Vegetable oil-based microemulsions using carboxylate-based extended surfactants and their potential as an alternative renewable biofuel Chodchanok Attaphong, Linh Do, David A. Sabatini ⇑ School of Civil Engineering and Environmental Science, The University of Oklahoma, 202 West Boyd Street, Norman, OK 73019, USA Institute for Applied Surfactant Research, The University of Oklahoma, Norman, OK, USA article info Article history: Received 10 June 2011 Received in revised form 23 September 2011 Accepted 20 October 2011 Available online 7 November 2011 Keywords: Microemulsion Phase behavior Fuel Ethanol Hybrid fuel abstract Recently, vegetable oils have received increased attention as a source of renewable fuels. However, the high viscosity of vegetable oils makes them problematic for long-term use in diesel engines. Therefore, vegetable oil reverse micelle microemulsions have been evaluated as an alternative method of reducing vegetable oil viscosity while eliminating the trans-esterification reaction and avoiding the unpurified glycerol and its environmental problems. Since extended-surfactants can form microemulsions with a high solubilization capacity and with a wide range of oils, extended-surfactant-based reverse micelle microemulsion systems were evaluated in this research. The objective of this research is to study the phase behavior of carboxylate-based extended surfactant microemulsion systems with the goal of formu- lating optimized systems for biofuel. It was found that carboxylate-based extended surfactants were able to form reverse micelle microemulsions without salt addition, thereby eliminating the phase separation and precipitation which had been observed with sulfate-based extended surfactants. In addition, fuel properties such as viscosity and temperature dependence were favorable and thus support the continued development of these surfactant-based fuel systems for use in diesel engines. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction The depletion of petroleum energy resources as well as their inherent environmental concerns have led to the pursuit of renew- able biofuels. Vegetable oils are being considered as such an alter- native fuel source. For example, during World War II, it was shown that several different vegetable oils could be used in diesel engine under normal operating conditions [1]. Research has evaluated the use of sunflower, safflower, soybean, cottonseed, rapeseed and peanut oils as potential renewable fuel sources [2–4]. Several characteristics of vegetable oils make them attractive for use as biofuel, including their renewable and non-toxic nature. Moreover, upon combustion vegetable oils emit less green house gases and other harmful emissions than fossil fuels [5,6]. Triglycer- ides are the primary components of vegetable oils causing the high viscosity of vegetable oils. Owing to the high viscosity, long-term use of neat vegetable oils in direct-injection diesel engines causes engine durability problems. For instance, fuel droplet size increases with viscosity which results in poor fuel atomization during the injection process. Furthermore, the long term operation of an en- gine with a viscous fuel results in deposit formation, ring sticking, and fuel dilution from excessive lubricant oils [4,7–10]. Therefore, four technologies have been evaluated to reduce the high viscosity of vegetable oils in order to overcome these problems: (1) vegeta- ble oil/diesel blends, (2) pyrolysis, (3) vegetable oil transesterifica- tion to fatty alkyl esters or biodiesel, and (4) vegetable oil-based microemulsifications [11]. Transesterification to produce biodiesel is the most common method to reduce the viscosity of vegetable oils because fuels from this process have properties comparable to No. 2 diesel (e.g. kine- matic viscosity, specific gravity, cetane number and gross heat of combustion). However, biodiesel also has many drawbacks includ- ing cold weather limitations due to relatively higher cloud point and pour point, and increasing nitrogen oxides (NO x ) in the ex- haust emissions [12,13]. In addition, biodiesel is formed by the transesterification reaction of triglycerides with alcohols in the presence of a catalyst and produces glycerol as a co-product. Since glycerol is expensive to purify or convert to a value-added product, it causes problems of disposal and environmental concern [14,15]. As an alternate approach, when produced from agricultural feedstocks, ethanol is a renewable energy source. For example, it can be made from very common crops such as sugar cane and corn. Therefore, ethanol–diesel blends, or E-diesel, can be used in diesel engines without modification. However, ethanol–diesel blends are limited by the fact that they are immiscible over a wide range of temperatures [1,2,16–18]. Surface active agents, or surfactants, 0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.10.048 ⇑ Corresponding author at: School of Civil Engineering and Environmental Science, The University of Oklahoma, 202 West Boyd Street, Norman, OK 73019, USA. Tel.: +1 405 325 4273; fax: +1 405 325 4217. E-mail addresses: c_attaphong@ou.edu (C. Attaphong), dieulinh@ou.edu (L. Do), sabatini@ou.edu (D.A. Sabatini). Fuel 94 (2012) 606–613 Contents lists available at SciVerse ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel