Langmuir 2008, 24, 12253-12259
12253
Hexagonal Phase Based Gel-Emulsion (O/H1 Gel-Emulsion): Formation and Rheology Mohammad Mydul Alam and Kenji Aramaki* Graduate School of EnVironment and Information Sciences, Yokohama National UniVersity, Tokiwadai 79-7, Hodogaya-ku, Yokohama 240-8501, Japan ReceiVed July 8, 2008. ReVised Manuscript ReceiVed September 1, 2008 The formation, stability, and rheological behavior of a hexagonal phase based gel-emulsion (O/H1 gel-emulsion) have been studied in water/C12EO8/hydrocarbon oil systems. A partial phase behavior study indicates that the oil nature has no effect on the phase sequences in the ternary phase diagram of water/C12EO8/oil systems but the domain size of the phases or the oil solubilization capacity considerably changes with oil nature. Excess oil is in equilibrium with the hexagonal phase (H1) in the ternary phase diagram in the H1+O region. The O/H1 gel-emulsion was prepared (formation) and kept at 25 °C to check stability. It has been found that the formation and stability of the O/H1 gel-emulsion depends on the oil nature. After 2 min observation (formation), the results show that short chain linear hydrocarbon oils (heptane, octane) are more apt to form a O/H1 gel-emulsion compared to long chain linear hydrocarbon oils (tetradecane, hexadecane), though the stability is not good enough in either system, that is, oil separates within 24 h. Nevertheless, the formation and stability of the O/H1 gel-emulsion is appreciably increased in squalane and liquid paraffin. It is surmised that the high transition temperature of the H1+O phase and the presence of a bicontinuous cubic phase (V1) might hamper the formation of a gel-emulsion. It has been pointed out that the solubilization of oil in the H1 phase could be related to emulsion stability. On the other hand, the oil nature has little or no effect on the formation and stability of a cubic phase based gel-emulsion (O/I1 gel-emulsion). From rheological measurements, it has found that the rheogram of the O/H1 gel-emulsion indicates gel-type structure and shows shear thinning behavior similar to the case of the O/I1 gel-emulsion. Rheological data infer that the O/I1 gel-emulsion is more viscous than the O/H1 gel-emulsion at room temperature but the O/H1 gel-emulsion shows consistency at elevated temperature.
1. Introduction High-internal-phase-ratio emulsions (HIPREs) have been the object of intensive research over many years.1-5 These emulsions are also called gel-emulsions,4,6 biliquid foams,7 and simply highly concentrated emulsions.8 The droplet shape of a gel-emulsion appears polyhedral (internal phase > 74%)1 and cannot move due to a very small amount of continuous phase. HIPREs have stimulated research because of their characteristic properties (rheological, structural, optical) which make them useful in practical applications such as cosmetics, foods, pharmaceuticals, aviation fuels, emulsion explosives, reaction media, and so forth.3,9-12 Recently, another kind of gel-emulsion, that is, liquid crystal (LC) based gel-emulsion, has attracted more attention.13,14 HIPREs and LC based gel-emulsions manifest some common properties, that is, high viscosity and prolonged stability. In contrast to HIPREs, LC based gel-emulsions are formed in the * To whom correspondence should be addressed. E-mail: aramakik@ ynu.ac.jp. Telephone and fax: 045-339-4300. (1) Lissant, K. J. J. Colloid Interface Sci. 1966, 22, 462. (2) Princen, H. M. J. Colloid Interface Sci. 1979, 71, 55. (3) Sagitani, H.; Hattori, T.; Nabeta, K.; Nagai, M. Nippon Kagaku Kaishi 1983, 1399. (4) Kunieda, H.; Solans, C.; Shida, N.; Parra, J. L. Colloids Surf. 1987, 24, 225. (5) Aronson, M. P.; Pekto, M. F. J. Colloid Interface Sci. 1993, 159, 134. (6) Ravey, J. C.; Stebe, M. J.; Sauvage, S. Colloids Surf., A 1994, 91, 237. (7) Sonneville-Aurbun, O.; Bergeron, V.; Gulik-Krzywicki, T.; Jonsson, B.; Wennerstrom, H.; Lindner, P.; Cabane, B. Langmuir 2000, 16(4), 1566. (8) Babak, V. G. Food Hydrocolloids 1992, 6(1), 45. (9) Rocca, S.; Muller, S.; Stebe, M. J. J. Controlled Release 1999, 61, 251. (10) Attwood, D.; Florence, A. T. Surfactant Systems; Chapman and Hall: New York, 1983. (11) Ishida, H.; Iwama, A. Combust. Sci. Technol. 1984, 37, 79. (12) Solans, C.; Esquena, J.; Azemar, N. Curr. Opin. Colloid Interface Sci. 2003, 8, 156. (13) Kunieda, H.; Tanimoto, M.; Shigeta, K.; Rodriguez, C. J. Oleo Sci. 2001, 50(8), 633. (14) Rodriguez, C.; Roman, G. M.; Kunieda, H. Langmuir 2004, 20, 5235.
region of the ternary phase diagram in which the isotropic phase coexists with the LC and also shows high viscosity even at low volume fraction of the disperse phase. The LC surrounds the emulsion droplets and reduces their creaming and coalescence. A LC based gel-emulsion often looks translucent to transparent, depending on the composition and the temperature of the system. It was reported that lamellar (LR) and reverse hexagonal liquid crystals (H2) are able to stabilize O/W and W/O emulsions.15 Now, there is growing interest in cubic phase based gel-emulsions (O/I1 gel-emulsion) because of high viscosity, high volume fraction of the internal phase, and advanced application as a template for the synthesis of macroporous silica materials.16 The cubic phase is the most viscous phase (elastic modulus > 104 Pa)17 among the liquid crystals and is used as the continuous phase to prepare gel-emulsions. Cubic phase based gel-emulsions have been mostly studied in the past.13,14,18,19 The rheological behavior of the cubic phase and related gel-emulsions were also studied.14,20-23 On the other hand, the hexagonal phase is also highly viscous, anisotropic and shows non-Newtonian behavior.21,24-26 It was (15) Suzuki, T.; Tsutsumi, H. J. Oleo Sci. 1987, 36, 588. (16) Esquena, J.; Izquierdo, P.; Kunieda, H.; Solans, C. Proceedings of the 1st Iberian Meeting on Colloids and Interfaces, Salamanca, Spain, 2005. (17) Castelletto, V.; Hamley, I. W.; Yang, Z. Colloid Polym. Sci. 2001, 279, 1029. (18) Rodriguez, C.; Shresta, L. K.; Varade, D.; Aramaki, K.; Maestro, A.; Lopez-Quentala, A.; Solans, C. Langmuir 2007, 23, 11007. (19) Watanabe, K.; Kanei, N.; Kunieda, H. J. Oleo Sci. 2002, 51(12), 771. (20) Jones, J. L.; McLeish, T. C. B. Langmuir 1995, 11, 785. (21) Rodriguez, C.; Achariya, D. P.; Aramaki, K.; Kunieda, H. Colloids Surf., A 2005, 269, 59. (22) Wang, H.; Zhang, G.; Du, Z.; Li, Q.; Wang, W.; Liu, D.; Zhang, X. J. Colloid Interface Sci. 2006, 300, 348. (23) Montalvo, G.; Valiente, M.; Rodenas, E. Langmuir 1996, 12, 5202. (24) Richtering, W.; Lauger, J.; Linemann, R. Langmuir 1994, 10, 4374. (25) Siddiq, M. A.; Radiman, S.; Jan, L. S.; Muniandy, S. V. Colloids Surf., A 2006, 276, 15.
10.1021/la8021547 CCC: $40.75 2008 American Chemical Society Published on Web 10/11/2008
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found that the formation of the hexagonal phase is very common in aqueous surfactant solutions, for example, poly(oxyethylene) alkyl ether-type surfactant,27 poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) triblock copolymer,28 poly(oxyethylene)-poly(oxypropylene) diblock copolymer,29 sodium dodecyl sulfate,30 and sucrose monododecanoate surfactant.31 It is generally formed between the micellar and lamellar phases in a binary water-surfactant system. It should be noted that it is necessary to get the H1+O phase in a ternary phase diagram to study a O/H1 gel-emulsion. Sometimes, addition of oil (some polar oils, triglycerides, aromatic hydrocarbons) induces a H1-LR transition in a ternary phase diagram,32-35 so it is difficult to get the H1+O phase, which is important to study O/H1 gel-emulsions. On the other hand, saturated hydrocarbons or middle or long chain triglycerides induce H1-I1 transition at low surfactant concentration, but at high surfactant concentration excess oil is in equilibrium with the H1 phase in the H1+O region,13,34 which makes it easy to study O/H1 gel-emulsions. Kunieda et al.13 studied the phase behavior and the formation of the O/I1 gel-emulsion in the water/C12EO8/decane system. They have shown that the addition of polyols decreases the viscosity of the O/I1 gel-emulsion but simultaneously changes the appearance from milky to transparent due to the change of the refractive index of the I1 phase. They have pointed out that a O/H1 gel-emulsion could form in the H1+O region in the ternary phase diagram, but they did not study this further. To our knowledge, there is no study on the hexagonal phase based gelemulsion (O/H1 gel-emulsion). In this contribution, we present for the first time results about the O/H1 gel-emulsion. We have investigated the partial phase behavior in water/C12EO8/hydrocarbon oil systems by optical observation, and the structure of the liquid crystal was determined by small-angle X-ray scattering technique. The formation (preparation) and stability of the O/H1 gel-emulsion have been compared with those of the O/I1 gel-emulsion in different hydrocarbon oils. The rheological behavior of the H1 phase and related O/H1 gel-emulsions were also evaluated. Viscoelastic properties of the O/H1 and O/I1 gel-emulsions were also compared.
2. Experimental Section 2.1. Materials. Monodisperse octa(oxyethylene) dodecyl ether (C12EO8) was purchased from Nikko Chemicals, Japan. Heptane, octane, decane, dodecane, tetradecane, hexadecane, and squalane were purchased from Tokyo Chemical Industry. Liquid paraffin (LP 70) was purchased from Kishida Company. All chemicals were used without further purification. Deionized and Milli-Q filtered water was used for all samples. 2.2. Methods. 2.2.1. Determination of the Phase Diagram. The samples were prepared individually by weighing the appropriate amounts of components in glass ampules. The ampules were flamesealed immediately, and the sample mixtures were homogenized using a vortex mixer. Viscous samples were homogenized by repeated (26) Cordobes, F.; Munoz, J.; Gallegos, C. J. Colloid Interface Sci. 1997, 187, 401. (27) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; Mc Donald, M. P. J. Chem. Soc., Faraday Trans. 1983, 79(1), 975. (28) Svensson, B.; Alexandridis, P.; Olsson, U. J. Phys. Chem. B 1998, 102, 7541. (29) Alexandridis, P.; Olsson, U.; Lindman, B. Langmuir 1998, 14, 2627. (30) Li, X.; Kunieda, H. Langmuir 2000, 16, 10092. (31) Aramaki, K.; Kabir, M. H.; Nakamura, N.; Kunieda, H.; Ishitobi, M. Colloids Surf., A 2001, 371, 183–185. (32) Kunieda, H.; Horii, M.; Koyama, M.; Sakamoto, K. J. Colloid Interface Sci. 2001, 236, 78. (33) Yamashita, Y.; Kunieda, H.; Oshimura, E.; Sakamoto, K. Langmuir 2003, 19, 4070. (34) Alam, M. M.; Varade, D.; Aramaki, K. J. Colloid Interface Sci. 2008, 325, 243. (35) Kunieda, H.; Shigeta, K.; Suzuki, M. Langmuir 1999, 15, 3118.
Alam and Aramaki centrifugation in both directions through narrow constriction in the sample tubes. The samples were kept in a temperature-controlled bath at 25 °C for several hours or days to bring the system to equilibrium. The phase equilibrium was determined by visual inspection and with a cross polarizer. Small-angle X-ray scattering (SAXS) technique was used to identify liquid crystal structure. 2.2.2. Preparation (Formation) of Gel-Emulsion. All components were added together at a final composition, melted at 80 °C, and then instantly mixed with a vortex mixer for around 2 min at 2500 rpm in open air-cooling. After 2 min, we observed a highly viscous emulsion that did not flow when the glass tube was turned upside down. 2.2.3. Small-Angle X-ray Scattering Measurements. Small-angle X-ray scattering technique was performed by using a small-angle scattering camera equipped with a rotating anode and a CCD detector (Rigaku, Nanoviewer). The samples of liquid crystals were filled in a hole of an iron plate and covered with plastic films for the measurement (Mylar seal method). The type of liquid crystal was determined by the SAXS peak ratio (e.g., the SAXS peak ratio for the hexagonal (H1) phase is 1:1/3:1/4).36,37 2.2.4. Rheological Measurements. Samples were homogenized and kept in a thermostatted bath at 25 °C for at least 24 h before the measurements. Measurements were performed in a ARES7 rheometer (Rheometric Scientific) using cone plate (diameter 25 mm with cone angle of 0.1 rad) geometry. Dynamic frequency sweep measurements were performed in the linear viscoelastic regime, as determined by dynamic strain sweep measurements.
3. Results and Discussion 3.1. Phase Behavior of Water/C12EO8/Heptane and Water/ C12EO8/Liquid Paraffin Systems at 25 °C. In order to understand the phase behavior, we have constructed partial phase diagrams of water/C12EO8/heptane and water/C12EO8/liquid paraffin systems at 25 °C as shown in Figure 1. In the water/C12EO8 system, the micellar solution (Wm) and hexagonal liquid crystal (H1) phase are successively formed with increasing surfactant concentration. It should be noticed that the I1 phase does not appear in the water/C12EO8 binary system at 25 °C. However, due to the solubilization of oil in the Wm or the H1 phase, the I1 phase appeared. A similar observation has been reported in the literature.13,32 A detailed phase diagram of the water/C12EO8/decane system at 25 °C was already documented.13 SAXS measurements were used to distinguish the type of liquid crystals (H1 or I1) with the help of peak ratios, that is, the SAXS peak ratio for the hexagonal (H1) phase is 1:1/ 3:1/4.36,37 The I1 phase has a face-centered cubic structure (Fm3m), judging from the SAXS peak ratios, 1:3/4:3/8.13 It was found from the previous report13 that there was also no I1 phase in the water/C12EO8 binary system and the phase sequence was Wm-H1-V1-LR. However, the I1 phase has appeared with the addition of decane in the Wm and H1 phases. A similar observation was also made in triglycerides34 and polar oil.32 Here, we have constructed a partial phase diagram up to 72 wt % C12EO8 concentration, because we are not interested beyond the H1 phase. It has been found that the surfactant layer curvature of the binary system at the water-surfactant axis is changed from highly positive to less positive with the surfactant concentration. Note that a positive curvature is defined as the curvature of a surfactant layer that is convex toward water. In both ternary systems in Figure 1, similar phase sequences are found (Wm-I1-H1), but the domain size of the phases or oil solubilization limit is different in each phase. From Figure 1, one could find that 8 wt % heptane is solubilized at 30 wt % surfactant solution whereas only 3 wt % liquid paraffin is solubilized. We (36) Kunieda, H.; Ozawa, K.; Huang, K. L. J. Phys. Chem. B 1998, 102, 831. (37) Kanei, N.; Tamura, K.; Kunieda, H. J. Colloid Interface Sci. 1999, 218, 13.
Hexagonal Phase Based Gel-Emulsion
Langmuir, Vol. 24, No. 21, 2008 12255 Table 2. O/H1 Gel-Emulsion (Observation after 2 min)a oil content (wt %)
30
35
40
45
50
60
70
80
90
heptane octane decane dodecane tetradecane hexadecane squalane liquid paraffin
O O O O O O O O
O O O O O × O O
O O O O × × O O
O O O O × × O O
× O × × × × O O
× × × × × × O O
× × × × × × O O
× × × × × × × O
× × × × × × × ×
a
(O) Gel-emulsion, (×) oil separated.
Table 3. O/H1 Gel-Emulsion after 24 h at 25 °Ca oil content (wt %)
30
35
40
45
50
60
70
80
90
heptane octane decane dodecane tetradecane hexadecane squalane liquid paraffin
× × × × × × O O
× × × × × × O O
× × × × × × O O
× × × × × × O O
× × × × × × × O
× × × × × × × O
× × × × × × × O
× × × × × × × O
× × × × × × × ×
a
(O) Gel-emulsion, (×) oil separated.
Table 4. O/I1 Gel-Emulsion after 24 h at 25 °Ca oil content (wt %)
30
35
40
45
50
60
70
80
90
heptane octane decane dodecane tetradecane hexadecane squalane liquid paraffin
O O O O O O O O
O O O O O O O O
O O O O O O O O
O O O O O O O O
O O O O O O O O
O O O O O O O O
O O O O O O O O
O O O O O O O O
× × × O O O O O
a
Figure 1. Partial phase diagram of (a) water/C12EO8/heptane and (b) water/C12EO8/liquid paraffin systems at 25 °C. Wm, micellar phase; I1, micellar cubic phase; H1, hexagonal phase. Wm+O, I1+O, and H1+O indicate two-phases, equilibrium of Wm, I1, and H1 with excess oil phase, respectively. Table 1. Maximum Oil Solubilization in the H1 Phase at 25 °C oil
solubilized oil in the H1 phase (wt %)
heptane octane decane dodecane tetradecane hexadecane squalane liquid paraffin
19 18 17 16 13 9 5 2
also studied the phase behavior with octane, dodecane, and squalane (not shown), but we did not find any significant difference in the phase sequences; only the oil solubilization limit differs considerably in the different surfactant aggregates. We determined the maximum solubilization of oil in the H1 phase (water/C12EO8 ) 40/60), and the results are presented in Table 1. It is noticeable that 19 wt % heptane is solubilized in the H1 phase whereas only 2 wt % liquid paraffin could be solubilized in it. We can observe a trend that oil solubilization increases in the H1 phase with decreasing molecular weight of oil.
(O) Gel-emulsion, (×) oil separated.
3.2. Formation and Stability of the Hexagonal (O/H1) and Cubic (O/I1) Phase Based Gel-Emulsion. We prepared a O/H1 gel-emulsion (formation) as described in the Experimental Section (section 2.2.2), and the results are presented in Table 2. One could observe from Table 2 that the O/H1 gel-emulsion is highly dependent on oil nature. Concentrations of 45 wt % heptane, 50 wt % octane, 45 wt % decane and dodecane, 35 wt % tetradecane, and 30 wt % hexadecane could form a O/H1 gel-emulsion. It is interesting to note that the O/H1 gel-emulsion formation is appreciably increased with squalane (70 wt %) and liquid paraffin (85 wt %). A trend is observed in linear chain hydrocarbon oils, namely the short chain oil (octane) is more susceptible to form a O/H1 gel-emulsion compared to the long chain oil (hexadecane), but this trend is not consistent with heptane. To check stability, the O/H1 gel-emulsion is kept at 25 °C and observed after 24 h, and the results are presented in Table 3. One could find that the O/H1 gel-emulsion separates within 24 h in all linear hydrocarbon oils, whereas squalane (up to 45%) and liquid paraffin do not separate. It could be pointed out that squalane and liquid paraffin show a more stable O/H1 gel-emulsion than linear chain hydrocarbon oils. This anomalous stability is difficult to explain, but we can find a correlation between the oil solubilization in the H1 phase (see Table 1) and the stability of the O/H1 gel-emulsion, that is, increasing oil solubilization in the H1 phase decreasing the stability of the O/H1 gel-emulsion. To compare the stability of the O/H1 gel-emulsion with that of the O/I1 gel-emulsion, we prepared the O/I1 gel-emulsion according to section 2.2.2 and observed it after 24 h at 25 °C. The observations are shown in Table 4. It is observed that a large amount of oil could incorporate into the O/I1 gel-emulsion and
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show good stability; the oil does not separate within 24 h. From Table 4, it should be noted that a long chain hydrocarbon oil is more susceptible to form a O/I1 gel-emulsion compare to the short chain oil. From Tables 2-4, it infers that the O/H1 gel-emulsion is highly dependent on the nature of the oil, whereas the oil nature has little or no effect on the O/I1 gel-emulsion. In the following section, we will explain why a long chain linear oil is not a good candidate for the O/H1 gel-emulsion. 3.3. Effect of Temperature and Oil Molecular Weight on Phase Behavior. We have studied the phase behavior with increasing temperature in different hydrocarbon oils at a constant water/C12EO8 ratio ) 40/60, and results are shown in Figure 2. It is observed that the H1 phase is formed at room temperature in the oil-free system and excess oil is in equilibrium with the H1 phase in the H1+O region. In the octane system (Figure 2a), different trends have been found with different concentrations of oil. In the oil-free system, the H1 phase changes to an isotropic phase with increasing temperature. At low octane concentration ( G′′) structure. Rheological data indicate that the O/I1 and O/H1 gel-emulsions show similar shear thinning behavior but the former is more viscous than the latter at 25 °C. It is also noticeable that temperature intensely affects the O/I1 gel-emulsion above 42 °C, whereas the O/H1 gel-emulsion is stable at that temperature and retains its structure even at 50 °C. Acknowledgment. We are thankful to the Ministry of Education, Culture, Sports, Science and Technology, Grant-inAid for Young Scientists (B), No. 18780094 and partly supported by Core Research for Evolution Science and Technology (CREST) of JST Corporation. M.M.A. acknowledges the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan for the Monbukagakusho Scholarship. LA8021547
