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Multiple O/W/O Emulsion Rheology Rajinder Pal Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada Received June 13, 1995. In Final Form: February 5, 1996X The rheological behavior of simple oil-in-water (O/W) emulsions and multiple oil-in-water-in-oil (O/W/O) emulsions is investigated. The simple O/W emulsions exhibit only marginal levels of shear thinning at high values of dispersed phase concentration. The multiple emulsions are found to be highly non-Newtonian in nature. The degree of shear thinning in multiple emulsions increases with the increase in primary O/W emulsion concentration. The oscillatory measurements indicate that multiple emulsions are predominantly viscous in that the loss modulus falls above the storage modulus over the entire frequency range investigated. Upon aging, the storage and loss moduli of the multiple emulsions show a significant increase. However, the increase in viscosity with aging is only marginal.
Introduction A multiple emulsion is an emulsion in which the dispersed droplets themselves contain even more finer droplets of a different phase. There may exist at least two types of multiple emulsions, namely oil-in-water-inoil (abbreviated O/W/O) emulsion and water-in-oil-inwater (abbreviated W/O/W) emulsion. In the case of an O/W/O emulsion, the water droplets have finer oil droplets dispersed within them, and the water droplets themselves are dispersed in a continuous oil phase. The W/O/W multiple emulsion, on the other hand, consists of tiny water droplets entrapped within large oil droplets, which, in turn, are dispersed in a continuous water phase. Multiple emulsions, both O/W/O and W/O/W types, find many applications in industries such as petroleum, cosmetics, pharmaceutical, agriculture, etc.1 The understanding of the rheological behavior of multiple emulsions is quite important in the formulation, handling, mixing, processing, storage, and pipeline transportation of such systems. Furthermore, rheological studies can provide useful information on the stability and internal microstructure of multiple emulsions. While there is a substantial amount of literature published on the rheology of simple emulsions,2-10 little attention has been given to multiple emulsions. Some research articles have been published recently on multiple emulsions,11-19 but for the most part, they are restricted to W/O/W multiple X
Abstract published in Advance ACS Abstracts, April 15, 1996.
(1) Py, C.; Rouviejre, J.; Loll, P.; Taelman, M. C.; Tadros, Th. F. Colloids Surf. 1994, 91, 215. (2) Pal, R.; Rhodes, E. J. Rheol. 1989, 33, 1021. (3) Sherman, P. In Encyclopedia of Emulsion Technology; Becher, P., Ed.; Marcel Dekker: New York, 1983. (4) Pal, R. AIChE J. 1995, 41, 783. (5) Pal, R. J. Rheol. 1992, 36, 1245. (6) Pal, R. Colloids Surf. 1993, 71, 173. (7) Pal, R. Colloids Surf. 1991, 64, 207. (8) Otsuba, Y.; Prud’homme, R. K. Rheol. Acta 1994, 33, 29. (9) Princen, H. M. J. Colloid Interface Sci. 1979, 71, 55. (10) Princen, H. M. J. Colloid Interface Sci. 1985, 105, 150. (11) Matsumoto, S.; Kita, Y.; Yonezawa, D. J. Colloid Interface Sci. 1976, 57, 353. (12) Kita, Y.; Matsumoto, S.; Yonezawa, D. J. Colloid Interface Sci. 1977, 62, 87. (13) Matsumoto, S.; Kohda, M.; Murata, S. I. J. Colloid Interface Sci. 1977, 62, 149. (14) Matsumoto, S.; Kohda, M. J. Colloid Interface Sci. 1980, 73, 13. (15) Matsumoto, S.; Kang, W. W. J. Dispersion Sci. Technol. 1989, 10, 455. (16) Oza, K. P.; Frank, S. G. J. Dispersion Sci. Technol. 1989, 10, 163. (17) Sela, Y.; Magdassi, S.; Garti, N. Colloids Surf. 1994, 83, 143. (18) Terrisse, I.; Seiller, M.; Grossiord, J. L.; Magnet, A.; Le HenFerrenbach, C. Colloids Surf. 1994, 91, 121.
S0743-7463(95)00475-6 CCC: $12.00
emulsions. To our knowledge, the present study is the first one to deal with the rheology of O/W/O multiple emulsions. It is important to note that, in crude oil production operations, it is the O/W/O type emulsions (rather than W/O/W emulsions) which are more frequently encountered. The major objectives of this study are as follows: (a) to study the rheological behavior of various differently concentrated O/W emulsions (referred to as primary emulsions); (b) to study the rheology of freshly prepared multiple O/W/O emulsions; and (c) to study the effect of aging on the rheological behavior of multiple O/W/O emulsions. Background Information Preparation of Multiple Emulsions. Multiple emulsions are generally prepared using a two-step procedure. In the first step, the primary emulsion is prepared. For the preparation of an O/W/O multiple emulsion, the primary emulsion is an ordinary oil-in-water (O/W) emulsion which is prepared using an oil and an aqueous solution of a high HLB (hydrophilic-lipophilic balance) surfactant. The primary emulsion in the case of a W/O/W multiple emulsion is an ordinary water-in-oil (W/O) emulsion which is prepared using water and a low HLB surfactant solution in oil. In the second step, the primary emulsion (O/W or W/O) is reemulsified in either an aqueous solution of a high HLB surfactant (to produce a W/O/W multiple emulsion) or an oil containing a low HLB surfactant (to produce an O/W/O multiple emulsion). The first step, that is, the preparation of the primary emulsion, is usually carried out in a high shear device so as to produce very fine droplets. The second emulsification step is carried out in a low shear device so as to avoid the rupturing of the multiple droplets. However, some of the internal phase of the multiple droplets is unavoidably lost to the external phase during the emulsification process.20 The key factors affecting the formation of the multiple emulsion are the chemical nature of various components, the concentration of the surfactants used in both steps of emulsification, the volume fraction of the primary emulsion in the whole multiple emulsion, and the mixing conditions.11,20 Matsumoto et al.11 found that the volume fraction of the inner phase in the primary emulsion has little or no effect on the yield of the multiple emulsion. (19) Grossiord, J. L.; Seiller, M.; Puisieux, F. Rheol. Acta 1993, 32, 168. (20) Florence, A. T.; Whitehill, D. Int. J. Pharm. 1982, 11, 277.
© 1996 American Chemical Society
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Table 1. Details of Rheological Measurements instrument
measuring system
Fann viscometer
coaxial cylinder
steady state
types of experiments
Bohlin CS-50
cone-and-plate
steady shear, oscillatory
Thus, multiple emulsions can be prepared using a wide range of the internal phase volume fractions. Viscosity of Multiple Emulsions. Previous studies12,14,15,20 indicate that the viscosity of the multiple W/O/W emulsions, in the Newtonian flow range, can be described adequately by the Mooney equation.21
ln ηr )
K1φprimary 1 - K2φprimary
(1)
where ηr is the relative viscosity of the multiple emulsion (defined as the ratio of the multiple emulsion viscosity to the external continuous-phase viscosity), φprimary is the volume fraction of the primary emulsion in the whole multiple emulsion, and K1 and K2 are empirical constants. Thus, the key factor governing the rheology of the multiple emulsion is φprimary, which is basically the volume fraction of the total dispersed phase in the multiple emulsion. Mooney’s equation can also be written as
φprimary ) ln ηr/(K1 + K2 ln ηr)
(2)
studies12,14,15,20
Previous on multiple W/O/W emulsions have found this expression to be very useful in calculating the swelling or shrinkage of the multiple droplets. The swelling or shrinkage of the multiple droplets occurs due to the transport of water from the internal aqueous phase to the external aqueous phase or vice versa. The driving force for the transport of water is the osmotic pressure gradient. It should be noted that the swelling or shrinkage of the multiple droplets results in a direct increase or decrease of φprimary; consequently, the relative viscosity of the multiple W/O/W emulsion changes significantly with time (aging). However, it is not clear if multiple O/W/O emulsions would behave in a similar manner. Experimental Work Materials. The emulsions were prepared using Shell CORENA 32 oil. This is a highly refined mineral oil which is widely used as a lubricant. The density and viscosity of CORENA 32 at 23 °C are 0.84 gm/mL and 64 mPa‚s, respectively. The water used throughout the experiments was deionized. The surfactant used for the preparation of primary O/W emulsions was Triton X-100, a commercially available nonionic surfactant manufactured by Union Carbide Chemicals & Plastics Technology Corporation, USA. Triton X-100 is an octylphenol ethoxylate with an average of nine to ten molecules of ethylene oxide. It is water-soluble and has a high HLB value of 13.5. In order to disperse the primary O/W emulsion into the oil phase, and hence form the multiple O/W/O emulsion, a low HLB surfactant was incorporated into the oil phase. The surfactant used was Emsorb 2500, a commercially available surfactant manufactured by Henkel Corporation, USA. The chemical name of Emsorb 2500 is sorbitan monooleate. It is oil-soluble and has a low HLB value of 4.6. Procedure. Five primary O/W emulsions were prepared having different oil concentrations, namely 25, 40, 50, 56, and 60% by weight. These emulsions were prepared in batches of 300 g. The known amounts of aqueous surfactant solution (2% by weight Triton X-100) and CORENA 32 oil were sheared together in a homogenizer (Gifford-Wood Model 1-LV) for about (21) Mooney, M. J. Colloid Interface Sci. 1951, 6, 162.
dimensions of measuring systems bob radius ) 1.7245 cm bob length ) 3.8 cm rotor radius ) 1.8415 cm gap width ) 0.117 cm plate diameter ) 60 mm cone diameter ) 40 mm cone angle ) 4° gap at cone tip ) 150 µm
10 min at a high speed. The primary emulsions produced were quite stable with respect to coalescence. The primary O/W emulsions were then re-emulsified in a surfactant/CORENA 32 oil solution (containing 1% by weight Emsorb 2500) so as to prepare the multiple O/W/O emulsions. The concentration (on a weight basis) of the primary emulsion in the multiple emulsion was kept the same as that of oil in the primary emulsion. For instance, the multiple O/W/O emulsion prepared from the 25% by weight primary O/W emulsion had the primary emulsion concentration of 25% by weight. In this manner, five multiple O/W/O emulsions having primary O/W emulsion concentrations of 25, 40, 50, 56, and 60% by weight were prepared in batches of 300 g each. In order to avoid rupture of the multiple emulsion (hence release of the inner oil droplets), the second emulsification process was carried out using a hand stirrer for about 5 min. The information regarding the droplets (size and structure) was obtained by taking photomicrographs of the emulsion samples. The emulsions were diluted with the same continuous phase before taking the photomicrographs. A Zeiss optical microscope equipped with a camera was used to take the photomicrographs. All the rheological measurements were carried out at 23 °C. For primary O/W emulsions, steady shear data were collected using a Fann coaxial cylinder viscometer. For multiple emulsions, the rheological measurements were carried out in a Bohlin controlled-stress rheometer (Bohlin CS-50) using a cone-andplate measuring system. Both steady shear and oscillatory data were collected. Table 1 gives the dimensions of the measuring systems utilized in the present study.
Results and Discussion Rheological Behavior of Primary O/W Emulsions. Figure 1 shows the flow curves for the primary O/W emulsions. The emulsions are Newtonian up to a dispersed phase volume fraction (φ) of 0.45. At higher values of φ, the emulsions exhibit only marginal levels of shear thinning at high values of shear stress. The relative viscosity, defined as the ratio of emulsion viscosity to continuous-phase viscosity, increases with an increase in the volume fraction of the dispersed phase (φ), as shown in Figure 2. Also, the relative viscosity (ηr) data can be described adequately by the following Mooney equation:
ln ηr )
K1φ 1 - K2φ
(3)
where φ is the volume fraction of the dispersed phase (oil) and K1 and K2 are constants. The solid curves in Figure 2 are representative of the above equation. Figure 3 shows the photomicrographs of the primary O/W emulsions. Clearly, the emulsions are polydisperse with respect to droplet size. The droplet size varies from about 2 to 50 µm. Rheological Behavior of Multiple O/W/O Emulsions. The flow curves for the fresh multiple O/W/O emulsions are shown in Figure 4. All the multiple emulsions investigated are non-Newtonian in nature. The flow curve for a multiple emulsion tends to exhibit three distinct regions: a lower Newtonian region at low stresses where the viscosity is constant, a shear-thinning region at intermediate stresses where the viscosity decreases
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Pal
Figure 1. Apparent viscosity as a function of shear stress for primary oil-in-water (O/W) emulsions.
Figure 2. Correlation of relative viscosity data of primary O/W emulsions at shear stresses of 2 and 10 Pa, respectively.
with the increase in shear stress, and an upper Newtonian region at high stresses where the viscosity becomes constant independent of shear stress. The shear-thinning behavior exhibited by the multiple O/W/O emulsions is likely due to the deformation of multiple droplets in shear flow. Under a steady macroscopic shear flow, the emulsion droplet is subjected to two opposing forces, namely (1) a viscous stress of magnitude ηcγ˘ , which tends to elongate the droplets (ηc is the continuous-phase viscosity, and γ˘ is the macroscopic shearrate) and (2) a stress of magnitude σ/R, which tends to minimize the surface energy and, hence, tends to maintain the spherical shape of the droplet (σ is the interfacial tension, and R is the droplet radius). Thus, the equilibrium shape of the droplet is governed by a dimensionless group: ηcγ˘ R/σ. With the increase in the shear rate, the droplets become more elongated in the direction of flow. This results in a decrease in the viscosity of the emulsion. The degree of shear thinning in multiple O/W/O emulsions is much more strong than that observed in the case of primary O/W emulsions (refer to Figure 1). This is mainly due to the fact that the droplets in the primary emulsions were relatively much smaller. Small droplets undergo little deformation in shear flow. Furthermore, the viscosity of the continuous phase (aqueous phase) of the primary emulsions was very low. In the case of the multiple O/W/O emulsions, the viscosity of the external oil phase is 64 mPa‚s at 23 °C (almost 65 times that of the aqueous phase). The higher the external-phase (continuous-phase) viscosity, the larger is the deformation of droplets. Therefore, the droplets of the multiple emulsions are expected to undergo a much larger deformation in shear flow. It should also be noted that part of the shear thinning observed in multiple O/W/O emulsions could be due to flocculation of droplets. The photomicrographs for the fresh multiple emulsions, at various different concentrations of primary O/W emul-
Figure 3. Photomicrographs of primary O/W emulsions: (A) φ ) 0.28; (B) φ ) 0.45; (C) φ ) 0.54; (D) φ ) 0.64.
Figure 4. Apparent viscosity as a function of shear stress for multiple O/W/O emulsions.
sion, are shown in Figure 5. The photomicrographs clearly indicate that most multiple droplets consist of a large number of small oil droplets entrapped within the large water droplets. However, there are a few water droplets which contain either a small number or none of the internal oil droplets. Figure 6 shows the photomicrographs of a multiple O/W/O emulsion, with a primary O/W emulsion concentration of 48% by volume, before and after shearing in the Bohlin rheometer. The shearing of the emulsion in the rheometer does not appear to alter the size of the droplets significantly. The relative viscosity (ηr) of multiple O/W/O emulsions is plotted as a function of the volume fraction of the primary O/W emulsion (φprimary) in Figure 7. As expected, the relative viscosity increases with an increase in φprimary
Multiple O/W/O Emulsion Rheology
Figure 5. Photomicrographs of multiple O/W/O emulsions: (A) φprimary ) 0.23; (B) φprimary ) 0.38; (C) φprimary ) 0.48; (D) φprimary ) 0.58.
although the rate of increase in ηr is slow at low values of φprimary. Also, the relative viscosity data can be described quite well by the Mooney equation (eq 1). The solid curves in Figure 7 are the plots of the Mooney equation with K1 and K2 values as indicated. The storage and loss moduli for the fresh multiple emulsions are plotted in Figure 8. For φprimary e 0.54, the loss modulus of the multiple emulsions falls above the storage modulus over the entire frequency range, indicating that these emulsions are predominantly viscous in nature. In the case of the multiple emulsion having φprimary ) 0.58, G′′ > G′ at low frequencies but, at frequencies greater than about 0.8 Hz, G′ > G′′. Clearly, this multiple emulsion is more viscoelastic than those with lower values of φprimary. The effect of the concentration of the primary emulsion (φprimary) on the storage and loss moduli of the multiple O/W/O emulsion is shown in Figure 9. For φprimary e 0.54, the increase in G′ and G′′ with an increase in φprimary is almost negligible, but a sudden dramatic increase in G′ and G′′ occurs at a φprimary of 0.58. The sudden increase in the moduli at a φprimary of 0.58 is likely due to crowding and jamming of droplets. Figure 10 compares the complex viscosity (η*) data with the steady shear viscosity data for multiple O/W/O emulsions. It appears that the complex viscosities agree with the steady shear viscosities only at low frequencies. At high frequencies (>1 rad/s), the complex viscosities generally fall below the steady shear viscosities except for the most concentrated multiple emulsion (φprimary ) 0.58), which exhibits the opposite trend (i.e., the complex viscosities fall above the steady shear viscosities).
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Figure 6. Photomicrographs of multiple O/W/O emulsion having φprimary ) 0.48: (A and B) before shearing; (C and D) after shearing in the viscometer.
Figure 7. Correlation of relative viscosity data of multiple O/W/O emulsions at shear stresses of 0.61 and 9.9 Pa, respectively.
Effect of Aging on the Rheological Behavior of Multiple O/W/O Emulsions. In order to study the effect of aging on the rheological behavior of multiple O/W/O emulsions, the rheological properties of a multiple emulsion having φprimary ) 0.58 were monitored as a function of time. The photomicrographs were also taken to see the changes in droplet size, etc. Figure 11 shows the effect of aging on the flow curves of a multiple O/W/O emulsion. The change in viscosity with aging is only marginal. This observation is in contrast with the behavior of multiple W/O/W emulsions,12,14,18 which show significant changes in the viscosity due to diffusion of water from the internal aqueous phase to the external aqueous phase or vice versa. Clearly, the exchange of oil between the internal and external oil phases of the present multiple O/W/O emulsions is
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Figure 8. Storage and loss moduli for multiple O/W/O emulsions as functions of frequency.
Pal
Figure 10. Complex viscosity data versus steady shear viscosity data for multiple O/W/O emulsions.
Figure 11. Effect of aging on the viscosity of multiple O/W/O emulsion.
Figure 9. Storage and loss moduli for multiple O/W/O emulsions as functions of volume fraction of primary O/W emulsion (φprimary).
negligible. In Figure 12, the viscosity is plotted as a function of aging time at two different stresses (low and high). At a low stress of 0.32 Pa, the viscosity increases with aging initially, but it tends to level off after 50 hours or so. At a high stress of 9.9 Pa, the changes in viscosity with aging time are negligible. The effect of aging on the storage and loss moduli of the multiple emulsion is shown in Figure 13. Both G′ and G′′ increase with aging. This can be seen more clearly in Figure 14, where the moduli are plotted as a function of aging time. Like viscosity (at low stresses), the moduli tend to level off after 50 h of aging. The observed changes in the rheological parameters upon aging could be explained as follows: with aging, the
Figure 12. Viscosity as a function of aging time for multiple O/W/O emulsion.
droplets of the multiple emulsion are expected to come closer together and form flocs. Consequently, at low stresses the viscosity and moduli (G′,G′′) tend to increase with aging. However, the emulsion droplets also have the tendency to coalesce together when they are brought into intimate contact with each other. Thus, the increase in the rheological parameters with aging is not indefinite
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Figure 15. Photomicrographs of a multiple O/W/O emulsion at different aging times: (A) aging time ) 5 h; (B) aging time ) 55 h. Figure 13. Effect of aging on the storage and loss moduli of multiple O/W/O emulsion.
Figure 14. Storage and loss moduli as functions of aging time for a multiple O/W/O emulsion.
because of the two opposing effects, i.e., formation of flocs and increase in droplet size. Figure 15 shows the photomicrographs of the multiple emulsion at different aging times. Clearly, the droplet size increases with aging due to coalescence of droplets. Concluding Remarks On the basis of the experimental results and the data analysis, the following conclusions can be reached: (1) The simple emulsions of oil-in-water type investigated in the present work are Newtonian up to a dispersedphase concentration of 45% by volume. At higher concentrations, the emulsions exhibit only marginal levels of
shear thinning. The relative viscosity of the emulsions can be adequately described by the modified Mooney equation. (2) The multiple O/W/O emulsions investigated in the present work are non-Newtonian in nature. The degree of shear thinning in these emulsions increases with an increase in the primary O/W emulsion concentration. Also, the relative viscosity of the multiple emulsions increases with an increase in primary O/W emulsion concentration. The modified Mooney equation is found to adequately describe the relative viscosity data of multiple O/W/O emulsions. (3) The photomicrographs of the multiple emulsions indicate that the majority of the multiple droplets consist of a large number of small oil droplets entrapped within the large water droplets. (4) The oscillatory measurements indicate that multiple O/W/O emulsions are predominantly viscous in nature (the storage modulus falls below the loss modulus). Both G′ and G′′ show a dramatic increase when the primary O/W emulsion concentration is increased from 54 to 58% by volume. (5) The aging of multiple emulsions has a significant effect on the storage and loss moduli; the values of the moduli increase with aging. The increase in viscosity with aging is only marginal. Acknowledgment. Financial support from the Natural Sciences and Engineering Research Council of Canada is gratefully appreciated. LA950475H
