J . Phys. Chem. 1991, 95, 2492-2498
2492
where
a,
and a, are characteristic functions given by
The parameter
CY~(K+;W)
= K-A’/(K_A’
CY*(K-A;W)
K+,W/(K-A’
K-A
+ w’) + w’)
(A31 (A41
can be expressed by K-A
= k,Pe + kd
(‘45)
if a Langmuir-type rate equation is accepted at stage I, where k, and kd are the adsorption and desorption rate constants, respectively. If the overall transport of molecules is controlled by the intracrystalline diffusion (in case l), the FR data may be described by’* fj(w) = ~
~ ~ ( b ; wj =) c or s
(All) provided the distribution is normal; the mean radius rm and the standard deviation u of Linde 5A were assumed to beI69l7
rm = 1.8 X 10” m u = 05, (A12) In case 2 of the finite rate of adsorbate supply, the characteristic functions are found to be given f j ( w ) = C Y ~ ~ ~ ( { , ~ ; Wj )= c or s (‘413)
(‘46)
where the characteristic functions, 63c and ij3,, which are derived from an isotropic sphere model are given by (A71
The analytical forms of the functions are as follows: a83c({&) = (aK_A/W)’(a + C83,(b;0)}/8 (A16) a8,,(t,b;o)= (aK-A/W)[ 1 - (aK-A/W)((aK-A/W) + c8,,(b;(L’))/e](A17) where
where
(A101
8 = {(uK-A/w) + c8,(b;w))* + (a + ~8~,(b;o))’ (A18) is introduced for simplicity. With commercial zeolites, a = lo-’ and then c = 1 are usually satisfied.28 Registry No. CH4, 74-82-8; C2H6,74-84-0; C,H8, 74-98-6.
where D denotes the Fickian diffusivity and r is the radius of the sphere. Since the size of microcrystals is not usually identical but distributed, the functions have to be modified as
(27) Yasuda, Y. Bull. Chem. SOC.Jpn., in press. (28) Suzuki, I.; Oki, S.; Namba, S. J . Caral. 1986, 100, 219. (29) Ruthven, D. M.; Loughlin, K. F. J . Chem. Soc., Faraday Truns. I 1972, 68, 696.
9
The parameter
= (2W/b)’/2
649)
b is the diffusional time constant defined by b = D/r2
Permeant Molecular Sieving with Electrochemically Prepared 6-nm Films of Poly(phenylene oxide) Robin L. McCarley: Eugene A. Irene, and Royce W.Murray* Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290 (Received: August 15, 1990) Ultrathin films (5-7 nm) of poly(pheny1eneoxide) produced by electrochemical polymerization of tetramethylammonium phenoxide onto Pt and Au electrodes from acetonitrile solutions exhibit molecular size-selective permeabilities to solution redox species that can be controlled by details of the film deposition process. The poly(pheny1ene oxide) films are stable in both organic and aqueous environments;permeabilities in water are at least 200 times less than in acetonitrile. Cross-linking of the poly(pheny1ene oxide) films is indicated at polymerization potentials greater than the peak oxidation of the phenoxide monomer (+0.4 V vs SSCE) and upon exposure to electrogenerated mediator/oxidants such as ferricinium. Extended electrochemical polymerizationyields poly(pheny1ene oxide) films with permeabilities of 6 X IO-’) cm2/s to 0.1 M [Fe(CN)J3 in aqueous solution; this corresponds to the passage of only 5 molecules/s across each molecule-sized (ca. 100 A*) section of film surface. Double-layer capacitances in both aqueous and acetonitrile solutions are decreased by only 3-10-fold at the poly(pheny1ene oxide)-coated electrodes, indicating that the polymer film does not appear to bind to the metal in a manner that blocks formation of an ionic space charge at the metal surface. There have been many recent investigations aimed at controlling surface properties by modification with molecular films and of the structure-property relationships of such films, on monomolecular assemblies and with polymeric microstructures.’ Designed
molecular surface modification has value in applications of molecular films in corrosion,tribometry, electrocatalysis, and biomimetic processes. A considerable structurefunction literature has accumulated for molecularly ordered films containing one or two
‘Resent address: Department of Chemistry, University of Texas, Austin,
S.;Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease,R. F.; Rabolt, J. F.;
TX 77812.
(1) Swalen, J. D.; Allara, D. L.;
Andrade, J. D.; Chandross, E. A,; Garoff,
Wynne, K. J.; Yu, H.Lungmuir 1987, 3, 932.
0022-3654/91/2095-2492%02.50/0 0 1991 American Chemical Society
Permeant Molecular Sieving molecular layers, Le., Langmuir-Blodgett and self-assembled films*-' and vesicular membranes,* and for the generally lessordered and sometimes much thicker molecular films on electrodes associated with electrocatalysis and chemically modified electrode~.~ A significant aspect of ultrathin molecular layer film behavior is its diffusion transport barrier to permeating molecular species; this barrier is measured by the permeability, PD, where P is the partition coefficient and D the diffusion coefficient of the permeant into and within the film. Permeabilities of ordered molecular films have been studied in several contexts, including free-standing lipid bilayers as in "black lipid membranes" and vesicles,'O polymerized vesicles," and self-assembled monolayers on Permeabilities of relatively thicker free-standing membranes and of electrode-supported polymeric films (with thicknesses equivalent to hundreds or more layers of monomer units) have also been widely i n ~ e s t i g a t e d . ' ~In~ ~thick polymeric films, in the absence of highly specific chemical interactions, the permeabilities of these polymeric molecular films can vary systematically with the molecular size of the permeant,'6-19*23 Le., the phenomenon called molecular sieving. Molecular sieving by polymer films of thickness equivalent to only a smaller number of monomer sites, Le., of thickness similar to that of the molecularly ordered lipid bilayers, has generally not been regarded as of interest, since such ultrathin films, being relatively poorly ordered, are likely to be riddled with pinhole defects, through which non-size-selective transport of permeant would occur in preference to permeant dissolving in and diffusing through the polymer. Exceptions can occur, however, for electrochemically polymerized films on electrodes; thus prepared 2-3-monolayer-thick films of poly(aminotetrapheny1porphyrin) exhibit permeabilities that vary systematically with permeant molecular volume.23 We describe here the first example of an ultrathin (5-7 nm) molecular film that acts as a molecular sieve with permeability adjustable, at ca. constant film thickness, via (cross-linking) details of the film preparation. This report describes the molecular sieving property of the ultrathin (5-7 nm), organic dielectric poly(pheny1eneoxide) (PPO) films that we have discovered24grow and passify (stop growing)
(2) Roberts, G.G. Ado. Phys. 1985,34, 475. (3) Peterson, I. R. J . Mol. Electron. 1987, 3, 103. (4) Vandevyver, M.; Barraud, A. J . Mol. Electron. 1988, 4, 207. (5) Vandevyver, M. Thin Solid Films 1988, 159, 243. (6) Bain, C. D.; Whitesides, G. M. Angew. Chem., Inr. Ed. Engl. 1989, 28, 506. (7) Whitesides, G. M.; Ferguson, G. S.Chemtracts 1988, I , 171. (8) Ringsdorf, H.; Schlarb, B.; Venzmer, J. Angew. Chem., Inr. Ed. Engl. 1988,27, 113. (9) Murray, R. W. Annu. Rev. Mater. Sci. 1984, 14, 145. (IO) Laiiger, P. Angew. Chem., Inr. Ed. Engl. 1985, 24, 905. (1 1) (a) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J . Am. Chem. Soc. 1987, 109, 3559. (b) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J . Am. Chem. SOC.1990, 112, 4301. (1 2) (a) Finklea, H. 0.;Avery, S.;Lynch, M.; Furtsch, T. Lungmuir 1987, 3, 409. (b) Finklea, H. 0.;Snider, D. A.; Fedyk, J. Ibid. 1990, 6, 371. (1 3) Sabatani, E.; Rubinstein, 1.; Moaz, R.; Sa&, J. J. Elecrroanal. Chem. 1987, 219, 365. (14) Chidsey, C. E. D.; Loiacono, D. N. Lungmuir 1990, 6, 682. (15) Finklea, H. 0.; Robinson, L. R.; Blackburn, A.; Richter, B.; Allara, D.; Bright, T. Lungmuir 1986, 2, 239. (16) Crank, J., Park, G. S., Eds. Dif/usion In Polymers; Academic: London, 1968; Chapters 2 and 3. (17) Stern, S.A.; Frisch, H. L. Annu. Rev. Mater. Sci. 1981, 11, 523. (18) Ohnuki, Y.;Matsuda, H.; Osaka, T.; Oyama, N. J . Electroonal. Chem. 1983, 158, 5 5 . (19) Ohsaka, T.; Hirokawa, T.; Miyamoto, H.; Oyama, N. Anal. Chem. 1987, 59, 1758. (20) Leddy, J.; Bard, A. J. J . Elecrroanal. Chem. 1983, 153, 223. (21) Leddy, J.; Bard, A. J.; Maloy, J. T.; SavQnt, J. M. J . EkCtroCrMI. Chem. 1985, 187, 205. (22) Ikeda, T.; Schmehl, R.; Denisevich, P.; Willman, K.; Murray, R. W. J . Am. Chem. Soc. 1982, 104, 2683. (23) Raprich, K. A.; Maybury. S. G.; Thomas, R. E.; Linton, R. W.; Irene, E. A.; Murray, R. W. J . Phys. Chem. 1989,93, 5568.
The Journal of Physical Chemistry, Vol. 95, No. 6, 1991 2493 during the electrochemical polymerization of tetramethylammonium phenolate from its acetonitrile solutions. Our choice of solvent conditions and recognition that we can manipulate a cross-linking reaction" in the film at oxidizing potentials (or with chemical mediator/oxidants) more positive than ca. +0.4 V vs SSCE, distinguish these PPO films from previousz5 electrolytic phenolic oxidations producing thick, low-density films. The 5-7-nm PPO films exhibit permeabilities that are both exceptionally small and that vary regularly with permeant molecular volume. All available criteria point to the absence of significant numbers of macroscopic pinhole defects in the films. Unlike the electropolymerized metalloporphyrin~,~~ the permeabilities of the PPO films are both reproducible and systematically variable, at constant film thickness, according to the details of the electropolymerization. The stability of the PPO film provides an opportunity for the first time to explore the sensitivity of ultrathin molecular film permeability to the solvent medium. Solvent effects have not been successfully studied with self-assembled films.26 The PPO film permeability to redox permeants from water solutions is much lower as compared to acetonitrile solutions; the aqueous permeability is in fact the smallest yet observed for films of roughly similar thickness, including electrode-supported self-assembled monolayer films"-14 and a recently reportedIzb8-10-nm-thick PPO film electrolytically prepared from aqueous acid. For example, a ferricyanide concentration gradient of 2 X lo5 M/cm across the PPO film produces passage of only 5 molecules/s across each molecule-sized (ca. 100 A2) section of film surface.
Experimental Section Chemicals, Electrodes, and Instrumentation. Tetramethylammonium phenoxide was made in situ by stoichiometric addition of solid tetramethylammonium hydroxide (Aldrich) to a 50 mM solution of phenol (EM Science) in 0.1 M Bu4NC104/CH3CN. Acetonitrile (Burdick and Jackson UV grade) was dried over 4-A molecular sieves. Bu4NC104was prepared and purified as described in the literat~re.~'All other chemicals were reagent grade or better. Water (>16 M a ) was purified with a Bamstead Nanopure filtration system. Electrodes were Teflon-shrouded F't disks (A = 0.1 cm2) polished to a mirror finish with 1-pm diamond paste and rinsed thoroughly with acetonitrile, water, and acetone. Potentials are cited relative to a saturated sodium chloride calomel electrode (SSCE). Electrochemistry was performed in glass cells in three-electrode mode with a PAR 273 galvanostat/potentiotat. Film permeabilities were measured by rotated disk voltammetry using a Pine Instruments ASR 2 analytical rotator. Film Deposition and Permeation Measurements. Tetramethylammonium phenoxide was oxidatively electropolymerizedU onto Pt electrode surfaces from 50 mM solutions in 0.1 M Bu4NC104/CH3CNby scanning (100 mV/s) the electrode potential to and holding at +0.8 V vs SSCE for a given amount of time or until a given number of coulombs of charge (QECP)had passed. The film-coated electrodes were copiously rinsed with acetonitrile and used immediately in permeation experiments. PPO films prepared at 0.2 V were chemically cross-linked by electrogeneration of oxidizing mediators such as ferricenium, as previously reported." The method employed to measure the film permeability to electroactive permeants"% is described in the text. Results and Discussion Permeation Measurement. Of the several methods previously2*23 employed to measure permeabilities of ultrathin, electrodesupported films, we view rotated-disk electrode voltammetry (24) McCarley, R. L.; Thomas, R. E.; Irene, E. A,; Murray, R. W. J. Elecrroanal. Chem., in press. (25) Mengoli, G.; Musiani, M. M. J . Elecrrochcm. Soc. 1987,134.643C. (26) The permeability of alkylthiol films on Au has been reported to increase to an immeasurably large value in acetonitrile solvent,lh whereas we find that PPO films exhibit readily measurable permeabilities in that solvent, Le., 1 X IO-" cm2/s for bis(pentamethylcyclopentadieny1)iron. (27) Sawyer, D. T.; Roberts, Jr., J. L. Experimental Elecrmchemistry for Chemists; Wiley-Interscience: New York, 1974; p 212. (28) Gough, D. A.; Leypoldt, J. K. Anal. Chem. 1979. SI. 439.
2494 The Journal of Physical Chemistry, Vol. 95, No. 6, 1991
McCarley et al.
TABLE I: Penmrbilities of Redox Molecules through Poly(phenylene oxide)-Coated Pt Electrodes, Measured in AcetoniMle/Electrolyte Solutions, for Films Prepared at +0.8 V vs SSCE with Different Polymerization Charges 101oPD,,N: cm2/s 106DsoLN, VOI, A3/ QECp = 23.2 QEcp = 24.6 QBcp 26.6 QEcp = 74.5 redox permeant cm2/s molecule mC/cm2 mC/cm2 mC/cm2 mC/cmz' 260 180 90 21 34 ferrocene 20 74 140 89 43 1 ,l'-dimethylferrocene 50 34 16 15 150 TCNQ 11 160 37 22 11 diferrocenylbenzene 0.82 200," 41: 23d 13 370 9.0 4.8 bis( pentamethylferrocene) [WbPY )2C121 7.9 510 4.3 2.0 0.30
"Estimated error of 10% for PDmL values induced by error of film thickness. bElectropolymerized at +0.2 V. 'Electropolymerized at +0.2 V, followed by potentiostating at +0.8 V in the absence of PhO- but in the presence of 1.0 mM ferrocene for 5 min. dElectropolymerized at +0.2 V, followed by potentiostating at +0.8 V in the absence of PhO- but in the presence of 1.0 mM (o-hydroxybenzoy1)ferrocene for 5 min. TABLE 11: Permeabilities of Redox Species through Poly(pheny1ene oxide)-Coated Pt Electrodes, from Aqueous Electrolyte Solutions, for Films Prepared at +0.8 V vs SSCE in 0.05 M PhO-/O.l M BurNCIOr/CHICN, with Different Polymerization Charges 10-12PD,I, cmz/s min PDpI QECP= 23.9 Q ~ c p= 26.7 redox permeant value obsd' mC/cm2 mC/cm2 H+ 800' [Fe(H20)sl2+ 23' 8.5' [Fe(CN)6130.6' tetramethylphenylene0.62' 14 0.6 diamine CpFeCpCH2N(CH3)z 4.0' 100 4.0 CpFeCpCH2N(CH3)j' 28 000 8700
'At meant meant 0.1 M
films prepared with long polymerization times. b0.5 mM perin 0.2 M KCI/H20 with QEcp = 27.7 mC/cm2. '0.1 M perin 1 .O M KCI/H20 with tECp= 45 min. d0.5 mM permeant in Bu,NClO,/CH$N.
as quantitatively most effective. The permeability PDPI of an electrode-supported film is measured by observing the transport rate of an electrochemically reactive permeant through the film to the underlying, rotated electrode. The permeant transport rate through the solution to the solution/film interface is conveniently separated out by extrapolating the dependence of the permeant limiting current ilim on w , the electrode rotation rate (rad/s), to infinite rotation rate, according to the relation2* C, and DmIare the permeant concentration and diffusion coefficient in the solution phase, respectively, d is film thickness, and v is kinematic viscosity.29 Provided the film thickness is known, permeabiIity3OPD I is measured from the extrapolated intercept of a I /ilim vs o-l/f"piot. This method requires that the polymer film transport barrier not exhibit any electrochemical or chemical reactivity itself, i.e., be passive in the electron-transfer sense. Film electroactivity would allow reaction of the permeant with the film rather than the electrode, making the current insensitive to the permeation rate. Chemical reactions of the permeant or its electrochemical reaction product with the film could alter permeability in the act of measuring it. T h e poly(pheny1ene oxide) films satisfy these requirements, being electroinactive at potentials less positive than +1.4 V vs SSCE (where the film is oxidatively degraded) and being unreactive toward permeants and their products provided the Eo' of the permeant used is not more positive than +0.8 V vs SSCE. The redox permeants employed in Tables I and I1 were selected with these and other criteria in mind. Permeationfrom Acetonitrile Solutions. The currents observed at bare and at PPO-coated electrodes, in both cyclic voltammetry (29) Bard, A. J.; Faulkner. L. F. Elecrrmhemical Merhods; Wiley: New York, 1980. (30)Knowing P and DN separately requires an independent measurement; in the p m n t case the extreme thinness of the films has frustrated independent determination of P and Dp,.
I +
0.6
0.0
-
+0.5
0.0
E(V) vs. SSCE Figure 1. Cyclic (A and C) and rotated-disk (B and D) voltammograms at bare (solid line) and PPO-coated (dashed line) Pt electrodes (QEcp = 21.2 mC/cmz). A and B: 0.5 mM ferrocene in 0.1 M Bu4NC104/ CH3CN, v = 100 and 20 mV/s, respectively. C and D: 0.5 mM tetracyanoquinodimethane in 0.1 M Bu,NCI04/CH3CN, u = 100 and 20 mV/s, respectively; S = IO MA. T
O
0
2.0
4.0
w-1/2(rpmj1/2 x
T 6.C
io2
Figure 2. Plots of eq 1, with RDE voltammetry currents normalized for electrode area and permeant concentration at PPO-coated Pt electrodts for 0.5 mM ferrocene in 0.1 M Bu4NCI04/CH3CN; QEcp = 0 (bare), 23.2, 24.6, and 26.6 mC/cm2 for a-d respectively.
and in rotated-disk electrode voltammetry, are always smaller at the coated electrode (Figure 1,-vs ---) and are consistently
The Journal of Physical Chemistry, Vol. 95, No. 6,1991 2495
Permeant Molecular Sieving
E
A
., -6.0
MOLECULAR VOLUME
(A3)
0 200 400 600 MOLECULAR VOLUME (k3)
d-’W-v2 X 10-4(d’2/cm)
to solution diffusion constant, for &O-coated Pt electrodes as a function of permeant volume and electropolymerizationcharge; QecP = 23.2,24.6, and 26.6 mC/cm2 for 0 , A, and respectively. Permeants are as in Table I.
smaller for the permeant with the larger molecular volume (Figure 1 A,B (ferrocene) vs Figure 1 C,D (tetracyanoquinodimethane, TCNQ)). The currents are also smaller when the film is subjected to a more rigorous cross-linking protocol during preparation, as exemplified by the progressively larger intercepts for curves b-d, for as QECp is increased, in the Figure 2 plots of l / i l i mvs the permeant ferrocene. Note in eq 1 that the permeability PD,, ‘ is inversely proportional to the intercept. Also, as anticipated from eq 1, the slopes of the l / i l i mvs plots (at equal Cml)in Figure 2 are the same at film-covered electrodes (curves b-d) and at naked electrodes (curve a). The curve b-d intercept changes seen in Figure 2 demonstrate a capability for systematic variation of film permeability at constant thickness. The intercepts show that the PPO film permeability decreases as larger QEcpcharges are allowed to pass during the electropolymerization of phenoxide solutions at +0.8 V vs SSCE. Measurements of film thickness, by several techniques, reported elsewhere” show that the films deposited at +0.8 V vs SSCE have thicknesses d = 5-7 nm that do not change further as additional increments of polymerizing charge are passed. The charge increments are instead consumed (we assume) by intrafilm cross-linking reactions that lower film permeability PDpd without significantly changing thickness, d. To fully illustrate the point, if film thickness were proportional to charge passed (as judged by permeation rates), the increment of QECp from 23.2 to 26.6 mC/cm* should thicken a 5-nm-thick film to 14 nm; a change that had it occurred would have been readily detected in the thickness studies.24 We take the PPO film thickness as 5 nm in calculations of all PD I values reported in Tables I and 11. The measurements of figure 2 were repeated using a series of neutral permeants in acetonitrile solutions; the results are reported in Table I and Figure 3. The PllPI values are reproducible to within *IO% (the largest uncertainty is in film thickness) and show that the permeability PD,, is a strong function of permeant molecular volume.31 Figure 3B shows that the PDPI results, normalized to each permeant’s solution diffusion constant, Dsoln, approach an exponential relation to permeant volume. Figure 3B also emphasizes the observation that PD decreases most sharply with permeant molecular volume for f i g s that have been more extensively cross-linked (larger Q E c p ) . Our interpretation of the latter effect is that the mobility of the poly(pheny1ene oxide) chain segments, the frequency and amplitude of which govern diffusion of permeants through the polymer, is progressively diminished as additional cross-linking connections are electrochemically formed between chains. The molecular sieving property of a 5-7-nm-thick organic dielectric film, in which we have no reason to suspect strong ordering, is an exceptional property for films this thin. It is important at this point to reflect on the assumptions underlying (31) Molecular volume rather than cross section is used in this comparison, being less subject to arbitrary choice of dimension for nonspherical permeants and because molecular sieving effects in gel permeation chromatography are well established to depend on molecular volume.32 (32) Heftmann, E. Chromarography;Reinhold: New York, 1961; p 353.
100
50
0
Figure 3. (A) Permeability (PD, ) and (B) log ratio of permeability
0
0
10 SLOPE X10-l2 INTERCEPT
-20
XIO-14
Figure 4. (A) Equation 1 plots normalized for electrode area and film thickness. Curves a-c: C, = 0.25, 0.50, and 1.0 mM,respectively. (B) Plot of intercept and slope from A, vs C[l.
the permeability interpretation. There are two basic models for solute transport33 through a very thin molecular film: a homogeneous picture of partition into and diffusion through the film (membrane model, eq 1) and a heterogeneous one of diffusion through solvent-filled holes or defects in a membrane (pinhole model). Full adherence of the results to eq 1, the membrane model, is indicated by Figure 2, and further in Figure 4A, where plots of the quantity nFA/dilimvs &1w-1/2 for various concentrations of decamethylferrocene are linear with intercepts and s l o p inversely proportional to C,(Figure 4B). The linear relation between the intercept and CI; (Figure 4B, W) gives PDpl = 2.94 X cm2/s, and that between the slope and CI; (Figure 4B, 0 ) gives D, = 4.8 X loT5cm2/s, which agrees with D, obtained by using a bare electrode, 5.5 X 10” cm2/s. Agreement with eq 1 in and of itself is insufficient, however, to demonstrate membrane vs pinhole transport, because relations that have been proposed for various models of pinhole geometries have a similar form of ilim-lvs dependency.21 The observation of molecular sieving is a more definitive characteristic of membrane transport than is obedience to eq 1. A pinhole is taken to represent a space penetrating (either directly or with tortuosity) through the molecular film that has a lateral dimension large in comparison to the permeant molecule. For a neutral permeant, “large” means that the solvent phase that fills the pinhole resembles bulk-phase solvent, i.e., there are no wall effects of solvent s t r ~ c t u r i n g . Defining ~~ pinholes in this manner, it follows that if transport through a film occurs predominantly through pinholes, no discrimination with respect to permeant molecular volume can be expected; that is, PD,, should not vary systematically with the permeant molecular volume. In contrast, molecular size discrimination is a well-known phenomenon in permeation of molecular vapors through thick polymeric membranes,I6J7 in size exclusion ~hromatography?~2~.~ (33) We distinguish here solute trans rt permeability from electron tunnelling between reactant and electrode.”go *Izb Electron tunnelling would yield potential-dependent currents; currents on the plateaus of the permeation waves observed here are on the other hand quite independent of potential over a ca. 0.243-Vinterval. (34) For a charged permeant, “large“ must include absence of ionic interactions with the pinhole walls, which is more difficult to assess; for this reason the study in acetonitrile depends exclusively on neutral permeants.
2496 The Journal of Physical Chemistry, Vol. 95, No. 6, 1991
McCarley et al.
and in transport of macromolecules in porous media where the T pores and macromolecule hydrodynamic radius have similar dimension~.~'~~* Comparisons of the Table I PDPI results to available theories are complicated since P and DPI are not30 B C independently known, but it is worth noting that the extant theories variously contain exponential and analogously strong dependencies on the permeant dimension, qualitatively resembling those displayed in Figure 3. We regard the case therefore as quite strong that the molecular volume effects displayed in Figure 3 and Table I represent permeation of these solutes through 5-7-nm PPO films H H by a process of the permeant partitioning into and diffusing +Q5 0.0 +Q5 QO +a5 QO through the PPO film as a molecular phase. E(V) ys SSCE Remembering that the electrochemically formed PPO film is Figure 5. Rotated disk voltammetry for 0.1 M Fe(CN),'- in 1.0 M an organic dielectric only 5 nm in thickness, it is remarkable that KCI/H20. (A) Bare Pt, u = 20 mV/s, w = 400 rpm. (B) PPO-coated it acts as such an efficient transport barrier in an organic solvent. ( Q ~ c p= 64 mC) Pt, u = 20 mV/s, w = 0 rpm. (C) Same as B, w = 400 This behavior is probably a testimony to cross-linking of the PPO rpm. S = 2 mA for A and 2 pA for B and C. chains. The importance of oxidative cross-linking is brought out more explicitly in the following experiment, in which we compare 60,000 the permeability of a low-density film to one that is further b cross-linked by using an electrochemically generated chemical oxidant. The preceding films were grown at +0.8 V vs SSCE 40,00$ oxidizing potential. A poly(pheny1ene oxide) film can be grown at much less positive potentials39 (Le., +0.2 V vs SSCE) but requires passage of a much larger QEcp to produce a measurable transport barrier. Table I shows that PDPI = 2 X lo-* cmz/s C (measured by using decamethylferrocene and an applied potential of+0.2 V) for-a film grown at +0.2 V for QECp = 74.5 mC/cm2; this permeability is 200 times larger than PD for PPO grown grown at +0.2 at +0.8 V and QECP= 26.6 mC/cm2. The 0 4.0 6.0 V has a thicknessz4similar to that grown at +0.8 V, but we believe u-1'2(rpmj1'2 x io2 it is not extensively cross-linked and is thus more permeable. Subsequent electrolysis of the +0.2 V film at +0.8 V in a solution Figure 6. Koutecky-Levich plot normalized for electrode area and percontaining only electrolyte does not lower the permeability, because meant concentration for 0.5 mM [Fe(CN),J3- in 0.2 M KCI/H20 at a bare Pt electrode (a) and PPO-coated (QECp = 27.8 mC/cm*) electrode the film has no redox states to carry charge to un-cross-linked (b). chain segments. Electrolysis at +0.8 V, on the other hand, in the presence of the mediator/oxidants ferrocene ( E O ' = +0.408 V) The aqueous PDP1values in Table I1 decrease with increasing and (0-hydroxybenzoy1)ferrocene ( E O ' = +0.665 V), again permeant size, in a fashion similar to that shown in Figure 3, measuring PDpdwith decamethylferrocene, lowers the permeability (tetramethylphenylenediamine < CpFeCpCHZN(CH3), < Feto 4.1 X IO4 and 2.3 X cm2/s, respectively (Table I). The (CN)6-3 < Fe(HzO)6+Z< H+), but the situation is more compermeability can clearly be driven down without incorporating plicated than in acetonitrile because of superimposed variations additional phenoxide groups into thefilm; we interpret the effect in permeant charge, which has a less predictable effect on PDPl. as a cross-linking reaction, which is driven more effectively by We were unable to select a suitable series of neutral, suitably more potent oxidants, i.e., (0-hydroxybenzoy1)ferrocene. reversible, water-soluble permeants of significantly varied moWe have preliminary evidence suggesting that the molecular lecular size. Nonetheless, the trend with permeant size seems volume of the mediator/oxidant plays a role in determining the evident in Table 11, and application of the membrane model to resulting permeability but have not fully investigated this possible the permeability data is again reasonable. templating effect. Additional evidence limiting the possibilities of interfering Permeationfrom Aqueous Solutiotu. Quantitative permeability pinhole transport in these experiments is obtained from the bemeasurements of redox species through ultrathin molecular films havior of a more concentrated (0.1 M) solutiona of [Fe(CN),13such as those made with self-assembling have not at an electrode coated with PPO at a large QECPcharge (Figure been made in aqueous solutions, and only a few RDE and other studies have been made with PPO in aqueous s o l ~ t i o n . l ~ ~ J * J ~5). The small and rotation-rate-independent limiting currents of the rotated-disk voltammograms at PPO-coated electrodes Permeability results obtained in aqueous solutions by using eq 1 (curves B and C) are much smaller than at naked electrodes (curve are shown in Table 11. As in acetonitrile, the permeability of a A) and give a PDPI = 6 X cmz/s (which is the smallest PPO film to a redox probe entering the film from an aqueous phase permeability we have observed and smaller than the impressive is depressed according to how extensively poly(pheny1ene oxide) value reported by Finklea et al.12b). It is significant that, in spite chain cross-linking has been forced during polymerization. of this extremely efficient transport barrier, the [ Fe(CN),13Permeabilities are reproducibly much less in water than in acereduction waves at the coated electrodes exhibit the same tonitrile, and the effect of cross-linking on PD,, in water is also and a similar wave shape as that seen at a naked electrode (curve much larger than in acetonitrile. See the data for A). An expected property of pinhole transport is that the current CpFeCpCH2N(CH3)zin Table 11. The results in Table I1 influx for the electrochemical reaction would be forced through the dicated with an asterisk correspond to long electropolymerization small pinhole dimension, and the large current density flowing times, in order to display what we consider to be our most imat the electrode interface under the pinhole(s), which is effectively pervious films for reference to other dielectric films on electrode an ~ltramicroelectrode,'~,~~ can produce changes in the reversibility surfaces.
r"
w
fih
Altgelt, K . H.Ado. Chromatogr. 1968, 7 , 3. (36) Ouano, A. C.Ado. Chromatogr. 1977,IS, pp. 233. (37) Guo, Y.;Langley, K. H.;Karasz, F. E. Macromolecules 1990, 23,
(35)
2022. (38) Davidson, M.G.;Deen, W. M. J . Membr. Sei. 1988, 35, 167. (39) The usual phenoxide voltammetric peak potential is ca. +0.35 V vs SSCE in acetonitrile.
(40) At the usual millimolar electrochemical concentration levels, low permeabilities can yield currents that fall to apparent baseline level and are thus "undetectable". This has been noted in the l i t e r a t ~ r c " ~ ~ and ~ ~ ~was *-*~ seen in the present experiments. We observe that, in such situations, the continued existence of very low permeability is easily revealed by the simple expedient of Figure 5, of using much higher concentrations of the electroactive permeant. (41) Wightman, R. M. Science 1988, 240, 415.
The Journal of Physical Chemistry, Vol. 95, No. 6, 1991 2497
Permeant Molecular Sieving
TABLE III: Double-Layer Capacitances for Bare a d Poly(pheaylene oxide)-Coated Pt Electrodes for Various Electrolytes, Measured at +0.3 V vs SSCE
cd, P F 0.025 V I S
0.1 VIS
electrolyte (0.1 M)
acetonitrile Bu~NCIO~ Fe(phen)ACIO,), Bu4NBPh4 water LiCIO, NaC10, NaBPh,
1.8
1.2
0
20
4.0
w-'1/2(rpmj112
=? x io
0
coated
coated/ bare ratio
0.22 0.15 0.67
2.2 2.6 2.2
0.6 0.8 1.8
0.27 0.31 0.82
3.0 4.5 5.0 0.84
0.33 0.38 0.36
0.32
0.08
1.5 3.5 4.2 0.42
0.29 0.40 0.14
9.0 12 14 4.4
3.5
0.16
0.05
4.0
B
0
bare
5.2 8.8 11 2.9
poly(dimethyldiallylammonium) chloride, MW = 240000
-
0.40 0.35
2.2 1.8
poly(styrenesu1fonate) sodium salt, MW = 500000
A
coated
bare
coated/ bare ratio
20
4.0
WdR(rpmi'"
6.0
x 102
Figure 7. Equation 1 plot normalized for electrode area and permeant concentration. (A) 0.5 mM CpFeCpCH2N(CH3), in 0.1 M Bu,NCI04/CH3CN. (B) 0.5 mM CpFeCpCH,N(CH,), in 0.1 M NaCIO,; QECp= 0 and 23.4 mC/cm2 for a and b, respectively.
of the electrochemical reaction. We observe no irreversibility effects on the [Fe(CN)6]3-/4- couple from 0.1 M solutions at 10-50-pm microdisks but do see irreversible waveshapes a t 200nm-wide Au band electrodes (bare). Thus, if this small PDPl is due to pinhole defects, they must be between 200 nm and 10 pm in dimension, which is inconsistent with the permeant size selectivity. Comparison of the PDF, results in Tables I and 11 shows that PPO films are much less permeable to redox species dissolved in water than in acetonitrile. This is illustrated graphically in Figure 6, where a plot of eq 1 for the reduction of [Fe(CN),]'- at a PPO-coated (curve b) electrode has a very large intercept compared to those observed in acetonitrile in Figure 2 for electrodes coated at similar Q E C electropolymerization ~ charges. The permeability difference between acetonitrile and water is best studied by using the same permeant, and results for CPF~C~CH~N(C through H ~ ) ~films prepared with QECp = 23.9 mC/cm2 are given in Table I1 and in Figure 7. The much larger Figure 7 intercept shows that PDPIin water is 280 times smaller than in acetonitrile. Previous wettability studies24 show the PPO films to be only mildly hydrophobic ( 6 = 44'). Comparisons with films prepared from alkyl-substituted phenolates, which are more hydrophobic but also more permeable, suggested that the low permeability of PPO films to aqueous permeants is more a bulk film property than a surface-crossing barrier property.24 The PPO films are fully wetted by acetonitrile droplets and are more permeable in acetonitrile, which suggests that the solvent difference arises by a change in the bulk rather than the film surface. The most probable interpretation is that acetonitrile solvent effectively solvates the ether dipoles in the film and enhances the polyether chain segmental mobility upon which D I depends. We take special note of t t e measurement of the film's permeability to hydronium ion in Table 11. Although larger than the other permeabilities (being the smallest permeant), it is nonetheless an impressively small value. There may be special features of proton transport in the polyether that cause this, such as the basicity of the ether sites, which would act as traps in the transport process much like alkali-metal-polyether coordination
0.38
0.19
is important in transport of Li+ in polyether polymer Double-Layer Capacitance Studies. The double-layer capacitance C , at PPO-coated Pt electrodes was estimated (Table 111) from the charging currents flowing during potential sweeps in solutions containing only supporting electrolyte. These are rough estimates since the effect of solution resistance, which is in series with the cdl, is neglected; the CdI results at the slower scan rate in Table I11 are thus more reliable. Nonetheless, the major effects are obvious: (i) the films depress the capacitance by comparable amounts in water and in acetonitrile; (ii) the depression of CdI is (in contrast to the permeability results) quite modest, between 3- and 10-fold; (iii) the CdI results exhibit a small sensitivity to the size of the electrolyte ions, but even when polymeric electrolytes are used the capacitance is not "quenched". The smaller ion of the polymeric electrolyte, or short-chain impurities, probably dominates cdl in those cases. The most striking aspect of Table 111 is the small effect of the PPO films on C, relative to the large transport barriers reflected in the PDPl results of Tables I and 11. It is important to observe, however, that even the small observed permeabilities are capable of supplying a sufficient flux of ions to satisfy the observed capacitances. A PD,, = 6 X lo-" cm2/s and C, = 0.1 M,for mol/cm2 s through a 5-nm example, will supply a flux of 2 X PPO film, whereas charging a CdI of 3.5 X lo-' F (e.g., 0.1 M [Fe(phen)J2+, Table 111) requires a flux of only 1 X 10-" mol/cm2 s. That is, it is difficult to rationalize the observed changes in C, in PPO films solely on the basis of low film permeability, since such a small quantity of ions is required to satisfy a space charge a t the electrode/polymer interface. The modest changes in cd,and relative insensitivity to ion size are consistent with a PPO film that offers a not greatly changed dielectric medium or plane of closest approach for the ions that form the double layer. These results are in contrast to those reported in an elegant study of monomolecular self-assembled layers of alkylthiols on Au electrodes in aqueous electrolytes by Porter et al.," who observe, like us, much depressed permeabilities in water but, unlike Table 111, see large depressions of cdl and in fact values that are inversely proportional to the alkylthiol chain length as in the Helmholtz29 capacitor model. The difference in behavior between PPO and the alkylthiols may simply reflect the greater hydrophobicity and transport properties of the alkylthiol film relative to PPO, but we also note that the alkylthiol layers form strong chemical bonds to the metal electrode surface, whereas the poly(pheny1ene oxide) films represent a strongly adherant but nonbonding coating. The possible importance of the latter features is also suggested by experiments where gold oxide layers are formed at Au/film interfaces and measured by electrochemical reductions. The quantity of gold oxide formed a t Au electrodes coated with alkylthi01s'~J~ is attenuated by 99% as compared to bare Au, whereas that formed at PPO-coated electrodes24is de(42) MacCallum, J. R.,Vincent, C. A., Eds. Polymer Electrolyte Reuiews I; Elscvier: London, 1987.
2498
J . Phys. Chem. 1991, 95, 2498-2501
creased only to 60% of the original value. Thus, the ability of a molecular film on an electrode surface to alter double-layer capacitances or electrode kinetics may possibly rest on effecting changes in the outer monolayer of metal surface atoms as well as on the more commonly considered effect of providing a strong transport barrier between the metal and the electrolyte solution. In summary, the electrochemically formed PPO films exhibit permeabilities in acetonitrile and aqueous environments that can be controlled by details of the electrochemical polymerization and are relatively defect-free so as to act as permeant molecular volume-selective sieving barriers. Permeabilities are less in water
due presumably to poorer solvation of the film there. The permeabilities are comparable to those (less quantitatively) described for self-assembled alkylthiol monolayers and as such, the PPO films are the first example of a nonordered molecular layer combining extreme thinness with relative freedom from pinhole defects.
Acknowledgment. This research was supported in part by a Materials Chemistry Initiative grant from the National Science Foundation. R.L.M. acknowledges support from the Electrochemical Society (SS 1989).
Influence of Framework SVAI Ratio on the Raman Spectra of Faujasitic Zeolites Prabir K. Dutta* and Jen Twu Department of Chemistry, The Ohio State University, Columbus, Ohio 43210 (Received: August 20, 1990)
This paper examines the dependence of the Raman spectrum of the faujasitic family of zeolites on the Si/AI ratio. The vibrational spectrum of the completely siliceous faujasite is consistent with those of other silica polymorphs. Based on the correlation between the prominent Raman bands and the T O T angle, the strong, sharp bands at 488 and 510 cm-' in the completely siliceous faujasite correspond to average Si-0-Si angles of 141' and 147'. For the framework with Si/AI = 1 .O, the four bands in the 900-1 250-cm-' region are assigned to the S i 4 stretching vibrations of Si attached to the four different oxygens of the framework. At intermediate Si/AI ratios, bands in the 900-1250-cm-' region show an increase in frequency as the %/A1 ratio increases.
Introduction The faujasitic family of zeolites plays an important role in a wide variety of separation, chemical, and petrochemical processes.' Their thermal stability is important to their function as catalyst, and dealumination is an established procedure for the improvement of thermal stability.2 Direct synthesis of faujasitic-like zeolites is only possible for Si/Al ratios of the framework between 1 and 3.3 To achieve higher %/A1 ratios, secondary methods that remove AI from the framework are required. Many such dealumination techniques have been reported.2 Spectroscopic methods have played an important role in determining the framework structure upon dealumination. In particular, solid-state NMR spectroscopy has provided information about the framework composition, the nature of silanol defects, and the extra framework aluminum trapped in the zeolites.e6 Infrared spectroscopy, in both the mid-infrared and 0-H stretching regions, has been used to draw correlations with the Si/AI ratio, unit cell size, and silanol groups.+* In this paper, we present the first Raman spectroscopic study of the faujasitic zeolite as a function of Si/Al ratio, along the same lines as we did recently for zeolite A.9 The motivation behind this study is twofold. First, there is considerable interest in developing Raman spectroscopic data on zeolite frameworks, since this information complements the infrared data. This is especially true for faujasite, which because of its cubic symmetry is expected to have mutually exclusive infrared and Raman bands.1° Second, there are an (1) Ward, J. W. Appl. Ind. Catal. 1984, 3, 271. (2) Scherzer, J. ACS Symp. Ser. 1984, No. 248, 157. (3) Robson, H. ACS Symp. Ser. 1989, No. 398, 436. (4) Klinowski, J.; Thomas, J. M.; Fyfe, C. A.; Gobbi, G.C. Nature 1982, 296, 533. (5) Englehardt, G.;Lohse, U.; Samoson, A.; Magi, M.; Tarmak, M.; Lippmaa, E. Zeolites 1982, 2. 59. (6) Ray, G.J.; Nerheim, A. G.;Donohue, J. A. Zeolites 1988, 8, 458. (7) Pichat, P.;Beaumont, R.; Barthomeuf, D. J Chem. Soc., Faraday Trans. I 1974, 70, 1402. (8) Miecznikowski, A.; Honuza, J. Zeolites 1985, 5, 188. (9) Dutta, P. K.; Deibarco, B. J . Phys. Chem. 1988, 92, 354. ( 1 0 ) Maroni, V. A. Appl. Spectrosc. 1988, 42, 487.
0022-365419112095-2498$02.50/0
increasing number of calculations on zeolite framework vibrations, the veracity of which can only be verified by comparison with high-quality experimental data."J2 In this study, we have chosen to examine faujasitic zeolites with framework Si/Al ratios of 1, 1.3, 2.6,3.3,4.5, and m, as determined by X-ray fluorescence and NMR spectroscopy. This choice was also based on the relevance of these particular materials in separations and catalytic processes.
Experimental Section Faujasite with a Si/Al ratio of 1.0 was synthesized by following the procedure of Kuhl, using a mixed-cation system of NaOH and KOH." N o attempts were made to replace the K+ by Na+ in the framework, because of the susceptibility to hydrolysis. The frameworks with Si/Al ratios 1.3, 2.6, 3.5, and 4.5 were commercial samples. The first two samples were obtained from Union Carbide and the latter two from Katalistiks. The framework with Si/Al ratio was a gift from Dr. Jack Donohue of Amoco Chemical and prepared by SiC14dealumination as described by him and his co-workers.6 The Raman spectra were obtained from hydrated zeolite samples, using 457.9-nm radiation as the exciting source (Spectra Physics 171 laser), a Spex 1403 spectrometer, and a RCA C31034 GaAs PMT with photon counting. Slit widths were typically 6 cm-I, and a laser power of 20-50 mW was used. Sloping background in the Raman spectra were corrected for with the help of Spectra Calc programs. The curve deconvolution was also done with the help of this program. 0)
Results Figure 1 shows the Raman spectrum of completely siliceous faujasite. The spectrum can be divided into four regions: 200-400 (I), 450-550 (11), 750-900 (111), and 900-1250 cm-' (IV). Low-frequency torsional modes as well as cation-oxygen lattice (11) No, K. T.; Bae, D. H.; Jhon, M. S.J . Phys. Chem. 1986,90, 1772. (12) de Man, A. J. M.; van Beest, B. H. W.; Leslie, M.; van Santen, R. A. J . Phys. Chem. 1990, 94, 2524. (13) Kuhl, G.H. Zeolires 1987, 7 , 451.
0 1991 American Chemical Society
