7612
Langmuir 2000, 16, 7612-7617
Effects of Surfactants on Water Transport in W1/O/W2 Emulsions Lixiong Wen and Kyriakos D. Papadopoulos* Department of Chemical Engineering, Tulane University, New Orleans, Louisiana 70118 Received January 19, 2000. In Final Form: July 17, 2000 The effects of oil-soluble and water-soluble surfactants on water transport rates in W1/O/W2 emulsions under osmotic pressure were studied at the single-globule level by using capillary video-microscopy technique. By changing the constitution of the internal aqueous compartment W1, the intervening oil O, and the outer aqueous suspending phase W2, these effects were further studied for different controlling water transport mechanisms. Experiments were conducted at visual contact, when water is mainly transported via hydrated surfactants, and at no visual contact, in which case water migration occurs primarily in the forms of spontaneously emulsified droplets and reverse micelles. Water transport rates were found to rise linearly with increasing oil-soluble surfactant concentration in the oil phase over a significant range, irrespective of the transport mechanism, though the effects are more pronounced when water is transported via hydrated surfactants. In contrast to the system-stabilizing effect of the oil-soluble surfactant, water-soluble surfactants in both W1 and W2 phases always weaken the stability of the emulsion globules. Water-soluble surfactants in W1 exhibited different effects on water transport than those in W2. Although the eventual tendency is that the water transport rates increase with increasing water-soluble surfactant concentration in W1, a small amount of water-soluble surfactants in W1 will retard transport from W1 to W2 for all transport mechanisms as compared to when there is no surfactant in W1. Water-soluble surfactants in W2 phase always accelerate the water transport regardless of their concentrations.
Introduction Under osmotic pressure gradients between the two aqueous phases of W1/O/W2 emulsions, water may migrate either from W1 to W2 or vice versa, depending on the direction of the osmotic pressure gradient. Such water transport may be critical, either as an enhancing or a diminishing factor, in the various applications of multiple emulsions in the food industry,1 agricultural formulations,2 red blood substitutes,3,4 separation processes,5-7 cosmetics,8 pharmaceuticals,9-11 etc. It has previously been reported that water transport rates in multiple emulsions are affected by the magnitude of the osmotic pressure gradients between phases W1 and W2, the nature and concentrations of the surfactants used for the preparation of the emulsions, the nature and viscosity of the oil phase, etc.12-17 However, predicting and controlling the mechanisms of water migration as functions of important * Corresponding author. (1) Dickinson, E.; Evison, J.; Owusu, R. K. Food Hydrocolloids 1991, 5, 481. (2) Matsumoto, S.; Kita, Y.; Yonesawa, D. J. Colloid Interface Sci. 1976, 57, 353. (3) Zheng, S.; Beissinger, R. L.; Wasan, D. T. J. Colloid Interface Sci. 1991, 144, 72. (4) Zheng, S.; Zheng, Y.; Beissinger, R. L.; Wasan, D. T.; McCormick, D. L. Biochim. Biophys. Acta 1993, 1158, 65. (5) Li, N. N. U.S. Patent 1968, 3,410,794. (6) Raghuraman, B.; Tirmizi, N.; Wiencek, J. Environ. Sci. Technol. 1994, 28, 1090. (7) Larson, K.; Raghuraman, B.; Wiencek, J. Ind. Eng. Chem. Res. 1994, 33, 1612. (8) Tadros, T. F. Int. J. Cosmet. Sci. 1994, 14, 93. (9) Frankenfeld, J. W.; Fuller, G. C.; Rhodes, C. T. Drug Dev. Commun. 1976, 2, 405. (10) Florence, A. T.; Whitehill, D. Int. J. Pharm. 1982, 11, 277. (11) Florence, A. T. Chem. Ind. 1993, 20, 1000. (12) Matsumoto, S.; Kohda, M. J. Colloid Interface Sci. 1980, 73, 13. (13) Matsumoto, S.; Inoue, T.; Kohda, M.; Ikura, K. J. Colloid Interface Sci. 1980, 77, 555. (14) Magdassi, S.; Garti, N. Colloids Surf. 1984, 12, 367. (15) Magdassi, S.; Garti, N. J. Control. Release 1986, 3, 273. (16) Omotosho, J. A.; Whateley, T. L.; Law, T. K.; Florence, A. T. J. Pharm. Pharmacol. 1986, 38, 865.
system parameters is still a major challenge and only a few studies about the effects of surfactants on the water transport have been reported.13,18-20 In their work on the water permeability of the oil phase in W1/O/W2 emulsions under osmotic pressure gradients between the two aqueous phases, Kita et al.21 proposed two possible mechanisms for the permeation of water and water-soluble materials: water molecules (1) “pass through thin lamellae of surfactants which partially form in the oil layer due to fluctuation of its thickness” and (2) diffuse across the oil layer by being incorporated in “reverse micelles,” while Colinart et al.22 had earlier suggested reverse-micellar water transport as well as via hydrated surfactants as the two possible ways for water migration of multiple emulsions. By measuring the rate of change in the size of the dispersed globules of water/hydrocarbon/water emulsions, Matsumoto et al.13 found a trend of decreasing water permeation coefficients with increasing oil-soluble surfactant (Span 80) concentration from 10 to 50% w/w in the oil phase. This result was later explained by Garti et al.18 via the influence of increasing viscosity in the oil phase, caused by high Span 80 concentration. Garti et al.18 also reported that the water permeation coefficient increased with oil-soluble surfactant concentration in the oil phase increasing from 2 to 20% w/w and approached an asymptotic value. Jager-Lezer et al.20 verified that oilsoluble surfactant is a major factor for water migration (17) Omotosho, J. A.; Whateley, T. L.; Florence, A. T. J. Microencapsul. 1989, 6, 183. (18) Garti, N.; Magdassi, S.; Whitehill, D. J. Colloid Interface Sci. 1985, 104, 587. (19) Garti, N.; Romano-Pariente, A. Colloids Surf. 1987, 24, 83. (20) Jager-Lezer, N.; Terrisse, I.; Bruneau, F.; Tokgoz, S.; Ferreira, L.; Clausse, D.; Seiller, M.; Grossiord, J.-L. J. Controlled Release 1997, 45, 1. (21) Kita, Y.; Matsumoto, S.; Yonezawa, D. Nippon Kagaku Kaishi 1978, 1, 11. (22) Colinart, P.; Delepine, S.; Trouve, G.; Renon, H. J. Membr. Sci. 1984, 20, 167.
10.1021/la000071b CCC: $19.00 © 2000 American Chemical Society Published on Web 08/26/2000
Water Transport in W1/O/W2 Emulsions
in multiple emulsions and that water transport rates increase with increasing oil-soluble surfactant concentration. Their work also showed that water-soluble surfactant present in the water phases has no effect on water transport, except at excessive amounts when it destabilizes the emulsion. It should be noted that all the above studies were conducted in bulk double-emulsion systems, where water transfer between the aqueous phases can be caused not only by facilitated diffusion mechanisms but also by the breakdown of oil globules, and where one mechanism cannot be distinguished from the other. Therefore, water transported by facilitated diffusion alone, through one or more mechanisms, could not be quantitatively determined. The authors of the present paper recently reported23 a visualization study of the water transport in W1/O/W2 emulsions under osmotic pressure using capillary videomicroscopy, where W1 represents the internal pure-water droplets, O represents the intervening oil phase, and W2 the suspending saline-water medium. When W1 and W2 were in visual contact, water transport occurred mainly through the hydrated surfactant mechanism, whereas in the case of visually noncontacting W1 and W2, that is, when there exists a visible minimum distance of separation between W1 and W2, the mechanisms were those of spontaneous emulsification and reverse micellization. In the latter case the water transport rates were found to be significantly lower than when W1 and W2 were in visual contact. In that study it was also proposed that the water transport rates are controlled by interfacial processes and not by facilitated diffusion.23 Since different mechanisms control the water transport rates between visually contacting and noncontacting W1/O and O/W2 interfaces, the effects of surfactants on the water transport in multiple emulsions are expected to be different in these two cases. In this paper, a single W1/O/W2 globule was prepared by using our capillary video-microscopy technique to exclude water transport occurring simultaneously through facilitated diffusion and breakdown of the oil globules. By further manipulating the W1 and W2 at either visually contacting or noncontacting positions, the effects of oil- and water-soluble surfactants on water transport between visually contacting and noncontacting W1/O and O/W2 interfaces, that is, the surfactant effects on different water transport mechanisms, have been studied and significant differences have been found. The surfactants’ effects on the stability of the emulsion globules were also reported. Experimental Section
Langmuir, Vol. 16, No. 20, 2000 7613
Figure 1. Schematic illustration of the W1/O/W2 system inside a capillary. h is the visual minimum separation distance between the W1/O and O/W2 interfaces. It is assumed that h ) 0 when W1/O is at visual contact with O/W2. All experiments were conducted on an Olympus IMT-2 inverted microscope connected to a high-resolution monitor (SONY) and an S-VHS VCR through a COHU CCD camera. The microscope is also equipped with two three-dimensional hydraulic micromanipulators (Narishige, Japan), two microinjection systems (IM-200, Narishige, Japan), and a PC loaded with OPTIMAS image analysis software (Bioscan Inc., WA) for capturing images and measuring the size change of the droplets with time. For preparing the W1/O/W2 globules, an internally hydrophobic microcapillary was filled with n-hexadecane containing Span 80 at some desired concentration and fixed on a capillary holder, which was then placed onto the stage of the microscope. A nontreated micropipette filled with NaCl aqueous solution was inserted into the middle of the microcapillary with the micromanipulator. By controlling the microinjection system, two large saltwater drops were injected and thus formed a small oil bank between them in the middle of the microcapillary, as shown in Figure 1. Another micropipette with hydrophobic tip was then inserted into the oil bank from the other end of the microcapillary and injected an aqueous drop at a different position in the oil bank each time an experiment was conducted. This preparation takes advantage of the oil’s high viscosity thus making it easy to micromanipulate the formed globules. An osmotic pressure gradient between the aqueous W1 and the saline W2 phases existed, forcing water to be transported from W1 to W2. This preparation allows observation of a single globule throughout the entire experiment and thereby makes it possible to distinguish the water transported by facilitated diffusion from that by coalescence between W1 and W2. Water transport rate was defined as the radius change rate of W1:
water transport rate ) volume change of W1 ) (unit surface area of W1) (unit time) radius of change of W1 unit time
Highly purified water (18.1 MΩ-cm resistivity) was obtained from a Barnstead E-pure water purification system. n-Hexadecane (99% pure), NaCl (99% pure), and water-soluble surfactant Tween 80 (polyoxyethylene (20) sorbitan monooleate) were purchased from Aldrich. Oil-soluble surfactant Span 80 (sorbitan monooleate) was purchased from Sigma. All were used as received without further purification. The preparation of microcapillaries and micropipettes and the experimental setup are similar as reported before.23-25 Using a micropipette puller (Narishige PB7, Japan), microcapillaries (∼200 µm i.d., (10 µm) were fabricated from melting point tubes (1.1-1.2 mm i.d. × 100 mm, Corning) and injection micropipettes with fine tip, used to form water or oil droplets, were shaped by pulling microtubes (Microcaps, 0.688 mm i.d. × 78 mm, Drummond Scientific) from one of their ends so as to produce a tip of outside diameter 10-15 µm. The inside surface of the microcapillaries and the tip of the micropipettes were rendered hydrophobic by following the procedure described previously.23-25
The size of W1 can be measured with the OPTIMAS system by analyzing still frames of the captured images at set time intervals, which allow determination of the water transport rates. The OPTIMAS system has a measuring resolution of about 0.4 µm and the measurements were conducted at time intervals corresponding to ∼4 µm size changes. Therefore, the measurement error for the water transport rate is within a range of 10%. Another source of error is associated with the distortion of the captured images caused by the curving of the microcapillary wall. Careful calibrations have been done under different conditions and an extra error within 5% is introduced into the measurement by this distortion. All experiments were conducted at room temperature.
(23) Wen, L.; Papadopoulos, K. D. Colloids Surf. 2000, 174, 159. (24) Hou, W. Q.; Papadopoulos, K. D. Chem. Eng. Sci. 1996, 51, 5043. (25) Hou, W. Q.; Papadopoulos, K. D. Colloids Surf. 1997, 125, 181.
Effects of Oil-Soluble Surfactant on Water Transport. Oil-soluble surfactants have long been considered as the primary carriers for water transfer in W1/O/W2
Results and Discussion
7614
Langmuir, Vol. 16, No. 20, 2000
Figure 2. W1/O and O/W2 interfaces were managed to be at (a) visual contact and (b) no visual contact.
emulsions.22,26 As proposed previously,23 the hydratedsurfactant mechanism controls the water transport rates when W1 and W2 are in visual contact, while in the case of a visible minimum distance of separation between W1 and W2, water transport occurs mostly through spontaneous-emulsification and reverse-micellar mechanisms. These are completely different mechanisms and the effects of the oil-soluble surfactants on water transport are expected to be different in these two cases. Experiments were performed at different Span 80 concentrations in the oil phase with the same internal W1 drop and external suspending aqueous phase, which were pure water and 5 M NaCl solution, respectively. The W1 drop was micromanipulated so as to touch or to be separated from W2, as shown in Figure 2. The radius change rates of the W1 drops were measured as the water transport rates. As shown in Figure 3, water transport rates between both visually contacting and noncontacting W1/O and O/W2 interfaces increased near-linearly with increasing Span 80 concentration in the oil phase over a significant range of surfactant concentration. Therefore, as carriers of water, the oil-soluble surfactant enhances the water transport (26) Mok, Y. S.; Lee, W. K. Sep. Sci. Technol. 1994, 24, 743.
Wen and Papadopoulos
Figure 3. Effects of Span 80 concentration in oil on water transport rates in W1/O/W2, when the W1/O and O/W2 interfaces are at (a) visual contact and (b) no visual contact. W1 ) pure water; O ) n-hexadecane + Span 80; W2 ) 5 M NaCl solution.
rates considerably irrespective of the transport mechanism and is a major contributor to mass transfer in W1/O/W2 emulsions. Although for each given water transport mechanism the water transport rate increases linearly with increasing surfactant concentration, the relationship between the overall water transport rate and surfactant concentration in bulk systems are expected to be nonlinear as reported before,18,20 the reason being that all mechanisms make contribution to water migration in a bulk system. In Figure 3 it may also be seen that when Span 80’s concentration in the oil phase increased from 0.05 to 0.5 M, for the visually contacting W1/O and O/W2 interfaces when water is transported via hydrated surfactants, the transport rate rose almost 6 times, whereas at no visual contact when water is transported through spontaneousemulsification and reverse-micellar mechanisms, the rate increased approximately 3 times. This result indicates that, although oil-soluble surfactants always enhance the water migration, the extent of their effects is different when different mechanisms control the water transport rates, and the effects are greater when water is transported via hydrated surfactants. For visually contacting W1/O
Water Transport in W1/O/W2 Emulsions
Langmuir, Vol. 16, No. 20, 2000 7615
Figure 4. Radius change rate of W1 remained nearly constant at different oil-soluble surfactant concentrations, indicating that water transport rates are controlled by interfacial processes. W1 ) pure water; W2 ) 5 M NaCl solution; O ) n-hexadecane +Span 80: (a) 0.05 M Span 80, at visual contact; (b) 0.05 M Span 80, at no visual contact; (c) 0.5 M Span 80, at visual contact; (d) 0.5 M Span 80, at no visual contact. Table 1. Effects of Water-Soluble Tween 80 Surfactant in W1 on the Stability of W1/O/W2 Globulesa W1 (Tween 80, M)
W2 (NaCl, M)
rupture time
W1 (Tween 80, M)
W2 (NaCl, M)
rupture time
W1 (Tween 80, M)
W2 (NaCl, M)
rupture time
0 0.004 0.01 0.05 0.1
0 0 0 0 0
>2 h >2 h >2 h >2 h >2 h
0 0.004 0.01 0.05 0.1
1 1 1 1 1
b b b 30-40 min 8-15 min
0 0.004 0.01 0.05 0.1
5 5 5 5 5
b 5-10 min 1-6 min 4-20 s 1-5 s
a O ) n-hexadecane + 0.1 M Span 80; visual contact between W and W ; the diameter of the original W drops was 60-70 µm. b No 1 2 1 rupture of the oil film occurred until all water in W1 diffused into W2.
and O/W2 interfaces, when water transport rates are controlled by a hydrated surfactant mechanism, the processes of hydration and dehydration of the surfactants are enhanced with increasing Span 80 concentration, thus accelerating the water transport. In the case of non visually contacting W1/O and O/W2 interfaces, adding oil-soluble surfactant speeds up the formation and detachment processes of spontaneously emulsified droplets and reverse micelles from W1/O interface to enhance water transport. Although the water transport rates changed with increasing oil-soluble surfactant concentration, the radius change rate of W1 drop remained nearly constant during each experiment as shown in Figure 4, indicating that the water transport rates are always controlled by interfacial processes at the various surfactant concentrations. Effects of Water-Soluble Surfactant in W1 on Stability. By changing the concentration of Tween 80 in the W1 drops and the NaCl concentration in the W2 phase, while keeping the oil phase the same (n-hexadecane + 0.1 M Span 80), it was found that the stability of W1/O/W2
globules is affected greatly by both the water-soluble surfactant concentration and the osmotic pressure gradient between the two aqueous phases. Adding either Tween 80 in W1 or NaCl in W2 can weaken the stability. Table 1 shows that when both W1 and W2 were pure water, the system was stable with 0.1 M Span 80 in the oil and no coalescence between W1 and W2 occurred. Even if there was some Tween 80 in W1 when W2 was pure water, the globules were still stable. When W2 was a 1 M NaCl solution, though no coalescence was seen at low Tween 80 concentrations, it did happen when Tween 80 concentration was above a certain value and adding more Tween 80 in W1 shortened the rupture time. In the case when W2 was a 5 M NaCl solution, coalescence occurred at all Tween 80 concentrations and was intensified with increasing Tween 80 concentration in W1. The experiments also showed that with constant Tween 80 concentration in W1, increasing the osmotic pressure gradient between the two phases could enhance the rupture of the oil film between W1 and W2. For example, with Tween 80 in W1 remaining
7616
Langmuir, Vol. 16, No. 20, 2000
0.1 M, no coalescence was seen when W2 was pure water, while oil film rupture occurred shortly after preparation when W2 was 5 M NaCl solution. It is well-known that the presence of oil-soluble surfactant in the oil is the primary contributor to stability in W1/O/W2 emulsions.24-27 When there are water-soluble surfactants in the aqueous phases, especially at high concentrations, it is possible that the water-soluble surfactants can solubilize the oil-soluble surfactants from the oil phase thus reducing the stability of the system.20 That explains why coalescence occurred in the presence of Tween 80 in W1 under conditions that produced no coalescence in its absence. Effects of Water-Soluble Surfactant in W1 on Water Transport. Before rupture of the oil film occurred as described in the previous section, or when the W1 drop was micromanipulated to keep a visible minimum separation distance from W2 phase, the change of the size of W1 drop could be measured as a function of time and the water transport rate was thereby obtained. Experiments at different Tween 80 concentrations in W1 were conducted between both visually contacting and noncontacting W1/O and O/W2 interfaces with 0.1 M Span 80 in the oil and 1 M NaCl solution as W2, and the results were shown in Figure 5. In both cases of visually contacting and noncontacting W1/O and O/W2 interfaces, the water transport rates were lower over a significant Tween 80 concentration range compared to transport in the absence of Tween 80 in W1. Though the tendency is to increase the water transport rates eventually with increasing Tween 80 concentration and, as shown in Figure 5a, the water transport rate at 0.1 M Tween 80 exceeded that of pure water, this result has little practical significance at such high water-soluble surfactant concentrations. Therefore, at concentrations of water-soluble surfactant in W1 which do not cause the globules to rupture, the water transport rate from W1 to W2 will decrease. As a consequence, those applications of multiple emulsions that demand sustained (slow) release of a water-soluble substance from W1 to W2 may benefit from minute amounts of water-soluble surfactants in W1. As is already known, no matter what mechanism the water is transported through, the oil-soluble surfactants are acting as carriers and the water transport rates increase with increasing oil-soluble surfactant concentration in the oil. A small amount of Tween 80 in W1 can solubilize Span 80 from the oil phase20 while it may not participate directly in the water-transport process, therefore decelerating water migration. It is speculated that at high concentrations, while still solubilizing Span 80, Tween 80 may also facilitate the hydration of surfactants and the formation of reverse micelles and spontaneously emulsified droplets, thus promoting water transport. Effects of Water-Soluble Surfactant in W2 on Stability and Water Transport. When compared to W1, the presence of water-soluble surfactant in W2 has more drastic effects on globule stability, and also some different effects on water transport from W1 to W2. For the same oil phase (n-hexadecane + 0.1 M Span 80) and W1 droplet (pure water), experiments were conducted at different Tween 80 concentrations in the outer aqueous phase, W2 (1 M NaCl solution). The results are shown in Table 2. When the W1 drop was placed in visual contact with the W2 phase containing Tween 80, coalescence happened shortly after preparation at such low Tween 80 concentrations, which did not produce coalescence when in W1, (27) Ficheux, M.-F.; Bonakdar, L.; Leal-Calderon, F.; Bibette, J. Langmuir 1998, 14, 2702.
Wen and Papadopoulos
Figure 5. Minute amounts of water-soluble surfactant in W1 can retard the water transport in W1/O/W2 emulsions. W1 ) H2O + Tween 80; O ) n-hexadecane + 0.1 M Span 80; W2 ) 1 M NaCl solution. W1/O and O/W2 interfaces are at (a) visual contact and (b) no visual contact. Table 2. Effects of Water-Soluble Tween 80 Surfactant in W2 on Stability and Water Transport in W1/O/W2 Globulesa W2b 0 0.005 0.01
rupture timec >2 h ∼1 min 5-20 s
water transport rated 0.17 0.31 0.40
a O ) n-hexadecane + 0.1 M Span 80; W ) pure water; the 1 diameter of the original W1 drops was ∼100 µm. b Tween 80, M, in c d 1 M NaCl solution. W1 contacts with W2. -dR/dt, µm/min, no contact between W1 and W2.
as shown in Table 1. This suggests a greater destabilizing effect of this surfactant when it is present in W2 than in W1. When we prevented the W1 drop from approaching W2 by using appropriate micromanipulation, water was transported from W1 to W2 via spontaneous emulsification and reverse micellar mechanisms. Unlike the presence of water-soluble surfactant in W1, which retarded the water transport at very low concentrations, Tween 80 in W2 always increased the water transport rates irrespective of its concentration. Finally, because Tween 80 increased the fluidity of the system dramatically, thus causing
Water Transport in W1/O/W2 Emulsions
significant deformation and hindering micromanipulation, W1/O/W2 globules were not possible to prepare in this study at high concentrations in W2. Multiple emulsion applications may benefit from the findings of this study through the following considerations. In cases where controlled and sustained release is desired, if the W1 droplets can be engineered so as not to be in contact with the W2 phase, very low mass transfer rates can be achieved. Small amounts of water-soluble surfactant in W1 as well as low oil-soluble surfactant concentrations should be kept in such cases. If high release rates are needed, then the W1 droplets should be designed so as to contact as much of the O/W2 interface as possible, while both the oil- and the water-soluble surfactants should be present at high concentrations. Conclusions Using capillary video-microscopy on the water transport in W1/O/W2 emulsion globules, it was found that for visually contacting and noncontacting W1/O and O/W2 interfaces alike, water transport rates increased nearlinearly with increasing Span 80 concentration in the oil phase over a significant range of surfactant concentration. This behavior is much more pronounced in visually contacting W1/O and O/W2 interfaces where water is transported via hydrated surfactants than in noncon-
Langmuir, Vol. 16, No. 20, 2000 7617
tacting interfaces where the mechanisms are those of spontaneous emulsification and reverse micellization. Contrary to the action of the oil-soluble surfactant in the oil phase, water-soluble surfactant in both W1 and W2 weakens the stability of the emulsion globules and, above certain concentrations, causes the rupture of the oil film, leading all materials in W1 to be released to W2. The speed of the oil film rupture increases with increasing watersoluble surfactant concentration. It was also found that the stability of double-emulsion globules is affected by the osmotic pressure gradient between the two aqueous phases, as higher osmotic gradients produced lower stability. Although, in general, water transport rates increase with increasing water-soluble surfactant concentration in W1, minute amounts of Tween 80 in W1 retard water transport from W1 to W2 irrespective of the transport mechanism as compared to when there is no surfactant in W1. Unlike when present in W1, water-soluble surfactant in W2 always increases the water transport rates, even at very low concentrations. Acknowledgment. The authors gratefully acknowledge the financial support provided by The American Chemical Society’s Petroleum Research Fund. LA000071B
