J. Phys. Chem. 1990, 94, 3180-3185
3180
Structure and Transformation in Polymorphism of Petroselinic Acid (cis-w-l2-Octadecenoic Acid) Kiyotaka Sato,* Noriyuki Yoshimoto, Faculty of Applied Biological Science, Hiroshima University, 724, Higashi- Hiroshima, Japan
Masao Suzuki, Research Laboratory, Nippon Oil and Fats Co., 580, Amagasaki, Japan
Masamichi Kobayashi, and Fumitoshi Kaneko Faculty of Science, Osaka University, 560, Toyonaka, Japan (Received: August 2, 1989)
Polymorphic behavior of petroselinic acid (cis-0-1 2,Ad-octadecenoic acid) was examined by differential scanning calorimetry (DSC), X-ray diffractometry (XRD), vibrational spectroscopy, and optical microscopy. To examine the effects of the position of a cis double bond on the polymorphism, a comparison was made to oleic acid (w-9) and asclepic acid (w-7) that are two positional isomers of octadecenoic acids. Two polymorphs were observed in petroselinic acid: a high-melting (HM) form and a low-melting (LM) form, the melting points being 30.5 OC (HM) and 28.5 O C (LM). The two forms always crystallized at the same time from the melt phase, yet the concentration of the HM form increased as the crystallization temperature increased. The thermodynamic stability of the two polymorphs was determined based on the solubility data and the transformation behavior: the HM form is more stable than the LM form above 18.7 O C , and vice versa below 18.7 O C , since Gibbs free energies of the two forms have the same value at 18.7 O C . An irreversible solid-state transformation was observed; the LM form converted slowly to the HM form above 18.7 O C , but the HM LM transition did not occur below that temperature. Single crystals of the LM form were grown only from acetonitrile solution, but the HM form was grown both from acetonitrile solution and from the melt just below its melting point. The XRD short spacing patterns of the LM form quite resembled those of the B form of stearic acid (octadecanoic acid) which is a typical saturated fatty acid. Vibrational spectroscopic data showed that the acyl chains in the LM form were packed in an 0, subcell, as in the B form of stearic acid. The Occurrence of the 0, subcell in cis-unsaturated acids was observed for the first time in this polymorph. The HM form revealed a triclinic parallel packing (TII)of the aliphatic chains. Hence, the aliphatic packing converted from 0, (LM) to TI,(HM). No interfacial melting was observed in the two forms, unlike oleic and asclepic acids, in both of which the aliphatic chain segment between the double bond and the methyl end group became disordered on heating below the melting point. Consequently, thermal and molecular structural behavior of the polymorphism of petroselinic acid is largely different from those of oleic acid and asclepic acid. This leads us to speculate that the polymorphism of the cis-unsaturated fatty acids is critically influenced by the position of the cis double bond in the aliphatic chain.
-
Introduction Cis-unsaturated fatty acyl chains are abundantly present in biolipids of organisms and play important roles in physiological activities.' They occupy about one-half of the whole acyl chains constructing biomembrane lipids, promoting fluidity and permeability of the membrane through conformational flexibility of the cis-unsaturated acyl chains. Specific interactions between the cis-unsaturated membrane lipids and membrane proteins have been known to modify the membrane activity.2 Therefore, the cis-unsaturation has attracted much attention in biophysical science^.^ In other fields such as colloidal ~ h e m i s t r y food ,~ c h e m i ~ t r ynutrition, ,~ and metabolism,6 the fats and lipids or surfactants involving cis-unsaturation reveal specific functionalities. To elucidate the functional roles of the cis-unsaturation of the aliphatic chains, polymorphism must be studied on a series of mono cis-unsaturated fatty acids in the crystalline phase. Polymorphism means Occurrence and transformation of different crystal structures of a substance, under varying sets of thermodynamic and kinetic conditions (temperature, pressure, supercooling, supersaturation, etc.).' The total behavior of polymorphism is directly related ( 1 ) Silver, B. L. The Physical Chemistry . of . Membranes; Solomon Press: New York, 1985; pp 75-99. (2) Spector, A. A.; Yorek, M . A. J . Lipid Res. 1985, 26, 1015-1035. (3) Small, D. M. The Physical Chemistry. of_Lipids: . Plenum: New York, 1986; pp 507-510. (4) Duncan, D. P. J . Am. Oil Chem. SOC.1984, 61, 233-241. ( 5 ) Ishinaga, M.; Sato, J.; Kitagawa, Y.; Sugimoto, E.; Kito, M. J . Eiochem. 1982, 92. 252-262. (6) Sugano, M.; Lee, J . H. The Proceedings of Nara Workshop on Functional Fats and Lipids; Sato, K., Kobayashi, M. A,, Eds.; (special issue of J . Disp. Sci. Techno/.);Marcel Dekker: New York, 1989; Vol. 10, pp 643-665.
0022-3654/90/2094-3180$02.50/0
to the nature of molecular interactions in the crystal. Thus, comparative studies for the polymorphism of various cis-unsaturated fatty acids enable us to highlight the critical influences of cis-unsaturation on physical chemical properties of cis-unsaturated fats and lipids. Despite its critical importance, the polymorphism of cis-unsaturated fatty acids has been rather poorly understood: e.g., crystal and Raman study.'* We have so far dealt with oleic acid (cis-w-9-octadecenoic acid),I3-l6 asclepic acid (cis-w-7-octadecenoic acid)," erucic acid (cis-w-9-docosenoic acid),18 palmitoleic acid (cis-w-7-hexadecenoic acid),'* and methyl oleate,19 using 99.9% purity samples. We have isolated three polymorphs in oleic acid, four forms in asclepic acid, four forms in erucic acid, two forms in palmitoleic acid, and two (7) Sato, K. Crystallization and Polymorphism of Fats and Fatty Acids; Garti, N., Sato, K., Eds.; Marcel Dekker: New York, 1988; pp 3-7. (8) Lutton, E. S. Oil Soap 1946, 23, 265-266. (9) Craven, B. M . J . Phys. Chem. 1959, 63, 1296-1298. ( I O ) Abrahamsson, S.; Ryderstadt-Nahringbauer, 1. Acta Crystallogr. 1962, 1 5 , 1261-1268. (1 1) Ernst, J.; Sheldick, W. S.; Fuhrhop, J.-H. 2.Naturforsch. 1979, 346, 706-7 1 1. (12) Koyama, Y.; Ikeda, K. Chem. Phys. Lipids 1980, 149-159. (13) Suzuki, M.; Ogaki, T.; Sato, K. J . Am. Oil Chem. Soc. 1985, 62, 1600-1604; Ibid 1986, 63, 553 (Erratum). (14) Kobayashi, M . ; Kaneko, F.; Sato, K.; Suzuki, M . J . Phys. Chem. 1986, 90, 6371-6378. (15) Sato, K.; Suzuki, M. J . Am. Oil Chem. Soc. 1986, 63, 1356-1359. (16) Hiramatsu, N.; Inoue, T.; Suzuki, M.; Sato, K. Chem. Phys. Lipids 1989, 47-53. ( 1 7 ) Suzuki, M.; Yoshimoto, N.; Sato, K. Abstracts of 1989 AOCS Annual Meeting; J. Am. Oil Chem. SOC.1989. 66, 442-443. (18) Suzuki, M.; Sato, K.; Yoshimoto, N.; Tanaka, S.; Kobayashi, M. J . Am. Oil Chem. SOC.1988, 65. 1942-1947. (19) Kaneko, F.; Nishi, I.; Kobayashi, M.; Sato, K.; Suzuki, M. Rep. Progr. Polym. Phys. Jpn. 1987, X X X , 135-138.
0 1990 American Chemical Societv
Polymorphism of Petroselinic Acid forms in methyl oleate. For all forms of each substance, thermodynamic stability, polymorphic transformations, molecular structures, single-crystal morphology, and effects of hydrostatic pressure on the polymorphic transformation were examined by means of differential scanning calorimetry (DSC), differential thermal analysis (DTA), X-ray diffractometry (XRD), Raman and FT-IR spectroscopy, solubility measurement, and optical microscopy. A reversible order-disorder transformation of a new type was discovered, occurring between y (order) and a (disorder) polymorphs in oleic, asclepic, erucic, and palmitoleic acids and also between y l (order) and aI (disorder) of erucic acid. This transformation induced, in the disordered forms, conformational disordering at an aliphatic chain segment between a cis double bond and a CH3 end group (w-chain), keeping the conformation of the aliphatic chain segment between the double bond and the COOH end (A-chain) ordered. We called this phenomenon "interfacial melting",I4 which assumes molecular fluidity at the w-chain segment due to an introduction of the cis double bond, as reflected in Raman scattering and entropy of fusion. (a) After the transformation from y to a, a Raman C-C stretching band exhibiting the conformation of the w-chain drastically decreased in intensity, whereas the corresponding Raman band of the A-chain ~ n c h a n g e d . ' ~(b) The entropy of fusion of the a-form of oleic acid was smaller than that of the @form which assumes all-trans conformation by 2596, due to the disordering in the o-chain.13 The a-form, however, may not be equivalent to a lamellar-type lyotropic liquid crystal of polar lipids, in which gauche conformation is extended over all the aliphatic chains.' The major difference is that the a-form contains a rigidly packed A-chain and a "liquidlike" w-chain which is located at the lamellar interface in the dimerized crystalline structure. Hence, the "melting" behavior of the a-form has so far been observed only with spectroscopic and thermal methods. No "interfacial melting", however, occurred in some other polymorphs and also in the two forms of methyl oleate. Besides the order-disorder transformation of this kind, a total feature of polymorphism was different from one acid to another. For example, the P-form of oleic acid did not occur in the other four acids. The y- and a-polymorphs are metastable and the &POlymorph is most stable at ambient temperatures in oleic acid.I3 Conversely, in erucic acid, the y- or a-forms are more stable than yI-or a'-forms below 25.9 OC.l8 The crystal habit and XRD short spacing patterns of aI of erucic acid were similar to those of p of oleic acid, but no interfacial melting occurred in @. The side-by-side packing mode of the aliphatic chains expressed by a subcel120and the olefinic conformation which are typically also revealed some expressed as skew-cis-skew' or ~kew-cis-skew'~ similarity or dissimilarity in the polymorphic modifications so far examined. Detailed crystal structures of some polymorphs were analyzed with four-cycle XRD using single crystals.2'*22 The complexity of this kind in the polymorphism of cis-monounsaturated fatty acids has never been observed in that of saturated fatty acids.23 The problem arises that the position of cis double bond in the aliphatic chain must be one of the key parameters to determine physical behavior in the p ~ l y m o r p h i s m . ~In~ this regard, one must remember the fact that the ability of a lipase from microorganisms to hydrolyze triacyl glycerols is dependent on the position of the cis double bond.25
The Journal of Physical.Chemistry, Vol. 94, No. 7, 1990 3181 I
I
I
0
I
1
20
1
I
I
I
I
I
40
I "C
Figure 1. DSC thermograms of melt cooling and subsequent heating of petroselinic acid at a rate of 2 OC/min.
Figure 2. Single crystals of two polymorphs of petroselinic acid (a) and (b), a-form of oleic acid (c), and B-form of stearic acid (d).
To approach this, we have dealt with petroselinic acid which has a cis double bond at the w-12 position, in comparison to the polymorphism of other cis-monounsaturated fatty acids as well as stearic acid, a typical saturated acid having 18 carbons. Particular attention was paid to the three positional isomers of octadecenoic acid, since petroselinic acid places the double bond at w-12 which is three carbons and five carbons closer to the -COOH group than oleic acid and asclepic acid, respectively.
319-350. (23) Sato, K.; Kobayashi, M. In Organic Crystals; Karl, N., Ed.; Springer
Materials and Methods The sample (>99.9% purity) was supplied by Nippon Oil and Fats Co. The purity was determined by capillary gas liquid chromatography (Shimadzu GC- 14A; column SP-2560, Supelco Inc.). Acetonitrile (>99% purity, Nakarai Chemical Co.) was used as solvent for growing single crystals. The thermal behavior of solidification, melting, and polymorphic transformation were examined by DSC (Seiko-SSC 580). The XRD patterns (Rigaku, CuK,:X = 0.1542 nm, Ni filter) were employed to obtain short and long spacings of powdered samples. FT-IR spectrum was taken with JASCO spectrometer (Model 8000) using a grid-wire polarizer. Polarized FT-IR spectrum was taken by using large single crystals which were grown from acetonitrile solutions. The solubility of each polymorph was determined by measuring the temperature at which the crystal became in equilibrium with solution; a single crystal was put in acetonitrile solution in a growth cell whose temperature was controlled by thermostated water (h0.05"C). Using a polarizing optical microscope, we observed the dissolution and growth process of the single crystal by fluctuating the solution temperature around the equilibrium condition. As to the metastable form, we completed the experiment before the more stable form started to crystallize.
Verlag: Berlin, in press. (24) Sato, K.; Kobayashi, M.; Kaneko, F.; Suzuki, M.; Sagi, N. The Proceedings of 15th Scandinaoia Lipid Symposium; Shukla, V. K. S., Holmer, G., Eds.; Lipidforum: Denmark, 1989; pp 67-85. (25) Jensen, R. G.; Gordon, D. T.; Heinermann, W.H.; Holman, R. T. Lipids 1972, 7, 738-741.
Results Figure 1 shows a DSC thermogram of melt-cooling and subsequent heating between 50 and -50 "C at a rate of 2 OC/min. On cooling, a large peak appeared at 22 "C. This corresponds
(20) Hernqvist, L. Crystallization and Polymorphism of Fats and Fatty Acids; Garti, N., Sato, K., Eds.; Marcel Dekker: New York, 1988; pp 97-137. (21) Kaneko, F.; Doctoral Thesis, Faculty of Science, Osaka University, 1989. (22) Kobayashi, M.;Kaneko, F. The Proceedings of Nara Workshop on
Functional Fats and Lipids; Sato, K., Kobayashi, M., Eds.; (J. Disp. Sci. Technol., special issue); Marcel Dekker: New York, 1989; Vol. 10, pp
3182
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990
Sat0 et al.
HM form
10.397
LM form 10.419
1
start
b
2min
0.394
1 . , 1 , 1 1 2o 2 8 (deg.) 25
Figure 4. X-ray diffraction short spacing spectra of H M form and LM form of petroselinic acid, and the B-form of stearic acid (unit, nm).
C
e
29.0
0.377
-
min
Figure 3. Melt-mediated transformation from LM form to HM form of petroselinic acid, just after the single crystal of the LM form was put at 29.0 "C (a); crystallization of the HM form at the expense (melting) of LM form (b); and crystal growth only of the HM form (c).
to crystallization of one polymorph. On heating, a small peak appeared around 26 "C followed by a large peak around 30 "C. This small peak may correspond to a transformation from one polymorph to another which melts at 30.5 "C. The two polymorphs are referred to as the LM and HM forms. We observed, using the optical microscope, that the LM form has its own melting point at 28.5 "C. We grew single crystals of both the forms. Figure 2a shows single crystals of the HM form grown from the melt at 29 OC. The same single crystal also could be grown from acetonitrile solutions at 20 "C. The LM form was grown from acetonitrile solution around 15 "C (Figure 2b). It is worth noting that, while growing the single crystals of the LM form from acetonitrile solutions, the HM form also crystallized, particularly when the solute concentration was raised. This is due to rather small differences in the solubility between the HM and LM forms and also in the nucleation rate of crystal.
The single crystals of the two forms showed platelike shape, the basal plane being parallel to dimerized bimolecular lamella. The interplanar angles between the lateral faces are 130" and 1 15" in the HM form, and 105" and 75" in the LM form. The interplanar angles of the HM form are the same as those of the a-form of oleic, asclepic, erucic, and palmitoleic acids (Figure 2c). The difference in the crystal morphology between Figure 2a and Figure 2c may be ascribed to a difference in growth rates of the lateral planes, presumably reflecting subtle differences in the molecular structures. The crystal shape of the LM form showed no similarity to the three forms of oleic acid. Conspicuously, this crystal shape is identical with that of the B form of stearic acid, Figure 2d. This indicates a similarity in an aliphatic side-by-side packing between the two forms. Using the single crystal of the LM form, we measured its melting point under the polarizing optical microscope. We put the single crystal on a glass cell whose temperature was rapidly changed by thermostated water. Soon afer the single crystal of the LM form was put at 29 "C,it rapidly melted (Figure 3b), and the HM form was nucleated and grew at the expense of the LM form (Figure 3c). Sequential photographs in Figure 3 clearly demonstrate that the melting point of the LM form is lower than that of the HM form and that the rate of a melt-mediated LM HM transformation is very high. By changing the temperature of the melt-mediated transformation, we determined the melting point of the LM form, 28.5 "C. We observed that, through the melt-mediated transformation from LM to HM, the single crystal of the HM form could easily be grown from the melt phase. Figure 4 shows the XRD short spacing patterns of the two forms of petroselinic acid and the B form of stearic acid. There are distinctive differences between the two polymorphs of petroselinic acid. Long spacing, d(001) = 4.14 nm of the HM form, is shorter than that of the LM form, d(001) = 4.40 nm, meaning that the long-chain axis is more inclined in the HM form, and that d(001) of the LM form is very close to that of stearic acid B, 4.38 nm.
-
Polymorphism of Petroselinic Acid
The Journal of Physical Chemistry, Vol. 94, No. 7, I990 3183
Petroselinic acid: L M form 6-1
.
petroselinic acid
wove number (cm-' )
Figure 5. Polarized FT-IR spectra of the LM form of petroselinic acid taken at 15 " C .
As to the short spacing spectra of the H M form, two strong peaks at 0.410 and 0.397 nm resemble those of the a-form of erucic acid (0.41 1 and 0.392 nm),I8and of the a-form of oleic acid (0.412 and 0.394 nm)." Therefore, the subcell structure of the HM form of petroselinic acid and the a-forms of oleic and erucic acids may be similar. This is consistent with the crystal morphological property (Figure 2). The XRD short spacing spectra of the LM form did not resemble any of the cis-monounsaturated fatty acids; instead they were rather similar to the B form of stearic acid as expected from the crystal morphology; major short spacing values and their diffraction intensities are quite similar (for example, 0.377 nm (LM) and 0.372 nm (B); 0.393 nm (LM) and 0.398 nm (B); 0.419 nm (LM); and 0.415 nm (B)). All of these properties indicate the same aliphatic side-by-side packing of the LM form of petroselinic acid and the B form of stearic acid which is packed according to an 0, subce11.26 This speculation was confirmed by the polarized FT-IR spectra taken for the single crystal shown in Figure 5 . We detected the typical Davidov splitting at the 1473- and 1463-cm-I bands with a polarization direction along the a axisz1(bisectrix of 75O), and at the 73 1 - and 720-cm-I bands with a polarization direction along the b axis, which are characteristic of the 0, subcell for the CH2 scissoring and CH2 rocking modes, respectively. The fine structural analysis using four-cycle XRD of the LM form of petroselinic acids confirmed that the LM form is exactly packed according to the 0, packing.2'q22 Figure 6 shows the solubility of the two forms in acetonitrile as a function of temperature. The solubilities of the LM and HM forms could easily be measured separately at 10-19 OC, since the more stable LM form occurred rather slowly, while measuring the solubility of the metastable H M form. However, the solubility of the metastable LM form was not measurable above 19 OC, since the occurrence of the H M form was enhanced even if only the single crystal of the LM form was put in the solution. Hence, we examined a solution-mediated transformation between the HM and LM forms to precisely determine a crossing temperature of the two solubilities, as exhibited in erucic acid.l* The crystal powders of the two forms were added in nearly saturated acetonitrile solution. While keeping the temperature of solution constant and prohibiting solvent evaporation, the crystal-suspended solution was stirred to induce the growth of the more stable form at the expense of the less stable one. A direction of the transformation was determined by observing characteristic XRD long spacing patterns of the crystals which were taken out from the solution. The solution-mediated transformation occurred from LM to HM above 18.7 OC, and vice versa below 18.7 OC. No transformation occurred at 18.7 OC, meaning that the solubilities of the HM and LM forms have the same value at this temperature, indicated by an arrow in Figure 6. The logarithm of the solubility expressed by molar fraction ( X ) was plotted against 1 / T (K-I), giving straight lines for the two polymorphs. Then, the enthalpy (AHd) ~~~
~
~
~~~
~
~
(26) Goto. M.; Asada, E. Bull. Chem. Soc. Jpn. 1978, 51, 2456-2459.
I
o'! . ' 10' ' ' ' ' 2'0 ' ' ' ' : I T Figure 6. Solubilities values of HM form and LM form of petroselinic acid in acetonitrile. An arrow indicates a crossing point of the two
solubilities.
Petroselinic acid
(
LM
-.
HM I
I
I
I
5
10
15
hour
Time
Figure 7. Solid-state transformation from LM form to HM form.
and entropy ( a d ) of dissolution were calculated according to an equation, In = - A H d / R T &&/R. The rate of transformation in a crystalline state from the LM to HM form was measured at various temperatures above 18.7 "C (Figure 7). The initial sample was formed by chilling the melt containing ca. 85% of the LM form at liquid Nztemperature (see below). The concentration of the LM form was decreased with increasing time for incubation and increasing temperature for transformation, as measured by intensity ratios of the characteristic XRD long spacing spectra. No solid-state transformation was detectable from H M to LM below 18.7 OC. Finally, the crystallization from the melt was examined in a wide range of crystallization temperature ( T , ) (Figure 8). The two forms always crystallized at the same time. At T, = -196 O C , the concentration of the H M form was approximately 15%. With increasing T,, the concentration of the LM form decreased. The rate of melt crystallization was extremely low at T, = 26 OC, where the LM and H M forms crystallized at the same ratio. Hence, it was impossible to obtain the single polymorph of petroselinic acid by simply chilling the melt. The melt growth of the HM form was singly possible above 28.5 "C.
x
+
Discussion Nomenclature of Polymorphs. We have so far employed Greek letters for the nomenclature of the polymorphs in the cis-monounsaturated fatty acids,I3-l9 since A, B, C, and A', B', C' were employed in even- and odd-numbered saturated acids, respect i ~ e l y . ~ 'For more detail, y and a were employed for the poly(27) OConnor, R. T. Fatty Acids; Markely, E. s.,Ed.; Interscience: New York, 1960; Part 1, pp 285-378.
3184
The Journal of Physical Chemistry, Vol. 94, No. 7, I990
Sato et al.
TABLE I: Melting Point (Tm, "C), Enthalpy (AH, kJ mol-'), and Entropy ( A S , J K-' mol-') of Fusion (f) and Dissolution (d) (in Acetonitrile) of HM and LM Forms of Petroselinic Acid, a- and &Forms of Oleic Acid, and C-Form of Stearic Acid petroselinic acid T"' AH 30.5 47.5 28.5
HM(f)
a(f)
13.3 16.2
T"'
AS 138.4 179.3 222.9 279.7
Tm
156.4
Petraselinic
I
oleic acid AH
As
39.6 51.9 59.4 76.0
C
69.6
stearic acid AH 61.3
As 178.8
acid
i&
$1
f 0 - LN4 ) O1
I
I
10
20
,
4 ,I, 30
0
IO
T
20 ("Ci
30
40
Figure 9. A qualitative illustration of Gibbs free energies of the HM form, LM form, and melt of petroselinic acid against temperature.
T, i"C1 T
Figure 8. Melt crystallization of HM form and LM form of petroselinic acid. The two arrows indicate the melting points of the two polymorphs.
morphs undergoing the order-disorder transformation. p was used for the most stable form of oleic acid; yI and a i in erucic acid were those which undergo another orderdisorder transformation. In the present case, however, we gave the nomenclature not common to any of the other cis-unsaturated fatty acids: HM (high melting) and LM (low melting). This is because (a) the HM form has the subcell and crystal morphology similar to the a-forms of erucic, asclepic. oleic, and palmitoleic acids. However, no conformational disordering in the w-chain was detectable in HM. Hence, referring to this polymorph as the a-form is misleading. (b) The LM form quite resembles the B-form of stearic acid, but calling the LM form B is also misleading. Thermal and Crystallization Behavior. Table I summarizes the thermal data of fusion and dissolution of petroselinic, oleic acid, and stearic acids. No fusion data of the LM form are avaitable to us because it converts irreversibly to the HM form by the melt-mediated transformation. From Table I, it follows that all of the thermal parameters of petroselinic acid are larger than those of oleic acid, except for the &form of oleic acid whose enthalpy and entropy values of fusion are larger than those of the HM form of petroselinic acid. T,,, and AHr of the a-form of asclepic acid are 13.8 "C and 39.8 kJ/mol, respectively, both of which are close to those of oleic acid, a-form, and smaller than those of the two forms of petroselinic acid. This may be attributed to more dense molecular packing in petroselinic acid caused by shifting the position of the cis double bond toward the -COOH group. As to &form of oleic acid, its molecular packing may be extraordinarily dense. It is interesting to compare the LM form to the B form of stearic acid. Neither solubility datum in acetonitrile nor fusion datum can be obtained for B, since B is sparingly soluble in acetonitrile and irreversibly transforms to C on heating. So, we evaluated AHdB) as 65.6 kJ/mol by adding AH,,(B-*C) = 4.3 kJ/mol to AIV~(C).*~Similarly, by adding the difference in AHdbetween the LM and HM forms, 4.4kJ/mol, to AHdHM) = 47.5 kJ/mol, we evaluated AHf(LM) = 51.9 kJ/mol, which is quite smaller than the estimated value of AHdB) of stearic acid. This is due to unstable molecular packing caused by the cis double bond, irrespective of its position. The thermodynamic stability was confirmed by the solubility data and the melt-mediated transformation, as drawn in a (28) Kunihisha, K . K . Thermochim. Acta 1978, 35, 1-5
?
o/;
T//
01
Figure 10. Three typical subcells.
qualitative relationship between Gibbs free energy (G) and temperature (G-T relationship) in Figure 9. According to this G T relationship, one may argue the following transformation pathH M via the solid-state, solutionways: above 18.7 "C LM mediated, and melt-mediated transformations, all of which were observed in the present study. Below 18.7 "C, however, the transformation from HM to LM was observed only via solution mediation, since no solid-state transformation was observed from H M to LM. Hence, the transformation feature between the two forms of petroselinic acid may be characterized by the irreversible solid-state LM HM conversion and its ease of conversion above 18.7 "C (Figure 7). This is compared to two examples: B C in stearic acid occurred at 54 "C, although two G values cross at 32 0C,29and a a , in erucic acid occurred at 31.2 "C while 25.9 "C was the crossing point of the two G values.'8 The reversibility in the solution-mediated transformation comes from its lower activation energies in the growth of the more stable form at the expense (dissolution) of the less stable By contrast, the irreversibility in the solid-state transformation is primarily ascribed to steric hindrance associated with the changes in the molecular structures. In the present case, we presume the steric hindrance due to olefinic conformation, subcell packing, methyl end packing, and -COOH conformation. As to the subcell packing (Figure lo), the LM and H M forms are in the 0, and T,, subcells. respectively. However, the conversion in the subcell does not solely justify the irreversibility from H M to LM, since 0, T occurs irreversibly in p'(0,) /3(TI,)in tristearoylglycerol,*il but reversibly in A'(T,,) C'(0,) in tridecanoic acid.31
-
-
-
-
-
- -
(29) Sato, K.; Kobayashi. M.; Morishita, H. J . Cryst. Growth 1988,87, 236-242. (30) Cardew, P. T.;Davey, R. J . Proc. R. Soc. London 1985, 398A, 41 5-428.
The Journal of Physical Chemistry, Vol. 94, No. 7, I990 3185
Polymorphism of Petroselinic Acid
TABLE 11: Molecular Properties of Polymorphs in Cis-Monounsaturated Fatty Acids acid palmitoleic oleic asclepic erucic
erucic oleic
petroselinic petroselinic methyl oleate methyl oleate
polymorph Y Y Y Y
2LM HM Low High
w-chain
A-chain
7 9
9 9
7
II
ni‘
9 9 9
13 13 9 6 6
SCS’
12 12 9 9
olefinic conformn SCS’b SCS’ SCSd
01 ‘1 0 1 ’1 01 ‘1 TI,
scs
(157O,
subcell 0’I1
cis, -160’)
ni
scs scs’
[[-like 0, Il-tYPe Tl,-like Tl,-like
T,,,”OC -18.4 (+CY) -2.2 ( p a ) -1 5.0 ( p a )
-1 .o (‘CY) 9.0 ( p a , )
AH,,,= kJ mol-) 7.5 8.8 7.8 8.8 8.9
ref 14 13
17 18, 22 18, 22 13, 22 19 19
Polymorphic transformation. bSkew-cis-skew’. eNot identified. dSkew-cis-skew The reversibility or irreversibility in the polymorphic transformation of saturated fatty acids is discussed in terms of collective rotational movement of the aliphatic chains and conformational gauche-trans conversion^.^^ In petroselinic acid, however, the transformation properties may be explained not only by the aliphatic chain interactions but also by olefinic interactions. Molecular Properties. Table I1 summarizes the major molecular properties observed in the cis-unsaturated fatty acids. As for the LM form, a recent single-crystal analysis21-22 clarified that the olefinic Cll-C12=C13-C14 conformation is expressed by the internal rotation angels of ( 1 5 7 O , cis -160’) which differ by far from those having the conformations of skew-cis-skew (SCS) or skew-cis-skew’ (SCS’) (oleic acid, ?,lo (erucic acid, y12’v22) erucic acid, ~ ~ ‘ 3 We ~ ~assume ) . that there would be a specific interrelationship between the olefinic conformation and the subcell structure in the cis-monounsaturated fatty acids, since SCS’ is observed in the crystals having the O’,, subcell (Figure IO), whereas T,,is revealed in the crystals with the SCS conformation. Hence, we infer that the peculiar olefinic conformation of the LM form might enable a total of the aliphatic chain to be packed according to the 0, subcell. From this, it is worth noting that the polymorphism of petroselinic acid, particularly the LM form, stands on a specific position in the structural properties of cis-monounsaturated fatty acids. First, the LM form belongs to an orthorhombic system, both in the unit cell and in the subcell. Second, there is no order-disorder transformation in the HM and LM forms, in contrast to other four cis-monounsaturated fatty acids. The absence of the interfacial melting in the LM and H M forms was confirmed by the fact that (a) there were no DSC peaks, which a, are indicative of the order-disorder transformation like y for the LM and H M forms and (b) Raman and FT-IR spectra also indicated no characteristic change. The third peculiarity is a unique olefinic conformation which may stabilize the 0, subcell packing as discussed above. Finally, note the easy solid-state transformation from the 0,-packed LM form to the T,,-packed H M form. These properties may be ascribed to the position of the cis double bond, w-12. The orderdisorder transformation always occurred in the acids whose w-chain length is shorter than or the same as the A-chain length. In the ordered y-phase, the shorter w-chain may induce a modified molecular vibrational mode at the methyl-sided chain
segment which is somewhat different from that in the interior chain portion involving -(COOH)2 in a dimerized unit cell. This was reflected in the single-crystal XRD structure data in which the thermal factors at the w-chain in the y-form of erucic acid were remarkably larger than those of the A-chain.21,2z Hence, on heating, many gauche conformations may be introduced only in the w-chain segment, transforming to the interfacially melted in a-form, prior to the melting of the whole molecule. We estimated that 30-40% of hydrocarbons in the w-chain in the a-form may be in the gauche conformation by comparing the entropies per CH2 for the y-a transformation and fusion of the a-form of oleic acid to that of the fusion of n-alkane crystal having all-trans c o n f o r m a t i ~ n . ~In~the ? ~ longer ~ w-chain, however, the molecular motion may be hindered as in the interior chain portion. Therefore, interfacial melting may not occur in the two forms of petroselinic acid, although the HM form revealed the molecular structure similar to the interfacially melted a-form. Finally, we assume that the effect of parity, even or odd, of the position of the double bond may also be influential. This problem arises because (a) the orderdisorder transformation occurred in the odd-number-positioned unsaturated acids (Table 11), and (b) the melting points of triacylgly~erols~~ and AH,rof major chain transformation of phosphatidylcholine^,^^ both of which have cis-monounsaturated fatty acyl chains, significantly depend on the parity of the double bond position. This phenomenon must be related to the fact that the parity of the carbon numbers of the saturated fatty acids remarkably influences the melting behavior; Le., the methyl end packing mode differs between oddand even-numbered acids, stabilizing the total of the crystal cohesive energy in a different way. Accordingly, this influence may also occur in the cis-monounsaturated fatty acid where the acyl chain is separated into two parts by the cis double bond. The parity of the w-chain varies with the position of the double bond. The melting point of an odd-numbered saturated acid with (2n 1 ) carbons is lower than that of an even numbered saturated fatty acid with (2n) carbons, because of the less stable CH3 end group packing in the odd-numbered acid. Accordingly, the odd-numbered w-chain in the cis-monounsaturated fatty acids may behave like the odd-numbered saturated fatty acid, presumably causing the interfacial melting. Registry No. Petroselinic acid, 593-39-5; acetonitrile, 75-05-8.
(3 1 ) Kobayashi, M. Crystallization and Polymorphism of Fats and Fatty Acids; Garti, N., Sato, K. Eds.; Marcel Dekker: New York, 1988; pp
(32) Hagemann, J. W.; Tallent, W. H.; Barve, J . A,; Ismail, 1. A.; Gunstone, F. D. J . Am. Oil Chem. SOC.1975, 52, 204-207. (33) Barton, D. G.; Gunstone, F. D. J . Bioi. Chem. 1975,250,447C-4476.
-
139-1 87.
+
