized electrochemically by the Diaz technique5 of cycling the potential between -0.2 and 0.72 V vs SCE. The electrolyte was 1 M HC1 and contained approximately 5% aniline on a volume basis. Synthesis time was varied between 30 minutes and 4 hours, resulting in adhering films of various thicknesses. After synthesis the films were immersed in a 1 M HCl solution and the potential cycled between 0.35 and -0.2 V vs S C E for 30 min. Resistiuity. Pellet resistivities were measured by using platinum wires as probes. Film resistivities were measured by using segmented gold film electrodes. These electrodes were made by vapor deposition of ca 50-A chromium on rigorously cleaned glass slides followed by vapor deposition of 1000-2000-Agold. Segmentation was achieved by using thin fibers as bamers during the deposition process. This procedure resulted in gold segments ca 3 mm wide and separated by gaps of ca. 60 pm. Polyaniline films grown on these electrodes bridged the gaps between the segments. Before measurements were taken, the electrodes were first held at -0.2 V vs S C E for 30 min. The potential was then stepped in 50-mV increments toward anodic potentials. A minimum of 10 min were allowed for the system to reach a quasi-equilibrium (8) (a) Honzl, J.; Ulbert, K.; Hadek, V.; Tustakova, M.; Metalova, M. J . Pol. Sci., Part C 1969, 16, 4465. (b) Kopylov, V. V.; Pravednikov, A. N.; Vozzhennikov, V. M.; Bel'skii, V. K. Russ. J. Phys. Chem. (Engl. Transl.) 1978, 52, 305. (c) Wnek, G. E. Synrh. Me?. 1986, 15, 213. (d) McManus, P. M.; Yang, S.C.; Cushman, R. J. J. Chem. SOC.,Chem. Commun. 1985, 1556. ( e ) Epstein, A. J.; Ginder, J. M.; Zuo, F.; Bigelow, R. W.; Woo, H. S.;Tanner, D. B.; Richter, A. F. Synth. Me?. 1987, 18, 303. (9) Surville, R. De; Jozefowicz, M.; Yu, L. T.; Perchon, J.; Buvet, R. Electrochim. Acta 1968, 13, 1451. (IO) Paul, E. W.; R i m , A. J.; Wrighton, M. S.J . Phys. Chem. 1985, 89,
1441. ( 1 1) Doriomedoff, M.; Hautiere-Cristofini, F.; Surville, R. De; Jozefowicz, M.; Yu, L.-T.; Buvet, R. J . Chim. Phys. Phys.-Chim. Biol. 1971; 68, 1063. (12) Kobayashi, T.; Yoneyama, H.; Tamura, H. J. Electroanal. Chem. 1984, 161, 419.
A W - 0.2
0.0
02
0.4
E / C V I vs
0.6
0.8
I
I .o
SCE
Figure 1. Typical cyclic voltammogram of polyaniline in 1 M HCI. Scan rate 100 mV/s. Polyaniline coverage: 38 mC/cmz. A dry film thickness of ca. 0.2 pm was estimated from the data of Paul, Ricco, and Wrighton.I0
before a resistance measurement was made. The electrodes were temporarily disconnected from the potentiostat during this measurement. Immediately following the resistance measurement, the potential was measured under galvanostatic conditions to ensure that no drift in potential had occurred. Potential stepping in the anodic direction was continued through the resistance minimum. The scan direction was reversed as soon as a significant resistance increase was observed. Irreversible resistance changes are known to occur if the potential is increased too far into the nonconductive region on the anodic side. I o The resistivity of chemically synthesized PAn was measured in the dry state (equilibrated with ambient air) as well as in solution. In the latter case optimum potentials determined for electrochemically synthesized PAn films were applied for 24 h before a measurement was taken. Cyclic Voltammetry. Cyclic voltammograms were obtained with platinum as well as gold electrodes. The scan rate was varied between 1 and 100 mV/s. Potentiometric Titrations. For this experiment relatively thick films were synthesized on platinum electrodes by using a long synthesis time of 2 h. A special cell was used and purified nitrogen was bubbled continuously through the electrolyte to exclude oxygen. The solution was stirred with a magnetic stirbar to ensure rapid mixing. The initial electrolyte was 0.4 M HC1. The desired potential was imposed on the film over a period of several hours to ensure equilibrium. After this treatment the film potential showed no tendency to drift under zero current galvanostatic conditions. Zero current was applied for the remainder of the experiment. The acid solution was then titrated with a HCl/KCl buffer followed by a series of citric acid/NaOH buffers to a final pH of approximately 6 . The film potential was followed with respect to the solution pH. The initial measurements starting at a potential of 0.275 V vs SCE showed that the potential-pH trace was reversible (the same curve being obtained by titrating from high to low p H ) if the titration rate was kept below 1.5 pH units per hour. Additional curves were then obtained by titrating at this rate from low to high pH only.
Results and Discussion Cyclic Voltammetry. Detailed cyclic voltammetry (CV) data were previously reported by Huang, Humphrey, and MacDiarmid.I3 In their study they used a scan rate of 50 mV/s. However, Kobayashi et al.14 used scan rates between 10 and 50 mV/s and observed that the peak positions (for the first redox process) varied (13) Huang, W.-S.; Humphrey, B. D.; MacDiarmid, A. G. J. Chem. SOC., * Faraday Trans. 2 1986, 82, 2385. (14) Kobayashi, T.; Yoneyama, H.; Tamura, H. J . Electroanal. Chem. 1984, 177, 28 1.
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 5815
Conductivity of Polyaniline OS
?
RADICAL CATION
~~~~~
->9
R“
0.4-
L-I
Figure 3. Structural formulae for dimeric repeat units used to rationalize electrochemistry. Note that the species A and Q can exist in various protonated forms that are not shown here.
10.3-
W 0.2
-
0
t
* 0
1 1
2
3
4
5
6
7
PH Figure 2. Phase diagram for polyaniline: Locus of E I l 2for first -A- and second -V- redox processes define the area in the potential-pH plane where radical cations predominate. Broken line - -0-defines the locus of the most conductive state as a function of pH. Solid curves - are lines of constant redox composition as determined by potentiometric titration.
-
with scan rate for pH > 1. In addition, the scan rate dependence of the reduction and oxidation peaks were different. Therefore we decided to investigate in more detail the pH and scan rate dependence of the cyclic voltammograms in the hope that more reliable formal potentials may be estimated. A typical voltammogram obtained in 1 M HC1 is shown in Figure 1 . It shows well-resolved peaks corresponding to two reversible redox processes as well as an irreversible process for which only an oxidation peak is observed. Depending on synthesis conditions, additional peaks may be observed between 0.8 and 1 V vs SCE. In some cases a third redox process featuring sharp oxidation and reduction peaks is observed in this potential range. Huang, Humphrey, and MacDiarmid13 have also reported the presence of “a small peak at 0.85 V vs SCE”. The peak potentials for the first redox process were found to vary linearly with the square root of scan rate. For this reason, values were estimated by extrapolating a plot of the potential (average of the anodic and cathodic peak potentials) vs the square root of scan rate, to zero scan rate. The slope of these plots varied with pH. At pH -0.2 (1 M HCl) the slope was approximately 1.5 m V / ( m v / ~ ) O and . ~ increased to 5 m v / ( m V / ~ ) O for . ~ p H 2. The average of the peak potentials for the second redox process was found to be scan rate independent a t lower scan rates. The experimentally determined E l I 1values are shown in Figure 2. To explain the observed electrochemistry of polyaniline, Huang, Humphrey, and MacDiarmidI3 proposed a set of half-reactions based on the octameric subunit (Le., consisting of eight aniline residues) of the polymer. They assumed that diions (doubly protonated quinone- diimine units or bipolarons) are formed at intermediate potentials. However, the electrochemical behavior of polyaniline shows strong similarities to that of the phenyl-capped dimer, N,N’-diphenyl-p-phenylenediamine.15The radical cations of this compound are known to be stable in acidic media.16 By analogy to the dimer, we believe that the cyclic voltammetry data for polyaniline are also consistent with the formation of radical cations (polarons) on initial oxidation of the polymer. For this purpose two repeat units of the polymer are sufficient to explain the data in a qualitative sense. The structural formulae of the different species are shown in Figure 3. First Redox Process. This redox process was investigated independently by cycling the potential between -0.2 V and a (15) (a) Cauquis, G.; Serve, D.Anal. Cbem. 1972,44,2222. (b) Durand, G.; Morin, G.;Tremillon, B. N o w . J. Cbim. 1979, 3, 463. (16) (a) Michaelis, L.; Hill, E. S . J . Am. Chem. SOC.1933,55, 1481. (b) Macero, D.J.; Janeiro, R. A. Anal. Chim. Acta 1962, 27, 585.
potential halfway between the first and second oxidation peaks. In addition, the potential was held constant at the switching potentials for 90 s before continuing the scan. With this procedure the potential peaks increased in sharpness. Above pH 3 the oxidation peak could not be resolved; it appeared to merge with the second oxidation peak at higher pH values. In agreement with the observations of Huang, Humphrey, and MacDiarmid,13 E l l z for the first redox process is almost independent of pH. However, our values are approximately 50 mV lower than those of Huang, Humphrey, and MacDiarmid,I3 reflexing the dependence of peak positions on scan rate. Below a potential of ca. -0.2 V vs SCE the polymer exists essentially in the fully reduced poly@-phenylamine) form (A in Figure 3). The first pair of CV peaks corresponds to the conversion to radical cations (R” in Figure 3 ) . The pH dependence of the half-wave potential is consistent with the following half-reaction: R’+ e- = A Eo = 0.050 V vs SCE
+
The half-reaction does not involve protons and as a result for the first redox process is independent of pH. Second Redox Process. For pH < 2 the present results are in good agreement with those of Huang, Humphrey, and MacDiarmid.13 The slope of E l I z vs pH is approximately -120 mV/pH. However, for pH > 2 the present data show a weaker dependence on pH with a gradual change of slope to ca -90 mV/pH. The second redox process leads to the fully oxidized form of the polymer (pernigraniline, Q in Figure 3). The proposed half-reaction is Q 2H+ e- = R” E, = 0.750 V vs SCE
+
+
The oxidation process results in deprotonation and consequently the formal potential ( E l l 2 )for this process decreases with increasing pH. As the proposed half-reaction involves two protons and one electron, the Nernst equation predicts an E l I 2vs pH slope of -118 mV/pH at 25 O C . The actual data are consistent with this prediction for pH < 2 but deviations are apparent at higher pH values. From the Eo data for the two redox processes, the equilibrium constant
for the polaron formation reaction A
+ Q + 2H+ = 2R‘+
is estimated to be K , = 7 X 10” mol-z-L2. Other Redox Processes. The other cyclic voltammetry peaks observed may represent redox reactions at the chain ends, degradation reactions, and/or redox activity of degradation products. The absence of a reduction peak corresponding to the third oxidation peak is consistent with an irreversible degradation reaction. The primary degradation product at high potentials is benzoquinone.17 Repeated cycling of the film beyond 0.6 V vs SCE leads to the growth of a new set of peaks located between the redox (17) Kobayashi, T.; Yoneyama, H.; Tamura, H.J . Electroanal. Chem. 1984, 177, 293.
Focke et al.
Q
A
P
i
BUFFER SOLUTIONS BUFFER t O . 2 M KCI PELLET
I
j
pH=2.0
I I 0.4
0.I
0.2
0.3
E/[VI
vs
0
SCE
Figure 4. Influence of the nature of the anions on the resistivity of
polyaniline in pH 2 buffers.
0
1
2
3
4
5
6
PH Figure 6. Minimum resistivity as a function of pH.
0.6
0.5
0.4
03
0.2
0.1
0.0
E l [VI vs SCE Figure 5. Influence of pH on the resistivity of polyaniline.
peaks of polyaniline that are characteristic of the hydroquinone-benzoquinone couple." Resistiuity. The resistivity of the chemically synthesized polyaniline in pellet form was 0.0018 Q m. When submerged in l M HCl the resistivity decreased to 0.0013 R m. The resistivity of PAn (as a function of potential) shows hysteresis with respect to scan direction. Resistivities measured during the reverse scan were lower than those obtained on scanning in the anodic direction. Data obtained during the scan from anodic to cathodic potentials are reported below. The dependence of the resistivity (of electrochemically synthesized PAn) on the nature of the anions was determined at pH 2 (Figure 4). The resistivity shows the typical U-type functional dependence on the electrochemical potential as first observed by Paul, Ricco, and Wrighton.Io This means that the resistivity features a broad minimum inside a small potential window and increases rapidly by several orders of magnitude outside the potential window. At pH 2 the resistivity minimnum is centered around E l / z= 0.28 V vs SCE. The effect of anions is relatively small; the resistivity in the Hydrion buffer (which contains tartaric, phosphoric, and phthalic acid) is only 60% higher than that of the HCl/KCI buffer at E = 0.30V. Figure 5 shows the effect of solution pH on the resistivity of polyaniline. In an attempt to suppress the effect of anions, the buffer solutions were diluted with an equal amount of 0.2 M KCl solution. Note that the potential window narrows as the pH increases; the resistivity minima become sharper and better defined. This is mostly due to the anodic "wall" moving toward more
cathodic potentials. The cathodic wall practically coincides for all pH values. These results are in qualitative agreement with the values of for the f m t and second redox processes observed by cyclic voltammetry. Thus the width and position of the potential window (where resistivity is low) is determined by the location of the first two redox processes observed in polyaniline. This implies that the conductive state corresponds to an intermediate oxidation state. The minimum in resistivity, at a given pH, was obtained by fitting the data to a quadratic equation. The locus of minimum resistivity on the potential-pH plane is shown in Figure 2. This line falls roughly halfway between the loci of E l l 2 for the first and second redox processes. At first sight, this is suggestive of a definite composition being associated with the most conductive state. To check this hypothesis, potentiometric titrations were performed as described in the Experimental Section. The results of these experiments are also shown in Figure 2. Each curve corresponds to a single oxidation state of the polymer, Le., a constant ratio of imine to amine nitrogens in the unprotonated form. The curvature of these curves is due to deprotonation of the polymer as pH is increased. As shown in Figure 2, the locus of minimum resistivity cuts across these curves. This indicates that at higher pH values the most conductive state corresponds to less oxidized forms of the polymer. The resistivity behavior of polyaniline in equilibrium with aqueous electrolyte of low pH can now be rationalized in terms of the electrochemistry discussed above. At low potentials ( E < -0.2 V vs SCE) PAn exists in the insulating, fully reduced, poly@-phenyleneamine) form. In this state PAn is an insulator since no charge carriers are present. Initial oxidation at low pH leads to the formation of radical cations which act as charge carriers. The resistivity at higher potentials decreases owing to deprotonation. At even higher potentials the polymer is converted into the pernigraniline structure consisting entirely of quinone diimine units. In this state the polymer is again an insulator and, in addition, it appears to be hydrolytically unstable. The disappearance of the first oxidation peak for pH > 3 indicates that, at higher pH, the stability of radical cations is decreased owing to deprotonation. Resistivity minima, normalized with respect to the resistivity at pH 1 and E = 0.35 V vs SCE, are shown in Figure 6 as a
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 5817
Conductivity of Polyaniline I5
IO
'
io4
=
10'
IO
I
IOi
I n Y
-3
IO '
Ip Y
4
-
0
lo 0
5
50 HUMIDITY
100
, 7.
Figure 8. Effect of humidity of air in equilibrium with polyaniline on
resistivity. Data from Doriomedoff et al."
0 0
10
20
30
40
50
H i DEGREE OF PROTONATION, CllN '1.
1
PROTONAT ION
Figure 7. Resistivity of the emeraldine oxidation state as a function of
pH. Degree of protonation obtained from MacDiarmid et a1.'* function of pH. (Note how the scatter in the data appears to be reduced for the buffers with KCl solution added.) For comparison, data obtained with a pellet of chemically synthesized polyaniline under similar conditions are also shown. The resistivity varies by less than an order of magnitude over the pH range 1-5. This is in sharp contrast to the data of MacDiarmid et al.' phich pertain to vacuum-dried samples of chemically synthesized PAn. In this case the resistivity rises by at least 3 orders of magnitude between pH 2 and 3. The reason for this difference in resistivity behavior for vacuum-dried and wet polyaniline are explored below. Effect of Protonation. MacDiarmid et al.ls have recently determined the degree of protonation of chemically synthesized polyaniline (emeraldine oxidation state) as a function of equilibrium solution pH. Since the acid used was HCl, the degree of protonation could conveniently be expressed as the mole ratio of chlorine to nitrogen. This ratio was determined by elemental analysis. An important conclusion of this study was that the degree of protonation decreases from nearly 50% at pH 0 to less than 10% at p H 3. The sharp decrease in protonation between pH 2 and 3 suggests that high degrees of protonation are a prerequisite for low resistivity in the dry state. MacDiarmid et al.'* have also established that the emeraldine oxidation state in 1 M HCl is characterized by a potential of 0.356 V vs SCE. This value corresponds closely to one of the curves in Figure 6 and allows us to relate the resistivity in the wet state to the degree of protonation. This was done as follows: the potential corresponding to a given pH was first determined from the curve (labeled with an E) in Figure 2. The resistivity corresponding to this potential was then obtained from the curve for the appropriate pH in Figure 5. The resistivity was then plotted against the degree of protonation measured by MacDiarmid et al.IS This plot is shown in Figure 7 and reveals that the resistivity in the wet state is not as sensitive to the degree of protonation as the resistivity in the completely dry state. It would therefore appear that, for PAn in contact with electrolyte, a marginal degree of protonation is sufficient to induce the conductor-insulator transition. Effect of Moisture. The large difference between the resistivity of polyaniline in contact with electrolyte and in the vacuum-dried state implies that the moisture content of the polyaniline should be an important parameter influencing resistivity. This has been known for a long To illustrate this effect, the data of Doriomedoff et al." are replotted as Figure 8. Notice that on ~
~~
(18) MacDiarmid, A. G.; Chiang, J.-C.; Richter, A. F.; Somasiri, N. L.
D. In Conducting Polymers; Alcacer, L., Ed.; Reidel: Dordrecht, The Netherlands, 1986.
5 -HI
VALENCE RESONANCE
I
DEPROTONATION
Figure 9. Migration of oxidation states along the polymer backbone by proton exchange and valence resonance.
HNC
N
HN
+NH
H Nt
H+
HN
PROTONATION
'NH
HN+
ELECTRON TRANSFER
"NH
NH
N
'"NH +NH
+HN
NH
-Ht
DEPROTONATION
Figure 10. Intermolecular charge transfer facilitated by proton-exchange
reactions. exposure to moisture the resistivity increases by 1 order of magnitude for PAn equilbriated at pH 0.3 but by more than 2 orders for PAn equilibrated at p H 3.5. We now propose a speculative mechanism to explain the dependence of resistivity on the protonation level and the moisture content. Assume that for pH > 1 only imine nitrogens are protonatedI8 and consider a partially oxidized and partially protonated sample of the polymer. The presence of unprotonated basic sites implies a dynamic equilibrium of protonation/deprotonation in the presence of a protic solvent. In fact, proton exchange in polyaniline has recently been demonstrated by NMR.I9 Unprotonated imine nitrogens are expected to act as barriers to conduction along a chain as well as between the chains. Owing to proton-exchange reactions the defects (barriers) are not fixed in time and space but actually fluctuate in position. Figures 9 and 10 show how a protonation/deprotonation cycle may facilitate intramolecular and intermolecular charge transport. Intramolecular charge transport involves translation of quinoidal structures ~
(19) Nechtstein,
~~
M.; Santier, C. J . Phys. (Les Ulis, F r . ) 1986, 47, 935.
5818
J . Phys. Chem. 1987, 91, 5818-5825
along the polymer backbone. In the fully deprotonated polymer of an intermediate oxidation state, no translation of quinoidal structures is possible. However, as soon as an imine nitrogen becomes protonated, translation over a distance of two rings becomes possible by valence resonance. Further translation then requires a deprotonation/protonation cycle as illustrated in Figure 9. Of course, if both imine nitrogens are protonated, translation over a larger number of rings becomes possible. However, any deprotonated imine nitrogen or protonated amine nitrogen will act as a barrier for translation along the polymer backbone. It follows that considerable mobility along the polymer chain is possible on protonation of only one of the imine nitrogens of the quinoidal structures. For interchain transport a double protonation (at least initially to allow the formation of radical cations) is required (see Figure 10). The model implies a coupling of electronic and ionic transport which increases as the degree of protonation of the imine nitrogens decreases. Since proton exchange depends on the presence of a source of protons, these concepts may explain the considerable decrease in resistivity that is observed when initially dry PAn is exposed to moisture. Also, H 2 0 molecules may aid in proton transport (via the hydronium ion) between chains. It is anticipated that the resistivity will be more sensitive to the degree of protonation in the “dry” state vs the “wet” state, and in fact this is what is found (Figure 8). The fact that the effect of moisture on the resistivity increases with increasing pH is also in agreement with the predictions of the proton-exchange model: At low pH a majority of imine nitrogens are protonated and only a few barriers (unprotonated imine nitrogens) exist. Therefore proton-exchange
reactions will have a relatively small impact on resistivity. At high pH many barriers to conduction exist and proton exchange has a greater influence on mobility along, as well as between, chains
Conclusions The conductive state of polyaniline is associated with an intermediate oxidation state. The electrochemical data obtained by cyclic voltammetry show that, at low pH, this state is consistent with radical cations. The radical cations are stable over a limited potential range only and, as a result, only low resistivity exists over the same limited potential range. As the stability range of the intermediate state decreases with increasing pH, the potential range of low resistivity also decreases with increasing pH. The protonation of quinone diimine units is incomplete at higher pH. However, even partial protonation is apparently sufficient to decrease resistivity by more than 3 orders of magnitude in the presence of electrolyte. In addition the sensitivity of the resistivity to moisture content increases with increasing pH. These observations imply that proton-exchange reactions play a central role in the conduction mechanism in polyaniline. Acknowledgment. This work was supported in part by a grant from the Office of Naval Research and an A R C 0 Career Development Award (to G.E.W.). W.W.F. is grateful for a graduate fellowship from the NIMR/CSIR. Registry No. PAn, 25233-30-1; HCI, 7647-01-0; KCI, 7447-40-7; CH~C~HSOI-, 16722-51-3;BFd-, 14874-70-5;CFBCOO-, 14477-72-6; tartaric acid, 87-69-4;phosphoric acid, 7664-38-2;phthalic acid, 88-99-3.
Is Liquid Water Really Anomalous? M.-P. Bassez,? J. Lee, and G. W. Robinson* Picosecond and Quantum Radiation Laboratory, Texas Tech University, Lubbock, Texas 79409 (Received: May 12, 1987)
A temperature dependence of the effective potential barrier hindering rotational motions of water molecules in liquid water has recently been invoked as the key to a detailed understanding of water’s “anomalous” characteristics. In this paper we demonstrate in greater detail how a large number of thermodynamic and transport properties of this remarkable liquid, supercooled to superheated, can be interrelated through empirically determined temperature-dependent activation barriers. It is suggested that the height of these barriers is a quantitative measure of the departure from local ice-Hike structure of the hydrogen-bonded network. Influences of temperature and pressure in liquid water are explained, and deuterium effects are reproduced. Specific volumes at supercooled temperatures and elevated pressures are calculated, and the origin of the wayward heat capacities becomes clear. In fact, from the analysis, a “water-like substance“ emerges that in all respects seems capable of approaching the properties of real water within a few percent.
1. Introduction
For some
it has been thought that rotational motions
rotational barriers decrease.1° It immediately follows that contributions to the partition function from the temperature-dependent
of water molecules play an essential role in chemical and dy-
namical effects in liquid water. Dynamical properties of pure liquid water, such as viscosity, dielectric relaxation, and N M R spin-lattice relaxation, are known to be related to the rates of molecular r e ~ r i e n t a t i o n . ~ ~ A theory described considers liquid water to be a network of hydrogen-bonded hindered rotor molecules that experience librations below a temperature-dependent activation barrier, AH( T ) , and rotational diffusion above it. As the temperature rises and the hydrogen bonds weaken, the forces between the molecules become more isotropic, and the heights of the ‘Permanent address: Institut Universitaire de Technologie, Laboratoire de Chimie-Physique, Universitt d’Angers, Belle-Beille, 49045 Angers Cedex,
France.
0022-365418712091-5818%01SO10
(1) Danneel, H. Z . Elektrochem. Angew. Phys. Chem. 1905, 1 1 , 249. (2) Bernal, J. D.; Fowler, R. H. J. Chem. Phys. 1933, 1 , 515. (3) Conway, B. E.; Bockris, J. OM.; Linton, H. J . Chem. Phys. 1956, 24,
834.
(4) Eisenberg, D.; Kauzmann, W. The Structure and Properties of Water, Oxford University Press: New York, 1969. ( 5 ) Hasted, J. B. Aqueous Dielectrics; Chapman and Hall: London, 1973. Collie, C. H.; Hasted, J. B.; Ritson, D. M. Proc. Phys. SOC.,London 1948, 60, 145. (6) Abragam, A. The Principles of Nuclear Magnetism; Oxford University Press: New York, 1961; pp 298-300. (7) Robinson, G. W.; Lee, J.; Casey, K. G.;Statman, D. Chem. Phys. Lett. 1986, 123,483.
(8) Robinson, G. W.; Lee, J.; Bassez, M.-P. Trans. Faraday SOC.1986, 82, 2351. (9) Robinson, G. W.; Lee, J.; Bassez, M.-P. Chem. Phys. Lett., 1987, 137, 316.
0 1987 American Chemical Society
