Rotator Phases of n-Heptane under High ... - American Chemical Society

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Rotator Phases of n-Heptane under High Pressure: Raman Scattering and X-ray Diffraction Studies Chunli Ma, Qiang Zhou, Fangfei Li, Jian Hao, Jingshu Wang, Liyin Huang, Fengxian Huang, and Qiliang Cui* State Key Laboratory of Superhard Materials, Jilin University, Changchun, 130012, People's Republic of China ABSTRACT: We performed high-pressure Raman scattering and angle-dispersive synchrotron X-ray diffraction measurements on n-heptane at room temperature. It has been found that n-heptane undergoes a liquid to rotator phase III (RIII) transition at 1.2 GPa and then transforms into another rotator phase RIV at about 3 GPa. As the pressure reaches 7.5 GPa, a transition from an orientationally disordered RIV phase to an ordered crystalline state starts and is completed around 14.5 GPa. Our results clearly present the high-pressure phase transition sequence (liquid RIII RIV crystal) of n-heptane, similar to that of normal alkanes.

1. INTRODUCTION The normal alkanes comprised of linear chains of saturated hydrocarbons are one of the most common elements in complex molecular series for the formation of membranes, micelles, selfassembled monolayers, molten salts, and biomarkers.1 4 In addition, the normal alkanes are important components of petroleum and play a key role in its derivative products such as fuels and lubricants.5 Therefore, the study of alkane molecules is of fundamental interest in understanding the physical and chemical properties of the complex molecules and is of practical interest for oil and gas exploration in the deep earth. High-pressure investigations are eligible to comprehensively understand the phase behavior of the normal alkanes. As a member of the alkane family, n-heptane is a straight-chain alkane with the chemical formula H3C(CH2)5CH3 or C7H16. The phase transition of n-heptane has recently been studied under high pressure at room temperature. The high-pressure infrared (IR) spectra of n-heptane up to 7 GPa demonstrated a liquid to solid transition between 1.2 and 1.5 GPa and a solid to solid transition around 3 GPa.6 In the high-pressure Raman scattering study, the authors showed that the liquid to solid transition occurred around 1.2 GPa and the solid to solid transition emerged at 7.5 GPa.7 Such a liquid to solid transition had also been predicted by the highpressure molecular dynamic (MD) simulations,8 but the solid to solid transition was not expected up to 7 GPa in this calculation. Therefore, there is still debate about the high-pressure phase transition of n-heptane so far. In this work, we report a more extensive study of the phase transition of n-heptane at room temperature using both highpressure (up to 20.78 GPa) Raman scattering and angle-dispersive synchrotron X-ray diffraction (ADXRD) techniques. The analyses of the ADXRD spectra allow us to reveal two rotator phases (R) of n-heptane under high pressure: RIII (triclinic symmetry) at 1.2 GPa and RIV (monoclinic symmetry) around 3 GPa. Furthermore, a phase transition to the crystalline n-heptane (orthorhombic symmetry) r 2011 American Chemical Society

has also been found when the pressure is in the range between about 7.5 and 14.5 GPa. Our work makes clear the pressureinduced phase changes of n-heptane, which will advance our fundamental understanding of the structural, physical, and chemical properties of alkane molecules.

2. EXPERIMENT The n-heptane samples were bought from Sigma Aldrich (purity of 99%) without further purification. High-pressure Raman scattering and ADXRD measurements were carried out under high pressure up to 20.78 GPa at room temperature. The sample together with a chip of ruby was loaded into a gasketed high-pressure diamond anvil cell (DAC) with a 400 μm culet face diameter. A 120 μm hole in diameter was drilled at the center of a 90-μm-thick preindented T301 stainless steel gasket to form a sample chamber. The pressure was calibrated by the frequency shift of the ruby R1 fluorescence line with a precision of ∼0.02 GPa.9 By monitoring the separation and the width of the ruby R1 and R2 lines, we confirmed that our experiments were performed under the quasi-hydrostatic condition. The high-pressure Raman spectra were recorded in a backscattering geometry using Acton SpectraPro 500i spectrometer with a liquid nitrogen-cooled CCD camera (500 mm focal length). The 532 nm excitation light with an output power of 400 mW was generated by a frequency-doubled diode-pumped neodymium:vanadate laser (Coherent Co.).10 Note that the Raman spectra of n-heptane are independent of the output power. The acquisition time of each spectrum is 60 s for the liquid n-heptane and is 30 s for the solid phase. The high-pressure ADXRD experiments were carried out at the beamline X17C of the National Synchrotron Light Source, Received: December 28, 2010 Revised: August 4, 2011 Published: August 09, 2011 18310

dx.doi.org/10.1021/jp112333m | J. Phys. Chem. C 2011, 115, 18310–18315

The Journal of Physical Chemistry C

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Figure 1. Room-temperature Raman spectra of n-heptane under pressures up to 20.78 GPa at three separated regions of Raman shift (a c). New Raman peaks in the low-frequency region in a at 1.2 GPa are marked by pound signs (#) and 3 GPa by asterisks (*).

Brookhaven National Laboratory. The monochromatic beam wavelength was 0.4066 Å. The diffraction data were collected using a MAR165 CCD detector. We analyzed the two-dimensional X-ray diffraction images using the FIT2D software and yielded one-dimensional intensity versus diffraction angle 2θ patterns.11 The average acquisition time of the spectrum was 400 s. The sample detector distance and geometric parameters were calibrated using a CeO2 standard (National Institute of Standards and Technology, Gaithersburg, MD). The XRD patterns at different pressures were simulated and analyzed by Material Studio software.

3. RESULTS AND DISCUSSION The dependence of Raman spectra with increasing pressure up to 20.78 GPa are shown in Figure 1. The ambient-pressure Raman spectrum of liquid n-heptane just agree well with that reported by Mizushima.12 The assignment of our Raman vibrational modes are exhibited in Table 1.6,13,14 Shown in Figure 1 are illustrative examples of the n-heptane Raman spectra change at 1.2 GPa, which obviously indicates that n-heptane undergoes a phase transition. In previous studies, this phase transition has been defined as a liquid to solid transition.6 8 However, according to the phase transition sequence of normal alkanes,15 in this paper, the phase transition around 1.2 GPa is studied with new explanations. It has been known that the application of pressure increases the population of gauche conformers in liquid short-chain alkanes (the maximum number of carbons is 16) rather than the trans conformers that represented the lowest energy for isolated molecules. Moreover, due to several intramolecular degrees of freedom that exist in liquid normal alkanes, increasing the pressure will perturb the distribution of molecular conformations.6,16 For longer chains, in spite of the energy advantage of alltrans conformation, the number of this conformation was still small because of its small statistical weight, so that the possible gauche conformations number becomes quite large. These have been comprehensively investigated by using Raman as the probe to study the conformations of normal alkanes in the liquid state as

a function of temperature and pressure.14 Liquid normal alkanes existed in a dynamic mixture of conformations; i.e., the liquid phase under high pressure preferred to show in globular conformations with some gauche bonds.16 18 The n-heptane has 13 possible distinct conformations: the straight chain, all-trans conformer, and kinked chain conformers with up to four gauche bonds distributed along its length,14 the 11 lowest lying having been calculated by density functional theory (DFT).6 The relative number of the all-trans conformer is about 15% in the gaseous state19 at ambient temperature and pressure. By using Boltzmann distribution, one can calculate the number of the various conformers; each gauche bond needs about 500 cal mol 1 of energy.6 Due to the greater statistical weight of the gauche conformers, the energy barrier is overcome, and due to the small number of trans conformers (for liquid alkanes longer than nonane), hence, the spectroscopic observation of the trans conformer is quite difficult.14 As stated above, liquid n-heptane under high pressure is just like liquid n-hexane20 mixed with several different conformers, all of which were gauche conformers. As shown in Figure 1, the Raman spectra at 0.61 GPa are broad and consist of many vibrational modes.6 With increasing pressure, several new Raman peaks appear at 1.2 GPa that can be seen from Figure 1 and Figure 2. Especially, the new Raman peak appears at 122.7 cm 1 (longitudinal acoustic mode, LAM), and its overtones are at 328.7 and 495 cm 1, which are due to the existence of the rotator phase.21 The LAM mode vibrates just like the accordion that combines with the C C C bond angle expansion or contraction in phase along the fully extended chain length.16 The sharp peak at 311.9 cm 1 in ambient pressure is also a “longitudinal acoustic mode”, which is due to a vibration of the all-trans conformers (TTTT) along their chain axes.14,16 These newly appeared longitudinal acoustic modes suggest that the molecules seem to adopt the most elongated trans conformation and with their long axes perpendicular to the packed layers7,8 which are also the main characteristics of the rotator phases. Moreover, the high-pressure stable conformation of n-heptane is an all-trans conformation.21,22 Therefore, we consider that the observed phase transition at about 1.2 GPa is a liquid to rotator phase 18311

dx.doi.org/10.1021/jp112333m |J. Phys. Chem. C 2011, 115, 18310–18315


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Table 1. Raman Vibrational Mode Assignments (cm 1) for Different Phases of n-Heptane liquid (0 GPa)

RIII (1.2 GPa) 50 250

311.9

328.7

RIV (3 GPa)

crystalline (14.5 GPa)

vibrational mode assignment

50 250

50 370

lattice and acoustic mode

271.2

427.4

CH3 rotating

342

396.1

longitudinal acoustic mode (LAM)

358.7

GTTT conformer

397.1

superposition of GTTT and TTGG

456.6

439.5

449.5

523.1

508.5

492.1

501.2

545.6

longitudinal acoustic mode (LAM) single gauche TGTT

742.4 777.1

739.1 774.6

747.7 778.2

809

CH2 CH3 rock and torsion CH2 CH3 rock and torsion

838.2

840.4

844

865.4

C C stretching CH3 rocking

854.1 889.1

887.2

896.9

924.1

CH3 rocking

902.5

913.3

917.5

936.6

CH3 rocking

909.5

CH3 rocking

934.4

CH2 rocking

950.2 961.6

CH2 rocking CH2 rocking

991.5 1029.6

998.8

1008.2

1015.6

1023.8

1035.3

1044

1045.5

C C stretching C C stretching

1085.7

1045.7


1065.8

1072.3

1109.4

C C stretching

1068.8

1080.1

1116.5

C C stretching

1082.5

1090.3

1143.9


1138.8

1144.3

1149.2

1175.2

C C stretching

1164

1169.5

1170.1

1059.1 1072.3 1082.5

C C stretching CH2 twisting-rocking CH2 wagging

1210.3

CH2 wagging

1238.8

1400 1550

1400 1550

1269.4 1400 1550

2800 3000

2850 3000

2860 3020

Figure 2. Raman peak shifts of n-heptane in the range between 50 and 575 cm 1 as a function of pressure.

1262.1

CH2 wagging

1400 1550

CH2 wagging CH2 and asymmetric CH3 bending-twisting

2950 3130

C H stretching

Figure 3. Raman peak shifts of n-heptane in the range between 700 and 1300 cm 1 as a function of pressure. 18312



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transition. The rotator phases, which are intermediate phases, occur between the crystalline and the isotropic liquid phases in normal alkanes and have been well studied by using neutron scattering,23 computer simulations24,25 and X-ray scattering;26,27 the molecules around their long axes exhibit rotation and disorder.21 23,28 And the rotator phase is still a freezing of different conformations. With the pressure increased to 3 GPa, as shown in Figure 1, Figure 2 and Figure 4, new Raman peaks appear at 89.8, 182.7, 271.2, and 2916.2 cm 1, and the Raman shifts (1445.4, 1455.2, 1457.4, and 2900.1 cm 1) as a function of pressure show discontinuities. Through the d-spacing dependence on pressure shown in Figure 5b, the new X-ray diffraction peak is noticed at 3.48 GPa. The n-heptane has two molecules in one unit cell,29 so that Pawley refinement is used for the X-ray diffraction peaks of n-heptane at different pressures to obtain the symmetry. This refinement results are shown in Figure 6 and the lattice parameters in Table 2. We observe from Figure 6a that the rotator phase of n-heptane is triclinic symmetry at 1.2 GPa. From the Pawley refinement results in Figure 6b, the symmetry of n-heptane at

3.48 GPa is monoclinic. Both Raman and X-ray diffraction suggest that there is a rotator to another phase transition around 3 GPa. In most cases, the general phase sequence of normal alkanes is liquid RII RI crystal in shorter chain length and liquid RIV RIII crystal in C27 and longer chain length.15

Figure 4. Raman peak shifts of n-heptane in the range between 1430 and 3180 cm 1 as a function of pressure.

Figure 6. Pawley refinement of n-heptane at 1.2 (a), 3.48 (b), and 14.63 GPa (c).

Figure 5. (a) Angle-dispersive X-ray diffraction patterns of n-heptane at pressures up to 19.5 GPa. A plus (+) and two asterisks (*) indicate new Bragg diffraction peak and a pound sign (#) denotes a peak from the Fe diffraction. (b) d-spacing of n-heptane as a function of pressure at room temperature. 18313



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Table 2. Variation of the Lattice Parameters of n-Heptane with Pressure pressure (GPa)

a (Å)

b (Å)

c (Å)

α (deg)

triclinic

1.2

4.323 49

9.133 61

18.354 47

25.536

monoclinic

3

8.236 52

4.213 92

7.809 93

8.134 41

8.004 2

4.053 13

symmetry

orthormbic

14.41

For the existence of a series of rotator phases in normal alkanes between fully ordered crystalline phases and the isotropic liquid phase, the triclinic symmetry phase of n-heptane is rotator phase RIII and the monoclinic symmetry is rotator phase RIV.26 Raman scattering and X-ray diffraction results fully certify that n-heptane transforms from liquid phase into RIII phase at 1.2 GPa and then into RIV phase at about 3 GPa. The latter phase transition is also found previously by infrared absorption study under high pressure up to 7 GPa.6 However, our high-pressure Raman scattering and X-ray diffraction suggested that there are still some interesting phenomena between about 7.5 and 14.5 GPa. As shown in Figures 2 4, the external mode (122.7 cm 1), CH3 rotating mode (271.2 cm 1), and CH3 rocking mode (889.1 cm 1) show great changes at 7.65 GPa as a function of pressure. When the pressure is higher than 14.41 GPa, the ambient pressure Raman peaks at 358.7 cm 1 belong to the GTTT conformer, 397.1 cm 1 results from a superposition of bands from GTTT and TTGG conformations, and 508.5 cm 1 is caused by single gauche TGTT; all of them do not exist. But the ambient pressure Raman peak at 311.9 cm 1 (LAM), which is caused by TTTT conformer, still exists up to 20.78 GPa. As shown in Figure 5a, it is found that the new X-ray diffraction peak that appeared at 7.57 GPa is weak and cannot be observed obviously above this pressure. And the X-ray diffraction refinement result (Figure 6c) shows that the structure at 7.57 GPa is still monoclinic, the same as the RIV phase at 3.48 GPa. The Pawley refinement is also provided at 14.63 GPa (in Figure 6d), and the symmetry is orthorhombic. When the pressure is higher than 7.5 GPa, there are still some gauche conformers that exist in the RIV phase,7 and the molecules around their long axes adopt an orientational disorder.30 However, the RIV phase which is composed of local long-range herringbone ordered regions begins to grow.31 Up to the pressure of 14.5 GPa, gauche conformers have totally disappeared and the conformation of n-heptane is complete a trans conformer; that is, n-heptane exhibits an orientational ordered crystalline phase.22,27 The conformation of n-heptane is an all-trans conformer of high pressure (higher than 14.5 GPa). Hence, n-heptane transforms from RIV to crystalline phase around 14.5 GPa, which is an orientational disorder order phase transition. Furthermore, the major difference between rotator and crystalline phase is the absence of the orientational disorder.31 Therefore, along with the disorder around the chain axes, n-heptane finally transformed into crystalline phase.15,24

4. CONCLUSION We successfully describe the structural phase transitions of nheptane under pressures up to 20.78 GPa at room temperature by the analysis of both high-pressure Raman scattering and ADXRD spectra. With increasing pressure the liquid n-heptane is transformed into an orthorhombic crystalline state via three phase transitions: liquid RIII at 1.2 GPa, RIII RIV around 3 GPa, and RIV crystal in the range of about 7.5 and 14.5 GPa. At 7.5 GPa, long-range herringbone-ordered structures in the RIV

β (deg)

γ (deg)

98.844

98.512

88.986

phase start to grow and the remaining gauche conformers gradually diminish with increasing pressure. Around 14.5 GPa, n-heptane is in the all-trans conformation, forming an orthorhombic crystal. Therefore, the RIV crystal phase transition corresponds to an orientational disorder order transition. Our results demonstrate that the high-pressure Raman scattering spectra combine with the ADXRD analysis provides a powerful tool for the exploration of structural phases in complex molecules.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported partially by the National Natural Science Foundation of China (Grant Nos. 10574054, 10976011, and 11004074), the specialized Research Fund for the Doctoral Program of Higher Education (SRFDP; Grant No. 20100061120093), and the National Basic Research Program of China (Grant No. 2011CB808200); and partially by COMPRES, the Consortium for Materials Properties Research in Earth Sciences under NSF Cooperative Agreement EAR 06-49658. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC0298CH10886. ’ REFERENCES (1) Sirota, E. B.; King, H. E., Jr.; Hughes, G. J.; Wan, W. K. Phys. Rev. Lett. 1992, 68, 492–495. (2) Chapman, D; Jones, M.; Micelles, M. N. Monolayers and Biomembranes; John Wiley & Sons: New York, 1984. (3) Hyun, B.-R.; Dzyuba, S. V.; Bartsch, R. A.; Quitevis, E. L. J. Phys. Chem. B 2002, 106, 7579–7585. (4) Kuang, H. W.; Li, Y. X.; Zeng, Y. T.; Meng, X. H.; Ge, M. Chin. J. Geochem. 2004, 23, 334–338. (5) Dagaut, P.; Reuillon, M.; Cathonnet, M. Combust. Sci. Technol. 1994, 95, 233–260. (6) Yamaguchi, M.; Serafin, S. V.; Morton, T. H.; Chronister, E. L. J. Phys. Chem. B 2003, 107, 2815–2821. (7) Kavitha, G.; Narayana, C. J. Phys. Chem. B 2006, 110, 8777–8781. (8) Krishnan, M.; Balasubramanian, S. J. Phys. Chem. B 2005, 109, 1936–1946. (9) Mao, H. K.; Xu, J.; Bell, P. M . J. Geophys. Res. 1986, 91, 4673–4676. (10) Jia, R.; Cui, Q. L.; Li, F. F.; et al. J. Light Scattering 2008, 20, 212–217. (11) Hammersley, A. P.; Svensson, S. O .; Hanfland, M.; Fitch, A. N.; Hausermann, D. High Pressure Res. 1996, 14, 235–245. (12) Mizushima, S.; Simanouti, T. J. Am. Chem. Soc. 1949, 71, 1320–1324. (13) Mayo, D. W.; Miller, F. A.; Hannah, R. W. Course Notes on the Interpretation of Infrared and Raman Spectra; John Wiley & Sons: New York, 2004. 18314



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Rotator Phases of n-Heptane under High ... - American Chemical Society

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