Lyotropic Liquid Crystals in Lubrication - ACS Symposium Series

Chapter 6

Lyotropic Liquid Crystals in Lubrication 1

1

2

Stig E. Friberg , Anthony J. Ward , Selda Gunsel , and Francis E. Lockwood 2

1

Chemistry Department, Clarkson University, Potsdam, NY 13699-5810 Pennzoil Products Company, The Woodlands, TX 77387

Downloaded by UNIV OF AUCKLAND on May 3, 2015 | http://pubs.acs.org Publication Date: October 10, 1990 | doi: 10.1021/bk-1990-0441.ch006

2

The basis for the use of lyotropic liquid crystals lubricants is discussed and representative results illustrate essential phenomena are reviewed.

as to

The e s s e n t i a l f u n c t i o n o f l u b r i c a n t s i s t o s e p a r a t e two moving s u r f a c e s from each other thereby r e d u c i n g the f r i c t i o n a l energy. I n t h e s i m p l e s t form, hydrodynamic l u b r i c a t i o n ; this i s achieved by t h e r e l a t i v e movement i n a l i q u i d g i v i n g s u f f i c i e n t v e r t i c a l p r e s s u r e t o keep t h e surfaces separated. F o r a s y s t e m o f s l o w movement, t h i s mechanism does n o t f u n c t i o n ; t h e hydrodynamic f o r c e s a r e n e g l i g i b l e , and t h e s e p a r a t i o n must be achieved by a c e r t a i n r i g i d i t y o f t h e l u b r i c a t i n g medium. I n t h e t r a d i t i o n a l l u b r i c a t i n g g r e a s e s t h i s r i g i d i t y i s c a u s e d by d i s p e r s i n g m i c r o s c o p i c soap c r y s t a l s i n t o an o i l i n a t h r e e - d i m e n s i o n a l network. T h i s d i s p e r s i o n i s m a n u f a c t u r e d by c o o l i n g a s o l u t i o n o f t h e soap, u s u a l l y a l i t h i u m compound, from t h e m e l t i n g t e m p e r a t u r e , ~ 3 0 0 ° C t o room temperature i n a c a r e f u l l y e s t a b l i s h e d time p a t t e r n t o o b t a i n t h e o p t i m a l s t r u c t u r e o f t h e soap c r y s t a l l i t e s . O t h e r p o s s i b i l i t i e s t o c o n f e r l o a d - c a r r y i n g c a p a c i t y t o o i l s i n c l u d e adding polymers, g r a p h i t e , o r molybdenum d i s u l f i d e . A lamellar liquid crystal a c t s i n a d i f f e r e n t manner. I t s s t r u c t u r e , Figure 1, c o n s i s t s of amphiphilic l a y e r s organized with the h y d r o c a r b o n c h a i n s b a c k - t o - b a c k , and t h e p o l a r groups s e p a r a t e d by a p o l a r s o l v e n t . The c h o i c e o f s o l v e n t was c o n s i d e r e d l i m i t e d t o w a t e r f o r a l o n g t i m e ( 1 ) , making t h e s e compounds v i r t u a l l y i m p o s s i b l e f o r l u b r i c a t i o n o f m e t a l s . T h i s o p i n i o n changed when M o u c h a r a f i e h ( 2 ) showed l a m e l l a r l i q u i d c r y s t a l s t o be formed a l s o by g l y c e r o l as t h e p o l a r s o l v e n t . T h i s d i s c o v e r y was f o l l o w e d by a l a r g e number o f p u b l i c a t i o n s showing l y o t r o p i c l i q u i d c r y s t a l s w i t h p o l a r o r g a n i c s o l v e n t s ( 3 - 5 ) o r with the " s o l v e n t " connected t o a long hydrocarbon c h a i n ( 6 ) . A c o n c e p t o f l i q u i d c r y s t a l s i n l u b r i c a t i o n has s u b s e q u e n t l y been t r e a t e d b o t h f r o m a t h e o r e t i c a l ( 7 , 8 ) and e x p e r i m e n t a l v i e w p o i n t ( 9 - 1 2 ) . L u b r i c a t i o n by l i q u i d c r y s t a l s i s , hence, a new s u b j e c t w i t h i n t r i b o l o g y , a l t h o u g h t h e o r g a n i z a t i o n o f a l a m e l l a r phase has been i n v o k e d t o d i s c u s s s t r u c t u r a l p a c k i n g i n a monolayer on a s o l i d s u r f a c e ( 1 3 ) .

0097-6156/90/0441-0101$06.00/0 © 1990 American Chemical Society

In Tribology and the Liquid-Crystalline State; Biresaw, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

102

TRIBOLOGY AND THE LIQUID-CRYSTALLINE

STATE


C o n s i d e r i n g t h e r e c e n t i n t e r e s t i n t h e s u b j e c t and t h e f a c t t h a t nonaqueous l y o t r o p i c l i q u i d c r y s t a l s have been known f o r o n l y t e n y e a r s , a s h o r t r e v i e w o f t h e p r o p e r t i e s o f t h e s e may be i n o r d e r . Non-Aqueous L y o t r o p i c L i q u i d C r y s t a l s . The l a m e l l a r l i q u i d c r y s t a l , F i g u r e 1 , i s e a s i l y i d e n t i f i e d from i t s o p t i c a l p a t t e r n when viewed between c r o s s e d p o l a r i z e r s i n a m i c r o s c o p e . The p a t t e r n , F i g u r e 2, i s c l e a r l y d i s t i n g u i s h e d from t h a t , F i g u r e 3, o f t h e e q u a l l y common v a r i e t y o f c l o s e - p a c k e d c y l i n d e r s , F i g u r e 4. The r e s e a r c h so f a r has been m o s t l y c o n c e r n e d w i t h t h e l a m e l l a r phase and i t s p r o p e r t i e s a r e a l s o b e s t known. The e s s e n t i a l c h a r a c t e r i s t i c s a r e t h e i n t e r a c t i o n between t h e s o l v e n t and t h e a m p h i p h i l e s and t h e o r d e r o f t h e s e two compounds. The polar solvent penetration i n t o the space between the a m p h i p h i l e s i s e v a l u a t e d from t h e l o w - a n g l e X - r a y d i f f r a c t i o n p a t t e r n s . These g i v e i n t e r l a y e r s p a c i n g , F i g u r e 1 , d i r e c t l y from Bragg's law. The v a r i a t i o n o f i n t e r l a y e r s p a c i n g w i t h p o l a r s o l v e n t c o n t e n t would be d - d

'

G

(1

+

*

(1)

)

( i n which d i s the i n t e r l a y e r spacing, d i s i t s v a l u e e x t r a p o l a t e d t o z e r o s o l v e n t c o n t e n t , and R i s volume r a t i o o f s o l v e n t t o a m p h i p h i l e ) i f a l l t h e s o l v e n t s were l o c a t e d between t h e p o l a r goup l a y e r s , F i g u r e 1 . T h i s i s n o t t h e c a s e ; a volume f r a c t i o n p e n e t r a t e s i n t o t h e space between t h e h y d r o c a r b o n c h a i n and t h e v a r i a t i o n o f i n t e r l a y e r s p a c i n g w i t h t h e amount o f s o l v e n t now d e s c r i b e d as Q

d - d

[1

Q

+

(1-a) R]

(2)

where a i s t h e volume f r a c t i o n p e n e t r a t e d i n t o t h e space between t h e h y d r o c a r b o n c h a i n s . T h i s e q u a t i o n p r o v i d e s two i m p o r t a n t q u a n t i t i e s : t h e i n t e r l a y e r s p a c i n g o f t h e a m p h i p h i l e s , p e r s e , d , and t h e f r a c t i o n of solvent penetrated. The second c h a r a c t e r i s t i c i s t h e i n d i v i d u a l o r d e r o f t h e s o l v e n t m o l e c u l e s and t h e a m p h i p h i l e s . I n f o r m a t i o n i s o b t a i n e d from t h e NMR s p e c t r a o f d e u t e r i a t e d groups i n t h e m o l e c u l e s . The s i g n a l from a CD group i s c h a r a c t e r i z e d by a d o u b l e t s p l i t o f v i n a n o n - a l i g n e d sample. Q

v = 3E S / 4 Q

(3)

i n w h i c h E Q i s t h e q u a d r u p o l e c o u p l i n g c o n s t a n t and S t h e o r d e r p a r a m e t e r . A typical system o f i n t e r e s t f o r l u b r i c a t i o n i s triethanolamine (TEA)-oleic acid (OLA)-glycerol. Experimental The o l e i c a c i d and t r i e t h a n o l a m i n e were F i s h e r " p u r i f i e d " and used w i t h o u t f u r t h e r p u r i f i c a t i o n . G l y c e r o l ( F i s h e r S c i e n t i f i c ) was s t o r e d over molecular s i e v e s . D e t e r m i n a t i o n o f Phase Diagram. The i s o t r o p i c s o l u t i o n r e g i o n s were d e t e r m i n e d by a d d i t i o n o f g l y c e r o l t o t h e t r i e t h a n o l a m i n e / o l e i c a c i d combination or by addition of oleic acid to the mixture of t r i e t h a n o l a m i n e and g l y c e r o l . The samples were s t o r e d i n a t h e r m o s t a t e a t 25°C f o r t h r e e days t o e n s u r e i n s i g n i f i c a n t changes o f s o l u b i l i t y .


FRIBERGETAL.

Lyotropic Liquid Crystals in Lubrication

103


6.

F i g u r e 2. The o p t i c a l m i c r o s c o p y p a t t e r n o f a l a m e l l a r l i q u i d c r y s t a l between c r o s s e d p o l a r i z e r s c o n t a i n s " o i l y s t r e a k s " and M a l t e s e c r o s s e s .



104

TRIBOLOGY AND THE LIQUID-CRYSTALLINE STATE

F i g u r e 3. The o p t i c a l p a t t e r n o f t h e h e x a g o n a l l i q u i d c r y s t a l , F i g u r e 4, i s d i s t i n c t l y d i f f e r e n t from t h a t o f t h e l a m e l l a r one, F i g u r e 2.

F i g u r e 4.

The h e x a g o n a l l i q u i d c r y s t a l c o n t a i n s c l o s e - p a c k e d c y l i n d e r s .


6.

FRIBERGETAL.


105

The l a m e l l a r l i q u i d c r y s t a l r e g i o n was d e t e r m i n e d by m i c r o s c o p y w i t h samples between c r o s s e d p o l a r i z e r s and t h e i r e x t e n s i o n c o n f i r m e d by l o w - a n g l e x - r a y d i f f r a c t i o n . IR Measurements. IR measurements were made on 1430 R a t i o R e c o r d i n g I n f r a r e d S p e c t r o p h o t o m e t e r ( P e r k i n - E l m e r ) . N a C l c e l l s were u s e d .

2


NMR Measurements. HNMR s p e c t r a were o b t a i n e d u s i n g a F o u r i e r T r a n s f o r m S p e c t r o m e t e r (JEOL FX90Q) and a t 2 5 ° C Low-Anale X-Ray D i f f r a c t i o n Measurements. Low-angle X - r a y d i f f r a c t i o n measurements were o b t a i n e d by a K i e s s i g l o w - a n g l e camera from R i c h a r d S e i f e r t , Germany. N i f i l t e r e d Cu r a d i a t i o n was u s e d and t h e r e f l e c t i o n s d e t e r m i n e d by T e n n e l e c P o s i t i o n S e n s i t i v e d e t e c t o r system (Model PSD-1100).

Results The p e r t i n e n t phase r e g i o n s a r e shown i n F i g u r e 5 d i s p l a y i n g two s o l u t i o n s and a l a m e l l a r l i q u i d c r y s t a l . The i n f r a r e d s p e c t r a g i v e t h e r a t i o between i o n i z e d and n o n i o n i z e d c a r b o c y l i c groups v e r s u s t h e TEA/OLA r a t i o , F i g u r e 6, u s i n g a c a l i b r a t i o n c u r v e from t h e sodium o c t a n o a t e / o c t a n o i c a c i d system (15) t o t r a n s f e r IR i n t e n s i t i e s t o c o n c e n t r a t i o n s . These bands were b u t l i t t l e i n f l u e n c e d by t h e a d d i t i o n o f g l y c e r o l , F i g u r e 7. The l o w - a n g l e X - r a y d i f f r a c t i o n d a t a , F i g u r e 8, showed a s t r o n g i n c r e a s e o f i n t e r l a y e r s p a c i n g w i t h added g l y c e r o l and a c a l c u l a t i o n u s i n g E q u a t i o n 2 gave a p e n e t r a t i o n o f 0.27. Discussion These r e s u l t s have g i v e n a d e s c r i p t i o n o f t h e s y s t e m w i t h a few n o t i c e a b l e f a c t s . The TEA/OLA system i n r e a l i t y c o n t a i n s a r a t h e r h i g h amount o f n o n i o n i z e d o l e i c a c i d , and h i g h amounts o f g l y c e r o l a r e accepted i n t o the l i q u i d c r y s t a l . At equimolar r a t i o o n l y one-half of t h e a c i d i s i o n i z e d and the system i n r e a l i t y c o n s i s t s o f t r i e t h a n o l a m i n e 0.5 mol, o l e i c a c i d 0.5 mol, and t r i e t h a n o l a m m o n i u m o l e a t e 0.5 mol. T h i s means t h a t t h e l i q u i d c r y s t a l i n r e a l i t y i s an example o f a t r a d i t i o n a l c o m b i n a t i o n o f a p o l a r s o l v e n t (TEA), an i o n i c s u r f a c t a n t (TEAOLA), and a h y d r o p h o b i c a m p h i p h i l e (OLA). The s i m i l a r i t y t o c l a s s i c aqueous systems o f E k w a l l and c o l l a b o r a t o r s i s s t r i k i n g ( 1 6 ) . E a r l i e r t r i b o l o g i c a l measurements (12) make an e v a l u a t i o n p o s s i b l e o f t h e s t r u c t u r a l i n f l u e n c e on t h e l u b r i c a t i o n . A t f i r s t the s t a b i l i t y of the s t r u c t u r e versus temperature i s important f o r the l u b r i c a n t f u n c t i o n and t h e p e r t i n e n t p r o b l e m i n t h i s c a s e i s t h e r m a l s t a b i l i t y w i t h added g l y c e r o l ( 1 2 ) . T a b l e I shows t h e i n t e r e s t i n g f a c t t h a t t h e t e m p e r a t u r e f o r t h e t r a n s i t i o n t o l i q u i d form i s i n c r e a s e d by the a d d i t i o n of g l y c e r o l . The l u b r i c a t i o n p r o p e r t i e s o f t h i s system was i n v e s t i g a t e d under two s e t s o f c o n d i t i o n s , e l a s t o - hydrodynamic and s l o w s l i d i n g l o w - l o a d c o n d i t i o n s , T a b l e I I . In t h e f i r s t c a s e , w i t h h i g h s h e a r r a t e s , the i n f l u e n c e of the l i q u i d c r y s t a l i s i n s i g n i f i c a n t ; the c e n t r a l f i l m t h i c k n e s s depended on t h e v i s c o s i t y o f t h e components o n l y . The sample w i t h most g l y c e r o l gave t h e g r e a t e s t v a l u e o f t h e f i l m t h i c k n e s s . The f a c t t h a t t h e TEA and t h e OLA, when combined under low s h e a r r a t e s , g i v e a " g e l l y " s t r u c t u r e had no i m p o r t a n c e ; t h e i r i n d i v i d u a l v i s c o s i t i e s were v e r y low, and t h e g l y c e r o l c o n t r i b u t i o n was t h e e s s e n t i a l one.


106



Glycerol

TEA

OLA

F i g u r e 5. The system t r i e t h a n o l a m i n e (TEA), o l e i c a c i d (OLA) and g l y c e r o l i n c l u d e s two i s o t r o p i c s o l u t i o n s and a l a m e l l a r l i q u i d c r y s t a l (L.L.Cr.)•

0

10

20

30

40

50

60

70

80

90

100

TEA/(TEA+OLA) , mole percent F i g u r e 6. The v a r i a t i o n nolamine f r a c t i o n .

of c a r b o x y l i c

group i o n i z a t i o n w i t h


trietha-


6. FRIBERGETAL.


107

X

o o o b o

1.1 Lam.

1.0

L i q . C r y s t a l region

0.9 0.8

DC

c o CO CD O

c o O

0.7

molar ratio

0.6

OLA/TEA

0.5

1.6

0.4 0.3 0.2 0

0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

Glycerol/(TEA+OLA) , mol./mol. F i g u r e 7. The r a t i o between i o n i z e d and n o n i o n i z e d c a r b o x y l i c group d i d n o t change s i g n i f i c a n t l y w i t h t h e g l y c e r o l c o n t e n t i n t h e l i q u i d c r y s t a l , F i g u r e 5.



108


300.4

0.8

1.2

1.6

2.0

c/p/(1-c/p) F i g u r e 8 . The v a r i a t i o n o f i n t e r l a y e r s p a c i n g , d, F i g u r e 7 , w i t h volume ratio of glycerol. c = weight f r a c t i o n o f g l y c e r o l p = density of glycerol


6. FRIBERGETAL.


109

T a b l e I . L i q u i d C r y s t a l / I s o t r o p i c L i q u i d T r a n s i t i o n Temperatures

Composition

Transition Temperature

TEA/OLA/Glycerol (mol

ratio)

67.6


1/1.6/0 1/1.60/1.63

98

1/1.60/4.25

111

1.1.60/9.80

131

T a b l e I I . C e n t r a l F i l m T h i c k n e s s , EHD. 1 0 " D i a m e t e r S t e e l on G l a s s ,

Composition LC

R o l l i n g Speed 7 . 9

F i l m T h i c k n e s s \m

(Weight R a t i o )

- [OLA/TEA ( 3 / 1 , wt r a t i o ]

0.145

LC/Glycerol

(4/1)

0.163

LC/Glycerol

(7/3)

0.190

LC/Glycerol

(4/6)

0.225 0.250

Glycerol

SOURCE:

Ball

inch/s

Reference 12.

These r e s u l t s a r e c o r r o b o r a t e d by v i s c o s i t y measurements a t v a r i o u s shear r a t e s . A t a shear r a t e o f 3 0 s " t h e v i s c o s i t y o f t h e l i q u i d c r y s t a l i s o f t h e o r d e r o f 1 0 0 P. An i n c r e a s e t o 2 . 1 0 s " c a u s e s a r e d u c t i o n t o 5 P. A t t h i s s h e a r r a t e t h e a s s o c i a t i o n s t r u c t u r e s t i l l has an i n f l u e n c e ; t h e v i s c o s i t y o f o l e i c a c i d i s 0 . 2 6 P a t 3 0 ° C and triethanolamine i s a t a lower l e v e l . G l y c e r o l , by c o m p a r i s o n , has a v i s c o s i t y a t 2 0 ° C o f 1 5 P and i t s i n f l u e n c e on t h e hydrodynamic l u b r i c a t i o n i s t h e e x p e c t e d one. In summary i t may be s a f e l y c o n c l u d e d t h a t t h e l i q u i d c r y s t a l l i n e s t r u c t u r e p e r se has o n l y i n s i g n i f i c a n t i n f l u e n c e on t h e l u b r i c a t i o n under hydrodynamic c o n d i t i o n s . Low l o a d s and slow m o t i o n , on t h e o t h e r hand, gave r e s u l t s , which d e m o n s t r a t e d t h e l o a d c a r r y i n g c a p a c i t y o f t h e l i q u i d c r y s t a l l i n e phase as w e l l as t h e low f r i c t i o n a l o n g t h e m e t h y l group p l a n e s . A s t e e l b a l l ( 1 / 2 " d i a m e t e r ) on s t e e l w i t h a s l i d i n g 1

3

1


110

TRIBOLOGY AND THE LIQUID-CRYSTALLINE STATE 2

rate of 1.7»10" i n c h / s gave a f r i c t i o n c o e f f i c i e n t t h a t showed a pronounced drop i n f r i c t i o n c o e f f i c i e n t when t h e l o a d was r e d u c e d from 4 0 0 t o 3 0 0 g, T a b l e I I I . The r e s u l t d e m o n s t r a t e s t h e e x t r e m e l y low f r i c t i o n c o e f f i c i e n t s when t h e l o a d i s s u f f i c i e n t l y low t o a l l o w t h e s t r u c t u r e t o c a r r y i t without deformation o f the planes.

Steel

Table I I I . F r i c t i o n C o e f f i c i e n t ( 5 2 1 0 0 ) 1 / 2 " Diameter B a l l on S t e e l 1 . 7 * 1 0 " i n c h / s s l i d i n g Rate 2


Load

Friction

g

Coefficient

500

0.100

400

0.100

300

0.045

200

0.025

100

0.020

A l t h o u g h no s t r u c t u r a l i n f o r m a t i o n from t h e s l i d i n g zone i s a v a i l a b l e , i t i s tempting t o a t t r i b u t e t h i s r e d u c t i o n t o the s t r u c t u r a l i n f l u e n c e o f the l a m e l l a r s t r u c t u r e . D i r e c t proof i s , o f course, r a t h e r d i f f i c u l t , b u t i n d i r e c t methods a r e p o s s i b l e and a r e a t p r e s e n t p u r s u e d . The importance o f t h e m e t h y l group l a y e r s as a s l i d i n g p l a n e i s d e m o n s t r a t e d b y t h e r e s u l t s u s i n g l i q u i d c r y s t a l s w i t h d i f f e r e n t amounts o f g l y c e r o l . The low-angle X - r a y d i f f r a c t i o n d a t a , F i g u r e 8 , show g l y c e r o l l a y e r s o f a p p r o x i m a t e l y 5 0 A t h i c k n e s s . T h i s v a l u e means t h a t t h e p o l a r s o l v e n t l a y e r , F i g u r e 1 , now has d i m e n s i o n s e x c e e d i n g t h o s e o f t h e a m p h i p h i l e s (d - d > d , F i g u r e 1 ) . The g l y c e r o l i n t h i s l a y e r i s c l o s e t o an i s o t r o p i c l i q u i d as shown by t h e q u a d r u p o l a r s p l i t o f t h e NMR s i g n a l s from d e u t e r i a t e d g l y c e r o l . However, t h e f r i c t i o n c o e f f i c i e n t d i d n o t v a r y s i g n i f i c a n t l y w i t h g l y c e r o l c o n t e n t ; t h e v a l u e s were a l l i n t h e range 0 . 0 8 5 - 0 . 1 0 0 w i t h t h e h i g h e r v a l u e s f o r t h e samples w i t h h i g h g l y c e r o l c o n t e n t . These v a l u e s s t r o n g l y i n d i c a t e t h a t t h e i m p o r t a n t p a r t o f t h e low f r i c t i o n s h e a r t a k e s p l a c e i n t h e m e t h y l group l a y e r . F u t u r e e x p e r i m e n t s i n w h i c h d i f f e r e n t compounds a r e combined w i l l c l a r i f y t h e s e phenomena. a

a

Acknowledgments T h i s r e s e a r c h was s u p p o r t e d by a g r a n t from t h e O f f i c e o f N a v a l R e s e a r c h # N 0 0 0 1 4 - 8 4 K 0 5 0 9 and from t h e N a t i o n a l S c i e n c e F o u n d a t i o n G r a n t CT-8819140.


6. FRIBERG ETAL.


111

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RECEIVED May 21, 1990


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