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Destabilization mechanisms in a triple emulsion with Janus drops
Journal of Colloid and Interface Science 361 (2011) 581–586
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Destabilization mechanisms in a triple emulsion with Janus drops Hida Hasinovic a, Stig E. Friberg b,⇑ a b
Ashland Consumer Markets/Valvoline, Lexington, KY, United States Clarkson University, Potsdam, NY, United States
a r t i c l e
i n f o
Article history: Received 17 March 2011 Accepted 23 May 2011 Available online 13 June 2011 Keywords: Janus emulsions Triple emulsions Double emulsions Silicone emulsions Vegetable oil emulsions
a b s t r a c t The destabilization mechanism was investigated of a triple Janus emulsion. The inner part of the emulsion consisted of Janus drops of a vegetable oil (VO) and a silicone oil (SO) in an aqueous (W) drop, (VO + SO)/W. This drop, in turn was dispersed in a VO drop forming a double emulsion (VO + SO)/W/ VO. Finally, these complex drops generated a complex Janus (SO + VO)/W/VO/SO triple emulsion by being dispersed in a continuous SO phase. The observations were limited to the time dependence of the over-all creaming/sedimentation processes, to the separation of layers of the compounds and to optical microscopy of the drop configuration with time. In the destabilization process the rise of the complex drops, (SO + VO)/W/VO, caused crowding in the upper part of the emulsion, which in turn led to enhanced coalescence, inversion and separation of a dilute vegetable oil emulsion. As a consequence of the separation of VO in the process, the remaining drops contained a greater W fraction and greater density. This change, in turn, resulted in sedimentation of the complex drops to form several high internal ratio morphologies in an SO continuous emulsion in the lower part of the test tube, among them a W/VO/SO emulsion. Finally, an inversion took place into an SO/VO/W double emulsion forming a separate bottom layer. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Emulsions [1–5] are unquestionably among the most important of all colloid and macro-dispersed systems; not only from a commercial point of view, but also of great interest scientifically. They are prevalent in foods as illustrated by frequent publication of monographs over the years [6–9], in cosmetics and personal care in general [10,11], as well as pharmaceutics [12]. The most essential property of emulsions is their ‘‘stability’’, by which is meant the rate of destabilization, of which the primary processes of flocculation and coalescence have early been extensively analyzed [13,14] with recent breakthroughs [15,16]. In parallel with these two initial processes, gravitational forces also cause creaming/sedimentation to take place [17,18] and Ostwald ripening [19–21] becomes relevant for emulsions with long term stability. Recent contributions have attempted a unification of these processes [22,23]. Of the different classes of emulsions, the double emulsions have attracted exceptional scientific interest, because their preparation and stabilization pose several problems. The original preparation method [24] was direct; i.e. for a final W/O/W configuration an initial W/O emulsion was subsequently emulsified into an aqueous phase; an area of later significant progress [25–27] in which Garti retains a leading position [3]. ⇑ Corresponding author. Address: 3856 Mountaintop Crkl, Cedar Hills, UT 84062, United States. E-mail address: [email protected] (S.E. Friberg). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.05.069
As an alternative, one-step methods to prepare double emulsions were early introduced by Aveyard et al. [28], who demonstrated the potential of preparing stable double emulsions in a one-step process using stabilization by two kinds of solid particles with slightly different wettability. Another essential advance was achieved with the later introduction of one step processes using traditional surfactants by Rocha-Filho and collaborators [29] resulting in a series of publications [30]. As a variation on the theme of particle stabilized emulsions, the stabilization of isotropic emulsions by Janus particles should be mentioned [31], because its development [26,32] attracted attention at the most lofty scientific level [33]. A substantial advance in the area of double emulsions was accomplished with the introduction of microfluidic devices emulsions in their preparation, which meant a remarkable increase as well as a radical redirection of the activities in the field. Early review articles evaluated the mechanism for breakup of large drops in such devices [34], clarified the basic conditions of wall wettability, demonstrated the formation of double emulsions with two aqueous drops of different composition within an oil drop and gave a general perspective of the potential for the preparation and engineering of new materials [35]. The research on the preparation of complex multiple emulsions and even particles has developed extremely rapidly in the few years since its beginning [36–38]. These methods led to a development to form Janus dispersions [39] and for that purpose the anisotropy of the drop interface has been utilized to prepare Janus nanoparticles [40,41] achieving
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particles of advanced complexity and refinement [42]; even for direct use as catalysts [43]. The elegance of these methods, combined with the unsurpassed adaptability to prepare the most complex emulsions, make them one of the most important developments in emulsion science ever. Nevertheless, the method is limited to a production maximum of kg/day and the need is still pronounced for a one-step method to attain volumes at a magnitude useful for general applications. One of the most elegant achievements of the microfluidics method is to form double emulsions combining several mutually insoluble oils within the dispersed drops. Such emulsions were not prepared by the bulk method except for the large number of attempts using oils modified by pegylation [44] until the recent publications by the authors [45,46]. The present investigation is concerned with a triple Janus emulsion in which aggregated VO + SO drops in W drops inside VO drops are dispersed in SO as the continuous phase, (VO + SO)/W/ VO/SO. The only surfactant, a nonionic commercial Tween 80R, exclusively soluble in water is present at 3% by weight on the water and at 0.3% of the total emulsion. As is obvious, such an emulsion is highly unstable, since the surfactant is located in the aqueous drop, leaving both the W/VO and the VO/SO interfaces without stabilizer. Hence, determining stability by a traditional analysis of the flocculation and coalescence rates [47] would be extremely complex, if even possible. Realizing this fact, the attention of the authors was instead focused onto a qualitative description of the combined destabilization processes of the triple emulsion configuration (VO + SO)/W/VO/SO. The morphology of the emulsion, Fig. 1, caused an initial creaming of the complex drops, (VO + SO)/W/VO, apparently of less density than SO. The subsequent enhanced coalescence of the creamed emulsion; brought about extremely intricate drop configurations. From this complex mixture a VO continuous emulsion separated and among the remaining structures a number of drops contained sufficient fraction of W to sediment through the SO emulsion layer, subsequently separating an O/W emulsion in the bottom part of the test tube. These different phenomena are with necessity rather complex and a preliminary qualitative charting of them was considered useful to offer material for subsequent fundamental treatment of the destabilization processes of complex triple Janus emulsions.
2. Experimental 2.1. Materials DC Fluid 100 CST, SO, from Dow Corning, Density 0.970 g/cm3, Viscosity: 102cP, High Oleic Acid Sunflower Olive Oil, VO, Density
0.914 g/cm3, Viscosity 70c, from Rita Corporation80-NV-LQ-(AP), Polysorbate 80 from Croda Inc., Keyacid Blue GS, 201-145-50, from Keystone Aniline Corporation, Automate Red IK HF Liquid Dye from Rohm and Haas, distributed by Keystone Aniline Corporation, Deionized water. 2.2. Emulsion composition The emulsion consisted of the following compounds given in weight fractions. Water (W) 0.097, Surfactant (S) 0.003, Vegetable oil (VO) 0.18, Silicone oil (SO) 0.72. 2.3. Emulsion preparation and observation Two-gram emulsion samples with the weight fractions given were weighed into 10 ml. test round bottomed tubes. A water soluble blue dye, Keyacid Blue GS, 201-145-50 was added to the water in trace amounts and a vegetable oil soluble red dye, Automate Red HF Liquid Dye, added to the vegetable oil. These additions were made to facilitate identification of the different phases in the microscope pictures and pictures with and without dyes were compared to ensure that the presence of the dyes did not distort the drop morphology. Emulsification was made by vibration in a Mini Vortexer from VWR Scientific at a speed of 10 for 2 min. A small amount of sample was placed on a pre-cleaned J. Melvin Freed Brand microscopic slide, 75 25 mm, and gently covered with VWR micro cover glass, 18 18 mm. All the pictures were taken by Axio Imager 4.6.3 Upright Microscope by Carl Zeiss AG using the red coded objective, EPIPLAN 5X/0.13HD with a magnification of 50. 3. Results The results are viewed against the complex configuration of the emulsions, Fig. 1A. The essential features of the configuration are identical to those presented in an earlier publication [46]. SO forms the continuous phase due to its sizeable volume ratio with the compound second in volume, VO, dispersed in it. The water in turn is located as drops within the VO liquid and, finally, Janus combination drops of VO and SO are found inside these water drops, Fig. 1B; giving a total triple emulsion configuration of (VO + SO)/ W/VO/SO. These characteristics have been described [45,46] and the present contribution is focused on the changes during the destabilization process. The surfactant is soluble only in the water and as a result, both the outer VO/SO interface and the W/VO interface were unstable, leading to fast coalescence of the (VO + SO)/W/VO drops. In
Fig. 1. Microphotograph of sample from the middle of the emulsion layer.
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Fig. 2. The volume fraction of the separated top VO layer. (The results are from two independent determinations).
addition extensive creaming took place, since the density of a dispersion of the W/VO combination with a weight ratio of approximately 0.56, comes to 0.944; significantly less than the density of SO, 0.97. The creaming, in turn, led to a high internal volume emulsion in the top part with increased flocculation/coalescence rate and subsequent separation of a top layer of VO. Fig. 2 reveals this division to be fast; after 20 min the separation was virtually complete, which is realistic considering the size of the drops and the viscosity of the continuous phase. The spread of the results from two determinations is substantial, as expected from the complexity of reactions, but the trend is undisputable. In addition a bottom layer of an aqueous emulsion separated, but the time dependence of its formation showed such an extensive spread that a numerical analysis was not deemed realistic. The excessive spread was anticipated as a result of composite and interconnected events; the creaming of the original drops, the coalescence in the top part of the emulsion, the sedimentation of the drops from this reaction and the final coalescence. Before entering into an account of the mechanisms, it is useful to compare the magnitude of the separated layer with the composition of the emulsion. The weight fraction of the VO is 0.18, corresponding to a volume fraction of 0.19, indicating virtually all of the VO as separated within 20 min. In itself, such results of a fast separation of the VO layer would be no novelty and of no general interest. Any emulsion, which is not protected against coalescence, would show similar behavior, unless the processes were retarded by high viscosity of the continuous
A
C
phase. The reason to draw attention to the present results is the complex and novel configuration of the emulsion, Fig. 1. The destabilization of such a multifaceted emulsion involving both coalescence and creaming resulted, as anticipated, in a separation of a layer continuous in VO, Fig. 2. However, that summary process must, with necessity, be composed of a series of complex configurations and developments and a preliminary examination of these is the main raison d’etre for this publication. The thorough quantitative treatments of the flocculation/ coalescence processes progressing from Ivanov [47] to recent advanced analysis [48] are not useful, or even feasible, for the emulsions in Fig. 1, before more experimental information is available about the intricate morphological changes. Hence, the attention is limited to the relationship between the drop configuration and the simultaneous coalescence and the gravitational movements. In fact the difference in densities of VO, 0.914 g/cm3 and W, 0.995 g/cm3 versus that of the continuous phase of 0.970 g/ cm3, means a potential exists for both creaming and sedimentation. The attention was accordingly centered on the configurations in the top and bottom part of the emulsion and the variation in the microphotograph features from these layers with time offers initial information, Fig. 3. The fast events in the top layer, Fig. 2, are reflected in the radical change after 15 min, Figs. 3A and B, revealing an inversion from the original (VO + SO)/W/VO emulsion, Fig. 3A, to a combination emulsion, Fig. 3B, of single drops of SO and W as well as Janus drops in a continuous VO phase. In addition to these short term changes in the top part of the emulsion, a new layer appeared in the bottom, which from its color obviously was a water continuous dispersion. The kinetics of its formation could not be enumerated, because the times for its emergence strongly varied from half an hour to several days. The configuration in the top and bottom layers of the freshly prepared emulsions, Fig. 3A and C, is similar to the pattern in Fig. 1. However, after 15 min the top layer is radically changed, as anticipated from the fact that a VO continuous emulsion had separated. Instead of the complex emulsion (VO + SO)/W/VO/SO, Fig. 3A, the system now consists of a continuous VO liquid with small independent drops of W and SO and a few Janus drops, Fig. 3B. An inversion has taken place; VO is now the continuous phase instead of SO. It is of note that the inversion of the outer VO (marked as VO, Fig. 3A) to become continuous phase also strongly impinged on the inner emulsion (VO + SO)/W, whose large
W
B
D
E
Fig. 3. Micrographs of the top, (A and B) and bottom, (C and D) parts of the emulsion immediately after preparation, (A and C), after 15 min, (B and D) and 2 h of storage, E.
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Fig. 4. Two patterns of the upper emulsion part (Top) after 6 min storage and of the lower part after 60 and 120 min (Btm).
drops were transformed into small and separate W and SO droplets. In a subsequent event, drops with substantial fraction of W sedimented towards the bottom,forming a double emulsion, W/ VO/SO after 15 min, Fig. 3D, followed by coalescence and inversion to a W continuous emulsion, Fig. 3E. These inclusive trends are evident from the microphotographs; the magnitude of the volume of VO in the original (VO + SO)/W/ VO drops, Fig. 1, resulted in a sufficiently low density for the combination drops to rise in the emulsion according to the analysis in the Discussion section. The ensuing crowding of drops in the top part of the emulsion caused coalescence followed by inversion. The coalescence was intrusive as made evident by the difference between Figs. 3A and B and as a result W drops and/or drops with sufficient W content were found in the bottom part of the emulsion, Fig. 3D. Although it was not possible to record these transformations in the top part of the emulsion in detail, configuration modifications in the emulsion immediately beneath and on top of the respective separated layers leave some indication of the complexity of intermediate stages. The image of the drops top part of the emulsion after 6 min as a realistic intermediate between 0 and 15 min and between 60 and 90 min of the bottom layer is useful to illustrate potential transitional configurations, Fig. 4. The patterns in the figures do exemplify the complexity of the drop arrangement, but the shape of the individual drops may be distorted, because of the effect of the cover glass. The emulsion top level at 6 min and its bottom height between 60 and 90 min after formation are both distinguished by the large continuous VO and SO areas, Fig. 4; calling attention to an ongoning coalescence. The preponderance of VO in the top compared to the pattern of the bottom layer is an expected outcome of the creaming/sedimentation events. As a matter of fact there are small W drops in the VO liquid, Fig. 4A and B, which prove that the coalescence of the (VO + SO)/W/VO drops also involved the inner parts
of these complex globules. In brief, the convoluted patterns in Fig. 4 are rational intermediates both for the minimal inversion on top levels, which only reach to the first layer of the drops and for the more in-depth inversion in the bottom emulsion layer. The authors are aware that the microscopy images may not truly reflect the detailed bulk morphology of the emulsions, but felt the patterns in Fig. 4 to be useful as illustrations of the potential complexity of the intermediate configurations. 4. Discussion The destabilization process gave rise to a number of phenomena, which merit a discussion, but the truly fundamental issue is the mutual effects in the creaming/sedimentation/coalescence combination. Since the initial emulsion is a triple Janus emulsion, (VO + SO)/W/VO/SO, Fig. 1, the vital condition is the density of the (VO + SO)/W/VO drops versus 0.97 g/cm3, the density of the continuous phase SO. The density of such a drop in g/cm3 is
q ¼ ðX W þ X VO þ X SO ðiÞ þ X S Þ=ððX W =0:995 þ ðX VO =0:914Þ þ ðX SO ðiÞ=0:97Þ þ ðX S =1:03ÞÞ
ð1Þ
in which XSO(i) is the weight fraction of SO located within the W drop and the densities were computed by assuming a partial molar volume equal to the one for the pure compounds. The direction of the creaming or sedimentation is decided by Dq = qC 0.97 g/cm3 with a negative sign indicating creaming and the opposite sedimentation. As is immediately evident, the presence of SO within the W drop has no effect on the direction per se and was left out in the continued analysis. For an ‘‘average’’ drop, for which XW = 0.097, XVO = 0.18, XS = 0.003, in which expressions X represents weigh fraction of the compound indicated by the subscript. The resulting q is 0.941 g/ cm3 and creaming will obviously take place leading to an increase
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585
A conservative estimate from Eq. (3) with cos b 0 and
c13 = 1 m Nm1 shows drops to be of dimensions far in excess of those found in the VO emulsion and a conclusion of all the drops at the bottom level originating in the top SO emulsion part appears well founded. 5. Conclusions Fig. 5. The shape and dimensions of pendant drop of compound 1 at the interface between compounds 2 and 3 (From [49] with permission).
of the number of the complex drops at the top of the original emulsion, where a high internal phase ratio emulsion will form, Fig. 4. This emulsion, in turn, changes in two ways; the increased fraction of dispersed drops leads to enhanced coalescence rate and the separation of the VO emulsion, Fig. 3B. The loss of VO from the complex drops during coalescence leads to increased density of the residual drops and, when the W/VO ratio has reached a level to bring the density in excess of 0.97, the drops sediment. It is of interest to calculate the remaining fraction, kXVO, of the total VO content to reach a density of 0.97 g/cm3.
Acknowledgment
0:97 ¼ ð0:1 þ 0:18kÞ=ðð0:097=:995Þ þ ð0:003=1:03Þ þ ð0:18k=0:914ÞÞ
ð2Þ
Solving gives k = 0.24, indicating three quarters of the VO must be separated before sedimentation commences of the complex drops. Fig. 3D illustrates the fact the drops found towards the bottom of the emulsion contain but little VO in comparison with the W content. These heavier drops will sediment through the entire SO emulsion layer prior to coalescence and separation as an aqueous emulsion at the bottom of the test tube. This fact in combination with the smaller density difference is reflected in the difference in time scale for the formation of the top VO and the bottom W emulsions, as well as the huge variation in the times for the latter to materialize. The picture actually also contains small drops of VO, which obviously have not sedimented from the higher levels of the VO continuous emulsion. Instead the origin of these drops is assumed as intermediates in the coalescence process, Fig. 4, where small drops of VO is not an unrealistic outcome of such a process in highly complex configurations. In summary, the average W/VO/SO ratio leads to creaming of (VO + SO)/W/VO drops, which leads to crowding in the top part of the emulsion and increased coalescence of these drops, which sequentially leads to inversion and separation of VO into a top emulsion layer. The coalescence leaves drops rich in W, because some VO is removed forming a separate VO layer, Fig. 2. The W drops with a greater fraction of W, Fig. 4, sediment and coalesce to form a separate (SO + VO)/W emulsion, Fig. 3, picture 7. These (SO + VO)/W drops (or (SO + VO)/W/VO drops with sufficient W/ VO ratio for sedimentation) may emanate from the upper SO emulsion layer or even from the top VO emulsion. In the latter case the drop must pass through the VO/SO interface and it is of interest to estimate the feasibility of such a transit. The fundamentals of the equilibrium between gravitational and interfacial tension forces for a pendant drop at a liquid interface was given by Kaptay [49], who determined the maximum drop size of a Cu2S–FeS, matte, floating on liquid low iron wollastonite slag in the copper matte smelting process. With the interfacial tensions (c), densities (q) and contact angle (b) in Fig. 5 [49], the horizontal projection of a drop of critical size was given as
Rcr ¼ ½ð2 c1=3 ð1 þ cos bÞ=ðg ðq1 q3 Þ1=2
The destabilization processes in a triple Janus emulsion (vegetable oil + silicone oil)/water/vegetable oil/silicone oil was investigated using visual observation of the separation of layers and microscopy patterns of the drop configuration. The results revealed the destabilization to be a complex process with the initial step a creaming of the (VO + SO)/W/VO drops in the SO continuous emulsion. The creaming led to a high internal volume ratio emulsion in the top part of the SO emulsion with increased coalescence. This process in turn resulted in an inversion to an (SO + VO)/W/VO emulsion, while leaving complex drops with sufficient density from an increased W/VO ratio to sediment in the SO emulsion. The ensuing coalescence at lower levels of the emulsion layer gave rise to a separated W continuous emulsion.
ð3Þ
The authors are grateful to Ashland Consumer Markets/Valvoline for support and to the reviewers for constructive criticism. References [1] P.B. Binks (Ed.), Modern Aspects of Emulsion Science, Royal Soc. Chem, Cambridge, UK, 1998. [2] J. Sjoblom (Ed.), Emulsion and Emulsion Stability, Taylor & Francis, NY, 2006. [3] A. Aserine (Ed.), Multiple Emulsions, John Wiley & Sons, Hoboken, NY, 2008. [4] J. Bibette, F. Leal-Calderon, V. Schmitt, Emulsion Science, Springer, 2007. [5] Th. Tadros (Ed.), Emulsion Science and Technology, WILEY-VCH, Amsterdam, NL, 2009. [6] M. Peleg, E.B. Bagley (Eds.), Physical Properties of Foods, Avui, Westport CT, 1983. [7] R.D. Bee, P. Richmond, J. Mingins (Eds.), Food Colloids, Royal Soc. Chem., Cambridge, UK, 1989. [8] S.E. Friberg, K. Larsson, J. Sjobolom (Eds.), Food Emulsions, Marcel Dekker, NY, 2004. [9] D.J. McClements, Food Emulsions, CRC Press, Boca Raton, FL, 2005. [10] L.D. Rhein, M. Schloss, A. O’Lenic (Eds.), Surfactants in Personal Care Products and Decorative Cosmetics, Marcel Dekker, NY, 2006. [11] Th. Tadros (Ed.), Colloids in Cosmetics and Personal Care, Marcel Dekker, NY, 2008. [12] F. Niellod, G. Marti-Mestres (Eds.), Pharmaceutical Emulsions and Suspensions, Marcel Dekker, NY, 2000. [13] D.T. Wasan, A.D. Nikolov, J. Sjoblom (Eds.), Encyclopedic Handbook of Emulsion Technology, Marcel Dekker, New York, 2001. [14] S. Dukhin, Oe. Saether, J. Sjoblom (Eds.), Encyclopedic Handbook of Emulsion Technology, Marcel Dekker, New York, 2001. [15] N. Bremond, A.R. Thiam, J. Bibette, Phys. Rev. Lett. 100 (2008) 0254501. [16] B.A. AU Grimes, C.A. Dorao, S. Simon, E.L. Nordgard, J. Sjoblom, Colloid Interface Sci. 348 (2010) 479–490. [17] J. Bibette, D. Roux, B. Pouligny, J. Phys. II, France 2 (1992) 401. [18] C. Dickinson, J. Ritzoulis, Colloid Interface Sci. 224 (2000) 148. [19] P. Taylor, Colloids Surfaces A 99 (1995) 175. [20] A.S. Kabalnov, E.D. Shchukin, Adv. Colloid Interface Sci. 38 (1992) 69. [21] S. Mun, D.J. McClements, Langmuir 22 (2006) 1551. [22] S. Pasalic, P.B. Jovanic, B. Bugarski, Mater. Sci. Forum 555 (2007) 177. [23] G. Urbina-Villalba, Int. J. Mol. Sci. 10 (2009) 761. [24] S. Matsumoto, Y. Kita, D. Yanesawa, J. Colloid Interface Sci. 51 (1976) 353. [25] N. Garti, A. Aserini, Adv. Colloid Interface Sci. 65 (1996) 37. [26] V.B. Menon, D.T. Wasan, Sep. Sci. Technol. 23 (1988) 2131. [27] R. Pala, J. Colloid Interface Sci. 307 (2007) 509. [28] R. Aveyard, B.P. Binks, J.H. Clint, Adv. Colloid Interface Sci. 100–102 (2003) 503. [29] J.M. Morais, O.D.H. Santos, J.R.L. Nunes, C.F. Zanatta, P.A. Rocha-Filho, J. Disper. Sci. Technol. 29 (1) (2008) 63. [30] J.M. Morais, P.A. Rocha-Filho, D. Burgess, Langmuir 25 (2009) 7954. [31] C. Casagrande, M. Veyssie, C.R. Acad. Sci. (Paris) 306 (1988) 1423. [32] V.B. Menon, D.T. Wasan, Colloids Surf. 23 (1987) 353. [33] P.-G. de Gennes, Angew. Chem. Int. Ed. EngI. 31 (1992) 842. [34] T. Nisisako, S. Okushima, T. Torii, Soft Matter 1 (2005) 23. [35] G. Muschiolika, Curr. Opin. Colloid Interface Sci. 12 (2007) 213.
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Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Destabilization mechanisms in a triple emulsion with Janus drops Hida Hasinovic a, Stig E. Friberg b,⇑ a b
Ashland Consumer Markets/Valvoline, Lexington, KY, United States Clarkson University, Potsdam, NY, United States
a r t i c l e
i n f o
Article history: Received 17 March 2011 Accepted 23 May 2011 Available online 13 June 2011 Keywords: Janus emulsions Triple emulsions Double emulsions Silicone emulsions Vegetable oil emulsions
a b s t r a c t The destabilization mechanism was investigated of a triple Janus emulsion. The inner part of the emulsion consisted of Janus drops of a vegetable oil (VO) and a silicone oil (SO) in an aqueous (W) drop, (VO + SO)/W. This drop, in turn was dispersed in a VO drop forming a double emulsion (VO + SO)/W/ VO. Finally, these complex drops generated a complex Janus (SO + VO)/W/VO/SO triple emulsion by being dispersed in a continuous SO phase. The observations were limited to the time dependence of the over-all creaming/sedimentation processes, to the separation of layers of the compounds and to optical microscopy of the drop configuration with time. In the destabilization process the rise of the complex drops, (SO + VO)/W/VO, caused crowding in the upper part of the emulsion, which in turn led to enhanced coalescence, inversion and separation of a dilute vegetable oil emulsion. As a consequence of the separation of VO in the process, the remaining drops contained a greater W fraction and greater density. This change, in turn, resulted in sedimentation of the complex drops to form several high internal ratio morphologies in an SO continuous emulsion in the lower part of the test tube, among them a W/VO/SO emulsion. Finally, an inversion took place into an SO/VO/W double emulsion forming a separate bottom layer. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Emulsions [1–5] are unquestionably among the most important of all colloid and macro-dispersed systems; not only from a commercial point of view, but also of great interest scientifically. They are prevalent in foods as illustrated by frequent publication of monographs over the years [6–9], in cosmetics and personal care in general [10,11], as well as pharmaceutics [12]. The most essential property of emulsions is their ‘‘stability’’, by which is meant the rate of destabilization, of which the primary processes of flocculation and coalescence have early been extensively analyzed [13,14] with recent breakthroughs [15,16]. In parallel with these two initial processes, gravitational forces also cause creaming/sedimentation to take place [17,18] and Ostwald ripening [19–21] becomes relevant for emulsions with long term stability. Recent contributions have attempted a unification of these processes [22,23]. Of the different classes of emulsions, the double emulsions have attracted exceptional scientific interest, because their preparation and stabilization pose several problems. The original preparation method [24] was direct; i.e. for a final W/O/W configuration an initial W/O emulsion was subsequently emulsified into an aqueous phase; an area of later significant progress [25–27] in which Garti retains a leading position [3]. ⇑ Corresponding author. Address: 3856 Mountaintop Crkl, Cedar Hills, UT 84062, United States. E-mail address: [email protected] (S.E. Friberg). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.05.069
As an alternative, one-step methods to prepare double emulsions were early introduced by Aveyard et al. [28], who demonstrated the potential of preparing stable double emulsions in a one-step process using stabilization by two kinds of solid particles with slightly different wettability. Another essential advance was achieved with the later introduction of one step processes using traditional surfactants by Rocha-Filho and collaborators [29] resulting in a series of publications [30]. As a variation on the theme of particle stabilized emulsions, the stabilization of isotropic emulsions by Janus particles should be mentioned [31], because its development [26,32] attracted attention at the most lofty scientific level [33]. A substantial advance in the area of double emulsions was accomplished with the introduction of microfluidic devices emulsions in their preparation, which meant a remarkable increase as well as a radical redirection of the activities in the field. Early review articles evaluated the mechanism for breakup of large drops in such devices [34], clarified the basic conditions of wall wettability, demonstrated the formation of double emulsions with two aqueous drops of different composition within an oil drop and gave a general perspective of the potential for the preparation and engineering of new materials [35]. The research on the preparation of complex multiple emulsions and even particles has developed extremely rapidly in the few years since its beginning [36–38]. These methods led to a development to form Janus dispersions [39] and for that purpose the anisotropy of the drop interface has been utilized to prepare Janus nanoparticles [40,41] achieving
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particles of advanced complexity and refinement [42]; even for direct use as catalysts [43]. The elegance of these methods, combined with the unsurpassed adaptability to prepare the most complex emulsions, make them one of the most important developments in emulsion science ever. Nevertheless, the method is limited to a production maximum of kg/day and the need is still pronounced for a one-step method to attain volumes at a magnitude useful for general applications. One of the most elegant achievements of the microfluidics method is to form double emulsions combining several mutually insoluble oils within the dispersed drops. Such emulsions were not prepared by the bulk method except for the large number of attempts using oils modified by pegylation [44] until the recent publications by the authors [45,46]. The present investigation is concerned with a triple Janus emulsion in which aggregated VO + SO drops in W drops inside VO drops are dispersed in SO as the continuous phase, (VO + SO)/W/ VO/SO. The only surfactant, a nonionic commercial Tween 80R, exclusively soluble in water is present at 3% by weight on the water and at 0.3% of the total emulsion. As is obvious, such an emulsion is highly unstable, since the surfactant is located in the aqueous drop, leaving both the W/VO and the VO/SO interfaces without stabilizer. Hence, determining stability by a traditional analysis of the flocculation and coalescence rates [47] would be extremely complex, if even possible. Realizing this fact, the attention of the authors was instead focused onto a qualitative description of the combined destabilization processes of the triple emulsion configuration (VO + SO)/W/VO/SO. The morphology of the emulsion, Fig. 1, caused an initial creaming of the complex drops, (VO + SO)/W/VO, apparently of less density than SO. The subsequent enhanced coalescence of the creamed emulsion; brought about extremely intricate drop configurations. From this complex mixture a VO continuous emulsion separated and among the remaining structures a number of drops contained sufficient fraction of W to sediment through the SO emulsion layer, subsequently separating an O/W emulsion in the bottom part of the test tube. These different phenomena are with necessity rather complex and a preliminary qualitative charting of them was considered useful to offer material for subsequent fundamental treatment of the destabilization processes of complex triple Janus emulsions.
2. Experimental 2.1. Materials DC Fluid 100 CST, SO, from Dow Corning, Density 0.970 g/cm3, Viscosity: 102cP, High Oleic Acid Sunflower Olive Oil, VO, Density
0.914 g/cm3, Viscosity 70c, from Rita Corporation80-NV-LQ-(AP), Polysorbate 80 from Croda Inc., Keyacid Blue GS, 201-145-50, from Keystone Aniline Corporation, Automate Red IK HF Liquid Dye from Rohm and Haas, distributed by Keystone Aniline Corporation, Deionized water. 2.2. Emulsion composition The emulsion consisted of the following compounds given in weight fractions. Water (W) 0.097, Surfactant (S) 0.003, Vegetable oil (VO) 0.18, Silicone oil (SO) 0.72. 2.3. Emulsion preparation and observation Two-gram emulsion samples with the weight fractions given were weighed into 10 ml. test round bottomed tubes. A water soluble blue dye, Keyacid Blue GS, 201-145-50 was added to the water in trace amounts and a vegetable oil soluble red dye, Automate Red HF Liquid Dye, added to the vegetable oil. These additions were made to facilitate identification of the different phases in the microscope pictures and pictures with and without dyes were compared to ensure that the presence of the dyes did not distort the drop morphology. Emulsification was made by vibration in a Mini Vortexer from VWR Scientific at a speed of 10 for 2 min. A small amount of sample was placed on a pre-cleaned J. Melvin Freed Brand microscopic slide, 75 25 mm, and gently covered with VWR micro cover glass, 18 18 mm. All the pictures were taken by Axio Imager 4.6.3 Upright Microscope by Carl Zeiss AG using the red coded objective, EPIPLAN 5X/0.13HD with a magnification of 50. 3. Results The results are viewed against the complex configuration of the emulsions, Fig. 1A. The essential features of the configuration are identical to those presented in an earlier publication [46]. SO forms the continuous phase due to its sizeable volume ratio with the compound second in volume, VO, dispersed in it. The water in turn is located as drops within the VO liquid and, finally, Janus combination drops of VO and SO are found inside these water drops, Fig. 1B; giving a total triple emulsion configuration of (VO + SO)/ W/VO/SO. These characteristics have been described [45,46] and the present contribution is focused on the changes during the destabilization process. The surfactant is soluble only in the water and as a result, both the outer VO/SO interface and the W/VO interface were unstable, leading to fast coalescence of the (VO + SO)/W/VO drops. In
Fig. 1. Microphotograph of sample from the middle of the emulsion layer.
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Fig. 2. The volume fraction of the separated top VO layer. (The results are from two independent determinations).
addition extensive creaming took place, since the density of a dispersion of the W/VO combination with a weight ratio of approximately 0.56, comes to 0.944; significantly less than the density of SO, 0.97. The creaming, in turn, led to a high internal volume emulsion in the top part with increased flocculation/coalescence rate and subsequent separation of a top layer of VO. Fig. 2 reveals this division to be fast; after 20 min the separation was virtually complete, which is realistic considering the size of the drops and the viscosity of the continuous phase. The spread of the results from two determinations is substantial, as expected from the complexity of reactions, but the trend is undisputable. In addition a bottom layer of an aqueous emulsion separated, but the time dependence of its formation showed such an extensive spread that a numerical analysis was not deemed realistic. The excessive spread was anticipated as a result of composite and interconnected events; the creaming of the original drops, the coalescence in the top part of the emulsion, the sedimentation of the drops from this reaction and the final coalescence. Before entering into an account of the mechanisms, it is useful to compare the magnitude of the separated layer with the composition of the emulsion. The weight fraction of the VO is 0.18, corresponding to a volume fraction of 0.19, indicating virtually all of the VO as separated within 20 min. In itself, such results of a fast separation of the VO layer would be no novelty and of no general interest. Any emulsion, which is not protected against coalescence, would show similar behavior, unless the processes were retarded by high viscosity of the continuous
A
C
phase. The reason to draw attention to the present results is the complex and novel configuration of the emulsion, Fig. 1. The destabilization of such a multifaceted emulsion involving both coalescence and creaming resulted, as anticipated, in a separation of a layer continuous in VO, Fig. 2. However, that summary process must, with necessity, be composed of a series of complex configurations and developments and a preliminary examination of these is the main raison d’etre for this publication. The thorough quantitative treatments of the flocculation/ coalescence processes progressing from Ivanov [47] to recent advanced analysis [48] are not useful, or even feasible, for the emulsions in Fig. 1, before more experimental information is available about the intricate morphological changes. Hence, the attention is limited to the relationship between the drop configuration and the simultaneous coalescence and the gravitational movements. In fact the difference in densities of VO, 0.914 g/cm3 and W, 0.995 g/cm3 versus that of the continuous phase of 0.970 g/ cm3, means a potential exists for both creaming and sedimentation. The attention was accordingly centered on the configurations in the top and bottom part of the emulsion and the variation in the microphotograph features from these layers with time offers initial information, Fig. 3. The fast events in the top layer, Fig. 2, are reflected in the radical change after 15 min, Figs. 3A and B, revealing an inversion from the original (VO + SO)/W/VO emulsion, Fig. 3A, to a combination emulsion, Fig. 3B, of single drops of SO and W as well as Janus drops in a continuous VO phase. In addition to these short term changes in the top part of the emulsion, a new layer appeared in the bottom, which from its color obviously was a water continuous dispersion. The kinetics of its formation could not be enumerated, because the times for its emergence strongly varied from half an hour to several days. The configuration in the top and bottom layers of the freshly prepared emulsions, Fig. 3A and C, is similar to the pattern in Fig. 1. However, after 15 min the top layer is radically changed, as anticipated from the fact that a VO continuous emulsion had separated. Instead of the complex emulsion (VO + SO)/W/VO/SO, Fig. 3A, the system now consists of a continuous VO liquid with small independent drops of W and SO and a few Janus drops, Fig. 3B. An inversion has taken place; VO is now the continuous phase instead of SO. It is of note that the inversion of the outer VO (marked as VO, Fig. 3A) to become continuous phase also strongly impinged on the inner emulsion (VO + SO)/W, whose large
W
B
D
E
Fig. 3. Micrographs of the top, (A and B) and bottom, (C and D) parts of the emulsion immediately after preparation, (A and C), after 15 min, (B and D) and 2 h of storage, E.
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Fig. 4. Two patterns of the upper emulsion part (Top) after 6 min storage and of the lower part after 60 and 120 min (Btm).
drops were transformed into small and separate W and SO droplets. In a subsequent event, drops with substantial fraction of W sedimented towards the bottom,forming a double emulsion, W/ VO/SO after 15 min, Fig. 3D, followed by coalescence and inversion to a W continuous emulsion, Fig. 3E. These inclusive trends are evident from the microphotographs; the magnitude of the volume of VO in the original (VO + SO)/W/ VO drops, Fig. 1, resulted in a sufficiently low density for the combination drops to rise in the emulsion according to the analysis in the Discussion section. The ensuing crowding of drops in the top part of the emulsion caused coalescence followed by inversion. The coalescence was intrusive as made evident by the difference between Figs. 3A and B and as a result W drops and/or drops with sufficient W content were found in the bottom part of the emulsion, Fig. 3D. Although it was not possible to record these transformations in the top part of the emulsion in detail, configuration modifications in the emulsion immediately beneath and on top of the respective separated layers leave some indication of the complexity of intermediate stages. The image of the drops top part of the emulsion after 6 min as a realistic intermediate between 0 and 15 min and between 60 and 90 min of the bottom layer is useful to illustrate potential transitional configurations, Fig. 4. The patterns in the figures do exemplify the complexity of the drop arrangement, but the shape of the individual drops may be distorted, because of the effect of the cover glass. The emulsion top level at 6 min and its bottom height between 60 and 90 min after formation are both distinguished by the large continuous VO and SO areas, Fig. 4; calling attention to an ongoning coalescence. The preponderance of VO in the top compared to the pattern of the bottom layer is an expected outcome of the creaming/sedimentation events. As a matter of fact there are small W drops in the VO liquid, Fig. 4A and B, which prove that the coalescence of the (VO + SO)/W/VO drops also involved the inner parts
of these complex globules. In brief, the convoluted patterns in Fig. 4 are rational intermediates both for the minimal inversion on top levels, which only reach to the first layer of the drops and for the more in-depth inversion in the bottom emulsion layer. The authors are aware that the microscopy images may not truly reflect the detailed bulk morphology of the emulsions, but felt the patterns in Fig. 4 to be useful as illustrations of the potential complexity of the intermediate configurations. 4. Discussion The destabilization process gave rise to a number of phenomena, which merit a discussion, but the truly fundamental issue is the mutual effects in the creaming/sedimentation/coalescence combination. Since the initial emulsion is a triple Janus emulsion, (VO + SO)/W/VO/SO, Fig. 1, the vital condition is the density of the (VO + SO)/W/VO drops versus 0.97 g/cm3, the density of the continuous phase SO. The density of such a drop in g/cm3 is
q ¼ ðX W þ X VO þ X SO ðiÞ þ X S Þ=ððX W =0:995 þ ðX VO =0:914Þ þ ðX SO ðiÞ=0:97Þ þ ðX S =1:03ÞÞ
ð1Þ
in which XSO(i) is the weight fraction of SO located within the W drop and the densities were computed by assuming a partial molar volume equal to the one for the pure compounds. The direction of the creaming or sedimentation is decided by Dq = qC 0.97 g/cm3 with a negative sign indicating creaming and the opposite sedimentation. As is immediately evident, the presence of SO within the W drop has no effect on the direction per se and was left out in the continued analysis. For an ‘‘average’’ drop, for which XW = 0.097, XVO = 0.18, XS = 0.003, in which expressions X represents weigh fraction of the compound indicated by the subscript. The resulting q is 0.941 g/ cm3 and creaming will obviously take place leading to an increase
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585
A conservative estimate from Eq. (3) with cos b 0 and
c13 = 1 m Nm1 shows drops to be of dimensions far in excess of those found in the VO emulsion and a conclusion of all the drops at the bottom level originating in the top SO emulsion part appears well founded. 5. Conclusions Fig. 5. The shape and dimensions of pendant drop of compound 1 at the interface between compounds 2 and 3 (From [49] with permission).
of the number of the complex drops at the top of the original emulsion, where a high internal phase ratio emulsion will form, Fig. 4. This emulsion, in turn, changes in two ways; the increased fraction of dispersed drops leads to enhanced coalescence rate and the separation of the VO emulsion, Fig. 3B. The loss of VO from the complex drops during coalescence leads to increased density of the residual drops and, when the W/VO ratio has reached a level to bring the density in excess of 0.97, the drops sediment. It is of interest to calculate the remaining fraction, kXVO, of the total VO content to reach a density of 0.97 g/cm3.
Acknowledgment
0:97 ¼ ð0:1 þ 0:18kÞ=ðð0:097=:995Þ þ ð0:003=1:03Þ þ ð0:18k=0:914ÞÞ
ð2Þ
Solving gives k = 0.24, indicating three quarters of the VO must be separated before sedimentation commences of the complex drops. Fig. 3D illustrates the fact the drops found towards the bottom of the emulsion contain but little VO in comparison with the W content. These heavier drops will sediment through the entire SO emulsion layer prior to coalescence and separation as an aqueous emulsion at the bottom of the test tube. This fact in combination with the smaller density difference is reflected in the difference in time scale for the formation of the top VO and the bottom W emulsions, as well as the huge variation in the times for the latter to materialize. The picture actually also contains small drops of VO, which obviously have not sedimented from the higher levels of the VO continuous emulsion. Instead the origin of these drops is assumed as intermediates in the coalescence process, Fig. 4, where small drops of VO is not an unrealistic outcome of such a process in highly complex configurations. In summary, the average W/VO/SO ratio leads to creaming of (VO + SO)/W/VO drops, which leads to crowding in the top part of the emulsion and increased coalescence of these drops, which sequentially leads to inversion and separation of VO into a top emulsion layer. The coalescence leaves drops rich in W, because some VO is removed forming a separate VO layer, Fig. 2. The W drops with a greater fraction of W, Fig. 4, sediment and coalesce to form a separate (SO + VO)/W emulsion, Fig. 3, picture 7. These (SO + VO)/W drops (or (SO + VO)/W/VO drops with sufficient W/ VO ratio for sedimentation) may emanate from the upper SO emulsion layer or even from the top VO emulsion. In the latter case the drop must pass through the VO/SO interface and it is of interest to estimate the feasibility of such a transit. The fundamentals of the equilibrium between gravitational and interfacial tension forces for a pendant drop at a liquid interface was given by Kaptay [49], who determined the maximum drop size of a Cu2S–FeS, matte, floating on liquid low iron wollastonite slag in the copper matte smelting process. With the interfacial tensions (c), densities (q) and contact angle (b) in Fig. 5 [49], the horizontal projection of a drop of critical size was given as
Rcr ¼ ½ð2 c1=3 ð1 þ cos bÞ=ðg ðq1 q3 Þ1=2
The destabilization processes in a triple Janus emulsion (vegetable oil + silicone oil)/water/vegetable oil/silicone oil was investigated using visual observation of the separation of layers and microscopy patterns of the drop configuration. The results revealed the destabilization to be a complex process with the initial step a creaming of the (VO + SO)/W/VO drops in the SO continuous emulsion. The creaming led to a high internal volume ratio emulsion in the top part of the SO emulsion with increased coalescence. This process in turn resulted in an inversion to an (SO + VO)/W/VO emulsion, while leaving complex drops with sufficient density from an increased W/VO ratio to sediment in the SO emulsion. The ensuing coalescence at lower levels of the emulsion layer gave rise to a separated W continuous emulsion.
ð3Þ
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