J. Phys. Chem. 1988, 92, 1928-1936
1928
Microporous Alumlnum Oxide Films at Electrodes. 4. Lateral Charge Transport in Self-organized Bilayer Assemblies Cary J. Miller, Cindra A. Widrig, Deborah H. Charych, and Marcin Majda* Department of Chemistry, University of California, Berkeley, Berkeley, California 94720 (Received: July 13, 1987; In Final Form: October 20, 1987)
Porous aluminum oxide films, developed previously to serve as electrode coatings, have been used as microstructural templates in the spontaneous assembly of amphiphilic bilayers. The perpendicular arrangement of the cylindrical pores of the aluminum oxide films attached to the electrode surface results in the perpendicular orientation of bilayer assemblies with respect to the electrode. This allows direct electrochemical measurements of the lateral charge transport in bilayer assemblies. The systems consisted of octadecyltrichlorosilane (OTS), coating the inner surfaces of the oxide films, and octadecyl derivatives of 4,4'-bipyridyl (Cl8MVZ+)or ferrocene. In the case of OTS/C,8MV2+bilayers, the maximum coverage of the bipyridyl amphiphile on the OTS-treated A1203surface, 3.9 X IO-'" mol/cm2, was found to be equal to the coverage of these molecules at the water/air interface under equilibrium spreading pressure conditions. Long-term stability of the bilayer system in aqueous electrolyte solutions free of C18MV2+was observed at half this coverage. The diffusion coefficients of lateral charge transport, obtained for the OTS/C18MV2+system by chronocoulometry, were in the range 2.3 X to 1.3 X lo-' cm2/s,depending on the oxidation state of the bipyridyl amphiphile and the presence of octanol in the electrolyte solution. The latter intercalates into the CI8MV2+monolayer, as shown by fluoroescence spectroscopy of pyrene acting as a probe of microenvironmental polarity of the bilayer interior. Octanol intercalation increases the bilayer's microfluidity and results in a twofold increase of the diffusion coefficients of the lateral charge transport by inducing counterion dissociation and increasing electrostatic repulsive interaction in the C18MV2+monolayer assembly.
Introduction We have recently reported our preliminary results of the first direct electrochemical measurements of the lateral charge transport kinetics in organized bilayer assemblies.' The present report constitutes a full account of our investigationsof the lateral electron transport in amphiphilic bilayers containing octadecyl derivatives of 4,4'-bipyridyl and ferrocene. The most important element of our experimental approach which led to the direct electrochemical measurements of lateral charge transport in amphiphilic bilayers is a microporous template of aluminum oxide which is used as a substrate for bilayer formation. The structure of the porous aluminum oxide template is shown in Figure 1. A detailed description of the oxide preparation, involving electrooxidation of aluminum, its structural characteristics, and the related fabrication procedures have been published in a separate report.* The organized bilayers discussed here are produced by two consecutive self-assembly steps involving octadecyltrichlorosilane and an octadecyl derivative of methylviologen (C18MV2+).This approach and the unique geometry of the oxide template produce organized amphiphilic bilayers which are oriented perpendicularly to the electrode surface (a gold layer vapor-deposited at the oxide film surface as shown in Figure 1). This allows us, as shown below, to apply electrochemical techniques in the investigations of lateral processes taking place in bilayers. The nature of lateral transport processes in organized bilayers and the understanding of their dynamics are of fundamental importance. Organized bilayer systems resemble biological membranes in their structure and behavior and, as a result, have been a subject of wide i n t e r e ~ t . ~ Information concerning translational diffusion of amphiphiles in biological membranes and biomimetic systems can be inferred from the analysis of ESR and NMR experiments based on spin exchange4s5and relaxation More direct measurements involve time measurement^.^-^ monitoring translational diffusion of a fluorescent label tagged ( 1 ) Miller, C. J.; Majda, M. J . A m . Chem. SOC.1986, 108, 3118.
(2) Miller, C. J.; Majda, M. J . Electroanal. Chem. 1986, 207, 49. (3) Fendler, J. H. Membrane Mimetic Chemistry; Wiley: New York, 1982. (4) Devaux, P.; IMcConnell, H . M. J . A m . Chem. SOC.1972, 94, 4475. ( 5 ) Trauble, H.; Sackmann, E. J . A m . Chem. SOC.1972, 94, 4499. ( 6 ) Lindblom, G.; Wennerstrom, H. Biophys. Chem. 1977, 6. 167. (7) Cullis. R. P. FEBS Lett. 1976. 70. 223. (8) Nery,'H.; Soderman, 0.;Canet, D.; Walderhang, H.; Lindman, 9. J . Phys. Chem. 1986, 90, 5802. (9) Soderman, 0.;Henriksson, M.; Olsson, M . J . Phys. Chem. 1987. 91, 116. I
i
~~~
0022-3654/88/2092-1928$0l.50/0
to a bilayer component in fluorescence recovery after photobleaching experiments.I0-l2 A number of techniques based on this principle were used and have produced a large body of data concerning translational diffusion of lipids and proteins, temperature-induced phase transitions, and other questions related to the structure and behavior of biological membrane systems. Several recent reviews on this subject are a ~ a i l a b l e . ' ~ - ' ~ The significance of the work described here, besides its relevance to bilayer membrane systems, extends also to the area of chemically modified In recent years, research concerned with chemical microstructures at electrodes has become a prominent area of electrochemistry. Studies of their physical structure, the chemical reactivity of their components, and the dynamics of electron and mass transport processes related to their function have been of primary importance."-20 Besides fundamental interest in the behavior and the properties of these multimolecular interfacial systems, their design for applications in electrocatalysis2' and as electrochemical sensor^^^.^^ has provided much of the driving force in this research. The bilayer system spontaneously assembled in porous electrode films described here is also important as an example of a general approach to reagent immobilization at electrode surfaces. In view of the applications mentioned above, this approach presents several distinct advantages. First, the high internal surface area of the oxide coatings allows the immobilization of large quantities of electrocatalysts or sensing molecules in molecularly well-defined assemblies. Second, reagent immobilization by self-assembly in porous aluminum oxide films produces electrode
( I O ) Poo, M.; Cone, R. A. Nature (London) 1974, 247, 438. ( I 1 ) Peters, R.; Peters, J.; Tews, K. H.; Bahr, W. Biochim. Biophys. Acta 1979, 367, 282. (12) Yguerabide, J.; Foster, M. C. In Membrane Spectroscopy; Grell, E..
Ed.; Springer-Verlag: West Berlin, 1981; Chapter 5 , p 199. (13) Cherry, R. J. Biochim. Biophys. Acta 1979, 559, 289. (14) Peters, R. Cell Biol. I n t . Rep. 1981, 5, 733. ( I 5 ) Edidin, M. In Membrane Structure; Finean, J. B.. Micheli, R. H., Eds.; Elsevier: Amsterdam, 1981; Chapter 2, p 37. (16) Hoffmann, W.; Restall, C. J. In Biomembrane Structure and Function; Chapman, D., Ed.; Verlag Chemie: Weinheim, 1984; Chapter 5 , p 257. (17) Murray, R . W. In Electroanalytical Chemistry; Bard, A . J., Ed.; Marcel Dekker: New York, 1984; Vol. 13, p 191. (18) Faulkner, L . R. Chem. Eng. News 1984, 62, 2 8 . (19) Chidsey, C. E. D.; Murray, R. W. Science 1986, 231. 25, (20) Wrighton, M. S. Science 1986, 231, 32. (21) Collman, J. P.; Denisevich, P.; Konai, Y.; Marrocco, M . ; Kovai, C.; Anson, F. C. J . A m . Chem. Sor. 1980, 102, 6027.
0 1988 American Chemical Society
Microporous Aluminum Oxide Films at Electrodes /
I
/J-)
Figure 1. A drawing showing the microscopic structure of a porous aluminum oxide film at a gold electrode. The oxide films used here were
2.7-3.6 pm thick, the average pore diameter was 800 A,and the thickness of the vapor-deposited gold layer was ca. 3 pm.
coatings of high permeability, which is necessary for efficient interactions of a solution reactant with the immobilized species. Third, surfactant self-assembly provides a means of stable yet dynamic immobilization; rapid lateral charge transport to the electrode surface allows electrooxidation and electroreduction of the monolayer components. This is advantageous, as the regeneration of the catalytically active oxidation state of immobilized catalysts is often a rate-limiting step and/or may require special mediators.22 In this report we focus on the bilayer structure and describe the effect of octanol intercalation on the lateral diffusion coefficients measured for these systems. In the companion paper in this series, we describe our investigations of the mechanism of the lateral charge transport in the bilayer a~semblies.,~Electrocatalytic applications of this system will be described in separate reports.
Experimental Section Reagents. N-Alkyl-N-methyl-4,4’-bipyridinium iodides were synthesized following a literature pr~cedure.,~The iodide salts were converted to the chloride salts by first preparing the perchlorate salt via precipitation with NaC104 in aqueous solution, followed by dissolution in acetonitrile and subsequent precipitation with tetramethylammonium chloride. The (ferrocenylmethy1)dimethyloctadecylammonium chloride salt was synthesized according to a published p r ~ c e d u r e . , ~ Octadecyltrichlorosilane (OTS) (Petrarch Systems Inc.) was vacuum-distilled into glass ampules; a fresh ampule was opened for each series of experiments involving a new batch of aluminum oxide films. Hexadecane (Aldrich) was passed through a column of activated alumina (ICN Biochemicals grade Super I neutral). All other chemicals were reagent grade and used as received. Oxide Films. The aluminum oxide films were produced from high-purity aluminum foils or vapor-deposited aluminum on glass slides by anodization at 65 V in 4% H3P04as described previ0us1y.~Fabrication of electrodes coated with OTS-treated oxide films involved the following modifications to the previously reported procedure.’ After the chemical dissolution of the barrier layer, the oxide films were dried with a heat gun and then transferred into a freshly made 2% solution (v/v) of OTS in hexadecane. The oxide films remained in the OTS hexadecane solution for 60 min, after which they were rinsed thoroughly with toluene and 2-propanol. Vapor deposition of gold onto one side of an oxide film fallowed the OTS treatment. Surfactant in(22) Anson, F. C.; Ni, C.-L.; Saveant, J. M. J . Am. Chem. Soc. 1985,107, 3442. (23). Goss, C. A.; Miller, C. J.; Majda, M. J . Phys. Chem., following paper in this issue. (24) Pileni, M.-P.; Braun, A. M.; Gratzel, M. Photochem. Photobiol. 1980, 31, 423. (25) Facci, J. S.; Falcigno, P. A,; Gold, J. M. Langmuir 1986, 2, 732.
The Journal of Physical Chemistry, Vol. 92, No. 7, 1988 1929 corporation into the OTS-treated porous aluminum oxide coated electrodes involved exposing the electrodes wetted with 2-propanol to an aqueous solution of a surfactant in 0.1 M KCI. Spectroscopic and Electrochemical Procedures. UV-vis spectra of octadecyl derivatives of 4,4’-bipyridyl in monolayer assemblies in OTS-treated oxide films were obtained in a diffuse reflectance mode. A free-standing section of porous aluminum oxide film with the assembled bilayer was placed in front of the gold working electrode pressed against a quartz window of a thin-layer electrochemical cell. The cell solution contained a reducing and an oxidizing mediator, methylviologen and p-benzoquinone, respectively, each at 50 1 M concentration level in either 0.1 M KC1 or 0.1 M KCl saturated with 1-octanol. The reflectance spectra were obtained after equilibration of the system with the working electrode polarized to the potentials positive and negative of the formal potentials of the two mediators. The spectrum obtained under oxidizing conditions was used as a background. The quantity of the methylviologen mediator in the thin-layer cell did not contribute significantly to the observed spectra; it was found to give rise to absorption bands which are less than 10 times the intensity of those due to octadecyL4,4’-bipyridyl immobilized in the oxide film. Fluorescence experiments were done with porous aluminum oxide films obtained by exhaustive electrooxidation of vapor-deposited aluminum films (2 Mm in thickness) on thin glass slides. The electrolysis produced transparent, aluminum oxide coated glass substrates which were then treated with OTS as described above. The assembly of a water-soluble surfactant completing the bilayer formation and the incorporation of pyrene (carried out from methanol solutions) and 1-octanol were done by submerging the entire oxide-coated substrate into appropriate solutions. The substrates were always sandwiched with a thin quartz plate prior to their emervion from solutions in order to prevent solvent evaporation and to limit the volume of the liquid phase in contact with the bilayer during recording of fluorescence spectra. The sandwiched samples were positioned at a 40° angle relative to the excitation beam, with the quartz side facing the beam and directed away from the emission monochromator. Fluorescence light passing through the sandwiched sample was measured. A spectral bandwidth of 1.1 nm was used to record emission spectra. The determination of cl&iv2+quantities in monolayers assembled on OTS-treated planar SiO, substrates involved the thin-layer cell, mediative, coulometric assay developed previously.26 The S i 0 2 substrates (600-nm-thick oxide films) were obtained by thermal oxidation of single-crystal silicon wafers (1,0,0 orientation). The SiO, substrates were cleaned in a 2:l v/v mixture of concentrated H2SO4/30% H 2 0 2 ,rinsed with deionized water, and dried with a heat gun just prior to the self-assembly of OTS. The latter process followed the same procedure used for the porous aluminum oxide films. The exposure of the substrates to a C18Mv2+solution of a given concentration (also 0.1 M in KCl) and the subsequent rinsing with 0.1 M KCI were done in a partially closed thin-layer cell (ca. 0.1 cm in thickness) in which the SiOz substrate is sandwiched against a glass slide carrying a vapordeposited gold electrode. It is important to note that this allowed the rinsing step to be carried out without exposing the surface monolayer to the liquid/air interface. Otherwise, the C18MV2+ monolayer was rapidly lost from the OTS surface. Over 10 cell volumes of the surfactant-free KC1 electrolyte were passed through the partially closed cell (a process taking ca. 2 min) to assure proper rinsing. The initial assembly of the partially closed thin-layer cell was done in a way which does not allow exposure of the gold electrode to the C18MVz+loading solution, in order to prevent amphiphile adsorption at the gold surface. After the rinsing step, the glass slide with the evaporated electrode pattern was adjusted to position the electrode surface directly opposite the S i 0 2 substrate. Then the cell was flushed with the electrolyte solution carrying the reducing and the oxidizing mediator precursors (dibenzylviologen and (dimethy1amino)methylferrocene) and closed to its ultimate 10-wm thickness. The cell design and (26) Widrig, C. A,; Majda, M. Anal. Chem. 1987, 59, 754.
1930 The Journal of Physical Chemistry, Vol. 92, No. 7, 1988
Miller et al.
B
A
A
Figure 2. Schematic representation of the OTS/CI8MV2+bilayer assembled on the inner surfaces of the porous aluminum oxide template (see Figure 1). The OTS monolayer is shown to be attached to the A1203 surface and cross-linked laterally via siloxane bridges according to the literature data.30
the electrode fabrication procedures were described previously.26 The coulometric assays of octadecyl-4,4'-bipyridyl adsorbed onto the OTS-treated porous aluminum oxide coated electrodes were performed either by integration of slow scan cyclic voltammograms or by extrapolation of Anson plots derived from long pulse length (typically 5-30 s) chronocoulometric experiments. In the latter method, correction was made for the material adsorbed directly on the electrode by subtracting the intercept value of the Anson plot extrapolated from the linear region (50-300 ms). The intercepts of the Anson plots for the reoxidation step were typically higher than those for the reduction step. This is due to surface precipitation of the electrogenerated C18MV'+ species. This phenomenon is fully reversible and does not cause interferences when the experiments were done in the absence of the viologen in the electrolyte solution. However, electroreduction of C18MV2+when present in solution led to the accumulation of substantial amounts of the precipitate. This would interfere with the chronocoulometric experiments and was avoided by carrying out the self-assembly step at open circuit. Instrumentation. Electrochemical experiments were done with the PAR Model 173, 175, and 179 instruments as described earlier and a BAS 100 electrochemical analyzer. An IBM Model EC/219 rotating disk electrode system was used in the film porosity measurements. Diffuse reflectance spectra were recorded with a Perkin-Elmer Lambda 9 spectrometer with a 60-mm integrating sphere attachment. An Aminco Ratio I1 spectrofluorometer (American Instrument Co.) was used in the fluorescence studies. The Langmuir-Blodgett experiments were done with the KSV 2200 LB Langmuir trough (KSV-Chemicals, Helsinki, Finland), equipped with a surface electrobalance and a filmtransfer system.
Results and Discussion Exposure of polar metal oxide surfaces to OTS solutions in hydrocarbon solvents results in an irreversible covalent binding of the silane molecules to the oxide ~urface.~'-~OThe lateral formation of the siloxane bridges and weak dispersive interactions between hydrocarbon chains of the surface-attached molecules result in nearly perfectly organized monolayers. Evidence of a high level of organization was obtained by Sagiv and co-workers using infrared spectroscopic methods, ellipsometry, and contact angle measurement^.^^.^^ We have used the same immobilization procedures to attach a monomolecular layer of octadecyltrichlorosilane to the internal surfaces of porous aluminum oxide films (Figure l).30 Exposure of the OTS-treated aluminum oxide films to an aqueous solution of an amphiphile leads to its spontaneous assembly in a monolayer with the tail-to-tail conformation with respect to the underlying OTS films as shown in Figure 2. In (27) Sagiv, J. J . Am. Chem. SOC.1980, 102, 92. (28) Netzer, L.; Sagiv, J. J . Am. Chem. SOC.1983, 105,674. (29) Netzer, L.; Iscovici, R.; Sagiv, J. Thin Solid Films 1983, 99, 2 3 5 ; 1983, 100, 67. (30) Maoz, R.; Sagiv, J. J . Colloid Interface Sci. 1984, 100, 465.
--
0
-0.3
-0.6
0
-0 3
-0 6
E / V v s Ag/AgCI,O I M C I '
Cyclic voltammograms of CL8MV2*self-assembled in an OTS-treated porous aluminum oxide film at a gold electrode ( A = 0.071 cm2): (A) 0.1 M KCI electrolyte; u = 50 mV/s. (B) 0.1 M KCI electrolyte nearly saturated with 1-octanol;u = 50 mV/s The change in the voltammetric behavior is reversible upon transfer of the electrode from one solution to the other. Figure 3.
the case of electrochemically active amphiphiles, their adsorption can be monitored electrochemically. This is illustrated in Figure 3A showing a cyclic voltammogram recorded at a gold electrode coated with porous aluminum oxide treated with OTS (see Figure 1 and the Experimental Section). The electrode was exposed to a well-stirred 50 pM aqueous solution of C18MV2+in 0.1 M KCl for approximately 10 min. Oxide-coated electrodes that have not been derivatized with OTS do not incorporate C18MV2+under these conditions. In separate investigations, we have observed adsorption of C18MV2+on A1203 above pH 8.0. The voltammogram in Figure 3A was recorded after transfer of the rinsed electrode to a pure electrolyte solution. The voltametric peak shape is characteristic of electrochemical processes conducted under conditions of a limited thickness of the diffusion layer.31 This is expected, since Cl,MV2+ molecules are confined within a thin film of the porous aluminum oxide. Under such conditions, all the electroactive surfactant bound within the electrode film can be reduced and reoxidized in the time of the voltammetric scan. In order to verify that all immobilized Cl8MV2' molecules are indeed electroactive, methylviologen and p-benzoquinone (which can act as reducing and oxidizing mediators for C18MV2+)were added to the electrolyte solution. No additional electroactivity of the viologen surfactant assembled in the porous oxide layer was observed. This is consistent with our assertion of the complete electroactivity of the C18MV2+monolayer. The mechanism of the lateral charge transport which leads to the complete reduction and reoxidation of C18MV2+molecules in the monolayer may involve one or both of two processes. One is translational, lateral diffusion of the amphiphilic molecules along the OTS-treated oxide pores toward the underlying electrode surface. The rate of this process is expected to depend on the microfluidity of the monolayer. The other possible process involves electron hopping between adjacent stationary C18MV2+molecules along the monolayer. In this case, the rate of charge propagation either could depend on the assembly's macrofluidity or could be limited solely by the kinetics of the electron self-exchange reaction. A detailed analysis of the mechanism of the lateral charge transport is presented in the companion paper in this series, where the translational diffusion is favored as the existing mechanism.23 (31) Hubbard, A. T.; Anson, F. C . In Electroanalytical Chemistry; Bard, A . J., Ed.; Marcel Dekker: New York, 1970; Vol. 4, p 129.
The Journal of Physical Chemistry, Vol. 92, No. 7. 1988 1931
Microporous Aluminum Oxide Films at Electrodes TABLE I: Change of Porosity of the Porous Aluminum Oxide Films upon Self-Assembly of ClnMV2+' % ' porosity
electrode no.
with OTS
with OTS/C18MV2'
layer
bilayer
1 2 3 4 5
52 64 45 32 38
46 59 40 30 30
7'% decrease 12 8 11 6 216
av9k3
"The data were obtained from the rotating disk electrode measurements (see text). bThis measurement was considered anomalous and was not included in the calculation of the average.
I I I I I ( I ( ( I ( I I I t0.6 t0.4 t0.2 0.0 -0.2 -0.4 -0.6
Surface Coverage and Stability of Bilayer Assemblies. Complete electroactivity of all self-assembled molecules via lateral charge transport allows us to measure their surface coverage. A coulometric assay involves integration of the voltammetric response such as that in Figure 3A or potential-step chronocoulometry described in the Experimental Section. Calculation of the surface coverage in both of these measurements involves an assumption of monomolecular character of the c18MV2+assembly on the inner surfaces of the oxide film and an estimate of its internal surface area. To verify this assumption, we have measured the porosity of the OTS-treated porous aluminum oxide films before and after the self-assembly of the electroactive half of the bilayer. These measurements rely on the rotating disk electrode methods and the data analysis described in detail previously.2 Using standard Koutecky-Levich analysis, one can extract the film's porosity from the diffusion-limited current due to the diffusion of an electroactive probe species through the electrode film.2 p-Benzoquinone was used as the solution probe because (i) its redox potential is conveniently 250 mV more positive than that of Cl8Mv2+and because (ii) it is uncharged, thus eliminating possible interferences due to ion pairing with the amphiphile species in the monolayer. The results are listed in Table I. The porosities of the OTS-treated oxide films prior to C18MVz+assembly depend primarily on the customary chemical treatment of the films before the OTS assembly and fall in the range typically observed for the underivatized oxide films, i.e., 35-60%.2 Upon exposure to c I 8 M v 2 + solutions, the measured porosity decreases, on average, by only 9%. For oxide films with an average 800-Apore diameter, this decrease corresponds to a ca. 20-A constriction of the radius of each cylindrical pore. This is in good agreement with the expected decrease of the film porosity resulting from the assembly of a single monolayer of Cl8MV2+on the inner surfaces of the oxide film. Thus, although these measurements are not sufficiently precise to allow a reliable assessment of the thickness of the immobilized amphiphilic layer, they do substantiate the monomolecular character of the C18MV2+assembly on the oxide surfaces. The surface coverage of C18MV2+on the OTS-treated oxide surface depends on the solution concentration of the surfactant used in the self-assembly step. The maximum coverage of 3.3 X mol/cmz, observed in the amphiphile-free solution, can be obtained through exposure to a solution of at least 0.6 m M in cl8MvZ+. At this level there is a rapid desorption of surfactant into the electrolyte (complete within several minutes) to the stable coverage of ca. 1.9 X 1O-Iomol/cm2. This surface concentration of C18MV2+can be also obtained by exposure of a fresh OTStreated oxide electrode to at least 50 p M solution of c18Mvz+. Thus, the ultimate coverage of adsorbed viologen surfactant showing long-term stability (less than 1 5 % loss per hour under voltammetric cycling) in the surfactant-free electrolytes is independent of the surfactant concentration in the loading solution above the 50 p M level. The coverages given above are the equilibrium values requiring, typically, several minutes exposure in the stirred loading solutions of C18MV2+.Also, they are subject to *20% error due to uncertainty in the determination of the inner surface area of the oxide films. The latter cannot be measured directly; its calculation is based on the analysis of transmission electron micrographs.2
E vs.
A g / A g C I , sot. K C I
B
V l
+0.6
1
1
1
1
+0.4 +0.2
1
1
t
I
I
I
I
I
I
0.0 -0.2 -0.4 -0.6
E vs. A g / A g C I , s o l . K C I
Figure 4. (A) Thin-layer cell cyclic voltammograms of the mediated, coulometric determination of Cl8MVZtmonolayer self-assembled on the
OTS-treated planar Si02 substrate. (Dimethy1amino)methylferrocene (-0.2 mM, Eo' = 0.38 V) and dibenzylviologen (-0.2 mM, Eo' = -0.54 V) are used as mediators in 0.5 M KCI, 0.1 M phosphate buffer (pH 7.4); scan rate 10 mV/s. (B) Blank run of the same experiment, conducted to determine the ratio of the mediators' concentration. The stability of the amphiphilic monolayers in electrolyte solutions depends also on the length of the surfactant's hydrocarbon chain. For hydrocarbon chains less than 16 carbons in length, the bilayer assemblies showed an almost immediate loss of the immobilized surfactant upon medium transfer to the surfactant-free solution. Determination of Surface Coverage by Mediative, Thin-Layer Cell Coulometry. In order to obtain more precise surface coverage data, we have examined the assembly of C18MV2+on OTS-treated, planar S i 0 2 surfaces obtained by thermal oxidation of singlecrystal silicon wafers. The same experimental protocol of selfassembly was followed as for the aluminum oxide films. In this case, however, the electrochemical assay of the C18MV2+involved the thin-layer cell, mediative, coulometric method developed and described earlier.25 In this technique, a section of Si02-coated silicon wafer with the self-assembled bilayer is sandwiched against a gold electrode to form a thin-layer cell ca. 10 pm in thickness as described in the Experimental Section. The electrochemical measurement consists, in essence, of the coulometric titration of the C18MV2+immobilized on the opposing wall of the thin-layer cell by electrogenerated mediators (the viologen cation radical and the ferrocenium ion).z6 The results of these experiments are shown in Figures 4 and 5 . The first of them (part A) shows a thin-layer cell voltammogram recorded during the surface coverage determination. The charge under the cathodic wave at -0.55 V is characteristically larger than that under the corresponding anodic wave. This is because the overall cathodic process consists of the electroreduction of the mediator and the mediated reduction of surface-immobilized c18MV2+.Since the redox potential of dibenzylviologen is 85 mV more negative than that of c18MV2+,
Miller et al.
1932 The Journal of Physical Chemistry, Vol. 92, No. 7, 1988 81
4 0
,
I
,
,
,
,
I
,
,
I
,
I
,
,
I
,
p 2 , s112
I
0
,
0.5 [C,, MV'+]/mM
I .o
Figure 5. Adsorption isotherm of C18MV2+on the OTS-treated planar Si02surfaces determined by mediated, thin-layer cell, coulometric assay. Error bars represent standard deviations of six to eight determinations done on different sections of thermally oxidized single-crystal Si substrates. The self-assembly step was carried out in a 0.1 M KC1 electrolyte.
the oxidized form of the viologen mediator is thermodynamically incapable of completely reoxidizing the now C18MV" monolayer. The mediated oxidation of the latter is accomplished by the electrooxidized form of the ferrocene mediator. Hence, the charge under the anodic wave at 0.38 V is significantly larger than that under the corresponding cathodic wave. The ferrocene mediator acts also as the internal standard, which allows us to carry out the quantitative data analysis without the knowledge of the exact thickness of the thin-layer cell. The required value of the ratio of the two mediators' concentration is obtained separately in a blank experiment (Figure 4B).26 Figure 5 shows the self-assembly isotherm for C18MVz+on OTS-treated planar SiO, substrates. The error bars refer to standard deviation (&lo%) based on six to eight individual measurements on different SiO, substrates. The maximum coverage, 3.9 X lo-'' mol/cm2 obtained at 1.0 mM C & f V 2 + ,is, to within the experimental error, in agreement with the direct electrochemical experiments described above for the porous aluminum oxide films. Further evidence that the viologen amphiphile assembly in porous aluminum oxide films leads to the same coverage as in the case of the planar Si02surfaces is provided by the mediative coulometric assay done directly on a porous aluminum oxide layer. In this case, porous oxide film was produced on a planar glass substrate coated with a thick film of AI. Without the usual separation of the oxide film from unoxidized aluminum, the film was derivatized with an OTS layer. Subsequently, it was exposed to the 0.7 mM C18MV2+loading solution, rinsed, and subjected to the thin-layer cell, coulometric assay as described above. The surface coverage obtained in this experiment was 3.7 X mol/cm2 (&lo%based on seven separate trials), again in agreement with the data in Figure 5. The agreement between the coverage of c18Mv2+assembled on planar substrates and in the porous aluminum oxide films substantiates our estimate of the inner surface area of the porous aluminum oxide films. Whether the C18MVZ+monolayers in the oxide films are less stable than those assembled on planar substrates is not clear, because the rinsing procedures are not comparable in these two experiments. It is likely that the transfer of an aluminum oxide coated electrode to the surfactant-free electrolyte is a more demanding experiment with respect to the C 18MV2+monolayer stability and hence results in the loss of the viologen amphiphile at high coverages. All the following experiments involving c 1 8 M v 2 +monolayers in aluminum oxide films were done at the mol/cm2, which is the loading level coverage of ca. 1.9 X of substantial stability. It is interesting to visualize the coverage values obtained above relative to a fully packed monolayer of Cl8MV2+.The latter can be measured by compressing the viologen amphiphile molecules under controlled surface pressure conditions at the water/air interface of a Langmuir trough. The limiting coverage, obtained by extrapolation from the steepest part of the pressure area isotherm to zero pressure, is 3.8 X lo-'' mol/cm2. A coverage of 4.2 X mol/cm2 was obtained at 22.0 dyn/cm, the highest
Figure 6. A typical Anson plot of a 10-s potential-step chronocoulometric experiment done at a porous aluminum oxide Au electrode with the self-assembled OTS/C,,MV2' bilayer. The experimental conditions are such as those in Figure 3A
pressure allowing a transfer of a c18Mvz+ monolayer onto a gold-coated glass slide. The subsequent electrochemical measurements confirmed the unity transfer ratio obtained from the Langmuir trough. These coverages indicate that the 4,4'-bipyridyl groups are oriented with its N-N axis perpendicular to the water/air interface. Any other orientation would result in a much smaller coverage. Based on simple geometric calculations, the surface coverage in this orientation under close-packed conditions still allowing for the free rotation along the 4-4' C-C bond of the bipyridyl is 3.8 X IO-'' mol/cm2. Hence, the maximum coverage of the self-assembled C18MV2+monolayers in porous aluminum oxide films and on the planar substrates appears to be limited by the tight packing of the molecules in the head group region, excluding the counterions. Because the coverages of this magnitude result from exposures to the surfactant solutions well below the critical micelle concentration for c18Mv2+ (reported to be 2.5 mM),32 the main driving force for the self-assembly process must stem from the reduction of the surface energy of the OTS/water interface. Measurements of Diffusion Coefficients by Potential-Step Chronocoulometry. The perpendicular orientation of the OTS/C18MV2+bilayer with respect to the electrode surface in porous aluminum oxide films (see Figures 1 and 2) allows us to use standard electrochemical methods in the studies of the dynamics of the lateral charge transport. The apparent diffusion coefficients were measured by potential-step chronocoulometry. A typical Anson plot (Le., a plot of chronocoulometric charge Q vs r112;see eq I), obtained as a result of such experiments, is shown in Figure 6. The intercept is substantially larger than that measured in the absence of the viologen amphiphile. This is due to CI8MV2+ adsorption directly on the electrode surface during the self-asmol/cm2 in 0.5 mM c18Mv2+ sembly step (up to ca. 4 X solutions). Similar intercept values were obtained in a control experiment, where a bare gold electrode, produced by complete dissolution of the aluminum oxide layer, was exposed to the same C18Mv'' loading solution. The linearity of the Anson plots at times shorter than 1.0 s (Figure 6) demonstrates the diffusion control of the process. The curvature of the plot at times longer than 1 .O s stems from the fact that by this time the front of the diffusion layer has extended to the opposite side of the oxide film, away from the electrode surface. This marks the end to the semiinfinite diffusion condition valid only in the rising linear region of the Anson plots. The slopes of Anson plots in that region can be interpreted, as shown below, in terms of the apparent diffusion coefficient, Dapp. of the lateral charge transport in c I 8 M v 2 +monolayers. The chronocoulometric charge-time transient is described by the following equation Q = 2nFACD,pp1/2t1~2/,1/2
where A is the electrode surface area and C is the concentration of c I 8 M v 2 +in the porous aluminum oxide film. It is convenient to calculate the concentration of CI8MV2+as an apparent value averaged over the entire oxide film volume. This allows us to use the full projected surface area of the electrode in contact with the oxide film. This is done by introducing into eq 1 the measured (32) Krieg, M.; Pileni, M.-P.; Braun, A. M.; Gratzel, M. J . Colloid Interfuce Sei. 1981,83,202.
The Journal of Physical Chemistry, Vol. 92, No. 7, 1988 1933
Microporous Aluminum Oxide Films at Electrodes TABLE 11: Average Diffusion Coefficients of Lateral Charge Transport in OTS/C&iV*+ Bilayer Assemblies' D~~~x lo8, cm2/s oxidation state
of the system
0.1 M KC1
C,8MV2+ CI8MV'+
7 . 0 24% 2.3 f 43%
*
0.030
0.1 M KCI + octanol 13.0 f 28% 4.7 36%
*
0.006
Based on 12 different electrodes.
0
charge, Q f ,due to complete reduction of all C18MV2+molecules within the oxide layer, and the thickness of the aluminum oxide layer, d , based on the following relationship:
C = Qr/nFAd
(2)
After the substitution, one obtains
Q = 2QrDapp1/2t'/2/dal/2
(3)
Using eq 3 and the experimental value of the slope, S, of the Anson plots, we can calculate D,, without the explicit knowledge of the viologen amphiphile concentration in the OTS/C18MV2+bilayer and the exact surface area of the electrode actually involved in the heterogeneous electron transfer:
Dapp= S 2 d ' a / 4 Q ~
0.018
R
(4)
Diffusion Coefficients of Lateral Charge Transport. The results
of the chronocoulometric experiments summarizing the behavior of 12 electrodes are shown in Table 11. For each set of conditions, the standard deviation contains a compounded statistical error of &25% due to the error in the three measurements involved in the calculation of Dappbased on eq 4. The standard deviation values in Table I1 may also reflect a range of differences in the system's behavior associated with the structure of the OTS layer. The OTS structure is likely to have an influence on the lateral charge transfer, particularly if the latter involves, as mentioned above, translational diffusion of the amphiphile molecules along the OTS-treated surface. The Dappfor C18MV'+ was obtained in the electrooxidation process after the equilibration of the system at a potential negative of the viologen amphiphile reduction potential. The observed threefold decrease of Dappis due primarily to dimerization of the viologen cation radical and the expected size increase of the diffusing species. The dimerization of methylviologen cation radical in aqueous solutions has been studied e ~ t e n s i v e l y . ~The ~ reported value of the association constant is ca. 380 M-'.33 Given the high concentration of the viologen amphiphile on the OTS surface, one would anticipate virtually complete dimerization of the reduced CI8MV2+monolayer. Experimental evidence of the dimerization in this system was obtained by recording the absorption spectrum of the viologen surfactant in a free-standing aluminum oxide film after its mediated electroreduction in a thin-layer spectroelectrcchemical cell (see Experimental Section). Figure 7 shows the spectrum of C18MV'+ in a bilayer assembly. The absorption bands at 365 and 540 nm are characteristic for the dimerized cation radical of the viologen s ~ r f a c t a n t . ~Ab~ sorption bands of the monomeric viologen at 395 and 620 nm are not observed.34 An additional reason for the observed decrease of the diffusion coefficients upon reduction of the C18MV2+ molecules may be related to the overall decrease of the charge in the head group region of the monolayer. The related effects of ion pairing and decrease in electrostatic repulsions will be discussed in more detail below. The Effect of Octanol. The presence of 1-octanol (a near saturation level concentration, approximately 3 mM was used to avoid presence of octanol droplets in the solution) in the electrolyte solution results in some distinct changes in the voltammetric behavior of the C18MV2+/0TSsystem, as seen in Figure 3B. Hexanol and decanol have the same effect when present in the (33) Gaudiello, J. G.; Ghosh, P. K.; Bard, A. J. J. A m . Chem. SOC.1985, 107, 3027 and references therein. (34) Watanabe, T.; Honda, K. J . Phys. Chem. 1982, 86, 2617.
200
300
400
500
600
700
800
X (nm) Figure 7. UV-vis reflectance spectrum of the OTS/C18MV'+bilayer assembly in a free-standingporous aluminum oxide film in contact with 0.1 M KC1 electrolyte. This spectrum is also obtained when the electrolyte is nearly saturated with octanol. TABLE III: Average Diffusion Coefficients of Lateral Charge Transport in OTS/CI8Fc+ Bilayer Assemblies"
oxidation state of the system C18Fc+ CI8Fc2+
Dappx lo8, cm2/s 0.1 M K N 0 3 0.1 M K N 0 3
2.7 f 27% 3.5 f 16%
+ octanol
7.7 f 19% 8.1 f 26%
"Based on four different electrodes. electrolyte solution near their saturation level. The narrowing of the peak shape (width at half-height of the cathodic peaks decreases from 170 to 125 mV in Figure 3) suggests more rapid lateral charge transport in the bilayer assembly as the shape of the voltammograms approaches that expected for the "thin-layer cell behavior".,' More detailed analysis reveals an approximately 20% decrease of the coverage of the assembled CI8MV2+,upon transfer of an electrode loaded to a stable coverage (ca. 1.9 X mol/cm2) from a 0.1 M KC1 solution to the same solution containing octanol. The stability of the remaining CI8MV2+is not decreased by the presence of octanol in the electrolyte. Upon return of the electrode to the octanol-free electrolyte, the original voltammetric behavior (Figure 3A) is reproduced essentially instantaneously. The change of voltammetric behavior illustrated in Figure 3 can be observed reproducibly many times by transferring an electrode between the solutions with and without octanol. Importantly, only the initial transfer of the system to the octanol containing electrolyte results in the decrease of the ClsMV2+coverage noted above. The apparent increase in the rate of lateral charge transport mentioned above is indeed observed in the chronocoulometric measurements. As seen in Table 11, the Dappvalues increase on average by a factor of 2 in the presence of octanol. The ratio of the diffusion coefficients of the CI8MV2+and CI8MV'+ under these conditions is 2.8, indicating that octanol does not prevent dimerization of the cation radical species. The fact that an identical absorption spectrum for the reduced bilayer to that shown in Figure 7 was also obtained in the presence of octanol in the electrolyte solution confirms this observation. A similar effect of octanol can be observed in cases of other bilayer systems analogous to OTS/C18MV2+.For example, Figure 8 shows a pair of voltammograms, recorded in 0.1 M KNO, solution with and without octanol, for a bilayer system consisting of OTS and an octadecyl derivative of ferrocene, CI8Fc+. Qualitatively, the same changes in the shape of the voltammetric wave are induced by the presence of octanol as in the case of the C18MV2+system. The data illustrating the effect of octanol on the diffusion coefficients for this system are listed in Table 111. Both the magnitude of the changes induced by octanol and the nature of the effect of the charge increase in the head group region upon C18Fc+oxidation to C18Fc2+are similar to the pattern of behavior represented in Table I1 by the CI8MV2+system. The Dappvalues in Tables I1 and I11 are of similar magnitude to the translational diffusion coefficients found for lipids in artificial membranes by fluorescence recovery after photobleaching techniques. (Those values are -5 X cmz/s above the transition temperature.14) Translational diffusion coefficients of lipids in
1934
The Journal of Physical Chemistry, Vol. 92, No. 7 , 1988
Miller et al.
C"3
-k--------------1
&
\A
CIBFc*
CH3
A
A,
/nm
Figure 9. Emission fluorescence spectrum of pyrene partitioned into OTS/CTAB bilayers in porous aluminum oxide film in contact with 0.1 M KCI electrolyte. A,, = 328 nm, spectral bandwidth 1.1 nm. TABLE IV: Ratios of Fluorescence Intensity at 373 and 384 nm, 1,/13,for Pyrene Partitioned in OTS and OTS/CTAB Assemblies in Porous Aluminum Oxide Films
,
I
I
I
l
/
0.5
/
I
I
I
0
1
8
I
\I t
type of system AI,O,/OTS/O.l M KCI AI,O,/OTS-CTAB/O.I M KCI AI,03/OTS-CTAB/0.1 M KCI '
0.5
!
!
I
1
1
0
E / V vs. Ag/AgCI, sot. K C I
Figure 8. Cyclic voltammograms of CI8Fct self-assembled in an OTStreated porous aluminum oxide film at a gold electrode ( A = 0.071 cm2): (A) 0.1 M KN03electrolyte; u = 50 mV/s. (B) 0.1 M K N 0 3 electrolyte nearly saturated with 1-octanol; L' = 50 mV/s. The change in the voltammetric behavior is reversible upon transfer of the electrode from one solution to the other.
cellular membranes are in general smaller, typically 10-9-10-8 cm2/s.14 Fluorescence Studies of Octanol Partitioning into Bilayer Assemblies. On the basis of the results presented thus far, it is apparent that octanol increases fluidity within the bilayer. We postulate that octanol intercalates into the C18MV2+and C18Fc+ monolayers. Partitioning of long-chain aliphatic alcohols in micelles leading to an increase of the counterion dissociation in the head group area has been reported by a number of investig a t o r ~ . ~In~our . ~ system, ~ the observed 20% decrease of CI8MV2+ coverage upon exposure to the octanol solution can be explained by postulating partitioning of octanol into the viologen amphiphile monolayer and the enhancement of counterion dissociation. This would then result in the observed lower stability of the CI8MV2+ molecules in the monolayer at the original coverage, leading to their subsequent loss from the assembly. In order to test the hypothesis of octanol intercalation, we used pyrene as a fluorescent probe of microenvironmental polarity. Pyrene was partitioned into a bilayer of OTS and cetyltrimethylammonium bromide (CTAB) assembled in porous aluminum oxide films (see the Experimental Section). CTAB was used instead of the viologen amphiphile, to avoid quenching of pyrene fluorescence. Figure 9 shows the emission spectrum of pyrene in the OTS/CTAB bilayer. The ratio of the first to the third vibronic bands, Z,/Z3, at 373 and 384 nm, respectively, was used to infer relative polarity changes upon addition of octanol to the electrolyte s o l ~ t i o n . ~In~ the , ~ ~course of these studies, we have observed that the absolute magnitude of the Z1/13 ratio depended on the pyrene concentration in the bilayer assembly. To minimize pyrene's influence on the micropolarity of the system, its concentration in the bilayers was decreased below the level at (35) Russell, J. C.; Wild, U. P.; Whitten, D. G. J . Phys. Chem. 1986. 90, 1319 and references therein. (36) Manabe, M.; Kawamura, H.; Yamashita. A,; Tokunaga. S. J . Colloid Interface Sei. 1987, 115, 147. (37) Kalyanasundaram, K.; Thomas, J. K. J . Phys. Chem. 1977,81. 2176. ( 3 8 ) Dong. D. C.; Winnik, M . A. Photochem. Photobiol. 1982, 35, 17
II/I~
+ octanol
0.96 i 0.05 1.35 i 0.04 0.81 f 0.03
which the excimer band at 475 nm is detectable. As seen in Table IV, the Z1/Z3 ratio for pyrene adsorbed in the OTS film in contact with 0.1 M KCI solution increases from 0.96 to I .35 when the porous oxide film is exposed to a dilute (50 gM) solution of CTAB in 0.1 M KCI and then transferred to the CTAB-free electrolyte. The increased polarity upon self-assembly of the CTAB monolayer suggests that pyrene is able to diffuse in the entire OTS/CTAB bilayer which presents, on average, a significantly more polar environment than the OTS layer itself. When the electrolyte in contact with the bilayer is replaced by 0.1 M KCI saturated with octanol, the ratio Z1/Z3 decreases from 1.35 to 0.81. This result demonstrates that octanol does intercalate into the CTAB layer and may possibly increase its organization to exclude water to a larger extent than would be possible in the absence of octanol. In agreement with the electrochemical experiments discussed above, the octanol-induced decrease in the fluorescence intensity ratio persists only when octanol is present in the electrolyte. The Z1/13 ratio returns to 1.35 upon exposure of the CTAB/OTS bilayer in the oxide film to the octanol-free electrolyte. Medium-transfer experiments of this type can be repeated several times, with consistently reversible results in terms of the 11/13 ratio. This demonstrates, in addition to showing the reversible partitioning of octanol, considerable stability of the CTAB monolayer. The Counterion Effects. After demonstrating the intercalation of octanol into the bilayer assembly and its effect on the rate of lateral charge transport, we wanted to probe in more detail the effect of octanol on the structure, organization, and ion pairing of the CI8MV2+assembly. The role of long-chain aliphatic alcohols on micellar aggregates has been investigated recently by a number of group^.^^,^^ There seems to be an agreement that incorporation of alcohols in micelles increases the charge density in the head group region by promoting counterion dissociation. Whitten et al. postulated that the role of alcohols is to increase the level of organization of SDS surfactant, increasing the separation between the hydrophobic and the hydrophilic regions of a micelle.35These effects of alcohol intercalation lead consequently to a decrease of aggregation number. Similar effects can be observed by investigating the role of counterions and their s o l ~ a t i o n .Evans ~ ~ and co-workers showed, (39) Berr, S. S.; Coleman, M. J.; Marriott Janes, R. R.; Johnson, Jr., J. S . J . Phys. Chem. 1986, 90,6492. (40) Brady, J. E.; Evans, D. F.: Grieser, F.; Warr, G. G.; Ninham, B. W. J . Phys. Chem. 1986, 90, 1853. (41) Brady, J. E.; Evans, D.F.: Kadir, B.; Ninham, B. W. J . A m . Chem. SOC.1984, 106, 4279
Microporous Aluminum Oxide Films at Electrodes
The Journal of Physical Chemistry, Vol. 92, No. 7, 1988
1935
TABLE V: Binding of Fe(CN)6e in CI8MV2+Monolayers; Effect of Octanol on the Diffusion Coefficients of Fe(CN)6" and C18MV2+
8 16
0.34
32
0.42 0.43
64
0.37
1.1 1.1 1.2 1.1
for example, that the aggregation number of dodecyltrimethylammonium micelles was lower in the presence of acetate and hydroxide ions than in a medium of less well solvated chloride Better solvation of counterions leads, again, to increased ion dissociation and consequently to higher electrostatic repulsion forces in the head group region. This in turn is directly responsible for the lower aggregation number. In our case, we can postulate that the extent of Coulombic interactions in the head group region of the C18MV2+monolayers is an important factor influencing both the monolayer structure and the dynamics of the lateral transport processes. Based on the literature reports discussed above, both the presence of octanol in the electrolyte solution and the nature of counterions will strongly influence the extent of Coulombic interaction in the amphiphilic monolayer. The first experimental evidence of counterion effects on the dynamics of lateral processes was observed when perchlorate ions were introduced at a millimolar level to the KCl solution where the voltammetric behavior of the OTS/C&fV2+ system was being investigated. This caused a virtually complete annihilation of the voltammetric signal described previously in Figure 3. The C104ions strongly associate with the viologen groups of the Cl8Mv2+ monolayer. This is consistent with the fact that in aqueous solutions the perchlorate salt of the viologen amphiphile is sparingly soluble. This results in freezing of the lateral charge transfer. The effect is reversible in that full electroactivity is restored upon medium transfer to pure 0.1 M KCI solution. An opposite extreme is observed when the oxide-coated electrode with the assembled OTS/CI8MV2+bilayer is transferred to a 0.1 M CH3COONa solution. One observes a rapid leaching of the Cl8MV2+,to a final coverage roughly half of that stable in the 0.1 M KC1 electrolyte. These observations are consistent with the counterion effects seen in the dodecyltrimethylammonium micellar solutions discussed above. Binding of Ferrocyanide in O T S / C 1 8 M p +Bilayers. In an attempt to quantify the counterion effects and the related role of octanol, we have investigated the behavior of the system in the presence of a low concentration of electroactive counterions. The ferricyanide/ferrocyanide couple was chosen as a model system because its redox potential is well-removed from that of the CI,MV2+/Cl8MV'+ couple. Even at a micromolar level in the electrolyte solution, ferrocyanide ions partition strongly to form ion pairs with the viologen groups in the head group region of the C,,MV2+ monolayer. This serves as a means of Fe(CN)64- immobilization, which can be detected electrochemically via lateral transport of the bound ferrocyanide ions to the electrode surface. The pertinent cyclic voltammograms recorded with and without octanol in the 0.1 M KC1 electrolyte are shown in Figure 10. The shape of the voltammetric waves for the Cl8MV2+monolayer and its dependence on the octanol presence are analogous to those described in Figure 3. The binding of ferrocyanide is easily recognized by the waves at 0.15 V. Integration of the wave due to the ferricyanide reduction gives us an assessment of its partitioning coefficient to be in order of lo5. Integration of the current due to C&v2+ reduction in the same experiment yields the ratio of the bound ferrocyanide to the C18MV2+in the monolayer assembly. In addition, two independent potential step chronocoulometric experiments give us the diffusion coefficients of the (42) Ninham, B. W.; Hashimoto, S . ; Thomas, J. K. J . Colloid Interface Sei. 1983, 95, 594. (43) Evans, D. F.; Sen, R.:Warr, G. G. J Phys. Chem. 1986, 90, 5500. (44) Quintela, P. A.; Reno, R. C. S.; Kaifer, A . E. J . Phys. Chem. 1987, 91, 3582.
14 7.7 7.7 7.6 7.6
4.7 4.0 5.2 5.3
14 14 14 13
A
I
I I I B I I I I I I I I I I +0.6 +0.4 +0.2 +O.O -0.2 -0.4 -0.6 -0.8 E vs. A g l A g C l , sat. K C I
B
I
I
I
I
I
I
+OB +0.4 +0.2
I
I
I
I
I
I
I
I
I
I
0.0 -0.2 -0.4 -0.6 -0.8
E vs. A g / A g C I , s o l . KCI
Figure 10. (A) Cyclic voltammograms of OTS/C18MV2+self-assembled in a porous aluminum oxide film at a gold electrode recorded in the presence of 17 pM K,Fe(CN), in 0.1 M KCI electrolyte. The background voltammograms (dashed line) were recorded in the same solution after nearly complete desorption of CI8MV2+from the electrode film by rinsing with methanol. (B) The same experiments done in the presence of octanol.
lateral charge transport for the bound Fe(CN)64-and CI8MV2+. In the former case, the Anson slopes were corrected for the small contribution due to the ferrocyanide diffusion in the electrolyte solution, as described before.2 The results of these experiments are collected in Table V. Strong interactions of ferrocyanide ions with the C18MV2+ groups result in a 0.3-0.4 ratio of these ions in the assembly, depending on the ferrocyanide concentration in solution. The ratio of bound Fe(CN)6C/C18MV2+is independent of octanol presence in the system. The most remarkable difference in the system's behavior in the presence and in the absence of octanol is the extent of the decrease of the C&V2+ diffusion coefficient upon addition of potassium ferrocyanide. In the absence of octanol, a fivefold decrease is observed, while in the presence of octanol, the diffusion coefficient of C,8MV2fdecreases only by a factor of 1.8 compared to the initial value found prior to the addition of ferrocyanide ions. In both cases, we associate the decrease in the c I 8 M v 2 +lateral mobility with the electrostatic binding of the head groups with
1936 The Journal of Physical Chemistry, Vol. 92, No. 7, 1988
ferrocyanide ions; these interactions involve most probably more than one C18MV2+molecule per one Fe(CN)t- ion. This comparison of the D,, values for C18MV2+with and without octanol clearly demonstrates that the extent of these interactions is decreased significantly by the presence of the intercalated octanol. Further analysis of the data in Table V leads to the conclusion that the lateral diffusion of ferrocyanide ions involves physical motion along the CI8MV2+monolayer since the ferrocyanide diffusion coefficient is higher than that for the viologen amphiphile. Charge transfer via electron hopping could not account for this fact because the rate constant of electron self-exchange for the ferricyanide/ferrocyanide couple is smaller than the one for the MV2+f MV" couple. By comparing the differences between the ferrocyanide and the C18MV2+Dappvalues with and without octanol, we can see that the motion of Fe(CN)64-ions along the C18MV2+assembly is accelerated upon the addition of octanol. This effect is treated in more detail e l s e ~ h e r e .This ~ ~ observation is an additional argument consistent with our contention that the intercalation of octanol decreases the extent of counterion binding in the head group region of the CI8MV2+monolayer.
Conclusions We have described in this report a novel electrochemical method for the direct measurement of lateral charge transport diffusion coefficients in organized bilayer assemblies. In these measurements, we took advantage of the pore structure geometry of the porous aluminum oxide films which serve as templates for the bilayer assembly. This geometry results in the perpendicular orientation of the bilayer assemblies with respect to the electrode surface and thus allows us to use standard electrochemical methods to investigate the kinetics of the lateral charge transport. The bilayer systems are obtained by self-assembly of CI8MV2+ or its ferrocene analogue from aqueous solutions onto the OTStreated inner surfaces of the porous aluminum oxide films. We found that the maximum coverage of the viologen amphiphile in the OTS/CI8MV2+bilayer is limited by the size of the closepacked C18MV2+head groups. However, bilayers showing long-term stability in the electrolyte solutions free of the viologen amphiphile are those with coverages approximately half this value. (45) Miller, C. J.; Majda, M. Anal. Chem., in press.
Miller et al. Bilayers with this coverage of C18MV2+exhibit diffusion coefficients of the lateral charge transport ranging from 2 X to 2 X lo-' cm2/s, depending on the character of the head group, its charge, and the presence of aliphatic alcohols in the electrolyte solution. Based on the fluorescence studies involving pyrene as a polarity probe, it was shown that octanol intercalates into the bilayer assemblies. On the basis of our experiments, we believe we have good evidence to propose the following model regarding the role of octanol in promoting a higher rate of lateral charge transport. It involves octanol intercalation which increases ordering of the amphiphilic molecules in the monolayer. This results in an increase of the environment polarity around the head groups of the individual surfactant molecules and promotes dissociation of counterions. The resulting electrostatic repulsive interactions in the head group region lead then to an increase of microfluidity of the monolayer and to the observed higher diffusion coefficients of charge transport. This model postulates, implicitly, that the C18MV2+molecules in the monolayer at a half-coverage are not uniformly distributed, in that they probably form aggregates with higher local surface concentration than the reported average coverage value. The intercalation of octanol, we postulate, promotes dispersion of these aggregates and leads to a more uniform spreading of the molecules in the monolayer of higher microfluidity. What this model is not capable of explaining is whether the primary source of the intermolecular interactions limiting the overall microfluidity of the system comes from within the hydrocarbon or from the head group region of the monolayer. Both of these are possibilities. We are currently carrying out investigations, some of which involve the dynamics of pyrene excimer formation, which address this question.
Acknowledgment. We gratefully acknowledge the National Science Foundation for supporting this research under Grant CHE-8504368. We also thank Prof. Joel M. Harris for his help and discussion of the initial fluorescence experiments and Mr. Charles A. Goss for synthesizing the electroactive amphiphiles. Registry No. OTS, 112-04-9; Cl,Fc+, 104716-46-3; CI8MVz+, 84458-72-0; CTAB, 57-09-0; AI2Ol, 1344-28-1; Au, 7440-57-5; Fe(CN):-, 13408-63-4; SO2, 763 1-86-9; 1-octanol, l l 1-87-5; C18MV'+, 113009-15-7; C,gFC2+,113009-14-6.
