J. Phys. Chem. 1995, 99, 3288-3293
3288
Kinetics of Dichloromethane Absorption into the Conductive Polymers Poly(N-methylpyrrole) and Poly(N-methylpyrrole/polystyrenesulfonate) Daniel L. Feldheim, Susan M. Hendrickson, Michael Krejcik, and C. Michael Elliott” Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523
Colby A. FOSS,Jr. Department of Chemistry, Georgetown University, Washington, D. C. 20057 Received: July 29, 1994; In Final Form: September 22, 1994@
Mass transients have been measured for the sorption of dichloromethane from aqueous/electrolyte solution into the organic conductive polymers poly(N-methylpyrrole-C10~) and poly(N-methylpyrrole-polystyrenesulfonate). The data are analyzed within the context of two sorption models: pseudo-first-order kinetics for single-site adsorption and diffusion-limited absorption in an infinite plane sheet. Rate constants and apparent diffusion coefficients for absorption differ by a factor of 2 for oxidized and reduced poly(N-methylpyrroleC104) but are very similar for the two oxidation forms of poly(N-methylpyrrole-polystyrenesulfonate). These observations are discussed in terms of the relative hydrophobicities of the oxidized and reduced forms of each polymer. A detailed study of the sorption rate constant vs film thickness reveals information on the rate-limiting step in the absorption process.
Introduction Organic conducting polymers are now being extensively researched for applications based on their chemical properties rather than solely on their conductive properties. Examples of these studies include gas separation membranes,’ liquidlliquid separations and extractions,2 and vapor sensor^.^^^ With the exception of the work of Josowicz et al.3 and Bartlett et al? on the interactions of methanol vapor with polypyrrole, the study of the interactions of gas and liquid solutes with conducting polymers has been fairly limited. In a previous paper we examined the equilibrium partitioning of halogenated hydrocarbons from aqueous solution into poly(N-methylpyrrole) (PNMP) and poly(N-methylpyrrole/polystyrenesulfonate) (PNMPPSS) filmsS5 In that paper we characterized the absorption as Langmuirian, and we showed that for PNMP the equilibrium amount of dichloromethane (DCM) absorbed into the reduced form of the film is roughly twice that of the oxidized form. In contrast, for PNMPPSS films the amount absorbed into the oxidized and reduced forms is nearly identical. The behavior of PNMP was rationalized by considering the differences in chemical and physical nature of its oxidized and reduced forms (Le., density, ionic composition, and morphology). In contrast to PNMP, the two oxidation state forms of the composite film are very nearly alike in chemical and physical makeup. In this paper we consider the time dependence of incorporation of halogenated hydrocarbons into the PNMP and PNMP/ PSS films. In addition to comparing the relative temporal response of these two film systems, we are also interested in examining the applicability of two general models for neutral molecule incorporation which have been applied recently to the conducting polymer/gas interfa~e.~These two models are reviewed briefly below. Theory: Surface Kinetic and Plane Diffusion Treatments The fist model we consider assumes that the rate-determining step in neutral molecule incorporation involves adsorption to
* Corresponding author. @
Abstract published in Advance ACS Abstracts, February 15, 1995.
specific sites in the polymer film. Hence, for dichloromethane incorporation we can define a rate constant k for the overall adsorption process DCM(aq)
+ ES A [DCM-SI
(1)
where “ES” represents an empty adsorption site in the film medium and [DCM-SI represents the specifically bound dichloromethane molecule. If one assumes that the back-reaction rate is low and that the solution concentration of DCM is constant with time, the pseudo-first-order expression is valid:6 rate = -d[ES]/dt = k[ES]
(2)
where [ES] represents the concentration of empty sites. Rearrangement and integration yield the familiar form [ES]J[ES], = exp(-kt)
(3)
where [ESIr is the concentration of empty sites at any time t, and [ES]o is the initial concentration. The ratio on the lefthand side of eq 3 is simply the fraction of sites that have not been filled by DCM at time t. This ratio can be related to the DCM concentration in the film via the relation [ES]J[ES], = 1 - [DCM-SlJ[DCM-S],G
(4a)
where [DCM-SI is the concentration of DCM in the film at time t and [DCM-SI, is the concentration of DCM in the film at equilibrium. The factor G is given by
G = K[DCM]/(K[DCM]
+ 1)
(4b)
where K is the equilibrium constant corresponding to eq 1. Assuming that DCM adsorption does not involve an appreciable change in film volume, the right-hand side of eq 4a can be directly related to the mass change due to DCM at time t (Mt) and at equilibrium (M-).Combining eqs 3 and 4a thus leads to the relation
0022-3654/95/2099-3288$09.00/0 0 1995 American Chemical Society
Kinetics of Dichloromethane Absorption 1 - MJ(M,G) = exp(-kt)
J. Phys. Chem., Vol. 99, No. 10, 1995 3289
(5)
It should be noted that the left-hand side of eq 5 approaches 1 - MJM, when K[DCMl>> 1 or when [DCM-SI, corresponds to complete film saturation. One or both of these conditions must be fulfilled in order for the determination of the rate constant k to be straightforward. Finally, a key prediction of eq 5 is that the time dependence of the mass change should be independent of film thickness. An equation useful for the calculation of desorption rate constants can be derived with the single assumption that the desorption reaction occurs independent of any reabsorption (i.e., the reverse of reaction 1 occurs faster than the forward reaction). Note that the pseudo-first-order approximation is not necessary. In this case d[DCM-Slldt = -kd[DCM-S]
(6)
0-Rtngs
where the subscript d indicates desorption. Rearrangement and integration yield (MJM,) = exp(-k,t)
(7)
In this case kd represents the desorption rate constant and does not depend on the factor G as the adsorption rate constant above. In the second model considered, a diffusion model, the time dependence of the film's mass change is assumed to reflect diffusion of the neutral species within the polymer and not sitespecific adsorption. In this model the film is treated as an infinite plane, with diffusion occumng only along the coordinate x normal to the plane surface. Crank has reviewed solutions to the problem of diffusion into a planar film for various conditions of surface diffusant c~ncentration.~While Crank does not address explicitly the situation of a permeable film on an impermeable electrode surface, the appropriate solution is the same as that for a film of thickness 2L bounded on each face by solutions containing an equal concentration of diffusant species: M
Mr -=l-C
M,
,
.=0(2n
Q
8 0
+ l)2d
Puartz Crystal
exp[-D(2n i- 1)2df/4L2] (8)
In eq 8, D is the diffusion coefficient of the diffusant within the film, and L is the film thickness. In contrast to the surface kinetic model embodied in eq 5, the key prediction of eq 6 is that the time dependence of the mass change upon DCM incorporation should be dependent on film thickness.
Experimental Section Chemicals. N-Methylpyrrole (Aldrich) was freshly distilled under nitrogen atmosphere prior to every experiment. Acetonitrile (Baxter, Burdick and Jackson) and dichloromethane (Mallinckrodt) were used as received. Sodium perchlorate (Aldrich) used for PNMP film growth was dried overnight at 130 "C. The tetrahexylammonium hexafluorophosphate (THA+) salt of polystyrenesulfonate was prepared as described previously.* 18 MS2 (Millipore) water was used for all experiments. Instrumentation. EQCM experiments were performed at room temperature with AT-cut quartz crystals with 5 MHz oscillation frequency. The crystals were coated on both sides with a thin layer of chromium followed by gold. The electrode area was 0.34 cm2. A Philips PM 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 W y ~ m i n g .ASYST ~ software was used for data collection.
SIDE VIEW
KeI.F
From Pump
\
Ag Wire Rel.
To WasIe
I
FACE VIEW
0-Ring
/
\
Pt Counler EleClrOde
Figure 1. Schematic representation of the electrochemical quartz crystal microbalance flow-through cell used for kinetic absorption measurements .
The counter electrode was a platinum wire and the reference electrode was SSCE for all experiments. Polymer Growth. Poly(N-methylpyrrole) films were grown from solutions of acetonitrile containing 0.1 M NaC104 and 1.0 M N-methylpyrrole monomer. Poly(N-methylpyrrole/polystyrenesulfonate) films were grown as above but with 0.1 M tetrahexylammonium polystyrenesulfonate as the supporting electrolyte. Both films were synthesized at a constant potential of +0.8 V vs SSCE. Visual inspection indicated that the polymer film covered the entire gold surface. Following synthesis the acetonitrile solution was replaced with an aqueous solution containing 0.1 M NaC104. The film was then cycled between + O S V and -0.4 V vs SSCE until no change in the voltammetry was observed (-20 cycles). For the EQCM experiments, once film stability was ensured, the film was removed from the three-compartment electrochemical cell and placed in a flow-through electrochemical cell. A brief description of this cell follows. Two kel-f blocks sandwich the quartz crystal between two O-rings (Figure 1). The face of one block contains an inlet for solution from an HPLC pump (Varian), an outlet to waste, a silver wire pseudoreference electrode, and a platinum wire counter electrode. With this apparatus the potential of the working electrode is held constant while solution containing 0.1 M NaClOd(aq) is pumped over the polymer. With the frequency at a constant value, the flow is changed to a solution containing 0.1 M NaC104(aq) with the indicated concentration
3290 J. Phys. Chem., Vol. 99, No. 10, 1995
Feldheim et al.
300
255 210-
2 -150 E
1
165-
B
-200 -
120-
,
-
a -250 -300
i
i
------
1
-400
4
I 0
-10
20
10
40
30
time (9)
-50
-100
1i
1
1 L
4
i
-200 -250
7-
-60
I 0
I
1
2
3
4
-- -
t---
I
5
6
7
time (s)
Figure 3. Frequency vs time for a saturated solution of SrClOl(aq) injected over a bare gold-coated quartz crystal.
of DCM. When the absorption of DCM reaches equilibrium the pump is switched back to the original solution and DCM desorption occurs. It is important to note that the frequency change of a quartz crystal in the absence of a polymer film due to DCM is less than ca. 10 Hz (due to solution density changes); and when the flow is changed between identical 0.1M NaC104(aq) solutions with a polymer film on the crystal, no change in frequency is observed. Finally, we note that the equilibrium DCM uptake increases with polymer thickness (Figure 2). This indicates bulk absorption of DCM into the polymer and constant rigidity of the polymer during the entire absorption process. Flow-Through Cell Characterization. Since the determination of absorption rate constants assumes a uniform and constant solution concentration of absorbent, the flow characteristics of any liquid flow-through cell are crucial to accurate kinetic measurements. To determine the extent of sample dilution between the sample loop and the quartz crystal, a solution of saturated SrClOd(aq) was placed in the loop and injected over a bare quartz crystal. This solution has a density of ca. 1.3 g/cm3 compared to l.0g/cm3 for the NaC104(aq) solution continually flowing over the crystal. If no spreading of the solution occurs from the sample loop to the crystal, then an immediate drop in frequency of the crystal would be observed due to the change in density of the solution. Figure 3 is a plot of frequency vs time for such an experiment. At flow rates of > 10 mL/min, the time for the total change in frequency to occur
is ca. 3 s, which means that only slight dilution of the sample occurs before it reaches the crystal. The rate constants reported within this paper reflect only data acquired past at least the first 3 s of DCM absorption. Furthermore, kinetic measurements on polymer films were performed at several flow rates to ensure that the absorption rate constant was independent of the solution flow rate.
Results EQCM Responses of PNMP and PNMPPSS to DCM. Absorption and desorption EQCM traces for a 3500 8,thicklO.ll PNMP film are shown in Figure 4A. The top and bottom traces in Figure 4A correspond to conditions where the film is oxidized and reduced, respectively. The first observation from Figure 4A is that the rate of DCM absorption is equal to the rate of desorption. This is in direct contrast to the observations of Josowicz et al. with respect to the absorption of methanol vapor into p ~ l y p y r r o l e .Figure ~ ~ 4A also emphasizes the difference in equilibrium partitioning of DCM into the two forms of PNMP as mentioned above. For this film, the DCM content in the reduced form is nearly 3x greater than the oxidized forms.
Kinetics of Dichloromethane Absorption
J. Phys. Chem., Vol. 99, No. 10, 1995 3291 1
1
-
7
-
7
-
7
-0.:
-
h
-2.5
0.775
c
+TI
1J
1
i
-3 0.1
-3.5 11
12
13
14
15
16
17
18
19
1
0.05
tkec)
Figure 5. In( 1 - M,/M=) vs time for DCM absorption into reduced PNMP. Each diamond represents a single data point from the transient data. The slope gives the overall rate constant for absorption (see Table 1). R2 = 0.99.
Typical differences in DCM content between the reduced and oxidized forms of PNMP are 1.5-2.5x, while little difference in partitioning is observed for the two oxidation forms of PNMP/ PSS (Figure 4B). Comparison with the Surface Adsorption Kinetic Model. We have found that for both films the absorption follows pseudo-first-order kinetics (eq 5). A typical plot of -ln(l M,/M,) vs time for DCM absorption into PNMPPSS, where Mt is the mass of sorbent at some time and M , the equilibrium mass uptake is shown in Figure 5 . The slope gives the overall rate constant for absorption. In Figure 6 the rate constants determined from the fit of experimental data to eq 5 are plotted against film thickness. For both the PNMP and PNMPPSS systems the apparent rate constant is independent of film thickness. Rate constants for absorption and desorption of DCM in PNMP and PNMPPSS films as a function of oxidation state are summarized in Table 1. Note that for the composite film, the rate constant is independent of oxidation state whereas for PNMP the rate constant is significantly lower for the oxidized film compared to the reduced film. Comparison with the Plane Diffusional Model. The transient data for DCM absorption into these films were also examined using the diffusional model given by eq 6 restated in the following form which includes the first three expansion terms: 1 - M J M , = 0.81 exp(-0.25At)
+ 0.09 exp(-2.25Ar) + 0.032 exp(-6.25At)
(9)
where A = Dn2/L2and D is the diffusion coefficient of DCM in the polymer phase. The coefficient A was evaluated for the different film systems by fitting eq 9 to the experimental data using an iterative approach. Table 2 summarizes the experimental A values for reduced PNMP films of different thicknesses and the inferred diffusion coefficients D. A note of caution must be expressed at this time with regard to any attempt to differentiate between the diffusion and the kinetic models with data obtained from a single film. The ambiguity arises in such attempts because both equations predict approximately exponential rates of sorption at long times. The quality of the data is not sufficient to distinguish between the subtle time-dependent differences in eqs 5 and 8. Therefore, information on the rate-limiting step in the sorption process can be obtained only through a study of the effect of film thickness on the rate of sorption (Figure 6 ) .
,
L L I
0.1
- - v ,L
- 1 -
0.15
0.2
_
L
L
A
0.25
-
0.3
Film Thickness (pn')
Figure 6. Rate constant for DCM abso,rption vs film thickness squared for six reduced PNMP films. Error bars represent the standard deviations of five trials for each film. TABLE 1. DCM Absorption and Desorption Rate Constants for a 350 nm Thick PNMP Film and a 380 nm Thick PNMPPSS Film (DCM Concentration Saturated in 0.1 M NaC104(aq)) PNMP PNMP-Clod PNMP/PSS-Na+ PNMPPSS a
0.40 f 0.12" 0.22 f 0.02 0.41 f 0.05 0.44 f 0.04
0.44 f 0.10 0.24 f 0.06 0.45 f 0.07 0.40 f 0.05
f values are the standard deviations of five trials for each film.
TABLE 2. A Parameters and Apparent Diffusion Coefficients for Five Reduced PNMP Films of Differing Thicknesses film thickness (pm) A (s-') D (cm2/s) x 1010 0.270 0.370 0.408 0.410 0.515
2.8 f 0.9 1.8 f 0.4 2.8 f 0.3 1.9 f 0.2 2.3 f 0.4
2.1 2.5 4.7 3.2 6.2
Discussion The difference in the adsorption rate constants and diffusion coefficients between the oxidized and reduced forms of PNMPC104 can be straightforwardly rationalized by considering the differences in their chemical and physical properties. PNMPC104 has a density of 1.5 g/mL vs 1.0 g/mL for reduced PNMP. The electrostriction present in the oxidized form reduces the free volume available for diffusion in this polymer. The diffusion coefficient is thus lowered compared to the less-dense reduced material. The kinetics of adsorption of DCM into oxidized PNMP are lowered relative to adsorption into reduced PNMP due to the chemical differences in these films. The high ionic content of PNMP-ClO4 provides a much less favorable environment for adsorption compared to the nonionic reduced PNMP. This is in agreement with the relative amounts of DCM adsorbed at equilibrium. The differences in the chemical and physical properties of the two oxidation forms of the composite material are, on the other hand, much less pronounced than for conventional PNMP. The two oxidation forms of the composite film have similar densities (-1.0 g/mL) and the polymer is ionic irrespective of its oxidation state. Consequently, it is reasonable that the diffusion coefficients and rate constants of adsorption are similar for the two forms as are the equilibrium mass uptakes. The fact that there appears to be no dependence on polymer thickness for the rates of DCM sorption and desorption is less
3292 J. Phys. Chem., Vol. 99, No. 10, I995
Feldheim et al.
straightfonvard to rationalize. Below we consider three simple cases and discuss the merits of each. Case 1. Nonporous Film; Diffusion-Limited Absorption. For this case the rate of uptake and loss of DCM is simply governed by diffusion of DCM through a uniform, homogeneous polymer phase. Here eq 8 would govern the rate. Consideration of eq 8 shows that were this the mechanism operative there should be a linear dependence of the rate on ULZ,where L is the polymer thickness. As can be seen from Figure 6 over a range of polymer thicknesses from ca. 200 to 500 nm there is no experimentally significant change in the rates of either uptake or loss, whereas eq 8 would predict a factor of 6 x difference between the rates of thinnest and thickest films. Case 2. Nonporous Film; Site SorptionlDesorption Rate Limiting. Like the case above, again the polymer phase is considered to be homogeneous but now the rate of diffusion through the polymer is assumed to be faster than the rate at which the DCM binds and releases from the sorption sites. Equation 5 , which is independent of film thickness, would govern this case. From a purely mechanical standpoint this model is consistent with the data presented. However, from a molecular level this case seems unreasonable on two counts. First, the partition equilibrium for DCM is large, yet the rates of sorption and desorption are comparable, which would not be expected if site-binding were rate limiting. Second, if sitebinding were rate limiting, a significant activation barrier would be necessary which would, in turn, imply a large structural and/ or electronic perturbation of the DCM molecule andor the polymer. We have previously reported the W-visible spectral changes of PNMP which accompany its saturation with DCM, and they are, at most, modest. Thus, from the perspective of the polymer, at least, there is no evidence of either major elecbonic or structural changes which accompany DCM adsorption. Case 3. Porous Film; Pure Diffusion Rate > Polymer Diffusion Rate. This case differs from either of the previous two models in that here the polymer is treated as nonhomogeneous and considered to contain open pores which are filled with solvent. There are, thus, two different diffusion coefficients involved in transporting DCM molecules. The solution diffusion coefficient, D,,governs diffusion in the open pores of the film while the polymer diffusion coefficient, Dp, governs diffusion in the bulk of the film. A simplified schematic illustration of this model is given in Figure 7. If diffusion of sorbent in solution is much faster than diffusion into the polymer ‘Ylhrils“, the pores quickly reach and maintain a constant concentration of sorbent (DCM). Slow diffusion of sorbent into the polymer film then follows. The rate constant of absorption will, consequently, be independent of film thickness provided that the macroscopic thickness is not so great that the time required for DCM to diffuse the length of the pores exceeds the time required for the sorbent to saturate the microscopic polymer phase (Le., more than ca. 2 s). Using the Einstein-Smoluchowski equation6
AL = (2DsAt)”2
(10)
and assuming a constant value of D, of cm21s, we can estimate that to decrease the rate of equilibrium mass uptake by as little as 2 s, films differing in thickness by tens of microns would he necessary. For practical reasons, this is not feasible with the EQCM experiment. As with case I, the rate of absorption will equal the rate of desorption. Considering the three cases above and the data reported within this paper, it appears that the sorption of DCM into PNMP and PNMPPSS is best described by case 3. It must he pointed out,
A
Pae
\\
DCM DCM DCM DCM DCM
C
Figure 7. Simplified schematic illustration or diffusion in a porous polymer membrane. (A) Introduction of DCM in solution next to polymer surface and fast diffusion into the pores. (B and C) Slower diffusion into the bulk of the polymer.
however, that these conductive polymers are complicated systems with complicated morphologies and that simple models such as those considered above are certainly oversimplifications of the real system. Summary and Conclusions Fundamental investigations of the interactions of neutral solutes and organic conducting polymers are necessary before future applications of these materials as separation membranes and sensors are fully possible. We have investigated the kinetics of dichloromethane absorption from aqueous solution into the conductive polymers poly(N-methylpyrrole) and poly(N-methylpyrrolelpolystyrenesulfonate). For PNMP films, the rate constant of absorption into the reduced form is approximately twice that of the perchlorate-doped, oxidized form. No trend was found between the rate constant and the square of the film thickness. However, we note that at thicknesses less than ca. 100 nm the kinetics of DCM absorption are too fast to measure. In contrast to PNMP, the rate constant of absorption of DCM into PNMPPSS is nearly identical to that of the oxidized form of the film. As with PNMP, however, the rate constant is independent of the square of the film thickness. Acknowledgment. The authors with to thank the CUBoulderAAB Center for Separations Using Thin Films and the National Science Foundation (CHE 9311694) for support of this work. S.M.H. wishes to thank the Depamnent of Education for additional financial support. D.L.F. gratefully acknowledges Procter and Gamble for financial support. References and Notes ( I ) (a) Andenon, M. R.; Mattes, B. R.; Reiss, H.: Kaner. R. B. Science 1991.252, 1412. (b) Liang, W.: Manin. C. R. Chem. Mater. 1991,3, 390. (2) (a) Feldheim. D. L.; Elliott. C. M. 3. Memhr. Sci. 1992, 70, 9. (b) Schmidt, V. M.: Tegtmeyer, D.: Heitbaum, H. Ad”. Mater. 1992, 4, 428. (3) (a) Topan, P. : Josowicr. M. J. Phys. Chem. 1992, 96. 7824. (b) Blackwood. D.; Josawicz, M. 3. Phys. Chem. 1991.95.493. (c) losowicz,
Kinetics of Dichloromethane Absorption M.; Janata, J.; Ashley, K. E.; Pons, S. Anal. Chem. 1987,59,253. (d) Topart, P.; Josowicz, M. J. Phys. Chem. 1992, 96, 8662. (e) Josowicz, M.; Janata, J. Anal. Chem. 1986, 58, 514. (4) (a) Bartlett, P. N.; Ling-Chung, S. K. Sensors Actuators 1989, 20, 287. (b) Bartlett. P. N.: Gardner. J. W.: Whitaker, R. G. Sensors Actuators 1990,A21, 911. ( 5 ) Feldheim, D. L.; Kreicik, M.; Hendrickson, S.M.; Elliott, C. M. J. Phys. Chem. 1994, 98, 5714: (6) Atkins, P. W. Physical Chemistry, 4th ed.; W. H. Freeman and Company: New York, 1990. (7) Crank, J. The Mathematics of DifSusion; Clarendon Press, Oxford, 1956.
J. Phys. Chem., Vol. 99, No. IO, I995 3293 (8) Elliott. C. M.: Kouelove. A. B.: Albery, W. J.; Chen, Z. J. Phys. Chem.’1991, 95 (4), 1743: (9) Orata, D.: Buttry, D. A. J. Am. Chem. SOC.1987, I09 (12), 3575. (10) F i thickness was calculated using the mass of the film (determined from the frequency change of the quartz crystal during synthesis), the densities of the films and the area of the working electrode (0.34 cm2). The density of PNMPPSS was determined using standard crystallographic techniques. The density of PNMP is given in ref 11. (11) Diaz, A. F.; Bargon, J. In Handbook of Conducting Polymers; Marcel Dekker, Inc.; New York, 1986; Vol. 1, pp 81-116. JP94 19858
