Role of the Middle Phase in Emulsions - American Chemical Society

12 Role of the Middle Phase in Emulsions 1

F. M. FOWKES, J. O. CARNALI, and J. A. SOHARA

Downloaded by PENNSYLVANIA STATE UNIV on September 9, 2013 | http://pubs.acs.org Publication Date: March 27, 1985 | doi: 10.1021/bk-1985-0272.ch012

Department of Chemistry, Lehigh University, Bethlehem, PA 18015

We have studied some soap-stabilized oil-in-water emul sions which flocculate at higher salinities without coalescense, developing flat planes of contact. The contact angles between the oil/water interfaces and the plane of contact increase with salinity up to 60°, indicating that the tensions in the plane of contact are less than the net oil/water interfacial tension. It is shown that these contact angles develop when middle phase films (M) coat the o i l droplets and fill the planar spaces between flocculated droplets. The net interfacial tension is γ + γ and the contact angle Θ allows calculation or each of these two tensions; MO

MW

W

cosΘ = γ / (γ W

MO

MO

+ γ ) MW

A small angle X-ray scattering study showed that the middle phase films between flocculated o i l droplets were about 90 nm in thickness, and spinning drop interfacial tension measurements were found to be able to centrifuge middle phase films off tne o i l drops along the axis of rotation. Such contact angle measurements are recommended as general tools for middle phase studies because they are sensitive to very low middle phase concentration and measure the relative hydrophilic or oleophilic character of the middle phase. In a number of studies of middle phase phenomena i n oil-wateremulsifier systems i t has been shown that middle phases form when the emulsifier i s about equally partitioned into the o i l and water phases Healy et a l . , ( l ) , Salager et a l . , (2), Shah et a l . , (2), Kuneida and Shinoda,(4). In these studies i t has been shown tnat 1

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0097-6156/85/0272-0173$06.00/0 © 1985 American Chemical Society

In Macro- and Microemulsions; Shah, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


174

M A C R O - A N D MICROEMULSIONS

the i n t e r f a c i a l tension of the middle phase versus oil (γ ) de creases as the emulsifier content of the oil increases, while decreases as the emulsifier content of the water increases (shown in Figure 1). In these studies the p a r t i t i o n c o e f f i c i e n t for the emulsifier has been systematically varied by changes in emulsifier carbon number or ethylene oxide number, by changes in the carbon number of the oil phase, by changes in the s a l i n i t y of the aqueous phase, by changes in temperature (with nonionic emuls i f i e r s ) or by changes in pH with soap or amine s a l t emulsifiers. In many of these studies the structure of the middle phase is not established, but it is c l e a r l y immiscible in water or oil and i t s e l e c t r i c a l conductivity is closer to water than oil. Phase diagram studies of oil-water-emulsifier systems Ekwall, (5), indicate that surfactant-rich phases immiscible in oil or water have rodshaped or lamellar micelles with some degree of o p t i c a l anisotropy or flow birefringence, and these phases have much greater elecr i c a l conductivity than oil. Figure 1 i l l u s t r a t e s that the middle phase composition varies smoothly from a water-rich composition to an oil-rich composition as the emulsifier p a r t i t i o n changes from mostly water-soluble to mostly oil-soluble. If lamellar structures are present the r e l a t i v e thickness of o l e o p h i l i c and hydrophilic layers must vary smoothly from the water-rich compositions to the oil-rich compositions. Middle phase studies are generally conducted with emulsifier concentrations high enough that the volume of the middle phase is e a s i l y observed, t y p i c a l l y 5-10%. Middle phases may be just as important at lower concentrations (less than î%) but have been d i f f i c u l t to observe. However, the i n t e r f a c i a l tension has been found to go through a minimum Chan and Shah, (6) j u s t as in Figure 1. In research unrelated to these middle phase studies, emulsions of mineral oil in water s t a b i l i z e d with O.5% of sodium soaps were observed to flocculate in an unexpected fashion upon increasing s a l i n i t y Princen et a l . , (7^), Aronson and Princen, (8). In O.1 M sodium chloride the emulsion droplets flocculated into clusters of spheres, but in O.3 to O.5 M s a l t solutions the f l o c culated droplets were separated by f l a t planes as shown in Figure 2. Where these planes met the oil/water interface a d i s t i n c t "contact angle" was observed (56° in O.5 M s a l t , as depicted in Figure 2). Although other explanations were offered i n i t i a l l y , we now know that these contact angles r e s u l t from thin films of middle phase which have spread spontaneously over a l l oil droplets and in the plane separating flocculated droplets. Such contact angles are easy to observe and become a sensitive measure for formation of middle phase films too thin to observe d i r e c t l y . In this paper we present three techniques f o r the study of thin middle phase films adsorbed on emulsion droplets. Film thicknesses have been measured by small angle X-ray scattering, contact angles of adjacent droplets have been measured in flocculated emulsions, and much d i r e c t evidence for such films has been observed v i s u a l l y in the spinning drop i n t e r f a c i a l tensiometer. Experimental Details Emulsions.

A white mineral oil of 125/135 Saybolt v i s c o s i t y


12.

FOWKES ET AL.


[-Middle Phase Rangea

F i g u r e 1. I n t e r f a c i a l tension versus oil/water emulsifier p a r t i t i o n c o e f f i c i e n t s f o r systems w i t h m i d d l e phases (M). (Healy e t a l , 1 9 7 6 ) . Reproduced w i t h p e r m i s s i o n from H e a l y , R. N . , R. L . R e e d , and D. G. Stenmark, S o c . P e t . E n g . J . , June 1976, p. 147; c o p y r i g h t owner: S o c i e t y o f P e t r o l e u m E n g i n e e r s o f AIME.

F i g u r e 2. F l o c c u l a t e d oil d r o p l e t s d i s p l a y i n g p l a n e o f and c o n t a c t a n g l e s 0y (O.4 M N a C l ) .

contact


176

MACRO- AND MICROEMULSIONS


(Fisher S c i e n t i f i c ) was emulsified into aqueous soap solutions of O.25 w% each of sodium laurate and sodium oleate prepared from sodium hydroxide, l a u r i c acid (Aldrich Gold Label) and o l e i c acid (Fisher P u r i f i e d ) . Coarse emulsions were used for microscopy (as in Figure 2), but fine emulsions (with droplet sizes of about O.2 microns), used for determination of middle phase f i l m thick nesses, were made by ultrasonication with a c e l l disruptor. Sodium chloride contents of these emulsions varied from O.1 M to O.5 M. I n t e r f a c i a l Tensions. I n i t i a l l y i n t e r f a c i a l tensions by the pendant drop technique. Later studies with an drop i n t e r f a c i a l tensiometer gave much information on films, as shown in the photographs made with a camera our tensiometer.

were measured EOR spinning middle phase mounted on

Small Angle X-Ray Scattering (SAXS). A Kratky camera using the 1.54 X CuKa l i n e in a 7.5 χ O.25 mm beam was made available by the Western E l e c t r i c Research Center in Princeton, New Jersey. Scat tering at angles of O.1 m i l l i r a d i a n increments were measured for 1000 seconds each. The s t a t i s t i c a l evaluation of the data, desmearing and data analysis were performed with the help of the program FFSAXS, Version 4 by C. G. Vonk of the DSM, Central Laboratory, Geleen, The Netherlands Fowkes and C a r n a l i , (9). Contact Angles. Figure 2 shows a photograph of a pair of emulsion droplets which have flocculated in O.5 M sodium chloride solution, and a drawing of such a system to show how the contact angle θ was determined. The r a d i i of the two drops (Rj and R ) and the o v e r a l l length of the doublet (L) were determined and from these measure ments X, the radius of t h e i r c i r c l e of contact,was calculated from: 2

(R

2

- X) 2

X

H

+ (R 2 2

-

X

2)^ =

L

-Ri-R

(1)

2

and the contact angle θ was calculated from:

1

2Θ = s i n ^X/Ri) + s i n " ( X / R )

(2)

2

For each contact angle reported several doublets were measured. Experimental Results and Discussion Contact Angle and SAXS Studies. The contact angles which developed at the junction of flocculated oil droplets, i l l u s t r a t e d in Figure 2, can be used together with the measured i n t e r f a c i a l ten sions (Table 1) in a vector diagram to demonstrate that the i n t e r f a c i a l tensions on the outer surface of the oil drops (Yow) appreciably greater than the i n t e r f a c i a l tensions (YQ^) in plane of contact between drops: a r e

t n e

Y

ow

=

Y

ow

c o s

^ w

The contact angles in Figure 2 are measured through the aqueous


(3)

12.

177

Role of the Middle Phase

FOWKES ET AL.

phase, and are therefore designated 6y. Similar contact angles for water droplets in oil would be measured through theoilphase ( 6 Q ) . Table I shows that Yow"~YOW i from 0 toO.49mJ/m as the s a l i n i t y increases toO.5M. n

Table I.

r

e

a

s

e

s

2

I n t e r f a c i a l Tensions and Contact Angles for O i l Droplets inO.5%Soap Solution vs. S a l i n i t y

NaCl Cone.


c

Y (mj/m

0 (degrees)

2

0 O.3 M O.4 M O.5 M

Y

w

0W

( m J / m Z )

0 O.25 O.35 O.49

0 32 45 56

1.6 1.2 1.1

Y

ow- ow

The above results are in good agreement with the o r i g i n a l findings of Princen and Aronson. Their expiantion for the lower i n t e r f a c i a l tensions in the plane of contact was that the aqueous layer between the two adsorbed monolayers might be so extremely thin that special a t t r a c t i v e forces between oil droplets diminished the e f f e c t i v e i n t e r f a c i a l tensions. We therefore set out to measure the thickness (H) of this aqueous layer by small angle X-ray scat tering (SAXS), using fine emulsions of various s a l i n i t i e s . The SAXS findings were analyzed by Guinier plots and by c o r r e l a t i o n functions in some d e t a i l ; a l l results indicated that the layers between f l o c culated droplets were not at a l l thin, but r e l a t i v e l y thick (90 nm). This measurement c l e a r l y demonstrated that no special short-range attractions between oil droplets can exist in this system. Another explanation for the contact angles and the decreased i n t e r f a c i a l tentions between oil droplets is that increasing s a l i n i t y has induced formation of middle phase (M) which has coated the oil droplets and f i l l e d the space between droplets in the plane of contact, as shown in Figure 3. The i n t e r f a c i a l tensions between droplets are those between oil and middle phase (Y^Q) whereas the tensions between oil and water are the sum of theoiland water i n t e r f a c i a l tensions with the thin i n t e r f a c i a l f i l m of middle phase (ΥΠΟ YMW). apparent Y tensions are γ + y tensions and these always exceed the tensions operating in the plane of con tact between flocculated oil droplets ( Y M Q ) . contact angle 8y is therefore a measure of the presence or thin films of middle phase around theoildroplets: +

T

h

u

s

t

h

e

q

w

Μ 0

T

C 0 S

Ύ

% • Μ0

/ ( Υ

Μ0

+

n

m

e

4

V

Table I I retabulates the data of Table I according to equation (4) and shows that the measured i n t e r f a c i a l tension of the oil/water in terface and the contact angle allow determination of γ and YMW Μ 0


178


Table I I .

I n t e r f a c i a l Tensions of Middle Phase vs. O i l or Water

\

NaCl Conc.

^M0

32° 45° 56°

O.3 M O.4 M O.5 M

2

1.6 mJ/m 1.2 1.1

^MW 2

O.25 mJ/m O.35 O.49

2

1.35 mJ/m O.85 O.61

The findings of Table II look remarkably l i k e those i l l u s t r a t e d in Figure 1, for increased s a l i n i t y has decreased Y Q and increased Ywr, as the emulsifier becomes partitioned more strongly into the oil phase. The contact angle information could become part of the same diagram, as i l l u s t r a t e d in Figure 4. The l e f t edge of the middle phase region in Figure 4 is where cos 6y f i r s t exceeds zero,(where the contact angle f i r s t appears). The mid-point of the diagram (where γ equals γ ^ ) is where is 60° and cos6 =O.5,as can be seen by equation (4). On the right side of the diagram water-in-oil emulsions are formed and contact angles 6 would be measured through the oil phase:


M

Μ 0

C

C O S 6

0

=

W

( Y

M0

+

(

V

5

)

On the right side of the diagram aqueous droplets w i l l be coated with middle phase and when the emulsifier is partitioned s u f f i c i e n t l y into the oil phase 6 approaches zero at the right boundary of the middle phase region. In systems with higher emulsifier concentrations the observed middle phase may well tend to hold trapped droplets. In the waterr i c h middle phase compositions on the l e f t side of Figure 4 (where cos6 exceeds 0 and is less than +O.5) oil droplets can e a s i l y become entrapped in the middle phase, for their i n t e r f a c i a l tensions (ΎΜΟ) than in the aqueous phase ( γ + γ ^ ) . Similarly the oil-rich middle phase compositions on the r i g h t side of Figure 4 w i l l tend to entrap aqueous droplets. Electron micrographs of middle phases do indeed show such entrapped droplets (Chan and Shah, 1982). Q

w

a

r

e l

e

s

s

Μ 0

Spinning Drop I n t e r f a c i a l Tensiometer Studies. In the foregoing studies with oil-in-water emulsions s t a b i l i z e d with O.5% of sodium soaps the volume of middle phase which develops with increased s a l i n i t y is o r d i n a r i l y too small to observe. However, i f a single oil drop is spun in the center of a large excess of the aqueous soap solution for some hours, middle phase films are seen to be centrifuged to the ends of the oil drop and to be ejected along the axis of r o t a t i o n as shown in Figure 5. In Figure 5a the aqueous phase is s a l t - f r e e and contacting droplets have a of 0°. In Figure 5b (O.3 M sodium chloride) the end of the oil drop after 24 hours is seen to be narrower and less r e f l e c t i v e as the middle phase f i l m is centrifuged towards that end. In Figure 5c (O.4 M sodium chloride), a f t e r 17 hours of spinning, middle phase material is seen to be spun o f f along the axis of rotation, and in Figure 5d (also O.4 M s a l t ) the droplets in contact show 0y values of 40-50°. As middle phase films form in the spinning drop i n t e r f a c i a l tensiometer, the i n t e r f a c i a l tension drops. However, as these films



12.

FOWKES ET AL.


Figure 3. Diagrammatic sketch of middle phase films surrounding oil droplets and f i l l i n g the plane of contact between them.

Figure 4. Contact angles and i n t e r f a c i a l tensions of oil-wateremulsifier systems with middle phases as a function of the emulsifier p a r t i t i o n between oil and water.



180


Figure 5. Photographs of oil drops in spinning drop i n t e r f a c i a l tensiometer with O.5% soap solution: (a) no s a l t present, 6 = 0° (b) O.3 M s a l t , spun 24 hours (c) O.4 M s a l t , spun 17 hours (d) O.4 M s a l t , 6 = 40-50° W

W


12.

FOWKESETAL.

181


are spun o f f , the i n t e r f a c i a l tension r i s e s again, as shown in Figure 6. Without s a l t present (triangles) no middle phase forms and there is no r i s e in i n t e r f a c i a l tension. However, with s a l t present the r e s u l t i n g middle phase is spun o f f and the net i n t e r f a c i a l tension r i s e s appreciably. We have sought to see whether these contact angles are observed in other systems with middle phases and find them to be a general phenomenon. For instance, in an emulsion system involving long chain amine s a l t s as emulsifiers, where middle phases occur, 6y values of 30-50° were observed.


Conclusions 1. The contact angles previously observed with flocculated oil droplets in saline emulsions are found to result from thin films of a middle phase which coats emulsion droplets and f i l l s the planes of contact between droplets. 2. In these systems the i n t e r f a c i a l tensions between oil droplets and the aqueous phase are the sum of the two tensions of the middle phase (γ^ο + YMW). Such measured tensions and measured contact angles allow c a l c u l a t i o n of γ^ο d Y M W a n

COS9

W

=

V

(

Y

M 0

+

:

V -

3. Small angle X-ray scattering measurements showed that in the system under investigation the middle phase films between f l o c culated oil droplets were about 90 ran in thickness. 4. Contact angles between contacting drops are a very s e n s i t i v e measure of middle phase behavior and can be used to detect middle phases in systems with low emulsifier content. 5. The magnitude of the contact angle is a measure of the r e l a t i v e oil and water content of the middle phase. 6. In the spinning drop i n t e r f a c i a l tensiometer the thin films of middle phase can in time be centrifuged o f f of oil drops. 7. Contact angles between emulsion drops are f a i r l y easy to observe in the spinning drop i n t e r f a c i a l tensiometer. Acknowledgments Support of this project was i n i t i a t e d by a starter grant from the Department of Energy (Fowkes and Carnali, 1983). The SAXS studies were made with a Kratky camera at AT&T Western E l e c t r i c Company's Engineering Research Center in Princeton, NJ; our thanks to Dr. John Emerson, who made these arrangements and provided technical advice to the project. The spinning drop measurements were made with the support of Atlas Powder Company. Our thanks also to IBM for a distinguished graduate fellowship for J . O. Carnali.



182


0

ï

5

Î5 Time

?T

2'0

24 "S"

28

(Hr.)

Figure 6. Time-dependence of i n t e r f a c i a l tensions of oil drops spinning in O.5% soap solutions (25°). Triangles - no s a l t . Hexagons and c i r c l e s - O.3 M s a l t . Squares - O.4 M s a l t . Interf a c i a l tensions r i s e as middle phase films are spun o f f .


12.

FOWKES ET AL.


183

Literature Cited 1. 2. 3. 4.


5. 6. 7. 8. 9.

Healy, R. N., R. L. Reed, and D. G. Stenmark, Soc. Pet. Eng. J., June, 1976, 147. Salager, J. L., J. C. Morgan, R. S. Schechter, W. Y. Wade, and E. Vasquez, Soc. Pet. Eng. J., April 1979, 107. Shah, K. D., D. W. Green, M. J. Michnick, G. P. Willhite, and R. E. Jerry, Soc. Pet. Eng. J., Dec. 1981, 763. Kundieda, Η., and K. Shinoda, Bull. Chem. Soc. Japan, 1982, 55, 1777. Ekwall, P., in "Advance in Liquid Crystals, Vol. 1 (Academic Press, 1975) 1-142. Chan, K. S., and D. O. Shah. In "Surface Phenomena in Enhanced Oil Recovery" Plenum, 1982; pp. 53-72. Princen, Η. Μ., M. P. Aronson, and J. C. Moser, J. Colloid Interface Sci., 1980, 75, 246. Aronson, M. P., and H. M. Princen, Nature, 1980, 286, 370. Fowkes, F. M. and J. O. Carnali, DOE Report #DE-FG19-80ET12267, 1983.

RECEIVED June 8, 1984


Role of the Middle Phase in Emulsions - American Chemical Society

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