Langmuir 1988,4, 1147-1151 experimental results recently reported for a series of fcc metals.lg Thus, the surface images of metals such as Pt, Pd, Au, and Rh as revealed through STM a t the nanometer s ~ a l show e ~ ~that ~ the early stages of electrochemical faceting imply a metal smoothing, which is later followed by the incipient formation of nuclei. The latter become centers for the development or preferred ~ r i e n t a t i o n . ~ These changes, which occur under preset fast electrodissolution-electrodeposition cycles, involve no change in real surface roughness. These facts are consistent with the conclusions derived from the model. Furthermore, the stable and reproducible preferred oriented surfaces resulting for real systems after a prolonged electrochemical faceting treatment keep an interesting parallelism with the quasi-stationary and reproducible configurations predicted by the model after a large number of ORCs. The correlations between the model and real systems can still be further extended t o supercluster-like metal electrode^^^^ consisting of an approximately uniform distribution of sticking nearly spherical preferred crystallographically oriented metal clusters of about 10 nm average diameter each. The STM images Of these surfaces at the 0.1 nm level exhibit a clear development
1147
of steps and terraces with kinks a t the atomic level which closely resemble the metal ion arrays depicted in Figure 8, where incipient step formation can be noticed. In conclusion, the simplified model described in this work constitutes the first quantum mechanical attempt to deal with the dynamics of metal surface crystallographic modifications promoted through ORC, in terms of both type of active centers and real surface-active area. The conclusions of the model can also be extended to account for atomic aspects related to electrocrystallization of metals,21*22 which are obviously implicit in the mechanism of electrochemical faceting of fcc metals.
Acknowledgment. This work was financially supporte$ by the Consejo Nacional de Investigaciones Cientificas y TBcnicas and the Comisidn de Investigaciones Cientificas de la Provincia de Buenos Aires. ,
(21) Budevski, E. B. In Comprehensiue Treatise of Electrochemistry; Conway, B., Bockris, J. O’M., Yeager, E., Khan, S. U. M., White, R. E., Eds.; New lga3; vO1. P 399. (22) Despic, A. R. In Comprehensiue Treatise of Electrochemistry; Conway, B.; Bockris, J. O’M., Yeager, E., Khan, S. U. M., White, R. E., Eds.; Plenum: New York, 1983; Vol. 7, p 518. 79
Substrate-Mediated Adsorbate-Adsorbate Interactions: Effect of Submonolayer Coverage and Coadsorbed Iodine on the Reversible Redox of 2,5-Dihydroxythiophenol Chemisorbed at Au and Pt Thomas Mebrahtu, Ginger M. Berry, Beatriz G. Bravo, Susan L. Michelhaugh, and Manuel P. Soriaga* Department of Chemistry, Texas A&M Uniuersity, College Station, Texas 77843 Received February 18, 1988. I n Final Form: April 22, 1988 A close-packed monolayer of 2,5-dihydroxythiophenol (DHT) chemisorbed on gold and platinum exclusively through the sulfur atom displays reversible two-electron quinonefhydroquinone redox, due to the pendant diphenol, at the same potential where the unadsorbed molecule reacts. However, the cyclic voltammetric peaks are approximately twice as broad at Pt as at Au. Since the DHT surface packing densities are identical at the two surfaces, the differences in the redox peak widths can only be rationalized in terms of substrate-mediated adsorbateadsorbate interactions on Pt. The aim of the present study is to obtain empirical information with regards to the origins of this substrate mediation. Experiments were performed in which the coverage of and composition within the chemisorbed DHT layer were varied at smooth Au and Pt surfaces in acid media. When DHT is chemisorbed at submonolayer coverages on Pt, no redox peaks are observed. This signifies that the diphenolic group is no longer pendant but is directly bonded to the surface; an adsorbed molecule orientation which allows DHT to behave like a surface chelate is a strong possibility. In comparision, redox activity is still observed when submonolayer DHT is chemisorbed on Au. Even on a sparsely populated Au surface, the diphenolic moiety remains pendant; this means that diphenol-Au reactivity is not enhanced even by entropic or chelate effects. Reversible redox peaks reappear when DHT is coadsorbed at submonolayer coverages onto an iodine-pretreated Pt electrode. In the presence of coadsorbed iodine, the diphenol group is again pendant; evidently,direct interaction between the diphenol moiety and Pt surface is blocked by the surface iodine. The redox peaks are sharpened when surface iodine is present, indicating that the substrate-mediated DHT-DHT interactions are also suppressed by iodine coadsorption. On Au, essentially no changes in the peak widths are observed for the iodinefDHT mixed layer. The present results suggest that the driving force in the substrate-mediated intermolecular interactions which occur within the close-packed DHT layer is the inherent strong reactivity of the diphenolic moiety with the Pt surface. Although the phenomenon of substrate-mediated adsorbate-adsorbate interactions is not well understood, it may be possible to view it in terms of traditional concepts of mixed-valence metal complexes in which two metal ions separated by a common ligand are still able to interact with each other through the mediation of the delocalized electrons in the ligand.
Introduction We recently reported a comparative study of the surface electrochemical behavior of aromatic mercapto compounds
* Author t o whom correspondence should be addressed.
a t smooth polycrystalline platinum and gold electrodes.’ In that investigation, it was found that the maximum (1) Bravo, B. G.;Mebrahtu, T.; Soriaga, M. P.; Zapien, D. C.; Hubbard, A. T.; Stickney, J. L. Langmuir 1987,3, 595.
0743-7463/88/2404-ll47$01.50/0 0 1988 American Chemical Society
1148 Langmuir, Vol. 4 , No. 5, 1988 surface packing density of 2,5-dihydroxythiophenol (DHT) is the same on gold (0.54 (4) nmol cm-2) as it is on platinum (0.56 (4) nmol cm-2). This value of r led to the postulate that DHT is attached exclusively through the sulfur moiety in a vertical S-al orientation. Support for this postulate was the observation of reversible quinone/diphenol redox a t the same potential where unadsorbed D H T reacts; no such redox reaction would have resulted if the diphenol moiety itself is directly bonded to the surface.2 There is, however, one very significant difference in the quinone/diphenol redox reaction of S-ql DHT at the two electrodes; the width of the redox peak at Pt is approximately twice as large as that at Au. Since the orientation and packing density of chemisorbed DHT are identical a t both surfaces, it was suggested that the adsorbate-adsorbate interactions which give rise to an unusually large redox peak width a t platinum are substrate-mediated.’ It was also found that hydroquinone itself chemisorbs on Pt but not on Au;’ hence, it was thought that the substrate mediation is driven by the inherently strong diphenol-Pt interaction. It is rigorously not valid to identify the behavior of free hydroquinone with that of the diphenol group in DHT since, in the latter, the possibility exists that appreciable Au-diphenol interactions may be induced after attachment of the SH moiety (“surface-chelate” effect). It is therefore important to study DHT itself under conditions, such as submonolayer coverages, which might be conducive to direct Au-diphenol interactions. In the present investigation, we explore further the nature of the Pt-substrate-mediated DHT-DHT interactions. In particular, we examine the influences of (i) surface coverage (submonolayer DHT coverage) and (ii) surface composition (submonolayer iodine coadsorption) on the reversible quinone-diphenol redox of DHT a t both Au and Pt electrodes. If indeed there is strong coupling between the pendant diphenol moeity and the Pt surface even a t full coverage, it is then anticipated that direct organic-metal interaction would result on a sparsely populated Pt surface. Surface chelation of DHT in which both the sulfur and diphenol groups interact directly with Pt becomes a distinct possibility; in this scenario, no redox peak would be observed because the direct Pt-diphenol bonding would lead to a large negative shift in its redox potential similar to that observed for hydroquinone chemisorbed a t Pt electrodes.2 At Au electrodes, only minimal change is to be expected even if the Au electrode is not fully covered with DHT if the diphenol-Au interaction remains weak and unenhanced by a surface-chelate effect. Iodine coadsorption is anticipated to disrupt adsorbate-adsorbate interactions through both spatial and electronic factors; it is known, for example, that iodine is more strongly bound to the surface than the diphenol group.2
Mebrahtu et al. Au) in 1 M H2S04. Electrode potentials reported here were referenced against a Ag/AgCl (1M C1-) electrode; all solutions
were prepared with pyrolytically triply distilled water.6 The surface area for the Pt electrode (1.14 cm2)was determined by underpotential hydrogen deposition: whereas that for Au (1.07 cm2)was based upon iodine chemisorption.6 2,5-Dihydroxythiophenol (DHT) was synthesized by D. C. Zapien, University of Cincinnati, using published procedure^.^ Surface coverage measurements of DHT were done by using two methods, both based upon thin-layercoulometry. The relevant equations are8
r = [(Q - Qb) - (Qi - Qib)1/pA re1
= (Q - Qb)el/ZFA
(1)
(2)
In eq 1, Q1 denotes the total faradaic charge measured for the unadsorbed quinone/diphenol redox obtained when the thin-layer cell is filled with a single aliquot of DHT solution. If the concentration is fairly high, chemisorption up to the maximum or limiting coverage occurs at this stage; the background charge Qlb is obtained in the absence of unadsorbed DHT after the single filling. Q and Qbr respectively, are the total and background charges measured when the cell is rinsed several times with DHT solution; no additional chemisorption occurs at this stage. F is Faraday’s constant, and A is the electrode surface area. In eq 2, re,is the surface coverage of reversibly electroactiue species and (Q - Qb)el is the electrolytic charge for the quinone/diphenol redox reaction measured in the absence of unadsorbed DHT. For DHT bound exclusively through the S moiety, the diphenol group is pendant and displays electroactivity at the same potential where unadsorbed DHT reacts; under these conditions, r is equal to the electroactive re,.Equations 1and 2 have been discussed in detail elsewhere.* Formation of a full-coverage layer of DHT is accomplished if the thin-layer electrochemical cell is rinsed several times with 1 mM DHT in 1M HaO,;measurements were performed to verify attainment of maximum coverage. Submonolayer DHT coverages are achieved if the thin-layer cell is filled with a single aliquot of a DHT solution of concentration C‘such that the submonolayer packing density is given by A r = VCo < Arm,
(3)
where Vis the cell volume (3.08 p L for Au, 3.56 p L for Pt) and rmsr is the maximum DHT coverage. Formation of the iodine/DHT mixed or coadsorbed layer is obtained by pretreatment of the surface with iodine followed by several rinses with dilute DHT. This procedure results in partial substitution or displacement of the preadsorbed iodine layer from the surface by DHT; ligand (adsorbate) substitution involving DHT and iodine is the subject of a separate study beyond the scope of the present article. The amount of iodine which remained on the surface following the DHT rinses was quantitated by oxidation at 1.2 V (during which the surface iodine is oxidized to aqueous iodate) followed by coulometric reduction at 0.7 V (during which aqueous iodate is reduced to aqueous iodine)?
where Qb is measured with an untreated surface.
Experimental Section Smooth polycrystalline platinum and gold thin-layer electrodes were utilized in this study; the fabrication of the thin-layer electrochemicalcells3and the preparation of the smooth electrodes Between experimental trials, are as described in earlier ~tudies.~ the electrodes were cleaned by electrochemical oxidation (1.2 V for Pt; 1.5 V for Au) and reduction (-0.2 V for Pt; -0.35 V for
Results and Discussion Figure 1shows cyclic current-potential curves for 2,5dihydroxythiophenol (DHT) chemisorbed at maximum coverage at smooth polycrystalline platinum (dashed curve) and gold (solid curve) electrodes in 1 M H2S04.
(2) (a) Soriaga, M. P.; Binamira-Soriaga, E.; Hubbard, A. T.;Benziger, J. B.; Pang, P. K. W. Inorg. Chem. 1985,24,65. (b) Soriaga, M. P.; White, J. H.; Song, D.; Chia, V. K. F.; Arrhenius, P. 0.; Hubbard, A. T. Inorg. Chem. 1985,24, 73. (3) Hubbard, A. T. Crit. Rev. Anal. Chem. 1973, 3, 201. (4) White, J. H.; Soriaga, M. P.;Hubbard, A. T. J.Electroanal. Chem.
(5) Conway, B. E.; Angerstein-Kozlowska, H.; Sharp, W. B. A.; Criddle, E. E. Anal. Chem. 1973,45, 1331. ( 6 ) Rodriguez, J. F.; Mebrahtu, T.; Soriaga, M. P. J. ElectroanaL Chem. 1987,233, 283. (7) Alcalay, W. Helu. Chim. Acta 1947, 30, 578. (8) (a) Soriaga, M. P.; Hubbard, A. T. J.Am. Chem. SOC.1982,104, 3937. (b) Soriaga, M. P.; Hubbard, A. T. J. Electroanul. Chem. 1982,142, 317.
1984, 177, 89.
Langmuir, Vol. 4,No. 5, 1988 1149
Substrate-Mediated Adsorbate-Adsorbate Interactions 1.8
3-
/YG0Id
2-
-3 -
&
1.4
-
1.0
-
0.6
-
0.2
-
I
/
Stan "
-".L
" 0.2 "
~ 0.4'
"
0.6 ~
~
"0.8 ~
~
E N vs. AgCl
Table I. Effect of Submonolayer Coverage and Iodine Coadsorption on the Reversible Quinone/Diphenol Redox Peak Width of 2,s-DihydroxythiophenolChemisorbed at Smooth Polycrystalline Gold and Platinum Electrodes nmol cmm2
r,,6nmol cm-2
'
0.0
0.2
0.4
0.6
0.8
E N vs. AgCl
Figure 1. Thin-layer cyclic current-potential curves for 2,5dihydroxythiophenol (DHT) chemisorbed at smooth polycrystalline gold (solid curve) and platinum (dashed curve) electrodes in 1 M H2S0,; the experiments were performed in the absence of unadsorbed DHT at 298 K. For the Pt experiments: volume of the thin-layer cell, V = 3.56 pL; area of the electrode, A = 1.14 cm2;sweep rate, r = 2 mV/s. For the Au experiments V = 3.08 IrL; A = 1.07 cm2;r = 3 mV/s. The difference in sweep rates for the solid and dashed curves must be noted.
Pnm,a
Half coverage
I
-2
40.0
-
fwhm,' V
Platinum 0.56 (max) 0.30 0.43 0.29
0 0 0.42 0.69
0.21 no redox activity 0.19 0.15
0.54 (max) 0.27 0.34
Gold 0 0 0.53
0.13 0.13 0.13
"The average relative standard deviation in rDmwas &6%. the vertical S-ql-DHT orientation is 0.57 nmol cm-*. *The average relative standard deviation in FI was f5%; full iodine coverage corresponded to 1.04 nmol cm-2. 'fwhm: full width at
ruledfor
half-maximum.
These voltammetric curves were obtained in the absence of unadsorbed species. The areas under these curves, with a pentafluorothiophenol-coatedsurface as blank, represent the quantity (8 - 8b)el in eq 2 and yield r values of 0.56 f 0.04 nmol and 0.54 f 0.04 nmol cm-2 a t Pt and Au, respectively. Such coverage indicates a vertical S-7' orientation for which rd is 0.57 nmol cm4; this orientational assignment* is supported by the observed reversible quinone/diphenol redox reaction in Figure 1. The most striking feature in Figure 1 is the fact that the half-width of the redox peak is approximately 50% larger for Pt than it is for Au. In determining the full width a t half-maximum (fwhm)values, i - ib curves were generated, where i and ib are, respectively, the sample and background currents; a pentafluorothiphenol-coatedsurface was used in the measurement of i,, Table I lists the full-coverage fwhm values as 0.21 V a t Pt and 0.13 V at Au. Since the modes of attachment and packing densities are identical a t both surfaces, the larger redox peak width a t Pt has been attributed to substrate-mediated (through-the-metal) rather than through-space adsorbate-adsorbate interactions. Because free hydroquinone interacts strongly with Pt but not with Au,l it was thought that the substrate
Figure 2. Thin-layer current-potential curves for DHT chemisorbed at smooth polycrystalline platinum: solid curve, full coverage; dashed curve, half coverage; sweep rate, 2 mV/s. Other experimental conditions were as in Figure 1. mediation arises from the inherently strong diphenol-Pt reactivity. The possibility exists that direct Au-diphenol interaction can be induced after attachment of the SH moiety, a process not unlike the chelate effect. The question may be asked if the larger peak width a t Pt is related to surface oxide formation since the latter is formed at less positive potentials on Pt than on Au. Past and present empirical results, however, indicate that surface oxide formation has little to do with the redox peak broadening. (i) It is known24 that, in molar acid, surface oxide formation on clean Pt does not start until about 0.6 V. Yet, as can be seen from Figure 1,the redox activity of DHT a t Pt is already appreciable a t potentials as low as 0.2 V. (ii) It is also ~ ~ o w Ithat I ~ -surface ~ oxide formation is significantly retarded by chemisorbed organics like DHT; chemisorbed pentafluorothiophenol, for example, completely blocks oxidation of the Pt surface even at 1.2 V. It is thus unlikely for surface oxidation to occur on the DHT-covered surface a t 0.2 V, the potential a t which DHT redox activity commences. (iii) Studies have been performed with 2,5-dihydroxybenzyl mercaptan (DHBM) chemisorbed a t smooth Pt; DHBM is similar to DHT except that a CH2 group lies between the diphenol and SH moieties.2 If surface oxide were responsible for the peak broadening of DHT, similar broadening would have resulted for DHBM since the packing densities and orientations of these two materials are the same on Pt. In fact, no band broadening was observed for DHBM. Evidently, insertion of the methylene group between the redox center and the surface anchor terminates the substrate-mediated intermolecular interactions. Figure 2 shows thin-layer current-potential curves, in the potential region where the diphenol-to-quinone oxidation occurs, for a Pt surface precoated with DHT at full (solid curve) and half (dashed curve) coverages. The principal result here is that the reversible quinone/diphenol redox reaction is absent at half coverage. Experiments were also performed a t other submonolayer coverages; no quinone/diphenol redox activity was observed a t 13 (I I'/Fmm) < 0.8. These results mean that, a t ODHT < 0.8, direct diphenol-Pt interaction has taken place; otherwise, pronounced quinone/diphenol redox peaks would have been observed.2 Figure 3 displays thin-layer voltammetric curves, also in the potential region where diphenol-to-quinone redox takes place, for a gold surface precoated with DHT a t full
1150 Langmuir, Vol. 4, No. 5, 1988
Mebrahtu et al.
141
I
i
I
1
1 4,
.-.
Full coverage
1.4
-
1.0
-
Full coverage DHT
Iwlth DHT
.
.-4
0.6
0.2
1
0.6 I
-
0.2 -
t[ f l 0.8 Start
-0.2 0.0
0.4
0.2
Start -0.2
0.6
I
1
0.0
0.2
EIV vs. AgCl
0.6
0.4
0.8
E N vs. AgCl
Figure 3. Thin-layer current-potential curves for DHT chem-
isorbed at smooth polycrystalline gold solid curve, full coverage; dashed curve, half coverage; sweep rate, 2 mV/s. Other experimental conditions were as in Figure 1. (solid curve) and half (dashed curve) coverages. In contrast to the resulk3 shown for Pt in Figure 2, comparatively sharp redox peaks are observed at Au even when DHT is present a t submonolayer coverages; fwhm measurements for the data in Figure 3 gave identical values of 0.13 V at both full and half coverages, Table I. This independence of the fwhm on DHT coverage signifies that adsorbate-adsorbate interactions, whether occurring through-space or substrate-mediated, do not exist in the close-packed layer of S-ql-DHT a t Au electrodes. The results shown in Figures 2 and 3 and Table I reflect the following chemisorption orientational states for full and submonolayer DHT on Au and Pt:
Figure 4. Thin-layer current-potential curves for DHT chemisorbed at smooth polycrystalline platinum in the presence (dashed
curve) and absence (solid curve) of coadsorbed iodine. Dottedsolid curve: Pt surface coated with pentafluorothiophenol. The respective coverages of iodine and DHT in the mixed or coadsorbed layer are given in Table I. Other experimental conditions were as in Figure 2.
0.8 -
4 . ,-
0.6
-
0.4
-
0.2
-
0.0
-
".-
0.0
With Iodine I
S
Start
0.2
0.4
0.6
0.8
HO H!
::::;::::t 1;
Submonolayer coverage o n Au
3 Submonolayer coverage o n PI
II
I
The orientational state I1 shows that, a t submonolayer coverages, DHT on Pt behaves as a surface chelate in which both sulfur and diphenol groups are bonded directly to the surface. The tilt angle of the chelated DHT is expected to vary with coverage; that is, at very low coverages, a virtually flat-oriented DHT layer may be formed. This is supported by the fact that quinone/diphenol redox activity starts to disappear at 6DHT < 0.8. Figure 4 shows the influence of coadsorbed iodine on the redox behavior of submonolayer DHT. In this figure, thin-layer voltammetric curves are shown for Pt covered with a full monolayer of D H T (solid curve) and an iodine/DHT mixed layer (dashed curve). The significant outcome of these experiments is that reversible quinone/diphenol redox reactivity for the submonolayer DHT has been restored by coadsorbed iodine. This reappearance of reversible redox activity indicates that preadsorbed iodine inhibits the formation of surface-chelated DHT by blocking direct diphenol-Pt bonding. The prevention of aromatic chemisorption by iodine has been demonstrated
E N vs. AgCl
Figure 5. Thin-layer current-potential curves for submonolayer DHT chemisorbed at smooth polycrystalline gold in the presence
(dashed curve) and absence (solid curve) of coadsorbed iodine. The respective coverages of iodine and DHT in the mixed or coadsorbed layer are given in Table I. Other experimental conditions were as in Figure 3.
previously.2 As can be seen in Table I, considerable sharpening of the redox peak is also induced by iodine coadsorption: fwhm values of 0.21 and 0.15 V were measured, respectively, for full-coverage DHT and for an iodine/DHT coadsorbed layer consisting of FI = 0.69 and r D H T = 0.29 nmol cm-2. Figure 5 shows thin-layer current-potential curves for gold electrodes containing submonolayer DHT (solid curve) and an iodine/DHT mixed layer (dashed curve). As can be observed, there is no change in the redox fwhm value in the presence (0.13 V) and absence (0.13 V) of surface iodine. This observation strongly suggests the absence of adsorbate-adsorbate interactions, whether of substrate-mediated or through-space origins, a t all coverages of DHT, at least of the type which leads to broadening of the reversible quinone/diphenol redox peak. The data in Figures 4 and 5 and Table I indicate a vertical S-q' orientation of DHT when coadsorbed onto an iodine-pretreated Pt or Au surface:
Langmuir 1988, 4, 1151-1156
bpy is 4,4’-bipyridine), the two metal centers are separated from each other by a bridging ligand. It is known that, even if the ligand is of considerable length, the two metal ions are still able to interact with one another provided the ligand contains delocalized electrons. This metalmetal interaction is therefore ligand-mediated. The coupling between the Ru(I1) and Ru(II1) ions in the above mixed-valence complex can be written schematically as
I N O H
HY ?//J,,,, ,
/
1151
Coadsorbed IodineiDHT on Pt and Au
Ru(II)-L-L-Ru(111) c* Ru(111)-L-L-Ru(I1)
Ill
where L-L denotes the bridging bpy ligand, the delocalized electrons of which mediate the coupling between the Ru(I1) and Ru(II1) ions. By analogy with eq 5, the DHT-DHT interactions mediated by the Pt surface may be represented by
The present results suggest that the driving force in the substrate-mediated intermolecular interactions which occur within the close-packed DHT layer is the inherent strong reactivity of the diphenolic moiety with the Pt surface. The interaction of adsorbates with each other through the mediation of the substrate is of considerable importance in connection with the coverage-dependence of binding energies and the formation of periodic adsorbate array^.^ The theoretical treatment, however, involves complicated many-body potentials which are presently not well u n d e r s t ~ o d .Qualitatively, ~ it is simpler to model the present case of Pt-substrate-mediated DHT-DHT interactions in terms of the established concepts of mixed-valence metal complexes.1° In a binuclear mixed-valence complex, such as (NH&Ru(II) (bpy)Ru(III)(NH3)5(where ~~
(9)(a) Somorjai, G. Archemistry in Two Dimensions: Surfaces; Cornel1 University Press: Ithaca, 1981. (b) Gomer, R.Interactions on Metal Surfaces; Springer-Verlag: Berlin, 1975. (c) Einstein, T. L.; Schrieffer, J. R. Phys. Reu. B: Condens. Matter 1973, B7, 3629. (d) Grimley, T. B.; Walker, S.M. Surf. Sci. 1969, 14, 396. (IO) Creutz, C. Prog. Inorg. Chem. 1983, 30, 1.
Q-S-Pt-Pt-S-HZQ c* HZQ-S-Pt-Pt-S-Q
(5)
(6)
where Pt-Pt symbolizes the Pt surface; for simplicity, only one diphenol (H,Q) and quinone (Q) substrate-mediated pair is shown. It is now easy to visualize how the coupling between the H,Q and Q centers can be mediated by the Pt surface. It is also easy to see how such mediation is disrupted in DHBM, where a CH2 “insulator” is placed between the S and Q/H,Q groups, or on Au, where the Q/H,Q centers have no affinities for the surface.
Acknowledgment is made to the Robert A. Welch Foundation, to the Center for Energy and Mineral Resources, and to the Regents of Texas A&M University for support of this research. Registry No. DHT, 2889-61-4; Au, 7440-57-5; Pt, 7440-06-4; 12,
7553-56-2.
Critical Behavior of the Liquid-Vapor Interface Near the LCST of the Water-2,6-Lutidine Liquid Mixture M. Privat,* L. Tenebre, R. Bennes, E. Tronel-Peyroz, J. M. Douillard, and L. Ghaicha U.A. 330, Laboratoire de Physicochimie des SystBmes Polyphash, CNRS, B P 5051, 34033 Montpellier Ceder, France Received November 12, 1987. I n Final Form: April 20, 1988 Bulk chemical potentials, surface tensions, and relative surface excesses have been determined (at various concentrations and temperatures) near the critical point (LCST) at the surface between the vapor and a monophase of water and 2,6-lutidine mixture. The variation of both chemical potential and surface tension obeys the critical law as xZa - x2ctends toward zero, the critical exponents being 6 and p / P . The relative excess follows the critical law with IT - ?“,I tending toward zero, with Y + P as the exponent, as expected. The same law is found for the ellipsometry coefficient measured as the same interface. It is suggested it is the thickness variations of the interface which determine the ellipticity variations with temperature. An understanding of critical behavior in physical systems having phase changes started with van der Waa1s.l Analyzing the transitions between a liquid phase and a gas phase, influenced by the existence of a large zone of a weak gradient of density, Van der Waals gave the first critical law via a simplified thermodynamic treatment. The theory dealt with the density of the liquid and the gas, near to the critical temperature. Later, Ornstein and Zernike, gave (1) van der Waals, J. D. Thesis, Leiden, 1873.
0743-746318812404-1151$01.50/0
the first statistical treatment of the critical state by use of the pair correlation method. Presently, the renormalization group theory is the more powerful way to study inhomogeneous but it is (2) Ornstein, L. S.;Zernike, R. Proc. Sec. Sci. K. Med. Akad. Wet. Amsterdam 1914, 17,793. (3) Muller, K. A.; Thomas, H. Structural Phase Transition I; Springer-Verlag: Berlin, 1981. Burkhardt, T. W.; Leewen, J. M. J. Real Space Renormalization; Springer-Verlag, Berlin, 1982. Pekalski, A.; Sznajd, J. Static Critical Phenomena in Inhomogeneous Systems; Proceeding Karpacz 1984, Springer-Verlag: Berlin, 1984.
0 1988 American Chemical Society
