Langmuir 1996,11, 1443-1447
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Articles Solubilization and Adsolubilization of Pyrrole by Sodium Dodecyl Sulfate: Polypyrrole Formation on Alumina Surfaces Gary P. Funkhomer,’ Maria P. Ar6valo,$Daniel T. Glatzhofer,*yt and Edgar A. O’Reart Department of Chemistry and Biochemistry, Department of Chemical Engineering and Materials Science, Laboratory for Electronic Properties of Materials, and the Institute for Applied Surfactant Research, The University of Oklahoma, Norman, Oklahoma 73019 Received October 7, 1994. I n Final Form: January 11, 1995@ The effect of added sodium chloride (0-0.5 M) on the solubilization behavior of pyrrole in micellar sodium dodecyl sulfate (SDS) solution was determined using semiequilibrium dialysis. The highest solubilization constant, K, was 2.85 M-l in the absence of salt. The minimum K observed was 1.95 M-l at 0.1 M salt. Static SDS adsorption isotherms and pyrrole adsolubilization on alumina powder were obtained with varying concentrations of pyrrole (0-0.016 M) and sodium chloride (0-1.5 M). Pyrrole was found to decrease surfactant adsorption in contrast to alcohols, alkanes, and aromatic hydrocarbons. Pyrrole adsolubilization values ranged from 7 to 25 pmoYg of alumina. A particular pyrrole adsolubilization value can be attained with a lower equilibrium SDS concentration by adding salt. The interfacial concentration of pyrrole is increased by the addition of salt, enabling the use of admicellar-assisted polymerization with ammonium persulfate t o produce thin, well-connected films of polypyrrole salts on alumina plates, demonstratingthe applicabilityofthe techniquet o monomers with moderate water solubility. The resistance of the film is comparableto much thicker films produced in the absence of salt and surfactant. No film formation was observed using solutions of SDS without added salt.
Introduction The use of surfactants adsorbed onto solid surfaces as a medium for the formation of ultrathin polymer films has been reported in recent years using a three-step process. 1-7 First, surfactants are adsorbed from aqueous solutions as admicelles (or hemimicelles or reverse hemimicelles) onto a solid substrate. By manipulation of factors governing adsorption behavior (e.g. surface charge and counterion concentration for ionic surfactants), nearly complete bilayer coverage of the surface can be achieved at the interface. 1-5,8-11 Second, a polymerizable monomer with low water solubility is allowed to partition into the admicellar layer (adsolubilization). Finally, polymerization on the solid surface is initiated. The surfactant is generally washed away with water, leaving a thin polymer coating. This process has been referred to as admicellar polymerization or the thin film via surfactant template (TFST) technique and has been described as a surface Department of Chemistry and Biochemistry. Department of Chemical Engineering and Materials Science. Abstract published in Advance A C S Abstracts, April 15,1995. (1)Wu, J.;Hanvell, J. H.; ORear, E. A,; Christian, S. D. MChE J . 1988,34, 1511. (2) Wu, J.;Hanvell, J . H.; O’Rear, E. A. Langmuir 1987, 3, 531. (3) Wu, J.:Hanvell, J. H.: O’Rear. E. A. J . Phvs. Chem. 1987,91,623. (4) Chen, H . Masters Thesis, University of”Oklahoma, 1992. (5) Lai, C. Masters Thesis, University of Oklahoma, 1992. (6) Waddell, W. H.; OHaver, J. H.; Evans, L. R.; Hanvell, J . H. J . Appl. Polym. Sci. 1995, 55, 1627. (7) O’Haver, J . H.; Hanvell, J. H.; O’Rear, E. A,; Snodgrass, L. J.; Waddell, W. H. Langmuir 1994, 10, 2588. ( 8 ) Soderlind, E. Langmuir 1994, 10, 1122. (9) Harwell, J. H.; Hoskins, J. C.; Schechter, R. S.; Wade, W. H. Langmuir 1985, 1, 251. (10)Hough, D. B.; Rendall, H. M. InAdsorptionfrom Solution at the Solid /Liquid Interface;Parfitt, G. D., Rochester, C. H., Eds.; Academic Press: New York, 1983; pp 275-277. (11)Bitting, D.; Hanvell, J. H. Langmuir 1987, 3, 500. +
@
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analogue of emulsion polymerization. The TFST method has potential for the formation of anticorrosion and lubricating coatings,12 and composite materials formed using this technique may be valuable for use as pigments, reinforcing fillers,6 and chromatographic packings.13 Formation of thin films of electrically conducting polypyrrole (PPy) salts14 has been of interest for the fabrication of electrodes,15J6 sensor^,^^-^^ and, more recently, modification of surfaces to facilitate cell attachmentez0In particular, selective adsorption with microlithography and admicellar polymerization offers a prospective route to patterned conducting polymeric structures. It is therefore desirable to apply the TFST technique to the formation of PPy films on surfaces. In principle, if sufficient pyrrole partitions into adsorbed surfactant bilayer structures in such a manner that it remains accessible to chemical oxidants, polymerization in TFST fashion should occur to form a thin layer of (12) Funke, W. In Polymeric Materials for Corrosion Control;Dickie, R. A., Floyd, F. L., Eds.; ACS Symposium Series 322;American Chemical Societv: Washineton. DC. 1986: D 224. (13jIvanov, AT E.: Saburov, V.; Zubov, V. P. Adv. Polym. Sci. 1992, 104, 135. (14) Kanazawa, K. K.; Diaz, A. F.; Geiss, R. H.; Gill, W. D.; Kwak, J. F.; Logan, J. A.; Rabolt, J. F.; Street, G. B. J . Chem. Soc., Chem. C o m m u i 1979,854. (15) Miller, L. L.; Zinger, B.; Zhou, Q. J . A m . Chem. SOC.1987,109, 2267. (16) Bull, R. A,;Fan, F. F.;Bard, A. J . J . Electrochem. Sac. 1982,129, 1009. (17) Nylander, C.;Armgarth, M.; Lundstrom, I . In ChemicalSensors; Seiyama, T., Fueki, K., Shiokawa, J., Suzuki, S., Eds.; Analytical Chemistry Symposia Series 17; Elsevier: New York, 1983; pp 203-
v.
207. -.
(18)Imisides, M. D.; John, R.; Riley, P. J.; Wallace, G. G. Electroanalysis N.Y. 1991, 3, 879. (19) Sadik, 0.A,; Wallace, G. G. Electroanalysis N.Y. 1993,5, 555. (20) Wong, J. Y.; Langer, R.; Ingber, D. E. Proc. Natl. Acad. Sci. U S A . 1994, 91, 3201.
0 1995 American Chemical Society
1444 Langmuir, Vol. 11, No. 5, 1995 conducting P P y salts according to eq 1. However, it was
anticipated that application of t h e TFST technique to polymerization of pyrrole would be problematic due to its water solubility (> 5%),emphasizing one of the limitations of t h e technique. In spite of numerous publications concerning polymerization of pyrrole in the presence of s ~ r f a c t a n t s , ~little l - ~ ~attention has been paid to the role of organized surfactant structures in t h e process, and little or no information on the solubilization or adsolubilization of pyrrole by surfactants exists. Such studies would be useful in interpreting results of polymerization of pyrrole in micellar media and in devising strategies to overcome the current monomer water solubility limitations of the TFST techniques. We report here the results of solubilization and adsolubilization (on alumina) of pyrrole by sodium dodecyl sulfate (SDS),the effect of adding sodium chloride on t h e solubilization behavior, and, finally, the results of initial studies on chemical polymerization of pyrrole in SDS media to form PPy salt thin films on the surface of alumina.
Experimental Section Sodium dodecyl sulfate (98%,Aldrich) was recrystallized from 95% ethanol. Pyrrole (98%,Aldrich) was purified by passing it through a short column ofbasic alumina, activity grade I (Sigma). Alumina plates (15 mm x 10 mm x 1.0 mm) were cleaned by soaking overnight in a solution comprised of equal volumes of 30%hydrogen peroxide and 30%aqueous ammonia, rinsing with distilled water, and drying in a vacuum oven at 60 "C. High surface area (155 m2/g)acidic alumina powder, activity grade I, was obtained from Aldrich. Reagent grade ammonium persulfate (Mallinckrodt) and sodium chloride (Baker) were used without further purification. Pyrrole concentrations were determined using aVarian Model 3700 gas chromatograph with a flame ionization detector interfaced with a Varian CDS l l l C integrator. An aqueous amine analysis column (Supelco; 60/80 Carbopack B, 4% Carbowax 20M, 0.8%KOH) was used at 160 "C with nitrogen as the carrier gas. Surfactant concentrations from the semiequilibrium dialysis experiments were obtained using a Dohrmann DC-180 Total Organic Carbon Analyzer (Rosemount Analytical), subtracting the contribution from the pyrrole. Surfactant concentrations from the adsorption studies were determined using a Perkin-Elmer Series 4 liquid chromatograph consisting of a Tracor model 951A pump, a Microsorb-MV 3 pm lOOA C18 reversed-phase silica gel column (Rainin Instrument Co.), and an Alltech 320 conductivity detector interfaced with a Varian 4270 integrator. Pyrrole Solubilization Measurements. The modified semiequilibrium dialysis method developed by Uchiyama et aLZ6 was used. Ordinary 5 mL dialysis cells were prepared using 6000 Da cutoff regenerated cellulose membranes (Fisher), presoaked in distilled water before use. The retentate side was filled with a solution of 0.200 M sodium dodecyl sulfate (SDS), 0.010 M pyrrole, and salt as required. The permeate side was filled with 0.050 M SDS and salt to equal the salt concentration (21) Warren, L. F.; Anderson, D. P. J . Electrochem. SOC.1987,134, 101. (22) Shimidzu, T.; Ohtani, A.; Iyoda, T.; Honda, K. J . Electroanal. Chem. Interfacial Electrochem. 1987,224,123. (23)De Paoli, M. A.;Panero, S.: Prosperi, P.; Scrosati. B. Electrochim. Acta 1990,35,1145. (24) John, R.; John, M. J.;Wallace, G. G.; Zhao, H. InElectrochemistry in Colloids andDisDersions: Mackav, .. R. A,. Texter., J.., Eds.:. VCH: New York, 1992; Chapter 17. (25)Schmidt,V.M.; Barbero, C.;Kotz,R. J . Electroanal. Chem. 1993, 352,301. (26) Uchiyama, H.; Christian, S. D.; Tucker, E. E.; Scamehorn,J. F. J . Phys. Chem. 1993,97,10868. I
Funkhouser et al. in the retentate. Typically, four cells were filled with the same retentate and permeate solutions. The cells were placed in a 30.0 "C bath for 18-24 h. Pyrrole and surfactant concentrations in the permeate and retentate were then determined using gas chromatography and total organic carbon analysis. Results were obtained in the form of the solubilization constant, K = X/corg where X is the mole fraction of pyrrole in the micelle and corgis the concentration in molarity of free pyrrole in the bulk aqueous phase. SDS Static Adsorption Isotherms and Pyrrole Adsolubilization. A stock solution of 0.016 M SDS was prepared using water adjusted to pH 4 with hydrochloric acid. This stock solution was used to prepare feed solutions. Feed solution (10 mL) was added to 16 mm x 125 mm screw cap test tubes containing 0.50 g of acidic alumina. The tubes were capped and allowed to equilibrate at 30.0 "C for 4 days. Then, 5 mL portions of a solution of pyrrole and NaCl (concentrations appropriate to achieve the desired final concentrations) in water adjusted to pH 4 were added to the tubes. The samples were allowed to equilibrate for 1 day. Sequential adsorption and salt addition prevented surfactant solubility problems at high salinity. The samples were centrifuged to settle the alumina particles and the pyrrole and SDS concentrations in the supernatant were analyzed using GC and HPLC (the mobile phase was water until the salt was eluted and then was switched to 80% vlv methanol in water to elute the surfactant). The adsolubilization constant, K&, used by Wu et aL2 was calculated as shown in eq 2.
Kads= (moles of adsolubilized pyrrole per mole of adsorbed SDS)/(pyrrole molarity in supernatant) (2)
Polymerization of Pyrrole on Alumina Surfaces. Alumina plates were placed in 4 dram vials and covered with 5 ml of one of the following four monomer solutions: 20 mM pyrrole; 20 mM pyrrole and 1.5 M NaC1; 20 mM pyrrole and 15 mM SDS; 20 mM pyrrole, 15 mM SDS, and 1.5 M salt. The vials were placed in a bath at 30 "C. After 30 min, an equimolar amount of ammonium persulfate (based on the pyrrole) was added as a concentrated solution. After 4 h, the plates were rinsed with distilled water and dried at 60 "C under vacuum. The resistance of the plates was measured along the length of the plate using a Keithley 610C electrometer.
Results and Discussion In order to form thin films of PPy on an alumina surface, the pyrrole m u s t be sufficiently concentrated at t h e interface and subsequently polymerized. The admicelle could provide this concentrating effect. Because pyrrole is miscible with most organic solvents but has limited water solubility (6 g/lOO g of water at 25 0C),27 solubilization in a micelle or admicelle was expected to be reasonably high. Nevertheless, semiequilibrium dialysis studies showed the solubilization ofpyrrole in SDS micelles to be unexpectedly low (solubilization constant, K = 2.8 M-I). To p u t this in perspective, cresols solubilized in 1-hexadecylpyridinium chloride micelles at the same mole fraction in the micelle have K values around 180 M-1.28 In order to facilitate t h e eventual admicellar-assisted polymerization, it seemed prudent to t r y to increase K. If pyrrole solubilizes in the hydrocarbon core of the micelle, increasing t h e micelle aggregation number by addition of neutral electrolyte should lower the Laplace pressure, allowing more pyrrole to solubilize in the micelle.29 However, polar organic solutes typically solubilize in the palisade region, which becomes less accessible to the solute upon increasing the aggregation number due to t h e closer packing of the surfactant head groups.29 The addition of (27)Vaughan, W. R. In Encyclopedia of Chemical Technology;Kirk, R. E., Othmer, D. F., Eds.; Interscience: New York, 1953;Vol. 11,p 340. (28)Bhat, S. N.; Smith, G. A.; Tucker, E. E.; Christian, S. D.; Scamehorn, J. F.; Smith, W. Ind. Eng. Chem. Res. 1987,26,1217. (29) Mukejee, P. Pure Appl. Chem. 1980,52,1317.
Polypyrrole Formation on Alumina
Langmuir, Vol. 11, No. 5, 1995 1445
3 ,
1000
I
s h
1 8
Pymle mnc. 100
'El
5.3mM
b 10
0
16mM
1 10
14
0
1000
100
Equilibrium SDS conc. WM)
100
200 300 NaCl conc. (mM)
400
500
Figure 3. Adsorption isotherm of SDS on alumina at various pyrrole concentrations with 0.33 M NaC1.
Figure 1. Effect of salt on the solubilizationconstant of pyrrole in SDS solution.
1000
NaCl conc.
1000
s 8 h
Pyrrole conc. 100
CI
OM
100
3
v)
.I
'P
=
OmM
0
6mM
+
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0
4
0.33 M 1.OM
VI
CI v)
0
1.5M
10
100
10
VI
0.1
1000
10000
EquilibriumSDS conc. (CLM) 10
100
1000
10000 100000
Equilibrium SDS conc. p M )
Figure 2. Adsorption isotherm of SDS on alumina at various
Figure 4. Adsorption isotherm of SDS on aluminawith various
salt concentrations. The pyrrole concentrationwas 16 mM in all cases.
pyrrole concentrations.
salt increases the activity coefficient of pyrrole in water, which may work to increase the solubilization in micelles. In order to determine the effect of sodium chloride on pyrrole solubilization by SDS, K values were obtained over a range of salt concentrations (Figure 1). Initially as the salt concentration increases, K decreases, which is consistent with the behavior expected for polar organic solutes. However, with salt concentrations above approximately 100 mM, this trend is reversed. This could be explained by the transition from spherical t o rodlike micelle^.^^^^^ Once the rodlike micelles form, further increases in salt concentration have a smaller effect on the surfactant packing density in the micelles, allowing the increase in the activity coefficient of the pyrrole in the bulk solution to become the dominant factor for the minor changes in K at high salt concentration. The K values over the entire range of salt concentrations tested remained lower than with no added salt. In light of the solubilization behavior of pyrrole in micellar solution, what to expect from the admicellar case was not obvious. In order to determine the adsolubilization of pyrrole in the admicelle, it was first necessary to ascertain if pyrrole itself is adsorbed on the alumina surface. In the absence of surfactant, no change in pyrrole concentration could be detected after adding alumina to an aqueous solution of pyrrole. The SDS adsorption isotherm in Figure 2 demonstrates the effect of pyrrole on SDS adsorption. The pyrrole causes a decrease in surfactant adsorption below the plateau region, atypical of most organic solutes studied. Upon addition of salt, the effect of the pyrrole on SDS adsorption is greatly reduced, as shown in Figure 3. As expected, the addition of salt dramatically improves SDS adsorption (Figure 4). This effect was reported in an earlier work by Bitting and HanvelLll although with much lower concentrations of added salt.
-
(30) Reiss-Husson, F.; Luzzati, V. J. Phys. Chem. 1964,68,3504. (31) Rusling, J. F. Acc. Chem. Res. 1991,24,75.
20
t
0
400 600 800 Equilibrium SDS conc. (CLM)
200
loo0
Figure 5. Effect of salt on the adsolubilization constant of pyrrole in SDS admicelles on alumina. The initial pyrrole
concentration was 16 mM in all cases. The adsolubilization behavior can be expected to be influenced by the surfactant head group packing density and pyrrole activity. The head group area of an SDS molecule in an admicelle in the plateau adsorption region has been reported to be 52-54 A2 in the absence of salt, assuming complete bilayer coverage.8J1 Head group areas calculated from plateau adsorption of SDS in salt solution show a decrease to approximately 35 A2 at a sodium chloride concentration of 0.15 M.ll The head group arFa of an SDS molecule in a hemisphere-capped rod is 48 A2, compared to 67 A2 for the head group area in a spherical micelle.32 Considering the packing density in the admicelle, one might predict the admicelle to have similar solubilizing behavior to rodlike micelles in solution such that the adsolubilization constant, Kads, increases with increasing salt concentration. Figure 5 shows the effect of the equilibrium SDS concentration on Kads,indicating a decrease in K& with increasing surface coverage. If the pyrrole can solubilize at the perimeter as well as the interior of the admicelle (two-site adsolubilization modeP), (32) Soderman, 0.; Jonstromer, M.; van Stam, J. J. Chem. SOC., Faraday Trans. 1993,89,1759. (33) Lee, C.; Yeskie, M. A,; Hanvell, J . H.; O'Rear, E. A. Langmuir 1990,6,1758.
Funkhouser et al.
1446 Langmuir, Vol. 11, No. 5, 1995
t
I
OJ
0
200
400
600
800
O
lSM
loo0
Equilibrium SDS conc. WM)
Figure 6. Effect of salt on pyrrole adsolubilization in SDS admicelles on alumina. The initial pyrrole concentration was 16 mM in all cases.
increasing surface coverage with its concomitantincrease in admicelle aggregation number would decrease the perimeter solubilization relative to the total solubilization. The porous nature of the substrate may also contribute to the decrease in K a d s with increasing surface coverage due to the negative radii of curvature for admicelles formed in pores. Both of these effects are consistent with the curves in Figure 5. Although K a d s decreases with increasing surface coverage, the total amount of pyrrole adsolubilized increases with the extent of the surface phase (Figure 6). It is interesting to compare the adsolubilization of pyrrole to the results for other species in SDS admicelles. The completely water soluble compounds ethanol and 2-propanol do not exhibit measurable adsolubilization nor do they influence surfactant a d s ~ r p t i o n .More ~ ~ hydrophobic alcohols (e.g. n-butan01,3~>~~ n - p e n t a n ~ land ~ ~ nhexan01~~) and aromatic hydrocarbons2all tend to enhance surfactant adsorption in the intermediate region below plateau adsorption, in stark contrast to the findings for pyrrole. For partial coverage with surfactant, adsolubilization sites for the alcohols are thought to be in the interior of the admicelle and at the periphery of the adsorption aggregate^.^^ As surface coverage becomes more complete, only interior sites are available and the ratio of adsolubilizate to surfactant approachesa lower constant value. Pyrrole adsolubilization exhibits similar behavior, with high values of K a d s a t low SDS concentrations decreasing to a constant at higher SDS concentrations. Because the supernatant pyrrole concentration change is low, changes in K a d s follow the trend in the ratio of adsolubilized pyrrole to adsorbed SDS. For example, in the system with 1.0 M NaCl and 16 mM pyrrole, &ds changes from 19.9 to 3.5 M-l as this ratio changes from 1:3to 1:19 across the range shown in the adsorption isotherm. Linear alcohols apparently orient with the hydroxyl moiety in the palisade layer and with the hydrocarbon chain aligned with the surfactant resulting in efficient packing and little effect on the plateau adsorption. The branched alcohol 2-methyl-2-hexanol disrupts this packing with an attendant decrease in plateau adsorption compared to n - h e ~ a n o l .Pyrrole, ~~ a much more hydrophilic species, probably adsolubilizes in the head group region of the admicelle. Aromatic hydrocarbons, such as benzene, adsorb at the micelle/water interface as well as solubilize in the interior.38 Pyrrole, being aromatic, could be expected to behave similarly, but with the added interaction of hydrogen bonding to the head groups. This (34)Yeskie, M.A.Ph.D. Dissertation, University of Oklahoma, 1988. (35)Sjoblom, J.;Blokhus, A. M.; Sun, W. M.; Friberg, S. E. J.Colloid Interface Sci. 1990,140, 481. (36)Myers, D.Surfactant Science and Technology;VCH: New York, 1988;p 168. (37)Barton, J.W.; Fitzgerald, T. P.; Lee, C.; O’Rear, E. A.; Harwell, J. H.Sep. Sci. Technol. 1988,23, 637. (38)Mukerjee, P.; Cardinal, J. R. J. Phys. Chem. 1978,82, 1620.
Figure 7. Alumina plates (from left to right) (a) no salt, no SDS; (b) 1.5 M salt, no SDS; (c) no salt, 15 mM SDS; (d) 1.5 M salt, 15 mM SDS. The pyrrole concentrationwas 20 mM in all cases.
may enable the pyrrole molecule to position itself partially inside the head group region. Our interpretation of the effect of pyrrole on surfactant adsorption is that the pyrrole molecules (perhaps with associated waters of hydration) insert in the admicelle and occupy space in the head group region but lack a sufficient hydrophobic group to act as a cosurfactant, thus reducing adsorption. The addition of sodium chloride augments counterion binding which mitigates electrostatic repulsion between head groups, decreasing the effective head group area to permit more adsorption and adsolubilization. Results show that pyrrole is not “salted out” of the supernatant. Even with the highest salt concentrations, nearly 95% of the pyrrole remains in solution. As in earlier studies, adsolubilization increases with the pyrrole concentration in the supernatant. Unlike previous studies, however, a saturation point for partitioning of pyrrole into the admicelles was not observed. Admicelle saturation of small molecules usually occurs at surfactant to adsolubilizate molar ratios on the order of 1:1,values which were not attained in the present system. Based on the adsolubilization data, we expected to attain better connected, more uniform PPy films due to the higher interfacial concentration of pyrrole resulting from the addition of salt to the system. This was shown to be the case in studies of the effects of salt and surfactant on PPy salt film formation on alumina plates. Samples were prepared from four different aqueous solutions containing (1)pyrrole only, (2) salt and pyrrole, (3) SDS and pyrrole, and (4) SDS, salt, and pyrrole. As discussed earlier, pyrrole does not adsorb on alumina powder in the absence of surfactant. By analogy, pyrrole adsorption will not likely occur on plates 1and 2, so any PPy formed on the surface does not result from pyrrole adsorbed before polymerization. For plates 3 and 4, where SDS was used, adsorption isotherms could not be obtained due to problems measuring extremely small changes in supernatant concentration of SDS and pyrrole caused by adsorption on the limited surface area of the plates. This is not a problem with the alumina powder, which has a very high specific surface area. Considering the high supernatant concentration of SDS, we can say qualitatively that SDS adsorption on the plates is in the plateau region, again by analogy to the adsolubilizationon alumina powder. As mentioned earlier, most of the pyrrole remains in the supernatant. Consequently, PPy forms in solution in all of the cases in addition to whatever happens on the surface, but this PPy does not adhere to the plates. The striking differences in the resulting alumina plates are shown in Figure 7 and Table 1. The darkest film was produced from the solution containing pyrrole only. The film was not well-adhered and could at times be scraped from the surface fairly easily. The film produced from the pyrrole and salt solution was very similar to that produced from the pyrrole solution, the only difference being its slightly less dark color. For the films produced in the absence of surfactant, polymer growth appears to
Polypyrrole Formation on Alumina
Langmuir, Vol. 11, No. 5, 1995 1447
Table 1. Resistance of Alumina Plates after Polymerization of Pyrrole with Ammonium Persulfatea plate SDS concn (mM) NaCl concn (MI resistance (kQ) 1
2 3 4
0 0 15 15
0 1.5 0 1.5
5.4 44.5 > 109 6.9
The pyrrole concentration was 20 mM in all cases. See Figure 7 for a description of the plates.
be occurring from nucleation sites on the alumina. This phenomenon is well-established for the growth of PPy on surfaces and results in globular patches of PPy on the sub~trate.~ Surprisingly, ~,~~ no PPy film formed on the plate using the solution of SDS and pyrrole. Since adsolubilization is low in the absence of salt, the concentration of radical cations in the admicelle after addition of oxidant may be too low to form polymer. Additionally, the adsorbed surfactant bilayer appears to protect the alumina surface from the nucleation phenomenon observed in the absence of surfactant. However, the film produced from the solution of SDS, salt, and pyrrole, although light in color, was well-adhered and uniform. This film was also quite shiny in contrast to the dull appearance of the other two films. The addition of salt increases the adsolubilization of pyrrole in the bilayer, so upon addition of oxidant, the concentration of radical cations in the admicelle is sufficient to form polymer. It is believed that admicelle formation occurs in a patchwise fashion, covering areas of high surface charge first. At surface saturation with surfactant, it is possible there are areas that are not covered with a surfactant bilayer, but the coverage is high enough to expect the bilayer to form a continuous phase.2 If polymer subsequently forms in the admicelle, it should be well connected across the alumina surface. The film appeared to be better connected, as indicated by the resistance measurements in Table 1. The resistance was comparable to the other two films in spite of visibly less polymer being present on the plate. Although these conditions are by no means optimized, the synergistic effect of salt and SDS on PPy film growth is clearly demonstrated. Moreover, the very different appearance and character of these films suggest that contrasting PPy structures are formed by the alternate methods of preparation. These differences are being studied by atomic force microscopy and the preliminary (39) Kittlesen,G.P.;White, H. S.; Wrighton, M. S.J.Am. Chem.SOC. 1984.106.7389. (40) k e s , S.P.;Gottesfeld, S.; Beery, J. G.;Garzon, F.;Agnew, S. F. Polymer 1991,32, 2325.
results are consistent with different polymer morphologies. Results of this work, along with further characterization of the electronic conductivity, will be reported in a future paper.41
Conclusion The admicellar polymerization method was demonstrated to be applicable using pyrrole, a monomer with moderate water solubility. We found that added neutral electrolyte was required to enhance the adsolubilization so polymerization could take place in admicelles on the surface of alumina. This is also the first example of using the TFST technique to produce polymer by a stepwise polymerization as opposed to a radical chain polymerization. Despite admicelles sometimes being considered as a surface analogue of micelles, predictions concerning the solubilizing properties of the admicelle from the measurements made on solution micelles obtained by semiequilibrium dialysis were not obvious. Further work is required to determine the actual conductivity of the films produced by this method due to the difficulty arising from the measurement of the film thickness on the plates. It may be possible to exploit the patchwise surface coverage obtained at lower surfactant concentrations as a mechanism to control the connectedness of the resulting PPy films. At present, the main drawback to using this method to form PPy salt films on alumina is the large amount of pyrrole wasted by polymerization in solutiop. Although the TFST technique is still somewhat limited by the low adsolubilization of pyrrole, preliminary results indicate that the addition of a cosolvent enhances the pyrrole adsolubilization. This process can also be used to form PPy film on alumina powder. Work is in progress to determine the optimum conditions for surface modification by PPy on chromatographic alumina for use as a conducting filler and a stationary phase for liquid chromatography.
Acknowledgment. The authors thank Russell Hooper and Oren Scherman for the initial semiequilibrium dialysis work and Professors S. Christian and J. Harwell for their valuable insight. We also thank the National Science Foundation for the grants which made this work possible (CTS-8912806 and EHR-9108771). LA940786C
(41)Yuan,W.L.;Funkhouser,G.P.;Cho,G.;Glatzhofer,D.T.;O'Rear, E. A. Manuscript in preparation.
