Sucrose Esters as Emulsion Stabilizers - ACS Symposium Series

Chapter 7 Sucrose Esters as Emulsion Stabilizers

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on October 12, 2013 | http://pubs.acs.org Publication Date: December 26, 1991 | doi: 10.1021/bk-1991-0448.ch007

Thelma M . Herrington, Brian R. Midmore, and Sarabjit S. Sahi Department of Chemistry, The University of Reading, Reading RG6 2AD, United Kingdom

Studies of the behaviour of some sucrose esters, both in the pure state and in solution in water and n-decane, have been undertaken. The sucrose esters used were sucrose mono- and di-laurate and mono- and di-oleate, all specially synthesised, and a purified commercial surfactant, sucrose monotallowate. The pure surfactants exhibited thermotropic liquid crystalline behaviour and lyotropic mesophases were formed with water and n-decane. The extent of micellar aggregation for sucrose monolaurate and monooleate was determined in aqueous solution over the temperature range 0 to 70°C by freezing point and vapour pressure methods. Model emulsion experiments on sucrose monolaurate and monotallowate, by studying the equilibrium thickness of an aqueous surfactant film between two oil droplets, showed that increasing the concentration of surfactant increased the repulsive forces. Finally bulk phase experiments were carried out to investigate the efficacy of some commercial sucrose esters in stabilising oil-in-water emulsions. Sucrose esters have been on the market for a number of years. Their manufacture initially involved the use of dimethylformamide which is a toxic solvent. Hence, they were unsuitable for many applications. Now they are produced by a new reaction technique avoiding the use of toxic solvents and are widely used as emulsifying agents and detergents. The molecule offers the unusual facility that both the degree of esterification and the chain length of the ester group can be altered to obtain a given hydrophile-lipophile balance, so that extensive permutations and combinations are possible. The nontoxic nature of the esters has led to their extensive use in the food industry (1). As sucrose itself stabilizes the conformation of various proteins in aqueous solution, there has been considerable interest in the interactions of sucrose esters (2,3). In our work a number of sucrose esters were synthesised and their properties studied, both in the pure state, and in the presence of aqueous and non-aqueous solvents. 0097-6156/91/0448-0082$06.25/0 © 1991 American Chemical Society In Microemulsions and Emulsions in Foods; El-Nokaly, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


7. HERRINGTON ET AL.

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83

The sucrose esters display both thermotropic and l y o t r o p i c l i q u i d c r y s t a l l i n e properties. They resemble the a l k y l glycosides and 1-thioglycosides (4) i n forming thermotropic l i q u i d c r y s t a l l i n e phases between room temperature and their melting points and form lyotropic l i q u i d c r y s t a l s i n water and n-decane. The phase diagrams were obtained of sucrose monolaurate, monooleate and d i l a u r a t e i n water and of the diesters i n n-decane. Sucrose monolaurate and monooleate are both f a i r l y water soluble with the formation of micellar solutions. The surfactants have sharp cmc*s and the micellar aggregation numbers were determined over the temperature range 0 to 70 C, using freezing point and vapour pressure methods. Fundamental studies of their emulsion s t a b i l i s i n g properties were c a r r i e d out using a l i g h t reflectance technique. The equilibrium thicknesses of oil-in-water films s t a b i l i z e d by sucrose surfactants were measured as a function of surfactant and added e l e c t r o l y t e concentration. The results were analysed i n terms of a simple three layer o i l - w a t e r - o i l model, which showed that increasing the concentration of surfactant increased the repulsive forces. MATERIALS AND METHODS Materials. The sucrose esters were prepared using a t r a n s e s t e r i f i c a t i o n procedure from sucrose and methyl laurate or oleate i n dimethyl formamide as solvent; the monoester was separated from sucrose and higher sucrose esters by l i q u i d chromatography and further p u r i f i e d by d i a l y s i s (5,6). Purity was estimated by gas and thin-layer chromatography as > 99.5%. The esters as prepared are mixtures of isomers; the sucrose molecule i s most r e a d i l y e s t e r i f i e d at the primary hydroxyl groups at the 6, 1' and 6* positions (7). GLC and NMR on methylated derivatives of the monoesters showed that the isomers 6':6:1' were present i n the proportions of 6:3:1. Sucrose monotallowate was prepared by purifying commercial sucrose tallowate (Tate and Lyle Industries Ltd.,Reading, U.K.) using s i l i c a gel column chromatography (6). Experimental. Phase structures were i d e n t i f i e d by p o l a r i z i n g microscopy using a L e i t z microscope f i t t e d with a hot-stage. Hexagonal and lamellar phases were i d e n t i f i e d by comparing t h e i r textures with l i t e r a t u r e photomicrographs (8), by their c h a r a c t e r i s t i c conoscopic figures and by low angle X-ray d i f f r a c t i o n . Cubic phases and micellar solutions may be distinguished by large differences i n v i s c o s i t y and r e f r a c t i v e discontinuity. DSC was used to confirm the t r a n s i t i o n s observed by o p t i c a l microscopy. Homogeneity of the mixtures was achieved using a vibromixer and repeated centrifugation through a narrow c o n s t r i c t i o n for more concentrated samples. Sealed, s t i r r e d samples i n a water bath were observed through cross polars, both f o r heating and cooling cycles; the phase sequence i n more complex regions of the phase diagram was checked by the microscope penetration technique. The aggregation numbers were determined over the concentration range 0 to 70 C using freezing point and vapour pressure methods (9). The osmolality, Θ, i s related to the freezing point depression, ΔΤ, i n d i l u t e solution by θ = AT/K , f

In Microemulsions and Emulsions in Foods; El-Nokaly, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

(1)

MICROEMULSIONS AND

84

EMULSIONS IN FOODS

where K^. i s the c r y o s c o p i c c o n s t a n t , and t o the r e s i s t a n c e changes, AR^,

o f the t h e r m i s t o r s of the vapour p r e s s u r e osmometer by

Θ = 2 C AR , i


where the c o n s t a n t s

i

(2)

i

are o b t a i n e d by c a l i b r a t i o n .

S o l u t i o n s were

made up by weight and the c o n c e n t r a t i o n c a l c u l a t e d from the m o l a l i t y u s i n g d e n s i t y data. The e q u i l i b r i u m d i s t a n c e s between two n-octane d r o p l e t s i n aqueous s o l u t i o n s o f the sucrose s u r f a c t a n t s were measured as a f u n c t i o n o f s u r f a c t a n t and o f added e l e c t r o l y t e c o n c e n t r a t i o n (10). The b a s i s of such a technique i s t o measure the r e f l e c t i o n s from the two i n t e r f a c e s o f a t h i n water f i l m sandwiched between two o i l d r o p l e t s u s i n g a p h o t o m u l t i p l i e r tube. The i n t e n s i t y o f the r e f l e c t e d l i g h t i s a f u n c t i o n o f f i l m t h i c k n e s s . The o p t i c a l arrangement i s shown s c h e m a t i c a l l y i n F i g u r e 1. The beam from a helium-neon l a s e r (λ = 632.8 nm) was r e f l e c t e d by means o f an a d j u s t a b l e m i r r o r so t h a t i t was i n c i d e n t on the f i l m c e l l a t an a n g l e θ o f l e s s than 5 . A beam s p l i t t e r i n the m i c r o s c o p e e n a b l e d the f i l m t h i n n i n g p r o c e s s t o be monitored v i s u a l l y . The image was f o c u s s e d a t the d e t e c t o r a p e r t u r e and o n l y the c e n t r a l a r e a o f the f i l m was recorded. The i n t e n s i t y o f the image was f o l l o w e d c o n t i n u o u s l y on a c h a r t r e c o r d e r . A c t u a l e m u l s i o n s t a b i l i t y was assessed by o b s e r v i n g creaming and s e d i m e n t a t i o n b e h a v i o u r and by s t u d y i n g changes i n d r o p l e t s i z e w i t h time. RESULTS Phase Behaviour. The t h e r m o t r o p i c l i q u i d c r y s t a l l i n e t r a n s i t i o n s o f the pure s u c r o s e e s t e r s are shown i n Table I. T a b l e I.

T r a n s i t i o n Temperatures o f the Sucrose E s t e r s Solid - L

a

Sucrose Sucrose Sucrose Sucrose

monolaurate monooleate dilaurate dioleate

55 33 38 -

L

Q

T/°C - L 138 94 81 52

L

- Isotropic 163 154 156 89

S u c r o s e monooleate and d i o l e a t e form a g e l - l i k e phase a t room temperature, but the monolaurate and d i l a u r a t e are c r y s t a l l i n e s o l i d s , w h i c h pass i n t o the g e l , L , phase on h e a t i n g . On f u r t h e r ρ h e a t i n g a l l f o u r e s t e r s form the l a m e l l a r , L^, phase. T h e r m o g r a v i m e t r i c a n a l y s i s showed t h a t the onset o f thermal d e g r a d a t i o n was around 180 C, but a d a r k e n i n g i n c o l o u r o c c u r r e d between 110 and 120 C. X-ray d i f f r a c t i o n gave l a y e r s p a c i n g s o f 37 and 43 Â f o r the monolaurate and monooleate r e s p e c t i v e l y i n the l a m e l l a r phase. S u c r o s e monolaurate i s v e r y s o l u b l e i n water and the i s o t r o p i c m i c e l l a r s o l u t i o n i s formed up t o c o n c e n t r a t i o n s o f 57 wt%; above 30 0



HERRINGTON ET AL.

Sucrose Esters

U p p e r drop! et Chart recorder

Photoriiultiplier

Lens

F i g u r e 1.

O p t i c a l components o f the apparatus.


86

MICROEMULSIONS AND EMULSIONS IN FOODS

wt% the m i x t u r e s show s t r e a m i n g b i r e f r i n g e n c e . The phase d i a g r a m i s shown i n F i g u r e 2. W i t h i n c r e a s i n g c o n c e n t r a t i o n o f s u r f a c t a n t a t 20 C the hexagonal, H^, phase i s formed, f o l l o w e d b y the g e l phase.


C o n c e n t r a t i o n s above 87 w t % formed the l a m e l l a r phase on h e a t i n g . The phase diagram f o r s u c r o s e monooleate and water i s shown i n F i g u r e 3. As e x p e c t e d t h i s i s not q u i t e so s o l u b l e i n w a t e r a s t h e l a u r a t e ; the c o n c e n t r a t e d m i c e l l a r s o l u t i o n s a l s o showed s t r e a m i n g birefringence. A r e g i o n o f v i s c o u s i s o t r o p i c phase, V^, i s i n t e r p o s e d between the hexagonal and g e l phases and a much l a r g e r l a m e l l a r phase r e g i o n e x i s t s . S u c r o s e d i l a u r a t e i s s p a r i n g l y s o l u b l e i n b o t h water and n-decane; the phase diagrams a r e shown i n F i g u r e s 4 and 5 respectively. B o t h show r e g i o n s o f l a m e l l a r and g e l phases. A s m a l l a r e a o f r e v e r s e d c u b i c i s o t r o p i c phase, V^, i s shown w i t h the h y d r o c a r b o n s o l v e n t . The l o n g e r a l k y l c h a i n s u c r o s e d i o l e a t e i s v e r y s o l u b l e i n n-decane; t h i s i s shown by the phase diagram i n F i g u r e 6. O n l y the l a m e l l a r and g e l phases a r e formed. S u c r o s e monotallowate i s a w h i t e powder a t ambient t e m p e r a t u r e s w i t h the f a t t y a c i d p r o f i l e : 35% o l e i c , 31% p a l m i t i c , 25% s t e a r i c , s m a l l amounts o f p a l m i t o l e i c and l i n o l e i c a c i d s . The phase diagram w i t h water i s shown i n F i g u r e 7. Below 39°C i t i s o n l y s l i g h t l y s o l u b l e i n water, but above t h i s temperature the s o l u b i l i t y i n c r e a s e s t o 60% and the l a m e l l a r phase i s formed on a d d i n g more water. The low s o l u b i l i t y would i n d i c a t e the p r e s e n c e o f c o n s i d e r a b l e amounts o f d i e s t e r , but GLC showed o n l y 5%. M i c e l l a r A g g r e g a t i o n . The cmc o f the s u c r o s e monolaurate -4 -3 ο (3.39x10 mol dm a t 25 C) was i n agreement w i t h the l i t e r a t u r e values(4.OxlO

- 4

3

4

3

mol dm" a t 25°C ( 2 ) ; 3.4χ1θ" mol dm" a t 27.1°C —6 —3 ( 1 2 ) ) . The cmc o f s u c r o s e monooleate was 5.13x10 mol dm a t 25°C. S i n c e the s u r f a c t a n t s b o t h had a s h a r p cmc, the thermodynamic d a t a o b t a i n e d f o r t h e i r s o l u t i o n s was a n a l y s e d by assuming t h e m i c e l l e s t o be e f f e c t i v e l y monodisperse w i t h a s i n g l e a g g r e g a t i o n number independent o f c o n c e n t r a t i o n . By c o n s i d e r i n g a s i n g l e - p h a s e m i c e l l a r s o l u t i o n t o be a two component system where the s o l v e n t c o n s i s t s o f water p l u s monomer a t the cmc and the m i c e l l e s a r e t h e s o l u t e , the t h e o r y o f M c M i l l a n and Mayer e n a b l e s the m i c e l l e - m i c e l l e i n t e r a c t i o n s t o be c a l c u l a t e d . The o s m o t i c p r e s s u r e TT, i s g i v e n a s a power s e r i e s i n the number d e n s i t y , p,

Π/kT = ρ • B V • B 22

2 2 2

V•

(3)

Thus, i n t h i s model, the n o n - i d e a l b e h a v i o u r ^ o f the system i s c h a r a c t e r i z e d by the v i r i a l c o e f f i c i e n t s B , etc. In d i l u t e * *o 0 solution, B * Β = -b , the s o l u t e - s o l u t e c l u s t e r i n t e g r a l . 9 9

9 9


Sucrose Esters


7. HERRINGTON ET AL.

weight % F i g u r e 2.

sucrose monolaurate

Phase diagram o f the s u c r o s e monolaurate + water system o v e r the temperature range 0-100 C. D o t t e d l i n e s i n d i c a t e b o u n d a r i e s not d e t e r m i n e d a c c u r a t e l y . (Reprinted with permission from r e f . 11. C o p y r i g h t 1988 A m e r i c a n O i l C h e m i s t s S o c i e t y )


87

88



weight % F i g u r e 3.

sucrose monooleate

Phase diagram o f the s u c r o s e monooleate + water system over the temperature range 0-100 C. Reproduced w i t h p e r m i s s i o n from r e f e r e n c e (11). (Reprinted with permission from r e f . 11. C o p y r i g h t 1988 American O i l C h e m i s t s S o c i e t y )


Sucrose Esters


IIERRINGTON ET AL.

90

LC1

60 L,

+

LC1

Ρ \ 30

20

40

60

80

100

weight % sucrose dilaurate F i g u r e 4.

Phase diagram o f the s u c r o s e d i l a u r a t e + water system o v e r the temperature range 0-100 C. ( R e p r i n t e d w i t h p e r m i s s i o n f r o m r e f . 11. C o p y r i g h t 1988 A m e r i c a n O i l C h e m i s t s S o c i e t y )



90


90

60

30

0

20

40

60

80

100

weight % sucrose dilaurate F i g u r e 5.

Phase diagram o f the s u c r o s e d i l a u r a t e + n-decane system over the temperature range 0-100 C. (Reprinted w i t h p e r m i s s i o n from r e f . 11. C o p y r i g h t 1988 A m e r i c a n O i l C h e m i s t s S o c i e t y )



7. HERRINGTON ET AL

90

Sucrose Esters

91

h

0

20

60

80

100

weight % sucrose dioleate F i g u r e 6.

Phase diagram o f the s u c r o s e d i o l e a t e + n-decane system over the temperature range 0-100 C. ( R e p r i n t e d w i t h p e r m i s s i o n f r o m r e f . 11. C o p y r i g h t 1988 A m e r i c a n O i l C h e m i s t s S o c i e t y )




LC1

Li

/

/

Κ] /

0

L

/

3

r S

0

20

40

60

80

weight % sucrose monotallowate r e 7.

Phase diagram o f the s u c r o s e monotallowate + water system over the temperature range 0-100 C. ( R e p r i n t e d w i t h p e r m i s s i o n from r e f . 1 1 . C o p y r i g h t 1988 American O i l Chemists S o c i e t y )


100

7. HERRINGTON ET AL

Sucrose Esters

93

The m i c e l l a r molar mass i s assumed t o be independent o f concentration. The o s m o t i c p r e s s u r e i s r e l a t e d t o t h e water a c t i v i t y by TTV = -RT I n a 1

and t h e o s m o l a l i t y Θ =

m

(4)

x

i s r e l a t e d t o t h e water a c t i v i t y by


Θ = -In aVM, 1 1 where

(5)

i s t h e molar mass o f water.

Thus

θ(Μ /V )/c = 1/M

2

where M

2

+ B

2 2

*

c / M

2

+

(

)

i s t h e m i c e l l a r molar mass, which i s assumed t o be

independent o f t h e c o n c e n t r a t i o n c. From eqn. (6) a p l o t o f 8 ( M / V ) / c a g a i n s t c has i n t e r c e p t 1/M and i n i t i a l s l o p e 1

6

2

1

2

#

2

Ai^ .

1

The m o l a l i t y r e g i o n s t u d i e d was 0.01 < m < 0.2 mol k g , w e l l below the c o n c e n t r a t i o n s f o r f o r m a t i o n o f mesophases. The b e s t f i t t i n g p o l y n o m i a l s t o t h e d a t a were found t o be l i n e a r i n c a t a l l temperatures. The v a l u e s o f t h e a g g r e g a t i o n number, η, and second v i r i a l c o e f f i c i e n t , B *»

a

r

e

22

S

i v e n

i n

T

a

o

l

e

1 1

·

T a b l e I I . A g g r e g a t i o n Numbers, η, and M c M i l l a n - M a y e r V i r i a l C o e f f i c i e n t s , B * , f o r Aqueous S o l u t i o n s o f 22

Sucrose Monolaurate and Sucrose Monooleate S u c r o s e Monolaurate

Sucrose Monooleate

T(°C) *

0 25 40 50 60

51±4 52±1 51±2 50±2 54±4

B

/ 7 i

22* 3 - 1 3 (cm mol x l O ) 1.47±0.08 1.40±0.04 1.34±0.05 1.30±0.06 1.12±0.08

11

B

/7

22* > (cm

97±8 99±6 101±9 104±10 96±13

3 - 1 3 mol x l O )

0.877±0.05 0.764±0.04 0.680±0.05 0.616±0.06 0. 572±0.06

( R e p r i n t e d w i t h p e r m i s s i o n f r o m r e f . 9. C o p y r i g h t 1986 E l s e v i e r S c i e n c e P u b l i s h e r s )


94


T h i n F i l m S t u d i e s . A s i n g l e l a y e r model was used t o c a l c u l a t e t h e f i l m t h i c k n e s s from t h e measured i n t e n s i t i e s , a s t h e p a r a f f i n c h a i n s o f t h e s u r f a c t a n t m o l e c u l e s a r e immersed i n t h e o i l phase and w i l l have almost t h e same index. F o r t h i s model

2

sin 0/2 =

J

J

^ min J - J . max min

J[

1

max J

, -1

(7)


where t h e f i l m t h i c k n e s s , h, i s r e l a t e d t o φ by φ/2 = 2Πη η/λ

(8)

1

J = ÎJ^/ÏQ»

t h e r a t i o o f r e f l e c t e d t o i n c i d e n t i n t e n s i t y ; n^ i s t h e

f i l m r e f r a c t i v e index.

The i n t e n s i t y o f t h e i n c i d e n t beam, I , was Q

d e t e r m i n e d by measuring t h e r e f l e c t i o n from a s m a l l p l a t e o f q u a r t z s u b s t i t u t e d f o r the f i l m ; J and J , a r e t h e l a s t maximum and max min minimum i n t e n s i t i e s recorded a s t h e f i l m t h i n s . The use o f t h i s f o r m u l a w i t h measurement o f J and J , a v o i d s t h e need t o know max min the r e f r a c t i v e index o f t h e o i l phase; a l s o , a s t h e l a s t term i s e f f e c t i v e l y u n i t y , measurement o f 1^ i s unnecessary. Measurements were c a r r i e d out on aqueous s o l u t i o n s o f s u c r o s e monolaurate and s u c r o s e monotallowate. The c o n c e n t r a t i o n o f t h e m o n o t a l l o w a t e i n t h e aqueous phase was kept c o n s t a n t a t j u s t above -3 -3 the cmc (7.41x10 mol dm ) and t h e KC1 c o n c e n t r a t i o n was v a r i e d -4 -3 -3 -3 between 5x10 mol dm and 7.5x10 mol dm . The r e s u l t s a r e shown i n F i g u r e 8. I t can be seen t h a t , a s t h e c o n c e n t r a t i o n o f e l e c t r o l y t e i s increased a t constant s u r f a c t a n t concentration, t h e e q u i l i b r i u m d i s t a n c e decreases, which i s c o n s i s t e n t w i t h d e c r e a s i n g r e p u l s i v e f o r c e s . The e s t i m a t e d e r r o r i n h i s ± 1 nm. Above an -3 -3 e l e c t r o l y t e c o n c e n t r a t i o n o f 7.5x10 mol dm b l a c k l u n e s formed a t the edges o f t h e e q u i l i b r i u m f i l m and i t t h i n n e d v e r y r a p i d l y t o t h e Newton B l a c k F i l m . F o r t h e monolaurate two s e t s o f measurements were made, one a t t h e cmc and t h e o t h e r a t one t e n t h o f t h e cmc. I n each case t h e e f f e c t o f e l e c t r o l y t e was t h e same a s f o r t h e m o n o t a l l o w a t e , b u t t h e e q u i l i b r i u m s p a c i n g s were g r e a t e r t h e g r e a t e r the c o n c e n t r a t i o n o f s u r f a c t a n t . Thus i n c r e a s i n g t h e s u r f a c t a n t c o n c e n t r a t i o n i n c r e a s e s t h e r e p u l s i v e f o r c e s and/or r e d u c e s t h e a t t r a c t i v e f o r c e s between t h e η-octane d r o p l e t s . The i n t e n s i t y v e r s u s time p l o t s a s t h e f i l m t h i n n e d showed a c o n t i n u o u s v a r i a t i o n w i t h v a r y i n g e l e c t r o l y t e concentration a t constant s u r f a c t a n t c o n c e n t r a t i o n . The p l o t s can be a n a l y s e d t o g i v e i n f o r m a t i o n on t h e i n t e r m o l e c u l a r f o r c e s . The f l o w o f l i q u i d between two s u r f a c e s i s g i v e n by t h e R e y n o l d s e q u a t i o n : e

2

d ( l / h ) / d t = 4Ap/3i)R

2


(9)

7.

HERRINGTON ET AL.

95

Sucrose Esters


100 -

F i g u r e 8.

E q u i l i b r i u m d i s t a n c e (h /nm) a s a f u n c t i o n o f t h e KC1 -3 -3 c o n c e n t r a t i o n (c/10 mol dm ) f o r : O , s u c r o s e monotallowate, cmc; • , s u c r o s e monolaurate, cmc/10; #, s u c r o s e monolaurate, cmc. ( R e p r i n t e d w i t h p e r m i s s i o n from r e f . 10. C o p y r i g h t 1982 R o y a l S o c i e t y of C h e m i s t r y ) 6

where AP i s t h e d i f f e r e n c e i n p r e s s u r e between t h e f i l m and t h e b u l k phase, h i s t h e f i l m t h i c k n e s s , *») i s t h e v i s c o s i t y , R i s t h e f i l m r a d i u s , AP = TT - TI^, where II i s the d i s j o i n i n g p r e s s u r e and n

= 2 3v/l

(sphere)

t

> 2vU

(rod)

>

(disc)

t vl. t

where l . / A = 1.5 + 1.265 n (n i s the number o f c a r b o n atoms t c c embedded i n the m i c e l l e core; f o r s u c r o s e monolaurate n = 11) c


v i s c a l c u l a t e d from the d e n s i t y of undecane (15). v a l u e s f o r s u c r o s e monolaurate are

The

(14);

limiting

2 A

> 68.6

A

c

68.6

> A

> 45.7

A

c

45.7 22.9

> A > A

c c

(spheres) 2

(rods) 2

> 22.9

A

(bilayers) ( r e v e r s e d phases)

Thus the t r a n s i t i o n s would be expected t o o c c u r i n the o r d e r : c u b i c ( I j ) -> hexagonal (U ) -> c u b i c ( V ^ -> l a m e l l a r (L ) -> r e v e r s e d ft

phases. C o n s i d e r i n g the m i c e l l e s as h a r d - c o r e p a r t i c l e s , the phase t r a n s i t i o n s w i l l be determined by p a c k i n g c o n s t r a i n t s . The l i m i t i n g volume f r a c t i o n s a r e 0.74 f o r f a c e - c e n t e r e d c u b i c ( s p h e r i c a l m i c e l l e s ) and 0.91 f o r the hexagonal phase (rod-shaped m i c e l l e s ) . The l i m i t i n g volume f r a c t i o n s are found e x p e r i m e n t a l l y t o be w i t h i n 70 t o 80% o f the c l o s e - p a c k e d v a l u e s . The phase diagrams a r e c o n s i s t e n t w i t h t h i s g e n e r a l p i c t u r e . The s t r e a m i n g b i r e f r i n g e n c e shown by s u r f a c t a n t - r i c h i s o t r o p i c s o l u t i o n s o f the monolaurate and monooleate a r e c o n s i d e r e d t o i n d i c a t e the presence o f rod-shaped m i c e l l e s and hence the i n c i p i e n t f o r m a t i o n o f the hexagonal phase. S u c r o s e monolaurate shows a more e x t e n s i v e hexagonal phase and l e s s l a m e l l a r phase t h a n the monooleate o f l o n g e r a l k y l c h a i n l e n g t h . T h i s i s s i m i l a r t o the behaviour o f the p o l y o x y e t h y l e n e s u r f a c t a n t s : comparison o f C E 0 , w i t h C E 0 , and o f C , E 0 w i t h C E 0 shows 1Z o 1U o ID O 1Z o t h a t the former e x h i b i t more l a m e l l a r and l e s s hexagonal phase t h a n the l a t t e r . However f o r the m o n o g l y c e r i d e s i n c r e a s i n g the a l k y l c h a i n l e n g t h d e c r e a s e s the s t a b i l i t y o f the l a m e l l a r phase (16). Most o f the p o l y o x y e t h y l e n e s u r f a c t a n t s show a lower c o n s o l u t e temperature, i m p l y i n g t h a t the headgroup h y d r a t i o n d e c r e a s e s w i t h i n c r e a s i n g temperature f o r a g i v e n a l k y l c h a i n l e n g t h ; t h i s would f a v o u r the l a m e l l a r phase a t h i g h e r temperatures and i s s u p p o r t e d by the phase diagrams. The s u c r o s e s u r f a c t a n t s showed no s i g n s o f a c l o u d p o i n t i n the temperature range s t u d i e d , c o n s i s t e n t w i t h the s t r o n g l y h y d r o p h i l i c c h a r a c t e r o f the s u c r o s e head group. F o r s u r f a c t a n t s w i t h a f a i r l y h i g h and i l l - d e f i n e d v a l u e o f the cmc, s u c h as o c t y l m e t h y l s u l p h o x i d e , i t i s p o s s i b l e t o s t u d y the changes o f the thermodynamic p r o p e r t i e s on m i c e l l i z a t i o n by u s i n g a mass a c t i o n model t o a n a l y s e the d a t a (17). However, when the cmc i s low, i t i s i m p o s s i b l e t o o b t a i n a c c u r a t e thermodynamic d a t a i n the p r e m i c e l l a r r e g i o n . R e c e n t l y , the m i c e l l a r p r o p e r t i e s o f the n - a l k y l p o l y e t h y l e n e o x i d e s u r f a c t a n t s , C^EOy w h i c h have low v a l u e s 1 o

o f the cmc,

iri

4

o

i O

o

but a l s o show a lower c o n s o l u t e temperature o r c l o u d


7.

HERRINGTON ET AL.

99

Sucrose Esters

p o i n t , have been s t u d i e d by n e u t r o n s c a t t e r i n g , NMR and dynamic l i g h t s c a t t e r i n g (18-20). However, the fundamental i s s u e o f t h e e f f e c t o f temperature i n c a u s i n g p r e d o m i n a n t l y m i c e l l a r growth o r i n c r e a s i n g the a g g r e g a t i o n o f s m a l l m i c e l l e s has n o t been r e s o l v e d . S u c r o s e monolaurate and monooleate a l s o have v e r y low v a l u e s o f t h e cmc, b u t d i f f e r from the j s e r i e s i n two r e s p e c t s ; t h e f o r m a t i o n c

E

O


n

o f l i q u i d c r y s t a l l i n e phases i n the pure s t a t e and t h e absence o f a c l o u d p o i n t below 100°. The a g g r e g a t i o n number o f the monolaurate i s c o n s i d e r a b l y l e s s than t h a t o f the monooleate a t a l l temperatures s t u d i e d . T h i s c a n be a t t r i b u t e d t o t h e l a r g e r hydrocarbon l e n g t h o f the o l e y l group. S i m i l a r b e h a v i o u r i s shown by the n - a l k y l p o l y e t h y l e n e o x i d e s u r f a c t a n t s ; f o r example a t 25 C the a g g r e g a t i o n number i s 400 f o r C

E 0

b

u

t

2

4

3

0

f

o

r

C

E 0

A

l

s

o

t

h

e

11 v

a

l

u

e

s

f

o

r

t

h

e

s

u

c

r

o

s

e

12 6 16 6 monesters a r e l e s s than those o f the C E 0 j s e r i e s w i t h comparable n

c h a i n l e n g t h and head group s i z e : f o r s u c r o s e monolaurate 7/ = 50, whereas f o r C E 0 . y = 400 a t 25 C. T h i s i s c o n s i s t e n t w i t h an 1 o

a g g r e g a t i o n number d e c r e a s i n g w i t h i n c r e a s i n g h y d r o p h i l i c n a t u r e o f the head group. Thus s m a l l e r m i c e l l e s a r e f a v o u r e d f o r t h e s u c r o s e esters. The s t r o n g l y h y d r o p h i l i c n a t u r e o f t h e s u c r o s e head group a l s o e x p l a i n s t h e l a c k o f a c l o u d p o i n t below 100 C. The p r e v i o u s e q u a t i o n (6) may be w r i t t e n as 2

TT/RTc = 1/M + B * c M + 2

22

(11)

2

The TT/c v e r s u s c p l o t s o f b o t h sucrose monoesters show a c o n s i s t e n t trend. I n c r e a s i n g temperature s t e a d i l y d e c r e a s e s t h e i n i t i a l p o s i t i v e slope, o f t h e curves, w h i c h r e f l e c t s s t e a d i l y d e c r e a s i n g values of B , the v i r i a l c o e f f i c i e n t f o r m i c e l l e - m i c e l l e 2 2

interaction. A n e g a t i v e v a l u e would i n d i c a t e m i c e l l e - m i c e l l e a t t r a c t i o n w i t h p o s s i b l e aggregation. This i n d i c a t e s that a c l o u d p o i n t may e x i s t above 100°C. F o r C E 0 , , B i s p o s i t i v e a t 18°C, iO

0 0

z e r o a t 20 C and n e g a t i v e a t 45 C as expected f o r a c l o u d p o i n t a t 50 C. T h i s i s an important d i f f e r e n c e between t h e s u c r o s e and p o l y e t h y l e n e o x i d e head groups. The e t h y l e n e o x i d e c h a i n s form a f l e x i b l e c o i l e d head group, a h e l i c a l c o i l , w h i c h c a n a l t e r s i z e and shape, whereas t h e s u c r o s e head group p r e s e n t s a r e l a t i v e l y r i g i d structure. T h i n l i q u i d f i l m s a r e o f t e n used as models f o r s t u d y i n g e m u l s i o n s t a b i l i t y as t h e i r e q u i l i b r i u m t h i c k n e s s i s d e t e r m i n e d by the same f o r c e s . The s t a b i l i t y o f emulsions depends on two processes. F i r s t l y , the r e v e r s i b l e f l o c c u l a t i o n o f the d r o p l e t s o f the d i s p e r s e d o i l d r o p l e t s , w i t h f o r m a t i o n o f a t h i c k "Common B l a c k Film". The t h i c k n e s s o f t h i s f i l m i s determined by the o p p o s i t i o n o f v a n d e r Waals a t t r a c t i v e and e l e c t r o s t a t i c r e p u l s i v e f o r c e s . These f i l m s a r e formed a t low i o n i c s t r e n g t h , t h e l o n g range r e p u l s i v e f o r c e s h i n d e r a g g r e g a t i o n . Secondly, i n s o l u t i o n s o f h i g h e r i o n i c s t r e n g t h , a second t h i n n i n g p r o c e s s may o c c u r and t h e d r o p l e t s " c o a l e s c e " t o form the v e r y t h i n "Newton B l a c k F i l m s " Here the v a n der Waals a t t r a c t i v e f o r c e s a r e b a l a n c e d b y t h e


100


r e p u l s i v e s t e r i c f o r c e s . Our measurements were r e s t r i c t e d t o t h e t h i c k e r "Common B l a c k F i l m s " . The energy o f i n t e r a c t i o n between t h e two o i l d r o p l e t s has 3 components: 1) t h e o v e r l a p o f t h e e l e c t r i c a l double l a y e r g i v e s a r e p u l s i v e p r e s s u r e , TT ; R

2) t h e v a n d e r Waals a t t r a c t i v e p r e s s u r e , IT^; 3) s t e r i c s t a b i l i s a t i o n produced by m o l e c u l a r a d s o r p t i o n a t t h e s u r f a c e , Tl . T h e i r sum i s t h e d i s j o i n i n g p r e s s u r e , lT . Thus Downloaded by UNIV OF CALIFORNIA SAN DIEGO on October 12, 2013 | http://pubs.acs.org Publication Date: December 26, 1991 | doi: 10.1021/bk-1991-0448.ch007

n

(12) The c o n t r i b u t i o n o f t h e v a r i o u s components as a f u n c t i o n o f f i l m t h i c k n e s s i s shown i n F i g u r e 10. At t h e e q u i l i b r i u m t h i c k n e s s t h e c a p i l l a r y p r e s s u r e , IT , which tends t o t h i n t h e f i l m i s e q u a l t o t h e

Sucrose Esters as Emulsion Stabilizers - ACS Symposium Series

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