Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 143–147
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
The investigation on the pressure-induced phase transition in linoleic acid by in situ Raman spectroscopy q Fan Ya (范雅) a,⇑, Zhou Jing (周静) b, Xu Da-Peng (许大鹏) c a
Department of Physics, Changchun University of Science and Technology, Changchun 130022, China National Engineering Research Center for Rare Earth Materials, General Research Institute for Nonferrous Metals, Beijing, China c College of Physics, Jilin University, Changchun 130012, China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Linoleic acid undergoes two pressure-
induced phase transitions below 1.29 GPa. The first transition from liquid phase to solid phase takes place in 0.07– 0.12 GPa. The second phase transition occurs in 0.31-0.53 GPa. Some conformational characters of the two high-pressure phases were determined. The pressure-induced phase transitions in linoleic acid are reversible.
a r t i c l e
i n f o
Article history: Received 14 November 2013 Received in revised form 11 March 2014 Accepted 11 March 2014 Available online 25 March 2014 Keywords: Linoleic acid Pressure-induced phase transition In situ Raman spectroscopy
a b s t r a c t With diamond anvil cell as a high-pressure apparatus, the in situ Raman spectra of linoleic acid from normal pressure to 1.29 GPa were measured to investigate the effect of pressure on the structural changes. In the pressure ranges of 0.07–0.12 GPa and 0.31–0.53 GPa, the significant changes in Raman spectra show that linoleic acid undergoes two pressure-induced phase transitions. Spectral analysis indicates that the polymethylene chain of linoleic acid molecule transforms from the disordered gauche conformation to the ordered trans conformation in the range of 0.07–0.12 GPa. And the polymethylene chain of linoleic acid molecule remains the ordered trans conformation whereas the conformation of the olefin group significantly changes and the degree of conformational order increases in the range of 0.31–0.53 GPa. The pressure-induced phase transitions in linoleic acid are reversible. Ó 2014 Elsevier B.V. All rights reserved.
Introduction As one well-known essential fatty acid, linoleic acid has many wholesome physiological functions, such as preventing arteriosclerosis, lowering blood fat, decreasing the incidence of coronary heart disease, and so on [1–5]. The application of ultra high pressure q Project supported by the National Natural Science Foundation of China (Grant Nos. 61006065 and 11204020). ⇑ Corresponding author. Tel.: +86 13596486395. E-mail address: [email protected] (F. Ya).
1386-1425/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2014.03.023
processing technology in food industry has wide perspective of development. As one of the most commonly appearing components of human diets, linoleic acid’s phase transition study under pressure is very helpful to improve food preservation technology. On the other hand, the presence of two carbon double bonds in its molecules makes the phase transition very easy to occur during compression [6]. Therefore, the study on the pressure-induced phase transition of linoleic acid has important theoretical values. So far some physical properties of linoleic acid under pressure have been investigated, such as viscosity, relative permittivity, and so on, which indicated the occurrence of the pressure-induced
144
F. Ya et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 143–147
phase transition in linoleic acid [7,8]. However, no high-pressure Raman or any other in situ experimental research on the pressure-induced phase transition in linoleic acid has been reported. Thus, the accurate transition point and structural characterization of the high-pressure phase are still unknown. Raman spectroscopy, equipped with a diamond anvil cell as a high-pressure apparatus, is a very powerful in situ technology for dynamically investigating the phase transition and conformational change of organic molecules [9–11]. In particular, for CH2 rich molecules such as linoleic acid, the spectral features in the CH2 stretching region are highly useful in spectral analysis because they dramatically change with the phase transition. In the present paper, high-pressure in situ Raman spectroscopy was used to investigate the pressure-induced phase transition in linoleic acid.
Experiment The liquid sample of linoleic acid (C18H32O2) with a purity above 99% was purchased from SIGMA Co., Ltd. High pressure apparatus used in this experiment is a Mao-Bell diamond anvil cell (DAC) with two diamonds of 500 lm culet size. The liquid sample with a small ruby chip (about 10 lm) was loaded in a hole (its diameter is 200 lm) drilled in a 250 lm thick T301 gasket pre-indented to 90 lm thickness. Then the cell was carefully pressurized in small steps and allowed to stabilize for a few minutes after each pressure change before Raman spectra were taken. The pressure calibration was carried out using the Ruby fluorescence [12]. No pressuretransmitting liquid medium was used in the experiment. We have monitored the separation between the R1 and R2 components of the Ruby fluorescence line which was found to remain invariant even at high pressure, indicating a negligible non-hydrostatic component. Both Raman spectra of linoleic acid and ruby fluorescence measurements were performed with a Raman microscope (Jobin Yvon, HR800) equipped with an Ar+ laser (514.5 nm) and a multiple track CCD detector. The laser output power was 20 mW and the integration time was 20 s. Frequency calibration of the Raman spectrum was realized with the characteristic 520 cm 1 line of silicon and the resolution was 1 cm 1. Therefore, the error of pressure calibration was ±0.07 GPa. Experiments were conducted up to 1.29 GPa and were reproduced several times. All measurements were carried out at room temperature. The frequencies of the Raman bands were determined by fitting with Gaussian function.
Results and discussion The in situ Raman spectra of linoleic acid were recorded at pressures up to 1.29 GPa at room temperature as shown in Figs. 1 and 2 and Table 1 give the frequency–pressure dependency relationships of the main modes and the pressure derivative (dx/dP) of the Raman modes in different pressure ranges as well as the mode assignments [13–19]. Several unmarked bands have not yet been assigned. In the presence of the predominant first-order diamond peak, we are not able to resolve the peaks around 1300 cm 1 region. And at normal pressure no Raman band was observed in the low wavenumber region of the Raman spectrum of the liquid sample so that we cannot compare it with the corresponding bands at high pressures, thus the spectral region covering the lattice modes was absent from the Raman spectra. It can be observed that the Raman spectra of linoleic acid totally do not change as the pressure increases to 0.07 GPa. From 0.07 to 0.12 GPa, the significant changes can be observed in the spectra, including the disappearance of two modes (t3, t9), appearance of three new modes (t8, t10, t14), and sudden changes in the slope of the frequency–pressure curves of some modes, which indicates the occurrence of one phase transition. With the increase in pressure from 0.12 to 0.31 GPa, there is only the linear shift of some Raman modes. But in 0.31–0.53 GPa, we have again found such changes in the spectra as the disappearance of t2 and appearance of new modes (t1, t5, t7), suggesting the occurrence of the second phase transition. Above 0.53 GPa, some bands linearly shift with the increase in pressure and no other change is observed. The above results indicate that the structural transformations of linoleic acid occur in the ranges of 0.07–0.12 GPa and 0.31–0.53 GPa, respectively. In linoleic acid molecules, the polymethylene chains are bent at two cis-formed C@C bonds and separated into several parts: the big olefin group in centre and the methyl-sided and carboxyl-sided chain in two sides. To analyze the structural transformations, we separate out the modes due to methylene, olefin, carboxylic groups and the modes unassigned. Methylene chain modes Below 0.07 GPa, the symmetric CH2 stretching mode (t13) remains unchanged. With the increase in pressure from 0.07 to 0.12 GPa, t13 weakens and broadens, so that the accurate peak position cannot be determined. The increase in the half-width of
Fig. 1. The Raman spectra of linoleic acid at normal and high pressures.
F. Ya et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 143–147
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Fig. 2. Frequency–pressure dependency relationships of the Raman modes of linoleic acid.
Table 1 dx/dP of the Raman modes of linoleic acid at different pressures and their mode assignments. Mode number
t1 t2 t3 t4 t5 t6 t7 t8 t9 t10 t11 t12 t13 t14 t15
dx/dP(cm
1
GPa
1
)
Assignments
(0–0.07 GPa) Liquid phase
(0.12–0.31 GPa) The first high-pressure phase
(0.55–1.29 GPa) The second high-pressure phase
– 48.57 38.57 44.29 – 0 – – 62.86 – 70.0 18.57 52.86 – 24.29
– 3.59 – 12.63 – –12.70 – 5.28 – 12.21 10.39 8.18 – 6.93 6.86
5.14 – – 4.41 4.84 3.97 3.98 4.77 – 5.60 1.98 2.92 6.18 9.66 8.81
t13 probably results from the perturbation spreading in the process of crystallization by pressure, or the distortion of lattice and lattice defect caused by the increase in pressure. As the pressure increases from 0.31 to 0.53 GPa, t13 abruptly appears at 2855 cm 1. With the further increase in pressure to 1.29 GPa, t13 linearly shifts from 2855 cm 1 to 2860 cm 1, indicating a decrease in the bond length and an increase in the force constant of the symmetric CH2 stretching mode.
C@CAH out of plane ts(CC) t(CC) t(CC), Ca-atom C@CAH out of plane
tas(CC)trans t(CC) ts(CC)trans CH2 scissoring
t(C@C) ts(CH2) tas(CH2) t(@CAH)
At normal pressure, the broad band from 2860 to 2950 cm 1 cannot be assigned. At 0.12 GPa, a strong peak abruptly appears at 2885 cm 1, which is assigned to the asymmetric CH2 stretching mode (t14) of the ordered all-trans polymethylene chains [17]. Both in the 0.12–0.31 GPa and 0.53–1.29 GPa pressure ranges, t14 linearly shifts toward higher wavenumber. This suggests that the molecule is compressed in the direction vertical to the main chains, which results in a decrease in the bond length and an increase in
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the force constant of the asymmetric CH2 stretching mode. Furthermore, in the 0.53–1.29 GPa pressure range the slope of the frequency–pressure curve of t14 is larger than that of t14 in the 0.12–0.31 GPa pressure range, which indicates that the second high-pressure phase’s compressibility in the direction vertical to the main chains is higher than that of the first high-pressure phase. At ambient pressure, the CH2 scissoring mode (t11) is located at 1440 cm 1, with a weak shoulder at around 1457 cm 1. With the pressure up to 0.12 GPa, we observe a shifting behavior of the main peak toward higher wavenumber and the splitting of the shoulder into the two ones at 1460 and 1480 cm 1. Above 0.53 GPa, in addition to the blue shift of the main peak, the obvious change of the shoulders can be observed, including the weakening of the 1460 cm 1 shoulder, the disappearance of the 1480 cm 1 and appearance of new shoulder at 1451 cm 1. The continuous blue shift of the main peak suggests that t11 stiffens because of the restriction in the CH2 stretching vibration in the direction perpendicular to the molecular main chain in the high-pressure phase. And the changes of t11 also indicate that the structure of subcell changes during phase transition. The CAC stretching modes are very sensitive to the conformational order in the alkane chain and conspicuously change after pressure is applied. At normal pressure, the symmetric CAC stretching mode is located at 724 cm 1 (t2), and the CAC stretching modes are located at 840 cm 1 (t3) and 1078 cm 1 (t9), in which the latter belongs to the disordered gauche methylene chains [14]. With the pressure up to 0.21 GPa, t3 and t9 disappear, and two new bands appear at 1064 (t8) and 1095 cm 1 (t10), which are both characteristic of the ordered all-trans polymethylene chains. The t8 is assigned to the asymmetric CAC stretching mode of the ordered all-trans polymethylene chains and the t10 is the symmetric CAC stretching mode of the ordered all-trans carboxyl-sided chains [17,18]. This indicates that polymethylene chains transform from the disordered gauche conformation (Fig. 3) into the ordered trans conformation (Fig. 4) during the transition from liquid phase to the first high-pressure phase. In the stable range of the first high-pressure phase (0.12–0.31 GPa), the three CAC stretching modes shift towards higher wavenumber to some extent, indicating that the molecules are compressed along the main chain, thereby resulting in an increase in the force constant and a decrease in the CAC bond length and thus, the length of the polymethylene chains. From 0.31 to 0.53 GPa, t2 disappears
whereas t8 and t10 totally remain unchanged, suggesting that the polymethylene chains remain ordered trans conformation during the second transition. In the stable range of the second high-pressure phase (0.53–1.29 GPa), t8 and t10 also show blue shift, but their slopes of the frequency–pressure curve are less than those in the 0.12–0.31 GPa pressure range, indicating that the second high-pressure phase’s compressibility in the main chains is lower than that of the first high-pressure phase. Olefin group modes Although the presence of the two cis-double bonds in linoleic acid molecules makes it very difficult to determine the conformation of olefin group, the Raman bands relative to this group all dramatically change with the increase in pressure. The C@C stretching mode (t12) continuously shifts towards higher wavenumber in the whole pressure range, indicating an increase in the force constant and a decrease in the bond length of C@C stretching mode. However, the sudden changes in the slope of the frequency–pressure curve can be observed in 0.07–0.12 GPa and 0.31–0.53 GPa pressure ranges, meanwhile similar features have been observed in the @CAH stretching mode around 3011 cm 1 (t15), suggesting the occurrence of phase transitions in these two pressure ranges. Furthermore, during the second phase transition in 0.31– 0.53 GPa, a new Raman band appears at 718 cm 1 (t1) due to the C@CAH out-of-plane bending mode [19] and t15 greatly sharpens. The above results suggest that the conformation of the olefin group changes, and the degree of conformational order increases in the second high-pressure phase. Carboxyl group modes The Raman band at around 908 cm 1 (t4) at normal pressure is due to the CAC stretching mode of the a carbon atom (in the vicinity of the carboxyl group). This band shifts towards higher wavenumber in the whole pressure range, indicating a decrease in the bond length of CAC in the vicinity of the carboxyl group. The pressure derivatives of this band in three different pressure ranges decrease with the increase in pressure, indicating that the compressibility of CAC in the vicinity of the carboxyl group decrease with the increase in pressure. In addition, during the transition from the first high-pressure phase to the second high-pressure phase, t4 abruptly shifts from 915 to 910 cm 1 (a red shift), suggesting an increase in the bond length of CAC in the vicinity of the carboxyl group caused by the change in the atomic position during phase transition. The modes unassigned
Fig. 3. The structure of gauche conformation.
Fig. 4. The structure of trans conformation.
During the second phase transition in 0.31–0.53 GPa, two new bands appear at 949 (t5) and 980 cm 1 (t7), which also gives evidence of the occurrence of phase transition although this two bands cannot be assigned now. By analyzing the pressure-dependent Raman spectra, we found that linoleic acid undergoes two pressure-induced phase transitions in the 0.07–0.12 GPa and 0.31–0.53 GPa ranges and the degree of molecular conformational order increases during the two phase transitions. It is interesting to note that de Sousa et al. have investigated the pressure-induced phase transitions in palmitic acid (C16H32O2, PA, saturated fatty acid) with the similar method [20]. They found that on applying pressure from ambient up to 21 GPa, the crystalline structure of PA shows a sequence of four phase transitions at about [0.0, 1.0], [3.0, 5.5], [8.0, 11.3] and [14.0, 15.4] pressure intervals. In comparison with palmitic acid, linoleic acid undergoes two transitions at lower pressures (below 0.6 GPa). The reason is that the presence of two carbon double
F. Ya et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 143–147
bonds in its molecules makes the molecular conformation of linoleic acid more flexible and phase transition easier to occur during compression. And more phase transition behavior in linoleic acid at higher pressures can be expected. Therefore, future works applied to higher pressures with more experimental means such as XRD will be carried out to investigate more high-pressure phases of linoleic acid and more information on those high-pressure phases. Finally, with the decompression, the Raman spectra of linoleic acid resemble those observed at comparable pressures during compression, and the sample is restored to the liquid state at normal pressure. This indicates that the pressure-induced phase transition for linoleic acid is completely reversible. Conclusions In summary, high-pressure in situ Raman studies up to 1.29 GPa are performed on linoleic acid at room temperature. Analysis on the changes of the Raman modes at different pressures indicates that linoleic acid solidifies and transforms to the first high-pressure phase in the 0.07–0.12 GPa range and further transforms to the second high-pressure phase in the 0.31–0.53 GPa range. During the first transition, the polymethylene chains transform from the disordered gauche conformation to the ordered trans conformation in linoleic acid molecule. During the second phase transition the polymethylene chains remain the ordered trans conformation whereas the conformation of the olefin group significantly changes and the degree of conformational order increases. The pressureinduced phase transitions in linoleic acid are reversible. The results give the accurate transition ranges and some conformational characters of the two high-pressure phases, which are important in
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understanding the physical and chemical properties of linoleic acid under high pressure and in improving food-preservation processes. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
Clarke, Br. J. Nutr. 83 (2000) 59–66. Clarke, Br. J. Nutr. 131 (2001) 1129–1132. S. Bellenger-Germain, J.P. Poisson, M. Narce, Lipids 37 (2002) 561–567. C. Le Jossic-Corcos, C. Gonthier, I. Zaghini, E. Logette, I. Shechter, P. Bournot, Biochem. J. 385 (2005) 789–794. C.M. Oomen, M.C. Ocké, E.J. Feskens, F.J. Kok, D. Kromhout, Am. J. Clin. Nutr. 75 (2001) 457–463. R.M. Siegoczyn´ski, High Press. Res. 13 (1994) 89–92. C.J. Schaschke, S. Abid, M.J. Heslop, High Press. Res. 27 (2007) 33–37. R. Kos´ciesza, R.M. Siegoczyn´ski, A.J. Rostocki, D.B. Tefelski, A. Kos, W. Ejchart, J. Phys.: Conf. Ser. 121 (2008) 142005-1–142005-4. W.L. Liu, Z.R. Zheng, R.B. Zhu, Z.G. Liu, D.P. Xu, H.M. Yu, W.Z. Wu, A.H. Li, Y.Q. Yang, W.H. Su, J. Phys. Chem. A 111 (2007) 10044–10049. M. Zhou, D.P. Xu, T.C. Liu, P. Zhang, S.Q. Gao, Z.W. Li, G.H. Lu, J. Phys. Chem. B 112 (2008) 15562–15568. Y. Fan, J. Zhou, S. Li, F.Y. Guan, D.P. Xu, Chin. Phys. Lett. 28 (2011) 110702-1– 110702-4. G.J. Piermarini, S. Block, J.D. Barnett, R.A. Forman, J. Appl. Phys. 46 (1975) 2774–2780. S.P. Verma, D.F.H. Wallach, Biochim. Biophys. Acta 486 (1977) 217–227. Y. Koyama, Ken-Ichi Ikeda, Chem. Phys. Lipids 26 (1980) 149–172. G.Z. Dongye, Q. Zhou, S.Q. Sun, G.D. Hu, Q. Wang, X.Y. Hu, Spectrosc. Spect. Anal. 20 (2000) 836–837. J.H. Shen, W.D. Wang, M.Y. Geng, Z.K. Xu, Z.L. Su, G.Y. Qin, J. Anhui Agri. Sci. 37 (2009) 6827–6828. M. Kobayashi, F. Kaneko, K. Sato, M. Suzuki, J. Phys. Chem. 90 (1986) 6371– 6378. Y. Kim, H.L. Strauss, R.G. Snyder, J. Phys. Chem. 92 (1988) 5080–5082. P. Tandon, G. Förster, R. Neubert, S. Wartewig, J. Mol. Struct. 524 (2000) 201– 215. F.F. de Sousa, G.D. Saraiva, P.T.C. Freire, J.A. Lima Jr, P. Alcantara Jr., F.E.A. Melo, J. Mendes Filho, J. Raman Spectrosc. 43 (2012) 146–152.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
The investigation on the pressure-induced phase transition in linoleic acid by in situ Raman spectroscopy q Fan Ya (范雅) a,⇑, Zhou Jing (周静) b, Xu Da-Peng (许大鹏) c a
Department of Physics, Changchun University of Science and Technology, Changchun 130022, China National Engineering Research Center for Rare Earth Materials, General Research Institute for Nonferrous Metals, Beijing, China c College of Physics, Jilin University, Changchun 130012, China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Linoleic acid undergoes two pressure-
induced phase transitions below 1.29 GPa. The first transition from liquid phase to solid phase takes place in 0.07– 0.12 GPa. The second phase transition occurs in 0.31-0.53 GPa. Some conformational characters of the two high-pressure phases were determined. The pressure-induced phase transitions in linoleic acid are reversible.
a r t i c l e
i n f o
Article history: Received 14 November 2013 Received in revised form 11 March 2014 Accepted 11 March 2014 Available online 25 March 2014 Keywords: Linoleic acid Pressure-induced phase transition In situ Raman spectroscopy
a b s t r a c t With diamond anvil cell as a high-pressure apparatus, the in situ Raman spectra of linoleic acid from normal pressure to 1.29 GPa were measured to investigate the effect of pressure on the structural changes. In the pressure ranges of 0.07–0.12 GPa and 0.31–0.53 GPa, the significant changes in Raman spectra show that linoleic acid undergoes two pressure-induced phase transitions. Spectral analysis indicates that the polymethylene chain of linoleic acid molecule transforms from the disordered gauche conformation to the ordered trans conformation in the range of 0.07–0.12 GPa. And the polymethylene chain of linoleic acid molecule remains the ordered trans conformation whereas the conformation of the olefin group significantly changes and the degree of conformational order increases in the range of 0.31–0.53 GPa. The pressure-induced phase transitions in linoleic acid are reversible. Ó 2014 Elsevier B.V. All rights reserved.
Introduction As one well-known essential fatty acid, linoleic acid has many wholesome physiological functions, such as preventing arteriosclerosis, lowering blood fat, decreasing the incidence of coronary heart disease, and so on [1–5]. The application of ultra high pressure q Project supported by the National Natural Science Foundation of China (Grant Nos. 61006065 and 11204020). ⇑ Corresponding author. Tel.: +86 13596486395. E-mail address: [email protected] (F. Ya).
1386-1425/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2014.03.023
processing technology in food industry has wide perspective of development. As one of the most commonly appearing components of human diets, linoleic acid’s phase transition study under pressure is very helpful to improve food preservation technology. On the other hand, the presence of two carbon double bonds in its molecules makes the phase transition very easy to occur during compression [6]. Therefore, the study on the pressure-induced phase transition of linoleic acid has important theoretical values. So far some physical properties of linoleic acid under pressure have been investigated, such as viscosity, relative permittivity, and so on, which indicated the occurrence of the pressure-induced
144
F. Ya et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 143–147
phase transition in linoleic acid [7,8]. However, no high-pressure Raman or any other in situ experimental research on the pressure-induced phase transition in linoleic acid has been reported. Thus, the accurate transition point and structural characterization of the high-pressure phase are still unknown. Raman spectroscopy, equipped with a diamond anvil cell as a high-pressure apparatus, is a very powerful in situ technology for dynamically investigating the phase transition and conformational change of organic molecules [9–11]. In particular, for CH2 rich molecules such as linoleic acid, the spectral features in the CH2 stretching region are highly useful in spectral analysis because they dramatically change with the phase transition. In the present paper, high-pressure in situ Raman spectroscopy was used to investigate the pressure-induced phase transition in linoleic acid.
Experiment The liquid sample of linoleic acid (C18H32O2) with a purity above 99% was purchased from SIGMA Co., Ltd. High pressure apparatus used in this experiment is a Mao-Bell diamond anvil cell (DAC) with two diamonds of 500 lm culet size. The liquid sample with a small ruby chip (about 10 lm) was loaded in a hole (its diameter is 200 lm) drilled in a 250 lm thick T301 gasket pre-indented to 90 lm thickness. Then the cell was carefully pressurized in small steps and allowed to stabilize for a few minutes after each pressure change before Raman spectra were taken. The pressure calibration was carried out using the Ruby fluorescence [12]. No pressuretransmitting liquid medium was used in the experiment. We have monitored the separation between the R1 and R2 components of the Ruby fluorescence line which was found to remain invariant even at high pressure, indicating a negligible non-hydrostatic component. Both Raman spectra of linoleic acid and ruby fluorescence measurements were performed with a Raman microscope (Jobin Yvon, HR800) equipped with an Ar+ laser (514.5 nm) and a multiple track CCD detector. The laser output power was 20 mW and the integration time was 20 s. Frequency calibration of the Raman spectrum was realized with the characteristic 520 cm 1 line of silicon and the resolution was 1 cm 1. Therefore, the error of pressure calibration was ±0.07 GPa. Experiments were conducted up to 1.29 GPa and were reproduced several times. All measurements were carried out at room temperature. The frequencies of the Raman bands were determined by fitting with Gaussian function.
Results and discussion The in situ Raman spectra of linoleic acid were recorded at pressures up to 1.29 GPa at room temperature as shown in Figs. 1 and 2 and Table 1 give the frequency–pressure dependency relationships of the main modes and the pressure derivative (dx/dP) of the Raman modes in different pressure ranges as well as the mode assignments [13–19]. Several unmarked bands have not yet been assigned. In the presence of the predominant first-order diamond peak, we are not able to resolve the peaks around 1300 cm 1 region. And at normal pressure no Raman band was observed in the low wavenumber region of the Raman spectrum of the liquid sample so that we cannot compare it with the corresponding bands at high pressures, thus the spectral region covering the lattice modes was absent from the Raman spectra. It can be observed that the Raman spectra of linoleic acid totally do not change as the pressure increases to 0.07 GPa. From 0.07 to 0.12 GPa, the significant changes can be observed in the spectra, including the disappearance of two modes (t3, t9), appearance of three new modes (t8, t10, t14), and sudden changes in the slope of the frequency–pressure curves of some modes, which indicates the occurrence of one phase transition. With the increase in pressure from 0.12 to 0.31 GPa, there is only the linear shift of some Raman modes. But in 0.31–0.53 GPa, we have again found such changes in the spectra as the disappearance of t2 and appearance of new modes (t1, t5, t7), suggesting the occurrence of the second phase transition. Above 0.53 GPa, some bands linearly shift with the increase in pressure and no other change is observed. The above results indicate that the structural transformations of linoleic acid occur in the ranges of 0.07–0.12 GPa and 0.31–0.53 GPa, respectively. In linoleic acid molecules, the polymethylene chains are bent at two cis-formed C@C bonds and separated into several parts: the big olefin group in centre and the methyl-sided and carboxyl-sided chain in two sides. To analyze the structural transformations, we separate out the modes due to methylene, olefin, carboxylic groups and the modes unassigned. Methylene chain modes Below 0.07 GPa, the symmetric CH2 stretching mode (t13) remains unchanged. With the increase in pressure from 0.07 to 0.12 GPa, t13 weakens and broadens, so that the accurate peak position cannot be determined. The increase in the half-width of
Fig. 1. The Raman spectra of linoleic acid at normal and high pressures.
F. Ya et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 143–147
145
Fig. 2. Frequency–pressure dependency relationships of the Raman modes of linoleic acid.
Table 1 dx/dP of the Raman modes of linoleic acid at different pressures and their mode assignments. Mode number
t1 t2 t3 t4 t5 t6 t7 t8 t9 t10 t11 t12 t13 t14 t15
dx/dP(cm
1
GPa
1
)
Assignments
(0–0.07 GPa) Liquid phase
(0.12–0.31 GPa) The first high-pressure phase
(0.55–1.29 GPa) The second high-pressure phase
– 48.57 38.57 44.29 – 0 – – 62.86 – 70.0 18.57 52.86 – 24.29
– 3.59 – 12.63 – –12.70 – 5.28 – 12.21 10.39 8.18 – 6.93 6.86
5.14 – – 4.41 4.84 3.97 3.98 4.77 – 5.60 1.98 2.92 6.18 9.66 8.81
t13 probably results from the perturbation spreading in the process of crystallization by pressure, or the distortion of lattice and lattice defect caused by the increase in pressure. As the pressure increases from 0.31 to 0.53 GPa, t13 abruptly appears at 2855 cm 1. With the further increase in pressure to 1.29 GPa, t13 linearly shifts from 2855 cm 1 to 2860 cm 1, indicating a decrease in the bond length and an increase in the force constant of the symmetric CH2 stretching mode.
C@CAH out of plane ts(CC) t(CC) t(CC), Ca-atom C@CAH out of plane
tas(CC)trans t(CC) ts(CC)trans CH2 scissoring
t(C@C) ts(CH2) tas(CH2) t(@CAH)
At normal pressure, the broad band from 2860 to 2950 cm 1 cannot be assigned. At 0.12 GPa, a strong peak abruptly appears at 2885 cm 1, which is assigned to the asymmetric CH2 stretching mode (t14) of the ordered all-trans polymethylene chains [17]. Both in the 0.12–0.31 GPa and 0.53–1.29 GPa pressure ranges, t14 linearly shifts toward higher wavenumber. This suggests that the molecule is compressed in the direction vertical to the main chains, which results in a decrease in the bond length and an increase in
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the force constant of the asymmetric CH2 stretching mode. Furthermore, in the 0.53–1.29 GPa pressure range the slope of the frequency–pressure curve of t14 is larger than that of t14 in the 0.12–0.31 GPa pressure range, which indicates that the second high-pressure phase’s compressibility in the direction vertical to the main chains is higher than that of the first high-pressure phase. At ambient pressure, the CH2 scissoring mode (t11) is located at 1440 cm 1, with a weak shoulder at around 1457 cm 1. With the pressure up to 0.12 GPa, we observe a shifting behavior of the main peak toward higher wavenumber and the splitting of the shoulder into the two ones at 1460 and 1480 cm 1. Above 0.53 GPa, in addition to the blue shift of the main peak, the obvious change of the shoulders can be observed, including the weakening of the 1460 cm 1 shoulder, the disappearance of the 1480 cm 1 and appearance of new shoulder at 1451 cm 1. The continuous blue shift of the main peak suggests that t11 stiffens because of the restriction in the CH2 stretching vibration in the direction perpendicular to the molecular main chain in the high-pressure phase. And the changes of t11 also indicate that the structure of subcell changes during phase transition. The CAC stretching modes are very sensitive to the conformational order in the alkane chain and conspicuously change after pressure is applied. At normal pressure, the symmetric CAC stretching mode is located at 724 cm 1 (t2), and the CAC stretching modes are located at 840 cm 1 (t3) and 1078 cm 1 (t9), in which the latter belongs to the disordered gauche methylene chains [14]. With the pressure up to 0.21 GPa, t3 and t9 disappear, and two new bands appear at 1064 (t8) and 1095 cm 1 (t10), which are both characteristic of the ordered all-trans polymethylene chains. The t8 is assigned to the asymmetric CAC stretching mode of the ordered all-trans polymethylene chains and the t10 is the symmetric CAC stretching mode of the ordered all-trans carboxyl-sided chains [17,18]. This indicates that polymethylene chains transform from the disordered gauche conformation (Fig. 3) into the ordered trans conformation (Fig. 4) during the transition from liquid phase to the first high-pressure phase. In the stable range of the first high-pressure phase (0.12–0.31 GPa), the three CAC stretching modes shift towards higher wavenumber to some extent, indicating that the molecules are compressed along the main chain, thereby resulting in an increase in the force constant and a decrease in the CAC bond length and thus, the length of the polymethylene chains. From 0.31 to 0.53 GPa, t2 disappears
whereas t8 and t10 totally remain unchanged, suggesting that the polymethylene chains remain ordered trans conformation during the second transition. In the stable range of the second high-pressure phase (0.53–1.29 GPa), t8 and t10 also show blue shift, but their slopes of the frequency–pressure curve are less than those in the 0.12–0.31 GPa pressure range, indicating that the second high-pressure phase’s compressibility in the main chains is lower than that of the first high-pressure phase. Olefin group modes Although the presence of the two cis-double bonds in linoleic acid molecules makes it very difficult to determine the conformation of olefin group, the Raman bands relative to this group all dramatically change with the increase in pressure. The C@C stretching mode (t12) continuously shifts towards higher wavenumber in the whole pressure range, indicating an increase in the force constant and a decrease in the bond length of C@C stretching mode. However, the sudden changes in the slope of the frequency–pressure curve can be observed in 0.07–0.12 GPa and 0.31–0.53 GPa pressure ranges, meanwhile similar features have been observed in the @CAH stretching mode around 3011 cm 1 (t15), suggesting the occurrence of phase transitions in these two pressure ranges. Furthermore, during the second phase transition in 0.31– 0.53 GPa, a new Raman band appears at 718 cm 1 (t1) due to the C@CAH out-of-plane bending mode [19] and t15 greatly sharpens. The above results suggest that the conformation of the olefin group changes, and the degree of conformational order increases in the second high-pressure phase. Carboxyl group modes The Raman band at around 908 cm 1 (t4) at normal pressure is due to the CAC stretching mode of the a carbon atom (in the vicinity of the carboxyl group). This band shifts towards higher wavenumber in the whole pressure range, indicating a decrease in the bond length of CAC in the vicinity of the carboxyl group. The pressure derivatives of this band in three different pressure ranges decrease with the increase in pressure, indicating that the compressibility of CAC in the vicinity of the carboxyl group decrease with the increase in pressure. In addition, during the transition from the first high-pressure phase to the second high-pressure phase, t4 abruptly shifts from 915 to 910 cm 1 (a red shift), suggesting an increase in the bond length of CAC in the vicinity of the carboxyl group caused by the change in the atomic position during phase transition. The modes unassigned
Fig. 3. The structure of gauche conformation.
Fig. 4. The structure of trans conformation.
During the second phase transition in 0.31–0.53 GPa, two new bands appear at 949 (t5) and 980 cm 1 (t7), which also gives evidence of the occurrence of phase transition although this two bands cannot be assigned now. By analyzing the pressure-dependent Raman spectra, we found that linoleic acid undergoes two pressure-induced phase transitions in the 0.07–0.12 GPa and 0.31–0.53 GPa ranges and the degree of molecular conformational order increases during the two phase transitions. It is interesting to note that de Sousa et al. have investigated the pressure-induced phase transitions in palmitic acid (C16H32O2, PA, saturated fatty acid) with the similar method [20]. They found that on applying pressure from ambient up to 21 GPa, the crystalline structure of PA shows a sequence of four phase transitions at about [0.0, 1.0], [3.0, 5.5], [8.0, 11.3] and [14.0, 15.4] pressure intervals. In comparison with palmitic acid, linoleic acid undergoes two transitions at lower pressures (below 0.6 GPa). The reason is that the presence of two carbon double
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bonds in its molecules makes the molecular conformation of linoleic acid more flexible and phase transition easier to occur during compression. And more phase transition behavior in linoleic acid at higher pressures can be expected. Therefore, future works applied to higher pressures with more experimental means such as XRD will be carried out to investigate more high-pressure phases of linoleic acid and more information on those high-pressure phases. Finally, with the decompression, the Raman spectra of linoleic acid resemble those observed at comparable pressures during compression, and the sample is restored to the liquid state at normal pressure. This indicates that the pressure-induced phase transition for linoleic acid is completely reversible. Conclusions In summary, high-pressure in situ Raman studies up to 1.29 GPa are performed on linoleic acid at room temperature. Analysis on the changes of the Raman modes at different pressures indicates that linoleic acid solidifies and transforms to the first high-pressure phase in the 0.07–0.12 GPa range and further transforms to the second high-pressure phase in the 0.31–0.53 GPa range. During the first transition, the polymethylene chains transform from the disordered gauche conformation to the ordered trans conformation in linoleic acid molecule. During the second phase transition the polymethylene chains remain the ordered trans conformation whereas the conformation of the olefin group significantly changes and the degree of conformational order increases. The pressureinduced phase transitions in linoleic acid are reversible. The results give the accurate transition ranges and some conformational characters of the two high-pressure phases, which are important in
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understanding the physical and chemical properties of linoleic acid under high pressure and in improving food-preservation processes. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
Clarke, Br. J. Nutr. 83 (2000) 59–66. Clarke, Br. J. Nutr. 131 (2001) 1129–1132. S. Bellenger-Germain, J.P. Poisson, M. Narce, Lipids 37 (2002) 561–567. C. Le Jossic-Corcos, C. Gonthier, I. Zaghini, E. Logette, I. Shechter, P. Bournot, Biochem. J. 385 (2005) 789–794. C.M. Oomen, M.C. Ocké, E.J. Feskens, F.J. Kok, D. Kromhout, Am. J. Clin. Nutr. 75 (2001) 457–463. R.M. Siegoczyn´ski, High Press. Res. 13 (1994) 89–92. C.J. Schaschke, S. Abid, M.J. Heslop, High Press. Res. 27 (2007) 33–37. R. Kos´ciesza, R.M. Siegoczyn´ski, A.J. Rostocki, D.B. Tefelski, A. Kos, W. Ejchart, J. Phys.: Conf. Ser. 121 (2008) 142005-1–142005-4. W.L. Liu, Z.R. Zheng, R.B. Zhu, Z.G. Liu, D.P. Xu, H.M. Yu, W.Z. Wu, A.H. Li, Y.Q. Yang, W.H. Su, J. Phys. Chem. A 111 (2007) 10044–10049. M. Zhou, D.P. Xu, T.C. Liu, P. Zhang, S.Q. Gao, Z.W. Li, G.H. Lu, J. Phys. Chem. B 112 (2008) 15562–15568. Y. Fan, J. Zhou, S. Li, F.Y. Guan, D.P. Xu, Chin. Phys. Lett. 28 (2011) 110702-1– 110702-4. G.J. Piermarini, S. Block, J.D. Barnett, R.A. Forman, J. Appl. Phys. 46 (1975) 2774–2780. S.P. Verma, D.F.H. Wallach, Biochim. Biophys. Acta 486 (1977) 217–227. Y. Koyama, Ken-Ichi Ikeda, Chem. Phys. Lipids 26 (1980) 149–172. G.Z. Dongye, Q. Zhou, S.Q. Sun, G.D. Hu, Q. Wang, X.Y. Hu, Spectrosc. Spect. Anal. 20 (2000) 836–837. J.H. Shen, W.D. Wang, M.Y. Geng, Z.K. Xu, Z.L. Su, G.Y. Qin, J. Anhui Agri. Sci. 37 (2009) 6827–6828. M. Kobayashi, F. Kaneko, K. Sato, M. Suzuki, J. Phys. Chem. 90 (1986) 6371– 6378. Y. Kim, H.L. Strauss, R.G. Snyder, J. Phys. Chem. 92 (1988) 5080–5082. P. Tandon, G. Förster, R. Neubert, S. Wartewig, J. Mol. Struct. 524 (2000) 201– 215. F.F. de Sousa, G.D. Saraiva, P.T.C. Freire, J.A. Lima Jr, P. Alcantara Jr., F.E.A. Melo, J. Mendes Filho, J. Raman Spectrosc. 43 (2012) 146–152.
