Langmuir 1987,3, 525-530
525
Modification of Platinum Electrode Surfaces with Adsorbed Monolayers of (Ferrocenylmethyl)dimethyloctadecylammonium Hexafluorophosphate John S. Facci Xerox Webster Research Center, Webster, New York 14580 Received October 15, 1986. In Final Form: February 3, 1987 The electrochemistry of an electroactive surfactant, (ferrocenylmethyl)dimethyloctadecylammonium hexafluorophosphate, adsorbed at iodine-coated Pt electrodes from highly dilute 1M HzS04and 1M HC104 solution is described. Absorption isotherms and cyclic voltammetry show that the electrochemistry of films of the title compound is strongly dependent on the electrolyte from which adsorption occurs. In H2S04 adsorption occurs in two monolayer stages whereas in HCIOl structural rearrangement of the initially adsorbed monolayer to a two-dimensional micellar structure occurs with increasing surface coverage. Excellent fits of the Frumkin isotherm with the data are demonstrated, illustrating a strongly attractive interaction between the amphiphilic hydrocarbon tails. Comparison with previous work on LangmuirBlodgett monolayers of this system on Au is made.
Introduction The bahavior of surfactant molecules a t electrode interfaces,’ especially electroactive surfactants,2has recently received increased attention particularly in connection with the renewed interest in Langmuir-Blodgett (L-B) multilayer films.3 It has been p e r ~ e i v e d ~that $ ~ modification ?~ of electrode surfaces with organized assemblies may offer a means of investigating the influence of spatial organization of redox centers on redox and photoelectrochemicalZf-’behavior. Redox functionalized surfactants were recently used to assemble organized electroactive monolayers a t electrode interfaces by transferring the preassembled monolayer from the air/water interface via the L-B t e c h n i q ~ e . ~SnOz ? ~ electrodes, for example, were modified with L-B films of surfactant-derivatized M(bpy),2+ (M = Ru, Os; bpy = 2,2’-bipyridine) and the heterogeneous electron transfer rate constants measured.za-c In our laboratory* the stability and properties of ordered monolayers of the title compound, (ferrocenylmethyl)dimethyloctadecylammonium hexafluorophosphate (I), a t the air/water and air/Na2S04(aq)interface were studied via pressure-area isotherms. Monolayers of I were also transferred to hydrophilic Au electrodes via the L-B transfer technique and examined electrochemically as a function of transfer pressure (molecular area). Voltammetric and adsorption characteristics of the system were described. In our laboratory, electrical (1) (a) Saji, T.; Hoshino, K.; Aoyagui, S. J. Am. Chem. Soc. 1985,107, 6865. (b) Sakai, K.; Saito, M.; Sugi, M.; Iizima, S. Jpn. J. Appl. Phys., Part 1 1985,24,865. (c) Finklea, H. 0.;Robinson, L. R.; Blackburn, A.; Richter, B.; Allara, D.; Bright, T. Langmuir 1986,2, 239. (d) McCaffrey, R. R.; Bruckenstein, S.; Prasad, P. N. Langmuir 1986,2,228. (e) Ellison, R. H. J. Phys. Chem. 1962, 66, 1867. (2) (a) Miller, C. J.; Majda, M. J.Am. Chem. SOC. 1986,108,3118. (b)
Daifuku, H.; Aoki, K.; Tokuda, K.; Matsuda, H. J. Electroanal. Chem. 1985,183, 1. (c) Daifuku, H.; Yoshimura, I.; Hirata, I.; Aoki, K.; Tokuda, K.; Matsuda, H., J. Electroanal. Chem. 1986, 199, 47. (d) Aoki, K.; Tokuda, K.; Matsuda, H. J.Electroanal. Chem .1986,199,69. (e) Fujihira, M.; Poosittisak, S. J . Electroanal, Chem. 1986, 199,481. (0 Sugi, M.; Sakai, K.; Saito, M.; Kawabata, Y.; Iizima, S. Thin Solid F i l m 1985,132, 69. (9) Fujihira, M.; Nishiyama, K.; Yamada, H. Thin Solid F i l m 1985, 132,77. (h) Fujihira, M.; Aoki, K.; Inoue, S.; Takemura, H.; Muraki, H.; Aoyagui, S.Ibid. 1985, 132,221. (i) Nakahara, H.; Fukuda, K.; Sato, M. Thin Solid Films 1985, 133, 1. (3) (a) Gaines, G. L. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience Publishers: New York, 1966. (b) See also: Thin Solid F i l m 1980, 68, 1-288, 1983, 99, 1-330, special issues devoted to L-B films. (4)Facci, J. S.; Falcigno, P. A,; Gold, J. M. Langmuir 1986,2,732-738. (5) Faulkner, L. R. Chem. Eng. News 1984, 28.
0743-7463f 87f 2403-0525$0l.50 f 0
and electrochemical applications of electroactive and electroinactive L-B films are being explored. Also, the well-defined thickness characteristics of L-B films are being employed to ascertain the distance dependence of electron-transfer behavior a t interfaces. It has been purported that the stability of L-B monolayers at the &/water interfaceh and electrode/electrolyte interface4 arises from the “cohesive” van der Waals interaction between adjacent stereotopically organized hydrocarbon tails. Despite the fact that hydrocarbon amphiphiles remain one of the most extensively studied classes of adsorbates, there appear to be few studies quantifying these interactions or assessing their stabilizing influence on the adsorption of hydrocarbon amphiphiles a t solid electrodes. This paper presents a description of the adsorption, morphological, and voltammertic characteristics of I absorbed from 1 M H2S04and 1 M HC104 electrolytes a t Pt electrodes coated with an adsorbed monolayer of iodine. It has been shown that long-chain ionized surfactants such as octadecyltrimethylaonium (ODTA) perchlorate may form self-assembling layers at interfaces and adsorb on surfaces which have been passivated by an adsorbed monolayer of iodine or other chemisorbed surfactants.6 Adsorption is irreversible and prolonged rinsing of the surface with pure electrolye does not remove the surfactant. Compound I may be viewed as a ferrocene derivative of ODTA in which the methyl group ODTA is replaced by a ferrocenylmethyl group. The ferrocene moiety, thus, acts as an electroactive tag from which the adsorption characteristics of the adsorbed layer may be inferred.
Experimental Section Materials. The synthesis of I from 1-iodooctadecane and (ferrocenylmethy1)dimethylamine was previously de~cribed.~ Water was purified by reverse osmosis (Millipore RO-15), deionization (Millipore Super Q), and double distillation under nitrogen from alkaline permanganate and dilute sulfuric acid and finally stored under a nitrogen atmosphere. H2S04and HCIOl (G.F.Smith) were double distilled from Vycor. Acetonitrile (Burdick & Jackson) was stored over Linde 4A molecular sieves. High-purity methanol (Burdick & Jackson) was used as received. Tetraethylammonium perchlorate (TEAP) was recrystallized 3 (6) Stickney, J. L.; Soriaga, M. P.; Hubbard, A. T.; Anderson, S. E. J. Electroanal. Chem. 1981, 125, 73.
0 1987 American Chemical Society
526 Langmuir, Vol. 3, No. 4, 1987
Figure 1. Steady-state cyclic voltammetric curve of a polycrystalline Pt bead in pure 1 M H2S04 (-); S = 36 pA/cm2. Integration boundaries for the determination of true area are given by the dashed line. Adsorption of iodine atoms on Pt yields the cyclic voltammetric curve shown in the dashed-doted line; S = 7.1 pA/cm2. times from water and dried in a liquid N2 trapped vacuum oven for several days at 50 "C. Tetraethylammonium iodide (TEA11 and K1 ("Baker Analyzed") were used as received. Instrumentation. A PAR Model 175was used in conjunction with a PAR Model 173/276 potentiostat. Current-potential curves were recorded on a Hewlett-Packard 7015B X-Y recorder. A conventional degassable two-compartment electrochemical cell with Luggin probe was employed for all electrochemical experiments. All potentials are referenced to a sodium chloride saturated calomel electrode (SSCE)of conventional de~ign.~ Argon used in blanketing and degassing of electrolyte solutions was purified via an Ace-Burlitch inert atmosphere system employing a BASF oxygen-scavenging catalyst. A 3-mm diameter monocrystalline Pt bead electrode was prepared from 0.5-mm diameter Pt wire (99.95%) by melting and slow cooling as described elsewhere.* Although the Pt bead was not etched as prescribed," the lustrous (111)facets were easily visible as previously reported. In addition, the bead was not cut to expose a single-crystalline face. Pt disk electrodes (Pine Instruments) and 0.5-mm-thick Pt foils were mirror polished with successively finer grades of diamond paste ending with 0.05-pm alumina.
Results and Discussion All adsorption experiments were done at Pt electrodes with an adsorbed monolayer of iodine (Pt/I). Prior to each experiment, Pt ws electrochemically cleaned by applying a vigorous oxygen evolution potential between +1.9 and 2.0 V in pure 1 M H,SO, for 4-5 min. This was followed by potential cycling between voltage limits corresponding to hydrogen evolution, -0.2 V, and water oxidation, +1.4 V, until the current potential pattern (Figure 1)characteristic of atomically clean polycrystalline Pt9J0was established. The cathodic charge Q, under the voltammetric waves for underpotential hydrogen deposition was used to calculate the true area of the Pt bead by assuming a value of 210 pC/cm2? The dashed lines in the figure were employed as integration boundaries.'O A true area, A , = 0.14 f 0.004 cm2, was obtained for the Pt bead electrode. (7) Adams, R. N. Electrochemistry at Solid Electrodes; Marcel-Dekker: New York, 1969; Chapter 9. (8)(a) Scortichini, C. L. Ph.D. Thesis, University of North Carolina, Chapel Hill, NC, 1982. (b) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal. Chem. 1980,107, 205. (9) Biegler, T.;Rand, A. J.; Woods, R. J.Electroanal. Chem. 1977,29, 269 and references therein. (b) Yeager, E.; Bockris, J. OM.; Conway, B. E.; Sarangapani, S.;Eds. Comprehensive Treatise of Electrochemistry; Plenum: New York, 1984; Vol. 9. (10) Facci, J. S.;Murray, R. W. JElectroanal. Chem. 1980,112, 221.
Facci
Figure 2. Cyclic voltammetric curves (steady state) at 20, 50, and 100 mV/s for the adsorption of I in H2S04: (A) 0.08 pM I in H2S04yields r = 6.4 X lo-" mol/cm2 S = 1.4 pA/cm2. (B) 0.2 pM in HzSO4 I, 'I = 1.4 X mol/cm2;S = 3.8 wA/cm2. Asterisk indicates shoulders for "wave A".
A roughness factor RF = 1.11was found based on an estimated projected area of 0.13 cm2. For Pt disk and foil electrodes, 1.3 C RF < 1.4. Following the cleaning procedure, K1 was added (electrode poised a t 0.2 V) to the electrolyte in sufficient quantity to bring the concentration of iodide ion to ca. 10 pM. This resulted in a monolayer of adsorbed iodine atoms" whose formation was confirmed via XPS measurements on Pt foils. As shown by the dashed-dotted curve in Figure 1 Pt/I electrodes in 1 M H2S04and 1 M HClO, exhibit a greatly reduced background current relative to unmodified Pt electrodes. Platinum oxide formation and rereduction reactions and underpotential hydrogen deposition reactions are also greatly suppressed at Pt/I between +OB5 and -0.2 V, the potential window of interest. Adsorption of the redox surfactant is conducted in a separate cell initially containing pure surfactant-free electrolyte. Increments of I are then dissolved in the electrolyte by adding microliter quantities of a dilute methanolic solution of I to the electrolyte. Mixing is accomplished by gentle Ar bubbling. I is found to adsorb rapidly and irreversibly upon immersion of Pt/I into the surfactant solution. Figure 2A illustrates the cyclic voltammetric waves for a Pt/I bead electrode at which I is adsorbed (designated I/H2S04throughout the rest of the paper) from a 0.060 pM solution in 1 M H2S04. The voltammetric waves are symmetrical about the formal potential E"' = 0.52 V and exhibit a peak potential splitting AE, = 10 mV. Such small peak potential splittings were common to all electrodes coated at monolayer and submonolayer levels. The charge under the anodic and cathodic current branches is equal and corresponds to a coverage r = Qa/nFA,= 3.3 X 10-l' mol/cm2. At all sweep rates examined (u 5 200 mV/s), the anodic and cathodic peak current were linear ( r = 0.9995) with sweep rate. This unambiguously demonstrates that the faradaic current results only from surface-confined ferrocene. Increasing the concentration of I in the 1 M H2S04 solution results in a higher ferrocene coverage. Surprisingly, however, the appearance of the surface voltammetric waves for I a t higher r is altered from the ideal shape shown in Figure 2A. This is exemplified by the voltammetric waves at several sweep rates in Figure 2B (r = 1.5 X 10-lomol/cm2). These voltammograms show the presence of a second wave ("wave A") characterized by anodic and cathodic shoulders at 0.42 V superimposed on the (11) (a) Hubbard, A. T. Acc. Chem. Res. 1980,13, 177. (b) Garwood, G. A.; Hubbard, A. T.Surf. Sci. 1980,92, 617. (c) Soriaga, M. P.; Hub1982, 104,2742. bard, A. T. J. Am. Chem. SOC.
Langmuir, Vol. 3, No. 4, 1987 527
Modification of Pt Electrode Surfaces
Table I. Summary of Absorption Parameters for the Adsorption of I at Pt/I Electrodes g" KO, M-I A.(AGoadS)* AGoR,adre AG
electrolyte 1 M H2S04
1 M HCIOl
-2.84 f 0.08 -1.78 f 0.20
(3.2 f 0.4) X lo6 (1.6 f 2) X lo6
-3.7 -2.0
Or,ad:
-5.2 -6.6
-8.9
-8.5
ported by the fact that in a medium such as 0.1 M TEAP/CH,CN, where I and I1 are not expected to adsorb, both compounds exhibit the same formal potential, E"' = 0.60 V (for reference, ferrocene E"' in 0.1 M TEAP/ CH&N is 0.36 V). The difference AE"' between the formal potential of adsorbed I (I/H2S04and I/HC104) and the formal potential of dissolved I (taken to be E"' of 11) may be interpretedl, in terms of the reaction scheme in eq la-c, Rsoln
-
Rads - eFigure 3. Steady-statecyclic voltammetric curves for the adsorption of I from 1 M HC104: (A) 50,100,200 mV/s; 0.27 pM I yields r = 6.4 X lo-" mol/cm2; S = 7.1 pA/cm2. (B) 20, 50, 100 mV/s; 3.7 pM I yields r = 3.5 X mol/cm2; S = 14.3 pA/cm2. Asterisk indicates location of "wave B".
main wave which has, a t this higher coverage, shifted to a more negative potential, +0.48 V. When transferred to surfactant-free 1M H2S04,I/H2S04electrodes in Figure 2A,B display no loss in coverge for many cycles. The excellent stability of these films in the reduced state is thus demonstrated. Extended cycling (15 000 cycles) results in a loss of ca. 70% of film electroactivity due to the slow reaction of the ferrocenium cation with water.12 Films of I similarly prepared by adsorption from 1 M HC104 surfactant solutions (I/HC104) result in somewhat different voltammetric behavior. The cyclic voltammetric waves for an electrode coated a t a submonolayer level (6 X 10-l1 mol/cm2) are illustrated in Figure 3A. The formal potential of the monolayer wave of I/HC104, 0.44-0.45 V, is 70-80 mV more negative than that of I/H2S04 a t a similar coverage level. Voltammograms at several sweep rates for I/HC104 a t a higher coverage (3.2 X mol/ cm2) are shown in Figure 3B. The figure shows the presence of a second anodic peak ("wave B")which has no cathodic counterpart. The charge under the cathodic wave, however, is equal to the charge under both anodic waves. An equilibrium apparently exists between two forms of adsorbed I a t the higher coverages. The peak potential of wave B is 60 mV positive of the formal potential of the main (monolayer) wave. As expected, the main wave (in 1 M HC104) a t 0.45 V exhibits peak currents which are linearly dependent on sweep rate. Wave B, however, shows a sublinear dependence of the sweep rate (log i vs. log u plots have a slope of 0.75 between 5 and 50 mV/s). Note that wave B almost disappears at 100 mV/s but is prominent a t 20 mV/s. The E"' of the short-chain analogue, (ferrocenylmethy1)trimethylammonium hexafluorophosphate (11),at Pt in 1 M H2S04and 1 M HC104 is 0.36 V. The formal potential of the low-coverage I/H2S04 and I/HC104 monolayer waves is therefore positive of this value by 160 and 90 mV, respectively. The E"' of I1 is taken to be the formal potential, of the nonadsorbed trialkyl(ferroceny1methy1)ammonium molecule. This assumption is sup(12) Szentrimay, R.;Yeh, P.; Kuwana, T. In Electrochemical Studies
of Biological Systems; Sawyer, D.
Washington, DC, 1977.
T., Ed.; American Chemical Society:
Rads oxads
(la) Ob)
(IC) Oxads OXsoln where R and Ox are respectively the reduced and oxidized forms of I and soln and ads refer respectively to solution and adsorbed forms of I. Denoting the free energy of adsorption of R and Ox from the solution (bulk) state by AG"R,"~and alows expressing AE"'in terms of the oxidation half-wave potentials Eo'adaand Eo'soln as shown in eq 2. AGOand AGOqada refer to eq l a and the reverse Ah'"'= EO'soln - E"'& = (AG"R,ads - AG"ox,ads)/J' (2)
of eq IC.Taking Eo'solnof I in 1M H2S04 and 1M HC104 as 0.36 V, one finds AE"' = -0.16 V and A(AG"ads) = AGoR,ads - AGOOx,ads = -3.7 kcal/mol for I in H2S04. Similarly A(AG"ads) = -2.0 kcal/mol in HC104 (see Table I). From this it is seen that R is more strongly adsorbed than Ox in both electrolytes. This may be rationalized in terms of a greater solubility of Ox in water due to its higher charge density and to greater electrostatic repulsion between more highly charged ferrocenium head groups. Similarly, Saji et al.lademonstrated disruption of micelles of the CI2 analogue of I, dodecyl(ferrocenylmethy1)dimethylammonium cation, in aqueous electrolyte by faradaically increasing the charge state of the surfactant. Micelle disruption was attributed to greater solubility and increased charge repulsion between the oxidized head groups. Thus, the surface waves in Figures 2 and 3 may be regarded as "postwaves" (solution wave negligibly small). A similar conclusion was reached for L-B monolayers of I transferred to hydrophilic Au surface^.^ For that system E"' = 0.50 V in 0.1 M Na2S04and A(AGoah) = -3.2 kcal/mol. Adsorption Isotherm Behavior. In order to better understand the nature of the additional waves, isotherms for the adsorption of I on P t / I from 1 M H2S04and from 1M HC104 were obtained along with parallel data on the coverage dependence of contact angle, formal potential, and peak width (Ehh). Isotherm data were collected both serially (without washing between incremental additions) and on renewed Pt/I surfaces (surfactant washed off between runs). Significant differences were not observed. Figure 4 demonstrates that adsorption depends strongly on the nature of the electrolyte anion. Curve A presents the isotherm data for the adsorption of I from 0.010-2.3 pM solution in 1M H2S04. Several features are pertinent. (13) (a) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, Fundamentals and Applications; Wiley: New York, 1980; Chapter 12. (b) Hupp, J, T.; Weaver, M. J. J.ElectroanaL Chem. 1983, 145, 43.
528 Langmuir, Vol. 3, No. 4 , 1987
Facci
i11 iilrM) Figure 4. Isotherms for the adsorption of I from 1 M H2S04(A) and 1 M HC104 (B). Inset shows an enlargement of the boxed area of curve A.
As the concentration is increased from 0.010 to 0.10 pM, a steep rise in surface coverage from 0.2-0.3 X to rl = 9.8 X mol/cm2 is observed (the plateau coverage is denoted rl). A short reproducible plateau at rl exists in which further addition of I yields no systematic increase in surface coverage. A second steep increase in coverage mol/cm2 results when from ca. 1 X to ca. 2 X the concentration of I is incremented from 0.15 to 0.30 pM. At this point, a second long plateau is observed which corresponds to a limiting coverage rz= 2 X mol/cm2, where r2 N 2rl. In 1 M HC104 (Figure 4B), a rather different isotherm behavior is obtained. As compared with the steep adsorption profile seen in HzS04,adsorption from 1M HC104 requires relatively higher solution concentrations to obtain the same coverage level in H2S04. The onset of the first saturation coverage in Fig. 4B, also at rl = 9.8 X 10-l’ mol/cm2, is delayed until ca. 0.4-0.5 pM, implying the adsorption driving force for the reaction Rsoln Radsis comparatively weaker in HC104. The first plateau in HCI04 also spans a comparatively larger range of solution concentrations. A long shallow increase in coverage is observed between 1 and 3 p M at which point a second mol/cm2 is attained. plateau at r3= 3.7 x rl = 9.8 X lowlomol/cm2 corresponds to a molecular area of 169 A2/molecule. A molecular area of 160 A2 was measured for Dreiding molecular models of I in an extended molecular configuration. Monolayer coverage with flatly adsorbed I is completed on Pt/I at I’ = Fl for adsorption from both 1 M HzS04 and 1 M HC104. The second coverage step in Figure 4A is very similar to the first (recs! 2r,) and therefore it appears reasonable to assume that in H2S04a second monolayer is deposited on top of the first. The hydrocarbon tails in the overlayer are also assumed to be flatly oriented on the basis of the differential coverage r2- rl. It is significant that the threshold for the observation of the shoulders designated “wave A” in 1 M H2S04is rl. Below this coverage only the monolayer wave is observed. Between and rZ,the height of wave A becomes more pronounced and the peak current for two waves become approximately equal at r2. Wave A is, thus, assigned to the second stage of adsorption from 1 M HzS04. Other studies of the adsorption of hydrocarbon amphiphiles at electrodes have also indicated a similar mode of adsorption. Recent evidence suggests that the first monolayer of calcium stearate transferred via the L-B technique to evaporated Au films from the air/water interface appears to orient flatly on the surface.ld This conclusion was based on a quartz crystal microbalance determination of the mass of material transferred to the Au-coated quartz crystal. In addition, T-A isotherms of stearic acid a t the air/Hg interface demonstrated that stearic acid occupies a large molecular area on mercury,
-
consistent with the molecule lying flat on the Hg surface.le In a fashion similar to adsorption from H2S04,a closepacked monolayer of flatly oriented I is adsorbed at P t / I at the rl plateau in HC104. The coverage on the second plateau, r3= 45 A2/molecule, corresponds closely to the area of the ferrocene head group. Molecular models of I indicate a cross-sectional molecular area of 49 f 9 A2/ molecule due to the ferrocene head group. In addition, the molecular area of I in a close-packed monolayer at the air/O.l M Na2S04interface was previously obtained from a T-A isotherm measurement and was found to be 51 A2/molecule.4 This suggests that the long transition from rl to r3results from a molecular orientation change from lying flat to “end on” (not necessarily perpendicular to the surface). The charged polar head group is assumed to be oriented toward the polar aqueous phase. This is supported by noting the resultant surface was hydrophilic. The threshold for the appearance of wave B is rl. As wave B becomes increasingly pronounced, between rl and r3, the original monolayer wave diminishes. It is proposed that the amphiphilic ferrocene molecules giving rise to wave B form a surface micellar phase in equilibrium with more a loosely held “monomer” phase which gives rise to the anodic wave at 0.45 V. As r increases (Le., solution concentration increases), the equilibrium at the surface is shifted toward the micellar phase (see eq 3). Since the appearance of the cathodic wave remains unchanged with r, it is postulated that oxidation of ferrocene corresponding to wave B results in the disruption of the micellar phase. Similarly, the functional surfactant dodecyl(ferroceny1methyl)dimethylammonium cation forms micelles in millimolar aqueous solution which are disrupted when oxidized to the ferrocenium state.la The equilibrium between micellar and “monomer” forms is also manifested in the sweep-rate dependence of the ratio of the two anodic waves. As the sweep rate decreases, wave B becomes more pronounced so that at very low sweeps ( < l o mV/s) and I’ r3,the wave at 0.45 V is almost entirely absent. During steady-state fast sweeps (e.g., Figure 31, there is insufficient time for reformation or reorganization of the disrupted micellar phase (Imic) and the film behaves as the monomeric phase (Imon), At steady state and longer time scales reformation of Imicis favored and peak currents for the disruption of Imicare relatively larger. The process for the anodic reaction may therefore be described by eq 3, where I,, is monomeric I and I’ is the oxidized form.
-
C *I,
Inlo,
(3)
Since the voltammetry of Pt in 1 M HC104 showed well-resolved underpotential hydrogen deposition waves,Sb the steeper rise in the adsorption profile in H2S04relative to HC104 likely stems from the charge difference in the electrolyte anions and not adventitious impurities in the HC104 electrolyte. Although the dominant anion in 1 M H,SO, is HS04- the ca. 10 mM S042-present from dissociation of HS04- may have a relatively greater influence on the shapes of the isotherms. Multiply charged anions may electrostatically compensate two adsorbate molecules whereas C104- may charge compenste only one. Thus a possible mechanism exists in H2S04 for the electrostatic attraction/binding of two molecules of I which is not possible in perchlorate electrolyte. Counterion effects on the stability of ionized surfactant films adsorbed at the air/water interface have long been For example, ionized n-octadecylammonium monolayers at the air/water interface are stabilized by the presence of SO4’-
Langmuir, Vol. 3, No. 4, 1987 529
Modification of P t Electrode Surfaces
J
c 4 -
-I
-
3
I00
2c3
, i
303
CONCENT9AT:CN
400
.
I
I
50C
0.40
(IM)
Figure 5. Fits of the Frumkin isotherm (-) to the adsorption isotherm data at r 5 rl for adsorption of I from 1 M H2SO4 ( 0 ) and 1 M HC104 (m).
relative to C1M H2S04);i.e., adsorption a t the interface is enhanced by multiply charged counterions.14 It is believed that a similar situation exists at the I/H2S04 interface. Somewhat analogous is the electrostatic crosslinking of polyelectrolyte films on electrodes by Donnan partitioning of multiply charged redox counterions into the fi1m.l5 In order to better understand intermolecular interactions in the submonolayer regions of curves A and B (r < F1), an attempt was made to fit the data to the Langmuir equation, eq 4, where 6 is defined in the present context 6 ( l - 6) - 1 = KC (4) as F/rl, K is the equilibrium constant for the reaction Rsoh + Rads,and c is the solution concentration. Plots of 6 / ( l - 6) vs. concentration were nonlinear, increasing monotonically with concentration. This demonstrates the existence of strong intermolecular attractive interactions. A better measure of intermolecular interaction was obtained by assuming a coverge-dependent equilibrium constant which varies exponentially with increasing coverage as embodied in the Frumkin isotherm,13 eq 5. In eq 5, the
KOC = [r/(r8- r)]exp(2grlRT) (5) saturation coverage rs = rl,KO is the equilibrium constant at zero coverage, and g is a measure of the coverage de-
pendence of the adsorption energy. Our results are expressed as g'values, where g' = 2grs/RT. Negative values of g ' signify attractive intermolecular interactions and vice versa. The Frumkin isotherm fit of eq 5 to the submonolayer data in Figure 4A,B was significantly better than a Langmuir isotherm fit and is shown in Figure 5. The data for adsorption from H2S04was best fit by KO = (3.2 f 0.4) X lo6 M-l and g' = -2.84 f 0.08 while adsorption from HC104was best fit by K" = (1.6 f 0.2) X lo6 M-l and g' = -1.78 f 0.20. These results are summarized in Table I. The equilibrium constant for the reaction Rsoln Rads is twice as large for adsorption from H2S04and interestingly g' is distinctly more negative. The larger KO for the I/H2S04adsorption reaction is likely due to the greater insolubility of the surfactant in 1 M H2S04due to the ca. 10 mM S042-as discussed above. The distinctly greater associative interactions may also be attributed to the trace presence of sulfate. Using the KO values above, one finds the free energies of adsorption for Oxsoln Oxadsfrom 1
-
-
(14) Davies, J. T.Proc. R. SOC.London, A 1951, 208, 224. (15) (a) Oyama, N.;Anson, F. C. J.Electrochem. SOC.1980, 127,247. (b) Oyama, N.;Shigehara, K.; Anson, F. C. Znorg. Chem. 1981, 20, 518. (c) Facci, J. S.; Murray, R. W. J. Phys. Chem. 1981, 85, 2870.
I
0
1.o 10'0 x
I
2.0 I'(rnole/cd)
3.0
Figure 6. Variation of Eo' with coverage for the monolayer wave of films of I adsorbed from 1M H2S04(A) and 1 M HC104 (B). A cathodic shift is observed in H2S04for r I rl. No systematic variation is seen for I adsorbed from HC104.
1
1
1 0 ' 0 ~I' (mole/cm2)
Figure 7. Variation of Efwhm with r for the monolayer wave of films adsorbed from 1 M H2SO4 (A) and 1 M HC104 (B). A broadening of the wave is observed in H2S04for r 5 rl in H2S04. No systematic variation is evident in HC104.
M HzS04and 1 M HC104 are -5.2 and -6.6 kcal/mol, respectively (see Table I). It can therefore be concluded that the greater value of A(AGoads) for adsorption from 1 M HzS04is not due so much to the stabilizing influence of Sod2-on the adsorption of R but more to the greater tendency for adsorption of the perchlorate salt of Ox. Coverage-Dependent Cyclic Voltammetric Behavior. Although the submonolayer (r < I?,) voltammetric waveshapes are nearly theoretical, an unexpected dependence of Eo' and Efwhm on r and electrolyte anion was observed. For example, in H2S04,E"' smoothly shifts negatively from +0.53 to +0.48 V as r increases from 1 X lo-" to 1 X mol/cm2 (Figure 6, curve A). For all r > rl,E"' of the monolayer wave remains at +0.48 V. Thus from eq 2, the adsorption energy A(AGoads) calculated for the first monolayer varies from 3.9 to 2.8 kcal/mol as r rises from 0 < r < rl. The adsorption energy for shoulders corresponding to wave A at +0.42 V (r,< r < r2)is found to be 1.4 kcal(mo1. In HC104 the formal potential of the monolayer wave remains at 0.44-0.45 V (A(AG"ads) = 2.0 kcal/mol), independent of surface coverage as shown in Figure 5B. When r > rl,wave B grows in at 0.48-0.49 V, independent of coverage (A(AG"ads) = 2.8 kcal/mol). Figure 7 shows the dependency of Ehhmof the cathodic wave of I/H2S04and I/HC104. Specific anion effects are again observed. In H2S04(Figure 7A), Ehh rises smoothly from a near ideal value16 of 94 mV at the lowest coverage mol/cm2. mol/cm2) to 150 mV at ca. 1 X (1X Estimates of Efwhm at higher coverages are made difficult (16) Brown, A. P.;Anson, F. C. Anal. Chem. 1977, 49, 1589.
530 Langmuir, Vol. 3, No. 4, 1987
V
Figure 8. Cyclic voltammetric waves (steady state) of films mol/cm2 I in transferred to alternate electrolytes: (i) 1.8 X 1 M HZSO,; (ii) transferred to 1 M HClO,; S = 3.8 rA/cm2, 50 mV/s. (iii) 3.2 X mol/cm2 I in 1 M HCIOl transferred to 1M H2S0,; (iv) 3.1 X mol/cm2. S = 11.9 ,uA/cm2, 100 mV/s.
by the overlapping wave A but appear to remain constant at 150-160 mV. Interestingly, the knee in curve B is observed at r = r l , analogous to the variation of Eo’ in H2S04 (Figure 6A). Efwbin HCI04 (Figure 7B) shows no significant systematic dependence on r. Peak widths of surface voltammetric waves invariably differ from the theoretically predicted 90.6/n mV13adue to surface activity effects. Ehhm< 90.6 mV, as found above, implies a “repulsive” interaction between electroactive sites.l8 Negligible electroactive center interactions (for which E h h = 90.6/n mV) are found only at the zero-coverage limit for I/HCI04 surfaces. Intersite repulsions are also minimal for films of I in HC104 where Efwhm = 105 & 10 mV. Although it is not possible to unambiguously interpret these results a t a molecular level, they would be consistent with a model in which adsorbate molecules are randomly positioned on the surface. In contrast, Efwbis generally higher in H2S04than in HC104, indicating a larger average intermolecular repulsive interaction. This may arise if the ferrocene moieties were brought into greater physical proimity via the electrostatic binding mechanism mentioned above. It must be remembered that in this context 9epulsiven refers to interactions between ferrocene sites and perhaps electrode-ferrocene interactions (reaction l b ) which are not understood in detail, whereas the “attractiveninteraction implied by the values of g’ are dominated by hydrocarbon tail-tail (reaction la) and Pt/I-tail interactions. Contact angles of a 10-clL droplet of clean (surfactantfree) electrolyte on surface-modified P t / I disk electrodes
Facci were estimated at various coverages for I/HC104. A freshly prepared unmodified Pt/I disk electrode exhibits a contact angle of 70-80’. A small increase in contact angle to 80-90’ is obtained when a 10-pL droplet of 1 M H$04 is placed on a P t / I surface containing 6 X mol/cm2 of I (6 = 0.65) adsorbed from H2S04. This is expected since the surface contains flatly oriented molecules which expose a large hydrophobic area of the molecule (this was also found to be true of I/HC104 electrodes). When r = Tz, the surface unexpectedly becomes markedly hydrophilic and the contact angle decreases to 20-30’. This points out that at rz,the overlaying monolayer may not be identical with the first (i.e., lying flat). Since the resultant surface exposes primarily the hydrophilic ammonium groups to the aqueous phase, the second layer may exist in an alternate configuration. Electrolyte-InducedSurface Reorganization. Films of I are irreversibly adsorbed (desorption is slow on the cyclic voltammetric time scale); they may be removed from the surfactant-containing electrolyte and transferred to a separate electrolyte solution containing the same anion but no surfactant with no loss in coverge (persistent immersion to an innocent electolyte ultimately results in desorption, however). Further, the cyclic voltammograms of I/H2S04 and I/HC104 are surface-morphology specific. Film structural reorganization thus becomes easily evident when a film prepared by adsorption from H2S04is transferred to pure 1 M HC104. A t > rl,for example, the cyclic voltammogram characteristic of I/H2S04immediately (on the first cycle) becomes that of I/HCI04 (Figure 8, curves i and ii, respectively). If the electrode of Figure 8, curve ii, is transferred back to H2S04,the voltammetry remains that associated with I/HC104. In contrast when a film is prepared by adsorption from HC104 and transferred to pure 1 M H2S04the voltammetry remains unchanged (Figure 8, curves iii and iv, respectively). Thus interconversion between the two surface structures does not occur. The “two-monolayer”structure of I/H2S04is easily converted to the mixed-micellar/monomer film structure at coverages l? > rl. However, the micellar structure of I/ HC104 does not convert to the two-monolayer I/H2S04 structure a t any coverage. Apparently the activation energy for the latter process is prohibitive. Similarly, the activation energy for the dissolution or reorganization of L-B mnolayers of I at Au4 and fatty acid monolayers at the airlwater interface14 was postulated to be high in a close-packed monolayer because of cohesive van der Waals interactions between hydrocarbon tails.
Acknowledgment. Helpful discussions with Dr. T. W. Smith are gratefully acknowledged. ESCA measurements made by Dr. D. H.-K. Pan are also appreciated. Registry No. I, 104716-47.4; Pt, 7440-06-4; I,, 7553-56-2.
