Permeability and Partitioning of Ferrocene Ethylene Oxide and

Oct 1, 1994 - Sharon A. Rothwell , Michael E. Kinsella , Zainiharyati M. Zain , Pier A. Serra , Gaia Rocchitta , John P. Lowry and Robert D. O'Neill. ...
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J. Phys. Chem. 1994, 98, 11129-11135

11129

Permeability and Partitioning of Ferrocene Ethylene Oxide and Propylene Oxide Oligomers into Electropolymerized Films from Acetonitrile and Polyether Solutions Radha Pyati and Royce W. Murray* Venable and Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill,North Carolina 27599-3290 Received: August IO, 1994@

We report the first electrochemically-based measurements of the rates of small polymer permeation into another polymer. The small polymer permeants are ferrocene ethylene oxide oligomers containing 2, 7, and 16 units and a propylene oxide oligomer containing 3 units. Their permeation into ultrathin microelectrode-supported films of the metal complex polymer poly[Ru(vbpy)3](C104)2 was measured from acetonitrile solutions and from solutions in a methyl-endcapped ethylene oxide oligomer, MPEG-400 (MW = 400). Permeations of other ferrocenes with bulky substituents were also measured from acetonitrile solutions. In all cases the permeability, PDPOL,into poly[Ru(vbpy)3](C104)2 films is strongly dependent on the permeant molecular volume. Direct measurement of the partition coefficient, P, in acetonitrile shows that permeability variations among a series of related permeants are controlled more by variations in P than by diffusivity in the poly[Ru(vbpy)3](C104)2 phase. Permeation of ferrocene polyether oligomers is much slower from MPEG-400 solutions than from acetonitrile; this appears to arise more from a decrease in D ~ Othan L from one in P .

Permeation into and diffusional transport in polymers is a topic of long-standing interest. lv2 The permeability of polymer membranes is the product, P D ~ o Lof , the permeant's partition coefficient P into and its diffusion coefficient D p o ~within the polymer. Small-moleculepermeability depends on the permeant size and shape and on the polymer hydrophilicity and functional group, among other variable^.^ Research on permeability into ultrathin polymer films coated by electropolymerization onto electrodes has uncovered4 behavior dominated by the permeant molecular volume, signifying that permeation occurs by permeant partition into the film and diffusion within it as opposed to transport through pinholes in the film much larger than the permeant molecular dimensions, or by electronic conduction.& Such observations are remarkable, since the ultrathin electrodesupported polymer films can be only tens of nanometers thick. Partition of polymers across polymer-polymer interfaces has been investigated in numerous combinations of polymers that are important in a d h e ~ i o n .Polymer-in-polymer ~ diffusion is often studied6 using fluorophore-labeled polymers and photopattern bleaching. This paper describes the first electrochemically-based measurements of polymer-into-polymerpermeation, in an investigation of permeation of redox-labeled small polymer (oligomeric) permeants into ultrathin, electrode-supported polymer films. The solution of permeant that contacts the ultrathin polymer film is in one instance based on a monomeric solvent (acetonitrile), and in another, the solvent is also oligomeric. Relatively little is known about small polymers partitioning at polymer-polymer interfaces. Specifically, using microelectrode techniques developed for electrochemicalexperimentationin polymeric solvents,' we have measured permeation of ferrocenes functionalized with polyether oligomers of varying chain length (Figure 1) into ultrathin electrode-supported films of the electropolymerized metal complexk poly[Ru(vbpy)3](C104)2 from their solutions in acetonitrile and in a 400 M W methyl-endcapped ethylene oxide oligomer (MPEG-400). Permeation as the product PDPOL,ED is detected electrochemically by oxidation of the ferrocene polyether oligomer, which requires its partition from acetonitrile @

Abstract published in Advance ACS Absrracts, October 1, 1994.

0022-3654/94/2098-11129$04.50/0

Fc FcEEM

FcPPPM

Fc350

Fc750 Fe

16

0 Figure 1. Ferrocene polyether oligomers. FcEEM, Fc350, and Fc750 are respectively methoxy(ethoxy),carbonylferrocene where n = 2, 7, and 16; FcPPPM is methoxy(propoxy)3carbonylferrocene.

or MPEG-400 solution into (coefficient P ) and diffusion through the poly[Ru(vbpy)3](C104)2 film to the (coefficient DPOL,MD) electrode surface. Additional experiments are conducted to characterize the size-selectivity of other bulky ferrocene derivatives and to estimate the value of P, the partition coefficient, and thereby that of Dpo~,Rm.The latter measurements depend on assuming an analogy between partitioning into polyvinylpyridine and into poly[Ru(vbpy)3](C104)2. This is a reasonable analogy given that both are polycationic pyridinium-like matrices. Our choices of poly[Ru(vbpy)3](C104)2 barrier films and polyether polymer electrolyte solvents for the present study were based on our previous experience with such electropolymerized films4" and with solid state voltammetry in polyether media.7

0 1994 American Chemical Society

11130 J. Phys. Chem., Vol. 98, No. 43, 1994 Our choice of ferrocene-labeled polyether permeants was based on our recent work on polyether-based redox molecular meltsga The present results shed further light on the behavior of these phases. The electrochemical methods described for measurement of permeant behavior should, however, be generally applicable to any other pair of polymeric phases where (i) the less-permeable phase can be fashioned into a thin, stable, electrode-supported form, (ii) both it and the contacting “solvent” polymer phase exhibit some ionic conductivity, and (iii) the investigated permeant bears a chemically reversible electroactive functionality. The accessible lower limits of permeability and of absolute P and D p o ~ , m values are not easily judged from the present work, since we envision that modifications of the method should give access to smaller values than those described.

Experimental Section Chemicals. Acetonitrile (Fisher) was distilled over calcium hydride. Polyethylene glycol dimethyl ether (MPEG-400, Polysciences) was stored in vacuo at 50 “C. LiC104 and EbNC104 were recrystallized and dried in vacuo overnight at 80100 “C. [Ru(vbpy)3]Cl2 was synthesized as described ear lie^-!^,^ Ferrocene (Aldrich), 1,4-bis(ferrocenylvinyl)benzene(Aldrich), and decamethylferrocene (Strem) were used as received; other ferrocene derivatives were synthesized by literature8procedures. Quatemized polyvinylpyridine (QPVP) was synthesized from polyvinylpyridine (MW = 50 000, Polysciences) and methyl p-toluenesulfonate (Aldrich) in ethanol.1° 62% of the pyridine sites were quaternized according to the elemental analysis. Found: C, 60.08; H, 6.21; N, 5.31; S , 9.04. QPVP was crosslinked by reaction with a,a’-dibromo-m-xylene (0.16: 1 mol Br/ Py) in a 10% QPVP solution in methanol. Electrodes and Polymer Film Preparation and Permeation Measurements. Measurements of PDPOL,MD values for ferrocene permeating into ultrathin poly[Ru(vbpy)3](C104)2films are based on differences between the reciprocals of steady state limiting currents for permeant oxidation at a naked Pt microdisk electrode and at a microdisk electrode coated with the ultrathin polymer film. Microdisk electrodes, 26 pm diameter Pt wires sealed in glass? were polished after each experiment with 0.05 p m alumina powder suspended in ultrapurified water and sonicated in methanol to remove traces of alumina. The auxiliary electrode was a 24 gauge Pt wire. The reference electrode was either a 0.5 mm diameter Ag wire (QRE) in the singlecompartment cell or, in all experiments in acetonitrile involving E112 measurements, a Ag/AgNO3 electrode isolated behind a Vycor frit. Electropolymerization and voltammetry were performed with a BAS lOOB electrochemical analyzer (BioAnalytical Systems, West Lafayette, IN) equipped with a preamplifier for low-current detection. Acetonitrile solutions were degassed with prepurified, solvent-saturated dry nitrogen and contained 0.1 M Et&TC104; MPEG-400 solutions were maintained under vacuum and contained LiC104 at a Li/ether 0 ratio of 1:16. Microelectrodes have properties’ that minimize uncompensated resistance effects and so are useful in polymer electrolyte solvents like MPEG-400 that even with dissolved LiC104 have modest ionic conductivity. The radial diffusion characteristic of microelectrode voltammetry encourages parasitic film deposition on the glass shroud around the Pt microelectrode, which is undersirable, so electropolymerization reaction conditions based on large potential scan are used to confine the electropolymerized films to the Pt surface with a linear diffusion condition. A typical electropolymerization involves 50 VIS

Pyati and Murray cyclical sweeps of the Pt microdisk potential between -0.5 and -1.5 V vs Ag QRE for 6.36 s in a 2 mM solution of [Ru(vbpy)3]Cl2 in 0.1 M Et&TC104/CH3CN. A typical surface coverage, r T , of deposited poly[Ru(vbpy)3](C104)2was 1.5 x mol/cm2 coverage, which, based on a 1.3 M Ru site concentration in poly[Ru(vbpy)3](C104)2, is a 112 nm thick film. Coverage was assessed for each film from the charge under a slow potential sweep Ru2+13+voltammetric oxidation wave in 0.1 M Et&TC104/CH3CN. The Ru site concentration is based on an ellipsometric measurement determined12on a structurally similar polymer, poly[Os(vpy)2(bpy)2](C1O4)2. Previous wore-g demonstrated that these films are generally devoid of macroscopic pinholes. The permeation measurement involved sweeping the poly[Ru(vbpy)3](Cl04)2 film-coated microelectrode potential through the ferrocene permeant oxidation potential but stopping short of that for the Ru2+13+oxidation of the film itself. A single electropolymerizedfilm was often used in comparisons between different permeants, in successive solutions of them, since on a given film are much relative measurements of PDPOL,MD more reliable and reproducible than are absolute values for different films. In acetonitrile solutions, permeation of FcPPPM, Fc350, and Fc750 (Figure 1) was measured serially, each in triplicate and using the same film, which was rinsed and soaked (5 min) in electrolyte after each measurement. Voltammograms taken during electrolyte soaking confirmed that permeant is rapidly and completely lost from the film. In MPEG-400 solutions, the rinsing step was omitted because of the fragility of the film. Following permeation measurements, the poly[Ru(vbpy)3](C104)2 film was polished off the microelectrode and the naked electrode limiting currents were measured in the different permeant solutions. Estimation of Partition Coefficient, P. A quaternized, cross-linked polyvinylpyridine polymer, QPVP, was used as a model cationic polymer for poly[Ru(vbpy)3](C104)2, to estimate ferrocene partition coefficients from the acetonitrile and MPEG400 polymer electrolyte solutions. This approximation was necessary because it was not possible to prepare poly[Ru(vbpy)3](ClO4)2 by electropolymerization in a sufficient bulk quantity for partitioning studies. The QPVP model was ground to ca. 50-100 p m diameter particles to increase its surface area and hasten equilibration with acetonitrile and MPEG-400 permeant soaking solutions. Polymer powder (0.1 g) and 0.5 cm3 of permeant and electrolyte solution were equilibrated in a microcentrifuge tube for 3.5-7 results) to allow days, times estimated (based on PDPOL,RED permeant diffusion throughout QPVP polymer particles of this size range. The equilibrated suspension was centrifuged. For supernatant acetonitrile solutions, supernatant was drawn into a disposable pipet tip which was plugged at the top with an assembly containing 20 pm radius carbon microdisk and Pt wire auxilary electrode^.^^ The pipet tip was then inverted and a microfritted tip connected to a Ag/AgNO3 reference electrode was inserted into the solution, forming a miniature electrochemical cell. For supernatant MPEG-400 solutions, 10-20 p L was drawn off and deposited onto the platform of an inverted three-electrode assembly7alike that used in permeation measurements, and the assembly was placed in a vacuum cell and evacuated. Microelectrode voltammetry gave from the steady state limiting currents the concentration of ferrocene permeant remaining in these solutions. The microelectrodes were Calibrated using standard acetonitrile and MPEG-400 solutions of the permeant.

Permeability into Electropolymerized Films For partition of Fc750 in the absence of electrolyte, the microelectrode measurement of the remaining permeant concentration required a later addition of LiC104 electrolyte to the supernatant. In calculating P, the volume of the QPVP phase was based on MPEG-400 and CHsCN-swollen densities of 0.67 and 1.22 g/cm3, respectively, values obtained by volume displacements upon adding QPVP to a graduated vial of the solvent. In order to measure how much solvent is imbibed by dry QPVP polymer upon contact with a permeant solution, correcting thereby the volume of permeant solution and concentration of permeant, the swollen density was compared to a dry density, 1.38 g/cm3, measured in heptane, a nonswelling solvent. In order to compare permeant behavior in otherwise constant polycationic polymer barrier (poly[Ru(vbpy)3](ClO4)2 and QPVP) and solvent (acetonitrile and MPEG-400) phases, C104- was employed as the barrier counterion throughout the present measurements. The QPVP phase is initially in the tosylate form, but during partition-equilibration with electrolyte-containing solvent, tosylate is diluted to nor more than 20% of the total counterion population. (The experiment with Fc750 in MPEG400 with no added LiC104 was an exception). Permeant Molecule Volume. Molecular volumes of permeants that are crystalline solids were calculated from structure and bond length data, with the aid of Chem3D Plus molecular modeling software (Cambridge Scientific Computing, Cambridge, MA). Molecular volumes for the higher molecular weight, tar-like ferrocene polyethers were calculated from densities measured by drawing the material into a weighed, volume-calibrated pipet tip.

J. Phys. Chem., Vol. 98, No. 43, 1994 11131 E (V) vs. AgiAgNO, 1.2 1.0 0.8 0.6 0.4 0.2 0.0

T

0:6

I_i E (V)vs. AgQRE

1 5 pA

0:O

(a)

500 pA

B

Figure 2. Panel A: (a) voltammetry of electropolymerized poly[Ru(vbpy)3](C104)~film; (b) voltammogram of 1 mM ferrocene/ EtWlOdCH3CN permeating through poly~u(vbpy)3](ClO4)2 film, (c) ferrocene at bare electrode. For all three, rpt && = 13 ym, scan rate = 10 mV/s. Panel B: 20 mM FcEEM/MPEG-400/LiC104(O/Li = 16: 1) (a) permeating through poly[Ru(vbpy)3](C104)2film and (b) at naked electrode. For both, r R disk = 13 ym, scan rate = 1 mV/s.

A permeability measurement is illustrated in Figure 2A, for ferrocene permeant in acetonitrile solution. Curve a is the film’s Ru2+I3+voltammetry, from which the electroactive coverage and film thickness are obtained. Curve c represents ferrocene oxidation at a naked microdisk; its limiting current iNAK gives14 DSOLN= 2.4 x cm2/s. The first limiting current plateau Results in curve b is the permeation-limited ferrocene current iExp. Permeability from Acetonitrile Solutions. The measureApplication of eq 1 gives PDPOL,RED = 6.4 x IO-* cm2/s for ment of permeability P D P O L of , ~ an electroactive permeant the permeability of ferrocene from acetonitrile into poly[Ruthrough a thin, electrode-supported film of thickness 6 is based (vbpy)3](C104)2. Application of eq 2 to the difference between on comparing steady state, transport-limitedcurrents for reaction the E112values of curves b and c of Figure 2A gives PDpoL,wd of the electroactive probe at film-covered ( i m ) and naked (i”) = 4.3, indicating unsurprisingly that ferrocene has a PDPOL,OX electrodes l3 larger permeability into the cationic poly[Ru(vbpy)s](C104)2 film than does its cationic counterpart ferrocenium. The more positive wave in curve b of Figure 2A rises at the potential of the Ru2+I3+reaction (curve a) and corresponds to the rapid oxidation of ferrocene by Ru3+ sites at the filmwhere n is electrons/molecule, F the Faraday constant, A the solution boundary. The plateau current for this wave is limited electrode area, and C ~ O Lthe N permeant concentration in the solely by the diffusion of ferrocene through the solution, so solution from which permeation occurs. Microdisk electrodes this current is the same as that for curve c. readily yield steady state, transport-limited currents at slow The preceding measurements were repeated for a series of potential sweep rates and are convenient tools for permeability other ferrocenes, selected for gradations in steric bulkiness measurements.” Equation 1 applies to a ferrocene permeant (Table l), and for the four ferrocene polyether oligomers shown in the present case (e.g., PDPoL,=D). The permeability of the in Figure 1. The results of the measurements, DSOLN, P~L,RED, product of the electrochemical reaction, a ferrocenium species DSOLN/PDPOL,RED, ~ D P O L , Oand X , PDPoL,mdPDpoL,ox,and the in the present case, e.g., PD~oL,ox,can be obtained from the estimated molecular volumes for the permeants are given in half-wave potentials of microdisk voltammograms of the Table 1. permeant at naked (E~/~,NAK) and at film-covered (E~R,PERM) Permeability from MPEG-400 Solutions. Figure 2B shows electrodes, using the relationkSd an analogous example for measurement of permeation of the dimer ethylene oxide-tailed ferrocene FcEEM (Figure 1) into poly[Ru(vbpy)3](C104)2 from MPEG-400 solution. Even at large (20 mM) permeant concentrations, the currents are small because of the slower transport in the polymer solvent. Also, the permeation currents are much smaller relative to the naked electrode currents, signifying a much smaller permeability than seen from acetonitrile solutions. Table 2 gives permeability where r is the microdisk radius and we assume DSOLN,RED = results for five ferrocene probes, giving DSOLN,PDPOL,RED, DSOLN,OX. The relative values of P&L,RED and P&L,OX reflect The values of PDPOL,RED are DSOLN,and DSOLN/PDPOL,RED. how thermodynamically favorable the formation of the oxidized several orders of magnitude smaller than the corresponding species is within the film. results from acetonitrile solutions. The relatively poor definition

Pyati and Murray

11132 J. Phys. Chem., Vol. 98, No. 43, 1994 TABLE 1: Diffusion Coefficients and Permeability Data for Ferrocene Derivatives from CHJCNSolutions

ferrocene FcEEM FcPPPM Fc350 Fc750 (ferrocenylmethy1)trimethylammonium cation ferrocenecarboxylate methyl ester decamethylferrocene p-nitrophenylferrocene 1,1'-bis@-bromophenyl)ferrocene 1,1',2,2'-tetrahenzylferrocene 1,4-bis(ferrocenylvinyl)benzene

1.5 (f0.3) x 2.2 (Zt0.9)x 8.3 (f2.9) x 9.3 (f4.8) x lo3 3.9 (fl.O) x lo3 1.6 (f0.8) x 10-o

60 100 600 820 1000 81

2. (Zt1.1) lO(f4.5) l.0(f0.3) 0.6(+0.4) 0.1 (f0.4) 2.3 (f0.5)

76

6.8 (f1.8)

10-5 10-5 10-5 10-5 10-6 10-5

6.4 (f0.4) x 8.0 (12.6) x 2.4 (f0.5) x 3.3 (11.4) x 10-9 2.0 (fo.5) x 10-9 3.4 ( H . 3 ) x

3.8 x 2.3 x 6.2 x 4.5 x 4.1 x 5.4 x

2.3

10-5

4.0 (11.2) x

6 x lo2

1.8 x 1.9 x 1.6 x 10-5

1.6 (f0.2) x 1.7 (51.5) x 2.1 (10.4) x lo-*

1.2 x 103 1.1 x 103 8 x lo2

120 95 110

1.2 x 10-5

1.4 (f0.2) x 10-9

8.5 x 103

1400

6.4 (11.0) x 10-9

1.7 x 103

130

1.1 x 10-5

lo2 lo3 lo3 lo3

4.3 (f0.2) 3.6 (10.5) 3.2 (Zt0.4) 3.7 (f0.7) 5.2 (f0.4) 2.6 (11.3)

2.4 x 1.6 x 1.5 x 1.2 x 7.5 x 1.5 x

2.7 x 7.6 x 10-1° 2.4 x 5.6 x 1.6 x low8 1.5 x

9 x lo2 2.1 x 104 6 x lo3 2 x lo3 5 x lo2 1.2 x 104

5.9 x

4 x lo3

eq 1. Calculated from either density Calculated from ~'NAK. Calculated from ~ N A Kand iEm, eq 1. Calculated from Eln and P&OL,RED, measurement or crystal structure data.16 e Calculated from partition data into quatemized polyvinylpyridine. TABLE 2: Diffusion Coefficients and Permeability Data for Ferrocene Derivatives from MPEG-400 Solutions

molecular

DSOLN'

permeant ferrocene FcEEM F~PPPM

Fc350 Fc750

(cm2/s)

2.1 x 1.3 x 1.1 x 10-7 7.3 x 3.7 x 10-8

PDPOL.RED~ (cm2/s) 9.8 (f0.9) x 2.6 ( f 0 . 2 ) x 2 . 2 ( f o . 3 ) x 10-12 1.8 ( f 0 . 2 ) x 1 ( f O . l ) x 10-11

DSOLN/PDPOL,RED volume' 22

103

50 x 103 49 x 103 41 x 103 3.6 x 103

DPOL,RED

(A3)

60 100 600 820 1000

(cm2/s)

P 0.35

2.8 x lo-"

DSOLN~ &OL.RED

8 x lo3

9 x 103

2.3 (f1.4)d

a Calculated from iNAK. Calculated from iNAK and iEm, eq 1. From Table 1. Equilibrated MPEG-400 solution did not contain LiC104; electrolyte needed to measure ferrocene concentration was added after equilibration.

of the permeation waves in MPEG-400 and variability in the Ag wire QRE unfortunately prevented application of eq 2 in the polymer solutions. Partition Coefficient Measurements. These are based on the change in concentration of the reduced penneant (the ferrocene state) upon equilibration of its solution with the polymer phase. Since the partition coefficient P into the polymer phase is not expected to be large and may be smaller than unity, the experiment required contacting comparable volumes of polymer and solution phase. Available film volumes of electropolymerized poly[Ru(vbpy)3](C104)2 are inadequate for this purpose, so a model cationic phase, a cross-linked, quatemized polyvinylpyridine (QPVP), was employed. Microelectrodes were employed to measure the concentrations of remaining permeant within the small volumes of equilibrated solutions (see Experimental Section). Figure 3A shows voltammograms of an acetonitrile solution of ferrocene at a (naked) microelectrode before and after, respectively, its equilibration with QPVP. Figure 3B shows analogous measurements on ferrocene in MPEG-400 polymer solution. The value of P is calculated as

E (V)

i----l

0.2 v

/A

(3)

Figure 3. Panel A: voltammograms used to calculate P for 1 mM ferrocene/E@JClO.&H3CN (a) after and (b) before equilibration with QPVP. Scan rate = 10 mV/s. Panel B: voltammograms used to calculate P for 20 mM ferrocene/MPEG-400/LiClO4(O/Li = 16:l) (c) after and (d) before equilibration with QPVP. Voltammograms occur on a rising background current. Scan rate = 1 mV/s. For both, rc disk = 20 pm.

where &fore and idterare proportional to the before- and afterequilibration concentrations, V~OL is the solvent-swollen polymer phase volume, and VSOLNis the difference between the initial volume of permeant solution and that volume of solvent imbibed to swell the polymer phase. The latter correction is modest in the acetonitrile solvent but large (ca. 15% of the initial volume) in MPEG-400. Table 1 displays results for partition coefficient P for the four ferrocene polyether oligomers and for ferrocene and ferrocenecarboxylate methyl ester (FcC0OCH-J in acetonitrile. Table

1 also shows values thus derived from the PDPOL,KD measurements for DPOL,KD and for the ratio DSOLNIDPOL,RED. Table 2 displays analogous data in the MPEG-400 solvent, where unfortunately the partition measurement was successful for only two of the group examined. For all four ferrocene ethylene oxide oligomers, the microelectrode-measured limiting currents in the MPEG-400 solution following equilibration with the QPVP phase were larger than those before equilibration, producing small but negative values (-0.2 to -1). The source of error is uncertain,16 but it seems certain that P for these compounds has a rather small value.

Permeability into Electropolymerized Films

J. Phys. Chem., Vol. 98, No. 43, 1994 11133

-7 ,

lo

100

loo0

Molecular Volume (A3 ) Figure 4. Permeability vs molecular volume for ferrocene derivatives from CH3CN solutions. Discussion Permeation from Acetonitrile Solutions. The molecular volumes of the permeants examined in acetonitrile solutions vary by 20-fold. The diffusivities in solution (DSOLN) are consistent with previous data4a and, while trending with diffusant size (Table I), vary only over a 3-fold range. The permeabilities PD~OL,RED vary much more widely with permeant, lying between ca. and 1 x and appear to be strongly related to permeant molecular volume, as shown in Figure 4 (drawn with log axes to accommodate the large dynamic range). Among the smaller ferrocenes, permeability decreases rapidly (with some “noise”) with increasing molecular volume; the decrease is more gradual for the largest ferrocenes. The diffusion retardation, D I P D P O L , likewise ~, varies systematically with permeant molecular volume, showing that transport of a large permeant to the electrode is retarded relatively more by the poly[Ru(vbpy)3](C104)~film than is that of a small one. The permeant volume correlation of course ignores aspects of conformational flexibility, which may vary widely among the permeants chosen. The results in Table 1 also show the effects of permeant charge. The cation (ferrocenylmethy1)trimethylammonium exhibits a P & L , ~ D permeability that is roughly the same as that of a IO-fold larger (but neutral) permeant (Fc350) and that is ca. IO-fold smaller than that of the similarly sized (but neutral) ferrocenecarboxylate methyl ester. The lowered permeability to the cationic ferrocene is most plausibly due to its Donnantype charge exclusion from the cationic poly[R~(vbpy)3]~+ film. Also, the permeability ratios PDpoL,REDIPDpoL,ox (from eq 2 ) in Table 1 are all > 1, signifying unsurprisingly that the oxidative formation of a femcenium cation at a metal-fixed-site-cation polymer interface is thermodynamically disfavored relative to that at a metal-dilute electrolyte interface. There seems to be no trend with molecular volume in this effect. The thermodynamic penalty of producing the dicationic (ferricenylmethy1)trimethylammonium from a monocation is less than that for producing a monocation from a neutral ferrocene species, i.e., is smallest for the former couple. PDPOL,REDIPD~OL,OX Permeability as an important transport parameter is invoked in a wide range of circumstances ranging from membrane-based separations17to ion selective electrodes.18 The current relation between permeant molecular bulk and permeability (Figure 4) has been found for other ultrathin electropolymerized polymer films4a-f in other studies of permeation into thicker polymer phases. It is important to recognize that the permeant size selectivity in Figure 4 in itself justifies analysis of transport

through the ultrathin film as a succession of partition and polymer phase diffusion, as opposed to transport through pinhole defects in the polymer film. Permeability is however a product of two effects and does not by itself fully reveal the underlying basis for thin film transport selectivity among a series of permeants. The partition coefficient measurements reported in Table 1 help in that regard, since these data vary in a manner suggesting that ahother effect is influential as well. P does vary with molecular volume among the four ferrocene ethylene oxide oligomers in a monotonic manner, spanning a 102-fold range for a IO-fold variation of molecular volume. Clearly a longer oligomeric chain length suppresses partition transfer of these permeants from acetonitrile into the QPVP model of the poly[Ru(vbpy)3](C10~)2film. The series ferrocene, ferrocenecarboxylate methyl ester, and FcEEM shows, on the other hand, an increase in P with increasing molecular volume. This series has an increasing content of polar functionalities, and the variation in P is well outside the experimental uncertainties. This series of results suggests that dipolar interactions may dominate the partitioning of small permeants whereas molecular volume becomes dominant for partitioning of larger permeants, as in the ferrocene polyether oligomers. The diffusivities D ~ o L ,of~ the D permeants in the poly[Ru(vbpy)31(C104)~film, calculated from the measured PDPOL,MD and P values (Table I), are best inspected in relation to their counterpart diffusivities in the acetonitrile solution phase, DSOLNI & L , ~ D (Table 1, right side). This ratio is large, as would be expected for transport through a monomeric fluid as opposed to a polymeric environment. The DSOLNIDPOL,RED ratio shows that diffusivities in the solution and polymer phases are least different (9 x 102-fold) for small hydrophobic diffusants (i.e., ferrocene) and become more different as polar substituents are added to the permeant (Le., ferrocene, ferrocenecarboxylate methyl ester, FcEEM). This trend is consistent with that of dipolar effects seen in P; an increase in partitioning due to dipolar interactions between the polymer phase and permeant will at the same time decrease the diffusivity of the permeant since, the dipolar interactions act as deeper site-localizing wells in the polymer phase transport process. The D S O L N I D ~ O ratio L , ~ Ddecreases with steric bulk in the oligomeric ferrocene series. The most bulky oligomers diffuse least rapidly in acetonitrile yet most rapidly in the poly[Ru(vbpy)3](C104)~phase. That steric bulk does not retard mass transport in the polymer barrier film was unexpected.19 The possibility exists and the caveat must be mentioned that the QPVP-poly[Ru(vbpy)3](C10~)2partitioning analogy is imperfect in a manner that artificially accentuates the dependencies of P and D P O L , ~onDmolecular volume. In summary of this analysis, while molecular volume would appear from Figure 4 to be the dominant factor in controlling permeability P&L,MD, the variation of its component partition coefficient, P , seems to contain dipolar interaction as well as molecular volume as factors influencing permeability. Permeability is more complex than simply permeant molecular volume. Permeation from MPEG-400 Solutions. An example of permeation voltammetry in MPEG-400 was shown in Figure 2B. The ferrocene oligomer diffusion coefficients in MPEG400 solutions are of the expected order of magnit~de’~ and decrease with increasing oligomeric chain length, as do the measured permeabilities (Table 2). Permeabilities from MPEG400 polymer solution are smaller than those from CH3CN s o l ~ t i o n sby~ ~factors ~ ~ ~ of ~ 103-104. The limited partition coefficient data obtained in MPEG-400 (Table 2 ) differ from the partition results in acetonitrile by much smaller factors.

11134 J. Phys. Chem., Vol. 98, No. 43, 1994 Comparison of the permeability and partition results for ferrocene and Fc750 in Tables 1 and 2 indicates that the primary reason for the lowered permeabilities from MPEG-400 solutions must be that the permeant diffusivities DPOL,MD are much smaller in poly[Ru(vbpy)3](C104)2 films swollen by MPEG400 relative to those in acetonitrile. Slow film diffusivity is seen for both a small (ferrocene) and bulky (Fc750) permeant. Possible reasons for the slow diffusivities are (i) that MPEG400 does not solvate the poly[Ru(vbpy)3](C104)2film well and (ii) that the dipolar interactions between the film sites and imbibed MPEG-400 solvent serve to stiffen the film lattice. The substantial partition coefficient for Fc750 into the poly[Ru(vbpy)3](C104)2 film (Table 2) suggests that the smaller oligomer MPEG-400 should penetrate the film rather freely and that reason i is not the major factor. Density measurements also show (see Experimental Section) that QPVP imbibed more MPEG-400 than CH3CN. The origin of the small permeant diffusivity in poly[Ru(vbpy)3](C104)2films swollen by MPEG400 is from this analysis indicated as reason ii, a polymer latticestiffening driven by MPEG-400-poly[Ru(vbpy)~](C10~)~ interactions. The data in Table 2 show that Fc750 exhibits a larger permeability than ostensibly less bulky permeants. This is a reproducible but as yet unexplained result. Fc750 is the only oligomeric permeant with a longer polyether chain than the MPEG-400 solvent. The presence of lower molecular weight ferrocene oligomers in the Fc750 sample can be ruled out on the basis of repeated flash chromatographic fractionation and of the more consistent results seen in acetonitrile (Table 1). The in MPEG-400 (Table 2 ) are fairly values of DSOLNIPD~OL constant for the oligomers, except for Fc750. Finally, the behavior of the failed partition measurements (vide supra) suggests that P is small ( 1 for Fc750 in the absence of LiC104 electrolyte. The change in P can be rationalized on the basis of the nature of polyether polymer electrolytes; Le., Li+ salts dissolve because of strong ether-metal ion interactions. Lif interactions with the ethylene oxide oligomer chain of Fc750 would favor its retention in the MPEG-400 phase relative to poly[Ru(vbpy)3](c104)~.On the other hand, when LiC104 is not present in the equilibrating solvent, the QPVP phase contained only tosylate counterions, so a specific counterion effect on the polymer phase behavior may also be important.

Conclusions This work shows that separation of partition and diffusion coefficients adds significant insights into factors that control permeation rates of bulky ferrocene derivatives and oligomers into a cross-linked cationic redox polymer. Most notably, the conclusion could be drawn that variations among related permeants from acetonitrile solutions are controlled more by variations in the degree of partitioning of the permeant into the poly[Ru(vbpy)3](C104)2film than by actual diffusion rates within it. Partition coefficients from acetonitrile appear to depend both on permeant molecular volume and on its dipolar interactions with the polymer film. Partition coefficients differ according to whether acetonitrile or MPEG-400 is the permeant’s originating solvent, but permeabilities differ much more between these two solvents. The inference is that permeant diffusion rates (Le., D~OL,RED) in poly[Ru(vbpy)3](C104)2 are much slower when this polymer is swollen with MPEG-400 relative to actonitrile. On the basis of limited data, this latter effect appears to be caused by polymer-stiffeningMPEG-400-poly[Ru(vbpy)3](C104)2interactions.

Pyati and Murray

Acknowledgment. This research was supported in part by grants from the National Science Foundation and the Department of Energy. R.P. acknowledges a Department of Education Fellowship. We acknowledge the assistance of Dr. C. S. Vellzquez for synthesis of the ferrocene ethylene oxide oligomers and the assistance of R. H. Temll with data analysis software and acknowledge Dr. M. Collinson and Prof. R. Mark Wightman for donation of carbon fibers. References and Notes (1) Elias, H.-G. Macromolecules;Plenum Press: New York, 1984;Vol. 1. (2) (a) Yi-Yan, N.; Felder, R. M.; Koros, W. J. J . Appl. Polym. Sci. 1980, 25, 1755. (b) Meares, P. J . Am. Chem. SOC. 1954, 76, 3415. (c) Hayes, M. J.; Park, G. S. Trans. Faraday Soc. 1955, 51, 1134. (3) (a) Rogers, C. E. In Polymer Permeability; Comyn, J., Ed.; Elsevier: New York, 1985; pp 1-74. (b) Berens, A. R.; Hopfenberg, H. B. J . Membr. Sei. 1982, IO, 283. (c) Rogers, C. E.; Stannett, V.; Szwarc, M. J. Phys. Chem. 1959,63, 1406. (d) Felder, R. 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Permeability into Electropolymerized Films (15) (a) Dunitz, J. D.; Orgel, L. E.; Rich, A. Acta Crystallogr. 1956, 9, 373. (b) CRC Handbook of Chemistry and Physics, 65th ed.; Weast, R. C., Ed.; CRC: Boca Raton, FL, 1984; pp F166-F170. (16) A relatively small bias in parameters used in the large correction for solvent volume imbibed by the polymer phase (vide supra) could cause negative P values, provided the "real" P values were rather small. Another possible source of error would be uptake of trace water by the hygroscopic MPEG-400 during its manipulation; water plasticizes MPEG-400, increasing DSOLN and thereby the apparent ferrocene derivative concentration. (17) (a) Giddings, J. C. (Inijed Separation Science; John Wiley & Sons: New York, 1991; pp 31-35. (b)Meares, P. In Membrane Separation

J. Phys. Chem., Vol. 98, No. 43, 1994 11135 Processes; Meares, P., Ed.; Elsevier: New York, 1976; pp 1-38. (c) Membrane and Ultrafiltration Technology: Developments Since 1981 ; Torrey, S., Ed.; Noyes Data Corporation: Park Ridge, NJ, 1984; pp 166196; (d) pp 423-450. (18) Lmdner, E.; Toth, K.; Pungor, E. Dynamic Characteristics of ZonSelective Electrodes; CRC: Boca Raton, FL,1988; pp 68-70. (19) (a) Our analysis of permeation neglects any kinetics of the partitioning step, which has been previously shown to appear as small effects in permeability studies.lgb Any interfacial rate control will in the present data cause a minor bias in D p o ~ , m (not P). (b) Leddy, J.; Bard, A. J.; Maloy, J. T.; Saveant, J. M. J . Electroanal. Chem. 1985, 187, 205.