The Polymorphism of Tristearin

1 The Polymorphism of Tristearin EDWARD

M.

BARRALL

II

and

J.

C.

GUFFY

Downloaded by 94.142.242.84 on August 2, 2016 | http://pubs.acs.org Publication Date: January 1, 1967 | doi: 10.1021/ba-1967-0063.ch001

Chevron Research Co., Richmond, Calif.

Tristearin, like other simple triglycerides, has been reported to exhibit a complex phase behavior on heating. Earlier publica tions have indicated that under certain circumstances a transi tion sequence of solid I — liquid I — solid II — liquid II can occur. This work has established the temperature limits and the transition heats for this sequence using differential thermal analysis and depolarized light intensity measurements. The sample pretreatments required to obtain the various phase se quences have also been studied. The structure of the semiliquid interphase between the two solid phases has been studied by high resolution nuclear magnetic resonance (NMR), and a trans formation mechanism has been proposed. For the first time, good photomicrographs have been published of the α , β inter phase and other crystal forms of tristearin. L

L

T P h e p o l y m o r p h i s m of t h e m o n o - , d i - , a n d t r i g l y c e r i d e s has been the s u b ject of m a n y studies for over a c e n t u r y . T h e s e m a t e r i a l s are of p a r t i c u l a r interest i n studies of ordered fluids a n d l i q u i d crystals because of t h e p e c u l i a r n a t u r e of t r i g l y c e r i d e p o l y m o r p h i s m . A s e m i l i q u i d phase exists i n t e r m e d i a t e between t w o t r u e s o l i d phases as some of these c o m pounds are heated. A s l o n g ago as 1849, t r i s t e a r i n was observed t o m e l t at 5 1 - 5 2 ° C . a n d t h e n to resolidify a n d o n f u r t h e r h e a t i n g to m e l t a g a i n at 6 2 - 6 2 . 5 ° C . (14)A n u m b e r of times, t h e triglycerides h a v e been t h e s u b ject of c o n t r o v e r s y i n the l i t e r a t u r e concerning the existence of phases a n d the significance of c e r t a i n x - r a y d a t a . T h e subject of t r i g l y c e r i d e p o l y m o r p h i s m a n d m u c h of the l i t e r a t u r e u p to 1961 has been reviewed i n d e t a i l b y C h a p m a n (4). T h e triglycerides h a v e been s t u d i e d b y x - r a y d i f f r a c t i o n (18), cooling a n d h e a t i n g curves (15), m i c r o s c o p y (16), d i l a t o m e t r y (4), dielectric constant measurement (9), i n frared s p e c t r o p h o t o m e t r y (6), b r o a d - l i n e nuclear m a g n e t i c resonance ( N M R ) (7), a n d differential t h e r m a l a n a l y s i s ( D T A ) (4, IS). T h e ex tensive x - r a y d i f f r a c t i o n l i t e r a t u r e d e a l i n g w i t h triglycerides has been t h o r o u g h l y reviewed u p to 1964 b y G u n s t o n e (12). 1 Porter and Johnson; Ordered Fluids and Liquid Crystals Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

2

ORDERED FLUIDS A N D LIQUID CRYSTALS

T h e m o s t d e t a i l e d x - r a y s t r u c t u r e s t u d y of these m a t e r i a l s t o d a t e is of t r i l a u r i n i n t h e fi f o r m b y V a n d (18). T h i s s t u d y was i n s t r u m e n t a l i n c l a r i f y i n g t h e confused p i c t u r e of t h e m o l e c u l a r a r r a n g e m e n t of t h e t r i glycerides i n t h e c r y s t a l l a t t i c e . C h a p m a n ' s r e v i e w (4) m a d e a s p e c i a l effort t o w a r d s c l a r i f y i n g t h e complex phase n o m e n c l a t u r e w h i c h h a d s u r r o u n d e d t h e triglycerides w i t h confusion a n d p r e v i o u s l y m a d e i t a l l b u t impossible to compare E n g l i s h a n d A m e r i c a n w o r k i n t h e field. C h a p m a n ' s phase designations are f o l l o w e d as closely as possible i n t h i s w o r k .


L

T r i s t e a r i n , t h e t r i g l y c e r i d e of octadecanoic a c i d , has received some a t t e n t i o n f r o m a n u m b e r of w o r k e r s . T o date o n l y one precise c a l o r i m e t r i c i n v e s t i g a t i o n of t h e phase t r a n s f o r m a t i o n s has been c a r r i e d out (8). The p r e v i o u s differential t h e r m o g r a m s (4, 13), a l t h o u g h s h o w i n g good d e t a i l , were a p p a r e n t l y m a d e i n s u c h a w a y t h a t q u a n t i t a t i v e heat d e t e r m i n a t i o n s were impossible. O n l y b r o a d - l i n e N M R spectra h a v e been t a k e n (7). T h e s e N M R spectra were m a d e to a i d i n i n t e r p r e t i n g the glass phase, f r o m 20°K. to r o o m t e m p e r a t u r e , a n d n o t i n the region of the higher t e m p e r a t u r e t r a n s i t i o n s where h i g h r e s o l u t i o n N M R is possible. A l t h o u g h t h e m i c r o scope has been one of t h e f a v o r i t e tools i n s t u d y i n g t r i g l y c e r i d e p o l y m o r p h i s m (4) j no charts of d e p o l a r i z e d l i g h t i n t e n s i t y ( D L I ) as a f u n c t i o n of t e m p e r a t u r e h a v e been p u b l i s h e d . E x p e r i m e n t s were c a r r i e d o u t i n t h i s laboratory to supply the calorimetric, D T A , N M R , a n d D L I d a t a w h i c h are n o t a v a i l a b l e i n t h e l i t e r a t u r e o n t r i s t e a r i n . Experimen

tal

A sample of zone-refined t r i s t e a r i n w a s o b t a i n e d f r o m the H o r m e l I n s t i t u t e , 801 N . E . 1 6 t h A v e . , A u s t i n , M i n n . T h i s m a t e r i a l h a d a p u r i t y of > 9 9 % b y thin layer chromatography. A sample of stearic a c i d w a s o b tained from Matheson, Coleman and Bell, Cincinnati, Ohio. After recryst a l l i z a t i o n f r o m a l c o h o l , a p u r i t y of 9 9 % ( d e t e r m i n e d b y t i t r a t i o n a n d t h i n layer chromatography) was obtained. Since p r e l i m i n a r y s u r v e y w o r k h a d i n d i c a t e d t h a t the c r y s t a l forms of t r i s t e a r i n were e x t r e m e l y sensitive t o i m p u r i t y a n d p r e v i o u s t h e r m a l h i s t o r y , several samples were p r e p a r e d : a c e t o n e - r e c r y s t a l l i z e d t r i s t e a r i n , a n nealed m e l t (cooled at 0 . 5 ° C . / m i n u t e to 10°C.) t r i s t e a r i n , a n d 9 7 % (mole %) t r i s t e a r i n - 3 % stearic a c i d t r e a t e d i n a l l of t h e a b o v e fashions. D T A Instrumentation. T h e differential t h e r m o g r a p h used i n t h i s s t u d y has been described i n d e t a i l (1, 2, 3). T h e microcalorimeter cell w h i c h u s e d ~ 0 . 0 0 5 g r a m of sample w a s used. T h e t h e r m o g r a m s were r e c o r d e d o n a n x-y recorder w i t h the differential t e m p e r a t u r e , AT, o n the 2/-axis a n d the sample t e m p e r a t u r e , T, o n the x - a x i s . T h e sample t e m perature w a s m e a s u r e d w i t h the same t h e r m o c o u p l e as the AT. This pro duces t h e r m o g r a m s w i t h p e a k l o c a t i o n s independent of h e a t i n g r a t e . T h e h e a t i n g rate w a s 4 ° C . / m i n u t e . T h e c a l o r i m e t e r w a s c a l i b r a t e d w i t h z o n e p u r i f i e d d o t r i a c o n t a n e , AH + AH = 51.7 c a l . / g r a m . Microscope and D L I Apparatus. T h e a p p a r a t u s is s h o w n i n F i g u r e 1 i n b l o c k f o r m . A Zeiss U l t r a p h o t I I p h o t o m i c r o s c o p e e q u i p p e d w i t h a p o l a r i z e r a n d a n a l y z e r , P o l a r o i d filters, a n d strain-free o p t i c s was u s e d t o o b t a i n a l l p h o t o g r a p h s discussed here. T h i s microscope w a s m o d i f i e d for r

f

Porter and Johnson; Ordered Fluids and Liquid Crystals Advances in Chemistry; American Chemical Society: Washington, DC, 1967.


1.

BARRALL AND GUFFY

Figure 1.

Polymorphism

of

Tristearin

3

Depolarized light intensity apparatus A. Microscope optics B. Photocell, cadmium telluride C. Polarizer D. Analyzer E. Hot stage F. Temperature regulator G. Recorder H. Photocell amplifier I. Sample J. Filter K. Light source L. Premeller

D L I measurement b y i n t r o d u c i n g a c a d m i u m t e l l u r i d e p h o t o c o n d u c t i v e cell a t t h e m o v i e c a m e r a focus. T h e l i g h t - d e p e n d e n t resistance changes of t h e p h o t o c o n d u c t i v e cell were m e a s u r e d a n d a m p l i f i e d i n t h e u s u a l m a n n e r a n d recorded o n t h e ?/-axis of a 1 m v . p e r i n c h M o s e l e y x-y recorder. T h e p h o t o m e t e r was a r r a n g e d so t h a t a 7-mv. s i g n a l a t the recorder corresponded t o complete t r a n s m i s s i o n t h r o u g h t h e p o l a r i z i n g filters o r i e n t e d p a r a l l e l t o one another. T h e recorder zero w a s a d j u s t e d t o c o r r e s p o n d t o t h e l i g h t t h r o u g h p u t w h e n t h e p o l a r i z e r s were crossed. D L I measurements were c a r r i e d o u t w i t h t h e p o l a r i z e r s crossed. S a m p l e s were heated o n a Zeiss h o t stage, m o d i f i e d b y r e p l a c i n g t h e p l a t i n u m sample t e m p e r a t u r e t h e r m o c o u p l e b y a m o r e sensitive c o p p e r c o n s t a n t a n couple, l o c a t e d as close as possible b e n e a t h t h e sample cover slip. A second t e m p e r a t u r e p r o g r a m t h e r m o c o u p l e w a s p l a c e d i n c o n t a c t w i t h t h e ceramic heater f r a m e of t h e h o t stage. T h e stage w a s p r o g r a m m e d at 2 ° C . p e r m i n u t e w i t h a s l o p e - p r o p o r t i o n a l b a n d c o n t r o l l e r . Temperature was controlled to ±0.05°C. I c e reference j u n c t i o n s were used o n b o t h samples a n d p r o g r a m thermocouples. T h e o u t p u t of t h e s a m p l e - i c e j u n c t i o n t h e r m o c o u p l e w a s recorded o n t h e 0.5 m v . p e r i n c h x-axis of t h e M o s e l e y x-y recorder.


4


N M R Apparatus. N M R s p e c t r a were o b t a i n e d o n a V a r i a n A - 6 0 e q u i p p e d w i t h a V a r i a n V - 6 0 4 0 v a r i a b l e t e m p e r a t u r e c o n t r o l l e r a n d probe. S c a n s were m a d e a t 7-second i n t e r v a l s . C a r e w a s t a k e n t o a v o i d r a d i o frequency s a t u r a t i o n of t h e p r o t o n resonance. T h e temperatures at the b e g i n n i n g a n d e n d of t h e r u n s were d e t e r m i n e d i n t h e s t a n d a r d m a n n e r f r o m t h e c h e m i c a l shift of t h e t w o ethylene g l y c o l p e a k s . C a l i b r a t i o n of t h e h e a t i n g c h a r a c t e r i s t i c s of t h e a p p a r a t u s as a f u n c t i o n o f t i m e , heater c u r r e n t , a n d t e m p e r a t u r e was c a r r i e d o u t p r i o r t o t h e t r i g l y c e r i d e s p e c t r a d e t e r m i n a t i o n s w i t h t h e ethylene g l y c o l p e a k s . K n o w i n g heater c u r r e n t a n d t i m e elapsed f r o m t h e onset of h e a t i n g p e r m i t t e d t e m p e r a t u r e s t o be d e t e r m i n e d t o ± 1 ° C . i n 10 t r i a l r u n s . Results


Differential Thermal Analysis.

A c e t o n e - r e c r y s t a l l i z e d t r i s t e a r i n has a

v e r y complex differential t h e r m o g r a m .

F i g u r e 2 shows a trace w h i c h is

representative of three separate d e t e r m i n a t i o n s .

T h i s curve

those p u b l i s h e d b y C h a p m a n (6) i n gross features.

T h r e e endotherms a n d

one e x o t h e r m are e v i d e n t .

resembles

T h e t e m p e r a t u r e s of t r a n s i t i o n a r e g i v e n i n

Table I.

I

A

1 1 1

A. Differential thermogram 0.00531 gram sample. Heating rate -4°C/min. B. Depolarized light intensity curve. Heating rate - 2°C/min.

I — ι — ι

0

1 — ι

5

ι

ι

ι

ι

ι

10 15 20 25 30 35 40

« •

'

•

45 50 55

Cooling

ι

ι

60

65

ι

ι

ι

70 75 80

ι

85

ι

ι

90 95

Sample Temperature Figure 2.

Tristearin

T h e agreement between t h e t w o sets of d a t a o n t r i s t e a r i n is g o o d w h e n t h e p r o b a b l e differences i n i n s t r u m e n t a t i o n are considered. O n l y the t e m peratures of t h e e n d o t h e r m a l m i n i m a , T , s h o u l d be considered since t h e b e g i n n i n g of t h e e n d o t h e r m depends g r e a t l y o n t h e s e n s i t i v i t y a n d s a m p l e shape used i n some i n s t r u m e n t s . T h e l o c a t i o n of t h e e x o t h e r m depends o n h e a t i n g rate t o some extent. H e a t i n g rate d a t a were n o t g i v e n w i t h C h a p m


1.

BARRA L L A N D G U F F γ

Table I.

Polymorphism

of

Tristearin

Temperatures of Transition for Tristearin and Stearic Acid Thermograph Features" °C.

Compound Tristearin

Reference This work

T

T 44.5 54.5 58.9 64.8

m

b

—

Chapman (6)

—


Melting point, capillary (6) Stearic acid

a

This work Void (19)

Process Endotherm Endotherm Exotherm Endotherm

—

Not noted Endotherm Exotherm Endotherm

57 63 76

56 58 73

51 57 66

T 51.5 58.9 63.4 76.8 e

47.1 57.5 60.3 70.3

54 64 73.1 68.6 69.0

57.5 Not given

Endotherm Endotherm

71.4 Not given

Tb. Temperature at beginning of endotherm or exotherm. T . Temperature at endothermal or exothermal inflex. T . Temperature at end of endotherm or exotherm. Phase nomenclature is that suggested by Chapman (4). m e

6

m a n ' s curves (4).

T h e i n i t i a l e n d o t h e r m seen i n t h i s w o r k a n d n o t n o t e d

i n C h a p m a n ' s curves c o u l d be caused b y t h e close p a c k o r t h o r h o m b i c —» o p e n h e x a g o n a l (OIL) t r a n s i t i o n .

C h a p m a n ' s i n f r a r e d studies h a v e

c a t e d t h e presence of these phases (5).

indi

F o r t h e r e m a i n i n g t r a n s i t i o n s close

agreement b e t w e e n l i t e r a t u r e m e l t i n g p o i n t s a n d D T A d a t a c a n n o t be ex pected.

T h e literature m e l t i n g points given for the monoacid triglycerides

were d e t e r m i n e d b y p l u n g i n g capillaries c o n t a i n i n g t h e esters i n t o p r e h e a t e d b a t h s a n d n o t i n g t h e presence o r absence of a change.

Timmer-

m a n s (17) has described t h i s m e t h o d i n d e t a i l a n d g i v e n precise i n s t r u c t i o n s for i t s a p p l i c a t i o n t o t r i g l y c e r i d e s .

D y n a m i c h e a t i n g d a t a w o u l d show

c o r r e s p o n d i n g effects a t l o w e r t e m p e r a t u r e s , as d o t h e D T A d a t a s h o w n i n Table I.

T h i s effect is generally observed w h e n t h e t r a n s i t i o n s m e a s u r e d

are b r o a d — i . e . , n o t i s o t h e r m a l . Table II. Fig. 2 Peak No. I

Heats of Transition of Tristearin and Stearic Acid AH, Substance Tristearin

II III IV

Transition Orthorhombic —» hexagonal (4) triclinic (4) βζ, Triclinic β —> liquid Solid -> liquid ί

Stearic acid

Cal./G.

This work 2 .0 6 .4 —12 .4 31 .1 47 .6


Lit.

-1S.7(14) 54.5(19) 47.6(11) 49.2(15)

6


T h e heats i n v o l v e d i n t h e v a r i o u s D T A endotherms a n d exotherms s h o w n i n F i g u r e 2 are g i v e n i n T a b l e I I . T h e d a t a are c o m p a r e d w i t h t h e l i t e r a t u r e where possible. T h e p r e v i o u s studies of t r i s t e a r i n h a v e e m p l o y e d c o n v e n t i o n a l c a l o r i m e t r i c techniques. C l o s e agreement is n o t e d b e t w e e n the w o r k of C h a r b o n n e t a n d S i n g l e t o n (8) a n d o u r D T A for t h e e x o t h e r m a l p o r t i o n of t h e a —^fi t r a n s i t i o n . T h e e n d o t h e r m a l p o r t i o n of t h i s t r a n s i t i o n appears t o h a v e been o v e r l o o k e d i n t h e classical c a l o r i m e t r i c t r e a t m e n t . H o w e v e r , these same w r i t e r s give large v a l u e s for t h e a —• l i q u i d t r a n s i t i o n (38.9 c a l . per gram) a n d t h e fi —τ l i q u i d t r a n s i t i o n (54.5 c a l . per g r a m ) . F r o m t h e o r e t i c a l considerations a n d general s i m i l a r i t y of s t r u c t u r e , t h e heat of f u s i o n of the fi f o r m s h o u l d be c o m p a r a b l e to t h a t of stearic a c i d . T w o previous v a l u e s for stearic a c i d are g i v e n : 47.6 c a l . per g r a m (15) a n d 49.2 c a l . per g r a m (16) b y c a l o r i m e t r y a n d D T A , respec t i v e l y . A v a l u e of 47.6 c a l . per g r a m u s i n g n - d o t r i a c o n t a n e as c a l i b r a t i o n s t a n d a r d was o b t a i n e d i n the present w o r k . T h e s e d a t a i n d i c a t e t h a t t h e D T A m e t h o d is capable of g i v i n g excellent c a l o r i m e t r i c results o n a s y s t e m s i m i l a r t o t r i s t e a r i n . I f peak I V , F i g u r e 2, is t h e fusion of the pure /3z,form, t h e heat of f u s i o n is 31.1 a n d n o t 54.5 cal. per g r a m as p r e v i o u s l y g i v e n (8). F r o m t h e D T A results i t is possible to calculate a n a p p r o x i m a t e heat of f u s i o n for t h e p u r e a f o r m , a l i q u i d : 31.1 + 6.4 — 12.4 or 26.1 c a l . per g r a m . C h a r b o n n e t a n d S i n g l e t o n o b t a i n e d 38.9 c a l . per g r a m (8) u s i n g a n analogous m e t h o d of c a l c u l a t i o n . B o t h heats of f u s i o n neglect heat c a p a c i t y effects i n v o l v e d i n t h e c a l c u l a t i o n . W e are u n a b l e t o account for the differences between earlier heat of t r a n s i t i o n a n d f u s i o n d a t a a n d t h e present D T A results. L

L

L

L


L

L

L

T h e p u r e t r i s t e a r i n s a m p l e w h i c h was annealed s l o w l y f o r m e d o n l y t h e βζ, phase. N a t u r a l l y , no exotherms were observed. O n l y a single peak o w i n g to the f u s i o n of t h e βζ, f o r m was observed at 70.3°C. T h e heat of f u s i o n of t h e βζ, f o r m m e a s u r e d f r o m t h i s s a m p l e was 31.9 c a l . per g r a m . T h e l i q u i d n i t r o g e n - q u e n c h e d s a m p l e of p u r e t r i s t e a r i n d i d n o t show t h e o r t h o r h o m b i c —> h e x a g o n a l t r a n s i t i o n . A l l other t h e r m o g r a p h i c c h a r acteristics were i d e n t i c a l to t h e acetone-recrystallized m a t e r i a l . T h e t r i s t e a r i n s a m p l e c o n t a i n i n g 3 m o l e % of stearic a c i d gave o n l y the a f o r m , w h i c h m e l t e d d i r e c t l y to t h e l i q u i d irrespective of t h e s a m p l e p r e t r e a t m e n t . T h e presence of i m p u r i t i e s appears to i n h i b i t t h e f o r m a t i o n of t h e β L f o r m even o n slow a n n e a l i n g f r o m t h e m e l t . T h e h e a t of fusion of the a f o r m was 29 c a l . per g r a m , w i t h t h e 3 % stearic a c i d b e i n g t a k e n i n t o account i n t h e c a l c u l a t i o n . L

L

Depolarized Light Study. T h e samples used for D L I were m e l t e d between cover slips a n d r a p i d l y quenched. T h e cover slips were h e l d 0.01 m m . a p a r t w i t h a glass spacer. T h i s t r e a t m e n t s h o u l d result i n t h e h e x a g o n a l , aL, phase as t h e r o o m t e m p e r a t u r e s o l i d . T h e d e p o l a r i z e d l i g h t i n t e n s i t y curve, F i g u r e 2, shows a sharp loss i n i n t e n s i t y b e g i n n i n g at 57°C. a n d approaches e x t i n c t i o n at 58°C. This in-


1.

Polymorphism

BARRALL AND GUFFY

of

Tristearin

7

dicates a loss of anisotropic s t r u c t u r e w h i c h w o u l d n o r m a l l y a c c o m p a n y a phase t r a n s i t i o n f r o m s o l i d t o l i q u i d . O n f u r t h e r h e a t i n g t h e i n t e n s i t y i n creases t o f o r m a shoulder at 64.8°C. a p p r o x i m a t e l y one-half as intense as t h e i n i t i a l solid-phase r o t a t i o n . A n e w p e a k r o t a t i o n , one-half of t h e o r i g i n a l i n t e n s i t y , is reached a t 76°C. T h i s corresponds t o t h e f i n a l m e l t i n g of the compound. T h e conclusions r e g a r d i n g these t r a n s i t i o n s are i n s u b s t a n t i a l agreement w i t h those d r a w n o n t h e basis of t h e differential t h e r m o grams. T h e first t r a n s i t i o n i n t o a disordered state is t h e onset of m e l t i n g of t h e a phase. T h e r e - f o r m a t i o n of t h e r o t a t i o n of l i g h t is t h e exo t h e r m a l conversion of a i n t o t h e β' phase. T h e increase i n d e p o l a r i z a t i o n above 64.8°C. is p r o b a b l y caused b y t h e f o r m a t i o n of t h e βL phase f r o m β'ζ,. T h e final e x t i n c t i o n a t 76°C. corresponds t o t h e m e l t i n g of t h e s o l i d . T h e m o l e c u l a r arrangement of t h e β' phase is n o t k n o w n w i t h certainty. L

L

L


L

Since t r a n s i t i o n t h r o u g h a n o p t i c a l l y isotropic phase, cubic, is excluded b y x - r a y d a t a (12), t h e 57°C. e x t i n c t i o n of r o t a t i o n indicates t h e f o r m a t i o n of a phase w i t h disorder n o r m a l l y associated w i t h a m e l t . T h i s indicates that the chain separation a n d dislocation during the transition from the p e r p e n d i c u l a r l y oriented a hexagonal s t r u c t u r e t o t h e o b l i q u e l y oriented fi t r i c l i n i c s t r u c t u r e , F i g u r e 2, are larger t h a n has p r e v i o u s l y been d e scribed or i m p l i e d . L

L

Microscopic Examination. S e v e n c r y s t a l types h a v e been described f r o m o p t i c a l microscope e x a m i n a t i o n . A l l of these phases were seen i n t h i s w o r k . F i g u r e s 3 a n d 4 show some examples. A n e w O>L f o r m , « L spherulite, c r y s t a l l i z e d o n slow cooling of t h e m e l t (1°C. p e r m i n u t e o r less).

Figure 3.

Interpénétration of a-spherulite and mosaic forms of tristearin

A t faster rates, t h e p r e v i o u s l y reported αζ,-mosaic f o r m e d . F i g u r e 3 shows a field w i t h b o t h forms c o c r y s t a l l i z e d . T h e αζ,-mosaic a n d αζ,-spherulite were i d e n t i c a l i n D T A a n d D L I b e h a v i o r t o t h e s o l v e n t - r e c r y s t alii zed c r y s tals. F i g u r e 4 shows t h e t r a n s f o r m a t i o n of a - s p h e r u l i t e s i n t o d u l l β''Lspherulites. O n f u r t h e r h e a t i n g , t h e βL phase forms a n d t h e c r y s t a l s b r i g h t e n somewhat. T h e extinctions of these crystals correspond t o those L


8


p r e v i o u s l y g i v e n i n the l i t e r a t u r e (16) a n d p e r m i t the assignment of p r o b able phases. R a p i d cooling D L I curves show t h e r e f o r m a t i o n of t h e s o l i d αχ, phase i n one step. T h e fi or β'' phases do n o t r e f o r m f r o m t h e m e l t . T h i s f u r t h e r confirms t h e i r r e v e r s i b i l i t y of these t r a n s f o r m a t i o n s . Nuclear Magnetic Resonance. P r e v i o u s N M R studies h a v e been c o n cerned w i t h b r o a d b a n d spectra at s u b a m b i e n t t e m p e r a t u r e s (7). These d a t a i n d i c a t e d a large a m o u n t of free r o t a t i o n i n t h e l o w t e m p e r a t u r e o r t h o r h o m b i c phase. F i g u r e 5 shows a series of r a p i d h i g h r e s o l u t i o n scans


L

L

Figure 4.

Crystal forms of tristearin

Upper, β 69°C. Left, β interphase 62°C. Right, a 55°C.

m a d e f r o m 55° to 73°C. u s i n g a s a m p l e m a d e b y r a p i d l y freezing a l i q u i d m e l t . T h r e e p o i n t s are significant i n these c u r v e s : t h e appearance of C H p r o t o n resonance f r o m 55.4° to 6 0 ° C , t h e disappearance of t h i s resonance f r o m 60.3° t o 6 4 . 6 ° C , a n d t h e r a t i o of t h e resonance m a x i m u m at 59.0°C. to t h a t at 72.3°C.

2

T h e appearance of a h i g h r e s o l u t i o n p r o t o n resonance i m m e d i a t e l y i n dicates t h e f o r m a t i o n of a n open, n o n r e s t r i c t i v e s t r u c t u r e . T h e t e m p e r a t u r e range over w h i c h t h i s s i g n a l is seen corresponds t o e n d o t h e r m I I o n t h e differential t h e r m o g r a m a n d t h e first e x t i n c t i o n of t h e D L I c u r v e . The t r a n s i t i o n between OLL a n d β'L forms consists of a disordered state.


BARRALL AND GUFF Y

Polymorphism

of

Tristearin


1.


9

10

O R D E R E D FLUIDS A N D LIQUID CRYSTALS

T h e disappearance of the p r o t o n signal f r o m 60.3° to 64.6°C. indicates the f o r m a t i o n of a new restricted or solid phase, βζ, or β'ζ, phase. This range corresponds to t h e e x o t h e r m of r e c r y s t a l l i z a t i o n o n t h e differential t h e r m o g r a m a n d the second D L I m a x i m u m . T h e r a t i o of t h e p r o t o n s i g n a l m a x i m u m at 59°C. to t h e p r o t o n s i g n a l f r o m t h e isotropic l i q u i d phase at 72.3°C. is 4 2 % . T h i s indicates t h a t some w h a t less t h a n h a l f of t h e t o t a l protons are free to resonate. T h i s c o n d i t i o n c a n be most easily e x p l a i n e d b y p o s t u l a t i n g a s e p a r a t i o n of the i n t r a c h a i n c r y s t a l l i n e zones. T h e l a c k of Ο II —CH C— Downloaded by 94.142.242.84 on August 2, 2016 | http://pubs.acs.org Publication Date: January 1, 1967 | doi: 10.1021/ba-1967-0063.ch001

2

signal at 59°C. indicates t h a t the s e p a r a t i o n does n o t extend u p to the c a r b o n before t h e ester group. T h e t e r m i n a l C H protons are smeared i n t o t h e p r i n c i p a l c h a i n C H peak because of p o o r resolution at 59°C. This m o d e of t r a n s i t i o n , a l t h o u g h n o t p r e v i o u s l y suggested, does n o t disagree w i t h a n y previous evidence. F i g u r e 6 shows t h e percent free protons as a f u n c t i o n of t e m p e r a t u r e . T h e c u r v e is d e r i v e d f r o m the areas s h o w n i n F i g u r e 5. T h e shape a n d t e m p e r a t u r e of the inflexes correspond d i r e c t l y t o D T A a n d D L I features. 3

2

40

50

60 SAMPLE

Figure 6.

70 TEMPERATURE,

80 -

90

C

Percent of rotatable hydrogen as a function of temperature for tristearin

Conclusions D T A indicates t h a t after some i n i t i a l a b s o r p t i o n of heat, t h e az,form i n the hexagonal base plane p e r p e n d i c u l a r arrangement translates t o t h e 62° i n c l i n e d t r i c l i n i c βζ, f o r m ( F i g u r e 7) w i t h a n emission of heat. T h e βζ,


1.

Polymorphism

BARRALL AND GUFFY

of

11

Tristearin

f o r m t h e n m e l t s a t 73°C. t o g i v e t h e l i q u i d phase.

S u c h a m o d e l is i n s u b

s t a n t i a l agreement w i t h t h e l i t e r a t u r e ( 4 , 1 0 , 1 2 ) a n d t h e observed t h e r m a l phenomena.

3Σ 3S

90° Inclination to the Base Plane (9)

3E

62° 7' Inclination to the Base Plane (18)


a

where C H -0-H -0-C-(CH ) CH 22

2

16

3

— ν

CH-0-0-(CH ), CH is 2

I

6

3

ο CH -0-C-(CH ) CH 2

Figure 7.

2

16

3

Chain arrangements in tristearin crystals

Assuming that the structure of the JSL phase is the same as that of trilaurin the only triglyceride for which single-crystal data are available

a

f

O n t h e basis of D T A , D L I , a n d N M R studies, t h e interphase between the a a n d $ o r β' phases of t r i s t e a r i n appears t o be a s e m i l i q u i d . T h e heat of c r y s t a l l i z a t i o n i n t o t h e βι phase is large enough t o cause t h i s effect. T h e direct a s s u m p t i o n t h a t t h i s interphase is s i m p l y m e l t e d a phase is r u l e d o u t b y t h e absence of p r o t o n signals other t h a n t h e c h a i n C H a n d CH . T h e open interphase s t r u c t u r e also explains p r e v i o u s l y observed i n f r a r e d doublets (6). L

L

L

L

2

3

T h e t h e r m a l h i s t o r y of t r i s t e a r i n as w e l l as t h e p u r i t y is i m p o r t a n t i n d e t e r m i n i n g t h e t y p e of c r y s t a l phase f o r m e d o n cooling t h e m e l t o r o n r e c r y s t a l l i z a t i o n . A c e t o n e r e c r y s t a l l i z a t i o n produces t h e f o r m stable a t l o w est t e m p e r a t u r e , the o r t h o r h o m b i c . R a p i d q u e n c h i n g of t h e m e l t produces the next most stable phase, t h e hexagonal o r αχ, phase. S l o w c o o l i n g per m i t s t h e βι phase t o f o r m , w h i c h does n o t revert t o t h e «ζ, f o r m at r o o m t e m p e r a t u r e b u t remains metastable u n t i l t h e n o r m a l m e l t i n g p o i n t is reached o n reheating. S m a l l a m o u n t s of i m p u r i t y cause t h e a f o r m t o appear irrespective of t h e r m a l t r e a t m e n t . T h e i m p u r e a crystals m e l t d i r e c t l y w i t h o u t c h a n g i n g i n t o t h e / 3 f o r m . T h i s last effect is p r o b a b l y caused b y t h e l o w e r i n g of t h e free energy of t h e c r y s t a l s t r u c t u r e . L

L

L


12


Acknowledgznen

t

The authors thank John Q. Adams for obtaining the N M R spectra and structure assignments.


Literature

Cited

(1) Barrall, Ε. M . , II, Gernet, J. F., Anal. Chem. 35, 1837 (1963). (2) Barrall, Ε. M . , II, Porter, R. S., Johnson, J. F., Anal. Chem. 36, 2172 (1964). (3) Barrall, Ε. M . , II, Porter, R. S., Johnson, J. F., J. Phys. Chem. 68, 2810 (1964). (4) Chapman, D., Chem. Revs. 62, 433 (1962). (5) Chapman, D., J. Chem. Soc. 1957, 4489. (6) Chapman, D., Nature 176, 216 (1955). (7) Chapman, D., Richards, R. E., Yorke, R. W., J. Chem.Soc.,1960, 436. (8) Charbonnet, G. H., Singleton, W. S., J. Am. Oil Chemists' Soc. 24, 140 (1947). (9) Crowe, R. W., Smyth, C. P., J. Am. Chem. Soc. 72, 4427 (1950). (10) Fox, D., Labes, M. M . , Weissberger, Α., eds., "Physics and Chemistry of the Or ganic Solid State,'' Vol. I, p. 135, Interscience, New York, 1963. (11) Garner, W. D., Madden, F. C., Rushbrooke, J. E., J. Chem. Soc. 1926, 2941. (12) Gunstone, F. D., Chem. Ind. (London) 1964, 84. (13) Haighton, A. J., Hannewijk, J., J. Am. Oil Chemists' Soc. 35, 344 (1958). (14) Heintz, W., Jahresber. 2, 342 (1849). (15) Malkin, T., "Progress in Chemistry of Fats and Other Lipids," Vol. II, Pergamon Press, London, 1954. (16) Quimby, O. T., J. Am. Chem. Soc. 72, 5063 (1950). (17) Timmermans, J., "Chemical Species," Chemical Publishing Co., New York, 1940. (18) Vand, V., Bell, I. P., Acta Cryst. 4, 104 (1951). (19) Vold, M . J., Anal. Chem. 21, 683 (1949). RECEIVED April 18, 1966.


The Polymorphism of Tristearin

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