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Effect of molecular weight of triglycerides on the formation and rheological behavior of cubic and hexagonal phase based gel emulsions
Journal of Colloid and Interface Science 336 (2009) 329–334
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Effect of molecular weight of triglycerides on the formation and rheological behavior of cubic and hexagonal phase based gel emulsions Mohammad Mydul Alam, Kenji Aramaki * Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai 79-7, Hodogaya-ku, Yokohama 240-8501, Japan
a r t i c l e
i n f o
Article history: Received 25 December 2008 Accepted 26 March 2009 Available online 5 April 2009 Keywords: Triglycerides Rheology Liquid crystal based gel emulsion Phase behavior Cubic phase Hexagonal phase
a b s t r a c t The effect of triglyceride molecular weight on the formation and rheology of cubic (O/I1) and hexagonal (O/ H1) phase based gel emulsions has been studied in water/C12EO8 systems. It was found that the addition of TDG (1,2,3-tridecanoyl glycerol) in the micellar solution leads to the formation of the I1 phase, which can solubilize some added oil. From SAXS data, it is revealed that the interlayer spacing (d) and the length of hydrophobic part (rI) increase with increasing TDG concentration in the I1 phase, whereas the effective cross-sectional area (as) decreases. After the oil solubilization limit, the d value remains nearly constant, indicating the I1+O phase appears. The high viscosity of the I1 phase facilitates the formation of the O/I1 gel emulsion. It has been observed that the formation and stability of the O/I1 and O/H1 gel emulsion is highly dependent on the molecular weight of triglycerides, namely, the high molecular weight triglycerides show better performance (formation and stability) compared to the low molecular weight triglycerides. The rheological behavior of the I1 phase was found to change from viscoelastic to elastic nature with TDG content. The values of the complex viscosity, jgj show different trends at different fixed frequencies within the I1 phase, whereas it decrease monotonically in the O/I1 gel emulsions. The increasing values of the jgj (at lower frequency) could be due to the neighboring micellar interaction and decreasing values of the jgj in the O/I1 gel emulsion could relate to the volume fraction of the I1 phase in the system. It is also figured out that the rheological parameters (elastic modulus, viscous modulus, and jgj) of the O/I1 the gel emulsion do not depend on the oil nature, whereas the O/H1 gel emulsion shows oil nature dependency. Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction Highly concentrated or high internal phase ratio emulsions (HIPREs) have been the objects of many studies for several years [1–8]. It is known that HIPREs can form in the water (O/W) and oil (W/O) rich region under certain conditions [2,9–11]. Due to their characteristic features such as high volume fraction of the internal phase, high viscosity, and transparency, they are also referred to as gel emulsions [1,3,12,13]. Rheological properties of HIPREs were also studied and found that range from viscoelastic to elastic nature depending on the system composition and temperature [6,14,15]. HIPREs have been used for practical applications such as aviation fuels, emulsion explosives, and cosmetics [16–19]. Another kind of gel emulsion that is lyotropic liquid crystal based gel emulsion has been taken attention. In contrast with HIPREs, LC based gel emulsion shows high viscosity even at low volume fraction of internal phase and prolonged stability [20– 23]. Among the liquid crystals, cubic phase (I1) is the most viscous phase and, thus commonly used to prepare O/I1 gel emulsion [20,21,24]. Generally, the I1 phase is formed in aqueous system with very hydrophilic nonionic surfactant such as poly(oxyehtyl* Corresponding author. Fax: +81 045 339 4300. E-mail address: [email protected] (K. Aramaki). 0021-9797/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2009.03.054
ene) alkyl ether [25], poly(oxyethylene)-poly(oxypropylene)poly(oxyethylene) triblock copolymer [26,27], polyglycerol fatty acid ester [28], and sucrose fatty acid ester [29]. Besides, the solubilization of oil in the micellar (Wm) or hexagonal phase (H1) often induces the I1 phase [21,30]. The I1 phase is normally formed between the micellar and the hexagonal phase as a function of surfactant concentration. Several authors studied the rheological behavior of the I1 phase [31,32] and O/I1 gel emulsion [22,24]. On the other hand, hexagonal phase (H1) is also highly viscous and is very common in aqueous surfactant solution, e.g., poly(oxyethylene) alkyl ether [25], sodium dodecyl sulfate [33], poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) triblock copolymer [34], and poly(oxyethylene)-poly(oxypropylene) diblock copolymer [26], sucrose monododecanoate [35]. Recently, we have found that the H1 phase could form O/H1 gel emulsion similarly to the O/I1 gel emulsion [36]. We have extensively studied the formation, stability, and rheological behavior of the H1 phase and O/H1 gel emulsion, which was compared to the O/I1 gel emulsion. In our study, we mainly concentrated on hydrocarbon oils. To our knowledge, there are few studies counted, in which triglyceride is used to study the gel emulsion instead of hydrocarbon oil as a dispersed phase [37]. Triglycerides or triacylglycerols are a simple class of lipids consisting of glycerol esterifies with three fatty acid moieties. Moreover, natural fats and oils including all vegetable oils, such
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structures. vL is the volume of the lipophilic part of one surfactant molecule. /O and /L are the volume fraction of oil and lipophilic part of surfactant in the system, respectively.
as palm oil or sunflower oil and animal fats, consist almost exclusively of triglyceride. These are commonly used for cosmetics, foods, and so on. Although triglyceride is an important class of food oil, detail rheological study of triglyceride-containing gel emulsion is still missing. Aramaki and Yanagimachi [37] reported the phase behavior and rheological properties of the I1 phase and the O/I1 gel emulsion as a function of triglyceride concentrations (mainly 1,2,3trioctanoylglycerol, TOG) in water/C12EO25 system. However, they did not show the effect of the molecular weight of triglycerides on the formation and stability of O/I1 gel emulsion containing triglycerides as a dispersed phase. The present paper is the supposed to guide for the gel emulsion with triglycerides in the same way as previous detailed investigation of hydrocarbon oils [36]. In this contribution, we have mainly studied the formation, stability, and rheology of the O/I1 and O/H1 gel emulsions with different molecular weight triglycerides. The changes of the microstructural parameters (d, rI, and as) of the I1 phase with the addition of TDG were also evaluated.
2.2.3. Rheological measurements Rheological measurements (oscillatory) on a ternary mixture of water/surfactant/triglycerides in the samples were carried out as a function of oil concentrations. All samples for rheological measurements were homogenized with repeated centrifugation and kept in a water bath at 25 °C for 24 h before measurements to ensure equilibration. Rheology of the gel emulsion was done within 24 h after preparation. The rheological measurements were performed in ARES rheometer (Rheometric Scientific) using cone plate geometry (/ = 25 mm, cone angle = 0.1 rad) in the linear viscoelastic regime, as determined previously by dynamic strain sweep measurements. A sample cover provided with the instrument was used to minimize the change in sample composition by evaporation during measurements.
2. Experimental
3. Results and discussion
2.1. Materials
3.1. Effect of TDG on the structural parameters of the I1 phase
Monodisperse octa(oxyethylene) dodecyl ether (C12EO8) was purchased from Nikko Chemicals, Japan. 1,2,3-Trihexanoylglycerol (tricaproin, THG) and 1,2,3-tridecanoylglycerol (tricaprin, TDG) were purchased from Tokyo Chemical Industry. 1,2,3-Trioctanoylglycerol (tricaplyrin, TOG) was purchased from Sigma–Aldrich Chemicals. Olive oil (average carbon number 17) was purchased from Wako Chemical Industries Ltd. Structure of triglyceride was presented elsewhere [30]. All these chemicals were highly pure and were used as received. Millipore-filtered water was used to prepare all the samples.
The partial ternary phase diagram of the water/C12EO8/TDG is already reported [39], here we would examine the change of interlayer spacing, d, length of the hydrophobic part rI, and the effective cross-sectional area per surfactant molecule, as, of the I1 phase with the addition of TDG. It should be noticed that the d value could give us information regarding the phase boundary between the I1 phase and the I1+O region and also to determine r1 and as. To evaluate the structural changes (d, rI, as) in the I1 phase, we have performed SAXS measurement at a constant water/C12EO8 ratio (60/40). We have calculated the structural parameters by using Eqs. (1) and (2) and presented in Fig. 1. It is inferred in Fig. 1 that the values of the interlayer spacing (d) increase with the TDG content and after a certain concentration, it show nearly constant. Intersection of these two lines is considered as phase boundary between the I1 phase and the I1+O region. Visual inspection could also ascertain phase boundary that changes from transparent (I1 phase) to turbid (I1+O). Simultaneously, the length of the hydrophobic part (rI) of the aggregates increases with the oil content but the effective cross-sectional area per surfactant molecule (as) decreases. It is already pointed out that the as decreases in the I1 phase if oil is solubilized in the aggregate cores
2.2. Methods 2.2.1. 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. Around room temperature, we observed a highly viscous emulsion that did not flow when the glass tube was turned upside down. 2.2.2. Small-angle X-ray scattering measurements The interlayer spacing of liquid crystals was measured using small-angle X-ray scattering, performed on a small-angle X-ray scattering camera equipped with rotating anode and a CCD detector (Rigaku, Nanoviewer). The samples of liquid crystals were filled in a hole of an iron plate and covered by plastic films for the measurement (Mylar seal method). The type of liquid crystal was determined by the SAXS peak position [38]. According to the geometry of liquid crystals, the effective cross-sectional area per surfactant molecule, as, is calculated by the following equations using the interlayer spacing, d, obtained from the SAXS measurement. For the I1 phase [30,38],
1=3 3 rI ¼ d ð/L þ /O Þ C; 4pnc 3vL /L þ /O as ¼ ; rI /L
ð1Þ ð2Þ
where, rI is the radius of lipophilic core of spherical micelle in the I1 phase, nc is the number of micelles in a unit cell of cubic phase, and C is a constant (C = (h2 + k2 + l2)1/2), where h, k, and l are miller indices. The values of the constants (nc, C) are (1, 1) for simple cubic, p p (2, 2) for body-centered cubic, and (4, 3) for face centered cubic
Fig. 1. Change in the interlayer spacings (d), effective cross-sectional area (as), and the length of hydrophobic part (rI), of the I1 phase are plotted as a function of TDG (wt.%) in a fixed water/C12EO8 ratio = 60/40.
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[35]. So, one could speculate that the TDG is mainly solubilized in the aggregates core. A similar observation was found in the previous article that high molecular weight triglycerides are mainly solubilized in the aggregates core [30]. The rI increases with the TDG content, indicating the micelles of the I1 phase swell with the adding TDG. After the oil solubilization limit, the d value shows nearly constant, indicating the excess oil is in equilibrium with the I1 phase or the I1+O phase appears. Due to high viscosity of the I1 phase, allow us to prepare O/I1 gel emulsion in the I1+O region. It should be noted that the melting of the I1 phase is necessary for adequate mixing. 3.2. Formation and stability of cubic (O/I1) and hexagonal (O/H1) phase based gel emulsions In the previous report [36], we have found that the formation and stability of the gel emulsion is dependent on the hydrocarbon oil nature. Here, we would see the effect of the molecular weight of triglycerides on the formation and stability of the O/H1 and O/I1 gel emulsion, which could prove useful in a variety of application. In Tables 1a and 1b, the formation (preparation) and stability of the O/H1 gel emulsion is presented. In table, values are indicating wt.% of oil at a fixed water/C12EO8 ratio (40/60). One can find that the formation of the O/H1 gel emulsion is highly dependent on the oil nature that is 40 wt.% of THG, 60 wt.% of TOG, 80 wt.% of TDG and olive oil can incorporate in gel emulsion. We can see that the high molecular weight triglycerides are more susceptible to form O/H1 gel emulsion compared to the low molecular weight oil. In our previous report [30], it was revealed that the low molecular weight triglycerides has more penetration tendency to the palisade layer in the micelles compared to the high molecular weight oil, so it could be speculated that the penetration of oil in the palisade layer makes the H1 phase flexible. As a result, the H1 phase could not form gel emulsion in case of low molecular weight triglycerides at higher oil concentration. To check stability, we kept all samples (O/H1 gel emulsion) at 25 °C and observed after 24 h and the observations are presented in Table 1b. In Table 1b, one can see that the stability of the O/H1 gel emulsion depends on the molecular weight of oil, namely the low molecular weight oil shows less stability compared to the high molecular weight triglycerides. Similar trends have been observed in the water/C12EO8/hydrocarbon oil systems [36]. It is known that the low molecular weight oil has higher coalescence rate compared to that of high molecular weight oil [40]. Therefore, it is possible that low molecular weight oil separates earlier than the high molecular weight oil. The formation (not shown, since same data observed after 24 h) and stability of the O/I1 gel emulsion have been studied and only stability data have been presented in Table 2. It is shown in Table 2 that the formation and stability of the O/I1 gel emulsion also depends on the molecular weight of oil. The prolonged stability of the O/I1 gel emulsion could be explained with the viscosity of the I1 phase. It is known that the I1 phase is the most viscous phase, so it is reasonable to assume that the I1 phase could reduce the emulsion breaking processes effectively and shows stable gel emulsion.
Table 1a Formation (preparation) of the O/H1 gel emulsion at room temperature.
Table 1b Stability of the O/H1 gel emulsion after 24 h at 25 °C. Oil
30
40
50
60
70
80
THG TOG TDG Olive
s s s
s s s
s s s
s s
s s
s
(s) Gel emulsion, () oil separated.
3.3. Rheological behavior of the I1 phase and O/I1 gel emulsion More information on the network structure of the cubic phase could be obtained from oscillatory shear frequency sweep measurements. Furthermore, the rheological behavior of the gel emulsion is important for the stability, processing, and storage. Representative data of the dynamic frequency sweep measurements in different concentrations of TDG at a fixed water/C12EO8 ratio = 60/40 is shown in Fig. 2. At lower concentration of TDG (3 wt.%), the elastic modulus, G0 and the viscous modulus, G00 increase with the applied frequency (x), indicating non-Newtonian fluid like behavior. In Fig. 2a, it is shown that the values of the G0 reach more than 105 Pa, which is consistent with a hard gel cubic structure [32]. There are no cross-over observed between the elastic and viscous moduli, however, judging from the trend, it could be assumed that the crossover presents at lower frequency (around 103 rad s1). There are no appreciable changes in the rheogram of the I1 phase with increasing oil content; the values of the G0 and G00 slightly increase at lower frequency. The G00 in the I1 phase reaches a minimum and then increases with the frequency. Mezzenga et al., presented a detail rheogram of the structured fluid in different frequencies [41]. They explained that in a certain region, the viscous modulus (G00 ) of the fluid steeply increases compared to the elastic modulus (G0 ) with increasing frequency, which was defined as a plastic nature or a rubbery state of the fluid. In Fig. 2a, we can see a similar trend at higher frequency that is the G00 steeply increases compared to that of the G0 can be supposed as the rubbery or the plastic nature. In simple way, we can say the I1 phase shows viscoelastic property at lower frequency but plastic nature at higher frequency. In Fig. 2b (O/I1 gel emulsion), the G0 increases or nearly constant within frequency range but monotonically decreases with increasing oil concentration. It should be noticed that the G0 dominating over G00 in all samples within measured frequency indicating solid or gel type structure. To determine the hardness or the brittleness of the I1 phase and the O/I1 gel emulsion, the complex viscosity, jgj was determined by the following equation and presented in Fig. 2c.
jg j ¼
1=2 G02 þ G002 ð3Þ
x
It has found that the values of the jgj in all samples decreases with applied frequency indicating shear-thinning nature. Minute observation could be seen at lower frequency, the jgj first increases with the oil content within the I1 phase but monotonically
Table 2 Stability of the O/I1 gel emulsion after 24 h at 25 °C.
Oil (wt.%)
30
40
50
60
70
80
Oil
30
40
50
60
70
80
THG TOG TDG Olive
s s s s
s s s s
s s s
s s s
s s
s s
THG TOG TDG Olive
s s s s
s s s s
s s s
s s s
s s
s s
(s) Gel emulsion, () oil separated.
(s) Gel emulsion, () oil separated.
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Fig. 2. The storage (elastic) modulus, G0 , and the loss (viscous) modulus, G00 , with various concentrations of TDG, (a) 3–5 wt.% for the I1 phase, (b) 40–80 wt.% for the O/I1 gel emulsion, (c) the complex viscosity, jgj. Water/surfactant ratio is fixed at 60/40 for all samples. Open symbols indicating G0 whereas filled symbols G00 in Fig. 2a and b.
decreases in the O/I1 emulsion region. No plateau regions are observed in the measured frequency range, similarly to the reported data of the water/C12EO25/decane system [22]. Since we did not find plateau value of the jgj at lower frequency, just for comparison, we plotted the jgj at different fixed frequencies as a function of TDG concentrations in Fig. 3. In Fig. 3, it has shown that the jgj increases in the single I1 phase region at lower frequency (x = 0.01 and 1 rad s1), whereas it decreases at higher frequency (x = 100 rad s1) and nearly constant value observed at x = 10 rad s1. The increasing tendency
Fig. 3. The complex viscosity, jgj, of the I1 phase and related O/I1 gel emulsions as a function of TDG concentration at different fixed frequencies, square (0.01 rad s1), circle (1 rad s1), triangle (10 rad s1), diamond (100 rad s1).
of the jgj (at lower frequency) could be attributed to the neighboring micellar interaction. Since the micelle swells with the TDG (Fig. 1, rI increases), so it is reasonable that the neighboring micellar interaction also increases. Similar behavior could be seen in the water/C12EO25/decane system [22]. However, this trend is not consistent at higher frequency, which could be due to the shear breaking interaction. In the O/I1 gel emulsion, the jgj monotonically decreases with increasing TDG concentration in the system could relate to the volume fraction of the I1 phase in the system. If the oil concentration increases in the gel emulsion, simultaneously the volume fraction of the continuous phase (I1 phase) decreases. Since, the overall viscosity of the gel emulsion is mainly controlled by the I1 phase, so due to lowering the volume fraction of the I1 phase in the system, the jgj decreases. To study the effect of oil nature on the rheological properties of the O/I1 gel emulsion, we have carried out frequency sweep measurements with different oil (TDG and olive oil) but a fixed composition of water/surfactant/oil = 12/8/80. Representative rheological data have shown in Fig. 4. In Fig. 4a, one could find that the G0 and G00 show similar trend in both the systems as a function of frequency. If we look in Fig. 4b, we can find that both the systems show shear-thinning behavior and no plateau value at lower frequency. Although oil is different, we did not find any noticeable difference between TDG and olive oil systems. It could be said that the disperse phase has little or no effect on the rheological behavior of the O/I1 gel emulsion. In another word, the rheology of the O/I1 gel emulsion is mainly controlled by the I1 phase. Details rheological behavior of the H1 phase and O/H1 gel emulsion have been shown in our previous report [36], here we would
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Fig. 4. Comparative rheological data of the O/I1 gel emulsion containing 80 wt.% oil, (a) the elastic modulus, G0 , and the viscous modulus, G00 (b) the complex viscosity, jgj. Open symbol corresponds to G0 whereas filled symbol G00 in Fig. 4a.
see the oil nature effect on the O/H1 gel emulsion at a fixed composition of water/surfactant/oil = 8/12/80 and the results are presented in Fig. 5. If we look Fig. 5a, one could find a clear difference, which manifests that oil nature changes the rheological behavior of the O/H1 gel emulsion. The values of the elastic modulus, G0 and the viscous modulus, G00 in olive oil system show higher than that of the TDG system, but both the systems show elastic or gel type structure (G0 remain higher than G00 ). The complex viscosity, jgj of the olive oil system remains higher than TDG system throughout the frequency, however shear-thinning behavior could find in both the systems. It is interesting that oil nature has no effect on the rheological behavior of the O/I1 gel emulsion but a clear effect has been observed in the O/H1 gel emulsion. To find the fact that is the oil nature effect on the O/I1 and O/H1 gel emulsions, we studied rheological behavior of the continuous phase (I1 and H1 phase) containing maximum oil (TDG and olive oil) at a fixed w/s = 60/40 for I1 phase and 40/60 for H1 phase, and the data presented in supporting material (Figs. X and Y). It is considered that the viscosity of the continuous phase (I1 or H1) is mainly controlled the viscosity of the gel emulsion. It has found that the amount of solubilized oil in the I1 and the H1 phases differs with oil nature; namely 5 wt.% TDG and 2 wt.% olive oil are solubilized in the I1 phase whereas, 8 wt.% TDG and 0.5 wt.% olive oil are solubilized in the H1 phase, which changes the rheogram of the phases. In rheology, the values of the rheological parameters (G0 , G00 ), and the jgj of the I1 phase containing TDG or olive oil do not differ so much (except lower fre-
quency) (Fig. X), whereas in the H1 phase (Fig. Y), a clear difference can be seen. This could be the reason that the O/I1 gel emulsion shows similar rheological behavior in both the TDG and olive oil systems, whereas rheology of the O/H1 gel emulsion vary with oil nature. Minute observation could see in Fig. Y (Supporting material) that the cross-over between the G0 and G00 in the H1 phase presents at lower frequency in case of olive oil system compared to the TDG system and the jgj also shows higher value within the measured frequency, indicating longer relaxation time or more structured system that is why olive oil system shows prolonged stability and higher viscosity than TDG system. 4. Conclusion The effect of the molecular weight of triglycerides on the formation, stability, and rheological behavior of the cubic (O/I1) and hexagonal (O/H1) phase based gel emulsions have been studied in the water/C12EO8/triglycerides system. It has been found that the aqueous solution of nonionic surfactant octa(oxyethylene dodecyl ether) could form micellar cubic phase (I1 phase) with the addition of triglycerides, which could solubilize small fraction of oil, manifesting from the increasing the values of d and rI. After oil solubilization limit, excess oil is in equilibrium with the I1 phase (d value is nearly constant) in the two-phase region (I1+O region), which gives opportunity to form O/I1 gel emulsion with adequate mixing. It has been observed that the formation (preparation) and stability of the O/H1 and the O/I1 gel emulsions are highly dependent on the
Fig. 5. Comparative rheological data of the O/H1 gel emulsion containing 80 wt.% oil (TDG and olive oil) at a fixed w/s ratio = 40/60, (a) the elastic modulus, G0 , and the viscous modulus, G00 (b) the complex viscosity, jgj. Open symbol corresponds to G0 whereas filled symbol G00 in Fig. 5a.
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molecular weight of triglycerides, namely, the high molecular weight triglycerides give better performance (formation and stability) compared to the low molecular weight triglycerides. In rheological study, it has been shown that the I1 phase changes from viscoelastic to plastic nature depending on applied frequency. The complex viscosity, jgj increases within the I1 phase but decreases in the O/I1 gel emulsion with increasing TDG concentration. The increasing trend of the jgj (lower frequency) in the I1 phase is ascribed with the interaction of the neighboring micelles (since the I1 phase swells with the solubilization of the TDG) and decreasing the jgj in the O/I1 gel emulsion simply relates to the volume fraction of the I1 phase in the system. It has also revealed that the rheology of the O/I1 gel emulsion is not dependent on the oil nature but oil nature dependency observed in the O/H1 gel emulsion. To sum up our study, it has been figured out that triglycerides could form liquid crystal based gel emulsion similarly to hydrocarbon oil. We hope this study will explore the basic understanding of the surfactant aggregates in presence of triglycerides, which is a basic phenomenon of the colloid and interface science. It has also evaluated how the rheological properties of the LC based gel emulsions are modified with the oil nature, which could improve the formulation concept of food, cosmetic, and pharmaceutical products. Acknowledgments The authors acknowledge the Ministry of Education, Culture, Sports, Science and Technology, Grant-in-Aid for Young Scientists (B), No. 18780094. In particular M.M. Alam acknowledges the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan for scholarship. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jcis.2009.03.054. References [1] H. Kunieda, C. Solans, N. Shida, J.L. Parra, Colloids Surf. 24 (1987) 427. [2] H. Kunieda, N. Yano, C. Solans, Colloids Surf. 36 (1989) 313.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Effect of molecular weight of triglycerides on the formation and rheological behavior of cubic and hexagonal phase based gel emulsions Mohammad Mydul Alam, Kenji Aramaki * Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai 79-7, Hodogaya-ku, Yokohama 240-8501, Japan
a r t i c l e
i n f o
Article history: Received 25 December 2008 Accepted 26 March 2009 Available online 5 April 2009 Keywords: Triglycerides Rheology Liquid crystal based gel emulsion Phase behavior Cubic phase Hexagonal phase
a b s t r a c t The effect of triglyceride molecular weight on the formation and rheology of cubic (O/I1) and hexagonal (O/ H1) phase based gel emulsions has been studied in water/C12EO8 systems. It was found that the addition of TDG (1,2,3-tridecanoyl glycerol) in the micellar solution leads to the formation of the I1 phase, which can solubilize some added oil. From SAXS data, it is revealed that the interlayer spacing (d) and the length of hydrophobic part (rI) increase with increasing TDG concentration in the I1 phase, whereas the effective cross-sectional area (as) decreases. After the oil solubilization limit, the d value remains nearly constant, indicating the I1+O phase appears. The high viscosity of the I1 phase facilitates the formation of the O/I1 gel emulsion. It has been observed that the formation and stability of the O/I1 and O/H1 gel emulsion is highly dependent on the molecular weight of triglycerides, namely, the high molecular weight triglycerides show better performance (formation and stability) compared to the low molecular weight triglycerides. The rheological behavior of the I1 phase was found to change from viscoelastic to elastic nature with TDG content. The values of the complex viscosity, jgj show different trends at different fixed frequencies within the I1 phase, whereas it decrease monotonically in the O/I1 gel emulsions. The increasing values of the jgj (at lower frequency) could be due to the neighboring micellar interaction and decreasing values of the jgj in the O/I1 gel emulsion could relate to the volume fraction of the I1 phase in the system. It is also figured out that the rheological parameters (elastic modulus, viscous modulus, and jgj) of the O/I1 the gel emulsion do not depend on the oil nature, whereas the O/H1 gel emulsion shows oil nature dependency. Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction Highly concentrated or high internal phase ratio emulsions (HIPREs) have been the objects of many studies for several years [1–8]. It is known that HIPREs can form in the water (O/W) and oil (W/O) rich region under certain conditions [2,9–11]. Due to their characteristic features such as high volume fraction of the internal phase, high viscosity, and transparency, they are also referred to as gel emulsions [1,3,12,13]. Rheological properties of HIPREs were also studied and found that range from viscoelastic to elastic nature depending on the system composition and temperature [6,14,15]. HIPREs have been used for practical applications such as aviation fuels, emulsion explosives, and cosmetics [16–19]. Another kind of gel emulsion that is lyotropic liquid crystal based gel emulsion has been taken attention. In contrast with HIPREs, LC based gel emulsion shows high viscosity even at low volume fraction of internal phase and prolonged stability [20– 23]. Among the liquid crystals, cubic phase (I1) is the most viscous phase and, thus commonly used to prepare O/I1 gel emulsion [20,21,24]. Generally, the I1 phase is formed in aqueous system with very hydrophilic nonionic surfactant such as poly(oxyehtyl* Corresponding author. Fax: +81 045 339 4300. E-mail address: [email protected] (K. Aramaki). 0021-9797/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2009.03.054
ene) alkyl ether [25], poly(oxyethylene)-poly(oxypropylene)poly(oxyethylene) triblock copolymer [26,27], polyglycerol fatty acid ester [28], and sucrose fatty acid ester [29]. Besides, the solubilization of oil in the micellar (Wm) or hexagonal phase (H1) often induces the I1 phase [21,30]. The I1 phase is normally formed between the micellar and the hexagonal phase as a function of surfactant concentration. Several authors studied the rheological behavior of the I1 phase [31,32] and O/I1 gel emulsion [22,24]. On the other hand, hexagonal phase (H1) is also highly viscous and is very common in aqueous surfactant solution, e.g., poly(oxyethylene) alkyl ether [25], sodium dodecyl sulfate [33], poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) triblock copolymer [34], and poly(oxyethylene)-poly(oxypropylene) diblock copolymer [26], sucrose monododecanoate [35]. Recently, we have found that the H1 phase could form O/H1 gel emulsion similarly to the O/I1 gel emulsion [36]. We have extensively studied the formation, stability, and rheological behavior of the H1 phase and O/H1 gel emulsion, which was compared to the O/I1 gel emulsion. In our study, we mainly concentrated on hydrocarbon oils. To our knowledge, there are few studies counted, in which triglyceride is used to study the gel emulsion instead of hydrocarbon oil as a dispersed phase [37]. Triglycerides or triacylglycerols are a simple class of lipids consisting of glycerol esterifies with three fatty acid moieties. Moreover, natural fats and oils including all vegetable oils, such
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structures. vL is the volume of the lipophilic part of one surfactant molecule. /O and /L are the volume fraction of oil and lipophilic part of surfactant in the system, respectively.
as palm oil or sunflower oil and animal fats, consist almost exclusively of triglyceride. These are commonly used for cosmetics, foods, and so on. Although triglyceride is an important class of food oil, detail rheological study of triglyceride-containing gel emulsion is still missing. Aramaki and Yanagimachi [37] reported the phase behavior and rheological properties of the I1 phase and the O/I1 gel emulsion as a function of triglyceride concentrations (mainly 1,2,3trioctanoylglycerol, TOG) in water/C12EO25 system. However, they did not show the effect of the molecular weight of triglycerides on the formation and stability of O/I1 gel emulsion containing triglycerides as a dispersed phase. The present paper is the supposed to guide for the gel emulsion with triglycerides in the same way as previous detailed investigation of hydrocarbon oils [36]. In this contribution, we have mainly studied the formation, stability, and rheology of the O/I1 and O/H1 gel emulsions with different molecular weight triglycerides. The changes of the microstructural parameters (d, rI, and as) of the I1 phase with the addition of TDG were also evaluated.
2.2.3. Rheological measurements Rheological measurements (oscillatory) on a ternary mixture of water/surfactant/triglycerides in the samples were carried out as a function of oil concentrations. All samples for rheological measurements were homogenized with repeated centrifugation and kept in a water bath at 25 °C for 24 h before measurements to ensure equilibration. Rheology of the gel emulsion was done within 24 h after preparation. The rheological measurements were performed in ARES rheometer (Rheometric Scientific) using cone plate geometry (/ = 25 mm, cone angle = 0.1 rad) in the linear viscoelastic regime, as determined previously by dynamic strain sweep measurements. A sample cover provided with the instrument was used to minimize the change in sample composition by evaporation during measurements.
2. Experimental
3. Results and discussion
2.1. Materials
3.1. Effect of TDG on the structural parameters of the I1 phase
Monodisperse octa(oxyethylene) dodecyl ether (C12EO8) was purchased from Nikko Chemicals, Japan. 1,2,3-Trihexanoylglycerol (tricaproin, THG) and 1,2,3-tridecanoylglycerol (tricaprin, TDG) were purchased from Tokyo Chemical Industry. 1,2,3-Trioctanoylglycerol (tricaplyrin, TOG) was purchased from Sigma–Aldrich Chemicals. Olive oil (average carbon number 17) was purchased from Wako Chemical Industries Ltd. Structure of triglyceride was presented elsewhere [30]. All these chemicals were highly pure and were used as received. Millipore-filtered water was used to prepare all the samples.
The partial ternary phase diagram of the water/C12EO8/TDG is already reported [39], here we would examine the change of interlayer spacing, d, length of the hydrophobic part rI, and the effective cross-sectional area per surfactant molecule, as, of the I1 phase with the addition of TDG. It should be noticed that the d value could give us information regarding the phase boundary between the I1 phase and the I1+O region and also to determine r1 and as. To evaluate the structural changes (d, rI, as) in the I1 phase, we have performed SAXS measurement at a constant water/C12EO8 ratio (60/40). We have calculated the structural parameters by using Eqs. (1) and (2) and presented in Fig. 1. It is inferred in Fig. 1 that the values of the interlayer spacing (d) increase with the TDG content and after a certain concentration, it show nearly constant. Intersection of these two lines is considered as phase boundary between the I1 phase and the I1+O region. Visual inspection could also ascertain phase boundary that changes from transparent (I1 phase) to turbid (I1+O). Simultaneously, the length of the hydrophobic part (rI) of the aggregates increases with the oil content but the effective cross-sectional area per surfactant molecule (as) decreases. It is already pointed out that the as decreases in the I1 phase if oil is solubilized in the aggregate cores
2.2. Methods 2.2.1. 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. Around room temperature, we observed a highly viscous emulsion that did not flow when the glass tube was turned upside down. 2.2.2. Small-angle X-ray scattering measurements The interlayer spacing of liquid crystals was measured using small-angle X-ray scattering, performed on a small-angle X-ray scattering camera equipped with rotating anode and a CCD detector (Rigaku, Nanoviewer). The samples of liquid crystals were filled in a hole of an iron plate and covered by plastic films for the measurement (Mylar seal method). The type of liquid crystal was determined by the SAXS peak position [38]. According to the geometry of liquid crystals, the effective cross-sectional area per surfactant molecule, as, is calculated by the following equations using the interlayer spacing, d, obtained from the SAXS measurement. For the I1 phase [30,38],
1=3 3 rI ¼ d ð/L þ /O Þ C; 4pnc 3vL /L þ /O as ¼ ; rI /L
ð1Þ ð2Þ
where, rI is the radius of lipophilic core of spherical micelle in the I1 phase, nc is the number of micelles in a unit cell of cubic phase, and C is a constant (C = (h2 + k2 + l2)1/2), where h, k, and l are miller indices. The values of the constants (nc, C) are (1, 1) for simple cubic, p p (2, 2) for body-centered cubic, and (4, 3) for face centered cubic
Fig. 1. Change in the interlayer spacings (d), effective cross-sectional area (as), and the length of hydrophobic part (rI), of the I1 phase are plotted as a function of TDG (wt.%) in a fixed water/C12EO8 ratio = 60/40.
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[35]. So, one could speculate that the TDG is mainly solubilized in the aggregates core. A similar observation was found in the previous article that high molecular weight triglycerides are mainly solubilized in the aggregates core [30]. The rI increases with the TDG content, indicating the micelles of the I1 phase swell with the adding TDG. After the oil solubilization limit, the d value shows nearly constant, indicating the excess oil is in equilibrium with the I1 phase or the I1+O phase appears. Due to high viscosity of the I1 phase, allow us to prepare O/I1 gel emulsion in the I1+O region. It should be noted that the melting of the I1 phase is necessary for adequate mixing. 3.2. Formation and stability of cubic (O/I1) and hexagonal (O/H1) phase based gel emulsions In the previous report [36], we have found that the formation and stability of the gel emulsion is dependent on the hydrocarbon oil nature. Here, we would see the effect of the molecular weight of triglycerides on the formation and stability of the O/H1 and O/I1 gel emulsion, which could prove useful in a variety of application. In Tables 1a and 1b, the formation (preparation) and stability of the O/H1 gel emulsion is presented. In table, values are indicating wt.% of oil at a fixed water/C12EO8 ratio (40/60). One can find that the formation of the O/H1 gel emulsion is highly dependent on the oil nature that is 40 wt.% of THG, 60 wt.% of TOG, 80 wt.% of TDG and olive oil can incorporate in gel emulsion. We can see that the high molecular weight triglycerides are more susceptible to form O/H1 gel emulsion compared to the low molecular weight oil. In our previous report [30], it was revealed that the low molecular weight triglycerides has more penetration tendency to the palisade layer in the micelles compared to the high molecular weight oil, so it could be speculated that the penetration of oil in the palisade layer makes the H1 phase flexible. As a result, the H1 phase could not form gel emulsion in case of low molecular weight triglycerides at higher oil concentration. To check stability, we kept all samples (O/H1 gel emulsion) at 25 °C and observed after 24 h and the observations are presented in Table 1b. In Table 1b, one can see that the stability of the O/H1 gel emulsion depends on the molecular weight of oil, namely the low molecular weight oil shows less stability compared to the high molecular weight triglycerides. Similar trends have been observed in the water/C12EO8/hydrocarbon oil systems [36]. It is known that the low molecular weight oil has higher coalescence rate compared to that of high molecular weight oil [40]. Therefore, it is possible that low molecular weight oil separates earlier than the high molecular weight oil. The formation (not shown, since same data observed after 24 h) and stability of the O/I1 gel emulsion have been studied and only stability data have been presented in Table 2. It is shown in Table 2 that the formation and stability of the O/I1 gel emulsion also depends on the molecular weight of oil. The prolonged stability of the O/I1 gel emulsion could be explained with the viscosity of the I1 phase. It is known that the I1 phase is the most viscous phase, so it is reasonable to assume that the I1 phase could reduce the emulsion breaking processes effectively and shows stable gel emulsion.
Table 1a Formation (preparation) of the O/H1 gel emulsion at room temperature.
Table 1b Stability of the O/H1 gel emulsion after 24 h at 25 °C. Oil
30
40
50
60
70
80
THG TOG TDG Olive
s s s
s s s
s s s
s s
s s
s
(s) Gel emulsion, () oil separated.
3.3. Rheological behavior of the I1 phase and O/I1 gel emulsion More information on the network structure of the cubic phase could be obtained from oscillatory shear frequency sweep measurements. Furthermore, the rheological behavior of the gel emulsion is important for the stability, processing, and storage. Representative data of the dynamic frequency sweep measurements in different concentrations of TDG at a fixed water/C12EO8 ratio = 60/40 is shown in Fig. 2. At lower concentration of TDG (3 wt.%), the elastic modulus, G0 and the viscous modulus, G00 increase with the applied frequency (x), indicating non-Newtonian fluid like behavior. In Fig. 2a, it is shown that the values of the G0 reach more than 105 Pa, which is consistent with a hard gel cubic structure [32]. There are no cross-over observed between the elastic and viscous moduli, however, judging from the trend, it could be assumed that the crossover presents at lower frequency (around 103 rad s1). There are no appreciable changes in the rheogram of the I1 phase with increasing oil content; the values of the G0 and G00 slightly increase at lower frequency. The G00 in the I1 phase reaches a minimum and then increases with the frequency. Mezzenga et al., presented a detail rheogram of the structured fluid in different frequencies [41]. They explained that in a certain region, the viscous modulus (G00 ) of the fluid steeply increases compared to the elastic modulus (G0 ) with increasing frequency, which was defined as a plastic nature or a rubbery state of the fluid. In Fig. 2a, we can see a similar trend at higher frequency that is the G00 steeply increases compared to that of the G0 can be supposed as the rubbery or the plastic nature. In simple way, we can say the I1 phase shows viscoelastic property at lower frequency but plastic nature at higher frequency. In Fig. 2b (O/I1 gel emulsion), the G0 increases or nearly constant within frequency range but monotonically decreases with increasing oil concentration. It should be noticed that the G0 dominating over G00 in all samples within measured frequency indicating solid or gel type structure. To determine the hardness or the brittleness of the I1 phase and the O/I1 gel emulsion, the complex viscosity, jgj was determined by the following equation and presented in Fig. 2c.
jg j ¼
1=2 G02 þ G002 ð3Þ
x
It has found that the values of the jgj in all samples decreases with applied frequency indicating shear-thinning nature. Minute observation could be seen at lower frequency, the jgj first increases with the oil content within the I1 phase but monotonically
Table 2 Stability of the O/I1 gel emulsion after 24 h at 25 °C.
Oil (wt.%)
30
40
50
60
70
80
Oil
30
40
50
60
70
80
THG TOG TDG Olive
s s s s
s s s s
s s s
s s s
s s
s s
THG TOG TDG Olive
s s s s
s s s s
s s s
s s s
s s
s s
(s) Gel emulsion, () oil separated.
(s) Gel emulsion, () oil separated.
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Fig. 2. The storage (elastic) modulus, G0 , and the loss (viscous) modulus, G00 , with various concentrations of TDG, (a) 3–5 wt.% for the I1 phase, (b) 40–80 wt.% for the O/I1 gel emulsion, (c) the complex viscosity, jgj. Water/surfactant ratio is fixed at 60/40 for all samples. Open symbols indicating G0 whereas filled symbols G00 in Fig. 2a and b.
decreases in the O/I1 emulsion region. No plateau regions are observed in the measured frequency range, similarly to the reported data of the water/C12EO25/decane system [22]. Since we did not find plateau value of the jgj at lower frequency, just for comparison, we plotted the jgj at different fixed frequencies as a function of TDG concentrations in Fig. 3. In Fig. 3, it has shown that the jgj increases in the single I1 phase region at lower frequency (x = 0.01 and 1 rad s1), whereas it decreases at higher frequency (x = 100 rad s1) and nearly constant value observed at x = 10 rad s1. The increasing tendency
Fig. 3. The complex viscosity, jgj, of the I1 phase and related O/I1 gel emulsions as a function of TDG concentration at different fixed frequencies, square (0.01 rad s1), circle (1 rad s1), triangle (10 rad s1), diamond (100 rad s1).
of the jgj (at lower frequency) could be attributed to the neighboring micellar interaction. Since the micelle swells with the TDG (Fig. 1, rI increases), so it is reasonable that the neighboring micellar interaction also increases. Similar behavior could be seen in the water/C12EO25/decane system [22]. However, this trend is not consistent at higher frequency, which could be due to the shear breaking interaction. In the O/I1 gel emulsion, the jgj monotonically decreases with increasing TDG concentration in the system could relate to the volume fraction of the I1 phase in the system. If the oil concentration increases in the gel emulsion, simultaneously the volume fraction of the continuous phase (I1 phase) decreases. Since, the overall viscosity of the gel emulsion is mainly controlled by the I1 phase, so due to lowering the volume fraction of the I1 phase in the system, the jgj decreases. To study the effect of oil nature on the rheological properties of the O/I1 gel emulsion, we have carried out frequency sweep measurements with different oil (TDG and olive oil) but a fixed composition of water/surfactant/oil = 12/8/80. Representative rheological data have shown in Fig. 4. In Fig. 4a, one could find that the G0 and G00 show similar trend in both the systems as a function of frequency. If we look in Fig. 4b, we can find that both the systems show shear-thinning behavior and no plateau value at lower frequency. Although oil is different, we did not find any noticeable difference between TDG and olive oil systems. It could be said that the disperse phase has little or no effect on the rheological behavior of the O/I1 gel emulsion. In another word, the rheology of the O/I1 gel emulsion is mainly controlled by the I1 phase. Details rheological behavior of the H1 phase and O/H1 gel emulsion have been shown in our previous report [36], here we would
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Fig. 4. Comparative rheological data of the O/I1 gel emulsion containing 80 wt.% oil, (a) the elastic modulus, G0 , and the viscous modulus, G00 (b) the complex viscosity, jgj. Open symbol corresponds to G0 whereas filled symbol G00 in Fig. 4a.
see the oil nature effect on the O/H1 gel emulsion at a fixed composition of water/surfactant/oil = 8/12/80 and the results are presented in Fig. 5. If we look Fig. 5a, one could find a clear difference, which manifests that oil nature changes the rheological behavior of the O/H1 gel emulsion. The values of the elastic modulus, G0 and the viscous modulus, G00 in olive oil system show higher than that of the TDG system, but both the systems show elastic or gel type structure (G0 remain higher than G00 ). The complex viscosity, jgj of the olive oil system remains higher than TDG system throughout the frequency, however shear-thinning behavior could find in both the systems. It is interesting that oil nature has no effect on the rheological behavior of the O/I1 gel emulsion but a clear effect has been observed in the O/H1 gel emulsion. To find the fact that is the oil nature effect on the O/I1 and O/H1 gel emulsions, we studied rheological behavior of the continuous phase (I1 and H1 phase) containing maximum oil (TDG and olive oil) at a fixed w/s = 60/40 for I1 phase and 40/60 for H1 phase, and the data presented in supporting material (Figs. X and Y). It is considered that the viscosity of the continuous phase (I1 or H1) is mainly controlled the viscosity of the gel emulsion. It has found that the amount of solubilized oil in the I1 and the H1 phases differs with oil nature; namely 5 wt.% TDG and 2 wt.% olive oil are solubilized in the I1 phase whereas, 8 wt.% TDG and 0.5 wt.% olive oil are solubilized in the H1 phase, which changes the rheogram of the phases. In rheology, the values of the rheological parameters (G0 , G00 ), and the jgj of the I1 phase containing TDG or olive oil do not differ so much (except lower fre-
quency) (Fig. X), whereas in the H1 phase (Fig. Y), a clear difference can be seen. This could be the reason that the O/I1 gel emulsion shows similar rheological behavior in both the TDG and olive oil systems, whereas rheology of the O/H1 gel emulsion vary with oil nature. Minute observation could see in Fig. Y (Supporting material) that the cross-over between the G0 and G00 in the H1 phase presents at lower frequency in case of olive oil system compared to the TDG system and the jgj also shows higher value within the measured frequency, indicating longer relaxation time or more structured system that is why olive oil system shows prolonged stability and higher viscosity than TDG system. 4. Conclusion The effect of the molecular weight of triglycerides on the formation, stability, and rheological behavior of the cubic (O/I1) and hexagonal (O/H1) phase based gel emulsions have been studied in the water/C12EO8/triglycerides system. It has been found that the aqueous solution of nonionic surfactant octa(oxyethylene dodecyl ether) could form micellar cubic phase (I1 phase) with the addition of triglycerides, which could solubilize small fraction of oil, manifesting from the increasing the values of d and rI. After oil solubilization limit, excess oil is in equilibrium with the I1 phase (d value is nearly constant) in the two-phase region (I1+O region), which gives opportunity to form O/I1 gel emulsion with adequate mixing. It has been observed that the formation (preparation) and stability of the O/H1 and the O/I1 gel emulsions are highly dependent on the
Fig. 5. Comparative rheological data of the O/H1 gel emulsion containing 80 wt.% oil (TDG and olive oil) at a fixed w/s ratio = 40/60, (a) the elastic modulus, G0 , and the viscous modulus, G00 (b) the complex viscosity, jgj. Open symbol corresponds to G0 whereas filled symbol G00 in Fig. 5a.
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molecular weight of triglycerides, namely, the high molecular weight triglycerides give better performance (formation and stability) compared to the low molecular weight triglycerides. In rheological study, it has been shown that the I1 phase changes from viscoelastic to plastic nature depending on applied frequency. The complex viscosity, jgj increases within the I1 phase but decreases in the O/I1 gel emulsion with increasing TDG concentration. The increasing trend of the jgj (lower frequency) in the I1 phase is ascribed with the interaction of the neighboring micelles (since the I1 phase swells with the solubilization of the TDG) and decreasing the jgj in the O/I1 gel emulsion simply relates to the volume fraction of the I1 phase in the system. It has also revealed that the rheology of the O/I1 gel emulsion is not dependent on the oil nature but oil nature dependency observed in the O/H1 gel emulsion. To sum up our study, it has been figured out that triglycerides could form liquid crystal based gel emulsion similarly to hydrocarbon oil. We hope this study will explore the basic understanding of the surfactant aggregates in presence of triglycerides, which is a basic phenomenon of the colloid and interface science. It has also evaluated how the rheological properties of the LC based gel emulsions are modified with the oil nature, which could improve the formulation concept of food, cosmetic, and pharmaceutical products. Acknowledgments The authors acknowledge the Ministry of Education, Culture, Sports, Science and Technology, Grant-in-Aid for Young Scientists (B), No. 18780094. In particular M.M. Alam acknowledges the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan for scholarship. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jcis.2009.03.054. References [1] H. Kunieda, C. Solans, N. Shida, J.L. Parra, Colloids Surf. 24 (1987) 427. [2] H. Kunieda, N. Yano, C. Solans, Colloids Surf. 36 (1989) 313.
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