Self-Assembled Monolayers from Organosulfur Compounds: A

and voltammetry. Film formation is described by Langmuir adsorption kinetics and is found to be fastest .... Also, investigations based on time-of-fli...
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Langmuir 1998, 14, 1103-1107

1103

Self-Assembled Monolayers from Organosulfur Compounds: A Comparison between Sulfides, Disulfides, and Thiols† Ch. Jung,‡ O. Dannenberger,§ Yue Xu,| M. Buck,*,⊥ and M. Grunze*,X Lehrstuhl fu¨ r Angewandte Physikalische Chemie, INF 253, 69120 Heidelberg, Germany Received August 6, 1997. In Final Form: October 16, 1997 Film formation of decanethiol [CH3(CH2)9SH], didecyl disulfide [CH3(CH2)9S-S(CH2)9CH3], and didecyl sulfide [CH3(CH2)9-S-(CH2)9CH3] on polycrystalline gold from ethanol solutions was studied in situ by second harmonic generation. The completed films were characterized by X-ray photoelectron spectroscopy and voltammetry. Film formation is described by Langmuir adsorption kinetics and is found to be fastest for the thiol. Referenced to the number of thiolate units the disulfide adsorbs about 40% more slowly. In sharp contrast, the rate of alkyl sulfide adsorption is smaller by more than 3 orders of magnitude. Films formed from the thiol and disulfide are indistinguishable and virtually free of contaminations. In contrast, the maximum coverage achieved with the sulfide is considerably lower and substantial amounts of contamination remain on the substrate. Differences between thiol/disulfide films and sulfide films are also indicated by their electrochemical behavior.

Introduction Among the class of self-assembling monolayers (SAMs), organosulfur compounds have received steadily increasing attention. The vast majority of work reported refers to thiols (R-SH).1-5 Disulfides (R′-S-S-R) have been studied in less detail even though they were the first to be studied.6,7 Despite the higher stability of sulfides (RS-R′) against nucleophilic reactions or oxidation as compared to thiols or disulfides, which is of advantage in the synthesis of the compounds, this class has been studied less thoroughly. It is commonly accepted that disulfides adsorbing on gold form the same surface species as observed for thiols.8-12 For sulfides the situation is less clear. In a set of experiments different sulfides, such as diphenyl sulfide and ethyl phenyl sulfide, adsorbed on gold were investigated by linear voltage sweeps (LVS), X-ray photoelec* Corresponding authors. † Dedicated to Professor Gottfried Huttner on the occasion of his 60th birthday. ‡ Present address: ETH-Zu ¨ rich, Inst. f. Terrestrische O ¨ kologie, Grabenstr. 3, CH-8952 Schlieren. § Present address: Department of Bioengineering, University of Washington, Box 357962, Seattle, WA 98195. | Permanent address: Department of Applied Chemistry, Harbin Institute of Technology, Harbin, 150001, People’s Republic of China. ⊥ [email protected]. X [email protected]. (1) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (2) Grunze, M. Phys. Scr. 1993, 7, 711. (3) Jung, D. R.; Czanderna, A. W. CRC Crit. Rev. Solid State Mater. Sci. 1994, 19, 1. (4) Ulman, A. Chem. Rev. 1996, 96, 1533. (5) Finklea, H. O. Electroanal. Chem. 1996, 19, 105. (6) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (7) Taniguchi, I.; Toyosawa, K.; Yamaguchi, H.; Yasukouchi, K. J. Electroanal. Chem. 1982, 140, 187. (8) Weisshaar, D. E.; Lamb, B. D.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 5860. (9) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389. (10) Biebuyck, H. A.; Whitesides, G. M. Langmuir 1993, 9, 1766. (11) Thome, J.; Himmelhaus, M.; Grunze, M. Submitted for publication. (12) Biebuyck, H. A.; Bain, C. D.; Whitesides, G. M. Langmuir 1994, 10, 1825.

tron spectroscopy (XPS), and IR reflection absorption spectroscopy (IRRAS).13 The results were interpreted in terms of a cleavage of the C-S bond leading to the formation of a gold thiolate. Studies of disulfides and sulfides on silver by surface-enhanced Raman spectroscopy (SERS) also suggested the cleavage of the C-S bond.14 Contrasting results were reported later on for the SERS experiments on silver and the cleavage of the C-S was explained by a surface photoreaction.15 An earlier systematic investigation of the adsorption of symmetrical and unsymmetrical dialkyl sulfides on gold by XPS, IRRAS, ellipsometry, and contact angle measurements reported strong evidence that SAMs of alkyl sulfides are different than those obtained from thiol solutions;16 i.e., the alkyl chains in films obtained from alkyl sulfides are less ordered than those in thiol films. Other work on unsymmetrical sulfides confirms this finding.18 However, the differences between thiol and sulfide films cannot be generalized. Dependent on the molecular composition and the analytical technique applied, films prepared from sulfides appear either identical or different to thiols. For example, symmetrical dialkyl sulfides yield the same ellipsometric film thickness as the respective thiol, whereas COOH-terminated analogues did not.16 With respect to their electrochemical behavior no significant differences between films made from di-n-alkyl sulfides and n-alkanethiols were reported, whereas disulfides containing one COOH group can be distinctly different depending on the chain length.18 These results were interpreted by the inability of the electrolyte solution to permeate the hydrocarbon layer in the case of di-nalkyl sulfides and that these layers are more prone to solvent permeation when carboxylic acid groups are present. The experiments reporting differences in the order of hydrocarbon chains between films formed from sulfides (13) Zhong, C. J.; Porter, M. D. J. Am. Chem. Soc. 1994, 116, 11616. (14) Sandroff, C. J.; Herschbach, D. R. J. Phys. Chem. 1982, 86, 3277. (15) Lee, S. B.; Kim, K.; Kim, M. S. J. Phys. Chem. 1992, 96, 9940. (16) Troughton, B.; Bain, C. D.; Whitesides, G. M. Langmuir 1988, 4, 365. (17) Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1989, 93, 1670. (18) Zhang, M.; Anderson, M. R. Langmuir 1994, 10, 2807.

S0743-7463(97)00885-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/03/1998

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and thiols16,18 are in contradiction to the work claiming a thiol type bonding due to C-S bond cleavage.13 Since longer chain alkanethiols replace thiols with shorter chains,10 a uniform thiol type layer is expected using unsymmetrical sulfides containing different chain lengths. However, this is not observed experimentally.16 Also, investigations based on time-of-flight ion mass spectrometry (TOF-SIMS) find the desorption of moieties containing the mass of the sulfide molecule19,20 and coverage measurements based on radiolabeled thiols concluded as well that C-S cleavage must be minimal if present at all.21 Both results seriously challenge the interpretation of a cleavage of the C-S bond for dialkyl sulfides. Along our interests in understanding the kinetics of SAM formation from organosulfur compounds, a comparative study of alkanethiols (docosanethiol (DCT), decanethiol (DT)), didecyl disulfide (DDDS), and didecyl sulfide (DDS) was performed using a combination of in situ and ex situ techniques such as second harmonic generation (SHG), X-ray photoelectron spectroscopy (XPS), and voltammetry.

Jung et al.

Figure 1. XP C 1s spectra of gold substrates immersed in (a) pure ethanol and ethanol solutions of (b) purified didecyl sulfide (DDS2), (c) didecyl sulfide as purchased (DDS1), (d) didecyl disulfide (DDDS), and (e) decanethiol (DT). Spectra f-i show the difference between the respective organosulfur compound and decanethiol.

Experimental Section The substrates were prepared by thermal evaporation of a gold layer of about 1000 Å onto Si(100) wafers. A titanium interlayer (∼10 Å) served as an adhesion promoter. The gold substrates were stored in a controlled nitrogen atmosphere in order to minimize contamination. For the adsorption experiments ethanol (p.A., Riedel de Hae¨n) was used as solvent. Decanethiol (DT, purity >95%) which was used as received and didecyl sulfide (DDS1, specified purity >97%) were purchased from Lancaster. Analysis of DDS1 by gas chromatography-mass spectroscopy (GC-MS) showed different kinds of impurities. As determined by gas chromatography flame ionization detection (GC-FID) the total amount of impurities is about 0.7% of which didecyl disulfide was the prominent one (0.6%). To pinpoint possible effects in the kinetic studies due to impurities, DDS1 was carefully cleaned by liquid-solid chromatography (silica gel, hexane) and recrystallization from hexane. The GC-MS and GC-FID analyses of the purified sulfide (DDS2) yielded a purity of 99.8% and a disulfide concentration below detection level. Didecyl disulfide (DDDS) was synthesized by standard procedures23 and recrystallization from hexane yielded a purity of >99% as checked by GC-MS and GC-FID. Films were prepared by immersion of the substrates in dilute solutions of the respective organosulfur compounds in ethanol. For the ex situ studies typical immersion times ranged from 12 to 15 h. Variation of the concentration between 10 µmol/L and 1 mmol/L had no effect on the results. XP-spectra were aquired with a Leybold MAX200 using nonmonochromatized Mg KR radiation (1253.6 eV). The pass energy of the energy analyzer was set to 48 eV resulting in a resolution of 0.9 eV. The quantitative evaluation of the XPS spectra is based on the comparison of ratios of the C 1s and the Au 4p and Au 4f intensities, respectively. The evaluation procedure has been described in detail elsewhere.24 The electrochemical measurements were carried out with a home-built electrochemical glass cell and a potentiostat (Bank Elektronik, LB81M) interfaced to a PC. The substrates were attached to the cell with carefully cleaned Viton rings leaving an electrode diameter of 10 mm. A calomel electrode was used as (19) Hagenhoff, B.; Benninghoven, A.; Spinke, J.; Liley, M.; Knoll, W. Langmuir 1993, 9, 1622. (20) Beulen, M. W. J.; Huisman, B. H.; van der Heijden, P. A.; van Veggel, F. C. J. M.; Simons, M. G.; Biemond, E. M. E. F.; de Lange, P. J.; Reinhoudt, D. N. Langmuir 1996, 12, 6170. (21) Schlenoff, J. B.; Li, Ming; Ly, Hiep J. Am. Chem. Soc. 1995, 117, 12528. (22) Peterlinz, K. A.; Georgiadis, R. Langmuir 1996, 12, 4731. (23) Fava, A.; Reichenbach, G.; Peron, U. J. Am. Chem. Soc. 1967, 89, 6696. (24) Wesch, A.; Dannenberger, O.; Wolff, J. J.; Wo¨ll, Ch.; Buck, M. Langmuir 1996, 12, 5330.

reference and a solution of K3[Fe(CN)6] (2 mM) and KCl (0.2 M) in Millipore water served as electrolyte. Scan rates were 50 mV/s. In situ SHG experiments give direct access to the processes occurring at the substrate/solvent interface, i.e., the adsorption of molecules.25,26 All measurements were performed with ppolarization for both the incident fundamental and the detected second harmonic light (pp-polarization) using the fundamental of a Nd:YAG laser (Continuum YG 671C10, 10 Hz, 7 ns). A home-built liquid flow cell was used for the kinetic studies.25 The optics were adjusted when the cell contained pure solvent only. Adsorption was started by switching the flow from pure solvent to the solution containing the organosulfur compound and the resulting change of the SHG signal from the substrate/solvent interface was monitored in real time. Due to the small volume of the cell the uncertainty of the zero point of the adsorption process is less than 5 s.

Results and Discussion Before presenting the in situ SH studies of film formation we first describe the XPS and CV measurements of the films formed after immersion times of 12-15 h. The XP spectra of the C 1s and O 1s regions are shown in Figure 1 and Figure 2, respectively. The spectrum from a freshly evaporated gold substrate after immersion in pure ethanol (Figure 1a) demonstrates that the native gold substrate is inevitably contaminated by contact with the solvent. Beside hydrocarbons (∼284.5 eV) contributions from oxidized carbon species (286-289 eV) are present. The C 1s peaks of Figure 1 mainly differ in position and intensity. The gold substrate (Figure 1a) and the purified didecyl sulfide (Figure 1b) are very similar but differ from the data of decanethiol (Figure 1e), didecyl disulfide (Figure 1d), and the nonpurified didecyl sulfide (Figure 1c). The deviations of the C 1s data from the spectrum of a monolayer of decanethiol are revealed by the difference spectra of films of the respective organosulfur compound and decanethiol (Figure 1f-i). In the C 1s region DDDS (Figure 1f) yields a flat line and thus indicates the identity of SAMs formed from decanethiol and the disulfide. Even though the differences are very small, the shape of the DDS1 (Figure 1g) curve indicates a deficiency of intensity around 285 eV and additional (25) Dannenberger, O. Ph.D. Thesis, University of Heidelberg, 1996. (26) Buck, M.; Eisert, F.; Fischer, J.; Grunze, M.; Tra¨ger, F. Appl. Phys. A 1991, 53, 552.

SAMs from Organosulfur Compounds

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Figure 2. XP O 1s spectra of gold substrates immersed in (a) pure ethanol and ethanol solutions of (b) purified didecyl sulfide (DDS2), (c) didecyl sulfide as purchased (DDS1), (d) didecyl disulfide (DDDS), and (e) decanethiol (DT). Spectra f-i show the difference between the respective organosulfur compound and decanethiol.

intensity around 284.2 eV. This is interpreted as evidence for an incomplete DT-like film with the incorporation of contaminations due to incomplete displacement of surface contaminations. In principle, such a shape of the difference spectrum can be caused also if the energy scales are slightly shifted against each other. From the positions of the gold peaks, such shifts are excluded; i.e., they must be smaller than 0.05 V. A simulation shows that, at most, only part of the feature could be explained by a shift of the energy scales. In contrast, DDS2 exhibits a pronounced difference (Figure 1h) and is almost identical to the difference spectrum produced by subtracting the spectrum of the gold substrate from the DT spectrum (Figure 1i). The data show that adsorption of DT and DDDS alters the spectra from the native substrate significantly. The total amount of carbon is increased and the dominating part of the C 1s signal is slightly shifted to about 285 eV. The origin of the shift is not clear. Several contributions, which are discussed in more detail in ref 11, e.g., conformational effects, dipole effects, or chemical shifts, can be involved at the same time making an interpretation impossible at present. The O 1s signals (532-533 eV), which are convoluted with the Au 4p peak (547 eV) and a satellite at 538 eV from the nonmonochromatized X-rays (Figure 2), confirm the C 1s spectra. Whereas films from alkanethiols and disulfides are equivalent as expected,12 the results for the sulfide depend on its purity. Films formed from the nonpurified DDS1 appear identical with DT and DDDS, in contrast to the purified substance DDS2, which exhibits distinct differences in both the O 1s and the C 1s region. Oxygen-containing compounds are still present and the essentially identical shapes of the signals as compared to the native gold substrate suggest that the contaminations present prior to adsorption have not been displaced by exposure to DDS2. Another important aspect is the difference in coverage between the purified sulfide and the other compounds. Figure 3 summarizes the film thicknesses determined by the ratio of the C 1s and Au 4f signals according to

IC 1 - e-dC/λC ) K -d /λ IAu e CS Au

(1)

Figure 3. Adsorption of decanethiol (DT), didecyl disulfide (DDDS), and didecyl sulfide of different purities (DDS1, DDS2) on gold from ethanol solution. Final film thickness dC as determined from the ratios of the XP signals of C 1s and Au 4p according to eq 1.

K is an instrument specific constant which is determined by calibration with a film of docosanethiol. dCS denotes the thickness of the film seen by the gold electrons, i.e., the layer of hydrocarbon chains plus the sulfur headgroups, whereas dC refers to the thickness of the layer of hydrocarbon chains, only. λC and λAu are the escape lengths of the carbon (C 1s) and gold (Au 4p) electrons, respectively. Details of the evaluation procedure can be found in ref 24. In agreement with the conclusions derived for the spectra presented in Figures 1 and 2, the film thicknesses are identical for DT, DDDS, and DDS1 within the experimental error of (0.53 Å. The averaged value of 11 Å for the thickness of the hydrocarbon layer agrees exactly with the expected thickness for a densely packed monolayer which results from the tilt angle of thiols on gold (30°) and a length of a methylene unit of 1.27 Å.17 The film thickness after exposure to DDS2 is significantly lower and resembles that of the contamination layer. However, this does not imply that no sulfide is adsorbed, since an S 2p signal is clearly detected. Unfortunately, a precise quantitative determination based on the sulfur signal has an inherently large error due to the poor signal-to-noise ratio. Two conclusions can be drawn based on the XPS measurements: First, from the difference between DDS2 and DT/DDDS the sulfide seems to form films different from thiols and disulfides, and second, the difference between DDS1 and DDS2 implies that an impurity, e.g., the disulfide, determines the composition of the films formed by immersion of the gold substrate in DDS1. The interpretation of the XPS results are substantiated by the voltage sweeps shown in Figure 4. Whereas films of DT, DDDS, and the nonpurified DDS (DDS1) show an indistinguishable blocking behavior, the SAM formed from the purified sulfide DDS2 exhibits a significant permeability causing the reduction of [Fe(CN)6]3- for potentials more negative than -0.2 eV. Furthermore, the first three SAMs are not affected by repetitive cycling whereas the charge waves increase with the number of cycles in the case of DDS2 indicating a progressing destruction of the film (not shown). However, the lower electrochemical stability does not seem to be general for sulfide films. As shown by Zhang and Anderson the permeability is strongly

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Jung et al. Table 1. Parameters of the Fit Curves of Figure 4 Based on Equation 2a substance

R

k (L mol-1 s-1)

krel

c (µmol/L)

kc (s-1)

DCT DT DDDS

0.29 0.295 0.295 0.265 0.21

4100 4561 5263 2631b 5.6 1

2 2 1 2b 1000 1000

0.0047 0.0052 0.003

DDS1 DDS2

2337 2600 3000 1500b 3.2 0.57

0.0032 0.00057

a DCT, docosanethiol; DT, decanethiol; DDDS, didecyl disulfide; DDS1, didecyl sulfide as purchased, DDS2, DDS1 purified; R, ratio of susceptibilities of the substrate and the interaction, k, rate constant of adsorption; krel, normalized rate constant; c, concentration. b Values of the adsorption process expressed in number of thiol units.

Figure 4. Potential sweeps in an aqueous electrolyte of K3[Fe(CN)6]/KCl: SAMs of (a) decanethiol (DT), (b) didecyl disulfide (DDDS), (c) didecyl sulfide as purchased (DDS1), and (d) didecyl sulfide (DDS2) on polycrystalline gold. The potential is referenced to the calomel electrode. The scan rate is 50 mV/s.

Assuming Langmuir adsorption kinetics the measured intensity of the SHG signal is phenomenologically described by26

ISHG ∝ |1 - R(1 - e-kct)|2 Iω2 with

R)

Figure 5. Adsorption of (a) purified didecyl sulfide (DDS2), (b) didecyl sulfide as purchased (DDS1), (c) didecyl disulfide (DDDS), (d) docosanethiol (DCT), and (e) decanethiol (DT) on gold from a solution of ethanol. The SHG intensity at 532 nm is recorded in pp-polarization. Note the large difference in concentration between DDS1/2 and DCT, DT, or DDDS. The solid lines are fits according to eq 2. Fit parameters are given in Table 1.

affected by the size of the sulfide molecule and the end group functionalization.18 For various dialkyl sulfides they found a blocking behavior equal to octadecanethiol. However, since in all cases the molecules were significantly larger than DDS the SAMs were thicker and thus are expected to be more resistant to ion permeation compared to DDS. The origin of the differences between DDS1 and DDS2 is revealed by the kinetics of film formation. Figure 5 summarizes the adsorption experiments where the adsorption process was monitored in situ and real time by SHG. In addition to the substances discussed above we included the adsorption curve of docosanethiol (DCT, Figure 5d) for reasons detailed below. The signal at t ) 0 represents the signal from the ethanol/gold interface. As established in earlier studies the change of the SHG signal is directly correlated with the thiolate coverage.

(2)

χint(θmax) χsub

(3)

where k is the rate constant of adsorption. c and t denote the concentration of the molecule in the ethanol solution and the time of adsorption, respectively. Iω refers to the intensity of the incident fundamental radiation. R is an empirical constant which is derived from the SHG intensities at zero and maximum coverage of the adsorbate. χsub describes the second-order optical properties of the gold surface prior to adsorption and χint(θmax) represents its alteration by the final adsorbate coverage. The empirical constant R is system dependent and is determined by the type of bonding of the adsorbate to the substrate. To be specific, an equal number of adsorbed thiol or sulfide molecules are likely to cause different changes of the SHG signal if the type of the moleculesubstrate bond is different for the thiol molecule and the sulfide. Conversely, if sulfides yielded films identical with those formed by thiols, the same R value would be expected. As shown in Figure 5, eq 2 satisfactorily describes phenomenologically all adsorption curves and Table 1 lists the corresponding fit parameters. The experimental uncertainty in the value of the rate constants is 5-10%. Even though the agreement of Langmuir fits with the experimental curves varies, we wish to note that this is the best simple model to fit our data. Other models such as a bulk diffusion controlled process or a secondorder process which were suggested in other kinetic studies21,22 fail completely to describe our curves. Since the present paper focuses on the differences in rate constants of adsorption for different classes of organosulfur compounds, rather than on the mechanistic details of thiol adsorption, we postpone a detailed discussion of the different models for the adsorption to a forthcoming paper.27 We first discuss the results for the thiol and disulfide adsorption. The fact that the R values are identical and thus films formed from the two different types of molecules appear identical is in full agreement with the XPS measurements presented above and in the literature.12 However, with respect to the kinetics of film formation there is a difference. One disulfide molecule is equivalent (27) Dannenberger, O.; Buck, M.; Grunze, M. In preparation.

SAMs from Organosulfur Compounds

to two adsorbed thiol molecules, i.e., a 1 mM solution of DDDS corresponds to a 2 mM solution of DT with regard to the total concentration of thiol units. Hence, the rate constant of adsorption of DDDS normalized to the thiol moiety is only 57% of the value for DT. This result seems to be in contrast to adsorption studies of hexadecanethiol and dihexadecyl disulfide.12 On the basis of ellipsometric and contact angle measurements Biebuyck et al. reported the same rates for the thiol and the disulfide. Beside the fact that the density of data points and the error bars given in their work would make it difficult to see a difference in the rate constants of adsorption by 40%, there is another crucial point concerning the sensitivity of the various techniques. Whereas SHG is sensitive to the formation of the sulfur-gold bond only and thus is a direct measure of the coverage of alkanethiolates, contact angle measurements and ellipsometry are determined by the hydrocarbon chains, i.e., their density and conformation. Furthermore, contact angle measurements and ellipsometry cannot differentiate between contributions from contaminations and the hydrocarbon chains of the organosulfur compounds. Contaminations which are continuously displaced by the thiol/disulfide26,28 are either present prior to adsorption and/or, as mentioned above, still adsorb during thiol/disulfide adsorption as long as film formation is incomplete. Since the techniques address different parts of the system, i.e., SHG probes the immediate goldmolecule interface, and ellipsometry and contact angle measurements yield information about the hydrocarbon layer, the measured kinetics is likely to be dependent on the technique. Whereas SHG sees changes right from the beginning of the adsorption process, the other techniques indicate changes only after a considerable delay time. The differences are too large to be explained by the difference in concentrations or by variations in the rate constant of adsorption. The difference in the rate constants between DT and DDDS raises the question of the rate-limiting step of adsorption. An effect due to the larger size of the DDDS molecule can be excluded since based on the comparison with DCT which is of comparable size as DDDS a change by only about 10% is expected. We take this as another indication that the rate-limiting step is not controlled by bulk diffusion. As outlined in detail in a forthcoming paper, we consider displacement of the adsorbed solvent and interaction of the SAM-forming molecules with the substrate the rate-limiting step.27 Because solvent molecules have to be displaced from the surface, the adsorption step is expected to occur faster for a thiol molecule than for a disulfide. Adsorption of thiol requires the availability of one chemisorption site, whereas a disulfide requires the simultaneous creation of two chemisorption sites. Solvent displacement as rate-limiting step agrees as well with the fact that the phenomenological model of Langmuir adsorption kinetics holds surprisingly well. At first glance the assumption of Langmuir kinetics, i.e., a negligible interaction between adsorption sites, seems not to hold. Due to the size and flexibility of the hydrocarbon chain, one would expect that the approach of a molecule to the surface is affected by the molecules already present on the surface. However, if the displacement of the solvent molecule from the surface is rate determining, a hindered approach by the adjacent molecules becomes negligible. The most striking result of the SHG experiment is the large difference of the rate constants between the sulfide and the thiol or disulfide by a factor of at least 500 for (28) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321.

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DDS1 and 2500 for DDS2. As discussed for DDDS, this result cannot be explained by the size of the sulfide molecule, indicating that the adsorption mechanism of the sulfide must be very different from the other two compounds. Three additional points concerning the sulfides of different purity are noteworthy. The first one relates to the difference in the values of R. Whereas the sulfide which was used “as received” yields almost the same value as the thiol and disulfide, the value of R of the purified sulfide is significantly smaller. The second one refers to differences in the rate constants. The explanation for the large difference in rates is obvious considering that the main impurity of the purchased sulfide is the disulfide. Large differences in the rate constants of adsorption are not restricted to simple alkane sulfides and thiols/ disulfides but are valid as well for more complex unsymmetrical sulfides with functionalized endgroups, e.g. carboxylic acid16 or isophthalic acid.29 Troughton et al. investigated film formation from unsymmetrical sulfides and found that thiol impurities produced dramatic changes of the wettability.16 The third point refers to the quantitative relation between the rate constants. The products kc of a 1 mM solution of DDDS and of a 1 mM solution of DDS are identical within the experimental error (see Table 1). In DDS1 the impurity of DDDS is 0.6%, which corresponds to a concentration of 6 mmol/L in a 1 mM solution of DDS. This is six times larger than that expected from the measurement. At present we can only speculate on this discrepancy. Due to the very large ratio of sulfide to disulfide in the solution, disulfide adsorption is paralleled by sulfide adsorption. Therefore, disulfides have to displace sulfides and in this sense sulfides behave like contaminations. As a result disulfide adsorption is slowed analogously to the retardation effect of normal contaminations. As seen by SHG for the adsorption of thiols the rate constant decreases with the time freshly evaporated gold films are exposed to ambient atmosphere.30 Conclusion The combination of SHG, XPS, and electrochemical experiments demonstrated that sulfides on gold substrates do not form films identical with those obtained from adsorption of thiols or disulfides. The results support other work16,19-21 that no C-S bond cleavage occurs upon adsorption of sulfides and is in contrast to the report of C-S cleavage.13 The kinetic studies by SHG revealed differences in the rate constant of adsorption for thiols and disulfides and imply that the rate-limiting step is not controlled by bulk diffusion. Furthermore, the SHG experiments show a strikingly low rate of adsorption for sulfides compared to thiols/disulfides and demonstrate that utmost care is required with respect to other organosulfur impurities when sulfides are used for experiments related to SAMs. Acknowledgment. The authors thank Georg Albert for preparing the substrates, Petra Schlickenrieder for the GC-MS analysis, and Paul Bagus for helpful discussions. Support of the Deutsche Forschungsgemeinschaft (Grant No. Bu 820/10-1) and the Fonds der Chemischen Industrie is gratefully acknowledged. LA9708851 (29) Dannenberger, O.; Jung, C.; Weiss, K.; Wo¨ll, Ch.; Buck, M.; Grunze, M.; Valyavettiil, S.; Mu¨llen, K. In preparation. (30) Dannenberger, O. Unpublished.