The Effect of AOT and Octanoic Acid on the Formation of Stable Water-in-diesel Microemulsion

Sodium bis(2-ethylhexyl)sulfosuccinate (AOT) and octanoic acid (OA) were used as surfactants to prepare water-in-diesel microemulsion. The effect of mixed surfactants ratio on the phase behavior of water-in-diesel microemulsion was investigated. The R0-T plot phase diagrams for the diesel/AOT and OA/water system with different surfactant ratios were constructed at 30-80 °C. The results indicate that the largest single phase region could be obtained when OA to AOT molar ratio was 1. The temperature had a significant influence on phase transformation behavior. The single phase separated into two immiscible phases with the increase of temperature when R0 value was above 10. Compared with applying AOT alone, mixing AOT with appropriate amount of OA is benefit to form smaller nanosized W/O droplets. The determination of particle size was performed to verify the phase transformation behavior, and the results were consistent with the phase diagrams.


Introduction
The increasing demand for fuel energy, continuing consumption of reserved petroleum and environmental pollution problems such as the emission of NO x , particulate matter (PM) and CO 2 due to the combustion of fuels have challenged the existing fuel energy [1][2][3]. Recently, more attention has been paid to the development of cleaner, alternative and sustainable fuels in order to meet higher emission standards and to reduce the dependence on pure fossil fuels.
The addition of water into diesel to form water-in-diesel microemulsion fuel exhibits several benefits. First of all, it has been proved that the vaporization of water and the mixing process can effectively reduce soot, PM and NO x emissions [4][5][6]. Another benefit is that water in the form of microstructure droplets can exert some positive influences on the combustion process of fuels. When microemulsion fuel is heated, the vaporization of water will cause continuous hydrocarbon phase to ''explode'' [7]. This phenomenon helps in improving fuel combustion process and combustion efficiency [8]. Furthermore, since water droplets are wrapped by diesel fuel, there is no direct contact between water and engine cylinder, which can eliminate the negative effect of water on lubricating oil contamination and engine wear [9]. There is no doubt that water-in-diesel microemulsion fuel is energy saving and environmental protection. Sharon  emulsion, water-in-diesel microemulsion and conventional diesel fuel [11]. Spray behavior studies demonstrated droplets of water-containing fuels penetrated further than droplets of regular diesel fuel. Combustion studies showed that water-in-diesel fuels yielded to flames with lower temperature and lowered soot concentrations than diesel fuel. Kannan and Anand reported that the emission characteristics like carbon monoxide (CO), carbon dioxide (CO 2 ), nitric oxide (NO) and smoke emissions for biodiesel and microemulsion fuels were lower than diesel fuel [12]. Bulent and Mudhafar investigated the effects of water concentration in a biodiesel nanoemulsion fuel on engine performance and exhaust emissions of a 4-cylinder diesel engine [13]. Biodiesel nanoemulsions containing 5%, 10% and 15% water were used for the engine tests. They concluded that increasing water concentrations in biodiesel nanoemulsions increased the engine brake specific fuel consumption and CO emissions. The rate of NO x reduction was greater than the rate of CO increase when the water concentration in biodiesel nanoemulsions increased from 10% to 15%. However, papers have been reported were mainly concerning combustion properties, spray behavior and reduction of emissions. In our previous studies, several kinds of nonionic surfactants were used to form water-in-diesel emulsions, whereas the stability of such emulsions was not satisfactory. Hence, we embarked on the study of phase behavior of water-in-diesel microemulsion. The objective of this research is to experimentally study the effect of AOT and octanoic acid on the formation of water-in-diesel microemulsion. AOT is a popular surfactant used for the formation of water-in-oil microemulsion droplets containing large amount of water under wide range of conditions (such as water content, temperature, solvent, electrolyte type and concentrations) [14][15][16]. OA is used as co-surfactant and its role is to enhance AOT surfactant property and compatibility with diesel at the interface.

Materials
Sodium bis(2-ethylhexyl)sulfosuccinate (AOT) with 98% purity was purchased from Fluka. Octanoic acid (OA) with 98% purity was purchased from Sigma-Aldrich. Double distilled de-ionized water was used in sample preparations. Diesel was purchased from local Petronas petrol station (national oil company of Malaysia).

Instrumentation
The water-in-diesel microemulsion samples were prepared in 3.5 mL vials sealed with caps. The phase behaviors of samples under different temperatures were investigated by changing temperature in the range of 30-80 °C in a Thermo Haake water bath, which accuracy is ±1.0 °C. Water content was measured by Karl Fisher Titrator (Mettler Toledo DL38). Dynamic Light scattering (DLS) experiments were performed to obtain the mean diameter of the water-in-diesel microemulsion droplets. The measurements were carried out by using a Zeta Sizer Nano Series (Malvern). All measurements were performed at the temperature of 40°C. Each sample was monitored for 3 times.

Water content and R 0 values
In order to confirm actual water content in the samples, Karl Fisher Titrator was employed to determine the water quantity, and then the water concentration could be calculated. After the water concentrations were figured out, R 0 values could be calculated according to formula 1. Results of R 0 values for all the samples with different surfactant ratios were listed in Table 1.

Construction of phase diagrams
The molar ratio of surfactant OA and AOT is an important factor that influences the phase behavior of microemulsion. The phase behavior was divided into several categories as shown in figure 1. The single phase seemed to be clear and transparent, while the phase behavior of two-phase or three-phase altered with variation of conditions, such as surfactant ratio, water content and temperature. To investigate the effect of surfactant ratios on the transformation of W/O microemulsion domain, R 0 -T plot phase diagrams of diesel/surfactants/water system for different surfactant molar ratios at temperature range of 30-80 °C were presented in figure 2.  (a) reveals that single phase separated into two immiscible phases with the increase of temperature for all cases. The highest R 0 value for single phase boundary was 20.50 when n OA /n AOT was 1 at 30 °C, and the corresponding water content was 10.8 %. Figure 2 (b) reveals the three-phase region (inside the boundary) transformation process with the increase of OA amount. For n OA /n AOT below 1, the three-phase region transformed from lower R 0 value to higher R 0 value. When n OA /n AOT was increased to 2, the phase diagram appeared the minimum three-phase region area. However, the three-phase region not only transformed to higher R 0 value but also expanded again when n OA /n AOT was 4. Furthermore, the single phase region disappeared at this ratio. The results illustrate that it is not suitable to form stable water-in-diesel microemulsion by applying AOT alone, and the addition of appropriate amount of OA is benefit to increase water content in water-in-diesel microemulsion. This is due to almost 70 % of the components in diesel are saturated linear alkanes with C 8 -C 16 [18][19], while the carbon number of AOT hydrophobic tail is six and the shape of AOT molecule is wedge. This may lead to an unfavorable interface between AOT and diesel oil. The presence of OA will effectively reduce the interfacial energy of water-oil surface and stabilize the water-oil surface [20]. Consequently, the optimum molar ratio of OA to AOT was 1 in this experiment.

Particle size and polydispersity index
The surfactant ratio has an important influence on the particle size, and then the particle size of the droplet determines the phase behavior of the samples. In order to identify the relation between particle size and phase behavior, the particle size of clear water-in-oil (W/O) region in samples has been measured. As shown in figure 3, the curves for different surfactant ratios were fluctuated with the increase of added water volume. The variation of particle size was consistent with phase behavior, the phase boundary of single phase and multiphase could be deduced. For example, the particle size at point 4 and point 5 in the case of n OA : n AOT = 1:1, corresponding to R 0 value of 17.34 and 20.50, were not linear increase. This tendency exactly indicates a phase transition, which also can be seen in figure  2. Point 4 was the phase boundary of single phase and multiphase, and point 5 was the phase boundary of two-phase and three-phase. The particle size at point 5 decreased to 21.7 nm, which was mainly due to the formation of a clear phase on the top of the sample (see figure 1 c). Figure 3 also reveals that the addition of excessive OA is more feasible to form larger size droplets. This may be due to AOT is interfacial active to increase the interfacial energy, which favors to form negative curvature. However, OA is not so interfacial active as AOT that it will lower the interfacial energy. With an overdose of OA is added to the mixture, there is an increase in the magnitude of the spontaneous curvature of the surfactant monolayer, and then the droplets swell in size [21].
In an emulsion-based solution, polydispersity index (PDI) is one of the key characteristics, as PDI contributes to physical stability and rheological properties of the solution [22]. PDI values of the samples with different surfactant molar ratios were listed in Table 2. The PDI value provides a measure of the narrowness of particle size distribution, with values ≤0.1 indicating a very narrow distribution [23]. That means it is more possible to obtain stable and monodisperse samples at lower PDI value. As shown in Table 2 amount. Moreover, the lowest PDI values for all the cases were exactly corresponded to particle size transition points in figure 3.

Figure 3.
Particle size of droplets in water-in-oil region as a function of water volume of addition. The samples were soaked in water bath with temperature of 40°C, and the measurement of particle size was done immediately after the sample was taken out.

Conclusions
In this work, the effect of mixed molar ratio of surfactant AOT and OA on the phase behavior of water-in-diesel microemulsion was tested. The results confirmed that nanosized water-in-diesel droplets formed by using the mixture of AOT and OA. The largest single phase region could be obtained when the molar ratio of OA to AOT was 1. The single phase separated into two immiscible phases with the increase of temperature when the R 0 value was above 10. It is not suitable to form homogeneous and stable water-in-diesel microemulsion by applying AOT alone. The addition of appropriate amount of OA is benefit to form clear and transparent microemulsion with smaller nanosized droplets. The measurement of particle size and PDI was performed to verify the phase transformation behavior, and the results were consistent with the phase diagrams.

Acknowledgments
Heilongjiang Academy of Sciences in China supported this work. We also gratefully acknowledge the guidance of Prof. Misni Misran in Malaya.

References
[1] Sheehan J, Cambreco V, Duffield J, Garboski M and Shapouri H 1998 A report by US Department of Agriculture and Energy 1-35