Langmuir 1993,9, 1404-1407
1404
Influence of Donnan Potentials on Apparent Formal Potentials Measured for Organized Thiol Monolayers with Attached Pentaamminepyridineruthenium Redox Centers Jody Redepenning,'J Harmon M. Tunison,? and Harry 0. Finkleat Department of Chemistry, University of Nebraska, Lincoln, Nebraska 68588-0304, and Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506-0045 Received November 20, 1992. In Final Form: March 2, 1993 The formal potential of pentaamminepyridinerutheniumcenters attached to gold electrodes by thiol linkages is measured in aqueous solutions with changing activities of sodium chloride. As the activity of the supporting electrolytechanges, the apparent formal potentials shift in a predictable manner because of changes in the Donnan potential at the monolayer/solution interface. Details of the potential measurements are provided. The results are consistent with a model in which one chloride is transferred at the interface between the monolayer and the solution for each electron transferred at the interface between the monolayer and the electrode. The concentration of supporting electrolyte has been found to influence formal potentials measured for some modified electrode surfaces.14J1J2 A similar concentration dependence has recently been discussed in terms of the role that Donnan potentials may play in determining apparent formal potentials measured for redox couples in polyelectrolytes on electrode surfaces."' The magnitude of the Donnan potential is determined by the relative activities of counterions in the solution and polyelectrolyte phases.8 Although in one limit a polyelectrolyte film can be thought of as a three-dimensional permselective phase in which the transport number for counterions approaches unity, the Donnan potential should be confined to the polyelectrolyte/solutioninterface. Consequently, the influence of the Donnan potential on apparent formal potentials should be independent of the film thickness. In the work described below we examine the influence of the Donnan potential on apparent formal potentials measured in the limit in which the redox couple is confined to a single monolayer on an electrode surface. A successful method for binding a high density of redox centers in a true monolayer is based on self-assembly of alkanethiol monolayers.g-20Self-assembled monolayers with attached
* To whom correspondence should be addressed. Nebraska. West Virginia University.
+ University of t
(1)Ellis, D.; Eckhoff, M.; Neff, V. D.J. Phys. Chem. 1981,85,1225-31. (2)(a)Inzelt, G.; Bacskai,J.; Chambers, J. Q.; Day, R. W.J. Electroanul. Chem. 1986,201.301-14. (b) Inzelt, G.; Horanyi, G.; Chambers, J. Q. Electrochim. Acta 1987,32,757-63. (c) Inzelt, G.;Szabo, L.; Chambers, J. Q.; Day, R. W. J. Electroanal. Chem. 1988,242,265-75. (3)Inzelt, G.; Szabo, L. Electrochim. Acta 1986, 31, 1381-87. (4)Tsou, T.-M.; Anson, F. C. J.Electrochem. SOC. 1984,131,595-601. (5)Inzelt, G.Electrochim. Acta 1989,34, 83-91. (6)(a) Redepenning, J.;Anson, F. C. J.Phys. Chem. 1987,91,4549-53. (b) Naegeli, R.; Redepenning, J.; Anson, F. C. J. Phys. Chem. 1986,90, 6227-32. (7)(a) Niwa, K.; Doblhofer, K. Electrochim. Acta 1986,31,549-53. (b) Doblhofer, K.; Armstrong, R. D. Electrochim. Acta 1988,33,453-60. (8)Helfferich, F. Ion Exchange; McGraw-Hilk New York, 1962;p 372. (9)(a) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990,6,371-6.(b) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. 1990,112,4301-6.(c) Chidsey, C. E. D. Science 1991,251, Chem. SOC. 919-22. (10)(a) Creager, S.E.; Collard, D. M.; Fox, M. A. Langmuir 1990,6, 1617-20. (b) Collard, D. M.; Fox, M. A. Langmuir 1991,7, 1192-7. (11)(a) Creager, S.E.; Rowe, G. K. A m l . Chin. Acta 1991,246,233-9. (b) Rowe, G. K.; Creager, S. E. Langmuir 1991,7,2307-2312. (12)(a) Uosaki, K.; Sato, Y.; Kita, H. Langmuir 1991,7,1510-14. (b) Shimazu, K.; Yagi, I.; Sato, Y.; Uosaki, K. Langmuir 1992,8,1385-87.(c) Uosaki, K.; Sato, Y.; Kita, H. Electrochim. Acta 1991,36,1799-801.
(pyridine)R~(NH3)5~+/~+ redox centers exhibit nearly ideal voltammetric behavior even at high coverages.lg Thus, we discuss how the apparent formal potentials of these electroactive (pyridine)R~(NH3)5~+/~+ monolayers are varied by changing the electrolyte activity in solution.
Experimental Section [HS (CH,) IoCONH-CHTPYRU(NH&] (PF& Preparation.
Synthesis of the electroactive thiol was based on the coupling of (4-AMP)Ru(NHa)s2+ (4-AMP= 4-(aminomethyl)pyridine)with w-thiolalkanecarboxylicacid. The preparation followedstandard literature procedures.S@-22The complex was purified by dissolving the majority of the crude product in a small amount of optical grade acetonitrile (Baxter) to produce a solution which contained a portion of undissolved suspended material. This insoluble materials was removed by centrifugation. After the supernate was decanted, the solid was washed twice with acetonitrileand all of the washings were combined. The dissolved complex was precipitated from the acetonitrile by addition of a large excess of tetrabutylammonium bromide (Aldrich). The bromide salt was converted to the hexafluorophosphate salt by dissolving it in a minimum amount of water and adding a large excess of ammonium hexafluorophosphate. Monolayers were prepared on polycrystalline gold wires (Aldrich)which had been cleaned by being heated to incandeecence in a gas-air flame. Cleaned electrodes were immersed for 17-24 h in a solution of the purified ruthenium thiol complex in (13)Lee, K. A. B. Langmuir 1990,6,709-12. (14)(a) De Long, H. C.; Buttry, D. A. Langmuir 1990,6,1319-22. (b) De Long, H. C.; Donohue, J. J.; Buttry, D. A. Langmuir 1991,7,21962202. (15)Obeng, Y.S.;Bard, A. J. Langmuir 1991, 7, 195-201. (16)(a) Hickman, J. J.; Ofer, D.; Laibinis, P. E.; Whitesides, G. M.; Wrighton, M. S. Science 1991,252,688-91. (b) Hickman, J.J.;Laibinis, P.E.;Auerbach,D. I.;Aou,C.;Gardner;T. J.; Whitesides,G. M.;Wrighton, M. S. Langmuir 1992,8, 357-9. (c) Frisbie, C. D.; Fritsch-Fades, I.; Wollman, E. W.; Wrighton, M. S. Thin Solid F i l m 1992,210/211,341-7. (17)Collinson, M.; Bowden, E. F.; Tarlov, M. J. Langmuir 1992,8, 1247-50. (18)Hong, H.-G.; Mallouk, T. E. Langmuir 1991, 7,2362-9. (19)(a) Finklea, H. 0.; Hanshew, D. D. J.Am. Chem. SOC. 1992,114, 3173-81. (b) Finklea, H. 0.;Hanshew, D. D. J. Electroanul. Chem., in Ravenscroft, M. S.; Snider, D. A. Langmuir press. (c) Finklea, H. 0.; 1993,9,223. (20)Hanshew, D. D. M.S. Thesis, West Virginia University, 1991. (21)(a) Ford,P.; Rudd, De F. P.; Gaunder, R.; Taube, H. J.Am. Chem. SOC.1968,90,1187-94.(b) Mataubara, T.; Ford, P. C.Inorg. Chem. 1976, 15,1107-10. (c) Ford, P. C.; Nalouf, G.; Petersen, J. D.; Durante, V. A. Adv. Chem. Ser. 1976,No.150,187-200. (22)(a) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988,4,365-85. (b) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. SOC.1989,111,321-35.
0743-7463/93/2409-1404$04.oo/o 0 1993 American Chemical Society
Apparent Formal Potentials for Thiol Monolayers optical grade acetonitrile. The surface coveragesobtained using this procedure are consistent with the 100% surface coverages (15-20 pC/cm2)observed in previous work.lg All electrochemistry was performed in degassed aqueous sodium chloride solutionsusing either asaturatedsodiumchloride calomel electrode (SSCE) or a silvedsilver chloride reference electrode. All concentrations are reported in units of molality. When the SSCE was used as the reference electrode, it was necessary to correct the measured potentials for the changing liquid junction potentials at the interace between the saturated sodium chloride solution in the reference electrode and the changing sodium chloride concentrations in the bulk of the solution to which the working electrode was exposed. The changingliquid junction potentials were calculated by measuring the cell potentials of the Ag/AgCl electrode versus the SSCE over concentrationsranging from 0.005to 2.0 m. The differences between the measured cell potentials and those calculated using the known sodium chloride activities in solution were attributed to the liquid junction potentials. At 0.10 m and abovethe activity coefficients used are those measured by Robinson and Stokes.23 A t 0.050 m and below the extended Debye/Huckel theory was used to calculate the activity coefficients. A PAR Model 273potentiostat/galvanostat was used toperform the electrochemical measurements. Experimental data were evaluated using a modified version of "Headstart" from EG&G/ PAR. Apparent formal potentials were determined using cyclic voltammetryat scan rates which were typically 100 mV/s. Under these conditions surfacewaves with facile heterogeneouskinetics and a = 0.5 were observed. The apparent formal potentials were assumedto be identical to the average of the anodic and cathodic peak potentials. We assume that shifts in the peak potentials associated with uncompensated resistance are nearly identical in magnitude but are opposite in sign for the anodic and cathodic peaks since the anodic and cathodic peak currents are nearly identical in magnitude. Consequently, modest amounts of uncompensated resistance should produce only small errors in the values measured for the apparent formal potentials. Peak separationswere observed to be lessthan 20mV even at electrolyte concentrations as low as 0.005 m. Formal potentials measured for individualelectrodesappear to be reproducible to better than f2 mV. All measurementswere carried out at room temperature (25 f 1 "C). Slopes reported for plots discussed below are obtained from linear regression. The errors listed for the slopes are 1 standard deviation.
Results and Discussion In some respects the interface between a charged monolayer on an electrode surface and an electrolyte solution is similar to the interface between a polyelectrolyte and an electrolyte solution. If a polyelectrolyte is placed in a dilute solution of a strong electrolyte, the concentration of counterions in the polyelectrolyte is typically considerably greater than that found in solution. Thus, under the influence of the concentration gradient, counterions should diffuse from the polyelectrolyte into solution until the concentrations become equal in the two phases. Because the counterions are charged, however, electroneutrality is violated and an electric potential begins to develop at the interface. This "Donnan potential" continues to increase until an equilibrium is reached in which it completely counteracts the tendency for counterions to diffuse into solution. Under these equilibrium conditions the net diffusion of counterions across the interface is zero and co-ions, i.e., those ions with the same charge sign as the polymer backbone, are excluded from the polyelectrolyte. A similar scenario can be envisioned for a charged monolayer on an electrode surface. Figure l a shows a cross section of a tightly packed monolayer of adsorbed (23) Robinson, R. A.; Stokes, R. H. Trans. Faraday SOC.1949, 45, 612-24.
Langmuir, Vol. 9, No. 5, 1993 1406 I I metal
I I
metal
surface
I
surface
I
I la
I
Figure 1. Drawing depicting cationic monolayers attached to metal surfaces: (a)monolayer of trivalent cations; (b)monolayer of divalent cations. Because the number of ions required to produce the Donnan potential is small compared to the total number of ions at the interface, the small deviation from electroneutrality associated with the Donnan potential is not shown. trications attached to the surface through a spacer.24Near each of these solvated trications are three univalent anions.1g The anions in this figureare represented as being small enough to nestle down near the plane of dications. The extent to which this occurs in real systems will be determined by the packing density of the adsorbed layer and on the size of the anions. Two extremes can be envisioned for what the actual structure might be: one in which all of the cations and anions lie in a single plane, and one resembling the Helmholtz model of the electrochemical double layer in which there is a plane of adsorbed cations and a separate layer with the necessary number of anions. Whatever the structure near the surface actually is, it should be true for high packing densities that there is a relatively high concentration of counterions inside the surface "plane" (represented by the dashed line in Figure la) compared to that in the bulk of the electrolyte. In a manner similar to the polyelectrolyte case, a Donnan potential should develop at the surface because a few mobile counterions have diffused away a short distance. Consequently,the surface should be slightly positive with respect to the adjacent electrolyte and electrolyte cations should be repelled. This Donnan potential should exist as long as the activity of counterions associated with the surface is greater than the activity of counterions in the bulk of solution. As the activity of counterions in solution approaches that of ions associated with the surface, complete charge compensation of the monolayer should be approached while the Donnan potential approaches zero. Many of the arguments that apply to the trivalent cationic monolayer in Figure l a also apply to the densely packed dicationic monolayer shown in Figure lb. Again it should be true that a Donnan potential should develop at the surface of the monolayer leaving it slightly positive with respect to the adjacent solution. Finally, if parta a and b of Figure 1accurately represent the structure of the oxidized and reduced forms of a monolayer, then it is evident that electron transfer between the electrode and the redox sites must be accompanied by transport of counterions across the monolayer/solution interface. Thus, we propose that the half reaction governing the voltammetric response of such systems includes not only the oxidized and reduced redox surface sites, (Oxn+),& and (Red(n-l)+),&, but also counterions (Cl- in this case) in the bulk of solution and at the surface of the monolayer: (24) Because it is firmly entrenched in the literature,we will continue to use the word 'monolayer" even though Figure 1 suggests it might be a misnomer in some cases.
Redepenning et al.
In this model migration of counterions is considered to be the exclusive means of carrying current across the monolayer/solution interface. Charge compensation by migration of cations is assumed to be negligibleat the monolayer/ solution interface. Given the half reaction shown in eq 1, the potential of the working electrode, E,, is given by eq
w 0.10-
2
RT
E, = EWo + -1n-nF %ed Qox
a~l,surf
RT %,surf (Ewo')app = E,,' + -In (3) n F aC1,sol In order to eliminate liquid junction potentials which might be convolved in the measured cell potentials, a junctionless cell is used in which a Ag/AgCl reference electrode is introduced directly into the same electrolyte solution contacting the working electrode. The potential for the reference half cell is given by eq 4 in which Erefo = +0.2223 V vs NHE.
-
1 Eref= Ere:+ RT -In -
(4) n F %l,sol By combination of eq 3 and 4, the potential of the cell represented in shorthand notation by eq 5 is given by eq 6. Agl AgCllX m NaCl~(C1-),[Ru~~-thioll~Au (5) E c e l l = %ernst
2
3
= (Ewo')app
4
- Ere?
-
* (4
%l,sol
where aoxand ared are the activities of the oxidized and reduced forms of the redox couple on the surface and where UCl,surf and ucl,solare the chloride activities at the surface and in solution, respectively. If the standard states of Ox and Red are chosen to be pure monolayers of Ox and Red, and if the electrode potential is measured when the mole fractionsof Ox and Red are equal,then the apparent formal potential, (Eo'lapp,can be expressed as is shown in eq 3 in which Eo' = Eo + (RT/nF) In (YoJYred).
1
.,
%
RT
+ 2In (aC1,surt) (6)
Note that the activity of chloride ions in solution does not appear in eq 6. Although the potentials of each of the two half cells exhibit a dependence on ucl,sol,the dependence is the same for each and thus does not appear in the cell potential. Consequently,if one chloride ion is transferred at the monolayer/solution interface for each electron transferred between the electrodeand the monolayer,then no change in the cell potential will be observed as the electrolyte activity is changed. The results of such a measurement performed on a monolayer of the Ru-thiol are shown in plot a in Figure 2. The slope of 3.7 f 1.4 mV is very near the value of 0 mV expected if eq 1accurately represents the half reaction which actually occurs at the working electrode. The small deviation from the ideal slope of 0 mV might be due to changingactivity coefficients at the surface or to a small deviation from ideal permselectivity. If monolayer permselectivity breaks down completely at high electrolyte concentrations, then the half reaction shown in eq 1 should not be valid and the only dependence of the formal potential on electrolyte concentration should be due to differences between the ion pairing of Oxn+and Red(n-l)+with C1-. As will be seen later, however, even at electrolyte concentrations as high as 2.0 m these monolayers retain a high degree of
permselectivity. In summary the results shown in plot a of Figure 2 suggest that the dominant form of charge transport at interfaces 1and 4 for the cell represented in eq 5 is electron transfer, while at interfaces 2 and 3 the dominant form of charge transport is chloride ion migration. These results are consistent with electrochemical measurements by Creager" and Uosaki12bon ferrocenylalkanethiol monolayers and with the conclusions reached by Buttry et al.14in their studies on ionic interactions for ferrocene and viologen monolayers. On one level the results discussed above are satisfying, because good agreement with the predicted behavior is obtained without the complications of liquid junction potentials. On another level the results are disconcerting since the 0 mV slope expected for these experiments is exactly the same result one would expect if the potential of neither half cell changed with changingua,,l. Although it seemed unlikely that the reference electrode was not responding reversibly to chloride ions in the electrolyte, cell potentials were also measured versus a SSCE reference electrode for which the potential is not dependent on the electrolyte activity. Such a measurement necessitates the introduction of liquid junction potentials, however, which make interpretation of the shifts in apparent formal potentials less straightforward, If the potential of the working electrode is measured versus a SSCE, the cell may be represented in shorthand notation by eq 7. HgolHg,C12pt. NaCllX m NaCl~(C1-),[Ru"-thioll IAu 1
3
4
6
(7) The potential of this cell, which cannot be equated with the potential calculated from the Nernst equation, is given by eq 8. In eq 8 Ej is the liquid junction potential at the interface between the saturated sodium chloride solution in the reference compartment and the solution containingsodium chloride in the working electrode compartment, Le., at interface 3 in eq 7. Given that the potential of the saturated sodium chloride calomel electrode, ESSCE, is 0.2360 V vs NHE, the cell potential can be described as is shown in eq 9 by substituting eq 3 into eq 8.
Langmuir, Vol. 9, No. 5, 1993 1407
Apparent Formal Potentials for Thiol Monolayers
Ecell= Ewe' -0.2360 V + RT -In aclcurf+Ej
nF
QCl,sol
(9)
If Ej is constant, then eq 9 predicts that a 59.2-mV shift in Eceushould be observed for each factor of 10 by which acl,solchanges. However, a plot of Emuvs log aClsOlfor the cell represented by eq 7 gives a slope which is only -46.1 f 0.4 mV (Figure 2b). The plot clearly indicates that if the validity of the half reaction in eq 1is to be determined, it is necessary for the liquid junction potentials to be measured. The cell which was used to determine the liquid junction potentials is represented by eq 10. HgolHg2Cl,lsat.NaCllX m NaCllAgCllAg 3
1
4
5
(10)
Although the liquid junction potentials for this cell can be calculated if certain assumptionsare made concerning the structure of the junction, we chose to measure the liquid junction potentials. Note that the liquid junction in this cell, junction number three, is identical to the liquid junction in the cell represented by eq 7. In fact, the same SSCE reference electrode was used in the monolayer experiments and in the junction potential measurements. So, if Eceufor the cell represented by eq 10 is measured experimentally and &ernst is calculated using eq 11,then the remaining unknown in eq 9, Ej, can be determined. ENernst
=
- EAn/AnC1 -I
=
RT
+0.2360 V - 0.2223 V + 3In (acl,sol)(11) When Ej is determined as described above for a number of different concentrations and is plotted versus log acl,sol, a straight line with a slope of -13.1 f 0.3 mV is obtained. This value is in good agreement with the value of -12.3 mV expected if the crude assumption that the liquid junction is a simple “type 1”junction2s and that the transport numbers of Na+ and C1- at infinite dilution, 0.396 and 0.604, respectively,26are used to calculate the liquid junction potential^.^^ Figure 2c is a plot of ENernst v8 log acl,solobtained after Ecauhas been corrected for changing Ej. Note that the slope is -59.3 f 0.6 mV, a value which is in excellent agreement with the value of -59.2 mV expected at 25 OC. It should be noted for the electrode described in Figure 2 that there is a slight discrepancy between the results obtained vs Ag/AgCl and those obtained vs SSCE. To be completely consistent these slopes should differ by exactly 59.2 mV regardless of whether or not the films are permselective. Although this does not appear to be the case if one considers this particular electrode alone, the apparent discrepancy can be explained if the slopes for several different electrodes with 100% coverages of the Ru-thiol complex are compared. In Table I the average slope for the measurements made vs SSCE is -57.9 mV and that the standard deviation calculated from these five different slopes is f1.3 mV. The average slope for measurementsmade vs Ag/AgCl is 2.4 mV with a standard deviation of f1.9 mV. After correcting for the 59.2 mV/ decade shift in the potential of the Ag/AgCl reference electrode, the average slope for the Ru-thiol half cell cell (25)Lingane, J. J. Electroanalytical Chemistry, 2nd ed.; Wiley-Interecience: New York, 1958; Chapter 3. (26)MacInnes,D. A. The Principles OfElectrochemistry;Dover: New
York, 1961;p 342. (27) Bard,A. J.; Faulkner,L. R. ElectrochemicalMethods;John Wiley and Sons: New York, 1980; p 69.
Table I. Slopes Obtained by plotting E,II vs log(acl.mln)for Six Different Electrolyte Concentrations Ranging from 0.005 to 0.20 IJr SSCEb -57.0 f 2.4d -59.2 f 0.4 -57.3 f 1.5 -56.5 f 3.8 -59.3 f 0.6 ~t=
Ag/AgClc 3.1 f 2.4d 3.4 i 0.3 e
-0.4 f 13.5 3.7 1.4
*
r = 2.4 i 1.9
-57.9 f 1.3
aErrors for the individual slopes are il standard deviation determined fromthe liiearE,uw log(aC1J plots. Errors associated with the average slopes,r ,are fl standard deviation calculatedfrom the scatterin the slopes. After correctionof changingliquidjunction potentials. Same electrode aa in preceding column. For this electrode the electrolyte activity ranged from 0.005 to 0.10 m. e Measurements w Ag/AgCl were omitted for this electrode. 0.40
3 Ia
-id j0.30
I B
1 0.20 -2.50
-1.50
-0.50
0. 0
log a Figure3. (a) Apparentformal potentiale (vsNHE) for @-AMP)Ru(NH&*+ monolayer on Au. Measurements were initially performed versus SSCE. (b) Apparent formal potentiala (vn NHE) for (4-AMP)Ru(NH3)b2+ monolayeron Au. Measurements were initially performed versus Ag/AgCl. is found to be -56.8 mV f 1.9 mV, a value which agrees with that determined from measurements made vs SSCE. To demonstrate the high degree of permselectivity that these tightly packed monolayers exhibit at high electrolyte concentrations, the apparent formal potentials for the monolayers are plotted versus log(aa,l) in Figure 3 for concentrations ranging from 0.005 to 2.0 m. These data happen to be for the same electrode shown in Figure 2. To show graphically the internal consistency of the results, in this case measurements made versus SSCE and Ag/ AgCl have been displayed versus NHE. Very little curvature is observed in these plots at high electrolyte concentrations. Over the activity range from 0.15 to 1.3 m,Le., over the concentration range from 0.2 to 2.0 m,the plots in Figure 3 still exhibit slopes of approximately -50 mV. Thus, even at relatively high electrolyte activities, the half cell depicted in eq 1appears to be consistent with the potential measurements and these monolayers appear to approach ideal permselectivity. Finally, similar plots for electrodes with low coverages of (~-AMP)Ru(NH&~+ give slopes whichare much flatter than the expected -59.2 mV. A detailed study of the permselectivity of electrodes with submonolayer coverages is the subject of a future investigation.
Acknowledgment. J.G.R. acknowledges the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research.
