J . Phys. Chem. 1994, 98, 5714-5720
5714
Interactions of Neutral Organics in Aqueous Solution with Conducting Polymer Films of Poly( N-methylpyrrole) and Poly (N-methylpyrrolelpolystyrenesulfonate) Daniel L. Feldheim, Michael Krejcik, Susan M. Hendrickson, and C. Michael Elliott’ Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523 Received: October 19, 1993; In Final Form: February 15, 1994’
W e have investigated the interactions between halocarbons in aqueous solution and conducting polymer films of poly(N-methylpyrrole) ( P N M P ) and poly(N-methylpyrrole/polystyrenesulfonate)(PNMP/PSS) employing cyclic voltammetry, UV-visible spectroelectrochemistry, and electrochemical quartz crystal microgravimetry (EQCM). The absorption of dichloromethane ( D C M ) into both films is adequately described by the Langmuir isotherm model for monolayer sorption. The partitioning of DCM from the aqueous phase into reduced P N M P is up to 2.5 times greater than the partitioning into the oxidized form of the polymer. The amount of DCM absorbed by both redox forms of the PNMP/PSS composite film is, however, identical. These observations are discussed in terms of the changes (or lack thereof) in chemical, physical, and morphological properties that accompany the redox processes of these films. Spectroscopicmeasurements suggest that there is slight chemical interaction between DCM and reduced P N M P and PNMP/PSS, but there is much less interaction with the oxidized polymers.
Introduction
Interest in organic conducting polymers has grown beyond such early applications as batteries,’ electrochromic displays,2 and sensor^^-^ into new fields such as gas separation,* ion exchange,6 and even drug delivery.’ These new applications are important examples of the versatility of conducting polymers in terms of their chemical rather than electrical properties. For example, in a single polymer film, two chemically very different materials are available, namely the oxidized and reduced forms. Even more chemical diversity exists when one takes into consideration the vast number of different dopant ions possible, each of which affects the chemical and morphological properties of the film. We have shown previously that the oxidation state and dopant ion of membranes of poly(N-methylpyrrole) and poly(3-methylthiophene) can significantly influence the rates of transport of neutral organic molecules across membranes of these materials.* Schmidt et al. found similar behavior of polyaniline and p~lypyrrole.~The potential thus exists for applications of conducting polymers in more economically efficient membrane separations. However, before any serious consideration can be given to such potential application, the interactions between conducting polymers and neutral molecules in solution must be better characterized. Below we describe our recent efforts a t characterizing the interaction of primarily one representative organic molecule, dichloromethane (DCM), with two different types of conductive polymer films. The first type of film is “conventional” poly(Nmethylpyrrole) (PNMP); the second is a so-called “molecular composite” of poly(N-methylpyrrole) and polystyrenesulfonate (PNMPIPSS (Figure 1). We chose these two films for our study because they differ in many parameters, the important ones for this study being structure and the mode by which the films are doped. Films of PNMP are ionic when oxidized and neutral when reduced. Films of PNMPIPSS, on the other hand, are ionic irrespective of the polymer oxidation state. Experimental Section Chemicals. N-Methylpyrrole (Aldrich) was freshly distilled under nitrogen atmosphere prior to each experiment. Acetonitrile
* Corresponding author. e Abstract
published in Advance ACS Abstracts, May 1, 1994
Reduced (non-ionic) Form
Oxidized (ionic) Form
Reduced (ionic) Fonn
Oxidized (ionic) Form
Figure 1. Two-dimensional chemical structures for PNMP (top) and
PNMP/PSS (bottom). (Baxter, Burdick and Jackson) and dichloromethane (Malinckrodt) were used as received. Sodium perchlorate (Aldrich) was used as received or after drying overnight at 100 O C . Tetrahexylammonium polystyrenesulfonate (THAPSS) was prepared as described previously.1° We used 18 MQ water (Millipore) for all experiments. Polymer Growth. All polymer films for the EQCM and UVvisible spectroscopy experiments were grown a t a constant potential of +0.8 V vs. Ag/Ag+ (0.1 M AgN03, DMSO). For the cyclic voltammetry experiments the polymer films were grown at constant potential or constant current. Similar results were obtained for either growth condition. PNMP-Clod was grown from acetonitrile solutions containing 1 M N-methylpyrrole and 0.1 M NaC104. Following film synthesis, the acetonitrile solution was replaced with an aqueous solution containing 0.1 M NaC104 and the potential applied to the film was cycled repeatedly between +0.5 and -0.4 V vs SSCE until stable voltammograms were obtained (- 10 scans).
0022-365419412098-5714$04.50/0 0 1994 American Chemical Society
Interactions of Neutral Organics with Polymer Films
I
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The Journal of Physical Chemistry, Vol. 98, No. 22, 1994 5715
I
+0.5
+0.5
/
II
I F . I-~ v ), -0.5
0
I
/
1 I I
I I
1 Figure 2. Cyclic voltammograms for a 100 mC PNMP film in contact with (solid line) 0.1 M NaC104 in water and (dashed line) 0.1 M NaC104 in water saturated in DCM. Scan rate is 20 mV/s. Potential is vs SSCE.
Figure 3. Cyclic voltammograms for a 10mC PNMP/PSS film in contact with (solid line) 0.1 M NaClOd in water and (dashed line) 0.1 M NaC104 in water saturated in DCM. Scan rate is 100 mV/s. Potential is vs SSCE.
PNMP/PSS was synthesized from acetonitrile solutions containing 0.1 M THAPSS and 1 M N-methylpyrrole. All other conditions were the same as those described above for PNMPC104. Instrumentation. EQCM experiments were performed with AT-cut quartz crystals having 5-MHz oscillation frequency. The crystals were coated on both sides in a "keyhole" pattern with thin layers of first chromium then gold. A Philips P M 6654c frequency counter was used to monitor the frequency of the crystals. The potentiostat and oscillator circuit were designed and built at the University of Wyoming." Spectroelectrochemistry. UV-visible spectroelectrochemical experiments were performed with a Hewlett-Packard Model 8452A photodiode array spectrometer. The spectroelectrochemical cell is described in detail elsewhere.lZ A few modifications were made in the basic design for use with polymer films. Briefly, the cell consists of a working electrode of transparent, vapor-deposited gold-coated calcium fluoride (6 X 6 mm), a platinum gauze counter electrode, and a silver wire psuedo reference electrode. The cell thickness was 1 mm. During film growth for spectroelectrochemical measurements, 40 mC of charge were passed. Cyclic Voltammetry. For the cyclic voltammetry experiments the working electrode was a 3 mm diameter glassy carbon disk and the counter electrode was a platinum grid (0.5cm X 1.O cm). Measurements were made using an EG&G PAR Model 173 potentiostat/galvanostat in conjunction with a PAR 175 programmer. Data collection was performed with a Yokogawa 3023 X-Y recorder. Scanning Electron Microscopy. Polymer films for scanning electron microscopy were grown under the conditions stated above but on tin oxide coated glass electrodes. Following film synthesis, the films were detached and mounted on SEM holders with the solution side of the film facing up. The samples were dried under vacuum for 4 h and sputtered with a thin layer of gold (5 mA for 10 min) to enhance conductivity. Micrographs were taken with a Philips 505 SEM.
TABLE 1: Cathodic Peak Shifts for PNMP in the Presence of 0.1 M NaC104 (aq) Saturated with the Indicated Organic Contaminant (Scan Rate = 100 mv/s)
Results Cyclic Voltammetry. Films of PNMP-C104 and PNMP/PSS both yield stable, well-behaved cyclic voltammetric responses in aqueous 0.10 M NaC104 electrolyte. After approximately 10 continuous potential scans, subsequent scans are superimposable (cf. Figures 2 and 3, solid curves). Saturation of the electrolyte with DCM vapor (- 1 mg/mL) produces an immediate and significant change in the voltammogram of the PNMP film (Figure 2, dashed curve). The voltammogram is shifted to more positive potentials relative to
organic DCM chloroform carbon tetrachloride 1,2-dichlorocthane 1,2-dichIorobenzene benzene acetoneb a
Ep,c(so~utc) - Epp.c(aq) (mv) 200 250 250" 250 0 15" 25
Not reversible. b Concentration was 1 mg/mL.
TABLE 2 Cathodic Peak Shifts for a PNMP Film in Contact with a Saturated Solution of DCM, 0.1 M in the Indicated Electrolyte (Scan Rate 50 mV/s) dopant p-toluenesulfonateClNOpclod-
Ep.c(DCM)
- E,,,(*,) (mV) 400 110 175 200
TABLE 3: Cathodic Peak Shifts for PNMP/PSS in the Presence of 0.1 M NaC104 (aq) Saturated with the Indicated Organic Contaminant (Scan Rate = 100 mV/s) DCM chloroform carbon tetrachloride 1,2-dichIoroethane benzene 1,1,2-trichIoroethane a
65 40 25" 35 30" 20
Not reversible.
that in DCM-free electrolyte. Viewed from the perspective of charge, it becomes thermodynamically more difficult to oxidize the polymer in the presence of DCM. The actual magnitude of the shift depends on the identity of the dopant ion, the concentration of solute, and the solute species itself. A summary of these data are presented in Tables 1 and 2. While data presented here focuses on DCM, the general phenomena discussed in this paper are observable for a wide variety of 1- and 2-carbon halocarbon compounds. The magnitude of the voltammetric shift differs slightly within this class of compounds, but the general nature of the shift is the same. Table 1 lists the cathodic peak shift relative to pure 0.1 M NaC104 electrolyte for several representative chlorocarbon compounds. The presence of other neutral organic molecules which are soluble a t similar, or greater, concentrations to DCM produces little or
5716 The Journal of Physical Chemistry, Vol. 98, No. 22, 1994
Feldheim et al.
42.100
+I ,680
t1.260
to. 8 4 0
t o .420
0.0000
300
500
400
WAVELENGTH
700
600
(r"
Figure 4. UV-visible spectra of a PNMP/PSS film as a function of applied potential (vs SSCE). The letters refer to the transitions shown in Figure 5 . At the far right-hand side of the figure, the uppermost spectrum was obtained at +0.5 V; the lowermost spectrum was obtained at -0.3 V.
no change in the voltammogram. For example, 1 mg/mL of acetone in 0.10 M NaC104 causes a shift in the cathodic peak potential of only 25 mV. Additionally, thevoltammetric response of PNMP-C104 films to DCM is sensitive to the dopant anion in the electrolyte. Table 2 lists cathodic peak shifts for a single PNMP film in a selection of electrolytes having different anions. Finally, the voltammetric shifts are both rapid and reversible. Purging the solution for several minutes with pure Nz returns the voltammogram to its original position and shape. EQCM studies in a flowing stream indicate that equilibrium for the interaction between the DCM and the film is reached within a few seconds.13 Qualitatively, the voltammetry for PNMP/PSS is similar to that of PNMP-C104 in the presence and absence of DCM. The changes which occur upon introduction of DCM into solution are, however, much more subtle. For PNMP, which is neutral in the reduced form and charged in the oxidized form, there is typically a peak shift of ca. 200 mV; for PNMP/PSS, which is charged in both forms, there is typically a shift of only ca. 65 mV (dashed curve in Figure 3). Table 3 lists peak-shift data, analogous to the PNMP data in Table 1, for a PNMP/PSS film. UV-Visible Spectroelectrochemistry. UV-visible spectra of a PNMP/PSS film, acquired as a function of applied potential, are shown in Figure 4. The qualitative features of these spectra are similar to those of polypyrrole which have been interpreted in detail e l ~ e w h e r e . ~ ~Briefly, Js the observed spectral features are attributed to transitions between three electronic levels associated with the conducting polymer bandgap (Figure 5 ) . These levels are the valence band, conduction band, and polaron or bipolaron band. The transitions, marked in Figure 5 , give rise to the various spectral features across the UV-visible region. UV-visible spectra for PNMP/PSS and PNMP films in contact with aqueous 0.1 M NaC104and DCM saturated aqueous NaC104 are shown in Figure 6. It is important to note that the spectra for the oxidized forms of each film in the presence and absence of DCM were collected at identical doping levels (Le., constant charge). Slight differences are observed in the spectra in the presence of DCM for both films. For reduced PNMP/PSS (Figure 6A), increases in absorbance are seen in the bands from 250 to 480 nm and 550 to 740 nm upon introduction of DCM. Smaller differences are observed for the oxidized film: an increase in absorbance of the band from 250 to 420 nm and a decrease in the 660- to 820-nm band. Similar behavior is observed for
1
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c
V n l e n c e Band I
Neutral Polymer
V a l e n c e Band
I
1
I
Valence Band
Doped P o l y m e r
Figure 5. Band diagrams for the neutral and doped forms of conducting polymers. Transitions are shown in the spectra of Figure 4.
PNMP (Figure 6B); however, the differences in the spectral response between the oxidized and reduced forms of PNMP to DCM are more pronounced than for the composite film. Difference spectra for oxidized and reduced PNMP are shown in Figure 7. Note that, in areas of the spectrum where DCM causes an absorption increase, the increase is greater for the reduced form of the film by a factor of 5. Electrochemical Quartz Crystal Microbalance. When using the EQCM in polymer film applications, it is essential that the linear mass to frequency response described by the Sauerbrey equation is maintained.l6 To this end we have chosen to study polymer films with thicknesses less than -0.5 Fm and have carefully monitored the mass to charge response during film growth. Smyrl et al. havenoted that a linear relationship between mass and charge during film growth signifies a rigid film free of viscoelastic effects.]' All films studied here showed the above relationship during synthesis. Frequency vs potential plots for a 200 nm thick PNMP film are presented in Figure 8. The thickness of the film was calculated using the frequency change of the quartz crystal during film synthesis, the area of the working electrode (0.34 cm*), and a density of 1.O g/cm3 for reduced PNMP.I8 Curve A is a frequency vs potential scan of a typical PNMP-C104 film in contact with 0.1 M aqueous NaC104. Curve B is a scan on the same film in contact with the same electrolyte which was saturated with DCM. Note that the sweep begins at a potential where the film is reduced and that sufficient time was allowed for DCM absorption to reach equilibrium. At potentials where the film is in its reduced form
Interactions of Neutral Organics with Polymer Films
The Journal of Physical Chemistry, Vol. 98, No. 22, 1994 5717 film. The 150-Hz decrease in frequency marks an uptake of 0.9 pg of DCM assuming that nothing is displaced from the film. Partitioning of DCM into the oxidized form of the polymer is also apparent (+OS V vs SSCE); however, the uptake, 0.6 pg, is significantly less than when the film is reduced. Typically, the amount of DCM absorbed into the reduced form of P N M P is 1.5-2.5 times greater than for the oxidized form. In addition, sweeping a t 100 mV/s does not allow sufficient time for the oxidized film to fully equilibrate with DCM in solution. The kinetics of absorption is the subject of a forthcoming publication.13 It is important to note also that the partitioning is completely reversible. The dashed curve in Figure 8 is a scan following the removal of DCM from solution. Finally, when no polymer film is present on the crystal surface the frequency change in the presence of a saturated solution of DCM is less than 10 Hz. At low concentrations of solute, the partition coefficient, K , can be calculated employing the following relation:
* l 680
+ l 260
tO E40
to.420
0.0000 WIVELENGTH lnal
+ l 900
B
0 0000
1
where Ah is the frequency change due to the mass change upon uptake of solute (DCM) (Le., the frequency of curve 8B a t -0.4 V subtracted from the frequency of curve 8A at -0.4 V), Afa the frequency change upon the original deposition of the polymer (Le., proportional to the total mass of the polymer), p the density of the polymer, and C, the concentration of solute (DCM) in solution.1g Stated differently, the term (Ahp)/Afais equal to the concentration of solute in g/mL within the polymer phase. Therefore, K is simply the ratio of the concentration of solute in the polymer to its concentration in solution. Figure 9A is a plot of DCM mass uptake vs solute concentration and Figure 9B is the corresponding plot of log Kvs. DCM concentration for a 125 nm thick reduced PNMP film. An inverse relationship is observed for this system; that is, the log K value decreases with increasing DCM concentration. This behavior is consistent with a gradual saturation of available absorption sites. The data of Figure 9 has also been fit to various sorption isotherms. Type I, or Langmuir, sorption isotherms are characteristic of “monolayer” or single-site absorption and are
__L
400
300
500 MbVELENGTH
600
700
EO0
hml
Figure 6. (a) UV-visible spectra for PNMP/PSS in the absence (solid line) and presence (dashed line) of DCM. (b) UV-visible spectra for PNMP in the absence (solid line) and presence (dashed line) of DCM.
(-0.4 V vs. SSCE), it is apparent that the frequency has decreased from curves A to B. This corresponds to an increase in the mass of the polymer as DCM partitions from the water phase into the
I I I
O.’OO
I
1
I
500
600
700
7
0.075
0.050
8
13
0.025
0.000
-0.025 300
400
BOO
HAVEIXNCTH (nm)
Figure 7. Difference spectra for a PNMP film. The spectra shown are the differences in the UV-visible spectrum for the polymer’ in the presence of DCM vs the absence of DCM for the indicated redox form. The solid lines are the best-fit lines to the difference spectra.
Feldheim et al.
5718 The Journal of Physical Chemistry, Vol. 98, No. 22, 1994
61 I
0
1
5 1 I -10
J
m -
s:
J/
4 1
*
d
I
-100
-150 /
I ‘ a 200
4 0 0 600 800 1000 1200 Concentration (pg/mL) Figure 10. Fit of the DCM sorption data of Figure 9 to the Langmuir isotherm equation. R = 0.998. C is equal to the DCM solution concentration and n is the DCM concentration in the film. The y-axis 0
-200
-250
I 0.2
0.4
0.6
I
0.0
-0.4
-0.2
Potentid (v)
Figure 8. Frequency vs potential for a 200 nm thick PNMP film. Top solid trace (A): 0.1 M NaC104(aq); (B) 0.1 M NaClOd(aq)/saturated DCM. Topdashed trace: 0.1 M NaC104following the removalof DCM. Scan rate = 100 mV/s. Potential is vs SSCE. Data were smoothed within an FFT smoothing routine available with the Asyst software.
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I
is therefore unitless.
TABLE 4 Dependence of the Absorption Equilibrium Constant on DCM Concentration for a 180 A Thick Reduced PNMP/PSS Film’ concn (mg/mL) log K 1.o 0.5 0.2
2.16 2.21 2.44
A film density of 1.0 g/mL was experimentally determined. The aqueous solution contained 0.1 M NaC104.
0.9 0.8
TABLE 5 Amount of Dichloromethane Absorbed into a 240 nm Thick PNMP/PSS Film and a 200 nm Thick PNMP Film as a Function of Oxidation State g of DCM absorbed/
polymer
0.2
I
I
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1
DCM absorbed (pg)
PNMP/PSS- Na+ PNMP/PSS PNMP PNMP-C104
I
0.78 f 0.06 0.72 0.04 0.9 f 0.05 0.6 f 0.03
*
mol of polymer 25 & 2.0 23 f 1.8 11 f 0.60 7.0 f 0.36
TABLE 6 Partition Coefficients for Some 1- and 2-Carbon Halocarbons into Poly(N-methylpyrrole)’ absorbate K
Y
2.8
1
I
I
3.2
tI e
e e
2.2
I
I
0
0.2
and desorption rate constants.*O Type I1 isotherms describe multilayer absorption with the BET equation
t
e
e
‘r
x/n(l - x ) - l/cn’+ ( c - l)x/cn’
,
I
I
I
1
(3)
where x is mole fraction, n and n’ have the same meaning as in eq 2, and
1
1
0.4 0.6 0.8 Concentration (mg/mL)
205 f 10 160 f 15 134 f 14 125 f 2.5
The potential was held at 0 V vs SSCE.
i
e
2.6
CH2C12 CHC13 ClzCHCHzCl CICHzCHzCI
I
1.2
Figure 9. (a) DCM mass uptake vs DCM solution concentration for a 125-nm reduced PNMP film. The potential was held at -0.4 V vs SSCE. Error bars represent the standard deviation of five trials at each concentration. R = 0.999. (b) log partition coefficient vs DCM solution concentration. Data are calculated from the mass uptakes of Figure 9A.
described by
C , / n = C,/n’
+ l/n’b
(2)
where C,is the DCM concentration in water, n is the concentration of DCM in the film at equilibrium, n’ is the concentration of sorption sites within the film, and b is the ratio of the absorption
where Qlis the adsorption energy for the first monolayer, QV the adsorption energy for all succeeding layers, and R and T have their usual meanings. Figure 10 is a fit of the DCM sorption data to the Langmuir type I model. Both models, above, appear to adequately describe the absorption phenomenon; however, when c is calculated from the slope and intercept of the type I1 isotherm, an unreasonably large number results. In the limit of c approaching infinity the BET isotherm collapses to the simple Langmuir model (e.g., Qv in eq 4,the driving force for adsorption of all succeeding layers
Interactions of Neutral Organics with Polymer Films
The Journal of Physical Chemistry, Vol. 98, No. 22, 1994 5719
A
B
D Figure 11. Scanning electron micrographs for (A) reduced PNMP (E) oxidized PNMP (C) reduced PNMP/PSS; and (D) oxidized PNMP/PSS. B are 526X. C and Dare 1200X.
A and
past monolayer coverage, is negligible compared to that for the first layer)?O Therefore, it appears that the absorption type can be reasonably described by the simple Langmuir model formonolayer coverage. Qualitatively then, it appears that DCM molecules have a much higher affinity for adsorption sites on the polymer than they do for each other. In contrast, for methanol absorption into polypyrrole films, absorption was found to fit either model and yield chemically reasonable parameters.' Values of log K vs DCM concentration for PNMP/PSS are listed in Table 4. The same behavior is observed with respect to log Kvs DCMconcentration for thecompositefilmas for PNMP itsell: increasing K with decreasing concentration. However, as shown inTable 5 , for similar film thicknessestheabsoluteamount of DCM absorbed into PNMP/PSS film is less than for PNMP. In contrast to PNMP, there appears to be little or no difference in the amount of DCM absorbed into the oxidized and reduced forms of the composite film. These data are also shown in Table 5. In addition. absorption of DCM into the PNMP/PSS copolymer film also fits the Langmuir isotherm. For both polymers, theamount of DCM absorbed increaseswith increasing film thickness indicating a bulk, rather than surface, absorption. This behavior is also strong evidence that the film maintains constant rigidity during the absorption process, a requirement for quantitative mass analysis with the EQCM.
Discussion The EQCM results provide potential explanations for the changes which occur in the voltammetry and spectra of PNMP and PNMP/PSS films when exposed to DCM or similar compounds. Consistent with thechangein PNMPpolarity upon oxidation/reduction, the partitioning of DCM into the reduced, neutral polymer issignificantlyfavoredoverpartitioningintothe oxidized. ionic polymer. The voltammetry, thus, could be interpreted as reflecting this difference in the partitioning. Qualitatively, starting with the DCM-saturated reduced film, in order to oxidize the film, energy is required to expel the DCM into the aqueous phase; this additional energy appears as a shift in the voltammetric peak to more positive potentials. Whilethispolarityargument isappealing it isalmost certainly an oversimplification. The fact that the partitioning behavior follows a Langmuir type isotherm may provide some insight into what is going on. In one extreme, the Langmuirian behavior could occur because of a classical specific interaction between the absorbate and a fixed number of binding sites. The spectroelectrochemical results suggest that if such specific interactions are occurring they do not perturb the electronic structure of the polymer very much. In the other extreme, Langmuirian behavior could result from partitioning of the
5720 The Journal of Physical Chemistry, Vol. 98, No. 22, 1994
absorbate molecules into a fixed void volume within the polymer. If the void volume were fixed and essentially filled with DCM in the reduced polymer, in order to provide room for dopant ions when the polymer is oxidized, DCM would have to be expelled from the polymer. The data in Table 2 is generally consistent with this argument: the largest anions produce the largest peak shifts. Also in agreement with this model is the trend in partition coefficient with halocarbon size reported in Table 6: the smaller halocarbons show the largest partition coefficient. Initial consideration of the data for PNMP/PSS in Table 3 appears to be consistent with the polarity argument since both forms of this film are nominally charged and, within experimental error, the same amount of DCM partitions into the oxidized and reduced films. On the other hand, the absolute amount of DCM which partitions into this ionic composite film is roughly midway between the amounts that partition, respectively, into the reduced and oxidized films of PNMP. There is, however, a significant difference in the morphology (as evidenced by electron microscopy, Figure 11) and density between the two polymers which might be responsible for thedifferences in absolute partition coefficients. The densities of oxidized and reduced PNMP are 1.5 and 1.O g/cm3, respectively, and for oxidized and reduced PNMP/PSS 0.9 and 1.O g/cm3, respectively. The shifts which occur in the visible spectrum of PNMP films, when placed in contact with a CHZClzsaturated aqueous solution (see Figures 6 and 7), may be an indication of some type of weak specific interaction between the solute and the polymer. While there is some shape change, the major effect is a shift in the bands to slightly lower energy. Thus, in the difference spectra of Figure 7, theappearantgreaterintensityexhibitedby thereduced polymer is actually a reflection of a larger spectral shift to lower energy of the CHZC12-saturated reduced polymer relative to the CHzClz-free reduced polymer. In considering potential modes of molecular-level interaction, Lewis acid/base or partial electrontransfer interactions between the polymer and CHzClz are the most obvious possibilities. Josowicz reported evidence for lonepair charge donation by Lewis-basic molecules to oxidized polypyrrole.14 Chlorocarbons are very weak Lewis bases but they are reasonably good acceptor molecules (the Guttmann acceptor number for CHzClz is 20.4).21 It is thus not unreasonable to consider the possibility that the reduced polymer may be donating charge to the CHZCl2. While this argument is consistent with the greater relative spectral shifts of the reduced films, it is hard to rationalize why that shift should be to lower energy. Another possible explanation for the spectral shifts is that the local dielectric constant of the polymer is changed by incorporation of CHZClz. It is difficult to surmise, in the absence of more detailed molecular structural information on the polymer, whether the local dielectric constant would increase or decrease. Consequently, little molecular-level insight is gained into the origins of the spectral shifts by such speculations. Thus, at this point there is no definitive molecular-level explanation for the interaction between DCM-like molecules and these polymer films. Probably, the reason there are no clear-cut explanations is that, to some degree each of the factors discussed above are contributing to the phenomena. In other words, sterics, polarity, and specific interactions are all important in the partitioning process. The fact that the spectrum of the polymer is perturbed, albeit modestly, supports the importance of some kind of weak-to-moderate specific interaction of DCM with the reduced PNMP, an interaction that is lessened by oxidation. Additionally, in a subsequent paper we will report results on the kinetics of DCM partitioning into these polymers.*3 These results also support the involvement of specific interactions between the PNMP films and DCM.
Feldheim et al.
Conclusions Irrespective of the detailed molecular-level origins of the interactions, we have shown here that it is possible to *tune” the partitioning behavior of DCM-like molecules between conducting polymer films and an aqueous phase by changes in polymer oxidation state, ionic character, and morphology. Conversely, differential partitioning of DCM-like molecules between the respective oxidation forms of these polymers produces dramatic changes in polymer voltammetry. These rapid reversible changes could, thus, provide a basis for the development of electrochemical sensors for these otherwise electrochemically-inactive molecules. Additionally, the fact that the oxidized and reduced forms of P N M P absorb significantly different amounts of DCM-like molecules suggests that these and similar polymers might beuseful in economically efficient extractions and/or separations. These are significant considerations given the present environmental importance of halocarbon molecules. Finally, the ability to electrochemically tune the chemical and physical properties of conducting polymers is an area that deserves additional attention. The structural diversity available within this class of conducting polymers is enormous when the variety of monomers available and potential for forming composites and copolymers are considered. Possibilities thus exist to tailor a polymer to favor a selective interaction with, for example, a particular molecule or class of molecules. While the moderate selectivity exhibited by the films studied here was, admittedly, serendipitous, this does not diminish the potential for a more premeditated, molecular-engineering based approach to future studies of conducting polymers and polymer composites. Acknowledgment. The authors thank the IAB/CU-Boulder Center for Separations Using Thin Films and the National Science Foundation (CHE-93 1 1694) for financial support. Additionally, we thank Professor Dan Buttry and his group a t the University of Wyoming for help with the EQCM. References and Notes (1) Chao, S.; Wrighton, M. S. J. Am. Chem. SOC.1987, 109, 6627. (2) Salmon, M.; Addy, J.; Diaz, A. Proc. First Eur. Electrochromic Display Res. Conf., Munich 1981, 111. (3) Topart, P.;Josowicz, M. J . Phys. Chem. 1992, 96, 7824. (4) Bartlett, P. N.; Ling-Chung, S. K. Sensors Actuators 1989,20,287.
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