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Far-infrared spectra of highly viscous liquids: glycerol and triacetin (glycerol triacetate)
Vibrational Spectroscopy 18 Ž1998. 149–156
Far-infrared spectra of highly viscous liquids: glycerol and triacetin žglycerol triacetate / T.S. Perova a , D.H. Christensen b, U. Rasmussen b, J.K. Vij c , O.F. Nielsen b c
b,)
a VaÕiloÕ State Optical Institute, St. Petersburg 199034, Russian Federation Department of Chemistry, UniÕersity of Copenhagen, 5-UniÕersitetsparken, DK-2100, Copenhagen, Denmark Department of Electronic and Electrical Engineering, UniÕersity of Dublin, Trinity College, Dublin 2, Ireland
Received 19 June 1998; revised 14 October 1998; accepted 19 October 1998
Abstract Far-infrared spectra in the region 25–500 cmy1 were obtained for glycerol and triacetin Žglycerol triacetate. at temperatures from 253 to 323 K. Far-infrared spectra of glycerol look more complicated in comparison with spectra of triacetin due to the hydrogen bonding. Band fitting performed for the frequency range 25–250 cmy1 shows that four bands have contributed to spectra of glycerol and triacetin in this region. The experimental results obtained for glycerol are in good agreement with normal mode analyses performed recently for crystalline glycerol wE.J. Bermejo, A. Criado, A. de Andress, E. Enciso, H. Schober, Phys. Rev. B 53 Ž1996. 5259.x and for glycerol in the liquid state wT. Uchino, T. Yoko, Science 273 Ž1996. 480.x. The far-infrared result are compared to the low-frequency Raman spectra of the two liquids. The difference in temperature behaviour revealed from these two kind of spectra is explained on the basis of different temperature contribution of relaxational and vibrational processes to the low-frequency vibrational spectra. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Far-infrared spectra; Glycerol; Triacetin
1. Introduction The understanding of dynamical processes in glycerol, a highly viscous liquid, has attracted much attention for many decades. Both static and dynamic measurements have been performed, including different thermal w1x and acoustic methods w2,3x, dielectric spectroscopy w4,5x, Raman w6–12x and inelastic neutron scattering w7,8,13x techniques. Despite the suggestion w6,9,10x that most of the atomic dynamics ) Corresponding author. Tel.: q45-35-320-300; Fax: q45-35350-609; E-mail: [email protected]
in the disordered state can be understood in terms of vibrations no far-infrared spectra have been studied for this highly viscous liquid. This is quite surprising since far-infrared spectra can yield a direct information about the vibrational modes and can be used as a supplementary information to the results obtained from low-frequency Raman and inelastic neutron scattering experiments w5,11,12,14,15x. Furthermore such an investigation becomes even more important since two papers have been published recently with the calculations of vibrational frequencies of glycerol using normal mode analysis based on the ab initio method and on the method of molecular modeling at
0924-2031r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 2 0 3 1 Ž 9 8 . 0 0 0 6 2 - 9
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T.S. PeroÕa et al.r Vibrational Spectroscopy 18 (1998) 149–156
medium-range order ŽMRO. w9,16x. A comparison of the calculated frequencies with the experimental ones is important in order to describe the mechanisms responsible for the low-frequency part of the vibrational spectra of highly viscous liquids and glasses. Another reason for such an investigations is to obtain additional information on low-frequency spectra in order to achieve a better understanding of the so called ‘boson peak’—the low-frequency peculiarity in Raman spectra of glasses Žincluding highly viscous liquids. and polymers. As was already mentioned glycerol has been studied using low-frequency Raman spectroscopy for many decades w6–12x. It was revealed that low-frequency Raman spectra of this compound in the liquid phase show a behaviour which is typical for many organic liquids w14,15,17x. However, with a temperature decrease the behaviour of the low-frequency Raman spectra of glycerol shows some differences in comparison with other organic liquids Ževen at temperatures quite far from the temperature of the glass transition ŽTg ... For glycerol a change of the shape of the experimentally observed low-frequency Raman spectra, I Ž n ., is quite obvious around 270 K Žnearly 100 K above the Tg s 182 K. w6,10,18x. At 263 K a noticeable peak appears at around ; 30 cmy1 , getting much more pronounced and shifted to ; 50 cmy1 at lower temperatures w6,10x. This low-frequency peak is the well-known boson peak observed in all glasses w7,19,20x. Variation of the temperature alters the relative contributions of the boson peak and of the central Rayleigh line Žwhich is caused by relaxational processes. to the total Raman spectrum I Ž n .. Above 270 K the two peaks overlap, thus forming a uniform Rayleigh line wing. Despite a considerable number of experimental and theoretical studies performed for the last two decades the nature of the boson peak is still unclear. The reader can find a discussion about this matter in Refs. w19–21x. The most recent attempts to model this behaviour are Ži. a generalized hydrodynamic theory based on a dipole-induced dipole scattering mechanism w22x, Žii. vibrational anomalies and phonon localization w23x, Žiii. a disorder-induced scattering mechanism w10,24–26x, and Živ. soft phonons w27–29x. The disorder-induced scattering mechanism was successfully applied for a description of low-frequency Raman spectra of glycerol and o-terphenyl and a good
agreement between experimental and calculated I Ž n . spectra has been demonstrated in w10,12x. According to this model the boson peak is closely related to the acoustic longitudinal and transversal phonons. Here we present far-infrared spectroscopic studies of two highly viscous liquids: glycerol, and glycerol triacetate Žtriacetin. in the temperature range 253– 323 K. These two liquids have been chosen for the reason that despite similarities in molecular structure only glycerol has a possibility of forming hydrogen bonds. Thus a comparison of the far-infrared spectra of these two liquids can be very useful in order to understand the role of hydrogen bonding in lowfrequency spectra. Another interesting peculiarity of these two highly viscous liquids is the inhomogeneity of their structures w30x which can cause their unusual properties in FIR and low-frequency Raman spectra Žsee for example Ref. w17,31x.. These two liquids also have a different fragility in accordance with Angell’s classification w32x, thus a different behaviour of the low-frequency vibrational spectra can be expected w33x.
2. Experimental The chemicals, glycerol and triacetin, were purchased from Aldrich. Glycerol 99.5% with less than 0.1% of water and triacetin 99% purity were used without further purification. Glycerol and triacetin were placed in between two polyethylene Žor poly4-methyl-1-penten ŽTPX.. windows at room temperature. The thickness of such a cell was approximately a few micrometers. Far-infrared spectra were obtained on a Bruker 120-HR FTIR spectrometer. The main spectrometer was evacuated while the liquid cells were placed in a thermostated holder in a special made sample compartment. This compartment was equipped with polyethylene Žor TPX. windows and flushed with dry nitrogen. The spectra in the region 50–500 cmy1 were recorded using a DTGS detector, 6 mm beam splitter and 2 cmy1 resolution; 512 scans were taken for each spectrum. Spectra in the region 25–250 cmy1 were recorded using a 12 mm beam splitter and a resolution of 2 cmy1 Žor 4 cmy1 in some cases.. Here an average of 125 scans was determined. To increase the signal to noise ratio a liquid helium cooled Si bolometer
T.S. PeroÕa et al.r Vibrational Spectroscopy 18 (1998) 149–156
Žmodel LN6-C of Infrared Laboratories, USA. has been used for measurements in the region 25–250 cmy1 . Two sources were used for different regions: a globar in the region 50–500 cmy1 and a high pressure Hg-lamp in the region 25–250 cmy1 . A Hetofrig 04 PT 623 CB4 temperature controller ŽHolm and Halby, Denmark. has been used for recording spectra in the temperature range from 253 to 323 K. After recording the spectra a background correction due to the windows absorption was taken into account by subtracting the empty cell spectrum from the total absorption spectrum. This was particular important in order to remove the 72 cmy1 band of polyethylene. However, we did not measure the polyethylene windows at low temperatures. Thus the polyethylene sharp peak at 72 cmy1 was not fully compensated at temperatures below 293 K. However, this peak appears on the slope of the broad bands and does not influence the analysis of the results as shown below.
151
Fig. 2. Far-infrared spectra of triacetin at room temperature in the region 50–500 cmy1 . ) shows the uncompensated peak of polyethylene windows.
ence of hydrogen-bonded molecules with a wide distribution of different H-bond stretching and bending vibrations, lattice translations and librations as shown from the normal mode analysis performed in Ref. w9x for crystalline glycerol.
3. Results and discussions 3.2. 25–250 cm y 1 range 3.1. 50–500 cm
y1
range
Figs. 1 and 2 show spectra of glycerol and triacetin in the region 50–500 cmy1 at room temperature. The spectra of glycerol in this range show much broader bands than the spectra of triacetin. In some sense the spectrum of glycerol looks much more like the spectrum of water in the same range Žsee for example Ref. w31x.. This probably reflects the pres-
Fig. 1. Far-infrared spectra of glycerol at room temperature in the region 50–500 cmy1 .
3.2.1. Glycerol We concentrated our attention mainly on the low-frequency part of the far-infrared spectra, in particular, in the range 25–250 cmy1 . Fig. 3a shows spectra of glycerol at temperatures 323, 293 and 253 K. As we can see from this figure there is no great difference between far-infrared spectra obtained at different temperatures. This result being different from the behaviour of low-frequency Raman spectra of glycerol in the same temperature range Žsee Refs. w6,10,18,31,34x and Fig. 3b .. However, for a more detailed analysis it is necessary to perform a band fitting of this complicated spectrum in order to find the contribution of different components. Fitting of the glycerol spectra at different temperatures has been made by use of the GRAMS RESEARCH programme w35x. The best result was obtained by using four Gaussian bands. The fitting parameters are listed in Table 1 and also shown in Fig. 4 as an example for the temperature at 293 K. As one can see from Table 1 four bands with the frequency maxima: ; 55, ; 95, ; 135 and ; 182 cmy1 have contributed to the far-infrared spectra of
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Fig. 3. Far-infrared (a) and low-frequency Raman (b) Žin RŽ n . representation. spectra of glycerol in the region 20–250 cmy1 at different temperatures.
glycerol in the range 25–250 cmy1 . These frequencies are in a good agreement with the calculated and experimental data obtained in Ref. w9x for Raman spectra of crystalline glycerol near the melting point Žat 295 K.. These frequencies can also be compared with the calculated values obtained for liquid glycerol by Uchino and Yoko w16x. The results show an amazing coincidence in particular for the frequencies at 55 and 95 cmy1 . The molecular modeling calculations in Ref. w16x were performed on the basis of a normal-mode analysis at medium-range order ŽMRO. for cyclic glycerol trimers. The authors of Ref. w16x found two kinds of vibrational modes for model I Žsee Fig. 2 from Ref. w16x.. The modes at ; 51 cmy1 , ; 77 cmy1 and ; 93 cmy1 are mainly attributed to the collective motions of the OH groups, whereas the modes at ; 95 cmy1 result from the translational motions Žespecially those of the CH 2 and CH groups. in each glycerol molecule. Nevertheless the fitting procedure of our experimental data shows only two bands Žat ; 55 and
; 95 cmy1 . in the low-frequency part Ž- 100 cmy1 . instead of the three calculated ones of Ref. w16x. The reason might be that the relative intensity of these bands can be different for infrared and Raman spectra. It should be noted here that the frequency maximum position with temperature for all other bands does not change as much as for the band around ; 55 cmy1 Žsee Table 1.. For example for the bands at 182, 137 and 95 cmy1 the difference in maximum position is only a few cmy1 in the temperature interval from 253 K up to 323 K whilst for the band at 55 cmy1 this change is about 12 cmy1 . Probably it means that we have two bands with different relative intensities or the intensity of the band at 77 cmy1 is very low in the far-infrared spectra. The analysis of the second derivatives shown in Fig. 5 also shows that probably two bands Ž; 50 and ; 70 cmy1 . with different relative intensity can influence the low-frequency part of this spectrum. Concerning the peak intensities, only a small difference in the behaviour of these four bands is found in the temperature range 293–323 K Žsee Table 1.. In particular the intensity of the low-frequency band at ; 55 cmy1 increases with increasing temperature. This result shows the same tendency as was revealed from the low-frequency Raman study of glycerol w6x. However, the intensity variation of the Raman spectra is much more pronounced in this temperature range. The behaviour of the far-infrared spectra in the low temperature range Ž253–293 K. substantially differ from the behaviour of the low-frequency Raman spectra. In particular as was shown in Refs.
Fig. 4. Band fitting of the far-infrared spectra of glycerol with four Gaussian bands.
T.S. PeroÕa et al.r Vibrational Spectroscopy 18 (1998) 149–156
Fig. 5. The second derivative spectrum Žtop. obtained from the experimental far-infrared spectrum of glycerol Žbottom. after the smoothing procedure ŽFFT filter..
w6,10,18x in Raman spectra of glycerol a noticeable peak at around ; 33 cmy1 developed at temperatures below ; 270 K. The peak getting even more pronounced at lower temperatures w6,10x. However, there is no sign of the presence of this peak in the far-infrared spectra in the temperature range studied here. It could not be a band with the frequency maximum at ; 50 cmy1 Žobtained from the fitting., since as was shown in Ref. w9x the band with the frequency maximum at 54 cmy1 persists in the low-frequency Raman spectra of crystalline glycerol and it is assigned to the lowest optical phonon. As it was noticed in Ref. w9x the calculated mode frequencies for the isolated molecule in this region belong to the torsional and mainly bending modes. Calculations performed for the condensed phase Žin particular for the liquid state. w16x show that these modes arise from the modes involving the OH groups in a trimer arranged hydrogen bonded complex of glycerol. However, as it follows from Ref. w9x the acoustic vibrations will be located at frequencies comparable to or below ; 33 cmy1 . Thus our far-infrared results show that lowfrequency vibrations Žat ; 55 cmy1 . in glycerol exist in the whole temperature range from 253 K Žsupercooled state. up to the 323 K Žliquid state.. From a first view this result seems to be different from the results observed by low-frequency Raman spectroscopy where an essential difference in experimentally obtained spectra was observed in the same
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temperature interval. However, this change was mainly observed in the Raman intensities which included basically two main contributions: relaxational and vibrational processes. Thus, the fact that at temperatures close to Tc Žwhich is 278 K for glycerol w33x. the low-frequency peculiarity getting much more pronounced in Raman spectra probably tells us only that the relaxational process decays faster Žthe Rayleigh line getting more narrow. than changes in vibrational processes. In order to perform a more proper comparison of the results one should compare the reduced Raman spectra Žso-called RŽ n . representation w14,15x. with far-infrared spectra. After these corrections the low-frequency Raman spectra look much closer to the far-infrared spectra as seen from Fig. 3a and b Žsee also Refs. w18,31x for details.. In particular our Raman data show that for liquid glycerol we can see quite similar broad bands in the low-frequency region with two main maxima at ; 85 and ; 125 cmy1 . However, it can also be seen from the spectra at room temperature that the shoulder at around 50–60 cmy1 exists. Moreover, the comparison of Fig. 3a and b shows quite clear that the low-frequency Raman spectra of glycerol even in the RŽ n .-representation are still more sensitive to temperature changes than far-infrared spectra. Probably the difference between far-infrared and low-frequency Raman behaviour of glycerol is due to the fact that these two kinds of spectra yield a different information about intermolecular interactions and local structure in liquids w36,37x.
Fig. 6. Far-infrared spectra of triacetin in the region 25–250 cmy1 at different temperatures. ) shows the uncompensated peak of polyethylene windows.
T.S. PeroÕa et al.r Vibrational Spectroscopy 18 (1998) 149–156
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intensity of the two bands at ; 87 and ; 135 cmy1 , respectively, slightly changes with temperature but it is not so pronounced as for the Raman spectra w18,31x. The absence of the boson peak in the lowfrequency Raman spectra of triacetin is possibly due to the fact that the measurements w18,31x were carried out very close to the melting point of triacetin Žin comparison with the situation for glycerol.. Probably to get a boson peak in the Raman spectrum of triacetin one should make measurements at temperatures below 253 K as was done in Ref. w18,31x. A comparison of the far-infrared spectra of glycerol and triacetin reveals that both molecules show bands with frequencies at ; 135 cmy1 and around ; 90 cmy1 Ž; 87 cmy1 for triacetin and ; 95 cmy1 for glycerol.. The relative intensity of these bands is quite different. For glycerol the intensity of the band at 135 cmy1 is much higher than the intensity of the band at 95 cmy1 whereas for triacetin the situation is opposite. Probably this is because the band at 95 cmy1 in accordance with Ref. w16x may belong to OH vibrations whereas the band at ; 87 cmy1 for triacetin must belong to other vibrational modes, because for triacetin there are no OH groups. As it was shown in Ref. w16x CH translations may also appear at ; 95 cmy1 in glycerol molecules arranged into trimers. A band with a frequency maximum around ; 55 cmy1 in the farinfrared spectra of glycerol does not show up in the triacetin spectrum, supporting the assignment of this band to hydrogen bond modes.
Fig. 7. Band fitting of the far-infrared spectra of triacetin with three Gaussian and one Lorentzian bands.
3.2.2. Triacetin Triacetin is a highly viscous liquid with a melting point at around 276 K and Tg s 230 K. The far-infrared spectra of triacetin in the region 25–250 cmy1 are very similar to the low-frequency Raman spectra Žsee Fig. 6 Žthis work. and Fig. 2A from Ref. w33x.. The temperature dependence of the far-infrared spectra in the range 253–293 K shown in Fig. 6 is less pronounced in comparison with the behaviour of low-frequency Raman spectra in RŽ n . representation Žsee Refs. w18,31x.. Fitting of the far-infrared spectra of triacetin has also been made using four bands Žthree Gaussian and one Lorentzian.. The result of the fitting procedure is shown in Fig. 7 and listed in Table 2. The relative
Table 1 Parameters of fitting of the far-infrared spectra of glycerol at different temperatures Temperature, K
n 1 , cmy1
A1
n 2 , cmy1
A2
n 3 , cmy1
A3
n4 , cmy1
A4
323 313 303 293 287 282 276 271 266 264 253
182.4 181.6 184.5 183.6 181.6 182.7 184.6 182.5 182.6 182 180.2
0.15 0.2 0.18 0.18 0.23 0.21 0.12 0.17 0.19 0.19 0.21
138.2 137.7 137.3 135.9 136 137 135.5 136.6 137.7 137.1 139.4
0.72 0.78 0.77 0.78 0.91 0.84 0.83 0.83 0.83 0.84 0.77
95.3 97.4 95.6 95.1 96 96.6 93.9 95.3 96.5 95 94.6
0.18 0.29 0.19 0.23 0.46 0.31 0.19 0.33 0.39 0.4 0.51
64.2 60.7 64.2 61 59.6 61.4 62 60 59.2 58 52
0.49 0.42 0.44 0.37 0.4 0.39 0.37 0.33 0.33 0.31 0.2
A is the peak intensity in absorbance units.
T.S. PeroÕa et al.r Vibrational Spectroscopy 18 (1998) 149–156
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Table 2 Parameters of fitting of the far-infrared spectra of triacetin at different temperatures Temperature, K
n 1 , cmy1
A1
n 2 , cmy1
A2
n 3 , cmy1
A3
n4 , cmy1
A4
292 288 278 272 269 264 261 253
87.5 87.5 88 88 87.8 87.2 87.4 87.2
0.73 0.72 0.76 0.76 0.76 0.75 0.76 0.75
134.3 135.2 135.9 135.8 135.9 135.6 136.2 136
0.22 0.22 0.24 0.27 0.29 0.30 0.32 0.33
190 190.5 190.5 190.7 190.7 191 191 191.3
0.4 0.34 0.34 0.35 0.37 0.38 0.39 0.40
216 216 216 216.4 216.5 216.8 216.7 217
0.89 0.85 0.79 0.77 0.81 0.84 0.88 0.90
A is the peak intensity in absorbance units.
Anyway it is quite obvious from this study that strong H-bonding can yield a considerable difference between far-infrared and low-frequency Raman spectra. This is also concluded from our FIR and lowfrequency Raman study of acetonerchloroform mixtures w37x.
4. Conclusion Far-infrared spectra Ž25–500 cmy1 . of two highly viscous liquids, glycerol and triacetin Žglycerol triacetate., have been studied in the temperature range from 253 K to 323 K. The presence of H-bonding in glycerol leads to a big difference in the shape of the far-infrared spectra of these two molecules. The spectra of glycerol show very broad bands, which possibly are connected with different relative intensities of a big number of overlapping vibrational bands. The analysis of spectra in the frequency range 25– 250 cmy1 performed using a band fitting shows that four bands contributed to the far-infrared spectra of both liquids in this region. However, the frequency maxima of these bands for glycerol and triacetin are different due to the contribution from different vibrational modes. The frequency maxima obtained for glycerol in this frequency region are in good agreement with results of normal mode analyses performed for crystalline w9x and liquid w16x glycerol. The temperature behaviour below 270 K of far-infrared spectra of glycerol show substantial differences as compared with the low-frequency Raman spectra. These differences may be explained as consequence of different temperature dependences of
relaxational and vibrational contributions to the low-frequency spectra.
Acknowledgements The authors thank the European Commission for funding this work through the INTAS-96-1411 grant. DHC and OFN are greatful to the Danish National Science Research Council for general funding during this project. JKV thanks Forbairt, Ireland for partial funding under its basic programme of research.
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w26x M. Kruger, M. Soltwisch, I. Petscherizin, D. Quitmann, J. Chem. Phys. 96 Ž1992. 7352. w27x V.G. Karpov, M.I. Klinger, F.N. Ignat’ev, Zh. Eksp. Teor. Fiz. 84 Ž1983. 760; Sov. Phys.-JETP 57 Ž1983. 439. w28x M.A. Il’in, V.G. Karpov, D.A. Parshin, Zh. Eksp. Fiz. 92 Ž1987. 291; Sov. Phys.-JETP 65 Ž1987. 165. w29x U. Buchenau, Y.M. Galperin, V.I. Gurevich, D.A. Parshin, M.A. Ramos, H.R. Schober, Phys. Rev. B 46 Ž1992. 2798. w30x E.W. Fischer, Physica A 201 Ž1993. 183. w31x T.S. Perova, J.K. Vij, J. Mol. Liquids 69 Ž1996. 1. w32x C.A. Angell, in: K.-L. Ngai, G.B. Wright ŽEds.., Relaxation in Complex Systems, NRL, Washington, 1984, p. 3. w33x C. Alba-Simionesco, V. Krakoviack, M. Krauzman, P. Migliardo, F. Romain, J. Raman Spectrosc. 27 Ž1996. 715. w34x T.S. Perova, D.H. Christensen, O.F. Nielsen, J.K. Vij, J. Mol. Struct., in press. w35x GRAMSr386e User’s Guide, Galactic Industries, NH, USA, 1991–1994. w36x E. Knozinger, O. Schrems, in: J.R. Durig ŽEd.., Vibrational Spectra and Structures, A Series of Advances, Vol. 16, Elsevier, Amsterdam, 1987. w37x T.S. Perova, D.H. Christensen, O. Faurskov Nielsen, Vibrational Spectrosc. 15 Ž1997. 61.
Far-infrared spectra of highly viscous liquids: glycerol and triacetin žglycerol triacetate / T.S. Perova a , D.H. Christensen b, U. Rasmussen b, J.K. Vij c , O.F. Nielsen b c
b,)
a VaÕiloÕ State Optical Institute, St. Petersburg 199034, Russian Federation Department of Chemistry, UniÕersity of Copenhagen, 5-UniÕersitetsparken, DK-2100, Copenhagen, Denmark Department of Electronic and Electrical Engineering, UniÕersity of Dublin, Trinity College, Dublin 2, Ireland
Received 19 June 1998; revised 14 October 1998; accepted 19 October 1998
Abstract Far-infrared spectra in the region 25–500 cmy1 were obtained for glycerol and triacetin Žglycerol triacetate. at temperatures from 253 to 323 K. Far-infrared spectra of glycerol look more complicated in comparison with spectra of triacetin due to the hydrogen bonding. Band fitting performed for the frequency range 25–250 cmy1 shows that four bands have contributed to spectra of glycerol and triacetin in this region. The experimental results obtained for glycerol are in good agreement with normal mode analyses performed recently for crystalline glycerol wE.J. Bermejo, A. Criado, A. de Andress, E. Enciso, H. Schober, Phys. Rev. B 53 Ž1996. 5259.x and for glycerol in the liquid state wT. Uchino, T. Yoko, Science 273 Ž1996. 480.x. The far-infrared result are compared to the low-frequency Raman spectra of the two liquids. The difference in temperature behaviour revealed from these two kind of spectra is explained on the basis of different temperature contribution of relaxational and vibrational processes to the low-frequency vibrational spectra. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Far-infrared spectra; Glycerol; Triacetin
1. Introduction The understanding of dynamical processes in glycerol, a highly viscous liquid, has attracted much attention for many decades. Both static and dynamic measurements have been performed, including different thermal w1x and acoustic methods w2,3x, dielectric spectroscopy w4,5x, Raman w6–12x and inelastic neutron scattering w7,8,13x techniques. Despite the suggestion w6,9,10x that most of the atomic dynamics ) Corresponding author. Tel.: q45-35-320-300; Fax: q45-35350-609; E-mail: [email protected]
in the disordered state can be understood in terms of vibrations no far-infrared spectra have been studied for this highly viscous liquid. This is quite surprising since far-infrared spectra can yield a direct information about the vibrational modes and can be used as a supplementary information to the results obtained from low-frequency Raman and inelastic neutron scattering experiments w5,11,12,14,15x. Furthermore such an investigation becomes even more important since two papers have been published recently with the calculations of vibrational frequencies of glycerol using normal mode analysis based on the ab initio method and on the method of molecular modeling at
0924-2031r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 2 0 3 1 Ž 9 8 . 0 0 0 6 2 - 9
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T.S. PeroÕa et al.r Vibrational Spectroscopy 18 (1998) 149–156
medium-range order ŽMRO. w9,16x. A comparison of the calculated frequencies with the experimental ones is important in order to describe the mechanisms responsible for the low-frequency part of the vibrational spectra of highly viscous liquids and glasses. Another reason for such an investigations is to obtain additional information on low-frequency spectra in order to achieve a better understanding of the so called ‘boson peak’—the low-frequency peculiarity in Raman spectra of glasses Žincluding highly viscous liquids. and polymers. As was already mentioned glycerol has been studied using low-frequency Raman spectroscopy for many decades w6–12x. It was revealed that low-frequency Raman spectra of this compound in the liquid phase show a behaviour which is typical for many organic liquids w14,15,17x. However, with a temperature decrease the behaviour of the low-frequency Raman spectra of glycerol shows some differences in comparison with other organic liquids Ževen at temperatures quite far from the temperature of the glass transition ŽTg ... For glycerol a change of the shape of the experimentally observed low-frequency Raman spectra, I Ž n ., is quite obvious around 270 K Žnearly 100 K above the Tg s 182 K. w6,10,18x. At 263 K a noticeable peak appears at around ; 30 cmy1 , getting much more pronounced and shifted to ; 50 cmy1 at lower temperatures w6,10x. This low-frequency peak is the well-known boson peak observed in all glasses w7,19,20x. Variation of the temperature alters the relative contributions of the boson peak and of the central Rayleigh line Žwhich is caused by relaxational processes. to the total Raman spectrum I Ž n .. Above 270 K the two peaks overlap, thus forming a uniform Rayleigh line wing. Despite a considerable number of experimental and theoretical studies performed for the last two decades the nature of the boson peak is still unclear. The reader can find a discussion about this matter in Refs. w19–21x. The most recent attempts to model this behaviour are Ži. a generalized hydrodynamic theory based on a dipole-induced dipole scattering mechanism w22x, Žii. vibrational anomalies and phonon localization w23x, Žiii. a disorder-induced scattering mechanism w10,24–26x, and Živ. soft phonons w27–29x. The disorder-induced scattering mechanism was successfully applied for a description of low-frequency Raman spectra of glycerol and o-terphenyl and a good
agreement between experimental and calculated I Ž n . spectra has been demonstrated in w10,12x. According to this model the boson peak is closely related to the acoustic longitudinal and transversal phonons. Here we present far-infrared spectroscopic studies of two highly viscous liquids: glycerol, and glycerol triacetate Žtriacetin. in the temperature range 253– 323 K. These two liquids have been chosen for the reason that despite similarities in molecular structure only glycerol has a possibility of forming hydrogen bonds. Thus a comparison of the far-infrared spectra of these two liquids can be very useful in order to understand the role of hydrogen bonding in lowfrequency spectra. Another interesting peculiarity of these two highly viscous liquids is the inhomogeneity of their structures w30x which can cause their unusual properties in FIR and low-frequency Raman spectra Žsee for example Ref. w17,31x.. These two liquids also have a different fragility in accordance with Angell’s classification w32x, thus a different behaviour of the low-frequency vibrational spectra can be expected w33x.
2. Experimental The chemicals, glycerol and triacetin, were purchased from Aldrich. Glycerol 99.5% with less than 0.1% of water and triacetin 99% purity were used without further purification. Glycerol and triacetin were placed in between two polyethylene Žor poly4-methyl-1-penten ŽTPX.. windows at room temperature. The thickness of such a cell was approximately a few micrometers. Far-infrared spectra were obtained on a Bruker 120-HR FTIR spectrometer. The main spectrometer was evacuated while the liquid cells were placed in a thermostated holder in a special made sample compartment. This compartment was equipped with polyethylene Žor TPX. windows and flushed with dry nitrogen. The spectra in the region 50–500 cmy1 were recorded using a DTGS detector, 6 mm beam splitter and 2 cmy1 resolution; 512 scans were taken for each spectrum. Spectra in the region 25–250 cmy1 were recorded using a 12 mm beam splitter and a resolution of 2 cmy1 Žor 4 cmy1 in some cases.. Here an average of 125 scans was determined. To increase the signal to noise ratio a liquid helium cooled Si bolometer
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Žmodel LN6-C of Infrared Laboratories, USA. has been used for measurements in the region 25–250 cmy1 . Two sources were used for different regions: a globar in the region 50–500 cmy1 and a high pressure Hg-lamp in the region 25–250 cmy1 . A Hetofrig 04 PT 623 CB4 temperature controller ŽHolm and Halby, Denmark. has been used for recording spectra in the temperature range from 253 to 323 K. After recording the spectra a background correction due to the windows absorption was taken into account by subtracting the empty cell spectrum from the total absorption spectrum. This was particular important in order to remove the 72 cmy1 band of polyethylene. However, we did not measure the polyethylene windows at low temperatures. Thus the polyethylene sharp peak at 72 cmy1 was not fully compensated at temperatures below 293 K. However, this peak appears on the slope of the broad bands and does not influence the analysis of the results as shown below.
151
Fig. 2. Far-infrared spectra of triacetin at room temperature in the region 50–500 cmy1 . ) shows the uncompensated peak of polyethylene windows.
ence of hydrogen-bonded molecules with a wide distribution of different H-bond stretching and bending vibrations, lattice translations and librations as shown from the normal mode analysis performed in Ref. w9x for crystalline glycerol.
3. Results and discussions 3.2. 25–250 cm y 1 range 3.1. 50–500 cm
y1
range
Figs. 1 and 2 show spectra of glycerol and triacetin in the region 50–500 cmy1 at room temperature. The spectra of glycerol in this range show much broader bands than the spectra of triacetin. In some sense the spectrum of glycerol looks much more like the spectrum of water in the same range Žsee for example Ref. w31x.. This probably reflects the pres-
Fig. 1. Far-infrared spectra of glycerol at room temperature in the region 50–500 cmy1 .
3.2.1. Glycerol We concentrated our attention mainly on the low-frequency part of the far-infrared spectra, in particular, in the range 25–250 cmy1 . Fig. 3a shows spectra of glycerol at temperatures 323, 293 and 253 K. As we can see from this figure there is no great difference between far-infrared spectra obtained at different temperatures. This result being different from the behaviour of low-frequency Raman spectra of glycerol in the same temperature range Žsee Refs. w6,10,18,31,34x and Fig. 3b .. However, for a more detailed analysis it is necessary to perform a band fitting of this complicated spectrum in order to find the contribution of different components. Fitting of the glycerol spectra at different temperatures has been made by use of the GRAMS RESEARCH programme w35x. The best result was obtained by using four Gaussian bands. The fitting parameters are listed in Table 1 and also shown in Fig. 4 as an example for the temperature at 293 K. As one can see from Table 1 four bands with the frequency maxima: ; 55, ; 95, ; 135 and ; 182 cmy1 have contributed to the far-infrared spectra of
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Fig. 3. Far-infrared (a) and low-frequency Raman (b) Žin RŽ n . representation. spectra of glycerol in the region 20–250 cmy1 at different temperatures.
glycerol in the range 25–250 cmy1 . These frequencies are in a good agreement with the calculated and experimental data obtained in Ref. w9x for Raman spectra of crystalline glycerol near the melting point Žat 295 K.. These frequencies can also be compared with the calculated values obtained for liquid glycerol by Uchino and Yoko w16x. The results show an amazing coincidence in particular for the frequencies at 55 and 95 cmy1 . The molecular modeling calculations in Ref. w16x were performed on the basis of a normal-mode analysis at medium-range order ŽMRO. for cyclic glycerol trimers. The authors of Ref. w16x found two kinds of vibrational modes for model I Žsee Fig. 2 from Ref. w16x.. The modes at ; 51 cmy1 , ; 77 cmy1 and ; 93 cmy1 are mainly attributed to the collective motions of the OH groups, whereas the modes at ; 95 cmy1 result from the translational motions Žespecially those of the CH 2 and CH groups. in each glycerol molecule. Nevertheless the fitting procedure of our experimental data shows only two bands Žat ; 55 and
; 95 cmy1 . in the low-frequency part Ž- 100 cmy1 . instead of the three calculated ones of Ref. w16x. The reason might be that the relative intensity of these bands can be different for infrared and Raman spectra. It should be noted here that the frequency maximum position with temperature for all other bands does not change as much as for the band around ; 55 cmy1 Žsee Table 1.. For example for the bands at 182, 137 and 95 cmy1 the difference in maximum position is only a few cmy1 in the temperature interval from 253 K up to 323 K whilst for the band at 55 cmy1 this change is about 12 cmy1 . Probably it means that we have two bands with different relative intensities or the intensity of the band at 77 cmy1 is very low in the far-infrared spectra. The analysis of the second derivatives shown in Fig. 5 also shows that probably two bands Ž; 50 and ; 70 cmy1 . with different relative intensity can influence the low-frequency part of this spectrum. Concerning the peak intensities, only a small difference in the behaviour of these four bands is found in the temperature range 293–323 K Žsee Table 1.. In particular the intensity of the low-frequency band at ; 55 cmy1 increases with increasing temperature. This result shows the same tendency as was revealed from the low-frequency Raman study of glycerol w6x. However, the intensity variation of the Raman spectra is much more pronounced in this temperature range. The behaviour of the far-infrared spectra in the low temperature range Ž253–293 K. substantially differ from the behaviour of the low-frequency Raman spectra. In particular as was shown in Refs.
Fig. 4. Band fitting of the far-infrared spectra of glycerol with four Gaussian bands.
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Fig. 5. The second derivative spectrum Žtop. obtained from the experimental far-infrared spectrum of glycerol Žbottom. after the smoothing procedure ŽFFT filter..
w6,10,18x in Raman spectra of glycerol a noticeable peak at around ; 33 cmy1 developed at temperatures below ; 270 K. The peak getting even more pronounced at lower temperatures w6,10x. However, there is no sign of the presence of this peak in the far-infrared spectra in the temperature range studied here. It could not be a band with the frequency maximum at ; 50 cmy1 Žobtained from the fitting., since as was shown in Ref. w9x the band with the frequency maximum at 54 cmy1 persists in the low-frequency Raman spectra of crystalline glycerol and it is assigned to the lowest optical phonon. As it was noticed in Ref. w9x the calculated mode frequencies for the isolated molecule in this region belong to the torsional and mainly bending modes. Calculations performed for the condensed phase Žin particular for the liquid state. w16x show that these modes arise from the modes involving the OH groups in a trimer arranged hydrogen bonded complex of glycerol. However, as it follows from Ref. w9x the acoustic vibrations will be located at frequencies comparable to or below ; 33 cmy1 . Thus our far-infrared results show that lowfrequency vibrations Žat ; 55 cmy1 . in glycerol exist in the whole temperature range from 253 K Žsupercooled state. up to the 323 K Žliquid state.. From a first view this result seems to be different from the results observed by low-frequency Raman spectroscopy where an essential difference in experimentally obtained spectra was observed in the same
153
temperature interval. However, this change was mainly observed in the Raman intensities which included basically two main contributions: relaxational and vibrational processes. Thus, the fact that at temperatures close to Tc Žwhich is 278 K for glycerol w33x. the low-frequency peculiarity getting much more pronounced in Raman spectra probably tells us only that the relaxational process decays faster Žthe Rayleigh line getting more narrow. than changes in vibrational processes. In order to perform a more proper comparison of the results one should compare the reduced Raman spectra Žso-called RŽ n . representation w14,15x. with far-infrared spectra. After these corrections the low-frequency Raman spectra look much closer to the far-infrared spectra as seen from Fig. 3a and b Žsee also Refs. w18,31x for details.. In particular our Raman data show that for liquid glycerol we can see quite similar broad bands in the low-frequency region with two main maxima at ; 85 and ; 125 cmy1 . However, it can also be seen from the spectra at room temperature that the shoulder at around 50–60 cmy1 exists. Moreover, the comparison of Fig. 3a and b shows quite clear that the low-frequency Raman spectra of glycerol even in the RŽ n .-representation are still more sensitive to temperature changes than far-infrared spectra. Probably the difference between far-infrared and low-frequency Raman behaviour of glycerol is due to the fact that these two kinds of spectra yield a different information about intermolecular interactions and local structure in liquids w36,37x.
Fig. 6. Far-infrared spectra of triacetin in the region 25–250 cmy1 at different temperatures. ) shows the uncompensated peak of polyethylene windows.
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intensity of the two bands at ; 87 and ; 135 cmy1 , respectively, slightly changes with temperature but it is not so pronounced as for the Raman spectra w18,31x. The absence of the boson peak in the lowfrequency Raman spectra of triacetin is possibly due to the fact that the measurements w18,31x were carried out very close to the melting point of triacetin Žin comparison with the situation for glycerol.. Probably to get a boson peak in the Raman spectrum of triacetin one should make measurements at temperatures below 253 K as was done in Ref. w18,31x. A comparison of the far-infrared spectra of glycerol and triacetin reveals that both molecules show bands with frequencies at ; 135 cmy1 and around ; 90 cmy1 Ž; 87 cmy1 for triacetin and ; 95 cmy1 for glycerol.. The relative intensity of these bands is quite different. For glycerol the intensity of the band at 135 cmy1 is much higher than the intensity of the band at 95 cmy1 whereas for triacetin the situation is opposite. Probably this is because the band at 95 cmy1 in accordance with Ref. w16x may belong to OH vibrations whereas the band at ; 87 cmy1 for triacetin must belong to other vibrational modes, because for triacetin there are no OH groups. As it was shown in Ref. w16x CH translations may also appear at ; 95 cmy1 in glycerol molecules arranged into trimers. A band with a frequency maximum around ; 55 cmy1 in the farinfrared spectra of glycerol does not show up in the triacetin spectrum, supporting the assignment of this band to hydrogen bond modes.
Fig. 7. Band fitting of the far-infrared spectra of triacetin with three Gaussian and one Lorentzian bands.
3.2.2. Triacetin Triacetin is a highly viscous liquid with a melting point at around 276 K and Tg s 230 K. The far-infrared spectra of triacetin in the region 25–250 cmy1 are very similar to the low-frequency Raman spectra Žsee Fig. 6 Žthis work. and Fig. 2A from Ref. w33x.. The temperature dependence of the far-infrared spectra in the range 253–293 K shown in Fig. 6 is less pronounced in comparison with the behaviour of low-frequency Raman spectra in RŽ n . representation Žsee Refs. w18,31x.. Fitting of the far-infrared spectra of triacetin has also been made using four bands Žthree Gaussian and one Lorentzian.. The result of the fitting procedure is shown in Fig. 7 and listed in Table 2. The relative
Table 1 Parameters of fitting of the far-infrared spectra of glycerol at different temperatures Temperature, K
n 1 , cmy1
A1
n 2 , cmy1
A2
n 3 , cmy1
A3
n4 , cmy1
A4
323 313 303 293 287 282 276 271 266 264 253
182.4 181.6 184.5 183.6 181.6 182.7 184.6 182.5 182.6 182 180.2
0.15 0.2 0.18 0.18 0.23 0.21 0.12 0.17 0.19 0.19 0.21
138.2 137.7 137.3 135.9 136 137 135.5 136.6 137.7 137.1 139.4
0.72 0.78 0.77 0.78 0.91 0.84 0.83 0.83 0.83 0.84 0.77
95.3 97.4 95.6 95.1 96 96.6 93.9 95.3 96.5 95 94.6
0.18 0.29 0.19 0.23 0.46 0.31 0.19 0.33 0.39 0.4 0.51
64.2 60.7 64.2 61 59.6 61.4 62 60 59.2 58 52
0.49 0.42 0.44 0.37 0.4 0.39 0.37 0.33 0.33 0.31 0.2
A is the peak intensity in absorbance units.
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Table 2 Parameters of fitting of the far-infrared spectra of triacetin at different temperatures Temperature, K
n 1 , cmy1
A1
n 2 , cmy1
A2
n 3 , cmy1
A3
n4 , cmy1
A4
292 288 278 272 269 264 261 253
87.5 87.5 88 88 87.8 87.2 87.4 87.2
0.73 0.72 0.76 0.76 0.76 0.75 0.76 0.75
134.3 135.2 135.9 135.8 135.9 135.6 136.2 136
0.22 0.22 0.24 0.27 0.29 0.30 0.32 0.33
190 190.5 190.5 190.7 190.7 191 191 191.3
0.4 0.34 0.34 0.35 0.37 0.38 0.39 0.40
216 216 216 216.4 216.5 216.8 216.7 217
0.89 0.85 0.79 0.77 0.81 0.84 0.88 0.90
A is the peak intensity in absorbance units.
Anyway it is quite obvious from this study that strong H-bonding can yield a considerable difference between far-infrared and low-frequency Raman spectra. This is also concluded from our FIR and lowfrequency Raman study of acetonerchloroform mixtures w37x.
4. Conclusion Far-infrared spectra Ž25–500 cmy1 . of two highly viscous liquids, glycerol and triacetin Žglycerol triacetate., have been studied in the temperature range from 253 K to 323 K. The presence of H-bonding in glycerol leads to a big difference in the shape of the far-infrared spectra of these two molecules. The spectra of glycerol show very broad bands, which possibly are connected with different relative intensities of a big number of overlapping vibrational bands. The analysis of spectra in the frequency range 25– 250 cmy1 performed using a band fitting shows that four bands contributed to the far-infrared spectra of both liquids in this region. However, the frequency maxima of these bands for glycerol and triacetin are different due to the contribution from different vibrational modes. The frequency maxima obtained for glycerol in this frequency region are in good agreement with results of normal mode analyses performed for crystalline w9x and liquid w16x glycerol. The temperature behaviour below 270 K of far-infrared spectra of glycerol show substantial differences as compared with the low-frequency Raman spectra. These differences may be explained as consequence of different temperature dependences of
relaxational and vibrational contributions to the low-frequency spectra.
Acknowledgements The authors thank the European Commission for funding this work through the INTAS-96-1411 grant. DHC and OFN are greatful to the Danish National Science Research Council for general funding during this project. JKV thanks Forbairt, Ireland for partial funding under its basic programme of research.
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