Roles of Deeply Supercooled Ethanol in Crystallization and Solvation

Feb 9, 2008 - Ryutaro Souda , Jens Günster. The Journal of Chemical Physics 2008 129 (9), 094707. Article Options. PDF (99 KB) · PDF w/ Links (180 KB...
0 downloads 0 Views 99KB Size
Download PDF
J. Phys. Chem. B 2008, 112, 2649-2654

2649

Roles of Deeply Supercooled Ethanol in Crystallization and Solvation of LiI Ryutaro Souda† Nanoscale Materials Center, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ReceiVed: October 24, 2007; In Final Form: December 18, 2007

The mechanisms of glass-liquid transition and crystallization of amorphous solid ethanol were investigated through detailed analyses of the interaction with LiI using time-of-flight secondary ion mass spectrometry and reflection absorption infrared spectroscopy. The LiI species adsorbed on the surface are incorporated into the bulk of ethanol at temperatures higher than 100 K, concomitantly with the reorganization of the ethanol molecules at the surface. This behavior is explicable by self-diffusion of the ethanol molecules as a consequence of the glass-liquid transition. The resulting liquid is a distinct phase, as revealed from the similarity of the IR absorption band to that of amorphous solid ethanol rather than liquid ethanol. The liquidliquid phase transition occurs at 130 K, and a supercooled liquid ethanol is formed, as evidenced by formation of a metastable LiI solution when ethanol is deposited on the LiI film. The supercooled liquid ethanol is unstable, so that it crystallizes immediately at 130 K on the Ni(111) substrate. The film morphology changes continuously, even after crystallization, and the film abruptly becomes smoother before film evaporation. This behavior implies that crystallization is not completed and that a liquidlike phase coexists.

Introduction To date, a considerable research effort has been devoted to the study of the relaxation mechanism of a metastable amorphous solid into its equilibrium structure. The dynamics of the amorphous solid are nearly arrested below its glass transition temperature Tg. Some liquid phase should evolve at temperatures higher than Tg because molecular motions are activated. For fragile glass formers, such as water and alcohols, crystallization occurs abruptly following the glass-liquid transition. For that reason, very little is known about the properties of the liquidlike phase. It also remains unresolved whether the liquid phase in which crystallization occurs is a normal (supercooled) liquid or a distinct phase.1 We have been investigating the molecular motions in thin amorphous films of water, methanol, and ethanol using time-of-flight secondary ion mass spectrometry (TOFSIMS) as a function of temperature.2-4 Results showed that the glass-liquid transition is a two-step process, suggesting that two liquid phases might exist in the deeply supercooled region: the translational molecular diffusion commences at Tg; then the film morphology changes abruptly after some aging because of the delayed evolution of fluidity (the latter was observed at around T ) 1.2Tg to 1.3Tg in the temperatureprogrammed TOF-SIMS measurements). Further insights into the properties of these liquids might be obtained from observations of their interaction with electrolytes. Among various electrolytes, lithium halides are highly soluble in water and alcohols at room temperature. For that reason, the presence of the supercooled liquid might be confirmed if their solutions are formed in the deeply supercooled region. In fact, we have demonstrated the formation of aqueous lithium halide solutions at around 160 K concomitantly with crystallization when amorphous solid water is deposited on thin films of LiCl, LiBr, and LiI.5 Nevertheless, very little is known about the properties of polar, nonaqueous solvents such as alcohols in the deeply †

E-mail: [email protected].

supercooled region. In this paper, we describe the interaction of LiI with amorphous solid ethanol (ASE) as a function of temperature and discuss the mechanisms of its glass-liquid transition and crystallization. The surface composition of alkali-metal halide solutions has recently attracted considerable attention in terms of heterogeneous chemistry.6,7 In contrast to the traditional view that the ions are depleted from the surface, molecular dynamics (MD) simulations8,9 indicate that heavier, polarizable, halide ions tend to segregate to the surface of aqueous electrolyte solutions because of their imperfect hydration relative to that of the unpolarizable cations. In contrast, the computed density profiles of NaI in a methanol solution exhibited much less surface propensity of the iodide ion than that in an aqueous solution.10 Experimentally, the surface composition of CsI deposited on an amorphous solid methanol surface has been investigated in comparison to that on the amorphous solid water surface using metastable impact electron spectroscopy (MIES).11 It has been suggested that amorphous solid water and methanol reflect the properties of the respective liquids because good agreement is obtained between experimental results and simulations of the solutions. In reality, however, thermodynamic connections between the amorphous solid and normal liquid remain an open question because of the occurrence of crystallization; it has been revealed that the local order of amorphous solid water resembles that of crystalline water rather than that of liquid water.12 A previous study11 includes no discussion related to the properties of two liquids that might evolve successively by heating of the amorphous solids to temperatures higher than Tg. Here we investigate the interaction of LiI with ASE to clarify the roles of two liquids in the crystallization and solvation of LiI on the basis of the comparison of the experimental results between TOF-SIMS and reflection-absorption infrared spectroscopy (RAIRS) as a function of temperature.

10.1021/jp710263m CCC: $40.75 © 2008 American Chemical Society Published on Web 02/09/2008

2650 J. Phys. Chem. B, Vol. 112, No. 9, 2008

Souda

Experimental Section The TOF-SIMS experiments described in this paper were performed in an ultra-high-vacuum (UHV) chamber with a base pressure of less than 1 × 10-8 Pa. A He+ beam with a primary energy of 2.0 keV was extracted from a differentially pumped electron-impact-type ion source and was chopped with an electrostatic deflector. The pulsed ion beam was incident onto a sample surface, which was floated with a bias voltage of +500 V. Positive ions were extracted into a field-free TOF tube through a grounded stainless steel mesh that was placed 4 mm above the sample surface. The secondary ions were detected using a channel electron multiplier and were pulse-counted using a multichannel scaler (LN-6500R, Laboratory Equipment Inc.). The substrate was a Ni(111) surface that was heated to 1200 K by electron bombardment from behind. A thick LiI film was prepared using the following procedure: a continuous film of aqueous LiI solution was formed on the Ni(111) substrate in air by adsorption of micrometer-sized-particle mists that had been created using a nebulizer. After dehydration of the film by heating at around 200 °C, the sample was introduced into the UHV chamber through an interlock system, which was evacuated using oil-free pumps. The sample was finally annealed up to ca. 400 °C in UHV until no contaminants were detected in the TOF-SIMS spectra. Formation of a continuous LiI film was confirmed from the absence of a Ni+ signal sputtered from the substrate. The Ni(111) and LiI samples were cooled using a closed-cycle helium refrigerator. Thin films of ASE were deposited on these surfaces at 70 K by backfilling the UHV chamber with the C2H5OH molecules. Then LiI was evaporated in situ from a Ta basket placed immediately in front of the sample. The coverage of ASE (LiI) was determined from the evolution curves of sputtered ion intensities as a function of exposure (deposition time). One monolayer (1 ML) of ASE was attained by exposure of 5 langmuirs (1 langmuir ) 1.3 × 10-4 Pa s) of ethanol molecules. The TOF-SIMS spectra were taken continually every 30 s at a ramping speed of 5 K min-1. The RAIR spectra were collected in a separate UHV chamber (base pressure of 3 × 10-8 Pa) using a Bio-Rad FTS 40A spectrometer equipped with a liquid-nitrogen-cooled mercury cadmium telluride detector. The spectra were taken over the wave number range of 400-4000 cm-1 using a 4 cm-1 resolution. The LiI film was deposited on a gold substrate at room temperature by prolonged thermal evaporation from the LiI source placed in front of the surface. The sample was cooled using liquid nitrogen; then the ASE film was deposited on it at 90 K. The temperature was increased at the same ramping speed as that used for TOF-SIMS. Experimental Results For the TOF-SIMS analyses of the solvation and film morphology, it is important to clarify the probing depth that is obtainable using this technique. Figure 1 presents the intensities of typical secondary ions sputtered from the (a) Ni(111) and (b) LiI (2 ML)/Ni(111) surfaces as a function of the coverage of the ASE film. The exposure at which the C2H3+ intensity is saturated is assigned to 1 ML of ethanol. The saturation of the H+(C2H5OH) ion is delayed relative to that of the C2H3+ ion because the former is sputtered, provided that hydrogen bonds are created between the adsorbed ethanol molecules. The Ni+ ion is sputtered together with the Ni+(C2H5OH)n ions by adsorption of ethanol onto the Ni(111) surface. They exhibit a maximum in intensity at around 1 ML coverage and then decay almost exponentially with increasing coverage. No Ni+ ion is sputtered from the clean surface. The resemblance of the

Figure 1. Evolution of typical secondary ions sputtered from (a) Ni(111) and (b) LiI (2 ML) adsorbed Ni(111) surfaces as a function of the coverage of the C2H5OH molecules. Measurements were made at 70 K.

evolution curves between the Ni+ and Ni+(C2H5OH) ions indicates that they are formed via the same mechanism: ionization occurs during collisions of sputtered Ni atoms with the adsorbed C2H5OH molecules immediately before leaving the surface. The Li+ and Li+(LiI) ions are two of the main species sputtered from the adsorbed LiI. The Li+ ion is detectable through a considerably thick ASE film. The ion adducts Li+(C2H5OH)n are sputtered by adsorption of the C2H5OH molecules; they decay more steeply than the Li+ dose. It might be considered that SIMS is an extremely surface-sensitive technique, but this is not true as far as the Li+ and Ni+(C2H5OH)n ions are concerned; they are detectable from the ASE films as thick as 10 ML in the present experiment. Figure 2 portrays TOF-SIMS intensities of typical secondary ions sputtered from the LiI (1 ML) adsorbed ASE film (100 ML) as a function of temperature. The Li+ intensity decreases at temperatures higher than 100 K, concomitantly with the increase in the H+ and H+(C2H5OH) intensities. These behaviors can be attributed to self-diffusion of the ethanol molecules because ethanol’s Tg is 97 K.13 The LiI species is incorporated in the bulk as a result of the translational molecular diffusion following the solvation at the surface. The increase of the H+ and H+(C2H5OH) intensities at 100 K is independent of the disappearance of LiI from the surface because essentially similar evolution curves are obtained when the pure ASE film is deposited on the Ni(111) surface (not shown). This behavior is attributable to the reorganization of the ethanol molecules at the surface associated with the glass-liquid transition. The H+ and H+(C2H5OH) intensities decay abruptly at 130 K. At this temperature, the ASE film dewets the Ni(111) substrate, as evidenced by the evolution of the Ni+(C2H5OH) ion. After the occurrence of this “phase transition”, the film morphology changes gradually, as evidenced by the continuous increase in the Ni+(C2H5OH) intensity. It is noteworthy that the ion intensities drop sharply at 165 K. Probably, this phenomenon can be interpreted thusly: the droplets developed at temperatures of 130-165 K are transformed into a smoother layer before complete evaporation of the film. The morphological change

Ethanol Roles in LiI Crystallization and Solvation

Figure 2. Intensities of secondary ions sputtered from the LiI (1 ML) adsorbed C2H5OH film (100 ML) as a function of temperature. The film was deposited at 70 K; then the temperature was increased at a rate of 5 K min-1.

at 165 K occurs much more drastically than that at 130 K; in fact, the complete absence of the Ni+(C2H5OH) ion is observed for 165-170 K, when the measurements were made with a shorter time span (10 s, not shown). In this temperature range, the mean film thickness of ethanol becomes greater than 10 ML, as estimated from the results shown in Figure 1a. A depth of origin of the Li+ ion can also be discussed using the Li+ intensity relative to that after ethanol evaporation (Figure 2); the coverage vs Li+ intensity relation is shown in Figure 1b. Immediately after deposition at 70 K, LiI is located on the surface (mean depth of 0.6 ML of ethanol). Subsequently, the mean depth becomes 3.8 ML at 120 K after the uptake of LiI into the bulk. The smallest Li+ intensity is observed after the phase transition occurs at 130 K, which corresponds to the mean depth of 5.0 ML. The Li+ intensity increases above 130 K because of the morphological change of the film. The diffusion of LiI into the bulk might not be completed up to 130 K, as inferred from its shallower location than the original ASE film thickness (100 ML). Temperature-programmed TOF-SIMS measurements were made for the ASE film (100 ML) deposited on the thick LiI film as well; those results are presented in Figure 3. Similar to the pure ASE film deposited on the Ni(111) substrate, the H+ and H+(C2H5OH) intensities increase at 100 K because of the glass-liquid transition. However, the ion evolutions at higher temperatures differ greatly from those of the pure ASE film because of the interaction of ethanol with LiI. The Li+ ion is detected prior to the film dewetting at 130 K because of intermixing at the interface or the surface segregation of LiI. Diffusion of LiI into the bulk before the phase transition at 130 K is consistent with the result shown in Figure 2. At temperatures higher than 130 K, the H+(C2H5OH) intensity drops considerably relative to that in Figure 2 because the ethanol molecules forming the hydrogen bonds are reorganized to solvate the LiI species. The solvating ethanol molecules remain after evaporation of the free molecules at 170 K, as evidenced by the presence of Li+(C2H5OH) and Li+(C2H5OH)2 species. The Li+(LiI) intensity above 190 K indicates that LiI is

J. Phys. Chem. B, Vol. 112, No. 9, 2008 2651

Figure 3. Intensities of secondary ions sputtered from the C2H5OH film (100 ML) deposited on the thick LiI film as a function of temperature. The ASE film was deposited at 70 K; then the temperature was increased at a rate of 5 K min-1.

Figure 4. RAIR spectra of the OH and CH stretching regions of the C2H5OH molecules as a function of temperature. The 100 ML of ethanol molecules was deposited on the Au film at 90 K; then the temperature was increased at a rate of 5 K min-1.

desolvated gradually at the surface (within 2 ML; see Figure 1). The ethanol molecules are persistent at higher temperatures; complete desolvation is not attained up to 270 K. Figure 4 shows the IR absorption band of OH and CH stretching regions of ethanol (100 ML) deposited on Au as a

2652 J. Phys. Chem. B, Vol. 112, No. 9, 2008

Souda LiI film as a function of temperature. The OH stretching band is almost unchanged at temperatures below 130 K; then its leading edge shifts to a higher wavenumber side. The peak from the crystal (3250 cm-1) evolves slightly above 130 K. The blueshifted band, which is dominant at temperatures of 130-140 K, is assignable to a supercooled ethanol solution of LiI. The hydrogen bonds of liquid ethanol might be weakened relative to those of ASE and crystals, thereby causing the blue shift in the band. Liquid ethanol is known to self-associate highly via hydrogen bonding;15,16 two main bands are observed for liquid ethanol diluted in CCl4 at 3500 and 3370 cm-1 corresponding, respectively, to ethanol dimers and multimers, in addition to a narrow band characteristic of the free OH groups at 3634 cm-1. According to this interpretation, the blue-shifted band observed in this study is mainly attributable to the multimers in liquid ethanol. A narrow peak evolves from this band at 3400 cm-1 at temperatures higher than 142 K because of the formation of a crystalline ethanolate of LiI. This species remains in the vacuum at temperatures higher than 230 K after the disappearance of the crystalline ethanol (3250 cm-1) at around 165 K. The decay of both the H+ and H+(C2H5OH) intensities in TOFSIMS at around 142 K (see Figure 3) appears to be related to the formation of the crystalline ethanolate. Discussion

Figure 5. RAIR spectra of the OH and CH stretching regions of the C2H5OH molecules as a function of temperature. The 100 ML of ethanol molecules was deposited on the LiI film at 90 K; then the temperature was increased at a rate of 5 K min-1.

function of temperature. A broad OH stretching band (31003400 cm-1) is observed immediately after deposition at 90 K: two broad peaks centered at around 3290 and 3200 cm-1 are distinguishable. The band shape changes drastically at temperatures higher than 130 K: a main peak appears at 3250 cm-1, together with at least three subpeaks at 3350, 3290, and 3170 cm-1. The CH stretching band (2850-3000 cm-1) shape also changes above 130 K, and fine structures become apparent. Here we do not assign the origins of these peaks specifically, but the red shift and narrowing of the OH stretching band have been explained as crystallization.14 According to this interpretation, we conclude that the ASE crystallizes at 130 K in the present experiment. However, we must address the film crystallinity because the film morphology changes gradually after crystallization and a more drastic change occurs in morphology immediately before film evaporation. The crystal is the most stable form thermodynamically; therefore, this behavior is not expected if the film is crystallized completely. The morphological change of the film after crystallization is most likely to result from the coexisting liquidlike phase. Given that the main peak at 3250 cm-1 is attributable to crystals, the subpeaks might be assignable to the liquidlike phase. In fact, the OH stretching band at 165 K is apparently broadened because of the main peak reduction; the dominance of the liquidlike phase relative to the crystals might be responsible for the smoothing of the film immediately before evaporation of the film. Another noticeable point that is apparent in Figure 4 is that the IR absorption band is not changed before and after the glass-liquid transition at 97 K, indicating that the liquidlike phase formed at temperatures above Tg resembles ASE in its local structure. Figure 5 depicts the IR absorption band of the OH and CH stretching regions of ethanol (100 ML) deposited on the thick

The ASE film exhibits an apparently liquidlike nature at temperatures higher than Tg ) 97 K; the adsorbed LiI molecules dissolve in the bulk because of the self-diffusion of the ethanol molecules. The ASE surface structure also changes because of the glass-liquid transition, as revealed by the abrupt increase in the TOF-SIMS H+ and H+(C2H5OH) intensities, but the local hydrogen-bond structures of ethanol are fundamentally unchanged at Tg, as manifested by the invariance of the IR absorption band. This liquid might be assignable to a distinct phase, exhibiting translational diffusion without apparent fluidity (ultraviscous liquid). The phase transition occurs at 130 K, and the film crystallizes. However, the continuous change in the film morphology after crystallization strongly suggests that the liquidlike phase coexists with crystallites. The relation between the liquid phases before and after crystallization remains unclear, but we tentatively infer that they have the same properties. The viscous liquid formed after crystallization has much higher affinity for the crystal ethanol surface than the Ni(111) surface, thereby resulting in gradual dewetting or the viscous droplet formation around the crystal grains. This liquid has a considerable wettability on the Ni(111) surface as well, as evidenced by its spreading over the surface after disappearance of the crystal grains at 165 K. The liquid phase observed here appears to have a strong resemblance to thin liquid layers that are formed on the crystal surface during surface premelting.17 Both are thought to be strongly correlated liquids with a local structural similarity to crystals rather than normal liquid. Another liquid is probably created temporarily at 130 K immediately before crystallization occurs, as manifested by the preferential formation of the LiI solution on the LiI film (see Figure 4); the IR absorption band of this solution is distinct from that of the ultraviscous liquid and rather resembles that of liquid ethanol. The high solubility of LiI is characteristic of liquid ethanol. Therefore, we explain this phenomenon as dissolution of LiI into supercooled liquid ethanol. The supercooled liquid ethanol was not identified in a previous study14 because it crystallizes immediately when deposited on the metal substrate. Its presence is detected herein by quenching the

Ethanol Roles in LiI Crystallization and Solvation crystallization using the LiI substrate. Consequently, what occurs at 130 K is the transformation of ultraviscous liquid into supercooled liquid, which should be assigned to the liquidliquid phase transition (L-L transition). Crystallization occurs as a result of the spontaneous nucleation in supercooled liquid ethanol. To date, the presence of two liquids has been hypothesized for water on the basis of the fact that two amorphous solids exist at temperatures lower than Tg;1 according to this second critical point hypothesis, crystallization occurs directly from the distinct liquid phase (ultraviscous liquid) and the L-L transition occurs at higher pressure and temperature. Our result conflicts with this hypothesis because the L-L transition might occur after some aging time following the glass-liquid transition, even at ambient pressure.5 Probably, because spontaneous nucleation occurs immediately after the L-L transition, the occurrence of the supercooled liquid has not been identified; moreover, it has been misinterpreted as crystallization. Regarding the interaction of ethanol with LiI, both viscous and supercooled liquids appear to be good solvents of LiI. The LiI dissolves in the bulk of ultraviscous ethanol formed at temperatures of 100 K < T < 130 K (see Figures 1 and 2) without changing the local hydrogen-bond structures of the ethanol molecules, as confirmed from the invariance of the IR absorption band (see Figure 5). The concentrated LiI solution is formed at 130 K as a result of the L-L transition, which is responsible for stabilization of the liquid phase in the deeply supercooled region. On the other hand, dilute solutions might undergo phase separation into the concentrated solution and crystals, on the basis of analogous characteristics of the quenched aqueous lithium halide solutions.18,19 Probably, the incomplete diffusion of the adsorbed LiI species into the bulk of the thick ASE film, like that portrayed in Figure 2, is caused by the phase separation that occurs immediately after the L-L transition at 130 K. The LiI species are thought to be solvated dissociatively in normal (and supercooled) liquid ethanol, although the solvation structure of LiI in ultraviscous ethanol is unknown. No answer to this question can be offered from results of a previous study, but a comparison with the hydration behavior of LiI is instructive.5 Similar to the present study, there exist two liquid phases of water: ultraviscous water and supercooled liquid water. In contrast to the present result, however, it was shown that the adsorbed LiI remains on the surface of ultraviscous water; LiI is incorporated into the bulk after the evolution of supercooled liquid water. This result was explained in terms of the surfactant activity of LiI: the molecularly adsorbed LiI cannot enter the bulk of water because it includes both hydrophilic (Li+) and hydrophobic (I-) moieties. Consequently, the formation of a contact ion pair is thought to be responsible for their persistence on the surface of ultraviscous water, whereas LiI dissolves in the bulk of supercooled liquid water by the formation of a solvent-separated Li+-I- ion pair. From this analogy, the contact ion pair of LiI might be formed on the surface of ultraviscous ethanol. In contrast to water, however, molecular LiI is thought to be miscible with ethanol because both the solute and solvent are amphiphilic in nature. It is noteworthy that the crystallization (supercooled LiI solution) occurs in the bulk (at the interface) of supercooled liquid ethanol, but the evolution of the crystalline ethanol peak in Figure 5 is faint after the L-L transition even though a considerably thick ASE film (100 ML) is used. This result might not be achieved

J. Phys. Chem. B, Vol. 112, No. 9, 2008 2653 if the mixing between LiI and ethanol were limited to the interface, suggesting the high solubility of LiI in ultraviscous ethanol. According to the MD simulations,8-11 the polarizable iodide ion tends to segregate to the surface of aqueous solutions, but no surface propensity is recognized for the methanol solutions. Therefore, the fact that LiI segregates to the surface (diffuses into the bulk) of ultraviscous water (ultraviscous ethanol) appears to be consistent with the MD simulation. However, the results of the MD simulation for the normal liquid cannot be applied straightforwardly to the distinct phase. The LiI molecules might be solvated without ionic dissociation at the surfaces of ultraviscous water and ethanol. The miscibility arises from the amphiphilic nature of both ethanol and LiI molecules, whereas the surface propensity of LiI on ultraviscous water is attributable to the surfactant properties of the LiI molecule. Conclusion Combined TOF-SIMS and RAIRS analyses of the interactions between the LiI solute and ethanol solvent revealed the presence of at least two distinct liquid phases in the deeply supercooled region. The ultraviscous liquid is formed at around Tg ) 97 K, where LiI adsorbed on the surface is incorporated into the bulk and the ethanol molecules at the surface are reorganized. However, the local hydrogen-bond structures between ethanol molecules in the bulk are fundamentally unchanged before and after the glass-liquid transition. This liquid phase should be assigned to the distinct phase because its IR absorption band differs largely from that of normal liquid. Ethanol crystallizes at 130 K, but the film morphology changes continuously after the crystallization finishes. In addition, the film morphology changes drastically at around 165 K, immediately before the film’s evaporation. These behaviors are explainable by the coexistence of the liquidlike phase with the crystallites. The crystallization is quenched and the supercooled LiI solution is formed at 130 K when the ASE film is deposited on the thick LiI film. This result strongly suggests that supercooled liquid ethanol is formed temporarily prior to crystallization. Therefore, the phase transition at 130 K should be assigned to the liquidliquid phase transition. Crystallization occurs as a result of spontaneous nucleation in supercooled liquid ethanol. The solvation structure of LiI in the bulk of supercooled liquid ethanol differs from that of ultraviscous ethanol; the solventseparated ion pair can be formed in the supercooled liquid, whereas the contact ion pair is thought to be the main species in the ultraviscous liquid. The amphiphilicity of both the solute and solvent are thought to be responsible for the high solubility of LiI in ultraviscous ethanol. References and Notes (1) Mishima, O.; Stanley, H. E. Nature 1998, 396, 329. (2) Souda, R. Phys. ReV. Lett. 2004, 93, 235502. (3) Souda, R. Phys. ReV. B 2005, 72, 115414. (4) Souda, R. J. Chem. Phys. 2005, 122, 134711. (5) Souda, R. J. Chem. Phys. 2007, 127, 214505. (6) Oum, K. W.; Lakin, M. J.; DeHaan, D. O.; Brauers, T.; FinlaysonPitts, B. J. Science 1998, 279, 74. (7) Knipping, E. M.; Lakin, M. J.; Foster, K. L.; Jungwirth, P.; Tobias, D. J.; Gerber, R. B.; Dabdub, D.; Finlayson-Pitts, B. J. Science 2000, 288, 301. (8) Jungwirth, P.; Tobias, D. J. J. Phys. Chem. B 2000, 104, 7702. (9) Jungwirth, P.; Tobias, D. J. J. Phys. Chem. B 2002, 106, 6361. (10) Dang, L. X. J. Phys. Chem. A 2004, 108, 9014. (11) Hofft, O.; Borodin, A.; Kahnert, U.; Kempter, V.; Dang, L. X.; Jungwirth, P. J. Phys. Chem. B 2006, 110, 11971. (12) Finney, J. L.; Hallbrucker, A.; Kohl, I.; Soper, A. K.; Bowron, D. T. Phys. ReV. Lett. 2002, 88, 225503.

2654 J. Phys. Chem. B, Vol. 112, No. 9, 2008 (13) Sugisaki, M.; Suga, H.; Seki, S. Bull. Chem. Soc. Jpn. 1968, 41, 2586. (14) Ayotte, P.; Smith, R. S.; Teeter, G.; Dohnalek, Z.; Kimmel, G. A.; Kay, B. D. Phys. ReV. Lett. 2002, 88, 245505. (15) Luck, W. A.; Schrems, O. J. Mol. Struct. 1980, 60, 333.

Souda (16) 19268. (17) (18) (19)

Schwager, F.; Marand, E.; Davis, R. M. J. Phys. Chem. 1996, 100, Souda, R. To be published. Angell, C. A.; Sare, E. J. J. Chem. Phys. 1968, 49, 4713. Suzuki, Y.; Mishima, O. Phys. ReV. Lett. 2000, 85, 1322.