Energy Fuels 2010, 24, 1854–1859 Published on Web 02/26/2010
: DOI:10.1021/ef9014026
Effect of Physical Properties on Microexplosion Occurrence in Water-in-Oil Emulsion Droplets Yoshio Morozumi* and Yuuto Saito Department of Intelligent Mechanical Systems Engineering, Kochi University of Technology, 185 Miyanokuchi, Tosayamada-cho, Kami-shi, Kochi 782-8502, Japan Received November 18, 2009. Revised Manuscript Received February 8, 2010
To investigate the effects of emulsifier type and content on a microexplosion occurrence, we measured the viscosity, diameters of dispersed water droplets, and interfacial tension between water and oil phases in nhexadecane emulsions prepared under varying emulsifying conditions, as well as microexplosion behavior and emulsion-droplet temperature during heating in an electrical furnace. With increasing emulsifier content, the viscosity of the oil phase remains almost constant, while that of the emulsion increases. Interfacial tension between the water and n-hexadecane phases varies with the emulsifier type but not with the emulsifier content above the saturation adsorption of the emulsifier. Interfacial tension depends very little upon droplet diameter, regardless of emulsifier conditions. Microexplosion temperature and waiting time are affected by emulsifier type, which probably arises from the thermal decomposition of the emulsifiers. In addition, increasing the emulsifier content has a negative influence on microexplosion occurrence, which might also be related to thermal decomposition of the emulsifiers.
conditions on microexplosions has been studied for O/W emulsion droplets and compared to the effects of normal gravity conditions,8 while the effects of various ambient temperatures and water contents have also been investigated.9-11 Water-fuel emulsion properties, such as volatility of base fuel oils, type of emulsion, and water content, are another factor that affects the occurrence of microexplosions and has been investigated both experimentally and theoretically. Avedisian and Andres12 investigated bubble nucleation in superheated water-hydrocarbon emulsions and reported that the superheat limit of water must be less than the boiling point of the hydrocarbon for the microexplosions to occurr. Yamasaki et al.13 experimented on microexplosions in W/O and O/W emulsion fuels using glass capillary tubes and confirmed that the rate of microexplosion is independent of the type of emulsion. Tarlet et al.14 developed a mathematical model to predict the microexplosion delay of emulsified fuel droplets numerically. Fu et al.15 proposed a mathematical model to describe the microexplosion occurrence in both W/O and O/W emulsion droplets and discussed the effect of dispersed droplet sizes in the emulsion on the microexplosion strength. Both experiments16 and
1. Introduction A water-fuel emulsion is a mixture of base fuel and water with a small amount of emulsifier and is usually classified as either a water-in-oil (W/O) and an oil-in-water (O/W) type, depending upon the hydrophilic-lipophilic balance (HLB) of the emulsifiers. In spray combustion of water-fuel emulsions, because of the difference in volatility between fuel and water, both types of emulsion droplets exhibit explosive evaporation of the water contained in the emulsion and induce secondary atomization, producing a number of fine secondary droplets. This secondary atomization is known as a microexplosion. Microexplosions enhance the mixing of fuels and air during combustion, resulting in improved combustion efficiency and lower pollutant emissions.1-5 Therefore, a fundamental understanding and phenomenological investigations of microexplosions in emulsion fuel droplets can lead to important improvements in spray combustion systems. The effect of ambient conditions on microexplosions in an emulsion fuel droplet has been studied extensively. The effect of the ambient pressure on the occurrence of microexplosions for freely falling droplets6 and suspended single droplets7 has been investigated experimentally. The effect of microgravity
(8) Tsue, M.; Yamasaki, H.; Kadota, T.; Segawa, D.; Kono, M. Proceedings of the 27th International Symposium on Combustion, 1998; pp 2587-2593. (9) Jeong, I.; Lee, K.-H.; Kim, J. J. Mech. Sci. Technol. 2006, 22, 148– 156. (10) Morozumi, Y.; Kitamura, Y.; Saito, Y. J. Chem. Eng. Jpn. 2010, in press. (11) Ocampo-Barrera, R.; Villasenor, R. Combust. Flame 2001, 126, 1845–1855. (12) Avedisian, C. T.; Andres, R. P. J. Colliod Interface Sci. 1978, 64, 438–453. (13) Yamasaki, H.; Tsue, M.; Katoda, T. Trans. Jpn. Soc. Mech. Eng., Ser. B 1993, 59, 1362–1367 (in Japanese). (14) Tarlet, D.; Bellettre, J.; Tazerout, M.; Rahmouni, C. Int. J. Therm. Sci. 2009, 48, 449–460. (15) Fu, W. B.; Hou, L. Y.; Wang, L.; Ma, F. H. Fuel Process. Technol. 2002, 79, 107–119. (16) Yoshiomoto, Y.; Tsukahara, M.; Murayama, T. Trans. Jpn. Soc. Mech. Eng., Ser. B 1989, 55, 3538–3542 (in Japanese).
*To whom correspondence should be addressed. Telephone: þ81887-57-2311. Fax: þ81-887-57-2320. E-mail: morozumi.yoshio@ kochi-tech.ac.jp. (1) Dryer, F. L. Proceedings of the 16th International Symposium on Combustion, 1977; pp 279-296. (2) Lasheras, J. C.; Fernandez-Pello, A. C.; Dryer, F. L. Combust. Sci. Technol. 1979, 21, 1–14. (3) Gollahalli, S. R.; Rasmussen, M. L.; Moussavi, S. J. Proceedings of the 18th International Symposium on Combustion, 1981; pp 349-360. (4) Ballester, J. M.; Fueyo, N.; Dopazo, C. Fuel 1996, 75, 695–705. (5) Kadota, T.; Yamasaki, H. Prog. Energy Combust. Sci. 2002, 28, 385–404. (6) Wang, C. H.; Law, C. K. Combust. Flame 1985, 59, 53–62. (7) Tsue, M.; Kadota, T.; Yamasaki, H.; Hamaya, H. Trans. Jpn. Soc. Mech. Eng., Ser. B 1996, 62, 3488–3493 (in Japanese). r 2010 American Chemical Society
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: DOI:10.1021/ef9014026
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Table 1. Emulsifying Conditions case
emulsifier
A B
Solgen 40 Solgen 40 (oil phase) Noigen TDS-30 (water phase)
emulsifier content (wt %)
water content (wt %)
0.02-5.0 0.02-5.0
20.0 20.0
calculations15 indicate that very small dispersed droplets exhibit only weak expansion or puffing, whereas a microexplosion will occur with larger dispersed droplets. Although these factors characterize the properties of emulsions, there are very few studies focusing on the role of emulsifiers, which are indispensable in the production of water-fuel emulsions. One such study is by Kitamura et al.,17 who experimentally investigated the flash-point behavior of W/O emulsions in a capillary tube and the influence of emulsifiers on the superheat limit for W/O emulsions by measuring the interfacial tension between the continuous oil phase and dispersed water phase. These studies notwithstanding, however, little is known about the effects of the emulsifier type and content on micoexplosion occurrence. Combustion processes greatly depend upon the atomization and spray characteristics of liquid fuels, and atomization is significantly influenced by the viscosity and surface tension of the fuel. In general, water-fuel emulsions exhibit greater viscosity than pure liquid fuels, and both water content and size of the droplets dispersed in the emulsion significantly influence its viscosity. However, measurements of the viscosity of emulsion fuels have been performed only for watermethanol and diesel emulsions with three compound emulsifiers,18 emulsified heavy crude oil,19 and animal fat emulsions blended with methanol or ethanol.20 The present study examines the emulsion properties of a water-in-n-hexadecane emulsion and the microexplosion characteristics of the emulsion droplet, focusing especially on the emulsifier type and content. We measured the viscosity, dispersed-droplet diameter, and interfacial tension between water and oil phases for emulsions prepared under varying emulsifying conditions, as well as microexplosion behavior and emulsion-droplet temperature under heating in an electrical furnace.
Figure 1. Schematic diagram of experimental apparatus.
2.2. Emulsification Measurements. Viscosity was measured by the vibration method using a viscometer (SV-10, A&D Co., Ltd.). The droplet size was measured from microscopic photographs of the emulsions. Interfacial tension between oil and water phases was measured by the pendent drop method, where a water droplet is injected in the continuous oil phase using a contact angle meter (DM-300, Kyowa Interface Science Co., Ltd.). 2.3. Heating Experiments. Figure 1 shows a schematic diagram of the experimental apparatus used for the heating experiments. A water-in-n-hexadecane emulsion droplet is suspended on a R-type thermocouple wire (droplet diameter, 1.5-1.8 mm; wire diameter, 0.1 mm). The furnace is lifted quickly, and the droplet is positioned at the center of the furnace and then heated. The furnace temperature is kept constant at 1073 K by a temperature controller. An insulating board is placed above the furnace to prevent the thermal radiation and natural heat convection to the suspended droplet. During heating, sequential images of the droplet are taken using a high-speed video camera. The frequency of the high-speed video camera is set to 1000 Hz throughout the experiment. The temperature of the droplet center is measured with the thermocouple and recorded with a data logger. Microexplosion occurrence is determined from the video images and correlated with thermocouple data to obtain the microexplosion temperature. Experiments were performed on 25 samples for each emulsifying condition. The microexplosion temperature is taken as the temperature below which half of the samples undergo microexplosion. Because the thermocouple wire located in the emulsion droplet may generate heterogeneous nucleation, which will be a trigger for the microexplosion, the effect of the thermocouple on the micoexplosion should be verified and, therefore, will be described later.
2. Experimental Section 2.1. Emulsification Procedure. Water-in-n-hexadecane emulsions were prepared using distilled water. Varying emulsifier types and contents were added as follows. Two types of nonionic surfactant emulsifier were used: Solgen 40 and Noigen TDS-30 (Dai-ichi Kogyo Seiyaku); their HLB values are 4.3 and 8.0, respectively. Solgen 40 was added to the n-hexadecane phase, and Noigen TDS-30 was added to the water phase, with the same ratio of the two emulsifiers. The emulsifying conditions for case A (one-emulsifier emulsion, Solgen 40 only) and case B (two-emulsifier emulsion, both Solgen 40 and Noigen TDS-30) are listed in Table 1. W/O emulsions were then produced; the mass fraction of the water content in the emulsion, Yw, was set to 0.2 throughout the experiment. The emulsions were prepared using a homogenizer (HG-200, Hsiangtai Machinery Industry Co., Ltd.) for 5 min at 10 000 rpm and room temperature.
3. Results 3.1. Emulsification Properties. Tables 2 and 3 summarize the emulsification properties of viscosity, mean diameter of water droplets, and interfacial tension between water and nhexadecane phases measured in this study. Figure 2 shows the viscosity of the oil phase, water phase, and emulsions as a function of the emulsifier content for cases A and B. As the emulsifier content increases, for the oil phase, viscosity remains constant (cases A and B), for the water phase, viscosity increases (case B), and for the emulsion, viscosity increases (case A) or remains constant (case B). The viscosities of the emulsions differ slightly and are higher than that of the respective oil phase (cases A and B). Therefore, the increase emulsion viscosity arises from the dispersion of water droplets.
(17) Kitamura, Y.; Huang, Q.; Oka, Y.; Takahashi, T. J. Chem. Eng. Jpn. 1990, 23, 711–715. (18) Sheng, H.; Wu, D.; Zhang, H.; Wei, X. Atomization Sprays 2006, 16, 1–13. (19) Shigemoto, N.; Al-Maamari, R. S.; Jibril, B. Y.; Hirayama, A.; Sueyoshi, M. Energy Fuels 2007, 21, 1014–1018. (20) Kerihuel, A.; Snthil Kumar, M.; Bellettre, J.; Tazerout, M. Fuel 2006, 85, 2640–2645.
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Table 2. Measured Emulsion Properties (Viscosity and Mean Diameter of Water Droplets) viscosity (mPa s) emulsifying condition
case A
case B
emulsifier content oil waster (wt %) emulsion phase phase 0.5 1.0 2.0 5.0 0.5 1 2 4
3.58 4.07 5.67 6.51 3.89 4.02 3.97 4.37
2.58 2.44 2.56 3.78 2.34 2.54 2.44 2.57
1.27 1.89 2.74 7.9
mean diameter of water droplets (mm) 1.58 1.84 1.51 1.56 2.06 2.17 1.92 1.88
Table 3. Measured Interfacial Tension between n-Hexadecane and Water emulsifying condition
case A
case B
emulsifier content (wt %)
interfacial tension (10-3 N/m)
0.0255 0.1 0.2 0.3 0.4 0.5 1.0 2.0 5.0 0.02 0.1 0.2 0.5 1.0 2.0 4.0
13.6 8.98 6.81 6.36 6.05 6.07 5.59 5.42 5.21 10.7 7.1 3.9 2.8 2.8 1.9 2.3
Figure 2. Viscosity as a function of the emulsifier content for cases A and B, measured for the oil phase, water phase, and emulsions.
Figure 3 shows the interfacial tension between water and n-hexadecane phases as a function of the emulsifier content for cases A and B. As the emulsifier content increases, the interfacial tension decreases. The saturation adsorption of the emulsifier, evaluated from the slope of the plots, is affected by the emulsifier type. For case B, the emulsifier adsorbs both sides of the water-oil interface and lowers the interfacial tension. Therefore, interfacial tension and saturation adsorption are both smaller for case B than for case A. Figure 4 shows the cumulative size distribution of water droplets for cases A and B. Arithmetic mean diameters of the droplets are also shown. A slight dependence upon the emulsifier content is observed in the size of water droplets. When the emulsifier type is compared, the size of water droplets in case A is seen to be smaller than that in case B. Because the state of emulsification for case A is insufficient and shows agglomeration of water droplets in the emulsion, the rest of the dispersed small droplets are counted for the calculation of the droplet size. In contrast, for case B, water droplets are clearly dispersed and agglomerates are rarely observed. In general, because increasing the emulsifier content lowers the interfacial tension between water and oil phases, water droplets disperse in the oil and maintain a metastable condition. This behavior is directly related to the surface energy of the dispersed phase. That is, increasing the emulsifier content decreases the interfacial tension; thus, smaller dispersed droplets are expected to form. However, this decrease is not observed; rather, interfacial tension remains almost constant above a certain value of the emulsifier content. The inflection point of the interfacial tension is
Figure 3. Interfacial tension between water and oil phases as a function of the emulsifier content for cases A and B.
regarded as the saturation adsorption or the surface excess of the emulsifier. The emulsifier adheres to the interface between the water and oil phases and lowers the interfacial tension. If the emulsifier is added beyond the saturation adsorption, the excess is isolated in the continuous or dispersed phase and contributes less to the decrease in the interfacial tension. Saturation adsorptions for cases A and B are 0.3 and 0.4 wt %, respectively. Because the emulsifier content is sufficiently higher than the saturation adsorption during measurements of the droplet diameter (Figure 3), the content has little influence on the droplet diameter for both cases. Although interfacial tension is lower in case B than in 1856
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Figure 4. Cumulative size distribution of water droplets for cases A and B.
Figure 5. Cumulative distribution of the microexplosion temperature and waiting time for case A.
case A, the diameter of water droplets are smaller for case A because of the agglomeration of water droplets. Although viscosity of emulsion increases with increasing the emulsifier content for both cases, there is little connection between viscosity and either size or the aggregation state of water droplets. 3.2. Microexplosion Occurrence. Because microexplosions occur over a broad range of droplet temperatures and waiting times, we used 25 samples to obtain a good distribution of these parameters. Figure 5 shows the cumulative distribution of the microexplosion temperature and waiting time for case A. A microexplosion is rarely observed when the emulsifier content is 2.0 wt %. Therefore, in this figure, the temperature and time are plotted for emulsifier contents of 0.5 and 1.0 wt %. Emulsion droplets are heated above the boiling point of water to the point of microexplosion. As the emulsifier content increases, both the microexplosion temperature and waiting time increase. Therefore, an increase in the emulsifier content would have a negative influence on the occurrence of microexplosion. There are limits of emulsifier contents on the microexplosion temperature for each emulsifier; the limits are about 1.0 wt % for case A and 2.0 wt % for case B, respectively. Because microexplosion occurs over a broad range of temperatures and waiting times, as previously mentioned, we characterize the microexplosion temperature and waiting time by the 50% microexplosion temperature and waiting time, below which half of the samples exhibit microexplosion. Figure 6 shows these value as a function of the emulsifier content for cases A and B. As the emulsifier content increases, the microexplosion temperature and waiting time
increase for both cases but microexplosions occur at a lower temperature in case B. The waiting time at the emulsifier content, Ye = 0.5 wt %, is shorter for case B but, at Ye = 1.0 wt %, is almost the same for the two cases. Thus, at Ye = 1.0 wt %, droplets tend to explode at lower temperature but with equal heating time. The microexplosion temperatures obtained in the experiment are around 200 °C, which is significantly larger than the temperature for heterogeneous nucleation. For example, for water at atmospheric pressure, the temperature for the heterogeneous temperature is typically 10-15 °C, which is commonly observed for the boiling of water from heated surfaces. Therefore, the effect of heterogeneous nucleation on the thermocouple wired will be negligible in the present experiment. 4. Discussion As mentioned previously, an increase in the emulsifier content results in the increase in both the microexplosion temperature and waiting time for its occurrence. However, the diameter of the water droplets remains almost constant. In this section, we discuss what properties of the water-fuel emulsion would affect the occurrence of microexplosion. Some experimental studies have reported that microexplosions are enhanced by increasing the water-droplet diameter.16,21 In our experiments, a smaller diameter of water (21) Marrone, N. J.; Kennedy, I. M.; Dryer, F. L. Combust. Sci. Technol. 1983, 33, 299–307.
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Figure 7. Variation of the calculated homogeneous bubble nucleation rate as a function of the temperature.
of the bubble nucleus. The superheat limit is taken as the temperature at which the nucleation rate exceeds 1012. Avedisian and Andres12 investigated the superheat limit of various water-fuel emulsions and found that the superheat limit is determined by the base oil volatility. In a waterin-n-hexadecane emulsion, the measured superheat limit is 263 °C, which is rather high compared to the microexplosion temperature measured in the present study. However, in reality, the temperature is expected to differ between the droplet center and surface in an emulsion droplet, and the difference is numerically predicted to be about 60 °C based on the present experimental conditions of the droplet diameter and thermal conductivity of n-hexadecane.10 The microexplosion model based on homogeneous bubble nucleation cannot properly explain the effect of the emulsifier type and content on the microexplosion temperature. In particular, for case B, the microexplosion temperature is considerably lower than the superheat limit, which is inconsistent with the calculated homogeneous bubble nucleation rates shown in Figure 7. In addition, the increase in the emulsifier contents may cause the rise in the superheat limit temperature of water for both cases A and B. The possible reason is that the superheat limit temperature would be affected by the bubble nucleation at the water/oil interface. However, the effect of emulsifier contents as well as emulsifier types is not clarified, which should be investigated quantitatively in future studies. Microexplosion occurrence is thought to be derived from homogeneous bubble nucleation in superheated water droplets. Avedisian and Andres12 developed a theoretical model for the rate of bubble nucleation to explain superheating in water-fuel emulsions. In their model, the rate of bubble nucleation is given as a function of vapor pressures and surface tensions for two liquid phases. In addition, homogeneous bubble nucleation is assumed to take place at the interface between two pure liquids, and there are three possibilities for the location of bubble formation, which depends upon the relative magnitudes of the interfacial tension of the two liquids and the surface tensions of each individual liquid. For our experiments, the surface tension of n-hexadecane is about 23 mN/m and the surface tension of water is 72 mN/m, while the interfacial tension between water and n-hexadecane is about 2-3 mN/m (case A) and 5-6 mN/ m (case B). Consequently, the interfacial tension of two liquids and the surface tensions of water and n-hexadecane satisfy the following equation, indicating that the bubble is a spherical
Figure 6. 50% microexplosion temperature and waiting time, each as a function of the emulsifier content for cases A and B.
droplets and lower microexplosion temperature are observed in case B. However, waiting times differ significantly for the two cases, and the measured droplet diameter is smaller for case A than that in case B. For case A, water droplets tend to aggregate in the emulsion and the aggregations easily coalesce into large water droplets, a significant number of which are probably larger than those measured. In addition, as the emulsifier content increases, the microexplosion temperature also increases, although the diameter of the water droplets remains almost constant for both cases. Therefore, it is hard to find a correlation between water-droplet size and microexplosion occurrence. Fu et al.15 developed a mathematical model that predicts the microexplosion strength of an emulsion fuel droplet, using the homogeneous bubble nucleation rate in the superheated water phase. The homogeneous bubble nucleation rate J is expressed as ! kTw 4πrcr 2 γ exp ð1Þ J ¼ nT 3kTw h where Tw is the water-phase temperature and rcr is the critical diameter of the bubble nuclei. Because the homogeneous bubble nucleation rate is highly sensitive to superheating and increases greatly near the superheat limit of the water phase, microexplosion is considered to occur when the waterphase temperature reaches the superheat limit. Figure 7 shows typical homogeneous bubble nucleation rates in the water phase as a function of the temperature, calculated by eq 1. The rate is sensitive to the phase temperature and the critical radius 1858
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form resting entirely within the n-hexadecane phase: σw g σo þ σ wo
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internal convection within the emulsion droplet. However, how the droplets coalescence will induce the homogeneous nucleation, and then the microexplosion is not still unknown. Therefore, interfacial behavior between water and fuel oils, including the thermal decomposition and stability of emulsifiers, is a subject for future investigation.
ð2Þ
In this case, the surface tension of n-hexadecane is substituted into the rate of bubble nucleation, which means that the bubble nucleation depends upon the surface tension of nhexadecane and the interfacial tension has no influence on bubble nucleation. The factors mentioned above are insufficient to explain the influence of emulsifier types and contents on the occurrence of microexplosions. Our experimental results demonstrate two features of emulsifiers: the microexplosion temperature increases with increasing the emulsifier content, and the microexplosion temperature is lower for the two-emulsifier (case B) than for the one-emulsifier (case A) emulsion, even though their waiting times are comparable. The second feature is also of interest; the rise in droplet temperature for case B is relatively slow, even though heating conditions for the two cases are almost the same. One possible explanation for the differing microexplosion characteristics is that the physical states of the emulsifiers change during heating. Changes in the physical state during heating are usually associated with thermal decomposition. The flash points of Solgen 40 and Noigen TDS-30 are 239 and 162 °C, respectively. Because emulsifier evaporation is not observed because of its low vapor pressure, thermal decomposition of an emulsifier should occur below its flash point. The thermal decomposition temperature of Solgen 40 may be higher than that of Noigen TDS-30, and this difference in temperature may explain the difference in the microexplosion temperature in cases A and B. In addition, thermal decomposition is generally endothermic; hence, the rise in temperature slows if the thermal decomposition of the emulsifiers occurs during heating. Thus, if the emulsifier content is sufficiently high, the thermal stability of the emulsifier would be maintained during the heating of the emulsion droplet, even though the thermal decomposition of the emulsifier occurs. The emulsion droplet is then further heated, resulting in a higher temperature for microexplosion. Although the effect of the emulsifier type and content on microexplosion occurrence is explained by thermal decomposition of the emulsifiers as discussed here, there are still some doubts. The change in thermal stability is significant and likely to reveal more on the effect of the emulsifier content. In addition, the thermal decomposition of emulsifiers may decrease the thermal stability, indicating the decrease in the interfacial tension between water and oil. This would lead to the size increase in the water droplets because of the coalescence of both the decreases in the interfacial tensions and
5. Summary and Conclusion We have investigated the properties of water-in-n-hexadecane emulsions in the present study. To determine the effect of emulsifiers on microexplosion occurrence, we measured the viscosity, diameter of dispersed water droplets, and interfacial tension between the water and oil phases for different emulsifer types and contents, as well as microexplosion behavior and droplet temperature during heating in an electrical furnace. Our major conclusions are summarized as follows. In the experiment, the water-in-n-hexadecane emulsion is produced by varying the emulsifier type and content. Two types of non-ionic surfactants with different HLB values were used as emulsifiers. Regardless of the emulsifier type, the viscosity of the oil phase remains almost constant with varying the emulsifier content, while the viscosity of the emulsion increases with increasing the emulsifier content. The viscosity of the emulsions is higher than that of the oil phase. Therefore, the increased viscosity of the emulsion is due to dispersion of water droplets. The saturation adsorption of the emulsifier is evaluated from the slope of the measured interfacial tensions between the water and oil phases. Because the emulsifier content is sufficiently higher than the saturation adsorption in the present conditions, droplet diameters are influenced very slightly by the emulsifier content. The interfacial tension is lower and the diameter of the water droplets is larger for the two-emulsifier than for the one-emulsifier emulsion. The difference in droplet size is due to droplet agglomeration in the one-emulsifier case. The microexplosion temperature and waiting time both increase with increasing the emulsifier content. Therefore, an increase in the emulsifier content would have a negative influence on the occurrence of a microexplosion. In contrast, the microexplosion temperature is lower for the two-emulsifier than for the one-emulsifier emulsion, which is likely due to the thermal decomposition of the emulsifiers; thermal decomposition differs for the two emulsifiers but occurs a bit below the microexplosion temperature. However, the effect of the emulsifier content on the microexplosion temperature is still uncertain, and thermal stability of the emulsifiers may have some influence and should be further investigated.
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