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Langmuir 1995,11, 1860-1863
Solid-state NMR Studies of Self-Assembled Monolayers Wei Gao and Linda Reven* Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec, Canada H3A 2K6 Received January 4, 1995. I n Final Form: March 14, 1995@ Octadecylsiloxane (OTS) and zirconium octadecylphosphonate(ODPA)partial monolayers on nonporous silica were characterized by 13C and 31PCPMAS NMR. Well-orientedmonolayers display an intense peak at 33 ppm, the characteristic 13Cchemical shift of the interior methylene carbons of the alkane chains in an all trans conformation,in addition to a smaller component at 30 ppm correspondingto conformationally disordered chains. In the case of the OTS self-assembled monolayers, both the NMR and FTIR spectra showed an increased extent of ordering of the chains with lower deposition temperatures and a reversible disordering with heating after deposition. The ODPA monolayer on zirconated silica represents the first example of a well-ordered metal phosphonate monolayer prepared by self-assembly techniques alone. Self-assembled monolayers are currently the subject of intensive study due to their potential applications as protective coatings, optoelectronic devices, catalysts, and modified electrodes, as well as for fundamental studies of wetting, adhesion, and the behavior of two-dimensional systems in genera1.l The popularity of these organic films is due to the potential of exerting molecular level control over structure and composition for design of films for specific functions. However, if the goal of tailoring thin films with molecular level control for practical applications is to be realized, these films will have to be actually characterized a t the molecular level. The current characterization techniques, which include contact angle measurements, ellipsometry, infrared spectroscopy, and atomic force microscopy,have failed to fully answer many fundamental questions regarding structural order, the presence of surface defects, and dynamic behavior of these monolayers. The alkylsilane films have been one of the most widely used classes of self-assembled monolayers due to the fact that they can be deposited on a variety of substrates and are both thermally and chemically highly stable. However, questions in regards to the mechanism of film formation, the nature of the interfacial region, and the degree of ordering continue to arises2 In this Letter, we report a solid-state NMR study of self-assembled monolayers to explore the potential of this technique to resolve some of these outstanding structural and dynamic issues. Nuclear magnetic resonance spectroscopy,the technique of choice for characterization of bulk organic samples, should likewise be useful for organic monolayers. Extensive 13Cand 29Sisolid-state NMR data concerning the dynamic behavior of the bonded phase in chemically modified silica gels can be correlated with their separation characteristics in high-performance liquid chromatograp h ~ These . ~ porous silica gels have very large surface areas, and highly resolved spectra can be obtained using magic-angle spinning and cross polarization, CPMAS NMR. Except in the case of self-assembled monolayers Abstract published in Advance ACS Abstracts, J u n e 1, 1995. (1)(a)Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A,; Garoff, S.; Israelachvili, J.; McCarthy, T. J.;Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987,3,932. (b) Bain, C. D.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1989,28,506. (c)Whitesides, G. M.; Ferguson, G. S.;Allara, D.; Scherson, D.; Speaker, L.; Ulman, A. Crit. Rev. S u r f Chem. 1993,3,49.(d) Katz, H.E. Chem. Mater. 1994,6 , 2227. (2) (a) Ulman, A. A d v . Mater. 1990, 2, 573. (b) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir Blodgett to Self-Assembly; Academic: New York, 1991. (3) Albert, K.; Bayer, E. J . Chromatogr. 1991,544,345.
synthesized on porous silica gel as a stationary phase,4 the poor sensitivity of NMR has precluded its use to characterize these films which are usually deposited on low surface area planar substrates. Several reports of syntheses of self-assembled monolayers on high surface area, nonporous substrates have recently appeared. Zirconium phosphonate multilayers have been grown on fumed silica5and monodispersed silica colloids have been coated with alkylsilane monolayers.6 We have carried out a similar synthesis of octadecyltrichlorosilane (OTS)and zirconium octadecylphosphonate monolayers on fumed silica in order to provide enough surface area for a solid-state NMR study. For the OTS monolayers on silica, a synthesis similar to that of the self-assembled monolayer coatings on silica monospheres by Margel and co-workers was followed.6 Briefly, 2.5 g of a commercial fumed silica (Cab-0-Sil, BET surface area 100 m2/g, Cabot Corp.), previously calcined a t 380 "C then stored in air, was degassed a t 95 "C (0.5 Torr) for 3 h, added to dry toluene in a nitrogen atmosphere, and sonicated for 30 min. OTS (10.7 mL) was added to the suspension a t the desired temperature and stirred under nitrogen for 12 h. The modified Cab0-Si1was repeatedly washed with dry toluene under a Nz atmosphere, followed by washings and centrifuging with methylene chloride and acetone before final drying at room temperature. These conditions were used to avoid excessive polymerization of the OTS in solution; however, the low surface water content resulted in an incomplete monolayer of 10% coverage as estimated from elemental analysis and the measured Nz BET surface area of 100 m2/g.' As a model for surface polymerized OTS, bulk polymerized OTS was produced by simply allowing neat OTS to be slowly hydrolyzed by atmospheric water. The resulting white powder was washed with water to remove generated HC1 and dried overnight a t 60 "C. Zirconium octadecylphosphonate monolayers were synthesized according to the procedure developed by Mallouk and co-workers for depositing zirconium phosphonate multilayers on Cab-O-Sil.5 Octadecylphosphonic acid (ODPA) was prepared by the Michaelis-Arbuzov reaction
@
(4)Fatunmbi, H. 0.;Bruch, M. D.; Wirth, M. J. Anal. Chem. 1993, 65,2048. (5)Hong, H.G.; Sackett, D. D.; Mallouk, T. E. Chem. Mater. 1991, 3,521. (6) Brandriss, S.;Margel, S. Langmuir 1993,9,1232. (7) Coverages were estimated from an area of 20 A2for each bound OTS molecule and 2% C weight as determined from elemental analyses of OTS/Cab-o-Si1samples (Atlantic Microlab). For ODPA, an area of 24 A2 for each molecule was used with 4% C weight (Galbraith Laboratories).
0743-7463/95/2411-1860$09.00/00 1995 American Chemical Society
Letters
Langmuir, Vol. 11, No. 6, 1995 1861 a 1'5
'14
1c,
ll
C
50
25
0
PPm
Figure 1. 13C NMR spectra of (a) OTS in solution, (b) OTS deposited at 0 "C on fumed silica, and (c) bulk polymerized OTS. The solid-state 13C CPMAS NMR spectra of the bulk polymerized OTS and OTShilica samples were obtained with a 5-ms contact time, a 5-s recycle time, and a 2.2-kHz sample rotation frequency. of octadecyl bromide with triethyl phosphite.s Elemental analysis indicated a coverage of 26%.7 Self-supported pellets of the modified Cab-0-Si1 and polymerized OTS dispersed in a KBr pellet were used for FTIR measurements. Variable-temperature FTIR spectra were collected from 25 to 180 "C with the substrate temperature held constant to within f l "C. The 13Cand 31PCPMAS NMR spectra were recorded on a Chemagnetics CMX-270 NMR spectrometer, operating a t a frequency of 67.92 MHz for 13C and 121.279 MHz for 31P, respectively. Normally, a spin rate of 2.2 kHz, a contact time of 5 ms, a recycle time of 5 s, and high power proton decoupling were used with a n average of 10 000 scans for each 13C CPMAS NMR spectrum. In Figure 1, the 13C NMR spectra of bulk polymerized OTS, OTS on fumed silica, and OTS in solution are shown along with the peak assignments which are based on previous studies of OTS grafted to porous s i l i ~ a . ~The .~ relative populations of trans and gauche conformations influence the 13Cchemical shift of the interior methylene carbons of the alkyl chains. For the n-alkanes, these carbons resonate a t 30 ppm in solution where there are equilibrium populations of trans and gauche conformations, but in the crystalline state they shift downfield to 33-34 ppm for a n all trans conf~rmation.~ This chemical shift difference has been recently used to characterize the chain conformation of ionic surfactants absorbed on silica and alumina surfaces.1° In the 13C CPMAS spectrum of bulk polymerized OTS in Figure IC,the interior methylene carbons, C3-Cl6, resonate a t 33.5 ppm. This downfield chemical shift, together with the frequencies of the methylene antisymmetric and symmetric stretching modes of 2918 cm-l (fwhh of 20 cm-l) and 2850 cm-l (fwhh of 10 cm-l) respectively, indicates that the alkane chains have crystallized into a n all trans conformation.2 The partial monolayer of OTS on fumed silica in Figure l b displays two components a t 33.6 and 30.5 ppm, arising respectively from domains of chains with a high population of trans (8)Bhattacharya, A.K.; Thyagarajan, G. Chem. Rev.1981,81,415. (9)(a)Earl, W. L.; VanderHart, D. L. Macromolecules 1979,12,762. (b) Tonelli, A. E.; Schilling, F. C. Acc. Chem. Res. 1981,223. (10)Saderlind, E.; Stilbs, P. Langmuir 1993,9,1678.
conformers and domains containing a significant number of gauche conformers. The methylene carbon in n-alkanes moves upfield by 4-6 ppm if any carbon atom three bonds away is in a gauche rather than trans conformation, the " y e f f e ~ t " .In ~ the case of rapid exchange between trans and gauche conformers, a n increase in the gauche population leads to a n upfield shift of the methylene carbon signal. This spectrum is reminiscent of semicrystalline polyethylene which exhibits distinct peaks at 30.9 and 34.1 ppm, arising from the amorphous and all trans crystalline domainsga and supports the view that an incomplete self-assembled monolayer consists of islands of ordered chainsll rather than a uniform distribution of disordered molecules. A dipolar dephasing experiment showed the 30.5 ppm component to be more mobile, with dipolar dephasing times, T d d , of 20 and 80 ps respectively for the components a t 33.6 and 30.5 ppm. These times are somewhat longer than that reported for highly crystalline polyethylene13(-10 ps) but are much shorter than that for amorphous polyethylene (-200 ps) and probably represent a range of restricted mobilities along the chain length. The antisymmetric and symmetric stretching mode frequencies of the OTS deposited on silica a t 0 "C were 2918 cm-l (fwhh of 23 cm-l) and 2850 cm-l (fwhh of 14 cm-I), respectively. While these frequencies are close to those typically reported for OTS monolayers on silicon wafers, the line widths are much larger, reflecting a larger degree of disorder. Mallouk and co-workers have synthesized shorter chain zirconium phosphonate multilayers on this surface and estimated that since fumed silica consists of agglomerated particles, only approximately 50% of the NZBET surface area is actually accessible to the ~ u r f a c t a n t .The ~ disorder in the OTS monolayer on the nonporous silica is due to both low coverages and the curvature ofthe surface which creates free volume between the chains. In the case of OTS polymerized on porous silica gel, the reported coverages are high, but the 13C CPMAS spectra show the chains to be conformationally disordered on this substrate since the interior methylene carbons' shift is 6 30 ~ p m . ~ The influence of the deposition temperature on the FTIR and 13CCPMAS spectra of the OTS monolayer is shown in Figure 2. In the infrared spectra, a peak at 2917 cm-l for the methylene antisymmetric stretchingmode is taken as the signature of a n all trans conformation which shifts to 2924 cm-I in the disordered liquid state. The 13CNMR spectra, shown along with the fiequency of the methylene antisymmetric stretching mode, reveal that the trans component a t 33 ppm of OTS on fumed silica is maximized a t a deposition temperature of 0 "C. Similar critical temperature behavior has also recently been observed for OTS deposited on silicon wafers and may be due to retardation of polymerization of the OTS in solution prior to attachment to the surface andor to a formation mechanism in which mobile alkylhydroxysilane species first form a Langmuir-like monolayer film before crosslinking.14 At deposition temperatures lower than 0 "C, below the freezing point of OTS, the OTS monolayer becomes more disordered.
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(11)(a)Cohen, S. R.; Naaman, R.; Sagiv, J. J. Phys. Chem. 1986,90, 3054. (b)Schwartz, D. K.; Steinberg, S.;Israelachvili, J.; Zasadzinkski, J. A. N. Phys. Rev.Lett. 1992,69,3354. (12)(a)Wasserman, S.R.; Whitesides, G. M.; Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; Axe, J. D. J.Am. Chem. SOC.1989,111,5852.(b) Angst, D. L.; Simmons, G. W. Lungmuir 1991,7,2236. (13)Chen, Q.;Yamada, T.; Kurosu, H.; Ando, I.; Shiono, T.; Doi, Y. J. Polym. Sci., B: Polym. Phys. Ed. 1992,30, 591. (14)Brzoska, J. B.; Azouz, I. B.; Rondelez, F.Langmuir 1994,10, 4367.
Letters
1862 Langmuir, Vol. 11, No. 6, 1995
25°C 2924.9 em.'
,4
ip
,p
000 0 0 0 0 0 0
Zr
I\
25
1 C
+l-l--T
116OC
50
25
0
PPm
0 PPm
1
25
PPm
0
Figure 2. 13CCPMASNMR spectra of OTS depositedon fumed silica at 2 5 , 1 5 , 0 , and -20 "C. The frequency of the methylene antisymmetric stretching mode, va(CH2),is listed beside each spectrum.
A
Zr
15°C
29t9.1 cm.'
a
Zr
2921.6 cm-' 50
50
Zr
b
3000
ri
2900
2800
cm-1
Figure 3. Variable temperature (a) 13C CPMAS NMR of OTS deposited on fumed silica at 0 "C and (b)FTIR spectra showing the gradual disordering of the alkyl chains with heating. The frequency of the methylene antisymmetric stretching mode, va(CH2),increased from 2918 cm-I at 25 "C t o 2925 cm-I at 180 "C. The spectra were observed t o return to their original state upon cooling to 25 "C.
Variable temperature 13C CPMAS NMR was used to probe the thermal stability of the OTS monolayers. Figure 3a shows that as the sample is heated, the component at 33 ppm diminishes and the 30 ppm peak increases, indicating a gradual increase in the number of gauche defects. Above 120 "C, the chains become completely conformationally disordered, with only the 30 ppm component remaining. Below 180 "C, this disordering of the chains is a reversible process since the 13CNMR spectra were observed to return to their original state upon cooling. This behavior is also observed by infrared spectroscopy for OTS self-assembled monolayers on silicon wafers.lla,l5 However, as shown in Figure 3b, the overall change in the methylene antisymmetric stretching mode for the partial OTS monolayers on silica was 7 cm-l which is much greater than the 2-3 cm-l shift reported for tightly packed ~~
(15) Parikh, A. N.; Allara, D. L.; Azouz, I. B.; Rondelez, F.J.Phys. Chem. 1994,98, 7577.
-50 PPm
Figure 4. (a) 13C CPMAS NMR spectrum of octadecylphosphonic acid (ODPA)depositedon zirconated fumed silica: 5 ms cross polarizationtime; 5 s recycle time; 2.2 kHz sample rotation frequency; 10 000 scans. (b) 31PCPMAS NMR spectrum of ODPAon zirconatedsilica: 31Pobserve frequency of 121.2 MHz; 2 ms contact time; 5 s recycle time; 3.3 kHz sample rotation frequency; 400 scans. Asterisks indicate spinning sidebands. (c) 31PCPMAS NMR spectrum of bulk ODPA: 2 ms contact time; 20 s recycle time; 3.6 kHz sample rotation frequency; 4 scans.
monolayers on silicon wafers15where there is less excess free volume available for thermal disordering. The observation that the quality of OTS monolayers on silicon wafers is improved by curing at 150 "C is believed to be due to additional cross-linking.lZbThe fact that our spectra return to their original state may imply that the disorderorder behavior is independent of the underlying siloxy structure. However, it will be necessary to carry out 29Si NMR studies to determine if any additional cross-linking occurs during the variable-temperature NMR experiments. The solid-state 13C and 31PCPMAS NMR spectra of another self-assembling system, zirconium octad&ylphosphonate on Cab-0-Si1are shown in Figure 4. The interior methylene resonance a t 34 ppm in the 13C CPMAS spectrum of octadecylphosphonic acid (ODPA) deposited on zirconated silica reflects the all trans conformation of the alkyl chains and there is only a small, more mobile component for the disordered chains a t 30 ppm. The infrared spectrum of this sample exhibits narrow peaks a t 2 9 1 8 cm-l (fwhh of 18 cm-l) and 2850 cm-l (fwhh of 10 cm-l) for the methylene antisymmetric and symmetric stretching modes, respectively, indicating ordering equivalent to that of a ODPA monolayer on a silica wafer, prepared by combining Langmuir-Blodgett and selfassembly techniques.16 Other syntheses of zirconium phosphonate monolayers and multilayers on silicon wafers, which are produced by first attaching a molecule containing a phosphonic acid end group, have failed to produce conformationally ordered chains due to defects in the initial stage offilm growth and a mismatch between (16)(a) Byrd, H.; Pike, J. K.; Talham, D. R. Chem. Mater. 1993, 5, 709. (b) Byrd, H.;Whipps, S.; Pike, J. K.; Ma, J.;Nagler, S. E.; Talham, D. R. J. Am. Chem. SOC.1994,116,295.
Letters the organic and inorganic 1attices.l' Apparently, zirconation of the fumed silica provides a better initial substrate than a phosphonate derivatized surface and the lattice mismatch can be compensated for by tilting of the C18 chains.16a However, a small number of defects, which are not detected by infrared spectroscopy, are revealed by the peak at 30 ppm in the 13C NMR spectrum. From these initial results it appears that self-assembly of dense, highly ordered monolayers of metal phosphonates on nonplanar substrates appears to be a much more facile process than for the alkylsilanes where the formation of the siloxane network a t the interface is highly sensitive to the amount of trace water available. The interfacial region can be probed by solid-state NMR to determine the extent of cross-linking of the siloxane or metal phosphonate network a t the substrate surface. The 31PCPMAS spectra of octadecylphosphonic acid alone and deposited on zirconated fumed silica are shown in Figure 4b,c. Whereas the isotropic 31Pchemical shift of ODPA is a t 31.3 ppm, the spectrum of ODPA deposited in zirconated silica shows a n isotropic peak a t 7 ppm, the same shift observed for bulk zirconium alkylphosphonates.18 Since there is only a small downfield peak at 16.5 ppm in addition to the 7 ppm peak, most of the phosphonic acid groups have completely reacted with the zirconated surface. In the case of OTS on silica, the 29Si CPMAS NMR showed only very weak resonances in addition to the strong signal from bulk silica. The low coverages of these samples precluded characterization of the extent of cross-linking by 29SiCPMAS as has been done in the case of OTS polymerized on high surface area porous silica geL4 Infrared spectroscopy studies show that there is no direct reaction of OTS with the surface hydroxyl groups or the first water layer on fumed silica.lg Absorption of OTS occurs with the subsequent water layers, so no change in the relative intensities ofthe 29SiNMR signals
Langmuir, Vol. 11, No. 6, 1995 1863 arising from the surface silanol groups and bulk silica is expected upon OTS absorption. In summary, the solid-state 13CCP MAS NMR spectra reveal that a partial monolayer of OTS consists of domains of all trans chains and domains of disordered chains having a liquid-like population of trans and gauche conformations. The relative sizes of these domains are shown to be influenced by the deposition temperature. The thermal stability of the OTS film was studied by variabletemperature 13CCPMAS NMR and infrared spectroscopy which show a gradual, reversible disordering of the all trans chains. The zirconium octadecylphosphonate monolayer on silica, on the other hand, is observed to be very highly ordered, with only a small number ofgauche defects. The infrared spectra of these samples are similar to monolayers supported on planar substrates. Solid-state NMR, however, provides additional information, showing resolved peaks for both conformationally ordered and disordered regions of the film. In these initial studies, the syntheses were not optimized to obtain high coverages. Future work will involve NMR studies of complete monolayers to determine how structural variations such as the cross-linking, chain length, and terminal functional group effect the degree of order and thermal stability of these films. Variable contact time CPMAS NMR measurements will allow quantification of the ordered and disordered components as well a s the degree of cross-linking. Relaxation time and lineshape measurements are currently in progress to give insight into the nature of the cooperative behavior among the chains and it will be of interest to compare the dynamic behavior of self-assembled monolayers with other ordered systems such as liquid crystals and lipid bilayers.
Acknowledgment. Support for this research from the Natural Sciences and Engineering Council of Canada (17)(a)Schilling,M.L.;Katz,H.E.;Stein,S.M.;Shane,S.F.;Wilson, (NSERC) and the FCAR Nouveaux Chercheurs proW. L.; Buratto, S.; Ungashe, S. B.; Taylor, G. N.; Putvinski, T. M.; gramme of Quebec is gratefully acknowledged. The Chidsey, C. E. D. Lunggmuir 1993,9,2156. (b) Bent, S.F.; Schilling, authors also thank Dr. G. R. Brown for use of his M. L.; Wilson, W. L.; Katz, H. E.; Harris, A. L. Chem. Muter. 1994,6, 122. equipment for the variable temperature ETIR experiments. (18)Gao, W.; Reven, L. Unpublished results. (19)Tripp, C P.; Hair, M. L. Lungmuir 1992,8,1120.
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