Hexane

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Formation and Growth of Tetrahydrofuran Hydrate at the Ice/ Hexane Interface Maiko Muro,† Makoto Harada,† Takeshi Hasegawa,‡ and Tetsuo Okada*,† †

Department of Chemistry, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan Laboratory of Solution and Interface Chemistry, Division of Environmental Chemistry, Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan

‡

S Supporting Information *

ABSTRACT: The formation and growth of the tetrahydrofuran (THF) clathrate hydrate (CH) are studied at the interface between ice and hexane containing THF by a confocal Raman microscopy. Raman spectroscopic measurements across the ice/hexane interface have clearly detected the formation of the THF CH under an appropriate condition. The ring breathing mode of THF found around 915 cm−1 is the most efficient probe for a change in the molecular environment of THF; we can distinguish the THF CH, THF in hexane, and THF in water on the basis of the band shift of this mode. The temperature is a critical parameter governing the formation of the THF CH. The THF CH is formed at −2.0 °C, and its layer thickness increases with time; a layer thickness of ca. 30 μm is reached 160 min after the contact of ice with THF/hexane. This has been supported by the analysis of the Raman spectra in the higher wavenumber range as well. In contrast, an induction time longer than 100 min is necessary for the THF CH formation at −2.5 and −5.0 °C, and the CH formation is not confirmed at −10 °C. This temperature dependence is closely related to the interfacial quasi liquid layer (QLL), which was detected by ice chromatographic experiments in our previous work. According to the ice chromatographic study, an interfacial QLL of several nanometers in thickness emerges at a temperature higher than −2.0 °C and is developed as the temperature increases. The presence of this structurally perturbed thin layer is required for the facile nucleation of the THF CH on the surface of ice, and −2.0 °C is the critical temperature for the entire process to occur within hours.

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INTRODUCTION Various gases and polar organic compounds with hydrophobic moieties are encapsulated in the cages of water molecules to form crystalline compounds called clathrate hydrates (CHs) under certain conditions. Practical interests in CHs principally come from their energy and environmental usefulness. Methane, for example, forms type I CH consisting of a small cage of pentagonal dodecahedron (512) and a large cage of tetradecahedron (51262). The methane CH, which is naturally deposited on the ocean floor, is expected as a next generation energy resource, and efficient technologies for mining are extensively investigated that strongly rely on its physicochemical nature.1−3 Carbon dioxide (CO2) can also be a guest of type I CH. The CO2 CH has received much attention in environmental science and technology because of its potential utilization for the capture and storage of CO2 produced by the consumption of fossil fuel.4,5 Another form of the well-known CHs is type II composed of a small cage of 512 and a large one of hexadecahedron (51264). Typical guests of type II CH are the components of air, that is, N2, O2, and Ar, which are found in, for example, glaciers.6 The type of CH is, thus, basically determined by the molecular size of a guest.7 Gas CHs can be prepared, for example, by keeping a mixture of a gas and ice © 2012 American Chemical Society

under an appropriate pressure and temperature. The measurements of the growth of the gas CH layer, for example, by X-ray and neutron diffraction, have suggested that a thin CH film covers the entire surface of ice at the initial stage, the CH film is then developed in thickness by the gas and water transportation, and finally, ice is almost entirely transformed into the gas CH.7−11 Gas CHs are usually formed with applying pressures; at 273 K, the Xe hydrate forms at ca. 1.4 atm, whereas the CH4 hydrate formation requires a pressure of >25 atm.12 In contrast, some CHs, including the tetrahydrofuran (THF) CH, can be produced under ambient pressure. In this CH, the guest is accommodated in the large cage of the type II CH, and the small cages remain empty. Assuming the stoichiometric formation of the THF CH, the molar ratio of THF to water is 1/17. This hydrate is, in addition, stable even at moderately high temperature (the melting point is ca. 4 °C). For these reasons, the THF CH has been studied as an analogue of gas CHs of potential use, such as the methane CH, albeit the Received: April 18, 2012 Revised: May 28, 2012 Published: May 30, 2012 13296

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structures are obviously different as stated above.13,14 Studies of the THF CH as a proxy for the methane CH has been justified on the basis of the similarity of their physicochemical properties, for example, similar bulk properties such as heat capacity, adiabatic compressibility, and interfacial tension, and in addition, the weak dipolar interaction between THF and water molecules have been indicated.13 In addition, the THF CH itself is expected to be an efficient hydrogen reservoir in fuel cell applications; H2 is retained in the open spaces of the THF CH and, therefore, is safely stored and transported.15−20 The THF CH can be prepared in a simple way, that is, by freezing the 1:17 mixture of THF/water. It can also be formed at the temperature-controlled interface between water and THF.21,22 This interface is useful to study the mechanism of the growth of the THF CH; the interfacial CH layer grows by multinulceation rather than the progress of the faces of the formed crystals. However, this interface is even macroscopically unstable because of high mutual miscibility of these solvents. Considering that the gas CHs are often formed by mixing solid ice with a gas both in laboratories and in natural environments, the development of the THF CH on the ice surface is not only of fundamental interest but also of practical significance.7−11 Some studies have indicated that the initial stage of the gas CH film formation on the ice surface is related to the surface quasi liquid layer (QLL).7,8 The CH formation on the ice surface should be accompanied by the deformation of the ice crystal (Ih under usual conditions) because the arrangement of water molecules in ice Ih is entirely different from that of CHs. This molecular rearrangement requires a large energy expenditure, which hinders the nucleation of the CH crystal. However, if the surface of ice is liquidlike, the deformation of the ice crystal is not necessary; thus, the nucleation of the CH crystal is energetically much easier than in the absence of the QLL. We have, in our previous work, showed that the QLL is formed at the interface between ice and THF dissolved in hexane.23 The thickness of this interfacial liquid layer is very sensitive to temperature; it was estimated to be 2 nm at −2.0 °C and 8 nm at −1.4 °C, but no liquid layer was detected at lower temperatures. Thus, the liquidlike water layer exists at the interface between the ice and an organic liquid in the range from −2 to 0 °C. In the present work, we focus our attention on the formation and growth of the THF CH at the interface between the ice and an organic phase containing THF. If a water-miscible organic solvent, including THF itself, comes into contact with ice, the interface is invaded by the organic solvent; this makes difficult the fine observation of the growth of the THF CH at the interface. Hexane was therefore selected as the base organic solvent to form the ice/liquid interface because of its low solubility of water and the free miscibility with THF. This system allows us to study the THF CH on the surface of ice without the invasion of the interface. It is known that the diffusion of a gas molecule through the CH layer is more than several orders of magnitude lower than its gas phase diffusion, and as a result, the development of the CH layer takes a very long time ranging from hours to days. Therefore, a slow rate is expected for the growth of the THF CH as well. Raman microscopy has been selected in the present study, because high spatial resolution is expected to allow us to evaluate a small change in the interfacial structure resulting from the CH formation. Also, it is intriguing to discuss the formation of the THF CH layer related to the interfacial QLL previously evaluated at the same interface.

Article

EXPERIMENTAL SECTION

A Raman spectrometer model Nanofinder 30 (Tokyo Instruments) coupled with an Olympus confocal microscopy system was used for spectral measurements. The spectrometer had a diffraction grating with 600 g mm−1, which covered a wavenumber range of 2600 cm−1. The entrance slit width was 50 μm, which provided a wavenumber resolution of ca. 4 cm−1; the resolution was examined by measuring fwhm of an emission line of a Ne lamp. A solid-state laser (532 nm) of 50 mW was used for excitation. The laser power on a sample was 9.7 mW. The wavenumber was calibrated everyday using major peaks of a mixture of toluene/acetonitrile (1:1 v/v), which is recommended by NIST. The calibration curve was constructed using a third-order polynomial, and the regression calculation was performed on MATLAB 2007b (Math Works). The regression curve was validated by measuring the standard peaks of cyclohexane; the wavenumber precision proved to be within 1.4 cm−1. A 50× objective (NA = 0.8 with a working distance of 1.0 mm) was used throughout the present study. The theoretical spatial resolutions were ca. 0.34 μm in the horizontal direction and ca. 1.8 μm in the vertical direction. A sample was prepared on a homemade Cu sample cell of 20 mm × 20 mm in area and 2 mm in depth (details are given in Figure S1 in the Supporting Information). The cell was pasted on a Peltier array (the effective area was 30 mm × 30 mm) controlled by a Cell System Peltier controller model TDC2030R. The temperature of a sample was monitored with platinum resistance thermometers put in the Cu cell and in the sample. The temperature fluctuation was smaller than ±0.05 °C. The reversed side of the Peltier module was cooled by a chiller. The sample cell and the Peltier array were installed on a three-dimensional piezo stage with the traveling distances of 100 μm in the horizontal (xy) directions and 30 μm in the vertical (z) direction. An appropriate amount of water or an aqueous electrolyte was poured into the cell so that the space of ca. 0.5 mm was left above the aqueous layer in the cell. A Plexiglas slip was put on the water surface to keep a flat observation surface of a sample after freezing. The solution was frozen at −20 °C. After complete freezing, the temperature increased up to −10 °C, and the Plexiglas cover was removed. The space over the ice sample was filled with a precooled organic phase. Unless otherwise stated, the organic phase was 1:5 (v/v) THF in hexane. A cover glass slip was put on the organic phase, and its fringe was sealed with ice as the adhesive. The atmosphere over the sample cell was replaced by dried Ar to prevent dew formation on the objective lens and the coverslip surface. Raman spectra were acquired on 90 points by scanning along the z-axis. The acquisition of a spectrum over a wavenumber range of 2600 cm−1 took 1 s at each measurement point. The piezo stage was moved upward by 1.0 μm after the completion of a measurement at a particular point, and then, the next spectrum was recorded. This routine was repeated over the distance of 90 μm. Because of the limitation of the traveling distance of the piezo stage, a set of measurements over the distance of 30 μm were repeated three times using a finecontrollable manual z-stage together with the piezo stage. Thus, the entire measurements on 90 points required ca. 2 min. The starting position was selected in the organic phase so that the interface was detected around the midpoint of a series of 90 measurements. The lower wavenumber range (∼2500 cm−1) and higher wavenumber range (1900−4000 cm−1) were 13297

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breathing mode appears at 919 cm−1, which is 5−6 cm−1 higher than the corresponding one found in its hexane solution. We can thus detect the formation of the THF CH at the interface between ice and THF/hexane based on the wavenumber shift of the ring breathing mode. The higher wavenumber range also provides useful information. The spectrum of the THF CH involves the CH2 stretching vibration mode in 2800−3000 cm−1 and OH stretching vibration mode in 3000−3500 cm−1 and appears to be a simple superimposition of the spectra of THF and ice. Although the CH2 stretching modes are so complex that we cannot find useful bands to identify the formation of the THF CH in such a simple way as in the lower wavenumber range, the penetration of THF or hexane into the ice phase can readily be detected by the emergence of any band in this range, where pure ice gives no bands. The OH stretching vibration band of the THF CH is composed of at least two components, that is, a stronger band at 3150 cm−1 and weaker one at 3400 cm−1; this feature is similar to that of ice rather than that of liquid water. The OH stretching vibration bands of water have been well discussed in various cases.30,31 The peak at the lower wavenumber is often referred to the icelike structure, in which water is structurally arranged in a tetrahedral manner, whereas the peak at the higher one comes from poorly developed hydrogen bonding between water molecules. Naturally, the spectrum of the THF CH is similar to that of ice as shown in Figure 1. In contrast, the Raman intensity at 3400 cm−1 is higher than that at 3150 cm−1 for an aqueous THF solution, giving quite different spectral pattern from ice or the THF CH. Thus, the identification of the molecular circumstances of THF is feasible not only from the ring breathing mode but also from the OH stretching vibration mode. Formation of THF CH Layer on the Ice Surface. A series of Raman scanning in the z direction was started 10 min after the preparation of an interface between ice and THF/hexane (represented by t = 10 min). As described in the Experimental Section, consecutive spectrum measurements were carried out every 1 μm along the vertical axis over the distance of 90 μm. The z coordinate of the starting position was assigned z = 0, and the distance between this position and a measurement point is hereinafter reported as the z coordinate. The vertical Raman scanning was repeated at the same site on a sample every 30 min to study the growth of the THF CH at the interface. More extensive measurements were avoided to prevent the thermally induced CH formation; in actuality, continuous laser irradiation caused melting of ice, which was expected to affect the THF hydrate formation. Figure 2 shows the Raman spectra measured at −2.0 °C at t = 10 min and those at t = 160 min. The results obtained at selected depths, that is, z = 1 μm [in 1:5 (v/v) THF/hexane], 25 μm (around the interface), and 80 μm (in ice) are compared in this figure. At t = 10 min, no obvious difference is seen between the spectra measured at z = 1 and 25 μm except Raman intensities. A decrease in the Raman intensity with increasing depth is due to the attenuation of Raman scattering by the intervention layer. Although all of the spectra were examined in detail, no sign of the THF CH formation was found in any depth at t = 10 min. In contrast, the Raman spectrum of almost pure THF CH was recorded at z = 25 μm at t = 160 min. In the spectrum recorded at z = 25 μm, peaks from hexane are not found, but the ring breathing mode of THF is clearly seen at 917 cm−1, which is ca. 5 cm−1 higher than the wavenumber of the corresponding

measured separately with different samples prepared from the same solution in the same way. Chemometric analysis was performed based on the alternative least-squares (ALS) regression technique for quantitative spectral decomposition.24,25 ALS is an automated classical least-squares (CLS) technique requiring no a priori knowledge and simultaneously provides a set of purecomponent spectra and concentration variations. Because CLS has an analytical limit that the accuracy is largely degraded if an inappropriate number of components are taken into consideration, a more robust technique is often employed, which is the “augmented ALS” technique.25,26 The principal component analysis (PCA)27 yielded an eigenvalue plot, which suggested that the number of major components should be three in the present measurements (data not shown). An augmented matrix involving “four” components was used, however, and reproducible results were readily obtained. Here, four components are reasonable because the possible chemical constituents at the interface are considered: THF/hexane, THF CH, THF/water, and ice.

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RESULTS AND DISCUSSION Raman Spectra of the Components Consisting of the Interface between Ice and THF/Hexane. The literature has suggested that THF gives slightly different Raman spectra depending on its chemical environments.28 In the present study, at least three different states of THF should be distinguished to spatially resolve the THF CH formation at the ice/hexane interface; that is, THF dissolved in hexane, THF dissolved in water, and the THF CH. Figure 1 shows the

Figure 1. Raman spectra for hexane, neat THF, THF in hexane, THF in water, THF CH, ice, and liquid water.

Raman spectra of THF in different molecular environments together with those of hexane, ice, and liquid water for comparison. The Raman band specific of THF is found at 914 cm−1 in its neat or hexane solution; actually, no differences in Raman spectrum were detected between neat and hexane solution of THF because of no specific interaction between THF and hexane. This band, which has been assigned to the ring breathing (C−C−C−C stretching) mode,29 is well resolved from the nearby modes of hexane even when THF is dissolved in this solvent. In contrast, the peak of the ring breathing mode is split into the doublet at 890 and 919 cm−1 when THF is dissolved in water; this state is readily identified due to its characteristic feature. Also, when THF is encapsulated in the large cage of the type II CH, the ring 13298

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(1) The Raman intensities basically decrease with increasing z because of the attenuation of the Raman scattering as stated above. (2) In the hexane phase, the intensity at 912 cm−1 is higher than that at 917 cm−1. (3) No peaks appear at either wavenumber in the ice phase; therefore, the intensities of 912 and 917 cm−1 agree with each other within the noise level. (4) At t = 10 min, the peak intensity at 912 cm−1 is higher than or equal to that at 917 cm−1 in the entire z range, indicating that no THF CH is formed. Therefore, the I917/I912 ratio is always smaller than unity. In contrast, at 160 min, the intensity at 917 cm−1 is higher than that at 912 cm−1 in the middle z range, and the I917/I912 ratio becomes larger than unity and gives a maximum. This clearly indicates the formation of the THF CH around the interface region. The thickness of the hydrate layer is estimated to be ca. 30 μm at t = 160 min. After a series of measurements, the Raman spectra at different sites were also examined, which suggest the formation of the THF CH with 25−30 μm in thickness at the interfacial regions. This implies that the THF CH is formed at the entire interface and that the laser irradiation does not affect the growth of the CH layer. The growth of the THF CH has been confirmed by the spectra in the higher wavenumber range. Because peak overlapping made it difficult to find bands useful for probing the growth of the THF CH, entire spectra in this range were analyzed by the ALS analysis to separate individual spectral components. The components separated from the raw spectra measured at t = 10 and 160 min are illustrated in Figure 4. Two

Figure 2. Raman spectra at the ice and THF/hexane interface obtained at different depths and different times elapsed from the preparation of the interface.

mode of THF in hexane, suggesting the formation of the THF CH. The growth of the THF CH layer can thus be evaluated from the change in the wavenumber of the ring breathing mode. However, the corresponding peaks are not resolved when two different states, the THF CH and THF not in the CH cage, coexist. The dependence of the peak intensities on z was therefore examined to give more explicit proof of the growth of the THF CH layer. Figure 3 shows the changes in the peak intensities at 912 and 917 cm−1 as well as in their ratio (I917/ I912) with z at t = 10 and 160 min. General trends can be summarized as follows:

Figure 4. Pure component spectra obtained by the ALS analysis (A and C) and changes in the component quantities with z (B and D) at t = 10 (A and B) and 160 min (C and D).

spectra, those of ice and THF/hexane, were separated from the spectra measured at t = 10 min, which indicates that no detectable THF CH is formed on the ice surface. In contrast, three components were extracted from the spectra at t = 160 min. Each spectrum agrees well with the authentic ones of THF/hexane, the THF CH, or ice (see Figure 1). Thus, the formation of the THF CH at the ice/hexane interface is confirmed from the Raman spectra in the higher wavenumber region as well. The relative quantity plot depicted in Figure 4D

Figure 3. Dependence of the peak intensities at 912 and 917 cm−1 on the depth (z) at t = 10 min (top) and t = 160 min (bottom). The zone, in which the intensity at 917 cm−1 is higher than that at 912 cm−1, is assigned to the THF CH layer. To show the formation of the THF CH layer more clearly, the ratios of the Raman intensity at 917 cm−1 to that at 912 cm−1 are also depicted in blue curves. 13299

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−2.0 °C, the retention of some solutes drastically increased with increasing temperature past −2.0 °C. This drastic change in retention was interpreted by the partition of a solute into the interfacial QLL. The interfacial QLL is so thin (in the range of nanometers) that it, if any, is undetectable by the present Raman measurements. It is interesting that the threshold temperature for the apparent growth of the THF CH layer agrees with that for the development of the interfacial QLL. This strongly suggests that the formation of THF CH on the solid ice surface is facilitated by the presence of the structurally disturbed thin layer such as the interfacial QLL. When a salt is doped in ice, a liquid phase is developed at the temperature above the eutectic point (teu) of the system. A simple but important question is whether the THF CH layer is developed at the interface of salt doped ice to a greater or a lesser extent than at the interface of pure ice. Some compounds, including salts, are known as inhibitors for CHs formation. The inhibitors are classified into kinetic and thermodynamic ones according to their inhibition mechanisms.36−38 Salts are the latter type, which lowers the freezing point of CHs. However, because the limited availability of mobile water molecules on ice regulates the formation and development of the THF CH layer, it is possible that a salt facilitates the nucleation of the THF hydrate at the temperature sufficiently lower than the freezing point. Although time-dependent Raman spectra were measured at several points on the KCl-doped ice surface to avoid the effect of inhomogeneity of the surface, no obvious development of the THF CH layer was confirmed even at −2.0 °C in contrast to the pure ice interface. Because the developments of the liquid water phase were confirmed from the OH stretching vibration band, an aqueous THF phase should emerge in the interior of doped ice. Our previous work has revealed that the size of the liquid phase is in the micrometer range.39 In such a large liquid phase, the nucleation of the THF CH is difficult to occur unlike in the thin interfacial QLL. Thus, a salt acts as an inhibitor even on the ice surface. The growth of the THF CH layer at the interface between ice and THF/hexane was analyzed by the following model. The assumptions were that (1) THF penetrates through the THF CH layer already formed on the ice surface and (2) THF is readily transformed into the THF CH when it is transported onto the ice surface. According to these assumptions, we can derive the following differential equation (see the Supporting Information of detailed derivation).

also implies that the layer thickness at t = 160 min is ca. 30 μm. The formation and growth of the THF CH at the ice/hexane interface have thus been clearly detected with the ring breathing mode of THF and the change in the spectral pattern in the range of 2500−3700 cm−1 as the probes. Temperature Dependence of THF CH Growth. The thickness of the THF CH layer can be evaluated from the relative intensity at 912 and 917 cm−1 as discussed above. Figure 5 summarizes the time dependence of the thickness of

Figure 5. Time changes in the THF CH layer thickness at different temperatures. The broken curve represents the result of fitting based on eq 2 for the thickness data obtained at −2.0 °C. The thickness includes an uncertainty of ca. 5 μm.

the THF CH layer measured in this way with varying temperature. The formation of the THF CH layer was confirmed at −2.0 °C as discussed above; the THF CH layer thickness increases with time and appears to become almost constant at t > 100 min. In contrast, no hydrate layer was found at −10.0 °C even at t = 160 min. Further measurements were not conducted because it was difficult to avoid the evaporation of the organic phase for a long time period. At −5.0 °C, although a thin THF CH layer was detected at t = 160 min, the extensive formation was not observed. The initiation of the CH formation appears to take a long time even at −2.5 °C; the THF CH layer was not detected explicitly until t = 100 min. Thus, the induction time is required for the formation of the THF CH at temperatures slightly lower than −2.0 °C, while it is readily formed at −2.0 °C. Nada32 has suggested with a molecular dynamics simulation that the rate-determining step of the growth of the THF CH from an aqueous phase is the rearrangement of THF molecules on the surface of the CH crystal rather than the formation of the cage structure of water molecules. Typically, the nucleation of CHs requires the formation of prehydrates consisting of locally ordered water−guest clusters. Although a sufficient amount of water molecules is necessary to form such structures, the availability of water is limited on the surface of ice unlike the interface of liquid water. As noted already, the THF CH formation is related to the presence of the interfacial QLL. Although a number of studies of the QLL on the surface (at an ice/gas interface) have been reported,33−35 the information on the interfacial QLL has been very few. Our ice chromatographic work indicated that the liquid layer is developed at the interface between ice and hexane containing a small amount of THF.23 This interfacial QLL was detected by studying the temperature dependence of the retention of molecular probes. While the hydrogen-bonding adsorption of probe solutes on the ice surface explained their retention at the temperatures lower than

t

m THF ∫ F(t ) dt 0

ρ

=

Dc° F (t )

(1)

where mTHF, D, and c° are the molecular weight of THF, the diffusion coefficient of THF in the hydrate layer, and the concentration of THF in the organic phase, ρ is the density of THF in the hydrate layer, and F(t) is time-dependent flux of THF in the hydrate layer. The solution of this equation is

δ(t ) =

2m THFDc°t ρ

(2)

where δ(t) is the time-dependent thickness of the THF CH. The time dependence of the THF hydrate layer thickness was fitted by this equation as shown by the broken curve in Figure 5. The diffusion coefficient of THF in the THF hydrate layer was determined to be 4.7 × 10−14 m2 s−1, which is 4−5 orders of magnitude lower than the diffusion coefficient of THF in 13300

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water at the ambient temperature (1.1 × 10−9 m2 s−1 at the limiting dilution).40 The diffusion coefficient of THF in the THF CH layer is larger than the corresponding values determined for gas molecules in the corresponding CH layers, D = 7.4 × 10−16 m2 s−1 for CO210 and D = 4.1 × 10−16 m2 s−1 for Xe.41 Gulluru and Devlin have indicated that the ether CHs are formed with unexpected rapidness even at very low temperature, 120 K, and the enhanced rate is due to the defectfacilitated transport of ethers.42 This implies that the THF CH layer formed at the ice/hexane interface is not very dense and has a number of imperfect crystal surfaces that facilitate the diffusion of THF from the hexane phase to the reaction interface.

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CONCLUSION The formation of the THF CH at the interface between ice and hexane strongly depends on the existence of the interfacial QLL. This has first been verified by combined analyses of Raman microscopy and ice chromatography measurements. Although the former method cannot detect the presence of the interfacial QLL of several nanometers in thickness, the latter evaluates such a thin layer. In contrast, the THF CH layer is undoubtedly detected by Raman spectroscopy, while ice chromatography does not probe it. Combined applications of these methods obviously compensate for individual inadequacies. In general, measurements at the solid/liquid or liquid/ liquid interface are more difficult than those at gas interfaces. The present work demonstrates a significant example that a combined utilization of entirely different approaches can provide essential information that is never given by individual uses.

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ASSOCIATED CONTENT

S Supporting Information *

Schematic illustration of sample cell and Raman measurements and model for the growth of the THF CH layer. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: +81-3-5734-2612. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been in part supported by a Grant-in-Aid for the Scientific Research from the Japan Society for the Promotion of Science.

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REFERENCES

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dx.doi.org/10.1021/jp303713q | J. Phys. Chem. C 2012, 116, 13296−13301