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The relationship between surfactant phase behavior and the creaming and coalescence of macroemulsions
The Relationship between Surfactant Phase Behavior and the Creaming and Coalescence of Macroemulsions L. M. BALDAUF,* R. S. SCHECHTER, *'1 AND W. H. WADEt *Department of Chemical Engineering and tDepartment of Chemistry, The University o f Texas at Austin, Austin, Texas 78712 AND
ALAIN GRACIAA University o f Pau, Pau, France Received May 4, 1981; accepted August 3, 1981 DEDICATED TO THE MEMORY OF PROFESSOR FRANK CHAUNCEY GOODRICH Macroemulsions are inherently thermodynamically unstable systems which exhibit lifetimes which depend on the initial state of the system and the rates of creaming and coalescence. A previous study has shown an apparent relationship of emulsion stability to the thermodynamic (equilibrium) phase behavior of systems containing surfactant/hydrocarbon/water (M. Bourrel, A. Graciaa, R. S. Schechter, and W. H. Wade, J. Colloid Interface Sci. 72, 161 (1979)). It was found that macroemulsions are most stable at the boundaries which separate two phase systems from those which form three phases consisting of a microemulsion in equilibrium with excess hydrocarbon and water phases. The present investigation shows that on approaching the threephase boundaries those factors, such as the density difference between phases, the continuous phase bulk viscosity, and the surface viscosity, which tend to influence the rate of creaming, all act to decrease it. Hence at the phase boundary creaming is shown to be a slow step, thereby causing the emulsion to appear to be stable. It was also found that, despite the fact that the surface viscosity is maximum at the boundaries, coalescence becomes more rapid as the boundaries are approached. Thus, while the time scale for creaming increases, that for coalescence decreases. Overall, then, macroemulsions formed well away from the phase boundaries cream first, but persist longest. Within the three-phase region where the surfactant system is balanced (equal concentrations of surfactant in the excess phases) coalescence proceeds at a remarkable, but unexplained, speed. In this region macroemulsions coalesce before they cream and emulsion stability is least. These observations are shown to apply to systems in the presence of varying quantities of alcohol. The phase behavior was adjusted by varying the electrolyte concentrations.
INTRODUCTION
A relationship between microemulsion phase behavior and macroemulsion stability has been reported. The existence of a correlation was thought to be important for both conceptual and practical reasons. 1 To whom all correspondence should be addressed.
Since microemulsions are thermodynamically stable mixtures of oil, electrolyte, and surfactant, the phase behavior is easily and reproducibly observed. The general trends are predictable. Thus it was decided to investigate this relationship in some detail. In particular the role of interfacial viscosity has been considered. It seems appropriate, therefore, to include this paper
187
Journalof Colloidand Interface Science, Vol.85, No. 1, January 1982
0021-9797/82/010187 - 11$02.00/0 Copyright© 1982by AcademicPress,Inc. All rightsof reproductionin any formreserved.
188
BALDAUF
in a symposium honoring Professor Frank Goodrich since he has played such a prominent role in the development of the theory and measurement of interfacial viscosity (2-4). Microemulsions can be classified as Type I, II, or III depending on the observed phase behavior. A Type I system consists of an oil-in-water (O/W) microemulsion in equilibrium with an almost pure oil phase. A Type II system is a microemulsion (W/O) in equilibrium with an excess aqueous phase. Type III systems exhibit three phases including a middle, surfactantrich phase in equilibrium with both oil and water phases. It has been noted (5) that the minimum interfacial tension and maximum solubilization occur for optimum Type III systems. The particular Type III system for which equal volumes of oil and water are contained in the middle phase are designated optimum. The phase behavior of microemulsions is a function of the tendency of the surfactant to partition between the oil and aqueous phases (6). It is, therefore, an accurate measure of the surfactant HLB in the presence of oil, of electrolytes, and of any other additives such as alcohols. It has been shown that the surfactant tends to partition equally between excess phases at or near the optimum conditions. In this context the surfactant can be said to be balanced at optimum. It seems reasonable then to expect that a correlation might exist between microemulsion phase behavior and macroemulsion stability. Bourrel et al. (1) have reported that maxima in macroemulsion stability occur at the phase boundaries separating Types I and III and II and III systems. Also a deep minimum in macroemulsion stability was observed when the system is adjusted so that it is optimum. These findings obtained by observing the initial rate of macroemulsion phase separation are good agreement with Boyd et al. (7) who were the first to report that upon increasing the surfactant HLB, macroemulsion stability, as measured Journal of Colloid and Interface Science, Vol. 85, No. 1, January 1982
ET AL.
by observing the initial droplet coalescence rate, exhibits two maxima--one corresponding to O/W and the other to W/O macroemulsions. Since Bourrel et al. (1) found the macroemulsion at the I - I I I boundary to be O/W and a W/O macroemulsion at the I I - I I I boundary, it is believed that the two maxima found by Boyd corresponds to those reported by Bourrel. In the present study, factors believed to contribute to macroemulsion stability are examined in relationship to the phase behavior of the system. Based on this study it has been possible to ascertain that the boundaries between Type III and Types I and II systems are positions of minimum creaming rates, and, therefore, correspond to maximum initial emulsion stability. However, coalescence at these boundaries has been found to be faster than for Type I and Type II systems removed from the boundaries. Thus the time required for complete macroemulsion coalescence is not in general a maximum at the phase boundaries. EXPERIMENTAL
The system under investigation in this study contained as surfactant Siponate DS-10, a blend of isomers of sodium dodecyl benzene sulfonate (SDBS). Secondary butanol (2 BuOH) and isopentanol (IC5OH) were used as cosurfactants. The concentrations of SDBS and IC~OH were maintained at 3 gpdl (grams per deciliter), while systems were studied which contained either 1 or 3 gpdl 2 BuOH. The concentration of sodium chloride was varied from 1 to 4 gpdl to produce the desired phase behavior. The hydrocarbon was normal nonane and a 1:I volume ratio of oil to aqueous phase was maintained. Interracial viscosity was measured using a deep-channel surface viscometer. The analysis of Wasan et al. (8) was used to calculate the interfacial and surface viscosities from the time periods of small
CREAMING AND COALESCENCE OF MACROEMULSIONS
189
5O
Lower Phase Boundary I-[ Lower Phase Boundary
46
.J 44 0 w ta 4 2 -
" J ~ 4o
L!
1: Nonane
"" "~ x
Water Oil Ratio: I : l
I1 3 38
1I I
36 LO
gpdl 5 gpd~
~,..
SDBS ICsOH
"~ ~-~.
----A---
I
gpdl
28uOH
----D--
5
gpdl
7' BuOH
I I.I
I 1.2
I 1.3 SODIUM
I 1.4
I 1.5
I 1.6
CHLORIDE CONCENTRATION
l 1.7
I 1.8
L 1.9
Z.O
(GPDL)
F1o. 1. Volumepercent of excess oil phase versus electrolyteconcentrationfor O/W microemulsions. Teflon particles floating on the gas-liquid and liquid-liquid interfaces. The bulk viscosities of the hydrocarbon and aqueous phases were measured by a Contraves low-shear 30 viscometer, which is a Couette-type rotational viscometer with variable shear rate capability. A Mettler/Parr DMA-46 digital density meter was used to measure the phase densities. All measurements were made at a temperature of 25°C. Microemulsion phase behavior was determined by preparing solutions of 5 ml aqueous phase and 5 ml hydrocarbon phase in sealed 10-ml pipettes. The pipettes were shaken and allowed to equilibrate several times until the volumes of the phases remained constant. The phase volumes of the aqueous and hydrocarbon phases were then recorded. The relative emulsion stabilities were determined by shaking the tubes uniformly and allowing the phases to separate. The time required for 20% of
the continuous phase to regenerate was recorded as an indication of the rate of creaming. The time required for total phase separation to occur was recorded as an indication of the rate of coalescence. RESULTS
Type I (O/W) Microemulsions At low electrolyte concentrations, the systems have been selected to that Type I phase behavior is observed. As the concentration is increased, at a certain electrolyte concentration, a Type III system is first observed. This is called the lower threephase boundary (LPB). The LPB for the nonane SDBS system was found at 1.85 gpdl NaC1 in the presence of 3 gpdl 2 BuOH and at 1.75 gpdl NaCl ff 2 BuOH is reduced to 1 gpdl. As the system approached the LPB and more oil was solubilized, the volume of the excess oil phase (Fig. 1) decreased. Also, Journal of Colloid and Interface Science, Vol. 85, No. 1, January 1982
190
BALDAUF ET AL. 0.7
Lower Phase Boundary [] Oil: Nonone Water Oir Ratio; I : l :3 gpdl SDBS
0.6
0.5
----m---- 3 gpdl
-
-
-
-
Lower Phase Boundary z~
~. ~'-I
j ,~/L
I
2BuOH
//[~'\
-
/
L \\\^~
A
0.4
o L} > 0.3
~: LLI 0.2 F Z_
0.1
GO
1.0
I t.I
I
t
I
I
I
1
I
I
LB
1.3
L4
1.5
L6
1.7
I.e
t.9
SODIUM CHLORIDE CONCENTRATION
2.0
(GPDL)
FIG. 2. Interfacial viscosity versus electrolyte concentration for O/W rnicroemulsions.
70
Lower Phase Boundary
60
u~
50
Oil : Nonane Water Oil Ratio; I :1 3 gpdl SDBS 3 gpdl ICsOH ---O---- I gpdl 2 BuOH ---A---- 3 gpdl 2 BuOH
/
i'=
Lower Phase Boundary ~---.~// ~ / rI " "1' ~ ~ // // / ~ / /
$
]I ii t
/
\
/ //
\\\
/ ~50
/
--
&
\\
/A" /
\
/
\\
I I I I I
mL tO
I ~1
I 1.2
I 1.3
I 1.4
I 1.5
I . 1.6
SODIUM CHLORIDE CONCENTRATION F I G . 3. E m u l s i o n
stability versus electrolyte concentration
Journal of Colloid and Interface Science, Vol. 85, No. 1, January 1982
I 1.7
~
[ 18
(GPDL) for O/W microemulsions.
r 19
20
CREAMINGAND COALESCENCEOF MACROEMULSIONS
191
420. Lower Phase Boundary 360. I.=E z
300.
0 240,
oz
180.
n,.
120.
OiL : Nonane Water Oil Ratio: L:I 3 gpdl
_~ 60. I-
O, 1.0
SDBS
3 gpdl
ICsOH
3 gpdl
2BuOH
I I.I
I 1.2
I 1.5
I 1.4
SODIUM CHLORIDE
I 1.5
I 1.6
CONCENTRATION
[ 1.7
I 1.8
[
I 1.9
] 2.0
(GPDL)
FIG. 4. Time required for complete coalescence versus electrolyte concentration for O/W microemulsions. the density of the aqueous phase decreased while the bulk viscosity of the aqueous phase increased. The density and bulk viscosity of the oil phase remained constant. To reduce the number of graphs, these data are not shown. The density of the microemulsion phase can, however, be accurately calculated based on the aqueous and oil phase densities and the volume of oil solubilized. The viscosity of the microemulsion phase increases substantially as the LP boundary is approached. This increase can be as much as a factor of 3 depending on the amount of oil solubilized. It should be noted that the system containing 1 gpdl 2 BuOH solubilized more than did the system containing 3 gpdl 2 BuOH, and therefore its aqueous phase possessed a lower density and higher bulk viscosity near the LPB. Smaller solubilizations with increased alcohol concentrations have been reported previously (9). The interfacial viscosity (Fig. 2) of both
the 1- and 3-gpdl 2 BuOH systems reached a maximum value at the LPB. When the surface and interracial viscosities were calculated simultaneously based upon the time periods of particles on the surface and interface, negative values were obtained for the surface viscosity. This was probably caused by the curvature of the gas-liquid interface. A surface viscosity of zero was therefore assumed, and the interfacial viscosity was calculated based upon the time periods of the particles at the liquid-liquid interface. At low salinity, the value of interfacial viscosity was about 0.20 sP (surface poise = 1 g/sec). The interfacial viscosity increased to 0.50 sP for the 1-gpdl 2 BuOH system and 0.59 sP for the 3-gpdl system at the LPB. It then dropped off sharply within the three-phase region. It should be noted, however, that within the three-phase region, the solutions would initially break into a microemulsion and an excess oil Journal of Colloid and Interface Science, Vol. 85, No. 1, January 1982
192
B A L D A U F E T AL. I I
I 44.0
Upper Phase BoundaryE}--,.~
I
~
Phase Boundary,',
,,~-Upper
43.5
43.o s
o ~42.5 w
J
ttl 4 2 . 0 /
/ f
/
I
/ t 1 &
/
bJ =E 41.5 /
0
/
Water Oil Ratio: Oil : Nonane 5 gpdl SDBS 3 gpdl ICeOH
/ /
41.0
---A----[3---
40.5
I gpdl 3 gpdl
I:1 2 BuOH
2 BuOH
I I 400 2.8
I 2.9
I 3.0
I 3,1
I :5.2 SODIUM
I 3.3
CHLORIDE
I 5.5
I 3.6
CONCENTRATION
(GPDL)
3.4
I 3.7
I 3.8
I 5.9
4.0
FIG. 5. Volume percent excess aqueous phase versus electrolyte concentration for W/O microemulsions.
phase. Only after a period of several days would a middle phase and an excess aqueous phase appear. This allowed the interfacial viscosity to be measured within the threephase region by the technique normally used for two-phase systems. Within the two-phase region, the first sign of emulsion breakdown to be observed was the creaming of the oil phase macroemulsion droplets to the top of the continuous fluid. Figure 3 shows the time required for approximately 20% of the continuous phase to regenerate. For both the I- and 3-gpdl 2 BuOH systems, the time required for the continuous phase to appear reached a maximum at the LPB. Also, the system containing 1 gpdl 2 BuOH was considerably more stable than the 3-gpdl system. Within the three-phase region, a layer of oil appeared at the top of the macroemulsion simultaneously with the appearance of the aqueous phase. This indicates that coalescence takes place in the Journal of Colloid and Interface Science, Vol. 85, No. l, January 1982
three-phase region, while creaming is dominant in the two-phase region. Because of this high rate of coalescence, large droplets quickly form and rise to the top of the continuous phase. The rate of appearance of the aqueous phase within the three-phase region was quite rapid. The time required for complet e coalescence (Fig. 4) was found to increase as the system moved away from the three-phase region. For the 3-gpdl 2 BuOH system, the solutions at low salinity which creamed most rapidly formed emulsions which persisted the longest--in excess of 24 hr. The solutions near the LPB which creamed very slowly separated in less than an hour, while those in the three-phase region broke in a few minutes. The solutions containing 1 gpdl 2 BuOH were very stable and formed emulsions which lasted for over 24 hr. Due to the long coalescence times involved, it was not possible to obtain reproducible results for this system. S011, it was evident
193
C R E A M I N G A N D C O A L E S C E N C E OF M A C R O E M U L S I O N S
that the emulsions broke most quickly in the three-phase region and most slowly at low salinity.
Type H (W/O) Microemulsions For Type II systems decreasing the system salinity results in more aqueous phase being solubilized until the upper phase boundary (UPB) is reached. When this occurs, an excess oil phase appears and the microemulsions will exist as the middle phase. For the nonane/water/SDBS system which contained 1 gpdl 2 BuOH, the UPB was found to occur at a salinity of 2.8 gpdl, while for the 3-gpdl 2 BuOH system, the UPB occurred at a salinity of 3.4 gpdl. As the system salinity decreased, the UPB was approached, and the volume of the excess aqueous phase (Fig. 5) decreased. The density and bulk viscosity of the oil phase increased as the amount of
Upper
r 0.8
.~v.~---upper
Phase
l ~J ~ 0.6
water solubilized into the oil phase increased. The aqueous phase bulk viscosity and density remained constant. Once again, the system containing 1 gpdl 2 BuOH solubilized more excess phase than did the 3-gpdl system, and therefore had a higher oil phase density and bulk viscosity near the UPB. The interfacial viscosity (Fig. 6) increased as the UPB was approached. At salinities far from the UPB, both the 1- and 3-gpdl systems had an interfacial viscosity of about 0.24 sP. Near the UPB, the 1-gpdl system reached a value of 0.52 sP while the 3-gpdl system reached a value of 0.38 sP. Unlike the O/W microemulsion systems, it was not possible to measure the interfacial viscosity within the three-phase region, because the solutions would separate very rapidly into three phases. It was therefore not possible to determine whether or not the interfacial viscosity decreased within the three-phase region.
Phase BoundaryB--...~ Oil: Nonone Water Oil Ratio: I:1
BoundoryA
75 gpdl SDBS 3 gpdI 1%OH - - - A - - - I gpdl - - - - [ 3 - - 5 gpdl
I I I I I
0.5
2BuOH 2 BuOH
\ \
8
\
~ 0.4
\ \ \
~
\ \\
0.3
A z -
0.2
O.I
QO - - M L 2.8
2.9
J
t
J
3.0
3.1
3.2
~ 3.3
~ _ _ J _ . . . _ . . . . . . . J ~ 3.4.
3.5
3,6
SODIUM CHLORIDE CONCENTRATION
(GPDL)
3.7
3,8
3.9
4.0
FIG. 6. Interfacial viscosity versus electrolyte concentration for W/O microemulsions. Journal of Colloid and Interface Science, Vol. 85, No. 1, January 1982
194
BALDAUF ET AL. I I ]
80
Upper I:~ose Boundory[]----.w~ ,.J~Upper
Oil: Nonane Water Oil Ratio: I : I 3 gpdl SDBS 5 gpdl ICsOH ---&-I gpdl 2 BuOH ----El---- 3 gpdl 2 BuOH
Phose Boundory~
70
~. 60 z v 50
40 _~ 30 w 2O
\
\
I0
Z
2.8
2.9
.5.0
&l
3.2
3,3
&4
3.5
5.6
57
3.8
3,9
4.0
SODIUM CHLORIDE CONCENTRATION (GPDL)
FIG. 7. Emulsion stabilityversus electrolyteconcentrationfor W/O microemulsions. The time required for 20% of the continuous phase to regenerate for Type II systems is shown in Fig. 7. The system containing 1 gpdl 2 BuOH behaved in the same way as did the O/W systems. Far from the UPB, the rate of creaming was rapid, while near the UPB, the rate of creaming was slow. Within the three-phase region, coalescence occurred simultaneously with creaming, so the rate of appearance of the continuous phase was rapid. The behavior of the 3-gpdl 2 BuOH system was somewhat different. Because the rate of coalescence even within the two-phase region was high, creaming always occurred simultaneously with coalescence. Therefore, the rate of appearance of the oil phase was lowest far from the UPB, and the rate increased as the system approached and passed through the UPB. The time required for complete coalescence (Fig. 8) shows that coalescence was extremely rapid for the 3-gpdl 2 BuOH Journal of Colloid and Interface Science, Vol. 85, No. I, January 1982
system. The rate of coalescence was highest at the three-phase boundary and was lowest at high salinity. Once again, the 1-gpdl 2 BuOH solutions required over 24 hr to coalesce, and the rate of coalescence was lowest far from the phase boundary where the rate of creaming was high. DISCUSSION The results indicate that the rate of creaming decreases as the three-phase boundary is approached, while the rate of coalescence increases. Within the three-phase region, coalescence is so rapid that the droplets coalesce before creaming can occur. The decrease in the creaming rate that is observed can be attributed to changes in the physical properties of the microemulsion which is found to be the external phase. As the microemulsion approaches the Type III region, the density difference between
195
CREAMING AND COALESCENCE OF MACROEMULSIONS
the phases decreases, and the bulk viscosity of the continuous phase and the interfacial viscosity increase. Also, the initial size of the emulsion droplets probably decreases, since the interfacial tension of microemulsions is known to decrease as the three-phase region is approached (6). Each of these factors contributes to the decreasing rate of creaming that is observed. In the three-phase region, these same factors, except for interfacial viscosity, should continue to cause the rate of creaming to decrease, yet the aqueous phase begins to appear at an extraordinary rate. At the same time, the appearance of a separate oil phase indicates that rapid coalescence is occurring. Apparently, due to the high rate of coalescence, the solutions in the three-phase region, which should cream the most slowly, rapidly form large droplets which quickly separate. Because the continuous phase appears rapidly
at low salinity due to rapid creaming and in the three-phase region due to rapid coalescence, an apparent maximum in the initial emulsion stability can be observed at the phase boundaries. The rate of coalescence of emulsion droplets is according to classical theories affected by electrical repulsion, steric repulsion, and the physical properties of the interfacial film (10). For anionic surfactants, electrical repulsion is important in O/W emulsions, where the charged head groups are oriented toward the continuous phases. On the other hand, steric repulsion is most important in W/O emulsions, where the long hydrocarbon tail extends into the continuous phase. In the present study macroemulsion coalescence rates decrease markedly as the microemulsion composition is varied so as to move away from the phase boundaries. This is an unexpected result because the
Phase Boundary
,~Upper 80
=;
ID 70
_z
~E
~, 6o z
w
0 '~ 3O
o w
Oil : Nonone Water Oil Ratio: I : I
g
3 gpdl 5 gpd[ 3 gpdl
~ 20 e~
ka
EJ
IO
3.4
3.5 SODIUM
3.6
3.7
3.8
CHLORIDE CONCENTRATION
3.9
SDBS ICsOH 2BuOH
4.0
(GPDL)
FIG. 8. Time required for complete coalescence v e r s u s electrolyte concentration for W/O microemulsions. Journal of Colloid and Interface Science, Vol. 85, No. 1, J a n u a r y 1982
196
BALDAUF
interfacial activity of the surfactant is maximum in the three-phase region. Thus macroemulsion droplets exhibit enhanced stability as interfacial activity decreases. The mechanisms responsible for this trend are not understood, but it seems likely that the hydrodynamic behavior (i.e., surfactant mass transfer, etc.) is a critical factor. The system containing 1 gpdl 2 BuOH was considerably more stable than the system containing 3 gpdl in the regions of both W/O and O/W macroemulsions. It therefore seems unlikely that either electrical or steric factors played a major role in enhancing the stability since they would be expected to contribute in only one of the two regions. These observations would appear to support the contention that the origins of the enhanced stability may be nonequilibrium factors. Apparently, the interfacial viscosity does not directly play an important role in the stabilization of these emulsions. In the Type I region, as the salinity increased and passed through the LPB, the rate of emulsion breakdown increased continuously. The interfacial viscosity passed through a maximum at the LPB, but this did not result in any noticeable increase in emulsion stability at this point. Also, for both O/W and W/O emulsion systems, the interfacial viscosity of the system containing 1 gpdl 2 BuOH was approximately the same as that of 3-gpdl system, although the 1-gpdl system was much more stable. Although these results indicate that the interfacial viscosity does not contribute greatly to the stabilization of the emulsions, the fact that a maximum is observed at the phase boundaries is significant. At the phase boundaries, the coalescence rate changes drastically, so that coalescence occurs before creaming. The maximum in interfacial viscosity at this point indicates a change in the structure of the interface. The difference between Journal of Colloid and Interface Science, Vol. 85, No. 1, January 1982
ET AL.
the stability of the 1- and 3-gpdl 2 BuOH systems suggests that interactions between the surfactant and co-surfactant molecules at the interface are quite important in determining the stability of the emulsion droplets. The high rate of coalescence observed in the three-phase region may be related to the fact that the interfacial tensions of such systems is ultralow (10 -3 dyne/cm). Natarajan and Schechter (11) have proposed that this phenomenon is due to the interaction of the capillary waves which spontaneously appear owing to thermal fluctuations. These waves can easily attain 100 ~ heights. This proposal is, however, tentative and not firmly established. CONCLUSIONS
(1) The rate of creaming of macroemulsions decreases as the three-phase b o u n d aries are approached. Factors such as the density difference between phases, the continuous phase bulk viscosity, and the interfacial viscosity all act to decrease the rate of creaming. (2) The rate of coalescence increases as the three-phase region is approached, and it increases dramatically in the threephase region. Therefore, the amount of time required for complete coalescence is high far away from the phase boundaries and is extremely low within the three-phase region. (3) The maxima in initial emulsion stability at the upper and lower three-phase boundaries observed by Bourrel are caused by the competing rates of creaming and coalescence. Creaming occurs more rapidly away from the phase boundaries in the twophase regions while coalescence occurs more rapidly in the three-phase region. (4) The interfacial viscosity of microemulsion systems reaches a maximum at the phase boundaries. This indicates that some type of change in the surfactant structure at the interface is taking place. Surfactant-
CREAMING AND COALESCENCE OF MACROEMULSIONS
cosurfactant interactions play an important role in determining the stability of hydrocarbon/water/surfactant macroemulsions. ACKNOWLEDGMENT The authors wish to thank the National Science Foundation for support of this project under Grant CPE-7813315. Professor Schechter holds the Dula and Ernest Cockrell, Sr. Chair in Engineering. REFERENCES 1. Bourrel, M., Graciaa, A., Schechter, R. S., and Wade, W. H., J. Colloid Interface Sci. 72, 161 (1979). 2. Goodrich, F. C., Proc. R. Soc. London Ser. A. 310, 359 (1969).
197
3. Goodrich, F. C., and Allen, L. H., J. Colloid Interface Sci. 40, 329 (1972). 4. Goodrich, F. C., Allen, L. H., and Poskanzer, A., J. Colloid Interface Sci. 52, 201 (1975). 5. Wade, W. H., Morgan, J. C., Schechter, R. S., Jakobson, J. K., and Salager, J. L., Soc. Pet. Eng. J. 18, 242 (1978). 6. Salager, J. L., Morgan, J. C., Schechter, R. S., and Wade, W. H., Soc. Pet. Eng. J. 19, 107 (1979). 7. Boyd, J., Parkinson, C., and Sherman, P., J. Colloid Interface Sci. 41,359 (1972). 8. Wasan, D. T., Gupta, L., and Vora, M. K., AIChE J. 17, 1287 (1971). 9. Salter, S. J., SPE 6843, presented at 52nd Annual Fall Technical Conference, October 9-12, 1977. 10. Rosen, N. J., in "Surfactants and Interfacial Phenomena," Chap. 8. Wiley, New York, 1978. 11. Natarajan, R., and Schechter, R. S., manuscript in preparation.
Journal of Colloid and Interface Science, Vol. 85, No. 1, January 1982
ALAIN GRACIAA University o f Pau, Pau, France Received May 4, 1981; accepted August 3, 1981 DEDICATED TO THE MEMORY OF PROFESSOR FRANK CHAUNCEY GOODRICH Macroemulsions are inherently thermodynamically unstable systems which exhibit lifetimes which depend on the initial state of the system and the rates of creaming and coalescence. A previous study has shown an apparent relationship of emulsion stability to the thermodynamic (equilibrium) phase behavior of systems containing surfactant/hydrocarbon/water (M. Bourrel, A. Graciaa, R. S. Schechter, and W. H. Wade, J. Colloid Interface Sci. 72, 161 (1979)). It was found that macroemulsions are most stable at the boundaries which separate two phase systems from those which form three phases consisting of a microemulsion in equilibrium with excess hydrocarbon and water phases. The present investigation shows that on approaching the threephase boundaries those factors, such as the density difference between phases, the continuous phase bulk viscosity, and the surface viscosity, which tend to influence the rate of creaming, all act to decrease it. Hence at the phase boundary creaming is shown to be a slow step, thereby causing the emulsion to appear to be stable. It was also found that, despite the fact that the surface viscosity is maximum at the boundaries, coalescence becomes more rapid as the boundaries are approached. Thus, while the time scale for creaming increases, that for coalescence decreases. Overall, then, macroemulsions formed well away from the phase boundaries cream first, but persist longest. Within the three-phase region where the surfactant system is balanced (equal concentrations of surfactant in the excess phases) coalescence proceeds at a remarkable, but unexplained, speed. In this region macroemulsions coalesce before they cream and emulsion stability is least. These observations are shown to apply to systems in the presence of varying quantities of alcohol. The phase behavior was adjusted by varying the electrolyte concentrations.
INTRODUCTION
A relationship between microemulsion phase behavior and macroemulsion stability has been reported. The existence of a correlation was thought to be important for both conceptual and practical reasons. 1 To whom all correspondence should be addressed.
Since microemulsions are thermodynamically stable mixtures of oil, electrolyte, and surfactant, the phase behavior is easily and reproducibly observed. The general trends are predictable. Thus it was decided to investigate this relationship in some detail. In particular the role of interfacial viscosity has been considered. It seems appropriate, therefore, to include this paper
187
Journalof Colloidand Interface Science, Vol.85, No. 1, January 1982
0021-9797/82/010187 - 11$02.00/0 Copyright© 1982by AcademicPress,Inc. All rightsof reproductionin any formreserved.
188
BALDAUF
in a symposium honoring Professor Frank Goodrich since he has played such a prominent role in the development of the theory and measurement of interfacial viscosity (2-4). Microemulsions can be classified as Type I, II, or III depending on the observed phase behavior. A Type I system consists of an oil-in-water (O/W) microemulsion in equilibrium with an almost pure oil phase. A Type II system is a microemulsion (W/O) in equilibrium with an excess aqueous phase. Type III systems exhibit three phases including a middle, surfactantrich phase in equilibrium with both oil and water phases. It has been noted (5) that the minimum interfacial tension and maximum solubilization occur for optimum Type III systems. The particular Type III system for which equal volumes of oil and water are contained in the middle phase are designated optimum. The phase behavior of microemulsions is a function of the tendency of the surfactant to partition between the oil and aqueous phases (6). It is, therefore, an accurate measure of the surfactant HLB in the presence of oil, of electrolytes, and of any other additives such as alcohols. It has been shown that the surfactant tends to partition equally between excess phases at or near the optimum conditions. In this context the surfactant can be said to be balanced at optimum. It seems reasonable then to expect that a correlation might exist between microemulsion phase behavior and macroemulsion stability. Bourrel et al. (1) have reported that maxima in macroemulsion stability occur at the phase boundaries separating Types I and III and II and III systems. Also a deep minimum in macroemulsion stability was observed when the system is adjusted so that it is optimum. These findings obtained by observing the initial rate of macroemulsion phase separation are good agreement with Boyd et al. (7) who were the first to report that upon increasing the surfactant HLB, macroemulsion stability, as measured Journal of Colloid and Interface Science, Vol. 85, No. 1, January 1982
ET AL.
by observing the initial droplet coalescence rate, exhibits two maxima--one corresponding to O/W and the other to W/O macroemulsions. Since Bourrel et al. (1) found the macroemulsion at the I - I I I boundary to be O/W and a W/O macroemulsion at the I I - I I I boundary, it is believed that the two maxima found by Boyd corresponds to those reported by Bourrel. In the present study, factors believed to contribute to macroemulsion stability are examined in relationship to the phase behavior of the system. Based on this study it has been possible to ascertain that the boundaries between Type III and Types I and II systems are positions of minimum creaming rates, and, therefore, correspond to maximum initial emulsion stability. However, coalescence at these boundaries has been found to be faster than for Type I and Type II systems removed from the boundaries. Thus the time required for complete macroemulsion coalescence is not in general a maximum at the phase boundaries. EXPERIMENTAL
The system under investigation in this study contained as surfactant Siponate DS-10, a blend of isomers of sodium dodecyl benzene sulfonate (SDBS). Secondary butanol (2 BuOH) and isopentanol (IC5OH) were used as cosurfactants. The concentrations of SDBS and IC~OH were maintained at 3 gpdl (grams per deciliter), while systems were studied which contained either 1 or 3 gpdl 2 BuOH. The concentration of sodium chloride was varied from 1 to 4 gpdl to produce the desired phase behavior. The hydrocarbon was normal nonane and a 1:I volume ratio of oil to aqueous phase was maintained. Interracial viscosity was measured using a deep-channel surface viscometer. The analysis of Wasan et al. (8) was used to calculate the interfacial and surface viscosities from the time periods of small
CREAMING AND COALESCENCE OF MACROEMULSIONS
189
5O
Lower Phase Boundary I-[ Lower Phase Boundary
46
.J 44 0 w ta 4 2 -
" J ~ 4o
L!
1: Nonane
"" "~ x
Water Oil Ratio: I : l
I1 3 38
1I I
36 LO
gpdl 5 gpd~
~,..
SDBS ICsOH
"~ ~-~.
----A---
I
gpdl
28uOH
----D--
5
gpdl
7' BuOH
I I.I
I 1.2
I 1.3 SODIUM
I 1.4
I 1.5
I 1.6
CHLORIDE CONCENTRATION
l 1.7
I 1.8
L 1.9
Z.O
(GPDL)
F1o. 1. Volumepercent of excess oil phase versus electrolyteconcentrationfor O/W microemulsions. Teflon particles floating on the gas-liquid and liquid-liquid interfaces. The bulk viscosities of the hydrocarbon and aqueous phases were measured by a Contraves low-shear 30 viscometer, which is a Couette-type rotational viscometer with variable shear rate capability. A Mettler/Parr DMA-46 digital density meter was used to measure the phase densities. All measurements were made at a temperature of 25°C. Microemulsion phase behavior was determined by preparing solutions of 5 ml aqueous phase and 5 ml hydrocarbon phase in sealed 10-ml pipettes. The pipettes were shaken and allowed to equilibrate several times until the volumes of the phases remained constant. The phase volumes of the aqueous and hydrocarbon phases were then recorded. The relative emulsion stabilities were determined by shaking the tubes uniformly and allowing the phases to separate. The time required for 20% of
the continuous phase to regenerate was recorded as an indication of the rate of creaming. The time required for total phase separation to occur was recorded as an indication of the rate of coalescence. RESULTS
Type I (O/W) Microemulsions At low electrolyte concentrations, the systems have been selected to that Type I phase behavior is observed. As the concentration is increased, at a certain electrolyte concentration, a Type III system is first observed. This is called the lower threephase boundary (LPB). The LPB for the nonane SDBS system was found at 1.85 gpdl NaC1 in the presence of 3 gpdl 2 BuOH and at 1.75 gpdl NaCl ff 2 BuOH is reduced to 1 gpdl. As the system approached the LPB and more oil was solubilized, the volume of the excess oil phase (Fig. 1) decreased. Also, Journal of Colloid and Interface Science, Vol. 85, No. 1, January 1982
190
BALDAUF ET AL. 0.7
Lower Phase Boundary [] Oil: Nonone Water Oir Ratio; I : l :3 gpdl SDBS
0.6
0.5
----m---- 3 gpdl
-
-
-
-
Lower Phase Boundary z~
~. ~'-I
j ,~/L
I
2BuOH
//[~'\
-
/
L \\\^~
A
0.4
o L} > 0.3
~: LLI 0.2 F Z_
0.1
GO
1.0
I t.I
I
t
I
I
I
1
I
I
LB
1.3
L4
1.5
L6
1.7
I.e
t.9
SODIUM CHLORIDE CONCENTRATION
2.0
(GPDL)
FIG. 2. Interfacial viscosity versus electrolyte concentration for O/W rnicroemulsions.
70
Lower Phase Boundary
60
u~
50
Oil : Nonane Water Oil Ratio; I :1 3 gpdl SDBS 3 gpdl ICsOH ---O---- I gpdl 2 BuOH ---A---- 3 gpdl 2 BuOH
/
i'=
Lower Phase Boundary ~---.~// ~ / rI " "1' ~ ~ // // / ~ / /
$
]I ii t
/
\
/ //
\\\
/ ~50
/
--
&
\\
/A" /
\
/
\\
I I I I I
mL tO
I ~1
I 1.2
I 1.3
I 1.4
I 1.5
I . 1.6
SODIUM CHLORIDE CONCENTRATION F I G . 3. E m u l s i o n
stability versus electrolyte concentration
Journal of Colloid and Interface Science, Vol. 85, No. 1, January 1982
I 1.7
~
[ 18
(GPDL) for O/W microemulsions.
r 19
20
CREAMINGAND COALESCENCEOF MACROEMULSIONS
191
420. Lower Phase Boundary 360. I.=E z
300.
0 240,
oz
180.
n,.
120.
OiL : Nonane Water Oil Ratio: L:I 3 gpdl
_~ 60. I-
O, 1.0
SDBS
3 gpdl
ICsOH
3 gpdl
2BuOH
I I.I
I 1.2
I 1.5
I 1.4
SODIUM CHLORIDE
I 1.5
I 1.6
CONCENTRATION
[ 1.7
I 1.8
[
I 1.9
] 2.0
(GPDL)
FIG. 4. Time required for complete coalescence versus electrolyte concentration for O/W microemulsions. the density of the aqueous phase decreased while the bulk viscosity of the aqueous phase increased. The density and bulk viscosity of the oil phase remained constant. To reduce the number of graphs, these data are not shown. The density of the microemulsion phase can, however, be accurately calculated based on the aqueous and oil phase densities and the volume of oil solubilized. The viscosity of the microemulsion phase increases substantially as the LP boundary is approached. This increase can be as much as a factor of 3 depending on the amount of oil solubilized. It should be noted that the system containing 1 gpdl 2 BuOH solubilized more than did the system containing 3 gpdl 2 BuOH, and therefore its aqueous phase possessed a lower density and higher bulk viscosity near the LPB. Smaller solubilizations with increased alcohol concentrations have been reported previously (9). The interfacial viscosity (Fig. 2) of both
the 1- and 3-gpdl 2 BuOH systems reached a maximum value at the LPB. When the surface and interracial viscosities were calculated simultaneously based upon the time periods of particles on the surface and interface, negative values were obtained for the surface viscosity. This was probably caused by the curvature of the gas-liquid interface. A surface viscosity of zero was therefore assumed, and the interfacial viscosity was calculated based upon the time periods of the particles at the liquid-liquid interface. At low salinity, the value of interfacial viscosity was about 0.20 sP (surface poise = 1 g/sec). The interfacial viscosity increased to 0.50 sP for the 1-gpdl 2 BuOH system and 0.59 sP for the 3-gpdl system at the LPB. It then dropped off sharply within the three-phase region. It should be noted, however, that within the three-phase region, the solutions would initially break into a microemulsion and an excess oil Journal of Colloid and Interface Science, Vol. 85, No. 1, January 1982
192
B A L D A U F E T AL. I I
I 44.0
Upper Phase BoundaryE}--,.~
I
~
Phase Boundary,',
,,~-Upper
43.5
43.o s
o ~42.5 w
J
ttl 4 2 . 0 /
/ f
/
I
/ t 1 &
/
bJ =E 41.5 /
0
/
Water Oil Ratio: Oil : Nonane 5 gpdl SDBS 3 gpdl ICeOH
/ /
41.0
---A----[3---
40.5
I gpdl 3 gpdl
I:1 2 BuOH
2 BuOH
I I 400 2.8
I 2.9
I 3.0
I 3,1
I :5.2 SODIUM
I 3.3
CHLORIDE
I 5.5
I 3.6
CONCENTRATION
(GPDL)
3.4
I 3.7
I 3.8
I 5.9
4.0
FIG. 5. Volume percent excess aqueous phase versus electrolyte concentration for W/O microemulsions.
phase. Only after a period of several days would a middle phase and an excess aqueous phase appear. This allowed the interfacial viscosity to be measured within the threephase region by the technique normally used for two-phase systems. Within the two-phase region, the first sign of emulsion breakdown to be observed was the creaming of the oil phase macroemulsion droplets to the top of the continuous fluid. Figure 3 shows the time required for approximately 20% of the continuous phase to regenerate. For both the I- and 3-gpdl 2 BuOH systems, the time required for the continuous phase to appear reached a maximum at the LPB. Also, the system containing 1 gpdl 2 BuOH was considerably more stable than the 3-gpdl system. Within the three-phase region, a layer of oil appeared at the top of the macroemulsion simultaneously with the appearance of the aqueous phase. This indicates that coalescence takes place in the Journal of Colloid and Interface Science, Vol. 85, No. l, January 1982
three-phase region, while creaming is dominant in the two-phase region. Because of this high rate of coalescence, large droplets quickly form and rise to the top of the continuous phase. The rate of appearance of the aqueous phase within the three-phase region was quite rapid. The time required for complet e coalescence (Fig. 4) was found to increase as the system moved away from the three-phase region. For the 3-gpdl 2 BuOH system, the solutions at low salinity which creamed most rapidly formed emulsions which persisted the longest--in excess of 24 hr. The solutions near the LPB which creamed very slowly separated in less than an hour, while those in the three-phase region broke in a few minutes. The solutions containing 1 gpdl 2 BuOH were very stable and formed emulsions which lasted for over 24 hr. Due to the long coalescence times involved, it was not possible to obtain reproducible results for this system. S011, it was evident
193
C R E A M I N G A N D C O A L E S C E N C E OF M A C R O E M U L S I O N S
that the emulsions broke most quickly in the three-phase region and most slowly at low salinity.
Type H (W/O) Microemulsions For Type II systems decreasing the system salinity results in more aqueous phase being solubilized until the upper phase boundary (UPB) is reached. When this occurs, an excess oil phase appears and the microemulsions will exist as the middle phase. For the nonane/water/SDBS system which contained 1 gpdl 2 BuOH, the UPB was found to occur at a salinity of 2.8 gpdl, while for the 3-gpdl 2 BuOH system, the UPB occurred at a salinity of 3.4 gpdl. As the system salinity decreased, the UPB was approached, and the volume of the excess aqueous phase (Fig. 5) decreased. The density and bulk viscosity of the oil phase increased as the amount of
Upper
r 0.8
.~v.~---upper
Phase
l ~J ~ 0.6
water solubilized into the oil phase increased. The aqueous phase bulk viscosity and density remained constant. Once again, the system containing 1 gpdl 2 BuOH solubilized more excess phase than did the 3-gpdl system, and therefore had a higher oil phase density and bulk viscosity near the UPB. The interfacial viscosity (Fig. 6) increased as the UPB was approached. At salinities far from the UPB, both the 1- and 3-gpdl systems had an interfacial viscosity of about 0.24 sP. Near the UPB, the 1-gpdl system reached a value of 0.52 sP while the 3-gpdl system reached a value of 0.38 sP. Unlike the O/W microemulsion systems, it was not possible to measure the interfacial viscosity within the three-phase region, because the solutions would separate very rapidly into three phases. It was therefore not possible to determine whether or not the interfacial viscosity decreased within the three-phase region.
Phase BoundaryB--...~ Oil: Nonone Water Oil Ratio: I:1
BoundoryA
75 gpdl SDBS 3 gpdI 1%OH - - - A - - - I gpdl - - - - [ 3 - - 5 gpdl
I I I I I
0.5
2BuOH 2 BuOH
\ \
8
\
~ 0.4
\ \ \
~
\ \\
0.3
A z -
0.2
O.I
QO - - M L 2.8
2.9
J
t
J
3.0
3.1
3.2
~ 3.3
~ _ _ J _ . . . _ . . . . . . . J ~ 3.4.
3.5
3,6
SODIUM CHLORIDE CONCENTRATION
(GPDL)
3.7
3,8
3.9
4.0
FIG. 6. Interfacial viscosity versus electrolyte concentration for W/O microemulsions. Journal of Colloid and Interface Science, Vol. 85, No. 1, January 1982
194
BALDAUF ET AL. I I ]
80
Upper I:~ose Boundory[]----.w~ ,.J~Upper
Oil: Nonane Water Oil Ratio: I : I 3 gpdl SDBS 5 gpdl ICsOH ---&-I gpdl 2 BuOH ----El---- 3 gpdl 2 BuOH
Phose Boundory~
70
~. 60 z v 50
40 _~ 30 w 2O
\
\
I0
Z
2.8
2.9
.5.0
&l
3.2
3,3
&4
3.5
5.6
57
3.8
3,9
4.0
SODIUM CHLORIDE CONCENTRATION (GPDL)
FIG. 7. Emulsion stabilityversus electrolyteconcentrationfor W/O microemulsions. The time required for 20% of the continuous phase to regenerate for Type II systems is shown in Fig. 7. The system containing 1 gpdl 2 BuOH behaved in the same way as did the O/W systems. Far from the UPB, the rate of creaming was rapid, while near the UPB, the rate of creaming was slow. Within the three-phase region, coalescence occurred simultaneously with creaming, so the rate of appearance of the continuous phase was rapid. The behavior of the 3-gpdl 2 BuOH system was somewhat different. Because the rate of coalescence even within the two-phase region was high, creaming always occurred simultaneously with coalescence. Therefore, the rate of appearance of the oil phase was lowest far from the UPB, and the rate increased as the system approached and passed through the UPB. The time required for complete coalescence (Fig. 8) shows that coalescence was extremely rapid for the 3-gpdl 2 BuOH Journal of Colloid and Interface Science, Vol. 85, No. I, January 1982
system. The rate of coalescence was highest at the three-phase boundary and was lowest at high salinity. Once again, the 1-gpdl 2 BuOH solutions required over 24 hr to coalesce, and the rate of coalescence was lowest far from the phase boundary where the rate of creaming was high. DISCUSSION The results indicate that the rate of creaming decreases as the three-phase boundary is approached, while the rate of coalescence increases. Within the three-phase region, coalescence is so rapid that the droplets coalesce before creaming can occur. The decrease in the creaming rate that is observed can be attributed to changes in the physical properties of the microemulsion which is found to be the external phase. As the microemulsion approaches the Type III region, the density difference between
195
CREAMING AND COALESCENCE OF MACROEMULSIONS
the phases decreases, and the bulk viscosity of the continuous phase and the interfacial viscosity increase. Also, the initial size of the emulsion droplets probably decreases, since the interfacial tension of microemulsions is known to decrease as the three-phase region is approached (6). Each of these factors contributes to the decreasing rate of creaming that is observed. In the three-phase region, these same factors, except for interfacial viscosity, should continue to cause the rate of creaming to decrease, yet the aqueous phase begins to appear at an extraordinary rate. At the same time, the appearance of a separate oil phase indicates that rapid coalescence is occurring. Apparently, due to the high rate of coalescence, the solutions in the three-phase region, which should cream the most slowly, rapidly form large droplets which quickly separate. Because the continuous phase appears rapidly
at low salinity due to rapid creaming and in the three-phase region due to rapid coalescence, an apparent maximum in the initial emulsion stability can be observed at the phase boundaries. The rate of coalescence of emulsion droplets is according to classical theories affected by electrical repulsion, steric repulsion, and the physical properties of the interfacial film (10). For anionic surfactants, electrical repulsion is important in O/W emulsions, where the charged head groups are oriented toward the continuous phases. On the other hand, steric repulsion is most important in W/O emulsions, where the long hydrocarbon tail extends into the continuous phase. In the present study macroemulsion coalescence rates decrease markedly as the microemulsion composition is varied so as to move away from the phase boundaries. This is an unexpected result because the
Phase Boundary
,~Upper 80
=;
ID 70
_z
~E
~, 6o z
w
0 '~ 3O
o w
Oil : Nonone Water Oil Ratio: I : I
g
3 gpdl 5 gpd[ 3 gpdl
~ 20 e~
ka
EJ
IO
3.4
3.5 SODIUM
3.6
3.7
3.8
CHLORIDE CONCENTRATION
3.9
SDBS ICsOH 2BuOH
4.0
(GPDL)
FIG. 8. Time required for complete coalescence v e r s u s electrolyte concentration for W/O microemulsions. Journal of Colloid and Interface Science, Vol. 85, No. 1, J a n u a r y 1982
196
BALDAUF
interfacial activity of the surfactant is maximum in the three-phase region. Thus macroemulsion droplets exhibit enhanced stability as interfacial activity decreases. The mechanisms responsible for this trend are not understood, but it seems likely that the hydrodynamic behavior (i.e., surfactant mass transfer, etc.) is a critical factor. The system containing 1 gpdl 2 BuOH was considerably more stable than the system containing 3 gpdl in the regions of both W/O and O/W macroemulsions. It therefore seems unlikely that either electrical or steric factors played a major role in enhancing the stability since they would be expected to contribute in only one of the two regions. These observations would appear to support the contention that the origins of the enhanced stability may be nonequilibrium factors. Apparently, the interfacial viscosity does not directly play an important role in the stabilization of these emulsions. In the Type I region, as the salinity increased and passed through the LPB, the rate of emulsion breakdown increased continuously. The interfacial viscosity passed through a maximum at the LPB, but this did not result in any noticeable increase in emulsion stability at this point. Also, for both O/W and W/O emulsion systems, the interfacial viscosity of the system containing 1 gpdl 2 BuOH was approximately the same as that of 3-gpdl system, although the 1-gpdl system was much more stable. Although these results indicate that the interfacial viscosity does not contribute greatly to the stabilization of the emulsions, the fact that a maximum is observed at the phase boundaries is significant. At the phase boundaries, the coalescence rate changes drastically, so that coalescence occurs before creaming. The maximum in interfacial viscosity at this point indicates a change in the structure of the interface. The difference between Journal of Colloid and Interface Science, Vol. 85, No. 1, January 1982
ET AL.
the stability of the 1- and 3-gpdl 2 BuOH systems suggests that interactions between the surfactant and co-surfactant molecules at the interface are quite important in determining the stability of the emulsion droplets. The high rate of coalescence observed in the three-phase region may be related to the fact that the interfacial tensions of such systems is ultralow (10 -3 dyne/cm). Natarajan and Schechter (11) have proposed that this phenomenon is due to the interaction of the capillary waves which spontaneously appear owing to thermal fluctuations. These waves can easily attain 100 ~ heights. This proposal is, however, tentative and not firmly established. CONCLUSIONS
(1) The rate of creaming of macroemulsions decreases as the three-phase b o u n d aries are approached. Factors such as the density difference between phases, the continuous phase bulk viscosity, and the interfacial viscosity all act to decrease the rate of creaming. (2) The rate of coalescence increases as the three-phase region is approached, and it increases dramatically in the threephase region. Therefore, the amount of time required for complete coalescence is high far away from the phase boundaries and is extremely low within the three-phase region. (3) The maxima in initial emulsion stability at the upper and lower three-phase boundaries observed by Bourrel are caused by the competing rates of creaming and coalescence. Creaming occurs more rapidly away from the phase boundaries in the twophase regions while coalescence occurs more rapidly in the three-phase region. (4) The interfacial viscosity of microemulsion systems reaches a maximum at the phase boundaries. This indicates that some type of change in the surfactant structure at the interface is taking place. Surfactant-
CREAMING AND COALESCENCE OF MACROEMULSIONS
cosurfactant interactions play an important role in determining the stability of hydrocarbon/water/surfactant macroemulsions. ACKNOWLEDGMENT The authors wish to thank the National Science Foundation for support of this project under Grant CPE-7813315. Professor Schechter holds the Dula and Ernest Cockrell, Sr. Chair in Engineering. REFERENCES 1. Bourrel, M., Graciaa, A., Schechter, R. S., and Wade, W. H., J. Colloid Interface Sci. 72, 161 (1979). 2. Goodrich, F. C., Proc. R. Soc. London Ser. A. 310, 359 (1969).
197
3. Goodrich, F. C., and Allen, L. H., J. Colloid Interface Sci. 40, 329 (1972). 4. Goodrich, F. C., Allen, L. H., and Poskanzer, A., J. Colloid Interface Sci. 52, 201 (1975). 5. Wade, W. H., Morgan, J. C., Schechter, R. S., Jakobson, J. K., and Salager, J. L., Soc. Pet. Eng. J. 18, 242 (1978). 6. Salager, J. L., Morgan, J. C., Schechter, R. S., and Wade, W. H., Soc. Pet. Eng. J. 19, 107 (1979). 7. Boyd, J., Parkinson, C., and Sherman, P., J. Colloid Interface Sci. 41,359 (1972). 8. Wasan, D. T., Gupta, L., and Vora, M. K., AIChE J. 17, 1287 (1971). 9. Salter, S. J., SPE 6843, presented at 52nd Annual Fall Technical Conference, October 9-12, 1977. 10. Rosen, N. J., in "Surfactants and Interfacial Phenomena," Chap. 8. Wiley, New York, 1978. 11. Natarajan, R., and Schechter, R. S., manuscript in preparation.
Journal of Colloid and Interface Science, Vol. 85, No. 1, January 1982