Langmuir 1989,5, 671-678
kinetics. Addition of one or two solvents to the study which do not follow the fortuitous correlation between AN and rL would clarify whether results for this system are in agreement with those discussed above. Finally, it should also be emphasized that the kinetic experiments which are discussed in this paper are difficult to carry out precisely. Each solvent must be carefully purified by specific procedures since small levels of impurities can result in serious errors in the heterogeneous rate constant. Often, the rate constants fall in a high range with respect to experimental techniquess1 available. In addition, the electrochemical cell and electrode configuration must be carefully designed to avoid problems with IR drop.80 In this regard, ac admittance techniques are ideally suited to measuring fast electrode kinetics. These experiments also provide the necessary capacitance data (60) Fawcett, W. R.; Yee, S. J. Electroanul. Chem. 1988, 249, 327.
67 1
for estimating double-layer effects when the admittance of the background electrolyte alone is measured.
Acknowledgment. The financial support of the Office of Naval Research, Washington, DC, is gratefully acknowledged. (61) Sluvters-Rehbach, M.: Sluvters, J. H. Comp. Treatise Electrochem. 1984,9, 177. (62) Calderwood. J. H.: Smvth. C. P. J. Am. Chem. Soc. 1956.78.1295. (63) Eloranta, J.'K.; h d a l k , P. K.Trans.Faraday SOC.1970,6&, 817. (64) Poley, J. P. Appl. Sci. Res., Sect. B. 1955,4, 337. (65) Hennelly, E. J.; Heston, W. M.; Smyth, C. P. J. Am. Chem. SOC. 1948, 70,4102. (66) Brownsell, V. L.; Price, A. H. J. Phys. Chem. 1970, 74, 4004. (67) Chandra, S.; Nath, D. J. Chem. Phys. 1969,51, 5299. (68) Payne, R.; Theodorou, I. E. J. Phys. Chem. 1972, 76, 2892. (69) Holland, R. S.; Smyth, C. P. J.Phys. Chem. 1955,59,1088. (70) Gaumann, T. Helu. Chim. Acta 1958,41, 1933. (71) Elie, V. Bull. SOC.Chim. Belg. 1984,93, 839. (72) Bass, S. J.; Nathan, W. I.; Meighan, R. M.; Cole, R. H. J. Phys. Chem. 1964,68, 509.
Adsorption and Micellization Influence the Electrochemistry of Redox Surfactants Derived from Ferrocene? John J. Donohue and Daniel A. Buttry* Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071 -3838 Received December 13, 1988. In Final Form: January 23, 1989
The electrochemically modulated adsorption of several cationic redox surfactants obtained by reaction of ((dimethy1amino)methyl)ferrocene and various 1-bromoalkanes is described. The adsorption and desorption processes on gold electrodes are studied as functions of chain length of the alkyl tail and concentration of the surfactant by using both electrochemical and quartz crystal microbalance (QCM) techniques. The reduced (ferrocene) forms of the surfactants are observed to be more strongly adsorbed than the oxidized (ferricenium) forms as judged by the more positive formal potentials for oxidation of the adsorbed complexes relative to those for the solution-phase complexes. These shifts in formal potential, in combination with the adsorption isotherms for the reduced forms of the surfactants, allow for the calculation of the AG values for adsorption of both forms of the surfactants. In addition, differences in the formal potentials for the surfactants in micelles and as monomers are shown to be functions of the AG and the aggregation number for micellization. These differences allow for the calculation of the AG values for micellization of the surfactants, a procedure unique to surfactants bearing redox groups which change their state of aggregation as a consequence of the redox group.
Introduction Interest in the adsorption of amphiphilic species at electrode surfaces has a long history, with much of the original work stemming from the influence of surface-active agents (surfactants) on the suppression of polarographic maximal and the demonstration that, at least in some cases, the maxima are related to adsorption of the depolarizer at the electrode surface.2 In recognition of the importance of understanding molecular interactions with metal surfaces in solution, a considerable body of literature developed on the adsorption of ionic and molecular species on electrodes.H The potential of ordered, adsorbed layers of amphiphiles on electrodes as model systems for the study of biological membranes also has been investigated.H Presented at the symposium entitled "Electrocatalysis",196th National Meeting of the American Chemical Society, Loa Angeles, CA, Sept 27-29, 1988.
In recent years, interest in the behavior of adsorbed organic layers (especially of amphiphilic molecules) at electrode surfaces has been rekindled both by the further development of an appreciation of the structural features which endow a molecule with specific interfacial properties (see, for example, ref 9-12) and by the evolution of the (1) See, for example: Heyrovsky, J.; Kuta, J. Principles of Polarography; Academic Press: New York, 1966; Chapter 19. (2) Barker, G. C.; Faircloth, R. L. Advances in Polarography; Langmuir, I. S., Ed.; Pergammon Press: London, 1960; Vol. 1, p 325. (3) Parsons, R. Adu. Electrochem. Electrochem. Eng. 1961,1, 1. (4) Damaskin,B. B.; Petrii, 0. A.; Batrakov Adsorption of Organic Compounds on Electrodes; Plenum Press: New York, 1971. (5) Anson, F. C. Acc. Chem. Res. 1975,8, 400. (6) Miller, I. R.; Blank, M. J. Colloid Interface. Sci. 1968,26,26,34. (7) Miller, I. R.; Bach, D. J. Colloid Interface. Sci. 1969, 29, 250. (8) Nelson, A.; Benton, A. J. Electroanal. Chem. 1986,202, 253. (9) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; A h a , D. L.; Porter, M. D. Langmuir 1988,4,365. (10) Gun, J.; Iscovici, R.; Sagiv, J. J.Colloid Interface Sci. 1984,101, 201.
0743-746318912405-0671$01.50/ 0 0 1989 American Chemical Society
672 Langmuir, Vol. 5, No. 3, 1989 instrumental methods used to characterize these adsorbed 1 a ~ e r s . l ~Detailed studies of the influence of adsorbed layers of ionic14 and nonionic16 surfactants on metal ion reduction have been reported. An in situ ellipsometric study of the potential-dependent adsorption of sodium dodecyl sulfate on Pt electrodes has been carried out.16 Several groups are pursuing studies of the self-assembly of monolayers of amphiphilic molecules containing redox groups at electrode s u r f a ~ e s . ~ J ~Also, - ~ ~ the use of Langmuir-Blodgett techniques to prepare electrodes coated with organized molecular assemblies recently has been As part of an ongoing investigation into the influence of molecular and supermolecular structure on the redox behavior of molecules at surfaces, we report here on the use of electrochemical and quartz crystal microbalance (QCM) techniques to elucidate the influence of adsorption and micellization on the redox behavior of a series of ferrocene-based redox surfactants having different alkyl chain lengths. A preliminary report of this work has already appeared in which the behavior of the dodecyl derivative was d e s ~ r i b e d . ~ ~
Experimental Section (Ferrocenylmethyl)heptyldimethylammoniumbromide (C7Br) was prepared by stirring an equimolar mixture of ((dimethylamino)methyl)ferrocene(Aldrich)with 1-bromoheptane(Aldrich)
(11)Hubbard, A. T.; Stickney, J. L.; Soriaga, M. P.; Chia, V. K. F.; S. D.; Schardt, B. C.; Solomun, T.; Song, D.; White, J. H.; Wieckowski, A. J. Electroanal. Chem. 1984,168,43. (12)Israelachvili, J. N.;Mitchell, D. J.; Ninham, B. W. Biochim. Biophys. Acta 1977,470,185. (13)Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980,Chapter 14. (14)Batina, N.; Cosovic, B.; Adzic, R. J . Ekctroanal. Chem. 1986,184, 427. (15)Pyzik, G.;Lipkowski, J. J . Electroanal. Chem. 1981,123, 351. (16)Besio, G.J.; Prud'homme, R. K.; Benziger, J. B. Lnngmuir 1988, 4,140. (17)Miller, C.J.; Widrig, C. A.; Charych, D. H.; Majda, M. J . Phys. Chem. 1988,92,1937. (18) Goes, C. A.; mer,C. J.; Majda, M. J.Phys. Chem.1988,92,1937. (19)(a) Miller, C. J.; Majda, M. J. Am. Chem. SOC.1986,108,3118. (b) Widrig, C. A.; Majda, M. Anal. Chem. 1987,59,754. (20)Widrig, C. A.; Majda, M., preprint. (21)Lee, H.; Kepley, J. K.; Hong, H. G.; Akhter, S.; Mallouk, T. E. J. Phys. Chem. 1988,92,2597. (22)Facci, J. S.;Falcigno, P. A.; Gold, J. M. Langmuir 1986,2,732. (23)Facci, J. S. Langmuir 1987,3,525. (24)Lee, C. W.; Bard, A. J. J. Electroanal. Chem. 1988,239,441. (25)Nishiyama, K.; Fujihira, M. Chem. Lett. 1987,1443. (26)Iyoda, T.; Ando, M.; Kancko, T.; Ohtani, A.; Schimidzu, T.; Honda, K. Tetrahedron Lett. 1986,27,5633. (27)Fujihira, M.; Araki, T. Bull. Chem. SOC.Jpn. 1986,59, 2375. (28)Fujihira,M.; Pooeittiaak, S. J. Ekctroanal. Chem.1986,199,481. (29)Fujihira, M.; Pooeittinak, S. Chem. Lett. 1986,251. (30)Daifuku, H.; Aoki, K.; Tokuda, K.; Matauda, H. J. Electroanal. Chem. 1986,183,1. (31)Metzger, R. M.; Panetta, C. A.; Heimer, N. E.; Bhatti, A. M, Torres,E.; Blackburn, G. F.; Tripathy, S. K.; Samuelson, L. A. J . Molec. Electron. l986,2,119. (32)Zaba,B. N.;Wilkinson, M. C.; Taylor, D. M.; Lewis, T. J.; Laidman,D. L. FEBS Lett. 1987,213,49. Robineon, L. R.; Blackburn, A.; Richfa, B.; Allara, (33)Finklea, H. 0.; D.; Bright, T. Langmuir 1986,2,239. (34)McCaffrey,R. R.; Bruckenstein, S.; Praaad, P. N. LangmLur 1986, 2,228. (35)Donohue, J.; Nordyke, L.; Buttry, D. A. In Chemically Modified Surfaces in Science and Indwtry; Leyden, D., Collins, W., E&.; Gordon and Breach New York, 1988; p 377. (36)(a) Van Huong, N.; Hinnen, C.; Lecoeur, J. J. Electroanal. Chem. 1980,106,185.(b) Scherson, D.; Kolb, D. J . Electroanal. Chem. 1984, 176,353. -0,
Donohue and Buttry
for 2 h at 40 "C. The resulting solid was recrystallized 3 times from acetone (Baker) to give a yellow crystalline product. The overall yield was 50%, mp 165-167 OC. The structures of this and all other compounds were verified by NMR and FTIR. (Ferrocenylmethy1)heptyldimethylammonium nitrate (C7N03) was prepared by adding C7Br to a solution of M/M ethanol/water saturated with sodium nitrate. A stoichiometricamount of silver nitrate then was added to remove the bromide. Filtration and extraction with dichloromethane,drying with magnesium sulfate, and evaporation gave an oil of C7NOs which was recrystallized from acetone. The overall yield was 60%, mp 123-126 OC. Surfactants with other chain lengths were prepared according to the same procedures by using other 1-bromoalkanes. Yields increased with increasing chain lengths. Melting points were 100-103 OC for C10N03,93-97 "C for C12NO9,and 90-92 OC for C14N09. The nitrate salts of the surfactants were used in preference to the bromide salts to avoid potential complications due to bromide adsorption on the gold electrode surfaces used in this study. Lithium sulfate was recrystallized 3 times from water. Other chemicals were reagent grade or better and were used as received. Solutions were prepared by using deionized water from a Millipore purification system. Critical micelle concentrations (cmc's) of the various Surfactants were measured with a du Nouy balance. The preparation of the quartz crystah and the instrumentation for the electrochemical/QCM experiment have been described p r e v i o ~ s l y . 3 ~The * ~ ~data ~ collection system has since been modified, consisting now of an IJ3M PC with a Data Translation DT2801-A data acquisition and control board programmed by using the ASYST programming environment. The voltage ramp for the cyclic voltammetric (CV) experimentswaa a true analogue ramp generated by using a BAS CV27 potentioatat. All potentials are quoted with respect to a Ag/AgCl reference electrode. Adsorption isotherms were constructed from the variation of the electrochemical charge for the surface waves of the adsorbed complexes with their solution concentrations. Corrections for background currentsand for the contribution of the solution-phase complexes to the currents in the CV were made. The QCM frequency data also were used to construct plots of the frequency increase observed for desorption of the complexes followingoxidation (see below) versus solution concentration. These were corrected for the frequency changes observed at bare gold electrode~(usually a a.0.5-2-Hz decrease) during the positive scans. The nature of the processes which cause these decreases is not known with certainty, although possibilities are the incipient oxidation of Au to form submonolayeroxide,%the formation of chromium oxides from Cr present at the surface due to diffusion of some Cr from the adhesion layer through the Au film, and electrokinetic coupling of the electrode surface charge with the diffuse double layer (see below).
Results and Discussion CV/QCM Experiments. References 35 and 37-43 give a list of recent publications representative of the use of the QCM to monitor mass changes at the electrode surface during electrochemical eltperiments. Briefly, the piezoelectric QCM is excited into a mechanically resonant shear mode oscillation by using a broad-band oscillator circuit41 designed to operate at the resonant frequency of the QCM c o m p i t e resonator. The gold electrodes, which are vapor deposited onto the QCM, are part of this composite resonator, so that mass changes which occur at the electrode exposed to the solution are reflected by changes in the resonant frequency of the device. A commercial frequency (37)Varineau, P. T.; Buttry, D. A. J. Phys. Chem. 1987,91, 1292. Buttry, D. A. J. Am. Chem. Soc. 1987,109,3574. (38)Orata, D. 0.; (39)Bruckenstein, S.;S h y , M. J. Electroanal. Chem. 1986,188,131. (40)Bruckenstein, S.;Shay, M. Electrochim. Acta 1986,30, 1295. (41)Melroy, 0. R.; Kanazawa, K. K.; Gordon, J. G.;Buttry, D. A. Langmuir 1986,2,697. (42)Feldman, B. J.; Melroy, 0. R. J . Electroanal. Chem. 1987,234, 213. (43)Deakin, M. R.;Melroy, 0. R. J. Ekctroanul. Chem. 1988,239,321.
Langmuir, Vol. 5, No. 3, 1989 673
Influence of Adsorption and Micellization
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behavior at Au electrodes. They adsorb rather strongly in their reduced forms, and oxidation induces some amount I of desorption. Since this desorption causes a mass change A 4.20 at the electrode surface, the QCM may be used to monitor U the change in the surface population of these surfactants. a This is shown in Figure 1for the C14 derivative, which in p 2.80 this experiment is present at such a low concentration (7 L LL pM) that the contribution to the electrochemical response Q of molecules diffusing to the electrode from solution is negligible. The CV shows a well-formed and symmetric surface wave for the adsorbed surfactant, with the formal -.000 potential of the adsorbed species given by the anodic peak potential of 0.53 V. In curve B of Figure 1, the QCM -080 .240 .400 -560 .720 frequency response is shown. The marked increase in the Potentiol. V v s Ag/AgCl QCM frequency indicates significant mass loss from the Figure 1. (A) CV scan for 7 pM C14 from 0.0 to 0.80 V in 1M electrode surface during the positive scan, which can be H3P04,50 mV/s, i, = 12pA/cm2. The current has been digitally attributed to the desorption of some of the adsorbed smoothed and corrected for background. (B)QCM frequency response measured concurrently with part A. surfactant as a consequence of its oxidation to a doubly charged state. This interpretation is supported by the counter is then used to measure these frequency changes. small current seen for reduction of the oxidized form on These measurements may be made concurrently with the the return scan, presumably due to loss of significant electrochemical experiment. amounts of the material to solution by diffusion away from A number of factors need to be considered when using the surface during the positive scan. Thus, at these low this frequency change to make quantitative calculations concentrations the delivery of surfactant from the solution of mass changes at the electrode surface. Many of these is too slow to provide enough material during the return have been d i s c u s ~ e d One . ~ ~of~the ~ ~most ~ important of scan to reattain the surface population which existed prior these is that the mass must behave as a perfectly elastic to the oxidation of the adsorbed species. This is also layer, i.e., it must be rigid, moving with the same amplitude reflected by the fact that the QCM frequency does not and frequency as the underlying quartz substrate and return to its original value but rather decreases only by experiencing no shear deformation. When rigid-layer an amount which indicates that 30% of the mass is rebehavior prevails, the frequency change (Af, in Hz) is gained during the scan. Waiting an additional few minutes linearly related to the mass change (Am, in g cm-2) through after the end of the scan allows the diffusion process a proportionality constant, which contains the materials sufficient time to repopulate the surface with the reduced properties of the quartz substrate and the crystal thickness. form of the surfactant, as indicated by a slow return to the This is shown by original QCM frequency. We defer a quantitative discussion of these frequency changes to a later section. Af = - ~ A ~ ~ O ~ / ( P , P , ) ' / ~ (1) The desorption of the oxidized form of the surfactant in which fo is the resonant frequency of the crystal in the is not unexpected, based on the previous observations of absence of the deposit (ca. 5 X lo6 Hz in the present case, micelle formation of the reduced form of the C12 derivative this value is inversely related to the crystal thickness), pq and disruption of these micelles upon oxidation to the is the shear modulus of quartz (2.947 X lo1' g cm-' s-~), doubly charged state.u That this behavior is mimicked and p,. is the density of quartz (2.648 g c m 3 . Rigid-layer in the adsorption of the surfactant at electrode surfaces behavior has been observed for a few systems in which is a reminder that simiiar forces operate in adsorption and adsorbates are present at monolayer coverages.35*39,41,43 micellization of amphiphilic species. However, even when rigid-layer behavior prevails, there Measurements of the charge consumed for oxidation of are other phenomena which can muddy the interpretation the adsorbed surfactant as a function of its solution conof the frequency changes that occur during redox reactions centration provide its adsorption isotherm, which is shown at electrodes having monolayer levels of adsorbates.% We shall discuss some of these in a later section. As previously reported,35 a general feature of these (44) Saji, T.; Joshino, K.;Aoyagui, S. J. Am. Chem. SOC.198S, 107, ferrocene derivatives is that they exhibit surface-active 6865. 5. 60
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674 Langmuir, Vol. 5, No. 3, 1989
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in Figure 2. The surface coverage, r (mol cm-2),is seen to increase monotonically with increasing concentration until a saturation value of 4.5 x mol cm-2 (r,)is attained at a concentration of ca. 20 pM. We believe that this represents saturation of the surface at monolayer coverage. Detailed examination of the shape of the isotherm reveals that it is of the Langmuir type, indicating little interaction between the adsorbed ionic surfactants or the presence of compensating effects which cancel the influence of interactions on the shape of the isotherm. By use of the projected geometric area of the electrode, the value of rsgives an area per molecule of 37 A2/molecule. However, if the surface roughness of the Au electrode (ca. 1.2) is taken into account, the value is 44 A2/molecule. In a previous study, Facci reported the head group limited area for the C18 derivative as 45 A2/molecule, based on ita adsorption isotherm on a Pt electrode,23in very good agreement with the present value. In addition, he reported that the surface area per molecule obtained from the equilibrium spreading pressure of the C18 derivative was 40-50 A2/molecule. The value of rSpredicted for head roup limited adsorption from molecular models is 50 molecule. These considerations provide good evidence for monolayer adsorption at saturation with the value of rsdetermined by head group area, implying an average orientation roughly perpendicular to the electrode surface. Figure 3 shows the CV/QCM data for the C12 derivative. Again the experiment was done at a C12N03 concentration (22 pM), well below that at which the contribution from diffusion of solution-phase complex becomes important, so the electrochemical response is dominated by the adsorbed complex. The smaller peak current on the reverse scan again indicates that there is some loss of adsorbed complex from the surface as a result of its oxi-
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Langmuir, Vol. 5, No. 3, 1989 675
Influence of Adsorption and Micellization
flO*PP
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1.0 mM. Thus, it appears that the plateau value of r corresponds to monolayer saturation. These data reveal an inverse relationship between the molecular area at saturation and the chain length. A possible explanation for this is that the energetics of aggregation for the alkyl chains in the adsorbed state are less effective a t overcoming the head group electrostatic repulsions as the chain length decreases. This type of relationship has been previously observed for limiting molecular areas in experiments of adsorption onto oxide surfaces from aqueous solutions with ionic surfactants of varying chain lengths.46 The adsorptive behavior of C 7 and C 2 also was examined by using the QCM to monitor changes in the mass of any adsorbates. These experiments gave no indication of adsorption of these two compounds at concentrations up to 10 mM. Thus, the onset of this adsorptive behavior occurs between C 7 and C10. Thermodynamicsof Adsorption and Micellization. Table I shows the apparent formal potentials @ (,,,, and flWp for the solution and adsorbed couples, respectively) of the various surfactant derivatives. These data show that both and Efaap, depend on chain length. The shifts in the d u e @a,app reflect changes in the free energy of adsorption of these species as a function of chain length. The shifts in the value of result from different extents of aggregation of the surfactants in solution. We discuss each below. As has been noted previously, the reduced forms of the C12& and the other derivatives%form micelles in aqueous solutions with cmc values that depend on chain length. Oxidation leads to disruption of these micelles for the C12 derivative,&and we have observed similar effects for some of the other derivatives. Thus, at the concentrations necessary to obtain,,@ ,, for the C12 and C14 derivatives, the reduced forms are present in micellar aggregates, while the oxidized forms exist as isolated ions in solution. The energetics of this disruption of the micellar aggregates are
,
(45) (a) Wakamatsu, T.; Fueretenau, D. W. In Adsorption from Aqueous Solution; Advances in Chemistry 79; American Chemical Society: Washington, D.C., 1968;Chapter 13. (b) Brode, P.F. Longmuir 1988,4,176.(c) Connor, P.;Ottewill, R. H. J. Colloid Znterfac.Sci. 1971, 37, 642.
Figure 6. c12.
Efappvs In concentration of added monomer (I,) for Scheme I
I,
, KM, Gads, I
I
reflected in @,,a p' The situation is shown in Scheme I. derived from this model is given The expression &r below: = E', - (AGM,I/nF) + ( R T / n F ) ( l - p-l) In I, ( R T / n F ) ( l - p-l) In p + ( R T / n F ) In (DR/Do)1/2(2)
where AGM,I is the free energy of micellization per mole of the reduced form of the surfactant, E', is the formal potential for the surfactant which would be observed in the absence of aggregation, Do is the diffusion coefficient of the oxidized surfactant, DR is the diffusion coefficient of the reduced form of the surfactant (which in this case is the micellar diffusion coefficient), p is the aggregation number, and I, is the concentration of micelles, which can be approximated by I J p , the total concentration of monomer added to the solution divided by the aggregation number, at concentrations high enough so that most of the monomer is present in micellar aggregates. In this equation, we explicitly account for the relatively large difference in the diffusion coefficients for the reduced (aggregated) and oxidized (unaggregated) forms of the redox couple. A prediction of this equation which can be tested experimentally is that E',,,, should depend logarithmically on the total concentration of added monomer, I,, a t concentrations much larger than the cmc. A plot of the results of such an experiment with the C12 derivative is shown in Figure 6. The slope of the line through the experimental points is 22.5 mV, to be compared with the theoretical value of 25.3 mV (assuminga value of p = 91 f 7).&
676 Langmuir, Vol. 5, No. 3, 1989
Similar experiments with C14 gave inconclusive results due to large ohmic resistance of the adsorbate layer and a consequent uncertainty in the precise value of E'w . The good agreement of the observed with the predictea slope lends support to the model presented in Scheme I. A t a certain concentration of total added monomer I,, if one has values for Efs,app,E',, DR,DO,and p, then the value of AGM,~may be obtained by using this equation. The value of E', may be approximated by the E?, of a shorter chain derivative, which is known not to form micelles a t the concentrations used to obtain E',. We chose the C2 derivative, obtained by reaction of the parent aminoferrocene compound and ethyl iodide, as this reference derivative, which has a value of @, = 0.390 V. This is the same value that the C10 derivative exhibited, in agreement with our du Nouy measurements, which indicated that it does not form micelles at concentrations up to 1.0 mM, the upper limit of the concentration range investigated. The values of DRand Do used were those given for the C12 derivative." The correction to from the diffusion coefficient term in the equation amounts to ca. 20 mV of the observed shift from E', and is constant for the series because the diffusion coefficients of the micelles and the isolated oxidized forms are nearly independent of chain length. The average value for AGM,I obtained from the points in the plot of Figure 6 is -30.9 kJ mol-', obtained by using the values for Ef,, DR,Do,and p discussed above. One can also use the cmc to calculate the value of AGw* For C12, the cmc value of 120 pM gives a AGM,I of -32.3 kJ mol-', in excellent agreement with the value predicted by using eq 2. A final way to obtain AGM is from the intercept of a linear regression of the data in the plot of versus In I,. This gives a value of -28 kJ mol-' but is tke least accurate method because of the length of the extrapolation required to evaluate the intercept. The values of AGM obtained from these calculations for both C12 and C14 are given in Table I. They clearly show that as the chain length increases, so does the tendency toward micellar aggregation. The value of the change on going from the C12 to the C14 derivatives is 2.5 kJ mol-' per methylene unit, based on the cmc values. (It is incorrect to evaluate the shift between the C10 and C12 derivatives, because micelle formation is not observed for the C10 derivative.) This is slightly lower than the range of previously obtained values for nonredox ionic surfactants of 2.8-3.5 kJ mol-1.48i48We speculate that this may arise from poor packing of the head groups and a consequently inefficient exclusion of water from the interior of the micelle due to the large mismatch between the head group and alkyl chain projected areas. Considering now the behavior of the adsorbed species, the observation of Langmuir adsorption behavior allows the extraction of AG,,J (the free energy of adsorption for the reduced form of the surfactant on a bare gold electrode, in kJ mol-') from the slope of the normalized adsorption isotherm in the limit of low surface coverages. Since r should depend to some extent on the surface charge on the electrode, this free energy of adsorption represents the average value for the range of potentials scanned during the CV experiment. (In a future publication, we will describe the use of potential step experiments with the QCM to measure the potential dependence of adsorption of thew (46) Mukerjee. P. Ado. Colloid Interfac. Sci. 1967, I, 241. (47) Clint, J. H.; Walker, T.J.Chem. SOC.,Faraday Tram. 1 1976, 71, 946. (48) h e n , M. J. Surfactants and Interfacial Phenomena; Wiley-Interscience: New York, 1978; p 115. (49) Reference 48, p 111.
Donohue and Buttry
surfactants at potentials away from the formal potential.) These values of AGd for the C10, C12, and c 1 4 derivatives are shown in Table I. The formal potentials of the adsorbed forms of these surfactants are shifted away from the E', value for the freely diffusing, unaggregated species in solution due to different free energies of adsorption for the reduced and oxidized forms of the surfactants. In this case, all of the adsorbed complexes have GWpvalues more positive than E?, because the reduced (and fess highly charged) forms of the redox couples have free energies of adsorption more negative than the oxidized forms. These values of ,?Za,ap are also given in Table I. A salient feature of these formd potentials is that they are strongly dependent on the chain length of the surfactant. The magnitude of the shift,along with the value of AG,,I obtained from the isotherm, may be used to obtain the value of AG,,II (the free energy for adsorption of the oxidized form of the surfactant) according to A G , u - AGa.1 n F ( E a , a p p - EfJ (3) in which all of the variables and constanta have their usual meaning. It is important to note the use of ,?Zsas the point of reference for calculating the potential shift rather than Ef,,>p,,which also includes a contribution from the energetics of the micellization process for the reduced form of the redox couple. These values for AGa may then be used to calculate thermodynamic values for the surface coverage of the oxidized form of the redox couple at the positive end of the CV scan for a given concentration, assuming a Langmuir adsorption isotherm for I1 and rapid equilibration of the adsorbed layer with the solution composition. These values of I.,' are also given in Table I as a percentage of rIand show that the desorption induced by oxidation should be nearly complete for C10 and C12. We have independently measured the extents of adsorption of the oxidized forms of the surfactants by chronocoulometry and find them to also indicate that the surface coverages of the oxidized forms are small under these conditions. The change in the values of AG, for the various chain lengths of both redox forms reveals an increasing tendency toward adsorption as chain length increases. However, the incremental change per methylene group is not constant either for the reduced or the oxidized forms of the surfactants. For example, the incremental change in AG, on going from C10 to C12 for the reduced form is only ca. 0.2 kJ mol-', while that on going from C12 to C14 for the oxidized form is nearly 2 kJ mol-'. That the incremental changes vary so widely with chain length and charge may be an indication that some aspect of the structure of the adsorbate layer is changing with these two variables. Recalling the dependence of area per molecule at saturation on chain length, which indicated stronger alkyl chain interactions for the longer chain lengths, we speculate that the longer chain derivatives adsorb in a more highly oriented fashion, allowing for stronger interactions between the alkyl chains and a consequently larger incremental change in AG, per methylene group. Comparison of QCM Frequency Changes with the Electrochemically Determined Surface Coverage Changes. As discussed above, a quantitative comparison of the QCM frequency changes with the electrochemically determined surface coverages can only be made when certain conditions prevail. The data in Figures 1 and 3 show that, in a qualitative sense, the oxidation of these redox surfactants induces desorption, which gives a characteristic increase in the QCM frequency. However, the frequency changes predicted for the electrochemically
Langmuir, Vol. 5, No. 3, 1989 677
Influence of Adsorption and Micellization
determined changes in surface coverage are not exactly equal to those determined experimentally, and the errors are in some cases considerably larger than the uncertainty in the frequency measurement. We discuss several potential causes for this below. In contrast to the situation in redox or conducting polymer films,where electroneutrality must be maintained over a macroscopic scale across the film thickness, charge neutrality on the solution side of the electrical double layer at an electrode surface need not be maintained, so long as the appropriate countercharge exists on the electrode surface. For example, coadsorption of supporting electrolyte ions or the counterion of the redox surfactant may occur nonstoichiometrically with respect to the surface coverage of the redox surfactant. In addition, the extent of coadsorption may change during the electrochemically induced desorption of the oxidized form of these redox surfactants. In such a case, the frequency changes which would be observed for these changes in ionic populations a t the electrode surface would not be negligible in comparison to those for the redox surfactant itself, leading to poor agreement between the predicted and observed QCM frequency changes. It is worth pointing out in this regard that counterion binding to micelles has been observed for many systems and is known to depend on such things as charge density at the surface of the micelle, the nature of the counterion (charge to radius ratio, structure maker versus structure breaker, ionic valency, etc.), polarizability, and hydration.m Another potential source of such discrepancies can be found in the state of adsorbed water at the surface. As was discussed p r e v i o u ~ l y if , ~the ~ tightness of binding of water molecules differs on the bare Au surface and the surfactant-covered surface, then the change in the mass of rigidly held water will contribute to the observed Af. For example, if water is more tightly bound on Au and less tightly bound on the surfactant-covered surface, then the observed Af for desorption of the redox surfactant will be slightly less than it would have been in the absence of such an effect. This effect will be observed if the component of motion of the water molecules parallel to the electrdde surface is diminished on the time scale of the experiment (5 MHz). They need not be completely immobilized at the surface.% Another way for solvent to influence the measurement is for solvent to be trapped within the surfactant layer (i.e., essentially rigidly held) and then to be released following disintegration of this layer. This effect would be conceptually similar in nature to the effects observed at rough electrode surfaces.62 Other potential sources for frequency changes which are not directly related to mass changes at the surface are the electrokinetic coupling between the surface charge and the double-layer charges1 and changes in the density and/or viscosity of the electrical double layer with potential. Either of these may be altered to some degree by changes in the surface coverage of the redox surfactant, although the magnitude of the effects is not predictable at this time. With all of these effects in mind, we reexamine the adsorption isotherms for C10, C12, and C14, plotting now the value of Af which is experimentally observed (Af,) and the value calculated for desorption of the amount of redox surfactant determined from the electrochemical measurements (Af,). This calculation is made assuming desorption of only the redox surfactant; the mass of any (50) Reference 48, p 102. (51) Hager, H., preprint. (52) Schumacher, R.; Gordon, J. G.; Melroy, 0.J . Electmaml. Chern. 1987,216,127.
6. 30
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'
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Figure 7. Isotherm from QCM for the adsorption of C10 from 1 M H3P04. Afe, 0;Af,, X . 0 s
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Figure 8. Isotherm from QCM for the adsorption of C12 from 1 M H3P04. Af,, 0;Af,, X . 18.0
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Figure 9. Isotherm from QCM for the adsorption of C14 from 1 M H3P04. AI,, 0; Aft, X .
accompanying ionic or solvent species is not included. Figures 7,8, and 9 show these data for C10, Cl2, and C14, respectively. These are the averaged results of at least five replicate experiments (eight in some cases), and it should be noted that the trends described below are visible in each individual experiment. For C10, 4, is significantly below the experimentalvalue in that part of the isotherm below monolayer coverage. This means that larger than expected frequency changes are observed when the molecules are loosely packed together. This seems to be best attributed to the presence of trapped solvent within the layer that is released as desorption occurs, thus giving rise to a value of Afe larger than Af,. The convergence of these values at monolayer coverage would be expected for such a model, because the
678 Langmuir, Vol. 5,No. 3, 1989
closer packing which occurs there should effectively exclude water from within the monolayer. For C14, Af, and af, agree relatively well until monolayer coverage is reached, at which point the value of Af, begins to exceed the calculated value. This indicates mass changes larger than those predicted under conditions in which the surfactant molecules are rather closely packed. This may be caused by counterion binding to the monolayer and their consequent detection by the QCM. The smaller molecular area, which causes a larger areal charge density, may be the cause of the more significant binding for C14, as compared to C10 and C12. For C12, there is good agreement between Af, and Afc at all values of .'I Given the arguments presented above, we speculate that this is due to intermediate behavior on the part of C12. Thus, molecular packing at submonolayer coverages may be efficient enough to effectively exclude solvent, but packing at saturation is not close enough to induce counterion condensation on the charged organic layer. However, it must be recognized that these arguments are highly speculative.
Conclusions The influence of micellar aggregation on the electrochemical response of solution-phase redox surfactants has been described. The apparent formal potential is shown to be linearly dependent on AGM, providing a way to obtain this value from electrochemical measurements. For C12, good agreement between the values of AGM obtained from the cmc and the formal potential lends strong support to this interpretation of the shifta of P-,, with concentration and chain length. In principle, this method allows one to obtain p , however, the logarithmic dependence of the observed formal potentials on p makes the value obtained very imprecise. The values of AGM obtained from the present experiments compare reasonably well with those determined for other cationic surfactants in high ionic strength solutions.63 For example, cetyltrimethylammonium nitrate (CTAN03)in 0.5 M NaN03 has a cmc of 0.8 mM compared with 0.016 mM for C14, with the caveat that C14 has a much more hydrophobic head group than does CTAN03, and the ionic strength in our experiments is twice that in the CTAN03 experiments. The phenomena observed here are reminiscent of the shifts in formal potential for cation and anion radical species in the presence of both monomers and micelles of anionic, cationic, and nonionic surfactants previously reported by McIntire and Blount" and others. These workers showed that the formal potential for oxidation of 10-methylphenothiazine(MPTH) depends strongly on the (53) Reference 48,pp 96, 97. (54) (a) McIntire, G.L.; Blount, H. N. J. Am. Chern. SOC.1979,101, 7720. (b) McIntire, G.L.; Blount, H. N. In Solution Behavior of Surfactants: Mittal, K. L. Fendler, E. J., Eds.; Plenum Press: New York, 1982; Vol. 2, pp 1101-1123. (55) Schuhmann,D.;Vanel, P.; Tronel-Peyroz, E.; Raous, H. In Solution Behavior of Surfactants; Mittal, K., Fendler, E. J., %.; Plenum Press: New York, 1982; Vol. 2, pp 1233-1247.
Donohue and Buttry
concentration of dodecyl sulfate (DS-) below the cmc for DS-, with the magnitude of the shift giving the stoichiometry of the complex formed between MPTH+ and DS-. In their case, a linear regression of the plot of the apparent formal potential versus the logarithm of the DSconcentration gave the stoichiometry from the slope and the equilibrium constant for complex formation from the intercept, a treatment quite similar to that presented above. The slopes of the early parts of the adsorption isotherms for C10, C12, and C14 give AGa,p This value used in combination with the shifts of the adsorption waves away from E', (the value of the formal potential in the absence of the micellization effects) was shown to give AG,,II. Considerable literature exists on the values of AG, for surfactants on oxide, halide, and polymeric surfaces. However, almost nothing is known about the values for the free energy of adsorption of surfactants onto metal surfaces. The few contributions which address the adsorption of surfactants onto metal s u r f a c e ~ ~ J do " ~not ~ ~ explicitly provide such values. For the case of Langmuir-Blodgett layers on electrode surfaces, the interpretation of shifts in formal potentials away from the solution values is clouded by the fact that the lateral interactions within the layer almost certainly dominate these shifts,not the interactions of the amphiphiles with the substrate material. Thus, while there are essentially no literature values with which to compare the present values of AGa, the incremental changes in AG, per methylene unit between C10 and C12 seem to be in reasonable accord with those expected from the work on oxide surfaces.ffi It is especially interesting that these incremental changes vary with the chain length, perhaps indicating a gradual change in the order within the adsorbate layer. The QCM frequency changes observed for the desorption/adsorption processes in this study also indicate a change in the behavior of the adsorbate layer, both as a function of surface coverage at a given chain length and as a function of chain length at a given surface coverage. It is intriguing that the QCM can give such hints as to the extent of solvent trapping within adsorbate layers and the extent of counterion binding at the surfaces of such layers. It is hoped that future work in this area will allow for less speculative explanations to be presented in this regard.
Acknowledgment. Acknowledgement is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for the partial support of this research. We thank Prof. David Jaeger for the use of the Du Nouy balance and Dr. Harold Hager and Prof. Marcin Mijda for making available to us preprints of their work. Registry No. C7Br,119638-20-9;C&r, 94149-10-7;ClZBr, 98778-40-6;C14Br, 114188-46-4; C7N03, 119851-06-8;CloN09, 119851-08-0;C1zNO3, 119851-10-4;CldNOs, 114267-76-4;Au, 7440-57-5;((dimethylamino)methyl)ferrocene, 1271-86-9. (56) Reference 48, pp 44-46.
