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Energy & Fuels 2007, 21, 222-226
Light-Scattering Investigation on Microemulsion Formation in Mixtures of Diesel Oil (or Hydrocarbons) + Ethanol + Additives Evandro J. Silva,†,§ M. Elisabete D. Zaniquelli,‡ and Watson Loh*,† Institute of Chemistry, UniVersidade Estadual de Campinas (UNICAMP), Caixa Postal 6154, 13084-970, Campinas, SP, Brazil, and Department of Chemistry, Faculdade de Filosofia, Cieˆ ncias e Letras de Ribeira˜ o Preto, UniVersidade de Sa˜ o Paulo, Ribeira˜ o Preto, SP, Brazil ReceiVed June 9, 2006. ReVised Manuscript ReceiVed September 20, 2006
Ethanol and diesel oil mixtures are potential candidates as fuels which display advantages such as a reduced emission of particulates and the use of a renewable fuel (ethanol). However, their use is hampered because of limited miscibility, especially at lower temperatures. This problem can be overcome with the use of certain additives, which produce increased miscibility. This work investigates the role of these additives in this mixing process, with particular emphasis on assessing earlier propositions that microemulsions are formed in such mixtures. Results obtained from light-scattering measurements provide the first direct evidence for microemulsion formation both in diesel oil and ethanol and in synthetic diesel (a mixture of hydrocarbons that mimic diesel oil properties) and ethanol and additives. The effects of the additive type and concentration, temperature, and volume fraction of the ethanol on these microemulsion droplets’ radii are presented, and their trends were found to follow those established for water-in-oil microemulsion systems.
Introduction Diesel fuel engines while durable and efficient in comparison with gasoline engines are also known for their excessive production of soot, particles, and nitrogen oxides and are currently the target of increasingly stringent environmental regulations, worldwide. The reduction of emissions from engines using diesel fuels, a priority in many urban and industrial environments, has motivated rapid developments in emission reduction science and technology. The addition of oxygenates to diesel oil provides the oxygen necessary for the formation of carbon oxides in place of carbonrich particles and thereby reduces the critical mass of carbon available for nucleation into particles and could result in the significant reduction of particulate matter emissions.1 The addition of alcohols, particularly of ethanol, constitutes an important alternative considering its suitable chemical characteristics; its large-scale production, at a relatively low cost; and its high volatility. Moreover, ethanol is one of the most important fuels from renewable sources available nowadays. However, these ethanol-diesel mixtures may present phase separation at certain compositions or as the temperature is reduced,2,3forming an ethanol-rich bottom phase that could stall the engine, preventing its proper performance. This problem may be overcome with the use of certain additives, which are capable of increasing their miscibility, ensuring homogeneous * Corresponding author. Phone: + 55 19 3521 3148. Fax: + 55 19 3521 3023. E-mail:
[email protected]. † Universidade Estadual de Campinas. ‡ Universidade de Sa ˜ o Paulo. § Present address: Department of Chemistry, Universidade Federal do Mato Grosso, Cuiaba´, MT, Brazil. (1) Choi, C. Y.; Reitz, R. D. Fuel 1999, 78, 1303. Song, J.; Cheenkachorn, K.; Wang, J.; Perez, J.; Boehman, A. L. Energy Fuels 2002, 16, 294. McCormick, R. L.; Ross, J. D.; Grabosky, M. S. EnViron. Sci. Technol. 1997, 31, 1144. (2) Gerdes, K. R.; Suppes, G. J. Ind. Eng. Chem. Res. 2001, 40, 949. (3) de Menezes, E. W.; da Silva, R.; Catalun˜a, R.; Ortega, R. J. C. Fuel 2006, 85, 815.
solutions over a wide temperature range, as has been shown in a series of reports.3-6 Most of these reports ascribe this additive effect to the formation of microemulsions where ethanol-rich droplets are stabilized by a surfactant-like additive, forming a macroscopically transparent and thermodynamically stable system.5-7 Despite its great importance, scientific investigations on these mixtures are still scarce in the open literature, and to the best of our knowledge, no direct confirmation of microemulsion formation has been reported yet. The knowledge of the exact mode of action of these additives is vital information for the development of new formulations capable of displaying greater efficiency, preferably at lower costs and with a reduced impact on fuel performance and on the environment. The present investigation was prompted by this relative lack of information and is focused on applying light-scattering measurements to verify the presence of microemulsions in such fuel mixtures, as well as to follow how their properties change with the use of different additives, with the system composition, and with the temperature. These investigations were carried out using basically two additives and their mixtures, which have been previously identified as the ones with the best capacities for increasing the miscibility between diesel oil and ethanol.8 In parallel, light-scattering measurements were also conducted using a mixture of three hydrocarbons that is capable of mimicking the miscibility behavior of diesel oil with ethanol (4) de Caro, S.; Mouloungui, Z.; Vaitlingom, G.; Berge, J. C. Fuel 2001, 80, 565. (5) Waterland, L. R.; Venkatesh, S.; Unanasch, S. Safety and Performance Assessment of Ethanol/Diesel Blends (E-Diesel). http://www.nrel.gov/docs/ fy03osti/34817.pdf (accessed June 2006). (6) Fernando, S.; Hanna, M. Trans. ASAE 2005, 48, 903. Fernando, S.; Hanna, M. Energy Fuels 2004, 18, 1695 (7) Schwab, A. W.; Fattore, R. S.; Pryde, E. H. J. Dispersion Sci. Technol. 1982, 3, 45 (8) Silva, E. J.; Caetano, T.; Mohamed, R. S.; Loh, W. VI Iberoamerican Conference on Phase Equilı´bria for Process Design, Foz do Iguac¸ u, Brazil, 2002.
10.1021/ef060264s CCC: $37.00 © 2007 American Chemical Society Published on Web 11/01/2006
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(and, for this reason, is referred to as synthetic diesel). This mixture presents two important advantages over commercial samples of diesel oil: it is colorless (displaying no absorption in the visible region of the spectrum, hence with no interference in the light-scattering measurements) and has a simple and welldefined chemical composition. Experimental Section Samples of commercial diesel oils were purchased at gas stations in the city of Campinas, Brazil. They were kept in tightly closed vessels and in the dark until use (typically within a few weeks of purchase). Because of their different chemical compositions, some differences were observed when comparing different samples (as will be discussed later), but all of the light-scattering measurements that are used for comparative analyses refer to the same sample. Two samples were used for light-scattering measurements, referred to as samples 1 and 2, which display, respectively, the following physicochemical properties at 25 °C: a density of 0.8473 and 0.8585 g cm-3 (measured in a digital densimeter model DMA 4500, Anton Paar, Austria) and a viscosity of 3.17 and 4.36 cP (determined using an Ostwald capillary viscometer inserted in a temperaturecontrolled water bath within (0.1 °C). Absolute ethanol, p.a., from Synth, was kept under molecular sieves. The organic solvents used to prepare the mixture called synthetic diesel were tetradecane, from Sigma, 99% purity; cyclohexane, from Acros, p.a.; p-xylene, from Acros, 99% pure; and Nujol, from Schering-Plough, of pharmaceutical grade. The additives employed were dodecylamine, 99+ % purity, from SigmaAldrich and oleic acid, p.a., from Synth. Mixtures were prepared by weight (except for ethanol content, which was expressed per volume). For the determination of miscibility curves, samples were kept in close vessels, under N2, inside a water compartment whose temperature could be controlled either by heating or by cooling with ice. These samples were continuously stirred and heated above their phase transition temperatures to become homogeneous and then cooled at ca. 1 degree/min until a visual detection of turbidity could be made. Measurements were repeated, at least, four times for each mixture, using three independent samples. The overall reproducibility of the measured transition temperatures was within 1°. This procedure follows the protocol suggested by ASTM (D2500).9 Prior to the light-scattering measurements, all solvents were filtered through 0.45 µm Millipore filters, and after mixture with the additives, they were centrifuged for a long time. Light-scattering measurements were performed using two instruments. Most of the measurements were obtained using a Zetasizer 3000 HAS device, from Malvern Inc., using a laser source of HeNe, at 633 nm with 10 mW of power. Measurements were made using an angle of 90°, with the instrument temperature control. Samples viscosities were previously determined by using appropriate capillary viscometers, in a temperature-controlled water bath (as described above). The addition of ethanol, within the studied range, caused a linear decrease in viscosity. Mixtures containing additives (oleic acid, dodecylamine, or both) displayed greater viscosity, typically with increases under 5% for samples containing 2 wt % additives and of ca. 10% in the presence of 4 wt % additives. In a general way, colored samples such as those containing commercial diesel oil may present absorbance problems in scattering experiments; however, they were minimized by the lower-power source used in these measurements. Several samples show a very small average size that is difficult to detect with equipment with a low-power source. Nevertheless, the obtained results show a good reproducibility, and the criteria of count rate, signal-to-background ratio, and in-range percentage were adequate. In addition, the samples were quite monodisperse, and the intensity and volume averages are practically co-incident. Polydispersity values for all (9) Standard Test Methods for Cloud Point of Petroleum Products; American Society for Testing and Materials D2500-02: West Conshohocken, PA, 2003.
Figure 1. Miscibility curves for two samples of commercial diesel oil and a synthetic diesel mixture (see text for description) with ethanol. Symbols: (0) commercial diesel oil #1, (*) commercial diesel oil #2, (O) synthetic diesel, and (2 diesel oil #1 + 2 wt % dodecylamine + 2 wt % oleic acid.
of the samples measured were always lower than 0.1 (meaning 10% dispersion around the average value quoted). Another favorable point was the reproducibility obtained using other different setups, as will be discussed later. Good reproducibility was also verified for measurements on the same sample within the same day, and for different preparations of the same composition. In order to validate results from this equipment, some measurements were performed using a more powerful instrument, at Lund University, Sweden, an ALV/DLS/SDS-5000F, CCF-8F compact goniometer system from ALV-GmbH, Germany. Its light source is a CW diode-pumped Nd:YAG solid-state Compass-DPSS laser with symmetrizer from COHERENT Inc., U. S. A., operating at 532 nm with a fixed output power of 400 mW. More details on this setup and on the data analyses performed may be found elsewhere.10 With this instrument, measurements on colored samples containing commercial diesel oil were not possible because of much greater effects of light absorption and emission by the sample. This instrument will be referred to as the ALV equipment, and only a few measurements were taken with this instrument, for the sake of comparison with the more detailed investigation conducted with the Malvern equipment.
Results and Discussion Light-Scattering Investigation on Microemulsion Formation. The miscibility behavior for ethanol and diesel oil mixtures is represented in Figure 1. A mixture of hydrocarbons containing 73 wt % tetradecane, 7 wt % nujol, 15 wt % cyclohexane, and 5 wt % p-xylene displays a very similar miscibility to that of ethanol and also contains compounds representing the main classes of hydrocarbons reported in diesel oils: aliphatic, naphtenic, and aromatic compounds, in compositions close to the average ones found in typical diesel oils.11 This mixture presents a series of advantages with respect to diesel oil, namely, its defined composition and, for studies using light-scattering techniques, such as the present one, its optical transparency in the visible region of the spectrum. Its defined composition is an advantage in terms of repeatability and comparison among experiments and also allows its use in thermodynamic modeling procedures that require the chemical definition of a mixture. For these advantages and considering its capability of mimicking the miscibility behavior of diesel oils with ethanol, we called it (10) Jansson, J.; Schille´n, K.; Olofsson, G.; da Silva, R. C.; Loh, W. J. Phys. Chem. B 2004, 108, 82. (11) Elvers, B.; Hawkins, S.; Schulz, G. Ullmann’s Encyclopedia of Industrial Chemistry, 5th ed.; VCH Publishers: Weinheim, Germany, 1990; Vol. A16.
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Table 1. Light-Scattering Measurements in Selected Mixtures of Ethanol-Diesel with and without Additivesa PTT (°C) % EtOH (v/v)
no additive
10 15 20
15.0 24.8 28.5
radius (nm) with additive 5.0b 6.0b 11.5b
7.2c 8.5c 11.5c
temperature (°C)
no additive
25 25/30 25/40
nd nd/nd */nd
with additive ndb ndb/ndb 1.2b/ 0.6b
ndc ndc/ndc 1.1c/0.7c
a PTT (phase transition temperature) indicates the temperature above which the mixture is homogeneous. nd refers to no indication of scattering species according to the equipment requirements; * refers to the mixture being macroscopically biphasic (turbid), not measured. b Mixtures containing 2 wt % dodecylamine c Mixtures containing 2 wt % oleic acid.
Table 2. Comparison between Light-Scattering Measurements on Mixtures of Synthetic Diesel Oil (SD), Ethanol, and Additives Obtained with Two Different Pieces of Equipmenta
“synthetic diesel oil”, and this mixture was used in some lightscattering measurements reported in the present investigation. The curves shown in Figure 1 separate homogeneous from biphasic systems with respect to temperature, in an upper critical solution temperature (UCST)-type behavior (miscibility increases with the temperature), with general features that agree well with previous reports for other diesel oils.2 We prefer not to refer to these curves as phase diagrams because these mixtures do not represent truly binary systems. An earlier screening on these mixtures by our group8 allowed the identification of the most efficient additives in increasing miscibility, as evaluated by the decrease caused in the transition (phase separation) temperatures, as exemplified by the results shown in Figure 1 for a mixture of dodecylamine and oleic acid. Light-scattering measurements were performed in mixtures containing these additives, as an attempt to clarify whether the increased miscibility in their presence should be ascribed to a cosolvent effect or to the formation of microemulsions. The first investigations were aimed at verifying the capability of lightscattering measurements to detect microemulsions in such mixtures. For this purpose, some mixtures of ethanol-diesel with and without additives were selected in order to provide systems that, at a certain temperature (25 °C), are macroscopically homogeneous without additives and others that should be biphasic but become homogeneous because of the presence of 2% dodecylamine or oleic acid. These results are summarized in Table 1. These data reveal that no scattering was detected in mixtures without additives that were above their phase transition temperatures (PTTs), as expected because they refer to totally miscible homogeneous systems. The only case investigated below its PTT was evidently biphasic and too turbid to be measured in this equipment because of the presence of large droplets. After a while, this system settled as two clear liquid phases. For systems with additives, again no scattering was detected when the respective ethanol-diesel mixture was already homogeneous, suggesting that the additive and ethanol are fully dissolved in the diesel oil. The only exception is a mixture containing 20% ethanol and 2% oleic acid, at 40 °C, which produced data corresponding to scattering particles of a radius equal to 0.7 nm, slightly below the nominal limit of this equipment. On the other hand, macroscopically homogeneous mixtures containing one of the two additives, which otherwise would be biphasic (because they are below their PTT), produced an indication of scattering particles with radii varying between 0.6 and 1.2 nm. These radii are rather small and below the values quoted for water-in-oil microemulsions (typically above 10 nm)12-14 and are also close to the detection limit of this instrument. Moreover, because of the color of diesel oil, the
measuring conditions are not the ideal conditions for the possibility of light absorption by the medium. In order to test whether these results are consistent, we performed a few measurements in a more powerful lightscattering instrument (see the Experimental Section for details). Because of the high-power laser used in this equipment, colored samples could not be investigated; hence, we used instead mixtures of ethanol and the above-mentioned synthetic diesel oil. Table 2 presents a comparison of results from light-scattering measurements on these mixtures with the two pieces of equipment, revealing that, within their respective uncertainties (which are also close), the determined radii are essentially the same. The only difference concerns a mixture containing 10% ethanol, which is macroscopically homogeneous, and for which the ALV equipment detected quite small scattering particles, with radii around 0.8 nm, which may arise from scattering due to the large hydrocarbon molecules contained in Nujol oil. The good agreement between both pieces of equipment confirms the capability of the bench equipment to detect scattering particles even of small radii. Additionally, the average radii values obtained for synthetic diesel oil are very close to the ones obtained with mixtures containing commercial diesel oil, validating its use to mimic properties of the more complex diesel oil. These results are consistent with microemulsion systems formed by ethanol-rich droplets, stabilized by the surfactant molecules within a hydrocarbon solvent. It is also worth mentioning that good reproducibility is observed with different samples of the same composition (as shown in Table 2), which is a confirmation of these values representing an equilibrium state, as expected for microemulsions. One striking point is the small size of these droplets, especially considering that their radii are smaller than the estimated length for fully stretched dodecylamine and oleic acid molecules (estimated as 1.6 and 2.1 nm, respectively).14 There are not many examples of microemulsions formed without water. The only publication located15 reports studies on mixtures of hydrocarbon and fluorocarbon oils in the presence of a surfac-
(12) Cebula, D. J.; Ottewill, R. H.; Ralston, J. J. Chem. Soc., Faraday Trans. 1 1981, 77, 2585. (13) Zulauf, M.; Eicke, H. F. J. Phys. Chem. 1979, 83, 480.
(14) Evans, D. F.; Wennerstrom, H. The Colloidal Domain: Where Physics, Chemistry, Biology, and Technology Meet; 2nd ed.; Wiley-VCH: New York, 1999.
mixture SD (10% EtOH) SD (20% EtOH + 2% dodecylamine) SD (20% EtOH + 2% dodecylamine + 2% oleic acid)
average radius/nmb
average radius/nmc
0.8 ( 0.2 1.5 ( 0.1 1.4 ( 0.2
nd 1.5 ( 0.2 1.7 ( 0.2
a nd ) no scattering detected. Uncertainty values quoted in the Table refer to deviations among replicates (two or three independent measurements). b Measurements performed with the ALV equipment. c Measurements performed with the Malvern instrument (see experimental section for details).
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Figure 2. Variation of the average radii of microemulsion droplets upon increasing the ethanol content. Symbols: (9) commercial diesel + 2% dodecylamine, (O) synthetic diesel + 2% dodecylamine, (4) commercial diesel + 2% oleic acid, (1) synthetic diesel + 2% oleic acid, (0) commercial diesel + 2% dodecylamine + 2% oleic acid, and (+) synthetic diesel + 2% dodecylamine + 2% oleic acid. All measurements performed at 25 °C. Uncertainty in radius values is estimated as ca. 10% (from their polydispersity).
tant-like molecule composed of hydrocarbon and fluorocarbon blocks. That binary mixture displays a UCST-type phase behavior, with phase transition temperatures that are reduced in the presence of the amphiphilic, very similar to the behavior observed for ethanol-diesel mixtures shown in Figure 1, including the additive effect. Light-scattering measurements performed in that system produced evidence for scattering particles with a radius of around 3 nm, which those authors explained as a consequence of significant interdigitation of the copolymer lyophobic moieties, in that case, the hydrocarbon chains. The same argument can be evoked in the present investigation, also indicating that the scattering domains are more of a micellar type (surfactant-rich, rather than ethanolrich). Small droplets (in the same range of values in Table 1) have also been reported to occur in microemulsion systems containing only traces of water,13 as is the case in the present investigation. Other systems with small droplets are water-insupercritical-CO2 microemulsions,16 for which radii in the range of a few nanometers have been reported. Again, these values are close to the extended lengths of the surfactant molecules employed. With respect to the present results, one has to bear in mind that a significant fraction of ethanol present in the mixture is miscible with hydrocarbon (both commercial and synthetic diesel)sfrom the miscibility curves shown in Figure 1, this amount can be estimated as ca. 10-15 vol %, at 25 °C. These data cannot be directly extracted from these curves because they are not true binary phase diagrams because the hydrocarbon component is, in fact, a mixture of other compounds. Therefore, the results presented in Tables 1 and 2, represent direct evidence that small scattering particles are present in ethanol-hydrocarbon (commercial or synthetic diesel)-additive mixtures. These particles should be constituted by the nonsoluble portion of ethanol stabilized because of interaction with the surfactant-like additives. Dissolved in these particles, there (15) Lo Nostro, P.; Ku, C. Y.; Chen, S. H.; Lin, J. S. J. Phys. Chem. 1995, 99, 10858. (16) Ryoo, W.; Webber, S. E.; Johnston, K. P. Ind. Eng. Chem. Res. 2003, 42, 6348. Xu, B.; Lynn, G. W.; Guo, J.; Melnichenko, Y. B.; Wignall, G. D.; McClain, J. B.; DeSimone, J. M.; Johnson, C. S. J. Phys. Chem. B 2005, 109, 10261.
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Figure 3. Variation of the average radii of microemulsion droplets upon increasing the additive content. Symbols: (9) commercial diesel + dodecylamine, (O) synthetic diesel + dodecylamine, (4) commercial diesel + oleic acid, (1) synthetic diesel + oleic acid, (0) commercial diesel + dodecylamine + oleic acid, and (+) synthetic diesel + dodecylamine + oleic acid. All measurements performed at 25 °C. Uncertainty in radius values is estimated as ca. 10% (from their polydispersity).
should also be a small amount of water that comes with the hydrocarbons and alcohol. This amount, however, should be quite small, considering that especially dried ethanol, containing less than 0.5 wt % water, has been used throughout. Hence, these data also contribute to the little information available on microemulsion formation in water-poor (or water-less) mixtures. More importantly, however, they constitute, to the best of our knowledge, the first direct evidence of microemulsion formation in diesel oil (or hydrocarbons), ethanol, and additives mixtures. Mixtures of the so-called E-diesel are usually referred to as microemulsions, mostly for the need for a surfactant-like additive, their optical transparency, and thermodynamic stability (normally inferred by their persistence without phase separation), which are characteristics of microemulsions. However, the same features could have been ascribed to a cosolvent effect of the additive acting to help dissolve the ethanol. In one of the investigations dealing with the additive-induced miscibility of such mixtures, Schwab and co-workers7 reported that fatty acids, alkylamines, and their mixtures are capable of forming dieselethanol microemulsions based on viscosity measurements which were compared to the behavior predicted for dispersions, as a function of the water content. They also observe homogenization of this mixture by using butanol as an additive, in this case, without firmly proposing microemulsion formation, which leaves room for doubts as to whether this could be alternatively ascribed to a cosolvent effect. More recently, Fernando and Hanna reported investigations on ethanol-diesel blending with biodiesel (soybean oil methyl esters),6 mapping their phase diagrams and identifying homogeneous regions. These were ascribed as microemulsions, though some of the evidence produced, such as signs of Ostwald ripening, is characteristic of emulsions, systems that are not in thermodynamic equilibrium. Anyway, these studies illustrate cases where homogeneous regions were identified in certain ethanol-diesel-additive mixtures, and these were assumed to be microemulsions. Effects of Varying Mixture Composition and Temperature. Increasing the amount of ethanol in the mixtures of both synthetic and commercial diesel oils leads to an increase in the radii of the scattering particles detected using the Malvern equipment, as shown in Figure 2. This increase is consistent with the typical behavior observed for water containing microemulsions, at a fixed surfactant concentration, as reported, for
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Figure 4. Temperature effect on the average radii of microemulsion droplets. Symbols: (9) commercial diesel + 2% dodecylamine, (O) synthetic diesel + 2% dodecylamine, (1) synthetic diesel + 2% oleic acid, and (0) commercial diesel + 2% dodecylamine + 2% oleic acid. Measurements performed with mixtures containing 20 vol % ethanol. Uncertainty in radius values is estimated as ca. 10% (from their polydispersity).
instance, by Zulauf and Eicke13 and Eicke and Rehak.17 For a fixed amount of surfactant, as is the case in Figure 2, this can be understood as a result of the necessity to incorporate more of the polar solvent in the droplets, at a rather fixed total interfacial area, which leads to an increase in their radii. When the same reasoning is followed, the effect of increasing the surfactants’ concentration at a fixed ethanol content is, therefore, opposed, as confirmed by the decrease in the determined radii shown in Figure 3. A temperature increase also leads to a decrease in the droplets’ radii, as shown in Figure 4. The miscibility curves shown in Figure 1 reveal that miscibility between these hydrocarbons and ethanol increases with temperature. For instance, the maximum amount of ethanol miscible with commercial diesel oil increases from ca. 10 vol % at 10 °C to ca. 20 vol % at 30 °C. Therefore, in a system at fixed composition, an increase in temperature leads to a decrease in the amount of ethanol that is not miscible with the hydrocarbon, consequently decreasing the volume of this solvent to be contained within the microemulsion droplets, which causes a decrease in their radii, as previously discussed when analyzing the results presented in Figure 2. (17) Eicke, H. F.; Rehak, J. HelV. Chim. Acta 1976, 59, 2883.
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Comparison between Commercial and Synthetic Diesel Samples. One interesting comparison that the present results allow is between microemulsions formed with commercial versus synthetic diesel oils, especially for assessing possible contributions from surface-active components present in samples of commercial diesel oil, which may affect microemulsion formation or stability. The comparison of samples with similar compositions (in terms of ethanol and additive content) in Figures 2-4 does not allow a discrimination of distinct different trends between microemulsions formed by commercial and synthetic diesel oils, at least in terms of their radii. This observation is of relevance with respect to the proposition of a synthetic mixture that may mimic not only the macroscopic miscibility of diesel oils with ethanol (as shown in Figure 1) but also their capacity of forming thermodynamically stable microemulsions. Conclusion In summary, the present results constitute a direct evidence for the formation of microemulsions in mixtures of ethanol, diesel oil (or synthetic diesel), and additives. This mechanism explains the capacity of certain additives to produce homogeneous mixtures of these, otherwise immiscible, liquids. Trends observed for variation in the droplets’ radii with respect to changes in the ethanol and additive content as well as changes in temperature agree with those reported for other microemulsion systems. Moreover, no diverging trend was observed when comparing the radii of microemulsions formed with commercial diesel with those determined with synthetic diesel, suggesting that this mixture of hydrocarbons may potentially reproduce other properties of the complex diesel oil mixture. Acknowledgment. The authors thank the Brazilian agency CNPq, through its CTPetro program, for financial support, as well as for a research productivity grant to W.L. and M.E.D.Z. E.J.S. thanks UFMT for a leave of absence, which allowed his participation in this investigation. We also thank Dr. Karin Schille´n, from Lund University, for the use of the light-scattering equipment under her responsibility and for important discussions on the results obtained there. We would like to dedicate this publication to the memory of Prof. Rahoma S. Mohamed, who first introduced us to this interesting theme of research. EF060264S
