7458
J . Phys. Chem. 1990, 94, 7458-7464
Rotatlonal and Vibrational Relaxation of Small Molecules in Porous Silka Gels L. Nikiel, B. Hopkins, and T. W. Zerda* Physics Department, Texas Christian University, P.O. Box 32915, Fort Worth, Texas 76129 (Received: April 4, 1990)
Samples of transparent silica gels of controlled porosities are produced stabilized at 800 "C. Raman spectra of samples impregnated with CS2, CHCI,, CH3CN, or acetone are recorded in order to obtain rotational and vibrational correlation functions and correlation times for those liquids. The effect of pore diameters on vibrational dephasing and rotational diffusion is discussed. It is shown that surface interactions, in particular, hydrogen bonding between the imbedded molecules and silanol groups, are responsible for slowing down the rotational relaxation within small pores. A simple model for reorientational motion of molecules hydrogen bonded to the silica surface is proposed. The vibrational modulation times are obtained from the Kubo theoretical function and used to analyze molecular interactions near the silica surface. The number of silanol groups on the surface is estimated from the C=O band of acetone.
1. Introduction Recent developments in the sol-gel technology enable production of gels of controlled pore diameters. Using the manufacturing process developed by Hench and co-workers,' we produced samples of glass of uniform pore sizes, in which the mean diameter can be varied between 20 and 100 A. The large number of pores, of an order of I Oi9 pores per gram, results in large surface area, in some cases reaching 900 m2/g. In addition,2 it is possible to control the number of O H and CH3 groups on the surface of pores, and the gels constitute excellent matrices for the investigation of molecular motion inside small volumes. Such investigations are important in view of possible applications in building optical switches and other devices. Since the sol-gel samples are transparent, one can use optical techniques to study molecular motion of liquids trapped inside the pores. Awschalom and Wamock*s analyzed the time-resolved birefringence signal from liquids confined within pores of various diameters, and they found that the decreasing pore diameter slows down reorientational motion and that molecular motion of the first monolayer is drastically different from that of the liquid in the center of the pore. Molecular reorientation of nitrobenzene, a polar liquid, was found to be slowed down more than that of CS2, a nonpolar liquid. The nature of slowing reorientational motion has not been completely explained, and it may originate from surface interactions as well as from geometrical restrictions. In order to separate those effects, we will study Raman spectra of liquids that can form strong hydrogen bonds with hydroxyl groups on the silica surfaces, such as acetone and acetonitrile, and we will compare the results with the spectra obtained for less active liquids such as CS2, nitromethane, and chloroform. Raman spectroscopy is especially well-suited for such investigations since the analysis of the spectra may provide information on rotational and vibrational correlation functiom6 The frequency splitting between the polarized Raman bands,'+ between the VH and VV polarization, reflects the magnitude of intermolecular coupling and indicates the presence of the resonance energy transfer. Additional information on intermolecular interactions may be obtained from the analysis of Fermi coupled vibrations. First, we will briefly discuss experimental procedures to determine the vibrational and reorientational correlation functions.
For an isolated Raman band the vibrational correlation function is given by Gvib(?) = I ( I V V - 4/dVH) exp(-iwt) dw/I(IVV - 4/3IVH) dw =
Ifi,exp(-iwt) d w / I Z i s 0 dw
and the rotational correlation function is given by Grot(t) =
I I V H
0022-3654/90/2094-7458$02.50/0
exp(-iwt) dw/Gvib(t)
(2)
where Ivvand IVH are the VV and VH polarized components of the band and Ii, = Ivv- 4/& is the isotropic component. For bands that have close neighboring bands it may be impossible to calculate correlation functions, and then relaxation processes are characterized by the vibrational correlation time obtained from the isotropic band width Tvib
= [2Tc(Awl/2)isol-]
(3)
and by the rotational correlation time that can be calculated from Trot-'
= [2Tc(Awl/2)VH] - 7vib-I
(4)
There are several mechanisms contributing to the isotropic bandshape,l0 namely, collision dephasing, vibrational population relaxation, resonant transfer of vibrational energy, vibrationalrotational interactions, and intramolecular couplings. For the systems discussed in this paper the dominating contributions originate from the vibrational dephasing for which Kubo has developed the general theory." In the frame of this model the theoretical vibrational correlation function has the form @ ( t ) = expl-(w(o)2)[T,2(eXp(-t/T,)
- 1)
+ t ~ ~ l (l5 )
where T~ is the vibrational modulation time and ( ~ ( 0 ) is ~ the ) second moment. The modulation time is defined by the normalized correlation function of the vibrational frequency as (4t) w(o))/(~(o)= ~ )exp(-t/Tc)
(6)
Within a simple binary collision model 7c may be interpreted as ~) for the a time between collisions. In eqs 5 and 6 ( ~ ( 0 ) stands vibrational second moment defined as ( ~ ( 0 )= ~ )sZi,(w
( I ) Hench, L. L.; West, J. Annu. Rev. Mater. Sci. 1990, 20. (2) Zerda, T. W.; Hoang, G. In Proceedings of the 4th International Conference on Ulrrastructure Processing of Ceramics, Glasses and Composites, Tucson, AZ, 1989. (3) Awschalom, D. D.; Warnock, J. In Molecular Dynamics in Restricred Geometries; Klafter, J., Drake, J. M., Eds.; Wiley: New York, 1989. (4) Warnock, J.; Awschalom, D. D.; Shafer, M . W. Phys. Reo. B 1986, 34, 415. (5) Warnock, J.; Awschalom, D. D. Phys. Reo. B 1987, 35, 1962. (6) Bartoli, F. J.; Litovitz, T. A. J . Chem. Phys. 1972, 56, 413. (7) Doge, G.; Arndt, R.; Khuen, A. Chem. Phys. 1977. 21, 53. (8) Schindler, W.; Zerda, T. W.; Jonas, J. J . Chem. Phys. 1984,81, 4306. (9) Logan, D.E. Mol. Phys. 1986, 58, 97.
(1)
- w0)' d w / l I i ,
dw
(7)
where wo is the frequency at the maximum of the band. In this paper we also discuss the frequency splitting for acetone adsorbed on silica gels with different surfaces and as a function of pore diameter. When the frequencies of the VV and VH polarized bands are different, it is usually the result of the intermolecular dipolar resonant transfer of vibrational energy due (IO) Oxtoby, D. W. Adv. Chem. Phys. 1981, 40, 1 . (1 1) Kubo, R. In Flucruations, Relaxations and Resonance in Magnetic Systems; TerHaar. D.,Ed.; Plenum Press: New York, 1962.
0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94. No. 19, 1990 7459
Small Molecules in Porous Silica Gels
to transition dipoledipole coupling. The theory predicts that when the coupling is weakened, the isotropic band shifts toward the anisotropic frequency, and eventually, when the coupling is a m pletely removed the frequencies of both components coincide. This effect has been observed previously in polar liquids, a c e t ~ n e , ' ~ - ' ~ ethylene carbonate,I5 and others.16 We also discuss Fermi resonance in acetonitrile and the effects induced by surface interactions on the Fermi coupling. The Fermi resonance occurs when two molecular vibrations of the same symmetry are coupled by the anharmonic part of the intramolecular potential. In effect, the frequencies of both vibrations shift and the Raman bands are observed at frequencies wg and different from the uncoupled frequencies.16 Usually the Fermi coupling occurs between a fundamental and a combination mode. The mixing of the vibrational modes not only alters the frequencies but also affects the intensities of both bands in resonance. Two experimental quantities, the separation between the observed bands, A = a,-ob, and the intensity of the two bands in resonance, R = l a / l b , allow the evaluation of the Fermi coupling coefficient, W, from
w = A R ' / ~ ( R+ 11-1
(8)
The above equation has been derived by assuming that the unperturbed intensity of the combination oscillation is small and can be ignored.
2. Experimental Section Samples of silica gels were prepared by using the sol-gel proess developed by Hench and his group.' Hydrolysis and polymerization reactions of tetramethyl orthosilicate in acidic water were followed by aging, drying, and firing processes.I8 Gels densified at low temperatures break when in contact with liquids, and to avoid this problem the samples were stabilized at 800 OC. This has one unique property: it allows pore diameters to remain unchanged throughout the densification process. Only the number of pores and their connectivity decrease with increasing temperature, but it was shown by Vasconcel~s'~ that the dramatic changes take place at temperatures above 800 OC. For example, for a 24-A-diameter gel, the number of pores decreases from about I O l 9 at 200 "C to less than 5 X 10l8per gram a t 800 O C , but further increase of temperature to IO00 OC decreases the number of pores to IO" per gram. Gels fired at 800 OC have a large number of open pores, and the connectivity between pores has been only slightly reduced.20 It means that the liquids selected for this study are able to penetrate the samples, as the number of branches and connectivity are basically the same as those of the dried gel. For Raman experiments we selected samples of average pore diameter 24, 33,60, and 120 A. The pore diameters, surface area, and pore volume were measured by using the BET technique. Depending on pore sizes the surface area varied between 400 and 700 m2/g. Since the samples were fired only at 800 'C, the pore surfaces were covered to some extent with OH groups. Silanol groups are very active and may form hydrogen bonds with adsorbed liquids. To reduce surface interactions, selected samples were boiled in methanol. This simple procedure allows replacement of OH groups by less polar CH3 groups. In order to check the effectiveness of the removal of the hydroxyl groups, we analyzed the (12) Fini, G.;Mirone, P. J. Chem. Soc.,Foraday Trans. 2 1974, 70, 1776. (13) Schindler, W.; Sharko, P. T.; Jonas, J. J. Chem. Phys. 1982, 76, 3493. (14) Dybal, J.; Schneider, B. Spectrochim. Acto 1985, 41A, 691. (15) Schindler, W.; Zerda, T. W.; Jonas, J. J. Chem. Phys. 1984,81,4306. (16) Mirone, P.; Fini, G. J . Chem. Phys. 1979, 71, 2241. (17) Bertran, J. F.; La Serna, B.; Doerffel, K.; Dathe, K.; Kabish, G. J . Mol. Struct. 1982, 95, I . (18) James, P. F. J . Non-Cryst. Solids 1988, 100, 93. (19) Vasconcelos, W. Ph.D. Dissertation,University of Florida, Gainesville, 1989. (20) Vasconcelos, W.; DeHoff, R. T.; Hench, L. L. In Proceedings of rhe 4th International Conference on Ultrastructure Processing of Ceramics, Glosses and Composites, Tucson, A Z , 1989.
2800
2900 Frequency
3000
3100
(cm-1)
Figure 1. Raman spectra of CH,vibrations for the sample fired at 800 O C before (A) and after alcohol treatment (B).
2900-3000-cm-' region of Raman spectra of gels after the alcohol treatment; examples are shown in Figure 1. The bottom spectrum in Figure 1 does not show even a trace of methoxy groups and was obtained for a nontreated sample; the upper spectrum is of the same sample after boiling in methanol. The two strong peaks are assigned to C-H stretching and bending modes of the methoxy group. Carefully measured amounts of liquid CS2, CHCl,, acetone, nitromethane, and acetonitrile were allowed to be adsorbed by the gels. The smallest quantities used were about one-fourth of that necessary for a monolayer coverage. Although adsorption was completed within an hour, we allowed an additional 24 h for the equilibrium to be reached. Raman spectra were recorded on a Spex double monochromator with slits opened to about 1.5 cm-I. The laser power was set to 0.5 W. 3. Results CS2. The v I vibrational mode of CS2 was measured in pure liquid and in the silica gel of 24-A diameter. No changes in the bandshape have been detected, and the reorientational relaxation times for CS2 in bulk liquid and inside the pores were both equal to 1.5 ps. This result is independent of pore sizes and is in agreement with picosecond experiments by Awschalom and W a r n ~ kwho , ~ did not detect any change in time dependence of the signal from CS2 confined in porous silica. It is worth mentioning that CS2exhibits a Raman band at about 397 cm-l that is symmetry forbidden and appears in the spectrum only due to collision-induced effects. This band has been the subject of intensive studies in pure liquid as a function of pressure and temperature which provided valuable information on intermolecular interactions.21 It is expected that induced bands also can be used to study surface interactions. Unfortunately, the 397-cm-l band overlaps with the S i 0 2 vibrational modes and cannot be analyzed. CHCI,. The C-H mode of chloroform is well-isolated from the neighbors, and its Raman VV and VH spectra can provide information on both vibrational and rotational relaxation processes. Previously, both processes have been studied for chloroform as a function of pressure and t e m p e r a t ~ r e . ~It~ ,is~now ~ well-established that the vibrational relaxation of chloroform in the liquid state is dominated by vibrational dephasing, and the rotational relaxation can be approximated by the collision model. In Figure 2 we show experimental rotational correlation functions for chloroform in the pure liquid and in the silica gel (2 1 ) Birnbaum, G. Phenomena Induced by Intermolecular Interactions; Plenum Press: New York, 1985. (22) Schroeder, J.;Schiemann, V. H.; Jonas, J. Mol. Phys. 1977, 34, 1501. (23) Rothschild, W.; Rosasco, G. J.; Livingstone, R. C. J . Chem. Phys, 1975, 62, 1253.
7460 The Journal of Physical Chemistry, Vol. 94, N o . 19, 1990
Nikiel et al.
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