6986
J. Phys. Chem. 1995, 99, 6986-6993
Adsorption-Desorption Dynamics of Amphiphilic Ferrocenes on Electrodes Studied by Flow Voltammetry Wenfeng Peng, De-Ling Zhou, and James F. Rusling* Department of Chemistry (U-60), University of Connecticut, Storrs, Connecticut 06269-3060 Received: October 21, 1994; In Final Form: February 22, 1995@
Adsorption-desorption kinetics and equilibria were measured by using flow voltammetry. The principle involves flowing an adsorbate-free blank electrolyte solution past a working electrode and switching the flow to a solution of adsorbate, or beginning with the adsorbate solution and switching to the blank. Changes in surface concentration of adsorbate with time are monitored by fast cyclic voltammetry. Mathematical models assuming the Langmuir isotherm were developed for adsorption-desorption kinetics under hydrodynamic conditions and employed for nonlinear regression of peak current versus time data to obtain adsorption parameters. Alkylammonium surfactants having ferrocene (Fc) attached to cationic head groups and alkyl chain lengths from C8 to C16 were investigated. Results suggest that these adsorbates achieve monolayer coverage from submicromolar solutions under flow conditions. Adsorption rates are controlled by both intrinsic adsorption kinetics and diffusion rates. Residence times on the electrode were 14 s for Fc-C8 and Fc-C12 and 66 s for Fc-C16. Free energies of adsorption @Goads)increased with the length of the hydrocarbon chain by about 0.75 kJ mol-' per ?HZgroup, in accord with a hydrophobic component to the driving force. AGoadsdepended weakly on electrode potential.
Introduction Surfactants adsorbed on electrodes can have a profound influence on electrochemical reactions. For example, rates of mediated electrolytic dechlorination can be enhanced in aqueous surfactant solutions in a film of cationic surfactant adsorbed on an Rates of electron transfer between electrodes and the protein myoglobin were about 1000-fold larger in films of surfactants than in aqueous solution^.^ Surfactant solutions have been suggested as less toxic, less costly substitutes for organic solvents in synthetic application^.^ Adsorbates exerting kinetic control have been applied in industrial electrochemical synthesis. For example, adiponitrile is synthesized by electrochemical dimerization in concentrated aqueous tetraethylammonium p-toluene~ulfonate.~~~ Tetraethylammonium ions adsorb on Hg or Pb cathodes, creating a surface reaction environment favoring rapid dimerization of acrylonitrile to adiponitrile and inhibiting the competing two-electron reduction. Similar results are obtained with nonionic surfactants.lc Electroactive surfactants containing ferrocene are useful to elucidate details of adsorption and electron transfer on Adsorption of a homologous series of alkyldimethyl(ferrocenylmethy1)ammonium ions on gold and platinum electrodes was studied in aqueous solutions by using cyclic voltammetry and quartz crystal microbalance (QCM) techniques. Adsorption isotherms were fit by the Langmuir equation.*%l0 The thermodynamics for adsorption of these compounds were established, but only qualitative information about adsorptiondesorption rates was reported. Abbott et al.12 found that electron transfer rates for tetraalkylammonium ions containing ferrocene on the C12 hydrocarbon chain decreased with the distance of the ferrocene from the head group in cationic micellar solutions, suggesting a head down orientation on the electrode prior to electron transfer. The classic approach to adsorption isotherms on solid materials is to measure the change in concentration of solute at a series of initial solution concentrations.lc This requires a high @Abstractpublished in Advance ACS Abstracts, April 15, 1995.
0022-365419512099-6986$09.00/0
ratio of adsorbent area to solution volume to ensure that the changes in solution composition are sufficiently large to be measured. Unwin and Bard13 recently employed ultramicroelectrode voltammetry for measuring such isotherms in a drop of solution. A related approach has been used by Hubbard and Anson,14 who measured the amount of adsorbate on electrodes by employing thin-layer electrochemical cells in refilling experiments. In contrast to the study of adsorption isotherms, dynamic techniques must be adopted to obtain adsorption and desorption rate constants. Unwin and Bard used dynamic scanning electrochemical microscopy to measure adsorption-desorption kinetics and surface diffusion rates. l5 Laviron developed analytical equations for rotating disk electrodes in adsorbate solutions, which can provide adsorption-desorption rate cons t a n t ~ . ~Sawamoto ~J~ et al. used a flow injection system to measure differential capacity-time curves and reported qualitative information about adsorption dynamic^.^*^^^ Abruna et aLZo showed that adsorption of osmium bipyridine complexes occurs under kinetic control by using fast cyclic voltammetry (CV). In this paper, flow voltammetry was used to estimate kinetic and thermodynamic parameters for adsorption-desorption of ferrocene surfactants. The method involves flowing a blank electrolyte solution past the working electrode which is held at fixed potentials and then switching the flow to an electrolyte solution containing adsorbate. This is the adsorption mode. Conversely, a flowing adsorbate solution can be switched to a blank solution in a desorption mode. A series of fast CV scans after switching solutions provide the time dependence of the surface concentration of adsorbate. Data are analyzed by nonlinear regression to provide parameters for adsorption and desorption.
Theory The electrode considered is a disk of diameter d embedded in an insulating planar wall of a channel flow-through cell (Figure 1). The cell has height b and a width w long enough to ensure laminar flow across the electrode. Linear diffusion 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 18, I995 6987
Adsorption-Desorption Dynamics of Ferrocenes
tZ Combining eqs 4-6 gives
/
O
C
Xd
GC electrode Figure 1. Schematic diagram giving coordinates and dimensions of the thin-layer flow cell with a disk electrode (see text for definition of symbols).
is the only mode of mass transport considered in the direction perpendicular to flow, while convection is the only mode of mass transport parallel to flow.21,22With these assumptions, the diffusion layer thickness at the electrode surface is directly proportional to x1I3, where x is the distance from the leading edge in the direction of The total diffusion-limited current ( i s ) for the reaction of reductant R,
-
-
which can be solved by the Runge-Kutta method.25c As t the surface concentration will reach a saturation value (r,) given by KCbr,
rs= KCb+1
where K = kdkd is the adsorption equilibrium constant, related directly to the free energy of adsorption. This is a form of the Langmuir equation. By integrating eq 7 with respect to r and t, we obtain
R--+ne where n is the number of electrons transferred, is given byz2 is = 1.234nFCbd5/3(D/b)U3(U/w)"3
(1)
where U is the volume flow rate, 6" is the bulk concentration of adsorbate in solution, D is its diffusion coefficient, and F is Faraday's constant. Equation 2 shows that the average diffusion layer thickness (6) is
When switching between blank and adsorbate solutions, 6 will undergo a 'flux step'.23 The transient nature of the flux step (ca. 1-3 s)24can be neglected when the adsorption process occurs over hundreds of Thus, diffusion layer thickness is considered constant during the solution exchange.25b Adsorption-Desorption under Nonelectrolytic Conditions. In the adsorption mode the electrode surface is initially free of adsorbate in a blank electrolyte and is suddenly exposed to an adsorbate solution. The surface concentration of adsorbate r in mol cm-2 at time t is given by
where c is a constant of integration. For the initial condition r = 0 at t = 0,
q6 c b + 1/K) = 0
(10)
In the desorption mode, adsorbate solution has flowed continuously over the electrode such that saturation adsorption has been achieved. At t = 0, we switch the flowing liquid to a solution containing no adsorbate (Le. 6" = 0). Equations 7 and 9 still hold, but the initial condition at t = 0 is now given by eq 8. Thus,
(3) where z is the distance in a perpendicular direction away from the electrode. Differentiating eq 3 with respect to t gives
(4)
Adsorption-Desorption under Electrolytic Conditions. Suppose R is initially present in the solution at concentration C Rand ~ can be adsorbed onto the electrode. Both the dissolved (Rsol)and adsorbed (Rads)species can be converted to oxidized forms (osol, o a d s ) in the electrode reactions
-
Rsol We assume that adsorption obeys the Langmuir isotherm and that the adsorption rate is controlled by both adsorptiondesorption kinetics and mass transport. Under these conditions, the adsorption rate can be expressed as17
dr = kac(r,- r) - k d r dt
(5)
where ka is the rate constant for adsorption, k,j is the rate constant for desorption, r, is the surface concentration for full coverage, and CS is the solution concentration of adsorbate at the electrode surface (z = 0). Under hydrodynamic conditions the mass flux is given by
+
Osol ne
and
Rad,
-
+
Oads ne
It is assumed that 0 and R have equal diffusion coefficients and diffusion layer thicknesses. Under diffusion-limited conditions, fluxes of 0 and R at the electrode surface are
D r 2 )z=o = D
C,b - e,"
D r 2 )z=o = D
0 - cos
According to the law of conservation of mass, the total Faradaic current is
6988 J. Phys. Chem., Vol. 99, No. 18, 1995
Peng et al.
It follows that Adsorbate solution
' Blank solution Equation 15 shows that when adsorption takes place, the sum of absolute concentrationsof dissolved 0 and R at the electrode surface decreases. If the potential applied is much more positive than E"', so that Cos>> C R and ~ To >> rR, eq 15 becomes
The adsorption rate of 0 can also be expressed by eq 5. Thus, we obtain the same r o - r expressions as in eqs 10 and 11 for adsorption and desorption modes, respectively. Surface Concentration Measurements. An equation relating r to the measured Faradaic current is needed to extract kinetic and thermodynamic parameters from flow voltammetry data. If eq 10 is divided by eq 8, we obtain
Dt -(e + l/K) = 0 (17) rm6 Because the adsorption current is directly proportional to r regardless of the reversibility of the heterogeneous reactions,26-28 we can express eq 17 as
Dt -(d'+
rm6
l / K ) = O (18)
where ip is the peak current at time t and ips is the peak current at saturation coverage. Likewise, for the desorption mode, eq 11 can be rewritten as
Kd'
$
Experimental Section Chemicals and Reagents. ((Dimethy1amino)methyl)ferrocene (98+%), 1-chlorooctane (99%), 1-bromododecane (97%), and 1-bromohexadecane (97%) were from Aldrich. All other chemicals were ACS reagent grade. Surfactants derived from ferrocene8-' (Fc), FcCHzN(CH&(CH2)$H3X, where Fc denotes ferrocene, n = 7, 11, 15, and x = Br or C1, were synthesized by procedures described below. (Fc-C8, Fc-C12, and Fc-C16 denote these surfactants.) The supporting electrolyte in the electrochemical experiments was 0.2 M NaCI. Water was distilled and then purified with a Sybron-BarnsteadNanopure system to specific resistance > 15 MQ-cm. Syntheses of Ferrocene Surfactants. ((Dimethylamino)methy1))ferrocene was reacted with an excess of alkyl halide under nitrogen and in the dark. The reaction mixture was stirred and heated with an oil bath. Yellow crystals were precipitated, after which the reaction flask was cooled to room temperature. The crystals were collected by filtration (hexane was added
I
1
Collector
Figure 2. Schematic diagram of the flow voltammetry system.
before filtration of Fc-C8). Two additional recrystallizations were done from acetone. Final products were dried in vacuo ovemight. Dimethyl(ferrocenylmethy1)hexadecylammonium bromide. 1-Bromohexadecane(1.2 equiv) was used at 45-50 "C for 5 h. Yield: 92%. Brilliant light-brownish yellow platelets. Mp: 137.0-137.5 "C. 'H NMR: 6 4.854 (s, ZH), 4.519 (t, ZH), 4.342 (t, ZH), 4.301 (s, 5H), 3.338 (m, 2H), 3.260 (s, 6H), 1.735 (m, 2H), 1.257 (m, 26H), 0.881 (t, 3H). Dimethyl(ferrocenylmethy1)dodecylammonium bromide. 1-Bromododecane (1.2 equiv) was used at 45-50 "C for 5 h. Yield: 92%. Brilliant light-brownish yellow platelets. Mp: 137.5-138.0 "C. 'H NMR: 6 4.855 (s, ZH), 4.530 (t, 2H), 4.338 (t, ZH), 4.301 (s, 5H), 3.360 (m, 2H), 3.262 (s, 6H), 1.742 (m, ZH), 1.298 (m, 18H), 0.884 (t, 3H). Dimethyl(ferrocenylmethy1)octylammonium chloride. 1-Chlorooctane (6.0 equiv) was used at 55-65 "C for 3 days. Yield: 17%. Yellow platelets. Mp: 150.0-151.5 "C. 'H NMR: 6 4.799 (s, ZH), 4.494 (t, 2H), 4.342 (t. ZH), 4.285 (s, 5H), 3.341 (m, 2H), 3.272 (s, 6H), 1.735 (m, ZH), 1.302 (m, lOH), 0.882 (t, 3H). NMR spectra were taken on a Bruker AF-270 spectrometer (in CDCl3 with 1% TMS as the intemal standard). Chemical shifts 6 reported in ppm were consistent with the expected structures of the compounds. Apparatus. The flow voltammetry system (Figure 2) has a similar arrangement to flow injection analyzers with amperometric d e t e c t i ~ n . A ~ ~two-channel ,~~ Sage syringe pump (Orion research, USA) was used to deliver solutions to a Model CC-5 (Bioanalytical Systems) electrochemical flow cell equipped with a glassy carbon disk working electrode, a stainless steel counter electrode, and a RE-4 Ag/AgCWaCl (3 M gelled) reference electrode. The stainless steel block was separated from an insulating block containing the working electrode by a 127 pm thick TG-5M Teflon gasket. The dead volume of the cell was 0.0071 cm3. A three-way switching valve (Rainin Instrument Co.) between the pump and the thin-layer cell was used to select between two solutions at the cell inlet. The valve is solenoid-actuated (12 V dc power) and has a small dead volume. It was connected to the cell by about 2 cm of tubing. These features were chosen to minimize dispersion upon switching solutions. Tubing was 0.8 mm I.D. Teflon. Connection of stainless steel syringe needles to tubing was made by using 0.8 mm i.d. Tygon tubing (Norton Performance Plastics Corp.). Cyclic voltammetry (CV) and chronocoulometry were done with a PARC Model 273 Electrochemistry system. Chronocoulometry and off-line CV were done with a glassy carbon working electrode (A = 0.071 cm2), a saturated calomel reference electrode (SCE), and a platinum counter electrode. Procedures. Glassy carbon electrodes were ground initially on 240 grit Sic paper and then polished successively on billiard cloth with 6, 1,0.3, and 0.05 pm alumina suspensions,followed by 1 min ultrasonic cleaning in pure water for each step.31For
J. Phys. Chem., Vol. 99,No. 18, 1995 6989
Adsorption-Desorption Dynamics of Ferrocenes replicate experiments, polishing with 0.3 and 0.05 pm alumina and ultrasonic cleaning were used. Adsorption and desorption were studied under nonelectrolytic conditions. The initial flow solution was the blank electrolyte, which was switched to an adsorbate solution. Every 20 or 25 s after switching, a CV scan was taken at 40 V s-l. The working electrode potential was maintained at the initial value between scans. Cyclic voltammograms were smoothed by the moving average techinque. When peak currents in successive cyclic voltammograms showed no further change, the flowing solution was switched back to the blank, and a new series of CV scans were done. Cell resistance, as measured by the PARC system, was fully compensated electronically in all cyclic voltammograms. Before each experiment, oxygen was removed from solutions by purging with purified nitrogen for at least 20 min. Experiments were done at ambient temperature (23 f 2 “C). Analyses of data were done by using nonlinear regression analysis on an IBM type 486 personal computer with a general program employing the Marquardt-Levenberg algorithm.32 Initial attempts focused on analyzing flow voltammetry data by using the Runge-Kutta solution of eq 7 with idips (proportional to TK,) as the dependent variable and t as the independent variable. Using an integration step time (0.001 s) small enough to provide accurate fits resulted in data analysis times > 10 h for 15 point data sets. With eqs 18 and 19 as models, regression analyses could be completed within 60 s. Although there might be statistical errors because the use of these equations reverses the roles of independent and dependent variables,33comparison of results from this approach to regression onto eq 7 showed that the parameters agreed within 1%. Since standard errors in parameters were on the order of 15-25%, a maximum 1% error from the data analysis was considered acceptable in view of the large savings in computation time. Values of 6 were calculated from eq 2. D for monomer amphiphiles was determined by chronocoulometry by using a previously described p r ~ c e d u r e . ~Cmc ~ , ~values ~ were estimated by using square wave voltammetry peak currents, from break points in plots of peak current vs amphiphile concentration.lc
Results and Discussion Validation and Predictions of Models. The general theory above reduces to well-known equations for special conditions. When Kdo 100D/(6rm),the total adsorption rate will be limited solely by diffusion. For desorption (Figure 3b), the surface concentration decays faster when k, increases. When k, is small, only a small amount of adsorbate is adsorbed or desorbed, even after a long time. In this case the adsorption or desorption rate is governed by the intrinsic adsorption and desorption kinetics. Therefore, the ratio (dk,T,)/D can be used to find the rate-determining step(s). Voltammetry of Amphiphilic Ferrocenes. Cyclic voltammograms of amphiphilic ferrocenes at a low scan rate (v) immediately upon immersion of the electrode into solution are shown in Figure 4. All voltammograms at 50 mV s-l had separations between anodic (Epa) and cathodic peaks (Epc) around 59 mV, consistent with diffusion-controlled, one-electron transfer. Formal potentials (EO’) as (Epa E,)/2 were 0.43 V for Fc-C8, 0.44 V for Fc-C12, and 0.56 V for Fc-C16 (vs SCE). These voltammograms differ somewhat in shape from that of pure diffusion control. A shoulder is superimposed on each main anodic peak. This ‘buried’ peak is due to adsorbed rather than dissolved species. Cathodic peaks are smaller than the anodic peaks. Similar behavior was observed on gold8 and platinumg electrodes for ferrocene amphiphiles. A study by QCM showed that there is some loss in mass after ferrocene surfactants are oxidized.8 For a diffusion-limited process, the peak current i, is directly proportional to v112.36 If the peak is caused by electrolysis of an adsorbed species, i, depends linearly on v. Neither the i, vs v1/2plot nor the i, vs v plot for Fc-C8 is linear (Figure 5 ) ,
+
Equation 21 is the expression obtained by assuming a linearized adsorption isotherm under diffusion Case 2: DI6 >> kJm,
160
Peng et al.
6990 J. Phys. Chem., Vol. 99, No. 18, 1995
0.80
1.20
1
t
a
I
0.40
-0.40
'
-2.00 0.76
L
--s
0.46
0.66
E 1.60
I \
0.66
0.36 0.26
0.16
SCE (VI
VI).
- 1.20
0.95
I
0.75
.*
,
0.55
0.35
0.15
E vs. SCE (VI Figure 6. Cyclic voltammograms at 40 V s-l of Fc-C12 film on GC electrode in solute-free 0.2 M NaCl with (-) and without (- - -) background subtraction. The background (- - -) was obtained in 0.2 M NaCl using a bare GC electrode. The film was formed by storing a GC electrode in 99.6 pM Fc-C12/0.2 M NaCl ovemight.
-0.10
TABLE 1: Characteristics of Ferrocene Amphiphiles from Voltammetry and Chronocoulometry" cmcb C E"' 106D iolorR compound &M) &M) (V vs SCE) (cm2 s-l) (mol cm-2) 0.43 5.48 f 0.19 2.32 f 0.13 Fc-C8 >500 107.3 0.44 4.51 & 0.24 2.91 f 0 . 2 1 Fc-C12 '500 99.6 0.56 3.11 f 0.32 3.66 f 0.45 38 25 Fc-C16 Potential stepped from 0.3 to 0.5 V vs SCE for Fc-C8 and FcC12 and from 0.4 to 0.65 V vs SCE for Fc-C16 at the concentrations given in column 3. D and r R are from nonlinear regression onto eq 23 of 50 Q-values equally spaced on the t axis. Each data set was the average of three measurements with average background charge ~ubtracted.~~ Estimated from dependence of square wave voltammetry peak current on concentration of probe (see Experimental Section)."
On disk electrodes, the chronocoulometric Q-t response has the fonn34,35
+
Q = bo f b,t1I2 b2t
+
0
,
3
6
9
"
1I2
12
15
18
(mVl/P/sll2)
Figure 5. Dependence of peak current (i,) for 107.3 pM Fc-C8 on ~ ; i, vs v. potential scan rate (v): (0)i, vs v ~ ' (0) suggesting that the current is controlled by diffusion and adsorption. Similar results were found for Fc-C12 and FcC16. We required conditions under which surface concentrations of adsorbates could be measured reliably and quickly during flow voltammetry. Adsorbed films of ferrocene surfactants gave relatively symmetric cyclic voltammograms at 40 V s-l (Figure 6), and above 20 V s-l the i, vs Y plot was linear. These results reflect the oxidation of adsorbate.36 Thus, a scan rate of 40 V s-l was chosen for flow voltammetry. Chronocoulometry. Potential-step chronocoulometry was used to measure diffusion coefficients of monomer ferrocene amphiphiles. To ensure that concentrations used were representative of monomers, critical micelle concentrations (cmc's) of the ferrocene amphiphiles were measured (Table 1).
(23)
where bo = Qdl nFArR, bl = 2nFAD112Cn-112 b2 = anFADC(nr)-1'2, Qdl is the amount of electricity required to charge the double layer, A is the electrode area, r is the electrode radius, and a is a constant accounting for nonlinear diffusion at the edge of the electrode. Experiments were done at amphiphile concentrations in which no micelles were present and are listed in Table 1. Nonlinear regression of background-subtracted Q-t data35 onto eq 23 gave 4.51(f0.24) x cm2 s-l for the diffusion coefficient of Fc-C12, in agreement with that reported for a similar C12-ferro~ene.~' Values of D and r R are listed in Table 1. Kinetics and Thermodynamics of Adsorption-Desorption. Flow voltammetry was done in adsorption and desorption modes for each surfactant. Cyclic voltammograms during adsorption (Figure 7a) reflect the increase of the surface concentration with time. After about 250 s, no further changes in cyclic voltammograms occurred, indicating attainment of steady state. The anodic peak current from steady state cyclic voltammograms is ips. Good fits of eq 18 to experimental data were obtained (Figure 7b). Desorption was initiated by switching the flow from adsorbate to adsorbate-free solution. Cyclic voltammograms indicate the decrease in surface concentration (Figure 8a). Good fits of eq 19 to these data were obtained (Figure 8b). Nonlinear regression analysis was used to obtain ka, K , and rmfor redox surfactants (Table 2) from the flow voltammetry 9
AdsoiTtion-
J. Phys. Chem., Vol. 99, No. 18, 1995 6991 120
a
#/.'--,.>&
/+; - +$& pi,'
&/--
30 -
I
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R.
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