J. Phys. Chem. 1992,96, 10565-10571 (19) Famngton, J. A.; Ebert, M.; Land, E. J. J. Chem. Soc., Faraday Trans. 1 1978, 74,665. (20) (a) Goerner, H. J. Phys. Chem. 1987,91,1887. (b) Goerner, H. J . Phys. Chem. 1985,89,4112. (21) Jachimowin. F.:. Levin.. G.:. Szwarc. M. J. Am. Chem. Soc. 1976.100. . .
(29) Ramamurthy, V.; Caspar, J. C.; Corbin, D. R. J. Am. Chem. Soc. 1991, 113, 594. (30) Shida, T.; Haselbach, E.; Bally, T. Acc. Chem. Res. 1984, 17, 180. (31) Toriyama, K.; Nunome, K.; IwasaLi, M. J. Am. Chem. Soc. 1987, 109. 4496. (32) Chen, F.; Cuo, X. J. Chem. Soc., Chem. Commun. 1989, 1682. (33) (a) Quin, X. Z.; Trifunac, A. D. J. Phys. Chem. 1990,944751. (b) Oelkrug, D.; Krabichler, G.; Honnen, W.; Wilkinson, F.; Willsher. C. J. J . Phys. b e m . 1988, 92, 3589. (34) Woitellier, S.; Launay, J. P.; Spangler, C. W. Inorg. Chem. 1989, 28, 758. (35) Reimers, J. R.; Hush, N. S.Inorg. Chem. 1990, 29, 4510. (36) Ihara, Y.; Blanchard-Desce, M.; Lehn, J.-M., unpublished rcsults. (37) (a) Cohen, H.; Meyentein, D. Inorg. Chem. 1974, 13, 2434. (b)
5426. ‘ (22) Almgren, M.; Thomas, J. K. Photochem. Phorobiol. 1980,31, 329. (23) McVie, J.; Sinclair, R. S.; Tait, D.; Truscott, T. G.; Land, E. J. J . Chem. Soc., Faraday Trans. 1 1979, 75, 2869. (24) Chauvet, J.-P.; Vlovy, R.; Land, E. J.; Santus, R.; Truscott, T. G. J. Phys. Chem. 1983,87,592. (25) (a) Ding, R.; Grant, J. L.; Metzger, R. M.; Kispert, L. D. J. Phys. Chem. 1988, 92, 4600. (b) Grant, J. L.; Kramer, V. J.; Ding, R.; Kispert, L. D. J. Am. Chem.Soc. 1988, 110, 2151. (26) (a) Lewis, F. D.; Dykstra, R. E.; Gould, I. R.; Farid, S. J . Phys. Chem. 1988,92,704. (b) Lewis, F. D.; Bedell, A. M.; Dykstra, R. E.; Elbert, J. E.; Gould, I. R.; Farid, S.J. Am. Chem. Soc. 1990,112, 8055. (27) (a) KUriyama, Y.; Arai, T.; Sakuragi, H.; Tolrumaru, K. Chem. Phys. Lett. 1990,173,253. (b) Kuriyama, Y.; Arai, T.; Sakuragi, H.; Tokumaru, K. Chem. Lett. 1989,251. (28) Gewner, F.; Olea, A.; Lobaugh. J. H.; Johnston, L. J.; Scaiano, J. C. J. Org. Chem. 1989,54,259.
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Association of Electroactlve Counterlons with Polyelectrolytes. 5. Electrostatic Binding of Ru(NH,):+ and CO(NH,):’ to Polyacrylated Rongzhong Jiang and Fred C. Anson* Arthur Amos Noyes Laboratories, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91 125 (Received: August 19, 1992)
The spontaneous association of M(NH3)l+complexes (M = Ru,Co) with polyacrylate anions (PAAs) in aqueous solution was investigated by means of cyclic and rotating disk voltammetry at graphite electrodes. Equilibrium constants, Kb,for the associationreactions were estimated and their dependences on pH, ionic strength, and the molecular weight of the polyacrylate were established. The values of Kb ranged between lo‘ and 104 M-’ for both M(NH3)(Wcomplexes. The M(NH3)t+countdons induce substantial adsorption of PAA-M(NH3)63+complexes on the electrode surface over a range of ratios of [PAA]: [M(NH3)l+]. The adsorbed layer both mediates electron transfer to PAA-M(NH3)63+complexes in solution and impedes the rate of reduction of the complexes directly at the electrode surface. Methyl viologen dications, MVZ+,exhibited no tendency toward association with PAA but the MV2+/+couple served as an efficient accelerator of the rate of the diffusion-concomplexes by homogeneous electron transfer vection-controlled reduction of PAA-Ru(NH&~+(but not PAA-CO(NH~)~~+) between the slowly and rapidly diffusing reactants.
Previous studies in this series have employed water-soluble polyelectrolytes with a fixed density of charged sites14 or protonated poly(4vinylpyridine) in which some variation in the density of charged sites was obtained within a restricted range or pH where the extent of protonation was sufiicient to maintain the solubility of the p~lyelectrolyte.~ In the present study, solutions of poly(acrylic acid) were employed over a range of pH sufficient to vary the density of the charged carboxylate groups on the polymer. One objective was to examine the effect of the polyelectrolyte charge on the extent of exchange of multiply-charged, elearoactivecations for the unipmitive counterions associated with the poly(acry1ic acid)-polyacrylate molecules. In addition to changes in charge density, the effects on the association equilib rium of variations in the molecular weight of the polyelectrolyte and in the ionic strength of the supporting electrolyte were also measured. Extensive adsorption of the complexes of the polyanion with Ru(NH3)d+ and C O ( N H ~ ) ~occurred ~+ under most experimental conditions, and the effects of the adsorbed layers on electron transfer to unadsorbed complexes in solution were investigated. The coupling of the diffusion of bound and unbound counterions by electron transfer between them was examined by comparing the behavior of the reversible Ru(NH~)~’+/’+ couple with that
of the irreversible reduction of C O ( N H ~ ) ~ ~ + . The purpose of the present study, as well as its predecessors, has been to exploit the electrochemical responses obtained from simple redox couples in the presence of polyelectrolytes to learn about the structures, mobilities, and counterion binding equilibria of the various polyelectrolytes. Studies with similar objectives have been carried out with DNA as the polyelectrolyte by Bard and co-workers.610
Experimental Section M8terirls. Samples of poly(acry1ic acid, sodium salt) having molecular weights of 20000,10000, 5000 and 2100 were obtained from Polysciences, Inc. The white, freeflowing powders were used as received. Other chemicals were reagent grade and were used without purification. Solutions were prepared from laboratory distilled water which had been passedthrough a purification train (Sybron Bamsted Nanopure). Appuahm and P”e.Previously described electrochemical instrumentation and procedures4were employed. Pyrolytic graphite electrodes were mounted to expose 0.32 cm2 of the edge planes of the graphite and were attached to stainlea steel shafts with heat-shrinkable tubing. The electrodes were polished with 0.3-pm alumina and sonicated in water before each experiment. Experiments were conducted at ambient laboratory temperatures, 20 f 2 OC. Potentials were measured and are reported with
Contribution No. 8710.
0022-3654192 , ,12096-10565S03.00/0 Q 1992 American Chemical Societv I
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10566 The Journal of Physical Chemistry, Vol. 96, No. 25, 1992
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Jiang and Anson
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E / V vs. SCE
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PH Figure 1. (A) Titration of 20 mL of a 25 mM solution of poly(acry1ic acid) in the absence and presence of added NaC1. (B) Fraction of the carboxylate groups which are protonated as a function of pH.
respect to a saturated calomel electrode. Cyclic voltammetric peak currents and rotating disk voltammetric plateau currents for the reduction of Fe(CN)63- were unaffected by the addition of up to 6 mM polyacrylate to s u p porting electrolyte solutions, which was taken as evidence that changes in the viscosity of the test solutions produced by the addition of the polyelectrolyte did not influence the magnitude of the measured cumnts. The concentration of the polyacrylatm is e x p d in termsof the quantities of COOH and Coo- groups present. The values quoted are, therefore, independent of the molecular weight of the polyelectrolyte. Unless otherwise specified, the experiments were conducted with the polyelectrolyte having a molecular weight of 20000.
Results a d Discussion Coaversioa of Pdy(acrylk acid) to Polyacryhte.Solutions of sodium polyacrylate were converted to poly(acry1ic acid) by passage through a cation-exchange column (Amberlite IR- 120) in its acid form. The resulting solutions of poly(acrylic acid) were titrated with standard solutions of NaOH which served both to establish the concentrations of the sodium polyacrylate solutions and to measure the extent of neutralization of the poly(acry1ic acid) as a function of pH. In Figure 1A are shown titration curves for poly(acrylic acid) in the absence and presence of a supporting electrolyte (NaCl). The drawn-out shape of the curves (compared with the carresponding curves for acrylic acid) and the dependence on the supporting electrolyte concentration are familiar characteristics of polymeric acids and basca."J2 The fractionalamversion of the original poly(acry1ic acid) into polyacrylate as calculated from the data in Figure 1A is shown in Figure 1B. As expected, the conversion of poly(acrylic acid) into the polyanion occurs over an extended range of pH values. This characteristic of the pdy(acrylic acid)-polyacrylate system was utilized in what follows to probe the influence of pol 'on charge on countercation binding. Amodatiall of Ru(NH3)( d Co(NH3)63+ with Polyacrylate A&m. The high affinity of polyacrylate anions (PAAs) for multiply-charged cations is clearly revealed by the decrease in the diffusion-limited reduction currents of the cations in the presence of PAAs. In Figure 2A are shown current-potential curves for the reduction of R u ( N H ~ ) ~and ~ +C O ( N H ~ ) at ~ ~a+
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MW/kDa m e 2. (A) Rotating disk voltammetry with a solution of 0.05 mM Ru(NH3)?+ or 0.05 mM CO(NH~)~~+ in the absence (solid line) and presence (dashed line) of 1.0 mM PAA. Supporting electrolyte: 0.05 M Na2B407,pH = 9.2. Electrode rotation rate: 900 rpm. Scan rate: 10 mV s-I. (B) Dependence of the plateau currents from A on the concentration of PAA. (C) Effects of the molecular weight of the PAAs on the plateau currents for the reduction of 0.05 mM Ru(NH3)2+in 0.05 M Na2B407.
rotating graphite disk electrode in the absence (solid lines) and in the presence (dashed lines) of PAAs at a pH (9.2) where essentially all of the carboxylic acid groups are deprotonated. The extent of the decrease in the plateau currents depends upon both the concentration and the molecular weight of the PAAs as shown in Figure 2B and 2C, respectively. The leveling of the plateau currents to a nearly common value in Figure 2B is the behavior to be expected if essentially all of the electroactive counterions become associated with the PAA molecules and diffuse to the electrode surface at the rate detennined by the difFusion d i e n t of the polyanion. As in previous reports,"s curves such as those in Figure 2B can be analyzed in detail to obtain estimates of both the equilibrium constant governing the association of the counterions with the polyelectrolyte and the diffusion coefficient of the complex. However, before doing so, it is nccesary to evaluate the effect of the adsorption of PAAs and the PM-M3+ co111plexes (M3+ = R u ( N H ~ ) ~C~ O + ,( N H ~ ) ~on ~ +the ) electrochemical responses of the unadsorbed complexes. Adsorptloa d PAA-W coarpkxcr 011 tbe ElactrodeSurirea The adsorption of the PAA-M3+ complexes on the graphite electrode was strong and only slowly rcvcrsibk so that the adsorbed complexes remained on the electrode surface when it was transferred to supporting electrolyte solutions containing neither PAA nor M3+to examine the cyclic voltammetry of the adsorbed
Counterion/Polyelectrolyte Association
E I V vs. SCE
Figure 3. Consecutive cyclic voltammetric scans in 0.01 M Na2B407 supporting electrolyte solution (pH = 9.2) with an electrode on which PAA-Ru(NH,),'+ (A) or PAA-Co(NH3)
