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Characterization of surfactants used for monodispersed oil-in-water microspheres production by microchannel emulsification
Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) '^c) 2001 Elsevier Science B.V. All rights reserved.
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Characterization of surfactants used for monodispersed oil-in-water microspheres production by microchannel emulsification Jihong Tong, Mitsutoshi Nakajima/ Hiroshi Nabetani, and Yuji Kikuchi National Food Research Institute, Ministry of Agriculture, Forestry and Fisheries, Kannondai 2-1-2, Tsukuba, Ibaraki 305-8642, Japan Super-monodispersed oil-in-water (O/W) microspheres (MS) were produced using a microchannel (MC) emulsification technique. The effect of the surfactant on the behavior of the O/W-MS formation was investigated using various surfactants for the MC emulsification process. It was found that the super-monodispersed O/W-MS production depends on the ionic type of the surfactants used. The results indicated that it is very important to maintain the hydrophilicity of the MC surface during the MC emulsification process. 2. INTRODUCTION Microspheres (MS), which are emulsion cells or solid particles dispersed in a continuous phase, have been utilized in various industries such as foods, cosmetics and pharmaceuticals, etc. Emulsions are dispersed multiphase systems consisting of two or more almost mutually insoluble liquids, with the dispersed phase present in the form of droplets in a continuous phase. Recently Kawakatsu et ai (1) have proposed a novel microchannel (MC) emulsification technique for super-monodispersed MS production. It is anticipated that the MS with monodispersibility may exhibit higher quality and stability than that produced by conventional techniques. It is known that surfactant is indispensable for forming and stabilizing emulsions or MS (2). In order to understand the mechanism of how a surfactant functions in the MC emulsification process, we investigated the surfactant effect on the behavior of the O/W-MS formation by using various kinds of surfactants. 2. MATERIALS AND METHODS 2.1. Materials High-oleic sunflower oil (triolein, >90% purity) was obtained from Nippon Lever B.V., Tokyo, Japan. Sodium oleate, oleic acid, sodium dodecyl sulfate (SOS), tri-noctylmethylammonium chloride (TOMAC) and polyoxyethylene (20) sorbitan monooleate (Tween 80, HLB: 15.0) were purchased from Wako Pure Chemical Ind., Osaka, Japan. Di-2ethylhexyl sodium sulfosuccinate (AOT) was purchased from Sigma Chemical Co., St. Louis, MO, USA. All materials were reagent grade and were used without further purification.
Corresponding author, E-mail: [email protected]
1056 2.2. Experimental system and procedure. An MC plate with dimension of 15 mm x Bottom view 15 mm X 0.5 mm, 600 Liquid channels around the 4 chamber sides, and an equivalent diameter of 8.9 ^im for one channel was used. SiUcon MC Plate The experimental apparatus was shown in Glass Plate Fig. 1. A module Oil phase Microscope installed with an MC flow plate adhering to a flat direction glass plate was filled Partition wall with a water phase. An ' Terrace oil phase chamber Microscope video system connected to the module Schematic b) Schematic diagram by a silicone tube (a) MC plate supplied the dispersed Fig. 1. Experimental apparatus of the MC emulsification method phase to the module. A a) MC plate; b) schematic diagram microscope video system and a monitor were employed to record and observe the MC emulsification process. The surfactants used in the MC emulsification process were dissolved into either the oil or water phase in a given concentration. The oil phase was pressurized by raising the oil phase chamber, and when the height difference between the chamber and the MC was large enough, the oil phase broke through the channels and began to form MS. The pressure applied at this point was defined as the breakthrough pressure. The behavior of the MS formation was analyzed from the video images recorded by a 3 CCD video camera, while the MS size and their distribution were determined by counting over 200 droplets using image processing software (MAC SCOPE, Mitani Co., Fukui, Japan) on a Macintosh computer. The interfacial tension was measured by an automatic interfacial tensiometer (PD-W, Kyowa Interface Science Co., Saitama, Japan) with the pendant drop method. 3. RESULTS AND DISCUSSION The different surfactants show different interfacial activities for lowering the interfacial tension. Figure 2 shows the effect of surfactant concentration on interfacial tension. It was found that the hydrophobic surfactants, AOT, TOMAC, and oleic acid, which were dissolved in the oil phase, showed higher interfacial tension than the hydrophilic surfactants. All data in those cases give an interfacial tension greater than 5 mN/m (solid keys). On the other hand, the interfacial tension of systems with water-dissolved surfactants was lower than 5 mN/m
1057 (open keys) at concentrations higher than CMC (critical micellar concentration). Also, the CMC values were different from each other. Solid keys: surfactants in the oil phase It was reported that the faster the 30 rOpen keys: in the water phase surfactant diffuses and is absorbed at newly formed interfaces, the easier the MS formed and the smaller the size of the formed MS (3). One can probably produce O/W-MS using the > Sodium oleate AB water-dissolved surfactants more easily than using the hydrophobic surfactants. The effect of surfactant concentration on the breakthrough 1 2 3 4 5 6 pressure was investigated. The breakthrough pressure decreased Surfactant concentration (wt%) with an increase in surfactant Fig. 2. Effect of surfactant concentration on interfacial tension concentration, which is attributed to See text for abbreviation the decrease in the interfacial tension. Contact angle is usually used to represent the wetting propensity of a surface by a liquid phase. We conduct the measurement of the contact angle of several systems on mm. :^m the MC surface in this study. ^•WE^iSSi Aqueous droplets containing sodium oleate, SDS and CTAB in 0.3 wt% concentration were formed on the MC surface in the presence of the oil phase, and the contact angle of 41.6°, 57.6°, and 30.3° were obtained respectively. The contact angles of the water droplet on the MC surface in 0.3 wt% AOT and 0.3 wt% TOMAC oil phase were 60.3° and 130° degrees, respectively. Except for the TOMAC-containing system, which made the MC surface highly Fig. 3 Behavior of the super-monodispersed O/W-MS hydrophobic, the other systems kept formation for triolein / wafer (0.3 wt% sodium oleate) the MC surface hydrophilic. The system, W: the water phase; O: the oil phase, a) MC platefilledwith water phase; b) full contact to the terrace different contact angle values are due with the oil phase; c) intrusion into the terrace; d) full to the different interaction between contact to the MC entrance; e) breaking through the MC the MC surface and the different and forming MS; 0 super-monodispersed O/W-MS. surfactants.
mmm
msssm
' lii^iiiasii; 5111^11
1058 The behavior of the 0/W-MS formation was investigated by using anionic surfactants in the MC emulsification process. For each surfactant, several concentration conditions in the range 0.05-2.0 wt% were tested, and the breakthrough pressure for each condition was recorded. Figure 3 shows the behavior of the OAV-MS formation for triolein (the oil phase) 0.3 wt% sodium oleate in water system (the water phase). With the increase in applied pressure, the oil phase was fully pressed up to the terrace (b, 0.71 kPa), then intruded into the terrace (c, 0.89 kPa), then reached the entrance of the channels (d, 1.07 kPa), and finally broke through the channels and started to produce MS (e, 1.33 kPa). Figure 3 (0 shows a picture of the MS produced. The average MS diameter was calculated from the statistics of over 200 droplets. In another system containing sodium dodecyl sulfate (SDS) in the water phase, the same results as for sodium oleate were obtained for the super-monodispersed OAV-MS production. In case of 0.3 wt% SDS in water, OAV-MS with an average diameter of 30.8 ^m and a standard deviation of 0.44 |im were produced. Up to about 50 % of the channels were in operation during the emulsification process. Among the three anionic surfactants, SDS has a CI2:0 hydrophobic saturated chain, while sodium oleate has a CI8:1 chain with an unsaturated bond. On the other hand, Di-2ethylhexyl sodium sulfosuccinate (AOT) has two much shorter main chains with two subchains, which means that the cross section of the hydrophobic tails of AOT is probably larger than that of its hydrophilic group. AOT has been used to form reversed micelles, a kind of W/O microemulsion used for protein extraction (4). AOT did not function as well as sodium oleate and SDS in this study probably because of the differences in the molecular structure, the hydrophobic property, and the interfacial tension. The anionic surfactants were suitable for the production of monodispersed OAV-MS using the MC emulsification technique. Surfactant dissolved into the water phase (continuous phase) gave excellent behavior of MS formation. Hydrophobic surfactant dissolved in the oil phase (dispersed phase) was inferior to that of the hydrophilic ones. Nonionic surfactants can also be used to produce monodispersed OAV-MS. Dissolving a surfactant (Tween 80) into the water phase gave better results than dissolving the same surfactant into the oil phase for the production of monodispersed OAVMS. Also, dissolving Tween 80 into both phases produced MS with better monodispersibility. Cationic surfactants are difficult to use for the production of monodispersed OAV-MS using the MC emulsification method. Besides the system interfacial tension, the hydrophilic property of the MC surface is another important factor, which affects the 0/W-MS production during the MC emulsification process. As surfactants are involved in the wetting process, the surfactant hydrophilic headMC interaction must be considered. We analyzed the functions of various kinds of surfactants in the MC emulsification process. Under the conditions of this study, the MC surface is negatively charged and hydrophilic. The negatively charged hydrophilic head of an anionic surfactant will be repulsed from the MC surface, and so no surfactant molecules will be adsorbed to the MC surface. This maintains the hydrophilicity of the MS surface during the emulsification process. Moreover, anionic surfactants can diffuse in the water phase, be absorbed at the
1059 interfaces speedily, and show excellent interfacial active ability. The OAV-MS formed can easily detach from the terrace outside the MC. Therefore, using anionic surfactants, monodispersed O/W-MS can be produced by the MC emulsification technique. For the case of the nonionic surfactants, between the surfactant hydrophilic group and the MC surface, there is no strong repulsion as there is for anionic surfactants, but still the MC surface can be kept hydrophilic. They can also be used to form monodispersed OAV-MS when the system interfacial tension has a low value. On the other hand, the positively charged hydrophilic head of cationic surfactant molecules will be attracted by the negatively charged MC surface, which may allow the MC surface to be wetted by the oil phase more easily than for the other two cases. When TOMAC was dissolved in the oil phase, the MC surface was wetted entirely and no MS were produced. Therefore, it can be concluded that super-monodispersed OAV-MS cannot be produced using cationic surfactants. From the above analysis, it is concluded that it is very important to keep the MC surface hydrophilic during the MC emulsification process. The interaction between the surfactant hydrophilic group and the MC surface significantly affects the super-monodispersed OAV-MS production. ACKNOWLEDGMENT This work was supported by the Program for Promotion of Basic Research Activities for Innovative Biosciences of Japan (MS-Project). REFERENCES 1. Kawakatsu, T., Y. Kikuchi, and M. Nakajima, J. Am. Oil Chem. Soc. 74 (1997) 317. 2. Schubert, H. and H. Armbruster, Intel Chem. Eng., 32 (1992) 14. 3. Schroder, V., O. Behrend, and H. Schubert, J. Colloid Interface Sci.. 202(1998) 334. 4. Tong, J. and S. Furusaki, Sep. Sci. Tech., 33 (1998) 899.
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Characterization of surfactants used for monodispersed oil-in-water microspheres production by microchannel emulsification Jihong Tong, Mitsutoshi Nakajima/ Hiroshi Nabetani, and Yuji Kikuchi National Food Research Institute, Ministry of Agriculture, Forestry and Fisheries, Kannondai 2-1-2, Tsukuba, Ibaraki 305-8642, Japan Super-monodispersed oil-in-water (O/W) microspheres (MS) were produced using a microchannel (MC) emulsification technique. The effect of the surfactant on the behavior of the O/W-MS formation was investigated using various surfactants for the MC emulsification process. It was found that the super-monodispersed O/W-MS production depends on the ionic type of the surfactants used. The results indicated that it is very important to maintain the hydrophilicity of the MC surface during the MC emulsification process. 2. INTRODUCTION Microspheres (MS), which are emulsion cells or solid particles dispersed in a continuous phase, have been utilized in various industries such as foods, cosmetics and pharmaceuticals, etc. Emulsions are dispersed multiphase systems consisting of two or more almost mutually insoluble liquids, with the dispersed phase present in the form of droplets in a continuous phase. Recently Kawakatsu et ai (1) have proposed a novel microchannel (MC) emulsification technique for super-monodispersed MS production. It is anticipated that the MS with monodispersibility may exhibit higher quality and stability than that produced by conventional techniques. It is known that surfactant is indispensable for forming and stabilizing emulsions or MS (2). In order to understand the mechanism of how a surfactant functions in the MC emulsification process, we investigated the surfactant effect on the behavior of the O/W-MS formation by using various kinds of surfactants. 2. MATERIALS AND METHODS 2.1. Materials High-oleic sunflower oil (triolein, >90% purity) was obtained from Nippon Lever B.V., Tokyo, Japan. Sodium oleate, oleic acid, sodium dodecyl sulfate (SOS), tri-noctylmethylammonium chloride (TOMAC) and polyoxyethylene (20) sorbitan monooleate (Tween 80, HLB: 15.0) were purchased from Wako Pure Chemical Ind., Osaka, Japan. Di-2ethylhexyl sodium sulfosuccinate (AOT) was purchased from Sigma Chemical Co., St. Louis, MO, USA. All materials were reagent grade and were used without further purification.
Corresponding author, E-mail: [email protected]
1056 2.2. Experimental system and procedure. An MC plate with dimension of 15 mm x Bottom view 15 mm X 0.5 mm, 600 Liquid channels around the 4 chamber sides, and an equivalent diameter of 8.9 ^im for one channel was used. SiUcon MC Plate The experimental apparatus was shown in Glass Plate Fig. 1. A module Oil phase Microscope installed with an MC flow plate adhering to a flat direction glass plate was filled Partition wall with a water phase. An ' Terrace oil phase chamber Microscope video system connected to the module Schematic b) Schematic diagram by a silicone tube (a) MC plate supplied the dispersed Fig. 1. Experimental apparatus of the MC emulsification method phase to the module. A a) MC plate; b) schematic diagram microscope video system and a monitor were employed to record and observe the MC emulsification process. The surfactants used in the MC emulsification process were dissolved into either the oil or water phase in a given concentration. The oil phase was pressurized by raising the oil phase chamber, and when the height difference between the chamber and the MC was large enough, the oil phase broke through the channels and began to form MS. The pressure applied at this point was defined as the breakthrough pressure. The behavior of the MS formation was analyzed from the video images recorded by a 3 CCD video camera, while the MS size and their distribution were determined by counting over 200 droplets using image processing software (MAC SCOPE, Mitani Co., Fukui, Japan) on a Macintosh computer. The interfacial tension was measured by an automatic interfacial tensiometer (PD-W, Kyowa Interface Science Co., Saitama, Japan) with the pendant drop method. 3. RESULTS AND DISCUSSION The different surfactants show different interfacial activities for lowering the interfacial tension. Figure 2 shows the effect of surfactant concentration on interfacial tension. It was found that the hydrophobic surfactants, AOT, TOMAC, and oleic acid, which were dissolved in the oil phase, showed higher interfacial tension than the hydrophilic surfactants. All data in those cases give an interfacial tension greater than 5 mN/m (solid keys). On the other hand, the interfacial tension of systems with water-dissolved surfactants was lower than 5 mN/m
1057 (open keys) at concentrations higher than CMC (critical micellar concentration). Also, the CMC values were different from each other. Solid keys: surfactants in the oil phase It was reported that the faster the 30 rOpen keys: in the water phase surfactant diffuses and is absorbed at newly formed interfaces, the easier the MS formed and the smaller the size of the formed MS (3). One can probably produce O/W-MS using the > Sodium oleate AB water-dissolved surfactants more easily than using the hydrophobic surfactants. The effect of surfactant concentration on the breakthrough 1 2 3 4 5 6 pressure was investigated. The breakthrough pressure decreased Surfactant concentration (wt%) with an increase in surfactant Fig. 2. Effect of surfactant concentration on interfacial tension concentration, which is attributed to See text for abbreviation the decrease in the interfacial tension. Contact angle is usually used to represent the wetting propensity of a surface by a liquid phase. We conduct the measurement of the contact angle of several systems on mm. :^m the MC surface in this study. ^•WE^iSSi Aqueous droplets containing sodium oleate, SDS and CTAB in 0.3 wt% concentration were formed on the MC surface in the presence of the oil phase, and the contact angle of 41.6°, 57.6°, and 30.3° were obtained respectively. The contact angles of the water droplet on the MC surface in 0.3 wt% AOT and 0.3 wt% TOMAC oil phase were 60.3° and 130° degrees, respectively. Except for the TOMAC-containing system, which made the MC surface highly Fig. 3 Behavior of the super-monodispersed O/W-MS hydrophobic, the other systems kept formation for triolein / wafer (0.3 wt% sodium oleate) the MC surface hydrophilic. The system, W: the water phase; O: the oil phase, a) MC platefilledwith water phase; b) full contact to the terrace different contact angle values are due with the oil phase; c) intrusion into the terrace; d) full to the different interaction between contact to the MC entrance; e) breaking through the MC the MC surface and the different and forming MS; 0 super-monodispersed O/W-MS. surfactants.
mmm
msssm
' lii^iiiasii; 5111^11
1058 The behavior of the 0/W-MS formation was investigated by using anionic surfactants in the MC emulsification process. For each surfactant, several concentration conditions in the range 0.05-2.0 wt% were tested, and the breakthrough pressure for each condition was recorded. Figure 3 shows the behavior of the OAV-MS formation for triolein (the oil phase) 0.3 wt% sodium oleate in water system (the water phase). With the increase in applied pressure, the oil phase was fully pressed up to the terrace (b, 0.71 kPa), then intruded into the terrace (c, 0.89 kPa), then reached the entrance of the channels (d, 1.07 kPa), and finally broke through the channels and started to produce MS (e, 1.33 kPa). Figure 3 (0 shows a picture of the MS produced. The average MS diameter was calculated from the statistics of over 200 droplets. In another system containing sodium dodecyl sulfate (SDS) in the water phase, the same results as for sodium oleate were obtained for the super-monodispersed OAV-MS production. In case of 0.3 wt% SDS in water, OAV-MS with an average diameter of 30.8 ^m and a standard deviation of 0.44 |im were produced. Up to about 50 % of the channels were in operation during the emulsification process. Among the three anionic surfactants, SDS has a CI2:0 hydrophobic saturated chain, while sodium oleate has a CI8:1 chain with an unsaturated bond. On the other hand, Di-2ethylhexyl sodium sulfosuccinate (AOT) has two much shorter main chains with two subchains, which means that the cross section of the hydrophobic tails of AOT is probably larger than that of its hydrophilic group. AOT has been used to form reversed micelles, a kind of W/O microemulsion used for protein extraction (4). AOT did not function as well as sodium oleate and SDS in this study probably because of the differences in the molecular structure, the hydrophobic property, and the interfacial tension. The anionic surfactants were suitable for the production of monodispersed OAV-MS using the MC emulsification technique. Surfactant dissolved into the water phase (continuous phase) gave excellent behavior of MS formation. Hydrophobic surfactant dissolved in the oil phase (dispersed phase) was inferior to that of the hydrophilic ones. Nonionic surfactants can also be used to produce monodispersed OAV-MS. Dissolving a surfactant (Tween 80) into the water phase gave better results than dissolving the same surfactant into the oil phase for the production of monodispersed OAVMS. Also, dissolving Tween 80 into both phases produced MS with better monodispersibility. Cationic surfactants are difficult to use for the production of monodispersed OAV-MS using the MC emulsification method. Besides the system interfacial tension, the hydrophilic property of the MC surface is another important factor, which affects the 0/W-MS production during the MC emulsification process. As surfactants are involved in the wetting process, the surfactant hydrophilic headMC interaction must be considered. We analyzed the functions of various kinds of surfactants in the MC emulsification process. Under the conditions of this study, the MC surface is negatively charged and hydrophilic. The negatively charged hydrophilic head of an anionic surfactant will be repulsed from the MC surface, and so no surfactant molecules will be adsorbed to the MC surface. This maintains the hydrophilicity of the MS surface during the emulsification process. Moreover, anionic surfactants can diffuse in the water phase, be absorbed at the
1059 interfaces speedily, and show excellent interfacial active ability. The OAV-MS formed can easily detach from the terrace outside the MC. Therefore, using anionic surfactants, monodispersed O/W-MS can be produced by the MC emulsification technique. For the case of the nonionic surfactants, between the surfactant hydrophilic group and the MC surface, there is no strong repulsion as there is for anionic surfactants, but still the MC surface can be kept hydrophilic. They can also be used to form monodispersed OAV-MS when the system interfacial tension has a low value. On the other hand, the positively charged hydrophilic head of cationic surfactant molecules will be attracted by the negatively charged MC surface, which may allow the MC surface to be wetted by the oil phase more easily than for the other two cases. When TOMAC was dissolved in the oil phase, the MC surface was wetted entirely and no MS were produced. Therefore, it can be concluded that super-monodispersed OAV-MS cannot be produced using cationic surfactants. From the above analysis, it is concluded that it is very important to keep the MC surface hydrophilic during the MC emulsification process. The interaction between the surfactant hydrophilic group and the MC surface significantly affects the super-monodispersed OAV-MS production. ACKNOWLEDGMENT This work was supported by the Program for Promotion of Basic Research Activities for Innovative Biosciences of Japan (MS-Project). REFERENCES 1. Kawakatsu, T., Y. Kikuchi, and M. Nakajima, J. Am. Oil Chem. Soc. 74 (1997) 317. 2. Schubert, H. and H. Armbruster, Intel Chem. Eng., 32 (1992) 14. 3. Schroder, V., O. Behrend, and H. Schubert, J. Colloid Interface Sci.. 202(1998) 334. 4. Tong, J. and S. Furusaki, Sep. Sci. Tech., 33 (1998) 899.