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Self-Assembled Monolayers of Alkanoic Acids: A Solid-State NMR Study Shane Pawsey, Kimberly Yach, Jamie Halla, and Linda Reven* Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec, Canada H3A 2K6 Received September 24, 1999. In Final Form: December 9, 1999 Long-chain alkanoic acids (CH3(CH3)nCO2H, n > 15) form conformationally ordered monolayers on zirconium oxide powder via a chelating bidentate zirconium carboxylate surface bond. ω-Hydroxyalkanoic acids (HO(CH3)15CO2H) produce hydrophilic monolayers, with no evidence of looping to the metal oxide surface. Variable temperature and two-dimensional solid-state NMR experiments demonstrate that alkanoate monolayers display chain dynamics similar to other self-assembled monolayers (SAMs). The thermal behavior of alkanoate SAMs is also compared to analogous fatty acid salt Langmuir-Blodgett monolayers which undergo pretransitional chain disordering.
Introduction Although the adsorption of long-chain carboxylic acids on metal oxides produced the first reported self-assembled monolayers (SAMs),1 relatively few studies have been carried out compared to other systems. The ionic interaction between carboxylic acids and most metal oxides is weak compared to the covalent surface bonds in thiol/gold or siloxane/silica SAMs. Furthermore, polar functional groups are easily incorporated into organothiol SAMs due to the specificity of the sulfur-gold interaction, but competition for surface sites between the carboxylic acid and other polar groups may occur in the case of functionalized alkanoic acids.2 The effect of structural variables (substrate, chain length, headgroup size) on the nature of the surface bond mode, chain conformation, and chemical stability of alkanoic acid monolayers on oxidized alumina, copper, and silver films has been characterized by vibrational spectroscopy.1b,3-8 One advantage that alkanoic acid SAMs have over other types of monolayers is the ability to directly monitor the nature of the interaction between the headgroup and the substrate via the CO stretching bands in the infrared spectrum. The binding geometry, which depends strongly on the metal oxide substrate, determines the packing density and chain conformation. These studies indicate that alkanoic acids bind most strongly to AgO through a bidentate surface bond and more weakly to aluminum or copper oxide via a monodentate attachment.3,4 Alkanoic acids adsorbed on metal oxides are of interest as a system which provides a link between Langmuir* To whom correspondence may be addressed. Tel: (514) 3988058. Fax: (514) 398-3797. E-mail:
[email protected]. (1) (a) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45. (b) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52. (2) Laibinis, P. E.; Hickman, J. J.; Wrighton, M. S.; Whitesides, G. M. Science 1989, 245, 845. (3) Schlotter, N. E.; Porter, M. D.; Bright, T. B.; Allara, D. L. Chem. Phys. Lett. 1886, 132, 93. (4) (a) Tao, Y. T. J. Am. Chem. Soc. 1993, 115, 4350. (b) Tao, Y. T.; Lee. M. T.; Chang, S. C. J. Am. Chem. Soc. 1993, 115, 9547. (5) Smith, E.; Porter, M. D. J. Phys. Chem. 1993, 97, 8032. (6) (a) Songdag A. H. M.; Raas, M. C. J. Chem. Phys. 1989, 91, 4926. (b) Songdag, A. H. M.; Touwslager, F. J. Langmuir 1994, 10, 1028. (c) Touwslager, F. J.; Sondag, A. H. M. Langmuir 1994, 10, 1028. (7) (a) Tao, Y. T.; Hietpas, G. D.; Allara, D. L. J. Am. Chem. Soc. 1996, 118, 6724. (b) Tao, Y. T.; Lin, W. L.; Hietpas, G. D.; Allara, D. L. J. Phys. Chem. B 1997, 101, 9732. (8) Lee, S. J.; Kim, K. Vib. Spectros. 1998, 18, 187.
Blodgett (LB) films and self-assembled monolayers.9-11 The large number of studies concerned with the structure, phase transitions, and thermal stability of fatty acid salt LB films can be usefully compared to alkanoate SAMs. Decoupled phase transitions for the hydrophilic and hydrophobic parts of fatty acid salt LB monolayers have been observed in which the hydrocarbon chains disorder below the main melting point of the ionic headgroup.12-21 Whether or not these transitions are reversible has not been completely resolved. Early on, Swalen suggested that as long as the headgroup remains intact, the thermal chain disordering should be reversible.12 In a variable temperature study of the electron diffraction patterns of cadmium fatty acid salt monolayers, Riegler concluded that the chain-length-dependent pretransitional disordering is irreversible.16 A later attenuated total reflectance Fourier transform infrared (FTIR-ATR) study of Cd, Ca, and Pb stearate LB monolayers heated to 130 °C reported complete recrystallization with no hysteresis upon cooling.17 Pretransitional disordering has also been reported for LB monolayers of functionalized alkanoic acids22 and other types of aliphatic molecules.23,24 Ulman proposed that the irreversibility of the chain disordering in LB (9) Ulman, A. Chem. Rev. 1996, 96, 1533. (10) Ulman, A. Adv. Mater. 1991, 3, 298. (11) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: Boston, MA, 1991. (12) (a) Naselli, C.; Rabolt, J. F.; Swalen, J. D. J. Chem. Phys. 1985, 82, 2136. (b) Rabe, J. P.; Swalen, J. D.; Rabolt, J. F. J. Chem. Phys. 1987, 86, 1601. (13) (a) Saperstein, D. D. J. Phys. Chem. 1986, 90, 1408. (b) Saperstein, D. D. J. Phys. Chem. 1987, 91, 2922. (14) Cohen, S. R.; Naaman, R.; Sagiv, J. J. Phys. Chem. 1986, 90, 3054. (15) Rothberg, L.; Higashi, G. S.; Allara, D. L.; Garoff, S. Chem. Phys. Lett. 1987, 133, 67. (16) Riegler, J. E. J. Phys. Chem. 1989, 93, 6475. (17) Ahn, D. J.; Franses E. I. J. Phys. Chem. 1992, 96, 9952. (18) Blasie, K. J. Phys. Rev. B 1989, 39, 12165. (19) (a) Hasegawa, T.; Kamata, T.; Umemura, J.; Takenaka, T. Chem. Lett. 1990, 1543. (b) Umemura, J.; Takeda, S.; Hasegawa, T.; Takenaka, T. J. Mol. Struct. 1993, 297, 57. (c) Umemura, J.; Takeda, S.; Hasegawa, T.; Kamata, T.; Takenaka, T. Spectrochim. Acta 1994, 50A, 1563. (20) Rapp, G.; Koch, M. H. J.; Ho¨hne, U.; Lvov, Y.; Mo¨hwald, H. Langmuir 1995, 11, 2348. (21) Vierheller, T. R.; Foster, M. D.; Wu, H.; Schmidt, A.; Knoll, W.; Satija, S.; Majkrzak, C. F. Langmuir 1996, 12, 5156. (22) (a) Kamata, T.; Umemura, J.; Takenaka, T.; Koizumi, N. J. Phys. Chem. 1991, 95, 4092. (b) Taniike, K.; Matsumoto, T.; Sato, T.; Ozaki, Y.; Nakashima, K.; Iriyama, K. J. Phys. Chem. 1996, 100, 15508. (23) Kawai, T. Bull. Chem. Soc. Jpn. 1997, 70, 771.
10.1021/la991273e CCC: $19.00 © 2000 American Chemical Society Published on Web 01/28/2000
Self-Assembled Monolayers of Alkanoic Acids
monolayers reported by some groups may be due to loss of material.10 In the case of the extensively studied cadmium arachidate multilayers, desorption begins at 70 °C, which is only slightly above the pretransitional disordering temperature for this chain length. Order/disorder transitions have been also detected in self-assembled monolayers on both planar25 and nonplanar substrates.26 Self-assembled monolayers on nonplanar substrates (thiols/gold nanoparticles,26 phosphonic acids/ metal oxides,27 and siloxanes/silica28-30) have been recently characterized by solid-state NMR spectroscopy to probe the previously unknown dynamic properties and the surface bonding states. A detailed picture of the chain motions present above and below the reversible order/ disorder transitions which occur in these monolayers has been derived from wide-line 2H NMR and 13C NMR relaxation studies.26 Alkanoate SAMs represent a convenient system for NMR studies of more complex structural problems since isotopically labeled and functionalized carboxylic acids are commercially available. Phosphonic acids bind more strongly to metal oxides, but functionalized long-chain phosphonic acids can be difficult to synthesize. Recent studies show that the adhesion of carboxylic acids to alumina and other substrates is greatly increased by pretreating the surface with zirconium alkoxides.31 Likewise, we report here that highly ordered self-assembled alkanoate monolayers form directly on ZrO2 in contrast to Al2O3 or TiO2 powders where most of the surfactant is removed during the washing steps. Ordered alkanoate SAMs on high surface area substrates have not been reported prior to this study. A number of papers concerning carboxylic acid functionalized silver colloids have appeared, but neither the average chain conformation nor the surface binding state was characterized.32 The purpose of this work is to establish whether alkanoate SAMs exhibit structural and dynamic features similar to previously studied thiol and phosphonic acid SAMs. Siloxane SAMs have also been extensively studied,28-30 but both their self-assembly and final structure are intrinsically different from the thiol and organic acid monolayers. Whereas the latter form monolayers (24) (a) Terashita, S.; Ozaki, Y.; Iriyama, K. J. Phys. Chem. 1993, 97, 10445. (b) Wang, Y.; Nichogi, K.; Iriyama, K.; Ozaki, Y. J. Phys. Chem. 1996, 100, 17232. (25) Badia, A.; Back. R.; Lennox, R. B. Angew. Chem., Int. Ed. Engl. 1994, 33, 2332. (26) (a) Badia, A.; Singh, S.; Demers, L.; Cuccia, L. A.; Brown, G. R.; Lennox, R. B. Chem. Eur. J. 1996, 96, 2657. (b) Badia, A.; Gao, W.; Singh, S.; Demers, L.; Cuccia, L.; Reven, L. Langmuir 1996, 12, 1262. (c) Badia, A.; Cuccia, L.; Demers, L.; Morin, F. G.; Lennox, R. B. J. Am. Chem. Soc. 1997, 119, 2682. (d) Badia, A.; Demers, L.; Dickinson, L.; Morin, F. G.; Lennox, R. B.; Reven, L. J. Am. Chem. Soc. 1997, 119, 11104. (e) Schmitt, H.; Badia, A.; Dickinson, L.; Reven, L.; Lennox, R. B. Adv. Mater. 1998, 10, 475. (27) (a) Gao, W.; Dickinson, L.; Grozinger, C.; Morin, F. G.; Reven, L. Langmuir 1997, 13, 115. (b) Gao, W.; Dickinson, Grozinger, C.; Morin, F. G.; Reven, L. Langmuir 1996, 12, 6429. (28) Gao, W.; Reven, L. Langmuir 1995, 11, 1860. (29) Fatunmbi, H. O.; Bruch, M. D.; Wirth, M. J. Anal. Chem. 1993, 65, 2048. (30) (a) Albert, K.; Lacker, T.; Raitza, M.; Pursch, M.; Egelhaaf, H. J.; Oelkrug, D. Angew. Chem., Int. Ed. Engl. 1998, 37, 778. (b) Pursch, M.; Strohschein, S.; Ha¨ndel, H.; Albert, K. Anal. Chem. 1996, 68, 386. (c) Pursch, M.; Brindle, R.; Ellwanger, A.; Sander, L. C.; Bell, C. M.; Ha¨ndel. H.; Albert, K. Solid State NMR 1997, 9, 191. (d) Pursch. M.; Sander, L. C.; Egelhaaf, H. J.; Raitza, M.; Wise, S. A.; Oelkrug, D.; Albert, K. J. Am. Chem. Soc. 1999, 121, 3201. (31) (a) Aronoff, Y. G.; Chen, B.; Lu, G.; Seto, C.; Schwartz, J.; Bernasek, S. L. J. Am. Chem. Soc. 1997, 119, 259. (b) Bernasek, S. L.; Schwartz, J. Langmuir 1998, 14, 1387. (c) VanderKam, S. K.; Bocarsly, A. B.; Schwartz, J. Chem. Mater. 1998, 10, 685. (d) VanderKam, S. K.; Gawalt, E. S.; Schwartz, J.; Bocarsly, A. B. Langmuir 1999. (32) Patil, V.; Mayya, K. S.; Pradhan, S. D.; Sastry, M. J. Am. Chem. Soc. 1997, 119, 9281.
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because of the correspondence between the substrate and the surfactant, the siloxanes may cross-link prior to, or after, adsorption and the orientation and packing of the organic groups are dictated by the siloxane backbone rather than the substrate.33 While the particular choice of the fatty acids on zirconia was mainly motivated by our desire to find a convenient system for fundamental studies of the dynamic properties of self-assembled monolayers, this particular surface modification of zirconia may have practical applications. Relatively little research concerning the surface modification of zirconia has been reported. As chromatographic supports34 or ultrafiltration membranes35 for the purification of biomolecules, zirconia-based materials offer the advantage of a very high mechanical strength combined with a much greater stability toward extremes in acidity and alkalinity as compared to silica or alumina. Carr and co-workers have taken advantage of the strong interaction between a protein’s carboxylic acid groups and zirconia to prepare a regenerable, protein-based, affinity chromatographic stationary phase.34 Experimental Section Materials. Nonporous zirconia (ZrO2) powder (monoclinic, VP zirconium dioxide, Degussa Corp.) was calcinated at 400 °C overnight to eliminate any residual organic impurities. The reported average primary particle size of this ZrO2 powder is 30 nm, and the BET surface area is 40 m2/g. The n-alkanoic acids, CH3(CH2)nCO2H, (n ) 9, 13, 14, 15, 16, 17, 18, 20), and ω-hydroxyalkanoic acids, HO(CH2)nCO2H (n ) 11, 15), were obtained from Aldrich and used without further purification. Sample Preparation. The ZrO2 powder was dispersed in the appropriate solvent by sonication for approximately 15 min. Surfactant (3 mM solutions) was added to the dispersed ZrO2 powder under stirring. Either acetone or ether was used as solvents and generally both gave similar results, but better coverages were obtained with ether for the two longest chain lengths (n ) 18, 20). Unless stated otherwise, enough surfactant was used so as to give an estimated 3-5 times excess of a complete monolayer. The resulting mixture was refluxed with stirring for 2 days and then annealed for two additional days by warming the solution to just below the boiling point without stirring. The solid was then removed by centrifuging and filtering. To remove any unbound surfactant, the samples were washed thoroughly 3-5 times by redispersing the powder in the appropriate solvent, filtering, and then drying under vacuum after the final washing step. The coverages, listed in Table 1, were calculated from the % carbon from elemental analysis, the 40 m2/g ZrO2 surface area, and a 30 Å2 area/molecule. NMR. Solid-state 13C NMR spectra (67.92 MHz) were recorded on a Chemagnetics CMX-270 NMR spectrometer with a 7 mm double-tuned fast magic angle spinning (MAS) Doty probe. A spin rate of 4 kHz and pulse delays of 3 s and 8000 scans were typically used to acquire the 13C CP-MAS spectra. For 1H-13C cross polarization (CP) experiments, 1H 90° pulse widths between 4 and 4.25 µs and a contact time of 3 ms were used. For the variable-temperature CP-MAS experiments, the sample temperature was controlled to within (2° by a Chemagnetics temperature controller. For the 2D wide-line-separation experiments (WISE),36 a 1H 90° pulse was followed by a proton evolution period, t1, consisting of 128 increments of 1 µs. After each t1 period, cross polarization followed by carbon detection with proton decoupling gives a carbon spectrum that is modulated as a function of t1 by the free induction decay of the associated protons. Scans (256 or 512) were acquired (33) Allara, D. L.; Parikh, A. N.; Rondelez, F. Langmuir 1995, 11, 2357. (34) Nawrocki, J.; Dunlap, C. J.; Carr, P. W.; Blackwell, J. A. Biotechnol. Prog. 1994, 10, 561. (35) Randon, J.; Blanc, P.; Paterson, R. J. Membr. Sci. 1995, 98, 119. (36) (a) Schmidt-Rohr, K.; Clauss, J.; Spiess, H. W. Macromolecules 1992, 25, 3237. (b) Clauss, J.; Schmidt-Rohr, K.; Adam, A.; Boeffel, C.; Spiess, H. W. Macromolecules 1992, 25, 5208.
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Table 1. Coverage,
13C
Pawsey et al.
Methylene Chemical Shifts, C-H Stretching Frequencies, and Order/Disorder Temperatures as a Function of Chain Length for CH3(CH2)nCO2/ZrO2 and HO(CH2)nCO2/ZrO2 % coveragea
alkanoic acid CH3(CH2)9CO2/ZrO2 CH3(CH2)13CO2/ZrO2 CH3(CH2)14CO2/ZrO2 CH3(CH2)15CO2/ZrO2 CH3(CH2)16CO2/ZrO2 CH3(CH2)17CO2/ZrO2 CH3(CH2)18CO2/ZrO2 CH3(CH2)20CO2/ZrO2 CH3(CH2)16CO2H (bulk) HO(CH2)11CO2/ZrO2 HO(CH2)15CO2/ZrO2
83 61 71 63 79 75 89 65 67 76
13C methylene chemical shifts (ppm)b
νs(C-H), νas(C-H) (cm-1)b
order/disorder temp (°C)c
30 30.4 33, 30 33, 30 33, 30 (sh) 33 33 33 33 33, 30 33
2921, 2852 2920, 2851 2919, 2850 2918, 2949 2917, 2849 2917, 2849 2916, 2848 2916, 2848 2917, 2848 2922, 2851 2917, 2850
∼30 ∼45 ∼50 ∼65 ∼70 ∼60
a
Estimate of the surface coverage is based on %C from elemental analyses and 30 Å2 area/molecule. b At room temperature. c The order/disorder temperature was arbitrarily chosen to be the temperature at which the transoid and gauchoid 13C NMR resonances are of equal intensity.
for each t1 evolution period. A short contact time of 0.5 ms was used to minimize any proton spin diffusion, which will equalize the proton line widths. The processed data sets contained 512 points in the F2 (13C) dimension and 256 data points in the F1 (1H) dimension. 13C two-dimensional (2D) exchange experiments37 are used to probe ultraslow motions. The pulse sequence consists of a preparation period where cross polarization is used to create 13C transverse magnetization which is allowed to precess during the subsequent evolution period and then stored along the z axis by applying a 90° pulse to the 13C spins. During the ensuing mix time, if any molecular reorientations occur, the 13C precessional frequencies will be altered after a second 90° pulse restores the magnetization to the transverse plane. The 13C signal is acquired during the second evolution period. If no molecular reorientations occur, the frequencies during the two evolution times will be identical and the 2D spectrum will contain intensity only along the main diagonal where f1 ) f2. By varying the mix times, motions with correlation times in range of 10-5 s < τc
