THEPOLYMORPHISM OF TRIOLEIN
June, 1947
1445
[CONTRIBUTION FROM THE PROCTER & GAMBLE CO.]
The Polymorphism of Triolein BY R. H. FERGUSON AND E. S. LUTTON I n developing the essential background for understanding the properties of solidified fats, much has been discovered and reported concerning the polymorphism of trisaturated glycerides. On the other hand, information relating to the triunsaturated glycerides is quite limited. Thermal and X-ray diffraction data for the different crystalline states of trielaidin, trierucin, and tribrassidin have been given by Malkin, et al.' Confirmatory thermal data, but diffraction data a t some variance with that of Malkin, on the lowest and highest melting forms of trielaidin have been published by Filer, et d 2 n'ith respect to the important glyceride triolein, Wheeler, et u Z . , ~ have presented melting point data and limited thermal curve evidence for three forms. They have done the same for two forms of trilinolein. N o X-ray diffraction data have been reported for either of these latter glycerides. Accordingly, the present paper on triolein is chiefly concerned with diffraction patterns and their use (1) to characterize more fully the triolein forms reported by Wheeler, and (2) to relate them properly to already accepted triglyceride structure types.
Experimental Preparation of Triolein.-The preparation of oleic acid of 98.5% purity has been previously d e ~ c r i b e d . ~Nearly all the impurity was saturated fatty acid. Triolein was prepared by direct esterification of oleic acid (in excess) and glycerol. Excess fatty acid was removed by Wesson refining. The triolein of Wheeler, et al., was perhaps more nearly pure, but the present sample (iodine value, 84.8; acid value, 0.6-theoretical iodine value, 86.0; acid value 0.0) appeared to be of good quality as judgecl principally by similarity in polymorphic behavior to that observed by \Vheeler, r f al., and by sharpness of melting.
If triolein is chilled t o -70" in a capillary of less than 1.5 rnm. diameter a translucent, almost transparent, state is obtained which gives so little visual change on melting that accurate melting points cannot be obtained. The degree of translucency is unique among solid triglycerides thus far examined by the authors. These have included numerous trisaturated glycerides in the alpha form, which they believe to be essentially similar to the so-called vitreous or gamma form reported by Clarkson and Malkin.5,6 It was finally observed that by chilling, as Wheeler, et al., had done, only t o -55" a satisfactorily opaque state for melting point study could be obtained. That the translucent and opaque states represent a single polymorphic form, alpha, was indicated by X-ray. The translucent state is stiff, like a solid, and is anisotropic as shown by its brightness between crossed polaroids. It can be maintained a t -40" for at least one hour. The melting point procedure and values obtained are recorded in Table I in comparison with the data of Wheeler, et al. Softening points are recorded for the two metastable forms; i t was found, however, that both these forms melted to clear liquid only slightly above the reported temperatures of softening. The difference in melting point between stabilized and unstabilized beta is not uncommon for glycerides. I t is presumably due to a difference in size or perfection of crystallites. Time-Temperature Curve.-Time-temperature curvcs have been used by several investigators in the study of triglyceride polymorphism. Such a curve a t its best is a very satisfying supplement to melting point study and may give additional information as t o phase change. I t niay be misleading, however, especially with regard to metastable forms unless breaks in the curve are checked with a variety of imposed conditions. Generally a slight trend in the teniperature of a break follows a trend in external temperature, and, unavoidably, a personal factor is introduced iii drawing a curve from the experimental points. The experimental method for the present study was just such 'isimple one as is described by Clarkson and Malkin.5 \Vith a potentiometer, thermocouple readings were made on a 1 g., usually jacketed, sample pretreated and submitted to the desired cooling or warming environment. A
TABLE I MELTING BEHAVIOR OF TRIOLEIN Preparation (F.& L.)
Form
Alpha Beta prime Beta 4
Melted, Melted, (a) XIelted, (b) Melted, Uncorrected reading.
10 sec. 10 sec. 10 sec. 10 sec.
-55" -55". 10 sec. -30' -55', gradually warmed - 5 5 " , gradually warmed to O " , 1 wk. 0"
Melting Point.-In studying the melting behavior of different glyceride forms one must, by almost as much art as science, discover conditions of treatment which lead to reproducible values. Frequently, several procedures are satisfactory for a given form. No difficulties were encountered with the two higher melting forms of triolein, but a real obstacle had t o be overcome in studying the lowest melting form. (1) Malkin, et d., Hilditch's "Chemical Constitution of Natural Fats," Chapman and Hall (D. Van Nostrand Co., Inc., New York, N. Y . ) ,1940, p. 358. (2) Filer, el d.,THIS JOURNAL, 68, 167 (1946). (3) Wheeler, et d., J . Bid. Chem., 135, 687 (1940). (4) Lutton, Oil b S o a p , 59, 265 (1946).
-32
M. p. or
J.
p.
w.
F. & L.
* 1.0' (S. P.)" * 1.0" (S. P,)
-13 3.8" (C. hl. P,) 5 5" (C. 1%. P,)
-32' - 12" 4 . 7 to 5.0"
single junction iron-constantan thermocouple with 0 O reference junction was used. By this method the curves of Figs. 1 and 2 mere obtained. They illustrate certain helpful procedures for thermal curve study, and a t the same time point up some of the pitfalls. Satisfactory agreement between thermal curve breaks and melting point data was secured for all three forms. In the case of the beta form (Fig. 2 ) the break a t 5.2 ', representing the temperature of complete liquefaction, agrees well with the beta melting point of 5.5". The beta form was prepared for this run by chilling the melt t o -40" (five minutes without jacket) then holding a t 0 " ( 5 ) Clarkson and Malkin, J . Chem. SOC.,666 (1934).
(6) Lutton, THISJ O U R N A L , 6'7, 524 (1945).
R.H.FERGUSON AND E.s. LUTTON
1446 0.00
Vol. 69
COOLING CURVES
(0.00)
-
1.00 v; 2 (- 19.4')
-
.*
3
-2.00 (-39.4')
0 5 10 15 10 15 20 Time, minutes. Fig. 1.-Thermal curves for the a-form of triolein: cooling curves, sample melted, no j a c k e t - 0 , -25'; X, -30'; Heating curves are X, sample melted, chilled five minutes -70' no jacket, -10" @, -35'; 0, -45'; A, -55'. with jacket; 0 , sample melted, chilled five minutes -70' no jacket, -20' with jacket.
0
5
(one hundred minutes with jacket). The pretreated sample was then placed (with jacket) a t 15O , and millivolt readings were taken. During heating curve runs of the metastable forms, transformation accompanies melting and results in different sorts of inflection a t the melting point. The beta prime form was prepared by chilling a melted sample five minutes a t -40' (without jacket). Readings were take: 10 or - 15 , with the sample in a bath at approximately i. e., near the expected thermal point. On approaching the beta prime melting point, the sample began t o transform from beta prime to beta. . Very near the melting point, these three processes probably occurred-beta prime to beta, beta prime to liquid, liquid t o beta. Evolution of heat from rapid formation of beta resulted in rapid temperature rise above that of the environment and lhen a decline toward environment level. Sharp breaks in each beta prime curve, marking initiation of the rapid tern; perature rise, are shown in Fig. 2. These breaks a t - 11 are in reasonable agreement with the melting point figure of -13' since neither value is accurate to better than 1'. The pretreatment t o obtain alpha was simply a chilling of the melt for five minutes or more a t -70". Several runs were made with the jacketed 5ample placed at various temperatures above the expected break for alpha rndtirig. In Fig. 1breaks are shown at -31 a;d - 3 3 O in good agreement with the melting point at -32 . The breaks are not particularly sharp, however. Melting is associated with, perhaps t o some extent preceded by, transformation t o beta prime. Other inflections occur a t high temperatures but these are interpreted to be due t o transformation of beta prime to beta a t lower temperatures and slower rates than those indicated in Fig. 2. Since for triolein as for most triglycerides, the supercooling limit is nearly identical with the alpha melting point, the cooling curve can be used to confirm the level of the alpha melting point. As the melt (without jacket) was cooled (Fig. 1) the temperature dropped; and if environment temperature was above the alpha melting point, a
-
flat in the curve appeared before the temperature rise (due to beta prime crystallization) could occur. \\.'hen environment temperature was just low enough for the flat to disappear the temperature rise was a maximum as the alpha + beta prime sequence occurred rapidly, liquid (plus liquid + beta prime). The minimum for this particular curve, a t -33 ', agrees closely with -32" obtained from heating curves and melting point test. Further lowering of the environment temperature lowered the minimum and also lowered the temperature rise above the minimum. In Table I1 is shown a comparison of thermal points determined by melting and thermal curve study.
-
TABLE I1 THERMAL POINTS,'C., FOR TRIOLEIN Form
Alpha Beta Prime Beta
B y m. p. or s. p.
-32 - 13 5.5
From timetemperature curves
-32 -11 5.2
X-Ray Diffraction.-The procedure for X-ray diffraction was, in general, as previously described.6 Samples i n 1a m . thin-walled Pyrex capillaries were pretreated as for the melting point study. To obtai; the pattern of the stable form, the sample was held a t 0 ; t o obtain patterns of metastable forms, samples were held about 15" below the respective melting points. It was possible to maintain the desired temperatures by keeping the samples in a "cold block" chilled by circulation of cold alcohol. The diffraction data are shown in Table I11 in comparison with data for prominent spacings previously published for tristearin .e The triolein patterns were slightly weak as a consequence of the difficulties of long time exposure a t low temperatures. The main lines were clearly.shown, however, and indeed more fairly strong lines appeared for the two higher melting forms than are obtained for the corresponding
THEPOLYMORPHISM OF TRIOLEIN
June, 1947
1447
0.50 (9.6' i
0.00 (0.00)
d 0
c
-0.50 (-9.6')
-1.00 (- 19.4')
k/
1
1
I
I
I
10 15 20 25 Time, minutes. Fig. 2.-Heating curves for 8'- and p-forms of triolein: p', @, sample melted five minutes -40' with no jacket, approximately -15' with jacket; X, sample melted five minutes -40' with no jacket, approximately -10" with jacket. For 8 , 0 , sample melted five minutes -40" with no jacket, one hundred minutes 0' with jacket, 15" with jacket. 5
TABLE I11
X-RAYDIFFRACTION DATAFOR Triolein dln
Lowest (Alpha)
4.36 S Also Diff. darkening Corresponding to smaller spacing than 4.36
Tristearin d/n
4.14 V S
TRIOLEIN, COMPARISON WITH Intermediate (Beta Prime) Triolein Tristearin d/n d/n
Main Short Spacings 5.22 Wf 4.35 s 4.18 v 4.09 W+
3.78 S
3.87.M
s
TRISTEARIN (IN A,) Triolein d/n
5.28 M 4.57 v s 4.34 4.15 W 3.97 M 3.84 M 3.71 M 3.55 3.41 V W 3.28 W
w
w
3.41 V W 3.18VVW
Stable (Beta) Tristearin d/n
5.24 M 4,61 V S
3.84 S 3.68 S
Long Spacings d/n
n
44.8V S
1
15.00M
3
9.14W
5
45.2
d/n
n
44.8 V S 23.0 1- I[15.13 s 9.15 W 7.6.5 v 50.6
46.8
w
d/n
43.0 V S 21.8 \V 14.45 M 10.75 V W 8.75 w
1 2 3
5 6 Average d 46.8
43.3
45.16
1448
R. H. FERCUSON AND E. S. LUTTON
Vol. 69
beta although it does exhibit several extra lines. While the single strong spacing of the lowest melting form of triolein is off in magnitude and is associated with a certain amount of diffuse darkenDiscussion ing, its singleness and its proximity (4.36 vs. Triolein polymorphism resembles that of tri- 3.15) to the usual alpha value recommend classistearin. In both cases, there are three forms each fication of this form as alpha. The large value of having a different complete melting point. There this single spacing and the accompanying diffuse are similarities in short spacing pattern as will be darkening presumably indicate a looser, less well discussed more fully later. There is also a simi- organized hexagonal structure than that characlarity in association of diffraction pattern type teristic of most alpha forms. The intermediate with order of melting. melting form is less easy to classify as beta prime The long spacings of triolein indicate the com- !nd yet its characterizing spacings, 4.35 and 3.87 mon double chain length structure such as has A., are nearer the usual values, 4.2 and 3.8 fi., than been found5 for tristearin. This is of interest in the case of the alpha spacing and there are no since the mixed glyceride 2-oleyldisteariri shows more extra lines than the beta form exhibits. triple (or sextuple) chain length structure'+* pre- Because of the pattern features noted, it seems sumably due to sorting of chains. Long spacings justifiable to classify the three triolein forms in are somewhat shorter for triolein than for corre- order of increasing melting point, as alpha, beta sponding forms of tristearin. The alpha long prime and beta (or alpha-2, beta prime-2 and spacing is small enough to suggest a tilted struc- beta-2 in the more specific nomenclature recently ture, uncommon for alpha type forms. proposed) Further evidence for the validity The triolein diffraction data correspond fairly of this broad classification of minor variations closely to the data of Malkin, et aZ.,lfor trierucin, under a single name must await single crystal the CZ2homolog of triolein, Malkin reported for studies. alpha long spacing -55.0 A., no data for the beta Acknowledgment.-The authors express their prime form, for beta long spacing -51.0, no data. for alpha short spacings and for the five strongest appreciation t o Dr. A. S. Richardson and others beta short spacings-5.24, 4.60, 4.03, 3.81, 3.70. of this Laboratory for advice and assistance in Classification of triolein forms as alpha, beta this study. Summary prime and beta requires perhaps some further Observation 'by Wheeler, et al., of three polyjustification. As has been said, in order of melting and most other features of thermal behavior morphic forms melting a t (approximately) - 32, the triolein forms resemble the three tristearin -13 and 5.5' has been confirmed. On the basis forms that have been correspondingly designated. of these melting points and newly reported eviBut use of the same designations for apparently dence from time-temperature curves, and parcorresponding forms is not defensible unless there ticularly from X-ray diffraction patterns, these forms may be regarded as alpha, beta prime and are sufficienl similarities in X-ray pattern. I n naming glyceride forms one could, by dwell- beta. It is a matter of fundamental interest that ing on differences in diffraction patterns of differ- alpha, beta prime, and beta structure types, of ent glycerides, come up with dozens of distinctive double chain length, not only persist through the terms. At present i t seems far more conducive saturated series, tristearin, tripalmitin, etc., but to a grasp of the whole rather complex glyceride appear also for the triunsaturated glyceride triosituation to follow the lead of Malkin in empha- lein. Triolein long spacings are slightly shprter than sizing similarities of pattern without remaining those of tristearin; the spacing 45.2 A. for the blind to distinctive differences. Alpha-, beta prime- and beta-type forms of tri- alpha form is short enough to suggest a tilted glycerides can be characterized, respectively, by structure, which would be unusual for a n alpha a single stropg ring corresponding to approxi- form. A unique feature of triolein behavior is the mately 4.15 A., two strong rings corresponding to 4.2 (strongest) and 3.8 A. and a strongest ring formation of a stiff, translucent, anisotropic solid corresponding to 4.6 A. Only in $he case of the when capillary samples of the melt are rapidly The diffraction pattern of this highest melting form with its 4.6 A. spacing does chilled to -70'. triolein fit well into the picture; this spacing is so translucent solid is identical with that of the characteristic as to seem sufficient basis for classi- opaque alpha form obtained by less rigorous chillNo comparable tfanslucent state fication of the highest melting triolein form as ing to -55'. has been observed for other triglycerides. (7) Lutton, TRISJOURNAL, 68, 676 (1946). IVORYDALE, OHIO RECEIVED JAVUARY 2, 1947 (8) Filer, et al., ibrd , 68, 167 (1946). forms of tristearin, indicating somewhat greater complexities of structure. A comparison of triolein and tristearin patterns is shown in Fig. 3.
0
f I
1:
w:,,
O
neta Fig. 3.-Comparison of S-r;,y rliffrxtion piittenis of the three forms of triolein with the three forms of tristearin: 0.triolein; S, tristearin. The pattern strips are printed from flat films taken with a central strip of nickel foil to eliminate CuK8 radiation.
