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O emulsions
Development of W/O/W-Type Dispersion during Phase Inversion of Concentrated W/O Emulsions SACHIO Department
of Agricultural
MATSUMOTO
Chemistry, College of Agriculture, Sakai-shi, Osaka 591, Japan
University
of Osaka
Prefecture,
Received November 1, 1982; accepted December 17, 1982 It has been found that the development of a W/O/W-type dispersion precedes phase inversion of concentrated W/O emulsions in the presence of hydrophilic emulsifier in the aqueous phase. This phenomenon was examined with a mixture of liquid paraffin and water using Span 80 in the oil phase and SDS, CTABr, or Tween 80 in the aqueous phase. The W/O/W emulsion formation was assessed by comparing the extent of the oil layer in a unit volume of the samples with each other, which was estimated from the viscosity changes in the diluted sample under the concentration gradients of glucose between the inner aqueous phase and the aqueous suspending fluid. The development of a W/O/W-type dispersion could be observed when the volume fraction of the aqueous phase was higher than 0.7, while another factor affecting the W/O/W emulsion formation was the molar ratio of the Span 80 to the hydrophilic emulsifier in the sample. In the case of SDS system, the range of the W/O/W emulsion formation appeared more widely in the molar ratio (i.e., from 4 to over 60) than that of CTABr or Tween 80 systems, so that the use of SDS facilitated the development of a W/O/W-type dispersion. The oil layer in the W/O/W emulsions provided by the phase inversion phenomenon was stable for at least a month when stored. INTRODUCTION
Some experimental studies (1-S) suggested that the two stage procedure of emulsification is reliable when preparing water-in-oilin-water-type multiphase emulsions (denoted as W/O/W emulsions herein). The first stage of emulsification is made for providing an ordinary W/O emulsion by successive addition of water into an oil phase containing hydrophobic emulsifier. The W/O emulsion is then mixed with an aqueous solution of hydrophilic emulsifier as the second stage procedure so as to obtain a W/O/W-type dispersion. Sherman and his co-workers (9, 10) emphasized that the development of a W/O[W emulsion structure precedes the thermal induced phase inversion of concentrated O/W emulsions. This may suggest that the state of multiphase emulsions can be generalized as one of the mesophases between O/W and
W/O emulsions, and also that there is a possibility of greater simplification of the technique for preparing W/O/W emulsions than the two stage procedure. More recently, the author has found that if the first stage procedure described above is carried out by introducing an aqueous solution of hydrophilic emulsifier into liquid paraffin containing sorbitan monooleate (Span SO),the W/O/W emulsion does appear in course of such the first-step process of the two stage procedure due to the phase inversion of W/O emulsions. This phenomenon has stimulated the development of a simplified method in preparing W/O/W emulsions together with some further insights into how W/O/W emulsions are formed. Therefore, the author has examined the reproducibility of the above phenomenon using the aqueous solutions of sodium dodecyl sulfate (SDS), cetyl trimethyl ammoniumbromide (CTABr), and polyoxyethylene sorbitan monooleate
362 0021-9797/83
$3.00
Copyright 0 1983 by Academic Press, Inc. All rights of reproduction in any form reserved.
Journa/ of Cobid
and Interface Science, Vol. 94, No. 2, August 1983
DEVELOPMENT
OF
W/O/W-TYPE
(Tween 80) at a variety of concentrations. The results obtained indicate the possibility of applying the phase inversion phenomenon in preparing W/O/W emulsions possessing the same functional properties as those provided by the two stage procedure of emulsification. This paper deals with some factors affecting the development of a W/O/W-type dispersion during the phase inversion of W/O emulsions. The W/O/W emulsion formation has been assessed by the extent of oil layer on the surface of the dispersed aqueous compartments using a viscometric method developed in the author’s laboratory (1 l), i.e., the viscosity changes in the sample have been measured under the glucose concentration gradients between the inner aqueous phase and the aqueous suspending fluid in order to estimate the area of the oil layer separating the two aqueous phases.
DISPERSION
363
and Span 80 used were of commercially available form delivered by Kao-Atlas Co., Tokyo. b. Procedure for inducing phase inversion of W/O emulsions. An assembly for examining reproducibility of the phase inversion phenomenon was composed of a pin-mixer and a peristaltic pump. It was essentially identical to that used for providing W/O emulsions in the first stage procedure of the W/O/W emulsion preparation (4). A welldefined volume of a mixture of liquid paraffin and Span 80 was introduced into a vessel of the pin-mixer, and an aqueous mixed solution of glucose and hydrophilic emulsifier was then added successively to the oil phase in the vessel at a rate of 5 ml/min by use of a vinyl pipe connected to the peristaltic pump. The pin-mixer was rotated steadily at 88 rpm at room temperature providing a constant shear force to the mixing system. The volume fraction of the aqueous phase introduced ranged from 0.50 to 0.86 at the EXPERIMENTAL end of the procedure. The pin-mixer used was diverted from a a. Sample composition. A series of the part of a Mixograph, which is an assembly samples to be tested consisted of an aqueous for the testing of dough consistency, made solution of 0.05 M glucose and liquid parby the National Mfg. Ltd., Lincoln, Neaffin in all instances in this study. The glucose braska. This mixer consisted of two units: an was a necessary component for providing an aluminium vessel with 4.2 cm depth and 7.5 osmotic pressure gradient between the inner aqueous phase and the aqueous suspending cm inner diameter, and a stirrer unit ccnfluid in the sample. sisting of a four-pins rotor. The length and The type of the samples could be divided diameter of each metal pin were 3.9 and 0.26 into three categories according to the kind cm, respectively. The vessel also possessed of hydrophilic emulsifiers in the aqueous three metal pins fixed vertically from the botphase such as anionic (SDS) type, cationic tom of the vessel, while the rotor was driven (CTABr) type, and nonionic (Tween 80) steadily at rotational speed of 88 rpm by type, respectively. One of these emulsifiers means of a speed reducer motor. When the was dissolved previously in the aqueous stirrer unit was immersed in the vessel and phase with a variety of the concentrations made to revolve in the oil phase, the latter from 0.004 to 0.05 A& while liquid paraffin circulated steadily through the gap between as the oil phase of the samples usually con- the four-pins rotor and the fixed three pins, tained Span 80 with the concentrations rang- thus providing a constant shear force to the ing from 0.23 to 0.93 M. system. Glucose, SDS, and CTABr were purchased c. Assessment of the W/O/W emulsion from Wako Pure Chemicals Co., Osaka, as formation. A series of the samples prepared an analytical grade, and were used in that were more or less the mixture of W/O/W and form without further treatment. Tween 80 O/W emulsions, although the ratio for the
364
SACHIO
MATSUMOTO
' 5pm ‘Of(
Maid
and Inlerface Science, Vol. 94, No. 2, August 1983
'
DEVELOPMENT
OF W/O/W-TYPE
W/O/W emulsion to the O/W one in each sample was influenced by the sample composition, as will be described. One could distinguish the W/O/W emulsion globules from the ordinary oil droplets using a phase contrast microscope, as shown in Fig. 1. However, the visual assessment could not lead to a quantitative estimation of the W/O/W emulsion formation. Therefore, the viscometric method (11) was employed to assessthe degree of the formation from the viewpoint of the total extent of the oil layer separating the inner aqueous phase from the aqueous suspending fluid in unit volume of the sample. First, the sample was diluted with pure water for providing a definite volume fraction of oil phase (denoted as &), i.e., &, was 0.1 for all cases in this study. The viscosity change in the diluted sample was then followed for 15 min. This commenced immediately after the dilution at a fixed shear rate of 38.3 set-’ at 25°C by means of a coneand-plate viscometer, made by Tokyo Keiki Ltd., Tokyo ( 12). The dilution brought a concentration gradient of glucose between the two aqueous phases, as pure water only distributed over the aqueous suspending fluid. Thus, the water started to migrate to the inner aqueous phase from t.he suspending fluid due to the osmotic pressure gradient. This resulted in an increase of the volume fraction of the inner aqueous phase (denoted as &,), so that the viscosity of the diluted sample also increased with increasing time after the dilution (see Fig. 2A). As reported previously (12, 13), the viscosity of W/O/W emulsions can be expressed by Mooney’s type equation ( 14), as follows:
ln vrd= a(& + Ad/[ 1 - X(4,+ ddl
[ 11
where qrel is the relative viscosity of the sam-
.
365
DISPERSION
I
I 0
2
4
6
8
2
4
6
8
10
12
CB)
3 22
1 0
Time after
the dilution
10
12
in minutes
FIG. 2. Changes in viscosity of W/O/W emulsions at 38.3 set-’ for shear rate (A), and in volume fraction of inner aqueous phases calculated from Eq. [ 1] (B), caused by the dilution of emulsions with pure water (& = 0.1 in all cases).Curve 1: 0.23 M Span 80 in liquid paraffin/ 0.01 M SDS in aqueous phase. Curve 2: 0.47 M Span 80 in liquid paraffin/O.01 M SDS in aqueous phase. Curve 3: 0.7 M Span 80 in liquid paraffin/O.01 M SDS in aqueous phase.
ple, and a and h are the constants. The values of these constants were already examined experimentally with the various W/O/W emulsions as 2.5 for a and 1.5 for h (12, 13). It is, therefore, possible to calculate the volume fraction I& from the viscosity of W/O/W emulsions using Eq. [ 11. Figure 2B shows the plots of the volume fraction against time after the dilution calculated from the viscosity change in the diluted samples, which is shown in Fig. 2A. The increasing rate of the volume fraction d&,/dt obviously corresponds to the volume flux of water permeating through the oil layer in a unit vol-
FIG. 1. Photomicrographs of dispersed globules in O/W emulsion (A) and W/O/W emulsions (B, C): (A) 0.7 MSpan 80 in liquid paraffin/O.01 MTween 80 in aqueous phase; volume fraction of total aqueous phase was 0.86; (B) 0.47 M Span 80 in liquid paraffin/O.02 M CTABr in aqueous phase; volume fraction of total aqueous phase was 0.75; (C) 0.7 M Span 80 in liquid paraffin/O.01 M SDS in aqueous phase; volume fraction of total aqueous phase was 0.75.
366
SACHIO
MATSUMOTO
ume of the diluted sample. This is also deeply related to the existence of the aqueous compartments surrounded by the oil layer in a unit volume of the diluted sample. According to the membrane studies (e.g., 15) the volume flux of water permeating through the oil layer can be expressed by &wldt
= L,~RT(m
- cm)
PI
where L, is the hydrodynamic coefficient of the oil layer, 2 is the total extent of the oil layer separating the two aqueous phases in a unit volume of the diluted sample, R is the gas constant, T is the absolute temperature, cl and c2 are the glucose concentrations of the suspending fluid and the inner aqueous phase, and gl and g2 are the osmotic pressure coefficients of glucose in the two aqueous phases. The mole flux of water is also given by h%.,ldtY p = L,~Wm =
RJkzc2
- ml)/ -
p
The mixing system was initiated by forming an ordinary W/O emulsion for all test systems evaluated. When the volume fraction of an aqueous solution of hydrophilic emulsifier (denoted as 4,) added successively to the mixing system was over 0.7, the continuous phase of the system was substituted by the aqueous phase, as shown in Fig. 3. This induced the development of a W/O/W-type dispersion in the system. It then followed that the maximal extent of the oil layer in the newly developed W/O/W emulsion could be observed at around 0.75 for &, (see Fig. 3), because an increasing amount of the aqueous suspending fluid containing hydrophilic emulsifier perhaps prompted the dispersement of the aggregates of the aqueous compartments. The apex of the plot in Fig. 3C when using Tween 80 as the hydrophilic emulsifier is much broader than that in Figs. 3A and B when using SDS or CTABr, respectively.
[31
WJ
where vis the partial molar volume of water, and P,, is defined as the water permeation coefficient of the oil layer. The value of PO was also examined experimentally in the previous work (16), and was obtained as about 5 X 10e4 cm/set for a series of the W/O/W emulsions. Therefore, if the volume flux of water d&/dt can be evaluated by the graphical differentiation on the volume fraction 4, vs time curve in an initial period of time after the dilution, one can estimate the total extent of the oil layer in an unit volume of the diluted sample from the relation of Eq. [3]. Thus, the formation of a W/O/W-type dispersion attributed to the phase inversion phenomenon can be assessed by the degree of the extent of the oil layer in each sample.
2 0 o 0.2 E f
volm
RESULTS
AND DISCUSSION
The phase inversion of emulsions generally occurs when the dispersed globules are packed very closely in the suspending fluid (17). It seems that the same thing happened during the mixing procedure in this study. Journal oJ’CoNoid and Interface Science, Vol. 94, No. 2, Augut
1983
0.70
0.75
0.80
0.85
0.90
(Cl
fraction
of (~amus solution of hydrmhilic emulsifier,
I
+Oaq
FIG. 3. Development of W/O/W-type dispersion as a function of volume fraction of aqueous phase (&J: (A) 0.7 M Span 80 in liquid paraffin/O.01 M SDS in aqueous phase: (B) 0.47 M Span 80 in liquid paraffin/ 0.03 M CTABr in aqueous phase; (C) 0.7 M Span 80 in liquid paraffin/O.0 1 M Tween 80 in aqueous phase.
DEVELOPMENT
OF W/O/W-TYPE
This may be brought about by the deficient role of Tween 80 in the dispersement of the aggregates. Additional increments of the aqueous phase, however, caused the rupture of the oil layer due to the solubilization of the oil layer components in the micelles of the hydrophilic emulsifier in the aqueous suspending fluid, so that the extent of the oil layer decreased with increasing amounts of the aqueous phase. It was confirmed experimentally that the Tween 80 system inverses completely to an ordinary O/W emulsion when the value for &, is higher than 0.85. Figure IA is an example of this case. Therefore, the development of a W/O/W-type dispersion in the course of the emulsification procedure may be characterized as a mesophase between the W/O and the O/W-type dispersions during the phase inversion process. It should be noted that the necessary conditions for developing W/O/W emulsion in the phase inversion process are not only the closely packed state of the dispersed globules in the W/O emulsion but the presence of a certain amount of hydrophilic emulsifier in the aqueous phase. Figure 4 shows the effect of the molar ratio of Span 80 to hydrophilic emulsifier on the formation of W/O/W emulsions, when the volume fraction &q is fixed at 0.75. The results obtained indicate that the range of the W/O/W emulsion formation as a function of the molar ratio is influenced by the kind of hydrophilic emulsifiers used. It is clear that the use of SDS as a hydrophilic emulsifier facilitates in a relative manner the development of a W/O/W-type dispersion, while the formation range of the W/O/W emulsions for CTABr or Tween 80 systems scatters in a comparative narrow region of the molar ratio. In any case, the left-hand side of the W/O/W emulsion range in Fig. 4 is occupied by the O/W-type dispersion, and the righthand side yields the W/O-type dispersion. Thus, the W/O/W emulsion may be stated as one of the emulsion phases in the phase diagram of the multicomponents of oil, water, and emulsifiers.
361
DISPERSION
20
30
40
50
60
(B1 CTABr SYStem
0 i.,
10
Molar ratlo
20
3
40
of Span 80 to hYdrmhilic
50
60
emulsifier
FIG. 4. Correlation between the W/O/W emulsion formation and the molar ratio of Span 80 to hydrophilic emulsifier in the whole system (6, = 0.75 in all cases).
An attempt was made to measure the changes in the extent of the oil layer in W/O/W emulsions developed as a function of storage. Each sample was aged for 30 days in an air-sealed glass flask at 25 “C in all cases, while a small part of the sample was diluted periodically with pure water so as to measure the viscosity change in the diluted sample described previously. Within the experimental error, the extent of the oil layer for SDS and CTABr systems calculated from the increasing rate of the viscosity in each period was not influenced by the period of incubation time (see Figs. 5A and B). Accordingly, the dispersion state of these systems may be stable and compared to a series of the W/O/W emulsions prepared by the two stage procedure (11). In the Tween 80 system, the extent of the oil layer decreased about 50% of that in the freshly prepared sample in an initial period of aging, and then reached a steady state, as shown in Fig. 5C. Optical microscopic observation suggests that the decrease of the oil layer in the Tween 80 system was brought Journal of Co/hid and Inler/hce Scrence, Vol. 94, No. 2, August 1983
368
SACHIO
2 z 0 o 0.2 E
5
15
20
25
30
CC) Tween 80 system
E z m 0.1 6 2 izYTrr4 0
10
MATSUMOTO
0
on the kind of the emulsifier employed, i.e., the use of SDS facilitates formation of W/O/W emulsions more than that of CTABr or Tween 80. (3) These results suggest that the state of W/O/W emulsions can be generalized as a mesophase in the phase diagram of the mixture of oil, water, and emulsifiers. This may lead to further insights into how W/O/W emulsions are formed. (4) On the other hand, another possible way for preparing W/O/W emulsions, i.e., the phase inversion technique, may be used in addition to the two stage procedure of emulsification.
0
REFERENCES 5
10
15
20
25
30
Aging period of time in doe
FIG. 5. Stability of the oil layer in W/O/W emulsions: (A) 0.7 M Span 80 in liquid paraffin/O.01 A4 SDS in aqueous phase (&,q = 0.75); (B) 0.47 MSpan 80 in liquid paraffin/O.03 M CTABr in aqueous phase (baq = 0.79); (C) 0.7 M Span 80 in liquid paraffin/O.025 M Tween 80 in aqueous phase (&,,q = 0.65).
5. 6.
about by the formation of the aggregates among the dispersed aqueous compartments during the initial stage of aging. This was clarified by since the aggregation formation caused a decrease in the extent of the oil layer separating the inner aqueous phase from the aqueous suspending fluid. CONCLUSIONS
(1) The development of a W/O/W-type dispersion precedes phase inversion of concentrated W/O emulsions stabilized by Span SO in the presence of a certain amount of hydrophilic emulsifiers such as SDS, CTABr, or Tween 80 in the aqueous phase. (2) The molar ratio of Span 80 to the hydrophilic emulsifier affecting the development of a W/O/W-type dispersion depends
Journnl oJCol/oid and Interface Science. Vol. 94, No. 2, August 1983
7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
Herbert, W. J., Luncet 2, 771 (1965). Engel, R. H., Riggi, S. J., and Fahrenbach, M. J., Nature (London) 219, 856 (1968). Davis, S. S., Purewal, T. S., and Burbage, A. S., J. Pharm. Pharmacol. 27 (Supplement), 60P (1976). Matsumoto, S., Kita, Y., and Yonezawa, D., J. Colloid InterSace Sci. 57, 353 (1976). Matsumoto, S., Kohda, M., and Murata, S., J. Colloid Interface Sci. 62, 149 (1977). Matsumoto, S., Ueda, Y., Kita, Y., and Yonezawa, D., Agric. Biol. Chem. (Tokyo) 42, 739 (1978). Panchal, C. J., Zajic, J. E., and Gerson, D. F., J. Colloid Interface Sci. 68, 295 (1979). Pilman, E., Larsson, K., and Tomberg, E., J. Disp. Sci. Xechnol. 1, 267 (1980). Sherman, P., and Parkinson, C., Prog. Coiloid Polym. Sci. 63, 10 (1978). Dokic, P., and Sherman, P., Colloid Polym. Sci. 258, 1159 (1980). Matsumoto, S., Inoue, T., Kohda, M., and Ohta, T., J. Colloid Interface Sci. 77, 564 (1980). Kita, Y., Matsumoto, S., and Yonezawa, D., J. Colloid Interface Sci. 62, 87 (1977). Matsumoto, S., and Kohda, M., J. Colloid Interface Sci. 73, 13 (1980). Mooney, M., J. Colloid Sci. 6, 162 (195 1). Cass, A., and Finkelstein, A., J. Gen. Physiol. 50, 1765 (1967). Matsumoto, S., Inoue, T., Kohda, M., and Ikura, K., J. Colloid Interface Sci. 77, 555 (1980). Sherman, P., J. Sot. Chem. Ind. (London) 69 (Sup plement), No. 2, S70 (1950).
of Agricultural
MATSUMOTO
Chemistry, College of Agriculture, Sakai-shi, Osaka 591, Japan
University
of Osaka
Prefecture,
Received November 1, 1982; accepted December 17, 1982 It has been found that the development of a W/O/W-type dispersion precedes phase inversion of concentrated W/O emulsions in the presence of hydrophilic emulsifier in the aqueous phase. This phenomenon was examined with a mixture of liquid paraffin and water using Span 80 in the oil phase and SDS, CTABr, or Tween 80 in the aqueous phase. The W/O/W emulsion formation was assessed by comparing the extent of the oil layer in a unit volume of the samples with each other, which was estimated from the viscosity changes in the diluted sample under the concentration gradients of glucose between the inner aqueous phase and the aqueous suspending fluid. The development of a W/O/W-type dispersion could be observed when the volume fraction of the aqueous phase was higher than 0.7, while another factor affecting the W/O/W emulsion formation was the molar ratio of the Span 80 to the hydrophilic emulsifier in the sample. In the case of SDS system, the range of the W/O/W emulsion formation appeared more widely in the molar ratio (i.e., from 4 to over 60) than that of CTABr or Tween 80 systems, so that the use of SDS facilitated the development of a W/O/W-type dispersion. The oil layer in the W/O/W emulsions provided by the phase inversion phenomenon was stable for at least a month when stored. INTRODUCTION
Some experimental studies (1-S) suggested that the two stage procedure of emulsification is reliable when preparing water-in-oilin-water-type multiphase emulsions (denoted as W/O/W emulsions herein). The first stage of emulsification is made for providing an ordinary W/O emulsion by successive addition of water into an oil phase containing hydrophobic emulsifier. The W/O emulsion is then mixed with an aqueous solution of hydrophilic emulsifier as the second stage procedure so as to obtain a W/O/W-type dispersion. Sherman and his co-workers (9, 10) emphasized that the development of a W/O[W emulsion structure precedes the thermal induced phase inversion of concentrated O/W emulsions. This may suggest that the state of multiphase emulsions can be generalized as one of the mesophases between O/W and
W/O emulsions, and also that there is a possibility of greater simplification of the technique for preparing W/O/W emulsions than the two stage procedure. More recently, the author has found that if the first stage procedure described above is carried out by introducing an aqueous solution of hydrophilic emulsifier into liquid paraffin containing sorbitan monooleate (Span SO),the W/O/W emulsion does appear in course of such the first-step process of the two stage procedure due to the phase inversion of W/O emulsions. This phenomenon has stimulated the development of a simplified method in preparing W/O/W emulsions together with some further insights into how W/O/W emulsions are formed. Therefore, the author has examined the reproducibility of the above phenomenon using the aqueous solutions of sodium dodecyl sulfate (SDS), cetyl trimethyl ammoniumbromide (CTABr), and polyoxyethylene sorbitan monooleate
362 0021-9797/83
$3.00
Copyright 0 1983 by Academic Press, Inc. All rights of reproduction in any form reserved.
Journa/ of Cobid
and Interface Science, Vol. 94, No. 2, August 1983
DEVELOPMENT
OF
W/O/W-TYPE
(Tween 80) at a variety of concentrations. The results obtained indicate the possibility of applying the phase inversion phenomenon in preparing W/O/W emulsions possessing the same functional properties as those provided by the two stage procedure of emulsification. This paper deals with some factors affecting the development of a W/O/W-type dispersion during the phase inversion of W/O emulsions. The W/O/W emulsion formation has been assessed by the extent of oil layer on the surface of the dispersed aqueous compartments using a viscometric method developed in the author’s laboratory (1 l), i.e., the viscosity changes in the sample have been measured under the glucose concentration gradients between the inner aqueous phase and the aqueous suspending fluid in order to estimate the area of the oil layer separating the two aqueous phases.
DISPERSION
363
and Span 80 used were of commercially available form delivered by Kao-Atlas Co., Tokyo. b. Procedure for inducing phase inversion of W/O emulsions. An assembly for examining reproducibility of the phase inversion phenomenon was composed of a pin-mixer and a peristaltic pump. It was essentially identical to that used for providing W/O emulsions in the first stage procedure of the W/O/W emulsion preparation (4). A welldefined volume of a mixture of liquid paraffin and Span 80 was introduced into a vessel of the pin-mixer, and an aqueous mixed solution of glucose and hydrophilic emulsifier was then added successively to the oil phase in the vessel at a rate of 5 ml/min by use of a vinyl pipe connected to the peristaltic pump. The pin-mixer was rotated steadily at 88 rpm at room temperature providing a constant shear force to the mixing system. The volume fraction of the aqueous phase introduced ranged from 0.50 to 0.86 at the EXPERIMENTAL end of the procedure. The pin-mixer used was diverted from a a. Sample composition. A series of the part of a Mixograph, which is an assembly samples to be tested consisted of an aqueous for the testing of dough consistency, made solution of 0.05 M glucose and liquid parby the National Mfg. Ltd., Lincoln, Neaffin in all instances in this study. The glucose braska. This mixer consisted of two units: an was a necessary component for providing an aluminium vessel with 4.2 cm depth and 7.5 osmotic pressure gradient between the inner aqueous phase and the aqueous suspending cm inner diameter, and a stirrer unit ccnfluid in the sample. sisting of a four-pins rotor. The length and The type of the samples could be divided diameter of each metal pin were 3.9 and 0.26 into three categories according to the kind cm, respectively. The vessel also possessed of hydrophilic emulsifiers in the aqueous three metal pins fixed vertically from the botphase such as anionic (SDS) type, cationic tom of the vessel, while the rotor was driven (CTABr) type, and nonionic (Tween 80) steadily at rotational speed of 88 rpm by type, respectively. One of these emulsifiers means of a speed reducer motor. When the was dissolved previously in the aqueous stirrer unit was immersed in the vessel and phase with a variety of the concentrations made to revolve in the oil phase, the latter from 0.004 to 0.05 A& while liquid paraffin circulated steadily through the gap between as the oil phase of the samples usually con- the four-pins rotor and the fixed three pins, tained Span 80 with the concentrations rang- thus providing a constant shear force to the ing from 0.23 to 0.93 M. system. Glucose, SDS, and CTABr were purchased c. Assessment of the W/O/W emulsion from Wako Pure Chemicals Co., Osaka, as formation. A series of the samples prepared an analytical grade, and were used in that were more or less the mixture of W/O/W and form without further treatment. Tween 80 O/W emulsions, although the ratio for the
364
SACHIO
MATSUMOTO
' 5pm ‘Of(
Maid
and Inlerface Science, Vol. 94, No. 2, August 1983
'
DEVELOPMENT
OF W/O/W-TYPE
W/O/W emulsion to the O/W one in each sample was influenced by the sample composition, as will be described. One could distinguish the W/O/W emulsion globules from the ordinary oil droplets using a phase contrast microscope, as shown in Fig. 1. However, the visual assessment could not lead to a quantitative estimation of the W/O/W emulsion formation. Therefore, the viscometric method (11) was employed to assessthe degree of the formation from the viewpoint of the total extent of the oil layer separating the inner aqueous phase from the aqueous suspending fluid in unit volume of the sample. First, the sample was diluted with pure water for providing a definite volume fraction of oil phase (denoted as &), i.e., &, was 0.1 for all cases in this study. The viscosity change in the diluted sample was then followed for 15 min. This commenced immediately after the dilution at a fixed shear rate of 38.3 set-’ at 25°C by means of a coneand-plate viscometer, made by Tokyo Keiki Ltd., Tokyo ( 12). The dilution brought a concentration gradient of glucose between the two aqueous phases, as pure water only distributed over the aqueous suspending fluid. Thus, the water started to migrate to the inner aqueous phase from t.he suspending fluid due to the osmotic pressure gradient. This resulted in an increase of the volume fraction of the inner aqueous phase (denoted as &,), so that the viscosity of the diluted sample also increased with increasing time after the dilution (see Fig. 2A). As reported previously (12, 13), the viscosity of W/O/W emulsions can be expressed by Mooney’s type equation ( 14), as follows:
ln vrd= a(& + Ad/[ 1 - X(4,+ ddl
[ 11
where qrel is the relative viscosity of the sam-
.
365
DISPERSION
I
I 0
2
4
6
8
2
4
6
8
10
12
CB)
3 22
1 0
Time after
the dilution
10
12
in minutes
FIG. 2. Changes in viscosity of W/O/W emulsions at 38.3 set-’ for shear rate (A), and in volume fraction of inner aqueous phases calculated from Eq. [ 1] (B), caused by the dilution of emulsions with pure water (& = 0.1 in all cases).Curve 1: 0.23 M Span 80 in liquid paraffin/ 0.01 M SDS in aqueous phase. Curve 2: 0.47 M Span 80 in liquid paraffin/O.01 M SDS in aqueous phase. Curve 3: 0.7 M Span 80 in liquid paraffin/O.01 M SDS in aqueous phase.
ple, and a and h are the constants. The values of these constants were already examined experimentally with the various W/O/W emulsions as 2.5 for a and 1.5 for h (12, 13). It is, therefore, possible to calculate the volume fraction I& from the viscosity of W/O/W emulsions using Eq. [ 11. Figure 2B shows the plots of the volume fraction against time after the dilution calculated from the viscosity change in the diluted samples, which is shown in Fig. 2A. The increasing rate of the volume fraction d&,/dt obviously corresponds to the volume flux of water permeating through the oil layer in a unit vol-
FIG. 1. Photomicrographs of dispersed globules in O/W emulsion (A) and W/O/W emulsions (B, C): (A) 0.7 MSpan 80 in liquid paraffin/O.01 MTween 80 in aqueous phase; volume fraction of total aqueous phase was 0.86; (B) 0.47 M Span 80 in liquid paraffin/O.02 M CTABr in aqueous phase; volume fraction of total aqueous phase was 0.75; (C) 0.7 M Span 80 in liquid paraffin/O.01 M SDS in aqueous phase; volume fraction of total aqueous phase was 0.75.
366
SACHIO
MATSUMOTO
ume of the diluted sample. This is also deeply related to the existence of the aqueous compartments surrounded by the oil layer in a unit volume of the diluted sample. According to the membrane studies (e.g., 15) the volume flux of water permeating through the oil layer can be expressed by &wldt
= L,~RT(m
- cm)
PI
where L, is the hydrodynamic coefficient of the oil layer, 2 is the total extent of the oil layer separating the two aqueous phases in a unit volume of the diluted sample, R is the gas constant, T is the absolute temperature, cl and c2 are the glucose concentrations of the suspending fluid and the inner aqueous phase, and gl and g2 are the osmotic pressure coefficients of glucose in the two aqueous phases. The mole flux of water is also given by h%.,ldtY p = L,~Wm =
RJkzc2
- ml)/ -
p
The mixing system was initiated by forming an ordinary W/O emulsion for all test systems evaluated. When the volume fraction of an aqueous solution of hydrophilic emulsifier (denoted as 4,) added successively to the mixing system was over 0.7, the continuous phase of the system was substituted by the aqueous phase, as shown in Fig. 3. This induced the development of a W/O/W-type dispersion in the system. It then followed that the maximal extent of the oil layer in the newly developed W/O/W emulsion could be observed at around 0.75 for &, (see Fig. 3), because an increasing amount of the aqueous suspending fluid containing hydrophilic emulsifier perhaps prompted the dispersement of the aggregates of the aqueous compartments. The apex of the plot in Fig. 3C when using Tween 80 as the hydrophilic emulsifier is much broader than that in Figs. 3A and B when using SDS or CTABr, respectively.
[31
WJ
where vis the partial molar volume of water, and P,, is defined as the water permeation coefficient of the oil layer. The value of PO was also examined experimentally in the previous work (16), and was obtained as about 5 X 10e4 cm/set for a series of the W/O/W emulsions. Therefore, if the volume flux of water d&/dt can be evaluated by the graphical differentiation on the volume fraction 4, vs time curve in an initial period of time after the dilution, one can estimate the total extent of the oil layer in an unit volume of the diluted sample from the relation of Eq. [3]. Thus, the formation of a W/O/W-type dispersion attributed to the phase inversion phenomenon can be assessed by the degree of the extent of the oil layer in each sample.
2 0 o 0.2 E f
volm
RESULTS
AND DISCUSSION
The phase inversion of emulsions generally occurs when the dispersed globules are packed very closely in the suspending fluid (17). It seems that the same thing happened during the mixing procedure in this study. Journal oJ’CoNoid and Interface Science, Vol. 94, No. 2, Augut
1983
0.70
0.75
0.80
0.85
0.90
(Cl
fraction
of (~amus solution of hydrmhilic emulsifier,
I
+Oaq
FIG. 3. Development of W/O/W-type dispersion as a function of volume fraction of aqueous phase (&J: (A) 0.7 M Span 80 in liquid paraffin/O.01 M SDS in aqueous phase: (B) 0.47 M Span 80 in liquid paraffin/ 0.03 M CTABr in aqueous phase; (C) 0.7 M Span 80 in liquid paraffin/O.0 1 M Tween 80 in aqueous phase.
DEVELOPMENT
OF W/O/W-TYPE
This may be brought about by the deficient role of Tween 80 in the dispersement of the aggregates. Additional increments of the aqueous phase, however, caused the rupture of the oil layer due to the solubilization of the oil layer components in the micelles of the hydrophilic emulsifier in the aqueous suspending fluid, so that the extent of the oil layer decreased with increasing amounts of the aqueous phase. It was confirmed experimentally that the Tween 80 system inverses completely to an ordinary O/W emulsion when the value for &, is higher than 0.85. Figure IA is an example of this case. Therefore, the development of a W/O/W-type dispersion in the course of the emulsification procedure may be characterized as a mesophase between the W/O and the O/W-type dispersions during the phase inversion process. It should be noted that the necessary conditions for developing W/O/W emulsion in the phase inversion process are not only the closely packed state of the dispersed globules in the W/O emulsion but the presence of a certain amount of hydrophilic emulsifier in the aqueous phase. Figure 4 shows the effect of the molar ratio of Span 80 to hydrophilic emulsifier on the formation of W/O/W emulsions, when the volume fraction &q is fixed at 0.75. The results obtained indicate that the range of the W/O/W emulsion formation as a function of the molar ratio is influenced by the kind of hydrophilic emulsifiers used. It is clear that the use of SDS as a hydrophilic emulsifier facilitates in a relative manner the development of a W/O/W-type dispersion, while the formation range of the W/O/W emulsions for CTABr or Tween 80 systems scatters in a comparative narrow region of the molar ratio. In any case, the left-hand side of the W/O/W emulsion range in Fig. 4 is occupied by the O/W-type dispersion, and the righthand side yields the W/O-type dispersion. Thus, the W/O/W emulsion may be stated as one of the emulsion phases in the phase diagram of the multicomponents of oil, water, and emulsifiers.
361
DISPERSION
20
30
40
50
60
(B1 CTABr SYStem
0 i.,
10
Molar ratlo
20
3
40
of Span 80 to hYdrmhilic
50
60
emulsifier
FIG. 4. Correlation between the W/O/W emulsion formation and the molar ratio of Span 80 to hydrophilic emulsifier in the whole system (6, = 0.75 in all cases).
An attempt was made to measure the changes in the extent of the oil layer in W/O/W emulsions developed as a function of storage. Each sample was aged for 30 days in an air-sealed glass flask at 25 “C in all cases, while a small part of the sample was diluted periodically with pure water so as to measure the viscosity change in the diluted sample described previously. Within the experimental error, the extent of the oil layer for SDS and CTABr systems calculated from the increasing rate of the viscosity in each period was not influenced by the period of incubation time (see Figs. 5A and B). Accordingly, the dispersion state of these systems may be stable and compared to a series of the W/O/W emulsions prepared by the two stage procedure (11). In the Tween 80 system, the extent of the oil layer decreased about 50% of that in the freshly prepared sample in an initial period of aging, and then reached a steady state, as shown in Fig. 5C. Optical microscopic observation suggests that the decrease of the oil layer in the Tween 80 system was brought Journal of Co/hid and Inler/hce Scrence, Vol. 94, No. 2, August 1983
368
SACHIO
2 z 0 o 0.2 E
5
15
20
25
30
CC) Tween 80 system
E z m 0.1 6 2 izYTrr4 0
10
MATSUMOTO
0
on the kind of the emulsifier employed, i.e., the use of SDS facilitates formation of W/O/W emulsions more than that of CTABr or Tween 80. (3) These results suggest that the state of W/O/W emulsions can be generalized as a mesophase in the phase diagram of the mixture of oil, water, and emulsifiers. This may lead to further insights into how W/O/W emulsions are formed. (4) On the other hand, another possible way for preparing W/O/W emulsions, i.e., the phase inversion technique, may be used in addition to the two stage procedure of emulsification.
0
REFERENCES 5
10
15
20
25
30
Aging period of time in doe
FIG. 5. Stability of the oil layer in W/O/W emulsions: (A) 0.7 M Span 80 in liquid paraffin/O.01 A4 SDS in aqueous phase (&,q = 0.75); (B) 0.47 MSpan 80 in liquid paraffin/O.03 M CTABr in aqueous phase (baq = 0.79); (C) 0.7 M Span 80 in liquid paraffin/O.025 M Tween 80 in aqueous phase (&,,q = 0.65).
5. 6.
about by the formation of the aggregates among the dispersed aqueous compartments during the initial stage of aging. This was clarified by since the aggregation formation caused a decrease in the extent of the oil layer separating the inner aqueous phase from the aqueous suspending fluid. CONCLUSIONS
(1) The development of a W/O/W-type dispersion precedes phase inversion of concentrated W/O emulsions stabilized by Span SO in the presence of a certain amount of hydrophilic emulsifiers such as SDS, CTABr, or Tween 80 in the aqueous phase. (2) The molar ratio of Span 80 to the hydrophilic emulsifier affecting the development of a W/O/W-type dispersion depends
Journnl oJCol/oid and Interface Science. Vol. 94, No. 2, August 1983
7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
Herbert, W. J., Luncet 2, 771 (1965). Engel, R. H., Riggi, S. J., and Fahrenbach, M. J., Nature (London) 219, 856 (1968). Davis, S. S., Purewal, T. S., and Burbage, A. S., J. Pharm. Pharmacol. 27 (Supplement), 60P (1976). Matsumoto, S., Kita, Y., and Yonezawa, D., J. Colloid InterSace Sci. 57, 353 (1976). Matsumoto, S., Kohda, M., and Murata, S., J. Colloid Interface Sci. 62, 149 (1977). Matsumoto, S., Ueda, Y., Kita, Y., and Yonezawa, D., Agric. Biol. Chem. (Tokyo) 42, 739 (1978). Panchal, C. J., Zajic, J. E., and Gerson, D. F., J. Colloid Interface Sci. 68, 295 (1979). Pilman, E., Larsson, K., and Tomberg, E., J. Disp. Sci. Xechnol. 1, 267 (1980). Sherman, P., and Parkinson, C., Prog. Coiloid Polym. Sci. 63, 10 (1978). Dokic, P., and Sherman, P., Colloid Polym. Sci. 258, 1159 (1980). Matsumoto, S., Inoue, T., Kohda, M., and Ohta, T., J. Colloid Interface Sci. 77, 564 (1980). Kita, Y., Matsumoto, S., and Yonezawa, D., J. Colloid Interface Sci. 62, 87 (1977). Matsumoto, S., and Kohda, M., J. Colloid Interface Sci. 73, 13 (1980). Mooney, M., J. Colloid Sci. 6, 162 (195 1). Cass, A., and Finkelstein, A., J. Gen. Physiol. 50, 1765 (1967). Matsumoto, S., Inoue, T., Kohda, M., and Ikura, K., J. Colloid Interface Sci. 77, 555 (1980). Sherman, P., J. Sot. Chem. Ind. (London) 69 (Sup plement), No. 2, S70 (1950).