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Langmuir 1996, 12, 2798-2801
NMR Study of the Molecular Dynamics of Ethanol and 2,2,2-Trifluoroethanol Liquids Confined to Nanopores of Porous Silica Glasses Lance Ballard and Jiri Jonas* Department of Chemistry, School of Chemical Sciences, University of Illinois, Urbana, Illinois 61801 Received November 27, 1995. In Final Form: February 26, 1996X A dynamic nuclear magnetic resonance (NMR) study of the polar fluids ethanol (EtOH) and 2,2,2trifluoroethanol (TFE) confined to porous silica sol-gel glasses is reported. The 13C NMR spin-lattice relaxation times, T1, were measured in glasses with pore radii ranging from 18.9 to 54.8 Å, over a temperature range from -13.6 to 30.5 °C. The data were analyzed in terms of the two-state, fast exchange model, and surface layer relaxation times, T1s, were calculated. On the basis of surface enhancement factors, T1b/T1s, where T1b is the relaxation time of the bulk liquid, it was concluded that the more acidic TFE has a weaker hydrogen bond interaction with silica, due to the fact that the alcohols serve as hydrogen bond acceptors. The experiment shows that EtOH and TFE have nearly identical surface layer viscosities, originating from the differences in hydrogen bonding with the silica surface. Confinement was found to have little effect on the internal rotation of terminal CF3 or CH3 groups.
Introduction The surface layer interaction of liquids with silica hydroxyl groups is of fundamental interest to many important processes, including catalysis, chromatography, and membrane separation. Dynamic studies of molecules in the surface layer are important for detailed understanding of these processes, but NMR studies of solidliquid interface phenomena are difficult because one must separate contributions to the NMR signal from molecules both in the surface layer and in the bulk liquid. An approach based on our earlier work1-4 which allows this separation is that of confined geometries, using porous silica sol-gel glasses prepared with well-defined pore sizes. For liquids confined to high-purity silica glasses, the observed spin-lattice relaxation rate (T1-1) is affected by interaction with the surface and by pure geometric (topological) confinement. This leads to the general expression for T1 in a confined system1
[
] [
]
A(ω) 1 2 1 1 2 1 1 1 ) + - 2 + T1 T1b R T1s T1b R T1s T1b R2
(1)
where T1b is the bulk liquid spin-lattice relaxation time, T1s is the surface layer spin-lattice relaxation time, is the thickness of the surface layer, R is the average pore radius, and A(ω) represents the topological confinement. Our recent studies1,3,5 have dealt with assessing the effects of topological confinement on molecules of low polarity. When considering a polar molecule where surface interactions are strong, however, eq 1 reduces to the two-state, fast exchange model4,6
[
]
1 2 1 1 1 ) + T1 T1b R T1s T1b
(2)
As eq 2 shows, measuring relaxation rates over a range of pore radii allows one to separate the relaxation rate of the surface-layer liquid. One can proceed to calculate a * Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, May 1, 1996. (1) Korb, J.-P.; Delville, A.; Xu, S.; Demeulenaere, G.; Costa, P.; Jonas, J. J. Chem. Phys. 1994, 101, 1074. (2) Liu, G.; Li, Y.; Jonas, J. J. Chem. Phys. 1991, 95, 6892. (3) Korb, J.-P.; Xu, S.; Jonas, J. J. Chem. Phys. 1993, 98, 2411.
S0743-7463(95)01070-5 CCC: $12.00
surface enhancement factor,2 T1b/T1s or SEF, which emphasizes the relative differences in relaxation rates due to the surface. At the simplest level, SEF values indicate differences in reorientational correlation times between the bulk and the surface-layer liquid related to the surface interactions. As a first approximation one can relate these correlation times to shear viscosities via the Debye equation. There are several goals of our 13C NMR relaxation study of ethanol (EtOH) and 2,2,2-trifluoroethanol (TFE) confined to a series of porous silica glasses. Our primary goal in this study was to examine whether the SEF’s can be used to compare the relative strengths of surface interaction between two similar molecules. If this is correct, it could in effect allow a single NMR experiment to provide information regarding two important silica surface-layer phenomenasmolecular motion and hydrogen bonding. In addition, we were interested in determining the effect of fluorine substitution on the dynamic behavior of the confined EtOH and TFE liquids. Various physical properties7-11 are listed in Table 1, and the diffusion of confined polar liquids, including ethanol, has recently been examined by 1H NMR and 2H NMR.12 When considering the strength of surface interactions for EtOH versus TFE, it is known that the electronegative fluorine substituent makes the TFE more acidic and makes the TFE a stronger hydrogen bond donor. However, a hydrogen bond is also possible where the alcohol serves as the hydrogen bond acceptor. The relative strength of the hydrogen bond formed depends on the role the alcohol is assuming. Thus, our goal was to see if the (4) Liu, G.; Li, Y.; Jonas, J. J. Chem. Phys. 1989, 90, 5881. (5) Xu, S.; Ballard, L.; Kim, Y. J.; Jonas, J. J. Phys. Chem. 1995, 99, 5787. (6) Brownstein, K. R.; Tarr, C. E. J. Magn. Reson. 1977, 26, 17. (7) CRC Handbook of Chemistry and Physics, 68th ed.; Weast, R. C., Editor-in-Chief; CRC Press: Boca Raton, FL, 1987. (8) Smithsonian Physical Tables, 9th rev. ed.; Forsythe, W. E., Ed.; Smithsonian Miscellaneous Collections; Smithsonian Institute: Washington, DC, 1964; Vol. 120, p 302. (9) Matsuo, S.; Yamamoto, R.; Tanaka, Y.; Kubota, H. Int. J. Thermophys. 1993, 14, 835. (10) Kobayashi, K.; Nagashima, A. Bull. JSME 1985, 28, 1453. (11) Streitweiser, A., Jr.; Heathcock, C. H. Introduction to Organic Chemistry, 3rd ed.; Macmillan: New York, 1985; p 1156. (12) Kimmich, R.; Stapf, S.; Seitter, R.-O.; Callaghan, P.; Khozina, E. Mater. Res. Soc. Symp. Proc. 1995, 366, 189.
© 1996 American Chemical Society
Ethanols in Nanopores of Porous Silica Glasses Table 1. Molecular Properties of Ethanol and 2,2,2-Trifluoroethanol property
ethanol
2,2,2-trifluoroethanol
mol wt (g/mol) TB (°C) TM (°C) F (g/cm3) (20 °C) η (cP) (25 °C) σ (Å) pKa
46.07 78.5a -117.3a 0.789a 1.096b 4.5d 15.9e
100.04 74a -43.5a 1.468a 1.744c 4.7d 12.4e
Reference 7. b Reference 8. c Reference 9. d Calculated in this work using data from refs 8 and 10. For details, see text. e Reference 11. a
SEF values can clearly differentiate between the two potential types of surface interaction. Besides examining the SEF’s, this study also represents a continuation of our recent efforts5 on examining the role of fluorine in modifying the dynamic behavior of fluorocarbons versus hydrocarbons. These experiments should be of relevance to the lubrication process, as designers take advantage of the high thermal stability and chemical inertness of fluorocarbons but still need information regarding potential additives soluble in fluorocarbons.13-16 Alcohols represent boundary-layer lubricant additives,17-20 intended to interact with an oxide-coated surface and form a molecular layer film, thereby preventing contact between engine parts. In spite of the fact that we deal with a silica surface in the absence of shear, we also hoped this study could provide some insight into the surface-layer dynamics relevant for boundary-layer lubrication. Experimental Section NMR grade 2,2,2-trifluroethanol (Aldrich) and absolute ethanol (McCormick Distilling Co., Inc.) were used without further purification. The liquids were rigorously degassed prior to sample preparation by successive freeze-pump-thaw cycles. Porous silica sol-gel glasses, prepared by the procedure previously developed in our laboratory,2 were selected with average pore radii of 19, 28, 34, and 55 Å. The pore radii were determined by the Brunauer-Emmett-Teller (BET) method,21 on a Quantachrome Autosorb-1 BET instrument. The glasses were evacuated (10-6 Torr) and heated to 350 °C for at least 2 h to remove volatile impurities. The degassed liquids were transferred to the glasses for loading under vacuum via the bulb-to-bulb procedure. Excess liquid was removed by careful application of vacuum (10-1 Torr). The procedure for loading glasses has been described previously.2 Carbon-13 spin-lattice relaxation times, T1, were determined at 45.2 MHz using either proton (170 MHz) or fluorine (169.2 MHz) CW decoupling. Measurements were taken on a homebuilt spectrometer equipped with a 4.2 T Oxford superconducting magnet and controlled with GE software. Thermal stability was maintained by a regulated nitrogen stream thermostated with an MGW Lauda temperature bath. Temperatures were measured by an Omega thermocouple located near the sample and calibrated with a methanol standard.22 Temperature precision was maintained within (0.3 °C, while accuracy must be estimated as (1.0 °C. (13) Paciorek, K. J. L.; Kratzer, R. H. J. Fluorine Chem. 1994, 67, 169. (14) Sianesi, D.; Zamboni, V.; Fontanelli, R.; Binaghi, M. Wear 1971, 18, 85. (15) McFadden, C.; Gellman, A. J. Langmuir 1995, 11, 273. (16) Gschwender, L. J.; Snyder, C. E., Jr.; Fultz, G. W. Lubr. Eng. 1993, 49, 702. (17) Glaeser, W. A.; Erickson, R. C.; Dufrane, K. F.; Kannel, J. W. Lubr. Eng. 1992, 48, 867. (18) Studt, P. Tribol. Int. 1989, 22, 111. (19) McFadden, C. F.; Gellman, A. J. Langmuir 1995, 11, 273. (20) Parker, B.; Zhang, R.; Dai, Q.; Gellman, A. J. ACS Symp. Ser. 1993, 485, 181. (21) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309. (22) Van Geet, A. L. Anal. Chem. 1970, 42, 679.
Langmuir, Vol. 12, No. 11, 1996 2799 Table 2. Observed T1 Values (s) for EtOH and TFE at 21.3 °Ca EtOH
TFE
pore size
C-1
C-2
C-1
C-2
bulk 54.8 Å 34.4 Å 27.7 Å 18.9 Å
9.85(6) 4.43(9) 3.2(1) 2.66(7) 1.96(6)
7.1(2) 5.3(1) 4.6(2) 4.2(1) 3.3(2)
2.73(4) 1.48(6) 1.14(3) 0.99(2) 0.71(2)
6.45(6) 4.2(3) 3.5(1) 3.2(4) 2.3(2)
a Values in parentheses represent the error in the last digit, representing the error from multiple runs or the accumulated T1 fitting error.
The T1 determinations were made by the standard 180°-τ90° inversion recovery sequence. Fifteen separate delay times were used, with a >5T1 delay between successive scans. The number of scans varied from 4 to 128, depending on the sensitivity. All measurements were performed in triplicate, with reproducibility generally within (5%. Nuclear Overhauser enhancement factors (NOE’s), as well as some additional T1 values, were determined for the bulk liquid samples at 75 MHz on a commercial GE-300 NB instrument to assess chemical shift anisotropy contributions. The NOE’s were measured by comparing integrals with the decoupler alternated on and off,23 and are estimated to be accurate within (15%.
Results and Discussion Decoupled spin-lattice relaxation times for each individual carbon atom in TFE and EtOH were determined over the temperature range from -15 to 30 °C for bulk liquid samples and for samples confined to glasses with pore radii from 19 to 55 Å. Representative relaxation times at 21.3 °C are listed in Table 2, and relaxation times as a function of temperature for C-1 are shown in Figure 1. NOE’s determined at 75 MHz for the bulk liquid samples at the highest experimental temperatures exhibited 100% NOE (η ) 1.98) within experimental error, while T1 values measured at 75 MHz agreed within experimental error with values at 45.2 MHz. For 13C NMR, it has been established23,24 that spinlattice relaxation generally occurs by intramolecular dipole-dipole relaxation, with possible contributions from readily identifiable spin rotation and chemical shift anisotropy mechanisms. On the basis of the increase in T1 with temperature observed, one can rule out spin rotation over the temperature range studied. In addition, the NOE values and the high-field relaxation times tend to rule out chemical shift anisotropy contributions. This leaves intramolecular dipole-dipole relaxation as the most probable relaxation mechanism. With a relaxation mechanism determined, one can proceed with data analysis. For this analysis, a surface layer of one molecular diameter was assumed. Molecular diameters were calculated by the fluidity analysis method of Hildebrand and Lamoureaux25 using viscosity and density data from the literature, and are listed with various other general properties in Table 1. By using these values and plotting the relaxation rate (T1-1) versus the inverse pore radius (R-1), eq 2 can be used to calculate surface layer relaxation times, T1s. These plots are shown in Figure 2, and the calculated T1s values are listed in Table 3. The surface enhancement factors (SEF’s) are also listed in Table 3. One notes from Table 3 that the EtOH C-1 SEF is nearly twice that of TFE, while for both (23) Bovey, F. A. Nuclear Magnetic Resonance Spectroscopy, 2nd ed.; Academic: New York, 1988; Chapter 5. (24) Lyerla, J. R.; Levy, G. C. In Topics in Carbon-13 NMR Spectroscopy; Levy, G. C., Ed.; John Wiley & Sons: New York, 1974; Vol. 1, Chapter 3. (25) Hildebrand, J. H.; Lamoreaux, R. H. Proc. Natl. Acad. Sci. 1972 69, 3428.
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Figure 1. Ethanol (a) and 2,2,2-trifluoroethanol (b) 13C spinlattice relaxation times versus temperature (K) for C-1 of bulk liquid and liquid confined to porous silica sol-gel glass: (b) bulk; (9) 54.8 Å; (2) 34.4 Å; (1) 27.7 Å; ([) 18.9 Å.
liquids the C-1 SEF is greater than that for C-2. The TFE C-2 SEF is slightly greater than that of EtOH. Within experimental error, the SEF values are independent of temperature. To understand the different SEF trends, it is necessary to consider the molecular motions responsible for relaxation on the NMR time scale. For bulk liquid, mostly isotropic overall molecular tumbling should be involved in the relaxation of both C-1 and C-2, with the C-2 carbon also having the ability to exhibit internal rotation around the C-C bond. Previous studies of bulk liquid EtOH have shown the latter contribution to become significant in polymorphic solid forms such as crystal I (mp 159.0 K) where the overall molecular orientation is becoming fixed.26 In analogy with solid bulk liquid EtOH, one expects hydrogen bonding at a solid-liquid interface to influence the orientation12 of the liquid surface layer and increase the relative importance of C-2 C-C free rotation. In this context, we feel the experimental results can be readily explained. First, the SEF values for C-1 are consistent with the difference in hydrogen bond strengths between the alcohol and the surface. A stronger EtOH hydrogen bond to the surface may at first appear surprising, given the higher pKa value of EtOH compared to TFE (see Table 1) and the known ability of TFE to be a better hydrogen bond donor.27-29 Silica hydroxyl groups, however, are reasonably acidic with pKa values around 6.8,27 (26) Eguchi, T.; Soda, G.; Chihara, H. Mol. Phys. 1980, 40, 681. (27) Farcasiu, D.; Ghenciu, A. Catal. Lett. 1995, 31, 351. (28) Jursic, B.; Ladika, M.; Sunko, D. E. Tetrahedron Lett. 1985, 26, 5323. (29) Middleton, W. J.; Lindsey, R. V., Jr. J. Am. Chem. Soc. 1964, 86, 4948.
Figure 2. 13C spin-lattice relaxation rates, 1/T1, as a function of pore radius (1/R) at various temperatures for ethanol, C-1 (a), ethanol, C-2 (b), 2,2,2-trifluoroethanol, C-1 (c), and 2,2,2trifluoroethanol, C-2 (d): (b) -13.6 °C; (9) 0.7 °C; (2) 12.0 °C; (1) 21.3 °C; ([) 30.5 °C.
Ethanols in Nanopores of Porous Silica Glasses
Langmuir, Vol. 12, No. 11, 1996 2801
Table 3. Calculated T1s (s) and Surface Enhancement Factors for EtOH and TFEa EtOH T1s [SEF] -13.6 °C 0.7 °C 12.0 °C 21.3 °C 30.5 °C a
TFE T1s [SEF]
C-1
C-2
C-1
C-2
0.53(3) [10.1(6)] 0.69(3) [10.2(5)] 0.89(3) [9.7(3)] 1.09(5) [9.0(4)] na
1.00(6) [3.8(2)] 1.39(8) [3.5(2)] 1.8(1) [3.3(2)] 2.2(2) [3.2(3)] na
0.166(9) [4.2(3)] 0.212(8) [5.7(2)] 0.32(2) [5.6(5)] 0.44(2) [6.2(3)] 0.59(4) [5.9(4)]
na 0.75(6) [4.6(4)] 1.14(8) [4.3(3)] 1.5(2) [4.3(6)] na
Values in parentheses represent the error in the last digit based on propagation of error.
resulting in the alcohols serving as hydrogen bond acceptors. Thus the effect of the electronegative fluorine substituent is to withdraw electron density from the oxygen atom and weaken the hydrogen bond. We note that this interpretation is consistent with an earlier vibrational study of alcohols on silica.30 When considering the difference in C-1 versus C-2 SEF values, one must be reminded of the ability of internal rotation for the C-2 carbon. That the C-2 SEF’s should be smaller than those of C-1 indicates that internal rotation does not appear to be affected by confinement. We note that this result supports a similar observation reported earlier for acetonitrile.31 The higher C-2 SEF for TFE versus EtOH is probably related to the larger mass of fluorine, which would be expected to restrict internal rotation by increasing the barrier heights. As was stated in the Introduction, 13C SEF values can also be used to indicate relative differences in viscosities between bulk and surface-layer liquids. Assuming isotropic reorientation in the extreme narrowing regime (ωτC , 1 where ω is the Larmor frequency), dipole-dipole 13C relaxation can be described as23
γ2Hγ2Cp2 1 ) τC NT1 r6
(3)
where N is the number of directly attached hydrogen’s, γ is the gyromagnetic ratio of the indicated nucleus, p is Planck’s constant, r is the CH bond length, and τC is a single correlation time characterizing the molecular motion. Provided this single correlation time can be used to describe molecular reorientation in both the bulk and surface-layer liquids, eq 3 shows that a 13C SEF is a ratio of correlation times, or by application of the Debye equation,32
τC ∝ η
(4)
the 13C SEF is a ratio of viscosities (η). One has to realize that application of the Debye equation can only be regarded as a first approximation, as there are also factors such as potential anisotropy differences between bulk and surface-
layer molecular motions. Nevertheless one arrives at the rather surprising result from the values of the C-1 SEF that TFE and EtOH have nearly identical surface layer viscosities, despite the much higher bulk liquid viscosity of TFE (see Table 1 for bulk liquid viscosity values). The similar static viscosities of the surface layer are a direct result of different hydrogen bond interactions with the silica. The assumption of a single τC to describe both the bulk and surface-layer liquid necessarily rules out using the C-2 SEF values to compare viscosity, since it appears C-2 is influenced by both overall tumbling and C-C internal rotation. Conclusions This work shows that NMR studies of liquids in confined geometries can be used to compare surface layer interactions of EtOH and TFE with silica. On the basis of differences in surface enhancement factors, it was observed that TFE has a weaker interaction (hydrogen bond) with the surface, which is consistent with the alcohol being a hydrogen bond acceptor. Fluorine affects the molecular dynamics of the system by withdrawing electron density from the oxygen atom and creating weaker hydrogen bonds. These weaker bonds relative to those for EtOH suggest apparently similar surface layer viscosities for the two molecules, despite quite different bulk layer viscosities. In a more general sense, we feel this study demonstrates a promising approach for comparing polar liquid interactions with silica surfaces. Acknowledgment. The authors wish to thank S. Xu for preparation of the silica sol-gel glasses used in this study. This work was supported in part by the Air Force Office for Scientific Research (AFOSR) under Grant F49620-93-1-0241 and by the Augmentation Awards for Science and Engineering Research Training (AASERT) under Grant F49620-93-1-0555. LA9510700 (30) Saracual, A. R. A.; Pulton, S. K.; Rochester, C. H. J. Chem. Soc., Faraday Trans. 1 1982, 78, 2285. (31) Zhang, J.; Jonas, J. J. Phys. Chem. 1993, 97, 8812. (32) Bloembergen, N.; Purcell, E. M.; Pound, R. V. Phys. Rev. 1948, 73, 679.
