Microemulsions and Emulsions in Foods - American Chemical Society

Chapter 17 Effect of Polysaccharide on Flocculation and Creaming in Oil-in-Water Emulsions Margaret M . Robins

Downloaded by CORNELL UNIV on October 26, 2016 | http://pubs.acs.org Publication Date: December 26, 1991 | doi: 10.1021/bk-1991-0448.ch017

AFRC Institute of Food Research, Colney Lane, Norwich NR4 7UA, United Kingdom

Creaming of the droplets in 20% alkane-in-water emulsions in the presence and absence of non-adsorbing polymer is reported in the form of oil concentration-height profiles collected at intervals of time. Without polymer, the polydisperse droplets (stabilised by non-ionic surfactant) cream individually at a rate determined by their diameter. Measurement of creaming rates enables the distribution of hydrodynamic diameters to be inferred, and it agrees well with the size distribution from a light diffraction method. In the presence of hydroxyethylcellulose in the continuous phase at a concentration exceeding 0.03%w/w, the droplets flocculate, and the creaming rates are used to estimate the size of the pores in the flocculated droplet network. At 0.03% polymer, flocculated and individual particle phases are in coexistence. Direct evidence for a depletion mechanism of flocculation is presented. Many foods are emulsions during or after manufacture (1). In oil-in-water emulsions, the dispersed oil droplets generally possess a lower density than the continuous aqueous phase. Unless the droplets are very small or very concentrated the density difference leads to the accumulation of the droplets at the top of the container ("creaming") with consequent loss of perceived quality. This paper presents results on the creaming of oil-in-water emulsions in the absence and presence of the polysaccharide hydroxyethylcellulose (Natrosol 250HR). Methods Emulsion preparation and characterisation. Emulsions were prepared containing droplets of mixed alkanes (heptane and hexadecane in the volume ratio 90:10), stabilised to coalescence by the non-ionic surfactant Brij 35 at a concentration of 0.35%w/w in the final continuous phase. The emulsions were initially prepared in a Waring 0097-6156/91/0448-0230$06.00/0 © 1991 American Chemical Society

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

17. ROBINS

Effect of Polysaccharide on Flocculation and Creaming

231


B l e n d o r a t 60% o i l volume f r a c t i o n 4>, and d i l u t e d t o o b t a i n 4> = 20%. The d i l u e n t was an aqueous s o l u t i o n o f p r e s e r v a t i v e a l o n e (sodium m e t a b i s u l p h i t e a t 0.2%w/w i n t h e f i n a l c o n t i n u o u s p h a s e ) , o r p r e s e r v a t i v e and polymer ( N a t r o s o l 250HR). The d r o p l e t s i z e d i s t r i b u t i o n was measured u s i n g a M a l v e r n M a s t e r s i z e r l i g h t d i f f r a c t i o n instrument. S i n c e t h e same c o n c e n t r a t e d e m u l s i o n was used t h e d r o p l e t s i z e d i s t r i b u t i o n was i d e n t i c a l i n a l l t h e samples. C h a r a c t e r i s a t i o n o f d i s p e r s e and c o n t i n u o u s p h a s e s . The d e n s i t y o f t h e l i q u i d s was measured a t 20°C u s i n g an A n t o n Paar DMA602 v i b r a t i n g t u b e d e n s i t y meter. V i s c o s i t i e s were measured a t low s h e a r - r a t e a t 20°C u s i n g a double-gap m e a s u r i n g system f i t t e d t o a B o h l i n R e o l o g i C o n t r o l l e d S t r e s s rheometer. The c o n c e n t r a t i o n o f polymer i n t h e "sub-cream" l a y e r o f c o n t i n u o u s phase formed a f t e r each e m u l s i o n had creamed was d e t e r mined u s i n g a s p e c t r o p h o t o m e t r y method ( 2 ) . The sub-cream l a y e r s were d i l u t e d t o a nominal c o n c e n t r a t i o n o f 0.006% v / v and 0.5ml p i p e t t e d i n t o a s t o p p e r e d t e s t - t u b e , t o w h i c h was added 1.5ml 3% w/w p h e n o l s o l u t i o n and 5ml c o n c e n t r a t e d s u l p h u r i c a c i d . The m i x t u r e was shaken and l e f t t o s t a n d f o r 20 m i n u t e s , b e f o r e samples were t r a n s f e r r e d t o c u v e t t e s f o r measurement o f a b s o r b a n c e a t 488nm. S t a n d a r d s o l u t i o n s o f 0.005 and 0.007% w/w polymer were a l s o measured, f o r c a l i b r a t i o n p u r p o s e s . The e r r o r on polymer c o n c e n t r a t i o n was l e s s t h a n 10%. Measurement o f creaming i n e m u l s i o n s . Most t e c h n i q u e s t o m o n i t o r creaming (or sedimentation) a r e i n t r u s i v e , such as sampling ( 3 ) , o r a p p l i c a b l e o n l y t o d i l u t e systems ( 4 ) . R e c e n t l y s e v e r a l noni n t r u s i v e methods have been d e v e l o p e d ( 5 ) , i n c l u d i n g t h e u s e o f ultrasonics. We have d e v e l o p e d a t e c h n i q u e based on t h e v e l o c i t y o f u l t r a s o u n d t h r o u g h t h e d i s p e r s i o n , w h i c h may be d i r e c t l y r e l a t e d t o i t s c o m p o s i t i o n ( 6 ) . The t e c h n i q u e i s s u i t a b l e f o r n o n - a e r a t e d d i s p e r s i o n s w i t h a wide range o f c o n c e n t r a t i o n s ( > l % v / v ) , i t i s nond e s t r u c t i v e and n o n - i n t r u s i v e . The a p p a r a t u s , shown s c h e m a t i c a l l y i n F i g u r e 1, d e t e r m i n e s t h e v e l o c i t y o f u l t r a s o u n d from measurements o f t h e t i m e - o f - f l i g h t o f a p u l s e o f u l t r a s o u n d g e n e r a t e d from a c o n t i n u o u s wave o f f r e q u e n c y 6.4MHz. The t i m e i s d e t e r m i n e d t o a p r e c i s i o n o f 5ns i n a t y p i c a l p r o p a g a t i o n t i m e o f 25/is. The t r a n s d u c e r s a r e h e l d a t f i x e d s e p a r a t i o n and moved v e r t i c a l l y s o t h a t measurements a r e made a t a s e r i e s o f h e i g h t s w i t h a s p a t i a l r e s o l u t i o n o f l e s s t h a n 2mm. The samples a r e c o n t a i n e d i n p a r a l l e l s i d e d c e l l s ( d i m e n s i o n s t y p i c a l l y 16mm wide x 32mm deep x 160mm h i g h ) o f p o l y m e t h y l m e t h a c r y l a t e . The c e l l s and t r a n s d u c e r s a r e immersed i n a water b a t h h e l d a t 20°C t o m a i n t a i n a c o n s t a n t t e m p e r a t u r e and t o p r o v i d e a good u l t r a s o n i c c o n t a c t between t h e t r a n s d u c e r s and t h e c e l l . In g e n e r a l t h e v e l o c i t y o f u l t r a s o u n d through a d i s p e r s i o n a t a g i v e n f r e q u e n c y i s a complex f u n c t i o n o f t h e c o m p o s i t i o n , p a r t i c l e s i z e d i s t r i b u t i o n and t h e p h y s i c a l p r o p e r t i e s o f t h e d i s p e r s e d and c o n t i n u o u s phases (2)• However, i n many s i m p l e d i s p e r s i o n s t h e p a r t i c l e s i z e and s t a t e o f a g g r e g a t i o n o f t h e p a r t i c l e s have a n e g l i g i b l e e f f e c t on t h e u l t r a s o n i c v e l o c i t y , and t h e o v e r r i d i n g f a c t o r i s t h e p a r t i c l e concentration. Often a simple mixing e q u a t i o n (8) d e s c r i b e s t h e r e l a t i o n s h i p between t h e u l t r a s o n i c v e l o c i t y V and volume f r a c t i o n



232

MICROEMULSIONS AND EMULSIONS IN FOODS

F i g u r e 1: S c h e m a t i c diagram o f u l t r a s o n i c creaming

monitor.

(1)

w

w

[ h3][ ^'4 where V i s t h e v e l o c i t y o f u l t r a s o u n d t h r o u g h t h e d i s p e r s i o n o f volume f r a c t i o n , and p , p , V , V a r e t h e d e n s i t i e s o f and u l t r a sound v e l o c i t i e s t h r o u g h t h e c o n t i n u o u s and d i s p e r s e p h a s e s . Figure 2 shows t h e measured and p r e d i c t e d v e l o c i t y t h r o u g h a l k a n e - i n - w a t e r e m u l s i o n s w i t h a range o f o i l c o n c e n t r a t i o n . The s i m p l e m i x i n g e q u a t i o n (1) i s c l e a r l y a good model f o r t h i s system. c

T h e o r y o f Creaming

d

c

d

or Sedimentation

The t e r m i n a l v e l o c i t y v o f a s i n g l e s p h e r i c a l p a r t i c l e moving g r a v i t y i n a v i s c o u s l i q u i d i s g i v e n by S t o k e s ' Law s

v

s

=

2

Ap.d .g 18

r,

c


under

(2)


17. ROBINS

233

Velocity (v/ms ) H

1500


1485.70m/sec LOOIOOg/cm' 1174.00m/sec 0.694009/001"

1200-

30

40

50

60

Concentration (0/%) Figure 2: U l t r a s o n i c v e l o c i t y and o i l c o n c e n t r a t i o n . (1).

Line

from

equation

where Ap=p -p , t h e d e n s i t y d i f f e r e n c e between t h e c o n t i n u o u s and d i s p e r s e phases, d i s t h e d r o p l e t d i a m e t e r , g, t h e a c c e l e r a t i o n due t o g r a v i t y and r) , t h e v i s c o s i t y o f t h e c o n t i n u o u s p h a s e . When a p p l y i n g t h e s i m p l e a n a l y s i s t o n o n - d i l u t e e m u l s i o n s we have t o make a l l o w a n c e f o r t h e e f f e c t o f t h e o t h e r d r o p l e t s on t h e c r e a m i n g r a t e s of i n d i v i d u a l d r o p l e t s . T h i s i n f l u e n c e may be i n c o r p o r a t e d i n t h e t h e o r y i n two ways. I t i s w e l l known t h a t t h e p r e s e n c e o f p a r t i c l e s i n a d i s p e r s i o n increases i t sv i s c o s i t y . One a p p r o a c h i s t o q u a n t i f y t h e i n c r e a s e i n r) and a p p l y e q u a t i o n (2) t o o b t a i n t h e d e c r e a s e i n v . A meanf i e l d model o f d i s p e r s i o n v i s c o s i t y (9) p r o p o s e s a r e l a t i o n s h i p between t h e v i s c o s i t y r\ o f a d i s p e r s i o n o f volume f r a c t i o n ^ and t h e c o n t i n u o u s phase v i s c o s i t y TJQ c

d

c

c

s

1

-

no

( W M n )

-2.5 m = 70%, e q u a t i o n s (2) and (3) p r e d i c t v = 0.55v . There a r e a l s o e m p i r i c a l c o r r e l a t i o n s f o r t h e e f f e c t o f concent r a t i o n on t h e v e l o c i t y o f a monodisperse s u s p e n s i o n , where t h e p a r t i c l e s move a s a body. That o f R i c h a r d s o n and Z a k i (10) p r e d i c t s v = 0.35v . I t i s debatable which e x p r e s s i o n i s v a l i d f o r p o l y d i s p e r s e d r o p l e t s moving a t d i f f e r e n t speeds t h r o u g h an e m u l s i o n ; we u s e e q u a t i o n (3) i n p r e f e r e n c e t o an e m p i r i c a l correlation. m

s

s


234



Creaming o f E m u l s i o n s W i t h o u t Polvmer I n F i g u r e 1 a t y p i c a l o i l c o n c e n t r a t i o n p r o f i l e i s shown s c h e m a t i c a l l y a l o n g s i d e t h e creaming emulsion. The m a j o r i t y o f t h e o i l i s a t t h e t o p o f t h e c e l l i n a c o n c e n t r a t e d cream l a y e r , w i t h t h e slower-moving d r o p l e t s b e g i n n i n g t o c l e a r from t h e b a s e . This i s t y p i c a l b e h a v i o u r f o r a p o l y d i s p e r s e e m u l s i o n c o n t a i n i n g no polymer. F i g u r e 3 shows ( i n a h o r i z o n t a l f o r m a t ) c o n c e n t r a t i o n p r o f i l e s a t v a r i o u s stages d u r i n g t h e creaming o f a polymer-free emulsion. Creaming i s d e t e c t e d w i t h i n 2 h o u r s , a l t h o u g h t h e e m u l s i o n a p p e a r s u n i f o r m v i s u a l l y f o r o v e r 10 d a y s . I n i t i a l l y the o i l i s u n i f o r m l y d i s p e r s e d i n t h e c o n t a i n e r , a t a c o n c e n t r a t i o n Q = 20% v / v . A f t e r a few days t h e d r o p l e t s have s t a r t e d t o r i s e up t h e c e l l , s o t h e c o n c e n t r a t i o n a t t h e base has f a l l e n , and t h e r e i s a c o n c e n t r a t e d cream l a y e r a t t h e t o p . With time, a l l t h e d r o p l e t s a r r i v e at the top. I f t h e y were m o n o d i s p e r s e , t h e lower meniscus would be s h a r p , as t h e y would a l l cream a t t h e same speed. It i s p o s s i b l e t o u s e t h e o b s e r v a t i o n t h a t t h e y move a t d i f f e r e n t speeds ( r e s u l t i n g i n t h e hazy, d i f f u s e "meniscus" r i s i n g up t h e c e l l ) , t o o b t a i n t h e e f f e c t i v e hydrodynamic s i z e d i s t r i b u t i o n ( 1 1 ) .

Time (days)

Concentration (%v/v)

—

0.02 0.11

—

0.37

—

2.21

—

3.23

—

6.19

—

9.95

—

12.94

—

20.10

••• 48.25

Height (mm) Figure 3: Oil concentration polymer.

profiles

f o r 19%

alkane

emulsion

without


17.

ROBINS


Determination of droplet s i z e d i s t r i b u t i o n . We assume t h a t t h e d r o p l e t s t r a v e l i n d i v i d u a l l y t o t h e cream l a y e r a t a speed c o n s i s t e n t w i t h t h e i r d i a m e t e r and S t o k e s ' Law ( e q u a t i o n ( 2 ) , w i t h the mean-field correction f o r v i s c o s i t y , equation (3)). I t i s p o s s i b l e t o i d e n t i f y p a r t i c l e f r a c t i o n s o f d i f f e r e n t s i z e by t h e c h a n g i n g shape o f t h e lower, d i f f u s e meniscus. Consider the p o s i t i o n on t h e meniscus a t which t h e o i l c o n c e n t r a t i o n i s 10%, approximately h a l f t h e t o t a l o i l c o n c e n t r a t i o n . The r a t e a t w h i c h t h i s 10% c o n t o u r r i s e s up t h e c e l l i s r e l a t e d by S t o k e s Law t o a p a r t i c u l a r d r o p l e t diameter, d . S i n c e h a l f t h e o i l has a l r e a d y moved away from t h e meniscus, d r e p r e s e n t s t h e m i d - p o i n t d i a m e t e r , t h e median d i a m e t e r . S i m i l a r l y , the r a t e of r i s e of the contour at 5% o i l r e p r e s e n t s t h e lower q u a r t i l e d r o p l e t d i a m e t e r . The h e i g h t s o f t y p i c a l c o n t o u r s a r e shown as a f u n c t i o n o f t i m e i n F i g u r e 4. E a c h i s l i n e a r , showing a c o n s t a n t v e l o c i t y o f each f r a c t i o n o f d r o p l e t s o f d i a m e t e r d. The v e l o c i t y o f each c o n t o u r e n a b l e s a c u m u l a t i v e s i z e d i s t r i b u t i o n t o be i n f e r r e d , as shown i n F i g u r e 5. 1

m

m


235



236


Contour velocity

0

2

4

Diameter (fim)

Qjm/s)

6

8

10

12

14

16

Contour concentration (%)

18

20

Figure 5 : V e l o c i t y o f c o n t o u r s and i n f e r r e d hydrodynamic d i a m e t e r s f o r each concentration contour.


17.

ROBINS


237

The r e s u l t i n g s i z e d i s t r i b u t i o n i s shown i n F i g u r e 6, and compared w i t h t h a t o b t a i n e d from a l a s e r d i f f r a c t i o n t e c h n i q u e , t h e M a l v e r n Mastersizer. The agreement i s v e r y good, c o n s i d e r i n g t h a t t h e c r e a m i n g method i s s u b j e c t t o s e v e r a l assumptions as t o t h e independence o f t h e m o t i o n o f t h e p a r t i c l e s , and t h e p r o b l e m o f allowing c o r r e c t l y f o r the l o c a l droplet concentrations.


Creaming

o f E m u l s i o n s C o n t a i n i n g Polymer

P o l y s a c c h a r i d e s t a b i l i s e r s are f r e q u e n t l y used t o reduce t h e r a t e o f c r e a m i n g , as w e l l as t o i m p a r t t h e r e q u i r e d m o u t h f e e l p r o p e r t i e s t o t h e p r o d u c t . We a r e i n t e r e s t e d i n t h e mechanisms by w h i c h t h e polymers i n f l u e n c e the s e p a r a t i o n p r o c e s s . In p a r t i c u l a r , p r e v i o u s work (12-13) has shown t h a t i f i n s u f f i c i e n t polymer i s added, t h e d r o p l e t s become f l o c c u l a t e d and cream f a s t e r t h a n i n t h e absence o f the s t a b i l i s e r . Here we p r e s e n t c r e a m i n g r e s u l t s on 20% a l k a n e - i n water e m u l s i o n s c o n t a i n i n g t h e p o l y s a c c h a r i d e h y d r o x y e t h y l c e l l u l o s e ( N a t r o s o l 250HR). The d r o p l e t s i z e d i s t r i b u t i o n i s t h e same as i n the polymer-free emulsion (Figure 6). A t c o n c e n t r a t i o n s up t o and i n c l u d i n g 0.02%w/w i n t h e c o n t i n u o u s phase t h e c r e a m i n g i s not v i s i b l e t o t h e eye u n t i l n e a r l y complete, and t h e creaming p r o f i l e s a r e v e r y s i m i l a r t o t h o s e w i t h o u t polymer ( F i g u r e 3 ) . However, a t a l e v e l o f 0.03%, t h e r e i s a change i n t h e creaming b e h a v i o u r . The p r o f i l e s a r e shown i n F i g u r e 7. V i s u a l l y t h e e m u l s i o n a p p e a r s opaque u n t i l a f t e r about 10 days, when t h e base s t a r t s t o c l e a r o f o i l . However t h e u l t r a s o n i c p r o f i l e s show t h a t t h e r e a r e two d i s t i n c t t y p e s o f c r e a m i n g . The m a j o r i t y o f t h e o i l d r o p l e t s move up t h e c e l l r a p i d l y , w i t h a s h a r p m e n i s c u s , b u t about 4% o i l remains t o cream s l o w l y w i t h a d i f f u s e meniscus, as i n t h e e m u l s i o n s w i t h z e r o o r low l e v e l s o f polymer. The c o n t o u r h e i g h t s f o r t h e e m u l s i o n c o n t a i n i n g 0.03% polymer a r e shown i n F i g u r e 8. A t c o n t o u r c o n c e n t r a t i o n s below 4%, t h e d r o p l e t s a r e moving i n f r a c t i o n s , as t h e y d i d when no polymer was present. The range o f c o n t o u r v e l o c i t i e s e x h i b i t e d i n d i c a t e s t h a t t h e d r o p l e t s moving i n d i v i d u a l l y r e p r e s e n t a s i m i l a r range o f d i a m e t e r s as t h e o r i g i n a l e m u l s i o n . However, t h e y r e m a i n as i n d i v i d u a l d r o p l e t s w h i l e t h e m a j o r i t y o f t h e o i l creams v e r y f a s t , w i t h l i t t l e v a r i a t i o n i n speed w i t h c o n t o u r c o n c e n t r a t i o n . This f r a c t i o n i s c l e a r l y a g g r e g a t e d , and t h e d r o p l e t s a r e c r e a m i n g as a s i n g l e e n t i t y , presumably i n a network w i t h s u f f i c i e n t l y l a r g e v o i d s f o r the unaggregated f r a c t i o n t o d r a i n through. We have t h u s o b s e r v e d c o e x i s t e n c e o f two e m u l s i o n p h a s e s ; a f l o c c u l a t e d f r a c t i o n c o n t a i n i n g 80% o f t h e o i l , and an u n f l o c c u l a t e d f r a c t i o n w h i c h can cream as i n d i v i d u a l d r o p l e t s . When t h e polymer c o n c e n t r a t i o n i s i n c r e a s e d f u r t h e r , s e v e r a l e f f e c t s a r e a p p a r e n t . A l l t h e o i l a p p e a r s t o become a g g r e g a t e d , creaming r a p i d l y w i t h a sharp meniscus. Although t h e v i s c o s i t y o f t h e c o n t i n u o u s phase i n c r e a s e s w i t h polymer c o n c e n t r a t i o n , t h e c r e a m i n g i s f a s t e r t h a n w i t h o u t polymer. F i g u r e 9 shows t h e c o n c e n t r a t i o n p r o f i l e s f o r an e m u l s i o n c o n t a i n i n g 1% polymer i n t h e c o n t i n u o u s phase. The v i s c o s i t y ( a t low s h e a r - r a t e ) o f t h e polymer s o l u t i o n i s 3.5Pas, but t h e creaming r a t e o f t h e meniscus i s 0.18/im.s" , f a s t e r t h a n t h e 10% c o n t o u r i n t h e p o l y m e r - f r e e system.

Porous network model. The s h a r p n e s s o f t h e meniscus and t h e shape o f t h e c o n c e n t r a t i o n p r o f i l e s above t h e meniscus a r e r e m i n i s c e n t o f t h e b e h a v i o u r o f a s o l i d m a t e r i a l under c o m p r e s s i o n . We t h u s model



238


Weight in Band (%)

0

1

2

3

4

5

6

7

8

9

10

11

12

13

Size Band (microns) Figure 6a: Droplet s i z e d i s t r i b u t i o n i n f e r r e d contours.

from concentration


17. ROBINS


239


Weight in Band (%)

Size Band (microns) F i g u r e 6b: Droplet s i z e d i s t r i b u t i o n from l i g h t d i f f r a c t i o n s i z e r (Malvern M a s t e r s i z e r ) .

particle


240




Time (days)

80

0.02 0.07 0.17 1.13 2.01 3.00 6.06 10.28 21.19 o

Base

20

40

60

80

100

120

140

Height (mm)

Figure

160

38.15

Top

7:

O i l c o n c e n t r a t i o n p r o f i l e s f o r 20% a l k a n e 0.03%w/w polymer i n t h e c o n t i n u o u s phase.

emulsion

containing


ROBINS



Height (mm)

Time (Days) F i g u r e 8: H e i g h t o f o i l c o n c e n t r a t i o n c o n t o u r s and creaming t i m e f o r e m u l s i o n c o n t a i n i n g 0.03%w/w polymer i n t h e c o n t i n u o u s phase.


241


242



Time (days)

80 -i

0.19 3.15 4.20

i

7.20 8.02 9.04 11.19 14.14 20.92

0

Base

20

60

80

100

Height (mm)

120

140

160

Top

F i g u r e 9: O i l concentration p r o f i l e s for 20% alkane emulsion 1.0%w/w polymer i n the continuous phase.

containing



17.

ROBINS


243

t h e f l o c c u l a t e d e m u l s i o n as an open b u t c o n t i n u o u s network, which compresses under g r a v i t y . The c o m p r e s s i o n r e s u l t s i n t h e c o n t i n u o u s phase d r a i n i n g t h r o u g h c h a n n e l s i n t h e network. The c r e a m i n g / c o m p r e s s i o n r a t e i s l i m i t e d by t h e v i s c o u s r e s i s t a n c e o f t h e c o n t i n u o u s phase as i t f l o w s t h r o u g h t h e c h a n n e l s . The d r a i n a g e o f l i q u i d t h r o u g h a porous bed o f p a r t i c l e s has been s t u d i e d e x t e n s i v e l y , and t h e a p p l i c a b i l i t y o f t h e model t o f l o c c u l a t e d d i s p e r s i o n s has been demonstrated by M i c h a e l s and B o l g e r ( 1 4 ) , who i n v e s t i g a t e d the sedimentation behaviour of f l o c c u l a t e d k a o l i n suspensions. T h e i r system d i f f e r s i n two main r e s p e c t s from o u r s ; t h e i r p a r t i c l e s a r e r i g i d , and t h e y a r e v e r y s t r o n g l y f l o c c u l a t e d . E m u l s i o n d r o p l e t s a r e d e f o r m a b l e , and, i n o u r e x p e r i e n c e , t h e f l o e s formed a r e weak and e a s i l y d i s r u p t e d upon d i l u t i o n . However, we have found t h e g e n e r a l d e s c r i p t i o n s o f f l o c c u l a t e d systems t o be a u s e f u l s t a r t i n g p o i n t f o r a n a l y s i s o f our r e s u l t s . The system i s m o d e l l e d as a p o r o u s bed t h r o u g h which t h e background l i q u i d d r a i n s . The maximum s e d i m e n t a t i o n ( o r creaming) r a t e v g i v e s an e s t i m a t e o f t h e e f f e c t i v e hydrodynamic d i a m e t e r o f t h e d r a i n a g e c h a n n e l s ( p o r e s ) , assumed t o be smooth s t r a i g h t c y l i n d e r s

v =

2

^ .Ap.d .g 0

(1-^f)

p

32 rj

(4)

c

a

n

d

d

i

s

t

n

e

where 0f = volume f r a c t i o n o f f l o e s (1 - 0o/0 ) p e f f e c t i v e hydrodynamic p o r e d i a m e t e r . T h i s d i a m e t e r was o b s e r v e d by M i c h a e l s and B o l g e r (14) t o i n c r e a s e w i t h s t r e n g t h o f f l o c c u l a t i o n . A p p l y i n g t h e p o r e / c h a n n e l model, we can e s t i m a t e t h e d i a m e t e r o f t h e p o r e s d i n e q u a t i o n (4) from t h e c r e a m i n g r a t e , v, v i s c o s i t y rj and o i l c o n c e n t r a t i o n s Q, . We assumed t h a t t h e d r o p l e t s w i t h i n t h e f l o e s were " c l o s e - p a c k e d " , w i t h - 0.7. F i g u r e 10 shows t h e e s t i m a t e d p o r e d i a m e t e r as a f u n c t i o n o f polymer concentration. A t low c o n c e n t r a t i o n s , t h e e f f e c t i v e d i a m e t e r o f t h e p o r e s i s c o n s t a n t , a t about 37/xm. T h i s i s c o n s i s t e n t w i t h t h e o b s e r v a t i o n i n t h e c o e x i s t e n t e m u l s i o n (0.03% polymer) t h a t i n d i v i d u a l d r o p l e t s o f up t o 10/*m c o u l d p a s s t h r o u g h t h e f l o c c u l a t e d network. A t h i g h e r l e v e l s , t h e r e i s a l a r g e i n c r e a s e w i t h c o n c e n t r a t i o n up t o about 300/im i n 1% polymer. This i s consistent w i t h s t r o n g e r f l o c c u l a t i o n a t h i g h e r polymer c o n c e n t r a t i o n s ( 1 4 ) . The i n c r e a s e d s t r e n g t h o f t h e f l o e s w i t h h i g h e r l e v e l s o f polymer i s a l s o e v i d e n t i n t h e r e l u c t a n c e o f t h e cream l a y e r t o become c l o s e l y packed. The o i l c o n c e n t r a t i o n a t t h e t o p o f t h e samples i n t h e e a r l y s t a g e s o f c r e a m i n g i s i n d i c a t i v e o f t h e r e s i s t a n c e o f t h e f l o e s t o compaction under g r a v i t y . With l e s s than 0.03% polymer t h e cream i s a t 70% c o n c e n t r a t i o n from t h e s t a r t , b u t i n t h e f l o c c u l a t e d systems i t b u i l d s up a t much lower p a c k i n g density. A t t h e h i g h e s t polymer c o n c e n t r a t i o n , ( F i g u r e 9) t h e cream i s a l m o s t u n i f o r m a t -40% b e f o r e e x h i b i t i n g slow c o m p a c t i o n w i t h time, again uniformly. m

p

c

m

m

Depletion Flocculation. We have o b s e r v e d t h a t a t a c r i t i c a l c o n c e n t r a t i o n o f polymer, 0.03%w/w, a f l o c c u l a t e d phase and i n d i v i d u a l d r o p l e t s can c o e x i s t . This i s consistent with a depletion rather t h a n b r i d g i n g mechanism (15-16). U n l e s s a polymer i s a t t r a c t e d t o t h e s u r f a c e o f t h e d r o p l e t s , and t h u s becomes a d s o r b e d , g e o m e t r i c a l c o n s t r a i n t s n e a r t h e d r o p l e t r e q u i r e t h a t t h e d e n s i t y o f polymer segments n e a r t h e s u r f a c e i s lower t h a n i n t h e b u l k c o n t i n u o u s phase. The r e g i o n n e a r t h e d r o p l e t s i s t h u s d e p l e t e d o f polymer. I f one v i s u a l i s e s d e p l e t i o n f l o c c u l a t i o n as b e i n g d r i v e n by a need


244



t o r e d u c e t h e volume o f c o n t i n u o u s phase from w h i c h t h e polymer i s excluded, the occurrence o f f l o c c u l a t i o n a t the c r i t i c a l c o n c e n t r a t i o n s h o u l d depend on t h e volume f r a c t i o n o f d r o p l e t s . T h i s image i s s u p p o r t e d by an e x p e r i m e n t u s i n g 0.03% polymer b u t w i t h o n l y 5% o i l volume f r a c t i o n . No f l o c c u l a t e d f r a c t i o n was o b s e r v e d , a l l t h e d r o p l e t s c r e a m i n g i n d i v i d u a l l y a s i n t h e systems w i t h l e s s polymer. The c r i t i c a l c o n c e n t r a t i o n , around 0.03%, i s c o n s i s t e n t w i t h t h e e x p e r i m e n t s and c a l c u l a t i o n s o f S p e r r y (15-16), f o r t h e same polymer and t h e same mean p a r t i c l e s i z e . A n o t h e r consequence o f d e p l e t i o n f l o c c u l a t i o n i s t h a t t h e f i n a l c o n t i n u o u s phase, c o n t a i n e d i n t h e sub-cream l a y e r , s h o u l d c o n t a i n s l i g h t l y more polymer t h a n t h e o v e r a l l c o n c e n t r a t i o n , b e c a u s e t h e c o n t i n u o u s phase i n t h e cream l a y e r with depleted. For t h i s reason we a n a l y s e d t h e sub-cream l a y e r s f o r polymer u s i n g d e n s i t y and spectrophotometry. F i g u r e 11 shows t h e e s t i m a t e d polymer c o n c e n t r a -

Pore diameter (pm) 300

250

200

150 -

100 -

50 -

1

1 0.2

1

1 0.4

1

-

1 0.6

0.8

1

1.2

Polymer concentration (% w/w) Figure

10:

E s t i m a t e d d i a m e t e r o f p o r e s i n f l o c c u l a t e d e m u l s i o n s and polymer concentrat ion.



17. ROBINS

245

Sub-cream concentration (% w/w)


1.4

-i

0 Figure

0.2

0.4

0.6

0.8

Initial concentration (% w/w)

1

1.2

11:

Concentration o f polymer i n sub-cream l a y e r s polymer c o n c e n t r a t i o n . L i n e r e p r e s e n t s y=x.

and

original

t i o n i n t h e sub-cream l a y e r s . (The d e n s i t y r e s u l t s , w h i c h depend on 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 a d d i t i o n t o polymer c o n c e n t r a t i o n , have been c o r r e c t e d t o a l l o w f o r t h e s u r f a c t a n t p r e s e n t on t h e d r o p l e t s ) . T h e r e i s c l e a r l y an e n r i c h m e n t o v e r t h e i n i t i a l , o v e r a l l c o n t i n u o u s phase c o n c e n t r a t i o n o f polymer, a l t h o u g h t h e p r e c i s e amount i s debatable. Taken as a whole, t h e e v i d e n c e i s s t r o n g l y i n f a v o u r o f depletion flocculation. The weakness o f t h e f l o e s , and t h e i m p r o b a b i l i t y t h a t t h e polymer c o u l d a d s o r b o n t o t h e s u r f a c t a n t c o a t e d d r o p l e t s , a r e c o n s i s t e n t w i t h t h e d e p l e t i o n model. Conclusions T h e r e i s a c o n s i d e r a b l e amount o f i n f o r m a t i o n t o be g a i n e d from measurement o f c o n c e n t r a t i o n p r o f i l e s d u r i n g t h e c r e a m i n g o f emulsions. The r a t e o f r i s e o f t h e meniscus i n f l o c c u l a t e d systems


246


yields information on the strength of the flocculation, and we hr.ve fitted a simple network model to our data. The size of drainage pores in the network is inferred from the creaming/compression rates. The critical polymer concentration required to flocculate the droplets, the coexistent phases observed at that concentration and the absence of flocculation at lower oil concentration

Microemulsions and Emulsions in Foods - American Chemical Society

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