Structure of Liquid Propionitrile at Interfaces. 2 ... - ACS Publications

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Structure of Liquid Propionitrile at Interfaces. 2. Experiment Feng Ding,†,▽ Qin Zhong,†,▽ Katherine Manfred,† XiaoXiao He,† John S. Bender,† Michael R. Brindza,†,○ Robert A. Walker,*,†,‡,§ and John T. Fourkas*,†,‡,∥,⊥,# †

Department of Chemistry & Biochemistry, University of Maryland, College Park, Maryland 20742, United States Chemical Physics Program, University of Maryland, College Park, Maryland 20742, United States § Department of Chemistry & Biochemistry, Montana State University, Bozeman, Montana 59715, United States ∥ Institute for Physical Science and Technology, University of Maryland, College Park, Maryland 20742, United States ⊥ Maryland NanoCenter, University of Maryland, College Park, Maryland 20742, United States # Center for Nanophysics and Advanced Materials, University of Maryland, College Park, Maryland 20742, United States ‡

ABSTRACT: Propionitrile has been studied at the liquid/ vapor, silica/vapor, and silica/liquid interfaces using vibrational sum-frequency generation (VSFG) spectroscopy and optical Kerr effect (OKE) spectroscopy. VSFG studies show that the alkyl tail of propionitrile tends to point into the vapor phase at the liquid/vapor and silica/vapor interfaces. At the silica/liquid interface, all vibrational resonances except for the methylene symmetric stretch exhibit strong cancellation in the VSFG signal. This result supports the existence of a lipid-bilayer-like organization at the silica/liquid interface, in agreement with simulation. OKE data for propionitrile confined in porous sol−gel glasses indicate that there is a surface layer, with a thickness of roughly 4 Å, that experiences inhibited orientational dynamics. The OKE data thus corroborate the picture of interfacial organization suggested by the VSFG results.

I. INTRODUCTION The intermolecular structure and dynamics of interfacial liquid molecules can differ markedly from those in the bulk liquid due to the asymmetric forces and reduced dimensionality experienced at interfaces.1−6 The distinctive structure and dynamics of interfacial liquids can, in turn, lead to significant differences in chemical, physical, and biological properties. Developing a microscopic understanding of interfacial structure and dynamics remains a major challenge in the liquids community, from both experimental and theoretical perspectives. Alkyl cyanides, which are composed of a cyanide group with an alkyl tail, are widely used solvents whose behavior at interfaces can serve as a model for more complex interfacial solvent and surfactant systems.7−17 The alkyl moiety is nonpolar and does not interact strongly with other functional groups, while the cyanide group is highly polar and has some tendency to dipole pair in the liquid state.18−20 The interplay between dipolar and van der Waals interactions in alkyl cyanides can lead to interesting, temperature-dependent structural phenomena.21,22 Moreover, cyanide groups have the abilities to accept hydrogen bonds and to interact with surface polar groups. These associations can strongly influence the molecular organization of the interfacial liquid alkyl cyanides. Acetonitrile is the simplest of the alkyl cyanides, and its properties in the liquid state23−31 and at interfaces8−10,12−15,17,32−35 have been studied extensively. We recently reported the use of a combination of molecular dynamics (MD) simulations and vibrational sum-frequency generation (VSFG) spectroscopy experiments to study acetonitrile at three different interfaces: © 2012 American Chemical Society

acetonitrile liquid/acetonitrile vapor, silica/acetonitrile vapor, and silica/acetonitrile liquid.7,11 Our simulations demonstrated that liquid acetonitrile at a flat silica surface takes on a structure that is reminiscent of that of a lipid bilayer,7,11 as has also been found for acetonitrile in nanometer-diameter cylindrical silica pores.34,35 Because the strongest associations between a liquid nitrile and a silica surface arise through interactions with the nitrogen atoms of the cyanide groups, nitriles tend to form an organized layer on such surfaces. The second sublayer of acetonitrile completes the bilayer structure with its methyl groups approximately antiparallel to the methyl groups in the first sublayer. In simulations this bilayer structure persists for more than 20 Å from the interface.7 VSFG spectra obtained in the methyl-stretching region provide further evidence for the existence of antiparallel structures of molecules at these interfaces.7 The promotion of this structural arrangement in acetonitrile by a silica interface may have important consequences in applications, such as catalysis and chromatography, for which proposed mechanisms of solute−substrate interactions typically assume that interfacial solvent structure does not play a role.36−38 Whether this bilayer-like structure exists for larger, more structurally complex alkyl cyanides remains an open question. Longer chains in alkyl cyanides introduce additional steric hindrance and packing constraints that can influence intermolecular ordering at interfaces. At the same time, increasing Received: November 16, 2011 Revised: January 17, 2012 Published: January 18, 2012 4019

dx.doi.org/10.1021/jp211061p | J. Phys. Chem. C 2012, 116, 4019−4025

The Journal of Physical Chemistry C

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liquid interface, the top window of a completely filled cell with a 1 mm path length (Hellma 404-1-46) was used. For measurements at the silica/vapor interface, the same cell was used but with only a drop of liquid. For measurements at the liquid/vapor interface, a cell with a 1 cm path length (Hellma 404-10-46) that was roughly half full of liquid was used. The OKE apparatus has been described previously as well.49,50 A KMLabs Ti:sapphire oscillator produces 28 fs pulses with a center wavelength of approximately 800 nm and a repetition rate of about 80 MHz. The pulses are split into a strong pump beam and a weak probe beam polarized at 45° and 0° to the vertical, respectively. A quarter-wave plate is placed in the probe beam path after the polarizer to implement optical heterodyne detection. The two beams are focused to the same point in the sample through a single lens. After the sample, the probe beam passes through an analyzer polarizer set to pass horizontally polarized light. The signal is collected by an amplified photodetector. The pump and probe beams are chopped at different frequencies. A small portion of the chopped probe beam is picked off and sent to a separate detector. This signal is subtracted from the local oscillator using an analog preamplifier. The resultant difference signal is fed into a lock-in amplifier referenced to the sum of the pump and probe chopping frequencies. After the sample, the pump beam is frequency doubled and detected with another photodetector/ lock-in pair. The second-harmonic signal is used to correct for long-term drift in the laser intensity. Data are collected by scanning a delay line in the probe beam path, and successive data sets are collected at opposite heterodyne angles. Sol−gel monoliths were prepared as described previously.51 The four samples used here had average pore diameters of 25, 26, 43, and 73 Å, respectively, with an average deviation in pore diameter of about ±10% (as measured by nitrogen adsorption and desorption46). Cylindrical monoliths with a diameter of approximately 8 mm were sanded and polished to produce disks with a thickness of somewhat less than 2 mm. One sample of each pore diameter was sealed in a 2 mm path length cell filled with liquid propionitrile. OKE data were obtained for the different pore sizes by ensuring that the pump and probe beams overlapped spatially only within the appropriate monolith. Bulk OKE data were obtained in the same cell. All data were collected at a temperature of 295 K. At least 10 OKE decays were obtained for each pore size and for the bulk liquid, half at each heterodyne angle. After averaging the decays for each heterodyne angle independently, the averaged data sets for opposite heterodyne angles were subtracted to obtain the pure heterodyne decay. The decays were then integrated to obtain the collective orientational correlation function.52

the length of the alkyl tail changes the balance between polar and dispersive interactions among the liquid molecules, which can also impact intermolecular structure and dynamics at interfaces. The goal of the work discussed here and in the preceding paper39 is to investigate the effect that increasing the length of the alkyl tail (in this case by one methylene group to make propionitrile, also known as ethyl cyanide) has on microscopic liquid interfacial structure. The preceding paper39 presented MD simulations of interfacial liquid propionitrile. In these simulations, propionitrile at the silica/liquid interface exhibits a lipid-bilayer-like structure similar to that seen for acetonitrile, albeit with an entangled alkyl layer as opposed to the interdigitated alkyl layer observed in acetonitrile. As is the case for acetonitrile,7 the bilayer structure propagates a significant distance into the bulk liquid. At the liquid/vapor interface a major driving force for the organization of the liquid is the preference of the alkyl groups to be directed toward the vapor phase due to their weak interactions with other molecules in the liquid. Here we present experimental studies of these same interfaces, as well as of the silica/propionitrile vapor interface. We have used VSFG40−42 to study the structure of propionitrile at all three interfaces. In addition, we have used optical Kerr effect43−45 (OKE) spectroscopy to study the dynamics of liquid propionitrile in porous, silica sol−gel46 glasses. The combination of these two techniques provides a detailed picture of the microscopic interfacial behavior of propionitrile and reinforces the trends observed in MD simulations.

II. EXPERIMENTAL SECTION VSFG spectra were obtained using a broadband, counterpropagating VSFG spectrometer that was described in detail previously.47 Briefly, 130 fs pulses with a center wavelength of 800 nm are generated by a Ti:sapphire amplifier (Coherent Legend Elite) at a repetition rate of 1 kHz and an average power of 3 W, and 30% of the 800 nm light seeds and pumps an infrared optical parametric amplifier (TOPAS, Light Conversion) with a difference-frequency generation module, which generates infrared (IR) pulses that are tunable between 2 and 12 μm. For the experiments described in this work, the IR wavelength was centered around 3.5 μm (2850 cm−1) with a full-width-at-halfmaximum bandwidth of about 100 cm−1. The visible pulse is generated by another 30% of the amplifier output, which is frequency narrowed by an optical stretcher to a bandwidth between 14 and 18 cm−1. At the sample, the IR and visible pulse energies are about 10 and 45 μJ, respectively. The incident angles of the 800 nm and IR beams are 64.5° and −54° from the surface normal, respectively. The signal propagates at an angle of approximately −35° to the surface normal (the exact angle depends on the medium and the IR frequency). The sumfrequency signal is sent into a monochromator (Acton SP300i) and is then dispersed onto a thermoelectrically cooled CCD (Pixis:100, Princeton Instruments). A complete spectrum across the 250 cm−1 frequency region of interest is obtained by acquiring VSFG spectra at eight consecutive wavelengths around the center wavelength (3300 nm) with a spectral step size of 50 nm.48 The spectra are normalized using the summed SFG signal from a gold substrate and are calibrated according to the absorption lines of a polystyrene film. Spectrophotometric grade propionitrile (Aldrich) with a reported purity of 99+% was used for the studies reported here. The samples were held in IR quartz cells that were cleaned in an oxygen plasma prior to use. For measurements at the silica/

III. RESULTS AND DISCUSSION VSFG Spectroscopy. Shown in Figures 1, 2, and 3 are VSFG spectra in the C−H stretching region for propionitrile at the liquid/vapor, silica/vapor, and silica/liquid interfaces, respectively. The black, red, and green lines are data measured under the SSP, PPP, and SPS polarization conditions, respectively. We follow the IR and Raman assignments of Wurrey, Bucy, and Durig.53 For reference purposes, their Raman assignments are summarized in Table 1 for the gas, solid, and liquid phases. The methyl symmetric stretch is in Fermi resonance with the overtone of a methyl deformation mode. In Raman spectroscopy, one of the two resultant peaks appears in the neighborhood of 2900 cm−1 and the other in the neighborhood of 2950 cm−1. The methylene symmetric stretch appears in the region of 2950 cm−1 as well. The methyl asymmetric stretch 4020



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Table 1. Raman Mode Assignments for Propionitrile in Different Phases (from Ref 53) mode

gas (cm−1)

liquid (cm−1)

solid (cm−1)

methyl SS/deformation overtone Fermi resonance pair methylene SS methyl AS methylene AS

2902 and 2961

2895 and 2946

2892 and 2947

2960 3000 --

2940 2996 --

2941 2991 2967

stretching modes appears in the SPS spectrum, and all of the modes show up in the PPP spectrum. On the basis of the strengths and frequencies of the symmetric methyl stretch Fermi resonance pair, these results indicate that the alkyl ends of the propionitrile molecules tend to point into the vapor phase at this interface. The strength of the methyl asymmetric stretch in the SPS spectrum indicates that the alkyl groups pointing into the vapor phase have a relatively broad range of orientations. This picture agrees with the results of simulations presented in the preceding paper.39 We can compare these results with VSFG spectra obtained at the ethanol liquid/vapor interface by Gan et al.54,55 These authors observed a strong contribution from the symmetric methyl stretch Fermi resonance modes in the SSP spectrum, found contributions from all of the modes in the PPP spectrum, and saw the signature of the methyl asymmetric stretch in the SPS spectrum. We find a much stronger contribution from the symmetric methyl stretch Fermi resonance modes in the PPP spectrum than was found by Gan et al. for ethanol.54,55 This result suggests that propionitrile at the liquid/vapor interface has a greater projection of the methyl axis along the surface normal than does ethanol at its liquid/vapor interface. The tendency of this axis to point along the surface normal in propionitrile is in agreement with our simulations.39 We should note that the differences between our experimental geometry and that of Gan et al. may also play some role in the differences between the propionitrile and ethanol VSFG spectra.42 We next turn to the solid/vapor interface (Figure 2). In this case, the SSP spectrum shows a strong contribution from the symmetric methyl stretch Fermi resonance pair, plus a weak contribution from the asymmetric methyl stretch. The same modes contribute to the PPP spectrum, although the relative contribution from the asymmetric methyl stretch is somewhat greater in this case. The SPS spectrum is too weak to be interpreted. Our results for propionitrile at the silica/vapor interface bear strong resemblance to those of Liu et al. for ethanol at the silica/vapor interface,56 suggesting that the interfacial ordering is quite similar in these two systems. The methyl symmetric stretch Fermi resonances appear more strongly in the propionitrile PPP spectrum than in the corresponding ethanol spectrum, and the asymmetric stretch peaks in propionitrile are correspondingly weaker. These results again suggest that the axis of the methyl group in propionitrile has a somewhat larger projection along the surface normal than in the case of ethanol. These differences could stem from the different interactions of these molecules with the surface and with one another. Propionitrile can only accept hydrogen bonds from the silica surface and must balance the dipole−dipole interactions between adsorbed molecules. In contrast, an ethanol molecule can accept hydrogen bonds or donate them both with surface silanol groups and with adjacent adsorbed neighbors. These effects may be responsible for the greater tilt angle of the CN group of propionitrile39 at the silica interface as compared to acetonitrile.7,11

Figure 1. VSFG data and mode assignments for propionitrile at the liquid/vapor interface under SSP (black), PPP (red), and SPS (green) polarization conditions.

Figure 2. VSFG data and mode assignments for propionitrile at the silica/vapor interface under SSP (black), PPP (red), and SPS (green) polarization conditions.

Figure 3. VSFG data for propionitrile at the silica/liquid interface under SSP (black), PPP (red), and SPS (green) polarization conditions.

appears near 3000 cm−1, and the methylene asymmetric stretch is weak and appears at 2967 cm−1 in the solid. We begin by considering the liquid/vapor interface of propionitrile. As can be seen in Figure 1, all of the C−H stretching modes appear in the VSFG spectra at this interface. The SSP spectrum is dominated by the symmetric methyl stretch Fermi resonance pair and may also have a contribution from the methylene symmetric stretch. One or more of the asymmetric 4021



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and Γ2 are the linewidths of the two Lorentzians. Note that we did not include any nonresonant background in these fits, as our counter-propagating experimental geometry minimizes nonresonant contributions to the spectrum.47 The above results indicate that interaction with a silica surface has different effects on the vibrational frequencies of alcohols and nitriles. In the case of alcohols, the donation and acceptance of hydrogen bonds from silica has little to no effect on the C−H vibrational frequencies of the alkyl group adjacent to the hydroxyl group on the molecule.56,57 In agreement with this interpretation, the symmetric methylene stretch makes a strong contribution to the VSFG spectrum of the silica/ethanol vapor interface but disappears at the silica/ethanol liquid interface.56 When a cyanide group accepts a hydrogen bond from a silica surface, there is a noticeable shift of the C−H vibrational frequencies involving the carbon atom adjacent to the cyanide. In acetonitrile, this shift is to higher frequency.7 Thus, we propose that the frequency of the symmetric methylene stretch for propionitrile molecules that accept hydrogen bonds from the silica surface is higher than the frequency of this mode for the molecules in the bulk liquid. Accordingly, we would expect to be able to fit the VSFG spectra to the sum of two Lorentzians of opposite sign, as was the case for the symmetric methyl stretch in acetonitrile. As shown in Figure 4 for the SSP polarization

Finally, we consider the data for the silica/liquid interface (Figure 3). The SSP and PPP spectra are dominated by a single peak at about 2940 cm−1, and a small contribution in the asymmetric stretching region appears under SSP, PPP, and SPS polarization conditions. These spectra are remarkably different from those obtained for the silica/liquid interface of ethanol,56 in which the methyl symmetric stretch Fermi resonance peaks, the methylene symmetric stretch peak, and the asymmetric stretch peaks are all sizable under both SSP and PPP polarization conditions. To understand the appearance of the VSFG spectra at the silica/liquid interface for propionitrile, we must first assign the prominent peak in the SSP and PPP spectra. Typically, the largest feature in such a spectrum would be one of the peaks from the symmetric methyl stretch Fermi resonance pair. However, at most one peak of this Fermi resonance pair appears in the spectrum. As shown in Table 1, the methyl symmetric stretch Fermi resonance peaks do show significant environmental sensitivity, and so it is possible that the different shifts of the symmetric methyl stretch and the overtone mode at the silica interface weaken the Fermi resonance so much that it cannot be observed. Were this the case, however, we would expect to observe a substantial shift in the frequency of the remaining peak (which would correspond to the more strongly IR- and Raman-active methyl symmetric stretch). The observed shift of a few cm−1 would not be expected to lead to such a large amplitude change. Furthermore, the methyl asymmetric stretch is unlikely to shift by more than 20 cm−1 from its bulk liquid position.53 We can therefore assign the peak at 2940 cm−1 in the SSP and PPP spectra to the symmetric methylene stretch. Although this mode is weak in the IR, it has a strong, polarized Raman signal.53 The fact that this mode appears strongly in the corresponding VSFG spectrum of ethanol54,55 supports this assignment. We can gain insight into the prominence of the symmetric methylene stretch of propionitrile at the silica/liquid interface from the VSFG spectroscopy of the silica/liquid interface of two other liquids: methanol and acetonitrile. Liu et al. observed a vanishingly small VSFG signal at the silica/liquid interface of methanol.56 They attributed this result to the existence of a strongly hydrogen-bonded layer of methanol and a corresponding second layer in which the methyl groups have the opposite orientation, leading to cancellation of the signal. Our VSFG experiments on acetonitrile at the silica/liquid interface showed a single feature for the symmetric methyl stretch.7 Our simulations 7 and those of Thompson and co-workers 34,35 indicate that acetonitrile forms a lipid-bilayer-like structure at the silica/ liquid interface, supporting a similar argument for cancellation. In the case of acetonitrile, however, the frequencies of the methyl stretching modes are sensitive to the interactions between the cyanide group and its surroundings. Thus, the cancellation for acetonitrile is incomplete due to a spectral shift between the methyl groups in the liquid and the methyl groups of molecules that accept hydrogen bonds from the silica surface.7 We were therefore able to fit the spectra to the magnitude squared of the sum of two Lorentzian features of opposite sign and with different center frequencies7

A1 A2 S(ω) ∝ − (ω − ω1) + i Γ1 (ω − ω2) + i Γ2

Figure 4. VSFG spectrum for propionitrile at the silica/liquid interface under SSP polarization conditions (black solid line) along with the fit to two Lorentzian features of opposite sign (red dashed line).

combination and in Figure 5 for the PPP polarization combination, we are able to fit the VSFG data quite well to this function. Other functions, such as an individual Lorentzian or an individual Gaussian with or without a nonresonant background, cannot adequately match the precision of the data. The fitting parameters are given in Table 2. Note that the frequencies of the two Lorentzians in these fits are the same for both SSP and PPP polarization conditions but that the line widths and amplitudes change to some extent. This behavior is expected, as SSP and PPP data emphasize different aspects of the molecular hyperpolarizability.42 Additionally, the amplitude ratios of the two Lorentzians are relatively similar for the SSP and PPP data. Due to differences in orientational distributions and, possibly, to differences in orientational dynamics, these numbers are not directly representative of the relative populations of hydrogenbonded liquid molecules at the interface.58 However, these

2

(1)

Here S(ω) is the signal intensity as a function of frequency, A1 and A2 are the relative magnitudes of the two Lorentzians, and Γ1 4022



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Figure 5. VSFG spectrum for propionitrile at the silica/liquid interface under PPP polarization conditions (black solid line) along with the fit to two Lorentzian features of opposite sign (red dashed line).

Figure 6. Collective orientational correlation functions for liquid propionitrile in the bulk and confined in silica glasses with different pore diameters. The correlation functions have been offset for clarity.

Table 2. Parameters from Fits of Propionitrile Silica/Liquid VSFG Data to Equation 1 spectrum

ω1 (cm−1)

ω2 (cm−1)

Γ1 (cm−1)

Γ2 (cm−1)

|A1/A2|

SSP PPP

2937 2937

2948 2948

12.8 11.8

14.5 15.7

0.84 0.74

Table 3. Parameters from Fits of Integrated Propionitrile OKE Data to Equation 2 with τ1 = 2.98 ps (the Bulk Relaxation Time) and τ2 = 13.3 psa

numbers do indicate that these populations are of roughly the same magnitude. The methyl stretching modes are not entirely absent from the VSFG spectra at the silica/liquid interface. The asymmetric stretch appears clearly in the SPS and SSP spectra, and the stronger peak from the symmetric stretch Fermi resonance pair appears in the SPS spectrum and weakly in the PPP spectrum. Thus, the cancellation of the methyl stretching modes is not complete, in agreement with our simulations.39 The data for the propionitrile silica/liquid interface have two important implications for the interfacial organization of the liquid. First, the silica interface does not lead to strong net ordering of the methyl groups of the molecules in the surface layer. Second, there is a tendency for a significant projection of the methylene symmetric stretch dipole along the normal to the interface, for both the tethered surface sublayer and its companion second sublayer. Both conclusions agree well with the results from our simulations.39 OKE Spectroscopy. The collective orientational correlation functions corresponding to the OKE decays for bulk and confined propionitrile are shown in Figure 6. The collective orientational correlation time measured for bulk propionitrile was 2.98 ps, in good agreement with previous results.21 The diffusive orientational portion of the integrated OKE data for the confined propionitrile in each pore size was fit to a biexponential decay of the form

B1 exp( − t /τ1) + B2 exp( − t /τ1)

a

pore diameter (Å)

B1/B2

73 43 26 25

4.2(2) 1.8(1) 0.86(4) 0.90(4)

Numbers in parentheses are the uncertainty in the last digit.

slower decay increases considerably with decreasing pore size, which can be attributed to the corresponding increase in the surface-to-volume ratio of the pores. The good agreement between the data in 25 and 26 Å pores demonstrates the consistency of these measurements. For many different confined liquids (including acetonitrile), we have observed multiexponential OKE decays in which the fastest component of the collective orientational diffusion has a time constant that matches that of the bulk liquid.59 Our interpretation for this type of behavior is that liquid molecules in the pore centers have bulk-like dynamics, whereas liquid molecules that interact with the pore surfaces have inhibited orientational dynamics that result in slower decays. The inhibition of surface dynamics can arise from a combination of geometrical effects, an increased hydrodynamic volume for reorientation, and specific interactions with the surface. In the case of propionitrile, specific interactions arise from the polar and hydrogen-bond-donating nature of the silica surface.37 We can use the amplitudes of the two exponentials in the OKE decays to estimate the thickness of the surface layer with inhibited dynamics (rs) using the equation60

(2)

⎛ rs = R ⎜⎜1 − ⎝

where we define τ1 to be the faster of the two orientational correlation times. In all cases, the value of τ1 was close to the collective orientational time for the bulk liquid, and the slower exponential had nearly the same time constant in all pore sizes (13.3 ps). We therefore refit the data by constraining τ1 to be equal to the bulk collective orientational time and τ2 to be 13.3 ps for all pore sizes. The resultant amplitudes for the fits of the two exponentials are given in Table 3. The amplitude of the

B1 ⎞ ⎟⎟ B1 + B2 ⎠

(3)

where R is the pore radius. This formula assumes that the pores are cylindrical. The true surface area of the pores is undoubtedly greater than for a completely cylindrical system, and so this equation can be viewed as providing an upper limit to the surface-layer thickness. The estimated thicknesses for the 4023



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IV. CONCLUSIONS We have presented the results of combined VSFG and OKE studies of interfacial propionitrile. In conjunction with the simulation study presented in the preceding paper,39 this work provides a detailed picture of how interfaces influence the organization and dynamics of propionitrile. At the liquid/ vapor and silica/vapor interfaces, we find that the alkyl tail of propionitrile tends to point into the vapor phase, with a large projection of the methyl symmetry axis along the surface normal. The propensity for alignment along the surface normal is significantly larger than for the liquid/vapor and silica/vapor interfaces of ethanol, and these differences must arise from the differences in intermolecular interactions among cyanide groups versus those among hydroxyl groups. In the case of the silica/ liquid interface of propionitrile, the VSFG spectra are dominated by a single peak. This peak arises from the symmetric methylene stretch, which occurs at a different frequency for molecules in the liquid as opposed to molecules that interact directly with the silica surface. The near complete cancellation of the other VSFG peaks is strong evidence for the existence of a lipid-bilayer-like organization of the liquid at this interface, as seen previously for acetonitrile7,11 and in confirmation of our simulation results for propionitrile.39 This structure is quite different from that of liquid ethanol at a silica interface, however. The bilayer structure is manifested dynamically by a significant increase in the orientational correlation time of the adsorbed layer of molecules, as observed by OKE spectroscopy. Because the bilayer is highly entangled, no exchangeable population of molecules is observed for this liquid. The measured surface-layer thickness of dynamically inhibited molecules is also in good agreement with simulations. There are many similarities between the interfacial structure and dynamics of acetonitrile and propionitrile, the most notable of which is the persistence of a bilayer structure at the silica/ liquid interface. It will be of great interest to explore how longer and/or more bulky alkyl groups (and the corresponding changes in intermolecular forces) influence the interfacial behavior of other liquid nitriles.

different pore sizes are shown in Figure 7. The measured thicknesses are the same to within the uncertainty in the estimates, and the average thickness is about 4.0 Å.

Figure 7. Estimated thickness of the surface layer or propionitrile with inhibited orientational dynamics as a function of pore diameter.

For confined acetonitrile, we previously found that the thickness of the dynamically inhibited surface layer at room temperature was approximately the length of one molecule (slightly greater than 4 Å).12,13 The entire surface layer had dynamics that were slower than those of the bulk, and half of the surface layer had extremely slow dynamics. We proposed that about half of the surface molecules were hydrogen-bonded to the surface and that the other half were interdigitated among them, with the cyanide group pointing away from the surface.13 We further proposed that the former population of molecules is the one with the extremely slow dynamics, whereas the latter population can relax more rapidly by exchanging into the bulklike portion of the confined liquid.13 This picture was supported by subsequent simulations.34 We expect the structure and dynamics of propionitrile at the liquid/silica interface to differ from those of acetonitrile. As shown in the preceding paper,39 although the alkyl group of propionitrile is longer than that of acetonitrile, the thickness of the bilayer is about the same for the two liquids. This result implies that the alkyl tails of propionitrile are significantly entangled, which could prevent exchange of molecules from the second sublayer into the bulklike population of liquid. The thickness of the surface layer with inhibited dynamics is smaller in propionitrile than it is for acetonitrile but is in good agreement with the bilayer thickness observed in our simulations (although the full bilayer thickness in the simulation is about 4.9 Å, a vast majority of the molecules in the first density peak have their center of mass at a distance of 4 Å or less from the surface).39 We therefore conclude that both sublayers of the surface propionitrile bilayer have inhibited dynamics. Our simulations indicate that there are additional propionitrile bilayers at a flat silica interface,39 and this is also likely to be the case in silica pores based on simulations of acetonitrile.34,35 However, it is the strong interaction of the first bilayer with the pore surface that leads to inhibited orientational dynamics, and we expect additional bilayers to have orientational dynamics that are similar to those of the bulk liquid.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Present Addresses ▽

Polymers Division, National Institute of Standards and Technology, Gaithersburg, MD 20899. ○ Optical Sciences Division, Naval Research Laboratory, 4555 Overlook Ave. SW, Washington, DC 20375. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation Collaborative Research in Chemistry program, grant CHE0628178. K.M. was supported by a Beckman Scholars Award from the Arnold and Mabel Beckman Foundation.

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REFERENCES

(1) Benjamin, I. Annu. Rev. Phys. Chem. 1997, 48, 407. (2) Penfold, J. Rep. Prog. Phys. 2001, 64, 777. (3) Weeks, J. D. Annu. Rev. Phys. Chem. 2002, 53, 533. (4) Nathanson, G. M. Annu. Rev. Phys. Chem. 2004, 55, 231. (5) Kaplan, W. D.; Kauffmann, Y. Annu. Rev. Mater. Res. 2006, 36, 1.

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