Langmuir 1994,10, 1971-1979
1971
Adsorption Dynamics of Electroactive Self-Assembling Molecules Jorge D. Tirado, David Acevedo,t Richard L. Bretz, and HBctor D. Abruiia' Department of Chemistry, Baker Laboratory, Cornel1 University, Ithaca, New York, 14853-1301 Received November 29, 1993. In Final Form: March 7,1994"
The kinetics of adsorption of redox active self-assemblingmolecules of the type [Os(bpy)aLCl]+(bpy = 2,2'-bipyridine and L = 4,4'-bipyridine (pypy), trans-l,2-bis(4-pyridyl)ethylene(py=py), 1,3-bis(4pyridy1)propane (dipy), or 1,2-bis(4-pyridyl)ethane (dipyH2)) have been investigated as a function of concentration,applied potential, and solvent. For all the complexesstudied, the adsorption process appears to be under kinetic rather than diffusion control. Whereas the rate constant appears to be invariant with concentration at values below 1pM, at high concentrations the kinetics are much faster. The electrode potential appears to affect the equilibrium coverage (likely due to electrostatic repulsions) but not the kinetics of adsorption. At low concentrations, there appears to be an induction period that we have tentatively ascribed to variations in local concentration in order to achieve a critical value after which adsorption is induced. In nonaqueous solvents, the solution concentration needed to achieve a saturation coverage is at least an order of magnitude larger than that in aqueous media pointing to the importance of adsorption strength and solubility differences. Whereas the rate of adsorption was not very sensitive to the nature of the solvent, the equilibrium value of the surface coverage was.
Introduction Self-assembled monolayers are molecular assemblies spontaneously formed by the immersion of an appropriate substrate into a solution of an active surfactant.' This irreversible adsorption process can be used to modify surfaces with molecules possessing a head group that binds to the substrate and, in addition, may have a specific chemical functionality as a tail group. Examples of selfassembling systems include sulfur-containing (especially thiols) compounds on gold2and p l a t i n ~ misocyanides ,~ on platinum: organosilicon on hydroxylated surfaces,5 and pyridine derivatives on platinum.6 The interest in these systems lies in their ability to form very well defined twodimensional structures, allowing for the controlled modification of surfaces. When the substrate surface is an electrode, one can correlate electrochemical behavior with the microstructure of the monolayer. Electrochemical techniques have been used to study various properties of these assemblies including stability, permeability, and uniformity. Doblhofer et al. used cyclic voltammetry together with IR spectroscopy to study the stability and the adsorptionfdesorption reactions of adsorbates with terminal ionic groups.' Using capacitance measurements, Chidsey and Loiaconos investigated the permeability of thiol derivatives adsorbed on gold electrodes and which additionally had different functional + Current address: Abbott Laboratories,Barceloneta, Puerto Rico. Abstract published in Advance ACS Abstracts, May 15, 1994. (1)Ulman, A. An Introduction to Ultrathin Organic Films From Langmuir-Blodgett To Self-Assembly; Academic Press: San Diego, CA, @
1991. (2) Bain, C. D.; Whitesides, G. M. Angew. Chem. 1989, 101,506. (3) Mebrahtu, T.; Berry, G. M.; Bravo, B. G.; Michelhaugh, S. L.; Soriaga, M. P. Langmuir 1988,4, 1147. (4) Hickman, J. J.; Zon, C.; Ofer, D.; Harvey, P. D.; Wrighton, M. S.; Laibinis, P. E.; Bain, C. D.; Whitesides, G. M. J.Am. Chem. SOC. 1989, 111,7271. (5) Finklea, H. 0.; Robinson, L. R.; Blackburn, A.; Ritcher, B.; Allara, D.; Bright, T. Langnuir 1986, 2, 239. (6) Stern, D. A.; Laguren-Davidson, L.; Frank, D. G.; Gui, J. Y.; Lin,
C. H.: Lu, F.; Salaita, G. N.: Walton, N.; Zapien, D. C.; Hubbard, A. T. J. Am. Chem. SOC. 1989,111,877. (7) Doblhofer, K.; Figura, JBrg; Fuhrhop, Jiirgen-Hinrich Langmuir 1992,8, 1811. (8) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682.
0743-7463/94/2410-1971$04.50/0
groups facing the solution phase. There have been numerous studies of electron transfer reactions of electroactive self-assembled monolayers on electrodes using chronoamperometry and cyclic voltammetry.sll Studies of the adsorption kinetics of alkanethiol monolayers adsorbed on gold have been carried out by Whitesides and co-workers.12 Sagiv et al.13 and Whitesides et al.14 studied similar processes for alkyltrichlorosilane monolayers using contact angle measurements to establish surface structure and optical ellipsometry to estimate thickness. We previously" reported on self-assemblingmonolayers derived from osmium complexes of the type [Os(bpy)zClLl+, where L is a substituted dipyridyl group which allows adsorption onto a platinum surface. In this paper, we present a comprehensive study of the adsorption kinetics of these materials onto platinum electrodes. The osmium complexes employed have the following general structure [Os(bpy)zLCll+ with bpy = 2,2'-bipyridine and L = 4,4'-bipyridine, trans-1,2-bis(4-pyridyl)ethylene,1,3bis(4-pyridyl)propane, or 1,2-bis(4-pyridyl)ethane,which are referred to in the text as Os(pypy), Os(py=py), Os(dipy),and Os(dipyHz), respectively. Cyclic voltammetry was used to monitor the adsorption process. We have investigated the effects of concentration, solvent and applied electrode potential during adsorption on the kinetics of adsorption.
Experimental Section Reagents. Water was purified with a Hydro purification system connected in serieswith a Millipore Milli-Q system.KC104 (GFS Chemicals) was recrystallized twice from water and dried under vacuum at room temperature. HzSO4 (Ultrex,J.T. Baker) and the dipyridyl derivatives (Aldrich)were used as received. Tetra-n-butylammoniumperchlorate (TBAP) (GFSChemicals) was recrystallized 3 times from ethyl acetate and dried under (9) Chidsey, C. E. D. Science 1991,251, 919. (10) Finklea, H. 0.;Hanshew, D. D. J.Am. Chem.SOC. 1992,114,3173. (11) Acevedo, D.; Abrufla, H. D. J. Phys. Chem. 1991,95,9590. (12) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.;Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. SOC. 1989,111, 321. (13) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984,100, 465. (14) Wasserman, S.R.; Tao, Y.-T.;Whitesides, G. M. Langmuir 1989, 5, 1074.
0 1994 American Chemical Society
Tirado et al.
1972 Langmuir, Vol. 10, No. 6,1994 vacuum at 75 "C for -72 h. THF (Fisher) was distilled from sodium/benzophenone. Acetonitrile, DMF, DMSO (Burdick & Jackson, distilled in glass), methylene chloride, and acetone (Fisher) were dried over 4-A molecular sieves. The osmium complexes (adsorbates) were synthesized followinga procedure similar to that previouslydescribed." About 200 mg of [Os(bpy)&lz] (0.35 mmol) was heated at reflux under a nitrogen atmosphere in deaerated ethylene glycol (ca. 25 mL) for 3 h with 2 equiv of the ligand L (L = 4,4'-bipyridine, trans1,2-bis(6pyridyl)ethylene,1,3-bis(4-pyridyl)propane,or l,2-bis(4-pyridy1)ethane).After cooling, an equivalent volume of water was added and the solution was subsequently filtered. The complexwasprecipitated withNHzF8fromasaturated aqueous solution. The product was collected, washed with water, and dried with ether. Purification was effected by chromatography on alumina with 15%methanol in toluene as eluent. The desired complex eluted as a brownish band. Instrumentation. Cyclicvoltammogramswere obtained with either an IBM EC/225 voltammetric analyzer or a Princeton Applied Research Model 173 potentiostat and Model 175 universalprogrammer. Abipotentiostat ModelRDE4from Pine Instrument Co. was used in some experiments. All cyclic voltammogramswere recorded on a SoltecXY recorder. Potentials were measured against a sodium chloride saturated silver/silver chloride (Ag/AgCl) reference electrode without regard for the liquid junction potential. Procedures. The working electrode was a platinum disk sealed in glass and polished with 1-pmdiamond paste (Buehler) prior to use. The electrode was pretreated by continuous cycling betweenthe oxideformation and hydrogen adsorption potentials (about +1.4 and -0.2 V, respectively) in 1 M HzSO4 until the voltammetry of a clean polycrystalline platinum electrode was obtained. The electrochemicallyactive area of the electrode was obtained by integration of the hydrogen adsorption waves using a conversion factor of 210 pC/cm2. The electrode was rinsed with water and subsequently immersed in an aqueous 0.1 M solution of KC104or 0.1 M TBAP in nonaqueous solvents and cycled between 0.0 and +0.55 V vs Ag/AgCl until a steady voltammogram was obtained. The complex was injected as an acetone solution, typically 0.5 mM, and purged with prepurified nitrogen in order to homogenize the solution prior to immersing the electrode. Thevolumeinjectedvariedaccordingto the desired final concentrationof the complex. Thepotential of the electrode was held constant during deposition at 0.0, +0.2, or +0.4 V vs Ag/AgCl. Cyclicvoltammetry was used to determine the coverage of the complex on the surface of the electrode by integration of the wave corresponding to the metal based oxidation at about +0.35 V. Since the solution concentration of the complex was typically in the micromolar regime, ita contribution to the measuredcurrent (charge)was negligible. Thus, surfacecoverage measurements could be carried out in the deposition solution with minimal error. The coverage was monitored versustime for all the complexes at different concentrations and adsorption potentials. The monolayers proved to be quite stable, less than 5 % lossafter 15 min of continuous cyclingat 100mV/s in aqueous electrolyte solution free of the complex. Based on this experimental evidence, it can be assumed that at room temperature the desorption reaction is negligible during the time of the experiments. Figure 1shows a typical cyclic voltammogram for a platinum electrode modified with about a monolayer (1 monolayer is approximately 1 X 10-10 mol/cm* (uide infra)) of the Os(dipy) complex. The response is that anticipated for a surface immobilized redox couple and the contribution from the complex in solution (1.22 pM) is negligible. Similar responses were obtained for all the complexes studied. Kinetics of Adsorption An understanding of adsorption processes at the metalsolution interface requires a knowledge not only of the equilibrium situation, that is the thermodynamics, but also of the kinetics of the adsorption process. Two models were considered in order to explain the kinetics of adsorption of the osmium complexes under study. Although both models are based on an adsorption
0.0
+0.1 +0.2 +0.3 +0.4 +OS t0.6
V vs. AgIAgC1
Figure 1. Cyclic voltammogram for the Os(dipy) complex adsorbed from a 1.22 pM solution of the complex in 0.1 M KC10,. Charge corresponds to a coverage),'l( of 0.97 X 10-10 mol/cm2. Scan rate = 100 mV/s.
equilibrium, one assumes kinetic control of the system while the other assumes fast adsorption with mass transport or diffusion control. We will initially discuss the adsorption equilibrium and then develop the details for each model. The adsorption process can be envisioned as the competition of ions, solvent, and adsorbate molecules in solution for binding sites on the substrate surface. Such competition can be described as a simple equilibrium of the form A(aol) + S(ads)
== A(ads) + s(aol)
(1)
where A(aol)represents adsorbate molecules in solution, S(&) is the solvent and/or ions adsorbed on the substrate, A(a&)represents the adsorbate molecules on the surface, and S(=l)is the solvent and/or ions displaced into the solution by the adsorbate. The condition of adsorption equilibrium under this formalism is given by
where ii represents the electrochemical potential of the above mentioned species. Expanding the equation in terms of standard chemical potentials and activities, one obtains
which upon rearrangement yields
(4)
where aA(a&) and as(a&)represent the activities of species A(e&)and S(,&) on the electrode surface and ~ ( ~ and 1 ) as(sol)represent the activities of the adsorbate in solution
Langmuir, Vol. 10, No. 6,1994 1973
Adsorption Dynamics of Self- Assembling Molecules Adsorbed solvent molecules
k = k, exp(-p
ti
g)
whereas for the desorption process the expression is
0 0
(9)
where kf and kb are the charge independent factors. The quantity p, termed the charge parameter, plays the same role in adsorption kinetics as the charge transfer coefficient, cy, plays in electrode kinetics. In passing from the preadsorbed to the adsorbed state, the system passes through a transition state where there is a change in the electrical part of the standard free energy of some fraction, p (0 < p < l),of the total change, AGq. The rates of adsorption and desorption, respectively, may now be written as
reaction coordinate
Figure 2. Schematic representation of the adsorption process at the electrode/solution interface and correspondingfree energy change for adsorption.
and the solvent and/or ions in solution, respectively. The numerator in the exponential term represents the standard free energy of adsorption (AGO). At constant temperature, the entire term in brackets is constant so that eq 4 can be expressed as (5)
where fl is given by
Equation 5 expresses the relationship between the activities of the species in solution with those of the adsorbed species. It is a general expression that does not depend on a particular isotherm nor specific kinetic model. The adsorption reaction can be seen as taking place in several stages. First, the adsorbate molecule A(ml)moves from the bulk solution to a position near the surface of the substrate to what might be termed a preadsorption state. Only after reaching such a preadsorption state can the molecule be adsorbed. The process of adsorption is represented by the passage of the adsorbing system along a reaction coordinate over a standard electrochemical free energy barrier as shown schematically in Figure 2. Following Mohilner'sl5 treatment, the standard electrochemical free energy of the adsorbing system, Go, may be expressed as the sum of two parts. The first, Gn, is independent of the electrical state whereas the second, Gq, is dependent on the electrical state. Thus one obtains
Go = G" + Gq
ll
where rt is the surface coverage of A(ab) at time t and is the saturation coverage. For the solvent and/or ions, one obtains that
If one combines the equation of the rate of adsorption, eq 10, with the activity of S(ab), eq 13, and expresses the activity of A(ml)as ahd) = yh&* (where y is the activity coefficientand C* is the bulk concentration of A(ml)),then the rate of adsorption can be written as
(7)
Then, the forward (adsorption) and reverse (desorption) rate constants can be expressed as the product of two factors, one dependentand another independentof charge. For the adsorption process (forward) one obtains (15) Mohilner, D. M. Electroartal. Chem. 1966, I , 241.
One can now apply a particular isotherm model to determinethe form of the ratio alq,,,,)/as(h). We previously studied the adsorption thermodynamics of these complexes and found that although both Langmuir and Frumkin isotherms were in qualitative agreement with the data, a slightly better fit was obtained in the latter with a small interaction parameter (a = O.2).'6 In the present case we have opted to use a Langmuirian model to describe the kinetics of adsorption. We have done this since the magnitudeof the interactionparameter was small and most of the data were at relatively low coverage where the two models converge, especially when a is small. In addition, this makes the derivation of the pertinent equations much less cumbersomeand minimizes the number of adjustable parameters which could make the model less generally applicable. When using the Langmuir isotherm, ah,,,,)is typically expressed as 8 and as(&)as 1- 8, where 8 is the fractional coverage (I'/rS) of the surface. Thus
where kl contains the activity coefficient of A(ml)and the rate constant k. Upon integration, the time dependence of the coverage of A(ab) is obtained as (16) Acevedo, D.; Bretz, R. L.; Tirado, J. D.; A b d a , H. D. Langmuir 1994,10,1300.
Tirado et al.
1974 Langmuir, Vol. 10, No. 6,1994
rt = rs(1 - exp(- k'c*t))
(15) 1.5
This equation is a specific case of the more general expression
rt = re(1 - exp(- k'c*t))
1 .o
(16)
where reis the equilibrium surface concentration a t a given bulk concentration.17 As the bulk concentration increases, rewill increase to the maximum value, rs. However, until this concentration is reached, the equilibrium value will be controlled by the bulk concentration. Hence eq 16 describes the dependence of coverage with time under kinetic control of the adsorption process and under Langmurian adsorption conditions. It is important to point out that in the derivation of eq 16 it is implicitly assumed that the desorption reaction can be neglected. It has been shown that the free energy of adsorption of the complexes under study is of the order of -50 kJ/mol, which should be considered as chemisorption.'6 In addition, given the fact that the monolayers were stable to cycling in the absence of complex in solution (see Experimental Section) and that the kinetics measurements were done in the presence of complex in solution would lend further supporting evidence to our contention that the rate of desorption is, in fact, negligible. The second model is based on strong adsorption, fast kinetics, and mass transport or diffusion control. Reinmuth18 has previously derived the pertinent equations under these conditions and we will follow his treatment. The basic assumptions are semiinfinite linear diffusion of a species in solution to a stationary plane with Langmurian adsorption at the boundary. When the kinetics of adsorption are fast, the solution concentration of the adsorbate in the solution layer adjacent to the electrode is reduced to nearly zero until the surface is completely covered. The limiting case under such conditions is described by
where a is the isotherm constant, C* is the bulk concentration of the adsorbate, D is the diffusion coefficient of the adsorbate, t is the time, and rtand I's are the coverages at time t and saturation coverage, respectively. The equation can be rewritten as
where K is a constant equal to 2?r1f2. We have discussed two models that may be employed in describing the kinetics of adsorption of the osmium complexesunder study. The first describes a process under kinetic control and Langmuirian conditions and leads to eq 16. The second assumes fast adsorption once the adsorbate diffuses to the surface and is described by eq 18. We now consider which model better explains our experimental observations.
Results and Discussion A. Kinetics of Adsorption. Parts A and B of Figure 3 show two sets of kinetic data (rvs t ) for the complexes Os(dipyH2)and Os(py=py), respectively. The curves are fits to the data using eq 16 with reand k' as the adjustable (17)Parsons, R. Adu. Electrochem. Electrochem. Eng. 1961,1,1. (18)Reinmuth, W.H.J. Phys. Chem. 1961,65,473.
OsdipyH,
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100
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Figure 3. Adsorption kinetics at various concentrationsof (A) Os(dipyH2)and (B)Os(py=py) at an applied potential of 0.0 V vs Ag/AgCl on a polycrystalline platinum electrode in 0.1 M KC104. Table 1. Diffusion Coefficients ( D ) for Os(dipyH2) from the Analysis of Figure 4A concentration slope (s-112) D (cm2/s) 0.038 0.017 45 3.4x 103 0.071 0.018 13 9.0x lo-' 0.160 0.022 32 1.5 X lo-' 1.6 X lo-' 0.300 0.029 21 0.780 0.063 51 6.3 X 1W
parameters. As can be ascertained, the agreement of the data to the fits is excellent suggesting that a kinetically controlled model is applicable. However, plots of normalized coverage (rt/re) vs t1I2(Figure 4) are linear, which according to eq 18 would be expected under the diffusion control model. In order to differentiate between the two models, one needs to consider the details of each model more closely. From the slopes of the plots in Figure 4, one can obtain the diffusion coefficient (D)of the speciesin solution. Table 1 shows the results obtained from such analysis for the Os(dipyH2) complex. The values of D obtained are unphysical, since they are at least 1 order of magnitude higher than the typical value for a species in aqueous solution (ca. 9 X 1o-S cm2/s). In addition, eq 18, derived under conditions of mass transport control, predicts a linear relationship between the slope of a plot of (normalized coverage vs t V 2 ) vs concentration. Parts A and B of Figure 5 show said plots for the data in parts A and B of Figure 4, respectively. The experimental plots (open symbols) were obtained from the actual data shown in Figure 4. The dashed line is a best fit to the points calculated (closed symbols) using the diffusion controlled model. This plot was obtained by taking the slope at the
Langmuir, Vol. 10, No. 6, 1994 1975
Adsorption Dynamics of Self-Assembling Molecules 1.2
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and experimental variation of the slope of rt/r,vs t1I2 plot with concentration for (A) Os(dipyHz) and (B)Os(py=py). Open symbols are the slope values from Figure 4. Closed symbols are the calculated slopevalues based on the diffusion control model. and Lines are linear regression to the experimental (-1 calculated (- - -1 pointa. lowest concentration as a reference point (i.e. normalized to this point) and predicting the value of the slope based
Figure 6. Variationof coverage(rt)with time (A)and normalized vs t1I2 (B)for Os(pypy) at an applied potential coverage (rt/rn) of 0.0 V vs Ag/AgCl on a polycrystalline platinum electrode in 0.1 M KC10,. Table 2. Thermodynamic and Kinetic Parameterm for Oe(py=py) (Adsorbed at +0.20 V) from the Analysis of Figure 3B concentration rei 0.01 (PM) (10-10 mol/cm2) kl (M-15-1) 0.012 0.04 106300 f 7000 0.080 0.53 783 f 100 0.523 0.92 734 i 40 1.350 0.90 1560 i 100 (1
Not reliable.
on the C*/I', ratio a t each concentration (which is known or can be easily calculated). Although linearity is observed in the experimental plot, the deviations from the diffusion model are clear for both complexes. These observations argue against the appropriateness of the diffusion control model to describe the system. Figures 6 and 7 show two sets of kinetic data for the Os(pypy) and Os(py-py) complexes, respectively, that are much more detailed. Again, for both complexes, the agreement of the data with fits to eq 16, that is a system under kinetic control, is excellent as shown by the curves on Figures 6A and 7A. Figures 6B and 7B show fits of the two proposed models-diffusion control and kinetic control-to the data. Deviations from the diffusion control model are evident and the concordance with the kinetic control model is superior in both cases. Based on the above analysis, we conclude that the adsorption kinetics of the osmium complexes under study are better represented by a model where the kinetics of adsorption control the overall process. B. Concentration Dependence of the Kinetics of Adsorption. We have also investigated the concentration dependence of the kinetics of adsorption. Table 2 shows the kinetic parameters (re,kl) obtained from an analysis of the data in Figure 3B. It is clear that the equilibrium coverage (re)depends on the concentration of the complex in solution. This trend would be anticipated assuming that eq 1 describes the adsorption equilibrium process
Tirado et al.
1976 Langmuir, Vol. 10, No. 6,1994 N
I
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0.34; M. ' k=2800 M-lsec-'
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I
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0
Table 3. Thermodynamic and Kinetic Parameters for Os(py=py) Adsorbed at +0.40 V from the Analysis of Figure 9 concentration re 0.01 (PM) (10-10mol/cmZ) k (M-18-1) 0.020 0.04 806w h 2000 0.074 0.64 1151 100 0.154 0.76 974 70 0.324 0.96 1115 70 3.910 1.27 7480 200
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and the kinetics of adsorption obey the kinetic control model (eq 16). Qualitatively, the situation can be described as follows. The concentration in the immediate vicinity of the electrode approaches the bulk concentration after sufficient time has elapsed to allow the system to reach adsorption equilibrium. Then, re will reflect the bulk concentration of the adsorbate. One can observe that the equilibrium coverage, re, increases with the solution concentration of the adsorbate until the latter is sufficiently high so as to reach the saturation coverage, rs,on the surface of the electrode. The postulated mode of adsorption of these complexes to the electrode surface is through the free nitrogen of the pyridine not complexed to the metal center." In these complexesthe osmium head group (Os(bpy)&l) will control the packing density since its projected area on the platinum surface is much larger than that of the ligand. If we consider the complex to be a sphere (head group) of 7-A radius (estimated from van der Waals radii) attached to a rod (the ligand), the calculated cross section per molecule is 154 A2, which produces a monolayer with 1.0 x mol/cm2 assuming a hexagonal close packing structure. This value is what we shall consider to represent the saturation coverage, rs. As mentioned earlier, the values of the rate constant, kl, were dependent on concentration. A t concentrations above 1p M the kinetics of adsorption were quite fast so that the saturation coverage was reached in a short time. At intermediate concentrations, between 1and 0.070 p M , there was minimal variation in the values of the rate constant. Tables 2 and 3 summarize the kinetic information obtained for two different experiments with the Os(py=py) complex in which this trend appears to be followed. It is important to point out that at concentra-
Not reliable.
tions below 0.070 pM,the uncertainty in the measured rate is large so these data were not considered in the analysis. The concentration effects on the kinetics of adsorption can be summarized as follows. First, and as would be anticipated, the equilibrium coverage, re,depends on the bulk concentration of the adsorbate, increasing with concentration until the saturation coverage, rs,is reached. Second, at concentrations above 1pM the kinetics are fast and equilibrium coverage values are attained in a short time. However, at moderate concentrations (0.070 pM < C*< 1pM)the observed rate constants were independent of the bulk solution concentration of the complex. C. Effects of the Applied Potential on the Kinetics of Adsorption. In addition to the concentration dependence of the kinetics of adsorption, we have also investigated the effects of the potential applied during the deposition. Since all of the complexes studied are charged (+1and +2 in the reduced and oxidized states, respectively), one would anticipate that the potential during deposition could have an effect assuming that the chosen potential is not the potential of zero charge. Figure 1shows a cyclic voltammogram of the Os(bipy) complex adsorbed from a solution 1.22 pM in the complex and 0.1 M KC104. The formal potential for this complex is +0.35 V vs Ag/AgCl, and this corresponds to a metal based (osmium) oxidation from +2 to +3 with the net charge on the complex changing from +1 to +2, respectively. The point of zero charge for a polycrystalline platinum electrode in contact with aO.l M KC104 solution (pH = 5.5) can be estimated using Bockris'lg relationship to obtain a value of about +0.01 \r vs Ag/AgCl. By adjustment of the electrode potential to a specific value, the net charge of the electrode and the oxidation state of the osmium metal center in the complex can be controlled and the effects of these on the kinetics of adsorption can be ascertained. By use of a bipotentiostat, two electrodes were immersed in a solution of the complex at various concentrations and the kinetic data were obtained as before. The purpose of employing a bipotentiostat was to have two electrodes under identical conditions with the only difference being the potential a t which the electrodes were held during the deposition process and to see how this affected the kinetics of adsorption. The potentials applied to the electrodes were +0.20 and +0.40 V vs Ag/AgCl, respectively. In both cases, the net charge on the electrode was positive since both potentials are positive of the point of zero charge. It should be mentioned that the oxidation state of the osmium is +2 a t +0.20 V and +3 a t +0.40 V. In addition, the overall charge on the complex is +1at +0.20 V and +2 at +0.40 V, respectively. Based on electrostatic arguments, if the electrode potential plays a role, the equilibrium coverage observed at +0.40 V should be lower than that (19)Bockris, J. D.; Argade, S. D.; Gileadi, E. Electrochim. Acta 1969, 14, 1259.
Adsorption Dynamics of Self-AssemblingMolecules
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Langmuir, Vol. 10, No. 6,1994 1977
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300
400
Time (min.) Figure 8. Surface coverage vs time for Os(dipy) at different concentrations and two deposition potentials: (-) +0.20 V and (- - -) +0.40 V vs Ag/AgCl on a polycrystalline platinum electrode in 0.1 M KC104. Table 4. Thermodynamic and Kinetic Information for Os(dipy) from the Analysis of Figure 8 potential vs AgIAgC1 0.20v
0.40 V
1
concentration
(PM) 4.10 0.468 0.316 0.161 0.070 4.10 0.468 0.316 0.161 0.070
ref 0.01 (lo-“-‘ moVcm2) 1.04 1.19 0.76 0.53 0.40 0.86 1.03 0.65 0.43 0.34
k’
(M-18-1)
1416 i 100 6841 k lo00 1153 i 40 996 i 80 1723 i 200 1361 i 200 5810 i 400 1296 i 50 1447 i 60 1413 i 200
at +0.20 V since the complex has a higher charge and thus will be repelled more strongly by the net positive charge of the electrode as well as by near neighbors. Figure 8 shows a set of data and fits for the Os(dipy) complex, and Table 4 summarizes the results obtained. The anticipated trend was observed, that is, a t each concentration studied the equilibrium coveragewas lower for the electrode whose potential was a t +0.40 V when compared to that for the electrode held at +0.20 V. However, no apparent effect was observed on the rate constant. The two sets of data show values of the rate constant that are essentially the same within the error of the measurement. Based on these observations, we believe that the net effect of the potential at which the adsorption is carried out is on the adsorption equilibrium and not on the kinetics of adsorption. Thus, the equilibrium coverage (re)is lower a t the more positive applied potential due to repulsive interactions between the positively charged electrode and the oxidized metal center in the complexes. D. Induction Time. In our studies on the kinetics of adsorption of these complexes,we saw evidence that would
100
200
300
400
500
Time ( m i n . ) Figure 9. Surface coverage vs time for O s ( p m y ) at different Concentrations and at an applied potential of +0.40 V vs Ag/ AgCl on a polycrystalline platinum electrode in 0.1 M KCIO,.
suggest that the process may involve local concentration variations which gave rise to induction times. Figure 9 shows the kinetic data for the Os(py=py) complex at different concentrations when the potential during adsorption was +0.40 V. The curves are fits to the data using eq 16. It can be seen that at low concentrations (i.e. 0.154,0.074,and 0.020 pM), the curves do not pass through the origin, whereas at higher concentrations they do. We believe that this behavior might be due to local fluctuations in the concentration of the complex near the electrode surface which needs to reach a critical value before adsorption is induced. The time required would depend on the solution concentration and since this only affects the preadsorption state, the process would not be detectable by cyclic voltammetry. At high concentrations, the kinetics of such a process would be fast and thus not detectable on the time scale of the experiment. However, as the concentration decreases (below 0.324 pM), the time required is longer, probably because a t these lower concentrations the stochastic processes leading to local concentration variations are much less efficient. The fits to the data were greatly improved when a delay or induction time period was included. We are currently involved in further studies as well as in simulations geared a t a better understanding of this process. E. Solvent Effects on the Adsorption Kinetics and Thermodynamics. Most of the nonaqueous solvents used in electrochemistry interact very strongly with Pt surfaces. In particular, benzene,20 acetonitrile,21 pyridine, and chemisorb strongly to Pt surfaces. Studies in UHV at well-defined platinum surfaces have shown the formation of superlattices upon adsorption of these and other adsorbates.23~~~ In this section, we describe studies of various nonaqueous solvents on the adsorption kinetics and thermodynamics of the four osmium complexes. The effects of solubility and solvent competition were analyzed according to the kinetic model described previously. Typical plots of rt vs t are shown in Figure 10 together with theoretical fits to eq 16. The values for the equilibrium coverage for the four osmium complexes are presented in Table 5. One could anticipate some of the observed solvent effects on adsorption by inspection of eq 1. The presence of ~
(20) Song, D.; Soriaga, M. P.; Vieira, K. L.; Zapien, D. C.; Hubbard, A. T.J. Phys. Chem. 1985,89, 3999. (21) Song, D.; Soriaga, M. P.; Hubbard, A. T. J. Electroanol. Chem. Interfacial Electrochem. 1986,201,153. (22) Song, D.; Soriaga, M. P.; Hubbard, A. T. J. Electrochem. SOC. 1987,134,874. (23) Garwood, G.A., Jr.; Hubbard, A. T. Surf. Sci. 1982,118, 223. (24) Katekaru, J. Y.; Garwood,G . A., Jr.; Hershberger, J. F.; Hubbard, A. T.Surf. Sci. 1982, 121, 396.
Tirado et al.
1978 Langmuir, Vol. 10, No. 6,1994
CU
'(sol)
average concentration = 10.9 pM
E
0.8 a CH2Cl2I
1
THF
Y
+
(ads)
-
A (ads)
+
weaker solvation and/or less solvent adsorption
0 M
I
0 r3
v
0.4 -
\ L
0.2
o~ DMF acetone
-
stronger solvation
more solvent adsorption
DMSO
Reaction coordinate 0
20
Time
40
/
60
80
min
Figure 10. Surface coverage vs time in various nonaqueous solvents for the Os(py=py)complex at an applied potential of 0.0 V vs Ag/AgCl on a polycrystalline platinum electrode in 0.1 M TBAP.
Figure 11. Solvation and solvent adsorption effects on the free energy of adsorption AGO. I
"
'
)
"
"
.
Table 5. Values of the Equilibrium Coverage in Units of 1CP mol/cm2 in Various Nonaqueous Solvents for the Four solvent
THF CHzClz acetone ACN DMF DMSO
Osmium Complexes Studied O s ( p m y ) Os(dipyH2) 1.06 0.66 0.83 0.66 0.10 0.46 0.26 0.48 0.50 0.44 0.30 0.29 0.19 0.05 0.15 0.18
Os(pypy)
Os(dipy) 1.14 0.45 0.66 0.53 0.15 0.21
adsorbed solvent molecules on the left-hand side of the equation clearly indicates an important role for the solvent in the adsorption process. Furthermore, since A(Bol)is in solution, solvation of the complex should also influence the energetics of the system. Although solvation in the monolayer should also contribute to the overall process, we shall not consider it in the present discussion since one can argue that solvation in the monolayer is hindered due to geometric and spatial constrains. Therefore, its contribution to the energetics of adsorption is likely to be small relative to the combined effects of solvent adsorption and complex solubility mentioned above. Weaker solvation of the complex and/or less solvent adsorption on the surface results in an initial state with a higher free energy since it represents a condition of lower stability. This results in a larger value for AGO which will be reflected in a larger equilibrium coverage value for a given solution concentration of the complex provided that the solution concentration is below the value at which re = rs. The opposite would hold for the case of stronger solvation of the complex and/or solvent adsorption at the surface. Figure 11 depicts a schematic representation of these effects. The activation energy for the adsorption process has been kept constant for reasons that will be discussed later. Figure 12 shows a plot of re(equilibrium coverage) vs Ut8,where t, is the static dielectric constant of the solvent. Although there is some scatter in the data, the same trend is observed for all the complexes. In general, high values of I", are observed for solvents with alow dielectric constant such as THF and CH2Cl2, whereas low values are obtained for high dielectric constant solvents such as DMSO and DMF. This trend could be caused by solubility differences
a
0.000
d
0.075
Acetone
0.150
Figure 12. Variation of equilibriumcoverage with the dielectric constant of the solvent for the four osmium complexes under study in six different solventa at an applied potential of 0.0 V vs Ag/AgCl on a polycrystalline platinum electrode in 0.1 M TBAP. among the various solvents. The net charge on the osmium complexes should make them more soluble in more polar solvents than in low polarity 01188. Thus, from Figure 11, smaller equilibrium coverage values (at the same bulk concentration) are expected due to a decrease in AGO. However,this correlation is not absolute since the strength of adsorption for these solvents is usually correlated with their polarity.2s Since the effect of higher solubility and solvent adsorption cause similar changes in the adsorption energy, it is not possible to separate these effects based on the data in Figure 12. The effect of higher solubility of these complexes in nonaqueous solvents is made clear when one considers that a much higher concentration is needed in order to achieve a saturation coverage, i.e. 1.0
-
(25) Leidner, C.R.;Murray, R. W.J.Am. Chem. SOC.1985,107,551.
Adsorption Dynamics of Self-Assembling Molecules Table 6. Values of In fr for the Osmium Complexes in Different Solvents* osmium complex THF CH2Clz acetone ACN DMF DMSO water 6.2 b 6.6 c 6.9 OS(PYPY) 6.6 b (2.87) (2.87) (2.87) (0.028) Os(dipyH2) 6.2 5.5 6.4 5.9 7.1 6.3 9.0 (9.20) (10.8) (10.2) (11.2) (9.67) (10.2) (0.27) 6.1 5.8 6.8 7.0 7.9 Os(py=py) 6.8 7.4 (11.8) (9.20) (11.1) (11.8) (11.4) (9.92) (0.34) Os(dipy) 5.4 5.8 6.5 6.1 c c 7.7 (9.75) (10.9) (10.9) (10.5) (0.33) The values in parentheses correspond to the bulk concentration (pM).* Not measured. Measured but not reliable. X 10-lomol/cm2. For example, in THF, the concentration needed to achieve a saturation coverage was -10 pM, whereas in water it was only -0.5 pM. Values of In k‘ as obtained from the fits to eq 16 are presented in Table 6. In general, no significant changes ( f lunit) in the rate constants are apparent except for the case of Os(dipyH2) in aqueous solution. In this case the rate constant is significantly larger in water than in nonaqueous solvents. Furthermore, it is also significantly larger than the value for Os(pypy). The difference is likely due to solubility differences between the two complexes since competition with the solvent for surface sites should be essentially identical for both. Thus, in this particular instance, the solubility seems to have an effect on the activation energy barrier. The nearly constant values of In kl seem to indicate that the transition state resembles the “reactants” (i.e., A(801) S(*&))(early transition state) so that it is affected in much the same way (and by about the same magnitude) as the reactants are by the solvent. This observation was implicit in Figure 11where the activation energy barrier was kept constant. Clearly, additional studies are necessary. In particular, studies of the effects of temperature, potential, and charge on k’ are needed to further understand the details of this process. These studies are currently underway.
+
Conclusions Osmium complexes which self-assemble onto platinum electrodes were used to study concentration, electrode
Langmuir, Vol. 10, No. 6,1994 1979
potential during adsorption, and solvent effects on the kinetics of adsorption. The kinetics of adsorption can be represented by a model in which the process is kinetically controlled rather than by the diffusion of the adsorbate to the electrode surface. The rate constant appears to be invariant with concentration a t moderate concentrations (4pM). At higher concentrations, the kinetics are fast and the surface coverage reaches a saturation value in a short period of time. The value of the equilibrium coverage (re)depends on the bulk concentration of the complex. Saturation coverages (r,)of about 1.0 X 10-10mol/cm2can be reached with a bulk concentration of -0.5 pM in aqueous solution. In organic solvents, the complexes also adsorb but the process is dominated by solubility and competition for surface sites with solvent molecules. In general, a higher solution concentration of the complex is necessary to achieve a surface coverage similar to that in aqueoussolution,an effect that can be assigned to solubility differences and strength of adsorption. The rate of adsorption is not very sensitiveto the nature of the solvent; however, the fiial equilibrium value of the surface coverage varies considerably from solvent to solvent. Furthermore, our studies showed that the potential applied to the electrode during the adsorption process has an effect on the equilibrium coverage due to electrostatic repulsions but has no detectable effect on the rate of adsorption. Finally, our data appear to indicate that a t low concentrations induction times are involved in the adsorption process. Further work (currently in progress) is necessary in order to understand the mechanism of this process.
Acknowledgment. This work was supported by the National Science Foundation through Grant DMR9107116. J.D.T. acknowledgessupport by a NSF Minority Graduate Fellowship and a Corning Graduate Fellowship. D.A. acknowledges support by a MARC Fellowship of the National Institutes of Health. R.L.B. acknowledges support by the US. Department of Education through a fellowship administered by the Material Science Department of Cornell University. H.D.A. acknowledgessupport by the J.S. Guggenheim Foundation.
