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Phase transitions of Decylammonium chloride by Raman and infrared spectroscopic studies
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Solld State Communications, Vol.62,No.2, pp.73-78, 1987. Printed in Great Britain.
0038-]098/87 $3.00 + .00 ©1987 Pergamon Journals Ltd.
PHASE TRANSITIONS OF DECYLAMMONIUM CHLORIDE BY RAMAN AND INFRARED SPECTROSCOPl C STUDIES M Picquart and G. Lacrampe Universit¢ Rene Descartes, Laboratoire de Physique Mol~culaire et Biologique, 45 rue des Saints Peres, / 5 2 7 0 Paris Cedex 06, France
(Received 22 August, 1986 by J. Joffrin. In revised form 3 November, 1986)
Raman scattering and Infrared absoption were used to study plnase t r a n s i t i o n between a non intercalated structure and an intercalated one in Decylammonium chloride CIoH21NH3C1, in the temperature range from 79 to 370 K and to determine the t r a n s i t i o n temperatures The m e l t i n g of the cnalns seems to be the cause of the t r a n s i t i o n Unusual s p l i t t i n g of some v i b r a t i o n modes help us to ct~aracterize the intercalated structure. "[he entering of air into the s t r u c t u r e seems to be at the origin of the discrepancy that appears in the experimental r e s u l t s
Raman spectra of these compounds can be divided into three regions of interest. The low frequency region between Rayleigh line and 500 cm-], the region between 800 and 1500 cm-T and the region between 2800 and 3000 cm -I. These regions can also be found in all long chain molecules, surfactants, biomembranes, alkanes or polyethylene. Raman studies about these lates 7-9 can help us to interpret our experimental results. Decane is an alkane with even number n of carbon atoms. Alkanes wiht 6~n~26 have a triclinic structure with one molecule per unit cell. Their space group is Cil that means that vibrational
I. INTRODUCTION Decylammonium chloride (DACI) has been the subject of recent papers i-6 Crystals of DACI at room temperature are perfectly ordered with chains in the all-trans configuration and form an interdigitated bilayer. N - H C I hydrogen bonds (3.15 to :3.19 A) between adjacent chains give a quite dense structure. It is known i to be monoclinic with two molecules per unit cell, a=5.7 A, b=7.16 A, c = !5.49 A, 8=91.3 " and belongs to the space group C22. Previous studies by NMR, X ray diffraction and Infrared absorption have shown a phase transition at 48 "C from an intercalated structure to a non intercalated one. This transition was shown to be non reversible. 1-3.6 We report here results of the study of the phase transition by Raman scattering and Infrared absorption in order to study this lrreversibility arid to characterize the different phases. We give also some indications about the structure of each phases. In order to explain some bands observed in the experimental spectrum, it is necessary to compare DACI (CIoH21NI-13CI) spectrum to those of Decane
modes are either Raman active or Infrared active. Low temperature Raman and Infrared spectra of Decane lO are characteristic of triclinic structure. The main characteristics of the Raman spectrum of the solid phase are the two peaks of the CH2 stretching symmetrical mode at 2847 and 2855 cm-1 yet observed by Snyder et al. 9 in the Raman spectrum of Eicosane (C2oH42). The Infrared spectrum in the solid phase is marked by the splitting of the CH2 rocking mode at 7 t 5 and 725 cm -1. Decylamine, to our knowledge, has not been studied by X-ray scattering. In the raman spectrum of the solid phase we observe a band at 1417 cm -]
(C IOH22 ) ahd Decylamine (C 10H2 INH2) 73
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PHASE TRANSITIONS
OF DECYLAMMONIUM
CHLORIDE
Vol. 62, No. 2
due to the Ag component of the CH 2 bending mode, splitted by crystal effect. This band is characteristic of an orthorhombic structure,ii The NH 2 stretching modes give small bands at 3148, 3174 and 3328 cm -i while a small band at 1098 cm -i is probably due to NH bending modes. The other bands are usually observed in long chain molecules. In the Infrared spectrum we also observe a band at 1603 cm -I due to N-H vibrations and the splitting of some of the rocking CH 2 progressions bands characteristic of orthorhombic structure. 2. EXPERIMENTALMETHODS Decylammonium chloride (DACI) was prepared according to Radley and Saupe method i2. Raman experimental set-up has been described elsewhere i3 and was used in the same conditions. The 514.5 nm laser line power was 300 m W and the spectral resolution less than 4 cm -i. Infrared spectra were recorded on a 783 Perkin Elmer spectrophotometer at a resolution of 1.5 cm
-I
Pellets of DACl powder and KBr powder were prepared and.placed in a cryostat or in a heating cell controled by a temperature regulation. We used also DACI deposited on a KBr window from ethanol solution Liquid cells of 12 pm and 25 t~m were used for experiments with decane and decylamine. Decane was purchased from Aldrich-Chemie and Decylamine from Fluka. They were used without further purification. 3. RESULTS AND DISCUSSION A Raman experiments. As observed on many others surfactants, alkanes, etc, the number of observed vibrational modes is smaller than the theoretical predictions. We can analyze the vibrations of only one molecule to identify the bands. Figure I-A shows Raman spectrum of DAC1 at room temperature. In the low energy part we can observe two intense peaks :one is situated at 80 cm-I with a strong shoulder at 91 cm- t , and the other one at 220 cm-t with an important shoulder at 235 cm -1. This band at 220 cm -I is probably the longitudinal acoustic mode (LAM) observed in alkanes. The frequency of this mode is strongly dependent of the chain length. We note also some small bands between 320 and 520 cm- t
A
//
Y
2900
1500
1000
500
Fig.1 Raman spectrum of DAC1 powder at 30°C (A), 65°C (B) and cooled at 30"C 2 hours later (C).
The part between 850 and 1500 cm-1 contains the CH3 rocking mode (891 cm-l), the skeletal vibrational modes between 1050 and 1130 cm -1, the CH2 twisting mode (1297 cm - I ) and the CH2 bending modes (1420 - 1480 cm-1) The high energy part is the more complex because it results from the superposition of CH stretching vibration modes and Fermi resonance interaction of these modes with the overtones of the bending modes The main features are the CH2 symmetrical stretching m o d e (2850 cm- 1) and the antisymmetrical stretching mode (2885 cm- 11 We can note that the band at 1066 cm-~ in Decylamine gives two bands at 1059 and t066 cm-I in DAC1 with nearly the same intensity. We can see also two bands with same intensity at 2850 and 2856 cm-1 in the CH stretching modes region. The strong band at 1423 cm -I exists as in Decylamine ( 1417 cm- 1). Heating the powder up to 65'C we can see on Fig. I-B some changes in the vibration modes. In the low frequency region, the bands under 100 cm-I are no more resolved and make a large wing of diffusion near the Rayleigh line and the longitudinal acoustical mode at 230 cm-! broadens. This mode must disappears in the liquid isotropic phase but the fact is still observed at the same frequency shows that the structure of the high temperature phase is still tamellar with the majority of the chains in all-trans configuration, tn the 800-t500 cm -1 region we observe the band due to the rocking CH3 mode (892 cm -I) and small bands that appear at 845 and 872
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PHASE TRANSITIONS OF DECYLAMMONIUM CHLORIDE
cm-I Following 5nyder 8 these bands are attributed to gauche conformations. The mode at 845 cm-1 corresponds to TG+-Tn_5 conformations
3.
and the band at 872 cm- ! to G-+Tn_4 conformations In the same time the intensity of the band situated at 1080 cm -! increases in relation to the CC streching modes. The band at t080 cm-1 shows that qauche .. conformers are ,.,a.e~en. r ~ t in the . structure. It is to be noticed that the spiitting of the modes observed in the tow temperature phase doesn't exist at 65°C and that the mode at 1423 cm-; disappears. !n the high temperature phase the CH2 bending modes are at t440 and }450 cm- 1 !n the CH stretching vibration mode region, the antisymmetrical mode (2885 cm -t) decreases and broadens and fina!]y has the same peak height than the symmetrical modes (2850 cm- !). These results I, n d" r~ ~ {e " that the structure of the high temperature phase is stilt ordered with the molecules in a bilayer configuration, but in an hexagonal arrangement as indicated by the CH stretching modes ratio 9 The fact that the modes are no more splitted suggest that the interaction between chains is weaker than at low temperatures and the =~ u.... ~ could be non-, ,er~al~e, with a meltinq of the chains ,as indicated by the enhancement of the !080 cm- ; peak and the increase of the band at ~_93_, cm -~ This ~,~=,,~t seems ;n agreement with the !attice parameters proposed by Busico e t a ] 4 a = 5.1 A. c = 2 8 t A for the high temberatJre phase. The a parameter doesn't change very much between low and high temperature phases, but in this !ast phase the chains are not intercalated. The peak height ratio t(2880)/1(2850) of the antisymmetrical and the symmetrical stretching vibration modes is used to measure the lateral order and gwes information on chain packing and chain mobiiity 14-~s We studied this ratio over the temperature range 79 K to 450 K (Fig. 2) We see that the ratio has the value of 2.4 at low temperature and decreases gently tilt 2. I at 312 K, then it sharply decreases at 313 K. It seems that we obtain an intermediate phase till 322 K where the sample passes in the hexagonal phase. The temperatures are in good agreement with the results of Kind et al. 1 In the same time at 3 t3 K the modes situated at 2854, 2869, 2893, 2950 and 2970 cm-1 disappear and the modes at 2860, 2965 and 2987 cm -1 appear. At ~22 K the mode at 2847 cm-t shift to 2850 cm- 1 (Fig. 3). In the intermediate phase and in the high temperature phase the peak at 1059 cm-1
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Fig.2 Height intensity ratio 1(2880)/1(2850) of DACl powder between 230 and 370 K. cm-1 2990
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Fig.3 Variation of the CH stretching modes frequency versus temperature between 290 and 340 K. disappears, the rotation of the molecules is observed by the decrease of the peak at 2885 cm -~ and its broadening meanwhile the increase of the peak at 1080 cm -~ shows the appearance of gauche
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PHASE TRANSITIONS OF DECYLAMMONIUM CHLORIDE
Vol. 62, No. 2
conformers. The structural difference between these two phases could be in the t i l t angle of the chains with the polar heads. Cooling the sample down to 30"C and recording the spectrum gives Fig. I-C. If we compared with Fig. I-A we cannot observe noticeable changes neither in the position of the peaks nor in the intensities. The Raman spectrum is quite identical before heating and after cooling. The transition is reversible. Splitting of the CC antisymmetrical vibration mode has never been observed to our knowledge, neither in orthorhombic structure nor in monoclinic or in trictinic phases. It could result from the intercalated structure, where the chains of the upper layer are interdigitated between the chains of the lower layer. In this case the splitting of the CH symmetrical stretching mode and the component of the CH2 bending mode (1423 cm-I) could be due to crystal effect resulting from this more compact monoclinic structure where we have interactions between chains of different layers. Experiments with DAC1 urea clathrates are in progress to elucidate this point. We just want to notice now that same splitting is observed, on the perovskite-type layer compound (CioH21NH~)2ZnC14 where the chains are interdigitated ~o but not observed on (C~oH21NH4)2CdC14 where the chains are not interdigitated ~o-t7 B. Infrared experiments. On Fig. 4-A is presented the Infrared spectrum of DACi at 32'C We can observe the splitting of tl~e CH2 bending modes ( 1435 - 1475 cm- ~) and the CH2 rocking modes (715 - 726 cm -1) due to crystal effect The all-trans conformation is confirmed by the narrow and intense CH2 wagging (1200 - 1370 cm-l) and CH2 rocking (7!5 - t0!9 cm-i) band progressions. We see also that the symmetrical NH3 deformation mode is splitted in three components (1584, 1615 and 1628 cm -I) This splitting is due to different lengths of the N-HC1 hydrogen bonds 6 Same thing is observed for NH3 rocking modes (942 and 95.5 cm-I). The antisymmetrical NH3 deformation mode gives two bands at t508 and 1515 cm -~. Heating the sample up to 67°C we obtain the Fig. 4-B The NH3 deformation and rocking mode give respectively a band at 1600 and 960 cm -1 indicating a rearrangement of the polar heads in such a way that the hydrogen bonds have the same lengths. The CH2 rocking mode gives only one band at 720 cm-1, and the CH2 bending modes give a
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Fig.4 Infrared spectrum, of DAC1 powder at 32°C (A), 67"C (B), 36"C one hour later (C) and 30°C six days later (D). broad band centered at 1465 cm -i These modes are in agreement with the experimental results of alkanes in hexagonal lattice. Nearly all the bands are broad demonstrating gauche conformations. But there is still an order, as observed in the Raman spectrum. Cooling the sample down to 36°C gives Fig.4-C, where a~! the bands of the previous phase are narrow and intense but with an order quite different from the phase we have at 25°C on Fig. 4-A The bands at 720 cm-1 and 1468 cm-1 seems to indicate, as in the high temperature phase, an hexagonal packing of the chains. The N-H--.-C1 bonds seem also identical. Fig. 4-D shows the Infrared spectrum at 30°C ten days later. We can note that the general feature of the spectrum is identical to that of the phase at 25°C. The bands are splitted, narrow and intense. We have only small differences in the relative intensities of some of them like the CH2 rocking and the NH3 bending modes. This could be the consequence of a change in the t i l t angle of the chains with the polar heads, The Infrared experiments presented on Fig. 4 and
Vol. 62, No. 2
PHASE TRANS'TTIONS OF DECYLAMMONI~UM
described above were made with pellets of DAC1 and KBr. They are nearly identical to the results of Casal et al. 6 In order to study the CH stretching region we made other pellets with less DAC1 than previously. In this last case the spectrum after heating and cooling process stay as in the Fig. 4-C even two months later. Doing the experiments on DACI evaporated from ethanol solution and deposited on KBr window, the spectra and the behaviour of the samples are nearly identical to those presented on Fig. 4-A,B,C. Waiting nearly two weeks after cooling, we obtain a spectrum with the bands splitted as in Fig. 4-A, but With peaks less intense, and two peaks at 158,9 and t591 cm -i for the NH3 bending modes, tt seems that the preparation procedure of the samoie ~s responsabte of the differences between infrared results. Ricard et al. 17 mentionned that they didn't use KBr pellets but suspension in Nujol to prevent effects of bressure or chemical exchange in perovskite layer combounds. In order to understand why Raman scattering and infrared absorption gives conflicting results, we want to say a few words about the Raman experimental sample mounting. DAC1 powder ts enclosed in sealed Imm-diameter capiiiaries. These are nearly 5 cm high, and contain about a height of 3 cm of powder, the extra part contains confined air. The spectra presented here are obtained by focusing the iaser beam neart,l in the middle of the height of the powder. We made other spectra of DACi eowder after the cooling at 30°C Dut fOCusing the !aser beam on the powder at the interface air-DACl. Fig..5 presents the CH stretching mode and the skeletal mode region~ in these conditions. We can see that the s~ectrum has the same aspect that the one recorded from the high temperature phase (non intercalated structure), we have only one peak at 1063 cm-1 and the spectrum is still the same two months later. Kind et al. 1, in order to explain the no reversibility of the thermal dilatation, show that when air is entering the structure during heating process, the transition becomes irreversible. But, as we can see from our results, this transition is perfectly reversible when there is no contact (or much less contact) with air. In the same time this result seems confirm that the splitting of the CC antisymmetrical mode is due to the intercalated structure. 4. CONCLUSION We showed that the discrepancy between Raman scattering and Infrared absorption results seems
77
CELORIDE
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Fig.5 CH stretching modes and CC skeletal modes of DAC1 powder at 30"C at the interface air-DAC1 after heating and cooling process. due to the pener,ratien cf air between DAC] layers. ~n the first experiments the transition is reversib!e, in the second it doesn't. The fact that the Raman spectrum is different according to the focusing point of the laser seems confirm this hTpothesis. Experiments with inert gas should be necessary in order to prove it. We also shown that the sample preparation m Infrared absorption is also important. It seems that interaction of DAC1 with KBr prevent the return of the sample to an ordered phase when there is less DAC] than KBr At the contrary when there is more DAC1 than KBr the return to an ordered pnase ]s obtained !O days later. We showed also that the sample prepared by ethanoi evaporation gives also slight differences. The differences in the Infrared absorption measurements seem coming from a change in the t i l t angle of the chains with the polar head, as it is shown by the modification of the NH3 vibration modes. In both case, as it was shown by X ray diffraction experiments, the transition is between an intercalated structure at low temperature and a non intercalated structure at high temperature. Acknowledgments. Thanks are due to M. Germain from the Laboratoire de Physique des Solides (Orsay) for the making of DACI and to Prof. M. .Jaffrain for helpful discussions.
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CHLORIDE
Vol. 62, No. 2
REFERENCES 1. R.Kind, R.Blinc, H.Arend, P.Muralt, J.Slak, G.Chapuis, K.J.Schenk and B.Zeks, Phys. Rev. A 26, 1816 (1982). 2. Biozar, M.I.Burzar, R.Blinc, R.Kind and H.Arend, Solid State Comm. 44, 737 (1982). 3. J.Seliger, V.Zagar, RBlinc, H.Arendad and G.Chapuis, J. Chem. Phys. 78, 2661 (1983). 4. V.Busico, P.Cernicchiaro, P.Corradini and M.Vacatello, J. Phys. Chem. 87, 1631 (1983). 5. G.Chapuis, K.Schenk and J.Zuniga, MoZ Cryst Liq. Cryst. 113, 113 (1984). 6. H.L.Casal, H.H.Mantsch and D.G.Cameron, Solid State Comm. 49, 571 (1984). 7. R.G.Snyder, J. Chem. Phys. 42, 1744 (1965). 8. R.G.Snyder, J. Chem. Phys. 47, 1744 (1967). 9. R.G.Snyder, Si.Hsu and S.Krirnm, Spectrochim. Acre 34 A, 395 (1978).
I0. M.Picquart, to be published. 11. S.Abbate, G.Zerbi and S.L.Wunder, J. Phys. Chem. 86, 314(1982). 12. K.Radley and A.Saupe, MoZ Cryst. L iq. Cryst. 44, 227 (1978). 13. M.Ptcquart, J. Phys. Chem. 90, 243 (1986). 14. B.P.Galoerand Wl.Petitcolas, B/ochim. Biophys. Acre, 465,260 (1977). 15. R.G.Snyder, J.R.Scherer and B.P.Gaber, Biochim. Biophys. Acta, 601,47 (1980). 16. L.Ricard, M.Rey-Lafon and C.Biran, J. Phys. Chem. 88, 5614 (1984). 17. L.Ricard, Riavagnat and M.Rey-Lafon, J. Phys. Chem. 89, 4887 (1985).
Solld State Communications, Vol.62,No.2, pp.73-78, 1987. Printed in Great Britain.
0038-]098/87 $3.00 + .00 ©1987 Pergamon Journals Ltd.
PHASE TRANSITIONS OF DECYLAMMONIUM CHLORIDE BY RAMAN AND INFRARED SPECTROSCOPl C STUDIES M Picquart and G. Lacrampe Universit¢ Rene Descartes, Laboratoire de Physique Mol~culaire et Biologique, 45 rue des Saints Peres, / 5 2 7 0 Paris Cedex 06, France
(Received 22 August, 1986 by J. Joffrin. In revised form 3 November, 1986)
Raman scattering and Infrared absoption were used to study plnase t r a n s i t i o n between a non intercalated structure and an intercalated one in Decylammonium chloride CIoH21NH3C1, in the temperature range from 79 to 370 K and to determine the t r a n s i t i o n temperatures The m e l t i n g of the cnalns seems to be the cause of the t r a n s i t i o n Unusual s p l i t t i n g of some v i b r a t i o n modes help us to ct~aracterize the intercalated structure. "[he entering of air into the s t r u c t u r e seems to be at the origin of the discrepancy that appears in the experimental r e s u l t s
Raman spectra of these compounds can be divided into three regions of interest. The low frequency region between Rayleigh line and 500 cm-], the region between 800 and 1500 cm-T and the region between 2800 and 3000 cm -I. These regions can also be found in all long chain molecules, surfactants, biomembranes, alkanes or polyethylene. Raman studies about these lates 7-9 can help us to interpret our experimental results. Decane is an alkane with even number n of carbon atoms. Alkanes wiht 6~n~26 have a triclinic structure with one molecule per unit cell. Their space group is Cil that means that vibrational
I. INTRODUCTION Decylammonium chloride (DACI) has been the subject of recent papers i-6 Crystals of DACI at room temperature are perfectly ordered with chains in the all-trans configuration and form an interdigitated bilayer. N - H C I hydrogen bonds (3.15 to :3.19 A) between adjacent chains give a quite dense structure. It is known i to be monoclinic with two molecules per unit cell, a=5.7 A, b=7.16 A, c = !5.49 A, 8=91.3 " and belongs to the space group C22. Previous studies by NMR, X ray diffraction and Infrared absorption have shown a phase transition at 48 "C from an intercalated structure to a non intercalated one. This transition was shown to be non reversible. 1-3.6 We report here results of the study of the phase transition by Raman scattering and Infrared absorption in order to study this lrreversibility arid to characterize the different phases. We give also some indications about the structure of each phases. In order to explain some bands observed in the experimental spectrum, it is necessary to compare DACI (CIoH21NI-13CI) spectrum to those of Decane
modes are either Raman active or Infrared active. Low temperature Raman and Infrared spectra of Decane lO are characteristic of triclinic structure. The main characteristics of the Raman spectrum of the solid phase are the two peaks of the CH2 stretching symmetrical mode at 2847 and 2855 cm-1 yet observed by Snyder et al. 9 in the Raman spectrum of Eicosane (C2oH42). The Infrared spectrum in the solid phase is marked by the splitting of the CH2 rocking mode at 7 t 5 and 725 cm -1. Decylamine, to our knowledge, has not been studied by X-ray scattering. In the raman spectrum of the solid phase we observe a band at 1417 cm -]
(C IOH22 ) ahd Decylamine (C 10H2 INH2) 73
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PHASE TRANSITIONS
OF DECYLAMMONIUM
CHLORIDE
Vol. 62, No. 2
due to the Ag component of the CH 2 bending mode, splitted by crystal effect. This band is characteristic of an orthorhombic structure,ii The NH 2 stretching modes give small bands at 3148, 3174 and 3328 cm -i while a small band at 1098 cm -i is probably due to NH bending modes. The other bands are usually observed in long chain molecules. In the Infrared spectrum we also observe a band at 1603 cm -I due to N-H vibrations and the splitting of some of the rocking CH 2 progressions bands characteristic of orthorhombic structure. 2. EXPERIMENTALMETHODS Decylammonium chloride (DACI) was prepared according to Radley and Saupe method i2. Raman experimental set-up has been described elsewhere i3 and was used in the same conditions. The 514.5 nm laser line power was 300 m W and the spectral resolution less than 4 cm -i. Infrared spectra were recorded on a 783 Perkin Elmer spectrophotometer at a resolution of 1.5 cm
-I
Pellets of DACl powder and KBr powder were prepared and.placed in a cryostat or in a heating cell controled by a temperature regulation. We used also DACI deposited on a KBr window from ethanol solution Liquid cells of 12 pm and 25 t~m were used for experiments with decane and decylamine. Decane was purchased from Aldrich-Chemie and Decylamine from Fluka. They were used without further purification. 3. RESULTS AND DISCUSSION A Raman experiments. As observed on many others surfactants, alkanes, etc, the number of observed vibrational modes is smaller than the theoretical predictions. We can analyze the vibrations of only one molecule to identify the bands. Figure I-A shows Raman spectrum of DAC1 at room temperature. In the low energy part we can observe two intense peaks :one is situated at 80 cm-I with a strong shoulder at 91 cm- t , and the other one at 220 cm-t with an important shoulder at 235 cm -1. This band at 220 cm -I is probably the longitudinal acoustic mode (LAM) observed in alkanes. The frequency of this mode is strongly dependent of the chain length. We note also some small bands between 320 and 520 cm- t
A
//
Y
2900
1500
1000
500
Fig.1 Raman spectrum of DAC1 powder at 30°C (A), 65°C (B) and cooled at 30"C 2 hours later (C).
The part between 850 and 1500 cm-1 contains the CH3 rocking mode (891 cm-l), the skeletal vibrational modes between 1050 and 1130 cm -1, the CH2 twisting mode (1297 cm - I ) and the CH2 bending modes (1420 - 1480 cm-1) The high energy part is the more complex because it results from the superposition of CH stretching vibration modes and Fermi resonance interaction of these modes with the overtones of the bending modes The main features are the CH2 symmetrical stretching m o d e (2850 cm- 1) and the antisymmetrical stretching mode (2885 cm- 11 We can note that the band at 1066 cm-~ in Decylamine gives two bands at 1059 and t066 cm-I in DAC1 with nearly the same intensity. We can see also two bands with same intensity at 2850 and 2856 cm-1 in the CH stretching modes region. The strong band at 1423 cm -I exists as in Decylamine ( 1417 cm- 1). Heating the powder up to 65'C we can see on Fig. I-B some changes in the vibration modes. In the low frequency region, the bands under 100 cm-I are no more resolved and make a large wing of diffusion near the Rayleigh line and the longitudinal acoustical mode at 230 cm-! broadens. This mode must disappears in the liquid isotropic phase but the fact is still observed at the same frequency shows that the structure of the high temperature phase is still tamellar with the majority of the chains in all-trans configuration, tn the 800-t500 cm -1 region we observe the band due to the rocking CH3 mode (892 cm -I) and small bands that appear at 845 and 872
Vol. 62, No. 2
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PHASE TRANSITIONS OF DECYLAMMONIUM CHLORIDE
cm-I Following 5nyder 8 these bands are attributed to gauche conformations. The mode at 845 cm-1 corresponds to TG+-Tn_5 conformations
3.
and the band at 872 cm- ! to G-+Tn_4 conformations In the same time the intensity of the band situated at 1080 cm -! increases in relation to the CC streching modes. The band at t080 cm-1 shows that qauche .. conformers are ,.,a.e~en. r ~ t in the . structure. It is to be noticed that the spiitting of the modes observed in the tow temperature phase doesn't exist at 65°C and that the mode at 1423 cm-; disappears. !n the high temperature phase the CH2 bending modes are at t440 and }450 cm- 1 !n the CH stretching vibration mode region, the antisymmetrical mode (2885 cm -t) decreases and broadens and fina!]y has the same peak height than the symmetrical modes (2850 cm- !). These results I, n d" r~ ~ {e " that the structure of the high temperature phase is stilt ordered with the molecules in a bilayer configuration, but in an hexagonal arrangement as indicated by the CH stretching modes ratio 9 The fact that the modes are no more splitted suggest that the interaction between chains is weaker than at low temperatures and the =~ u.... ~ could be non-, ,er~al~e, with a meltinq of the chains ,as indicated by the enhancement of the !080 cm- ; peak and the increase of the band at ~_93_, cm -~ This ~,~=,,~t seems ;n agreement with the !attice parameters proposed by Busico e t a ] 4 a = 5.1 A. c = 2 8 t A for the high temberatJre phase. The a parameter doesn't change very much between low and high temperature phases, but in this !ast phase the chains are not intercalated. The peak height ratio t(2880)/1(2850) of the antisymmetrical and the symmetrical stretching vibration modes is used to measure the lateral order and gwes information on chain packing and chain mobiiity 14-~s We studied this ratio over the temperature range 79 K to 450 K (Fig. 2) We see that the ratio has the value of 2.4 at low temperature and decreases gently tilt 2. I at 312 K, then it sharply decreases at 313 K. It seems that we obtain an intermediate phase till 322 K where the sample passes in the hexagonal phase. The temperatures are in good agreement with the results of Kind et al. 1 In the same time at 3 t3 K the modes situated at 2854, 2869, 2893, 2950 and 2970 cm-1 disappear and the modes at 2860, 2965 and 2987 cm -1 appear. At ~22 K the mode at 2847 cm-t shift to 2850 cm- 1 (Fig. 3). In the intermediate phase and in the high temperature phase the peak at 1059 cm-1
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Fig.2 Height intensity ratio 1(2880)/1(2850) of DACl powder between 230 and 370 K. cm-1 2990
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Fig.3 Variation of the CH stretching modes frequency versus temperature between 290 and 340 K. disappears, the rotation of the molecules is observed by the decrease of the peak at 2885 cm -~ and its broadening meanwhile the increase of the peak at 1080 cm -~ shows the appearance of gauche
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conformers. The structural difference between these two phases could be in the t i l t angle of the chains with the polar heads. Cooling the sample down to 30"C and recording the spectrum gives Fig. I-C. If we compared with Fig. I-A we cannot observe noticeable changes neither in the position of the peaks nor in the intensities. The Raman spectrum is quite identical before heating and after cooling. The transition is reversible. Splitting of the CC antisymmetrical vibration mode has never been observed to our knowledge, neither in orthorhombic structure nor in monoclinic or in trictinic phases. It could result from the intercalated structure, where the chains of the upper layer are interdigitated between the chains of the lower layer. In this case the splitting of the CH symmetrical stretching mode and the component of the CH2 bending mode (1423 cm-I) could be due to crystal effect resulting from this more compact monoclinic structure where we have interactions between chains of different layers. Experiments with DAC1 urea clathrates are in progress to elucidate this point. We just want to notice now that same splitting is observed, on the perovskite-type layer compound (CioH21NH~)2ZnC14 where the chains are interdigitated ~o but not observed on (C~oH21NH4)2CdC14 where the chains are not interdigitated ~o-t7 B. Infrared experiments. On Fig. 4-A is presented the Infrared spectrum of DACi at 32'C We can observe the splitting of tl~e CH2 bending modes ( 1435 - 1475 cm- ~) and the CH2 rocking modes (715 - 726 cm -1) due to crystal effect The all-trans conformation is confirmed by the narrow and intense CH2 wagging (1200 - 1370 cm-l) and CH2 rocking (7!5 - t0!9 cm-i) band progressions. We see also that the symmetrical NH3 deformation mode is splitted in three components (1584, 1615 and 1628 cm -I) This splitting is due to different lengths of the N-HC1 hydrogen bonds 6 Same thing is observed for NH3 rocking modes (942 and 95.5 cm-I). The antisymmetrical NH3 deformation mode gives two bands at t508 and 1515 cm -~. Heating the sample up to 67°C we obtain the Fig. 4-B The NH3 deformation and rocking mode give respectively a band at 1600 and 960 cm -1 indicating a rearrangement of the polar heads in such a way that the hydrogen bonds have the same lengths. The CH2 rocking mode gives only one band at 720 cm-1, and the CH2 bending modes give a
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Fig.4 Infrared spectrum, of DAC1 powder at 32°C (A), 67"C (B), 36"C one hour later (C) and 30°C six days later (D). broad band centered at 1465 cm -i These modes are in agreement with the experimental results of alkanes in hexagonal lattice. Nearly all the bands are broad demonstrating gauche conformations. But there is still an order, as observed in the Raman spectrum. Cooling the sample down to 36°C gives Fig.4-C, where a~! the bands of the previous phase are narrow and intense but with an order quite different from the phase we have at 25°C on Fig. 4-A The bands at 720 cm-1 and 1468 cm-1 seems to indicate, as in the high temperature phase, an hexagonal packing of the chains. The N-H--.-C1 bonds seem also identical. Fig. 4-D shows the Infrared spectrum at 30°C ten days later. We can note that the general feature of the spectrum is identical to that of the phase at 25°C. The bands are splitted, narrow and intense. We have only small differences in the relative intensities of some of them like the CH2 rocking and the NH3 bending modes. This could be the consequence of a change in the t i l t angle of the chains with the polar heads, The Infrared experiments presented on Fig. 4 and
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PHASE TRANS'TTIONS OF DECYLAMMONI~UM
described above were made with pellets of DAC1 and KBr. They are nearly identical to the results of Casal et al. 6 In order to study the CH stretching region we made other pellets with less DAC1 than previously. In this last case the spectrum after heating and cooling process stay as in the Fig. 4-C even two months later. Doing the experiments on DACI evaporated from ethanol solution and deposited on KBr window, the spectra and the behaviour of the samples are nearly identical to those presented on Fig. 4-A,B,C. Waiting nearly two weeks after cooling, we obtain a spectrum with the bands splitted as in Fig. 4-A, but With peaks less intense, and two peaks at 158,9 and t591 cm -i for the NH3 bending modes, tt seems that the preparation procedure of the samoie ~s responsabte of the differences between infrared results. Ricard et al. 17 mentionned that they didn't use KBr pellets but suspension in Nujol to prevent effects of bressure or chemical exchange in perovskite layer combounds. In order to understand why Raman scattering and infrared absorption gives conflicting results, we want to say a few words about the Raman experimental sample mounting. DAC1 powder ts enclosed in sealed Imm-diameter capiiiaries. These are nearly 5 cm high, and contain about a height of 3 cm of powder, the extra part contains confined air. The spectra presented here are obtained by focusing the iaser beam neart,l in the middle of the height of the powder. We made other spectra of DACi eowder after the cooling at 30°C Dut fOCusing the !aser beam on the powder at the interface air-DACl. Fig..5 presents the CH stretching mode and the skeletal mode region~ in these conditions. We can see that the s~ectrum has the same aspect that the one recorded from the high temperature phase (non intercalated structure), we have only one peak at 1063 cm-1 and the spectrum is still the same two months later. Kind et al. 1, in order to explain the no reversibility of the thermal dilatation, show that when air is entering the structure during heating process, the transition becomes irreversible. But, as we can see from our results, this transition is perfectly reversible when there is no contact (or much less contact) with air. In the same time this result seems confirm that the splitting of the CC antisymmetrical mode is due to the intercalated structure. 4. CONCLUSION We showed that the discrepancy between Raman scattering and Infrared absorption results seems
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CELORIDE
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Fig.5 CH stretching modes and CC skeletal modes of DAC1 powder at 30"C at the interface air-DAC1 after heating and cooling process. due to the pener,ratien cf air between DAC] layers. ~n the first experiments the transition is reversib!e, in the second it doesn't. The fact that the Raman spectrum is different according to the focusing point of the laser seems confirm this hTpothesis. Experiments with inert gas should be necessary in order to prove it. We also shown that the sample preparation m Infrared absorption is also important. It seems that interaction of DAC1 with KBr prevent the return of the sample to an ordered phase when there is less DAC] than KBr At the contrary when there is more DAC1 than KBr the return to an ordered pnase ]s obtained !O days later. We showed also that the sample prepared by ethanoi evaporation gives also slight differences. The differences in the Infrared absorption measurements seem coming from a change in the t i l t angle of the chains with the polar head, as it is shown by the modification of the NH3 vibration modes. In both case, as it was shown by X ray diffraction experiments, the transition is between an intercalated structure at low temperature and a non intercalated structure at high temperature. Acknowledgments. Thanks are due to M. Germain from the Laboratoire de Physique des Solides (Orsay) for the making of DACI and to Prof. M. .Jaffrain for helpful discussions.
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I0. M.Picquart, to be published. 11. S.Abbate, G.Zerbi and S.L.Wunder, J. Phys. Chem. 86, 314(1982). 12. K.Radley and A.Saupe, MoZ Cryst. L iq. Cryst. 44, 227 (1978). 13. M.Ptcquart, J. Phys. Chem. 90, 243 (1986). 14. B.P.Galoerand Wl.Petitcolas, B/ochim. Biophys. Acre, 465,260 (1977). 15. R.G.Snyder, J.R.Scherer and B.P.Gaber, Biochim. Biophys. Acta, 601,47 (1980). 16. L.Ricard, M.Rey-Lafon and C.Biran, J. Phys. Chem. 88, 5614 (1984). 17. L.Ricard, Riavagnat and M.Rey-Lafon, J. Phys. Chem. 89, 4887 (1985).