5245
J. Phys. Chem. 1991, 95, 5245-5249
Anodlc Underpotential Depodtlon and Cathodic Stripping of Iodine at Polycrystalline and Slngie-Crystal Gold. Studies by LEED, AES, XPS, and Electrochemistry Beatriz C. Bravo, Susan L. Michelhaugh, Manuel P. Soriaga,*it Department of Chemistry, Texas A&M University, College Station, Texas 77843
Ignacio Villegas, D. Wayne Suggs, and J o b L. Stickney* School of Chemical Sciences, University of Georgia, Athens, Georgia 30602 (Received: October 3, 1990)
The oxidative chemisorption and cathodic stripping reductive desorption of iodine have been compared at smooth polycrystalline and well-defined Au( 1 1 1) singlacrystalelectrodes. Experimental measurements were based upon cyclic voltammetry, thin-layer coulometry, X-ray photoelectron spectroscopy, Auger electron spectroscopy, and low-energy electron diffraction. The results indicate that iodide is oxidatively adsorbed as zerovalent atomic iodine at potentials between -0.4 V and +0.4 V (Ag/AgCl reference); at lower potentials, surface iodine is reductively desorbed as aqueous iodide, while at considerably more positive potentials, it is oxidized to aqueous iodate. Studies with the Au( 1 1 1) electrode in dilute aqueous CsI solutions showed ordered adlayer structures at the selected potentials investigated. Below -0.4 V, the potential at which oxidative deposition of iodine starts to occur, a distinct (4x4) quarter-coverage CsI layer (0, = rCr/rAu = O1 rl/rAU 0.25) was formed. At -0.4 V < E < -0.2 V, increased to 0.33; this increase was coupled with the loss of adsorbed Cs, and the structure of this adlattice was Au( 111)(v'3Xt/3)R30°-I. At E > -0.2 V, the I coverage reached 0.4, a value made possible by a compression of the original (d3Xt/3)R30° structure in one dimension to form a nearly hexagonal iodine adlattice with a ( 5 x 4 3 ) unit cell. The amount of adsorbed iodine continues to increase as the potential is made still more positive until the surface is saturated with a monolayer of close-packed I atoms of coverage limited by van der Waals interactions; additional iodine forced into the already space-limited interfacial layer only leads to the formation of molecular iodine, which is evolved into the solution as I,(aq). The oxidative chemisorption process may be thought of as the oxidative underpotentialdeposition of I atoms, while the reductive desorption reaction may be viewed as the cathodic stripping of iodide ions.
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Introduction The interaction of surface-active supporting electrolyte with the electrode surface has always been regarded as an important aspect in electrochemical surface science. Adsorption-induced changes in the chemical and electrochemical characteristics of these materials, for example, provide valuable insights into the interfacial coordination' properties of the electrode surfam? such properties, in turn, may help formulate a molecular-level understanding of the critical interfacial events that control a given electrocatalytic reaction. The chemisorption of species derived from the iodide ion at typical solid-metal electrodes provides an interesting case study since the strong surface activity of this ligand brings about profound changes in the redox chemistry of the iodine/iodide couple in the surface-boundstate? This system is also important because: (i) the iodide ion can be used as a model for a strongly surfaceactive electrolyte, and (ii) the kinetics of redox processes of unadsorbed species can be altered when the working-electrode surface is pretreated with a full monolayer of iodine.) Spectroscopic and electrochemical studies of iodine chemisorbed at polycrystalline (Rh, Pd, Ir, Pt, and Au3v4),single-crystal (Pt5and Pd6), and bimetallic (Ag-on-Pt and Au-Pt') electrodes have been published. In this paper, we report results from recent studies that compare the surface chemical and electrochemical properties of chemisorbed iodine at smooth polycrystalline Au and at a well-defined Au(l11) single crystal. A study of this nature is important because, while it is realized that the use of single-crystal surfaces is essential in understanding electrocatalytic phenomena at the atomic level!$ it remains imperative to correlate results obtained from model (singlecrystal) surfaces with those from real (polycrystalline) electrodes.
Experimental Section Experimental measurements for the polycrystalline material were based upon thin-layer electrochemical (TLE) methods and X-ray photoelectron spectroscopy (XPS); experiments with the Au( 1 1 1) single-crystal electrode employed Auger electron spec-
* To whom correspondence should be addressed. 'NSF Presidential Young Investigator. 0022-3654/91/2095-5245$02.50/0
troscopy (AES), low-energy electron diffraction (LEED), and cyclic voltammetry. XPS and AES provide information on the chemical composition at the interface; XPS also gives data relating to the oxidation states of the elements at the surface.'O Information on adsorbate unit cells formed and the long-range crystallographic order of both adsorbate and suhstrate can be obtained with LEED.l0 The fabrication of thin-layer electrochemical cells was as described previously." The preparation of the polycrystalline Au electrodes included metallographic polishing, etching in hot concentrated HNO,,and high-temperature treatment to generate a smooth surface.'* Between experimental trials, the polycrystalline Au electrodes were cleaned by sequential oxidation (at potentials just below that for oxygen evolution) and reduction (at potentials well within the hydrogen evolution region). Surface cleanliness was verified initially by cyclic voltammetry. Au rods were used for the TLE experiments while foils were employed for (1) (a) Muettertis, E. L. Bull. Soc. Chim. Belg. 1975,84959, (b) Albcrt, M. R.; Yates, J. T. A Surface Scienrisr's Guide ro OrganometallicChemimy; American Chemical Society: Washington, DC, 1987. (c) Hoffman, R.
Surfaces and Solids; VCH Publishers: New York, 1989. (2) Soriaga, M. P. Chem. Rev. 1990, 90,771. (3) Berry, G.M.; Bravo, B. G.; Bothwell, M. E.; Cali, G. J.; Harris, J. E.; Mebrahtu, T.; Michelhaugh, S.L.; Rodriguez, J. F.; Soriap, M. P. Lungmuir 1989, 5, 707. (4) Rodriguez, J. F.; Soriaga, M. P. J . Electrochem. Soc. 1989, 135,616. (5) Lu, F.; Salaita, G.N.; Baltruschat, H.; Hubbard, A. T. J. Electrmnal. Chem. 1987,222,327. ( 6 ) Rodriguez, J . F.; Mebrahtu, T.; Soriaga, M. P. 1. Elecrrounal. Chem. 1989, 264, 291. (7) Cali, G.J.; Harris, J. E.; Bothwell, M. E.; Soriaga, M. P. hngnrulr 1990, 6, 74. (8) Faulkner, L. R. Characterizarion of Electrochemical Processes; National Research Council: 1985. (9) (a) Hubbard, A. T. Chem. Rev. 1988,88,633. (b) Yeager, E. Surf. Sci. 1980, 101, 1. (IO) (a) Somorjai. G. A. Chemistry in 7'wo Dimensions: Surloccs; Cornell University Press: Ithaca, NY, 1981. (b) Ertl, G.;Kuppers, J. Low Energy Electrons and Suflacc Chemistry: VCH Publishers: New York, 1985. (11) Hubbard, A. T. Crit. Reo. Anal. Chem. 1973, 3, 201. (12) Rodriguez, J. F.; Mebrahtu, T.; Soriaga, M. P. J. E l e ~ t m ~Chem. l. 1987, 233, 283.
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5246 The Journal of Physical Chemistry, Vol. 95, No. 13, 199'I
the XPS measurements; in both cases, the bulk Au purity was a t least 99.99%. The active surface areas of the polycrystalline electrodes were determined by the iodine chemisorption method;I2 the roughness factor of the test surfaces averaged less than 1.1. Determination of the iodine coverage by TLE has been discussed in detail elsewhere.'J2 Briefly, the method involves oxidation of the surface iodine to aqueous IO< followed by coulometric reduction of the 1O3-(aq) to aqueous 12.4J2 XPS measurements were carried out with a Hewlett-Packard 5950A ESCA spectrometer equipped with an AI Ka X-ray source and a hemispherical electron energy analyzer. The angle of incidence was 52O, the collection angle 38O. The beam power was 800 W, which did not damage the adsorbed I layer as ascertained by comparison of pre- and post-XPS electrochemical measurements. Data acquisition and processing were performed with software developed by Surface Science Laboratories. Binding energies were referenced to the Au Fermi level and calibrated with the 4f7p photoelectron peak at 83.9 eV. The intrinsic resolution of the AI K a anode is 0.8 eV. The Au single crystal, prepared according to the Bridgeman technique, was a disk 4 mm in diameter and 2 mm in thickness. It was oriented by Laub back-reflection to yield the (1 11) surface plane on both the front and back faces. The faces were metallographically polished with successively finer grades of diamond paste. Studies with this single crystal required the use of an ultrahigh-vacuum surface spectroscopy apparatus to which an ambient-pressure chamber was interfaced for electrochemical e~periments.~'Only LEED and AES were used for the present investigations with the Au( 111) single-crystal electrode. The antechamber was back-filled and continuously purged with oxygen-scrubbed, ultrahigh-purity Ar during the electrochemical experiments. The single crystal was initially cleaned by several cycles of Ar+ ion bombardment ( 5 X Torr and 15 pA ion current) and annealing (750 K a t 1 X lo4 Torr). Surface cleanliness was verified by the low background in the LEED pattern and the absence of peaks other than those of Au in the Auger spectrum. The atomically smooth (1 11) crystallographic plane surfaces were established by the appearance of distinct (1 X 1) LEED patterns of hexagonal symmetry, although the spots are slightly broadened by the (1 X23) reconstruction of the clean Au(ll1) surface.14 Surface coverages were derived from a combined analysis of AES and LEED data, as discussed in detail e l ~ e w h e r e . ' ~ + The '~ electrochemical cell consisted of a small glass cup divided into two compartments, one for the working and auxiliary (Pt) electrodes, the other for the reference electrode (Ag/AgCl in 10 mM Cl-). Electrolytic solutions were prepared with pyrolytically triply distilled wateri7 and deaerated for at least 30 minutes with ultrahigh-purity Ar. Analytical-grade reagents were used. CsI was chosen instead of KI since the Auger peak for K overlaps that for Au.
Results and Discussion Figure 1 shows X-ray photoelectron spectra of clean and Icoated polycrystalline Au. For these experiments, the Au foils were coated with iodine by either of two methods: (i) high-tcmperature annealing in a gaseous N2environment saturated with I2 vapor; (ii) immersion in aqueous 1 mM KI. It is important to note in Figure 1 that (i) the iodine 3d,/2 and 3d3/?XPS peaks for the KI(aq)-immersed electrode are identical with those for the 12(g)-annealed surface, and (ii) no peaks attributable to K are observed for the KI(aq)-exposed electrode. These results (13) Stickney, J. L.; Ehlers, C. B.; Gregory, B. W. In Electrochemical Surface Science; Soriaga, M.P., Ed.; ACS Symposium Series 378; American Chemical Society: Washington, DC, 1988; p 99. (14) (a) Nakai, Y.;Zei, M. S.; Kolb, D. M.;Lehmpfuhl, G. Eer. Eunsen-Ges. Phys. Chem. 1984, 88, 340. (b) Melle, H.;Menzel, E. 2.Nafurforsch. 1978, 33A, 282. (IS) Stickney, J. L.; Ehlers, C. B.; Gregory, B. W. Langmuir 1988, 4, 1368. (16) Schoeffel. J. A.; Hubbard, A. T. Anal. Chem. 1977, 49, 2330. ( I 7) Conway, B. E.;Angerstein-Kozlowska,H.; Sharp, W. B. A.; Criddle, E. E. Anal. Chem. 1973, 45, 1331.
Bravo et al.
1
I " " " " " " " ' I Au4f
83
93
I
t
I
618.4 A
630.0
1
Au
I 610
620
630
640
Binding Energy/eV
E - 1. X-ray photoelectron spectra for clean and iodine-coated smooth polycrystalline Au foil electrodes exposed to aqueous 1 mM KI solution or heated to 800 K in a stream of N, gas saturated with 1, vapor. The top spectrum shows the Au 4f5 and 4f7,2peaks, whereas the bottom spectrum details the I 3d3,, and3dsi2 peaks. Experimental procedures and conditions were as described in the text.
,
indicate that I-(aq) undergoes spontaneous oxidation upon chemisorption to form a layer of zeroualent I atoms the composition of which is identical with those formed from direct dissociative chemisorption of 12(g). Although it is possible for an adsorbed ion to be screened by its image charge, a t or below the potential of zero charge (pzc) this screening may be inefficient; hence, a counterion would be retained when an anion is specifically adsorbed.'" The oxidation of I-(aq) to adsorbed I atoms is accompanied by evolution of a stoichiometric amount of H2(g) in protic solutions. It is also important to point out that the Au 4fSl2and 4f712XPS peaks are unchanged after pretreatment with iodine. The binding energy EB data are more meaningful when compared with values obtained from reference compound^.'^ For example, EB for metallic Au is 83.8 eV, whereas EB for Au in AuCl, in which the Au is in the +1 oxidation state, is 86.2 eV. EB for Au in the Au-I(ads) interface presently studied is 83.9 eV; evidently, the surface Au atoms retain their zerovalency character after iodine chemisorption. With respect to the chemisorbed I atoms, reference and experimental EB values for I- and 12(multilayer)support the suggestion made above that iodine is adsorbed in the zerovalent state. For example, EB for I 3d,,, in CsI and in KIO4 are 618.0 and 624.2 eV, respectively. In comparison, values measured for Au-I(ads), 618.4 eV, fall in the range 624.2 eV > EB > 618.0 eV. These results, that the metal surface and the adsorbed iodine atoms are in their zerovalent states, are in agreement with those for other metal surfaces reacted with iodine.5-6.20 (18) Yeager, E. Surf. Sci. 1980, 101, 1. (19) Wagner, C. P.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.H a n d h k
of X-Ray Phormlecrron Spectroscopy; Perkin-Elmer: Eden Prairie, MN, I976 ._. -.
(20) (a) Dowbin. P. A,; Kima, Y.J.; Mueller, D.; Rhodin, T.N. J . Chem. Phys. 1988,89,4406. (b) Wertheim. G.K.;DiCenzo, S. B.; Buchanan, D. N. E. Phys. Reo. B 1982, 25, 326.
Anodic Deposition and Cathodic Stripping of Iodine 1.1
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-
0.7
-
0.5
-
0.3
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0.1
:
. ' . . . -0.1 -0.2
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Emersion PotentialN vs AgCl
Figure 2. Superimposed plots of the fractional coverage of iodine on smooth polycrystalline Au as measured by thin-layer coulometry (left ordinate) and X-ray photoelectron spectroscopy (right ordinate). The XPS data were based upon the I 3d5 peak at 618.2 eV. Experimental procedures and conditions were as described in the text. pH 6.8 (a)Clean
(b) 0.3 V
200
400
600
Energy/eV
Figure 3. Auger spectra: (a) clean Au(ll1) surface after ion-bombardment and high-temperature annealing; (b) Au( 1 1 1) surface emersed at 0.3 V from a 1.0 mM CsI solution buffered to pH 6.8 with 0.50 mM H3P04and 0.65 mM CsOH; (c) Au( 1 1 1) surface emersed at -0.6 V from a 1.0 mM Csl solution buffered to pH 6.8 with 0.50 mM H3P04 and 0.65 mM CsOH.
The surface coverage of I as a function of electrode potential at smooth polycrystalline Au electrodes, as measured by thin-layer coulometry, has been reported.4 In Figure 2, these values are compared with those obtained from XPS measurements. For the XPS-generated plot, the fractional coverage of iodine (e r,/ I'l,max) was taken as the area under the I 3dSl2peak for a given emersion potential divided by the area of the peak at 0.0 V where rr= rl,max; the areas were normalized with respect to the that of the Au 4f7/2 peak to compensate for variations in sample positioning. It is clear from Figure 2 that excellent agreement is obtained in the 8, vs E plots for the TLE and XPS measurements. The implications of the 8,vs E plot in terms of preferential chemisorption of iodine over iodide have been discussed in detail el~ewhere.~.~ To investigate the relationship between interfacial structure and adsorbate reactivity, a series of experiments were performed in which the Au( 11 1) single crystal was emersed (removed from solution) at selected potentials from a 1 mM CsI solution buffered at pH 6.8 and reexamined by AES and LEED; the results are given in Figure 3 for the AES measurements and Figure 4 for the LEED experiments. Figure 5 presents a summary plot that correlates the elemental Cs and I coverages with the observed
Figure 4. LEED patterns obtained for a Au( 1 1 1) surface upon emersion from a CsI solution as in Figure 3 (electron beam energy 60 eV; beam current 2 PA): (a, top) (4x4) at -0.6 V; (b, center) (43X43)R30° at -0.3 V; (c, bottom) ( 5 x 4 3 ) at 0.1 V.
Bravo et al.
5248 The Journal of Physical Chemistry, Vol. 95, No. 13, 199I 0.6,. . . , . . . , . . . , . . . , . . . , . . . , . . . ,
-1.0
-0.8
-0.6 -0.4
-0.2
0.0
0.2
0.4
Emersion PotentialN vs. AgCl
I
Figure 5. Iodine and cesium surface coverages at Au( 1 1 1 ) as a function of emersion potential from a CsI solution as in Figure 3.
c
. .
. '
.
. . . '
.
O A u
Cs
. . I
1 iI pH 6.8
-0.8
-0.6
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0.0
I
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E N vs. AgCl Figure 6. Cyclic voltammogram o f a Au crystal in a CsI solution as in Figure 3 (scan rate: 5 mV/s). As explained in the text, the edge of the Au( 1 1 1) disk electrode, which constituted about 30%of the total surface
area, was polycrystalline.
adlattice structures at a given emersion potential. In this figure, at least three distinct LEED/AES vs E regions can be identified: (i) at E < -0.4 V, in which a (4X4)-CsI LEED pattern at cov= e1= r1/rAu0.25 is observed; (ii) at erages eCs= rcs/rAu -0.4 V < E < -0.2 V,in which a (d3Xd3)R30°-I LEED pattern at el 0.33 is formed; and (iii) at E > -0.2 V, in which the 4 3 structure is compressed to allow for an increase in coverage of 0.4 forming a (5Xd3)-1 LEED pattern. The latter two structures had been seen by Cochran and Farrell in adsorption studies from gas-phase I2 at Au( 11 l).21 Figure 6 shows a cyclic current-potential curve for the Au( 111) electrode in 1 mM CsI buffered at pH 6.8. Two redox peaks, superimposed on a significant background charge, are discernible. Correlation of the two anodic peaks in Figure 6 with the transitions in Figure 5 suggests a close correspondencebetween (i) the surface oxidation processes, (ii) the changes in I coverage, and (iii) the transitions in the LEED patterns. Surface processes such as underpotential deposition (UPD) have been shown to be profoundly sensitive to surface structure; different deposition potentials have been found for different low-index ~ l a n e s . 2 2 The ~ ~ oxidative adsorption of iodine may thus be thought of as the oxidatioe underpotentialdeposition (oxidative UPD) of I atoms. As already mentioned, the single crystal employed here had only two faces oriented in the (1 1 1) plane; the rim of the disk-shaped electrode, which constituted up to 30% of the total surface area, remained polycrystalline. The low definition in the voltammetric peaks in Figure 6 can therefore be rationalized in terms of redox features unique to the (1 11) plane being superimposed on a slowly varying
background current arising from processes at the plethora of randomly oriented sites at the polycrystalline edge of the electrode. Plausible interfacial geometric structures can be deduced from the combined LEED and AES data. For example, an adlattice structure at E < -0.4 V that would be consistent with the findings of (i) a (4x4) LEED pattern, and (ii) equal onequarter coverages of Cs and I is given in Figure 7a. This structure implies that the CsI layer is formed by electrosorption of Cs+ ions at sufficiently negative potentials accompanied by coadsorption of I- ions. This process would not be unlike Cs+ electrosorption at Hg electrodes studied in the past.24 The possibility that surface CsI is due to crystallization from the emersion layer can be negated by the facts that (i) the emersion layer from a 1 mM solution does not contain and (ii) enough electrolyte to account for > 0.05
(21) Cochran, S. A.; Farrell, H. H. SurJ Sci. 1980, 95, 359. (22) Juttner, K.; Lorenz, W. J. Phys. Chem. N . F. 1980, 122, 1963. (23) Stickney, J. L.; Rosasco, S. D.; Soriaga, M. P.; Song, D.; Hubbard, A. T . Surf Sci. 1983, 130, 326.
(24) Delahay, P. Double h y e r and Electrode Kinetics; Wiley and Sons: New York, 1985. (25) Salaita, G. N.; Stern, D. A.; Lu, F.; Baltruschat, H.; Schardt, B. C.; Stickney, J. L.; Soriaga, M. P.; Frank, D. G.; Hubbard, A. T. Langmuir 1986, 2, 825.
-
-
-
-
-
Figure 7. Proposed surface structures: (a, top) ( 4 x 4 ) LEED pattern (e, =8 1 0.25); (b, center) ( 4 3 X 4 3 ) R 3 0 ° LEED pattern (0, 0.33); (c, bottom) ( 5 x 4 3 ) LEED pattern (e, 0.4).
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5249
J. Phys. Chem. 1991,95,5249-5255
-
,
. . .
.
. . . .
Figure 8. Calculated ( 5 x 4 3 ) LEED pattern.33 species derived from the phosphate buffer are not present on the surface. As the potential is made more positive than -0.4 V, the expected electrodesorption of Cs+ ions is borne out by the data in Figures 3 and 5. In addition, 8, increases while e, decreases. The absence of counter cations at the interface and the data afforded by the XPS experiments indicate that the iodine species which exist on the surface under these conditions is zerovalent. The zerovalency of the surface iodine and the fact that the increase in 8, is coincident with the emergence of the first anodic peak in the cyclic voltammogram in Figure 6 lead to the conclusion that formation of the new superlattice is via an oxidative chemisorption (anodic deposition) reaction. The (43X43)R30° LEED pattern observed here is typical for adlattices formed from the adsor tion of halogens on the (1 11) surface plane of transition The interfacial structure generally proposed for this particular
metal^!^^'*^^
(26) Erley, W. Surf. Sci. 1980, 94, 281. (27) Felter, T. E.; Hubbard, A. T. J. Electroanal. Chem. 1979,100,473. (28) Goddard, P. J.: Lambcrt. R. M. Surf Sei. 1977.67. . . 180. (29) Erley, W-.; Wagner, H. surf. Sci. 66, 371. (30) Salaita, G. B.; Lu, F.; Languren-Davidson, L.; Hubbard, A. T. J . ElectroanaI. Chem. 1987, 229, 1. (31) Stickney, J. L.; Ehlers, C. B. J. Vac. Sci. Techno/. A 1989, 7 , 1801. (32) Pauling, L. P. The Nature ofthe Chemical Bond, Comell University Res: New York, 1960, Chapter 7. (33) Schardt, B. C.; Thiescn. R. Unpublished LEED pttem calculations.
lw,
LEED pattern involves the adsorption of onethird of a monolayer of halogen atoms (e, 0.33) in which each halogen atom is situated in the 3-fold sites. Such a structure, drawn to atomic dimensions, is depicted in Figure 7b. It is essential to note in this figure that the I atoms are not fully closapacked, that is, enough space is still available on the Au(l1 1)(43x43)R3O0-I adlayer for additional iodine chemisorption. Coincident with the appearance of the second anodic peak in the voltammetric curves (Figure 6) are: (i) a still further increase in 8,to 0.4, and (ii) a change from a (43X43)R30° to a ( 5 x 4 3 ) LEED pattern (Figure 4c). The latter is also frequently observed on the (1 11) planes of transition metals exposed to hal0gens,2'*~l but is occasionally mislabeled as a (43X43)R30° =splitpattern" because of the characteristic groups of spots at the 4 3 positions and the absence of other fractional-order beams (Figure 8). A structure that contains a combination of 4 3 and (12x12) symmetry elements has previously been assigned, erroneously, to this particular LEED pattern." Figure 7c shows one structure that is consistent with the LEED and AES data. This structure is formed by (i) forcing some of the I atoms originally present in 3-fold sites into less ideal sites, and (ii) compacting the ( 4 3 X d3)R30° structure in one dimension. Three domains are formed of this structure, each rotated by 120'. The result of the compression is the formation of a close-packed, albeit slightly distorted, hexagonal structure. Under these conditions, saturation chemisorption of I atoms, limited by van der Waals dimensions, is reached. At still more positive potentials, such as at 0.4 V, additional iodine atoms forced into the already space-limited interfacial layer only leads to the formation of molecular iodine which is evolved into the solution as 12(aq). This, of course, represents the anodic part of the reversible I,(aq)/I-(aq) redox couple. At considerably higher potentials, the I(ads) atoms and the 12(aq)molecules are all oxidized to I03-(aq) ions. The results and conclusions of the present study closely parallel those for iodide/iodine at Pt( 111)5 which, in the area of electrochemical surface science, is a fundamentally significant trend. Acknowledgment. M.P.S.acknowledges the National Science Foundation (Presidential Young Investigator program) and the Robert A. Welch Foundation for partial support of this work. Registry NO. 12, 7553-56-2; Au, 7440-57-5; CSI, 7789-17-5; 103, 15454-31-6; I-, 20461-54-5; I, 14362-44-8.
Electrostatlc Forces between Charged Surfaces in the Presence of a Polyelectrolyte Chaln R. Podgornik J. Stefan Institute, P.O.B.100, 61 11 1 Ljubljana, Yugoslavia (Received: October 9, 1990)
We have derived the self-consistent field equations describing the configurational statisticsof an infinitely long polyelectrolyte chain confined between two charged macroscopic surfaces. We were able to obtain an analytic solution of the linearized SCF equations in the limit of the ground-state dominance. This permits us to investigate the connection between the polyelectrolyte conformation and the intersurface forces for different values of the parameters describing the system. The most important characteristics of the interaction free energy is a region of intersurface separations where the interactions are attractive. We show that the onset of attraction is connected with a conformational transition of the confined chain.
Introduction The study of elwtrogtatic interactions between macrogcopic surfaces with fixed charges immersed in an aaueous electrolvte has been extensive in the-pt. and the undersdnding reached-on the theOfCtiCa1 level is usually SUbSumd under the hading ofthe DLVO theory.' In this framework the electrostatic interactions 0022-3654/91/2095-S249$02.50/0
between macroscopic bodies are broken into two disjointed contributions. First of all there is a contribution that has its origin (1) Derjaguin. B. V.; Churacv, N. V.; Muller, V. M. Surfuce Fonvs; Consultants Bureau: New York, 1987. Isreelachvili, J. N. Inrermo/eculw and Surface Forces; Academic hew London, 1985.
Q 1991 American Chemical Society
