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Self-Assembled Monolayers of an Aryl Thiol: Formation, Stability, and Exchange of Adsorbed 2-Naphthalenethiol and Bis(2-naphthyl) Disulfide on Au Randall R. Kolega and Joseph B. Schlenoff* Department of Chemistry and Center for Materials Research and Technology (MARTECH), The Florida State University, Tallahassee, Florida 32306-4390 Received May 11, 1998. In Final Form: July 6, 1998 The adsorption of 2-naphthalenethiol and bis(2-naphthyl) disulfide onto bulk Au has been indirectly quantified from the adsorbate solution by liquid chromatography. A study of the kinetics of monolayer formation and exchange of these adsorbants has been carried out. Using chromatographic retention times and diode array spectroscopy, the products of desorption and exchange have been identified directly. For both aryl-derived monolayers, desorption in pure solvent yields bis(2-naphthyl) disulfide. Exchange of both aryl monolayers with decanethiol is observed, with 2-naphthalenethiol as the sole product, suggesting that exchange involves proton transfer at the Au surface rather than monolayer desorption followed by addition. Exchange of the aryl monolayers with didecyl disulfide is much slower and yields decyl 2-naphthyl disulfide. A comparison of the coverage of the aryl monolayers obtained by chromatography has been made with voltammetric reductive desorption. Confirmation of reductive thiolate desorption by voltammetry is obtained by in situ ultraviolet-visible spectroscopy. Thiolate oxidation of the polycyclic aromatic monolayers under aerobic conditions was not observed. Estimates of the free energies of adsorption for aliphatic and aromatic thiols provide similar values, indicating no strong preference by Au for either type of thiol. Similar conclusions are reached concerning the respective disulfides.
Introduction The chemisorption of organosulfur compounds on metal surfaces is of interest for many applications including electrode modification,1,2 biological sensors,3 photolithography,4 and corrosion protection.5 The most extensively studied monolayer assemblies on Au are those spontaneously formed from long-chain alkanethiol or disulfide solutions.6 Although these self-assembled monolayers (SAMs) are the most comprehensively studied monolayer films due to their high degree of structural order, recent interest has developed in organosulfur monolayers that are bound to Au via an aromatic moiety.7-24 It appears (1) Molecular Design of Electrode Surfaces; Murray, R. W., Ed.; Wiley: New York, 1992. (2) Finklea, H. O. Electrochemistry of Organized Monolayers of Thiols and Related Molecules on Electrodes in Electroanalytical Chemistry; Bard, A. J.; Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19, pp 109-335. (3) (a) Zhong, C.-J.; Porter, M. D. Anal. Chem. 1995, 67, 709A-715A. (b) Cornell, B. A.; Braach-Maksvytis, V. L. B.; King, L. G.; Osman, P. D. J.; Raguse, B.; Wieczorek, L.; Pace, R. J. Nature 1997, 387, 580-583. (4) (a) Tarlov, M. J.; Burgess, D. R. F.; Gillen G. J. Am. Chem. Soc. 1993, 115, 5305-5306. (b) Huang, J.; Dahlgren, D. A.; Hemminger, J. C. Langmuir 1994, 10, 626-628. (5) (a) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9022-9028. (b) Ohno, N.; Uehara, J.; Aramaki, K. J. Electrochem. Soc. 1993, 140, 2512-2519. (6) For reviews on SAMs see: (a) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self- Assembly; Academic: San Diego, CA, 1991; (b) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437-463; (c) Delamarche, E.; Michel, B.; Biebuyck, H. A.; Gerber, C. Adv. Mater. 1996, 8, 719-729; (d) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (7) (a) Bravo, B. G.; Michelhaugh, S. L.; Soriaga, M. P. J. Electroanal. Chem. 1988, 241, 199-210. (b) Mebrahtu, T.; Berry, G. M.; Bravo, B. G.; Michelhaugh, S. L.; Soriaga, M. P. Langmuir 1988, 4, 1147-1151. (8) Kwan, W. S. V.; Atanasoska, L.; Miller, L. L. Langmuir 1991, 7, 1419-1425. (9) Sabatani, E.; Cohen-Boulakia, J.; Bruening, M.; Rubinstein, I. Langmuir 1993, 9, 2974-2981. (10) (a) Chadwick, J. E.; Myles D. C.; Garrell, R. L. J. Am. Chem. Soc. 1993, 115, 10364-10365. (b) Garrell, R. L.; Chadwick, J. E. Colloids Surf. A: Physicochem Eng. Aspects 1994, 93, 59-72. (11) Creager, S. E.; Steiger, C. M. Langmuir 1995, 11, 1852-1854.
that the thiolate headgroup of alkanethiol SAMs is prone to oxidation under ambient conditions, resulting in the formation of sulfinates and sulfonates.25 These oxidized monolayers have compromised structural stability because they are no longer chemisorbed and can be washed away.4,14 It has been suggested that when the mercapto group is directly attached to an aryl, the thiolate is less prone to oxidation under ambient conditions.10 SAMs have been studied with numerous surface optical and spectroscopic techniques, such as ellipsometry,26,27 (12) Chang, S.-C.; Chao, I.; Tao, Y.-T. J. Am. Chem. Soc. 1994, 116, 6792-6805. (13) Tour, J. M.; Jones, II, L.; Pearson, D. L.; Lamba, J. J. S.; Burgin, T. P.; Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. V. J. Am. Chem. Soc. 1995, 117, 9529-9534. (14) Garrell, R. L.; Chadwick, J. E.; Severance, D. L.; McDonald, N. A.; Myles D. C. J. Am. Chem. Soc. 1995, 117, 11563-11571. (15) Tsutsumi, H.; Furumoto, S.; Morita, M.; Matsuda, Y. J. Colloid Interface Sci. 1995, 171, 505-511. (16) Wells, M.; Dermody, D. L.; Yang, H. C.; Kim, T.; Crooks, R. M.; Ricco, A. J. Langmuir 1996, 12, 1989-1996. (17) Hayes, W. A.; Shannon, C. Langmuir 1996, 12, 3688-3694. (18) Lee, Y. J.; Jeon, I. C.; Paik, W.-k; Kim, K. Langmuir 1996, 12, 5830-5837. (19) (a) Dhirani, A.-A.; Zehner, R. W.; Hsung, R. P.; Guyot-Sionnest, P.; Sita, L. R. J. Am. Chem. Soc. 1996, 118, 3319-3320. (b) Zehner, R. W.; Sita, L. R. Langmuir 1997, 13, 2973-2979. (20) Sato, Y.; Fujita, M.; Mizutani, F.; Uosaki, K. J. Electroanal. Chem. 1996, 409, 145-154. (21) Lamp, B. D.; Hobara, D.; Porter, M. D.; Niki, K.; Cotton, T. M. Langmuir 1997, 13, 736-741. (22) Bandyopadhyay, K.; Sastry, M.; Paul, V.; Vijayamohanan, K. Langmuir 1997, 13, 866-869. (23) Tao, Y.-T.; Wu, C.-C.; Eu, J.-Y.; Lin, W.-L.; Wu, K.-C.; Chen, C. Langmuir 1997, 13, 4018-4023. (24) Sastry, M.; Patil, V.; Mayya, K. S. J. Phys. Chem. B. 1997, 101, 1167-1170. (25) (a) Li, Y.; Huang, J.; McIver, R. T.; Hemminger, J. C. J. Am. Chem. Soc. 1992, 114, 2428-2432. (b) Tarlov, M. J.; Newman, J. G. Langmuir 1992, 8, 1398-1405. (c) Reiley, H.; Price, N. J.; White, R. G.; Blyth, R. I. R.; Robinson, A. W. Surf. Sci. 1995, 331-333, 189-195. (d) Horn, A. B.; Russell, D. A.; Shorthouse, L. J.; Simpson, T. R. E. J. Chem. Soc., Faraday Trans. 1996, 92, 4759-4762. (e) Scott, J. R.; Baker, L. S.; Russell Everett, W.; Wilkins, C. L.; Fritsch, I. Anal. Chem. 1997, 69, 2636-2639.
S0743-7463(98)00553-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/15/1998
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reflection-absorption Fourier transform infrared (FTIR) spectroscopy,26,28 surface-enhanced Raman spectroscopy,29 X-ray photoelectron spectroscopy (XPS),27,28 and electron and atom diffraction.30 Physical methods applied to these systems include wetting contact angle measurements27,28 and scanning probe microscopy.31 Most of these analytical tools, although providing information on the surface composition, the orientation, and the properties of monolayers, are either qualitative or semiquantitative in nature.2,6 With these techniques, a great deal of insight has been obtained on the structural characterization of alkanethiol SAMs, but the fundamentals of formation, stability, and exchange are not as well understood. Quantitative in situ tools for surface studies of SAMs deposited from the solution phase include second-harmonic spectroscopy,32 the quartz crystal microbalance,33-36 and radiolabeling.37 Electrochemical studies have been particularly successful at addressing some the fundamental issues of self-assembly.2 Redox-active molecules, such as alkanethiols derivitized with a pendant ferrocene group and immobilized onto electrode surfaces, have provided insight into competitive self-assembly of mixtures of sulfur-containing compounds onto Au, as well as exchange.38-43 The present work employs aromatic thiols with high optical extinction coefficients in the ultraviolet-visible (UV-vis) region. These UV “labels” can yield information on the formation, desorption, and exchange of single component and mixed monolayers. Our approach employs a chromatographic scheme for quantitative and qualitative analysis, using multichannel (diode-array) detection. We are able to track individual components (including thiols, disulfides, mixed disulfides, oxidized thiols, and cleavage products) of solutions in contact with SAMs on Au of the type shown in Figure 1. Further, our experiments have been performed with dilute solutions, where the quantities of possible impurities are limited to a few percent of a monolayer. Questions regarding the contribution of impurities, such as disulfides in thiols, and vice versa, in SAM formation have thus been eliminated. Our separations studies have been coordinated with complementary (26) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (27) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (28) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parikh, Y. T.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152-7167. (29) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 8284-8293. (30) (a) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546-558. (b) Camillone, N. III; Chidsey, C. E. D.; Liu, G.-y.; Putvinski, T. M.; Scoles, G. J. Chem. Phys. 1991, 94, 8493-8501. (31) (a) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2805-2810. (b) Alves, C. A.; Smith, E. L.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 1222-1227. (32) Buck, M.; Eisert, F.; Fischer, J.; Grunze, M.; Tra¨ger, F. Appl. Phys. A 1991, 53, 552-556. (33) Buttry, D. A.; Ward, M. D. Chem. Rev. 1992, 92, 1355-1379. (34) Schneider, T. W.; Buttry, D. A. J. Am. Chem. Soc. 1993, 115, 12391-12397. (35) Karpovich, D. S.; Blanchard, G. J. Langmuir 1994, 10, 33153322. (36) Schlenoff, J. B.; Dharia, J. R.; Xu, H.; Wen, L.; Li, M. Macromolecules 1995, 28, 4290-4295. (37) Schlenoff, J. B.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 12528-12536. (38) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301-4306. (39) Collard, D. M.; Fox, M. A. Langmuir 1991, 7, 1192-1197. (40) Hickman, J. J.; Ofer, D.; Zou, C.; Wrighton, M. S.; Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1991, 113, 1128-1132. (41) Uosaki, K.; Sato, Y.; Kita, H. Langmuir 1991, 7, 1510-1514. (42) Creager, S. E.; Rowe, G. K. J. Electroanal. Chem. 1994, 370, 203-211. (43) Rowe, G. K.; Creager, S. E. Langmuir 1994, 10, 1186-1192.
Kolega and Schlenoff
Figure 1. HPLC determination of two representative trials of the nominal surface coverage on Au of bis(2-naphthyl) disulfide versus time from 3.4 µM solution. The error bar is representative of two relative standard deviations of the HPLC injection variability. The insert is a qualitative schematic diagram of 2-naphthalenethiolate anchored to a Au interface. The dimension shown is the van der Waals diameter of naphthalene as drawn with the CsSsAu bond parallel to the surface normal.
electrochemical experiments to bring together a consistent picture of aromatic SAM behavior on Au. Experimental Section Materials. 2-Naphthalenethiol was obtained from Aldrich (99%) and was used as received. 1-Decanethiol (Aldrich, 97%) was distilled under reduced pressure and was >97% pure according to 1H NMR and HPLC with didecyl disulfide as the only impurity. Bis(2-naphthyl) disulfide, didecyl disulfide, and decyl 2-naphthyl disulfide were synthesized following a published procedure for the oxidation of thiols to disulfides with the only modification being the substitution of iodine in place of bromine.44 Briefly, the respective disulfides were synthesized from the corresponding thiol(s) in a two-phase system of CH2Cl2 and 10% aqueous KHCO3. A slight excess of iodine was slowly added to the respective mixtures, which were stirred for several hours at 25 °C. Bis(2-naphthyl) disulfide precipitated and was washed with water and ethanol. Repeated recrystallization from CH2Cl2 yielded 99.97% purity disulfide (by HPLC); mp 141.8-142.0 °C (lit. 141.8-142.6 °C).45 For the didecyl disulfide mixture, the organic reaction phase was separated, washed with water and dried with anhydrous MgSO4, and the solvent evaporated. Didecyl disulfide purification was done with silica column chromatography (hexanes), and the purity was >99.7%. 1H NMR and IR spectra of bis(2-naphthyl) disulfide and didecyl disulfide matched those of authentic samples.46 Decyl 2-naphthyl disulfide was obtained in the same manner as didecyl disulfide. Purification was done with silica column chromatography (hexanes) followed by recrystallization from MeOH; mp 40.6-41.5 °C; 1H NMR (300 MHz, CDCl3, δ ) ppm, J ) Hz): δ ) 7.98 (s, 1H); δ ) 7.9-7.7 (m, 3H); δ ) 7.66-7.60 (m, 1H); δ ) 7.47 (m, 2H); δ ) 2.71, J ) 7.2 (t, 2H); δ ) 1.68, J ) 7.2 (p, 2H); δ ) 1.4-1.2 (m, 14H); and δ ) 0.87, J ) 6.6 (t, 3H). Substrate and Monolayer Preparation. All monolayer formation, exchange, and desorption solutions were in acetonitrile (Fisher HPLC or Optima grade), unless noted otherwise, and the solvent was used without further purification. Acetonitrile was chosen for its optical transparency throughout the near UV range. For all experiments, fresh reaction solution was always prepared prior to use and all solutions were protected from light. Reaction volumes used were from 7.5 to 8.5 mL. Monolayers were formed by immediate immersion of cleaned Au foil into adsorbate solution. No effort was made to deoxygenate the adsorbate solutions or any of the solutions analyzed by HPLC. (44) Drabowicz, J.; Mikolajczyk, M. Synthesis 1980, 32-34. (45) Weinstein, A. H.; Pierson, R. M. J. Org. Chem. 1958, 23, 554560. (46) Sadtler Research Laboratories. Standard NMR Spectra Collection and Standard Infrared Spectra Collection; Division of Bio-Rad Laboratories: Philadelphia, PA.
Self-Assembled Monolayers of an Aryl Thiol The Au foil (>99.99%) was cleaned prior to each experiment as follows: treated with fresh piranha (7/3 v/v concentrated H2SO4/H2O2); polished successively with 5, 1, 0.3, and 0.05 µm Buehler alumina polish; ultrasonicated in water; retreated with piranha for a maximum of 10 min; ultrasonicated in water; rinsed extensively with water and acetonitrile; and finally dried in a steam of nitrogen. Water (18 MΩ) from a Barnstead NANOpure II purification system was used. Caution! Piranha solution should be handled with extreme care and should never be stored in a closed container. It is a very strong oxidant and reacts violently with most organic materials. After cleaning, the Au foil was completely wetted by water. Except for voltammetric desorption, all experiments were performed in piranha-cleaned, disposable, 20-mL scintillation vials that were rinsed extensively with water and acetonitrile.47 All experiments were performed at room temperature. Formation of the monolayers for desorption and exchange involved immersion of the Au substrates into 4-8 µM adsorbate solution for a minimum of 48 h, unless noted otherwise. Upon removal from the formation solution, SAMs were rinsed repeatedly with fresh solvent, dried under nitrogen, and immediately immersed into desorption or exchange solutions. HPLC. Reversed-phase HPLC separations were performed on a YWC C18 packed column (type: R-ODS-5) with a Beckman 168 UV diode array detector set in the scan range of 210 to 350 nm. The chromatograms were acquired at 241 nm with a sample loop size of 20 µL. Elution was isocratic at a flow rate of 1 mL min-1. The mobile phase was 86% acetonitrile (Fisher, Optima grade)/14% H2O (Fisher, Optima grade) for all experiments except for the polycyclic aromatic monolayer exchange with didecyl disulfide experiments in which case 100% acetonitrile was used. All species were baseline resolved except for decanethiol and bis(2-naphthyl) disulfide. Retention times with 86% acetonitrile/ 14% H2O as the mobile phase are as follows: 2-naphthalenethiol, tr ) 4.83 min; decanethiol, tr ) 15.0 min; and bis(2-naphthyl) disulfide, tr ) 15.18 min. Retention times with 100% acetonitrile as the mobile phase are as follows: 2-naphthalenethiol, tr ) 3.98 min; decanethiol, tr ) 6.55 min; bis(2-naphthyl) disulfide, tr ) 6.58 min; decyl 2-naphthyl disulfide, tr ) 14.00 min; and didecyl disulfide, tr ) 38.9 min. Due to their high UV transparency, retention times of decanethiol and didecyl disulfide were determined from millimolar solutions. The HPLC relative standard deviation for consecutive 3.5 to 9 µM aryl compound stock solution injections was ∼2%. The limit of detection (minimum detectable) for these experiments is ∼3% of a polycyclic aromatic monolayer. Evaporative loss for all the capped reaction vials was periodically monitored by weight loss and was determined to be insignificant. All coverages are reported as nominal values based on the geometric area of the foil, unless noted otherwise, and do not take into consideration the surface roughness of the polished Au, which was determined to be 1.7 ( 0.1 by radiolabeling37 and adsorbed iodine oxidation.48 The same piece of Au foil (19.36 cm2 geometric area) was used for all the HPLC experiments. This procedure gave a highly reproducible surface for adsorption regardless of the number of cycles of cleaning and use. All monolayer formation experiments examined by HPLC involved coexamination of blank solutions to check for any anomalous behavior of the solutions in the absence of Au. The monolayer formation and exchange solutions examined by HPLC were stirred with a Teflon-coated magnetic stir bar. Voltammetric Reductive Desorption. Voltammetry was performed with a Princeton Applied Research Model 362 scanning potentiostat. The electrochemical measurements were in a conventional three-electrode cell with 0.5 M KOH (Fisher, certified A. C. S.) in purified water with a platinum wire counter electrode and a Ag/AgCl (saturated KCl) double junction reference electrode, unless noted otherwise. The Au foil working electrodes, cut from the same foil as used for the HPLC experiments, were (47) Used as received or base-cleaned vials, no matter how extensive the rinsing, catalyzed the oxidation of dilute 2-naphthalenethiol as determined by HPLC. Base is well known to catalyze the oxidation of thiols. see Capozzi, G.; Modena, G. Oxidation of Thiols In The Chemistry of the Thiol Group; Patai, S., Ed.; Wiley: London, 1974; pp 785-839. (48) Rodriguez, J. F.; Mebrahtu, T.; Soriaga, M. P. J. Electroanal. Chem. 1987, 233, 283-289.
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Figure 2. Normalized UV spectra in acetonitrile of 2-naphthalenethiol, ) 42 000 @ 241 nm (dark solid line); bis(2naphthyl) disulfide, ) 62 800 @ 248 nm, ) 66 000 @ 215.5 nm (- - -); 2-naphthalenesulfonic acid, ) 100 000 @ 228 nm (- - -) and naphthalene, ) 99 000 @ 220 nm (light solid line). polished and cleaned with piranha as already described and had geometric areas of 3.38 cm2. Desorption solutions were purged for a minimum of 20 min with Ar prior to the desorption and a blanket of Ar was maintained over the solutions during the measurements. All voltammetric desorption curves were acquired as single-scan cyclic voltammograms at a scan rate of 100 mV s-1. The scan range for the Au foil electrodes was -0.4 to -1.4 V, starting at the anodic potential. The Au ball electrodes were prepared by exposing ends of 25µm Au wire (99.9%) to H2 flame and annealing in the cooler regions of the flame. The electrodes were used immediately after preparation. UV-Vis/In Situ Reductive Desorption. UV-vis spectra were acquired on the Cary 3E spectrophotometer. In situ UV reductive desorption was performed in a 1-cm quartz cuvette modified for electrodes and Ar purge. Reductive desorption was performed under the same conditions as already noted except an Ag wire was used as a pseudo-reference electrode in place of the Ag/AgCl double junction reference electrode and the desorption solution was constantly stirred with a Teflon-coated magnetic stir bar.
Results Formation of Aryl Monolayers. The kinetics of bis(2-naphthyl) disulfide adsorption to Au in acetonitrile were studied with HPLC by determining the concentration drop in the adsorbate solution. The nominal surface coverage can be obtained from the amount of disulfide depleted from the adsorbate solution. Representative curves of the surface coverage of bis(2-naphthyl) disulfide versus time are presented in Figure 1. The coverage reached (6.8 ( 0.4) × 10-10 mol cm-2 of naphthalene. (Surface coverage is given in terms of naphthalenethiolate, which is twice the number of disulfides adsorbed.) On the time scale of these experiments, no other species, aside from the disulfide, were observed in both the adsorbate and blank solutions. Also, it must be emphasized that inclusion of adventitious 2-naphthalenethiol is insignificant: given the purity of the disulfide, and the low concentration and volume used for adsorption, a maximum of 0.4% of the monolayer could be due to the thiol impurity. It should be noted that the identity of the chromatographic peaks was checked not only by retention time, but also by their UV spectra using the diode array detector. A component analysis of the UV spectra verified the purity of each peak because the naphthalene moiety is highly sensitive to any attached substituent, as seen in Figure 2. The adsorption of 2-naphthalenethiol from acetonitrile is depicted in Figure 3. An identical plot was obtained
5472 Langmuir, Vol. 14, No. 19, 1998
Figure 3. HPLC determination of the nominal surface coverage on Au of 2-naphthalenethiol versus time from 6.3 µM solution, corrected for disulfide formation ([) and uncorrected for disulfide evolution (0). HPLC determination of the nominal surface coverage on Au of 2-naphthalenethiol versus time from an equimolar mixture of 4.6 µM 2-naphthalenethiol and decanethiol corrected for disulfide evolution (4).
when cyclohexane was used as the adsorption solvent. Addition is complete within 45 min and a half dozen trials in acetonitrile yielded a saturation coverage of (7.8 ( 1.0) × 10-10 mol cm2. Disulfide formation in the adsorbate solution, although fairly slow, is observed and accounted for in these experiments.49 This result corresponded to an additional conversion in solution of ∼5% of a monolayer per day to the disulfide. Further oxidation of the adsorbate solutions to form sulfinic or sulfonic acid was not observed by HPLC. A plot of 2-naphthalenethiol addition to Au from an equimolar solution of naphthalenethiol and decanethiol is also presented in Figure 3. Approximately 1/3 of a complete naphthalene monolayer formed, which would suggest a slight preference for the alkanethiol. Note that the coverage remains fairly constant with time. A mixture of 4.2 µM 2-naphthalenethiol and 2.1 µM bis(2-naphthyl) disulfide showed no initial preference of either species for adsorption (plot of addition versus time not shown). The equilibrium composition of this monolayer, however, cannot be determined from the adsorbate solution (see following discussion). Exchange and Desorption of Aryl Monolayers. The kinetics of preformed 2-naphthalenethiol monolayer exchange with UV-transparent decanethiol are presented in Figure 4. The sole product of exchange was readily identified as 2-naphthalenethiol. This exchange is not due to thiol-disulfide interchange in solution because it is much too slow in the absence of base.50 Control solutions of 0.1 mM decanethiol with 1 µM bis(2-naphthyl) disulfide yielded, at most, 5% interchange over a period of a week. The exchange in Figure 4 shows a slight concentration dependence; however, the exchange rate does not fit simple order kinetics (initial order ) 0.2 in decanethiol). The final amount exchanged was 6.8 × 10-10 mol cm-2 and is believed to be quantitative. This value is taken to be the nominal monolayer coverage of 2-naphthalenethiol on the polycrystalline Au used in these experiments (vide infra). It is identical to the coverage of the aryl disulfide derived monolayer in Figure 1. A plot of the rate of exchange of a bis(2-naphthyl) disulfide-derived SAM with 0.13 mM decanethiol is essentially identical with that of the aryl thiol-derived monolayer (curve not shown). All of this (49) Disulfide formation was accounted for in the coverage by adding 2 mol of disulfide formed in solution to the moles of thiol missing. (50) Dalman, G.; McDermed, J.; Gorin, G. J. Org. Chem. 1964, 29, 1480-1484.
Kolega and Schlenoff
Figure 4. HPLC determination of the kinetics of exchange of 2-naphthalenethiol derived monolayers in 1.3 mM decanethiol ([); 0.13 mM decanethiol (0); and 0.013 mM decanethiol (4), with the sole exchange product identified as 2-naphthalenethiol. Identical behavior was observed with the aryl disulfide-derived monolayer (data not shown). Bis(2-naphthyl) disulfide monolayer exchange with 0.1 mM didecyl disulfide as decyl 2-naphthyl disulfide (O). For both types of aryl monolayers in millimolar solutions of didecyl disulfide at 115 h, 19% of the monolayers exchanged as the mixed disulfide and 4 ( 2% desorbed as bis(2-naphthyl) disulfide over this time.
monolayer exchanges with decanethiol, with the sole exchange product identified as 2-naphthalenethiol. Exchange with a disulfide, didecyl disulfide, showed markedly different behavior. The overall amount of aryl thiolate lost from the surface was very low. Analysis of either preformed aryl monolayer showed two new solution species following attempts at exchange with didecyl disulfide: bis(2-naphthyl) disulfide and decyl naphthyl disulfide. A few percent of a monolayer identified as disulfide in the exchange solution was attributed to spontaneous desorption. For example, immersing either aryl monolayer in pure solvent (acetonitrile, ethanol, methanol) for 72 h yielded ∼2% of the monolayer spontaneously desorbing as bis(2-naphthyl) disulfide.51 The slow appearance of decyl 2-naphthyl disulfide is attributed to aryl monolayer exchange with didecyl disulfide (see Figure 4). On termination of these experiments, the balance of the unexchanged naphthalenethiolate in the SAMs was accounted for by adding decanethiol and observing complete desorption of 2-naphthalenethiol. Control solutions of bis(2-naphthyl) disulfide-didecyl disulfide and 2-naphthalenethiol-didecyl disulfide showed that solution interchange is not significant. Electrochemistry. Voltammetric reductive desorption of sulfur-containing SAMs has been used to determine surface coverage. Desorption in strong alkali is understood to occur as a one-electron process as follows: RSAu(s) + e- f RS- + Au(s).2,52 Figure 5 presents direct spectroscopic evidence that voltammetric reductive desorption indeed involves loss of thiolate from the surface. The lower curve in Figure 5 is a UV spectrum in 0.5 M KOH of a reductively desorbed monolayer derived from 2-naphthalenethiol. The upper curve is of a standard addition of thiol into the base. Note that the absorption maximum of 2-naphthalenethiol in 0.5 M KOH experiences a bathochromic shift (compare with Figure 2). Reductive (51) Desorption solutions were preconcentrated with an N2 stream before analysis. (52) (a) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335-359. (b) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 26872693.
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Langmuir, Vol. 14, No. 19, 1998 5473
Figure 5. In situ UV spectrum of reductively desorbed 2-naphthalenethiol monolayer from Au foil (4.55 cm-2 nominal surface area) in stirred 0.5 M KOH with the potential held at -1.25 V and purged with Ar (lower curve). A silver wire was used as the pseudo-reference electrode and a platinum wire was the counter electrode. 2-Naphthalenethiol in 0.5 M KOH (upper curve).
removal of the monolayer as determined by UV spectroscopy was essentially identical to the coverage obtained by HPLC. Typical reductive desorption voltammograms of monolayers derived from micromolar solutions of 2-naphthalenethiol with various immersion times are presented in Figure 6. At least three peaks are observed. Growth of the most cathodic peak with time was observed with the aryl thiol. This behavior was not observed with the alkanederived SAMs at any concentration. Bis(2-naphthyl) disulfide-coated electrodes exhibited an almost identical peak shift (data not shown). After complete monolayer formation, within an hour, the growth of the most cathodic peak, at the expense of the others, occurs whether the electrode is immersed in the adsorbate solution or emersed in ambient. Monolayers formed from short exposure to millimolar solutions of aryl adsorbate yield desorption voltammograms similar to those for micromolar solutions at long time, and no cathodic shift is apparent. Figure 7 depicts reductive desorption curves of Au electrodes with monolayers from millimolar solutions of aryl thiol. Reductive desorption curves of decanethiol and didecyl disulfide monolayers are included for comparison and are typical for alkanethiol SAMs on rough polycrystalline Au substrates. The multiple peaks are believed to be due to the different desorption energetics of the aliphatic SAMs from various surface sites of polycrystalline Au substrates.53 Also, from Figures 6 and 7, note that the reductive removal of the naphthalenethiolate monolayers occurs away from the sloping current caused by the reduction of solvent. This should allow for a more accurate estimation of the surface coverage of the naphthalene monolayers than that of the decanethiolates. Figure 8, depicting the apparent coverage as a function of time, taken together with Figure 7, reveals no difference in electrochemical behavior between thiol- and disulfidederived aryl SAMs. The voltammetry experiments show that even in micromolar solutions of adsorbate, addition is rapid and monolayer formation complete within an hour, which is in agreement with the HPLC experiments. From (53) (a) Walczak, M. M.; Alves, C. A.; Lamp, B. D.; Porter, M. D. J. Electroanal. Chem. 1995, 396, 103-114. (b) Zhong, C.-J.; Zak, J.; Porter, M. D. J. Electroanal. Chem. 1997, 421, 9-13. (c) Yang, D.-F.; Wilde, C. P.; Morin, M. Langmuir 1997, 13, 243-249. (d) Sellers, H.; Ulman, A.; Schnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389-9401.
Figure 6. Voltammetric reductive desorption curves in 0.5 M KOH of Au electrodes modified by immersion for various times in 8.3 µM 2-naphthalenethiol. As-prepared clean Au foil (a); 2-min immersion time gave a coverage of 48% of the saturation value (b); 15-min immersion time gave a coverage of 77% of the saturation value (c); 1-h immersion time gave a coverage of 100% of the saturation value (d); and 66-h immersion time gave a coverage of 100% of the saturation value (e).
the electrochemistry, a complete monolayer forms within minutes from millimolar concentrations of either aryl or aliphatic adsorbate solutions. Discussion Formation, Desorption, and Stability of Naphthalenethiolate Monolayers. If the sulfur headgroup were sp hybridized (calculations show small energy differences between sp and sp3 S),53d as shown in Figure 1, with no intercalation of the naphthalene, the theoretical coverage corresponds to ∼35.5 Å2 per molecule, given that the van der Waals dimensions of naphthalene are 8.12 × 7.36 × 3.4 Å.54 An estimated coverage of 4.0 × 10-10 mol per real cm2 or 41.4 Å2 per naphthalene on smooth polycrystalline Au is obtained from the chromatography using a roughness factor of 1.7. This result suggests that the naphthalene end-group might lie somewhat canted toward the Au surface. A limited number of naphthalenethiolate SAMs on Au have been studied.12,22 Thiophenol has been shown to form a poor monolayer whereas benzyl mercaptan is more densely packed.9,23 Also, when more phenyl groups are linearly attached to thiophenol, rather well-ordered SAMs have been observed, with the aromatic rings being nearly perpendicular to the Au surface.9,19,23 A coverage of 4.4 × 10-10 mol cm-2 was obtained by Tao et al.23 for thiophenol on Au. From cyclic voltammetry at 0 V in 0.5 M KOH, we find a double-layer (54) Gland, J. L.; Somorjai, G. A. Surf. Sci. 1973, 38, 157-186.
5474 Langmuir, Vol. 14, No. 19, 1998
Kolega and Schlenoff
techniques.55 According to the reductive desorption measurements, a constant saturation coverage of aryl thiol was obtained using adsorption solutions with a concentration range of 4 orders of magnitude. Thus, it appears that these aryl monolayers can be described by a very flat adsorption isotherm. A high affinity isotherm, coupled with the fact that only a few percent of either aryl monolayer spontaneously desorbed as the bis(2-naphthyl) disulfide in pure solvent, suggests that these SAMs are very strongly adsorbed. If we define, for RSSR + 2Au(s) h 2RSAu(s),
Kads )
Figure 7. Voltammetric curves for the reductive desorption of monolayers (saturation coverage) on Au in 0.5 M KOH derived from 2mM 2-naphthalenethiol (a); 1 mM bis(2-naphthyl) disulfide (b); 2 mM decanethiol (c); and 1 mM didecyl disulfide (d). Within experimental error, voltammetric curves are typical of monolayers formed from millimolar adsorbate solutions with immersion times of minutes to >60 h.
Figure 8. Nominal surface coverage of monolayers on Au electrodes versus immersion time in 8.3 µM 2-naphthalenethiol ([) and 4.1 µM bis(2-naphthyl) disulfide (0) as determined by voltammetric desorption.
capacitance of decanethiol- and naphthalenethiol-coated Au foil of 3 and 7 µF cm-2, respectively. The immersion time necessary to form a complete aryl monolayer was found for both the in situ HPLC and ex situ electrochemical techniques to be from 30 to 60 min from micromolar adsorbate solutions, with most of the SAM forming within minutes. A review on the published kinetics of alkanethiol monolayer formation from the solution phase reports a wide range of times required for SAM formation, from 4 s to over 27 h, depending on a variety of experimental conditions and measurement
[RSAu(s)]2 [RSSR][Au(s)]2
(1)
then for 2% desorption in 10 mL of solvent from a 20 cm2 Au foil, Kads is ∼1011. By contrast, our previous work with radiolabeled alkanethiol SAMs indicated more extensive spontaneous desorption in pure solvent,37 although the desorption products were not determined. The only work involving the identification of the species of spontaneous desorption comes from temperature-programmed reaction spectroscopy, where at elevated temperatures the monolayer is removed as the disulfide, although this result can be complicated by a variety of decomposition products.56 It should be noted that aerobic conditions have been employed throughout our work. The role of oxidizing agents, such as oxygen, in facilitating desorption should be considered. Possible modes of oxidizer-assisted SAM desorption include those where disulfide formation is promoted: for example, Templeton et al.57 have recently found that iodine quantitatively removes adsorbed thiols as disulfides from Au nanoparticles.58 Alternatively, in light of several studies that have shown oxidative instability of SAMs,25 it may be that surface thiols are oxidized to weaker-binding sulfur in a higher oxidation state. We find evidence for limited desorption as disulfides for aryl thiols, but no oxidation products. In contrast, because alkane thiols are less resistant to oxidation,10 desorption as sulfinate or sulfonate cannot be ruled out in our previous work. Garrell et al.10,14 report that the benzenethiolate monolayer is more resistant to sulfur headgroup oxidation than are alkanethiolate SAMs under ambient conditions. Greater oxidative stability probably applies to all aromatic hydrocarbon thiolate monolayers, because no higher oxidation product of bis(2-naphthyl) disulfide was ever detected in any of our naphthalenethiolate adsorption, desorption, or exchange solutions. The ingrowth of bis(2-naphthyl) disulfide in aryl thiol solution is slightly enhanced by the presence of the Au surface.59 In contrast to this small catalytic effect, Mohri et al.60 found rapid conversion of 4-aminobenzenethiol to the corresponding disulfide under the influence of Au (55) Bensebaa, F.; Voicu, R.; Huron, L.; Ellis, T. H. Kruus, E. Langmuir 1997, 13, 5335-5340. (56) (a) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733-740. (b) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 588-569. (c) Jaffey, D. M.; Madix, R. J. J. Am. Chem. Soc. 1994, 116, 3020-3027. (57) Templeton, A. C.; Hostetler, M. J.; Kraft, C. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 1906-1911. (58) Analogous to the result found in ref 57, a series of Au thiolate compounds were synthesized and were found to be unstable in the presence of oxidizer or air, resulting in the evolution of disulfide. Schlenoff, J. B., Kolega, R. R., unpublished results. (59) Oxidation in the blank solutions occurred at an average rate of ∼1%/day conversion of 2-naphthalenethiol to the disulfide. In the presence of Au foil, conversion was ∼2.5%/day. (60) Mohri, N.; Inoue, M.; Arai, Y.; Yoshikawa, K. Langmuir 1995, 11, 1612-1616.
Self-Assembled Monolayers of an Aryl Thiol Scheme 1. Proposed Mechanism of Aryl Monolayer Exchange with Alkanethiol
powder. This more marked catalytic behavior is a consequence of higher specific surface area of the Au morphology employed. Aminothiophenol may also be inherently easier to oxidize. As seen in Figure 3, formation of a monolayer derived from an equimolar composition of decanethiol and 2-naphthalenethiol did not strongly favor decanethiol, both initially and over time, which suggests no strong thermodynamic preference for either species, assuming the system is at equilibrium upon termination of the experiment. The initial composition of mixed monolayers is, in general, controlled by the solution ratio and is diffusion limited. The final composition reflects a balance between a number of variables that are discussed in the literature.6,61 We believe that the uptake of 2-naphthalenethiol in Figure 3 constitutes slightly more than a monolayer. This view is supported by the identical reductive desorption of both types of aryl monolayers seen in Figure 8. In addition, an identical amount of desorption and exchange was observed by HPLC with either the aryl thiol or its disulfidederived monolayers. The implied quantitative similarity between the aryl thiol and disulfide monolayers was confirmed when a preformed bis(2-naphthyl) disulfide SAM was added to a 2-naphthalenethiol solution and resulted in no apparent uptake of adsorbent. Most prior work holds that alkanethiols and their respective disulfides produce indistinguishable monolayers,6 although contact angle measurements suggest slightly more efficient packing with thiols and the ability to complete a SAM formed from its corresponding disulfide.62 The higher apparent coverage of naphthalenethiol could be due to “etching” of the surface by thiols,63 but not disulfides, which could increase the consumption of 2-naphthalenethiol in solution to yield insoluble Au complexes. Physisorption of 2-naphthalenethiol onto an existing SAM is possible: multilayer formation of SAMs in the solution phase has been reported.18,34,64 Whatever the reasons, it appears that the disulfide adsorbs in a somewhat more predictable fashion. Exchange of Preformed Naphthalenethiolate SAMs. We have shown unequivocally that in place exchange with solution decanethiol, SAMs desorb as thiols. Exchange with disulfides is limited and much slower. Because no aryl disulfide or mixed disulfide was observed in solution, exchange of naphthalenethiolate SAMs with decanethiol clearly involves proton transfer at the Au surface followed by desorption as in Scheme 1. Exchange does not involve desorption of the SAM as the disulfide (61) (a) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155-7164. (b) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164-7175. (62) Biebuyck, H. A.; Bain, C. D.; Whitesides, G. M. Langmuir 1994, 10, 1825-1831. (63) (a) Edinger, K.; Go¨lzha¨user, A.; Demota, K.; Wo¨ll, Ch.; Grunze, M. Langmuir 1993, 9, 4-8. (b) Scho¨nenberger, C.; Sondag-Huethorst, J. A. M.; Jorritsma, J.; Fokkink, L. G. J. Langmuir 1994, 10, 611-614. (c) McDermott, C. A.; McDermott, M. T.; Green, J.-B.; Porter, M. D. J. Phys. Chem. 1995, 13257-13267 and references therein. (64) Kim, Y. T.; McCarley, R. L.; Bard, A. J. Langmuir 1993, 9, 19411944.
Langmuir, Vol. 14, No. 19, 1998 5475
followed by addition of the thiol to the surface, as we postulated for alkanethiol SAMs.37 We emphasize that solution thiols are not a consequence of the interchange of solution species, because comparison solutions containing thiols but no Au yielded little interchange over several days.50 Desorption of the thiolate anion is unlikely because alkanethiol SAMs have been shown to be fairly stable to acidic conditions as well as hydrogen-saturated solutions.27,37 Homolytic cleavage of the SsAu bond should be discounted because it is not consistent with experimental results.65 The bimolecular mechanism proposed by Scheme 1 is contrary to the commonly held view of exchange of alkanethiolate SAMs. Prior studies, including our own, have inferred that dissociation must be the rate-limiting step of exchange.37,39,40 Evidence that we cited to support this conclusion included a weak dependence of exchange rate on solution thiol concentration. After considering prior work showing (i) desorption of SAMs as disulfides in temperature-programmed desorption56 and (ii) diffraction evidence that headgroups adopt a disulfide configuration on Au,66 we postulated that SAMs desorb in a unimolecular fashion as disulfides, although analysis of desorbed species was not performed. Naphthalenethiolate exchange with decanethiol (Figure 4) exhibits an initial order of 0.2 in decanethiol. This finding, however, is not inconsistent with a bimolecular mechanism of exchange. The earliest model of exchange, proposed by Chidsey et al.,38 depicts exchange occurring at crystalline monolayer domain boundaries of well-packed systems, although substrate defect sites are suggested as well. Exchange is limited by the rate at which surface thiolates diffuse to defect sites (or the rate that defects migrate across the monolayers“hole” transport). An important difference between naphthalenethiol and long-chain alkanethiols is that exchange is complete with the former (Figures 1 and 4) whereas in the latter a significant fraction of the monolayer cannot be exchanged, remaining kinetically trapped over the time of the experiment.37-40 Complete displacement of short-chain alkanethiolate SAMs has been noted depending on the conditions of exchange.27,39,67 Thus, the extent of order, greater for long-chain thiols, appears to be a key factor in limiting the access of incoming displacer molecules. Alkanethiol SAMs formed on nanoparticles of Au, which are expected to have much more accessible sulfur headgroups, can be exchanged rapidly and quantitatively by other thiols.57 When didecyl disulfide is employed as exchanger, decyl 2-naphthyl disulfide is found in solution (Figure 4), suggesting an exchange mechanism analogous to that for the thiol. The slower exchange is due to the steric bulk of didecyl disulfide. The formation of bis(2-naphthyl) disulfide in solution is due to spontaneous desorption followed by replacement. SAMs derived from solutions of equal molar alkanethiol and almost structurally identical disulfide have a monolayer composition that is predominately that of the thiol, as shown by Bain et al.27,68 Further studies by the Whitesides group62 revealed that rates of alkanethiol or dialkyl disulfide addition to Au were indistinguishable. Also, it was found that alkanethiols (65) A novel stable biradical with a disulfide linkage was synthesized and used as a spin label for spontaneous desorption. Homolytic cleavage of the AusS bond would certainly destroy the spin. Schlenoff, J. B., Kolega, R., unpublished results. (66) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216-1218. (67) Hutt, D. A.; Leggett, G. J. Langmuir 1997, 13, 3055-3058. (68) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989, 5, 723-727.
5476 Langmuir, Vol. 14, No. 19, 1998
exchange with alkanethiolate SAMs much faster than with dialkyl disulfides.62 This result is entirely consistent with what is observed with our aryl compounds (compare Figures 1, 3, and 4). Biebuyck et al.62 conclude that the final composition of a SAM coadsorbed from a mixture of thiol and its disulfide reflects the greater ability of a thiol to displace an adsorbed thiolate or impurity. We observed no initial preference of either the aryl thiol or its disulfide during adsorption from an equimolar mixture of the two; however, the equilibrium composition of this monolayer cannot be determined from solution because exchange of this mixed monolayer with the 2-naphthalenethiol in solution produces the same compound. Electrochemistry and Thermodynamic Comparisons. The charge required for electrochemical desorption can be used to estimate the surface coverage of thiolates. For the aryl monolayer, the apparent coverage is (12 ( 1) × 10-10 mol cm-2 (115 ( 10 µC cm-2), which is significantly greater than the (6.8 ( 0.4) × 10-10 mol cm-2 determined from the HPLC experiments. The aliphaticcoated electrodes have an electrochemically determined coverage of (15 ( 1.5) × 10-10 mol cm-2 (144 ( 14 µC cm-2). When the surface roughness of 1.7 is accounted for, a coverage of 8.8 × 10-10 mol cm-2 is obtained, compared with 7.6 × 10-10 as generally accepted for Au(111).6 For both the aryl and alkane monolayers, the reported monolayer saturation coverage obtained from voltammetric reductive desorption was constant regardless of the adsorbate solution concentration from 0.004 to 4 mM. The use of adsorption/desorption charge as a measure of thiol coverage has been somewhat controversial. Schneider and Buttry34 suggest that reductive desorption is accompanied by additional nonfaradaic current approximately equivalent to the faradaic charge associated with the removal of alkanethiolates. The source of this extra current is reported to be due to the charging necessary to form the double layer of the stripped electrode. Furthermore, Schneider and Buttry34 and Krysinski et al.69 report that alkylmercaptan addition to Au is a partial electron transfer process. We are not able to identify whether the discrepancy in electrochemically determined coverage is a result of faradaic or nonfaradaic (charging) current. The reduction of naphthalene to the radical anion as the source of additional current in the voltammetric analysis should be discounted because this process occurs at significantly more negative potentials.70 Other faradaic sources are possible. We were concerned that an adventitious oxide layer could be contributing charge on reduction of our polished polycrystalline Au, but cyclic voltammetry according to the method of Ron and Rubinstein71 revealed no discernible oxide on as-prepared nascent Au foil. Other possibilities for additional charge include reduction of a weak oxygen-naphthalene chargetransfer complex or reduction of an oxide layer that forms after monolayer deposition. Although long-chain alkanethiols protect surfaces from oxidation,5 these aromatic thiols are more disordered. Whatever the cause, our conclusions from electrochemical desorption on mechanically polished Au are limited to a qualitative comparison of monolayers from aryl thiol and disulfide. The cathodic shift noted in Figure 6 is reminiscent of certain organosulfur compounds that exhibit cleavage of (69) Krysinski, P.; Chamberlain, II, R. V.; Majda, M. Langmuir 1994, 10, 4286-4294. (70) Perichon, J. Polycyclic Aromatic Hydrocarbons In Encyclopedia of Electrochemistry of the Elements; Bard, A. J.; Lund, H., Eds.; Marcel Dekker: New York, 1978; Vol. XI, pp 71-161. (71) Ron, H.; Rubinstein, I. Langmuir 1994, 10, 4566-4573.
Kolega and Schlenoff
Figure 9. Cyclic voltammagrams of Au ball electrodes in 0.1 M 2-naphthalenethiol (upper curve) and in 0.1 M decanethiol (lower curve). The electrolyte was 0.5 M KOH in 50/50% (v/v) ethanol/water. The “A” and “B” peaks are attributed to electroactivity at Au(111) and other crystallographic domains, respectively.
the SsC bond upon chemisorption to Au or exposure to negative potentials.7a,21,72 This sulfurization of the Au surface does not occur with 2-naphthalenethiol- or bis(2-naphthyl) disulfide-derived monolayers. Figure 5 shows that naphthalenethiolate is indeed reductively desorbed from a 2-naphthalenethiol monolayer in alkaline solution. To our knowledge, the only other spectroscopic evidence that SAMs are reductively desorbed as thiolates comes from a recent in situ vibrational study.73 Furthermore, voltammetric analysis of electrodes exposed to Na2S or 2-naphthalenesulfonic acid gave significantly different desorption peaks or almost no current for the respective adsorbates. The evolution of a more cathodic peak, which occurs after a full monolayer is formed, might indicate reordering of the surface thiolates such that they occupy higher-energy defect sites or they become better packed. In our hands, the only planar system that appeared to yield the expected coverage for alkanethiol SAMs on electrochemical desorption in aqueous base employed Au evaporated on mica followed by annealing to give predominantly Au(111). This substrate has been extensively probed by Porter et al.,52,53a,b and its reliability has been confirmed by others.23,74 Cyclic voltammetry was performed using Au wire that had been melted into balls using an H2 flame. This procedure is known to produce Au rich in (111) crystallographic domains (cd’s).75 Cyclic voltammetry of 1 mM decanethiol and 2-naphthalenethiol using such Au ball electrodes is depicted in Figure 9. The electrolyte was 0.5 M KOH in 50/50% (v/v) ethanol/water. The ethanol solubilizes the thiol and, possibly, solvates the monolayers so that the electrochemistry is more reversible. Two major pairs of peaks are observed, which we attribute to electroactivity at Au(111) (“A” peaks, less negative potentials) and other cd’s of Au (“B” peaks), including Au(110).53 The cyclic voltammetry reveals greater reversibility (faster electron-transfer kinetics) for naphthalenethiolate monolayers than for alkanethiolates, due, presumably, to greater disorder in the former. The (72) Weisshaar, D. E.; Walczak, M. M.; Porter, M. D. Langmuir 1993, 9, 323-329. (73) Yang, D.-F.; Al-Maznai, H.; Morin, M. J. Phys. Chem. B 1997, 101, 1158-1166. (74) Peanasky, J. S.; McCarley, R. L. Langmuir 1998, 14, 113-123. (75) (a) Hsu, T.; Cowley, J. M. Ultramicroscopy 1983, 11, 125. (b) Ross, C. B.; Sun, L.; Crooks, R. M. Langmuir 1993, 9, 632-636.
Self-Assembled Monolayers of an Aryl Thiol
Langmuir, Vol. 14, No. 19, 1998 5477
Table 1. Adsorption, Desorption, and Exchange Energetics of Thiols and Disulfides on Au reaction
E° (V vs SHE) e-
[i] [ii] [iii] [iv]
C12H25SsAu(s) + a C12H25S- + Au(s) C10H21SsAu(s) + e- a C10H21S- + Au(s) C10H9SsAu(s) + e- a C10H9S- + Au(s) C6H5SsSC6H5 + 2e- a 2C6H5S-
[v] [vi] [vii] [viii] [ix]
C6H13SsSC6H13 + 2e- a 2C6H13SH+ + e- a 1/2H2 C3H7SH a C3H7S- + H+ C6H5SH a C6H5S- + H+ C6H13SsSC6H13 + 2C6H5SH a C6H5S-SC6H5 + 2C6H13SH
[x] ) [vi] + [vii] - [ii] [xi] ) [vi] + [viii] - [iii] [xii] ) [v] - 2[ii] [xiii] ) [iv] - 2[iii] [xiv] ) [xi] - [x]
C10H21SH + Au(s) a C10H21SsAu(s) + 1/2H2 ArSH + Au(s) a ArSsAu(s) + 1/2H2 (C10H21S)2 + 2Au(s) a 2Au(s)sSC10H21 ArSsSAr + 2Au(s) a 2Au(s)sSAr ArSH + C10H21SsAu(s) a ArSsAu(s) + C10H21SH
∆G° (kcal mol-1)
reference
-0.88a
+20.2b,c
-0.88(-1.20)d -0.63(-1.02) -0.300e -0.315 -0.58 0.000
+20.2(27.6) +14.5(23.5) +13.8 s +26.7f
82 this work this work 80 83 s
+14.7g +8.8h +1.1i
84 85 50
-5.5(13) -5.7(14) -14(29) -15(33) ∼0
a The peak positions, E 1/2 from the voltammograms, have been converted to standard conditions (1 M, SHE reference) using E°) E1/2 - (0.059 × 3) + 0.197. b Au(s) ) Au(111) surface site. c For half cells, using hydrogen electrode (SHE) reference, ∆G° ) -nFE°. d Values in parentheses are approximate for higher energy sites. e This E°used in calculations. f [v] ) [iv] + [ix] + 2[vii] - 2[viii]. g Ka for propanethiol ) 1.4 × 10-11. h Ka for thiophenol ) 3.2 × 10-7. i K ) 0.16.
peak widths are 50-60 mV, which are less than the 90 mV expected for a one-electron process, which would suggest either a two-electron process, (ArSsAu(s))2 + 2ef 2ArS- + Au(s), or attractive site-site interactions for adsorbed species, as in underpotential deposition of metals.76 The electrochemistry is reminiscent of recently published voltammetry of alkanethiols on Ag(111)77 and mercury.78 Following on from prior estimates of the energetics of thiol binding to Au,37 we undertook a condensed analysis of the thermodynamics of aryl- and alkanethiol SAMs using some of our own and literature data. The various reactions considered are outlined in Table 1. The E° for naphthalenethiolate reduction is from cyclic voltammetry in Figure 9. The reduction potentials for alkanethiols are taken from the literature and from Figure 9. The E° for aliphatic disulfides, difficult to obtain directly via electrochemistry,79,80 was calculated using the mercaptandisulfide interchange equilibrium data of Dalman et al.50 In this thermodynamic analysis, the acid dissociation constant of propanethiol has been used to represent that of longer-chain alkanethiols, and the Ka for thiophenol has been employed in lieu of that for 2-naphthalenethiol. The reduction potential for phenyl disulfide is used in lieu of that for bis(2-naphthyl) disulfide. Ideally, of course, all of the values in Table 1 would be determined for the specific molecule of interest under the same conditions. Several noteworthy points are evident when the data are combined as shown. First, the adsorption free energies for aliphatic (reaction x, Table 1) and aromatic (reaction xi) thiols are similar. This result would indicate no strong preference (reaction xiv) for aliphatic over aromatic thiols, as confirmed by Figure 3. Although the AusS bond is often quoted as being “strong” (∼40 kcal mol-1),6,56b the overall magnitude of the adsorption free energy is not large for the Au(111) sites. Likewise, the adsorption free energies for aromatic and aliphatic81 disulfides are close and are significantly greater than those for thiols. (76) Zhang, J.; Sung, Y.-E.; Rikvold, P. E.; Wieckowski, A. J. Chem. Phys. 1996, 104, 5699-5712. (77) Hatchett, D. W.; Stevenson, K. J.; Lacy, W. B.; Harris, J. M. White, H. S. J. Am. Chem. Soc. 1997, 119, 6596-6606. (78) Stevenson, K. J.; Mitchell, M.; White, H. S. J. Phys. Chem. B 1998, 102, 1235-1240. (79) (a) Kolthoff, I. M.; Stricks, W.; Kapoor, R. C. J. Am. Chem. Soc. 1955, 77, 4733. (b) Jocelyn, P. C. Eur. J. Biochem. 1967, 2, 327-331. (c) Lees, W. J.; Whitesides, G. M. J. Org. Chem. 1993, 58, 642-647. (80) Nichols, P. J.; Grant, M. W. Aust. J. Chem. 1982, 35, 24552463.
However, in a competition between a thiol and a disulfide under ambient conditions, SAM formation from the thiol would prevail because reactions x and xi are irreversible due to loss of hydrogen. Our calculations compare 10carbon aromatic and aliphatic thiols. Additional stability would be imparted by longer alkyl chains due to favorable van der Waals packing6 or hydrophobic78 forces, amounting to increments of -0.5 kcal43 to -1 kcal78 per methylene unit per mole. The adsorption energies permit an estimate of the extent of spontaneous desorption of disulfide from SAMs. If Ψ is defined as the fraction of SAM desorbed from a particular cd of Au, A is the area of the Au (cm2), σ is the surface coverage (mol cm-2), and V is the volume of pure solvent used in the desorption experiment (L), using eq 1 we obtain
2V Ψ3 ) 2 AK (1 - Ψ) adsσ
(2)
If we use ∆G° ) 15 kcal mol-1 (63 kjoule mol-1, Kads ) 1.1 × 1011) from Table 1, reaction xiii, and let V ) 10 mL, A ) 20 cm2, and σ ) 1.7 × 10-10 (corresponding to 25% Au(111)), the value of Ψ is 3%. Thus, desorption in pure solvent of a few percent of a monolayer as disulfide, determined by the chromatography, is a reasonable finding. The rest of the monolayer is more strongly bound and would not desorb to an appreciable extent. Conclusions This work has identified the products of desorption and exchange of SAMs in the solution phase. 2-Naphthalenethiol and its disulfide add rapidly to Au to yield stable monolayers that are not prone to oxidation. In comparison with alkanethiol monolayers, the behavior of naphthalenethiol on Au appears to be easier to interpret, because it exchanges fully, desorbs in base as thiolate only, and shows no oxidation products (sulfones, sulfinates, sul(81) The adsorption energy calculated here for aliphatic disulfides is considered more reliable than our previous estimate (ref 37), because in the latter we used the reduction of hydroxyethyl disulfide to represent reaction [v]. (82) Weisshaar, D. E.; Lamp, B. D.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 5860-5862. (83) Liu, M.; Visco, S. J.; De Jonghe, L. C. J. Electrochem. Soc. 1990, 137, 750-759. (84) Lange’s Handbook of Chemistry, 13th ed., McGraw-Hill: New York, 1985; pp 5-54. (85) Kreevoy, M. M.; Eichinger, B. E.; Stary, F. E.; Katz, E. A., Sellstedt, J. H. J. Org. Chem. 1964, 29, 1641-1642.
5478 Langmuir, Vol. 14, No. 19, 1998
fonates). A few percent, at most, of the monolayers spontaneously desorb in pure solvent as bis(2-naphthyl) disulfide. These aryl monolayers completely exchange over a period of about a day in decanethiol, evolving as 2-naphthalenethiol, whereas they exchange very slowly with didecyl disulfide to give decyl 2-naphthyl disulfide. This result is consistent with a bimolecular-type of exchange with transfer of H from incoming thiol to the SAM at the surface. The identical surface coverage, as
Kolega and Schlenoff
determined from the electrochemistry, and identical exchange and desorption behavior, from HPLC, suggests that aryl thiol- and disulfide-derived monolayers are indistinguishable. Acknowledgment. This work was supported by the Florida State University Center for Materials Research. LA980553B
