Effect of Thermal History and Impurities on Phase Transitions in

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 19, 2018 | https://pubs.acs.org Publication Date: January 1, 1967 | doi: 10.1021/ba-1967-0063.ch002

2 Effect of Thermal History and Impurities on Phase Transitions i n Long-Chain Fatty A c i d Systems T . J . R. C Y R , W . R. J A N Z E N , and B . A .

DUNELL

The University of British Columbia, Vancouver 8, Canada

Broad-line nuclear magnetic resonance has been used to study melting in stearic acid and a mesomorphic (crystalline to waxy) phase transition in lithium stearate. Extensive motion, liquid-like, though less extensive than that in an isotropic free-flowing liquid, takes place within the system below the melting point of stearic acid or the crystalline to waxy phase transition of lithium stearate. The amount of liquid-like character, as measured by the intensity of a narrow component in the NMR spectrum relative to the total intensity of the whole spectrum, depends on the presence of impurities in the system and even more significantly on whether and how many times the sample has been melted.

" D r o a d l i n e n u c l e a r m a g n e t i c resonance ( N M R ) i n v e s t i g a t i o n of stearic a c i d (9) a n d o t h e r l o n g c h a i n f a t t y acids (3) has i n d i c a t e d t h a t a s i g n i f i c a n t f r a c t i o n of the p r o t o n s i n these substances is i n r a p i d m o t i o n — a l m o s t l i q u i d - l i k e — s e v e r a l o r e v e n tens of degrees below t h e accepted m e l t i n g p o i n t s of t h e a c i d . T h i s m o t i o n is m a d e e v i d e n t b y a n a r r o w c o m ponent, between 0.1 a n d 0.01 gauss w i d e , w h i c h appears i n t h e n o r m a l b r o a d - l i n e s p e c t r u m w e l l below t h e m e l t i n g p o i n t , a n d w h i c h , w i t h o u t a p p a r e n t change i n w i d t h , grows i n i n t e n s i t y a t t h e expense of t h e i n t e n s i t y of t h e b r o a d - l i n e c o m p o n e n t ( w h i c h r e m a i n s of c o n s t a n t w i d t h ) u n t i l t h e m e l t i n g p o i n t is r e a c h e d . T h e f a t t y acids used i n these experiments were c a r e f u l l y p u r i f i e d , so t h a t i t c a n be asserted c o n f i d e n t l y t h a t t h e p r o t o n s i n l i q u i d - l i k e m o t i o n , w h i c h c o n t r i b u t e to the n a r r o w l i n e , are n o t m a i n l y i n i m p u r i t y molecules. W e h a v e , h o w e v e r , suggested (3, 9) t h a t t h e l i q u i d - l i k e m o t i o n i n t h e f a t t y acids centers a b o u t , a n d w i t h increasing t e m p e r a t u r e grows o u t f r o m , i m p u r i t y centers or l a t t i c e defect centers i n t h e c r y s t a l . A n a t t e m p t has been m a d e i n the w o r k reported here to o b t a i n evidence t h a t t h i s l i q u i d - l i k e 13 Porter and Johnson; Ordered Fluids and Liquid Crystals Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

14

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

b e h a v i o r originates, i n p a r t at least, a r o u n d i m p u r i t y centers b y a d d i n g definite a m o u n t s of k n o w n i m p u r i t i e s to c a r e f u l l y p u r i f i e d stearic a c i d . D u r i n g t h i s w o r k i t became a p p a r e n t t h a t defect centers m u s t p l a y a n e q u a l l y o r m o r e i m p o r t a n t p a r t t h a n i m p u r i t y centers i n p r o m o t i n g t h e existence of l i q u i d - l i k e regions i n t h e c r y s t a l .

A s t u d y of l i t h i u m stéarate

has also been r e p o r t e d i n d e t a i l (7) ; i n t h i s p a p e r we note the influence of the t h e r m a l h i s t o r y of t h e sample o n one of t h e phase changes i n l i t h i u m


stéarate a n d consider i t as evidence for the i m p o r t a n c e of l a t t i c e defects i n p r o m o t i n g phase t r a n s i t i o n at t e m p e r a t u r e s below t h a t at w h i c h t h e t r a n s i t i o n is m a c r o s c o p i c a l l y e v i d e n t . Experimen

tal

Materials. T w o different lots of stearic a c i d were used i n t h e e x p e r i m e n t s reported here, one p u r i f i e d sample b e i n g used for the stearic a c i d ex p e r i m e n t s , the other for m a k i n g l i t h i u m stéarate. T h e i n i t i a l source for b o t h was E a s t m a n K o d a k W h i t e L a b e l grade, f u r t h e r p u r i f i e d b y r e c r y s t a l l i z a t i o n a c c o r d i n g to t h e m e t h o d of B r o w n a n d K o l b (6) f r o m freshly d i s t i l l e d reagent grade acetone at — 20°C. T h e b a t c h used to prepare l i t h i u m stéarate h a d a freezing p o i n t , after t w o recrystallizations, of 6 9 . 5 ° C ; a t h e r m o m e t e r c a l i b r a t e d at t h e ice a n d steam points was used a n d corrected ( + 0 . 4 8 ° C . ) for s t e m exposure. A hot, 5 0 % b y v o l u m e w a t e r - e t h a n o l m i x t u r e w a s s a t u r a t e d w i t h reagent grade l i t h i u m h y d r o x i d e a n d t i t r a t e d i n t o a h o t e t h a n o l s o l u t i o n of t h e purified stearic a c i d to a p h e n o l p h t h a l e i n end p o i n t , a n d a few drops of excess base were added. T h e p r e c i p i t a t e d soap was filtered o n a B u c h n e r funnel after i t h a d cooled a n d w a s d r i e d i n a v a c u u m desiccator o v e r p h o s phorus pentoxide o v e r n i g h t a n d t h e n i n a n o v e n at 110°C. for 4 h o u r s . T h i s sample was t h e n heated g e n t l y i n a 2 0 0 - m l . flask, fitted w i t h a s t o p cock, u n t i l the soap h a d m e l t e d a n d flowed freely for a few m i n u t e s . T h e stopcock was closed a n d the s a m p l e allowed to cool to r o o m t e m p e r a t u r e . T h e r e s u l t i n g glassy m a t e r i a l w a s g r o u n d u p i n a m o r t a r a n d k e p t i n a v a c u u m desiccator for 10 d a y s . A f t e r t h e s a m p l e h a d been l o a d e d i n t o a n N M R sample t u b e , i t was h e a t e d for 3 h o u r s at 125°C. u n d e r v a c u u m , a n d the glass sample t u b e w a s sealed off w h i l e s t i l l e v a c u a t e d . A second l i t h i u m stéarate s a m p l e was p r e c i p i t a t e d b y t i t r a t i o n as a b o v e b u t was a t no t i m e fused d u r i n g its p r e p a r a t i o n . W a t e r a n d o t h e r solvents were r e m o v e d b y s u c k i n g a i r t h r o u g h the p r e c i p i t a t e d soap o n the B u c h n e r f u n n e l for 1 1/2 hours, t h e n d r y i n g i n a n o v e n at 120°C. for 12 h o u r s , a n d c o o l i n g a n d k e e p i n g i n a v a c u u m desiccator o v e r P 0 for 5 d a y s . T h i s s a m p l e was also heated at 120°C. u n d e r v a c u u m for 4 hours after i t h a d been powdered a n d transferred to its N M R s a m p l e t u b e , a n d the t u b e w a s sealed off w h i l e s t i l l u n d e r v a c u u m . 2

5

T h e other sample of stearic a c i d was p u r i f i e d b y a n i n i t i a l d i s t i l l a t i o n of the a c i d at reduced pressure a n d b y subsequent r e c r y s t a l l i z a t i o n s , b y the m e t h o d of B r o w n a n d K o l b (6), of the m i d d l e f r a c t i o n of the d i s t i l l a t e . A f t e r three r e c r y s t a l l i z a t i o n s t h e freezing p o i n t , as o b s e r v e d f r o m a t i m e t e m p e r a t u r e cooling c u r v e o n 5 grams of the sample, w a s 69.5°C. ( w i t h t h e r m o m e t e r s t e m exposure c o r r e c t i o n of 0 . 4 8 ° C ) , a n d t h e p l a t e a u i n t h e cooling c u r v e at 69.5°C. c o n t i n u e d at t h a t t e m p e r a t u r e u n t i l the substance was c o m p l e t e l y solidified. A l t h o u g h t h i s freezing p o i n t is l o w e r t h a n t h a t

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


2.

CYR E T A L .

Phase

15

Transitions

of G u y a n d S m i t h , 69.60°C. (11), i t is h i g h e r t h a n t h a t of F r a n c i s a n d his co-workers, 69.32°C. (8). A p u r i t y check o n t h i s s a m p l e b y gas c h r o m a t o g r a p h y of t h e m e t h y l a n d e t h y l esters o n a C a r b o w a x 20 M c o l u m n showed t h a t t h e m i d d l e f r a c t i o n of d i s t i l l a t e c o n t a i n e d 1 % p a l m i t i c a c i d a n d 2 % l i n o l e n i c a c i d a n d t h a t after one r e c r y s t a l l i z a t i o n f r o m acetone no i m p u r i t y was detectable. T h e substances a d d e d to t h e stearic a c i d as c o n t r o l l e d i m p u r i t i e s were themselves c a r e f u l l y p u r i f i e d . T h e p a l m i t i c a c i d was E a s t m a n K o d a k W h i t e L a b e l grade, f u r t h e r p u r i f i e d b y repeated c r y s t a l l i z a t i o n . T h e oleic a n d elaidic acids were o b t a i n e d f r o m t h e H o r m e l I n s t i t u t e as b e t t e r t h a n 9 9 % p u r e a n d were used w i t h o u t f u r t h e r t r e a t m e n t . T h e w a t e r w a s d i s t i l l e d , outgassed, a n d a d d e d to the stearic a c i d as w a t e r v a p o r t h r o u g h a v a c u u m s y s t e m . F o r a l l samples i t was i m p o r t a n t to p r e v e n t c o n t a m i n a t i o n w i t h w a t e r or a d d i t i o n of m o r e t h a n t h e p u r p o s e l y a d d e d q u a n t i t y of w a t e r . T h e p u r i f i e d stearic a c i d was p a c k e d i n t o glass s a m p l e tubes t h a t h a d been cleaned a n d b a k e d out. I f i m p u r i t i e s o t h e r t h a n w a t e r were to be added, these were weighed i n t o the s a m p l e tubes at t h i s p o i n t , a n d t h e tubes were sealed i n t o a v a c u u m s y s t e m . T h e s a m p l e was h e l d i n a w a t e r b a t h at 7 0 ° - 7 1 ° C . for a b o u t 1 h o u r u n d e r v a c u u m a n d t h e n at 68°C. (just below t h e freezing point) u n d e r v a c u u m for 6 or 8 d a y s , after w h i c h t h e s a m p l e t u b e was sealed off w h i l e the s y s t e m was s t i l l u n d e r v a c u u m , a n d the sample was n o t exposed to t h e atmosphere d u r i n g a n y subsequent ex p e r i m e n t . I n t h e case where w a t e r was t h e a d d e d i m p u r i t y , a s a m p l e t u b e of p u r e stearic a c i d , p r e v i o u s l y p r e p a r e d as above, was opened a n d i m m e d i a t e l y sealed i n t o the v a c u u m s y s t e m . W a t e r v a p o r was t r a n s f e r r e d t h r o u g h t h e v a c u u m s y s t e m to the sample, a n d t h e s a m p l e t u b e was r e sealed at l i q u i d n i t r o g e n t e m p e r a t u r e . T h e sealed-off s a m p l e w a s t h e n m e l t e d a n d h e l d at 7 0 ° - 7 1 ° C . for a short t i m e a n d annealed at 68°C. for several d a y s . A l l samples were a l l o w e d to cool to r o o m t e m p e r a t u r e s t a n d i n g i n a p o l y s t y r e n e f o a m s a m p l e s u p p o r t a n d were t h e n r e a d y for N M R e x a m i n a t i o n . I n a d d i t i o n t o the samples p r e p a r e d a c c o r d i n g to t h e a b o v e d e s c r i p t i o n , t w o samples of stearic a c i d , m e n t i o n e d below, were p r e p a r e d w i t h o u t being m e l t e d . Procedure. A l l spectra were o b t a i n e d f r o m a V a r i a n m a g n e t a n d D P 6 0 spectrometer at 60 M c . per sec. T e m p e r a t u r e c o n t r o l was a c h i e v e d b y flowing heated a i r past t h e sample, w h i c h was p l a c e d i n a D e w a r vessel w i t h i n t h e probe insert of t h e spectrometer. T h e t e m p e r a t u r e of t h e s a m ple was m o n i t o r e d b y t w o thermocouples, one u p s t r e a m a n d one d o w n s t r e a m f r o m the sample. T h e t e m p e r a t u r e gradient between these t h e r m o couples depends o n the t e m p e r a t u r e sought, a n d t h e u n c e r t a i n t v of t e m perature is e s t i m a t e d to be =b0.5°C. at 50°C., ± 2 ° C . at 150°C., a n d ± 3 ° C . , at 210°C. O n e h o u r was allowed for the s a m p l e to come to t h e r m a l e q u i l i b r i u m before a s p e c t r u m was r u n at a new t e m p e r a t u r e .

A t the end of 45 m i n -

u t e s - 1 h o u r , t h e t i m e r e q u i r e d to r u n a w i d e - l i n e s p e c t r u m , a repeat r u n was m a d e at t h e same t e m p e r a t u r e a n d was f o u n d i n a l l cases to agree w i t h the first s p e c t r u m .

L i m i t a t i o n of m a c h i n e t i m e d i d n o t p e r m i t extension

of e q u i l i b r a t i o n t i m e s .

N o evidence of hysteresis was f o u n d i f t e m p e r a

tures were a p p r o a c h e d f r o m a b o v e or below, unless the s a m p l e h a d been fused i n the i n t e r v a l between observations.


16

ORDERED FLUIDS A N D LIQUID CRYSTALS

Second m o m e n t s were c a l c u l a t e d u s i n g t h e c o r r e c t i o n of A n d r e w (1) for m o d u l a t i o n a m p l i t u d e , n a m e l y , s u b t r a c t i o n of i / / 4 f r o m t h e v a l u e c a l c u l a t e d f r o m t h e e x p e r i m e n t a l d e r i v a t i v e c u r v e . T h e f r a c t i o n of p r o t o n s i n the sample i n rapid, liquid-like m o t i o n was estimated b y t a k i n g this frac t i o n t o be t h e r a t i o of t h e area u n d e r t h e n a r r o w c o m p o n e n t of t h e a b s o r p t i o n c u r v e t o t h e area u n d e r t h e w h o l e a b s o r p t i o n c u r v e . T h i s supposes t h a t t h e same s a t u r a t i o n f a c t o r applies t o b o t h components. This condi t i o n w a s checked b y r u n n i n g s p e c t r a at different r.f. p o w e r i n p u t s b u t otherwise h a v i n g t h e same o p e r a t i n g c o n d i t i o n s a n d same state of t h e s a m ple. W i t h i n t h e range of r.f. p o w e r i n p u t used i n these e x p e r i m e n t s t h e r a t i o of t h e height of t h e n a r r o w d e r i v a t i v e p e a k t o t h e h e i g h t of t h e b r o a d d e r i v a t i v e peak w a s independent of r.f. p o w e r i n p u t f o r t h e stearic a c i d . F o r l i t h i u m stéarate, p r e l i m i n a r y s p e c t r a were r u n o v e r a w i d e range of r.f. power, a n d a n r.f. l e v e l w e l l below t h e s a t u r a t i o n l e v e l w a s c h o s e n for t h e r u n n i n g of a l l r e p o r t e d spectra. F o r convenience, a c a l i b r a t i o n c u r v e w a s c o n s t r u c t e d for a specific m o d u l a t i o n a m p l i t u d e , c i r c u i t response t i m e , r e corder r a t e , a n d r a t e of scan t h r o u g h resonance, t h i s c a l i b r a t i o n b e i n g of t h e r a t i o , h e i g h t of n a r r o w d e r i v a t i v e p e a k t o h e i g h t of b r o a d d e r i v a t i v e p e a k , against f r a c t i o n of i n t e g r a t e d a b s o r p t i o n i n t e n s i t y l y i n g u n d e r t h e n a r r o w c o m p o n e n t — i . e . , f r a c t i o n of p r o t o n s i n l i q u i d - l i k e m o t i o n . A l l t h e stearic a c i d curves were t h e n r u n u n d e r t h e specific c o n d i t i o n s f o r w h i c h t h e c u r v e w a s c o n s t r u c t e d , a n d t h e r a t i o of d e r i v a t i v e p e a k heights w a s t a k e n as a d i r e c t measure of t h e f r a c t i o n of t h e s y s t e m w h i c h w a s i n l i q u i d l i k e m o t i o n . F o r a case i n w h i c h a s p e c t r u m w a s repeated a t a different scan r a t e , t h e r a t i o of p e a k heights w a s t h e same f o r b o t h scan rates. I t w a s t h u s confirmed t h a t t h e c i r c u i t a n d recorder response t i m e s were s h o r t c o m p a r e d w i t h t h e scan rate t h r o u g h resonance.


w

Results

and

2

Discussion

Stearic Acid. T h e g r o w t h of n a r r o w c o m p o n e n t a n d t h e q u a l i t y of the stearic a c i d spectra are s h o w n i n F i g u r e 1 b y a series of t y p i c a l s p e c t r a t a k e n a t v a r i o u s temperatures. A m o r e d e t a i l e d s t u d y of t h e s p e c t r u m

Figure 1.

Growth of narrow component in a pure stearic acid sample with increasing temperature

Ratio of derivative peak heights, narrow to broad component: lfi°C., no narrow component observable; 62°, 1.5 to 1; 68°, 23.6 to 1


2.

CYR E T A L .

Phase

17

Transitions

j u s t below the m e l t i n g p o i n t u s i n g a h i g h r e s o l u t i o n spectrometer ( V a r i a n A - 6 0 ) w i t h a t e m p e r a t u r e - c o n t r o l l e d p r o b e showed a l i n e w i d t h at h a l f height of a p p r o x i m a t e l y 5 m i l l i g a u s s or some 20 to 25 c.p.s.

T h e tempera

t u r e i n the p r o b e was measured as 66°C. b y o b s e r v i n g t h e c h e m i c a l shift of pure ethylene g l y c o l a n d k n o w i n g its dependence o n t e m p e r a t u r e .

Since

the w i d t h of a n o r m a l l i q u i d l i n e is a f r a c t i o n of a cycle per second,

one

concludes t h a t the n a r r o w component corresponds to a h i g h l y viscous l i q u i d


or to a state i n w h i c h d i f f u s i o n a l m o t i o n is seriously r e s t r i c t e d .

T h e re

sults of i n i t i a l experiments i n w h i c h i n c r e a s i n g a m o u n t s of p a l m i t i c a c i d were added as a n i m p u r i t y to stearic a c i d are s h o w n i n F i g u r e 2, together w i t h the c u r v e for the f r a c t i o n of protons i n l i q u i d - l i k e m o t i o n i n a stearic ι

1

1

1

1

1

1

1

1

1

1~~

0.31%

Temperature, °C. Figure 2.

Percent of protons in liquid-like motion as a function of temperature for pure and impure stearic acid samples Equilibrium amounts of liquid predicted from assumed phase diagram

a c i d sample t o w h i c h no i m p u r i t y h a d been a d d e d . A l s o s h o w n i n t h i s figure is t h e effect of a d d i n g t w o 18-carbon u n s a t u r a t e d a c i d s — o l e i c a n d elaidic. T h e f r a c t i o n of p r o t o n s i n v o l v e d i n l i q u i d - l i k e m o t i o n is f o u n d t o be a n order of m a g n i t u d e s m a l l e r i n t h e results reported here t h a n i n t h e results first o b t a i n e d i n t h i s l a b o r a t o r y a n d reported i n 1960 (9). W e have no e x p l a n a t i o n for t h i s difference. T h e observed freezing p o i n t of t h e stearic a c i d used i n t h e 1960 w o r k i n d i c a t e d t h a t if t h e i m p u r i t y were p a l m i t i c a c i d (a m o s t p r o b a b l e i m p u r i t y ) , t h e q u a n t i t y of i m p u r i t y i n t h e stearic a c i d was m u c h less t h a n 1 % . I t is possible t h a t t h e b r o a d - l i n e component i n t h e previous studies was b e i n g s a t u r a t e d whereas t h e n a r r o w


18

O R D E R E D FLUIDS A N D LIQUID C R Y S T A L S


c o m p o n e n t was n o t . N o o t h e r evidence was f o u n d , however, to i n d i c a t e a n y degree of s a t u r a t i o n i n t h e s y s t e m . A s we discuss below, t h e r m a l h i s t o r y is i m p o r t a n t w i t h respect to t h e i n t e n s i t y of l i q u i d - l i k e l i n e seen i n t h e spectra of these substances, a n d a l t h o u g h i t seems t o t a x t h e a p p l i c a b i l i t y of t h i s effect to e x p l a i n so large a d i s c r e p a n c y , i t is possible t h a t t h e earlier results c a n be a t t r i b u t e d to t h e effect of repeated fusion of the s a m p l e w i t h o u t subsequent a n n e a l i n g . T h e w o r k of F r a n c i s , C o l l i n s , a n d P i p e r o n m e l t i n g a n d freezing p o i n t s of stearic a n d p a l m i t i c a c i d m i x t u r e s (&) indicates t h a t a eutectic is f o r m e d a t a b o u t 55°C., b u t i t is n o t clear w h e t h e r t h e systems f o r m i n g t h e eutectic are t h e t w o p u r e acids or s o l i d solutions. I f t h e y are t h e p u r e acids, t h e n t h e a m o u n t s of l i q u i d - l i k e m o t i o n as g i v e n b y t h e n a r r o w components of the spectra a b o v e the eutectic t e m p e r a t u r e l i e a p p r o x i m a t e l y w i t h i n t h e l i m i t of the a m o u n t of l i q u i d one s h o u l d expect to be i n e q u i l i b r i u m w i t h pure stearic a c i d for s i m p l e eutectic f o r m a t i o n a n d t h e m e l t i n g p o i n t s g i v e n b y F r a n c i s . T h e s e percentages of l i q u i d c a n be c a l c u l a t e d b y s i m p l e a p p l i c a t i o n of the ' l e v e r l a w " to a s i m p l e eutectic phase d i a g r a m c o n s t r u c t e d o n the basis of F r a n c i s ' m e l t i n g or freezing p o i n t s . I n t h e cases where t h e l i q u i d f r a c t i o n observed b y N M R is s m a l l e r t h a n t h e l i q u i d f r a c t i o n p r e d i c t e d f r o m t h e phase d i a g r a m , one c a n assume t h a t t h e r m a l e q u i l i b r i u m was n o t achieved i n t h e N M R sample. T h e e q u i l i b r i u m m o l e f r a c t i o n of l i q u i d t o be expected at a n y t e m p e r a t u r e for a p a r t i c u l a r m o l e % of p a l m i t i c a c i d i m p u r i t y i n stearic a c i d , as d e t e r m i n e d f r o m t h e supposed s i m p l e eutectic phase d i a g r a m , is s h o w n b y dashed lines i n F i g u r e 2. T h e i r r e g u l a r i t y i n the order of t h e curves for p a l m i t i c a c i d i m p u r i t y is n o t u n d e r s t o o d . 4

A l t h o u g h one m a y be able to a t t r i b u t e t h e l i q u i d - l i k e m o t i o n a p p e a r i n g i n t h e N M R spectra to expected e q u i l i b r i u m a m o u n t s of l i q u i d present i n a t w o - c o m p o n e n t s y s t e m a b o v e a eutectic t e m p e r a t u r e , t h i s does n o t a c count for the 0 - 1 % l i q u i d character present at t e m p e r a t u r e s below 54° o r 56°C., t h e eutectic t e m p e r a t u r e . W e believe t h a t t h i s m u s t be a t t r i b u t e d to l i q u i d - l i k e m o t i o n of a few molecules centered about i m p u r i t y a n d s t r u c t u r a l defects i n t h e c r y s t a l l a t t i c e , or a l t e r n a t i v e l y (12) to m o t i o n of molecules i n defects o n the surface of the crystals. T h e m o r e extensive l i q u i d character f o u n d i n samples c o n t a i n i n g t h e 18-carbon u n s a t u r a t e d acids as i m p u r i t y appears also t o be s i m p l y e x p l a i n able i n terms of a phase d i a g r a m i n v o l v i n g eutectic f o r m a t i o n between t h e t w o p u r e components. W e assume t h a t the freezing p o i n t curves of S m i t h (14) for m i x t u r e s of stearic a n d oleic or elaidic acids c a n be c o m b i n e d w i t h s i m p l e eutectic f o r m a t i o n i n v o l v i n g the t w o pure components. The equi l i b r i u m m o l e percentages of l i q u i d t h a t c a n be c a l c u l a t e d for s u c h phase e q u i l i b r i a are s h o w n as dashed curves i n F i g u r e 2. A g a i n one m u s t a t t r i b u t e r e s i d u a l l i q u i d - l i k e m o t i o n below the 43°C.-eutectic of elaidic a n d stearic a c i d to d i s o r i e n t a t i o n at l a t t i c e defects. T h e eutectic for oleic a n d stearic acids lies at a b o u t 13°C.


2.

CYR E T A L .

Phase

19

Transitions


T h e i m p o r t a n c e o f t h e sample's t h e r m a l h i s t o r y is i l l u s t r a t e d b y o b servations m a d e o n some p u r e stearic a c i d s a m p l e — i . e . , p u r i f i e d a c i d t o w h i c h n o i m p u r i t y w a s a d d e d — a n d s h o w n i n F i g u r e 3. T w o stearic a c i d samples were prepared w i t h o u t being fused after r e c r y s t a l l i z a t i o n f r o m acetone, b u t t h e y were t h o r o u g h l y d r i e d i n a v a c u u m . T h e stearic a c i d

stearic acid

Δ*UNFUSED

• = U N F U S E D (second sample) O=FUSED

ONCE

• =FUSED

TWICE

• = 0.5 M O L E % W A T E R

ADDED

= 4

40 Figure 3. 1. 2. 3. 45.

50

60

70

Temperature, ° C .

Percent of protons in liquid-like motion as a function of temperature

Pure unfused stearic acid Pure unfused stearic acid (different sample) Pure stearic acid, fused once Pure stearic acid, fused twice Stearic acid containing 0.5 mole % water

was first d r i e d i n a v a c u u m desiccator over P 2 O 5 , t h e n p u t i n t o sample tubes w h i c h h a d been b a k e d o u t , a n d t h e tubes were t h e n sealed i n t o a v a c u u m s y s t e m a n d left u n d e r v a c u u m a t about 65°C. f o r a few d a y s . T h e p e r centage of t h e protons i n l i q u i d - l i k e m o t i o n , s h o w n i n curves 1 a n d 2, is lower for these samples t h a n for a n y other stearic a c i d samples observed. A t h i r d sample, w h i c h w a s fused d u r i n g p r e p a r a t i o n , as described above, showed m o r e l i q u i d - l i k e m o t i o n , as i l l u s t r a t e d b y c u r v e 3. T h i s s a m p l e was m e l t e d a second t i m e after the p o i n t s o n c u r v e 3 were o b t a i n e d . I t was t a k e n t o 7 0 ° - 7 1 ° C . f o r 1 h o u r d= 15 m i n u t e s a n d t h e n cooled over a p e r i o d of some 16 hours t o r o o m t e m p e r a t u r e . A large hysteresis effect was observed i n t h e c u r v e f o r percent l i q u i d character vs. t e m p e r a t u r e , t h e p o i n t s f o r descending t e m p e r a t u r e l y i n g above b o t h sets f o r ascending t e m p e r a t u r e — n a m e l y , curves 3 a n d 4. T h e sample w a s t h e n a g a i n o b served w i t h increasing t e m p e r a t u r e b e g i n n i n g 4 hours later, a n d as c u r v e 4 shows, t h e extent of l i q u i d - l i k e m o t i o n w a s greater t h a n t h a t observed o n


20



i n i t i a l h e a t i n g (curve 3). T h e r m a l h i s t o r y has i n t h i s example as m u c h effect as a d d i n g 0.3 m o l e % p a l m i t i c a c i d i m p u r i t y . V a r i a t i o n i n t h e r a t e of cooling of a sample f r o m the m e l t w o u l d no d o u b t influence the n u m b e r a n d n a t u r e of l a t t i c e defects a n d i m p u r i t y centers, a n d t h e l e n g t h of t i m e for w h i c h a sample is h e l d at a n elevated t e m p e r a t u r e s h o u l d affect t h e a n n e a l i n g out of these defects a n d the possible c o n s o l i d a t i o n of i m p u r i t y centers b y diffusion. W a t e r produces a m o r e p r o f o u n d effect t h a n a n y of the i m p u r i t i e s t r i e d , except possibly oleic acid (curve 5, F i g u r e 3). A l t h o u g h one m a y a g a i n be able to e x p l a i n t h i s result as t h e s t r a i g h t f o r w a r d p r e d i c t i o n of a phase e q u i l i b r i u m s t u d y , we are not aware of t h e possible n a t u r e of t h e stearic a c i d - w a t e r phase d i a g r a m . A n a l t e r n a t i v e e x p l a n a t i o n seems r e a d i l y a v a i l a b l e , however, i n t h a t h y d r o g e n b o n d i n g of w a t e r w i t h stearic a c i d s h o u l d o c c u r a n d d i s t u r b the r e g u l a r i t y of the p o l a r c a r b o x y l layers i n t h e stearic a c i d c r y s t a l a n d t h u s p r o d u c e m a n y defects a b o u t w h i c h ex tensive m o l e c u l a r m o t i o n m a y center. F u r t h e r evidence of the i m p o r t a n c e of t h e r m a l h i s t o r y i n d e t e r m i n i n g the extent to w h i c h some molecules are i n v o l v e d i n r a p i d a n d extensive m o t i o n before the b u l k of a s a m p l e is i n v o l v e d i n s u c h m o t i o n is offered by a s t u d y of l i t h i u m stéarate.

LiC

1Q

not fused

I

ι ι 111 ι ι ι 111 0 5 10 Figure 4-

ι 15

ι t 2 0 2 5 3 0 gauss

0

I I—I—I—I—I—I—I

5

I I

10 gauss

Typical spectra of unfused lithium stéarate sample at various temperatures



2.

CYR E T A L .

21

Phase Transitions

I ι ι ι ι 1

0

1 2

I

I

3

I

4

I

5 gauss

Figure 5. Absorption spectra of fused lithium stéarate obtained from a high resolution spectrometer

Lithium Stéarate. D i f f e r e n t i a l t h e r m a l analysis (16) a n d d i l a t o m e t r i c (5) studies of l i t h i u m stéarate h a v e s h o w n t h a t phase t r a n s i t i o n s o c c u r at 114° ± 1°C. a n d 180° ± 5°C. I t s m e l t i n g p o i n t is n e a r 229°C. (4, 5, 16). T h e first of these t r a n s i t i o n s is regarded as a change f r o m one c r y s t a l l i n e f o r m to another, t h e corresponding change i n l i t h i u m p a l m i t a t e at 103°C. h a v i n g been verified b y m i c r o s c o p y as a change i n c r y s t a l f o r m (17). An N M R s t u d y (7) places t h e first t r a n s i t i o n at 114°C. a n d indicates t h a t t h e second t r a n s i t i o n , w h i c h depends o n t h e r m a l h i s t o r y , occurs at a b o u t 187° ± 5 ° C . a n d has t h e character of a change f r o m a c r y s t a l f o r m to a w a x y c o n d i t i o n . F i g u r e 4 shows t y p i c a l l i n e shapes at several t e m p e r a t u r e s for a l i t h i u m stéarate sample w h i c h h a d never been fused. T h e b r o a d l i n e of t h e second (or h i g h temperature) c r y s t a l l i n e f o r m disappears between 184° a n d 191°C., a n d t h e n a r r o w l i n e w i t h significant i n t e n s i t y i n t h e w i n g s , w h i c h replaces t h e b r o a d line, is characteristic of w a x y phases of t h e soaps,


22



corresponding t o t h e N M R s i g n a l of s o d i u m stéarate a b o v e 114°C. (#), a n d of p o t a s s i u m , r u b i d i u m , a n d cesium stéarates a b o v e 170°, 143°, a n d 100°C., respectively {10, 13). W e do n o t i m p l y t h a t t h e t r a n s i t i o n f r o m c r y s t a l l i n e t o w a x y is n o t sharp b u t o n l y t h a t w e d i d n o t a t t e m p t t o specify t h e t r a n s i t i o n t e m p e r a t u r e more precisely, i n p a r t because of u n c e r t a i n t y of s a m p l e t e m p e r a t u r e {ca. ± 2 ° ) i n t h i s range. T h e a b s o r p t i o n s i g n a l f o r t h e w a x y phase of l i t h i u m stéarate, as o b t a i n e d f r o m t h e h i g h r e s o l u t i o n spectrometer, is s h o w n i n F i g u r e 5 f o r f o u r temperatures f r o m 196° t o 225°C., a t w h i c h p o i n t t h e s p e c t r u m appears t o be i n d i s t i n g u i s h a b l e f r o m t h a t of a n isotropic l i q u i d . T h e second m o m e n t of t h e w a x y phase is of t h e order of 1 gauss u p t o 215°C., above w h i c h i t decreases r a p i d l y t o a v a l u e of t h e order of 10~~ gauss a t 225°C. This b e h a v i o r corresponds t o t h e v i e w t h a t t h e w a x y phases i n t h e soaps i n v o l v e a n ordered a r r a n g e m e n t of t h e p o l a r ends of t h e molecules w i t h t h e h y d r o c a r b o n chains r e s t r i c t e d i n extent of m o t i o n o n l y b y t h e c o m p a r a t i v e i m m o b i l i t y of t h e p o l a r p a r t s . D i f f u s i o n a t a rate sufficiently great f u r t h e r t o decrease line w i d t h a n d second m o m e n t becomes i m p o r t a n t above 215°C. 2

4

ι ι ι ι ι ι ι ι ι ι ι

0 Figure 6.

5

10 gauss

ι

0

ι

ι

2

ι

ι—ι

5

ι

ι—ι—I—ι

10 gauss

Typical spectra of fused lithium stéarate sample at various temperatures

O n e s h o u l d also examine spectra f o r a sample w h i c h h a d been m e l t e d d u r i n g p r e p a r a t i o n . T h e s e are s h o w n i n F i g u r e 6, f r o m w h i c h one c a n see t h a t a n a r r o w component " a n t i c i p a t e s " t h e phase change t o t h e w a x y state



2.

Phase

CYR E T A L .

23

Transitions

I39±3°C. fused three times

I—I

Ο

I

I

I

I

5

I

I

I

I

I

I

10 gauss

I

I

15

20

Figure 7. Growth of narrow component with re peated fusion in lithium stéarate

m a n y degrees below 187°C., a t w h i c h t e m p e r a t u r e we h a v e placed t h e t r a n s i t i o n f r o m spectra f o r t h e unfused sample. H e r e , however, t h e i n t e n s i t y of t h e n a r r o w component increases so g r e a t l y t h a t t h e b r o a d c o m ponent becomes lost a t as l o w a t e m p e r a t u r e as 171°C. Finally, Figure 7 shows t h e g r o w t h of n a r r o w component, a t t r i b u t e d t o t h e g r o w t h of w a x y regions i n t h e l i t h i u m stéarate c r y s t a l phase, w i t h increase i n t h e n u m b e r of times t h e sample was fused. A t a b o u t 136°C. there is n o clear evidence of w a x y regions i n a n unfused sample, a s m a l l b u t finite a m o u n t of w a x y region i n a once-fused sample, a n d a s i g n i f i c a n t l y large a m o u n t i n a t h r i c e fused sample. I t seems reasonable t o suggest t h a t i n resolidification after m e l t i n g t h e sample develops l a t t i c e defects about w h i c h " w a x y " disorder a n d m o t i o n of t h e h y d r o c a r b o n chains c a n occur a t r a t h e r l o w t e m p e r a tures. T h e question arises as t o w h a t t h e b e h a v i o r is i f the l i t h i u m stéarate is heated o n l y t h r o u g h t h e c r y s t a l l i n e t o w a x y phase t r a n s i t i o n a n d t h e n



24

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

cooled. T h e s p e c t r a i n F i g u r e 8 were o b t a i n e d f r o m the s a m p l e w h i c h h a d never been m e l t e d b u t h a d been heated to 193°C. T h e 137°C. s p e c t r u m i n t h i s figure was r u n 45 m i n u t e s after t h e 193°C. s p e c t r u m of F i g u r e 4. O f t h e 45 m i n u t e s , 10 m i n u t e s w a s c o o l i n g t i m e a n d 35 m i n u t e s e q u i l i b r a t i o n t i m e . T h e 184°C. s p e c t r u m i n F i g u r e 8 w a s r u n after 2 hours at t h i s t e m p e r a t u r e a n d i n a series of spectra t a k e n a t progressively increasing t e m p e r a t u r e b e g i n n i n g at 137°C. (the first s p e c t r u m i n t h i s figure). T h e t h i r d s p e c t r u m i n F i g u r e 8 (191°C.) shows t h e c o m p l e t i o n of t h e second passage of t h i s l i t h i u m stéarate s a m p l e t h r o u g h t h e m e s o m o r p h i c phase t r a n s i t i o n , w h i c h is s h o w n to be a r e p r o d u c i b l e t r a n s i t i o n . B o t h the 137°

' « » » » ' ' ' '

0

5

ι ι

10 gauss

Figure 8. Spectra for unfused lithium stéarate sample taken through mesomorphic phase transition, cooled to 137°C., and heated again

a n d 184°C. spectra show s o m e w h a t b u t not g r e a t l y m o r e p r o n o u n c e d e v i dence of n a r r o w c o m p o n e n t t h a n corresponding spectra f r o m the s a m p l e w h i c h h a d been n e i t h e r fused n o r t a k e n t h r o u g h t h e m e s o m o r p h i c t r a n s i -



2.

CYR ET AL.

Phase Transitions

25

tion. Because the equilibration time for the 137°C. spectrum was short and 184°C. does not correspond exactly to the 182°C. spectrum of Figure 4, we only say that passage through the crystalline to waxy phase transition has little effect i n producing lattice defects about which liquid-like motion can occur compared with the effect of melting, and indeed, it may have no effect at all. This would agree with the work of Void and H a t tiangdi (15), who raised the temperature of a sample of lithium stéarate to 200°C. and then cooled it slowly to room temperature, taking x-ray diffraction patterns. Correspondence of x-ray patterns before and after heating indicated that the lithium salt is well crystallized and that the phase transitions are reversible. Acknowledémen

t

We are grateful to the National Research Council of Canada for the financial assistance of a grant-in-aid of this research. Literature

Cited

(1) Andrew, E . R., Phys. Rev. 91, 425 (1953). (2) Barr, M. R . , Dunell, Β. Α., Can. J. Chem. 42, 1098 (1964). (3) Barr, M. R . , Dunnell, Β. Α., Grant, R . F., Can. J. Chem. 41, 1188 (1963). (4) Benton, D. P., Howe, P. G., Puddington, I. E., Can. J. Chem. 33, 1384 (1955). (5) Benton, D . P., Howe, P. G., Farnand, R . , Puddington, I. E., Can. J. Chem. 33, 1798 (1955). (6) Brown, J . B., Kolb, D. K., "Progress in the Chemistry of Fats and Other Lipids," Vol. 3, p. 58, Pergamon Press, London, 1955. (7) Dunell, Β. Α., Janzen, W. R . , Wiss. Z. Friedrich-Schiller Univ., Jena 14, 191 (1965). (8) Francis, F., Collins, F. J. E., Piper, S. H., Proc. Roy. Soc. (London) A158, 691 (1937). (9) Grant, R . F., Dunell, Β. Α., Can. J. Chem. 38, 359 (1960). (10) Ibid., p. 1951. (11) Guy, J. B., Smith, J. C., J. Chem. Soc. 1939, 615. (12) Krynicki, K., Powles, J. G., Proc. Phys. Soc. 83, 983 (1964). (13) Shaw, D. J., Dunell, Β. Α., Trans. Faraday Soc. 58, 132 (1962). (14) Smith, J . C., J. Chem. Soc. 1939, 974. (15) Vold, R . D., Hattiangdi, G. S., Ind. Eng. Chem. 41, 2311 (1959). (16) Vold, R. D., Vold, M . J., J. Am. Oil Chemists Soc. 26, 520 (1949). (17) Vold, R . D., Vold, M . J., J. Phys. Chem. 49, 32 (1945). RECEIVED February 11, 1966.


Effect of Thermal History and Impurities on Phase Transitions in

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