- Email: [email protected]
Oil Ternary System in the Presence of Polymer Oil
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
186, 294–299 (1997)
CS964650
Phase Behavior of a Water/Nonionic Surfactant/Oil Ternary System in the Presence of Polymer Oil AJITH CHERIAN JOHN,* HIROTAKA UCHIYAMA,† KAZUYOSHI NAKAMURA,‡
AND
HIRONOBU KUNIEDA * ,1
*Department of Physical Chemistry, Division of Material Science and Chemical Engineering, Faculty of Engineering, Yokohama National University, Tokiwadai 156, Hodogaya-Ku, Yokohama 240, Japan; †Research and Development Department, Procter & Gamble Far East Inc., 17, Koyo-cho Naka 1-chome, Higashinada-ku, Kobe 658, Japan; and ‡Department of Living Science, Faculty of Education, Niigata University, Igarashi, Nino-cho 8050, Niigata 950-21, Japan Received May 29, 1996; accepted October 15, 1996
The effect of a polymer oil, polydimethyl siloxane, on the phase behavior of the water/C12EO6 /isopropyl myristate (IPM) system has been studied. Since the polymer oil is completely soluble in IPM, it was dissolved in IPM and the solutions were used as the oil phase to study its effect. The presence of polymer increases the hydrophobic nature of IPM and thereby decreases the solubilization of oil into the surfactant phase (microemulsion). Moreover, at a certain range of silicone oil concentration in the IPM, a fourphase body consisting of excess water, excess oil, and two surfactant phases is formed within the ternary system. The two surfactant phases are designated as D (with bicontinuous type of structure) and D * (with L3 type of structure). Careful phase behavioral studies revealed that, with increasing silicone oil concentration, a three-phase region containing water phase, D * phase, and an oilrich D phase develops within the system and this region overlaps with the normal three-phase region of the system containing the water, D, and oil phases to give a four-phase body. The order of various phases from top to bottom in the four-phase body is oil, D, D*, and water. q 1997 Academic Press Key Words: phase behavior; polymer oil; microemulsion; fourphase body.
INTRODUCTION
The property of nonionic surfactants whereby they change from water-soluble (aqueous micellar) solutions to oil-soluble (reverse micellar) solutions with increasing temperature is well documented (1–4). At a characteristic temperature, the solubilizing power of a surfactant reaches a maximum and a three-phase body in which a bicontinuous microemulsion phase equilibrates with excess water and oil phases originates. The interactions of the surfactant with oil and water are approximately balanced at a temperature called HLB temperature. In the majority of this type of phase behavior studies, the 1
To whom correspondence should be addressed.
JCIS 4650
/
6g1c$$$101
MATERIALS AND METHODS
Materials The surfactant hexoxyethylene dodecyl ether (C12EO6 ) was obtained from Nikko Chemicals Company and isopropyl myristate (IPM) was obtained from Tokyo Kasei Kogyo
294
0021-9797/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.
AID
oil phases used are simple saturated or unsaturated hydrocarbons; however, the possibility of using polymer oils as one of the components in formulating microemulsions has rarely been studied and the phase behavior of such systems are less known (5, 6). Silicone oils are polymer liquids widely used in cosmetics, lubricants, toiletries, etc. (7). Therefore, phase behavioral studies incorporating these oils are industrially important. Katayama et al. studied microemulsions with silicone oils. They found that the presence of hydrophilic groups (NH2 ) in the silicone oil is a criterion for the formation of single-phase microemulsions (5). Hydrophilic groups normally penetrate the surfactant palisade layer and thus stabilize the microemulsions. Interestingly, polydimethyl siloxane, as an oil phase, has difficulty forming single-phase microemulsions. In addition, phase behavioral studies using this oil are also difficult as the clean phase separation is seldom obtained. These difficulties can be minimized by dissolving this oil in a suitable solvent and then using it as an oil phase. In a similar study, Kabalnov et al. investigated the effect of water-soluble polymers on the three-phase behavior of a nonionic surfactant system (8, 9). They observed the formation of a four-phase equilibrium with increasing concentration of dextran, where a liquid crystalline phase coexists with the oil, water, and bicontinuous microemulsion phase. In this context, we report the phase behavior of a ternary system containing a nonionic surfactant (hexoxyethylene dodecyl ether) and the effect of polydimethyl siloxane on the phase diagrams. Isopropyl myristate, which is a good solvent for silicone oil, is used as the oil phase.
01-08-97 01:09:56
coidas
WATER/NONIONIC SURFACTANT/OIL IN PRESENCE OF POLYMER OIL
FIG. 1. Temperature–surfactant concentration phase diagram of the water/C12EO6 /IPM system. s, pure IPM; l, IPM containing 10% silicone oil. Oil:water ratio is 1:1. I, II, and III are one-, two-, and three-phase regions, respectively. Wm, aqueous micellar solution; Om, reverse micellar solution; D, bicontinuous surfactant phase; W, water; O, oil.
Company. Polydimethyl siloxane (silicone oil), commercially known as SH-200, obtained from Toray Dow Corning Silicone Company, has the general formula (CH3 )3SiO[(CH3 )2SiO]nSi(CH3 )3 . The weight-average molecular weight of this oil is about 2600 and the viscosity is 30 cS at 257C (supplied by the manufacturer). All these materials were of extra-pure grade and were used without further purification. Water used in the study was doubly distilled. Determination of Phase Diagrams Sealed ampoules of different compositions of water, oil, and surfactant were prepared and placed in a thermostated water bath (accuracy, 0.17C) to attain the temperature. On attaining the required temperature, they were well shaken and left for phase separation from several hours to few weeks depending on the stability of emulsions. Phase equilibria were determined by visual observation. The liquid crystalline phase was detected by crossing the polarizing films. To study the effect of silicone oil, this polymer oil was dissolved in IPM, and this solution was used as the oil phase instead of pure IPM. RESULTS AND DISCUSSION
Phase Diagrams at Constant Oil:Water Ratio The HLB temperature of a nonionic surfactant changes from one ternary system to another depending on the nature of the oil phase. Figure 1 shows the phase diagram of the
AID
JCIS 4650
/
6g1c$$$102
01-08-97 01:09:56
295
system water/C12EO6 /IPM as a function of surfactant concentration. The oil:water ratio is invariably kept constant at 1:1. The diagram indicated by open circles corresponds to pure IPM, whereas the diagram with filled circles represents IPM containing 10% silicone oil. For a pure IPM system, at lower temperatures the surfactant forms aqueous micelles and equilibrates with an excess oil phase [II (Wm / O)]. With rising temperature, increasing amounts of oil dissolve into the micellar core and they grow continuously. At intermediate temperatures, a bicontinuous microemulsion (or surfactant phase, D) equilibrates with excess oil at the top and excess water at the bottom (III). Further increase in temperature causes infinite swelling of the micelles with the oil phase, and ultimately the emulsion type changes to a water-in-oil type [II(Om / W)]. Toward higher surfactant concentration, a single isotropic microemulsion phase is formed between the two two-phase regions. The point marked a is the point of maximum solubilization at which a minimum amount of surfactant solubilizes almost equal amounts of oil and water phases into a single isotropic phase (3). This kind of phase behavior of IPM is similar to that of a hydrocarbon system. The solubility of the IPM and C12EO6 in the excess water phase is considered to be negligibly small, and thus the bottom tie line of the three-phase triangle is tilted from the water-rich corner (0% C12EO6 ) to 3.2% C12EO6 at 50/50 oil/water, judging from the initial point of the three-phase body along the surfactant concentration axis in Fig. 1. Hence, it is inferred that the monomeric solubility of surfactant C12EO6 in the oil phase is considerably high, i.e., É6%. In the case of IPM containing 10% (w/w) silicone oil (filled circles), apart from the shift in the whole phase diagram toward higher temperatures, the overall nature of the diagram remains more or less the same. Comparing the lower and upper three-phase boundaries individually, it is clear that the shift in the upper boundary of the three-phase regime toward higher temperature is more than that of the lower boundary. It means that, in the presence of silicone oil, the surfactant aggregates are more soluble in the water phase, whereas they are less soluble in the oil phase. This is because the lower boundary of the diagram corresponds basically to the clouding temperature of the surfactant in the presence of the oil phase and the upper boundary of the diagram is the mutual solubility curve of the oil and surfactant (1). Hence, surfactant aggregates tend to separate from oil on addition of the polymer oil. A similar tendency was reported in water-soluble polymer–surfactant systems (10, 11). Moreover, the point of maximum solubility moves toward higher surfactant concentration, whereas the monomeric solubility of the surfactant in the oil phase remains practically the same, because the boundaries of the three-phase region at low surfactant concentration remain the same. These observations are evidence of a significant decrease in the solu-
coidas
296
JOHN ET AL.
FIG. 4. Schematic representation of the shift of the three-phase triangle corresponding to Fig. 3 with increasing temperature. Notation as in Fig. 1. FIG. 2. Schematic representation of the shift of the three-phase triangle corresponding to Fig. 1 from the water-rich corner to the oil-rich corner with increasing temperature. Notation as in Fig. 1.
bilization of the oil phase in the surfactant phase. Figure 2 is a schematic representation of the change in the threephase tie triangle corresponding to the phase diagrams shown in Fig. 1 with increasing temperature. The triangle moves from the water-rich corner to the oil-rich corner with increasing temperature. Figure 3 is the phase diagram of the water/C12EO6 /oil system in which the oil phase contains 16% (w/w) silicone oil. At this concentration of silicone oil, the phase diagram registers a marked change in its shape. The one-phase region
at the higher surfactant concentration disappears completely and the three-phase body continues even up to high surfactant concentrations and high temperatures. This indicates that an infinite swelling of the micelles with the oil phase does not take place at this silicone oil concentration. Therefore, the middle surfactant phase in the three-phase body probably contains a water continuous structure similar to the L3 phase (D * ) observed in a water–nonionic surfactant binary system (4, 12). At higher temperatures these surfactant molecules dissolve into the excess oil phase as reverse micelles. The change in the three-phase triangle corresponding to this diagram with increasing temperature can hence be represented as shown in Fig. 4 (13). It is possible that the IPM/silicone oil ratio is different in microemulsion and oil phases, and then, one has to use a quaternary phase diagram; however, we focus attention mainly on the change in the water/oil ratio in each phase in Fig. 4. Initially a certain amount of oil dissolves into the micellar core and thereby the tie triangle moves slightly away from the water–surfactant axis; however, there is no infinite dissolution of the oil phase into the surfactant phase, and therefore the triangle remains within the water side, but the point intersecting the oil– surfactant axis moves upward (14, 15). This change in phase behavior is verified later in this paper. Four Coexisting Phases
FIG. 3. Temperature–surfactant concentration phase diagram of the water/C12EO6 /IPM (16% silicone oil) system. Oil:water ratio is 1:1. II and III are two- and three-phase regions, respectively. Notation as in Fig. 1.
AID
JCIS 4650
/
6g1c$$$102
01-08-97 01:09:56
Figure 5 is a phase diagram of the system in which the composition is fixed except for the silicone oil concentration in the oil phase. The oil:water ratio is 1:1 and the surfactant concentration in the total composition is 8% by weight. The two-phase regions at lower temperatures [II(Wm / O)] and higher temperatures [II(Om / W)] contain aqueous micelles and reverse micelles, respectively. Below the threephase region (III), a region of liquid crystal (LC present)
coidas
WATER/NONIONIC SURFACTANT/OIL IN PRESENCE OF POLYMER OIL
297
phase region containing the D * phase region (W / D * / Om), are formed. The observed four-phase body is a result of the overlap of these two-phase regions. We could not observe the other two three-phase regions in the present system which may be attributed to the narrowness of these regions. Therefore, in Fig. 5, the surfactant phase in the three-phase region is considered to be the normal type (D) toward the left side of the four-phase body (at lower silicone oil concentrations), whereas it is likely D * type toward the right side (at higher silicone oil concentrations) of the four-phase body. Formation of a liquid crystalline region below the three-phase region is also attributed to the low solubility of oil in the surfactant phase. The liquid crystal undergoes gradual melting with increasing temperature and is converted to the D * surfactant phase, which coexists with excess oil and water phases. At still higher temperatures, the surfactant dissolves in the oil phase as reverse micelles and a two-phase region is formed. FIG. 5. Phase diagram of the water/C12EO6 /IPM system as a function of temperature and silicone oil concentration in the IPM. Oil:water ratio is 1:1 and surfactant concentration in the system is 8% (w/w) of the composition. IV is the four-phase region and D * indicates L3-type surfactant phase. Other notation as in Fig. 1.
exists. In addition to these regions, a very narrow region of four different coexisting phases appears at a certain silicone oil concentration range which is shaded in the figure (IV(W / D * / D / O)). In this silicone oil concentration range, at the lower boundary of the three-phase region, excess water is separated out from the surfactant phase, and a three-phase (W / D / O) region is formed initially. This three-phase region is very narrow and is omitted in Fig. 5. With further temperature increase, a fourth phase appears at the boundary between the W and D phases and it grows gradually at the expense of the D phase. This phase is identified as the D * surfactant phase and it appears more bluish than the upper D phase. At a certain temperature, this D * surfactant phase attains maximum growth, then starts shrinking and finally disappears to give a three-phase again. In a water/nonionic surfactant/oil system in which the oil phase is a mixture of hydrocarbon and a polar or amphiphilic oil such as long-chain alcohols, fatty acids, and monoglycerides, up to four kinds of three-phase formation can be observed. This is because the polar oil behaves like amphiphilic compounds and adsorbs at the interface rather than remaining as free oil. Formation of a four-phase body has generally occurred when any two of the three-phase regions overlap (13, 14). In the present system, however, the polymer oil polydimethyl siloxane is not a polar oil. But the presence of this polymer in the IPM makes the oil phase less soluble in the surfactant aggregates, and this results in formation of the D * phase. Thus, two three-phase regions, a normal three-phase region (W / D / O) and a three-
AID
JCIS 4650
/
6g1c$$$102
01-08-97 01:09:56
Phase Diagrams at Constant Surfactant Concentrations To verify the formation of the four-phase body, we studied another set of phase diagrams. Figure 6 is a phase diagram of the system containing pure IPM (silicone oil-free) as a function of increasing oil content and temperature. The surfactant concentration in the aqueous phase is 5% by weight. I, II (Wm / O), and II (Om / W) have the same meaning as in the preceding figures. Above the isotropic one-phase region, there is a LC-present region. Beyond the
FIG. 6. Phase diagram of the water/C12EO6 /IPM system as a function of oil concentration. Surfactant concentration in the water phase is 5% (w/ w). Other notation as in the preceding figures.
coidas
298
JOHN ET AL.
one-phase region, at higher oil concentrations, there is a three-phase region, which is not marked in the diagram. Figure 7 is the corresponding phase diagram of the system in which the oil phase IPM contains 10% (w/w) silicone oil. In addition to the I, II(Wm / O), II(Om / W), LCpresent, and III phase regions, which appeared in the pure IPM system, a three-phase region containing the D * surfactant phase also appears above the LC-present region [III(W / D * / D)]. Above the LC-present region, the emulsion is fairly stable. With increasing temperature, this stable emulsion separates into an aqueous phase and a surfactant phase. At a certain stage, the upper surfactant phase again splits into two, an oil-rich phase at the top and D * phase at the middle. At the upper boundary of the three-phase region, the system reverts again to the emulsion [II(Om / W)]; however, at this silicone oil concentration, the two threephase regions are separated from each other. Further increase in the silicone oil concentration in IPM widens the three-phase body containing the D * phase. At 12% silicone oil concentration (Fig. 8), the two three-phase regions [i.e., III(W / D / O) and III(W / D * / D)] merge to form a continuous strip of three-phase regions. At almost the middle portion of this region, where there is prominent overlapping of the two three-phase regions, a four-phase region appears which is shaded in the figure. Thus, the four-phase formation is confirmed. Since two kinds of microemulsion phases are found, it is likely that the IPM/polymer oil ratio would be different in
FIG. 8. Phase diagram of the water/C12EO6 /IPM (12% silicone oil) system as a function of oil concentration. Surfactant concentration in the water phase is 5% (w/w). Other notation as in preceding figures.
microemulsion and oil phases. The partitioning of polymer oil in each phase will be reported in the near future. CONCLUSIONS
The water/C12EO6 /IPM system can form a single-phase microemulsion. The presence of polydimethyl siloxane (silicone oil) in the IPM enhances the lipophilic nature of the oil phase and thereby decreases the solubility of oil in the surfactant aggregation. The point of maximum solubilization, i.e., the point at which minimum surfactant solubilizes equal amounts of oil and water into a single isotropic phase, also increases in the presence of polymer oil. In the presence of silicone oil, due to the decreased solubilization of oil in the surfactant phase, a three-phase body containing water, D *, and D phases originates within the system. With increasing silicone oil concentration this three-phase region expands, and at a certain range of silicone oil concentration it overlaps with the normal three-phase region containing water, D, and oil phases to form a four-phase body. At higher concentrations of silicone oil, the surfactant phase in the three-phase body is D * type and the phase behavior becomes similar to that of a water/nonionic surfactant/amphiphilic oil system. REFERENCES FIG. 7. Phase diagram of the water/C12EO6 /IPM (10% silicone oil) system as a function of oil concentration. Surfactant concentration in the water phase is 5% (w/w). Other notation as in preceding figures.
AID
JCIS 4650
/
6g1c$$$102
01-08-97 01:09:56
1. Kunieda, H., and Friberg, S. E., Bull. Chem. Soc. Jpn. 54, 1010 (1981). 2. Kunieda, H., and Shinoda, K., J. Colloid Interface Sci. 107, 107 (1985).
coidas
WATER/NONIONIC SURFACTANT/OIL IN PRESENCE OF POLYMER OIL 3. Kunieda, H., Nakano, A., and Akimaru, M., J. Colloid Interface Sci. 170, 78 (1995). 4. Kunieda, H., and Shinoda, K., J. Dispersion Sci. Technol. 3, 233 (1982). 5. Katayama, H., Tagawa, T., and Kunieda, H., J. Colloid Interface Sci. 153, 429 (1992). 6. Messier, A., Schorsch, G., Rouviere, J., and Tenebre, L., Prog. Colloid Polym. Sci. 79, 249 (1989). 7. Disapio, A., and Fridd, P., Int. J. Cosmet. Sci. 10, 75 (1988). 8. Kabalnov, A., Olsson, U., and Wennerstro¨m, H., Langmuir 10, 2159 (1994).
AID
JCIS 4650
/
6g1c$$$102
01-08-97 01:09:56
299
9. Kabalnov, A., Olsson, U., Thuresson, K., and Wennerstrom, H., Langmuir 10, 4509 (1994). 10. Siano, D. B., and Bock, J., J. Colloid Interface Sci. 90, 359 (1982). 11. Wormuth, K. R., Langmuir 7, 1622 (1991). 12. Mitchell, D. J., Tiddy, G. J. T., Waring, L., Bostock, T., and McDonald, M. P., J. Chem. Soc. Faraday Trans. 1 79, 975 (1983). 13. Kunieda, H., Asaoka, H., and Shinoda, K., J. Phys. Chem. 92, 185 (1988). 14. Kunieda, H., J. Colloid Interface Sci. 133, 237 (1989). 15. Kunieda, H., and Haishima, K., J. Colloid Interface Sci. 140, 383 (1990).
coidas
186, 294–299 (1997)
CS964650
Phase Behavior of a Water/Nonionic Surfactant/Oil Ternary System in the Presence of Polymer Oil AJITH CHERIAN JOHN,* HIROTAKA UCHIYAMA,† KAZUYOSHI NAKAMURA,‡
AND
HIRONOBU KUNIEDA * ,1
*Department of Physical Chemistry, Division of Material Science and Chemical Engineering, Faculty of Engineering, Yokohama National University, Tokiwadai 156, Hodogaya-Ku, Yokohama 240, Japan; †Research and Development Department, Procter & Gamble Far East Inc., 17, Koyo-cho Naka 1-chome, Higashinada-ku, Kobe 658, Japan; and ‡Department of Living Science, Faculty of Education, Niigata University, Igarashi, Nino-cho 8050, Niigata 950-21, Japan Received May 29, 1996; accepted October 15, 1996
The effect of a polymer oil, polydimethyl siloxane, on the phase behavior of the water/C12EO6 /isopropyl myristate (IPM) system has been studied. Since the polymer oil is completely soluble in IPM, it was dissolved in IPM and the solutions were used as the oil phase to study its effect. The presence of polymer increases the hydrophobic nature of IPM and thereby decreases the solubilization of oil into the surfactant phase (microemulsion). Moreover, at a certain range of silicone oil concentration in the IPM, a fourphase body consisting of excess water, excess oil, and two surfactant phases is formed within the ternary system. The two surfactant phases are designated as D (with bicontinuous type of structure) and D * (with L3 type of structure). Careful phase behavioral studies revealed that, with increasing silicone oil concentration, a three-phase region containing water phase, D * phase, and an oilrich D phase develops within the system and this region overlaps with the normal three-phase region of the system containing the water, D, and oil phases to give a four-phase body. The order of various phases from top to bottom in the four-phase body is oil, D, D*, and water. q 1997 Academic Press Key Words: phase behavior; polymer oil; microemulsion; fourphase body.
INTRODUCTION
The property of nonionic surfactants whereby they change from water-soluble (aqueous micellar) solutions to oil-soluble (reverse micellar) solutions with increasing temperature is well documented (1–4). At a characteristic temperature, the solubilizing power of a surfactant reaches a maximum and a three-phase body in which a bicontinuous microemulsion phase equilibrates with excess water and oil phases originates. The interactions of the surfactant with oil and water are approximately balanced at a temperature called HLB temperature. In the majority of this type of phase behavior studies, the 1
To whom correspondence should be addressed.
JCIS 4650
/
6g1c$$$101
MATERIALS AND METHODS
Materials The surfactant hexoxyethylene dodecyl ether (C12EO6 ) was obtained from Nikko Chemicals Company and isopropyl myristate (IPM) was obtained from Tokyo Kasei Kogyo
294
0021-9797/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.
AID
oil phases used are simple saturated or unsaturated hydrocarbons; however, the possibility of using polymer oils as one of the components in formulating microemulsions has rarely been studied and the phase behavior of such systems are less known (5, 6). Silicone oils are polymer liquids widely used in cosmetics, lubricants, toiletries, etc. (7). Therefore, phase behavioral studies incorporating these oils are industrially important. Katayama et al. studied microemulsions with silicone oils. They found that the presence of hydrophilic groups (NH2 ) in the silicone oil is a criterion for the formation of single-phase microemulsions (5). Hydrophilic groups normally penetrate the surfactant palisade layer and thus stabilize the microemulsions. Interestingly, polydimethyl siloxane, as an oil phase, has difficulty forming single-phase microemulsions. In addition, phase behavioral studies using this oil are also difficult as the clean phase separation is seldom obtained. These difficulties can be minimized by dissolving this oil in a suitable solvent and then using it as an oil phase. In a similar study, Kabalnov et al. investigated the effect of water-soluble polymers on the three-phase behavior of a nonionic surfactant system (8, 9). They observed the formation of a four-phase equilibrium with increasing concentration of dextran, where a liquid crystalline phase coexists with the oil, water, and bicontinuous microemulsion phase. In this context, we report the phase behavior of a ternary system containing a nonionic surfactant (hexoxyethylene dodecyl ether) and the effect of polydimethyl siloxane on the phase diagrams. Isopropyl myristate, which is a good solvent for silicone oil, is used as the oil phase.
01-08-97 01:09:56
coidas
WATER/NONIONIC SURFACTANT/OIL IN PRESENCE OF POLYMER OIL
FIG. 1. Temperature–surfactant concentration phase diagram of the water/C12EO6 /IPM system. s, pure IPM; l, IPM containing 10% silicone oil. Oil:water ratio is 1:1. I, II, and III are one-, two-, and three-phase regions, respectively. Wm, aqueous micellar solution; Om, reverse micellar solution; D, bicontinuous surfactant phase; W, water; O, oil.
Company. Polydimethyl siloxane (silicone oil), commercially known as SH-200, obtained from Toray Dow Corning Silicone Company, has the general formula (CH3 )3SiO[(CH3 )2SiO]nSi(CH3 )3 . The weight-average molecular weight of this oil is about 2600 and the viscosity is 30 cS at 257C (supplied by the manufacturer). All these materials were of extra-pure grade and were used without further purification. Water used in the study was doubly distilled. Determination of Phase Diagrams Sealed ampoules of different compositions of water, oil, and surfactant were prepared and placed in a thermostated water bath (accuracy, 0.17C) to attain the temperature. On attaining the required temperature, they were well shaken and left for phase separation from several hours to few weeks depending on the stability of emulsions. Phase equilibria were determined by visual observation. The liquid crystalline phase was detected by crossing the polarizing films. To study the effect of silicone oil, this polymer oil was dissolved in IPM, and this solution was used as the oil phase instead of pure IPM. RESULTS AND DISCUSSION
Phase Diagrams at Constant Oil:Water Ratio The HLB temperature of a nonionic surfactant changes from one ternary system to another depending on the nature of the oil phase. Figure 1 shows the phase diagram of the
AID
JCIS 4650
/
6g1c$$$102
01-08-97 01:09:56
295
system water/C12EO6 /IPM as a function of surfactant concentration. The oil:water ratio is invariably kept constant at 1:1. The diagram indicated by open circles corresponds to pure IPM, whereas the diagram with filled circles represents IPM containing 10% silicone oil. For a pure IPM system, at lower temperatures the surfactant forms aqueous micelles and equilibrates with an excess oil phase [II (Wm / O)]. With rising temperature, increasing amounts of oil dissolve into the micellar core and they grow continuously. At intermediate temperatures, a bicontinuous microemulsion (or surfactant phase, D) equilibrates with excess oil at the top and excess water at the bottom (III). Further increase in temperature causes infinite swelling of the micelles with the oil phase, and ultimately the emulsion type changes to a water-in-oil type [II(Om / W)]. Toward higher surfactant concentration, a single isotropic microemulsion phase is formed between the two two-phase regions. The point marked a is the point of maximum solubilization at which a minimum amount of surfactant solubilizes almost equal amounts of oil and water phases into a single isotropic phase (3). This kind of phase behavior of IPM is similar to that of a hydrocarbon system. The solubility of the IPM and C12EO6 in the excess water phase is considered to be negligibly small, and thus the bottom tie line of the three-phase triangle is tilted from the water-rich corner (0% C12EO6 ) to 3.2% C12EO6 at 50/50 oil/water, judging from the initial point of the three-phase body along the surfactant concentration axis in Fig. 1. Hence, it is inferred that the monomeric solubility of surfactant C12EO6 in the oil phase is considerably high, i.e., É6%. In the case of IPM containing 10% (w/w) silicone oil (filled circles), apart from the shift in the whole phase diagram toward higher temperatures, the overall nature of the diagram remains more or less the same. Comparing the lower and upper three-phase boundaries individually, it is clear that the shift in the upper boundary of the three-phase regime toward higher temperature is more than that of the lower boundary. It means that, in the presence of silicone oil, the surfactant aggregates are more soluble in the water phase, whereas they are less soluble in the oil phase. This is because the lower boundary of the diagram corresponds basically to the clouding temperature of the surfactant in the presence of the oil phase and the upper boundary of the diagram is the mutual solubility curve of the oil and surfactant (1). Hence, surfactant aggregates tend to separate from oil on addition of the polymer oil. A similar tendency was reported in water-soluble polymer–surfactant systems (10, 11). Moreover, the point of maximum solubility moves toward higher surfactant concentration, whereas the monomeric solubility of the surfactant in the oil phase remains practically the same, because the boundaries of the three-phase region at low surfactant concentration remain the same. These observations are evidence of a significant decrease in the solu-
coidas
296
JOHN ET AL.
FIG. 4. Schematic representation of the shift of the three-phase triangle corresponding to Fig. 3 with increasing temperature. Notation as in Fig. 1. FIG. 2. Schematic representation of the shift of the three-phase triangle corresponding to Fig. 1 from the water-rich corner to the oil-rich corner with increasing temperature. Notation as in Fig. 1.
bilization of the oil phase in the surfactant phase. Figure 2 is a schematic representation of the change in the threephase tie triangle corresponding to the phase diagrams shown in Fig. 1 with increasing temperature. The triangle moves from the water-rich corner to the oil-rich corner with increasing temperature. Figure 3 is the phase diagram of the water/C12EO6 /oil system in which the oil phase contains 16% (w/w) silicone oil. At this concentration of silicone oil, the phase diagram registers a marked change in its shape. The one-phase region
at the higher surfactant concentration disappears completely and the three-phase body continues even up to high surfactant concentrations and high temperatures. This indicates that an infinite swelling of the micelles with the oil phase does not take place at this silicone oil concentration. Therefore, the middle surfactant phase in the three-phase body probably contains a water continuous structure similar to the L3 phase (D * ) observed in a water–nonionic surfactant binary system (4, 12). At higher temperatures these surfactant molecules dissolve into the excess oil phase as reverse micelles. The change in the three-phase triangle corresponding to this diagram with increasing temperature can hence be represented as shown in Fig. 4 (13). It is possible that the IPM/silicone oil ratio is different in microemulsion and oil phases, and then, one has to use a quaternary phase diagram; however, we focus attention mainly on the change in the water/oil ratio in each phase in Fig. 4. Initially a certain amount of oil dissolves into the micellar core and thereby the tie triangle moves slightly away from the water–surfactant axis; however, there is no infinite dissolution of the oil phase into the surfactant phase, and therefore the triangle remains within the water side, but the point intersecting the oil– surfactant axis moves upward (14, 15). This change in phase behavior is verified later in this paper. Four Coexisting Phases
FIG. 3. Temperature–surfactant concentration phase diagram of the water/C12EO6 /IPM (16% silicone oil) system. Oil:water ratio is 1:1. II and III are two- and three-phase regions, respectively. Notation as in Fig. 1.
AID
JCIS 4650
/
6g1c$$$102
01-08-97 01:09:56
Figure 5 is a phase diagram of the system in which the composition is fixed except for the silicone oil concentration in the oil phase. The oil:water ratio is 1:1 and the surfactant concentration in the total composition is 8% by weight. The two-phase regions at lower temperatures [II(Wm / O)] and higher temperatures [II(Om / W)] contain aqueous micelles and reverse micelles, respectively. Below the threephase region (III), a region of liquid crystal (LC present)
coidas
WATER/NONIONIC SURFACTANT/OIL IN PRESENCE OF POLYMER OIL
297
phase region containing the D * phase region (W / D * / Om), are formed. The observed four-phase body is a result of the overlap of these two-phase regions. We could not observe the other two three-phase regions in the present system which may be attributed to the narrowness of these regions. Therefore, in Fig. 5, the surfactant phase in the three-phase region is considered to be the normal type (D) toward the left side of the four-phase body (at lower silicone oil concentrations), whereas it is likely D * type toward the right side (at higher silicone oil concentrations) of the four-phase body. Formation of a liquid crystalline region below the three-phase region is also attributed to the low solubility of oil in the surfactant phase. The liquid crystal undergoes gradual melting with increasing temperature and is converted to the D * surfactant phase, which coexists with excess oil and water phases. At still higher temperatures, the surfactant dissolves in the oil phase as reverse micelles and a two-phase region is formed. FIG. 5. Phase diagram of the water/C12EO6 /IPM system as a function of temperature and silicone oil concentration in the IPM. Oil:water ratio is 1:1 and surfactant concentration in the system is 8% (w/w) of the composition. IV is the four-phase region and D * indicates L3-type surfactant phase. Other notation as in Fig. 1.
exists. In addition to these regions, a very narrow region of four different coexisting phases appears at a certain silicone oil concentration range which is shaded in the figure (IV(W / D * / D / O)). In this silicone oil concentration range, at the lower boundary of the three-phase region, excess water is separated out from the surfactant phase, and a three-phase (W / D / O) region is formed initially. This three-phase region is very narrow and is omitted in Fig. 5. With further temperature increase, a fourth phase appears at the boundary between the W and D phases and it grows gradually at the expense of the D phase. This phase is identified as the D * surfactant phase and it appears more bluish than the upper D phase. At a certain temperature, this D * surfactant phase attains maximum growth, then starts shrinking and finally disappears to give a three-phase again. In a water/nonionic surfactant/oil system in which the oil phase is a mixture of hydrocarbon and a polar or amphiphilic oil such as long-chain alcohols, fatty acids, and monoglycerides, up to four kinds of three-phase formation can be observed. This is because the polar oil behaves like amphiphilic compounds and adsorbs at the interface rather than remaining as free oil. Formation of a four-phase body has generally occurred when any two of the three-phase regions overlap (13, 14). In the present system, however, the polymer oil polydimethyl siloxane is not a polar oil. But the presence of this polymer in the IPM makes the oil phase less soluble in the surfactant aggregates, and this results in formation of the D * phase. Thus, two three-phase regions, a normal three-phase region (W / D / O) and a three-
AID
JCIS 4650
/
6g1c$$$102
01-08-97 01:09:56
Phase Diagrams at Constant Surfactant Concentrations To verify the formation of the four-phase body, we studied another set of phase diagrams. Figure 6 is a phase diagram of the system containing pure IPM (silicone oil-free) as a function of increasing oil content and temperature. The surfactant concentration in the aqueous phase is 5% by weight. I, II (Wm / O), and II (Om / W) have the same meaning as in the preceding figures. Above the isotropic one-phase region, there is a LC-present region. Beyond the
FIG. 6. Phase diagram of the water/C12EO6 /IPM system as a function of oil concentration. Surfactant concentration in the water phase is 5% (w/ w). Other notation as in the preceding figures.
coidas
298
JOHN ET AL.
one-phase region, at higher oil concentrations, there is a three-phase region, which is not marked in the diagram. Figure 7 is the corresponding phase diagram of the system in which the oil phase IPM contains 10% (w/w) silicone oil. In addition to the I, II(Wm / O), II(Om / W), LCpresent, and III phase regions, which appeared in the pure IPM system, a three-phase region containing the D * surfactant phase also appears above the LC-present region [III(W / D * / D)]. Above the LC-present region, the emulsion is fairly stable. With increasing temperature, this stable emulsion separates into an aqueous phase and a surfactant phase. At a certain stage, the upper surfactant phase again splits into two, an oil-rich phase at the top and D * phase at the middle. At the upper boundary of the three-phase region, the system reverts again to the emulsion [II(Om / W)]; however, at this silicone oil concentration, the two threephase regions are separated from each other. Further increase in the silicone oil concentration in IPM widens the three-phase body containing the D * phase. At 12% silicone oil concentration (Fig. 8), the two three-phase regions [i.e., III(W / D / O) and III(W / D * / D)] merge to form a continuous strip of three-phase regions. At almost the middle portion of this region, where there is prominent overlapping of the two three-phase regions, a four-phase region appears which is shaded in the figure. Thus, the four-phase formation is confirmed. Since two kinds of microemulsion phases are found, it is likely that the IPM/polymer oil ratio would be different in
FIG. 8. Phase diagram of the water/C12EO6 /IPM (12% silicone oil) system as a function of oil concentration. Surfactant concentration in the water phase is 5% (w/w). Other notation as in preceding figures.
microemulsion and oil phases. The partitioning of polymer oil in each phase will be reported in the near future. CONCLUSIONS
The water/C12EO6 /IPM system can form a single-phase microemulsion. The presence of polydimethyl siloxane (silicone oil) in the IPM enhances the lipophilic nature of the oil phase and thereby decreases the solubility of oil in the surfactant aggregation. The point of maximum solubilization, i.e., the point at which minimum surfactant solubilizes equal amounts of oil and water into a single isotropic phase, also increases in the presence of polymer oil. In the presence of silicone oil, due to the decreased solubilization of oil in the surfactant phase, a three-phase body containing water, D *, and D phases originates within the system. With increasing silicone oil concentration this three-phase region expands, and at a certain range of silicone oil concentration it overlaps with the normal three-phase region containing water, D, and oil phases to form a four-phase body. At higher concentrations of silicone oil, the surfactant phase in the three-phase body is D * type and the phase behavior becomes similar to that of a water/nonionic surfactant/amphiphilic oil system. REFERENCES FIG. 7. Phase diagram of the water/C12EO6 /IPM (10% silicone oil) system as a function of oil concentration. Surfactant concentration in the water phase is 5% (w/w). Other notation as in preceding figures.
AID
JCIS 4650
/
6g1c$$$102
01-08-97 01:09:56
1. Kunieda, H., and Friberg, S. E., Bull. Chem. Soc. Jpn. 54, 1010 (1981). 2. Kunieda, H., and Shinoda, K., J. Colloid Interface Sci. 107, 107 (1985).
coidas
WATER/NONIONIC SURFACTANT/OIL IN PRESENCE OF POLYMER OIL 3. Kunieda, H., Nakano, A., and Akimaru, M., J. Colloid Interface Sci. 170, 78 (1995). 4. Kunieda, H., and Shinoda, K., J. Dispersion Sci. Technol. 3, 233 (1982). 5. Katayama, H., Tagawa, T., and Kunieda, H., J. Colloid Interface Sci. 153, 429 (1992). 6. Messier, A., Schorsch, G., Rouviere, J., and Tenebre, L., Prog. Colloid Polym. Sci. 79, 249 (1989). 7. Disapio, A., and Fridd, P., Int. J. Cosmet. Sci. 10, 75 (1988). 8. Kabalnov, A., Olsson, U., and Wennerstro¨m, H., Langmuir 10, 2159 (1994).
AID
JCIS 4650
/
6g1c$$$102
01-08-97 01:09:56
299
9. Kabalnov, A., Olsson, U., Thuresson, K., and Wennerstrom, H., Langmuir 10, 4509 (1994). 10. Siano, D. B., and Bock, J., J. Colloid Interface Sci. 90, 359 (1982). 11. Wormuth, K. R., Langmuir 7, 1622 (1991). 12. Mitchell, D. J., Tiddy, G. J. T., Waring, L., Bostock, T., and McDonald, M. P., J. Chem. Soc. Faraday Trans. 1 79, 975 (1983). 13. Kunieda, H., Asaoka, H., and Shinoda, K., J. Phys. Chem. 92, 185 (1988). 14. Kunieda, H., J. Colloid Interface Sci. 133, 237 (1989). 15. Kunieda, H., and Haishima, K., J. Colloid Interface Sci. 140, 383 (1990).
coidas