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oil system
Colloids and Surfaces, 24 (1987) 225-237 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
225
The F o r m a t i o n of G e l - E m u l s i o n s in a Water/Nonionic Surfactant/Oil System H. KUNIEDA 1, C. SOLANS 2, N. SHIDA 1and J.L. PARRA 2
~Department of Applied Chemistry, Faculty of Engineering, Yokohama National University, Tokiwadai 156, Hodogaya-ku, Yokohama 240 (Japan) 2Instituto de Technologia Quimica y Textil, C.S.I.C., C. Jorge Girona Salgado, 18-26, 08034 Barcelona (Spain) (Received 26 May 1986; accepted 23 December 1986)
ABSTRACT A highly viscous and translucent gel, which forms m a very diluted water-rich region of a water/ nonionic surfactant/oil system, was investigated by phase study and microscopic observation. The gel consists of two isotropic phases; one phase is almost pure water and the other is an oil phase solubilizing a large amount of water. Therefore, the gel stabilized by a lipophilic nonionic surfactant is essentially a high-internal-phase-volumeW/O emulsion. In the process of gel formation, an unstable O/W-type emulsion is produced at the beginning due to the presence of the large amount of water, although the surfactant is lipophilic. Simultaneously, W/O emulsification occurs inside small oil droplets. Finally, all water is taken up into the oil phase and the gel is obtained. Hence, the size of water droplets in the gel is considerably smaller (submicrometer order) than in an ordinary W/O emulsion of the same system.
INTRODUCTION
It is known that highly viscous and clear gels form in very diluted regions of some nonionic surfactant systems [ 1-3 ]. These kind of gels are very important for practical purposes such as cosmetics [ 3 ], formulation of drugs [ 4 ], jet fuel [ 5 ], and so on. For example, the fact that very small oil droplets are produced in a gel forming in an oil-rich region has been put to use in preparing a fine O / W emulsion [3,6]. Most of the gels which have been found in nonionic surfactant systems are produced in oil-rich regions, but recently it was found that a gel is also produced in water-rich regions [7,8]. In other words, there are at least two types of gel in surfactant system, one in an oil-rich region and the other in a water-rich region. The solution behavior of the surfactant may be closely related to the mechanism of gel formation but the correlation has not been clarified yet. In this context, the relation between the gel formation and phase properties 0166-6622/87/$03.50
© 1987 Elsevier Science Publishers B.V.
226 of the surfactant has been studied in a water-rich region of water/pure nonionic surfactant/oil systems. MATERIALSAND METHODS
Materials Homogeneous polyethyleneglycol dodecyl ether (abbreviated as R12EOn) was obtained from Nikko Chemicals Co. Its purity, > 98-99%, was confirmed by chromatographic analysis. Extra-pure decane, heptane, and m-xylene were obtained from Tokyo Kasei Kogyo Co.
Determination of phase diagrams Procedures for the phase diagram determination are described elsewhere [9].
Preparation of gels or emulsions Two methods were used: (1) A test tube (diameter 18 m m ) containing the sample was shaken by hand for 10-15 min. (2) The same container was used, but the sample was shaken with seven glass balls (diameter 6 m m ) by hand for the same time interval. The results were unchanged by further agitation. When method (1) was used, a gel did not form in the systems investigated.
Determination of droplet type in the gel or emulsion In order to determine the types of emulsions and gels, the so-called Becke line effect was used [ 10 ]. This method is based on the fact that when rays pass through a prismatic or lenticular object they are bent inwardly or entripetally if the object (droplet) is of higher refractive index than the continuous phase and outwardly or centrifugally if the refractive index of the droplet is lower than that of the continuous medium. In order to apply correctly the Becke line effect, one must focus sharply on the droplet and then rack the microscope tube upwards while observing the fringe of light around the object. If the fringe appears to contract as the tube is racked up, it is concluded that the droplet has a higher refractive index than the continuous medium ( O/W emulsion or gel). If the fringe appears to expand, the droplet has a lower refractive index (W/O emulsion or gel).
227 TABLE 1 Gel formation in water/pure nonionic surfactant/oil systems (water-rich region) Surfactant
Oil
Gel formation
HLB-temp. ( ° C )
R12E04
heptane decane cyclohexane toluene heptane m-xylene heptane
yes yes yes no no no no
9.0 19 5 - 24 48 24 71
RI~E05 RI2E06 R~2EOs
R E S U L T S AND D I S C U S S I O N
Relation between gel formation and HLB temperature of the surfactant Nonionic surfactant changes from water soluble ( forming a micelle in water) to oil soluble ( forming a reversed micelle in oil ) with rise in temperature. The transition temperature of the solution behavior is called the hydrophile-lipophile-balanced ( H L B ) temperature, at which three phases consisting of surfactant, water, and oil phases appear and the type of emulsion changes from O / W type to W / O type. Therefore, it is also called phase inversion temperature ( PIT ) [ 11 ]. At temperatures above the PIT, the surfactant acts as a lipophilic one, i.e., a W / O emulsifier, whereas below the P I T it acts a hydrophilic one, i.e., an O / W emulsifier. The relation between HLB temperature and gel formation in a water-rich region is shown in Table 1. The HLB temperatures were calculated by using the HLB numbers according to the equation given in a previous paper [ 9 ]. W h e n a hydrophilic surfactant is used, the gel forms in an oil-rich region [1-3], whereas it is produced in a water-rich region by using lipophilic surfactants. In other words, the gel forms in a water-rich region at temperatures above the HLB temperature and vice versa. However, aromatic hydrocarbon systems are an exception, it being very difficult to form a gel in a diluted region of these systems. In the following section, we deal with a lipophilic pure surfactant system (water/R12EO4/oil) in which a gel is produced in a water-rich region.
Phase diagram of water/R~2EO4/decane system Gel forms in a water-rich region of a water/R12EO4/decane system at above 25 ° C. In order to understand the correlation between the phase behavior and
228 gel formation, phase diagrams of this system were determined at 25 and 50 °C and are shown in Figs 1 and 2. In the two-phase region (II), one of the phases is almost pure water containing less than 0.05 wt% of surfactant confirmed by HPLC. The other phase is a nonaqueous reversed micellar solution phase [ oil phase indicated by (I) ]. Therefore the tie lines are drawn from the water corner as shown in Figs 1 and 2. With a rise in temperature, one-phase region shifts towards the highly concentrated area. This means that the solubilization of water in the oil phase decreases with increase in temperature. The HLB temperature of this system is around 19°C and at the temperatures shown in Figs I and 2, the surfactant is relatively lipophilic [ 12,13] and acts as a W/O emulsifier. A liquid crystalline phase appears at higher concentrations above one isotropic phase (I).
Correlation between phase behavior and gel formation The phase diagram of a water/R12EO4/decane system as a function of temperature is shown in Fig. 3. The concentration of surfactant in water is 0.5 wt% and the concentration of decane in the system is plotted horizontally. A threephase region (III) consisting of water, oil and surfactant phases appears and the solubilization of oil reaches its maximum at the HLB temperature. Below the HLB temperature, there is a two-phase region consisting of an aqueous micellar solution phase and an excess oil phase. Above the HLB temperature, the gel is produced in a two-isotropic-phase region consisting of a nonaqueous reversed micellar solution phase and an excess water phase. The gel in this system is not so stable and breaks down with time. Finally, two phases were observed in the gel region. Although it is very difficult to distinguish the boundary of the gel region (highly viscous and translucent state), we classify the dispersion states in the two-phase region after shaking [methods (1) and (2) in the experimental section] as follows: A, highly viscous translucent gel; a filament of an electric bulb can be seen through the gel. A', highly viscous gel; it looks bluish-white but the filament cannot be seen. A", highly viscous gel much more turbid than A', which adheres to the glass walls of the test tube and looks translucent. B, viscous (but much less than A-A") white W/O emulsion. C, unstable O/W emulsion; after a while, the white flocks (or cream) remain above the water phase. As described later, oil drops in flocks contain small water droplets. D, very unstable emulsion; after shaking, the system splits completely into two clear phases in less than half an hour. The state of dispersion was also determined in water/R,2EO4/heptane and water R12EOs/m-xylene systems, shown in Figs 4 and 5. As described in the experimental section, we used two methods. Using method
229
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Fig. 1. Phase diagram of water/R~2EOJdecane system at 25 ° C. I and II are one- and two-phase regions. L.C. present indicates the region in which a liquid crystalline phase appears. The dotted lines are tie lines.
R12EO~
H20
C10H22
Fig. 2. Phase diagram of water/R,2EOJdecane system at 50 ° C. The notation is the same as that in Fig. 1.
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Fig. 3. The effect of adding decane on the types of dispersion in a water/RleEO4/decane system. Decane was added to 0.5 wt% RI~EO4aqueous solution: (a) by method (1); (b) by method (2). The notation is described in the text. (1), a gel was not produced and the W / O emulsion region is not large. Since there is an unstable two-phase region (region D) between the gel region and the liquid crystalline ( L.C.-present ) region, it suggests that a liquid crystalline phase is not related to the formation of gel. Using method (2), gel was produced and the most transparent gels were obtained at temperatures 10-20 °C above the H L B temperature. However, in the m-xylene system, a gel was not formed using method (2), as shown in Fig. 5. Since it is very difficult to produce W / O emulsions in an oil-rich region of this system, it is considered that a large amount of water cannot be taken up into an oil phase in an aromatic hydrocarbon system. For this reason, a gel cannot be produced. The gel regions in partial phase diagrams of a water/R12EOJdecane system at 25-50°C are shown in Fig. 6 ( a ) - ( d ) . As shown in Fig. 6, the oil/surfactant ratio is very important for the formation of gel. This fact suggests that the property or the structure of the oil phase may be responsible for the gel. With increase in temperature, the gel region shrinks and at 50 ° C a transparent gel. (A) cannot be obtained. In the vicinity of the one-phase region close to the w a t e r - s u r f a c t a n t axis (indicated by D ) , the coalescence rate of droplets in the emulsion is very fast and it separates into two clear phases completely within half an hour or so. It is considered that the existence of this very-unstable-emulsion region may be very important for
231
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Iqg. 4. The effect of adding heptane on the types of dispersion in a water/Rv2EO4/heptane system. Heptane was added to 0.5 wt% Rr,E04 aqueous solution: (a) by method (1) ; (b) by method ( 2 ). The notation is described in the text.
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232
an application such as breaking emulsion. This phenomenon is completely different from that in other areas of the two-phase region. Moreover, the boundary of region D is along one tie line and the change in coalescence rate along the border is very drastic. It means that the phase volume ratio is not related to these phenomena. Nor is the viscosity of the oil phase related to the coalescence rate, since the apparent viscosity of the oil phase in the region D is much higher than that in the other regions. For this reason, this may be related to 10
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233
the change in structure of the oil phase. At these temperatures, the surfactant is lipophilic and forms a reversed micelle solubilizing water in it. Therefore the total volume ratio of reversed micelle (surfactant + water) is increasing from the oil-rich region to the water corner. At a certain point (if the micelle is lO
~
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Fig. 6. The gel region m partial phase diagrams of a water/R~2EO4/decane system at (a) 25, (b) 30, (c) 40 and (d) 50 ° C. Method (2) was used for producing gel. The notation is the same as in previous figures.
234
Fig. 7. The process of gel formation. Composition: 99.3 wt% of 0.5 wt% R,2EO4aqueous solution, 0.7 wt% heptane.
Fig. 8. Photomicrograph of flocks in Fig. 7 (a). Small water droplets exist inside oil drops. monodispersed and spherical, the limit of close packing being about 70% of total volume) the oil phase changes from oil continuous to surfactant continuous, i.e., surfactant phase. In fact, the one phase close to the water-surfactant axis is connected to the axis at a higher temperature, and it is called the L3 phase. This change in structure of the solution is considered to be related to the sudden instability of the emulsion.
The mechanism of gel formation Pictures of the process of gel formation are shown in Fig. 7. W h e n the time of shaking is not long enough to form the gel, a white flock is separated, as shown in Fig. 7 (a) (state C ). As the shaking time gets longer, the flock volume
235
Fig. 9. Photomicrographs of gel in Fig. 7 (c). (a) Just after preparing gel. (b) 4 h after preparing gel. Water droplets become larger due to coalescence.
increases until finally all the water phase is taken up into flock and gel is produced. A microscopic picture of the flock is shown in Fig. 8, in which oil droplets containing fine water drops can be seen. Therefore, by agitating, O/W emulsification occurs (but the emulsion is unstable) and simultaneously, W/O emulsification is proceeding inside the oil droplet. In other words, W / O / W emulsification takes place at this time. In our method, it was necessary to add the glass balls or other solid surface to promote gel formation. Judging from the above results, this local agitation is necessary to promote W/O emulsification in the interface of oil drops, and for this reason the fine water droplet can be obtained in the gel. The micropictures for gel and ordinary W/O emulsion ( state B ) are shown in Fig. 9. It is almost impossible to observe the water droplets in the gel by optic microscopy, as the droplet size is of submicrometer order. This result is completely consistent with the results for O/W gel being produced in an oil-rich region [ 3 ]. With the increase in oil content, the size of
236
Fig. 10. Photomicrographsof W/O emulsion in a water/R12EO4/heptanesystem. (a) 96 wt% of 0.5 wt% RI2EO~aqueous solution, 4 wt% of heptane. (b) 90 wt% of 0.5 wt% R12E04 aqueous solution, 10 wt% of heptane. water droplet increases rapidly, as shown in Fig. 10 and the system looks like an ordinary white W / O emulsion. The gel in our system was not so stable and became gradually turbid with time. As shown in Fig. 10, the size of water droplets becomes larger due to coalescence and the gel looks more turbid. ACKNOWLEDGMENT Financial support from The Yam~da Foundation for Science is gratefully acknowledged.
237 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13
M.J. Groves, R.M.A. Mustafa and J.E. Carless, J. Pharm. Pharmacol., 26 (1974) 616. A.A. Ali and B.A. Mulley, J. Pharm. Pharmacol., 30 (1978) 205. H. Sagitani, T. Hattori, K. Nabeta and M. Nagai, Nippon Kagaku Kaishi, (1983) 1399. D. Attwood and A.T. Florence, Surfactant Systems, Chapman and Hall, New York. 1983, Ch. 11. H. Ishida and A. Iwama, Combus. Sci. Technol., 37 (1984) 79. H. Sagitani, Y. Hirai, K. Nabeta and M. Nagai, J. Jpn. Oil Chem. Soc., 35 (1986) 102. J.J. Garcia Dominguez and C. Solans, J. Disp. Sci. Tech., 6 (1985) 271. C. Solans, N. Azemar, F. Comelles, J. Sanchhez Leal and J.L. Parra, Proc. of the XVI! Jornadas CED/AID, Madrid, AID, Barcelona, 1986, pp. 109-122. H. Kunieda and K. Shinoda, J. Colloid Interface Sci., 107 (1985) 107. J.F. Carriere, Chem. Weekbl, 26 (1929) 413. K. Shinoda and H. Saito, J. Colloid Interface Sci., 26 (1968) 70. H. KuniedaandK. Shinoda, Bull. Chem. Soc. Jpn, 55 (1982) 1777. H. KuniedaandK. Shinoda, J. Disp. Sci. Tech.,3 (1982) 233.
225
The F o r m a t i o n of G e l - E m u l s i o n s in a Water/Nonionic Surfactant/Oil System H. KUNIEDA 1, C. SOLANS 2, N. SHIDA 1and J.L. PARRA 2
~Department of Applied Chemistry, Faculty of Engineering, Yokohama National University, Tokiwadai 156, Hodogaya-ku, Yokohama 240 (Japan) 2Instituto de Technologia Quimica y Textil, C.S.I.C., C. Jorge Girona Salgado, 18-26, 08034 Barcelona (Spain) (Received 26 May 1986; accepted 23 December 1986)
ABSTRACT A highly viscous and translucent gel, which forms m a very diluted water-rich region of a water/ nonionic surfactant/oil system, was investigated by phase study and microscopic observation. The gel consists of two isotropic phases; one phase is almost pure water and the other is an oil phase solubilizing a large amount of water. Therefore, the gel stabilized by a lipophilic nonionic surfactant is essentially a high-internal-phase-volumeW/O emulsion. In the process of gel formation, an unstable O/W-type emulsion is produced at the beginning due to the presence of the large amount of water, although the surfactant is lipophilic. Simultaneously, W/O emulsification occurs inside small oil droplets. Finally, all water is taken up into the oil phase and the gel is obtained. Hence, the size of water droplets in the gel is considerably smaller (submicrometer order) than in an ordinary W/O emulsion of the same system.
INTRODUCTION
It is known that highly viscous and clear gels form in very diluted regions of some nonionic surfactant systems [ 1-3 ]. These kind of gels are very important for practical purposes such as cosmetics [ 3 ], formulation of drugs [ 4 ], jet fuel [ 5 ], and so on. For example, the fact that very small oil droplets are produced in a gel forming in an oil-rich region has been put to use in preparing a fine O / W emulsion [3,6]. Most of the gels which have been found in nonionic surfactant systems are produced in oil-rich regions, but recently it was found that a gel is also produced in water-rich regions [7,8]. In other words, there are at least two types of gel in surfactant system, one in an oil-rich region and the other in a water-rich region. The solution behavior of the surfactant may be closely related to the mechanism of gel formation but the correlation has not been clarified yet. In this context, the relation between the gel formation and phase properties 0166-6622/87/$03.50
© 1987 Elsevier Science Publishers B.V.
226 of the surfactant has been studied in a water-rich region of water/pure nonionic surfactant/oil systems. MATERIALSAND METHODS
Materials Homogeneous polyethyleneglycol dodecyl ether (abbreviated as R12EOn) was obtained from Nikko Chemicals Co. Its purity, > 98-99%, was confirmed by chromatographic analysis. Extra-pure decane, heptane, and m-xylene were obtained from Tokyo Kasei Kogyo Co.
Determination of phase diagrams Procedures for the phase diagram determination are described elsewhere [9].
Preparation of gels or emulsions Two methods were used: (1) A test tube (diameter 18 m m ) containing the sample was shaken by hand for 10-15 min. (2) The same container was used, but the sample was shaken with seven glass balls (diameter 6 m m ) by hand for the same time interval. The results were unchanged by further agitation. When method (1) was used, a gel did not form in the systems investigated.
Determination of droplet type in the gel or emulsion In order to determine the types of emulsions and gels, the so-called Becke line effect was used [ 10 ]. This method is based on the fact that when rays pass through a prismatic or lenticular object they are bent inwardly or entripetally if the object (droplet) is of higher refractive index than the continuous phase and outwardly or centrifugally if the refractive index of the droplet is lower than that of the continuous medium. In order to apply correctly the Becke line effect, one must focus sharply on the droplet and then rack the microscope tube upwards while observing the fringe of light around the object. If the fringe appears to contract as the tube is racked up, it is concluded that the droplet has a higher refractive index than the continuous medium ( O/W emulsion or gel). If the fringe appears to expand, the droplet has a lower refractive index (W/O emulsion or gel).
227 TABLE 1 Gel formation in water/pure nonionic surfactant/oil systems (water-rich region) Surfactant
Oil
Gel formation
HLB-temp. ( ° C )
R12E04
heptane decane cyclohexane toluene heptane m-xylene heptane
yes yes yes no no no no
9.0 19 5 - 24 48 24 71
RI~E05 RI2E06 R~2EOs
R E S U L T S AND D I S C U S S I O N
Relation between gel formation and HLB temperature of the surfactant Nonionic surfactant changes from water soluble ( forming a micelle in water) to oil soluble ( forming a reversed micelle in oil ) with rise in temperature. The transition temperature of the solution behavior is called the hydrophile-lipophile-balanced ( H L B ) temperature, at which three phases consisting of surfactant, water, and oil phases appear and the type of emulsion changes from O / W type to W / O type. Therefore, it is also called phase inversion temperature ( PIT ) [ 11 ]. At temperatures above the PIT, the surfactant acts as a lipophilic one, i.e., a W / O emulsifier, whereas below the P I T it acts a hydrophilic one, i.e., an O / W emulsifier. The relation between HLB temperature and gel formation in a water-rich region is shown in Table 1. The HLB temperatures were calculated by using the HLB numbers according to the equation given in a previous paper [ 9 ]. W h e n a hydrophilic surfactant is used, the gel forms in an oil-rich region [1-3], whereas it is produced in a water-rich region by using lipophilic surfactants. In other words, the gel forms in a water-rich region at temperatures above the HLB temperature and vice versa. However, aromatic hydrocarbon systems are an exception, it being very difficult to form a gel in a diluted region of these systems. In the following section, we deal with a lipophilic pure surfactant system (water/R12EO4/oil) in which a gel is produced in a water-rich region.
Phase diagram of water/R~2EO4/decane system Gel forms in a water-rich region of a water/R12EO4/decane system at above 25 ° C. In order to understand the correlation between the phase behavior and
228 gel formation, phase diagrams of this system were determined at 25 and 50 °C and are shown in Figs 1 and 2. In the two-phase region (II), one of the phases is almost pure water containing less than 0.05 wt% of surfactant confirmed by HPLC. The other phase is a nonaqueous reversed micellar solution phase [ oil phase indicated by (I) ]. Therefore the tie lines are drawn from the water corner as shown in Figs 1 and 2. With a rise in temperature, one-phase region shifts towards the highly concentrated area. This means that the solubilization of water in the oil phase decreases with increase in temperature. The HLB temperature of this system is around 19°C and at the temperatures shown in Figs I and 2, the surfactant is relatively lipophilic [ 12,13] and acts as a W/O emulsifier. A liquid crystalline phase appears at higher concentrations above one isotropic phase (I).
Correlation between phase behavior and gel formation The phase diagram of a water/R12EO4/decane system as a function of temperature is shown in Fig. 3. The concentration of surfactant in water is 0.5 wt% and the concentration of decane in the system is plotted horizontally. A threephase region (III) consisting of water, oil and surfactant phases appears and the solubilization of oil reaches its maximum at the HLB temperature. Below the HLB temperature, there is a two-phase region consisting of an aqueous micellar solution phase and an excess oil phase. Above the HLB temperature, the gel is produced in a two-isotropic-phase region consisting of a nonaqueous reversed micellar solution phase and an excess water phase. The gel in this system is not so stable and breaks down with time. Finally, two phases were observed in the gel region. Although it is very difficult to distinguish the boundary of the gel region (highly viscous and translucent state), we classify the dispersion states in the two-phase region after shaking [methods (1) and (2) in the experimental section] as follows: A, highly viscous translucent gel; a filament of an electric bulb can be seen through the gel. A', highly viscous gel; it looks bluish-white but the filament cannot be seen. A", highly viscous gel much more turbid than A', which adheres to the glass walls of the test tube and looks translucent. B, viscous (but much less than A-A") white W/O emulsion. C, unstable O/W emulsion; after a while, the white flocks (or cream) remain above the water phase. As described later, oil drops in flocks contain small water droplets. D, very unstable emulsion; after shaking, the system splits completely into two clear phases in less than half an hour. The state of dispersion was also determined in water/R,2EO4/heptane and water R12EOs/m-xylene systems, shown in Figs 4 and 5. As described in the experimental section, we used two methods. Using method
229
R12EO4
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J
H20
C10H22
Fig. 1. Phase diagram of water/R~2EOJdecane system at 25 ° C. I and II are one- and two-phase regions. L.C. present indicates the region in which a liquid crystalline phase appears. The dotted lines are tie lines.
R12EO~
H20
C10H22
Fig. 2. Phase diagram of water/R,2EOJdecane system at 50 ° C. The notation is the same as that in Fig. 1.
230 (a) 70
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,
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Fig. 3. The effect of adding decane on the types of dispersion in a water/RleEO4/decane system. Decane was added to 0.5 wt% RI~EO4aqueous solution: (a) by method (1); (b) by method (2). The notation is described in the text. (1), a gel was not produced and the W / O emulsion region is not large. Since there is an unstable two-phase region (region D) between the gel region and the liquid crystalline ( L.C.-present ) region, it suggests that a liquid crystalline phase is not related to the formation of gel. Using method (2), gel was produced and the most transparent gels were obtained at temperatures 10-20 °C above the H L B temperature. However, in the m-xylene system, a gel was not formed using method (2), as shown in Fig. 5. Since it is very difficult to produce W / O emulsions in an oil-rich region of this system, it is considered that a large amount of water cannot be taken up into an oil phase in an aromatic hydrocarbon system. For this reason, a gel cannot be produced. The gel regions in partial phase diagrams of a water/R12EOJdecane system at 25-50°C are shown in Fig. 6 ( a ) - ( d ) . As shown in Fig. 6, the oil/surfactant ratio is very important for the formation of gel. This fact suggests that the property or the structure of the oil phase may be responsible for the gel. With increase in temperature, the gel region shrinks and at 50 ° C a transparent gel. (A) cannot be obtained. In the vicinity of the one-phase region close to the w a t e r - s u r f a c t a n t axis (indicated by D ) , the coalescence rate of droplets in the emulsion is very fast and it separates into two clear phases completely within half an hour or so. It is considered that the existence of this very-unstable-emulsion region may be very important for
231
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Iqg. 4. The effect of adding heptane on the types of dispersion in a water/Rv2EO4/heptane system. Heptane was added to 0.5 wt% Rr,E04 aqueous solution: (a) by method (1) ; (b) by method ( 2 ). The notation is described in the text.
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232
an application such as breaking emulsion. This phenomenon is completely different from that in other areas of the two-phase region. Moreover, the boundary of region D is along one tie line and the change in coalescence rate along the border is very drastic. It means that the phase volume ratio is not related to these phenomena. Nor is the viscosity of the oil phase related to the coalescence rate, since the apparent viscosity of the oil phase in the region D is much higher than that in the other regions. For this reason, this may be related to 10
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233
the change in structure of the oil phase. At these temperatures, the surfactant is lipophilic and forms a reversed micelle solubilizing water in it. Therefore the total volume ratio of reversed micelle (surfactant + water) is increasing from the oil-rich region to the water corner. At a certain point (if the micelle is lO
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Fig. 6. The gel region m partial phase diagrams of a water/R~2EO4/decane system at (a) 25, (b) 30, (c) 40 and (d) 50 ° C. Method (2) was used for producing gel. The notation is the same as in previous figures.
234
Fig. 7. The process of gel formation. Composition: 99.3 wt% of 0.5 wt% R,2EO4aqueous solution, 0.7 wt% heptane.
Fig. 8. Photomicrograph of flocks in Fig. 7 (a). Small water droplets exist inside oil drops. monodispersed and spherical, the limit of close packing being about 70% of total volume) the oil phase changes from oil continuous to surfactant continuous, i.e., surfactant phase. In fact, the one phase close to the water-surfactant axis is connected to the axis at a higher temperature, and it is called the L3 phase. This change in structure of the solution is considered to be related to the sudden instability of the emulsion.
The mechanism of gel formation Pictures of the process of gel formation are shown in Fig. 7. W h e n the time of shaking is not long enough to form the gel, a white flock is separated, as shown in Fig. 7 (a) (state C ). As the shaking time gets longer, the flock volume
235
Fig. 9. Photomicrographs of gel in Fig. 7 (c). (a) Just after preparing gel. (b) 4 h after preparing gel. Water droplets become larger due to coalescence.
increases until finally all the water phase is taken up into flock and gel is produced. A microscopic picture of the flock is shown in Fig. 8, in which oil droplets containing fine water drops can be seen. Therefore, by agitating, O/W emulsification occurs (but the emulsion is unstable) and simultaneously, W/O emulsification is proceeding inside the oil droplet. In other words, W / O / W emulsification takes place at this time. In our method, it was necessary to add the glass balls or other solid surface to promote gel formation. Judging from the above results, this local agitation is necessary to promote W/O emulsification in the interface of oil drops, and for this reason the fine water droplet can be obtained in the gel. The micropictures for gel and ordinary W/O emulsion ( state B ) are shown in Fig. 9. It is almost impossible to observe the water droplets in the gel by optic microscopy, as the droplet size is of submicrometer order. This result is completely consistent with the results for O/W gel being produced in an oil-rich region [ 3 ]. With the increase in oil content, the size of
236
Fig. 10. Photomicrographsof W/O emulsion in a water/R12EO4/heptanesystem. (a) 96 wt% of 0.5 wt% RI2EO~aqueous solution, 4 wt% of heptane. (b) 90 wt% of 0.5 wt% R12E04 aqueous solution, 10 wt% of heptane. water droplet increases rapidly, as shown in Fig. 10 and the system looks like an ordinary white W / O emulsion. The gel in our system was not so stable and became gradually turbid with time. As shown in Fig. 10, the size of water droplets becomes larger due to coalescence and the gel looks more turbid. ACKNOWLEDGMENT Financial support from The Yam~da Foundation for Science is gratefully acknowledged.
237 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13
M.J. Groves, R.M.A. Mustafa and J.E. Carless, J. Pharm. Pharmacol., 26 (1974) 616. A.A. Ali and B.A. Mulley, J. Pharm. Pharmacol., 30 (1978) 205. H. Sagitani, T. Hattori, K. Nabeta and M. Nagai, Nippon Kagaku Kaishi, (1983) 1399. D. Attwood and A.T. Florence, Surfactant Systems, Chapman and Hall, New York. 1983, Ch. 11. H. Ishida and A. Iwama, Combus. Sci. Technol., 37 (1984) 79. H. Sagitani, Y. Hirai, K. Nabeta and M. Nagai, J. Jpn. Oil Chem. Soc., 35 (1986) 102. J.J. Garcia Dominguez and C. Solans, J. Disp. Sci. Tech., 6 (1985) 271. C. Solans, N. Azemar, F. Comelles, J. Sanchhez Leal and J.L. Parra, Proc. of the XVI! Jornadas CED/AID, Madrid, AID, Barcelona, 1986, pp. 109-122. H. Kunieda and K. Shinoda, J. Colloid Interface Sci., 107 (1985) 107. J.F. Carriere, Chem. Weekbl, 26 (1929) 413. K. Shinoda and H. Saito, J. Colloid Interface Sci., 26 (1968) 70. H. KuniedaandK. Shinoda, Bull. Chem. Soc. Jpn, 55 (1982) 1777. H. KuniedaandK. Shinoda, J. Disp. Sci. Tech.,3 (1982) 233.