The Formation of Rhamnolipid-Based Water-Containing Castor Oil

Article pubs.acs.org/EF

The Formation of Rhamnolipid-Based Water-Containing Castor Oil/ Diesel Microemulsions and Their Potentiality as Green Fuels Ren Zhu,†,‡ Jie Liang,*,†,‡ Xing-zhong Yuan,*,†,‡ Le-le Wang,†,‡ Li-jian Leng,†,‡ Hui Li,§ Hua-jun Huang,∇ Xue-Li Wang,†,‡ Shan-Xing Li,†,‡ and Guang-ming Zeng†,‡ †

College of Environmental Science and Engineering, Hunan University, Changsha 410082, People’s Republic of China Key Laboratory of Environment Biology and Pollution Control (Hunan University), Ministry of Education, Changsha 410082, People’s Republic of China § Institute of Bio-energy, Hunan Academy of Forestry, Changsha 410004, People’s Republic of China ∇ School of Land Resources and Environment, Jiangxi Agricultural University, Nanchang 330045, People’s Republic of China ‡

ABSTRACT: The utilization of vegetable oils as a source of renewable fuels has attracted much attention. However, the high viscosity of vegetable oils limits their long-term application. The microemulsion technique of vegetable oils has the advantages of viscosity reduction and environment-friendly properties. In this study, the phase behavior of the microemulsion and the solubilization mechanism of water and castor oil in diesel were researched to evaluate the solubilization capacity of water and castor oil in diesel under given conditions. The proper concentration of rhamnolipid (RL) was 50 g/L. N-octanol was certified as an ideal co-surfactant with the optimal co-surfactant/surfactant (C/S) mass ratio (w/w) of 0.60. The optimum castor oil/diesel (V/D) volume ratio (v/v) was 0.18. Moreover, fuel properties of water-containing castor oil/diesel (WCD) microemulsion were identified, including density, dynamic viscosity, cloud point, pour point, water content, corrosivity, heating value, and elemental composition. The thermal and storage stability of WCD microemulsion were also conducted. Compared with castor oil, WCD microemulsion has lower viscosity, which presents similar fuel characteristics as diesel.

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INTRODUCTION In recent years, concerns over energy shortages and environmental pollution from fossil fuel have attracted more focus on the development of renewable biodiesels. Vegetable oils are considered as a suitable candidate for biodiesels production, because of their similar properties with diesel.1,2 In addition, vegetable oils induce lower pollutant (sulfur, polyaromatic hydrocarbons (PAHs)) emissions and a higher flash point, which meet the requirements of environmental and firefighting issues.3 A variety of vegetable oils have been evaluated as potential renewable fuel source, including castor, sunflower, soybean, cottonseed, rapeseed, algae, and peanut oils.1,2,4,5 Castor oil is widely used in the production of biodiesel. Mejiá et al. and Cneicao et al.6,7 used two methods (mixing castor oil with diesel and thermal oxidative degradation of castor oil) to convert castor oil into biodiesel, which has shown good fuel properties (good stability and low viscosity). Triacylglycerides are the major compositions of castor oil, containing more than 86% ricinoleic acid.8 Castor oil has good storage stability, as it has low oxidative rancidity property. Meanwhile, castor oil is named as a typical nondrying oil and has high pour point and flash point.9 However, bceause of the high viscosity of castor oil, longterm use of castor oil in direct-injection diesel engines leads to injector coking and ring sticking.10,11 Many techniques have been explored for generating lower viscosity biodiesels. Those techniques vary considerably in procedure complexity and product quality, as follows: simple dilution by mixing with diesel, pyrolysis or transesterification, and modification.10−13 In the present work, the diesel-based water/oil (w/o) (reverse © 2014 American Chemical Society

micelle) system was utilized to dissolve the castor oil in order to produce the water-containing castor oil/diesel (WCD) microemulsion. Microemulsions are thermodynamically stable systems that contain water and an oil phase. Microemulsions can exist in four forms as the well-known Winsor Type microemulsion phases, including Types I, II, III, and IV. The Type I (o/w) microemulsion solubilizes oil into spherical and normal micelles within a continuous water phase. The Type II (w/o) microemulsion solubilizes water in reverse micelles, which occurs in the oil phase.14 The Type III (middle phase) microemulsion exhibits three phases: excess oil and water phases in equilibrium with a bicontinuous phase in which lamellar micelles are formed.15 In the Type IV microemulsion (a single phase), in a middle phase microemulsion, the increment of biosurfactant concentration results in the middle phase volume increasing until both the oil and water coexist.16 The proposed mechanism of the reverse micelle microemulsion is based on the “like dissolves like” principle.17 To the Winsor Type II microemulsion, oil was used as the continuous phase and water was used as the inner core of the micelles simultaneously. Biosurfactant (rhamnoilpid) was adopted to realize our goal of constructing a surfactant-based system and also to demonstrate that if we can achieve our goal with the restricted selection of biosurfactants, it will be even easier to come into Received: June 12, 2014 Revised: August 22, 2014 Published: August 24, 2014 5864

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surfactant/surfactant (C/S) ratio (1:1). After then, diesel and RLcosurfactant blends were mixed with a total mass of 10 g at different mass ratios. The water was quantitatively injected into the mixtures until the solution became turbid. (2) The method based on the electric conductivity variation was performed using 10 mL of diesel and 1 g of RL−co-surfactant (1:1) blends. The concentration of water was gradually increased. The phase behavior was determined by measuring the electric conductivity variation of each phase in the test tube. The electric conductivity variation was determined by using a conductivity meter (Model DDSJ308, Biocotek, China). Water-Containing Castor Oil/Diesel Microemulsion Preparation. Microemulsion was prepared on a mass basis for the biosurfactant and co-surfactant and on a volumetric basis for the diesel and castor oil. First, RL was dissolved in diesel with different surfactant mass concentrations by ultrasonication. Co-surfactant (nalkanol) with a given C/S ratio and excessive water were injected afterward. After magnetic stirring at 1000 rpm for 15 min, the dieselbased w/o microemulsion was formulated. Second, castor oil and the above diesel-based w/o microemulsion were mixed in different V/D ratio with a total volume of 50 mL in thermostat water bath. Third, a high-speed centrifugal (5500 rpm centrifuge, 6 min) was used to accelerate the separation of mixtures. The mixtures then were divided into two phases: the supernatant liquid is WCD microemulsion, and the substratum for the turbid liquid was the excessive water and impurities. The parameter δ (%) was defined as the water content of the WCD microemulsion and E (reported in terms of L/g), which represents the RL efficiency, was defined as the volume of water dissolved in per mass RL. Water-Containing Castor Oil/Diesel Microemulsion Characterization. Elemental analyses were performed using a Vario EL III elemental analyzer (Elementar Analysen Systeme GmbH, Germany). The specific method of this study used to analyze C, H, N, and S elements was JY/T 017-1996 (“General Rules for Elemental Analyzer”, State Education Committee of PR China).13 Dynamic light-scattering study was performed using a nanometer particle size analyzer (Zetasizer Nano ZEN3600, Malvern, U.K.). Viscosity of WCD microemulsion was measured using a dynamic viscosity analyzer at 30 °C (SNB-2, China). Higher heating value was conducted using a calorimeter (SDACM500, China). The cloud point and pour point were determined by observing the samples to thicken and become cloudy at cold temperature.14 The water content was measured by Karl Fischer method (KF-1A, China). The corrosivity was conducted with copper strip corrosion test (CSCT) according to ASTM Standard Test Methods D 130,24 which was based on the discoloration of a standard copper strip immersed into a sample at 100 °C for 3 h (SYA-5096, China). Thermal Stability. The experiments were conducted using thermogravimetric analysis (TGA). TGA was performed in an integrated thermogravimetric analyzer (TG/DTA 7300, Japan). Approximately 25−45 mg samples were placed in the thermogravimetric analyzer’s crucible. The thermal analysis was conducted at a heating rate of 10 °C/min from 35 °C to 700 °C in a dry nitrogen atmosphere of 60 mL/min. The method can be adopted to analyze the thermal behavior of biodiesel.25 The kinetic analysis used for the thermal conversion of the oil is similar to that reported by Torrene and Galan26 and Jaber and Probert.27 Equation 1 was used to evaluate the kinetic parameters of the biodiesel samples:

effect with the abundant choice of synthetic surfactants. Biosurfactants have superior properties, relative to synthetic surfactants for specific applications, because of their biodegradability, structural diversity, and biocompatibility, relative to synthetic surfactants.14 Rhamnoilpid (RL) is a green and environmental additive, which has the advantages of no secondary pollution. Several studies have been carried out to evaluate the characteristics of vegetable oils/diesel microemulsion. Attaphong1 developed an approach to build stable vegetable oils/ diesel microemulsion with carboxylate-based extended surfactants. Fuel properties (such as stability, viscosity, heating values, and corrosivity) of those microemulsion systems were improved obviously compared with crude vegetable oils. Kibbey11 investigated the microemulsification of vegetable oils (canola oil and algae oil) in diesel phase with oleylamine surfactant. It was demonstrated that the viscosity of vegetable oils was reduced by microemulsification. It was reported by several researchers that the microwater-indiesel microemulsion had positive effects on reducing the combustion temperature and harmful emissions from diesel engines.18−20 As used in the 1−1.5 wt % water-containing fuelin-diesel engine, the indicated specific fuel consumption (ISFC) was reduced.19,21 Those can be due to the microexplosion of the w/o microemulsion, which caused fuel atomization. However, the high water content fuel would affect the engine durability, resulting in an increase in dynamic viscosity and a reduction in the fuel’s heating value.18 The microemulsion system can reduce the particle size of the micelles. The smaller the size of the micelles, the smaller the water content that can be obtained.22 The objective of this work was (i) to solubilize castor oil into diesel via microemulsion techniques and then (ii) to evaluate its potential as green fuels. That objective could be divided into five sections: (1) to study the phase behavior of WCD microemulsion; (2) to study the influencing parameters of solubilization water and castor oil in diesel; (3) to analyze the properties of WCD microemulsion as fuel, including heating value, elemental composition, viscosity etc.; (4) to discuss the thermal stability and storage stability of the WCD microemulsion with a disparate castor oil/diesel (V/D) ratio; and (5) to explain the mechanism of solubilizing water and castor oil into diesel by microemulsion technique.

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EXPERIMENTAL DETAILS

A series of normal alcohols (butanol, pentanol, hexanol, heptanol, and octanol) were used as a co-surfactant. Castor oil was provided by Sigma−Aldrich. Diesel (0#) was provided by a local petrol station in Hunan Province, China. Water was ultrapure water. Unless otherwise stated, chemicals were analytically pure. Rhamnolipid (RL) was provided by Sigma−Aldrich with 90 wt % active and average molecular weight of 577. The critical micelle concentration (CMC) is defined as the concentration of surfactant in the bulk when micelles start to form. A fluorescent probe method has provided one of the most popular means for measuring CMC historically.23 With the use of the fluorescent probe method (FluoroMax-4, Japan), it could be derived that the CMC of RL in diesel solution was 10 g/L. This observation means that diesel microemulsion would form spontaneously when the concentration of RL was above 10 g/L. Phase Behavior. In order to study the phase behavior of microemulsion, two different methods were used: (1) The principles of the pseudo-ternary phase diagram, representing a three-component system, were used. Initially, cosurfactant and biosurfactant (RL) were blended at a fixed co-

⎡ ART 2 ⎛ E 2RT ⎞⎤ ln[− ln(1 − x)] = ln⎢ ⎜1 − ⎟⎥ − a ⎢⎣ BEa ⎝ Ea ⎠⎥⎦ RT

(1)

The plot of ln[−ln(1 − x)] vs 1/T should give a straight line with slope −Ea/R, from which the activation energy Ea can be calculated.28 Long-Term Storage Stability. All the samples (50 mL each) were stored at ambient temperature (25−35 °C) for three months in glass bottles with a capacity of 100 mL and semiclosed conditions. The 5865

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samples were not stirred at intervals. The variable of dynamic viscosity was determined every 10 days.

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RESULTS AND DISCUSSION Before the results of the study have been presented, all the samples have been simply determined for the evidence of the water-in-oil microemulsion structure by the dynamic light scattering measurement. All experiments were performed in triplicate. Phase Behavior Study. The Winsor Type II microemulsion was formulated by solubilizing the hydrophilic component (water) into reverse micelles dispersed in the oil (diesel) phase. Figure 1 shows the pseudo-ternary phase

Figure 2. Phase behavior of the diesel, water, and amphiphile systems, based on the variation in electric conductivity.

Figure 1. Pseudo-ternary phase diagrams of the diesel, water, and amphiphile systems. The Winsor Type II microemulsion is denoted by the roman numeral II; other Winsor Type microemulsions are denoted by roman numeral X.

diagram of the diesel-based microemulsion systems. In that system, the Winsor Type II microemulsion was formed. The maximum solubilization of water in diesel (δmax) was increased steadily as the surfactant−co-surfactant concentration increased. The δmax value exceeded 1.38% with the mass ratio of surfactant−co-surfactant/diesel up to 0.25, suggesting that the water content of microemulsion is able to produce microexplosions with RL as the surfactant.19 The following transition occurred in Figure 2: Winsor Type II to Winsor Type III to Winsor Type I. The conductivity of microemulsion is mainly composed of the conductivity of the microemulsion droplets (dispersion phase) and the continuous ion-conductive phase.29 The conductive mechanism in the Winsor Type II system can be due to the swimming and collisions of droplet particles in electric field, and the jumping of surfactant molecule in the boundary layer. Those two activities improve the conductivity of microemulsion.30 At the Winsor Type II to Type III critical point (with the comparison between Figures 1 and 2), the same δmax was found. Figures 1 and 2 showed that RL enhanced the stability of the interfacial film and appropriate water was solubilized in diesel. Effect of Surfactant Concentration. The effect of surfactant concentration on water solubilization capacity was displayed in Figure 3. Surfactant concentration is one of the most important factors that affect the construction of the waterin-oil microemulsion. In Figure 3, as the concentration of RL increasing, parameter δ was increased continuously, while E had

Figure 3. Effect of rhamnolipid concentration on the water solubilization capacity. The concentration of co-surfactant (n-octyl) and the initial V/D ratio were fixed at 20 g/L and 0.15, respectively. Five different RL concentration gradients were investigated: 10, 30, 50, 70 and 90 g/L.

an opposite trend. In addition, when the concentration of RL exceeds 50 g/L, the δ is >1%. RL is constituted by monorhamnolipids and dirhamnolipids. Both of them contain hydrophilic and lipophilic linkers. In the phase behavior study, it was hypothesized that the microemulsion was a solution of swollen (reverse) micelles at lower volume fractions of solubilized polar substances (or water).31 Much research had been carried out based on this structure.32,33 Each single particle of microemulsion could be regarded as a micelle, which was divided into spherical micelle and reverse micelle (Figure 4).34 As the concentration of RL increased, the hydrophilic linker and superficial area of the hydrophilic core was increased. Therefore, more water can be solubilized. The molecular structure of surfactant affects the surfactant molecular and interaction packing density.35 The more bulky the structure of a sugar-based surfactant, the larger the areas per headgroup and the lower the solubilization enhancement. As a result, a greater amount of surfactant was required to solubilize the polar phase in the oil phase.36 RL has larger size of the 5866

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single-phase microemulsion formed. Using n-octanol as the cosurfactant, the value of δ was greater than that of n-heptanol. Linear alcohols with different carbon chain lengths had different effects on the solubilization capacity of castor oil. The standard Gibbs free energy of transfer (ΔGt) of alcohols from the continuous oil phase to an interfacial region was decreased as the chain length of linear alcohols increased.38 The smaller the value of ΔGt, the greater the spontaneity that the microemulsion could be formed. The correlation between ΔGt and the carbon number of linear alcohols has been investigated previously in other microemulsion systems.39,40 In addition, linear alcohols with different carbon chain lengths had different effects on the surfactant and the superficial area of the palisades region.34 It was suggested that the WCD microemulsion was formed almost spontaneously in n-octanol. Effect of C/S Ratio. The effect of C/S ratio on castor oil solubilization capacity was displayed in Figure 6. When the C/S

Figure 4. Proposed mechanism for the role of co-surfactant in increased solubilization capacity of diesel microemulsion. The w/o reverse micelles system was further divided to three parts: an oilcontinuous phase, a palisades layer, and a hydrophilic core.

hydrophilic linker, compared with synthetic surfactants.37 When the size of hydrophilic core is fixed, the hydrophilic linker of surfactant would take up more volume than that of a synthetic surfactant. Therefore, biosurfactant is used as a surfactant to form microemulsions, because its δ value is smaller than that of a synthetic surfactant. As the concentration of RL increased, the occupancy volume of surfactant was enhanced, and, thus, the E value of RL was reduced. On the basis of the above observations, an appropriate concentration of RL (50 g/ L) was suggested. Effect of Co-surfactant Type. The effect of co-surfactant type on castor oil solubilization capacity was displayed in Figure 5. When n-butanol, n-pentanol, and n-hexanol were used as co-

Figure 6. Effect of C/S ratio on the water and castor oil solubilization capacity. The RL concentration was fixed at 50 g/L and the initial V/D ratio was set at 0.15. N-octanol to Rh ratios varied from 0 to 1.0 (w/ w).

ratio was 0 and 0.2, two separate emulsion phases formed. However, with a C/S ratio of 0.4−1.0, all the castor oil was solubilized into the reverse micelle microemulsion. A singlephase microemulsion was formed. The value of δ increased as the C/S ratio increased, from δ = 0.4 to δ = 0.6. However, the value of δ decreased as the C/S ratio increasing from δ = 0.6 to δ = 1.0. In this study, a WCD microemulsion (w/o reverse micelles) formed with RL as the surfactant, and castor oil was solubilized into the oil phase and/or into the hydrophilic core (water pools) of the reverse micelles. It was well-defined that the location of a solubilized molecule into a micelle would be determined primarily by the molecule structure of the additives, relative to the structural components of the surfactant (Figure 7).41 Based on the properties of the additives and the theory of w/o reverse micelles system (Figure 4), the solubilization mechanism was proposed as below. In the w/o reverse micelle system, polar additives (water) were intimately associated with a hydrophilic micelle core (Figure 7a), while slightly polar additives such as alcohols and long-chain fatty acids were usually located in what was called the palisades layer (Figure

Figure 5. Effect of co-surfactant type on the water and castor oil solubilization capacity. The concentration of RL and V/D ratio were fixed at 50 g/L and 0.15, respectively. Five normal alcohols (n-butanol, n-pentanol, n-hexanol, n-heptanol, and n-octanol) were used as cosurfactants, and the C/S ratio was 0.4 (W/W).

surfactants, two separate emulsions were found. Some sections of castor oil was solubilized into diesel to form the water/castor oil/diesel emulsion layer, and a few sections of diesel may also be solubilized into castor oil to form the water/diesel/castor oil emulsion layer. In addition, as the carbon chain length of linear alcohols increased, the volume of the water/castor oil/diesel emulsion layer was also increased. However, with n-heptanol and n-octanol as co-surfactants, all of the castor oil was solubilized into the w/o microemulsion, and, consequently, a 5867

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Figure 7. Location for the solubilization of additives in the reverse micelles: (a) all of the solubilization of additives occurs in the hydrophilic core, (b) some the solubilization of additives occur in the hydrophilic core and some occur in the palisades layer, (c) all of the solubilization of additives occurs in the palisades layer, and (d) all of the solubilization of additives occurs in the oil-continuous phase.

7b). Except for the solubilization of additives in the hydrophilic micelle core and the core palisades boundary region, the additives may also be found entirely in the palisades region (Figure 7c) or on the surface of the micelles (Figure 7d). N-octanol is linear. The alcohol, which has medium chain length, can help the surfactant to reduce the interfacial tensions between the two immiscible phases and increase adsorption at the interface.41 Linear alcohol molecules are slightly polar materials, which could efficiently pack themselves between the larger biosurfactant chains at the palisades layer.34 RL is an anionic biosurfactant, because of the electrostatic repulsion that exists between the hydrophilic groups, which are not easy to associate into micelles. After adding a co-surfactant, the alcohol molecules could be inserted between the RL molecules. The distance between the hydrophilic groups was increased, while the electrostatic repulsion was reduced. As a result, the reverse micelles were formed. However, when the boundary of the micelles was saturated by alcohol, the excessive amounts of alcohol would be dissolved in the aqueous phase or the palisades layer. The RL molecules would be separated too far by excessive alcohol molecules. The palisades layer would become unstable and the water would become difficult to

solubilize. Based on an overall consideration of various factors, the C/S ratio of 0.6 was favorable. Effect of the Initial V/D Ratio. The viscosity in this study referred to dynamic viscosity and was measured at 30 °C. The effects of V/D ratio on the viscosity and the δ value of the WCD microemulsion are depicted in Figure 8. Note that castor oil was completely solubilized in the reverse micelle microemulsion of diesel where the V/D ratio was varied from 0 to 0.54, which is consistent with Attaphong’s work.36 While the V/ D ratio increased to 0.54, the viscosity increased to 23.6 mPa s. It is not suitable to use that type of diesel in existing diesel engines. Thus, the higher V/D ratio was unnecessary. As shown in Figure 8, the viscosity and δ varied dramatically at different V/D ratios. As the V/D ratio increased, the value of δ decreased. This may be due to the polar substances of castor oil dissolved in the hydrophilic core. The castor oil was composed of various acids and lipids, which contained the hydrophilic and hydrophobic groups (or polar groups and nonpolar groups). When castor oil and diesel were mixed directly, obvious delamination occurred. While the w/o (reverse micelles) system was used to solubilize castor oil, castor oil could be entirely solubilized to the system and form a 5868

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Table 1. Composition Content of Castor Oil, Diesel, and the WCD Microemulsion Elemental Analysis (wt %)

a

sample

C

H

Oa

N

castor oil diesel (0#) WCD microemulsionb

73.69 83.00 81.49

10.64 12.60 11.28

13.49 3.07 5.65

2.18 1.33 1.58

By difference. bWCD = water-containing castor oil/diesel.

doubled. The higher oxygen content can decrease the C2 species (C2H2, C2H3, C2H4, C2H5, and C2H6) interaction, and thus inhibit the formation of aromatic rings.42 In addition, the oxygen radicals and extra oxygen that are provided by the castor oil can be delivered to the thermal decomposition zone to oxidize the fuel and reduce the particulate matter (PM) formation.19 Fuel Properties. The fuel properties of castor oil, diesel, and WCD microemulsion were measured, including density, cloud point, pour point, dynamic viscosity, water content, corrosivity, and heating value. A summary of those measurements is shown in Table 2. The cloud point is the temperature at which fuel begins to thicken and become cloudy. The pour point is the temperature at which fuel begins to thicken and no longer can be poured. Both of them are cold properties of fuels. The cloud point and pour point of the WCD microemulsion were −4 and −15 °C, resepctively, which were close to those of diesel (−6 and −18 °C, respectively). The dynamic viscosity of the WCD microemulsion is 8.14 mPa s, which was significantly lower than that of castor oil (425.8 mPa s). According to the national standards of the People’s Republic of China (No. GB252− 2011), the dynamic viscosity of diesel (0#) should be 3.5−8.5 mPa s. The water content of the WCD microemulsion was 1.25%, which could reduce pollutant emissions and, thus, could improve energy efficiency via the microexplosion phenomenon.19 The results of the copper strip corrosion test (CSCT) indicated that the corrosivity of the WCD microemulsion was 1a, while the corrosivity of castor oil was 1b. The higher heating values (HHVs) of castor oil, diesel, and the WCD microemulsion were 36.62, 43.99, and 41.77 MJ/kg, respectively. Because of the solubilization of castor oil into diesel, the heating value of WCD microemulsion decreased slightly, compared with that of diesel. Compared to castor oil, the WCD microemulsion had more desirable fuel properties, which were similar to those of diesel. In addition, 100 mL of the WCD microemulsion under optimal conditions (RL concentration of 50 g/L, n-octanol as a cosurfactant, a C/S ratio of 0.60, and a V/D ratio of 0.18) was selected to calculate the expected production cost. Through comprehensive calculation, the expected production cost of WCD microemulsion was 8.73 ¥/L higher than that of the same amount diesel (7.59 ¥/L in Changsha City, Hunan Province of China). This is mainly due to the higher price of high-purity RL. As diesel production is gradually reduced, the price of diesel will increase substantially. However, vegetable oil and RL are renewable energy sources, so with mass production and improved craftsmanship, the prices of castor oil and RL will decline. So the cost of production of the WCD microemulsion will certainly be lower than the price of the same amount of diesel in the near future. The aforementioned results may indicate that the WCD microemulsion formed in this study has great potential to be utilized as a green fuel.

Figure 8. Effect of initial V/D ratio on the water solubilization capacity and viscosity of the WCD microemulsion. The concentration of Rh and C/S ratio were fixed at 50 g/L and 0.60, respectively. Initial V/D ratio varied from 0 to 0.54 (v/v) and n-octanol as a co-surfactant.

transparent and stable microemulsion. This indicate that the w/ o (reverse micelles) system constructed by the paper had a good compatibilization of hydrophilic and hydrophobic substances. In addition, the dynamic light scattering (DLS) measurement was used to measure the diameter of the reverse micelles. The diameters of reverse micelles (in the absence of castor oil) were in the range of 75−85 nm. However, when castor oil was solubilized, the diameters of the reverse micelles showed an increment as the V/D ratio increased. This was because castor oil and diesel could not dissolve into each other; the hydrophilic and hydrophobic substances of castor oil were dissolved in the reverse micelle system under the action of surfactant and co-surfactant. As a result, some was dissolved in reverse micelles internally (Figures 7a, 7b, and 7c) and some were dissolved on the outside (Figure 7d). Although the diameters of the reverse micelles increased, the w/o (reverse micelles) system remained the same and castor oil was really all melted into the reverse micelle microemulsion. The results showed that the model mentioned in Figures 4 and 7 are supported by the experimental results. The initial viscosity of castor oil is 428.5 mPa s. The viscosity would certainly increase when the V/D ratio increased. Fatty acids are the major components of castor oil, which have hydrophilic groups and lipophilic groups. In Figure 7, it can be inferred that the fatty acids may be located in the palisades layer and/or on the surface of the micelles. When excessive amounts of castor oil dissolved in the aqueous phase or the palisades layer, the RL molecules would be separated too far by excessive castor oil molecules. The palisades layer would become unstable, and, consequently, the viscosity values increase more quickly. Based on an overall consideration of various factors, the V/D ratio of 0.18 was favorable. Property Analysis. Properties of castor oil, diesel, and the WCD microemulsion were characterized and systematically compared. The examined WCD microemulsion was obtained under optimal conditions with an RL concentration of 50 g/L, n-octanol as a co-surfactant, a C/S ratio of 0.60 (w/w), and a V/D ratio of 0.18 (v/v). Elemental Analysis. It was indicated in Table 1 that elemental compositions of the WCD microemulsion were similar to those of diesel, except oxygen content. Compared to diesel, the oxygen content of the WCD microemulsion 5869

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Table 2. Properties of the Castor Oil, Diesel, and the WCD Microemulsion sample castor oil diesel (0#) WCD microemulsionb

density (g/cm3)

cloud point (°C)

pour point (°C)

dynamic viscosity @T = 30 °C (mPa s)

water content (wt %)

CSCTa

HHV (MJ/kg)

0.93 0.87 0.89

−6 −4

−7 −18 −15

428.50 4.88 8.14

0.00 0.00 1.25

1b 1a 1a

36.62 43.99 41.77

a A value of “1” denotes slight tarnish; a suffix of “a” denotes light orange color, almost the same as the freshly polished strip, and the suffix “b” denotes a dark orange color. bWCD = water-containing castor oil/diesel.

Thermal Stability. In order to identify the effect of castor oil content on the thermal stability of WCD microemulsion, thermogravimetric analysis (TGA) was carried out in this work. Thermogravimetric data was used to characterize the materials as well as investigate the thermodynamics and kinetics of the reactions and transitions.25 The direct Arrhenius plot method was used in TGA to assess thermal stability of fuels, which has been applied by many researchers.43,44 Equation 1 was used to determine the activation energy (Ea). The Arrhenius plot of the oil samples was presented in Figure 9, which was used to

Figure 10. Effect of V/D ratio and storage time on viscosity of WCD microemulsion. The tested samples were obtained under the Rh concentration of 50 g/L and a C/S ratio of 0.60, with different V/D ratios and n-octanol as a co-surfactant.

free fatty acid content, and UV absorption are observed.47 In addition, the viscosity has been found to increase with oxidation of unsaturated esters and with increasing degree of saturation. The changes of viscosity can be a good indicator to evaluate the long-term storage stability of the WCD microemulsion. After 90 days of storage, all of the oil samples were still clear liquid without precipitation. Figure 10 compared the variation in the viscosity of oil samples. In Figure 10, those variations in viscosity became more turbulent as the castor oil content increased. Therefore, the long-term storage stability of WCD microemulsion became worse as the castor oil content increased. In addition, the viscosity of all samples decreased slightly at the initial stage. This may be due to the influence of the temperature. Overall, the viscosity of the oil samples changed slightly from the initial stage to the end, which indicated that all of the oil samples had good storage stability.

Figure 9. Integral plot for different V/D ratios of the WCD microemulsion. The tested samples were obtained under a Rh concentration of 50 g/L and a C/S ratio of 0.60, with different V/D ratios and n-octanol as a co-surfactant.

calculate the kinetic parameters such as Ea and frequency factor. Figure 9 showed a linear relationship of ln[−ln(1 − x)] versus 1/T. According to eq 1, the Ea value could be calculated by the slope of the graphed lines, with the linear regression of the ordinate and abscissa parameters, respectively. Figure 9 compared the Ea value of the WCD microemulsion with different V/D ratios. The Ea values of the microemulsion without castor oil additive was 40.94 kJ/mol. As the V/D ratio increased, the reducing rate of Ea was decreased. When the V/ D ratio increased 0.07, the Ea value was only reduced by ∼1 kJ/ mol. Those results suggest that castor oil has a negative impact on the thermal stability of the WCD microemulsion. However, the negative impact is unconspicuous. Long-Term Storage Stability. Figure 10 showed the effect of castor oil content on the long-term stability of the WCD microemulsion. The resistance of biodiesel to oxidative degradation during storage is an important issue for the viability and sustainability of such alternative fuels.45,46 The viscosity of biodiesel changed dramatically during 90-day storage tests: significant increases in viscosity, peroxide value,

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CONCLUSION This study revealed the potential of water-containing castor oil/ diesel (WCD) microemulsion as a biodiesel for alternative of diesel. The following conclusions can be made. (1) The optimum conditions to form the WCD microemulsion were a rhamnolipid (RL) concentration of 50 g/L, n-octanol as a co-surfactant, a co-surfactant/ surfactant (C/S) ratio of 0.60 (w/w), and a castor oil/ diesel (V/D) volume ratio of 0.18 (v/v). (2) The value of δ increased as the RL content increased. The WCD microemulsion can maintain its stability with small amounts of water (1.2−1.5 vol %). (3) The concentration of n-octanol cannot be overdosed. Excessive amounts of n-octanol would enlarge the 5870

dx.doi.org/10.1021/ef501307e | Energy Fuels 2014, 28, 5864−5871

Energy & Fuels

Article

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distance between RL molecules, resulting in an unstable palisades layer. (4) The castor oil molecules may be located in the palisades layer and/or on the surface of the micelles. Poor fuel properties were obtained at higher V/D ratios. (5) The method of this study provides a more short-term and environmentally benign process to improve castor oil and can enhance energy efficiency or even reduce the pollutant emissions.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86-731-88821697. Fax: +86-731-88823701. E-mail: [email protected] (J. Liang). *Tel.: +86-731-88664182. Fax: +86-731-88823701. E-mail: [email protected] (X. Z. Yuan). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support, which provided by the National Natural Science Foundation of China (No. 21276069), the Natural Science Foundation of Hunan Province, China (No. 13JJ4118), Key Laboratory of Renewable Energy Chinese Academy of Sciences (No. y407k91001), and the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20120161130002).

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dx.doi.org/10.1021/ef501307e | Energy Fuels 2014, 28, 5864−5871