Geometric Effects in the Dynamics of a Nonwetting Liquid in

Pore-size dependence of rotational tunneling in confined methyl iodide. R. Dimeo , D. Neumann , Y. Glanville , D. Minor. Physical Review B 2002,104201...
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J. Phys. Chem. B 1998, 102, 10288-10294

Geometric Effects in the Dynamics of a Nonwetting Liquid in Microconfinement: An Optical Kerr Effect Study of Methyl Iodide in Nanporous Glasses Brian J. Loughnane and John T. Fourkas* Eugene F. Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467 ReceiVed: July 14, 1998

Optical Kerr effect spectroscopy has been used to study the orientational dynamics of liquid methyl iodide in bulk and confined in nanoporous glasses of several different pore sizes. Consistent with the behavior of other nonwetting or weakly wetting liquids, the decays of confined methyl iodide are described well by the sum of two exponentials, the faster of which has the same decay rate as the bulk liquid. The slower exponential is interpreted as arising from the hindered reorientational dynamics of liquid molecules on or near the surfaces of the pores. The rate of surface reorientation depends significantly on the pore diameter, which along with other evidence suggests that the retardation of surface dynamics arises through geometric effects.

1. Introduction A knowledge of the molecular details of the dynamics of liquids at solid/liquid interfaces is of great importance in areas of science and technology such as separations, heterogeneous catalysis, electrochemistry, and nanomachinery. Accordingly, numerous experiments, simulations, and theories have been directed toward understanding the solid/liquid interface in recent years.1-6 A significant portion of this work has been concerned with understanding the dynamics of nonwetting liquids near surfaces, which is of particular relevance to developing better lubricants. Surface-force apparatus (SFA) experiments have been an especially effective means of studying the behavior of nonwetting liquids confined near surfaces7-12 and have prompted a number of simulations related to this problem.13-20 A complementary experimental approach to the SFA is to confine nonwetting liquids within a nanoporous medium.21-30 High surface-to-volume ratios make such materials excellent systems with which to study dynamics at the liquid/solid interface. One class of nanoporous materials that has received considerable attention in recent years is silicate sol-gel glasses. These materials can be fabricated so as to have a narrow distribution of pore sizes, with the average pore diameter being controllable between approximately 20 and 100 Å.31-33 With appropriate care, high optical quality, monolithic glass samples can be produced that are amenable to study using any of a myriad of optical22,24,26,29-31,34-37 and other38-40 techniques. In addition, because the chemistry of silicate surfaces is well understood, the surfaces of the pores in these materials can be modified chemically to tune the interactions with confined liquids.41 On the basis of NMR33,42 and optical Kerr effect43-45 (OKE) experiments,21,29,37 a general picture of liquids confined in nanoporous silicate glasses has emerged. Within this picture, the confined liquid can be described in terms of two components, one of which has the same dynamics as the bulk liquid and the second of which has dynamics that are retarded due to interactions with the pore surfaces. While some studies46 have suggested that surface-layer effects are minimal in nonwetting liquids, recent Raman,26 NMR,28 and OKE29 studies on confined CS2 have shown evidence for a surface population of molecules with modestly retarded dynamics.

OKE spectroscopy offers the ability to measure directly the orientational time correlation function of a liquid and is therefore a powerful tool for elucidating the dynamics of confined liquids.30,37 One of the specific goals of our OKE studies of confined liquids is to develop a deeper understanding of the effects of confinement on nonwetting and weakly wetting liquids. While prototypical lubricant molecules are mediumchain-length alkanes, many facets of the dynamics at solid interfaces should be universal for nonwetting liquids. We have therefore chosen to focus on nonwetting liquids composed of small, spectroscopically simple molecules for our initial OKE studies. We have reported experiments on CS2 in pores 24 Å in diameter previously,29,30 and here we examine the reorientational dynamics of methyl iodide in bulk and confined in nanoporous sol-gel glasses with a range of different pore diameters. As in our previous studies of CS2, our results for methyl iodide are consistent with a two-state model of the liquid. The data here suggest that geometric confinement effects govern the behavior of the surface population of molecules. 2. Experimental Section A. Sol-Gel Synthesis. Monolithic nanoporous glass samples of high optical quality are prepared using the two-step acid basecatalyzed hydrolysis of tetraethyl orthosilicate (98% TEOS, Acros Organics).33 Deionized water is mixed rapidly into a solution of TEOS and ethanol in a 12:1:2 molar ratio. HCl in the amount of 0.001 mol is added, and hydrolysis proceeds under acidic conditions. The solution is stirred in a 40 °C heat bath for 40 min and eventually becomes clear and homogeneous. It is transferred to an ice bath, where hydrolysis is completed under basic condition with the addition of another 12 parts of H2O and an amount of NH4OH. The amount of base added is controlled carefully in order to produce samples with different pore sizes. For instance, 0.001 mol of NH4OH is added to produce the smallest pores; more basic conditions produce larger pores. Once the solution is well mixed, it is poured into cylindrical polystyrene containers (5 × 1.5 cm) that are capped tightly. Gelation occurs within 30 min. To create the smallest pores, samples are aged for 1 week at room temperature and are allowed to dry for a period of 1 month. In the drying stage, samples are uncapped and covered with Parafilm in which a

10.1021/jp9830169 CCC: $15.00 © 1998 American Chemical Society Published on Web 11/12/1998

Dynamics of a Nonwetting Liquid small pinhole is made to allow the liquid phase to evaporate slowly. For medium-sized pores, samples are aged at 55 °C for 2-4 weeks before drying. For samples with larger pores, samples are aged for 4 weeks at 55 °C and then placed in a bath of 0.1 M NH4OH for 24 h before drying. During the drying process, the gels shrink and harden. All gels are then placed in ceramic crucibles and heated to 800 °C at a rate of 0.5 °C per minute in a Lindberg/Blue programmable muffle furnace. Almost 90% of the samples survive the heating process without fracturing. The monolithic samples are ground to a thickness of 2 mm with various grit sandpaper and then are hand polished to optical quality using a TEXMET 100 polishing cloth and 24, 6, and 1 µm diamond paste (Buehler) consecutively. Samples are again heated to 450 °C to remove any water and organic material that may have permeated the pores during the polishing treatment. For surface-modified studies, dried samples are refluxed in a 10-fold excess of methanol for 72 h such that the surface hydroxyl groups are replaced by methoxy groups.47 The samples are dried at 150 °C under vacuum to remove any residual methanol and water. IR spectra show that reesterification results in the growth of a large peak centered at 2900 cm-1 that has been assigned to the C-H stretching band,47 while broad bands centered near 3400 cm-1 narrow significantly due to the removal of surface hydroxyls.47 Furthermore, BET adsorption results show a significant decrease in the apparent adsorption energy within the modified pores as evident from the change in the Brunauer, Emmett and Teller (BET) constant from 298 to 133.47 B. Sample Characterization and Preparation. The BET surface area, average pore radius, and pore size distribution48 were measured with a BET sorptometer (PMI/APP). The results of the Pierce adsorption isotherms47 show a highly connective porous network with a narrow pore size distribution and specific surface areas that range from 340 to 650 m2/g, depending on the size of the pores. Adsorption-desorption isotherms show characteristic hysteresis typical with mesopores48 (i.e., those with 20-100 Å diameters). Samples with roughly cylindrical pores of 24, 42, and 86 Å average diameter were used for this study. The samples are sealed in 2 mm path length quartz cells filled with methyl iodide that is first doubly filtered through 0.1 µm Millipore filters. The samples are allowed to soak for at least 24 h at room temperature before any experiments are performed. The quartz cell is mounted on a specially constructed brass sample holder that is mounted on a continuous-flow vacuum cryostat (Janis ST-100) that is cooled with liquid nitrogen. The silicon diode temperature probe of the temperature controller (LakeShore model 330) is placed directly on the quartz cell. C. Optical Experiments. Optical Kerr effect experiments were performed using an optical heterodyne detection polarization spectroscopy setup,49,50 which is shown in Figure 1. A modified commercial Ti:sapphire laser (Coherent Mira Basic) generates extremely stable mode-locked pulses at a wavelength of 800 nm that are externally recompressed to a duration of approximately 65 fs with a pair of LaFN28 prisms (CVI) set approximately 32 in. apart. The beam after recompression passes through a 90% beam splitter. The transmitted portion is used as the pump, and the reflected portion is used as the probe pulse. The pump beam passes through an achromatic half-wave plate and a Glan laser polarizer (which is set 45° to the vertical) and then is focused into the sample using a 75 mm focal length achromatic lens. The probe beam traverses an optical delay line and passes through a Glan laser polarizer set for vertical polarization. The polarizer is mounted to a high-resolution automatic rotation stage (Newport URM100 ACC with MM

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Figure 1. Heterodyne-detected OKE setup. DX ) doubling crystal; GLP ) Glan laser polarizer; GTP ) Glan-Thompson polarizer; HWP ) half-wave plate; PD ) photodiode; PDC ) prism dispersion compensator; QWP ) quarter-wave plate; SF ) spatial filter.

3000 DC controller). The beam is focused into the sample with the achromatic lens, recollimated after the sample, and passed through a Glan-Thompson polarizer set for horizontal polarization. The extinction of the polarizer pair is generally better than 5 × 105. The signal is spatially filtered and detected by a highbandwidth, amplified photodiode (New Focus model 2001). With the pump beam blocked and the quarter-wave plate removed, the angle of the analyzer polarizer is set to maximize the extinction of the probe beam. The quarter-wave plate is reinserted and adjusted to remaximize the extinction of the probe beam. The orientation of the quarter-wave plate then remains constant for the duration of the experiment. The “zerobackground” angle is optimized by ensuring that the intensity of the probe beam is exactly the same at the opposite heterodyne angles of the probe polarizer. The orientation of the Glan laser polarizer is adjusted to provide an in-quadrature local oscillator. Alternate data sets are taken with the polarizer rotated in one direction by no more than 2° and then rotated by the same amount in the opposite direction so that the homodyne contribution to the signal can be removed later. The stability of the local oscillator is monitored continuously with an oscilloscope. The pump and probe beams are chopped by the five-slot and seven-slot rings of the same chopper (New Focus Research model 300). After the chopper, a portion of the probe beam is picked off by a beam splitter. This beam passes through an iris and a set of polarizers (for intensity adjustment) and then is detected with another photodiode that is matched to that detecting the signal; we will refer to this as the reference diode. With the pump beam blocked, the intensity of the reference beam is adjusted with the polarizers to match exactly the intensity of the local oscillator in an oscilloscope. The iris is then positioned around the beam and closed to account for any small beam distortions that may arise from spatial filtering of the signal. The reference and signal diodes are connected to the differential inputs of a preamplifier (Stanford Research Systems SR560) that has its high-pass filter set at 10 kHz. The preamplifier output is fed into a digital lock-in amplifier (Stanford Research Systems SR810). The pump beam is then unblocked, and the lock-in is referenced to the sum of the pump and probe beam chopping frequencies (7.7 kHz). Not only does this scheme greatly improve the signal-to-noise ratio by enhancing the gain of the signal but it also allows us to take advantage of the full dynamic range of the lock-in. After the pump beam passes through the sample, it is frequency-doubled in a KDP crystal. The doubled light is

10290 J. Phys. Chem. B, Vol. 102, No. 50, 1998 detected with another photodiode and lock-in amplifier. Since the second harmonic generation (SHG) and the heterodyned OKE signals are both proportional to the square of the laser intensity,43 the OKE signal can be divided by the SHG signal to account for any small intensity fluctuations that may arise during data collection, which may take up to 24 h. Programs written in LabView (National Instruments) are used to collect data and to interface with all instruments used in the experiment. The temperature is monitored continuously, and no data are taken if the temperature changes by more than 0.5 °C from the set point. For every step on the delay line, both lockin amplifiers collect data for 100 ms. The lock-in data buffers are dumped into the computer, and the average of the signal for each lock-in is calculated. Each point in the OKE signal is divided by the corresponding point in the SHG signal and saved in an array. The delay line is then moved 50 µm, and the instrumentation is allow to come to equilibrium for 300 ms before data collection recommences. Generally, 50 scans are taken for each pore size and eight scans for the bulk liquid at each temperature, half at each heterodyned angle. In each case, the data obtained at the negative heterodyne angle are averaged and subtracted from the average of the data obtained at the positive heterodyne angle. The baseline is then zeroed carefully, and the data are fit. Preliminary fitting is done in LabView, followed by a nonlinear least-squares analysis with a Levenberg-Marquardt algorithm.51 D. Viscosity Measurements. Temperature-dependent viscosity measurements are performed with an Ubbelohde viscometer (ICL Calibration Laboratories) with an accuracy of 0.3%. With this instrument, the nominal constant used to convert the time required for the liquid to pass through the calibrated capillary into centipoise does not change with temperature, making it a good choice for a temperature-dependent study. CH3I in an amount of 15 mL is doubly filtered through a 0.1 µm Millipore filter and poured into the viscometer reservoir. The viscometer is then placed in a clear, cylindrical 4 L Dewar flask. A specially constructed holder ensures that the viscometer does not move and remains perpendicular to the bottom of the flask. A temperature sensor with an accuracy of 0.1 °C is placed in the middle of the Dewar. The Dewar is filled with ethanol, which is stirred vigorously with a mechanical stirrer. Once the desired temperature is reached by adding dry ice to the bath, thermal equilibrium is achieved within 20 min. Reproducibility of the data taken over a period of time is used to double check that thermal equilibrium has been achieved. The methyl iodide is drawn from the reservoir with suction, and the time required to pass through the capillary and fill the reservoir is measured. Each test takes about 4 min. This procedure is repeated 5 times at each temperature to ensure reproducibility. As a result of the unavailability of temperature-dependent density data for methyl iodide, the room-temperature value of the density52 was used in calculating the viscosities, which should lead to a small but systematic error in the viscosity data employed here. 3. Results A representative plot of methyl iodide OKE data in the bulk and the pores taken at 272 K is shown in Figure 2. In all data sets, the early part of the response arises from librational and collision-induced scattering.43 Since we are concerned with orientational diffusion, we will concentrate on the portions of the decays with delay times greater than approximately 2 ps. The long-time portion of the bulk data in Figure 2 is well described by a single exponential, as was the case for all other temperatures ranging from just above the freezing point to just

Loughnane and Fourkas

Figure 2. Representative data obtained at 290 K in CH3I in bulk and confined in pores 83, 42, and 24 Å in diameter. Hindered surface reorientational diffusion is evident in the data for the confined liquid, which show significant signal well after the bulk liquid has relaxed. Decays are offset for clarity.

Figure 3. Debye-Stokes-Einstein plot of the orientational correlation time versus η/T for bulk CH3I. The solid line is a linear least-squares fit to the data.

below the boiling point of methyl iodide. For a molecule as symmetric as methyl iodide, orientational diffusion should lead to a single-exponential decay in the OKE signal. According to the Debye-Stokes-Einstein equation,53 the orientational correlation time τ of a solute in a simple liquid is given by

τ)

4πηr3 3kBT

(1)

where η is the viscosity, r is the hydrodynamic radius of the solute, kB is Boltzmann’s constant, and T is the temperature. Figure 3 demonstrates that a plot of the rotational correlation time versus the measured viscosity divided by the temperature yields a straight line, which shows that methyl iodide behaves hydrodynamically over the entire temperature range studied. It is clear from the data in Figure 2 that relaxation in the pores is nonexponential and becomes increasingly more so with decreasing pore size. As was the case in our previous study of CS2,29,30 the data in confinement are well described by a biexponential decay in which the fastest decay constant matches that of the bulk liquid at the same temperature. To demonstrate the quality of the fits, a representative fit and residuals for methyl iodide in 24 Å diameter pores at 272 K is shown in Figure 4. As is often observed in bulk liquids,54 at each temperature studied, stripping the reorientational decay off the bulk data reveals an additional exponential component that arises from intermolecular dynamics. As additional evidence of the quality of the fits, an exponential decay is found after the biexponential

Dynamics of a Nonwetting Liquid

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TABLE 1: Reorientational Times and Bulk/Surface Amplitude Ratiosa T (K)

bulk decay time

surface decay time, 24 Å

surface decay time, 42 Å

surface decay time, 83 Å

amplitude ratio, 24 Å

amplitude ratio, 42 Å

amplitude ratio, 83 Å

207.3 212.3 222.1 235.6 254.3 270.4 290.6 311.0

9.61(10) 8.37(10) 6.93(10) 5.36(10) 3.94(10) 3.14(10) 2.52(10) 2.08(10)

38.1(19) 33.0(17) 27.4(17) 21.6 (11) 13.4(7) 10.7(3) 8.48(20) 8.02(20)

29.9(19) 28.5(17) 23.0(17) 17.1(11) 11.1(7) 9.41(3) 7.21(20) 6.54(20)

30.0(19) 23.1(17)

4.09 5.04 5.25 5.35 4.84 5.87 6.86 11.8

7.86 8.85 10.2 10.2 10.2 13.2 13.2 20.7

18.25 16.12

a

13.9(11) 12.1(7) 7.82(30) 6.38(20) 5.03(20)

13.43 17.9 18.2 21.3 24.6

All times are in picoseconds. Numbers in parentheses are approximate uncertainties in the last digit.

Figure 4. Representative CH3I data obtained at 272 K in the 42 Å pore diameter sample. The solid line through the data is the biexponential fit, with the residual shown at the bottom.

Figure 5. Arrhenius plot of the viscosity of the bulk liquid (circles) and the calculated effective viscosity of the surface layer for 24 Å (triangles), 42 Å (squares), and 83 Å (diamonds) pore-diameter samples. The solid lines are linear least-squares fits.

fit is stripped off the data for the confined liquid for each pore size at each temperature, and this decay has the same time constant seen in the bulk liquid at the same temperature. Parameters derived from the biexponential fits for all temperatures and pore sizes are given in Table 1. As with our previous study with a nonwetting liquid,29,30 we believe that the slower exponential observed in the pores arises from surface interactions. We will hereinafter refer to the faster of the exponentials as arising from a bulklike population of molecules and the slower exponential as arising from the surface population of molecules. One way to characterize the surface population is to calculate an effective viscosity by multiplying the bulk viscosity by the ratio of the surface decay time to the bulk decay time.30 While any change in the boundary conditions or molecular volume for reorientation at the surface are subsumed into the effective viscosity, this is still a useful diagnostic quantity for comparison. Figure 5 is an Arrhenius plot of the viscosity of the bulk liquid and of the effective

Figure 6. Relative populations of the bulklike and surface CH3I molecules as a function of temperature. Solid symbols represent the surface population, open symbols represent the bulk, and the line is a guide for the eye: squares, 83 Å pores; triangles, 42 Å pores, circles, 24 Å pores.

viscosity for each of the pores studied. As is often the case for simple liquids that the bulk viscosity is well described by the Arrhenius equation, which yields an activation energy of 6.38 kJ/mol. The activation energies calculated for the surface effective viscosities are approximately 7.1, 6.9, and 7.6 kJ/mol for 25, 42, and 83 Å diameter pore sizes, respectively. This modest increase in activation energy is consistent with previous studies of wetting37 and nonwetting30 liquids. The surface decay is slowest in the smallest pores and increases as the pore diameter increases. The amplitude of each exponential in the OKE signal is given by the relative population of molecules that relax at that rate divided by the time constant.55 We can extract the relative populations of the bulklike and surface molecules at each temperature by using the amplitudes and decay times of the two exponentials,30 which are given in Table 1. A plot of the temperature dependence of the surface-to-bulk population ratio for each pore size is shown in Figure 6. The surface population increases gradually with decreasing temperature for all pore sizes. The simplest possible model for the observed behavior of confined methyl iodide is that a layer of molecules with retarded dynamics exists at the pore surfaces. If we assume that the pores are cylindrical and are completely filled, we can calculate an effective thickness of this surface layer given the relative populations of bulklike and surface molecules. The calculated thickness as a function of temperature for each pore size is shown in Figure 7. While there is a modest increase in the calculated thickness of the surface layer with decreasing temperature, at any given temperature the surface thickness can be considered equal for all pore sizes within experimental error. As with our previous CS2 study,30 the results show that the implied surface thickness is less than the diameter of a single molecule of methyl iodide (which can be estimated from the

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Loughnane and Fourkas

Figure 7. The calculated thickness of the surface layer as a function of temperature for each of the pore sizes: solid triangles, 83 Å pores; open circles, 42 Å pores; solid circles, 24 Å pores. Error bars are estimated standard deviations. The solid line is a linear least-squares fit as a guide for the eye.

diameter of an iodine atom52 to be approximately 4.4 Å) for all temperatures. It is possible that the pore surfaces contain only a limited number of trapping sites for the liquid, which would lead to a surface layer less than a monolayer thick. A number of lines of evidence point to a more complicated model however. First, it is clear from Figure 4 that there is a systematic trend toward slower surface relaxation rates with decreasing pore radius. Second, the activation energy for reorientation in the surface population is only moderately higher than that in the bulk liquid while the rate of relaxation differs by a factor of 3 to 4. This suggests that energetics play only a minor role in surface dynamics. The evidence thus far implies that pure geometric considerations have an important influence on the surface dynamics, as has been suggested previously for confined nonwetting liquids.23,25,28,33,46 A number of theoretical,56,57 simulation,19,58,59 and experimental7,9,12 studies show that liquids tend to be more ordered near solid surfaces than in the bulk. In particular, cylindrical molecules have a tendency to lie flat on the surface. One can imagine that for a molecule lying parallel to a flat surface the rotational dynamics in a plane parallel to the surface can be quite different from the dynamics perpendicular to the surface. The dynamics of a molecule lying perpendicular to the surface may differ from those of a molecule lying parallel to the surface as well. Curved surfaces can further enhance such effects by influencing both the dynamics and the ordering of the liquid. Such a model suggests that both the proximity of a molecule to the surface and its orientation relative to the surface determine the molecule’s reorientational rate, which would explain the submonolayer surface layer thickness calculated here and in our previous work. Korb et al.28 recently proposed a model to describe the reorientational dynamics of confined nonwetting liquids that is based on the idea that reorientation parallel to pore surfaces is unaffected by confinement whereas reorientation perpendicular to the pore surface is hindered. This model predicts that the reorientational time perpendicular to the surface (τ⊥) is given by28

τ⊥ ) -

τb

[ (

χR ln 1 -

)]

1 2 I1(χR) 1χR χRI0(χR)

(2)

where τb is the bulk reorientational time, R is the pore radius, χ is related to the orientational correlation length (which is

Figure 8. Best fit (solid line) of surface reorientational times in confinement to the theory of Korb et al.28 for a temperature-independent value of χ. The optimum value of χ determined from this fit is 0.11 Å-1: solid triangles, 83 Å pores; open circles, 42 Å pores; solid circles 24 Å pores.

Figure 9. Comparison of data sets obtained in normal and surfacemodified samples with a 42 Å pore diameter at 290 K. The modified sample clearly exhibits slower surface reorientation.

assumed to be only weakly temperature dependent), and In is the nth-order modified Bessel function. Figure 8 shows a fit of the surface reorientation data to eq 2 using a temperatureindependent value for χ of 0.11 Å-1. The quality of our data is not high enough to determine any slight temperature dependence in the value of χ or to make a detailed assessment of this model at the present time. However, the agreement between the theory and experiment seems reasonable, and the optimal value of χ of 0.11 Å-1 is physically sensible, in that it implies that surface orientational effects propagate over a distance of approximately two molecular diameters. Further information on confined dynamics can be gained by surface modification of pores. Previous studies have shown that modification to make the surfaces hydrophobic greatly speeds up the surface reorientation of wetting liquids, while the influence on nonwetting liquids is minimal.33 Figure 9 demonstrates that in 42 Å diameter pores at 290 K replacing the surface hydroxyl protons with methoxy groups does indeed have some effect on the overall dynamics of methyl iodide. Surprisingly, however, the dynamics in the modified pores are slower than those in the unmodified pores. The data in the modified pores can also be fit to a biexponential function whose fastest decay time matches that of the bulk liquid. The time constant of the slower decay component is 9.7 ps, as opposed to 7.2 ps in the unmodified sample. This small but significant difference was reproducible in other samples. It is worthwhile to compare our results in the modified pores to those of Lee, Wallen, and Jonas, who studied the effects of confinement on the ν2 Raman band of methyl iodide.24 In their study, a modest reduction in the width of this band and an

Dynamics of a Nonwetting Liquid increase in the peak shift upon dilution with deuterated methyl iodide were found in going from unmodified to modified pores. One possible interpretation of the reduction in line width is that the dynamics are somewhat slower in the surface-modified samples. Lee, Wallen, and Jonas further observed a reduction in the contribution of resonant intermolecular vibrational coupling in the diluted liquid upon surface modification,24 which is also indicative of slower dynamics in the modified pores. Surface modification can affect both structure and dynamics at the surface. Both our results and those of Lee, Wallen, and Jonas imply that the dynamics of methyl iodide in pores are further inhibited by surface modification, which lends further credence to the idea that geometrical considerations play an important role in the dynamics of confined nonwetting liquids. We are currently studying the effects of surface modification on confined liquids in greater detail. 4. Conclusions We have studied the orientational dynamics of methyl iodide confined in nanoporous glasses over a broad range of temperatures. For each temperature and pore size studied, the dynamics are found to be bimodal, with one component matching the relaxation of the bulk liquid and the other component relaxing considerably more slowly. We have ascribed the slower relaxation process to hindered reorientation of methyl iodide molecules on or near the surfaces of the pores. The data reported here demonstrate that surface reorientation becomes increasingly hindered as the average pore diameter of the sample is decreased. We find that the activation energy for surface relaxation is only modestly larger than that in the bulk liquid, even though the bulk and surface rates vary by a factor of 3 to 4. Furthermore, if one assumes that all molecules at the pore surfaces exhibit hindered reorientation, the implied thickness of the surface layer is less than a monolayer. Taken together, these facts suggest that surface relaxation is retarded primarily through geometric effects. The observed surface relaxation rates are in reasonable agreement with a model proposed by Korb et al.28 in which only relaxation perpendicular to the pore surfaces is hindered in nonwetting liquids. We are currently reexamining the dynamics of confined CS2 in a range of pore sizes, which should provide data of high enough quality to make a critical assessment of this model. We also believe that molecular dynamics simulations could shed considerable light on confinement effects on nonwetting liquids composed of small molecules. Acknowledgment. This work was supported by the National Science Foundation, Grant CHE-9501598. J.T.F. is a Research Corporation Cottrell Scholar and thanks the Camille and Henry Dreyfus Foundation for a New Faculty Award and the Alfred P. Sloan Foundation for a Sloan Research Fellowship. We thank Prof. C. T. Moynihan for helpful advice and discussions on viscosity measurements. References and Notes (1) Molecular Dynamics in Restricted Geometries; Drake, J. M., Klafter, J., Eds.; Wiley: New York, 1989. (2) Drake, J. M.; Klafter, J. Phys. Today 1990, 43, 46. (3) Dynamics in Small Confining Systems Extended Abstracts; Drake, J. M., Klafter, J., Kopelman, R., Eds.; Materials Research Society: Pittsburgh, 1990; Vol. EA-22, p 248. (4) Dynamics in Small Confining Systems; Drake, J. M., Klafter, J., Kopelman, R., Awschalom, D. D., Eds.; Materials Research Society: Pittsburgh, 1993; Vol. 290, p 377.

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