Vibrational spectroscopic study on polymorphism and order-disorder

Polarized Raman and Infrared Spectroscopy and ab Initio Calculation of Palmitic and Stearic Acids in the Bm and C Forms. L. F. L. da Silva , T. Andrad...
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J. Phys. Chem. 1986,90, 6371-6378

6371

Vibrational Spectroscopic Study on Polymorphism and Order-Disorder Phase Transition in Oleic Acid Masamichi Kobayasbi,* Fumitoshi Kaneko, Department of Macromolecular Science, Faculty of Science, Osaka University, Toyonaka. Osaka 560, Japan

Kiyotaka Sato, Laboratory of Chemical Physics, Faculty of Applied Biological Science, Hiroshima University, Fukuyama, Hiroshima 720, Japan

and Masao Suzuki Oil and Fats Research Laboratory, Nippon Oil and Fats Co. Ltd., Ohama, Amagasaki 660, Japan (Received: April 21, 1986)

Infrared and Raman spectra of three crystal modifications, a,8, and y, of oleic acid were investigated. The spectral data of the y (low-melting) phase were analyzed on the basis of crystal structure determined by Abrahamsson and Ryderstedt-Nahringbauer. The reversible solid-state phase transition between y and a was followed by the infrared and Raman spectra, and it was concluded that the phase transition was of an order (y)-disorder (a)type accompanied by a conformational disordering in the methyl-sided alkyl segment. Therefore, the y a transition was recognized as a new type of interface melting process occurring in layer-formed organic long-chain compounds. The extent of conformational disorder in alkyl chains was compared among various types of disordered crystalline phase. From the spectral feature of phase p, it was supposed that the subcell structure of this phase differed from the already-known 0, and Trll(or OlIl) type.

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Introduction Unsaturated fatty acids are widely distributed in biological tissues as a main constituent of biomembranes. In most cases, they are linked at position 2 of phospholipids and generate a large variety of the characteristics of membranes; Le., they promote fluidity and control the phase transition behavior depending on the environmental condition. From the technological viewpoint, these compounds as well as their derivatives have attracted great attention by their unique activities or functions as medical and industrial materials. In spite of such importance of unsaturated fatty acids, only a little is known about molecular-order structures and physicochemical properties even for the most common case of oleic acid. The highest barrier that has hindered the fine physicochemical investigations of these compounds is the rather low purity of the commericially available samples. Various physical properties, such as polymorphism, phase transition, and electric and mechanical behaviors are greatly influenced by the contamination of very small amount of impurities. Quite recently, an ultrapure oleic acid sample was produced by Nippon Oil and Fats Co. Ltd. The purity was guaranteed to be more than 99.9%. With this sample, Suzuki et al. performed, for the first time, a comprehensive study on the polymorphism in oleic acid.' They found that a new modification, named a, in addition to the already-known low-melting (y) and high-melting (B) phases, showed the phase diagram and also clarified the crystal growth behaviors of the three modifications. It was newly found that a reversible phase transition took place between phases y and a by cooling and heating the sample. Based on their findings, the next Step is to reveal the molecular-order structures of the three modifications and clarify the mechanism of the y CY phase transition. As for the structure of oleic acid, Abrahamsson and Ryderstedt-Nahringbauer determined the crystal structure of the low-melting (y) phase.* Koyama and Ikeda investigated the Raman spectra of low- and high-melting phases of a series of unsaturated fatty acids and considered the relationship between the vibrational frequencies

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( 1 ) Suzuki, M.; Ogaki, T.; Sato, K. J . Am. Oil. Chem. SOC.1985, 62, 1600-1 604. (2) Abrahamsson, S.; Ryderstedt-Nahringbauer,I. Acru Crystallogr. 1962, 15, 1261-1268.

and the molecular conformation^.^ In the previous works, however, the presence of phase a was not mentioned, since this phase appears only in highly pure samples. The present paper deals with the vibrational spectroscopic information about the molecular and crystal structures of the three phases and the structural changes a phase transition. accompanying the y

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Experimental Section A . Samples. The oleic acid sample (Extra Oleic 99 with the purity higher than 99.9%) was supplied by Nippon Oil and Fats Co. Ltd. Samples of specific crystal modifications for the spectral measurement were prepared by the following ways. Phases a and y. Liquid oleic acid sealed in a glass ampule was cooled rapidly in a dry ice-methanol bath. The solidified material was ground to fine powder (in order to avoid the effect of polarization in the Raman measurement) in a room held at 4 O C , and then sealed again in a thin-walled glass capillary which was subjected to the Raman measurement. The modification of the obtained powder sample was checked to be a or y depending on the measuring temperature (see below). The sample for the infrared measurement was prepared as follows. Liquid sample was sandwiched between two KBr windows and mounted on the cold finger of an Oxford flow-type cryostat. The temperature was lowered at the rate of 1-2 OC/h. At a particular temperature, uniaxially oriented thin bundles of needle crystals began to grow between the windows. A region in which the needle crystals were well aligned was isolated by masking the extraneous area with aluminum foil. This sample was subjected to the measurement of the polarized infrared spectra. Phase p. The a phase crystals prepared at 5 OC in a glass ampule was remolten and kept at 13-14 OC for 1 day. Then, crystals of phase grew very slowly (ref 1). The crystals were ground into fine powder and sealed in a thin-walled capillary for the Raman measurement. Preparation of pure crystals for the infrared measurement was rather difficult because of contamination with a (or y) phase. Liquid sample was put between two KBr windows with a small amount of powdered seed crystal of @ and kept at 13-14 OC for 1 day. Thin-layered @ crystal formed. However, we did not succeed in preparing well-oriented samples by this way. (3) Koyama, Y.; Ikeda, K. Chem. Phys. Lipids 1980, 26, 149-172.

0022-3654/86/2090-6371$01.50/0 0 1986 American Chemical Society

6372 The Journal of Physical Chemistry, Vol. 90, No. 23, I986

Kobayashi et al.

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Wovenumber /cm -I Figure 1. Raman spectra of the three crystalline phases of oleic acid. E. Spectral Measurement. Raman spectra of the samples thus prepared were measured with a JASCO R500 double monochromator using the 5 14.5-nm line (Ar’ laser) as the excitation source. The scattered light was collected at right angle to the incident beam. Infrared absorption spectra were taken with a JASCO DS-402G and an A-3 spectrophotometer equipped with a wire-grid polarizer. The Raman and infrared spectra of three modifications in the whole frequency range are reproduced in Figures 1 and 2, respectively.

Structure and Molecular Vibrations of Phase y Abrahamsson and Ryderstedt-Nahringbauer2 determined the crystal structure of the “low-melting” phase of oleic acid by the x-ray diffraction method using the reflection data taken a t -20 O C . Comparison of our powder diffraction pattern with the data in ref 2 indicates that Abrahamsson’s low-melting phase corresponds to our y phase. The unit cell belongs to a pseudoorthorhombic system and contains four molecules (or two hydrogen-bonded dimers). The space group is P 2 , / a . The molecular structure is as follows: the C=C bond assumes the cis form and the polymethylene chains at both sides of the C=C bond take the all-trans conformation. The internal rotation angles of the C-C bonds linked to the C==C respectively. bond are f133O (nearly skew (S)) and -133’ Thus. the conformation of the - C - C 4 - C group is described

(s),

as SCS. There are centers of inversion at the midpoint of the dimerized carboxyl groups as well as of two terminal methyl groups of the neighboring bimolecular layers. The two dimers are related to each other by the symmetry of twofold screw axis (parallel to the b axis) or glide plane (normal to the b axis). The polymethylene chains form an O’,,-type subcell (belonging to an orthorhombic system) where all skeletal planes are parallel as in the triclinic polyethylene (t-PE or Tr,,)type subcell (Figure 3). The optically active molecular vibrations of this crystal lattice are divided into four symmetry species: the Raman active A, and B, and the infrared-active A, and B, species, each including 156 internal modes. The splitting of an intramolecular mode into the four symmetry species is ascribed to the intermolecular interactions (the Davydov splitting). The magnitude of the frequency gap caused by the splitting depends on the strength of the interaction. For example, large split gaps between the Raman-active and infrared-active modes are observed for the vibrations associated with the carboxyl group because of the strong hydrogen bond between two groups. In what follows we will discuss the relationship between the structure and the molecular vibrations of phase y of oleic acid. A . Conformation of Polymethylene Chains. In the Raman spectra of crystalline n-alkanes as well as crystalline polyethylene, there appear several sharp and strong bands associated with the all-trans conformation of alkane chains.+’ All these Raman bands

Polymorphism and Phase Transition in Oleic Acid

0

The Journal of Physical Chemistry, Vol. 90, No. 23, 1986 6373

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1400

Wavenumber / cm-a

0

4000

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3!Xm

2000

le00

1600

1000

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1400

W avenumber / cm-I Figure 2. Infrared spectra of the three crystalline phases of oleic acid: -, I needle axis; - - -, 11 needle axis.

bs

bs ,:;.

.'-'..... I.*

as

,.!,,!-&....



.I'

as

0 '//

f - PE

a

b

as

0 - PE

C

Figure 3. Subcell structures of polymethylene chains in organic long-chain compounds.

appear with strong intensity in phase y of oleic acid as listed in Table I. In the case of unsaturated hydrocarbon chains, the spectral feature of the C-C stretching, v(CC), modes differ from that of saturated ones. Around 1100 cm-l there appear two strong bands in oleic acid (at 1125 and 1095 cm-' in y and 1130 and 1108 cm-' in 8) instead of only one band in the cases of n-alkanes and saturated fatty acids. In oleic acid, the polymethylene chains are separated into two parts by a cis formed C=C bond, one being the methyl-sided chain and the other the carboxyl-sided chain. (4) Strobl, G. R.; Hagedorn, W. J. Polym. Sei., Polym. Pkys. Ed. 1978, 16, 1181-1193. ( 5 ) Cutler, D. J.; Glotin, M.; Hendra, P. J.; Jobic, H.; Holland-Moritz, K.; Cudby, M. E.A,; Willis, H. A. J. Polym. Sci., Polym. Phys. Ed. 1979, 17, 907-91 5 . (6) Kobayashi, M.; Tadokoro, H.; Porter, R. S . J. Chem. Phys. 1980, 73, 3635-3642. (7) Glotin, M.; Mandelkern, L. Colloid Polym. Sci. 1982, 260, 182-193.

TABLE I: Raman Bands Characteristic of AU-Trans Polymethylene Chains in Three Crystalline Phases of Oleic Acid band, cm-' &phase

y-phase

assignment

1122"

2884 1299 1187 1130

288 1 1298 1182 1125

1095

1108

1095

1063

1064

1063

CH2 antisym str CH2 twist CH2 rock C-C sym str (CH3 side) C-C sym str (COOH side) C-C antisym str

a-phase 2881 1298

a Weak

band (see text).

Both chains consist of nine carbon atoms but they are different in the terminal condition. For the methyl-sided chain, one end is free and the other is fixed, whereas for the carboxyl-sided chain

' i

Kobayashi et al.

6374 The Journal of Physical Chemistry, Vol. 90, No. 23, 1986

1

8-Phose

(-173°C)

I

lb I100

c

L

\

t

c I

10501

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\

1

il

\, \

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/

W a v e numb e r / cm-1

Figure 5. Raman spectrum in the C-C stretching region of @ and y

phases of oleic acid taken at liquid nitrogen temperature. 0

7.r

Phase Angle

4

Figure 4. Dispersion curve of the u4 branch of polyethylene (solid curve)

and observed frequency data of (0)oleic acid, (0)palmitoleic acid, (A) petroselinic acid, and (e) erucic acid. both ends are fixed because of dimerization of the carboxyl groups. In the approximation of the simple-coupled oscillators model for the skeletal stretching modes, the allowed phase angles of the C , chain which moves under two different boundary conditions are given approximately by the equations

ak= (2k - 1 ) ~ / 2 ( n- 1 )

with k = 1, 2, ..., n - 1 (1)

for the free-fixed-boundary model, and

ak= h / ( n - 1 )

with k = 1 , 2, ..., n - 1

(2)

for the fixed-fixed-boundary modeLs The difference in aPk causes a significant difference in the frequency of one of the most-in-phase modes (the mode with the smallest k value corresponding to the symmetric stretching, v,(CC), and that with the largest k value corresponding to the antisymmetric stretching, v,(CC)), since the dispersion curve of v(CC), Le., the u4 branch of the infinitely long polyethylene molecule, changes very sharply in the vicinity of = 0 (Figure 4)?,1° Based on this consideration, the Raman bands of phase y of oleic acid a t 1125 and 1095 cm-I are assigned to the v,(CC) mode of the methyl-sided and carboxyl-sided chains, respectively. This assignment is supported by the fact that the observed relationships between the frequency and the phase angle derived from eq 1 or 2 for various unsaturated fatty acids consisting of different chain lengths agree with the theoretically obtained v4 dispersion curve as is seen in Figure 4. This idea for assigning the v(CC) bands of the unsaturated fatty acids was applied to the progressive bands with k 2 2. The bands at 1052 and 1043 cm-' of oleic acid y phase (or those of 1038 and 1026 cm-' of B phase in Figure 5) may be assigned to the progressive bands with k = 2 for the methyl-sided and carboxyl-sided chains, respectively. The frequencies a t the corresponding phase angles on the v4 dispersion curve in Figure 4 are 1066 and 1027 cm-'. Both pairs of bands with k = 1 and 2 behave very similarly on the y a phase transition as will be discussed below. For other progressive band series, however, the assignment is difficult because the v4 branch shows a nonmonotonous change.

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(8)Zbinden, R.Infrared Spectroscopy of High Polymers; Academic: New York, 1964;pp 98-146. (9)Tasumi, M.;Shimanouchi, T. J . Chem. Phys. 1965, 43, 1245-1258. (10)Tasumi, M.;Krimm, S. J . Chem. Phys. 1967, 46, 755-766.

On the contrary, the frequency gap due to the difference in the boundary condition is very small for the v,(CC) mode, since the dispersion curve is rather flat in the vicinity of CP = T . In oleic acid the band splitting due to this origin becomes detectable when the sample is cooled a t a temperature as low as -185 OC (Figure 5). The v,(CC) mode splits into doublet at 1066 and 1062 cm-'. By the characteristic shape of the u4 branch, the v,(CC) Raman band is very sensitive to the conformational order in the alkane chain." This problem will be discussed below in relation to the structural change on the phase transition. The spectral pattern of the progressive bands reflects very sensitively the stem length of the trans-zigzag chain.12 The observed progressive infrared bands in the region 720-1000 cm-' (due to the CH2 rocking-twisting (v8) branch) and in the region 1200-1380 cm-I (the CH2wagging (vj) branch) correspond fairly well to those of n-CgH20.12All the spectral data due to the polymethylene chains in phase y are perfectly consistent with the all-trans conformation. It has been pointed out that the Raman spectrum around 900 cm-' of fatty acids reflects the conformation of the C-C bond in the vicinity of the carboxyl group.I3 The C , E, and A forms of even-numbered n-fatty acids, consisting of the all-trans chains, give rise to a simple doublet, while the B form with the chains twisted a t the C2-C3 bond (gauche form) gives rise to a complicated multiplet. In oleic acid, the three modifications give a doublet in this region although the frequencies vary by a few cm-I depending on modification. B. Subcell Structure. In most cases, alkyl chains in long-chain compounds form one of two basic subcell structures. One is the orthorhombic polyethylene (0-PE or 0,) type where the zigzag skeletal planes of the nearest neighboring chains are almost perpendicular to each other (Figure 3c), and the other is t-PE type where all the skeletal planes are parallel (Figure 3b). Spectral patterns characteristic of these subcell structures are as follows. 1 . 0-PE Subcell. The infrared CH2 rocking, r(CH2) (-720 cm-I), and CH, scissoring, @CHI)(- 1460 cm-I), bands split into doublet with differently polarized components (the B1,-B2, Davydov splitting). The Raman profile due to the 6(CH2)consists of a sharp 1416-cm-' band and a broad band with two main peaks at 1440 and 1465 cm-'. In the case of fatty acids, in addition to the above-mentioned bands, there is a band associated with 6(CH2) of the a-CH2 group at about 1410 cm-'. This band appears independently of the subcell structure. (11) Kobayashi, M.J . Mol. Struct. 1985, 126, 193-208. (12) Snyder, R.G.;Schachtschneider, J. H. Spectrochim. Acta 1963,19,

85-116. (13) Harada, I.; Tasumi, M.; Sato, K.; Okada, M. Prep. Mol. Struct. Symp. 1978, 2A09.

The Journal of Physical Chemistry, Vol. 90, No. 23, 1986 6375

Polymorphism and Phase Transition in Oleic Acid Oleic acid

y

(-1O'C)

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Oleic acid

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Oleic acid

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I-IO'C)

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TABLE II: Raman and Infrared Bands Associated with the Olefin Group in the Three Crystalline Phases of Oleic Acid (Frequencies in cui') 8-phase

a-phase IR

Raman (-2

"C)

3003 2980 1657 1268 966

(2

"C)

"C)

IR (-17 "C)

3015

3012

3020

1664

1642 1265 962 646 512

1645

695 587

Raman (-10

y-phase

645

2. t-PE Subcell. The infrared r(CH2) and 6(CH2) bands appear as singlet, and the 1416-cm-' band disappears. The Otll subcell is not so common and has been found for the first time in oleic acid y2and thereafter in peroxypelargonic acid,14 linoleic acid,'$ and linoleinic acid.I5 This belongs to the category of the t-PE type. In Figures 6 and 7, the infrared spectra in the 6(CHz) and r(CH,) regions, respectively, are compared among oleic acid y(Otll), oleic acid j3 (unknown), palmitic acid A2 (t-PE), and palmitic acid C(o-PE). Oleic acid y phase exhibits the spectral pattern close to that of the t-PE type. The Raman spectral pattern in the CH stretching region is also sensitive to the subcell structure (Figure 8).l6 Appearance of (14) Beiitskus, D.; Jeffrey, G. A. Acta Crystallogr. 1965, 18, 458-463. (1 5) Emst, J.; Sheldrick, W. S.; Fuhrhop, J.-H. 2.Naturforsch. 1979,346, 706-7 11.

Raman (-10

"C)

2996 2983 1661 1275 97 1 701 590

IR (-10

"C)

3008 1664

704

assignment

C-H str C=C str

C-H in-plane bend C-H out-of-plane bend C-H out-of-plane bend C=C-C bend

the doublet at about 2850 cm-' is characteristic of the t-PE subcell, as is observed in the case of palmitic acid A2.17 Except for the appearance of the bands due to the -CH=CH- group (around 3000 cm-I) the spectral pattern of oleic acid y is that of the typical t-PE subcell structure. The infrared 6(CH2) and r(CH2) bands in oleic acid y show clear dichromism parallel and perpendicular, respectively, to the needle axis of the crystals grown between two KBr windows. In most cases of long-chain molecular crystals grown by this way, the layer surfaces stack along the direction normal to the window surface. In the present case, the window surface is parallel to the (16) Snyder, R. G.; Hsu,S. L.; Krimm, S. Spectrochim. Acta, Parr A 1978, 34, 395-406. (17) Kobayashi, T.; Kobayashi, M.;Tadokoro, H. Mol. Cryst. Liq. Cryst. 1984, 104, 193-206. (18) Hayashi, S.; Umemura, J. J . Chem. Phys. 1975, 63, 1732-1740.

6376 The Journal of Physical Chemistry, Vol. 90, No. 23, 1986

Kobayashi et al.

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Figure 8. Raman spectra in the CH stretching region of fatty acid with different subcell structures. TABLE III: Raman and Infrared Bands Associated with the Carboxvl Group in the Three Crvstalline Phases of Oleic Acid (Freauencies in cm-') a-phase 8-phase y-phase

Raman "C)

(-2

(2

IR "C)

1643"

1692

b

1410

1414 922

141 1

91 1 527 a

Raman "C)

(-10

IR "C)

(-17

1703 1433 1413 954

525

IR "C)

(-10

1640

1692

1410

1414 922

91 1

905 690-670 532

Raman "C)

(-10

680 538

527

532

assignment C=O str C-OH str a-CH2bend 0-H bend C-C(carboxy1) str O=C-0 in-plane bend C-C-O bend

Broad band. *Overlapped by the C = C stretching band.

ab plane. The abovementioned dichroism is very consistent with Abrahamsson's structure, and we can conclude that the needle axis is parallel to the b axis. C. Bands due to OleJn Group. The infrared and Raman bands associated with the cis-formed -CH=CH- group of the three modifications are summarized in Table 11. The frequencies of these bands change significantly with the conformation of the C-C bonds linked to the olefin group. Based on Abrahamsson's structure, the frequencies ofJhe bands in phase y are recognized as characteristic of the SCS conformation. In phases a and B the corresponding bands are shifted more or less as described in the following two sections. D. Bands due to Carboxyl Group. The infrared and Raman bands associated with the carboxyl group of the three crystal modifications are summarized in Table 111. The frequency of the W-0 in-plane bending, S(C=O), is known to depend on the C1-C2conformation. In the case of form C of even-numbered n-fatty acids, the cis and trans forms give rise to a S(C-0) band a t 690 and 670 cm-I, respectively. In oleic acid y, the internal

rotation angle of the CI-C1 bond is 26O (nearly cis form), so that the band is expected to appear around 690 cm-I. However, the corresponding band cannot be detected, probably by the overlap of the strong CH out-of-plane bending, y(CH), band at 700 cm-'.

Phase Transition between y and a As described in the Introduction, phase y transforms to Q at -2.2 "C (on heating), and then phase a melts at 13.3 OC. On cooling, phase a transforms reversibily to y at -3.6 "C. The enthalpy and entropy changes of the y Q transition were measured as AH = 8.76 kJ/mol and A S = 32.2 J K-' mol-', respectively.' The polymorphism and the phase transition are depicted schematically in Figure 9. Thermodynamically, phase @ is the most stable with the highest melting point at 16.2 OC. It was confirmed that phases Q and y transforms irreversibly to p at a very slow rate as indicated by the broken arrows in the figure. This section deals with the vibrational spectral changes on the y a phase transition. The most conspicuous change was seen

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Polymorphism and Phase Transition in Oleic Acid

The Journal of Physical Chemistry, Vol. 90, No. 23, 1986 6377

MELT '-Pharrl-17%)

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!-Phaae (-2%)

Figure 9. Polymorphism and phase transitions in oleic acid.

I

1100

3

IO00

Wave number /cm-l a-Phase ( O X )

,'"*'.*c" /

//

Figure 11. Raman spectra in the C-C stretching region of y and a phases of oleic acid.

:I

y -Phase (-I0 "C

I /

J

,?.,,,..*yu.\wh.u'-*.~~~ *P',

in the low-frequency Raman spectrum as shown in Figure 10. All the sharp bands of y (measured at -1 1 "C) overlap each other and collapse to a broad band with a peak around 25 cm-' in a (measured at 0 "C). The smeared band profile of phase a originates from the lack of the translational symmetry, indicating that a is a disordered phase. In order to clarify at what position disordering takes place, spectral changes that occur during the phase transition were investigated for various bands associated with different parts of the molecule. A. Polymethylene Chains. As stated in the preceding section, the 1125- and 1095-cm-' bands are assigned to the v,(CC) mode of the methyl-sided and the carboxyl-sided polymethylene chains, respectively. As phase y transforms to a,the former band decreases drastically in intensity, while the latter remains unaltered (Figure 11). A similar spectral change is observed for the progressive bands with k = 2 at 1052 and 1043 cm-'; the former changes to a shoulder, while the latter remains unchanged. Sin@ the intensity of the v,(CC) band is a measure of the conformational order, this spectral change indicates that conformational disordering takes place preferentially in the methyl-sided chain. The changes in the infrared spectrum on the y Q transition are as follows. (1) The dichroism of the r(CH2) band decreases appreciably, (2) the ug progressive band series becomes obscured, and (3) the CH3 rocking band at 895 cm-' smears into the wing of the strong absorption (at 920 cm-')due to the OH out-of-plane bending of the carboxyl group. On the contrary, the v3 progressive band series almost remains unaltered. The infrared activity of this band series is due to the coupling with the carboxyl modes,lg and such coupling should be strong in the carboxyl-sided chain compared with the methyl-sided chain. Infrared and Raman bands due to the methyl C-H stretching are broadened (Figure 12). These spectral changes also indicate that conformational as well as orientational disordering occurs in the methyl-sided chain. Quite recently, we found similar reversible phase transition in palmitoleic acid and erucic acid by DSC and Raman spectral experiments. The terminal methyl groups constitute the lamellar

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(19) Umemura, J. J. Chem. Phys. 1978, 68, 42-48.

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interface of the bimolecular layers of phase y, so that the y a transition is recognized as an interfacial melting process. Such an order-disorder phase transition has not been found so far in saturated fatty acids. Thus, introduction of an olefin group inside an acyl chain induces an increase in chain mobility. This may be one of the origins of an increase in fluidity of biomembranes with an increase of the degree of unsaturation. It is known that an interface melting process is accompanied by the rotator phase transition of n-alkanes that takes place in a narrow temperature range immediately below the melting point. In this case, however, the amount of the gauche bonds induced is very limited and they are concentrated in the vicinity of the chain ends.zo-2' Actually, the intensity of the v,(CC) Raman band

(20) Zerbi, G.; Magni, R.;Gussoni, M. Holland-Moritz, K.; Bigotto, A,; Dirhkov. S . J . Chem. Phvs. 1981. 75. 3175-3194. (21) Maroncelli, M. Strauss, H. L:; Snyder, R.G. J. Phys. Chem. 1985, 89, 5260-5267; J. Chem. Phys. 1985, 82, 2811-2824.

Kobayashi et al.

6378 The Journal of Physical Chemistry, Vol. 90, No. 23, 1986 TABLE I V Entropy (J K-' mot' per C H I ) of phase Transition in Various Loag-chaii Compounds unsaturated fatty acids palmitoleic acid oleic acid erucic acid quaternary ammonium salts of alkyl halides alkyl trimethylamine chlorides alkyl-DABCO bromides n-alkanes (fusion)

4.28 3.59 3.47 6.03 -6 10.86

of n-alkanes remains unchanged through the rotator phase transition. 22 As another extreme of the order-disorder phase transition of long-chain compounds, we mention here the case of quaternary alkyl halide salts of amines. Quite recently, quaternary alkyl halides salts of 1,4-diazabicycl0[2.2.2]octane (DABCO) were found to undergo a reversible phase t r a n ~ i t i o n . ~Our ~ Raman spectral study showed that all the characteristic bands due to the all-trans alkyl chains appearing in the low-temperature phase disappeared completely in the high-temperature phase. Similar spectral change was measured in the case of trimethylammonium salts of alkyl halides. In these cases, a large amount of gauche conformation is introduced in the alkyl chains of the high-temperature phase. Thus, various extents of conformational disorder in alkyl chains were estimated by the spectroscopic method. They are consistent with the values of entropy of transition AS, per carbon atom as summarized in Table IV. Here, the values of unsaturated fatty acids were derived by assuming that disordered conformations are limited only in the methyl-sided chain. The entropy of fusion of n-alkanes has been measured as 10.9 J K-' mol-I per CH2. Compared with this, all the cases mentioned here exhibit far less ASt values, even in the case of quaternary ammonium salts. It is very important to clarify by what mechanism the conformational disorder in polymethylene chains is controlled and how it is related to the structure of the polar or ionic groups to which the chains are linked. B. Olefin Groups. As described in Table 11, the infrared and Raman bands associated with the olefin group display relatively significant frequency shift on the phase transition. This suggests that a conformational change around the olefin group is accompanied. Moreover, the Raman bands due to the C H stretching (at 3003 cm-'), the CH in-plane bending (1268 cm-I), and the (22) Barnes, J. D.; Fanconi, B. J. Chem. Phys. 1972, 56, 5190-5192. (23) Shimizu, J.; Nogami, T.; Mikawa, H.Solid Srare Commun. 1985, 54, 1009-101 1.

CH out-of-plane bending (-700 cm-l) modes are broadened in phase a,especially, the last one being smeared into the background (Figure la). This may be ascribed to a dynamical motion occurring in this disordered phase. C. Carboxyl Groups. As is seen in Table 111, the frequencies as well as the band shapes of the carboxyl group modes almost remain unchanged in the phase transition. Although the crystal structure of a phase is not known at a present, we have some X-ray diffraction data. In the y transition the long spacing (the b dimension) increases from 4.18 nm of y to 4.33 nm of a, while the relative intensities of the (001) reflections do not show significant changes. Furthermore, single-crystal specimens exhibit no morphological changes on observation under optical microscope. These experimental findings suggest that on phase transition no large molecular displacements take place in addition to the above-mentioned conformational disordering at the interface region.

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Spectral Feature of Phase p The crystal structure of phase @ has not been determined yet. The vibrational spectroscopic data indicate that the polymethylene chains of both sides of the olefin group assume the all-trans conformation as in phase y (see Table I). The most remarkable difference in vibrational frequency between /3 and y is found for the characteristic bands of the olefin group (see Table 11). This suggests that the conformation of the -C-C=C-Cgroup in j3 differs from SCS in y. Koyama and Ikeda3 proposed that phase j3 took SCS type conformation on the basis of the spectral data of two rotational isomers of cis-3-hexene. The infrared and Raman profiles of the r(CH2), 6(CH2), and v(CH,) modes of phase p are very characteristic. They are remarkably different from those of typical o-PE and t-PE subcell structures (see Figures 6-8). This fact indicates that phase p assumes a specific subcell structure. The Raman spectra in the C-C stretching region taken at liquid nitrogen temperature are compared between j3 and y (Figure 5). At this temperature, separation of bands associated with the methyl- and carboxyl-sided chains becomes detectable in both v,(CC) and v,(CC) modes. In phase y, each of these four bands appears as a sharp singlet, while the corresponding bands of B are broader or split clearly into doublet. Similar band splitting in phase is seen for the infrared progressive band series of the v3 branch. These spectral data indicate that in the subcell of phase /3 the C C zigzag planes of the neighboring chains are not parallel each other but inclined to a certain extent. We are now progressing with the X-ray work to determine the crystal structure of phase p. Registry No. Oleic acid, 112-80-1.