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J. Phys. Chem. B 2007, 111, 14130-14135
Raman Spectroscopic Investigations of Pressure-Induced Phase Transitions in n-Hexane G. Kavitha and Chandrabhas Narayana* Light Scattering Laboratory, Chemistry and Physics of Material Unit, Jawaharlal Nehru Centre for AdVanced Scientific Research, Jakkur, Bangalore 560 064, India ReceiVed: July 3, 2007; In Final Form: September 7, 2007
We report high-pressure Raman studies on n-hexane up to 16 GPa. n-Hexane undergoes solid-solid transition around 9.1 GPa along with an already reported liquid-solid transition around 1.4 GPa. The intensity ratio of the Raman modes relating the all-trans conformation (1147 and 2872 cm-1) to that of the gauche conformation (1074 and 2923 cm-1) shows a sudden change across 9.1 GPa, suggesting an increase in the all-trans population conformers above 9.1 GPa. The disappearance of the torsional modes suggests a steric hindrance to the methyl end group, similar to the n-heptane case, suggesting that the high-pressure phase (above 9.1 GPa) is an orientationally disordered phase. In general, the transition pressure for the solid-solid transition is inversely proportional to the length of the carbon backbone in the medium chain length n-alkanes.
1. Introduction Structural transitions of short and medium chain simple saturated hydrocarbons are extensively studied for their chemical stability. It is important to know the phase behavior and conformational equilibrium of simple saturated hydrocarbons due to their applications. The electron mobilities of methane and ethane differ by a factor of 500 even though the difference in the number of atoms is only 3 (one carbon and two hydrogen).1 The excess electron mobility in hydrocarbons depends on its structure.2 This mobility and chain-ordering of the hydrocarbons are useful in examining the perturbation effects of cholesterol or proteins when present within the lipid layer.3 To understand the two-dimensional ordering4-6 and dynamic processes,7-9 one can use the alkanes on metal as a model system. This has a variety of applications in adhesion, lubrication, molecular electronics, and catalytic surface reaction on platinum.10-13 n-Hexane and n-pentane account for most of the normal paraffins found in petroleum. High-pressure studies of n-hexane and n-pentane would then give an insight into the environment of petroleum deep inside the earth.14 it would also help in understanding the generation and retention of petroleum in geological structure. Other applications of n-hexane are as an extracting agent, a solvent, and a reaction medium in food chemistry.15-19 In general, the n-alkanes exhibit different kinds of conformational isomers in the liquid state. Their conformational change with high pressure and in the solid phase is essential in understanding the variety of phenomena found in polymer science and biology.20,21 Raman spectroscopy is a nondestructive tool for probing conformational changes of molecules in condensed phases. Raman spectroscopy is also ideally suited for structural investigation, and it probes an ideal wavelength region in the electromagnetic spectrum. Certain Raman modes of lipid bilayer systems are predominantly sensitive to hydrocarbon chain conformation.22 Alkane chain conformation is relevant to know about the precise structure of the lipid bilayer. High-pressure Raman spectroscopy can probe the conformational change in simple hydrocarbons. This makes high-pressure * Corresponding author. E-mail:
[email protected].
Raman studies on n-hexane important. n-Hexane undergoes a liquid to solid transition around 1.4 GPa and has been reported earlier.23 In this paper, we have focused on the solid-solid transition of n-hexane upon application of high pressure. The high-pressure Raman experiments on n-hexane have been carried out up to 16 GPa. On the basis of the Raman mode behavior, we observed a solid-solid phase transition around 9.1 GPa and the nature of this transition has been described in this paper. 2. Experimental Details The spectroscopic grade n-hexane was obtained from SigmaAldrich. This was used for the high-pressure Raman experiment without further purifications. The high-pressure cell used in this experiment is based on the Mao-Bell diamond anvil cell (DAC)24 having two low fluorescent diamonds with 500 µm culet size. A pre-indented stainless steel gasket was drilled to have a 200 µm hole at the center to load the n-hexane along with a small ruby chip (CH2 groups, is also triclinic. n-Hexane crystal structure is similar to that of n-heptane.36,37 3.1. LAM (C-C-C Angle Bending Mode). The lowfrequency Raman modes depend on the number of molecules per unit cell. Since the even paraffin starting from C6H14 to
14132 J. Phys. Chem. B, Vol. 111, No. 51, 2007
Figure 3. Raman modes of skeletal C-C stretching modes: (a) TTG + TGT and (b) TTT + TGT conformers as a function of pressure. The solid lines are the linear fit to the data.
Figure 4. Raman modes of (a) asymmetric >CH2 and -CH3 bending, (b) >CH2 rocking (TTT), and (c) -CH3 deformation (TTT + TTG) and >CH2 scissoring (TGT) as a function of pressure. The solid lines are the linear fit to the data.
C24H54 has only one molecule per unit cell, they have the smallest number of Raman bands in the low-frequency region.36 The low-frequency modes are very weak in the liquid phase in the case of both Raman and IR spectra. This coupled with a huge Rayleigh background made it difficult to observe any modes below 300 cm-1. The bending and torsional modes appear in the region of 200-600 cm-1 which overlaps with LAM at 328 cm-1 and other external modes. We observe only
Kavitha and Narayana
Figure 5. (a) (CH2)n and (b) -CH3 stretching modes as a function of pressure. The solid lines are the linear fit to the data.
the LAM (328 cm-1) in our experiment. This mode is called an accordion mode due to its in-phase expansion and contraction of the C-C-C bend angle.38,39 The frequency of this mode is inversely proportional to the length of the chain.40 In the case of n-hexane, this mode is related to GTT conformation (where G stands for gauche and T stands for trans).28-30 As shown in Figure 2a, there is an abrupt change in the frequency of this mode around 1.4 GPa. This is due to the liquid-solid transition reported earlier.23 This mode disappears beyond 9.1 GPa. This could signify the onset of a phase transition and will be discussed later. 3.2. Methyl Rocking Mode. The well-resolved mode at 908 cm-1 is associated with a single gauche conformation (GTT).28-30 Figure 2b shows the pressure dependence of this mode. This mode is related to the -CH3 end group and, hence, has an importance in chain packing and conformational randomness in the solid phase.41 There is a considerable change near the crystallization pressure at 1.4 GPa, as shown in Figure 2 and Table 1. Beyond the crystallization, this mode has a positive dω/dP. At higher pressure the intensity reduces and gradually disappears beyond 9.1 GPa. This could be an onset of a phase transition and is discussed later. 3.3. Skeletal C-C Stretching Region (1050-1200 cm-1). In the condensed state the chain packing is reflected by the carbon backbone skeletal stretching mode behavior as a function of pressure. Figure 3 shows the pressure dependence of these modes. These modes occupy the spectral region of 1050-1200 cm-1. In the case of n-hexane, the mode at 1075 cm-1 has contribution from both skeletal C-C stretching TTG and TGT isomers. The mode at 1146 cm-1 originates from -CH3 rocking (TTT) and C-C stretching (TGT) conformers.29,30 These modes become sharp and distinguishable beyond 1.4 GPa as a result of periodic arrangement of the molecules upon crystallization. Beyond this pressure both these modes (1075 and 1146 cm-1) show a positive dω/dP up to 9.1 GPa. As shown in Figure 3a, the mode which has only gauche conformers, namely, 1075 cm-1 mode, disappears beyond 9.1 GPa in comparison with the mode associated with the trans conformers (1146 cm-1 mode),
Pressure-Induced Phase Transitions in n-Hexane
J. Phys. Chem. B, Vol. 111, No. 51, 2007 14133
TABLE 1: Frequency (ω) of the Raman Modes of n-Hexane and Its Pressure Derivative (dω/dP) Observed in Different Phasesa phase I (1 GPa)
phase II (>1.4 GPa)
ω (cm-1)
dω/dP
ω (cm-1)
dω/dP
328 908 1075 1146 1445 1446 1465 2872 2902 2923 2946
3.3 7.5 4.8 3.6 3.8 0.2 4.4 4.0 9.9 2.6 3.1
2992
20.0
335 915 1081 1152 1436 1453 1474 2880 2913 2929 2952 2977 2993
5.5 1.8 3.5 2.9 4.7 3.9 1.8 10.4 8.5 8.4 8.3 11.3 11.8
a
phase III (>9.1 GPa) ω (cm-1)
dω/dP
1173 1472 1482 1485 2964 2968 3008 3020
2.8 2.8 2.3 1.9 5.9 5.8 6.0 5.2
3069
4.3
mode assignment LAM C-C-C angle bending methyl rocking mode (GTT) skeletal C-C stretching (TTG + TGT) CH3 rocking (TTT) + C-C stretching (TGT) asymmetric CH2 and CH3 bending CH2 rocking (TTT) CH3 deformation (TTT + TTG) and CH2 scissoring (TGT) symmetric (CH2)n stretching (TTT) symmetric (CH2)n stretching (TTT) symmetric (CH2)n stretching (TGT) in-plane asymmetric methyl stretching (TTT) Out-of-plane asymmetric methyl stretching (GTT) asymmetric C-H methyl stretching (TTT)
Their mode assignments are given on the basis of refs 27-35.
Figure 6. Intensity ratios (a) I1146/I1075 and (b) I2872/I2923 of trans and gauche conformational isomers of n-hexane as a function of pressure. The solid lines are the linear fits to the data.
which persists up to 16 GPa, as shown in Figure 3b. It is relevant to know about the intensity variation of these modes with pressure since it reflects the population of trans and gauche conformers in n-hexane in any phase. Figure 6a shows the intensity ratio of the 1146 and 1075 cm-1 mode. The intensity ratio (I1146/I1075) suggests an increase in all-trans conformers withsomegaucheconformersstillpersistingabovecrystallization.32-35 At 9.1 GPa, the gauche conformers cease to exist. This point and its implication on the possible phase transition will be discussed later. 3.4. Methylene and Methyl Bending Region (1400-1500 cm-1). The >CH2 and -CH3 bending mode region in n-alkane is around 1400-1500 cm-1, and Figure 4 shows its pressure dependence. In the case of n-hexane, the modes at 1445, 1446, and 1465 cm-1 are associated with asymmetric >CH2 and -CH3 bending, >CH2 rocking (TTT) and the contribution from -CH3 deformation (TTT + TTG), and >CH2 scissoring (TGT), respectively.29,30 These modes show a strong dependence on pressure and have a large positive dω/dP, as shown in Table 1. These modes persist up to the highest pressure but show a
change in slope at 9.1 GPa, suggesting a possible phase transition. The behavior of the bending modes of n-hexane is similar to that observed in the case of n-heptane reported in our earlier work.34 3.5. Polymethylene and Methyl Stretching Region. The C-H stretching region is influenced by large intramolecular interaction and interaction between symmetric C-H stretching fundamental and the overtone of the >CH2 scissor mode. This bunch of modes around 2800-3100 cm-1 plays a key role in the conformational chain packing at higher pressure. These modes have been classified into two categories:29,30,36 symmetric methylene stretching region comprising three bands ((a) sharp band at 2872 cm-1 (TTT), (b) broad band at 2902 cm-1 (TTT), and (c) a shoulder near 2923 cm-1 (TGT)); methyl stretching region involving another three modes ((a) in-plane asymmetric methyl stretching (TTT) at 2946 cm-1, (b) out-of-plane asymmetric methyl stretching (GTT) at 2977 cm-1, and (c) asymmetric C-H stretching of CH3 (TTT) at 2992 cm-1). Figure 5 shows the pressure dependence of these modes, and their dω/ dP values are given in Table 1. Large changes around 1.4 GPa are a result of the liquid-solid transition.23 There is a considerable mode broadening as well as changes in their intensity ratio around 9.1 GPa as shown in Figure 6b.42 This observation suggest a possible phase transition in n-hexane around 9.1 GPa. As is true in n-alkanes, upon crystallization the n-alkanes retain some amount of gauche conformers. In the case of n-hexane, as in n-heptane34 and n-pentane43 in the solid phase, we observe the presence of gauche conformers. This is confirmed by appearance of the 2977 cm-1 (GTT) mode beyond solidification. This mode is not well-resolved in the liquid phase of n-hexane. (See Figure 1c and the Supporting Informtaion) Above 9.1 GPa we observe an increase in the trans conformer, since the I2872/ I2923 (TTT/TGT) ratio shows a steep increase, as shown in Figure 6b. 4. Discussion Several dynamic mixtures of distinct conformations including straight chain all-trans isomer and kinked chain conformers with up to three gauche bonds will exist in pure liquid n-hexane. The conformers contributions to the n-hexane at ambient conditions are approximately 30%, TTT; 35%, TTG; 15%, TGT; 15%, GGT; and 5%, GGG.28,29 Among these conformers, TTT, TTG, and TGT conformers have strong features in Raman spectra. Therefore these conformers have been studied in detail in this paper. Detailed analysis of the peak position, intensity, and band shape can also reveal structure and conformational information. In the liquid phase the high-frequency region of
14134 J. Phys. Chem. B, Vol. 111, No. 51, 2007 the Raman spectrum, Figure 1c, shows a featureless, indistinguishable broad spectrum up to 1.4 GPa. The splitting of the modes above 1.4 GPa is consistent with increased molecular order in the solid phase compared to the liquid state44,45 Application of hydrostatic pressure to the molecular system causes a change in the interatomic distances, but lowering the temperature can have both reduction in molecular motion as well as changes in interatomic distances. It is evident from Figures 2-5 that the Raman modes gradually shifted to higher frequencies with increasing pressure, suggesting a reduction in bond lengths and atomic distances with pressure. The LAM (328 cm-1) and the -CH3 rocking mode (908 cm-1) can give insight into the intermolecular interaction at higher pressures.38,39,41 Above the liquid to solid transition at 1.4 GPa,23 these modes show a positive dω/dP (see Table 1). It is interesting to note that both LAM and -CH3 rocking modes disappear beyond 9.1 GPa. This suggests that at higher pressures n-hexane tries to be in all-trans conformation for achieving denser packing. This leads to two things: (a) the LAM mode cannot be executed due to steric hinderances and (b) -CH3 end group rotation is dampened. The gauche conformer at 1074 cm-1 and the all-trans conformer at 1147 cm-1 show very large pressure dependences in the solid phase as in the case of previously reported highpressure studies on n-heptane34 and n-pentane.43 This could be due to dense packing of n-hexane in the condensed state with pressure. It is interesting to see that, above 9.1 GPa, the 1074 cm-1 mode disappears, suggesting the conversion of end gauche to all-trans conformation beyond 9.1 GPa. The plot in Figure 6a corroborates this point.29 The bending modes of methyl and methylene groups in n-alkanes are related to in-chain configurations and molecular interactions.46 Figure 4 shows the pressure dependence of this mode for n-hexane. The mode behavior exhibited in the case of n-hexane is similar to that for n-heptane;34 this suggests that the high-pressure phases of n-hexane and n-heptane are similar. It is important to note that, beyond 9.1 GPa, the modes exhibit a smaller dω/dP value. This could be due to denser packing achieved in the high-pressure phase (>9.1 GPa) by an all-trans conformer similar to n-heptane.34 This also suggests that, as in n-heptane, phase II and phase III are structurally very similar. We now discuss the polymethylene stretching modes shown in Figure 5. These C-H stretch vibrations of hydrocarbons carry information on molecular conformation;23,31,46 these modes have the highest pressure derivative (dω/dP) values compared to the other bands, as shown in Table 1. This suggests that upon applications of pressure there is a large effect on the conformation of n-hexane (n-alkane in general).34 These modes in condensed phase are characterized by at least six individual sharp peaks, where four (2872, 2902, 2946, and 2992 cm-1) are associated with all-trans conformation and the other two (2923 and 2977 cm-1) modes are associated with single gauche conformation.30 It is interesting to note that the intensity of the 2946 cm-1 (TTT) mode increases steeply in comparison to other modes at high pressures beyond 9.1 GPa. The gauche modes decrease in intensity and cease to exist beyond 9.1 GPa. The quantitative intensity ratio plot in Figure 6b also suggests the same. In the high-pressure phase III, n-hexane has an all-trans conformation which is similar to that of n-heptane.34 This is different from what we observe in the case of n-pentane, where a gauche conformer is still present in phase III.43 The disappearance of the methyl rocking mode suggests random freezing of the methyl end group in the dense state of n-hexane above
Kavitha and Narayana 9.1 GPa, as in the case of n-heptane.34 Hence, we suggest that phase III is an orientationally disordered phase. In summary, high-pressure Raman spectroscopic studies on linear chain n-hexane was carried out up to 16 GPa. We observe a solid-solid transition at 9.1 GPa along with the earlier reported liquid-solid transition at 1.4 GPa. The n-hexane Raman mode behavior suggests that the high-pressure phase has all-trans configuration. Due to random freezing of the -CH3 end group, the high-pressure phase is an orientationally disordered phase. The behavior of n-hexane is similar to the n-heptane.34 Out of the three midchain alkanes, n-pentane43 shows a different behavior due to its small sizes. It is also seen that the solidsolid transition pressure is inversely proportional to chain length. Acknowledgment. C.N. thanks Prof. C. N. R. Rao, Jawaharlal Nehru Centre for Advanced Scientific Research and Department of Science and Technology, for financial support. Supporting Information Available: Raman spectra of the polymethylene and methyl stretching vibrational region of n-hexane in liquid and solid phases. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Allen, A. O. National Standard Reference Data System, Vol. 58 National Bureau of Standards, U.S. Government Printing Office: Washington, D.C., 1976. Nishikawa, M. Electrons in Condensed Media. In CRC Handbook of Radiation Chemistry; Tabata, Y., et al., Eds.; CRC Press: Boca Raton, FL, 1991; Chapter 5. (2) Kengo, I.; Nishikawa, M.; Richard, A. H. J. Chem. Phys. 1996, 104, 1545. (3) David, A. P; Trevor, J. G.; Dennis, C. Biochemistry. 1980, 19, 349. (4) Uosaki, K.; Yamada, R. J. Am. Chem. Soc. 1999, 121, 4090. (5) Marchenko, A.; Cousty, J.; Van, L. P. Langmuir 2002, 18, 1171. (6) Marchenko, O.; Cousty, J. Phys. Rev. Lett. 2000, 84, 5363. (7) Weckesser, J.; Barth, J. V.; Kern, K. J. Chem. Phys. 1999, 110, 5351. (8) Fichthorn, K. A.; Miron, R. A. Phys. Rev. Lett. 2002, 89, 196103. (9) Paserba, K. R.; Gellman, A. J. Phys. Rev. Lett. 2001, 86, 4338. (10) Jortner, J., Ratner, M., Eds. Molecular Electronics, Chemistry for the 21st Centry; Blackwell: Oxford, U.K., 1997. (11) Bhushan, B.; Israelachvili, J. N.; Landman, U. Nature 1995, 374, 607. (12) Bhushan, B., Ed. Handbook of Micro/nano Tribology, 2nd ed.; CRC Press: Boca Raton, FL, 1999; Vol. 1. (13) Yang, M.; Somorjai, G. A. J. Am. Chem. Soc. 2004, 126, 7698. (14) Qiao, E.; Zheng, H. Appl. Spectrosc. 2005, 59, 650. (15) Edmundo, C.; Valerie, C. D.; Alain, M. J. Am. Oil Chem. Soc. 1998, 75, 309. (16) Soumanou, M. M.; Bornscheuer, U. T. Eur. J. Lipid Sci. Technol. 2003, 105, 656. (17) Prycek, J.; Ciganek, M.; Simek, Z. J. Chromatogr. 2004, A1030, 103. (18) Ramadan, M. F.; Morsel, J. T. Photochem. Anal. 2003, 14, 366. (19) Komprda, T.; Zelenka, J.; Fajmonova, E.; Bakaj, P.; Pechova, P. J. Agric. Food. Chem. 2003, 51, 7692. (20) Snyder, R. G.; Scherer, J. R.; Gaber, B. P. Biochim. Biophys. Acta 1980, 601, 47. (21) Wallach, D. F. H.; Verma, S. P.; Fookson, J. Biochim. Biophys. Acta 1979, 559, 153. (22) Snyder, R. G.; Cameron, D. G.; Casal, H. L.; Compton, D. A. C.; Mantsch, H. H. Biochim. Biophys. Acta 1982, 684, 111. (23) Huai, W.; Haifei, Z.; Qiang, S. Appl. Spectrosc. 2005, 59, 1498. (24) Mao, H. K.; Bell, P. M. Year Book-Carnegie Institution of Washington; Carnegie Institution of Washington: Washington, D.C., 1978; Vol. 77, p 904. (25) Mao, H. K.; Xu, J.; Bell, P. M. J. Geophys. Res., [Solid Earth Planets] 1986, 91, 4673. (26) Pavan Kumar, G. V.; Ashok Reddy, B. A.; Arif, Md.; Kundu, T. K.; Chandrabhas, N. J. Phys. Chem. B 2006, 110, 16787. (27) Hibbon, J. H. Chem. ReV. 1935, 18, 11. (28) Patrick, T. T. W.; Mantsch, H. H. J. Chem. Phys. 1983, 79, 2369. (29) Huang, Y.; Wang, H. Langmuir 2003, 19, 9706. (30) Snyder, R. G. J. Chem. Phys. 1967, 47, 1316. (31) Snyder, R. G.; Scherer, J. R. J. Chem. Phys. 1979, 71, 3221.
Pressure-Induced Phase Transitions in n-Hexane (32) Yamaguchi, M.; Serafin, S. V.; Morton, T. H.; Chronister, E. L. J. Phys. Chem. B 2003, 107, 2815. (33) Schoen, P. E.; Priest, R. G.; Sheridan, J. P.; Schnur, J. M. J. Chem. Phys. 1979, 71, 317. (34) Kavitha, G.; Narayana, C. J. Phys. Chem. B 2006, 110, 8777. (35) Kint, S.; Scherer, J. R.; Snyder, R. G. J. Chem. Phys. 1980, 73, 2599. (36) Heinz, G.; Fanconi, B. J. Chem. Phys. 1973, 59, 534. (37) Norman, N.; Mathisen, H. Acta Crystallogr. 1960, 13, 1043. (38) Braden, D. A.; Parker, B. S.; Hudson, B. S. J. Chem. Phys. 1999, 111, 429.
J. Phys. Chem. B, Vol. 111, No. 51, 2007 14135 (39) Logan, K. W.; Danner, H. R.; Gault, J. D.; Kim, H. J. Chem. Phys. 1973, 59, 2305. (40) Snyder, R. G.; Strauss, H. L.; Alamo, R.; Mandelkern, L. J. Chem. Phys. 1994, 100, 5422. (41) Brown, K. G.; Bicknell-Brown, E.; Ladjadj, M. J. Phys. Chem. 1987, 91, 3436. (42) Snyder, R. G. J. Chem. Phys. 1982, 76, 3342. (43) Kavitha, G.; Chandrabhas, N. J. Phys. Chem. B 2007, 111, 7003. (44) Sheppard, N.; Szasz, G. J. J. Chem. Phys. 1949, 17, 86. (45) Mizushima, S.; Simanouti, T. J. Am. Chem. Soc. 1949, 71, 1320. (46) Cann, P. M.; Spikes, H. A. Tribol. Lett. 2005, 19, 289.
