pubs.acs.org/Langmuir © 2009 American Chemical Society
Influence of Solvation and the Structure of Adsorbates on the Kinetics and Mechanism of Dimerization-Induced Compositional Changes of Mixed Monolayers on TiO2 Jonathan R. Mann, Jeremy S. Nevins, Gregory R. Soja, David D. Wells, Seth C. Levy, David A. Marsh, and David F. Watson* Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, New York 14260-3000 Received May 16, 2009. Revised Manuscript Received August 5, 2009 Mixed monolayers of thiol-terminated (T) and methyl-terminated (Me) carboxylic acids on nanocrystalline TiO2 films underwent dimerization-induced compositional changes. At short reaction times, the compositions of mixed monolayers were kinetically controlled and mirrored the compositions of coadsorption solutions. On time scales up to several hours, well after the establishment of saturation surface coverages, the monolayers relaxed to thermodynamically controlled compositions through the displacement of Me by T. Equilibration was driven by the formation of intermolecular disulfide bonds between thiol groups of adsorbed T, which yielded polydentate dimeric adsorbates that were bound more strongly than monomeric adsorbates to TiO2. The rate of compositional changes increased with decreasing solvent viscosity and decreasing alkyl chain length of T, suggesting that the rate of adsorption of T to TiO2 strongly influenced the overall kinetics under certain conditions. Steric bulk within adsorbates and the strength of surface-attachment interactions also influenced the rate of compositional changes. A kinetic model, derived on the basis of Langmuir adsorption and desorption kinetics, accounts for key aspects of the mixed-monolayer compositional changes. The rate-determining step in the overall mechanism involved either the adsorption of T or the formation of disulfide bonds, depending on the conditions under which monolayers were prepared. Our findings illustrate that dimerization and other intermolecular interactions between adsorbates may dramatically influence the composition and terminal functionalization of mixed monolayers.
Introduction The physical properties and chemical reactivity of mixed monolayers can be controlled by varying the structure, conformation, terminal functionalization, surface coverage, and spatial distribution of adsorbates.1-14 Therefore, mixed-monolayerfunctionalized surfaces and nanoparticles may have applications *To whom correspondence should be addressed. (1) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 3665–3666. (2) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155– 7164. (3) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164–7175. (4) Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 10714– 10721. (5) Ostuni, E.; Yan, L.; Whitesides, G. M. Colloids Surf., B 1999, 15, 3–30. (6) Chapman, R. G.; Ostuni, E.; Takayama, S.; Holmlin, R. E.; Yan, L.; Whitesides, G. M. J. Am. Chem. Soc. 2000, 122, 8303–8304. (7) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103–1169. (8) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301–4306. (9) Atre, S. V.; Liedberg, B.; Allara, D. Langmuir 1995, 11, 3882–3893. (10) Beake, B. D.; Leggett, G. J. Phys. Chem. Chem. Phys. 1999, 1, 3345–3350. (11) Tan, J. L.; Tien, J.; Chen, C. S. Langmuir 2002, 18, 519–523. (12) Twardowski, M.; Nuzzo, R. G. Langmuir 2003, 19, 9781–9791. (13) Kr€amer, S.; Fuierer, R. R.; Gorman, C. B. Chem. Rev. 2003, 103, 4367– 4418. (14) Smith, R. K.; Lewis, P. A.; Weiss, P. S. Prog. Surf. Sci. 2004, 75, 1–68. (15) Mrksich, M. Chem. Soc. Rev. 2000, 29, 267–273. (16) Smith, J. C.; Lee, K.-B.; Wang, Q.; Finn, M. G.; Johnson, J. E.; Mrksich, M.; Mirkin, C. A. Nano Lett. 2004, 3, 883–886. (17) Capadona, J. R.; Collard, D. M.; Garcı´ a, A. J. Langmuir 2003, 19, 1847– 1852. (18) Martins, M. C.; Ratner, B. D.; Barbosa, M. A. J. Biomed. Mater. Res. A 2003, 67A, 158–171. (19) Drechsler, U.; Erdogan, B.; Rotello, V. M. Chem.;Eur. J. 2004, 10, 5570– 5579. (20) Arima, Y.; Iwata, H. Biomaterials 2007, 28, 3074–3082.
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in chemical sensing and biomolecular recognition,4,5,15-21 molecular electronics,22-27 catalysis,28-30 and as substrates and building blocks for materials assembly.31-37 Significant research has focused on characterizing and controlling the compositions of mixed monolayers. To date, most reports involve binary mixed monolayers of alkanethiols on gold and silver.1-3,7,9,38 Mixed (21) Choi, S.; Murphy, W. L. Langmuir 2008, 24, 6873–6880. (22) Shipway, A. N.; Willner, I. Acc. Chem. Res. 2001, 34, 421–432. (23) Napper, A. M.; Liu, H.; Waldeck, D. H. J. Phys. Chem. B 2001, 105, 7699– 7707. (24) Yue, H.; Waldeck, D. H.; Schrock, K.; Kirby, D.; Knorr, K.; Switzer, S.; Rosmus, J.; Clark, R. A. J. Phys. Chem. C 2008, 112, 2514–2521. (25) Holman, M. W.; Liu, R.; Adams, D. M. J. Am. Chem. Soc. 2003, 125, 12649–12654. (26) Yasutomi, S.; Morita, T.; Imanishi, Y.; Kimura, S. Science 2004, 304, 1944– 1948. (27) Liu, B.; Bard, A. J.; Mirkin, M. V.; Creager, S. E. J. Am. Chem. Soc. 2004, 126, 1485–1492. (28) Drechsler, U.; Fischer, N. O.; Frankamp, B. L.; Rotello, V. M. Adv. Mater. 2004, 16, 271–274. (29) Lu, X.; Lv, B.; Xue, Z.; Li, M.; Zhang, L.; Kang, J. Thin Solid Films 2005, 488, 230–235. (30) N€oll, G.; Kozma, E.; Grandori, R.; Carey, J.; Sch€odl, T.; Hauska, G.; Daub, J. Langmuir 2006, 22, 2378–2383. (31) Zamborini, F. P.; Hicks, J. F.; Murray, R. W. J. Am. Chem. Soc. 2000, 122, 4514–4515. (32) Jones, D. M.; Brown, A. A.; Huck, W. T. S. Langmuir 2002, 18, 1265– 1269. (33) Shenhar, R.; Rotello, V. M. Acc. Chem. Res. 2003, 36, 549–561. (34) Tognarelli, D. J.; Miller, R. B.; Pompano, R. R.; Loftus, A. F.; Sheibley, D. J.; Leopold, M. C. Langmuir 2005, 21, 11119–11127. (35) Loftus, A. F.; Reignhard, K. P.; Kapourales, S. A.; Leopold, M. C. J. Am. Chem. Soc. 2008, 130, 1649–1661. (36) Mann, J. R.; Watson, D. F. Langmuir 2007, 23, 10924–10928. (37) Sendroiu, I. E.; Schiffrin, D. J.; Abad, J. J. Phys. Chem. C 2008, 112, 10100– 10107. (38) Laibinis, P. E.; Fox, M. A.; Folkers, J. P.; Whitesides, G. M. Langmuir 1991, 7, 3167–3173.
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monolayers of alkyltrichlorosilanes on silicon and silica,39,40 alkanes on silicon,41-43 and alkylphosphonates and alkylcarboxylates on metal oxides25,36,44-47 have also been reported. Mixed monolayers are typically formed by coadsorption of components from mixed solutions. The compositions of some mixed monolayers are controlled kinetically.40,48,49 More often, compositions are controlled thermodynamically.3,38,50-52 The equilibrium compositions of mixed monolayers often differ from those of coadsorption solutions from which they are prepared. For example, alkanethiols with polar functional groups are preferentially adsorbed to gold surfaces from nonpolar solvents, and vice versa.2,3,52,53 Hydrogen bonding, dipole-dipole interactions, and dispersion forces can lead to the stabilization of adsorbates2,3,38,49,52-56 or the formation of single-component domains within mixed monolayers.9,54,55,57-60 We recently reported unusual mixed monolayers that underwent compositional changes after the initial coadsorption reaction.47 The monolayers consisted of 16-mercaptohexadecanoic acid (MHDA) and n-octanoic acid (OA) on nanocrystalline TiO2 films. Both components adsorbed as carboxylates. At short adsorption times, the compositions of mixed monolayers corresponded closely to those of coadsorption solutions, suggesting that initial compositions were controlled kinetically. With increasing adsorption times, the mole fraction of MHDA within the monolayers increased while that of OA decreased. The compositional changes occurred on time scales from minutes to hours, well after the formation of full monolayers. We attributed the compositional changes to the formation of disulfide bonds between thiols of MHDA and the resulting increase of the affinity of dimeric MHDA for the TiO2 surface. Control experiments indicated that thiols were oxidized by molecular oxygen. Mixed monolayers prepared under reducing, deaerated conditions did not undergo disulfide formation or compositional changes.47 The surface adduct formation constant (Kad) of dimeric disulfide-bridged (39) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074– 1087. (40) Offord, D. A.; Griffin, J. H. Langmuir 1993, 9, 3015–3025. (41) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145–3155. (42) Wagner, P.; Nock, S.; Spudich, J. A. J. Struct. Biol. 1997, 119, 189–201. (43) Liu, Y.-J.; Navasero, N. M.; Yu, H.-Z. Langmuir 2004, 20, 4039–4050. (44) Tosatti, S.; Michel, R.; Textor, M.; Spencer, N. D. Langmuir 2002, 18, 3537–3548. (45) Zwahlen, M.; Tosatti, S.; Textor, M.; H€ahner, G. Langmuir 2002, 18, 3957– 3962. (46) Sasahara, A.; Uetsuka, H.; Onishi, H. Langmuir 2003, 19, 7474–7477. (47) Soja, G. R.; Mann, J. R.; Watson, D. F. Langmuir 2008, 24, 5249–5252. (48) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992, 8, 1330– 1341. (49) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M.; Deutch, J. J. Phys. Chem. 1994, 98, 563–571. (50) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 6560–6561. (51) Shon, Y.-S.; Mazzitelli, C.; Murray, R. W. Langmuir 2001, 17, 7735–7741. (52) Choo, H.; Cutler, E.; Shon, Y.-S. Langmuir 2003, 19, 8555–8559. (53) Kang, J. F.; Liao, S.; Jordan, R.; Ulman, A. J. Am. Chem. Soc. 1998, 120, 9662–9667. (54) Smith, R. K.; Reed, S. M.; Lewis, P. A.; Monnell, J. D.; Clegg, R. S.; Kelly, K. F.; Bumm, L. A.; Hutchison, J. E.; Weiss, P. S. J. Phys. Chem. B 2001, 105, 1119–1122. (55) Lewis, P. A.; Smith, R. K.; Kelly, K. F.; Bumm, L. A.; Reed, S. M.; Clegg, R. S.; Gunderson, J. D.; Hutchison, J. E.; Weiss, P. S. J. Phys. Chem. B 2001, 106, 10630–10636. (56) Auletta, T.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Langmuir 2002, 18, 1288–1293. (57) Stranick, S. J.; Parikh, A. N.; Tao, Y.-T.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 7636–7646. (58) Dunbar, T. D.; Cygan, M. T.; Bumm, L. A.; McCarty, G. S.; Burgin, T. P.; Reinerth, W. A.; Jones, L., II; Jackiw, J. J.; Tour, J. M.; Weiss, P. S.; Allara, D. L. J. Phys. Chem. B 2000, 104, 4880–4893. (59) Tamada, K.; Hara, M.; Sasabe, H.; Knoll, W. Langmuir 1997, 13, 1558– 1566. (60) Chen, S.; Li, L.; Boozer, C. L.; Jiang, S. J. Phys. Chem. B 2001, 105, 2975– 2980.
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Mann et al. Chart 1. Names, Abbreviations, and Structures of Adsorbates
MHDA, which was coordinated to TiO2 through two carboxylates, presumably increased through a mechanism analogous to the chelate effect in coordination chemistry.61,62 Thus, MHDA displaced OA from the surface as the monolayers approached thermodynamically controlled equilibrium compositions. Because the compositional changes of mixed MHDA-OA monolayers on nanocrystalline TiO2 films occur on relatively long time scales, these and related systems may serve as models to evaluate the influence of structure, intermolecular interactions, and solvation on the mechanisms and kinetics by which mixed monolayers equilibrate. In this article, we report the influence of solvation, the composition of the coadsorption solution, the chain length and structure of adsorbates, and the surface-attachment mode on the kinetics and mechanism of compositional changes within mixed monolayers of thiol-terminated components (T) and methyl-terminated components (Me) on TiO2. The structures, names, and abbreviations of all T and Me are summarized in Chart 1. Our results provide further evidence that the temporal evolution of the composition of mixed monolayers, and the preferential adsorption of T to TiO2, are caused by dimerization through disulfide formation. We present rate law expressions that model the time-dependent compositional changes of mixed monolayers. Depending on the compositions of mixed monolayers and the surrounding solvent, the rate-determining step involves either the adsorption of components to TiO2 or the formation of disulfide bonds between T. An improved understanding of the kinetics and mechanism of compositional changes will enable the preparation of thiol-containing mixed monolayers with precisely controlled compositions. More generally, our results may be relevant to a range of mixed-monolayer systems for which intermolecular interactions between adsorbates influence equilibrium compositions and/or the rate at which compositions approach equilibrium.
Experimental Section Materials. Titanium(IV) tetraisopropoxide was obtained from Alfa Aesar or Sigma-Aldrich. Sodium borohydride was obtained from Fisher Scientific. MHDA, 11-mercaptoundecanoic acid (MUDA), 8-mercaptooctanoic acid (MOA), 6-mercaptohexanoic acid (MHA), 4-mercaptohydrocinnamic acid (MCA), the disulfidebridged dimer of MUDA, n-hexylphosphonic acid (HPA), n-hexadecanoic acid (HDA), n-undecanoic acid (UDA), OA, n-hexanoic acid (HA), n-propanoic acid (PA), titanium(IV) tetraisopropoxide, (61) Basolo, F.; Pearson, R. G. Mechanisms of Inorganic Reactions: A Study of Metal Complexes in Solution; John Wiley and Sons: New York, 1967. (62) Martell, A. E. Adv. Chem. Ser. 1967, 62, 272–294.
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Mann et al. and poly(ethylene glycol) were obtained from Aldrich. 3-Mercaptoisobutyric acid (MIBA) was obtained from TCI America. Nanocrystalline TiO2 particles (P25) were obtained from Evonik. 3-Mercaptopropionic acid (MPA), tetrahydrofuran (THF), ethyl ether (Et2O), ethanol (EtOH), 1-butanol (BuOH), heptane, toluene, methylene chloride, chloroform, and nitric acid were obtained from various sources. Reagents were used without further purification. Synthesis of TiO2 Films. Nanocrystalline TiO2 films were prepared following the method of Heimer et al.63 Titanium(IV) tetraisopropoxide (50 mL) was added slowly to rapidly stirred dilute nitric acid (300 mL, 0.5% v/v). The mixture was boiled until the total volume was reduced to 90 mL. The mixture was then heated at 200 °C in a sealed vessel for 15 h, after which poly(ethylene glycol) (PEG) (5.4 g) was added, and the mixture was stirred for an additional 8 h. Films were prepared by spreading this mixture onto glass slides using a horizontally oriented glass stirring rod or pipet and then annealing in air (430 °C, 30 min). PEG has been shown to increase the thickness and porosity of nanocrystalline TiO2 films and to promote crystallization in the anatase structure.64,65 Our IR data revealed that PEG was decomposed during the annealing step; the contribution of PEG peaks to the total C-H stretching absorbance of TiO2 films functionalized with T and/or Me was negligible. The projected surface area of films was 3.4 ( 0.2 cm2. Scanning electron microscopy (SEM) and X-ray powder diffraction (XRD) measurements have shown that the films were 4.1 ( 0.9 μm thick and consisted of anatase TiO2 with average particle diameters of 20-30 nm.36,66 IR Spectra. Surface amounts of adsorbates per projected area were determined by analysis of the C-H stretching region of IR spectra. Spectra were obtained with 2 cm-1 resolution using either a Perkin-Elmer 1760-X or a Nicolet Magna-IR 550 spectrometer. Transmission-mode spectra from 3200 to 2600 cm-1 were acquired by positioning monolayer-functionalized TiO2 films on glass substrates perpendicular to the IR beam. Spectra were baseline corrected by subtracting a Gaussian and a constant to minimize the absorbances from 3200 to 3000 cm-1 and 28002600 cm-1 using a least-squares method. Surface amounts of adsorbates per projected area were determined by dividing the absorbance at the maximum of the asymmetric CH2 stretching (νa(CH2)) band by the extinction coefficient (in cm2/mol), which was calculated from Beer-Lambert plots for the adsorbate dissolved in CCl4. (A sample calculation for OA is shown in eq S1 of the Supporting Information.) We assumed that extinction coefficients were unchanged upon adsorption to TiO2.
Adsorption Kinetics for Single-Component Monolayers. TiO2 films (16-24 per beaker) were immersed in freshly prepared solutions (0.02-2.0 mM, 100 mL) of the component for adsorption. Experiments were performed at room temperature (22 °C). At the desired immersion time, films were removed from solution, rinsed by swirling in the adsorption solvent for 3-5 s, and allowed to air-dry. (IR data revealed that 2-5% of adsorbates were removed from TiO2 films functionalized with full monolayers of T and/or Me during a typical rinsing procedure.) Films were stored in the dark until characterization by IR spectroscopy. Fractional surface coverages of adsorbates were calculated by dividing the νa(CH2) absorbance by the absorbance corresponding to the saturation surface amount of a given adsorbate per projected area, as determined from equilibrium binding experiments. A minimum of four TiO2 films were functionalized for each adsorption time. The data points in Figures S4 and S5 represent the average values of these films, and the error bars represent the standard deviation. (63) Heimer, T. A.; D’Arcangelis, S. T.; Farzad, F.; Stipkala, J. M.; Meyer, G. J. Inorg. Chem. 1996, 35, 5319–5324. (64) Srikanth, K.; Rahman, M. M.; Tanaka, H.; Krishna, K. M.; Soga, T.; Mishra, M. K.; Jimbo, T.; Umeno, M. Sol. Energy Mater. 2001, 65, 171–177. (65) Matsuda, A.; Kotani, Y.; Kogure, T.; Tatsumisago, M.; Minami, T. J. Am. Ceram. Soc. 2000, 83, 229–231. (66) Dibbell, R. S.; Soja, G. R.; Hoth, R. M.; Watson, D. F. Langmuir 2007, 23, 3432–3439.
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Equilibrium Binding for Single-Component Monolayers. TiO2 films (two per vial) were immersed in freshly prepared solutions (0.02-2.0 mM, 4.0 mL) of the component for adsorption. Experiments were performed at room temperature (22 °C). After immersion for a minimum of 4 h, films were removed from the adsorption solution and rinsed in the adsorption solvent. (Kinetics studies revealed that adsorption of PA from 0.02 mM solutions was complete within 10 min, adsorption of OA from 0.02 mM solutions was complete within 45 min, and adsorption of HDA from 0.2 mM solutions was complete within 60 min; therefore, we assumed that all reactions had reached equilibrium after 4 h of immersion.) Films were stored in the dark until characterization by IR spectroscopy. A minimum of four TiO2 films were functionalized for each solution concentration. The data points in Figure S6 represent the average values of these films, and the error bars represent the standard deviation.
Preparation and Characterization of Mixed Monolayers on Nanocrystalline TiO2 Films. TiO2 films (16-24 per beaker) were immersed in freshly prepared solutions (100 mL) containing the two components for adsorption. Experiments were performed at room temperature (22 °C). In all experiments, at least four TiO2 films were functionalized for each immersion time. Unless otherwise specified, the coadsorption solutions contained a 1-to-9 molar ratio of T (0.2 mM) and Me (1.8 mM) in THF. (Saturation surface amounts of alkanoic and mercaptoalkanoic acids per projected area were attained upon immersion of nanocrystalline TiO2 films in 2.0 mM solutions.36) Carboxylic acid-containing adsorbates typically bind to metal oxides as carboxylates.66-71 Monodentate, bidentate chelating, and bidentate bridging geometries have been reported. We presume that MCA also adsorbed to TiO2 as a carboxylate. For all coadsorption solutions, sufficient volumes were used to ensure that the amounts of both components in solution exceeded the amounts required to yield saturation surface amounts per projected area for single-component monolayers. Therefore, neither component was a limiting reagent in the adsorption process. TiO2 films were removed from the coadsorption solutions after the desired reaction time, then rinsed with solvent, and allowed to dry. Films that had been immersed in BuOH solutions were rinsed with EtOH or THF. Films were stored in the dark until characterization by IR spectroscopy.
Preparation and Characterization of Mixed Monolayers on Evonik P25 TiO2 Nanoparticles. TiO2 nanoparticles (4.0 mg) were suspended in 5.0 mL coadsorption solutions of MHDA (0.34 mM) and OA (3.06 mM) in THF. After the desired reaction time, the TiO2 was collected by centrifugation. The pellet was immediately rinsed with THF and dried in air at 75 °C. IR spectra were obtained of the monolayer-functionalized TiO2 in KBr pellets. Surface Coverage Calculations. Either of two methods was used to determine the compositions of mixed monolayers from C-H stretching absorbance of IR spectra. Both methods are described in Appendix S1 of the Supporting Information. Adsorbed Et2O, heptane, THF, EtOH, or BuOH contributed less than 0.005 to the asymmetric CH2 stretching (νa(CH2)) absorbance or 20-fold less than the νa(CH2) absorbance of TiO2 films functionalized with full monolayers of MHDA or HDA. We previously reported that oxalic acid displaced THF from TiO2 surfaces at concentrations greater than 0.18 mM.36 On the basis of these results, we neglected contributions of adsorbed solvent to the overall C-H stretching absorbance. Method 1 was used for (67) Meyer, T. J.; Meyer, G. J.; Pfennig, B. W.; Schoonover, J. R.; Timpson, C. J.; Wall, J. F.; Kobusch, C.; Chen, X.; Peek, B. M.; Wall, C. C.; Ou, W.; Erickson, B. W.; Bignozzi, C. Inorg. Chem. 1994, 33, 3952–3964. (68) Murakoshi, K.; Kano, G.; Wada, Y.; Yanagida, S.; Miyazaki, H.; Matsumoto, M.; Murasawa, S. J. Electroanal. Chem. 1995, 396, 27–34. (69) Tao, Y.-T.; Hietpas, G. D.; Allara, D. L. J. Am. Chem. Soc. 1996, 118, 6724–6735. (70) Duffy, N. W.; Dobson, K. D.; Gordon, K. C.; Robinson, B. H.; McQuillan, A. J. Chem. Phys. Lett. 1997, 266, 451–455. (71) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52–66.
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Figure 1. IR spectra of nanocrystalline TiO2 films as a function of immersion time in THF solutions containing 0.2 mM MHDA and 1.8 mM OA. binary mixed monolayers of linear mercaptoalkanoic acids and alkanoic acids for which the chain length of the mercaptoalkanoic acid was greater than that of the alkanoic acid and for mixed monolayers containing HPA. Method 2 was used for binary mixed monolayers of mercaptoalkanoic acids and alkanoic acids for which the chain length of the mercaptoalkanoic acid was less than that of the alkanoic acid. Method 2 was also used for mixed monolayers containing MCA and MIBA. The data points in all figures represent the average values of at least four films, and the error bars represent the standard deviation.
Time-of-Flight Secondary Ion Mass Spectrometry (ToFSIMS). ToF-SIMS analysis was performed on an ION TOF
5.100 ION TOF Gmbh (Muenster, Germany). Bi32þ primary ions (25 kV, 0.39 pA) were used for image acquisition of low-mass species, whereas Csþ primary ions (10 kV, 0.98 pA) were used to increase the relative yield of the molecular ions versus high-mass substrate clusters for spectral comparison. Negative secondary ions were collected and accelerated to 2 kV and mass analyzed in a reflectron-type time-of-flight tube. The reflectron voltage was adjusted to compensate for different surface potentials of the samples. A low-energy (20 V) electron flood gun was utilized for charge compensation. Ions were postaccelerated to 10 kV and detected with a multichannel plate detector. Spectra were analyzed with IONSPEC software; images were analyzed with IONIMAGE software (version 4.1.0.1).
Results and Discussion Dimerization-Induced Compositional Changes. Mixed monolayers of MHDA and OA on nanocrystalline TiO2 films were characterized first as a prototypical system. Our initial findings were reported previously.47 Mixed monolayers were prepared from coadsorption solutions with varying compositions. The sum of the concentrations of MHDA and OA was held constant at 2 mM, while the relative amounts of the two components were varied. The mole fractions of MHDA in solution (χT,soln) were 0.1, 0.25, 0.5, and 0.75. (Solvent was not included in the calculation of mole fractions.) IR spectra of mixed-monolayer-functionalized TiO2 films varied with the amount of time that TiO2 films were immersed in coadsorption solutions (Figure 1). The absorbances of the νa(CH2) band (2928 cm-1) and the symmetric CH2 stretching (νs(CH2)) band (2857 cm-1) increased with immersion time, while the absorbance of the νa(CH3) band (2961 cm-1) decreased. (IR (72) Arnold, R.; Azzam, W.; Terfort, A.; W€oll, C. Langmuir 2002, 18, 3980– 3992.
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Figure 2. Mole fractions of MHDA within mixed MHDA-OA monolayers (χT,surf) as a function of immersion time and mole fraction of MHDA in the coadsorption solution (χT,soln). Superimposed on the data are calculated χT,surf values from fits to eq 6.
bands were assigned on the basis of previously reported spectra of alkanoic acids adsorbed to metal oxides.71,72) These spectral changes are consistent with a decrease in surface coverage of OA and an increase in surface coverage of MHDA with increasing immersion time (Figure 2). The data in Figures 1 and 2 reveal several key aspects of the time-dependent compositional changes. First, for each composition of the coadsorption solution, the sum of the surface amount of OA per projected area (ΓOA) plus the surface amount of MHDA per projected area (ΓMHDA) was nearly constant (∼10-7 mol/cm2) for immersion times from 30 min to 6 h (Figure S1). These coverages are consistent with those of single-component monolayers of MHDA, OA, and HDA on nanocrystalline TiO2 films.36 Thus, full monolayers were established before the 30 min films were removed from coadsorption solutions. Second, for each composition of the coadsorption solution and at all immersion times from 30 min to 6 h, the mole fraction of MHDA on the TiO2 surface (χT,surf) was greater than χT,soln. Third, the initial rate of compositional changes of the mixed monolayers increased with χT,soln. Extrapolation of the plots of χT,surf vs t (Figure 2) to the y-axes yields a crude estimate of the initial compositions of mixed monolayers. For χT,soln of 0.1, 0.25, and 0.5, estimated y-intercepts (χT,surf,init values) did not differ significantly from χT,soln. (For χT,soln of 0.75, the mixed monolayers consisted essentially entirely of MHDA at all measured immersion times, indicating that any compositional changes were complete within 30 min.) The similarity between χT,surf and χT,soln at short immersion times suggests that initial compositions of mixed monolayers were controlled kinetically. It follows that the displacement of OA by MHDA at longer immersion times, after the initial formation of full monolayers with saturation surface amounts of adsorbates per projected area, corresponded to the evolution from kinetically controlled to thermodynamically controlled compositions. The preferential adsorption of MHDA appears to be driven by formation of disulfide bonds between adsorbed MHDA molecules (Scheme 1).47 (In Scheme 1, kad,Me and kad,T are the rate constants for adsorption of Me and T, respectively; kd,Me and kd,T are the rate constants for desorption of Me and T, respectively; and kSS is the rate constant for disulfide formation.) We hypothesize that the Kad values of disulfide-bridged MHDA dimers, which are tethered to the surface through two carboxylates, are greater than those of monomeric adsorbates, leading to enhanced stability on the surface. Successive desorption and Langmuir 2009, 25(20), 12217–12228
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Article Scheme 1. Proposed Dimerization-Induced Mechanism of Compositional Changes
Figure 3. Normalized intensities of ToF-SIMS peaks for MHDA and dimeric MHDA for mixed MHDA-OA monolayers as a function of immersion time in THF coadsorption solutions with χT,soln = 0.1. Inset: normalized intensity of ToF-SIMS peaks for dimeric MHDA.
readsorption should thus favor an increase of χT,surf with time. This dimerization-induced chelation mechanism was supported by several control experiments,47 which revealed that the compositional changes did not occur in the absence of disulfide formation for mixed monolayers prepared under reducing conditions. ToF-SIMS. Negative ion ToF-SIMS data were acquired for mixed MHDA-OA monolayers prepared at immersion times of 0.5, 1, 2, 4, and 6 h. The spectra contained peaks with the following central masses (assignments in parentheses): 271.2 Da (C16H31OS-), 289.2 Da (C16H33O2S-), 541.6 Da (C32H61O2S2-), 542.6 Da (C32H62O2S2-), and 543.5 Da (C32H63O2S2-). The peaks at 271.2 and 289.2 Da are associated with the molecular ion of monomeric MHDA, whereas the peaks at 541.6, 542.6, and 543.5 Da correspond to the molecular ion of disulfide-bridged dimeric MHDA. To compare intensities of a given peak from samples prepared at different immersion times, the sum of integrated intensities of the molecular ion peaks for monomeric or dimeric MHDA was divided by the sum of integrated intensities of substrate-related peaks centered at 399.7 Da (Ti6O7-), 479.6 Da (Ti7O9-), and 496.6 Da (Ti7O10H-). The normalized intensities of the molecular ion peaks for both monomeric and dimeric MHDA increased with immersion time (Figure 3). For monomeric MHDA, the normalized intensity increased by a factor of ∼3 from immersion times of 0.5-6 h; for dimeric MHDA, the increase was approximately 2-fold. The increased intensity of the peaks at 271.2 and 289.2 Da indicates that the coverage of thiol-containing adsorbates on TiO2 increased with immersion time, consistent with IR spectral data and our proposed mechanism. The increased intensity of the peaks at 541.6, 542.6, and 543.5 Da provides evidence that disulfide formation may have played a role in the mechanism of compositional changes. The ratios of the intensities of peaks assigned to molecular ions of monomeric and dimeric MHDA were not necessarily proportional to the ratios of these adsorbates on Langmuir 2009, 25(20), 12217–12228
TiO2. Determining the molar ratios of adsorbed monomeric and dimeric MHDA was complicated by probable differences in fragmentation patterns, which could arise from the different molecular weights and coordination modes (one versus two adsorbed carboxylates). In addition, cleavage of the S-S bond of dimeric MHDA could yield monomeric MHDA anions. In a control experiment, we obtained negative-ion ToF-SIMS data for the disulfide-bridged dimer of MUDA, which was deposited onto a silicon wafer by evaporation of a solution (Figure S2). The spectrum contained peaks associated with the molecular ions of both monomeric and dimeric MUDA, indicating that cleavage of only the S-S bond was possible under the conditions of our ToF-SIMS experiments. The lower mass region of negative ion ToF-SIMS data was used to compare the overall amounts of sulfur-containing fragments and their spatial distribution on TiO2 as a function of immersion time. Peaks with the following central masses were assigned to sulfur-containing fragments: 32.0 (S-), 33.0 (HS-), 35.0 (H3S-), 45.0 (CHS-), 59.0 (C2H3S-), and 89.0 (C4H9S-). These peaks were integrated into images colored red, while central masses of 79.9 (TiO2-) and 80.9 (TiO2H-) were assigned as substrate peaks and the corresponding integrated images were colored blue. Chemical images were normalized to the total ion image to account for any sample topography; these normalized images were then overlaid to create the images in Figure S3. Comparison of the images for immersion times of 1 and 6 h reveals that the amount of sulfur-containing fragments increased with immersion time, consistent with the displacement of OA by MHDA. The images provide no evidence for the formation of localized domains containing enriched coverages of MHDA with resolvable feature sizes. Adsorption Kinetics. Having established the mechanism of compositional changes, we sought to characterize the kinetics as a function of the structures of adsorbates and the properties of the surrounding solvent. As a first step, we measured the rates of adsorption of alkanoic acids to TiO2 as a function of the viscosity of solvents and the chain length of alkanoic acids. Singlecomponent monolayers of HDA were prepared by immersing nanocrystalline TiO2 films in 2.0 mM solutions of HDA in BuOH, THF, and Et2O for varying amounts of time (Figure S4). HDA adsorbed most rapidly from Et2O and least rapidly from BuOH. The adsorption reactions were precisely modeled by Langmuir kinetics:73,74 dθ dð1 - θÞ ¼ ¼ kad Cð1 - θÞ dt dt
ð1Þ
where θ is the fractional surface coverage of HDA on TiO2, dθ/dt (or equivalently -d(1 - θ)/dt) is the rate of adsorption, kad is the rate constant for adsorption, and C is the concentration of HDA in solution. The corresponding integrated rate law is lnð1 - θÞ ¼ -kad Ct
ð2Þ
(73) Langmuir, I. J. Am. Chem. Soc. 1918, 40, 1361–1402. (74) Kondo, T.; Takechi, M.; Sato, Y.; Uosaki, K. J. Electroanal. Chem. 1995, 381, 203–209.
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Table 1. Rate Constants for Adsorption of HDA to Nanocrystalline TiO2 Films solvent
η (mPa s)
kad (M-1 s-1)
BuOH THF Et2O
2.55 0.456 0.224
0.76 ( 0.02 5.5 ( 0.5 12 ( 1
where t is time. Equations 1 and 2 are predicated on the assumption that the rate of desorption was negligible compared to the rate of adsorption; these equations have been used to model the adsorption of alkanethiols to gold and electroactive metal complexes to platinum under such conditions.74-78 Plots of ln(1 - θ) vs t were linear for the adsorption of HDA to TiO2 (Figure S4b), indicating that it was unnecessary to include a desorption term in the integrated rate law. The rate of desorption of HDA from full monolayers into THF containing 0.1 mM HCl was ∼10-fold less than the rate of adsorption from 0.2 mM THF solutions and ∼100-fold less than the rate of adsorption from 2.0 mM THF solutions. (Desorption into nonacidified THF was slower still.) While some desorption of alkanoic acids undoubtedly occurred, the linearity of the data in Figure S4b and the slow desorption kinetics support our assumption that desorption did not significantly influence the measured adsorption kinetics. Values of kad (Table 1) were extracted from the slopes of linear fits to the data in Figure S4b. The value of kad was inversely proportional to solvent viscosity (η) (Figure 4a). Because the data were well-modeled by Langmuir kinetics, diffusion through bulk solution was probably not rate-limiting in the overall adsorption mechanism. Dannenberger et al. reported a similar dependence of kad on η for the adsorption of alkanethiols to gold, under conditions in which adsorption reactions were modeled more precisely by Langmuir kinetics than diffusional kinetics.77 They suggested that displacement of adsorbed solvent molecules was rate-limiting in the adsorption of alkanethiols. To assess the chain-length dependence of adsorption kinetics, single-component monolayers of PA, OA, and HDA were adsorbed to nanocrystalline TiO2 films from THF (Figure S5). The adsorption reactions were precisely modeled by eqs 1 and 2. The values of kad were inversely proportional to the number of carbons (N) in the alkyl chain of the alkanoic acids (Table 2, Figure 4b). The rate of adsorption of alkanethiols to gold surfaces has been shown to decrease with increasing chain length,59,77-80 and inverse proportionalities between kad and N have been reported.77-79 Dannenberger et al. attributed the effect to decreased mobility with increasing N, which gave rise to slower diffusion through a boundary layer at the surface.77 We speculate that solute mobility also limited the rate of adsorption of alkanoic acids to nanocrystalline TiO2 films. The dependence of kad on mobility, evidenced by its proportionality to both N-1 and η-1, may have arisen from diffusion of components through a boundary layer of physisorbed solvent or through the pore structure of nanocrystalline TiO2 films. Our measured kad values for alkanoic acids were within an order of magnitude of previously reported values for the adsorption of ruthenium(II) polypyridyl complexes (75) Tirado, J. D.; Acevedo, D.; Bretz, R. L.; Abru~na, H. D. Langmuir 1994, 10, 1971–1979. (76) DeBono, R. F.; Loucks, G. D.; Della Manna, D.; Krull, U. J. Can. J. Chem. 1996, 74, 677–688. (77) Dannenberger, O.; Buck, M.; Grunze, M. J. Phys. Chem. B 1999, 103, 2202– 2213. (78) Hong, H.-G.; Park, W. Electrochim. Acta 2005, 51, 579–587. (79) Peterlinz, K. A.; Georgiadis, R. Langmuir 1996, 12, 4731–4740. (80) Subramanian, R.; Lakshminarayan, V. Electrochim. Acta 2000, 45, 4501– 4509. (81) Kilsa˚, K.; Mayo, E. I.; Brunschwig, B. S.; Gray, H. B.; Lewis, N. S.; Winkler, J. R. J. Phys. Chem. B 2004, 108, 15640–15651.
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to nanocrystalline TiO2 films81 but were up to several orders of magnitude lower than literature values for the adsorption of alkanethiols to planar gold surfaces.77,82,83 We presume that the high surface area and high porosity of the nanocrystalline TiO2 films gave rise to slower adsorption. Equilibrium Binding. As a second step toward developing a kinetic model of compositional changes of mixed monolayers, we acquired equilibrium binding data for several adsorbates and solvents. Surface amounts of adsorbates per projected area were calculated from the νa(CH2) absorbances of functionalized films (eq S1 of the Supporting Information) or the integrated absorbance of the C-H stretching region.36 All equilibrium binding data were modeled by the Langmuir adsorption isotherm.73 Representative data for the adsorption of MHDA from EtOH and toluene are shown in Figure S6. Kad values and saturation surface amounts of adsorbates per projected area (Γ0) were extracted from linear plots of C/Γ vs C, where C is the concentration of adsorbate in the solution (Table 3). Γ0 values were on the order of 10-7 mol/cm2, similar to literature values for carboxylic acid-functionalized porphyrins and metal polypyridyl complexes on nanocrystalline TiO2.84-86 Kad values ranged from 3 103 to 3 104 M-1 and did not vary systematically with solvent viscosity or polarity. For a given solvent, the Kad values of HDA, OA, and MHDA were, within the precision of our measurements, identical. Kad equals the ratio of the rate constants for adsorption (kad) and desorption (kd). Because the equilibrium binding data and the adsorption kinetics were fit by the Langmuir model, we infer that the desorption of alkanoic and mercaptoalkanoic acids also followed Langmuir kinetics. Furthermore, the independence of Kad on solvent, chain length, and terminal functionalization suggests that the dependence of kad on η-1 and N-1 was offset by a similar dependence of kd on η-1 and N-1. Kinetic Model of Compositional Changes of Mixed Monolayers. A kinetic model was derived from the mechanism presented in Scheme 1. The irreversible displacement of Me by T involves a minimum of three steps: desorption of Me, adsorption of T, and formation of a disulfide bond. We assumed that adsorption and desorption of all Me and T followed Langmuir kinetics, as shown for the adsorption of alkanoic acids. We also assumed that dimerization of T occurred on the TiO2 surface, rather than in solution. This assumption is supported by the identical values of Kad for HDA, OA, and MHDA (Table 3). The model did not account for the possible influence of surface protonation-deprotonation equilibria on the kinetics or mechanism of adsorption and desorption reactions. For mixed monolayers of Me and T, the rate laws for adsorption and desorption are dθi ¼ kad, i Ci ð1 - θtot Þ dt
-
dθi ¼ kd, i θi dt
ð3Þ
ð4Þ
where θi is the fractional surface coverage of the ith component of the monolayer, dθi/dt and -dθi/dt are the rates of adsorption and (82) Karpovich, D. S.; Blanchard, G. J. Langmuir 1994, 10, 3315–3322. (83) Schessler, H. M.; Karpovich, D. S.; Blanchard, G. J. J. Am. Chem. Soc. 1996, 118, 9645–9651. (84) Trammell, S. A.; Meyer, T. J. J. Phys. Chem. B 1999, 103, 104–107. (85) Watson, D. F.; Marton, A.; Stux, A. M.; Meyer, G. J. J. Phys. Chem. B 2004, 108, 11680–11688. (86) Hoertz, P. G.; Staniszewski, A.; Marton, A.; Higgins, G. T.; Incarvito, C. D.; Rheingold, A. L.; Meyer, G. J. J. Am. Chem. Soc. 2006, 128, 8234–8245.
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Figure 4. (a) Rate constants for adsorption (kad) of HDA to nanocrystalline TiO2 films as a function of reciprocal of solvent viscosity (η-1).
(b) kad values for adsorption of alkanoic acids to nanocrystalline TiO2 films from THF as a function of reciprocal of alkyl chain length (N-1). Superimposed on both data sets are linear fits. Table 2. Rate Constants for Adsorption of Alkanoic Acids to Nanocrystalline TiO2 Films from THF Solutions Me
N
kad (M-1 s-1)
HDA OA PA
15 7 2
4.8 ( 0.6 77 ( 2 420 ( 20
Table 3. Equilibrium Binding Dataa adsorbate
solvent
Kad (M-1)
Γ0 (mol cm-2)
(1.5 ( 0.1) 10-7 HPA THF (3 ( 4) 104 b 3 HDA THF (4 ( 2) 10 (1.6 ( 0.1) 10-7 OA THF (5 ( 4) 103 (2.2 ( 0.1) 10-7 OA EtOH (3.0 ( 0.7) 103 (1.5 ( 0.1) 10-7 OA heptane (4 ( 3) 103 (3.8 ( 0.3) 10-7 b 3 MHDA THF (6 ( 4) 10 (1.2 ( 0.1) 10-7 MHDA EtOH (3 ( 1) 103 (2.5 ( 0.1) 10-7 MHDA heptane (7 ( 2) 103 (4.1 ( 0.2) 10-7 MHDA toluene (4.5 ( 0.7) 103 (2.9 ( 0.1) 10-7 MHDA CH2Cl2 (9 ( 3) 103 (2.3 ( 0.1) 10-7 MHDA CHCl3 (1.4 ( 0.3) 104 (1.8 ( 0.1) 10-7 a Standard deviations are from linear regression analysis of data in plots such as in Figure S6b,d; data were weighted by the standard deviations of absorbances of four or more samples. b These data were reported previously.36
desorption of the ith component of the monolayer, respectively; kad,i and kd,i are the rate constants for adsorption and desorption of the ith component, respectively; Ci is the concentration of the ith component in solution; and θtot is the total fractional surface coverage of adsorbates. Two integrated rate laws were derived for the three-step mechanism in Scheme 1. Although they were derived on the basis of potentially limiting assumptions, the rate laws provide simple analytical models for evaluating the influence of structure and solvation on the kinetics and mechanism of mixed-monolayer compositional changes. The derivations are presented in Appendix S2 in the Supporting Information. The first rate law (eq 5) was based on the assumption that the second step, adsorption of T, is rate-determining in the overall mechanism. The second rate law (eq 6) was based on the assumption that the third step, disulfide bond formation, is rate-determining in the overall mechanism. Data presented below indicated that the first step, desorption of Me, was not rate-determining. Both rate laws were derived by Langmuir 2009, 25(20), 12217–12228
assuming a rapid pre-equilibrium in the first step of Scheme 1. Because the molar ratio of Me to T in coadsorption solutions was 9-to-1, the readsorption of Me (reverse of step 1) was probably faster than adsorption of T (step 2). Therefore, it is not unreasonable that the second or third step of the overall mechanism was rate-limiting. rate-determining step = adsorption of T (step 2): kad, T CT t þ lnð1 - χT, surf , init Þ lnð1 -χT, surf Þ ¼ Kad, Me CMe
ð5Þ
rate-determining step = disulfide formation (step 3): Kad, T 1 ¼ kSS 1 -χT, surf Kad, Me
!2
CT CMe
2 tþ
1 1 - χT, surf , init
ð6Þ
where χT,surf is the mole fraction of T on the surface; χT,surf,init is the value of χT,surf at t = 0; kad,T is the rate constant for adsorption of T to TiO2; Kad,T and Kad,Me are the surface adduct formation constants for T and Me, respectively; CT and CMe are the concentrations of T and Me, respectively, in the coadsorption solution; t is the immersion time; and kSS is the rate constant for disulfide formation. Importantly, the integrated rate laws allow for graphical determination of the rate-determining step in the overall mechanism of compositional changes. If the second step is rate-determining, then a plot of ln(1 - χT,surf) vs t should be linear. If the third step is rate-determining, then a plot of 1/(1 - χT,surf) vs t should be linear. The quality of fits to integrated rate laws was determined on the basis of R2 values from linear regression analysis in which data points were weighted by standard deviations. If R2 values differed by less than 0.25%, then a weighted sum of squares of residuals (WRes)2 was instead used as the coefficient of determination: ðWRes Þ2 ¼
N X 1 ðχT, surf , meas - χT, surf , fit Þ2 2 σ i ¼1
ð7Þ
where N is the number of data points (each averaged from multiple measurements at a given t), χT,surf,meas and χT,surf,fit are the measured and fitted values of χT,surf, and σ is the standard deviation of measured χT,surf,meas values at a given t. Decreasing (WRes)2 corresponds to increasing quality of fit. DOI: 10.1021/la901740d
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Figure 5. 1/(1 - χT,surf) as a function of immersion time for mixed
Figure 6. Mole fractions of MHDA within mixed MHDA-OA monolayers (χT,surf) as a function of immersion time and coadsorption solvent. Superimposed on the data are fits to eq 5 (dashed lines) or eq 6 (solid lines).
Table 4. Rate Constants Derived from Eq 6 for Mixed MHDA-OA Monolayers from THF Solutions with Varying χT,soln
Values of kSS were calculated from kobs,6 and the Kad values for MHDA and OA (Table 4). The kSS values were on the order of 10-3-10-2 s-1; however, the standard deviations were large due primarily to the inherently large standard deviations of measured Kad values. Therefore, precise determination of kSS from the fit results was not possible. Solution Concentration. The kinetic model predicts that, regardless of the rate-determining step, the rate of mixed-monolayer compositional changes should depend only on the ratio of the concentrations of T and Me and not on the total concentration. To investigate this possibility, we prepared mixed MHDA-OA monolayers from two THF coadsorption solutions with χT,soln of 0.1 but different total concentrations. In the first solution, the concentrations of MHDA and OA were 0.2 and 1.8 mM, respectively; in the second solution, the concentrations were 10 and 90 mM, respectively. Plots of χT,surf vs t were nearly superimposable for the two sets of mixed monolayers (Figure S8). Extrapolation of the data to the y-axis reveals that the values of χT,surf,init were similar for the two data sets. The independence of χT,surf,init on the total concentration of components in solution is consistent with kinetic control of initial mixed-monolayer compositions. If the adsorption of MHDA and OA follows Langmuir kinetics, then the relative rates of adsorption from mixed coadsorption solutions should vary with the ratio of concentrations but not the total concentration. The rates of compositional changes were similar for mixed monolayers prepared from the two coadsorption solutions (Figure S8). The data were modeled more precisely by eq 6 than eq 5 (Figure S9, Table S2), suggesting that disulfide formation was rate-determining. Fit results from eq 6 are superimposed on the data in Figure S8. The relative independence of the data on the sum of the concentrations of MHDA and OA supports the proposed kinetic model. Solvent. Mixed MHDA-OA monolayers prepared from coadsorption solutions in BuOH, EtOH, THF, heptane, and Et2O underwent time-dependent compositional changes (Figure 6). Extrapolation of the data for BuOH (Figure 6) to the y-axes reveals that χT,surf,init was less than χT,soln. This decrease of χT,surf,init relative to χT,soln provides further evidence that initial compositions were controlled kinetically. The kad value of OA should be greater than that of MHDA due to its shorter chain length; therefore, kinetic control of mixed-monolayer compositions favors the adsorption of OA. The decrease of χT,surf,init relative
MHDA-OA monolayers with χT,soln of 0.1, 0.25, and 0.5. Superimposed on the data are linear fits to eq 6.
χT,soln 0.1 0.25 0.5
kobs,6 (s-1)
kSS (s-1)
(1.1 ( 0.4) 10-2 (6.2 ( 0.9) 10-3 (1.5 ( 0.3) 10-3
(8 ( 20) 10-3 (4 ( 11) 10-3 (1 ( 3) 10-3
Solution Composition. The kinetic model was applied to the data for mixed MHDA-OA monolayers prepared from THF solutions with χT,soln of 0.1, 0.25, and 0.5 (Figure 2). Plots of ln(1 - χT,surf) vs t (Figure S7) and 1/(1 - χT,surf) vs t (Figure 5) were constructed for each data set. (Data acquired after equilibration of the monolayers were not included in the fits.) The data for each of the mixed monolayers were fit more precisely by eq 6 than eq 5, as determined by comparison of R2 values (Table S1). Plots of χT,surf vs t were constructed from the fit results and are superimposed on the data in Figure 2. Values of kobs,6, or the observed rate constant for eq 6 where kobs,6 = kSS(Kad,T/Kad,Me)2, were extracted from the slopes of linear fits and are summarized in Table 4. (In principle, it should be possible to obtain values of χT,surf,init from y-intercepts of linear fits; however, the standard deviations were large.) The kobs,6 values decreased with χT,soln. Equation 6 predicts that the rate of compositional changes should be proportional to (CT/CMe)2, and kobs,6 should be independent of the ratio of concentrations. While the rate of compositional changes increased significantly with χT,soln (Figure 2), the increase was not proportional to (CT/CMe)2; therefore, kobs,6 varied with χT,soln. The smaller-than-expected increase of rate with χT,soln suggests that the rate of disulfide formation may not have been significantly greater than the rate of MHDA adsorption. Equation 5 predicts that, if the adsorption of MHDA is rate-determining, the rate of compositional changes should vary with CT/CMe. Thus, if neither disulfide formation nor MHDA adsorption is unambiguously rate-determining, an intermediate dependence of the overall rate of compositional changes on the ratio of concentrations would be anticipated. Therefore, while the integrated rate law of eq 6 accounts for the data more precisely than eq 5, the assumption that disulfide formation is rate-determining is probably an oversimplification. Further evidence that the rates of disulfide formation and MHDA adsorption are similar for mixed monolayers prepared from THF is presented below. 12224 DOI: 10.1021/la901740d
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Table 5. Rate Constants Derived from Eqs 5 and 6 for Mixed MHDA-OA Monolayers from Different Solvents solvent
ε
BuOH EtOH THF heptane Et2O
17.8 25.3 7.52 1.92 4.27
η (mPa s) 2.544 1.074 0.456 0.387 0.224
kobs,5 (s-1) (1.3 ( 0.5) 10-4 (2.3 ( 0.5) 10-4
kobs,6 (s-1)
(1.1 ( 0.1) 10-2 (9 ( 2) 10-3 (1.0 ( 0.1) 10-2
to χT,soln was most pronounced for BuOH, for which kad values were lowest. The rate of mixed-monolayer compositional changes decreased with increasing solvent viscosity (Figure 6). The rates were similar for Et2O, THF, and heptane and were much slower for BuOH and EtOH. The rate of compositional changes did not vary systematically with dielectric constant (ε) of the solvents, suggesting that solvent polarity was not a determining factor. The data for Et2O, heptane, and THF were modeled more precisely by eq 6 than eq 5 (Figure S10, Table S3), consistent with disulfide formation as the rate-determining step. For EtOH and BuOH, (WRes)2 values (eq 7) were significantly lower for the data constructed from fits to eq 5 than eq 6 (Figure S10, Table S3). Therefore, we infer that adsorption of MHDA to TiO2 was ratedetermining in the compositional changes of mixed monolayers prepared in EtOH and BuOH. Fit results (from eq 6 for Et2O, heptane, and THF and eq 5 for EtOH and BuOH) are superimposed on the measured compositional data in Figure 6. The fit results suggest that the rate-determining step switched from the adsorption of MHDA to the formation of disulfide bonds as the solvent viscosity decreased. This trend is consistent with the dependence of kad on η-1. The increase of kad,T with decreasing solvent viscosity apparently caused MHDA adsorption to become faster than disulfide formation in the least viscous solvents. Further decreases of the solvent viscosity did not dramatically impact the rate of compositional changes. Values of kobs,6 were extracted from the slopes of linear fits to eq 6 for mixed monolayers prepared in THF, heptane, and Et2O (Table 5). The values of kobs,6 were, within the precision of our measurements, equal for all three solvents. The independence of kobs,6 on viscosity implies that kSS did not vary significantly with the viscosity of the surrounding solvent. Values of kobs,5, where kobs,5 = kad,T/Kad,Me, were extracted from the slopes of linear fits to eq 5 for mixed monolayers prepared in BuOH, EtOH, and THF (Table 5). A plot of kobs,5 vs η-1 was linear for BuOH, EtOH, and THF (Figure 7). (THF was included in the plot for the sake of comparison, even though R2 was higher for the fit to eq 6 than eq 5.) The linearity of the data is consistent with the expected dependences of kad,T and, therefore, kobs,5 on η-1. Thus, the linear plot provides strong evidence that the rate of compositional changes of mixed monolayers prepared in BuOH, EtOH, and THF was strongly influenced by the rate of adsorption of MHDA to TiO2. The dependence of kad on η-1 was attributed to decreased mobility with increasing viscosity. We performed two experiments to investigate whether the observed dependence of kobs,5 on η-1 was also caused by variation of mobility. First, mixed MHDA-OA monolayers were prepared by immersing nanocrystalline TiO2 films in either unstirred or rapidly stirred THF coadsorption solutions. The rates of compositional changes were equal. Therefore, the diffusion of components through bulk solution, the rate of which was increased by stirring, was not rate-limiting in the overall mechanism of compositional changes. Second, we prepared mixed MHDA-OA monolayers on Evonik P25 TiO2 nanoparticles suspended in THF. The rate of Langmuir 2009, 25(20), 12217–12228
Figure 7. kobs,5 as a function of the reciprocal of solvent viscosity (η-1) for mixed MHDA-OA monolayers prepared from BuOH, EtOH, and THF. Superimposed on the data is a linear fit.
compositional changes of mixed monolayers on P25 TiO2 nanoparticles in unstirred THF suspensions was similar to that of mixed monolayers on nanocrystalline TiO2 films. In contrast, the compositions changed much more rapidly for mixed monolayers on P25 TiO2 nanoparticles in stirred THF suspensions (Figure S11). The increased rate of compositional changes with stirring suggests that the mobility of components near the TiO2 surface influenced the rate of the overall mechanism of compositional changes. We infer that the pore structure of nanocrystalline films limited mobility. Chain Length of Mercaptoalkanoic Acids. Mixed monolayers of HDA and the following mercaptoalkanoic acids were prepared, where N is listed in parentheses: MHDA (15), MUDA (10), MOA (7), MHA (5), and MPA (2). (Mixed monolayers containing MHDA included OA as the alkanoic acid, rather than HDA. Different alkyl chain lengths of mercaptoalkanoic and alkanoic acids were required to determine mixed-monolayer compositions by method 2.) Each of the mixed monolayers underwent time-dependent compositional changes (Figure 8). The value of χT,surf,init increased significantly with decreasing chain length, providing further evidence that initial compositions were controlled kinetically. The rate of adsorption of mercaptoalkanoic acids presumably increased with decreasing chain length, due to the dependence of kad on N-1, giving rise to the observed increase of χT,surf,init. The rate of compositional changes, as indicated by the initial slope of plots of χT,surf vs t, also increased with decreasing chain length. Therefore, desorption of HDA was clearly not rate-determining in the overall mechanism of dimerization-induced compositional changes. The data for mixed monolayers containing MUDA, MOA, and MPA were modeled more precisely by eq 5 than eq 6 (Figure S12, Table S4); therefore, adsorption of T was rate-determining. The data for MHA were modeled more precisely by eq 6 (Figure S12, Table S4). The data for MHDA were modeled somewhat more precisely by eq 6 than eq 5 (Figure S12, Table S4), though neither fit was optimal. The apparent variation of the ratedetermining step for MHA and the lack of a clear-cut ratedetermining step for MHDA suggest that the rates of adsorption and disulfide formation were nearly equal for mixed monolayers prepared in THF. We speculate that uncontrolled conditions, such as the morphology or surface area of the TiO2 substrates, may have caused the anomalous rate-determining step for mixed monolayers containing MHA. Fit results are superimposed on the measured data in Figure 8. For the sake of comparison, we used the fit results from eq 5, DOI: 10.1021/la901740d
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Figure 8. Mole fractions of MAAs within mixed monolayers (χT,surf) as a function of immersion time and MAA chain length (N). Superimposed on the data are fits to eq 5.
Figure 9. kobs,5 as a function of the reciprocal of the alkyl chain length of mercaptoalkanoic acids (N-1). Superimposed on the data is a linear fit.
Table 6. Rate Constants Derived from Eq 5 for Mixed Monolayers with T of Varying Chain Length T
N
kobs,5 (s-1)
MPA MHA MOA MUDA MHDA
2 5 7 10 15
(5.3 ( 0.1) 10-3 (1.3 ( 0.1) 10-3 (1.2 ( 0.1) 10-3 (9 ( 2) 10-4 (4 ( 2) 10-4
rather than eq 6, for all mercaptoalkanoic acids, including MHA and MHDA. The values of (WRes)2 in Table S4 provide an indication of the overall goodness of fit. Values of kobs,5 were extracted from the slopes of linear fits to eq 5 (Table 6). A plot of kobs,5 vs N-1 was linear (Figure 9). (The value of kobs,5 for MHA deviated from the linear trend due to the poorer fit of these data to eq 5.) The linear relationship between kobs,5 and N-1 is consistent with the expected dependence of kad on N-1. Together with the solvent-dependent data outlined above, these data provide compelling evidence that, under certain conditions, the overall rate of the dimerization-induced compositional changes was limited or strongly influenced by the adsorption of T to the TiO2 surface. Furthermore, the linearity of the data in Figure 9 and the quality of the fits in Figure 8 suggest that the kinetic model outlined above accounts for key elements of the kinetics of dimerization-induced compositional changes. Sterics. The dependence of the rate of compositional changes on the rates of adsorption and disulfide formation suggests that the structure of adsorbates may influence the overall kinetics. To investigate the role of sterics, we compared the compositional evolution of mixed MHDA-OA monolayers and mixed MCA-OA monolayers. We reasoned that the phenyl ring of MCA (Chart 1) may affect the rates of adsorption and/or disulfide formation. Mixed monolayers were prepared from coadsorption solutions in THF with χT,soln of 0.1 and 0.25 (Figure 10). MHDA-OA monolayers and MCA-OA monolayers underwent similar compositional changes, in which T displaced OA from the surface. For a given χT,soln, however, the compositions of MCA-OA monolayers changed more slowly than those of MHDA-OA monolayers, as determined by the initial slope of the data or by comparison of χMCA,surf and χMHDA,surf at any reaction time. The data were modeled more precisely by eq 6 than eq 5 (Table S5), consistent with disulfide formation as the rate-determining step. Fit results are superimposed on the data in Figure 10. Values of kobs,6 were extracted from the fits (Table 7). 12226 DOI: 10.1021/la901740d
Figure 10. Mole fractions of MHDA or MCA within mixed T-OA monolayers (χT,surf) as a function of immersion time and χT,soln. Superimposed on the data are fits to eq 6.
(The values of kobs,6 for MHDA-OA mixed monolayers in Tables 4 and 7 correspond to different sets of samples.) For both compositions of the coadsorption solutions, kobs,6 was greater for MHDA-OA monolayers than for MCA-OA monolayers, implying that kSS was greater for MHDA than MCA. Steric hindrance between phenyl rings of MCA presumably increased the activation barrier for disulfide formation and decreased kSS. Steric hindrance apparently exerted a more significant influence on the rate of compositional changes than favorable π-stacking interactions between MCA. The values of kobs,6 for mixed MCA-OA monolayers and mixed MHDA-OA monolayers varied with χT,soln. As discussed above, this variation suggests that disulfide formation is not unambiguously rate-determining in the overall mechanism of compositional changes. Thus, while we attribute the slow compositional changes of mixed MCAOA monolayers primarily to the decreased rate of disulfide formation, slower adsorption kinetics may also have played a role. In a second experiment to probe the influence of sterics, we prepared mixed monolayers consisting of OA and either MPA or MIBA (Chart 1). MIBA contains a methyl group at the C2 position. For both sets of mixed monolayers, χT,surf increased with reaction time (Figure S13). The compositional changes of MIBA-OA monolayers occurred slightly more slowly than those of Langmuir 2009, 25(20), 12217–12228
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Table 7. Rate Constants Derived from Eq 6 for Mixed MHDA-OA and MCA-OA Monolayers T
χT,soln
kobs,6 (s-1)
MHDA MCA
0.1 0.1
(2.2 ( 0.1) 10-2 (6 ( 2) 10-3
MHDA MCA
0.25 0.25
(1.0 ( 0.1) 10-2 (3 ( 2) 10-3
MPA-OA monolayers, although the MPA data had significant error bars at short immersion times. The data for MIBA-OA were well-modeled by eq 6, consistent with disulfide formation as the rate-determining step. Fit results are superimposed on the MIBA-OA data in Figure S13. (The corresponding MPA-OA data were not fit due to the variation at short immersion times.) The slower compositional changes for mixed MIBA-OA monolayers, and the apparently rate-limiting kinetics of disulfide formation, imply that steric hindrance between methyl groups decreased the rate of disulfide formation. Slower adsorption of MIBA than MPA to TiO2 may also have affected the relative kinetics of compositional evolution. Surface-Attachment Group. Carboxylate linkages to TiO2 are labile in organic solvents, as evidenced by the evolution of mixed monolayers from kinetically controlled to thermodynamically controlled compositions. Phosphonic acids adsorb more strongly than carboxylic acids to metal oxides.87-89 To investigate the influence of the surface-attachment interaction on the formation and compositional evolution of mixed monolayers, we prepared mixed monolayers of MHDA and HPA (Chart 1). Monolayers were prepared from coadsorption solutions in THF with χT,soln of 0.1, 0.5, and 0.9. Mixed MHDA-HPA monolayers did not undergo dimerization-induced compositional changes (Figure 11). Instead, HPA displaced MHDA from the TiO2 surface. Monolayers prepared from coadsorption solutions with χT,soln of 0.1 and 0.5 consisted essentially entirely of HPA at all immersion times. For χT,soln of 0.9, MHDA was displaced by HPA over several hours. The Kad value of HPA was approximately 10-fold greater than those of HDA, OA, and MHDA for adsorption from THF (Table 3). The data in Figure 11 imply that Kad of HPA was also greater than that of disulfide-bridged MHDA dimers. The slow compositional evolution of mixed MHDA-HPA monolayers is consistent with the adsorption kinetics of HPA, which adsorbed much more slowly than alkanoic acids to TiO2 (Figures S4, S5, and S14). The surfaceattachment mode of phosphonic acids to TiO2 may depend on the crystallinity, exposed crystal face, and adsorption conditions.90-95 A recent computational analysis indicated that phosphonic acid adsorbs to anatase TiO2 as a triply deprotonated anion in various bidentate coordination geometries.96 The high Kad value of HPA (Table 3) is consistent with polydentate (87) Yan, S. G.; Prieskorn, J. S.; Kim, Y.; Hupp, J. T. J. Phys. Chem. B 2000, 104, 10871–10877. (88) Pawsey, S.; Yach, K.; Reven, L. Langmuir 2002, 18, 5205–5212. (89) Cooper, R. J.; Camp, P. J.; Henderson, D. K.; Lovatt, P. A.; Nation, D. A.; Richards, S.; Tasker, P. A. Dalton Trans. 2007, 1300–1308. (90) Randon, J.; Blanc, P.; Paterson, R. J. Membr. Sci. 1995, 98, 119–129. (91) Gao, W.; Dickinson, L.; Grozinger, C.; Morin, F. G.; Reven, L. Langmuir 1996, 12, 6429–6435. (92) Guerrero, G.; Mutin, P. H.; Vioux, A. Chem. Mater. 2001, 13, 4367–4373. (93) Lafond, V.; Gervais, C.; Maquet, J.; Prochnow, D.; Babonneau, F.; Mutin, P. H. Chem. Mater. 2003, 15, 4098–4103. (94) Mutin, P. H.; Lafond, V.; Popa, A. F.; Granier, M.; Markey, L.; Dereux, A. Chem. Mater. 2004, 16, 5670–5675. (95) Adolphi, B.; J€ahne, E.; Busch, G.; Cai, X. Anal. Bioanal. Chem. 2004, 379, 646–652. (96) Luschtinetz, R.; Frenzel, J.; Milek, T.; Seifert, G. J. Phys. Chem. C 2009, 113, 5730–5740.
Langmuir 2009, 25(20), 12217–12228
Figure 11. Surface amounts per projected area (Γ) of HPA and
MHDA, and the sum of ΓHPA and ΓMHDA, for mixed MHDAHPA monolayers prepared from THF coadsorption solutions with χT,soln of 0.9.
coordination. The slower adsorption kinetics may arise from the occurrence of multiple deprotonation steps. The data in Figure 11 reveal that the composition of mixed monolayers depends strongly on the nature and strength of surface-attachment interactions. For mixed MHDA-HPA monolayers, dimerization-induced compositional changes were “turned off” by incorporating an adsorbate with a more stable (and probably less labile) surface-attachment interaction.
Conclusions Mixed monolayers of thiol-terminated and methyl-terminated carboxylates on nanocrystalline TiO2 films underwent compositional changes after the initial formation of full monolayers with kinetically controlled compositions. Initial mixed-monolayer compositions varied with the composition of the coadsorption solution, the chain length of adsorbates, and the viscosity of the coadsorption solvent. Equilibration of mixed monolayers involved the displacement of Me by T. This process was driven by the formation of intermolecular disulfide bonds and the increased stability of disulfide-bridged dimers on the surface relative to monomeric adsorbates. The mechanism outlined in Scheme 1 provides a basis for modeling the kinetics of compositional changes. The relative rates of the adsorption of T to TiO2 and the formation of disulfide bonds determined the overall rate of compositional changes. The slow kinetics of compositional changes enabled a detailed investigation of structure and solvation on the rates of adsorption, disulfide formation, and compositional changes. Rate constants for adsorption of mercaptoalkanoic acids to TiO2 were inversely proportional to solvent viscosity and chain length. The kinetics of disulfide formation appeared to be relatively independent of these parameters but varied with the steric bulk of adsorbates. The nature of the interaction between adsorbates and TiO2 also significantly influenced mixed-monolayer compositions. No compositional changes were observed for monolayers incorporating HPA, which was more stable and probably more inert on the TiO2 surface. In summary, the kinetics of compositional changes and the rate-determining step in the overall mechanism varied significantly with the conditions under which mixed monolayers were prepared. The fabrication of mixed monolayers of T and Me with precisely controlled compositions requires careful consideration of the interplay between the rates of adsorption and disulfide formation as well as the nature and strength DOI: 10.1021/la901740d
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of the surface-attachment interaction. Our findings highlight the influence of intermolecular interactions on the composition and terminal functionalization of mixed monolayers. Various intermolecular interactions within monolayers may dramatically impact the compositional evolution, physical properties, and chemical reactivity of mixed-monolayer-functionalized surfaces. Acknowledgment. We thank Prof. Joseph Gardella for the use of ToF-SIMS instrumentation and for insightful discussions, particularly with regard to the interpretation of ToF-SIMS data. We thank Meghan Kern for her contributions to desorption experiments. This work was funded, in part, by the National Science Foundation (CHE-0645678) and the University at Buffalo, State University of New York. In addition, acknowledgment
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is made to the donors of the American Chemical Society Petroleum Research Fund for partial support of this research. ToF-SIMS instrumentation (Gardella lab) was acquired through a National Science Foundation Major Research Instrumentation grant (CHE-0619728). Supporting Information Available: Adsorption kinetics and equilibrium binding data, ToF-SIMS data and chemical images, compositional evolution data and fits to eqs 5 and 6 for various mixed monolayers, goodness-of-fit parameters for kinetic modeling of various mixed monolayers, descriptions of methods for calculating mixed-monolayer compositions from IR spectra, and derivations of integrated rate laws (eqs 5 and 6). This material is available free of charge via the Internet at http://pubs.acs.org.
Langmuir 2009, 25(20), 12217–12228
