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Effects of the number of fatty acid residues on the phase behaviors of decaglycerol fatty acid esters
Journal of Colloid and Interface Science 296 (2006) 685–689 www.elsevier.com/locate/jcis
Effects of the number of fatty acid residues on the phase behaviors of decaglycerol fatty acid esters Sakiko Ai ∗ , Masahiko Ishitobi Food Ingredients Laboratory, Mitsubishi Chemical Corporation, Kamoshida-cho 1000, Aoba-ku, Yokohama 227-8502, Japan Received 21 June 2005; accepted 15 September 2005 Available online 26 October 2005
Abstract The effects of the number of fatty acid residues (n) in decaglycerol fatty acid esters, i.e., decaglycerol laurates (abbreviated to (C11 )n G10 ), on the phase behaviors of three laurate esters, (C11 )1.9 G10 , (C11 )2.7 G10 , and (C11 )3.4 G10 , were investigated. The unreacted decaglycerol remaining in each ester was removed by liquid extraction before use. (C11 )1.9 G10 formed hexagonal liquid crystals in aqueous solutions, while (C11 )2.7 G10 and (C11 )3.4 G10 , which are more hydrophobic than (C11 )1.9 G10 , formed lamellar liquid crystals. The cloud point in aqueous solution was measured for mixtures of these three esters. The cloud phenomenon was observed when the weight ratio of hydrophilic groups to the total surfactant (W H /W S ) was around 0.6. The cloud point shifted to a markedly higher temperature, even with a slight increase in the W H /W S ratio. The solubilization abilities of (C11 )n G10 for the oils m-xylene and (R)-(+)-limonene were also examined. When the W H /W S ratio was between 0.60 and 0.64, (C11 )n G10 formed microemulsions and lyotropic liquid crystals in the presence of water and the oils. These self-organized structures were stable, even above 90 ◦ C. It is concluded that the phase behavior of (C11 )n G10 are insensitive to temperature, but strongly dependent on both the W H /W S ratio and the number of fatty acid residues (n). © 2005 Elsevier Inc. All rights reserved. Keywords: Polyglycerol fatty acid ester; Cloud point; Microemulsion; Liquid crystal; Phase behavior
1. Introduction Polyglycerol fatty acid esters are widely used as surfactants for foods, cosmetics, pharmaceuticals, and toiletries. Many attempts have been made to clarify their functions. However, few basic studies have addressed their physicochemical properties because of the difficulties associated with their structural complexity. Specifically, they are mixtures of isomers with various polyglycerol chain lengths and/or various degrees of esterification [1], and moreover, commercially available polyglycerol esters contain large amounts of unreacted polyglycerol [2]. Ishitobi and Kunieda [3] studied the effects of the polyglycerol chain length distribution on the phase behaviors of polyglycerol laurates. They reported that esters with broadly distrib-
* Corresponding author. Fax: +81 45 963 3976.
E-mail address: [email protected] (S. Ai). 0021-9797/$ – see front matter © 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.09.029
uted polyglycerol chains were more tightly packed at the interface than esters with sharply distributed polyglycerol chains, and that the former showed much greater abilities for surface tension lowering and emulsification. In the case of conventional poly(oxyethylene)-type nonionic surfactants, it is well known that a phase inversion takes place due to dehydration of hydrophilic chains, i.e., ethylene oxide units, as the temperature increases [4]. Kunieda et al. [5] investigated the phase behaviors of polyglycerol dilaurates with various polyglycerol chain lengths, (C11 )2 Gn , and reported that their phase behaviors were not very sensitive to temperature. Most of the polyglycerol fatty acid esters used commercially as surfactants consist of polyglycerol with a polymerized number less than 10. In the present study, we investigated the phase behaviors of decaglycerol laurates with three different numbers of fatty acid residues, i.e., n = 1.9, 2.7, and 3.4, while fixing the polyglycerol number at 10. Hereafter, these laurates are abbreviated to (C11 )n G10 .
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2. Materials and methods
30 min at a given temperature, and the cloud phenomena were observed visually.
2.1. Materials 2.5. Phase diagram construction 2.1.1. Decaglycerol Decaglycerol was synthesized by polymerization of glycerol as follows. Glycerol was reacted with potassium carbonate at 250 ◦ C under a pressure of less than 33.3 kPa. The average degree of polymerization was 10.1, as calculated from the hydroxyl value (OHV), which represents the number of KOH milliequivalents to the hydroxyl content (in moles) in a 1-g sample [6].
Surfactants were mixed with water and an oil (m-xylene or (R)-(+)-limonene) in glass ampoules. The water/oil ratio was kept constant at 1:1. The ampoules were mixed completely using a vortex mixer or centrifuge. The phase states were determined by both visual inspection and a crossed polarizer. 3. Results and discussion 3.1. Phase behavior of (C11 )n G10 in aqueous solutions
2.1.2. Decaglycerol fatty acid esters Three decaglycerol laurates were synthesized according to the following procedure. Decaglycerol synthesized as described above and lauric acid (palmac98-12; Acidchem International) were mixed at three different lauric acid/polyglycerol molar ratios, namely 1, 2, and 3. Next, 0.0025 wt% of sodium hydroxide was added to the reaction mixtures as a catalyst. The esterification reaction was performed at 240 ◦ C for 3.5 h and then at 260 ◦ C for 4 h under a nitrogen stream at atmospheric pressure. Unreacted polyglycerol remaining in the crude surfactants was removed using a previously described extraction method [3]. The numbers of fatty acid residues (n) in the three crude surfactants were 1.9, 2.7, and 3.4, respectively. The free polyglycerol contents in the purified surfactants were less than 2 wt%. The purified surfactants, i.e., the three decaglycerol laurates, were abbreviated to (C11 )1.9 G10 , (C11 )2.7 G10 , and (C11 )3.4 G10 .
The three samples, (C11 )1.9 G10 , (C11 )2.7 G10 , and (C11 )3.4 G10 , were dissolved in water to the desired concentrations. Fig. 1 shows the phase diagrams as a function of the temperature. (C11 )1.9 G10 formed an aqueous micellar solution at low concentrations and liquid crystals at high concentrations (Fig. 1a). The intensity √ ratio of the SAXS peaks for the liquid crystals was 1:1/ 3, indicating that they were hexagonal H1 liquid crystals. In contrast, the liquid crystals formed by (C11 )2.7 G10 and (C11 )3.4 G10 , which are more hydrophobic than (C11 )1.9 G10 , showed SAXS peak intensity ratios of 1:1/2, indicating that they were lamellar liquid crystals. At high concentrations, (C11 )2.7 G10 and (C11 )3.4 G10 formed a single lamellar phase, while at low concentrations, the lamellar liquid crystals coexisted with water (Figs. 1b and 1c).
2.2. Construction of phase diagrams
3.2. Cloud points
Samples of different compositions were sealed in glass ampoules with a narrow constriction. Homogeneity of the viscous samples was attained by passing them through the narrow constriction by repeated centrifugation. After the homogenization, the ampoules were placed in a thermostat and phase separation was observed visually. The existence of liquid crystals was detected with a crossed Nicol polarizer, and the types of liquid crystals were determined by small-angle X-ray scattering (SAXS).
(C11 )1.9 G10 and (C11 )2.7 G10 were mixed to the desired compositions, and the cloud points were measured for different W H /W S ratios. The concentration of the surfactant in aqueous solution was fixed at 10 wt%. As shown in Fig. 2, the cloud phenomenon was observed at a W H /W S ratio near 0.625. It has been reported that the cloud point of commonly used poly(oxyethylene)-type nonionic surfactants shifts to a higher temperature as the W H /W S ratio increases [8]. The cloud point of (C11 )n G10 shifted to a higher temperature at relatively high temperatures above 70 ◦ C as the W H /W S ratio increased, but shifted to a lower temperature at temperatures below 70 ◦ C. As shown in Fig. 1b, (C11 )2.7 G10 formed lamellar liquid crystals in water, and these liquid crystals coexisted with water at low concentrations. Consequently, it is considered that the cloud phenomenon observed below 70 ◦ C is caused by separation of the lamellar liquid crystals from the water. In other words, it is hard to measure the cloud point because the separation of the lamellar liquid crystals from the water takes place simultaneously, even if the cloud phenomenon caused by dehydration of hydrophilic moieties takes place below 70 ◦ C. Accordingly, it is concluded that the true cloud point can be measured from the cloud phenomenon observed above 70 ◦ C. A comparison of the cloud points of (C11 )n G10 with those of poly(oxyethylene)-type surfactants [4] is shown in Fig. 3. As shown in Fig. 2, the cloud point of (C11 )n G10 appeared to depend on the temperature to a
2.3. SAXS measurements A small-angle scattering goniometer equipped with an 18kW Rigaku Denki rotating anode generator (RINT 2500) was used. The samples were sealed in a steel holder covered with a thin plastic film (Mylar seal method). The measurements were made at 25 ◦ C. The liquid crystal types were determined by the √ SAXS peak ratios, namely 1:1/2:1/3 for lamellar and 1:1/ 3:1/2 for hexagonal phases [7]. 2.4. Cloud point measurement Cloud points of aqueous solutions containing 10 wt% of the decaglycerol laurates were measured for different W H /W S ratios as follows. Glass ampoules were placed in a thermostat for
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(a) Fig. 2. Cloud points in the water/(C11 )n G10 system as a function of W H /W S .
(b)
Fig. 3. Cloud points of (C11 )n G10 (") compared with polyoxyethylene oleyl ethers (Q) [4].
(c) Fig. 1. Phase diagrams of the water/(C11 )n G10 systems as a function of temperature: (a) water/(C11 )1.9 G10 system, (b) water/(C11 )2.7 G10 system, (c) water/ (C11 )3.4 G10 system.
certain extent, but the extent of its temperature sensitivity was lower than those of the poly(oxyethylene)-type surfactants. The cloud point of (C11 )n G10 was clearly dependent on the W H /W S ratio. 3.3. Formation of microemulsions (C11 )1.9 G10 , (C11 )2.7 G10 , and (C11 )3.4 G10 were mixed to the desired compositions and their phase behavior were stud-
ied. Fig. 4a shows a phase diagram of the water/(C11 )n G10 /mxylene system as a function of the W H /W S ratio. The water/mxylene weight ratio was kept constant at 50:50, and the concentration of surfactant in the system was kept at 10 wt%. As shown in Fig. 4a, a single-microemulsion phase appeared when the W H /W S ratio was in the range of 0.60 to 0.64, and this phase became narrower as the temperature increased. The selforganizing structure obtained was stable above 90 ◦ C. After the phase behavior was measured at 90 ◦ C, the ampoules were frozen and placed in a thermostat. The surfactants were not decomposed and their phase behaviors were preserved. It has been reported that the solubilization ability of poly(oxyethylene)type surfactants is dependent on the temperature [4]. However, the results obtained in the previous section revealed that the cloud point of (C11 )n G10 was insensitive to temperature and dependent on the W H /W S ratio. Therefore, it is proposed from the present results that a temperature-insensitive microemulsion is formed when (C11 )n G10 is used at a suitable W H /W S ratio with the oil m-xylene. Fig. 4b shows a phase diagram of the water/(C11 )n G10 / (R)-(+)-limonene system as a function of the W H /W S ratio. Similarly to the case of the water/(C11 )n G10 /m-xylene system,
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(a) (a)
(b)
(b) Fig. 4. Phase diagrams of the water/(C11 )n G10 /oil systems as a function of W H /W S : (a) m-xylene, (b) (R)-(+)-limonene. The surfactant concentration is 10 wt%.
a temperature-insensitive microemulsion was formed. Therefore, it can be concluded that a stable self-organizing structure is formed, even in the case of the commercially used oil (R)(+)-limonene. 3.4. Effect of the surfactant concentration In the previous section, it was found that (C11 )n G10 formed a temperature-insensitive microemulsion at a W H /W S ratio of 0.625, when the oil was m-xylene Fig. 4a. Fig. 5a shows phase diagrams of the water/(C11 )n G10 /m-xylene system at a W H /W S ratio of 0.625 as a function of the surfactant concentration. The water/m-xylene weight ratio was kept constant at 50:50. As the temperature increased, the surfactant phase behavior changed from water-soluble to oil-soluble through a three-phase region or a single-microemulsion phase. In phase diagrams of poly(oxyethylene)-type surfactants, the three-phase region was reported to be horizontal and independent of the surfactant concentration [4], whereas in the water/(C11 )n G10 /m-xylene system, the three-phase region of (C11 )n G10 shifted to a high temperature as the concentration decreased Fig. 5a. This shift of the three-phase region
Fig. 5. Phase diagrams of the water/(C11 )n G10 /oil systems as a function of surfactant concentration: (a) m-xylene, W H /W S = 0.625, (b) (R)-(+)-limonene, W H /W S = 0.605.
of (C11 )n G10 appeared to depend on the distribution of the polyglycerol moieties. It is considered that relatively lipophilic moieties are dissolved in the oil at low concentrations, with the result that the polyglycerol laurates forming the microemulsion phase become more hydrophilic. As shown in Fig. 5a, it is possible to form a single-microemulsion phase at the low surfactant concentration of 5 wt%. Fig. 5b shows phase diagrams of the water/(C11 )n G10 / (R)-(+)-limonene system at a W H /W S ratio of 0.605 as a function of the surfactant concentration. In a manner similar to that of the water/(C11 )n G10 /m-xylene system, the threephase region of (C11 )n G10 shifted to high temperature as the concentration decreased. It is concluded that a singlemicroemulsion phase is formed at a low surfactant concentration of 5 wt%, even in the case of the commercially used oil (R)-(+)-limonene. 4. Conclusions The following conclusions were derived from this investigation into the effects of the number of fatty acid residues in decaglycerol laurates ((C11 )n G10 ) on their phase behavior:
S. Ai, M. Ishitobi / Journal of Colloid and Interface Science 296 (2006) 685–689
1. In aqueous solutions, (C11 )1.9 G10 forms hexagonal liquid crystals, while (C11 )2.7 G10 and (C11 )3.4 G10 , which are more hydrophobic than (C11 )1.9 G10 , form lamellar liquid crystals. 2. The cloud phenomenon is observed at a W H /W S ratio of around 0.6. However, the lamellar liquid crystals separate from the water below 70 ◦ C, and the cloud point caused by dehydration of hydrophilic chains is observed above 70 ◦ C. 3. A single-microemulsion phase is formed in both the water/(C11 )n G10 /m-xylene and water/(C11 )n G10 /(R)-(+)-limonene systems. These self-organized structures are stable above 90 ◦ C. The surfactants are not decomposed by heating at 90 ◦ C. 4. The phase behavior of (C11 )n G10 is insensitive to temperature, but dependent on both the W H /W S ratio and the number of fatty acid residues.
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References [1] K. Kosswig, in: N.M. van Os (Ed.), Nonionic Surfactants, Organic Chemistry, in: Surfactant Science Series, vol. 72, Dekker, New York, 1998, chap. 4. [2] J.J. Lewis, in: N.M. van Os (Ed.), Nonionic Surfactants, Organic Chemistry, in: Surface Science Series, vol. 72, Dekker, New York, 1998, chap. 8. [3] M. Ishitobi, H. Kunieda, Colloid Polym. Sci. 278 (2000) 899. [4] H. Kunieda, Y. Hasegawa, J. Oleo Sci. 45 (10) (1996) 1053. [5] H. Kunieda, A. Akahane, Jin-Feng, M. Ishitobi, J. Colloid Interface Sci. 245 (2000) 365. [6] H. William (Ed.), Official Methods of Analysis of the Association of Official Analytical Chemists, eleventh ed., Association of Official Analytical Chemists, Washington, DC, 1970, p. 444. [7] K. Fontell, G.W. Gray, P.A. Winsor (Eds.), Liquid Crystals and Plastic Crystals, vol. 2, Wiley, New York, 1974, pp. 80–109. [8] H. Sagitani, Y. Hayashi, M. Ochiai, J. Am. Oil Chem. Soc. 66 (1989) 146.
Effects of the number of fatty acid residues on the phase behaviors of decaglycerol fatty acid esters Sakiko Ai ∗ , Masahiko Ishitobi Food Ingredients Laboratory, Mitsubishi Chemical Corporation, Kamoshida-cho 1000, Aoba-ku, Yokohama 227-8502, Japan Received 21 June 2005; accepted 15 September 2005 Available online 26 October 2005
Abstract The effects of the number of fatty acid residues (n) in decaglycerol fatty acid esters, i.e., decaglycerol laurates (abbreviated to (C11 )n G10 ), on the phase behaviors of three laurate esters, (C11 )1.9 G10 , (C11 )2.7 G10 , and (C11 )3.4 G10 , were investigated. The unreacted decaglycerol remaining in each ester was removed by liquid extraction before use. (C11 )1.9 G10 formed hexagonal liquid crystals in aqueous solutions, while (C11 )2.7 G10 and (C11 )3.4 G10 , which are more hydrophobic than (C11 )1.9 G10 , formed lamellar liquid crystals. The cloud point in aqueous solution was measured for mixtures of these three esters. The cloud phenomenon was observed when the weight ratio of hydrophilic groups to the total surfactant (W H /W S ) was around 0.6. The cloud point shifted to a markedly higher temperature, even with a slight increase in the W H /W S ratio. The solubilization abilities of (C11 )n G10 for the oils m-xylene and (R)-(+)-limonene were also examined. When the W H /W S ratio was between 0.60 and 0.64, (C11 )n G10 formed microemulsions and lyotropic liquid crystals in the presence of water and the oils. These self-organized structures were stable, even above 90 ◦ C. It is concluded that the phase behavior of (C11 )n G10 are insensitive to temperature, but strongly dependent on both the W H /W S ratio and the number of fatty acid residues (n). © 2005 Elsevier Inc. All rights reserved. Keywords: Polyglycerol fatty acid ester; Cloud point; Microemulsion; Liquid crystal; Phase behavior
1. Introduction Polyglycerol fatty acid esters are widely used as surfactants for foods, cosmetics, pharmaceuticals, and toiletries. Many attempts have been made to clarify their functions. However, few basic studies have addressed their physicochemical properties because of the difficulties associated with their structural complexity. Specifically, they are mixtures of isomers with various polyglycerol chain lengths and/or various degrees of esterification [1], and moreover, commercially available polyglycerol esters contain large amounts of unreacted polyglycerol [2]. Ishitobi and Kunieda [3] studied the effects of the polyglycerol chain length distribution on the phase behaviors of polyglycerol laurates. They reported that esters with broadly distrib-
* Corresponding author. Fax: +81 45 963 3976.
E-mail address: [email protected] (S. Ai). 0021-9797/$ – see front matter © 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.09.029
uted polyglycerol chains were more tightly packed at the interface than esters with sharply distributed polyglycerol chains, and that the former showed much greater abilities for surface tension lowering and emulsification. In the case of conventional poly(oxyethylene)-type nonionic surfactants, it is well known that a phase inversion takes place due to dehydration of hydrophilic chains, i.e., ethylene oxide units, as the temperature increases [4]. Kunieda et al. [5] investigated the phase behaviors of polyglycerol dilaurates with various polyglycerol chain lengths, (C11 )2 Gn , and reported that their phase behaviors were not very sensitive to temperature. Most of the polyglycerol fatty acid esters used commercially as surfactants consist of polyglycerol with a polymerized number less than 10. In the present study, we investigated the phase behaviors of decaglycerol laurates with three different numbers of fatty acid residues, i.e., n = 1.9, 2.7, and 3.4, while fixing the polyglycerol number at 10. Hereafter, these laurates are abbreviated to (C11 )n G10 .
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2. Materials and methods
30 min at a given temperature, and the cloud phenomena were observed visually.
2.1. Materials 2.5. Phase diagram construction 2.1.1. Decaglycerol Decaglycerol was synthesized by polymerization of glycerol as follows. Glycerol was reacted with potassium carbonate at 250 ◦ C under a pressure of less than 33.3 kPa. The average degree of polymerization was 10.1, as calculated from the hydroxyl value (OHV), which represents the number of KOH milliequivalents to the hydroxyl content (in moles) in a 1-g sample [6].
Surfactants were mixed with water and an oil (m-xylene or (R)-(+)-limonene) in glass ampoules. The water/oil ratio was kept constant at 1:1. The ampoules were mixed completely using a vortex mixer or centrifuge. The phase states were determined by both visual inspection and a crossed polarizer. 3. Results and discussion 3.1. Phase behavior of (C11 )n G10 in aqueous solutions
2.1.2. Decaglycerol fatty acid esters Three decaglycerol laurates were synthesized according to the following procedure. Decaglycerol synthesized as described above and lauric acid (palmac98-12; Acidchem International) were mixed at three different lauric acid/polyglycerol molar ratios, namely 1, 2, and 3. Next, 0.0025 wt% of sodium hydroxide was added to the reaction mixtures as a catalyst. The esterification reaction was performed at 240 ◦ C for 3.5 h and then at 260 ◦ C for 4 h under a nitrogen stream at atmospheric pressure. Unreacted polyglycerol remaining in the crude surfactants was removed using a previously described extraction method [3]. The numbers of fatty acid residues (n) in the three crude surfactants were 1.9, 2.7, and 3.4, respectively. The free polyglycerol contents in the purified surfactants were less than 2 wt%. The purified surfactants, i.e., the three decaglycerol laurates, were abbreviated to (C11 )1.9 G10 , (C11 )2.7 G10 , and (C11 )3.4 G10 .
The three samples, (C11 )1.9 G10 , (C11 )2.7 G10 , and (C11 )3.4 G10 , were dissolved in water to the desired concentrations. Fig. 1 shows the phase diagrams as a function of the temperature. (C11 )1.9 G10 formed an aqueous micellar solution at low concentrations and liquid crystals at high concentrations (Fig. 1a). The intensity √ ratio of the SAXS peaks for the liquid crystals was 1:1/ 3, indicating that they were hexagonal H1 liquid crystals. In contrast, the liquid crystals formed by (C11 )2.7 G10 and (C11 )3.4 G10 , which are more hydrophobic than (C11 )1.9 G10 , showed SAXS peak intensity ratios of 1:1/2, indicating that they were lamellar liquid crystals. At high concentrations, (C11 )2.7 G10 and (C11 )3.4 G10 formed a single lamellar phase, while at low concentrations, the lamellar liquid crystals coexisted with water (Figs. 1b and 1c).
2.2. Construction of phase diagrams
3.2. Cloud points
Samples of different compositions were sealed in glass ampoules with a narrow constriction. Homogeneity of the viscous samples was attained by passing them through the narrow constriction by repeated centrifugation. After the homogenization, the ampoules were placed in a thermostat and phase separation was observed visually. The existence of liquid crystals was detected with a crossed Nicol polarizer, and the types of liquid crystals were determined by small-angle X-ray scattering (SAXS).
(C11 )1.9 G10 and (C11 )2.7 G10 were mixed to the desired compositions, and the cloud points were measured for different W H /W S ratios. The concentration of the surfactant in aqueous solution was fixed at 10 wt%. As shown in Fig. 2, the cloud phenomenon was observed at a W H /W S ratio near 0.625. It has been reported that the cloud point of commonly used poly(oxyethylene)-type nonionic surfactants shifts to a higher temperature as the W H /W S ratio increases [8]. The cloud point of (C11 )n G10 shifted to a higher temperature at relatively high temperatures above 70 ◦ C as the W H /W S ratio increased, but shifted to a lower temperature at temperatures below 70 ◦ C. As shown in Fig. 1b, (C11 )2.7 G10 formed lamellar liquid crystals in water, and these liquid crystals coexisted with water at low concentrations. Consequently, it is considered that the cloud phenomenon observed below 70 ◦ C is caused by separation of the lamellar liquid crystals from the water. In other words, it is hard to measure the cloud point because the separation of the lamellar liquid crystals from the water takes place simultaneously, even if the cloud phenomenon caused by dehydration of hydrophilic moieties takes place below 70 ◦ C. Accordingly, it is concluded that the true cloud point can be measured from the cloud phenomenon observed above 70 ◦ C. A comparison of the cloud points of (C11 )n G10 with those of poly(oxyethylene)-type surfactants [4] is shown in Fig. 3. As shown in Fig. 2, the cloud point of (C11 )n G10 appeared to depend on the temperature to a
2.3. SAXS measurements A small-angle scattering goniometer equipped with an 18kW Rigaku Denki rotating anode generator (RINT 2500) was used. The samples were sealed in a steel holder covered with a thin plastic film (Mylar seal method). The measurements were made at 25 ◦ C. The liquid crystal types were determined by the √ SAXS peak ratios, namely 1:1/2:1/3 for lamellar and 1:1/ 3:1/2 for hexagonal phases [7]. 2.4. Cloud point measurement Cloud points of aqueous solutions containing 10 wt% of the decaglycerol laurates were measured for different W H /W S ratios as follows. Glass ampoules were placed in a thermostat for
S. Ai, M. Ishitobi / Journal of Colloid and Interface Science 296 (2006) 685–689
687
(a) Fig. 2. Cloud points in the water/(C11 )n G10 system as a function of W H /W S .
(b)
Fig. 3. Cloud points of (C11 )n G10 (") compared with polyoxyethylene oleyl ethers (Q) [4].
(c) Fig. 1. Phase diagrams of the water/(C11 )n G10 systems as a function of temperature: (a) water/(C11 )1.9 G10 system, (b) water/(C11 )2.7 G10 system, (c) water/ (C11 )3.4 G10 system.
certain extent, but the extent of its temperature sensitivity was lower than those of the poly(oxyethylene)-type surfactants. The cloud point of (C11 )n G10 was clearly dependent on the W H /W S ratio. 3.3. Formation of microemulsions (C11 )1.9 G10 , (C11 )2.7 G10 , and (C11 )3.4 G10 were mixed to the desired compositions and their phase behavior were stud-
ied. Fig. 4a shows a phase diagram of the water/(C11 )n G10 /mxylene system as a function of the W H /W S ratio. The water/mxylene weight ratio was kept constant at 50:50, and the concentration of surfactant in the system was kept at 10 wt%. As shown in Fig. 4a, a single-microemulsion phase appeared when the W H /W S ratio was in the range of 0.60 to 0.64, and this phase became narrower as the temperature increased. The selforganizing structure obtained was stable above 90 ◦ C. After the phase behavior was measured at 90 ◦ C, the ampoules were frozen and placed in a thermostat. The surfactants were not decomposed and their phase behaviors were preserved. It has been reported that the solubilization ability of poly(oxyethylene)type surfactants is dependent on the temperature [4]. However, the results obtained in the previous section revealed that the cloud point of (C11 )n G10 was insensitive to temperature and dependent on the W H /W S ratio. Therefore, it is proposed from the present results that a temperature-insensitive microemulsion is formed when (C11 )n G10 is used at a suitable W H /W S ratio with the oil m-xylene. Fig. 4b shows a phase diagram of the water/(C11 )n G10 / (R)-(+)-limonene system as a function of the W H /W S ratio. Similarly to the case of the water/(C11 )n G10 /m-xylene system,
688
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(a) (a)
(b)
(b) Fig. 4. Phase diagrams of the water/(C11 )n G10 /oil systems as a function of W H /W S : (a) m-xylene, (b) (R)-(+)-limonene. The surfactant concentration is 10 wt%.
a temperature-insensitive microemulsion was formed. Therefore, it can be concluded that a stable self-organizing structure is formed, even in the case of the commercially used oil (R)(+)-limonene. 3.4. Effect of the surfactant concentration In the previous section, it was found that (C11 )n G10 formed a temperature-insensitive microemulsion at a W H /W S ratio of 0.625, when the oil was m-xylene Fig. 4a. Fig. 5a shows phase diagrams of the water/(C11 )n G10 /m-xylene system at a W H /W S ratio of 0.625 as a function of the surfactant concentration. The water/m-xylene weight ratio was kept constant at 50:50. As the temperature increased, the surfactant phase behavior changed from water-soluble to oil-soluble through a three-phase region or a single-microemulsion phase. In phase diagrams of poly(oxyethylene)-type surfactants, the three-phase region was reported to be horizontal and independent of the surfactant concentration [4], whereas in the water/(C11 )n G10 /m-xylene system, the three-phase region of (C11 )n G10 shifted to a high temperature as the concentration decreased Fig. 5a. This shift of the three-phase region
Fig. 5. Phase diagrams of the water/(C11 )n G10 /oil systems as a function of surfactant concentration: (a) m-xylene, W H /W S = 0.625, (b) (R)-(+)-limonene, W H /W S = 0.605.
of (C11 )n G10 appeared to depend on the distribution of the polyglycerol moieties. It is considered that relatively lipophilic moieties are dissolved in the oil at low concentrations, with the result that the polyglycerol laurates forming the microemulsion phase become more hydrophilic. As shown in Fig. 5a, it is possible to form a single-microemulsion phase at the low surfactant concentration of 5 wt%. Fig. 5b shows phase diagrams of the water/(C11 )n G10 / (R)-(+)-limonene system at a W H /W S ratio of 0.605 as a function of the surfactant concentration. In a manner similar to that of the water/(C11 )n G10 /m-xylene system, the threephase region of (C11 )n G10 shifted to high temperature as the concentration decreased. It is concluded that a singlemicroemulsion phase is formed at a low surfactant concentration of 5 wt%, even in the case of the commercially used oil (R)-(+)-limonene. 4. Conclusions The following conclusions were derived from this investigation into the effects of the number of fatty acid residues in decaglycerol laurates ((C11 )n G10 ) on their phase behavior:
S. Ai, M. Ishitobi / Journal of Colloid and Interface Science 296 (2006) 685–689
1. In aqueous solutions, (C11 )1.9 G10 forms hexagonal liquid crystals, while (C11 )2.7 G10 and (C11 )3.4 G10 , which are more hydrophobic than (C11 )1.9 G10 , form lamellar liquid crystals. 2. The cloud phenomenon is observed at a W H /W S ratio of around 0.6. However, the lamellar liquid crystals separate from the water below 70 ◦ C, and the cloud point caused by dehydration of hydrophilic chains is observed above 70 ◦ C. 3. A single-microemulsion phase is formed in both the water/(C11 )n G10 /m-xylene and water/(C11 )n G10 /(R)-(+)-limonene systems. These self-organized structures are stable above 90 ◦ C. The surfactants are not decomposed by heating at 90 ◦ C. 4. The phase behavior of (C11 )n G10 is insensitive to temperature, but dependent on both the W H /W S ratio and the number of fatty acid residues.
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